Atomically Thin-Layered Molybdenum Disulfide (MoS2) for Bulk

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Atomically Thin-layered Molybdenum Disulfide (MoS) for Bulk-Heterojunction Solar Cells 2

Eric Singh, Ki Seok Kim, Geun-Young Yeom, and Hari S. Nalwa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13582 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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ACS Applied Materials & Interfaces

Atomically Thin-layered Molybdenum Disulfide (MoS2) for Bulk-Heterojunction Solar Cells Eric Singh,†, ‡, § Ki Seok Kim,‡ Geun Young Yeom,*, ‡, § and Hari Singh Nalwa*,⊥ †

Department of Computer Science, Stanford University, Stanford, CA 94305, USA

‡

School of Advanced Materials Science and Engineering, Sungkyunkwan University,

2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, South Korea §

SKKU Advanced Institute of Nano Technology, Sungkyunkwan University,

2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, South Korea ⊥

Advanced Technology Research, 26650 The Old Road, Suite 208, Valencia, CA 91381, USA

Corresponding Authors

*E-mail: [email protected] (G. Y. Yeom). *E-mail: [email protected] (H. S. Nalwa).

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ABSTRACT Transition metal dichalcogenides (TMDs) are becoming significant due to their interesting semiconducting and photonic properties. In particular, TMDs such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), titanium disulfide (TiS2), tantalum sulfide (TaS2), and niobium selenide (NbSe2) are increasingly attracting attention for their applications in solar cell devices. In this review, we give a brief introduction to TMDs with a focus on MoS2; and thereafter, emphasize the role of atomically thin MoS2 layers in fabricating solar cell devices, including bulk-heterojunction, organic, and perovskites-based solar cells. Layered MoS2 has been used as the hole-transport layer (HTL), electron-transport layer (ETL), interfacial layer, and protective layer in fabricating heterojunction solar cells. The trilayer graphene/MoS2/n-Si solar cell devices exhibit a power-conversion efficiency of 11.1%. The effects of plasma and chemical doping on the photovoltaic performance of MoS2 solar cells have been analyzed. After doping and electrical gating, a power-conversion efficiency (PCE) of 9.03% has been observed for the MoS2/h-BN/GaAs heterostructure solar cells. The MoS2-containing perovskites-based solar cells show a PCE as high as 13.3%. The PCE of MoS2-based organic solar cells exceeds 8.40%. The stability of MoS2 solar cells measured under ambient conditions and light illumination has been discussed. The MoS2-based materials show a great potential for solar cell devices along with high PCE, however, in this connection, their long-term environmental stability is also of equal importance for commercial applications.

KEWORDS: molybdenum disulfide (MoS2), bulk-heterojunction solar cells (BHJ), photoactive layer, MoS2 electron- and hole-transport layers, stability of MoS2 solar cells

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INTRODUCTION The demand for renewal energy is rapidly growing due to the limited energy resources worldwide. Accordingly, new materials and technologies have been investigated for energy production, including the search for new semiconducting materials for harvesting clean energy from the sun for daily use in our life. Applications of solar energy range from communication systems to home electrification to automobiles and solar cookers, among many others. Platinum (Pt) and indium tin oxide (ITO or tin doped indium oxide) are the two most important materials that have been widely used as electrodes in developing different types of thin film solar cells, including silicon (Si), cadmium telluride (CdTe), gallium arsenide (GaAs), copper indium gallium selenide (CuInGaSe), copper-zinc-tin-chalcogenide (CZTSSe), organic, and dyesensitized solar cells. In addition, ITO thin films have been used as transparent semiconductors in mobile devices, flat screen displays, light-emitting diodes (LEDs), electroluminescent displays, infrared-reflective coating, sensors, and touch-screen technologies. The criticality has been identified of 62 metals and metalloids that include gold (Au), platinum (Pt), germanium (Ge), selenium (Se), tellurium (Te), gallium (Ga), chromium (Cr), and tungsten (W) for use in electronics, optoelectronics and photonics based industries.1 Although minuscule quantities of such metals are initially required in developing technologies, large-scale commercial production may face risks, due to their cost-effectiveness, depletion of resources, supply risks, toxicity, and other environmental concerns. For example, metals of criticality such as Ga, Cd, Se, Te, Pt, and In are of significant importance to the growth of solar cell industries. In particular, indium tin oxide (ITO) is one of the most conventional materials used in all types of semiconductor devices, as well as in fabricating solar cells. Indium (In) is a critically important and costly metal. Besides their scarcity, both Pt and In are expensive, need high temperature for processing, and are 3



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sensitive to corrosive chemicals. Therefore, new eco-friendly inexpensive electrode materials with low temperature processability, good mechanical strength, low toxicity, and good environmental stability are of great demand in solar cell industries. To meet these challenges, new organic and inorganic electrode materials have been explored as a replacement for these critically important metals, such as ITO in semiconductor industries, and Pt counter electrodes in dye-sensitized solar cells (DSSCs). With the advent of nanotechnology in the past decade, carbon nanomaterials, such as fullerene derivatives, carbon nanotubes, graphene, and their based composites, have been extensively explored for energy applications.2 After atomically layered graphene film was separa ted from graphite by mechanical exfoliation technique in 2004, two-dimensional (2D) graphene has attracted much attention. Novoselov et al.3 reported room temperature carrier mobilities of 10,000 cm2/Vs for few-layer graphene, and carrier mobilities of 15,000 cm2/Vs at 300 °K and 60,000 cm2/Vs at 4 °K for multilayer graphene. Nair et al.4 measured 97.7% optical transparency for a single-layer graphene, which reduces to 90.8% for 3-layer graphene, attributing 2.3% opacity to each graphene layer. Lee et al.5 reported a breaking strength of 42 newtons per meter (N/m) and Young’s modulus of 1.0 terapascals (TPa) for free-standing monolayer graphene film, evidencing graphene as one of the strongest materials. Balandin et al.6 measured room temperature thermal conductivity of single-layer graphene as high as 5300 W/mK suggesting graphene superiority over carbon nanotubes in thermal conduction. A sheet resistance of ~125 Ω/sq for monolayer of graphene, and 30 Ω/sq for 4-layer of graphene with 90% optical transparency was measured by Bae et al.7 Both the optical transmittance and the sheet resistance of graphene were found to decrease with increasing number of graphene layers. Therefore, the combination of high carrier mobilities, high optical transparency, low sheet resistance, and high 4


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mechanical strength makes graphene very promising for applications in solar cell devices. Graphene/n-silicon solar cells exhibit a power-conversion efficiency (PCE) of 10% for chemically doped graphene. The PCE further increases to 12.4% with oxide thickness of 15 Å, and to 15.6% after applying an antireflective coating.8 After a decade of research, graphene-base d solar cells have achieved sufficiently high PCE to be potentially used in solar cell devices. The graphene and graphene-based materials have a very wide range of applications in nanoelectronics,9,10

photonics,11,12

solar

cells,13-15

fuel

cells,16

supercapacitors,17-21

photocatalysts,22 sensors,23-26 and medical fields.27,28 The unique properties and broad range of applications of graphene have generated interest in exploring transition metal dichalcogenides (T MDs), which exhibit similar lamellar structure to that of graphite. One interesting difference between graphene and TMDs is the band gap. Graphene has no band gap; however, it can be introduced by applying nanostructuring, as well as different chemical and physical techniques. On the other hand, TMDs have a band gap, exhibit very interesting electrical and optical properties, and therefore these materials are suitable candidates for applications in electronic and optoelectronic devices. In particular, among the various TMD materials that have stable twodimensional crystal structures,29 such as molybdenum diselenide (MoSe2), molybdenum disulfide (MoS2), tungsten diselenide (WSe2), and tungsten disulfide (WS2), MoS2 has been intensively studied, because of its distinctive optical, electronic, and catalytic characteristics. MoS2 has covalent bonds between S atoms and M atoms, while the layers of MoS2 are bound to each other by van der Waals forces.30-33 In addition, the electronic properties significantly change depending on the number of MoS2 layers, by showing increased band gap energies from 1.29 (multilayer MoS2) to 1.9 eV (monolayer MoS2), and the change of band gap from indirect band gap to direct band gap as the layer number decreases.34-36 The variation of band gap energy and 5



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the change of band gap by varying the MoS2 layer thickness make the few-layer MoS2 film extremely beneficial as a photovoltaic material for absorbing solar energy. The first part of this review provides an introductory reference to solar cells, while the second part gives a general introduction to TMDs with an emphasis on synthesis, Raman spectroscopy, and the photoluminescence behavior of single and few-layers MoS2. The later parts highlight the role of single-layer and multilayer MoS2 in fabricating bulk-heterojunction (BHJ) solar cells, including their uses as photoactive layers, transparent electrodes, MoS2 based organic- and perovskite-solar cells, the effect of thermal annealing and doping on the photovoltaic performance of MoS2 solar cell devices, and their environmental stability. In summary, challenges in developing commercially viable MoS2 solar cells are outlined.

TRANSITION

METAL

DICHALCOGENIDES

(TMDs)

AND

MOLYBDENUM

DISULFIDE (MOS2) TMDs, which are composed of MX2, where M is a transition metal (Mo, W, Ti, Zr, Hf, V, Nb, Ta, Tc, Re, Pd, Pt, etc.), and X is a chalcogen (S, Se, Te), such as MoS2, WS2, MoSe2 and WSe2, form 2D layer structures, and are abundant in nature. In an MX2 monolayer structure (X-M-X), an atomic layer of transition metal (M) is sandwiched between two chalcogen (X) atomic layers, where the transition metal atom (M), such as Mo or W, is covalently bonded with chalcogen atoms (X), such as S, Se, or Te. Van der Waals interactions occur between two adjacent MX2 layers of TMDs.29,37-41 From an electrical point of view, MoS2 and WS2 are semiconductors, and WTe2 and TiSe2 are semimetals, while VSe2 and NbS2 are metals in their bulk crystalline forms.40

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One of the most common and convenient methods for preparing single and few-layer TMD materials is mechanical exfoliation from the bulk crystals of TMDs,42-44 which is a top-down approach that involves peeling off the layers from bulk crystalline material. This micromechanical cleavage technique could easily provide single and few-layer TMDs with minimum effort, compared with other chemical and electrochemical methods. Chemical vapor deposition (CVD) has been widely used as a bottom-up approach to precisely deposit consecutive monolayer and multilayers of TMDs of desired thickness.45-47 Other methods for preparing layered TMDs include chemical,40,48,49 lithium (Li) intercalation,50-52 and ultrasonicassisted liquid exfoliation in organic solvents.53-57 Niu et al.58 developed a salt-assisted liquidphase exfoliation approach for preparing single- and few-layer TMDs, such as MoS2, MoSe2, WS2, and WSe2. This method offers TMDs sheets in large quantity within a short time, and where 65% of the TMD sheets consist of 1-5 layers. Raman, XRD, atomic force microscopy (AFM), and transmission electron microscopy (TEM) confirmed the formation of single and few-layer films. Figure 1 shows the TEM images that reveal the single crystalline features of MoS2, MoSe2, WS2, and WSe2. Atoms are symmetrically arranged in a hexagonal structure with a d-spacing of 2.7 Å for MoS2, 2.8 Å for MoSe2, 2.7 Å for WS2, and 2.8 Å for WSe2, corresponding to the (100) planes.

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Figure 1. TEM images of few-layer TMD sheets prepared by a NaCl-assisted liquid-phase exfoliation approach: (a) MoS2, (c) MoSe2, (e) WS2, and (g) WSe2; insets show the SAED patterns. (b), (d), (f) and (h) are their corresponding HR-TEM images. Reprinted with permission from ref 58. Copyright (2014) Wiley-VCH. Photoluminescence (color lines) and absorption (gray lines) spectra of mechanically exfoliated single-layer MoS2, MoSe2, WS2, and WSe2. The photoluminescence peaks of TMDs generally coincide with the absorption peak. All materials show 10% absorbance at the band gap. Photoluminescence spectra were measured with an excitation wavelength of 472 nm. Absorption spectra were recorded by differential reflectance measurement. Reprinted with permission from ref 59. Copyright (2013) American Chemical Society. 8


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The photoluminescence (PL) and absorption spectra of single-layer MoS2, MoSe2, WS2, and WSe2 (Figure 1) were measured by Eda and Meier.59 The single-layers of TMDs were mechanically exfoliated from their counterpart bulk crystals, and the measurements showed that the single-layers of TMDs are direct band gap semiconductors, while the bulk crystals of TMDs are indirect band gap semiconductors. In addition, the band gap energy of TMDs becomes greater with decreasing number of layers. Therefore, photons are efficiently absorbed and emitted by the single layers of TMDs. Tongay et al.60 reported the bandgap energies of TMD monolayers using photoluminescence peaks as 1.84 eV for MoS2, 1.56 eV for MoSe2, and 1.65 eV for WSe2; therefore, the bandgap energies vary, depending on the particular monolayer TMD. Wang et al.61 reported a direct band gap of 1.48 eV (840 nm) of CVD-grown monolayer MoSe2 by photoluminescence measurements with a laser wavelength of 514.5 nm and n-type behavior, with a mobility of 50 cm2/Vs, and on/off current ratio of 106. Tonndorf et al.62 reported an indirect band gap of 1.2 eV (1.03 µm) for bulk WSe2, while strong photoluminescence peaks from monolayer, bilayer, and trilayer WSe2 were recorded at 1.65 eV (752 nm), 1.54 eV (806 nm), and 1.46 eV (849 nm), respectively. The bulk MoSe2 showed an indirect band gap at 1.1 eV (1.13 µm), while the photoluminescence peaks from monolayer and bilayer appeared at 1.57 eV (792 nm) and 1.54 eV (807 nm), respectively. The photoluminescence intensities from monolayer MoSe2 were 10 - 20 times stronger compared to bilayer MoSe2. The photoluminescence emission peak from trilayer MoSe2 exhibited two peaks at 1.53 eV (812 nm) and 1.35 eV (922 nm), while the photoluminescence emission peak for monolayer WS2 was observed at 1.97 eV. McCreary et al.63 reported in-plane (E12g at 357.5 cm−1) and out-of plane (A1g at 419 cm−1) Raman modes for monolayer WS2, where the frequency difference (∆) between 9



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E12g and A1g modes was 61.5 cm−1 for monolayer WS2. For TMDs, the frequency difference (∆) between E12g and A1g modes in Raman spectroscopy decreases with decreasing number of TMD layers; therefore in general, the frequency difference is used as a measure of TMD thickness.

Figure 2. (a) Schematic representation of the synthesis of MoS2 thin layers on sapphire and SiO2/Si substrates. (b) Raman spectra of bilayer and trilayer MoS2 grown on a sapphire substrate. Argon (Ar) and argon+sulfur (Ar+S) in the figure indicate bilayer and trilayer MoS2 prepared in pure Ar and in Ar+sulfur mixture, respectively, in the second annealing process. (c) Out-of-plane (A1g) and in-plane (E12g) Raman peaks as a function of the number of layers for micromechanically exfoliated MoS2 layers. The peak frequency differences (∆ = A1g - E12g) have 10


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been used to identify the number of MoS2 layers. (d) Photoluminescence (PL) of trilayer MoS2 films prepared in Ar and Ar+S, where the PL intensity in Ar is very week compared with that in Ar+S (Excitation laser 473 nm). Reprinted with permission from ref 86. Copyright (2012) American Chemical Society.

Single and few-layers MoS2 can be prepared by a number of methods, including micromechanical

exfoliation,42-44,64

chemical

vapor

deposition,47,65-69

physical

vapor

deposition,70,71 ultrasonic-assisted liquid exfoliation,53-57,72 Li-intercalation exfoliation,50,52,73 hydrothermal synthesis,74 chemical and electrochemical processes,58,75 thermolysis of the precursors such as alkyldiammonium tetrathiomolybdates,76 sulfurization of molybdenum trioxide (MoO3) and Mo,77-79 sulfurization of pre-annealed Mo foil,80 and radio-frequency (RF) sputtering method.81-85 Liu et al.86 reported a new synthetic method for preparing large-area MoS2 thin layers on sapphire and silicon substrates from ammonium thiomolybdate solution in the presence of argon (Ar) and sulfur (S) at very high annealing temperatures. In the first step, the precursor ammonium thiomolybdate [(NH4)2MoS4] was dip-coated on either sapphire or SiO2/Si substrate, followed by annealing at 500 oC for 1 hr in a gas mixture of argon/hydrogen (Ar/H2). Bilayer and trilayer MoS2 films were prepared by annealing at 1000 oC for 30 min either in pure Ar atmosphere, or in the presence of a gaseous mixture of Ar and sulfur. Figure 2 shows (a) a schematic representation for preparing MoS2 thin layers either on sapphire or SiO2/Si substrate, (b) the Raman spectra of bilayer and trilayer MoS2 grown on a sapphire, (c) Raman peaks of A1g and E12g modes and their difference as a function of MoS2 layers, and (d) PL of trilayer MoS2 films prepared in Ar and Ar+S. Figure 2b shows that the MoS2 films exhibit E12g 11



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mode at 383.1 cm-1 and the A1g mode at 405.4 cm-1 (∆ ~22.3 cm-1) for trilayer MoS2, and E12g mode at 384.6 cm-1 and the A1g mode at 404.4 cm-1 (∆ ~20 cm-1) for bilayer MoS2; and therefore, a smaller frequency difference (∆) for thinner MoS2 thickness. In addition, the MoS2 layers grown on sapphire show higher Raman and PL intensities compared with SiO2/Si substrate, due to their superior crystalline quality. The sulfur addition during the 1,000 oC annealing process noticeably enhanced the crystallinity of MoS2 layers, and can play a role in removing extra oxygen species, hence inhibiting oxidation. The sulfur annealing has a significant effect on the properties of field effect transistors (FETs), where the mobility increased from 10-2 to 4.7 cm2/Vs, and on/off current ratio increased from 2.4×103 to 1.6×105. The bottom-gate FETs fabricated with MoS2 trilayers show n-type behavior, where MoS2 grown on sapphire substrate has higher electron mobility and on/off ratios, compared with MoS2 grown on SiO2/Si substrates. Note that MoS2 films grown at 500 oC thermolysis do not exhibit any on/off current ratio.

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Figure 3. (a) 3D representation of MoS2 structure (S-Mo-S layer), where a single layer of Mo atoms (denoted in black) is sandwiched between two layers of sulfur (S) atoms (denoted in yellow). (b) Atomic force microscope (AFM) image of a monolayer MoS2, and (c) AFM height profile of a monolayer MoS2 corresponding to 6.5 Å interlayer distance. Single layers can be

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exfoliated using a micromechanical cleavage method. Reprinted with permission from ref 87. Copyright (2011) Nature Publishing Group.

Although MoS2 has been studied earlier with bulk and exfoliated monolayers, much research interest in this 2D material was generated when Radisavljevic et al.87 reported the high room temperature carrier mobility of 200 cm2/Vs with on/off current ratios of 108 with single-layer MoS2 FETs. Another interesting point from the electronics and optoelectronic viewpoints is that MoS2 can be transited from a direct band gap semiconductor to an indirect band gap semiconductor by controlling the layer thickness from monolayer to multilayer, respectively. Figure 3 shows 3D atomic representation of MoS2 structure (S-Mo-S layer), atomic force microscope (AFM) imagery, and the AFM height profile of a monolayer MoS2 that corresponds to 6.5 Å interlayer distance. Single layers of MoS2 were exfoliated using a micromechanical cleavage method. Other research88 also performed optical and AFM imaging on mechanically exfoliated monolayers of WSe2 and NbSe2, in addition to monolayer MoS2, deposited on 270 nm SiO2 substrates. The thicknesses of 6.75 Å for MoS2, 6.7 Å for WSe2, and 6.86 Å for NbSe2 were measured from the AFM imaging, which correspond to the interlayer distances in dichalcogenide crystals.

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Figure 4. (a) Optical images of mechanically exfoliated monolayer (1L), bilayer (2L), and trilayer (3L) of MoS2 on SiO2/Si substrates. (b) Raman spectra of 1L, 2L, and 3L MoS2. (c) Photoluminescence spectra of 1L, 2L, and 3L MoS2. The PL peak arising from indirect band gap transition is shown as I, while those appearing due to the direct band gap transition are shown as A and B peaks. Reprinted with permission from ref 89. Copyright (2013) American Chemical Society.

Mouri et al.89 reported the PL of a few-layer MoS2 and its tunable PL using chemical doping, and where 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 7,7,8,815



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tetracyanoquinodimethane (TCNQ) were used as p-type dopants, and nicotinamide adenine dinucleotide (NADH) as an n-type chemical dopant. Figure 4 shows optical images of mechanically exfoliated monolayer (1L), bilayer (2L), and trilayer (3L) of MoS2 and their Raman and PL spectra. The frequency differences (∆) between A1g and E12g Raman modes, which depend on the number of MoS2 layers, were found to be 19.2 cm−1 for monolayer (1L), 22.3 cm−1 for bilayer (2L), and 24.4 cm−1 for trilayer (3L) MoS2. (In fact, the frequency difference values measured for the specific MoS2 layers are higher in Figure 4 than those in Figure 2. Many researchers observed similar results to those in Figure 4; therefore, the results in Figure 4 are more accurate than those in Figure 2.) The PL spectra of monolayer (1L), bilayer (2L), and trilayer (3L) of MoS2 show two peaks: a high intensity peak around 1.85 eV (denoted by A), and a low intensity peak around 2.05 eV (denoted by B). These A and B photoluminescence peaks are related to the direct band gap transition at the K point. The weak peaks (denoted by I) are related to the indirect band gap transition in bilayer and trilayer MoS2, which disappears in the PL spectrum of monolayer MoS2, due to the transition of indirect to direct band gap energy. Mouri et al.89 found that the PL intensity of monolayer MoS2 was drastically increased by F4TCNQ and TCNQ dopants, while the PL intensity was found to decrease by NADH n-type dopant, showing that both the extraction, as well the injection of electrons in monolayer MoS2 are possible via chemical doping, which can tune the electrical and optical properties of atomic thin MoS2. The evolution of the electronic structure and optical properties of MoS2 was systematically studied by Mak et al.34 using the optical absorption, PL, and photoconductivity spectroscopy as a function of the number of MoS2 layers. The PL spectrum showed a direct band gap of ∼1.9 eV

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in monolayer MoS2, and an indirect band gap of ∼1.6 eV in bilayer MoS2, which also coincided with the onset of photoconductivity. The monolayer MoS2 strongly emits light, and its luminescence quantum efficiency was enhanced more than 1,000 times compared with the bulk MoS2. The monolayer MoS2 showed no photoconductive response well below the direct gap transition, but an abrupt increase in photoconductivity appeared only near the direct band gap, again confirming monolayer MoS2 as a direct band gap semiconducting material, while bilayer and few-layer MoS2 as indirect band gap semiconductors. Han et al.90 investigated the electronic structures of monolayer and bilayer MoS2 using scanning photoelectron microscopy (SPEM). For SPEM measurements, monolayer and multilayer MoS2 films were obtained by mechanically exfoliating bulk 2H-MoS2 crystals, and deposition onto a Si substrate. The Raman frequency difference (∆) of 18.9 and 21.4 cm−1 between in-plane E12g and out-of-plane A1g vibration modes confirmed single-layer and double-layer MoS2, respectively. First-principles calculations along with SPEM measurements showed that the occurrence of a band gap transition caused by the direct band gap of 1L MoS2 (S-Mo-S) can be changed to an indirect band gap through interlayer van der Waals interactions only after supplementing additional MoS2 layers. The measurements evidenced that monolayer MoS2 is a direct band gap semiconductor (1.70 eV), but the bilayer MoS2 becomes an indirect band gap semiconductor like bulk MoS2 crystal, due to the induced van der Waals interactions. Bulk MoS2 was characterized as an indirect band gap semiconductor of 0.9 eV. There are three phases of MoS2: 1T-MoS2, 2H-MoS2, and 3R-MoS2. 2H-MoS2 phase is semiconducting, while the 1T-MoS2 phase is metallic and hydrophilic, with a contact angle of < 30°.75,91-95

MOS2 BASED HETEROJUNCTION SOLAR CELLS 17



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The monolayer MoS2, MoSe2, and WS2 can absorb incident sunlight up to 5−10% in the visible wavelength region as demonstrated by Bernardi et al.96, which is an order of magnitude higher sunlight absorption compared with 50 nm Si film or 15 nm thick GaAs film, therefore confirming the large absorbance in monolayers of TMDs. The single TMD monolayer can absorb as much incident sunlight as 50 nm Si film, and produce photocurrents as high as 2.0–4.5 mA/cm2, as calculated using density functional theory, GW approximation (G is Green’s function and W is the screened Coulomb interaction), and the Bethe-Salpeter approach. 1 nm thick Si, GaAs, and poly(3-hexylthiohene) (P3HT) generate photocurrents of 0.1−0.3 mA/cm2. The solar cells based on bilayer of MoS2/graphene (0.9 nm) and MoS2/WS2 bilayer (1.2 nm) were fabricated, where photoactive layers can attain power conversion efficiencies of 1.0 – 1.5%. High power densities of 2.5 MW/kg for 0.9 nm thick MoS2/graphene layers and 1.8 MW/kg for 1.2 nm MoS2/WS2 bilayer were recorded, compared to 2.5 KW/kg for 35 µm thick Si and 54 KW/kg for 1 µm thick GaAs. Britnell et al.97 also reported large optical absorption greater than 107 m-1 over the visible range, suggesting that 95% of the sunlight can be absorbed by a 300 nm thick TMD film. The hexagonal boron nitride (h-BN)/graphene/MoS2/graphene heterostructure enhances the optical field in the photoactive layer, and shows 10-fold increase in the photocurrent intensity. An extrinsic quantum efficiency (EQE) above 30% and photoresponsivity above 0.1 A/W were achieved in the TMD/graphene stacks. Jariwala et al.98 summarized the band gap and field effect mobilities for extensively studied organic and inorganic semiconductors along with semiconducting TMDs, in order to explore their potential for photovoltaic applications (Figure 5). Both direct band gap and field effect mobility values of monolayer TMDs fall within a desirable range. As discussed above, 2D materials show very unique optoelectronic characteristics suitable for fabricating TMD-based solar cells. Even though 18


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TMDs generally have narrow band gaps, MoS2 exhibits very interesting electronic and photonic properties, and its band gap varies from 1.29 to 1.90 eV, depending upon the number of MoS2 layers. A number of studies on solar-energy conversion, light emission, and other optoelectronic properties of TMDs have been conducted.99-109 In this section, the roles of monolayer and multilayer MoS2 films as a hole transport layer (HTL), passivation layer, or Schottky-barrier active layer in solar cells are discussed.

Figure 5. A comparison of the band gap and field effect mobility for important organic and inorganic semiconductors in regard to photovoltaic applications. Semiconducting TMDs exhibit a band gap near the Shockley-Quessier limit and high mobilities, which are important parameters to be considered for applications in photovoltaic cells. Reprinted with permission from ref 98. Copyright (2014) American Chemical Society.

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Schottky-barrier solar cells were fabricated by Shanmugam et al.110 using a stack of layerstructured MoS2 nanomembranes as a photoactive layer that exhibits photo-absorption in the 350–950 nm range. The MoS2 nanomembrane-based ITO/MoS2/Au solar cells showed a PCE of 0.7% for 110 nm thick MoS2, and a PCE of 1.8% for 220 nm MoS2 stacked structures. The same research team111 also fabricated bulk-heterojunction (BHJ) solar cells using MoS2/titanium dioxide (TiO2) nanocomposite and poly(3-hexylthiophene) (P3HT) active layers. The ITO/TiO2/MoS2/P3HT/Au stacked structure exhibited a JSC of 4.7 mA/cm2, open circuit voltage (VOC) of 560 mV, and PCE of 1.3%. The TiO2/MoS2/P3HT interfaces affected the performance of solar cells as a result of the interfacial recombination effect. The heterojunction solar cells using n-type monolayer MoS2 on the p-Si substrate were developed by Tsai et al.112 MoS2 monolayers were fabricated by the CVD method. Figure 6 shows the structure and fabrication procedure of an Al/MoS2/p-Si/Cr-Ag heterojunction solar cell, AFM height profile, Raman spectrum, absorbance, and PL spectra of a monolayer MoS2. The Raman spectrum of monolayer MoS2 showed an E12g peak at 384.5 cm−1 and an A1g peak at 404.7 cm−1 separating two vibrating modes by 20.2 cm−1, thereby confirming the formation of monolayer MoS2. A height of 0.65 nm detected by AFM also supported a monolayer structure of MoS2. The absorbance spectrum of monolayer MoS2 deposited on glass substrate showed a direct optical band gap of 1.9 eV for monolayer MoS2. PL measurements under an excitation wavelength of 473 nm exhibited a peak at 680 nm. After depositing monolayer MoS2 on Si surface, the work function decreased from 4.45 eV for p-Si to 4.20 eV for MoS2. The energy difference between the Fermi level and the valence band was measured by ultraviolet photoemission spectroscopy (UPS) as 0.5 eV for p-Si, and 1.70 eV for monolayer MoS2. The current density-voltage (J-V) measurement of the MoS2/p-Si heterojunction solar cell resulted in 20


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a VOC of 0.41 V, photocurrent density (JSC) of 22.36 mA/cm2, fill factor (FF) of 57.26%, and PCE of 5.23%, compared to the VOC of 0.38 V, JSC of 21.66 mA/cm2, and FF of 56.02% for the Al/pSi Schottky solar cells.

Figure 6. (a) Device sketch of the Al/MoS2/p-Si/Ag heterojunction solar cell. (b) Fabrication process of the Al/MoS2/p-Si/Cr-Ag heterojunction solar cell. (c) AFM height profile, and (d) Raman spectrum of a monolayer MoS2. (e) Optical absorbance spectrum of a monolayer MoS2 on a glass substrate. (f) PL spectra of as-grown and transferred MoS2 films. Reprinted with permission from ref 112. Copyright (2014) American Chemical Society.

MoS2/Si p-n junctions were prepared by Hao et al.113 by depositing MoS2 thin films on ptype Si substrate using a magnetron sputtering technique. The Raman spectra showed E12g and A1g modes, and the current density versus voltage (J-V) measurement recorded a turn-on voltage 21



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of 0.2 V for MoS2 thin films. The MoS2/Si p-n junctions showed photovoltaic properties with Jsc of 3.2 mAcm−2 and Voc of 0.14 V, FF of 42.4%, and PCE of 1.3% under light illumination of 15 mW cm−2. The mechanism of photovoltaic behavior was suggested in connection with the energy band structure of formed MoS2/Si p-n junctions. Jiao et al.114 studied the role of MoS2 as an interfacial layer in graphene/Si solar cells, by varying the annealing temperature and thickness of MoS2. The PCE increased from 2.3 to 4.4% after MoS2 film was annealed at low temperature of 80 oC; however, the PCE dropped to 0.6% when annealed at 200 oC. The PCE value increased with decreasing MoS2 film thickness, but was saturated at 2 nm. The MoS2 films annealed at 80 o

C formed a type II structure that facilitated hole transport, whereas MoS2 films annealed at 200

o

C formed a valence band mismatch, due to the increased work function of MoS2 (which is

related to the phase transformation from 1T-MoS2 at 80 oC to 2H-MoS2 at 200 oC). The PCE was also found to increase to 6.6% after silicon surface passivation. The graphene/MoS2/n-Si solar cells were fabricated by Tsuboi et al.115 with inserting CVDgrown MoS2 thin film between graphene and n-Si. MoS2 thin film was used for passivation, as well as an electron-blocking/hole-transporting layer. CVD-grown MoS2 multilayer film and single layer graphene were deposited on n-type Si substrate. Figure 7e,f show the current density–voltage (J–V) curves of a monolayer-graphene/MoS2/n-Si, monolayer-graphene/n-Si, and monolayer-, bilayer-, and trilayer- graphene/MoS2/n-Si based solar cells under AM 1.5 illumination conditions. The characteristics of the MoS2 layers used for the CVD-grown MoS2 are also shown in Figure 7a-d. The MoS2 film was 17 nm thick in the solar cell devices. The suggested band diagrams of graphene/n-Si based solar cells with and without MoS2 layer are shown in (Figure 7g,h). The graphene/n-Si based solar cells without MoS2 layer showed a lower PCE of 0.07% compared with the high PCE of 1.35% for monolayer-graphene/MoS2/n-Si based 22


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solar cells, showing that photovoltaic performance was increased by the insertion of MoS2 film. The thickness of MoS2 layer, as well as the numbers of graphene layers in the graphene/MoS2/nSi solar cells, was changed. The PCE of graphene/MoS2/n-Si solar cells was found to increase as the number of graphene layers increased, and a PCE of 8.0% was measured for trilayer graphene. The optimized trilayer graphene/MoS2/n-Si based solar cells having 9 nm thick MoS2 layer exhibited the highest PCE of 11.1%. The MoS2 layer insertion into graphene/n-Si solar cells changed the band alignment, which positively impacted the photovoltaic performance.

Figure 7. (a) Raman spectra of CVD MoS2 films; two Raman peaks, that is, an in-plane (E12g at 383 cm−1) mode and an out-of-plane (A1g at 408 cm−1) mode are observed. (b) Optical absorption spectra; absorption peaks associated with A and B excitons are observed. (c) AFM image of an as-grown MoS2 films showing ~6.5 nm roughness due to assembly of small grains. (d) XPS data of the MoS2 film showing Mo and S peaks. The inset is the Mo 3d spectrum. (e) Current density– voltage (J–V) curves of a monolayer-graphene/MoS2/n-Si solar cell, where the gray line denotes 23



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data in dark, and the yellow line those under AM 1.5 illumination. Inset shows the J–V plot of a monolayer-graphene/n-Si solar cell under AM 1.5 illumination condition. (f) Current density– voltage (J–V) curves of monolayer-, bilayer-, and trilayer- graphene/MoS2/n-Si based solar cells, where the number of graphene layers was modified. The MoS2 film in the solar cell devices was 17 nm thick. Band diagrams of suggested solar cells (g) in the graphene/n-Si and (h) in the graphene/MoS2/n-Si solar cells. Reprinted with permission from ref 115. Copyright (2015) Royal Society of Chemistry.

The MoS2 thin films deposited on 3–5 nm thick SiO2 buffered p-type Si substrates (resistivity of 1.2–1.8 Ωcm) was used by Hao et al.116 to fabricate Pd/MoS2/(3~5nm)SiO2/Si/In solar cells. The photovoltaic properties of the solar cell were studied without, and with, a SiO2 buffer. The MoS2/Si junction solar cell shows a PCE of 1.4%; however, the PCE increased to 4.5% after the SiO2 buffer was incorporated, which is a 3-fold improvement on the reference solar cell device. The SiO2 buffer layer played an important role in enhancing the built-in field, and promoted the separation of photogenerated electron–hole pairs, which resulted in improved photovoltaic performance. Hao et al.117 also reported a 375% increase in the PCE of MoS2/Si hybrid solar cells via Pd chemical doping, due to the inclusion of Pd atoms in the MoS2 films. Typical semiconductor heterostructures are generally fabricated using epitaxial growth methods. These days, heterostructures have also been formed by the vertical stacking of 2D materials, such as graphene and related 2D materials. These heterostructural materials are bonded together by van der Waals forces, and have atomically sharp interfaces. Furchi et al.118 fabricated van der Waals heterojunction onto the Si/SiO2 substrate with mechanically exfoliated molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) monolayers. The heterojunction 24


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was electrically tunable, and by using the MoS2/WSe2 heterojuction, atomically thin heterojuction diode devices could be fabricated. By light illumination, charge transfer occurs across the planar interface, and the heterojuction device exhibited a photovoltaic effect. The J−V characteristics of the MoS2/WSe2 heterojunction were measured with incident optical power ranging between 180 and 6400 W/m2. A PCE of 0.2% and an external quantum efficiency (EQE) of 1.5% were estimated for the MoS2/WSe2 heterojunction. MoS2/cadmium sulfide (CdS) heterojunctions were prepared by CVD and chemical bath deposition (CBD) by Gu et al.119 The CdS films were deposited on fluorine tin oxide (FTO) substrate by CBD, and MoS2 films of 10 nm thickness were deposited by CVD. The MoS2/CdS heterojunction absorbs in a broad spectral range of 350 to 800 nm, which is an enhancement in the absorption of light compared to CdS. MoS2/CdS heterojunction solar cells show a Jsc of 0.227 × 10-6 A/cm2, Voc of 0.66 V, and FF of 22%. No PCE is reported for the MoS2/CdS heterostructure solar cells, but for monolayer MoS2/indium phosphide (InP) heterostructure solar cells, a PCE of 7.1% with a gate voltage of +6 V is reported by Lin et al.120 The CuInS2–CdSe quantum dot (QD) co-sensitized solar cells were fabricated by Barpuzary et al.121 with single crystalline ZnO nanowires using MoS2 nanosheets/Cu2ZnSnS4 (CZTS) microspheres as counter electrode. The CuInS2–CdSe QD solar cells exhibited better photovoltaic performance than that of bare CuInS2 and CdSe. The solar cell devices having counter electrodes based on 1.0 wt% of MoS2 contents in CZTS also showed better photovoltaic performance compared with bare CZTS, because MoS2 nanosheets create an electrical network between CZTS microspheres. A better band energy alignment between MoS2 and CZTS facilitated the charge transfer process, giving rise to high photovoltaic performance. Even though CZTS-based materials possess a high absorption coefficient (104 cm-1) and direct band gap of 1.5 25



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eV, and are therefore excellent absorber materials for solar cells,122 by using the prominent charge transfer network of MoS2 nanosheet and CZTS microspheres as a counter electrode, a PCE of about 4.6 % was obtained for the QD solar cell. However, in the case of CZTS-based solar cells, the formation of unwanted MoS2 layer with Mo back-contact between CZTS and Mo electrode during the annealing process can decrease the solar cell performance, which is one of the important factors causing lower PCE compared to that of CdTe and Cu(In,Ga)Se2-based solar cells. Yang et al.123 studied the role of MoS2 layer and defects in CZTS solar cells by using annealed Mo back-contact layers. The MoS2 layer formation was suppressed by the increase of oxygen content in the Mo layer. Furthermore, the defect density in the absorber–buffer interface as well in the absorber layer was found to decrease with an increase in Na diffusion during initial deposition of the absorber precursor. The JSC, VOC, and PCEs all improved with the increase of annealing temperature of the Mo layer, indicating the performance of CZTS solar cells can be enhanced by suppressing MoS2 layer formation, as well by increasing the Na content in the Mo layer. Liu et al.124 suggested that a layer of TiB2 thin films between the absorber and the Mo back-contact in CZTS solar cells prevents the formation of MoS2 layer. TiB2 thin films increase the PCE of solar cells by reducing the series resistance. The use of TiB2 thin films leads to the degradation of absorber crystal quality; therefore, control of TiB2 layer thickness is important for minimizing MoS2 thickness, and using a large grain CZTS absorber. Bulk black phosphorus is another interesting 2D layered material that shows a direct band gap of 0.3 eV, and a mobility as high as 10,000 cm2/Vs.125-127 Like TMDs, a stack of phosphorene monolayers in black phosphorus is bound together by van der Waals interactions. Deng et al.128 prepared 2D p-n heterojunctions using p-type black phosphorus prepared by mechanical exfoliation, and n-type monolayer MoS2 deposited by CVD on a silicon wafer, which 26


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exhibit a photodetection responsivity of 418 mA/W at 633 nm wavelength, 100 times higher compared with a black phosphorus phototransistor FF of 50% with an external quantum efficiency (EQE) of 0.3%.

THE EFFECT OF DOPING ON MOS2 SOLAR CELLS Chemical doping alters the electronic and optical properties of semiconducting materials, and as a consequence, impacts the performance of heterojunction solar cells.13,129 The PCE of monolayer graphene/n-Si Schottky junction solar cells increased from 1.9 to 8.6% after doping the graphene with bis(trifluoromethanesulfonyl)amide [(CF3SO2)2NH] (TFSA) because of charge transfer phenomena, as investigated by Miao et al.130 A similar behavior was observed by Li et al.131 for multilayer graphene (MLG)/n-Si solar cells. When MLG film was doped with SOCl2 and HNO3 vapors, the sheet resistance (Rs) of MLG films significantly decreased from 735 to 405 Ω/sq and from 550 to 210 Ω/sq, and the PCE values of the MLG/n-Si solar cells increased from 5.52 to 8.94 and to 9.27%, respectively. Likewise, the PCE of a CVD-grown graphene/n-Si Schottky heterojunction solar cell increased by 243% from an initial 2.45 to 5.95% after SOCl2 doping, as observed by Cui et al.132 In another study by Wu et al.133 after HNO3 doping of graphene for the improvement of graphene conductivity, 4-layer-graphene/P3HT/CH3−SiNWs and 5-layer graphene/P3HT/SiNH based solar cells showed PCE values of 9.73 and 10.34%, respectively.

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=

Figure 8. A comparison of CHF3 plasma-doped (treated) and untreated MoS2 solar cells in terms of (a) Voc, (b) Jsc, (c) FF, and (d) PCE data, where CHF3 plasma-treated MoS2-based devices are denoted by red circles, and untreated are denoted by blue triangles. The Voc, Jsc, FF, and PCE parameters for plasma treated and untreated solar cells are plotted as functions of MoS2 thickness. In any range of MoS2 thickness, the plasma-treated MoS2-based devices show higher 28


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photovoltaic response performance than that of untreated solar cells. (e) XPS peaks (Mo 3d5/2 and Mo 3d3/2) of the plasma-treated/untreated (pristine) MoS2 surfaces. The CHF3 plasma treated MoS2 shows the redshift of Mo 3d5/2 indicating the p-doping characteristics of the MoS2 layers. (f) Band energy diagram of a CHF3 plasma-treated MoS2 solar cell with Au/CHF3 plasma-treated (or p-doped) MoS2/untreated (or n-type) MoS2/ITO, which has a p-n junction with built-in potential between plasma treated (p-type) MoS2 and untreated (n-type) MoS2 of∼ 0.7 eV. Reprinted with permission from ref 134. Copyright (2014) American Chemical Society.

This section examines the effect of chemical doping on the photovoltaic performance of MoS2-based solar cells by comparing undoped and chemically doped MoS2-based solar cell devices. Like graphene, a correlation can be observed between the chemical doping and photovoltaic performance of MoS2-based solar cells, and such examples are discussed below. Wi et al.134 used plasma doping to improve the power-conversion efficiencies in multilayer MoS2 having different thickness of MoS2 photoactive layer. The ITO/multilayer MoS2/Au solar cell was fabricated where p–n junctions in MoS2 layers were formed using a plasma-induced pdoping. Figure 8a-d compares the photovoltaic performance of CHF3 plasma-treated and untreated MoS2 solar cells, where VOC, JSC, FF, and PCE data are plotted as functions of MoS2 thickness. The plasma-treated MoS2-based devices when compared with untreated solar cells show higher photovoltaic response as a function of MoS2 thickness. The vertically stacked MoS2-based solar cells (ITO/MoS2/Au) exhibited the highest JSC of 20.9 mA/cm2 and PCE of 2.8% for 120 nm thick MoS2 photoactive layers. The I-V characteristics of MoS2 solar cells doped with SF6, CF4, and CHF3 plasma yielded PCEs of 0.3, 1.2 and 1.9% for 60 nm thick MoS2 photoactive layers, respectively, under 532 nm illumination. The CHF3 plasma-doped MoS2 solar 29



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cells showed Voc of 0.20 V, Jsc of 7.43 mA/cm2, FF of 40%, and PCE of 0.59% for MoS2 layer of 50 nm thickness. The PCE was found to be thickness dependent, where CHF3 plasma-doped MoS2 solar cells showed PCEs of 0.25, 0.062, 0.06, 0.02, 0.018, and 0.012% for 210, 342, 356, 400, 420, and 500 nm thick MoS2 photoactive layers, respectively. The indication of p-type doping by CHF3 plasma treatment of untreated (or n-type) MoS2 layer and the band energy differences of ~0.7 eV at the interface between the CHF3 plasma treated (p-type doped) MoS2 and untreated (n-type) MoS2 are also shown in (Figure 8e,f).

Figure 9. (a) Schematic representation of the MoS2/h-BN/GaAs heterostructure solar cell. (b) MoS2/GaAs and J-V characteristics of MoS2/GaAs and MoS2/h-BN/GaAs heterojunction solar cells under AM1.5G illumination. (c) Linear fitting of dV/dLnI versus I data for calculating series resistance (Rs) of the MoS2/GaAs and MoS2/h-BN/GaAs heterojunctions. (d) J-V characteristics of undoped and AuCl3 doped MoS2/h-BN/GaAs solar cells in the dark, and under 30


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AM1.5G illumination. (e) Linear fitting of dV/dLnI versus I data for calculating the series resistance (Rs) of undoped and AuCl3 doped MoS2/h-BN/GaAs solar cells. (f) J-V curves of the MoS2/h-BN/GaAs solar cells for different gate voltage (Vgate) under AM1.5G illumination. Reprinted with permission from ref 139. Copyright (2015) Nature Publishing Group.

Gallium arsenide (GaAs) is a III-IV direct band gap semiconductor of 1.42 eV with an electron mobility of 8000 cm2/Vs at 300 °K.135 GaAs thin film based single junction solar cell exhibits a PCE as high as 28.8%, making it one of the top semiconductors for fabricating high performance solar cells.136-138 Lin et al.139 fabricated MoS2/GaAs heterojunctions, and then used 2D hexagonal boron nitride (h-BN) to develop MoS2/h-BN/GaAs heterojunctions based solar cells, where Au contacts were used on the rear surface of GaAs substrate, and on the front surface of MoS2. The ultrathin h-BN layer was inserted between the MoS2/GaAs heterostructure, in order to suppress the static charge transfer. The inserted h-BN layer helps in tuning of the Fermi level of MoS2, but does not inhibit the transport of holes from GaAs layer to MoS2, which eventually contributes to the photovoltaic performance of solar cells. The inserted ultrathin h-BN layer can contribute to enhancing the Voc, though the Jsc of the solar cells remains unchanged; therefore, Fermi level tuning of MoS2 gives rise to a better performance of MoS2/h-BN/GaAs solar cells. After inserting the ultrathin h-BN layer between MoS2 and GaAs layers, the photovoltaic properties of final MoS2/h-BN/GaAs solar cells was remarkably increased. The J-V characteristics of the MoS2/GaAs and MoS2/h-BN/GaAs heterostructure based solar cells were measured under AM1.5G illumination. The Jsc was found to decrease from 20.6 to 20.2 mA/cm2, while the Voc increased from 0.51 to 0.57 V, the FF increased from 45.9 to 47.0%; and using the correlation PCE = Voc × Jsc × FF, the PCE values of 4.82% for the MoS2/GaAs and 5.42% for 31



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MoS2/h-BN/GaAs heterojunction solar cells were obtained. The series resistance (Rs) increased from 56.8 Ω for the MoS2/GaAs solar cell device to 81.9 Ω for the MoS2/h-BN/GaAs solar cell. The transient PL showed decay time constants of 0.97, 0.59 and 0.52 ns for bare GaAs, MoS2/GaAs and MoS2/h-BN/GaAs heterostructure device, respectively, in the fast decay range. Figure 9 shows a schematic representation of the MoS2/h-BN/GaAs heterostructure, the J-V characteristics and series resistance (Rs) measurements of undoped and AuCl3-doped MoS2/hBN/GaAs solar cells under AM1.5G illumination, and the effect of doping and gate voltage (Vgate) change on PCEs. MoS2 was doped with an AuCl3 solution in nitromethane (1 mM) to examine the effect on chemical doping on the PCE of the MoS2/h-BN/GaAs solar cell. After the AuCl3 doping, the value of Rs decreased from 56.0 to 45.9 Ω, the Jsc value increased from 20.6 to 20.8 mA/cm2, Voc increased from 0.56 to 0.64 V, the FF increased from 46.6 to 53.7%, and the PCE increased from 5.38 to 7.15% for MoS2/h-BN/GaAs heterostructure based solar cell devices, after the doping of MoS2 with the AuCl3 solution. The electrical gating for the Fermi level tuning of MoS2 also improved the PCE of AlCl3 doped MoS2/GaAs solar cells by increasing the PCE values from 4.71% at 0 V to 5.81% at -0.5 V, and to 6.15% at -1.0 V. In the case of AlCl3 doped MoS2/h-BN/GaAs solar cells, when the Vgate was changed from 0 to -0.5 and to -1.0 V, the PCE values increased from 6.87% to 8.27% and to 9.03 %, respectively. The MoS2/h-BN/GaAs solar cells showed a PCE of 5.4%, which was significantly enhanced to 9.03% after electrical gating and chemical doping. Table 1 summarizes photovoltaic parameters including JSC, VOC, FF, and PCE (η) for different solar cell structures based on MoS2.96,111115,134,139

The PCE of MoS2 based solar cell devices ranges from 0.3% to 11%. The photovoltaic

parameters of solar cell devices without MoS2 were included for comparison with reference materials. The PCE value of MoS2 based solar cell devices range between 0.32% for 32


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ITO/MoS2/Au solar cell to the highest PCE of 11.1% for the trilayer graphene/MoS2/n-Si solar cell. The PCE value of the graphene/MoS2/n-Si solar cell device annealed at 80 oC is more than twice that of the similar graphene/MoS2/n-Si solar cell.114

Table 1. Performance of MoS2-based solar cells. The photovoltaic parameters of short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE; η) for different solar cell structures are summarized. _____________________________________________________________________________________ Solar Cell Device

JSC

VOC

FF

PCE

(mA/cm2)

(V)

(%)

(η, %)

Ref.

_____________________________________________________________________________________ MoS2/WS2 (Bilayer)

3.50

1.00

60.0

1.50

[96]

ITO/TiO2/MoS2/P3HT/Au

4.70

0.56

-

1.30

[111]

MoS2 (monolayer CVD)/p-Si

22.36

0.41

57.26

5.23

[112]

MoS2 (multilayer)/p-Si

3.20

0.14

42.4

1.30

[113]

Graphene/MoS2/n-Si (annealed at 80 oC)

27.2

0.55

32

4.8

[114]

Graphene/MoS2/n-Si

21.9

0.46

23

2.31

[114]

Monolayer graphene/MoS2/n-Si

13.1

0.41

25

1.35

[115]

Bilayer graphene/MoS2/n-Si

21.4

0.51

55

5.98

[115]

33



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Trilayer graphene/MoS2/n-Si

28.2

0.54

53

8.02

[115]

Trilayer graphene/MoS2/n-Si*

33.4

0.56

60

11.1

[115]

Graphene/n-Si

1.9

0.22

18

0.07

[115]

ITO/MoS2/Au

13.8

0.093

25.0

0.32

[134]

ITO/MoS2/Au (SF6 doped)

36.0

0.103

22.7

0.30

[134]

ITO/MoS2/Au (CF4 doped)

37.0

0.167

37.6

1.20

[134]

ITO/MoS2/Au (CHF3 plasma-doped)

21.0

0.26

47.0

2.56

[134]

MoS2/GaAs

20.6

0.51

45.9

4.82

[139]

MoS2/h-BN/GaAs

20.2

0.57

47.0

5.42

[139]

MoS2/h-BN/GaAs (AuCl3 doped)

20.8

0.64

53.7

7.15

[139]

MoS2/h-BN/GaAs (AuCl3 doped) Vgate = 0 V

20.2

0.64

53.1

6.87

[139]

MoS2/h-BN/GaAs (AuCl3 doped) Vgate = −0.5 V

20.7

0.72

54.9

8.27

[139]

MoS2/h-BN/GaAs (AuCl3 doped) Vgate = −1.0 V

21.1

0.76

56.3

9.03

[139]

____________________________________________________________________________________ Notes: Chemically exfoliated MoS2 nanosheets (ce-MoS2) *(9-nm thick MoS2 insertion layer) Gate voltage (Vgate)

MOS2 IN ORGANIC SOLAR CELLS The electron donor–acceptor blend active layer consisting of poly(3-alkylthiophenes) P3HT:([6,6]-phenyl-C61-butyric acid methyl ester) (PC61BM) has been used in fabricating bulk34


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heterojunction solar cells to improve the PCE. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) blend has been employed as a hole transport layer in organic solar cells. Carbon nanotubes, graphene and TMDs have been explored as a replacement for PEDOT:PSS blend. Materials such as bathocuproine (PCB), MoO3, LiF, V2O5, NiO, and polymers such as polypyrrole and polythiophene have been used as a buffer layer in the fabrication of solar cell devices. ITO has >90% optical transparency and high conductivity; therefore, it is the most commonly used transparent conducting electrode in organic solar cells, though ITO also exhibits some disadvantages, such as high production cost and lack of mechanical flexibility. TMDs including MoS2 have been used in fabricating organic solar cells as an interfacial layer, a hole transport layer, or an electrode material.

Figure 10. (a) Chemical structures of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61butyric acid methyl ester (PC61BM). (b) Schematic structure of the organic solar cell device Al/P3HT:PC61BM/MoSx/ITO, where MoSx consists of both MoS2 and MoS3. (c) Current density−voltage (I−V) curves of organic solar cells with different anode buffer layer, including bare ITO, PEDOT:PSS, and MoSx obtained at 150, 200, 300, and 400 oC. Reprinted with permission from ref 141. Copyright (2013) American Chemical Society.

35



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Page 36 of 82

MoS2 nanosheets were used as an efficient hole extraction layer (HEL) to P3HT:[6,6]-Phenyl C71 butyric acid methyl ester (PC71BM) and poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]

thieno[3,4-b]thiophenediyl})

(PTB7):PC71BM based organic solar cells by Gu et al.140, which showed the PCEs of 4.02 and 8.11%, respectively, higher than the PCE of the solar cells with traditional vacuum-evaporated MoO3 for the HEL. The PCEs of PTB7:PC71BM based MoS2 solar cells were found to be 6.97, 8.11, and 7.75% for 1-time, 2-time, and 3-time MoS2 spin-coated films, respectively, indicating PCE dependence on the different layers of MoS2 films. The PTB7:PC71BM based monolayer MoS2 heterojunction solar cell yielded a JSC of 15.78 mA/cm2, VOC of 0.69 V, fill factor of 64%, and PCE of 6.97%. Li et al.141 used ammonium thiomolybdate (NH4)2MoS4 to fabricate MoSx anode buffer layer for organic solar cells. Figure 10 shows the chemical structures of P3HT and PC61BM, the schematic structure of the organic solar cell device Al/P3HT:PC61BM/MoSx/ITO, where MoSx consists of both MoS2 and MoS3, and current density−voltage (I−V) curves with different anode buffer layers. The values for ITO, PEDOT:PSS, and MoSx were obtained at 150, 200, 300, and 400 oC. MoSx was considered beneficial for high photovoltaic performance, where MoS3 contributed to higher VOC, while MoS2 contributed to higher JSC. The organic solar cell with bare ITO showed a lower PCE of 2.14%, due to the leakage current between the ITO and PC61BM. MoSx buffer layers were prepared at different thermal decomposition temperatures of (NH4)2MoS4 from 150 to 400 oC. The PCEs of organic solar cells were found to increase from 2.91% at 150 oC to 3.15% at 200 oC, and to 3.90% at 300 oC; however, the PCE value decreased to 3.57% for MoSx obtained at 400 oC. The highest PCE of 3.90% was recorded when the ratio of MoS3 and MoS2 was balanced, which was higher compared to the PEDOT:PSS-based solar

36


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cell device (with PCE of 3.64%). Therefore, apart from conventional MoO3, as a replacement for PEDOT:PSS, molybdenum sulfides and Mo4+ also show potential as anode buffer materials.

Figure 11. (a) Transmission spectra of PEDOT:PSS, chemically exfoliated MoS2 nanosheets (ce-MoS2), and O-ce-MoS2 ultrathin film. (b) J-V characteristics, and (c) EQE spectra of the solar cells with bare ITO, PEDOT:PSS, ce-MoS2 as-deposited, and ce-MoS2 films annealed at 150 oC under AM 1.5G illumination. Reprinted with permission from ref 142. Copyright (2014) The Royal Society of Chemistry.

MoS2 sheets were used as hole transport layers (HTLs) by Yang et al.142 for fabricating organic solar cells, and studied the effect of different crystalline structures of MoS2 sheets to optimize and examine the performance of organic solar cells. The incorporation of oxygen atoms into MoS2 sheets was done by UV–ozone treatment, which enhanced the performance of the solar cells. The spin-coated PTB7:PC71BM was used as an active layer, while poly[(9,9-bis(3′(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) was used as an electron transporting layer (ETL). Figure 11 shows the transmission spectra of PEDOT:PSS, chemically exfoliated MoS2 nanosheets (ce-MoS2), and oxygen incorporated ce-MoS2 (O-ce37



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Page 38 of 82

MoS2) ultrathin film. Both ce-MoS2 and O-ce-MoS2 show high optical transparency over the 400–900 nm range, comparable to that of PEDOT:PSS thin film. The J-V characteristics, and external quantum efficiency (EQE) spectra of solar cells with bare ITO, PEDOT:PSS, ce-MoS2 ultrathin film as deposited and ce-MoS2 films annealed at 150 oC used as hole extraction layers are also shown. The ce-MoS2 films were annealed for 15 min in the glovebox, in order to avoid oxidation. The annealed ce-MoS2 films showed an enhanced PCE of 5.77%, compared with 4.99% for the as-deposited ce-MoS2 films.

Figure 12. (a) Solar cell structure having few-layer MoS2 as an HTL. (b) J–V characteristics of PEDOT:PSS and MoS2 HTLs prepared by film transfer and drop casting techniques. (c) J–V 38


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curves of PEDOT:PSS and MoS2 HTL based solar cells in the dark, and under illumination. (d) The transmittance of PEDOT:PSS MoS2 HTLs on ITO glasses. Reprinted with permission from ref 58. Copyright (2014) Wiley-VCH.

Nui et al.58 used exfoliated MoS2 sheets deposited onto ITO substrate via MoS2 film transfer after vacuum filtration, followed by annealing for 1 hr at 500 oC; and also via drop casting, using a MoS2 aqueous solution diluted in methanol, followed by a similar annealing process. The saltassisted liquid-phase exfoliation approach was used to prepare MoS2 layers for organic solar cells. The photoactive layer was made of (poly(3-hexylthiophene) P3HT and ([6,6]-phenyl-C61butyric acid methyl ester) PC61BM blend. The solar cell structure ITO/MoS2/P3HT:PC61BM/Al was used. Figure 12 shows the solar cell structure having few-layer MoS2 as an HTL, and J–V characteristics of MoS2 HTLs prepared by film transfer and drop casting techniques, and PEDOT:PSS. The optical transmittance of MoS2 HTL was 90.6% at 550 nm, 6% less compared with PEDOT:PSS. A PCE of 1.81% was recorded for the ITO/MoS2/P3HT:PC61BM/Al solar cells, where MoS2 was prepared by film transfer, compared to PCE of 1.59% for the MoS2 prepared by drop casting. For PEDOT:PSS as an HTL reference, a PCE value of 2.50% was obtained; while without HTL, the PCE was very poor, as low as 0.02% under AM 1.5 illumination.

39



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Page 40 of 82

Figure 13. (a) Schematic structure of the normal ITO/MoS2/P3HT:PC61BM/Ca-Al solar cell device. (b) The inverted organic solar cell ITO/MoS2/ P3HT:PC61BM/HTL/Ag having MoS2based interfacial layers. J–V characteristic curves of solar cells with (c) un-doped MoS2-based HTL in normal solar cell, and (d) un-doped MoS2-based ETL in an inverted solar cell configuration. J–V curves of cells with (e) gold chloride trihydrate (HAuCl4·3H2O) (p-doped) MoS2-based HTL in normal organic solar cells, and (f) sodium borohydride (NaBH4) doped (ndoped) MoS2-based ETL in inverted organic solar cells. Reprinted with permission from ref 143. Copyright (2013) The Royal Society of Chemistry.

MoS2 thin-films were used for both HTL and ETL layers by Yun et al.143 for fabricating organic solar cells. The MoS2-based HTLs were prepared by doping the MoS2 film with a pdopant gold chloride trihydrate (HAuCl4·3H2O). MoS2 solution was spin-coated onto UV/Ozone-treated ITO substrate. HAuCl4 solution was spin-coated onto MoS2 film already 40


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annealed at 150 oC. The n-dopant sodium borohydride (NaBH4) was used to prepare MoS2-based ETLs. The HTL was prepared by spin-coating PEDOT:PSS:iso-propyl alcohol solution onto the photoactive layer. The photoactive layer was fabricated in a similar manner to the invertedstructure organic solar cells. Ag was used as the top electrode. P3HT:PC61BM layer was spincoated onto the HEL. The final solar cell devices were annealed at 120 oC for 10 min. Figure 13 shows the schematic structure of the normal solar cell (ITO/MoS2/P3HT:PC61BM/Ca-Al), and the inverted organic solar cell (ITO/MoS2/P3HT:PC61BM/HTL/Ag) having MoS2-based interfacial layers, the J–V characteristics of solar cells with un-doped MoS2-based HTL in the normal solar cell and un-doped MoS2-based ETL in an inverted solar cell configuration, and the J–V curves of solar cells p-doped with gold chloride trihydrate (HAuCl4·3H2O) MoS2-based HTL in normal organic solar cells, and n-doped with sodium borohydride (NaBH4) MoS2-based ETL in inverted organic solar cells. Both p- and n-doping lead to a change in the work function of the MoS2 interfacial layers. The PCE of p-doped MoS2-based HTL based solar cells devices increased from 2.8 to 3.4%. Also the PCE of n-doped solar cells was significantly increased compared with pure MoS2 thin-films. In the case of Van Le et al.144 sonication exfoliated MoS2 nanosheets were used as a HEL in organic solar cells. Hu et al.145 used chemical exfoliated MoS2 nanosheets were used as interfacial layers and silver nanowires (AgNWs)/MoS2 composite as a transparent electrode to develop polymer solar cells.

Both raw (MoS2 nanopowder purchased from Signma-Aldrich) MoS2, as well as

exfoliated MoS2 nanosheets thin films deposited on a glass substrate have 80% transmittance in the 350-700 nm range, where the transmittance of the exfoliated MoS2 nanosheets thin film is slightly better than that of raw MoS2, due to the thickness difference. Although pristine MoS2 nanosheets are n-type semiconductor, they can be changed to p-type semiconductor after UV41



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Page 42 of 82

ozone plasma treatment. The work functions of n-MoS2 and p-MoS2 nanosheets were calculated as 4.3 and 5.0 eV, respectively. The solution-processed n-MoS2 nanosheets thin films were used as ETL, while p-MoS2 nanosheets were used as HTL. The optimized AgNW-MoS2 nanosheets transparent electrode exhibited high transmittance of 93.1% at 550 nm, and a low sheet resistance of 9.8 Ω/sq. The AgNWs/MoS2 nanosheets composite electrode is also better in terms of environmental stability, being more moisture and oxidation resistant than the AgNWs electrode. The ITO-free polymer solar cells based on AgNWs-MoS2 nanosheets/n-MoS2 nanosheets cathode and p-MoS2 nanosheets/Ag anode showed the highest PCE of 8.72% with poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T) and (6,6)-phenyl-C70 butyric acid methylester (PC70BM) (PBDTTT-C-T:PC70BM) hybrid system. Table 2 summarizes the performance of MoS2 nanosheets based PBDTTT-C-T:PC70BM solar cells. The thickness of the top Ag electrode was decreased from 100 to 30 nm in solar cells, while the effective area was increased from 0.06 to 0.65 cm−2. The control solar cell device (Glass/ITO/ZnO/PBDTTT-CT:PC70BM/MoO3/Ag (100 nm)) shows a PCE of 7.62%. After n-MoS2 and p-MoS2 were used as an interfacial layer, Glass/ITO/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (100 nm) showed a PCE of 8.43%. When ITO cathode was replaced by AgNWs and AgNW-MoS2 electrodes, the ITO-free

Glass/AgNWs/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag

(100

nm)

and

the

Glass/AgNW-MoS2/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (100 nm) solar cells achieved PCEs of 6.39 and 8.0%, respectively. Therefore, the replacement of ITO cathode by AgNWMoS2 electrode resulted in a PCE comparable to the control solar cell device. Organic solar cells were fabricated by Qin et al.146 using UV/ozone treated MoS2 film as an HTL and P3HT:PC61BM deposited on FTO coated glass substrate. PEDOT:PSS is generally used 42


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as the HTL, because it can smooth the anode surface. However, PEDOT:PSS as the HTL shows a long-term interface stability problem, because PEDOT:PSS has acidic and hygroscopic properties. Instead of PEDOT:PSS, transition metal oxides such as MoO3, V2O5, NiO, WO3, Cr2O3 were investigated, and among these, MoO3 is known to be one of the most promising candidates, because it is environmentally stable and optically transparent, and it has a relatively high hole-mobility. Therefore, the MoS2 film surface was UV/ozone treated to form a MoO3MoS2 double layer. Raman spectroscopy, ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission spectroscopy, and Hall-effect measurements indicated various valence states of Mo, due to the oxidation of MoS2 by UV/ozone treatment to double-layered MoO3–MoS2 films. MoS2 films show a p-type characteristic, due to the presence of Mo5+ and Mo6+ states. Organic solar cells based on double-layered MoO3–MoS2 films as an HTL show a PCE of 4.15%, after optimizing the sputtering deposition temperature and thickness of the HTL layer. The double layer MoO3–MoS2 films interface easily facilitates hole-transfer. The JSC value of solar cells having MoO3–MoS2 film HTL was found to be higher than that of pure MoO3 film as an HTL. Yang et al.147 used MoS2 nanosheets decorated with 20 nm Au nanoparticles as an HTL for developing plasmonics for organic solar cells. Organic solar cells having MoS2-Au nanoparticles as the HTL showed increased JSC and PCE, compared to MoS2 alone as the HTL. Table 2 summarizes the photovoltaic performances of MoS2-based organic solar cells. MoS2 nanosheets based PBDTTT-C-T:PC70BM solar cells145 under AM1.5G illumination show the best photovoltaic device parameters.

43



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Table 2. Photovoltaic performance of MoS2-based organic solar cells in terms of the short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE) for different solar cell structures are summarized. ____________________________________________________________________________________

JSC

Solar Cell Device

VOC

(mA/cm2) (V)

FF (%)

PCE

Ref.

(%)

____________________________________________________________________________________________ MoS2/PTB7:PC71BM

15.86

0.72

71.0

8.11

[140]

o

9.02

0.590

59.0

3.15

[141]

ITO/MoS2/P3HT:PC61BM/Al (MoS2 annealed at 300 C)

o

9.96

0.581

67.0

3.90

[141]

ITO/PEDOT:PSS/ P3HT:PC61BM/Al

9.47

0.611

62.0

3.62

[141]

ITO/P3HT:PC61BM/Al

8.91

0.472

51.0

2.14

[141]

ITO/MoS2/PTB7:PC71B/PFN/Al (ce-MoS2 as-deposited)

14.29

0.62

56.3

4.99

[142]

ITO/MoS2/PTB7:PC71B/PFN/Al (ce-MoS2 annealed at 150 oC)

14.10

0.65

63.0

5.77

[142]

ITO/MoS2/PTB7:PC71B/PFN/Al (ce-MoS2 annealed at 300 oC)

13.83

0.56

61.6

4.77

[142]

ITO/PEDOT:PSS annealed/PTB7:PC71B/PFN/Al

15.00

0.74

68.5

7.60

[142]

ITO /PTB7:PC71B/PFN/Al

14.14

0.55

59.1

4.60

[142]

ITO/MoS2/P3HT:PC61BM/Al (film transfer of MoS2)

7.1

0.58

44.0

1.81

[58]

ITO/MoS2/P3HT:PC61BM/Al (drop casting of MoS2)

7.1

0.58

38.9

1.59

[58]

ITO/MoS2/P3HT:PC61BM/Ca-Al

9.13

0.43

43.65

1.69

[143]

ITO/MoS2/P3HT:PC61BM/Ca-Al (annealed at 150 C)

9.00

0.55

57.46

2.84

[143]

ITO/MoS2/P3HT:PC61BM/Ca-Al (HAuCl4 doped)

8.62

0.59

66.24

3.38

[143]

ITO/P3HT:PCBM/LiF/Al

6.98

0.48

55

1.84

[144]

ITO/MoS2/P3HT:PC61BM/Al (MoS2 annealed at 200 C)

o

44


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ITO/MoS2/P3HT:PCBM/LiF/Al

8.03

0.29

46

1.08

[144]

ITO/MoS2/P3HT:PCBM/LiF/Al (UV/Ozone/15 min)

7.74

0.50

63

2.44

[144]

ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al

8.14

0.52

67

2.87

[144]

ITO/MoS2/PEDOT:PSS/P3HT:PCBM/LiF/Al

7.97

0.52

68

2.81

[144]

ITO/ZnO/PBDTTT-C-T:PC70BM/MoO3/Ag (100 nm)

15.79

0.76

63

7.62

[145]

ITO/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (100 nm)

15.96

0.76

70

8.43

[145]

AgNW/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (100 nm)

14.79

0.75

57

6.39

[145]

AgNW-MoS2/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (100 nm)

15.66

0.76

67

8.00

[145]

AgNW-MoS2/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (30 nm) (0.06 cm2) 12.66

0.75

64

6.02

[145]

AgNW-MoS2/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (30 nm) (0.65 cm2) 9.38

0.54

53

2.71

[145]

AgNW-MoS2/n-MoS2/PBDTTT-C-T:PC70BM/p-MoS2/Ag (30 nm) (0.65 m2)

0.39

40

1.20

[145]

7.43

____________________________________________________________________________________________

Notes: poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)

MOS2 IN PEROVSKITES-BASED SOLAR CELLS The organometal halide perovskites, CH3NH3MX3, where M is Pb or Sn, and X is Cl, Br, or I, based solar cells have significantly advanced in the past 5 years, because their PCE has grown from 9.7% in 2012 to over 22% in 2015.148-150 A PCE of up to 21.13%, which remains at 18% after 250 hr of aging, has been reported for cesium-containing triple cation perovskite solar cells.149 Only limited work has been done on the use of TMDs in perovskites-based solar cells. Kim et al.151 used MoS2, WS2, and graphene oxide (GO) as HELs in perovskites solar cells instead of PEDOT:PSS. MoS2 and WS2 thin layers were prepared by CVD using (NH4)MoS4 and (NH4)WS4 precursor solutions. The perovskites solar cells with p-n junction were fabricated using CH3NH3PbI3−xClx perovskite layer. 45



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Figure 14. (a) Energy band diagram of fabricated perovskites solar cells. (b) Current densityvoltage characteristics of PEDOT:PSS. MoS2, WS2, and graphene oxide (GO) as HELs in perovskites solar cells, which show maximum PCEs of 9.93, 9.53, 8.02, and 9.62%, respectively. (c) Photovoltaic parameters of the solar cell device analyzed for 5 different batch fabrications (20 solar cells). Reprinted with permission from ref 151. Copyright (2016) Elsevier.

Figure 14 shows energy band diagram, current density-voltage plots of PEDOT:PSS. MoS2, WS2 and GO used as HELs in perovskites solar cells and photovoltaic parameters analyzed for 5 46


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different batch fabrications. The measured work functions were 5.1 eV for GO, 5.0 eV for MoS2, and 4.95 eV for WS2. The energy band diagram indicates that all these materials are suitable as the HEL in fabricating the CH3NH3PbI3xClx perovskite solar cells. The Jsc of 14.89 mA/cm2, Voc of 0.96, and FF of 67%, resulted in a PCE of 9.53% for the MoS2 based perovskites solar cells. Maximum PCEs of 8.02% for WS2, 9.53% for MoS2, and 9.62% for GO-based perovskites solar cells were achieved, compared to a PCE of 9.93% for the PEDOT:PSS-based devices. The PCE of MoS2 based solar cells was comparable to the PEDOT:PSS based device; therefore, MoS2 HELs can be used as a HEL replacement for PEDOT:PSS. This study indicates MoS2 and WS2 are promising materials for application as the HELs in perovskites solar cells.

Figure 15. (a) Endurance test on 800 hr shelf life for glass/FTO/compact-TiO2/mesoporousTiO2/CH3NH3PbI3/spiro-OMeTAD/Au based solar cells with MoS2 and doped Spiro-OMeTAD as a hole transport layer (HTL), showing higher stability for the MoS2–based solar cells. Reprinted with permission from ref 152. Copyright (2015) IEEE. (b) I-V curves of the perovskite solar cells both Spiro-OMeTAD+MoS2, and spiro-OMeTAD alone, measured after ageing for 7

47



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and 23 days, where solar cell devices were stored in a desiccator. Reprinted with permission from ref 153. Copyright (2016) Wiley-VCH.

Capasso et al.152 used MoS2 flakes as HTL instead of (2,2',7,7'-tetrakis-(N,N-di-4methoxyphenylamino)-9,9'-spirobifluorene)

(spiro-OMeTAD)

to

fabricate

metal-organic

perovskite solar cells. MoS2 flakes were deposited on a perovskite layer by a spray coating method. The glass/FTO/compact-TiO2/mesoporous-TiO2/CH3NH3PbI3/spiro-OMeTAD/Au based solar cells were developed with MoS2 flakes instead of spiro-OMeTAD. The solar cells with MoS2 as HTL exhibited a PCE of 3.9%, compared with a PCE of 3.1% for spiro-OMeTAD as HTL. The CH3NH3PbI3/MoS2/spiro-OMeTAD/Au based solar cell device exhibited a PCE of 13.09%. The MoS2-based cells showed higher stability than that of doped spiro-OMeTAD-based solar cells, during an endurance test conducted on 800 hr shelf life (Figure 15a). Both types of solar cells, with, and without, MoS2 as HTL, show a drastic decrease in PCE values from 400 to 800 hrs, from the degradation of the perovskite layer due to air exposure. Capasso et al.153 demonstrated

that

in

glass/FTO/compact-TiO2/mesoporous-TiO2/CH3NH3PbI3/MoS2/Spiro-

OMeTAD/Au based solar cells, MoS2 flakes act both as a HTL from the perovskite to the SpiroOMeTAD, and as a protective layer between perovskite and Au electrode. Solar cells with MoS2 showed a PCE of 13.3%, and higher stability over 550 hr, compared with reference solar cells without MoS2. Perovskite solar cells with a large area of 1.05 cm2 showed scalability and a PCE of 11.5%. The use of MoS2 was found to improve the shelf life of large-area perovskite-based solar cells. The stability of perovskite solar cells was measured after ageing for 7 and 23 days filled by placing the devices in a desiccator having 30% relative humidity in dark conditions (Figure 15b). The PCE of perovskite solar cells having both spiro-OMeTAD+MoS2 showed 48


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higher stability, compared with those having spiro-OMeTAD alone. Ahmed et al.154 used TiO2/MoS2 nanocomposite to enhance the absorption in CH3NH3PbI3 based solar cells. The use of few-layer thick MoS2 increased the PCE from 3.7 to 4.3%.

STABILITY OF MOS2 SOLAR CELLS Solar cell devices can degrade or completely fail due to the degradation of electrode and photoactive materials by oxygen and humidity, ultraviolet light exposure, corrosion of electrodes and

interconnects,

high

temperature

operation,

and

breakage

from

the

poly(3,4-

ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) blend that is used as an interfacial material in organic BHJ solar cells. PEDOT:PSS is inherently acidic, and absorbs moisture that causes gradual degradation of the components used in solar cell devices, up to and including complete failure. Furthermore after long-term light illumination, solar cell devices also degrade due to photo-oxidation. An excellent review on the stability of graphene-based heterojunction solar cells has been written by Singh et al.14 In this section, the stability of MoS2 based solar cell devices is discussed with, and without, any encapsulation, and as a function of storage time under ambient conditions, and also under prolonged illumination time.

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Figure 16. Structures of organic solar cell devices (a) (Type I) ITO/P3HT:PCBM/LiF/Al, (b) (Type

II)

ITO/MoS2/P3HT:PCBM/LiF/Al,

ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al,

and

(c) (d)

(Type

III)

(Type

IV)

ITO/MoS2/PEDOT:PSS/P3HT:PCBM/LiF/Al. The stability of Type-III and Type-IV solar cell devices measured in air without any encapsulation: normalized values of (e) Jsc, (f) Voc, (g) PCE, and (h) FF, as functions of time in air. Reprinted with permission from ref 144. Copyright (2014) The Royal Society of Chemistry.

Organic solar cells were developed by Van Le et al.144 using sonication exfoliated MoS2 nanosheets as a HEL. Figure 16 shows structures of organic solar cell devices with the configurations: (a) ITO/P3HT:PCBM/LiF/Al (Type I), (b) ITO/MoS2/P3HT:PCBM/LiF/Al (Type

II),

(c)

ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al

(Type

III),

and

(d)

ITO/MoS2/PEDOT:PSS/P3HT:PCBM/LiF/Al (Type IV), and (e) – (h) the stability of Type-III and Type-IV solar cells measured as function of time in air atmosphere without any 50


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encapsulation. MoS2/PEDOT:PSS based solar cells exhibited a PCE of 1.08%, which was lower compared to that of solar cells without HEL (PCE of 1.84%). When MoS2 surface was treated with UV/ozone for 15 min, the PCE increased to 2.44%. The work function of MoS2 also increased from 4.6 to 4.9 eV after UV/ozone treatment, as evidenced by synchrotron radiation photoelectron spectroscopy. The improvement in the PCE value of MoS2 nanosheets based solar cells was caused by the alignment of band gap. The PCE of solar cell devices increased to 2.81% after a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer was inserted between the MoS2 and the P3HT:PCBM active layer; however, the PCE value was comparable to solar cell devices having only PEDOT:PSS. The MoS2/PEDOT:PSS layers (HEL) significantly improved the stability of organic solar cells. The Type III and Type IV solar cell devices showed no remarkable difference in VOC and FF; however, after the air exposure of the devices for 120 h, the values of Jsc and PCE decreased to 0.2 for Type III, and 0.6 for Type IV. Therefore, longer stability was achieved for the devices using MoS2/PEDOT:PSS layers, than the devices using only the PEDOT:PSS layer. Inserting the MoS2 layer between ITO and PEDOT:PSS stabilizes the organic solar cells, by protecting the surface of ITO from the hygroscopic PEDOT:PSS layer. The UV/ozone treatment of MoS2 may contribute to the stability of solar cells.

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Figure 17. J-V curves of organic solar cells fabricated by using m-MoS2 solution after devices were stored for (a) 6 days, (b) 9 days, and (c) 100 days. To prepare m-MoS2 solution, CTAC aqueous solution was added to the ce-MoS2 sheets dispersed in deionized water. Reprinted with permission from ref 155. Copyright (2014) The Royal Society of Chemistry.

Liu et al.155 used chemically exfoliated (CE) MoS2 nanosheets, where the surface was modified with a hydrophilic surfactant. The ce-MoS2 nanosheets were prepared by a Li intercalation method, where MoS2 crystals were absorbed by a butyllithium solution in hexane for a period of 2 days. The obtained LixMoS2 compound was washed to remove residues, filtered, and the MoS2 was then diluted with deionized water (DI). The cetyltrimethylammonium chloride (CTAC) aqueous solution was added to the deionized water containing ce-MoS2 dispersed sheets to prepare m-MoS2 nanosheets. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imagery confirmed the honeycomb crystalline structure of MoS2. The ce-MoS2 or m-MoS2 solution was used to fabricate organic solar cells on glass/ITO substrate. To fabricate organic solar cells, PTB7:PC71BM layer as an active layer and a PFN layer deposited on the active layer and Al cathode were used. The ceMoS2 colloidal solution exhibited long-term stability. Figure 17 shows the J-V characteristics of

52


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solar cell devices fabricated by using m-MoS2 solution where devices were stored for 6, 9, and 100 days, which indicated the long-term stability of organic solar cells.

Figure 18. Stability of PCE of the MoS2/h-BN/GaAs solar cell device as a function of the illumination time. Reprinted with permission from ref 139. Copyright (2015) Nature Publishing Group.

The stability of the AuCl3 doped MoS2/h-BN/GaAs heterostructure based solar cell devices sealed by poly(methyl methacrylate) (PMMA) was tested by measuring the variation in the PCE values over a period of 50 hrs under AM1.5G illumination (Figure 18).139 The initial PCE value of 6.73% for the MoS2/h-BN/GaAs solar cell increased to 6.96% after 50 hrs illumination, and no light-induced degradation was observed, compared to crystalline silicon based solar cells, which generally degrade with 24 hrs under similar conditions; therefore after encapsulation, MoS2/hBN/GaAs solar cell devices are fairly stable. Several strategies for improving the long-term stability of solar cell devices have been employed.14 For example, Brus et al.156 introduced high PCE and stability in graphene/n-Si heterojunction solar cells by using CH3-passivated Si surfaces. After 28 days under ambient 53



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conditions, the PCEs of CH3-terminated solar cell devices dropped from 2 to 1.5% at 100 mW, and from 4.2 to 3.7% at 20 mW. In another study, a MoO3 buffer layer was used to improve the stability of organic solar cells by Kanai et al.157 Approaches such as encapsulation,158 interfacial buffer layer,159 and crosslinker,160 have been employed, in order to induce stability of different types of solar cells. In addition to MoS2, other TMDs, such as WS2, TiS2, TaS2, and NbSe2, show excellent charge transport properties; therefore, a few reports have emerged of using these TMDs both as HTL and ETL in fabricating organic solar cells. 2D NbSe2 nanosheets used the HTL to replace MoO3

in

organic

solar

cells.161

When

NbSe2

was

used

as

HTL

in

the

ITO/NbSe2/P3HT:PC61BM/LiF/Al configuration, the solar cell exhibited a PCE of 3.05%, compared to a PCE of 2.70% for the PEDOT:PSS HTL based solar cell. When PTB7:PC71BM was applied as an active layer, a PCE of 8.10% was achieved, without any thermal treatment. Van Le et al.162 used UV-ozone treated TaS2 nanosheets as the HEL, and electron extraction layers (EEL) for organic solar cells. The TaS2 nanosheets were 1 nm in thickness and 70 nm in lateral size, as indicated by AFM. After UV-ozone treatment of TaS2 nanosheets, the work function increased from 4.4 to 5.1 eV. The PCE values of UV-ozone-treated TaS2 and TaS2 based inverted solar cells were 3.06 and 2.73%, respectively, noticeably higher compared to those of organic solar cells without TaS2 in normal configuration (PCE of 1.56%), and in an inverted configuration (PCE of 0.22%). Therefore, TaS2 shows potential as HEL and EEL layers in organic solar cells. Van Le et al.163 also used WS2 nanosheets prepared by sonication exfoliation as a HEL in organic solar cells. The PCE values of WS2 layers based solar cell were 1.84%, which increased to 2.4% after 15 min UV-ozone treatment, due to the band alignment. When a PEDOT:PSS layer was inserted between the WS2 layer and the active layer, the PCE 54


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value further increased to 3.07% which is higher than that of PEDOT:PSS based solar cells (PCE of 2.87%). Pradhan et al.164 developed p-n junctions using multilayered MoSe2 stacked onto hBN, and showed an EQE over 30 %, and a PCE of 5% with a FF of 70%. Finally, strain exists extensively in the 2D structures and strain engineering can significantly modulate the optical and electronic properties of atomically layered MoS2.165 Hui et al.166 and Zhang et al.167 described the modulation of the electronic and magnetic properties of various low-dimensional materials including MoS2. Especially, Hui et al.166 demonstrated that the change of direct bandgap of ~300 mV per 1% strain for trilayer MoS2. Therefore, the strain between the interfaces during the fabrication of atomically thin-layered MoS2 solar cells needs to be carefully managed and controlled.

CONSLUSION AND PERSPECTIVE The new developments in the PCEs of all types of solar cells are periodically updated by Green et al.168,169 A PCE of 25.6% for crystalline silicon heterojunction solar cell was reported by Masuko et al.170 from Panasonic. A PCE of 31.6% for one-sun tandem GaInP/GaAs cell was reported by Kayes et al.171. Green et al.172 reported a PCE of 34.5% for a 28 cm2 mini-module consisting of a GaInP/GaInAs/Ge triple junction cell. A PCE of 31.2% was reported for the InGaP/GaAs/InGaAs module by Takamoto.173 This is the highest PCE over 30% efficiency for a one-sun photovoltaic module. Larramona et al.174 reported a PCE of 9.8% for a 1 cm2 CZTSSe (Cu2ZnSnS4-ySey) solar cell. A PCE value of 22.1% for perovskite was reported by Yang et al.148 and of 11.2 ± 0.3% for an organic solar cell by Mori et al.175 A PCE of 46.0 ± 2.2% for GaInP/GaAs; GaInAsP/GaInAs by the Fraunhofer Institute for Solar Energy Systems,176 and of 45.7 ± 2.3% for GaInP/GaAs/GaInAs/GaInAs by the NREL177 were reported for multi-junction 55



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solar cells. The highest PCE values range between 10 to 46% for thin film solar cells based on crystalline silicon, GaAs, CuInGaSe, organic, perovskites and multi-junction solar cells. Although inorganic semiconductor thin film shows great promise for solar cell industries, the potential of organic and/or organic/inorganic hybrid solar cell devices has also been explored for future commercial applications. The graphene-based materials have been extensively studied for solar cell applications where graphene/n-silicon solar cells exhibit a PCE of 15.6% with an antireflective coating.8 Though rather new, TMDs were followed for solar cell devices taking a similar path. The trilayer-graphene/MoS2/n-Si based solar cell devices exhibit a PCE of 11.1%.115 The passivation of MoS2 surface with Al2O3 dielectric layer has been demonstrated to enhance the PCE of multilayer n-MoS2/p-Si solar cells from 2.21% (without Al2O3 passivation) to 5.6%.178 A maximum PCE of 9.03% has been measured for the MoS2/h-BN/GaAs solar cell devices, after AuCl3 doping and electrical gating.139 The PCE value of 8.43% was achieved for MoS2-based organic solar cells.145 The perovskites-based solar cells consisting of MoS2 show a PCE of 9.53%,151 which is comparable to the PEDOT:PSS device under similar experimental conditions. In another case, TiO2/CH3NH3PbI3/MoS2/Spiro-OMeTAD/Au based solar cells show a PCE of 13.3% and stability over 550 hr, where the MoS2 flakes contribute both as a protective layer, and HTL.153 The PCEs of MoS2 based solar cells have emerged to be competitive with conventional PEDOT:PSS based solar cells. Additionally, MoS2 dispersions show better longterm stability than that of the PEDOT:PSS blend, due to its acidic nature. In addition to the high PCE of MoS2-based BHJ solar cells, ease of fabrication, environmental stability against photooxidation, chemicals, cost-effectiveness and toxicity are still challenging factors for large-scale commercial production. Furthermore, nanostructured materials such as carbon nanotubes (CNTs), graphene, transition metal dichalcogenides (TMDs) are known to cause cytotoxicity to 56


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human179-183, therefore, the toxicity of MoS2 based materials should be studied in reference to their applications in BHJ solar cells. Among TMDs, MoS2 thin films have been mainly studied as hole transport layers, electron transport layers, and interfacial layers in solar cells, where PCE and stability issues are rather at the developmental stage. The field of MoS2 based solar cell devices is not up to par with that of graphene-based solar cells; therefore, much work needs to be done in the coming years. For practical applications in solar cell industries, a combination of high power conversion efficiency, ease of processing, cost-effective manufacturing, long-term environmental stability, and good compatibility with desired substrates are important requirements. These challenges can be meet by developing MoS2 hybrid heterostructures for solar cells, from chemical functionalization to composite formation, in order to match energy levels with other required material components. Very limited reports on the use of MoSe2, WS2, TaS2, TiS2 and NbSe2 layers in solar cell devices have appeared in the literature; therefore, other TMDs of Mo, W, Ti, Zr, Hf, V, Nb, Ta, Tc, Re, with S, Se, and Te chalcogens should be explored as HEL, ETL, buffer layer, etc. In conclusion, more research activities should be focused on incorporating TMDs in different types of solar cell devices to match the recent developments in graphene-based materials, and/or to find competitive TMDs in the near future that match photovoltaic performance with that of the best inorganic semiconductors like GaInP/GaAs, or emerging perovskite-based solar cells.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (G. Y. Yeom). *E-mail: [email protected] (H. S. Nalwa). Notes 57



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The authors declare no competing financial interest.

ACKNOWLEDGMENTS Eric Singh is grateful to Prof. Geun Young Yeom for offering him the summer research internship in his research laboratory at Sungkyunkwan University (SKKU). This work was supported by the Nano•Material Technology Development Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2012M3A7B4035323) and (2016M3A7B4910429).

DISCLAIMER The scientific data contained in this review are for information purposes only. The authors cannot accept liability of any kind whatsoever for the accuracy of contents and data, or any omissions, errors, or claims of completeness.

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