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Band-Diagram of Heterojunction Solar Cells through Scanning Tunneling Spectroscopy Uttiya Dasgupta, Abhijit Bera, and Amlan J. Pal ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00635 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 4, 2017

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Band-Diagram of Heterojunction Solar Cells through Scanning Tunneling Spectroscopy Uttiya Dasgupta, Abhijit Bera, and Amlan J. Pal * Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

AUTHOR EMAIL ADDRESS: [email protected] RECEIVED DATE TITLE RUNNING HEAD: Band-Diagram of Heterojunction Solar Cells through Scanning Tunneling Spectroscopy CORRESPONDING AUTHOR FOOTNOTE: Corresponding author. Tel: +91-33-24734971. Fax: +9133-24732805. E-mail: [email protected].

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ABSTRACT

Band-diagram in heterojunction solar cells is of utmost importance to visualize possibility of chargeseparation and carrier-transport. The diagram should in principle be drawn from the viewpoint of the charge-carriers in the devices. While considering solar cells based on conjugated organics and/or inorganic compound semiconductors, we have shown in this Perspective that scanning tunneling spectroscopy (STS) can draw appropriate band-diagrams, which the carriers would encounter during the separation and transport processes. Differential conductance (dI/dV) images in scanning tunneling microscopy (STM) moreover are capable of energy-mapping; one can therefore map domains of the materials in a bulk-heterojunction (BHJ). Correlation between morphology of the components in BHJs and device performance can therefore be envisaged through STS.


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ToC graphics

LUMO

HOMO Acceptor

Absorber Donor

Acceptor


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In solar cells, several materials have emerged over the last couple of decades as promising alternatives to silicon which at present is being used in solar cell modules. The materials that are seriously considered for commercialization are organic semiconductors and conjugated polymers,1-3 lower-dimensional structures of (compound) inorganic semiconductors,4-7 a combination between these two types of semiconductors,8 and more-recently several organic-inorganic lead or tin halide-based perovskitestructured compounds.9 As a result of extensive research, energy conversion efficiency ( ) of solar cells based on some of these materials first overcame the 10% efficiency barrier and then superseded the performance of thin-film and multicrystalline silicon solar cells by achieving an efficiency of about 23 % at the moment.10 Since there is still some room for further improvement considering the theoretically achievable efficiency limit, thrusts on research have shifted to study band-engineering of these new classes of materials, so that efficiency of charge-separation and charge-transport can be tuned separately.

Importance of Energy Levels in Organic/Polymeric Solar Cells It can be accepted that a junction is required in solar cell architectures for charge-separation to occur upon illumination. Energy levels of the two materials must form a type-II or staggered band-alignment so that photogenerated excitons in both the materials can dissociate and separate for onward transport to the two electrodes. These processes generate photocurrent in the external circuit. Bulk-heterojunctions (BHJs) between electron-donor and electron-acceptor materials11 and hybrid BHJs having polymer:QD (quantum dots) architectures8,12 with interpenetrating domains or networks have been proved to be efficient in this direction. To begin with, materials for solar cell applications are chosen by considering primarily their band gap, band positions, and carrier mobility. For efficient separation of photogenerated carriers in a heterojunction, a proper band-alignment at the interface is a matter of prime importance, since an improper band-alignment, such as type-I (straddling gap) or type-III (broken gap) hampers the charge


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separation efficiency. The interfaces with the electrodes need to be energetically optimized for charge collection as well as to act as blocking contact for the other type of carriers.13 Transport process of separated-carriers is often controlled by morphology of BHJs, that is, domains of electron-donor and electron-acceptor materials.3,14 Size and shape of the domains control the overall area of the interface, which is responsible for charge separation, and pathways for carrier transport to the opposite electrodes. An improper morphology, such as isolated domains, increases the possibility of recombination of separated carriers thereby reducing the photocurrent in the external circuit and accordingly

of solar cells.

Morphology of active layers, more specifically, a spatially percolating-network of donor and acceptor materials is extremely crucial to achieve a high performance in solar cells. With a limited exciton diffusion length in organic semiconductors, a trade-off has to be achieved between charge-separating interfaces and unhindered transport pathways for electrons and holes. In one hand, a finely homogeneous morphology with an overall large donor/acceptor interface area is preferred for charge carrier separation, a proper phase-separation is needed to reduce carrier trapping and thereby recombination, facilitating charge transport to the respective electrodes. Processing solvents, chemical structures of donor and acceptor materials, additives, and surface properties of the substrate can be critically important in determining the nanoscale phase separation and photocurrent of these solar cells. Optimized morphology of active layers alone however does not guarantee high performance of the devices. While spatial percolation is instrumental in yielding photocurrent (JSC), energy levels of the donor and the acceptor and their arrangements are crucial to generate photovoltage (VOC). Efficient exciton dissociation or charge-transfer at the donor/acceptor interfaces requires a large energy-offset, which however lowers the VOC. Therefore a balance between charge-separation and VOC is needed along with appropriate morphology of the donor and acceptor materials.


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Limitations of Conventional Methods in Drawing Band-Diagram A careful evaluation of band-diagram prior to fabrication of devices is therefore crucial to achieve an optimized performance. The widely practiced experimental method for drawing band-diagrams has been cyclic voltammetry (CV), which is a potentiodynamic electrochemical measurement done in solution phase, in conjunction with optical spectroscopy. The inherent disadvantages of CV measurements along with energy variation in solution and thin-film phases make the outcome inaccurate and thus the banddiagram deviates from the actual energy-scenario in sandwiched solar cell structures. To obtain energy levels of active materials in their thin-film form, it is apparent that the measurements should be done on films as used in solar cell architectures. With optical spectroscopy, electron and hole energy levels could not be separately investigated since transitions between two levels are inherently involved in the spectroscopy.

Limitations of Conventional Microscopic Methods in Identifying Morphology and Domains As has been stated, size and shape of domains control area of the interface between electron-donor and electron-acceptor materials. To control morphology of the domains, relative concentration of the components,4 annealing-temperature,15 diameter and functional groups of QDs,16 film-deposition conditions,17 and use of additives18 have been some of the viable options. Although the commonly-used imaging methods in solar cells, namely scanning electron microscopy (SEM)19 and atomic force microscopy (AFM)20 provide excellent information on film-quality, roughness, and thickness, they refrain from indentifying different materials in the nanoscale and thus domains or morphology of individual materials in thin-films. With high-resolution transmission electron microscopy (HR-TEM), materials having different crystalline phases can be differentiated; in composites with polymers and organic molecules, the materials however remained indistinguishable. AFM however provides complementary information of surface by use of vastly different tip-sample interaction schemes. Since charge separation in bilayer solar cells is directly correlated to the morphology at the interface, AFM is an appropriate 6 Environment ACS Paragon Plus

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instrument to measure surface-roughness with a nanometer resolution. The role of these microscopytechniques in BHJs thus remains limited.

Highlights of Ongoing Research on Scanning Tunneling Microscopy Scanning tunneling microscopy and spectroscopy (STM and STS) is a surface analytical technique that probes the local density of states (LDOS) with a high spatial and energetic resolution. In STM, a sharp metallic tip (mechanically cut or chemically etched) is brought into proximity (∼0.3−1.0 nm) to a conducting or semiconducting surface. The most commonly used tip materials are platinum−iridium (Pt/Ir) and tungsten (W) due to their chemical stability and mechanical strength. When an electrical bias is applied between the tip and the conducting substrate on which ultrathin-films are deposited, electrons tunnel through the tip−sample gap; while scanning the thin-film surface, the STM electronics monitor this current through a feedback loop to maintain a tip-sample distance. Polarity of the applied voltage determines the direction of tunneling current. In the two bias directions, the tip may tunnel electrons to empty states or accept electrons from filled states of the semiconductor; finally connection between the tip and the base-electrode completes the circuit. A tunneling process occurs when the magnitude of bias allows energetic alignment between the tip work-function and energy levels of the semiconductor. Density of states can be considered to be proportional to the tunneling conductance at low temperature and at low sample bias. Voltage dependence of tunneling current thus provides the location of the latter levels. To be precise, in the differential conductance (dI/dV) spectrum, conduction and valence band (CB and VB) edges appear as first peaks in the two bias directions. That is, a dI/dV spectrum allows derivation of the energy levels of the semiconductor at the point of measurement. Since STS is a localized mode of measurement, one records many such spectra on different points of the semiconductor-monolayer. Both the energies are plotted as histograms, which provide location of CB and VB-edges of the semiconductor. From the distribution of histograms extending within the band gap, one may deduce the density of surface states in the system. 7 Environment ACS Paragon Plus

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In addition to dI/dV versus voltage plots, STS allows one to record dI/dV images. Such images provide energy-level mapping over an area. Nanoscale domains can be identified and their local electronic properties can be probed simultaneously through dI/dV imaging. Since STS is a localized mode of measurement, energy levels applicable to a single domain can also be derived through this technique. Due to the shape of the tip, the size of domains in dI/dV images can be as low as sub-nanometer. Since specialized substrates are used in STM and since the morphology of materials may depend on the substrate, information of the domains on a desired substrate may however not always be exact as obtained from STS. It is now well-known that due to a strong quantum confinement effect, semiconducting QDs have fascinating optoelectronic properties to be used in a range of devices. With STS, electronic states of such lower-dimensional materials composed of few hundred atoms can be measured very precisely. Banin et al. was the first to characterize such atomic-like states of InAs nanocrystals having a diameter less than 3 nm by STM at around 4 K.21 Earlier, the size-dependent electronic states of PbSe quantum dots were recorded through optical spectroscopy, that is, transitions between filled and empty energy levels, thereby elucidating the optical gap instead of the energy of individual states involved in the transition.22 Diaconescu et al. have measured the electronic states of individual PbS QDs by STS using one-to-two monolayer of the nanocrystalline material treated with 1,2-ethanedithiols (EDT).23 Up to six individual CB and VB energies were resolved for a range of QD sizes. Jdira et al. similarly used STM to study and model energy levels of individual CdSe QDs in a multilayer array.24 Probability of electron injection into QD arrays depended strongly on the symmetry of the QD wave-functions and their response to local electric fields. In the following, we shall discuss some typical parameters that influence band-edges of nanostructures vis-à-vis characteristics of solar cells based on such materials.


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Diameter Variation It is now well-established that due to quantum confinement, the discrete electronic levels of QDs, which are sometimes called “artificial atoms,” shift away from the Fermi energy with decreasing particle diameter.21,25 STS has been used to probe the energy levels of CdSe QDs with a varied diameter.25 Apart from visualization of band-gap widening with a decrease in the diameter, the STS often yielded electron orbitals with s, p, d, and f symmetry in order of increasing energy. With the use of cryogenic STS, twoand six-fold single-electron-charging multiplets of the s- and p-levels, respectively, have been observed in InAs QDs.21 The multiplets in DOS spectra moreover led to derivation of charging energy, which again depended on the diameter of the QDs. The levels derived from STS could be correlated to the optical transitions through photoluminescence excitation spectra. While studying the effect of PbSe quantum dot size on the performance of Schottky-junction solar cells, smaller particles having a larger band gap led to a higher VOC with the efficiency optimizing at an intermediate diameter.26

Effect of Doping Introduction of dopants in a semiconductor is known to shift the Fermi energy which in effect controls carrier density and the conductivity; dopants can therefore alter the type of majority carriers. Use of doped semiconductors is hence welcomed in photovoltaic devices and thereby requiring detailed knowledge of the Fermi energy and concurrently the band-diagram.27,28 To observe the effect of doping via STS, we present a typical example of lead sulfide (PbS) thin-films.29 The metal chalcogenide could be doped at both the sites with heterovalent cationic or anionic elements. The site and valency of the dopants would dictate the nature of effects to be achieved. When pristine and doped PbS thin-films were characterized by STS, the undoped PbS resulted CB and VB at 0.66 and 0.58 eV away from the Fermi energy, respectively, in the form of peaks in the DOS spectra (Figure 1a). While drawing energy bands of the doped-semiconductors from STS, the Fermi energy could be seen to shift within the gap upon introduction of dopants. Fermi energy of the Ag@PbS 9 Environment ACS Paragon Plus

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shifted towards the VB since a monovalent cation (Ag+) introduced one extra hole in the system; Ag@PbS hence exhibited a p-type nature relative to the pristine semiconductor. The Bi@PbS, on the other hand, acted as an n-type semiconductor (even with respect to the pristine-PbS). Here Fermi energy shifted towards the CB since each Bi3+ brought a free-electron into the lattice. For aliovalent anion substitutions, halides converted the semiconductor to become n-type due to introduction of electrons; phosphide doping similarly shifted the Fermi energy towards the VB edge demonstrating p-type nature of the semiconductor. The results from STS and correspondingly histogram of CB and VB energies have been summed up as band-diagrams of the pristine and doped-semiconductors (Figure 1b). With ordinate of the plot being energy with respect to the Fermi level, the diagram clearly shows that STS can probe doping of a (binary) semiconductor through monitoring the shift in Fermi energy.

Energy (eV)

-1

CB

-2

-1

VB

0

1

2

PbS

Ag@PbS Bi@PbS P@PbS Cl@PbS

-0.5

-1.0

-0.5

(b) 0.0

Fermi Energy

0.0

0.5

0.5

1.0

1.0

Energy (eV)

-1.0

(a) PbS dI/dV (Ohm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Bias on the Tip (V)

Figure 1. (a) Numerical derivative of I−V characteristics versus tip-voltage plots of pristine PbS ultrathinfilms. The CB and VB edges of the semiconductor are marked with vertical broken and straight lines at the negative and the positive voltage, respectively. (b) Schematic band diagram of PbS and doped-PbS thin-films formed through the SILAR technique. Here SILAR represented successive ionic layer adsorption and reaction process. The dashed line at 0 V represents each semiconductor’s Fermi energy, which was aligned to 0 V. Reproduced from ref 29. Copyright 2017 Royal Society of Chemistry. The shift in Fermi energy has been substantiated by studying charge separation processes when two differently-doped semiconductors were allowed to form a junction (pn-junction) and sandwiched between ACS Paragon 10 Plus Environment

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two electrodes. Current-voltage characteristics of the pn-junction under illumination yielded improved photovoltaic characteristics when compared with the characteristics of Schottky diodes based on the components of the junction.

Effect of Surface Ligands Surface ligands, which are an integral part of QDs grown through colloidal synthesis method, often introduce mid-gap levels;30,31 they may also shift the Fermi energy without any change in the band gap.32,33 Thus, electronic properties of nanoparticles and therefore their associated roles in solar cell architectures change drastically with the introduction of different surface doping via ligand-exchange. By itself, ligands can influence carrier mobility in thin-films due to a change in the inter-QD dielectric environment and hopping distance.34 When other parameters remained invariant, mobility increases exponentially with decreasing ligand length.35 Passivation of electronic trap-sites arising out of structural aperiodicity and off-stoichiometry of the core can be achieved by introduction of appropriate ligands, thereby increasing the carrier and exciton lifetimes that are beneficial to solar cells.36-38 Since replacement of surface ligands changes the nature and density of chemical binding groups and also the dipole moment, a shift in the vacuum energy level and in turn the VB and CB-edges of QDs can be achieved upon such an exchange.39,40 These effects have been successfully studied through STS, while the optical spectroscopy has shown limitation to probe the levels. As an example, we may cite the report of PbS QDs treated with 12 different ligands. The ligands influenced energies of both the band-edges up to 0.9-1.0 eV (Figure 2).33 The impact of shifts in these energy levels on photovoltaic performance was determined through studies on devices employing 1,2-ethanedithiol (EDT), 1,2-benzenedithiol (1,2-BDT), and 1,3-benzenedithiol (1,3-BDT) ligands. Even between these chemically similar ligands, the shift in the VB-edge was more than 0.2 eV and thus yielding an increase in open-circuit voltage (VOC) by 0.2 V. The shift in VB-edge thus necessitated adjustments of the electron- and hole-extracting contacts to achieve further optimal performance. 11 Environment ACS Paragon Plus

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Figure 2. PbS QDs with a range of possible ligands (left panel). In the right panel, energy level diagrams of PbS QDs exchanged with the ligands are shown. Reproduced from ref 33. Copyright 2017 American Chemical Society.

Discussion on Band-Diagram of Heterojunction Solar Cells Drawn by STS Band-Diagram of Heterojunction Solar Cells The energy levels of a heterostructure are naturally more interesting when solar cell structures are formed. As such, energy levels of the two materials of heterostructure must form a type-II bandalignment. It is thus extremely important to form band-diagram at the donor-acceptor interface with energies that will be “seen” by electrons and holes. The energy-levels, namely the difference between HOMO of the electron-donor and LUMO of the acceptor material (or between the VB of p-type and CB of n-type material) in effect decides the VOC of solar cells. Here, HOMO and LUMO represent highest occupied molecular orbitals and lowest unoccupied molecular orbitals, respectively, of conjugated organics. In the following, we shall discuss band-diagrams formed in a couple of heterojunction solar cells. Bismuth-Doped PbS QDs in Poly(3-hexylthiophene-2,5-diyl) (P3HT) Matrix: Hybrid BulkHeterojunction Solar Cells. Bismuth-doped PbS QDs (Bi@PbS) in P3HT matrix form a hybrid bulkheterojunction structure. The dopants in PbS QDs transform the intrinsic p-type semiconductor into an ntype one. With P3HT being a well-known electron-donor, Bi@PbS:P3HT hybrid BHJ is hence expected ACS Paragon 12 Plus Environment

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to act as solar cells. We determined band-alignment at the P3HT:QD interface upon various level of doping.28 From the dI/dV spectra, CB-VB edges of the QDs and HOMO-LUMO levels of the polymers could be located in the form of peaks in the LDOS spectra. DOS of QDs as a function of dopantconcentration is shown in Figure 3a. The figure shows that in undoped QDs, Fermi energy was marginally closer to the VB edge inferring their pristine (mild) p-type nature. In STS of doped-QDs, we have observed a shift in Fermi energy towards the CB. With HOMO and LUMO levels of the polymer available from separate STS measurements, a shift in Fermi energy of QDs upon doping favored formation of a type-II band-alignment at the polymer:QD interface. In other words, energy diagrams of BHJs based on P3HT:QDs before and after bismuth doping in the QDs, as presented in Figure 3b, evidenced formation of a better type-II band-alignment for improved performance in P3HT:Bi@PbS hybrid BHJ solar cells. Formation of a favorable type-II band-alignment at the P3HT:QD interface upon bismuth doping can also be supported by recording photovoltaic characteristics of the devices. We have observed that upon bismuth doping, the shift in Fermi energy of QDs would result in a decrease in VOC, which could be correlated to the difference between HOMO of P3HT and CB of QDs. Here, the energies refer to levels as seen by the charge-carriers in the devices. The type-II band-alignment that the dopants in QDs yielded was however beneficial for charge separation thereby lowering recombination processes of separated carriers. Moreover, the increased conductivity in doped QDs (inferred from the shift in Fermi energy) facilitated extraction of electrons through the QDs resulting in an increase in short-circuit current (JSC). Hence,

of the solar cells responded to the doping content in PbS QDs and maximized at an optimum

bismuth concentration (Figure 3c). Such a behavior could therefore be correlated to the band-diagram of the hybrid BHJ solar cells as drawn from STS of the materials.


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0.2

(a)

Energy (eV)

0.3

0%

0.1 0.0

PbS

-4

-3

P3HT

Bi@PbS

-4

Fermi Energy

-5

-5

1%

0.2

-6

0.1 0.0

(b)

P3HT:PbS

0.1 0.0

6%

0.2 0.1 -1.5

-1.0

-0.5

0.0

0.5

Voltage (V)

1.0

1.5

P3HT:Bi@PbS

-6

Bi content in Bi@PbS QDs: 0% 1%

(c)

2

3%

Current Density (mA/cm )

4

0.2

0.0

P3HT

Energy (eV)

-3

dI/dV (nS)

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0

2% 3% 4% 5% 6% 9%

-4

-8 -0.2

0.0

0.2

0.4

Voltage (V)

Figure 3. (a) Differential conductance (dI/dV) plots of PbS and bismuth-doped PbS QDs. Dopant content in the QDs is specified as legends. CB and VB edges of the QDs are marked in the DOS spectra as vertical broken and continuous lines in the positive and the negative voltages, respectively. (b) Energy diagram of BHJs based on P3HT:QDs before and after bismuth doping in the QDs sandwiched between ITO and Al electrodes. (c) Current−voltage characteristics of P3HT:QD BHJ devices under a 1 Sun illumination condition. Content of bismuth in Bi-doped PbS (Bi@PbS) QDs was varied from 0 to 9%. Redrawn and reproduced from ref 28. Copyright 2017 American Chemical Society.

Band-Diagram in Layered Junctions. In the following, we will cite examples of a couple of multilayered-structures, such as (1) pn-junction solar cells based on p-type Cu2FeSnS4 (CFTS) and a range of chalcogenides as n-type compound semiconductors and (2) a p-i-n structure for perovskite solar cells.

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CFTS is an emerging photovoltaic material having structure, band-gap, and absorption coefficient analogous to Cu2ZnSnS(Se)4 (CZTSSe). We considered a range of chalcogenides as n-type compound semiconductors, in conjunction with the p-nature of CFTS to fabricate and characterize pn-junction solar cells.41 To establish a correlation between the band-alignment in pn-junctions and

of solar cells based on

these junctions, we have estimated the band-edges of individual materials and drawn a tentative energylevel diagram of the devices (Figure 4). Upon characterization, we observed that the photovoltaic parameters, as presented in Table 1, were well-correlated to the band-diagram. The VOC of any pnheterojunction solar cell is known to depend on the band offset between the VB of the p-type material and the CB of the other. As drawn from STS, the offset indeed dictated the VOC of the solar cells. Here a smaller offset (between VB of p-type and CB of n-type) led to a decrease in VOC. A larger offset in CB (and VB) between the materials on the other hand increase charge-separation and hence JSC of the solar cells. The CFTS|Bi2S3 heterojunction yielded optimized efficiency amongst these bilayer solar cell structures.

Figure 4. Schematic energy level diagram of CFTS|CdS, CFTS|Bi2S3, and CFTS|Ag2S heterojunctions. The line at 0 V represents the Fermi energy after contact. Redrawn and reproduced from ref 41. Copyright 2017 Elsevier Ltd.


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Table 1. Photovoltaic parameters of solar cells based on different pn-heterojunctions and the Schottky diode. Numbers in the parentheses denote the average value of respective parameters and their standard deviation. Reproduced from ref 41. Copyright 2016 Elsevier Ltd. Active Layer

VOC (V)

JSC (mA/cm2)

Fill Factor (FF) (%)

η (%)

0.61 (0.58, ±0.02)

9.3 (8.8, ±0.37)

52 (48, ±0.03)

2.95 (2.49±0.29)

CFTS | Bi2S3

Difference between VB of CFTS and CB of ntype material (eV) 0.97

CFTS | CdS

0.93

0.56 (0.53, ±0.03)

6.5 (5.6, ±0.87)

37 (33, ±0.05)

1.37 (0.96, ±0.21)

CFTS | Ag2S

0.90

0.40 (0.38, ±0.02)

6.0 (5.7, ±0.36)

32 (28, ±0.02)

0.77 (0.61, ±0.10)

-

0.30 (0.30, ±0.04)

2.0 (1.6, ±0.30)

33 (30, ±0.03)

0.20 (0.14, ±0.04)

CFTS

Introduction of inorganic, low cost, and stable hole-transport layer in perovskite (CH3NH3PbI3) solar cell has attracted a huge interest recently.42,43 In spite of the success of spiro-MeOTAD, whose chemical name is 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene, its high cost and low ambient-stability have motivated researchers around the globe to seek an alternative to this material. In this direction our group has incorporated thin-films of cuprous oxide (Cu2O) as an efficient hole-transport material in planar perovskite solar cells. To check whether the Cu2O|CH3NH3PbI3 interface is energetically favorable for charge separation, we again opted to draw a band-diagram of the p-i-n heterojunction using STS and the corresponding DOS (Figure 5a).43 PCBM (phenyl-C61-butyric acid methyl ester) has been the electron-transport material. The DOS spectra and histogram of energies thereof revealed that Cu2O would form a type-II band-alignment with CH3NH3PbI3 (MAPbI3); energy levels at the MAPbI3|PCBM substantiated the conventional usage of PCBM for easy electron-transport to the top electrode (aluminum). The device structure would provide efficient charge-separation possibilities and thereby reduce recombination pathways. In fact the photovoltaic parameters of the corresponding devices


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evidenced such a possibility (Figure 5b). The results hence show that band-diagram of solar cells drawn a priori through STS can be handy in predicting functionally of the devices. 15 2

(a)


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(b) Cu2O|CH3NH3PbI3|PCBM

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Figure 5. (a) Schematic energy level diagram of Cu2O|MAPbI3|PCBM heterojunction. The dashed line represents the Fermi energy after contact. (b) Current-voltage characteristics of Cu2O|MAPbI3|PCBM heterojunctions under dark and a white light illumination condition. Redrawn and reproduced from ref 43. Copyright 2017 American Chemical Society.

Depletion Region in a pn-Junction Nanorod vis-à-vis Charge Separation in Solar Cells. We may recall that nanorods containing a pn-junction, which offers a field-assisted charge separation, includes a depletion region along with p- and n-sections at the two ends. Upon illumination, separation of charge carriers occurs through a drift of minority carriers across the depletion region. Here we first formed pnjunction in a nanorod through a fine chemistry following a colloidal synthesis route.44,45 We showed that these nanorod-junctions were handy in charge separation upon illumination.46 We aimed to use nanorod-junctions, each of which containing an intimate contact between p-type Cu2S and n-type CdS sections, in a polymer matrix to form hybrid BHJ devices.46 This is in contrast to the conventional polymer:QD excitonic hybrid BHJs,8 which generally dissociate excitons generated within a much shorter length-scale, to the tune of exciton diffusion length in the two materials. When nanorodjunctions were put in a polymer matrix, each nanorod-junction acted as an efficient charge-separating 17 Environment ACS Paragon Plus

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channel in hybrid BHJ solar cells. The charge-separation occurred due to the perfect type-II bandalignment at the interface devoid of any organic ligands. It may further be stated that the length of nanorod-junctions should preferably be as long as the sum of the depletion width and exciton diffusion lengths from the two ends of the depletion-region. With such a length of nanorod-junctions, excitons photogenerated anywhere in the nanorod would diffuse to reach the depletion region for charge separation. Since the p-type Cu2S would not provide an efficient channel for electron-transport, we varied the comparative length of the segments in the nanorod-junctions to keep it to a minimum. We first mapped the nanorod-junction across its length by recording tunneling current at different points on the nanorods.47 From the DOS spectra, we could locate the CB and VB-edges across the Cu2S and the CdS section of the pn-junction. The band-edges at the two ends of a pn-junction nanorod resembled that of CdS and of Cu2S. The DOS spectra across the pn-junction yielded a band-bending along with the depletion region at the junction (Figure 6a). The band-diagram moreover showed that in a nanorodjunction, the depletion region extended to around 42 nm. We could moreover estimate the width of the depletion region extended to the p- and n-sections separately. We then characterized thin-film devices based on BHJ between pn-junction nanorods and P3HT. We also formed BHJs with the nanorods of individual materials for comparison. With 60 nm long nanorodjunctions, the length of Cu2S with respect to the CdS section was varied to three possible ratios. When we characterized the devices under a white light illumination, all the devices acted as solar cells (Figure 6b). While comparing the photovoltaic characteristics, we found that the parameters of solar cells improved drastically in the pn-junction nanorod based hybrid BHJs. This improvement was in comparison to the BHJs based on individual nanorods, that is, P3HT:CdS and P3HT:Cu2S. The improvement occurred since the pn-junction consisted of a depletion region that is instrumental in separating charge carriers through drift of minority carriers across the region. In fact, amongst the BHJs based on P3HT:nanorod-junctions, optimized when the depletion region could form fully in the junction (Cu2S|CdS = 40:60). Thus the nanorods introduced efficient channels for charge separation without substantial sacrifice in electron ACS Paragon 18 Plus Environment

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transporting CdS channels. Such a notion is supported by an improved VOC and fill-factor both in P3HT:pn-junction devices. It may be stated that the electric field in nanorod-junctions helped in separation of charge carriers only. Transport of carriers, on the other hand, occurred due to the conventional electric field that was developed because of dissimilar work-functions of ITO and aluminum electrodes. Y[nm]

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Figure 6. (a) Mapping of a CdS|Cu2S nanorod junction for CB and VB-edges along the length of the nanorod. The figure in addition shows the STM topography of the nanorod junction and the spots at which tunneling current was measured. CB and VB edges along with the Fermi energy (EF) across the junction and the depletion region of the pn-junction are also shown in the figure. (b) Current–voltage characteristics of BHJs based on Cu2S nanorods, CdS nanorods, and Cu2S|CdS nanorod-junctions in P3HT matrix under a 1 Sun illumination condition. For the BHJs based on nanorod-junctions, relative length p- and n-sections was varied. Redrawn and reproduced from refs 46,47. Copyright 2017 American Chemical Society and Elsevier Ltd.


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dI/dV Images of Heterojunctions As stated earlier, AFM is not capable to define the domains in a BHJ. For this purpose STS can be utilized to image the percolating network of electron-donor and acceptor in the active layer since the imaging process of STS relies on energy-levels of semiconductors. In a BHJ, morphological characterization through energy-mapping will be of great relevance. One of the most significant studies in this direction has been conducted in a 100 nm thick P3HT:PCBM BHJ through cross-sectional STS measurements (Figure 7).48 In contrast to the traditional cross-sectional SEM imaging, a sharp boundary appeared in the STS image. In addition to topographical images, the film was mapped by tunneling conductance obtained from differential tunneling current (dI/dV). The donor and acceptor regions hence returned different DOS depending on their set of energy levels. The interpenetrating phases could therefore be identified and indicated in the nanometer scale.

Figure 7. (a) Normalized dI/dV images of the P3HT:PCBM BHJ. (b) A magnified sliced image across the PCBM+(1)/(P3HT+)(2)/PCBM+(3) heterojunction. (c) Local density of states (LDOS) measurements from PCBM+ to P3HT+ across the interfacial region are indicated by red, green, and yellow curves. (d) Atomicscale evolution of band alignment across the P3HT+/PCBM+ heterointerface. Here P3HT+ and PCBM+ represent P3HT-rich and PCBM-rich sections, respectively. Reproduced from ref 48. Copyright 2017 American Chemical Society. ACS Paragon 20 Plus Environment

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Vertical segregation of interpenetrating donor-acceptor phases, which provide better charge-collecting pathways, could be visualized in dI/dV images. LDOS at the nanoscale interface expectedly indicated type-II band-alignment with an offset of 1.4 eV between HOMO of P3HT and LUMO of PCBM. The observed 1.6 nm spatial extent of the interface along with 10 nm diffusion length of excitons suggested dissociation of most of the photogenerated excitons. This report undoubtedly demonstrated the applicability of STS in viewing morphology of BHJs to optimize charge separation and collection processes in solar cells. Through STS, we have also imaged a heterostructure, namely a junction formed in a single nanorod.49 With DOS providing location of CB and VB-edges, we have recorded dI/dV images at different voltages. In Figure 8, we have presented the images of a Cu2S|CdS heterojunction nanorod. The section of the nanorod-junction having a nonzero DOS at a voltage could be found to be visible in dI/dV images at that voltage. In other words, selective sections of the heterojunction could be viewed in the images. The images could be correlated to the DOS spectra of the individual semiconductors. Such a voltage-selective viewing in dI/dV images allowed us to image the undepleted p- and n-sections and more importantly the depletion-region of the heterojunction extended to each of the semiconductor sections.


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(a) Cu2S|CdS

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Figure 8. (a) STM topography of a Cu2S|CdS nanorod junction along with its line profile; the z-scale bar is shown in (a). dI/dV images of the junction by probing (b) CB of n-type CdS, (c) between the CBs of the two semiconductors, (d) above both the CBs, (e) VB of p-type Cu2S, (f) between the two VBs, and (g) below both the VBs. Sample voltage have been specified as legends. Reproduced from ref 49. Copyright 2017 IOP Publishing Ltd.

Outlook and Future Challenges STS has recently been proven to be an unprecedented tool for determination of DOS of semiconductors and thereby band-mapping in junctions. In this context, band-diagram of various junctions for solar cell applications could be drawn precisely with energies as actually encountered by electrons/holes in a device. The required type-II band-alignment in a junction for photovoltaic devices based on upcoming materials can be drawn before actually forming the devices. Junctions based on organic and/or inorganic ACS Paragon 22 Plus Environment

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semiconductors in the form of organic/organic, organic/inorganic, and also inorganic/inorganic interfaces alike can be studied through STS. The band-diagram drawn through this method will infer on the separation of carriers and pathways for electrons and holes to the opposite electrodes. Morphology of the materials in a heterojunction can be studied elegantly through STS. With the dI/dV imaging technique, domains in the nanoscale can be identified due to the energy-level-mapping nature of the technique. Due to identification of domains in a heterojunction, the goal of correlating morphology and energetic characteristics with device performance could be achieved. This is due to the fact that bicontinuous pathways for electrons and holes lower the recombination loss and thereby yield efficient transport processes. By drawing energy band-diagrams and imaging domains of donor and acceptor materials, we expect to achieve new insights towards improvement of device performance. Application of STS in photovoltaics is relatively a new area of research. The band diagrams drawn from STS are however with respect to the aligned Fermi energy. CB and VB energies of the materials would have allowed us to draw the exact band diagram with respect to the vacuum level. Also, cross-sectional STS set-up needs to be a common experimental technique in deriving domains along the depth of a device.

Biographies Mr. Uttiya Dasgupta is a graduate student at the Indian Association for the Cultivation of Science (IACS) in the Department of Solid State Physics since November 2012. His research interest includes fabrication and characterization of photovoltaic devices based on inorganic nanocrystals and perovskite thin-films. Dr. Abhijit Bera was a final year graduate student at IACS during preparation of the manuscript. His Ph.D. thesis was on scanning tunneling spectroscopy of a range of nanostructures and characterization of


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molecular rectifier and magnetoresistive devices. Very recently, he moved to TU Wien, Vienna as a postdoctoral fellow. Prof. Amlan J. Pal is a Professor at Indian Association for the Cultivation of Science in the Department of Solid State Physics. His research group works on organic electronics, scanning tunneling spectroscopy, fabrication and characterization of opto-electronic devices based on inorganic nanocrystals and conjugated organics, and organic spintronics. (Web: http://iacs.res.in/faculty-profile.html?id=73).

Acknowledgements AJP acknowledges JC Bose Fellowship (SB/S2/JCB-001/2016) of SERB. The authors acknowledge financial assistance from DeitY and SERIIUS projects bearing grant numbers 12(1)/2012-EMCD and IUSSTF/JCERDC-SERIIUS/2012, respectively. UD and AB acknowledge CSIR Junior Research Fellowship Numbers 09/080(0843)/2012-EMR-I (Roll No. 519699) and 09/080(0779)/2011-EMR-I (Roll No. 510847), respectively.

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(27) Ning, Z. J.; Zhitomirsky, D.; Adinolfi, V.; Sutherland, B.; Xu, J. X.; Voznyy, O.; Maraghechi, P.; Lan, X. Z.; Hoogland, S.; Ren, Y.; Sargent, E. H. Graded Doping for Enhanced Colloidal Quantum Dot Photovoltaics. Adv. Mater. 2013, 25, 1719-1723. (28) Saha, S. K.; Bera, A.; Pal, A. J. Improvement in PbS-based Hybrid Bulk-Heterojunction Solar Cells through Band Alignment via Bismuth Doping in the Nanocrystals. ACS Appl. Mater. Interfaces 2015, 7, 8886-8893. (29) Bhunia, H.; Kundu, B.; Chatterjee, S.; Pal, A. J. Heterovalent Substitution in Anionic and Cationic Positions of PbS Thin-Films Grown by SILAR Method vis-a-vis Fermi Energy Measured through Scanning Tunneling Spectroscopy. J. Mater. Chem. C 2016, 4, 551-558. (30) Zillner, E.; Fengler, S.; Niyamakom, P.; Rauscher, F.; Kohler, K.; Dittrich, T. Role of Ligand Exchange at CdSe Quantum Dot Layers for Charge Separation. J. Phys. Chem. C 2012, 116, 16747-16754. (31) Kundu, B.; Chakrabarti, S.; Pal, A. J. Redox Levels of Dithiols in II-VI Quantum Dots vis-a-vis Photoluminescence Quenching: Insight from Scanning Tunneling Spectroscopy. Chem. Mater. 2014, 26, 55065513. (32) Millo, O.; Balberg, I.; Azulay, D.; Purkait, T. K.; Swarnakar, A. K.; Rivard, E.; Veinot, J. G. C. Direct Evaluation of the Quantum Confinement Effect in Single Isolated Ge Nanocrystals. J. Phys. Chem. Lett. 2015, 6, 3396-3402. (33) Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulovic, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863-5872. (34) Leschkies, K. S.; Beatty, T. J.; Kang, M. S.; Norris, D. J.; Aydil, E. S. Solar Cells Based on Junctions between Colloidal PbSe Nanocrystals and Thin ZnO Films. ACS Nano 2009, 3, 3638-3648. (35) Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960-1969. (36) Kim, D.; Kim, D. H.; Lee, J. H.; Grossman, J. C. Impact of Stoichiometry on the Electronic Structure of PbS Quantum Dots. Phys. Rev. Lett. 2013, 110, 196802. (37) Oh, S. J.; Berry, N. E.; Choi, J. H.; Gaulding, E. A.; Paik, T.; Hong, S. H.; Murray, C. B.; Kagan, C. R. Stoichiometric Control of Lead Chalcogenide Nanocrystal Solids to Enhance Their Electronic and Optoelectronic Device Performance. ACS Nano 2013, 7, 2413-2421.


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ACS Energy Letters

Quotes:

With scanning tunneling spectroscopy (STS), band-diagram of solar cells can be drawn from the viewpoint of charge-carriers in the devices.

The required type-II band-alignment in a junction for photovoltaic devices based on upcoming materials can be drawn before actually forming the devices.

Differential conductance (dI/dV) images are capable of energetically map domains of the materials in a bulk-heterojunction (BHJ).

Morphology of the components in BHJs and device performance can be correlated.


Band Diagram of Heterojunction Solar Cells ... - ACS Publications

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