Tetrahedrite (Cu12Sb4S13) Ternary Inorganic Hole Conductor for

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Tetrahedrite (Cu12Sb4S13) Ternary Inorganic Hole Conductor for Ambient Processed Stable Perovskite Solar Cells Muthusamy Tamilselvan, and Aninda Jiban Bhattacharyya ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Energy Materials

Tetrahedrite (Cu12Sb4S13) Ternary Inorganic Hole Conductor for Ambient Processed Stable Perovskite Solar Cells Muthusamy Tamilselvan and Aninda J. Bhattacharyya* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India KEYWORDS: metal sulphide, ternary semiconductor, perovskite, tetrahedrite, hole conductor, hydrophobicity

ACS Paragon Plus Environment

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ABSTRACT

Hole transport layer (HTL) in a solar cell is a key component for the optimization of photon to electron conversion efficiency and long-term device stability. So far, organic hole transport (HT) materials have been extensively used in mesoscopic perovskite (n-i-p) solar cell for achieving high efficiency. Generally, organic hole transporters are expensive and exhibit low chemical stability. This often results in rapid degradation in the photon to electron conversion efficiencies. In this report, copper antimony sulfide (CAS, Cu12Sb4S13) tetrahedrite crystal phase nanoparticles capped with organic ligands are synthesized using a simple hot injection method. The ternary semiconductor nanoparticles exhibit high carrier concentration (3.4×1018 cm-3) and electrical conductivity (0.043 Ω cm) with respect to the conventionally used organic HT e.g. spiro-ometad. Additionally, appropriate conduction band and valence band positions in the CAS nanoparticles assist in efficient separation of the photo generated holes from the perovskite materials and block the electrons from moving transport towards the counter metal contact. Incorporation of CAS in the MAPbI3 perovskite solar cell as an HTL assembled using ambient fabrication process, achieves a power conversion efficiency of approximately 6.5%. The efficiency degradation studied under controlled humidity (˂ 70%) conditions exhibited a slower rate of decrease for the MAPbI3-CAS (85% and 50% on the 4th and 15th day) compared to the MAPbI3-spiro solar cell (10% on the 4th day). The chemical stability and solar cell performance are attributed to the synergetic effect of crystal phase stability and hydrophobic nature of the organic ligand capped CAS HTL layer. This leads to high moisture resistance and nullifies the need for an additional encapsulation layer as demonstrated in reported literature.


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1. INTRODUCTION

Photovoltaic (PV) technologies provide great opportunities to meet the ever increasing worldwide demand for energy in an environmental friendly and sustainable manner. Till date, crystalline silicon has dominated the PV domain in spite of its high cost of manufacturing in comparison to other promising solar photon harvesting materials such as CZTS, CdTe.1,2 The third generation solar cell technology involving several inorganic quantum dots, organic polymers and dyes as light harvesters still lag far behind silicon technology in terms of efficiency and poor stability.2-4 In the recent times, the organic-inorganic hybrid perovskite materials with the general chemical formula, MAPbX3 (e.g. MA= CH3NH3, X= I) have been demonstrated as the most efficient light absorber materials and pose as a formidable competitor to silicon. Within a very short span of time, the perovskite light absorbers have demonstrated a very high light to electricity conversion efficiency. It has been recently demonstrated that perovskites assembled in the mesoscopic configuration (n-i-p) resulted in a record high light to electricity conversion efficiency of 22%.5 The remarkable efficiency of MAPbI3 based solar cells is attributed to the unique ns2 configuration properties of the perovskite materials viz. effective mass of charge, defects tolerance, low exciton binding energy, and long diffusion path length.6 In the presence of moisture and air however, the organic-inorganic hybrid perovskite materials undergo crystal phase degradation due to volatile nature of organic cations (CH3NH3I). This limits the long term stable power conversion efficiency leading to formidable challenges towards large scale device fabrication.7 Altering the chemical composition of perovskites with very stable organic cations or introducing a moisture protective layer in the solar cell device are some of the strategies that have been considered for enhancing the device stability.8-11 The former strategy results in undesirable changes in the optical and electrical properties of the light


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absorber.12 In the latter, a hydrophobic hole transporting layer (HTL) is placed on top of the light absorber to enhance the device stability.13 In the mesoscopic (n-i-p) solar cell configuration,

fluorene–dithiophene (FDT), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]

(PTAA) and 2,2´,7,7´-tetrakis(N,N-p-dimethoxyphenylamino)-9,90-spirobi-uorene (Spiro) are widely employed in the perovskite solar cell.14-17 Using organic hole conductors, solar cells with high power conversion efficiency have been demonstrated. However, the devices display poor chemical stability and has necessitated the inclusion of hydrophilic additives such as salts (e.g. LiTFSI).18 On the other hand, some of the inorganic metal oxides (NiO)19, metal halides/pseudohalides (CuI, CuSCN)

20,21

, and light absorber materials (CZTS, CIS, CGS)8,22-25

have been also used as hole conductors in n-i-p type solar cell. The photo conversion efficiency of the above materials is not as high as the organic hole conductors. In recent years, hole conductor free solar cells have demonstrated efficiencies comparable to HTL incorporated solar cells.26-28 However, retention of high light to electricity conversion efficiencies over a longer period is still a formidable challenge. The problem persists due to the non-trivialities associated with encapsulation of the high moisture sensitive perovskite light absorbing materials. Therefore, chemical design of efficient hole conducting materials, should satisfy the following criteria:29 (a) high hole mobility, (b) appropriate

valence band maximum (VBM) for the

absorber

materials, (c) long term stability under moisture and air, and (d) pin hole free layer formation. Thus, exploring new p-type inorganic hole conducting materials with the above-mentioned requirements are highly recommended. Group I-VI-VII semiconductors (e.g. I-Cu; VI- Sb: VII-S, Se) occur in different crystallographic phases and compositions. They are found in various naturally occurring mineral forms like chalcostbite (CuSbS2), tetrahedrite (Cu12Sb4S13), famatinite (Cu3SbS4), and skinnerite


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(Cu3SbS3).30,31 The optical band gap ranges from 1 to 2 eV depending on the elemental composition as well as crystallographic phase.31 Due to high optical coefficient, low band gap, non-toxicity and high earth abundant elements, these are considered as potential candidates in the field of optoelectronic devices.32-35 Like other copper based semiconductors, this group too has a p-type conductivity due to intrinsic copper vacancy defects formed in the crystal.36 Among various crystal forms, the tetrahedrite phase Cu12Sb4S13 (CAS) possesses a unique unit cell arrangement with multivalent states of copper-ions in the crystals resulting in a high carrier concentration and low electrical resistivity.37 Due to high thermal stability and electrical conductivity, the tetrahedrites have been widely studied in the field of thermoelectric materials. In this report, we have synthesized single phase tetrahedrite Cu12Sb4S13 (CAS) nanoparticles via hot injection method with organic ligands as the capping agent. The optical and electrical properties of the organic ligand capped CAS nanoparticle thin films have been extensively studied. Mesoscopic (n-i-p) perovskite solar cell using CAS nanoparticle thin film as the hole transporting layer assembled under ambient conditions showed exceptional stability when monitored for more than half a month. The long-term stability is attributed to the organic ligand capped CAS HTL which performs the dual function of extracting the photogenerated hole from the light absorber and retaining the stability of perovskite via prevention of penetration of moisture.

2. EXPERIMENTAL SECTION

2.1. Synthesis of copper antimony sulfide (Cu12Sb4S13): Single phase, tetrahedrite CAS nanocrystals were successfully synthesized by the hot injection protocol. In detail, 1 mmol of copper (I) iodide (CuI) and 0.3 mmol of antimony (III) chloride (SbCl3) were added into a 100


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ml three neck round bottom (RB) flask along with 15 ml of oleylamine. Here, oleylamine is used as a solvent and capping agent for nanoparticle synthesis. 1 mmol of sulphide source solution was prepared by sonication of thioacetamide in 1.5 ml of oleylamine at room temperature. The temperature of the solution in RB flask was raised to 120 oC at the rate of 5 oC/mins under constant stirring and maintained for 45 min to get a yellow colored solution of metal-oleylamine complex. Then, the temperature of the metal complex solution in the RB flask was further increased to 180 oC. The prepared thioacetamide solution was injected into the metal-oleylamine complex solution rapidly. The color of the solution turned into brown-red. After 10 mins the colloidal solution was rapidly cooled to room temperature by adding toluene and precipitating the CAS nanoparticles with addition of ethanol. The precipitate was collected by centrifugation and further subjected to washing with 1:1 toluene and ethanol mixture. The nanoparticles were dispersed in chlorobenzene for further use.

2.2. Device fabrication: Transparent FTO glass (8 ohm/sq) were etched in a desired pattern by using Zn powder with 2M HCl solution, followed by sonication with detergent, acetone and ethanol separately. Finally, the etched substrate was dried by flowing nitrogen gas. A dense TiO2 blocking layer was deposited on patterned FTO glass by spin coating titanium di isopropoxide bis-(acetylacetonate) in ethanol (1:8) v/v ratio. Mesoporous TiO2 film was deposited on the blocking layer by commercial available TiO2 paste (Dyesol NRT 3:1 volume ratio with anhydrous ethanol), spin coated at 2000 rpm for 60 s and sintered at 500 oC for 30 min. The mesopours TiO2 layer was sensitized by a light absorber material MAPbI3 based on previously reported protocol.49 The perovskite precursor solution was prepared by dissolving an equimolar amount (1.25 mmol) of PbI2 (0.593 g) and MAI (0.197g) in mixture of 700 µl γButyrolactone and 300 µl DMSO at 70 oC for 12 h. A clear yellow color solution of perovskite


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precursor was dropped on the FTO/TiO2 mesoporous film and spun via two successive processes at 1000 rpm for 10 s and 4000 rpm for 30 s, respectively. The perovskite coated FTO/TiO2 substrate was kept on a pre-heated hot plate for solvent removal and thermal annealing at 110 oC for 10 mins. Once the temperature of the perovskite coated FTO/TiO2 substrate reached to room temperature, inorganic hole conductor CAS (dispersed in chlorobenzene) was spin coated on top of it at variable spin rates for 30 s. This was followed by heating at 100 oC to remove the solvent from the CAS HTL. To compare the CAS hole conductor efficiency, a reference cell was fabricated with the well-known spiro-OMeTAD organic hole conductor.22 In detail, 73.2 mg of spiro-OMeTAD and 29 µL of 4-tert-butylpyridine dissolved in 1 mL of chlorobenzene. Later this solution is mixed with 18 µL of lithium-bis-(trifluoro methane sulfonyl) imide (Li-TFSI) solution (520 mg LiTFSI in 1 mL acetonitrile). The spiro layer was deposited on top of the pervoskite layer by spin-coating the above mixture solution at 3000 rpm for 30 s. The metal contact (Au 100 nm) was deposited on the hole conductor layer by thermal vaporization technique along with shadow mask to define the effective active area ( ≈ 0.09 cm2) of solar cell. 2.3. Characterization and measurements: PANanyltical Empyrean X-ray diffractometer; Cu-Kα radiation, 1.5418 Å with Nickel filter was used to record the

X- ray diffractogram (XRD) of

CAS nanoparticles. Chemical

composition of CAS sample was performed using EDS facility of FESEM. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were done using a JEOL-200 kV FETEM and the micrographic images were analysed using a Digital micrograph software. Thickness and cross section of each layers in solar cell was examined by scanning electron microscopy (SEM) (Ultra55 FESEM Karl Zeiss). Absorption spectroscopic diffuse reflectance spectra (DRS) were recorded using solid-state UV-visible spectrophotometer


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(PerkinElmer, Lambda 35). The X-ray photoelectron spectroscopy (XPS) and ultraviolet valence photoelectron spectroscopy data were recorded by AXIS-Ultra using monochromatic Al-Kα radiation (225 W, 15 mA, 15 kV). Contact angle measurements were done using sessile drop method with Holmarc optics contact angle meter (Holmarc optics, Cochin, India Model no: HOIAD-CAM-01A) of about 3 µl water droplet on the film and images were recorded with high performance CMOS sensor of image resolution 1280 X 960 at 10 fps. The pellet of CAS nanoparticles was placed in 4.13K gauss magnetic field

to perform the Hall measurement.

Keithely source meter unit (Model 2400) was used to perform the current vs voltage (I-V) characterizations under a simulated 1.5AM solar irradiation with power of 100 mW/cm2. Orial class 3A solar simulator was used to generate radiation and power of the lamp was calibrated by the mono crystalline silicon reference cell (NREL). All the I-V measurements were carried out in a conditioned room with humidity less than 70% at room temperature. 3. RESULTS AND DISCUSSION

Figure 1. a) Powder-XRD pattern of CAS nanoparticles shown along with the reference pattern (ICDD No. 00-042-0561). b) UV-Visible absorption spectrum. The inset shows the Tauc’s plot, used for the estimation of band gap of the CAS nanoparticles.


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Copper antimony sulfide based ternary compounds naturally occur in different crystallographic phases with various chemical compositions. Different synthesis methods have been employed to synthesis copper antimony sulfide in the tetrahedrite (Cu12Sb4S13) crystal form.30,31,38-40 We show here the synthesis of single phase CAS nanocrystals by using an optimum hot injection protocol using a simple sulfide source such as thioacetamide. Crystallographic phase purity of the CAS nanocrystals is characterized by PXRD and the obtained diffractogram is shown in Figure 1a. Prominent peaks at 2θ = 24.34, 29.95, 34.66, 48.87, 59.28° correspond to the planes (220), (222), (400), (440), (622) and are in good agreement with ICDD No. 042-0561 (Space group: I4̅3m; a = b = c = 10.323 Å and α = β = γ = 90°). In order to ensure a single phase CAS nanoparticle, the samples were further sintered at 450 °C for 2 h in nitrogen atmosphere (Figure S1). No extra peak appears in the diffraction pattern following the sintering procedure which confirmed the presence of single phase purity of CAS sample. Figure 1b shows the optical absorption spectrum of CAS nanocrystals dispersed in chlorobenzene. Two different distinct features are observed in the spectrum, indicated by arrows marked as 1 and 2. The small shoulder absorbance at 550 nm indicates (arrow-1) is attributed to the excitonic transition which typically occurs in nanocrystals due to weak quantum confinement effects.41 The absorbance band occurring at around 650 nm (arrow-2) is attributed to the combination of the absorption onset along with sub-band gap excitonic transitions due to copper deficiency.42 The direct and indirect band gap are calculated using the Tauc plot equation (αhυ)1/n = A (hυ-Eg); where h is the Planck's constant, υ the photon frequency, α the absorption coefficient, Eg is the band gap, and A is a proportionality constant. The direct and indirect band gap have been calculated via substitution of n value as ½ and 2 respectively in the above equation. From the calculation, the CAS nanoparticles reveal both direct (= 2.0 eV, figure S2)


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and indirect (= 1.83 eV) (Figure 1b inset,). So, based on these estimates we have concluded that CAS is an indirect band gap semiconductor. The estimated values reported here are comparable with the reported value.40 Generally, the hole transport materials (HTL) such as of spiro, CuSCN and CuI have a wide band gap. In the present work, CAS nanoparticles with lower band gap than spiro show significant light absorption in the visible range, as shown in figure 1b. However, this additional light absorption may not translate to enhanced photocurrent.

Figure. 2. a) Transmission electron micrographs of CAS nanoparticles. b) Size distribution of CAS by considering more than 100 nanoparticles. c) HRTEM lattice images with assigned interplanar d-spacing values. d) SAED pattern obtained by the TEM.


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Morphology and size distribution of the oleylamine capped CAS nanocrystals are examined by TEM. The micrograph (Figure 2a) reveal that the particles roughly spherical in shape with an average size of 9 nm (Figure 2b). The HRTEM image of a single nanocrystal is shown in Figure 2c. The observed lattice fringes have an interplanar distance of approximately 0.29 nm, representing the d-spacing corresponding to the (222) plane in the cubic tetrahedrite. The overall crystallinity of the nanocrystals is further confirmed by the SAED pattern of CAS particle (Figure 2d). The rings in the pattern can be assigned to the prominent planes of CAS nanocrystal and are in good agreement with P-XRD results shown in Figure 1a. Composition and uniform distribution of the three elements in CAS nanocrystals (Cu, Sb, S) are confirmed by the EDS and the atomic percentage of each element nearly match with that of the theoretical value Figure S3. Due to existence of the elements in multivalent states in the tetrahedrite crystal structure, the compounds were examined with XPS. The high-resolution core spectrum of the individual elements is fitted with Lorentzian to estimate the binding energy and oxidation states of Cu, Sb, S shown respectively in Figure 3a, b and c. The two peaks with the energy level of 161.6 eV and 162.8 eV represents the binding energy of S 2p3/2 and S 2p1/2, respectively which confirms the -2 oxidation state of sulfur. The doublet peaks of Sb 3d5/2 and Sb 3d3/2 are observed at 529.1 and 538.5 eV, respectively in the core spectrum of Sb. The separation between the peaks is 9.4 eV which confirms the +3 oxidation state of Sb in CAS. In general, oxygen (1s) peaks overlap with Sb (as shown by arrow in the Figure 3b) and represents the absorbed oxygen at the nanoparticle surface which matches with previously published reports.32 Figure 3a shows the high-resolution peak of Cu-atom with multiple peaks. The peaks at 932 and 951.8 eV representing Cu 2p3/2 and Cu 2p1/2 are for oxidation state of +1 and the small peaks at 934.2 and 953.5 eV confirms the presence of Cu (II) ions in the tetrahedrite form. The satellite peaks at around 945 eV also further


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ensures the +2 oxidation state of copper ions present in the tetrahedrite form. This confirms that there are two different oxidation states of Cu present in the tetrahedrite form, which is responsible for the efficient hole conductivity.43 In order to utilize the CAS nanocrystals as a HTL in the solar cell, it is important to estimate the valence band maximum (EVBM) and conduction band minimum (ECBM) positions of the compound. Figure 3d shows the wide spectrum of ultraviolet valence spectroscopy with He I photon energy source. The work function (Fermi level) of the CAS sample is EF = -4.61 eV obtained by subtracting secondary electron onset value (16.6 eV) from excitation energy (21.21 eV) used in the UPS measurement. The EVBM is calculated from the inset of Figure 3d, located at (-4.9 eV versus vacuum) and obtained by fitting the straight line. The ECBM is obtained at around -3.07 eV by adding the optical band gap (Eg = 1.83 eV) of the CAS nanocrystals.


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Figure 3. High resolution core spectrum of Cu 2p (a), Sb 3d (b), S 2p (c) respectively. d). wide range UPS (He I: 21.21 eV) spectrum of CAS nanoparticles. The inset represents the valence band edge spectrum from UPS spectrum. e) Energy band diagram of CAS hole conductor layer along with conventional layers used in perovskite solar cell. The EVBM is located at 0.30 eV just below the Fermi level suggesting that the CAS NPs nanoparticles are a good p-type semiconductor like other wellknown copper-based sulfide semiconductors because of low defect formation energy of intrinsic copper vacancy defects. 34,44. EVBM and ECBM positions of CAS NPs with respect to MAPbI3 and other metal oxides and


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conventional hole conductor e.g. spiro-OMeTAD used in the mesoscopic solar cells are depicted in Figure 3e .11,45 The higher ECBM of CAS NPs than the MaPbI3 act as a barrier for electron transport towards the dark cathode. The EVBM in CAS NPs is located above the EVBM of MAPbI3 and this can easily extract photogenerated holes. To further confirm hole conductivity in the CAS nanocrystals, Hall measurements are conducted based on the van der Paw method. The hole carrier concentration and the electrical resistivity are of the hydraulic pressed CAS nanocrystals at room temperature was obtained to be 3.4×1018 cm-3 0.043 Ω cm respectively. The resistivity value is lower compared to the well-known hole conductors such as spiro (4000 Ω cm), CuI (1 Ω cm), CuSCN (2000 Ω cm), we strongly feel that this will also be promising for perovskite solar cells.46 Figure 4a displays the XRD pattern of the perovskite along with different layers in the solar cell. The peaks at 2θ = 14.10o, 28.47o, 31.85o represent planes (110), (220), (310) in the tetragonal phase structure. No peak is observed at 2θ = 12.5o corresponding to the prominent peak of PbI2 crystal phase which is usually formed along with the MAPbI3.47 The absence of this peak confirms the formation of single phase MAPbI3. The additional peaks marked by star and


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Figure 4. a) XRD pattern of FTO/TiO2/MAPbI3/CAS. The peaks marked with *, o and [] represents the FTO glass, TiO2 layers, and CAS nanoparticle respectively. The inset shows the characteristic XRD peak of CAS nanoparticle. 2b) Transmittance spectra of FTO/TiO2/MAPbI3 with and without CAS layer along with CAS on FTO/TiO2. circle correspond respectively to the FTO glass and TiO2 layers which are placed at the bottom to the MAPbI3 layer. The peak shown with a red color box (Figure 4a inset) represents the (222) plane in CAS film formed by spin coating nanoparticles on the top of the MAPbI3 layer. After coating the CAS NPs layer, the MAPbI3 crystal structure is observed to remain the same which strongly proves that the deposition of oleylamine capped nanoparticles do not deteriorate the MAPbI3 structures under ambient condition fabrication process. Optical transmission spectrum of FTO/TiO2/MAPbI3 was recorded before and after deposition of HTL layer (CAS) including the FTO/TiO2/CAS film (Figure 4b). After incorporation of CAS HTL, there are slight transmission changes in the visible light regime due to smaller band gap (= 1.83 eV) of CAS nanoparticle. Therefore, incorporation of CAS in the devices would slightly improve the visible light absorption due to relatively lower band gap rather than most organic hole conductors. The photo conversion efficiency of solar cell is highly influenced by the morphology and thickness of each layer that are present in the solar cell. With regard to the hole conducting layer it is highly recommended to avoid the undesirable short paths between light absorber materials and metal contact by ensuring a pin hole-free hole transport layer (HTL) of optimum thickness on the perovskite layer. Figure 5a and b depicts the morphology of HTL on the perovskite layer as obtained from scanning electron microscopy (SEM) in the FTO/TiO2/MAPbI3/ CAS(spiro) solar cells.


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Figure 5.

SEM images of top view of

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film morphology a) CAS HTL.b) spiro HTL

onMAPbI3/TiO2/FTO. Inset in a and b represents the water contact angle measurements on respective thin films. High magnification cross section area of (c) with CAS HTL (d) spiro HTL. The MAPbI3 layer (on the TiO2 layer) comprises of small grains of size ≈ (30 -50) nm (Figure S4). After incorporating the CAS NPs on the MAPbI3 layer by spin coating at a desirable optimum spin rate (3000 rpm) and time (30 s) , the grains of MAPbI3 are covered by the hole conductor material resulting in smoothening of the surface compared to the bare MAPbI3 (Figure 5a ). The smooth surface of CAS ensures a pinhole-free layer as well as ensuring even deposition of metal (Au) contacts on the HTL. From the cross-sectional area of solar cell (as obtained from the SEM Figure 5c & d), the thickness of each layer is obtained to be 400 nm, 150 nm, and 850 nm for FTO, TiO2 blocking layer, and TiO2 mesoporous layer with MAPbI3, layers respectively. The thicknesses of CAS and spiro HTL layers are around 280 and 240 nm, respectively. As mentioned earlier, the crystal structure of organic-inorganic perovskite become


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unstable when exposed to moisture. To avoid this situation and improve the long term stability of devices, various strategies towards increasing the hydrophobicity of the HTL have been adopted.13 The hydrophobicity of CAS NPs coated on FTO glass are analyzed by the water contact angle measurements shown in the Figure 5a and 5b insets. CAS HTL shows higher hydrophobic nature with larger contact angle =98 o compared with spiro coated layer =70o. The hydrophobicity of the CAS HTL originate from the long chain capping agents (oleylamine) wrapped around the CAS NPs. Due to the hydrophobic nature of the CAS HTL layer, it is expected that moisture uptake from atmosphere into the perovskite layer will be significantly less compares to the Spiro as the HTL in the device.

Figure 6. J-V characteristic curve of FTO/TiO2/MAPbI3 (CAS/Spiro) devices fabricated at ambient condition. Figure 6 shows the characteristic current density (JSC) versus voltage (V) curve of best performing MAPbI3 solar cell incorporated with an optimized HTL thickness of CAS(spiro) measured under the simulated solar irradiation with the energy of 100 mWcm-2. The perovskite solar cell with as the CAS HTL shows a photon conversion efficiency (PCE ) of approximately


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6.5% along with an open circuit voltage (VOC), current density (JSC) and fill factor (FF) of 0.80 V, 18.08 mA/cm2, 0.45 respectively (c/f Table. 1). The reference cell with the standard organic Table.1. The photovoltaics device parameters of best performing solar cell with different hole transport layer.

Hole Transport layer

JSC (mA/cm-2)

VOC (V)

FF

PCE (%)

(HTL) CAS

18.08

0.80

0.45

6.5

Spiro - OMeTAD

16.1

1.05

0.62

10.5

hole conductor spiro- OMeTAD, fabricated under identical conditions shows an efficiency with 10.5 %. The CAS NPs show considerably higher JSC compared to the spiro hole conductor. This may be attributed to the additional light absorption in the visible light regime as well as due to lower resistivity of the CAS nanoparticles compared to that of spiro-HTL.23,29 The cell with Spiro as the HTL, the VOC and FF increases from 0.80 to 1.05 eV and 0.45 to 0.62 respectively. In general, the VOC of the devices is calculated by the energy difference between the Fermi level of the electron transport layer (ETL) and HTL in the solar cell.48 As discussed previously, the EVBM of CAS (-4.9 eV) is greater than of the Spiro EVBM value (-5.20 eV). This may introduce the VOC loss in the cell. With regard to the fill factor, the device with Spiro shows a better coverage of the perovskite layer and smooth interface with back metal contact, comparing with CAS HTL. The average photovoltaic parameters of five cells for each HTLs incorporated in the devices are shown in the Supporting Information Table S1. Inspite of lower PCE of perovskite/CAS compared to the perovskite/Spiro solar cell, the CAS HTL device displays promising values for


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the key photovoltaic parameters and good hole conducting characteristics. It is envisaged that with better device fabrication processes resulting in improved perovskite films, cells with CAS HTL films will expectedly produce improved PCE.

Figure 7. Normalized value of (a) JSC, (b) VOC, (c) FF and (d) PCE as a function of time To study the stability of the solar cells fabricated under ambient conditions, the devices are stored inside a desiccator for 15 days under laboratory light conditions. We carried out a systematic device stability test based on performance (PCE) for both MAPbI3/CAS and


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MAPbI3/Spiro forms at different time intervals. Figure 7 shows the degradation of normalized photovoltaic parameters such as JSC, VOC, FF, and PCE over a period of 15 days. The MAPbI3/ CAS device retains 85% and 75% of PCE ability on the 4th and 10th day of measurements respectively, with a further decrease to nearly 50% at end of the 15 days. The FF for the MAPbI3/CAS device remained stable for the entire measurement period. The degradation in VOC and JSC is around 72% and 70% at the end of 15days, respectively. In case of MAPbI3/Spiro device shows drastic degradation in PCE from 10.50% to 1.12% (4th day)

and it degrades

completely on 10th day. This despite the fact that is displays a higher 1st day PCE compared with MAPBI3/CAS devices. Based on the above observations, the MAPbI3/CAS has superior stability in terms of PCE when compared to the Spiro-based devices and the degradation in PCE is retained for over 10 days of measurement. This PCE stability over a long period of time is attributed to the synergetic effects of de-wetting and very stable inorganic structure in the CAS HTL. As a result of this, penetration of moisture into the light absorption perovskite layers are significantly less leading to enhanced long time chemical stability. The specific parameter value of the devices with CAS and Spiro are shown in the Supporting Information Table S2& S3. The PCE degradation can be accounted via the XRD pattern of MAPbI3 materials for both CAS and spiro devices. Time dependent XRD patterns of MAPbI3/spiro and MAPbI3/CAS are shown in Figure S5. From the XRD, we observe that the intensity of the prominent peak at 2θ = 14.10°, which is attributed to perovskite tetragonal structure, starts to decrease in MAPbI3/spiro device. At the same time, the intensity of the peak at 2θ = 12.5°, which represents the PbI2 crystal structure, starts to evolve and keeps increasing over successive days of measurements. At the end of the 10th day, the peak at 2θ = 14.10° completely diminishes because of degradation of the perovskite structure and PbI2 layer formation in the Spiro based device due to moisture. In


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the MAPbI3/CAS device, the XRD peak for the formation of PbI2 is not observed and perovskite crystal structure remains the same over 10 days of observation. Notable intensity of PbI2 peak appeared only after 15 days of the testing. Therefore, hydrophobic nature of CAS film on the perovskite layer slows down the degradation of MAPbI3 crystal structure compared to the organic hole conductors. 4. CONCLUSION In this work here, a ternary semiconductor viz, copper antimony sulphide is demonstrated as a low cost, chemically stable and hydrophobic inorganic hole conductor for application in mesoscopic perovskite (n-i-p) solar cell. The crystalline Cu12Sb4S13 (tetrahedrite) nanoparticles capped with organic ligands, exhibited high carrier concentration and electrical conductivity with favourable values for ECBM and EVBM. Solar cell in (n-i-p) configuration assembled under ambient conditions with an optimized thickness of CAS as the HTL showed very promising photon to electric conversion efficiency of nearly 6.5%. The lower value in VOC for MAPbI3/CAS in comparison to the conventional hole conductor spiro- OMeTAD is attributed to the low negative EVBM value of CAS nanoparticle. Due to higher degree of hydrophobicity, chemical stability and good electrical conductivity of CAS HTL, the device retained nearly 50% power efficiency over a period of 15 days under controlled humidity conditions. We strongly propose that the thin film of tetrahedrite phase nanoparticles have a very strong potential also in inverted hetero junction perovskite solar cells and other solar photon applications. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge:


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P-XRD pattern of CAS sample after sintering, direct band gap plot of CAS nanoparticle, Energy dispersive X- ray

spectra CAS sample, SEM image of MAPbI3 on mesoporous TiO2 surface,

Average photovoltaic device specific parameters of five solar cells with the CAS (spiro) HTL layer. photovoltaic parameters of MAPbI3/CAS and MAPbI3/Spiro device over a period of 15 days, Time dependent XRD pattern of MAPbI3 in CAS(Spiro) incorporated solar cell.

AUTHOR INFORMATION Corresponding author *[email protected] NOTES There are no conflicts to declare. ACKNOWLEDGEMENTS Authors acknowledge the CSIR (No. 01(2879)/17/EMR-II), New Delhi for financial support, CENSE, IISc. Bengaluru and SSCU, IISc. Bengaluru for infrastructural support.

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FIGURE FOR TOC:


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Tetrahedrite (Cu12Sb4S13) Ternary Inorganic Hole Conductor for

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