In2S3 Thin

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A Non-Vacuum Approach for Fabrication of CuZnSnSe/InS Thin Film Solar Cell and Opto-Electronic Characterization Dhruba B. Khadka, SeongYeon Kim, and Junho Kim

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03193 • Publication Date (Web): 08 May 2015 Downloaded from http://pubs.acs.org on May 11, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

A Non-Vacuum Approach for Fabrication of Cu2ZnSnSe4/In2S3 Thin Film Solar Cell and Opto-Electronic Characterization

Dhruba B. Khadka, SeongYeon Kim, JunHo Kim* Department of Physics, Incheon National University, 12-1 Songdo-dong Yeonsu-gu, 406-772 Incheon, South Korea

* Corresponding Author: JunHo Kim E-mail: [email protected] Phone: +82-32-835-8221

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Abstract Cd-free Kesterite-based Cu2ZnSnSe4 (CZTSe)/In2S3 champion solar cell of 5.74% efficiency has been fabricated by chemical spray pyrolysis. In this fabrication route, CZTSe absorber layer was sprayed by using a precursor solution, where metallic salts were dissolved in waterbased solvent, and subsequently selenized with Se powder at high temperature. In2S3 buffer as an alternative to CdS buffer was also deposited by chemical spray pyrolysis. The device characteristics were studied by measuring dark/light illuminated J-V-T, external quantum efficiency, temperature dependence of open circuit voltage (VOC) and series resistance (Rs), and admittance spectroscopy. The performance of sprayed CZTSe/In2S3 solar cell was found to be limited by high back contact barrier potential, poor carrier collection and detrimental intrinsic defect states in device.


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1. INTRODUCTION Chalcogen-based thin film photovoltaic cells have drawn considerable interest as an alternative energy source for safe and sustainable solution of energy crisis. Despite of the photovoltaic (PV) technology with Cu(In,Ga)Se2 (CIGSe) and CdTe thin films which have achieved highest efficiencies of 21.7% and 21.0%, respectively, the scarcity of elements such as In, Ga, Te and toxic Cd have raised high cost and environmental issues for large scale and long term production.1-3 To address these issues, quaternary chalcogenide Cu2ZnSnS4 (Se4) (CZTS (Se)) has gained much attention due to earth abundant, non-toxic and low cost constituents along with its high absorption coefficient α >104 cm-1 and tunable band gap 1.01.5 eV which is suitable for efficient light harvesting.3,4 Despite these promising properties of CZTS (Se), the device performance is still low as reference to CIGSe solar cell. Therefore, to make kesterite-based CZTS (Se) thin film solar cell as mature as CIGSe, extensive researches have been carried out on material fabrication, secondary phase control,2 defect formation, 5, band gap tuning,4,6-8 back contact barrier,9-11 and band alignment.12,13 The kesterite CZTSSe solar cell processed from a hydrazine-based precursor solution has recently achieved a record efficiency of 12.6% with CdS buffer layer and 12.4% with double In2S3/CdS emitter14,15 which implies further improvement to compete to CIGSe technology. Different fabrication approaches have been successfully employed to fabricate kesterite thin film solar cells. The CZTSSe solar cells adopting the vacuum based deposition showed power conversion efficiencies (PCE) of 11.6% via co-evaporation16 and 9.7% via sputtering.17 Since vacuum-based technologies need expensive high vacuum facilities, the potential of chalcogenide compound can be efficiently exploited when combined with a scalable, nonvacuum fabrication technique. A number of solution process techniques have been applied for



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chalcopyrite and kesterite solar cell fabrication.18 Although the performance of CZTSSe solar cell adopting hydrazine based hybrid slurry approach has been reported to be progressed to best performance, the hazards associated with highly toxic and explosive hydrazine may inhibit the adaptation of this technique.14,18 The high efficiency of CZTSSe solar cell has achieved PCE of 9.6% by using binary and ternary nanoparticle solution approach.19 The nanocrystal based fabrication techniques have also provided competitive CZTSSe solar cell of efficiency as high as 9.0% whereas devices with Ge alloyed CZTSSe (CZGTSSe) nanocrystal have achieved improved PCE as high as 9.4% with band gap tuning.7,20 Furthermore, CZTSSe solar cell of 8.3% efficiency has been reported using molecular ink prepared by dissolving metal salt with dimethyl sulfoxide (DMSO) solvent.21 The organic solvent used for precursor preparation can have safety issue and carbon residue may normally remain which is detrimental for solar cell performance. Thus, as an alternative to hydrazine or organic solvent, non-toxic and low cost solvent like water and ethanol are preferable for eco-friendly synthesis. The CZTSSe solar cell fabricated with precursor ink prepared using water and ethanol based solvent has been reported with high efficiency of 8.6% via spray deposition and 6.2% via spin coating.22,23 The solution based non-vacuum processes such as spin coating, doctor blade, electro-deposition, chemicals bath deposition (CBD), and spray pyrolysis have been successfully adopted for fabrication of absorber layer in kesterite thin film solar cell.14,18-26 Among these, the aqueous precursor solution route, chemical spray pyrolysis (CSP) is a viable approach for low cost and environment-friendly fabrication technique which has been employed for fabrication of kesterite solar cell reported in this communication. In addition to the absorber layer fabrication issues, Cd-free buffers for kesterite solar cell have also recently been the subject of investigation to avoid the use of toxic Cd and to get


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higher photo generated current. Even though Cd-buffer adopted devices have yielded the best performance until now, the Zn- and In-based buffers can be widely used for kesterite solar cell as in the CIGSSe solar cell.27,28 Only few studies on alternative buffers for CZTSSe solar cells have been reported. To date, the kesterite solar cell with efficiencies of 4.3% with ZnO buffer, 5.8% with ZnS(O,OH) buffer and 7.2% with In-based buffer have been reported.13,29 Recently, CZTSSe solar cell of efficiency 12.4% has been reported employing double buffer CdS/In2S3 with improved VOC deficit (Eg/q-VOC).15 Thus, the cost effective and environment friendly fabrication for high efficient kesterite solar cell with Cd-free buffer is one of the ultimate and long term goals in research community. In this article, we report the fabrication of CZTSe/In2S3 thin film solar cell by chemical spray pyrolysis (CSP) using cheap and safe water based precursor solution. The absorber layer CZTSe as well as In2S3 buffer layer are fabricated by electrostatic field assisted CSP technique. CSP technique is promising due to its simplicity, scalability and easy control of cation stoichiometry. This fabrication technique has been employed for synthesis of various chalcogenide thin films to study the surface morphology, structural, and optical properties in photovoltaic aspects.6,8,30-34 Our group has previously reported the CZTSe thin films solar cell with PCE of 2.4% using CSP technique.26 Here, we deal with improved approach of our earlier report26 by depositing CZTS absorber layer as well as In2S3 buffer layer adopting optimized precursor composition and solvent along with controlled layer thickness and selenization conditions. We have presented material growth properties of absorber and buffer layers and characterization of fabricated CZTSe solar cell device with champion PCE of 5.74%. The electrical characterizations are employed to understand the limitations of spray



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pyrolyzed CZTSe thin film solar cell taking account of interface recombination, back contact barrier, defect analysis. 2. EXPERIMENTAL DETAILS 2.1. Materials. Copper (II) chloride (CuCl2; ≥97%), zinc chloride (ZnCl2; ≥98%), tin (IV) chloride pentahydrate (SnCl4·5H2O; 98%), indium chloride (InCl3; 98%), thiourea (SC(NH2)2; 99.0%) and elemental selenium powder (99.99%, 75 μm) were used as received from SigmaAldrich. 2.1. Preparation of CZTSe Thin Film. The CZTSe absorber layer was prepared by selenization of as-sprayed CZTS thin film deposited on Mo-coated soda lime glass (Mo-SLG) substrate by CSP technique. For deposition of sprayed CZTS thin film, aqueous solution was prepared by dissolving copper (II) chloride (CuCl2), zinc chloride (ZnCl2), tin (IV) chloride pentahydrate (SnCl4) and thiourea ( SC(NH2)2) in alco-aqueous solvent (de-ionized water (DIW) and ethanol,1:1) with molar ratios as CuCl2:ZnCl2:SnCl4·5H2O:SC(NH2)2 = 1.85:1.2:1:14 to make a copper poor and zinc rich precursor composition. We used cheap chemicals rather than expensive extra pure ones. In our earlier report, CZTS precursor solution was prepared only in DIW with copper poor and zinc correct precursor composition.26 It is point to be noted that the CZTS precursor solution prepared in mixed solvent (DIW+ethanol) was found to be stable for longer time compared to that of prepared only in DIW. The excess amount of sulfur source i.e. thiourea (SC(NH2)2) was used to minimize oxidation as a consequence of sulfur loss during spray deposition. The prepared precursor solution was deposited onto Mo-SLG at substrate temperature of 450 °C using homemade electrostatic field assisted ultrasonic spray system34 in which high electrostatic field was applied between outlet of aerosol flow pipe and substrate to accelerate the molecular


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CZTS mist flow. It is to be noted that our spray system is nozzle-free spray, which is good for deposition of ultrafine aerosol by minimizing the droplets coalescence before reaching to the substrate. The CZTS film was prepared by spraying the precursor mist onto the substrate under a rate of 1.2 ml/min by means of carrier gas of nitrogen at pressure of 0.1 MPa. The assprayed films deposited on Mo-SLG were selenized by rapid thermal annealing 50 oC/min under selenium vapor ambient and dwelled at ~ 500-520 °C for 30 min keeping the sample inside of rectangular graphite box filled with selenium powder (99.99%). The schematic diagrams of annealing furnace and temperature profile for selenization are as our earlier reports.8,26,36 2.2. Device Fabrication. To complete thin film device structure, indium sulfide (In2S3) was used as Cd-free buffer which was also deposited by CSP method. For the deposition of indium sulfide buffer, the precursor solution, which was prepared by dissolving InCl3 and SC(NH2)2 with the molar ratio of 1:7 in alco-aqueous (DIW (50%) + ethanol (50%)) solvent, was sprayed onto CZTSe absorber layer at substrate temperature of 360 oC with a flow rate of 0.8 ml/min by N2 as carrier gas. Prior to buffer deposition, surface of postselenized CZTSe absorber layer was etched for 3 minutes in 10% of potassium cyanide DIW solution and immediately washed with DIW, then dried under gentle flow of nitrogen gas. Finally, the CZTSe device was completed by subsequent deposition of intrinsic zinc oxide (i-ZnO) layer and indium tin oxide (ITO) layer via radio frequency (RF) magnetron sputtering. No metal grid and anti-reflection coating were further deposited on the solar cell device. 2.3. Film and Device Characterization. The stoichiometry, surface morphology and crosssectional image of samples were studied by field emission scanning electron microscopy (FESEM, JEOL, JSM-7001F) equipped with energy dispersive X-ray spectroscopy (EDS,



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Oxford, INCA). The crystal structure and phase were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns were obtained in 2θ scan at a scanning rate of 3° per min by XRD system (Rigaku, Smart Lab) equipped with a Cu Kα source of λ=1.5412 Å operated at 45 kV and 200 mA. The Raman spectroscopy measurements were conducted using a Spectro Raman system equipped with spectrometer (Mmac 750) and laser of excitation wavelength λ=532 nm (irradiation power < 1 mW) and spot size of approximately 1μm. The fabricated CZTSe solar cell device was characterized by current density- voltage (JV) characteristics, external quantum efficiency (EQE) and admittance spectroscopy (AS). J-V measurement was performed with a source meter (Keithley 2400) and under AM 1.5G filtered illumination of 1000 Wm-2 Xe lamp (Abet Technology), which has been calibrated with Si reference solar cell. The J-V characteristics were also investigated in the temperature range 300-90 K under dark, white light and filtered light illumination. The quantum efficiency measurement was carried out with Xe light source; monochromator combined with light chopper and locked in amplifier system (Mc Science). Admittance spectroscopy (AS) was performed to study the defect in the fabricated device with an LCR meter (E4980A, Agilent) which probes from 20 Hz to 2 MHz in the temperature range of 300-90 K. All C-f scans were carried out at an AC voltage 30 mV under dark condition. Capacitance–voltage (C-V) measurements were also performed with an LCR meter same as used for AS measurement to estimate the space charge width, built in potential and free carrier density. For J-V-T and C-fT measurements, liquid nitrogen cryostat was used with temperature controller with error of ± 0.05 K. 3. RESULTS AND DISCUSSION 3.1. Crystal Quality and Phase Analysis


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The as-sprayed and postselenized films were characterized by XRD and Raman spectroscopy to study the crystal quality and to probe out possible secondary phases. The XRD and Raman spectroscopy results of fabricated films are shown in Figure 1. The XRD pattern of as-sprayed CZTS film (Figure 1a) shows small and broad diffraction peaks corresponding to (112), (220), (204), and (116) planes assigning polycrystalline CZTS structure along with two noticeable peaks from Mo which are assigned with ICDD No. 01075-4122 (CZTS) and ICDD No. 04-003-2919 (Mo).35 The XRD pattern of as-sprayed CZTS thin film implicates poor crystallinity.8 The typical XRD pattern of postselenized CZTSe film (Figure 1b) exhibits sharp and enhanced peaks corresponding to dominant orientation (112) and other characteristic smaller diffraction peaks of (002), (101), (110), (103), (200), (004), (211), (220), (204), (312), (116), (314), (316), and (208) orientations.35 The diffraction peaks of (101) and (110) planes of MoSe2 phase35 are also observed where weak CZTSe peak (200)/(004) are convoluted with MoSe2 peaks (101). The XRD pattern result indicates that the crystallinity of as-sprayed film is drastically improved as a result of postselenization at high temperature in selenium vapor ambient. Since XRD pattern of CZTSe is very close to that of possible secondary phases such as cubic ZnSe and ternary Cu2SnSe3 (CTSe), the detection of these secondary phases is very difficult.35 Being sensitive tool for phases identification, Raman spectroscopy is usually combined with the XRD patterns to investigate the possible secondary phases in fabricated samples. Raman spectrum of as-sprayed film (Figure 2c) assigns broad dominant peak (A1 peak) at 335 cm-1 which is in the regime of kesterite CZTS phase.4,8 Raman spectrum of as-sprayed film also reflects poor crystal quality. Raman spectrum of postselenized CZTSe is depicted in Figure 1d where the dominant Raman peak at 195 cm-1 and other peaks at 172 cm-1 and weaker characteristic peaks at about ~ 233 and 244



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cm-1 (within broad range 230-252 cm-1) stem from kesterite CZTSe phase.4,6,7,26,36 Raman spectrum of postselenized CZTSe with broad peak at 230-252 cm-1 (Figure 1d) has also been observed in CZTSe Raman spectra in earlier reports.4,26,36 ZnSe phase is assigned with dominant Raman peak at 250 cm-1 and additional Raman peak at ~ 210 cm-1.36 CTSe shows dominant characteristic Raman peak at 180 cm-1 along with another small peak at 236 cm-1.36 Hence, Raman peaks of secondary phase ZnSe and CTSe can be superimposed within the above mentioned broad range 230-253 cm-1. Since no additional peak at 180 cm-1 is observed in Figure 1d, CTSe phase does not exist in postselenized CZTSe film. MoSe2 exhibits dominant Raman peak at 242 cm-1 with others peaks at 162, 285, 352 cm-1.37 Even though MoSe2 phase observed in SEM cross-sectional image, because of the limitation of the penetration depth of the laser beam, there is less possibility to detect MoSe2 phase. No Raman peaks for other secondary phases; Cu2Se (270 cm-1), Cu2-xSe (147, 260 cm-1),38 SnSe (108, 150 cm-1), and SnSe2 (116, 186 cm-1)36,39 are observed in Figure 2d. Thus, from above discussion, the XRD patterns and Raman spectroscopy results consolidate the facts that assprayed CZTS film can be well selenized under selenium vapour ambient to get well crystallized CZTSe absorber layer. 3.2. Stoichiometry and Morphology of Prepared Thin Films and Device The stoichiometry and surface morphology of the as-sprayed and postselenized films were investigated by SEM and EDS measurements. EDS results of as-sprayed CZTS film show elemental stoichiometry, Cu/(Zn+Sn)=0.85, Zn/Sn=1.13, S/(Cu+Zn+Sn)=0.93. A significant portion of oxygen content was observed in as-sprayed thin film as our earlier reports.8,30 This is quite usual in as sprayed films because the sprayed deposition was carried out in air ambient at elevated temperature The postselenized CZTSe thin film shows atomic


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percentages of constituent elements; Cu (18.6± 1%), Zn (11.6± 1%), Sn (9.8± 1%), Se (50.2± 5%) and oxygen (below the detection limit), which corresponds to stoichiometries of Cu/(Zn+Sn)=0.87, Zn/Sn=1.18, Se/(Cu+Zn+Sn)=1.25. This indicates that as-sprayed film is well selenized to form CZTSe which is strongly supported by XRD and Raman results as depicted in Figure 1. The fluctuations in cations composition observed in as-sprayed and postselenized films fall within the error limit of the EDS analysis. EDS results of postselenized CZTSe thin film demonstrate Cu-poor and Zn-rich stoichiometry which have been reported to be beneficial for high efficiency kesterite solar cell in both vacuum and nonvacuum approach.4,14,17,21 The surface morphologies of as-sprayed and postselenized thin films are shown in Figures 2a and 2b, respectively, where cross-section images of respective films are depicted as insets. The surface image of as-sprayed CZTS film (Figure 2a) does not show any crystalline texture whereas that of postselenized CZTSe film (Figure 2b) exhibits compact and well grown crystalline texture with noticeable grains as a consequence of postselenization. Inset in Figure 2a shows cross-section of as-sprayed CZTS film with the thickness of ~ 1 μm without any grain feature whereas that of postselenized CZTSe film as shown in inset in Figure 2b exhibits large grain of crystalline texture in vertical direction having thickness of about ~ 0.7 μm. An interfacial layer of MoSe2 between absorber layer and Mo layer has been observed which has formed during selenization at high temperature in Se ambient. Thick MoSe2 has detrimental effect on device performance imposing potential barrier for hole carriers transport.9,10 Figure 2c shows a cross section image of the complete solar cell device which consists of multilayers; transparent conducting layer of ITO/i-ZnO (~300 nm)/ buffer layer of In2S3 (~100 nm)/ absorber layer of CZTSe (~ 0.7 μm)/ back contact layer of MoSe2/Mo ACS Paragon Plus Environment


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(1.5 μm)/SLG. A sprayed buffer layer of In2S3 is found to be well buried in CZTSe absorber layer. Raman spectra of In2S3 buffer assigned peaks at 136, 209, 254, 308, 368 cm-1 and optical band gap estimated to be ~2.76 eV (Figure S1, Supporting Information). 3.3. Device Characteristics Figure 3a depicts the current density-voltage (J-V) characteristics of best CZTSe/In2S3 solar cell device measured in the dark and illumination under AM 1.5G. The champion solar cell device exhibits a PCE (η) of 5.74% with an open circuit voltage (VOC) of 0.43 V, a short circuit current density (JSC) of 28.27 mAcm-2, a fill factor (FF) of 47.07%. The device performances of some solar cell devices fabricated by same approach and conditions as those of best device have shown in supporting information (Figure S2 and Table S1). Compared with our previous report,26 all device parameters; VOC, JSC and FF, are much enhanced leading to improvement in PCE. These enhanced device parameters are believed to be due to improved quality of CZTSe absorber layer, In2S3 layer and better interface formation. The performance of sprayed CZTSe/In2S3 solar cell is promising through which we can open a path to enhance the efficiency by further improving the multilayer quality and to advance understanding of corresponding material system. Moreover, we compared the solar cell performance of our champion CZTSe solar cell with other best CZTSSe thin film solar cells fabricated by different solution approaches as listed in Table 1. The reported kesterite solar cell devices (Table 1) show much lower solar cell parameters in reference to the theoretical estimation which indicates the requirement of deeper investigation of the limiting factors to achieve high efficiency device. As we compare reported kesterite devices, the VOC of our device is found to be comparable whereas the JSC and FF show lower values than those of the devices listed in Table 1. The overall efficiency of this device is observed to be limited mainly


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by low Jsc and FF as consequence of low Rsh, high Rs, and high diode ideality factor (A=1.91) analogous to the report by Xin et al..21 The dark and illuminated J-V curves (Figure 3a) exhibit crossover behavior which might be associated with the electrical barrier either in the buffer/absorber interface, or back contact barrier of MoSe2 interfacial layer9,41 as shown in the SEM image (Figure 2c). Figure 3b shows the external quantum efficiency (EQE) measured at 0 V bias and EQE ratio at -1 V and 0 V bias for the corresponding solar cell. The maximum EQE (56%) corresponds to around 675 nm and then gradually decays for longer wavelength regime. However, the EQE is enhanced in the entire wavelength compared to our earlier report,26 it is still lower than other reports.21,22,24 The poor EQE response in the short wavelength regime is due to absorption losses in the window and buffer layer (ZnO, In2S3).42 A weak EQE response in longer wavelength regime is likely caused by high recombination losses in the bulk and depletion region as well as severe back contact recombination. The ratio of EQE at -1 V and 0 V bias (EQE(-1V)/EQE(0V)) increases towards long wavelength regime, which also indicates strong recombination losses in the bulk CZTSe absorber layer and near to the back contact. These factors result in short minority carrier life time, and hence device parameters degrade.3,42,43 Furthermore, the band gap of the CZTSe absorber layer is estimated to be 1.08 eV from linear extrapolation of plot (hν.EQE)2 vs. Energy (hν) as depicted in the inset of Figure 3b. This estimated band gap is close to the earlier reports.23,24,26 As mentioned above, the device parameters in Figure 3a show high series resistance (Rs) and crossover of dark and illuminated J-V curve (at point J

In2S3 Thin

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