Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition

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Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition Prepared by Selenization of Sputtered Novel Precursors Fang-I Lai, Jui-Fu Yang, Wei-Chun Chen, and Shou-Yi Kuo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11346 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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1 Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition Prepared by Selenization 2 of Sputtered Novel Precursors 3 4 5 6 Fang-I Lai1, Jui-Fu Yang1,2, Wei-Chun Chen3, Shou-Yi Kuo2,4* 7 8 9 1. 10 Department of Photonics Engineering, Yuan-Ze University, 135 Yuan-Tung Road, Chung-Li, 32003, Taiwan 11 12 2. Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Taoyuan 333, 13 14 Taiwan. 15 16 3. Instrument Technology Research Center, National Applied Research Laboratories, 20 R&D Road V1, Hsinchu 17 18 19 Science Park, Hsinchu 300, Taiwan 20 21 4. Department of Nuclear Medicine, Chang Gung Memorial Hospital, Linkou, No.5, Fuxing Street, Kwei-Shan, 22 23 Tao-Yuan 333, Taiwan 24 25 26 *Corresponding author: [email protected] 27 28 Keywords: CZTSe, gradient, novel precursors, sputter, solar cell 29 30 31 Abstract 32 33 In this study, we proposed a new method for synthesis of the target material used in two-stage process for preparation 34 35 of high quality CZTSe thin film. A target material consists of mixture of CuxSe and ZnxSn1-x alloy has been 36 37 38 synthesized, providing quality CZTSe precursor layer for highly efficient CZTSe thin film solar cells. The CZTSe 39 40 thin film can be obtained by annealing the precursor layers through a 30-minute selenization process under selenium 41 42 atmosphere at 550 ℃. The CZTSe thin films prepared by using the new precursor thin film were investigated and 43 44 45 characterized using X-ray diffraction (XRD), Roman scattering and photoluminescence (PL) spectroscopy. It has 46 47 been found that, diffusion of Sn has occurred and formed CTSe phase and CuxSe phase in the resultant CZTSe thin 48 49 film. By selective area electron diffraction transmission electron microscopy (SAD-TEM) images, the crystallinity of 50 51 52 CZTSe thin film has been verified to be single crystal. By secondary ion mass spectroscopy (SIMS) measurements, it 53 54 has been confirmed that a double-gradient band gap profile across the CZTSe absorber layer has been successfully 55 56 achieved. The CZTSe solar cell with CZTSe absorber layer consists of the precursor stack exhibit high efficiency of 57 58 2 59 5.46 %, high short circuit current (JSC) of 37.47 mA/cm , open circuit voltage (VOC) of 0.31 V, and fill factor (F.F.) of 60 ACS Paragon Plus Environment


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1 47 %, at device area of 0.28 cm2. No crossover of the light and dark current-voltage (I-V) curves of CZTSe solar cell 2 3 4 have been observed, and also, no red-kink has been observed under red light illumination, indicating low defect 5 6 concentration in the CZTSe absorber layer. Shunt leakage current with characteristic of metal/CZTSe/metal leakage 7 8 current model has been observed by temperature dependent I-V curves, which has led to the discovery of metal 9 10 11 incursion through the CdS buffer layer on CZTSe absorber layer. Such leakage current, also known as 12 13 space-charge-limited (SCL) current, has grown larger as the measurement temperature increases and completely 14 15 overwhelm the diode current at measurement temperature of 200 ℃. This is due to inter-layer diffusion of metal that 16 17 18 increases the shunt leakage current and decrease the efficiency of the CZTSe thin film solar cells. 19 20 21 22 Introduction 23 24 25 Photovoltaic has been one of the most important renewable energies, in which, thin film solar cells possess great 26 27 potential to become the major solar cell products, due to their low material cost. To date, many thin film solar cells 28 29 has been widely commercialized, such as amorphous Si (a-Si), CdTe, and Cu(In,Ga)Se (CIGS) photovoltaic 2 30 31 1 32 technologies. In particular, the record efficiency of CIGS thin film solar cells has been pushed to 22.3 %, closing the 33 34 record efficiency gap between CIGS solar cells and crystalline Si solar cells. The breakthrough on record efficiency 35 36 of CIGS solar cells has prompted the development of thin film solar cell technologies based on the quaternary 37 38 kesterite material Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe). CZTS and CZTSe are structural analogue of CIGS, 39 40 41 which contain abundant earth material Zn and Sn as the substitutional element to rare earth material of In and Ga in 42 43 CIGS. CZTS and CZTSe material possess similar optical properties as those of CIGS, such as high absorption 44 45 coefficient, intrinsic p-type semiconductor and direct band gap. By changing the concentration of sulfur (S) or 46 47 48 selenium (Se), the band gap value of this material can be varied between 1 eV to 1.5 eV, which are respectively the 49 50 band gap values of CZTSe and CZTS. These material characteristics of CZTS and CZTSe have made themselves 51 52 perfect candidate for high efficiency thin film solar cell at a very low cost. When compared with CIGS thin film 53 54 55 devices, those made of CZTSSe show rapid progress in performance. Up to now, CZTSSe thin-film solar cells have 56 57 already achieved 12.7% power conversion efficiency. However, no further improvements in efficiencies have been 58 59 made since the champion efficiency of 12.7% was reached since 2014. 2 Some problems exist as regards the 60 ACS Paragon Plus Environment

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1 achievement for high-efficiency for CZTS-based thin-film solar cells. One of the main obstacles is primarily because 2 3 4 of a severe open-circuit voltage (Voc) deficit in CZTSSe-based devices, which is believed to originate from the 5 6 potential fluctuation and non-radiative recombination centers in CZTSSe thin films. 3 Many efforts have been made to 7 8 overcome the Voc deficit issue, particularly working on the CZTSSe absorber itself. 4-7 Unfortunately, CZTSSe-based 9 10 11 thin film solar cells still have relatively lower power conversion efficiency than CIGS. Our previous study has 12 13 demonstrated that the double-gradient band profile in CIGS solar cell is beneficial for its photovoltaic performance. 14 15 In this work, we have attempted to specifically address the fabrication of high-crystallinity CZTSe absorber layer 16 17 18 with double-gradient band profile to simplify the process for high-efficiency CZTSe solar cell. Traditionally in CIGS 19 20 solar cells, it usually needs one or more additional fabrication process during the deposition of the absorber layer to 21 22 achieve a double-gradient band gap profile, such as adjustment on vapor flow of the source element during 23 24 25 co-evaporation process, or surface sulfization of the absorber layer after selenization. Utilizing the CuxSe/ZnxSn1-x 26 27 bi-layer precursor, a double-gradient band gap profile can be directly generated due to the nature of each element, 28 29 which causes the inter-diffusion during the sputtering and the selenization process. 30 31 To date, common deposition method for CZTSe thin film are evaporation, 8 sputter, 9 electro-chemical deposition, 32 33 10 11 34 and ink printing. CZTSe thin film solar cells prepared by ink printing method holds the highest record efficiency. 35 36 11 However, hydrazine is used during the preparation of ink-printed CZTSe thin film for the champion efficiency 37 38 CZTSe solar cell, which has made the fabrication process become more expensive, highly risky and highly toxic, due 39 40 41 to that hydrazine is a controlled substance that is highly corrosive, flammable and explosive. The use of hydrazine is 42 43 needed to be proceeded in an oxygen-free environment, and can cause the molybdenum (Mo) electrode back contact 44 45 layer being partially etched, which can decrease the yield rate during mass production and also bring scale-up issues. 46 47 48 These make ink-printed CZTSe solar cells more difficult to be commercialized. Therefore, vacuum process is more 49 50 suitable for production of CZTSe solar cells at industrial level, due to that vacuum process is stable, non-toxic and is 51 52 also capable of producing high efficiency CZTSe solar cell. Among various vacuum processes, sputtering takes 53 54 12 which make itself the most feasible method for the 55 advantages of large area, large throughput, and low cost, 56 57 industry. By far, efficiency of 9.7 % is the highest record efficiency of CZTSe thin film solar cell fabricated by two 58 59 stage process, for which three different materials, Cu Sn , Zn, Cu, were used as precursor layers, and a selenization 10 90 60 ACS Paragon Plus Environment


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1 process under H2Se atmosphere at 460 ℃ were conducted. Though this method delivers a CZTSe thin film solar cell 2 3 4 with a very high efficiency, it however brings issues, such as the use of H2Se gas, which is highly corrosive and toxic, 5 6 and also the potential problem of precursor stacks. 9 Additionally, three sputter guns and three target source materials 7 8 are needed for this methods, which drastically increases the fabrication cost. With the presence of multiple metal 9 10 11 stacking precursor layers, the potential problem of precursor stack is illustrated as follows. In the precursor stacks, if 12 13 Zn layer and Cu layer were next to each other, there will be inhomogeneous distribution of Zn and Cu due to their 14 15 self-assembly at liquid phase during selenization process. This leads to inhomogeneous composition of a CZTSe thin 16 17 13 18 film. The choose of materials of each precursor layers is crucial for a CZTSe thin film of a high efficiency device. 19 20 In order to develop an effective strategy for this goal, Zn60Sn40 alloy has been used as the target material for the 21 22 sputtering. 14 Additionally, it has been found that, during the formation of a CIGS thin film, the presence of CuSe as a 23 24 25 liquid phase mediating substance can benefit the nucleation of CIGS chalcopyrite grain, and help the growth of high 26 15 27 quality CIGS thin film. Also, a precursor layer stack containing Se can improve the homogeneosity of selenization 28 29 by providing selenium internally in the precursor layers during selenization. 16 The same scheme can also be applied 30 31 32 to CZTSe thin film growth, since CIGS and CZTSe share similar thin film properties. Considering the development 33 34 on engineering of precursor layers for high quality CZTSe thin film, this work proposes an improved precursor 35 36 scheme using two different target materials, CuxSe and ZnxSn1-x alloy, for the sputtering of precursor layer. The use of 37 38 CuxSe and ZnxSn1-x alloy stacking layers provides CuxSe material to enhance the growth of CZTSe grain, while 39 40 41 ZnxSn1-x alloy can solve the problem of self-assembled Zn and Sn metal clusters and promote the compositional 42 43 homogeneosity of CZTSe thin film. The bi-layer precursor also contains selenium and is able to enhance the quality 44 45 of selenization outcomes. After selenization, the CZTSe thin film has been fabricated into a CZTSe thin film solar 46 47 2 48 cell, with efficiency of 5.46 %, open circuit voltage (Voc) of 0.31 V, short circuit current of 37.47 mA/cm , and fill 49 50 factor (F.F.) of 47 %. Qualitative characterizations including X-ray diffraction (XRD), Raman scattering, 51 52 photoluminescence (PL), scanning electron microscope (SEM), transmission electron microscope (TEM), and 53 54 55 secondary ion mass spectroscopy (SIMS) have been conducted, and revealed the high quality of the resultant CZTSe 56 57 thin film, including the double-gradient bandgap profile and low defect concentration in the CZTSe thin film. 58 59 60 ACS Paragon Plus Environment

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1 Experimental details 2 3 4 This research uses method of selenization after sputter, also known as two stage process, to prepare CZTSe thin film. 5 6 First, a glass substrate with area of 2.5 x 2.5 cm2 was coated with a layer of molybdenum (Mo) with thickness of 1 7 8 µm as the back contact layer. Then, different precursor layers prepared by using ZnxSn1-x and CuxSe target material 9 10 11 were separately deposited onto the Mo/glass substrate. The sputtering of the precursor layers was proceeded under 12 13 Argon (Ar) atmosphere with pressure of 5 x 10-4 torr, while RF power of 100 W for ZnxSn1-x, and that of 200 W for 14 15 Cu Se were used. The target-to-substrate distance was kept at 13.5 cm, and the targets of. 3 inches in diameter was x 16 17 2 2 18 used in this study. The RF power densities for ZnxSn1-x and CuxSe targets are 2.19 W/cm and 4.39 W/cm at room 19 20 temperature. Subsequently, the thickness of individual layers was 0.5 μm, and 0.25 μm. Following the sputtering, 21 22 the samples were placed in a quartz tube with 700 mg selenium tablet, and sent into a furnace. The quartz tube was 23 24 25 purged with Ar gas during the selenization process. The highest temperature during the selenization was 550 ℃, at 26 27 which the temperature has been maintained for 30 minutes. The CZTSe thin films were prepared after the selenization 28 29 process was finished. The furnace was then cooled in air after the selenizaiton. An as-deposited CZTSe thin film had 30 31 32 been reserved for characterization measurements, while other thin films were sent to device fabrication process. An 33 34 n-type CdS buffer layer with thickness of 50 nm was prepared by chemical bath deposition (CBD) using 35 36 Cd(CH3CO2)2•2H2O and CH4N2S solution. A layer of intrinsic ZnO (i-ZnO) with thickness of 50 nm was sputtered 37 38 onto the CdS layer, and followed by an Al doped ZnO (Al:ZnO, AZO) layer with thickness of 300 nm and Al 39 40 41 concentration of 3 % as window layer, deposited by sputtering. The CZTSe thin film solar cell was finished at the 42 43 sputtering of Ni with thickness of 50 nm and Al with thickness of 1 µm as the front grid. The device was then 44 45 characterized by current-voltage (I-V) measurements and external quantum efficiency (EQE) measurement, while the 46 47 48 as-deposited CZTSe thin films were characterized by XRD, field emission scanning electron microscope (FE-SEM), 49 50 transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), Raman scattering, SIMS and PL, to 51 52 investigated the structural, optical and electrical characteristics of the CZTSe thin films and CZTSe thin film solar 53 54 2 55 cells. The active area of the device is 0.28 cm (excluding front grids area). 56 57 XRD analysis was conducted by a SIEMENS D500 X-ray diffractometer with a CuKa (1.5418 Å) radiation. The 58 59 diffracted beam was scanning with a step of 0.005°, an angular θ-2θ range from 10° to 80°, and with integration time 60 ACS Paragon Plus Environment


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1 of 1 s. The produced CZTSe film has also been characterized by micro-Raman spectroscopy under room temperature 2 3 4 by using Ar laser (514 nm) and a triple monochromator (Horiba Jobin-Yvon Triax) combined with a TE-cooled CCD 5 6 detector. For photoluminescence (PL) measurements, the excitation source that was used was a 635-nm line 7 8 semiconductor laser (PicoQuant LDH-D-C-635M dual mode laser, 30mW) under continuous-wave (CW) excitation. 9 10 11 The signal was dispersed using a 0.5-m monochromator with a 600-groove mm-1 grating (Jobin Yvon iHR) and was 12 13 detected using an infrared-photo multiplier (HAMAMATSU H10330A-75). The sample was mounted inside a 14 15 vacuum chamber integrated with a closed-loop helium cryostat cooling system and measured at 10K. EQE spectra 16 17 18 were acquired by using a 300 W xenon lamp (Newport 66984) light source and a monochromator (Newprot 74112). 19 20 The beam spot size projected on the sample was approximately 1 mm × 3 mm. The temperature was controlled at 25 21 22 ±1℃ during the measurements. 23 24 25 26 27 Results and discussion 28 29 Figure 1 shows the XRD spectra of different precursor thin films: Zn Sn , Cu Se and Cu Se/Zn Sn . The XRD x 1-x x x x 1-x 30 31 32 spectra of the CZTSe thin film prepared by the bi-layer CuxSe/ZnxSn1-x precursor thin films have also been displayed. 33 34 From the XRD spectra, it can be seen that, the CuxSe/ZnxSn1-x bi-layer precursor are sharing the same diffraction 35 36 peaks as shown in the XRD spectrum of Zn/Sn alloy, indicating that the CuxSe/ZnxSn1-x bi-layer precursor contains 37 38 the same Zn/Sn metal alloy as that in the Zn/Sn precursor layer. It can also be seen that, except for the observed ZnSn, 39 40 41 CuxSe binary secondary phases, the presence of CTSe phase can also been observed in the XRD spectrum of 42 43 CuxSe/ZnxSn1-x bi-layer precursor, showing strong formation tendency of kesterite CZTSe phase. After selenization, 44 45 the resultant CZTSe thin film obtained from the CuxSe/ZnxSn1-x bi-layer precursor shows strong (112), (204) and 46 47 48 (312) orientation of CZTSe. Except for XRD diffraction peaks of MoSe2 and Mo phase, no appearance of secondary 49 50 phases has been observed. The full width at half maximum (FWHM) of XRD diffraction peak of (112) oriented 51 52 CZTSe phase is 900 arcsec. 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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Fig.1 XRD patterns of precursor and CZTSe samples taken on Mo/SLG.

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1 For a further confirmation of the composition of the CZTSe thin film, Raman scattering and photoluminescence 2 3 4 spectroscopy measurement has been conducted on characterization of the CuxSe/ZnxSn1-x bi-layer precursor, and the 5 6 CZTSe thin film obtained by the bi-layer precursor. The left spectrum displayed in Figure 2(a) shows the Raman 7 8 spectrum obtained from the CuxSe/ZnxSn1-x bi-layer precursor, in which it not only shows the Raman peak of CuxSe 9 10 -1 17 it also shows Raman peak of CTSe phase at around 180 cm-1. 18 The result coincides with the 11 at around 262 cm , 12 13 observation of XRD diffraction peak of CTSe phase in the XRD diffraction spectrum of the CuxSe/ZnxSn1-x bi-layer 14 15 precursor, as shown in Figure 1, and has as well confirmed the existence of CTSe phase in the Cu Se/Zn Sn x x 1-x 16 17 18 bi-layer precursor before the selenization process. The spectrum displayed on the right side of Figure 2(a) is the 19 20 Raman spectrum of the CZTSe thin film obtained from the CuxSe/ZnxSn1-x bi-layer precursor after selenization 21 22 process. The Raman spectrum of the CZTSe thin film shows clear Raman peak at 170 cm-1, 194 cm-1 and 242 cm-1. 23 24 19 25 Those are due to the CZTSe phase. No Raman peak related to binary secondary phase has been observed. 26 Figure 2(b) shows the normalized photoluminescence (PL) spectra of both the CuxSe/ZnxSn1-x bi-layer precursor 27 28 29 and the CZTSe thin film obtained from the bi-layer precursor. Both of the PL spectra were acquired under 10 K. In 30 31 32 the PL spectrum of the CuxSe/ZnxSn1-x bi-layer precursor (shown in black solid line), a PL peak with wide FWHM 33 20 34 and strong intensity has been observed at photon energy of 0.84 eV, whereat the bandgap energy of CTSe is located. 35 36 The PL signal of CuxSe has been fully quenched by the dominant CTSe PL emission. The PL spectrum of the 37 38 CuxSe/ZnxSn1-x bi-layer precursor also shows the CTSe phase, as those observed in XRD spectra and Raman spectra. 39 40 41 In Figure 2(b), the PL spectrum of CZTSe thin film (shown in red solid line) shows a PL emission peak at photon 42 43 energy of 0.90 eV, indicating the transformation of CTSe to CZTSe, which also increases the bandgap energy of the 44 45 alloy. The formation of kesterite CTSe phase in the CuxSe/ZnxSn1-x bi-layer precursor might be due to that, during the 46 47 48 deposition of CuxSe thin film on the ZnxSn1-x layer, the bombardment of plasma at ZnxSn1-x surface provides surface 49 50 energy that is adequate for the temperature at the surface to be increased to a level around the growth temperature of 51 52 CTSe. The ionic bombardment during the deposition of CuxSe thin film on the ZnxSn1-x layer can also induce surface 53 54 55 diffusion of the atoms at the surface of ZnxSn1-x layer, and alters the elemental compositions at the surface. 56 57 While the surface diffusion is occurring, Sn element in ZnxSn1-x layer can be easily re-evaporated due to its low 58 59 boiling point, and acts as a gaseous reactant in the formation of CTSe at the surface of Cu Se or the interface between x 60 ACS Paragon Plus Environment

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1 CuxSe and ZnxSn1-x. The CuxSe/ZnxSn1-x bi-layer precursor has been completed as the form of Cu2SnSe3 2 3 4 CuxSe after the deposition, and directly transferred into kesterite CZTSe phase after selenization. 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 ACS Paragon Plus Environment

+ ZnxSn1-x +


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Fig.2 (a) Raman spectra of precursor and CZTSe samples taken on Mo/SLG (b) PL spectra of precursor and CZTSe samples taken on Mo/SLG at 10 K. ACS Paragon Plus Environment

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1 2 3 The morphology and composition of the CZTSe thin film have been investigated by SEM and, TEM and EDS. 4 5 6 The results are shown in Figure 3. Figure 3(a) and 3(b) respectively shows top view SEM image and cross sectional 7 8 SEM image of the CZTSe thin film, while Figure 3(c) shows a TEM image of the CZTSe thin film, in which an area 9 10 11 in bulk area has been chosen to obtained selected area electron diffraction pattern, as shown in Figure 3(d). In order to 12 13 prevent the CZTSe thin film from being damage by electron beam while obtaining TEM images, all TEM images 14 15 were taken under acceleration voltage of 120 kV. The top view SEM image of the CZTSe thin film shown in Figure 16 17 18 3(a) presents a surface of CZTSe thin film with dense and homogeneous CZTSe grains. The CZTSe grains have 19 20 consistent diameters of around 2-3 µm, and are aligned closely to each other without pin holes between the CZTSe 21 22 grains. The thickness of the CZTSe thin film has been found to be 1 µm. No obvious air voids or particles have been 23 24 25 observed between CZTSe thin film and Mo layer. Particles at the interface between CZTSe and Mo are usually due to 26 21 27 segregation of ZnSe phase, which, in a CZTSe thin film, is usually presented as grains with smaller size. The large, 28 29 dense and homogeneous CZTSe grains without pin holes at the surface is crucial for thin film solar cells with 30 31 32 multi-crystalline material as the absorber layer. This feature provides a longer diffusion length for minority carriers 33 34 and a stronger built-in potential. In the TEM image shown in Figure 3(c), excellent coverage of CZTSe thin film on 35 36 the Mo substrate has been observed, while no air void or deep gap can be seen between CZTSe grains or between 37 38 CZTSe layer and Mo layer. The SAD pattern shown in Figure 3(d) shows a diffraction pattern of pure single crystal 39 40 41 CZTSe, indicating the high quality of the CZTSe thin film. The elemental composition of the CZTSe thin film has 42 43 been obtained by EDS under 15 keV, which gives a copper poor and zinc rich CZTSe composition with a Cu/(Zn+Sn) 44 45 ratio of 0.9, and a Zn/Sn ratio of 1.27. 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


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Fig. 3. Scanning electronic microscopy images of (a) plan and (b) cross-sectional view image of a typical CZTS layer (c) 、(d) TEM images and SAD patterns.


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1 2 3 SIMS measurement has been conducted to examine the depth-resolved elemental composition distribution of the 4 5 6 CZTSe thin film. The results are shown in Figure 4(a), in which intensity of Cu, Zn, Sn, Se and Mo has been plotted 7 8 as a function of depth. As shown in Figure 4(a), the SIMS plot can be divided into two areas, which are separately 9 10 11 distinguished by the green shade (left half) and purple shade (right half). The boundary of the two area is located at 12 13 around 1 μm. The SIMS plot covered by the green shade, which is obtained near the surface of the sample, is the 14 15 intensity of the CZTSe thin film, while the SIMS plot covered by the purple shade obtained at deeper area of the 16 17 18 sample is the intensity of a thick MoSe2 layer. The MoSe2 layer can be identified by an abrupt increase of the 19 20 intensity of Se at 1 μm depth, and also the appearance of Mo at the same depth. A thin layer of MoSe2 between 21 22 CZTSe thin film and Mo layer and help the formation of Ohmic contact at the interface and benefit carrier 23 24 25 transportation. However, a thick layer of MoSe2 has high resistance, and can hinder carrier transportation, and 26 27 deteriorate the performance of a CZTSe thin film solar cell. This issue of carrier collection will be investigated by 28 29 current-voltage (I-V) measurement illustrated in the later part of this paper. From the SIMS plot, it shows 30 31 32 homogeneous distribution of Se in CZTSe and Mo layer, revealing that a successful selenization process has been 33 34 achieved. In particular, gradient distribution of copper, zinc and tin has been observed. The intensity of both copper 35 36 and tin have been gradually increased from the surface of CZTSe thin film and have reached their maxima values at a 37 38 point around 300 nm depth. Following the depth point where the maxima concentrations of copper and tin are 39 40 41 detected, gradually decreased concentrations of copper and tin have been subsequently observed, in which the 42 43 concentration of both elements have reached zero around the interface between CZTSe and Mo layer. The intensity of 44 45 zinc exhibits monotonic increase from the surface to the bottom of the CZTSe thin film. Figure 4(b) shows the ratio 46 47 48 of intensity of copper, to the summary of intensity ratio of zinc and tin (Cu/[Zn + Sn]), as a function of depth of the 49 50 CZTSe thin film. 51 52 The intensity ratio of Cu/(Zn+Sn) can directly determine the band gap value of CZTSe material, probably due to 53 54 55 orbital hybridization of the d-orbital of copper and p-orbital of selenium, which leads to a new band gap profile. As a 56 57 result, the band gap value of CZTSe can be decreased by an increasing value of Cu/(Zn+Sn) ratio. 22 As shown in 58 59 Figure 4(b), the Cu/(Zn+Sn) ratio exhibits a reverse double-gradient distribution profile, giving a double-gradient 60 ACS Paragon Plus Environment


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1 band gap value of the CZTSe thin film. A double-gradient band gap profile is widely used in CIGS solar cells to 2 3 4 provide benefits on collection of photon generated carrier and absorption of incident light. A gradient band gap 5 6 profile at the surface of absorber layer of a solar cell can enable broad band absorption of incident light, while a 7 8 gradient band gap profile at the bottom of absorber layer of a solar cell can generate a back scattering electrical field 9 10 11 that accelerate the transportation of photo generated holes. In order to obtain a double-gradient band gap profile, it 12 13 usually needs one or more additional fabrication process during the deposition of the absorber layer to achieve, such 14 15 as adjustment on vapor flow of the source element during co-evaporation process, or surface sulfization of the 16 17 18 absorber layer after selenization. By using the CuxSe/ZnxSn1-x bi-layer precursor, a double-gradient band gap profile 19 20 can be directly generated utilizing the nature of each element, which causes the inter-diffusion during the sputtering 21 22 and the selenization process. 23 24 Figure 4(c) shows schematic illustration of the CZTSe formation process by CuxSe and ZnxSn1-x precursor 25 26 27 during selenization. Experimental results have verified the formation of kesterite CTSe phase in the CuxSe/ZnxSn1-x 28 29 bi-layer precursor at step 1. As we increased the selenization temperature, Zn Sn x 1-x and CuxSe decomposed 30 31 32 sequentially because of different melting point. Meanwhile, the existence of CTSe interlayer plays an important role 33 34 for the CZTSe grain growth. Taking the diffusion ability, reaction affinity and CTSe-assisted grain growth into 35 36 consideration, 23-28 inhomogeneous distribution of the concentration of copper, zinc and tin was formed. Finally, the 37 38 CZTSe film with double-gradient bandgap profile was fabricated. 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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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 Fig. 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. (a) and (b) Depth-dependent elemental distribution and Cu/(Zn+Sn) profiles obtained by SIMS for CZTSe thin film prepared on Mo substrate. (c) Schematic formation process of the CZTSe layer.



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1 2 3 An as-deposited CZTSe thin film has been fabricated into a CZTSe thin film solar cell. In order to unveil its 4 5 6 electrical characteristic, current-voltage (I-V) measurements have been conducted. Dark I-V measurements, light I-V 7 8 measurements and I-V measurement under filtered red light illumination. A long pass filter at 650 nm has been used 9 10 11 during the I-V measurement. The results are shown in Figure 5(a), in which the inset shows a schematic illustration of 12 13 the CZTSe device and the I-V measurement under filtered red light illumination, wherein a long pass filter cutoff at 14 15 650 nm is used. In the I-V curve diagram, no crossover of the dark I-V curve and light I-V curve has been observed, 16 17 18 indicating no significant potential barrier existed. A significantly large potential barrier is usually caused by high 19 20 concentration defect or band offset at the interfaces in a heterojunction, which could induce a difference between the 21 22 current behavior of dark I-V measurement and light I-V measurement, and subsequently results in a crossover of the 23 24 25 two I-V curves. 26 The effect on transportation of photo generated carriers can also be revealed by I-V measurement under 27 28 29 illumination of long wavelength light for a CZTSe solar cell. The n-type CdS buffer layer is transparent to the 30 31 32 incident photons with longer wavelengths, therefore only the CZTSe absorber layer in a CZTSe solar cell can 33 34 generate carriers by absorption of those incident photons with longer wavelengths. This limitation ensures that the 35 36 extracted photo-current is collected from thermionic emission over the band offset, by which the thermionic current 37 38 can be prevented from being quenched by large photo generated current, which is usually the case in light I-V 39 40 41 measurement. Such measure can solely obtain the behavior of the current restricted by the potential barrier built by 42 43 band offset at the interface between CdS and CZTSe. If a distortion of the I-V curve has been exhibited during I-V 44 45 measurement under illumination of light with longer wavelength, a large potential barrier is very probable to exist at 46 47 29 The I-V curve of the 48 the interface between CdS and CZTSe. Such distortion of I-V curve is known as ‘red kink’. 49 50 CZTSe solar cell under illumination of light with wavelength of 650 nm shown in Figure 5(a) exhibits no distortion, 51 52 indicating no significant potential barrier is playing a role. These results of dark I-V measurement, light I-V 53 54 55 measurement and I-V measurement under illumination of light with 650 nm wavelength shows that no large band 56 57 offset or high concentration of defects exist in CZTSe absorber layer or at the interface between CdS and CZTSe. 58 59 In order to investigate the contribution of CZTSe absorber layer to the device performance of the CZTSe solar 60 ACS Paragon Plus Environment

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1 cell, comparison between the electrical characteristics of the CZTSe solar cell obtained from light I-V measurement 2 3 4 with and without 650 nm long pass filter has been made. The device efficiency, Voc, Jsc, and series resistance of the 5 6 CZTSe solar cell obtained from light I-V measurement are 5.46 %, 0.31 V, 37.53 mA/cm2, and 2.83 Ω • cm2, 7 8 respectively, while the device efficiency, Voc and Jsc of the CZTSe solar cell obtained from light I-V measurement 9 10 2 11 using 650 nm long pass filter respectively are 3.87 %, 0.30 V, and 26.83 mA/cm , respectively. From the comparison, 12 13 no obvious change has been observed on the values of Voc obtained by light I-V measurements with and without 650 14 15 nm long pass filter. However, an evident decrease in J by 28.51 % has been observed in the light I-V measurements sc 16 17 18 with 650 nm long pass filter, comparing to the case without the long pass filter. These results have revealed that, since 19 20 the photon generated current is solely collected from the CZTSe absorber layer as the 650 nm long pass filter is used, 21 22 the V of the CZTSe solar cell is dominated by the CZTSe absorber layer due to that no significant change in Voc has oc 23 24 25 occurred when no photon current is generated in CdS buffer layer. The 28.52 % decrease in Jsc in the case where the 26 27 650 nm long pass filter is used has shown that, roughly 28.52 % of Jsc under whit light illumination is contributed by 28 29 the CdS buffer layer. In this analysis, the contribution of AZO window layer has been neglected, due to that the light 30 31 32 intensity within spectral range of AZO absorption in a solar spectrum is negligible. Also, no built-in potential exists in 33 34 AZO layer to collect carrier generated by light absorption at AZO. Therefore, photo-current generated in AZO layer 35 36 can be excluded without effecting the analysis results. 37 38 The decrease in the efficiency of CZTSe solar cell measured by I-V measurements using 650 nm long pass filter 39 40 41 can be presented as η-ηLPF, where η is the device efficiency of the CZTSe solar cell obtained by light I-V 42 43 measurement without 650 nm long pass filter, and ηLPF is the device efficiency of the CZTSe solar cell obtained light 44 45 I-V measurement with 650 nm long pass filter (LPF). An efficiency decreased of 25 % can been obtained, which is 46 47 48 similar to the 28.51 % decrease in Jsc calculated by the same method using Jsc obtained from light I-V measurement 49 50 with and without 650 nm long pass filter. This match indicates that the performance degradation under illumination of 51 52 long wavelength light is mainly attribute to the decreased Jsc. Additionally, due to that the CdS absorber layer starts to 53 54 55 dominate the light absorption of a CZTSe solar cell at wavelength around 500 nm will be shown in EQE spectrum in 56 57 later part of this paper, the long pass filter at 650 nm has filtered out part of the incident photons that will contribute 58 59 photon current generated at CZTSe absorber layer. Given this fact, it is expected that the contribution of 60 ACS Paragon Plus Environment


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1 photo-current generation of CdS is less than the observed value around 25 %, which reveals that the generation of 2 3 4 photo-current for a CZTSe solar cell is heavily depending on the CZTSe absorber layer. The dominance of CZTSe 5 6 absorber layer on photo-current generation could be due to that, CdS share a similar absorption coefficient as that of 7 8 CZTSe, however the thickness of CdS in a CZTSe solar cell is only 50 nm, which is significantly thinner than the 9 10 11 CZTSe absorber layer with thickness of the 1 µm, therefore a major part of incident photons has penetrated through 12 13 the n-type CdS buffer layer and has been absorbed by the CZTSe absorber layer. Also, the n-type buffer layer of a 14 15 CZTSe solar cell is usually deposited by chemical bath deposition (CBD), which is a method producing massive 16 17 18 impurities in the resultant thin film that leads to a layer of compound mixture containing O, OH, S, and others. This 19 20 inevitable consequence can decrease the carrier collected from the n-type buffer layer prepared by CBD, due to the 21 22 high concentration carrier recombination center existing in the n-type buffer layer. However, the effect of n-type 23 24 25 buffer layer is still important for obtaining a strong built-in potential, surface passivation of CZTSe absorber layer, 26 27 and protection at the surface of CZTSe layer from damage of plasma bombardment during the sputtering of AZO 28 29 window layer. Alternative material for n-type buffer layer can be used to increase the photo-current generation of a 30 31 32 CZTSe solar cell, such as Zn(O,S). 33 In order to investigate the spectral response of the CZTSe thin film solar cell prepared from the CuxSe/ZnxSn1-x 34 35 36 bi-layer precursor, EQE measurement has been conducted. The result is shown in Figure 5(b), in which a typical EQE 37 38 spectrum of a CZTSe solar cell is shown, and an inset to the EQE spectrum, showing the first order derivative of the 39 40 41 EQE spectrum curve, is also displayed to indicate the spectral ranges where the generation of photo-current is 42 43 dominated by each thin film layer in the CZTSe solar cell. The curve shown in the inset obtained from first order 44 45 derivative of the EQE spectrum shows three extreme values at 1.05 eV, 2.44 eV, and 3.31 eV, which respectively are 46 47 48 the band gap energies of CZTSe, CdS and ZnO. It can be seen that, the CZTSe absorber layer dominates spectral 49 50 range from the beginning of the EQE spectrum curve to 2.44 eV, ranging from wavelength in infra-red (IR) region to 51 52 wavelength of visible light. In this spectral range where CZTSe dominates the generation of photo-current, the 53 54 55 decrease of EQE is attribute to the carrier recombination in CZTSe bulk, the point defects in CZTSe, and a narrow 56 57 space charge region, which causes failure on collection of photo generated carrier, hence the decrease in EQE. 30 The 58 59 CdS buffer layer on CZTSe absorber layer is dominating EQE in the spectral range from 2.44 eV to 3.31 eV. In this 60 ACS Paragon Plus Environment

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1 spectral range, the EQE is generally lower than those within the spectral range where CZTSe dominates photo current 2 3 4 generation. This is due to the higher defect concentration in CdS layer, which is highly related to the compound 5 6 mixture impurities in the CdS thin film. As mentioned before, those compound mixture impurities are a consequent 7 8 side product of CBD process and is inevitable. The EQE at photon energies larger than 3.31 eV corresponds to where 9 10 11 AZO dominates photo current generation. The EQE of the CZTSe solar cell obtain in this region exhibits the lowest 12 13 EQE comparing to those observed in other spectral range. This is due to the extremely weak carrier collection caused 14 15 by the absent of built-in electric field in AZO layer. 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 ACS Paragon Plus Environment


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Fig. 5. (a) The dark and light I-V measurements of CZTSe solar cell, the blue line represents the result under filtered red light illumination. (b) The EQE spectra of CZTSe solar cells prepared on Mo substrate. ACS Paragon Plus Environment

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1 2 3 4 5 6 A temperature dependent I-V measurement has been conducted on the CZTSe thin film solar cell for 7 8 weatherability test. Temperature of 30 ℃, 80 ℃, 100 ℃, 150 ℃ and 200 ℃ have been chosen for temperature 9 10 11 dependent I-V measurement. At each temperature, the temperature has been dwelled for one hour to reach a balance 12 13 of thermal conduction during the measurement. Both light I-V measurement and dark I-V measurement have been 14 15 conducted. Figure 6(a) shows the dark I-V curves of the CZTSe solar cell obtained at different temperature, while 16 17 18 Figure 6(b) shows the light I-V curves of the CZTSe solar cell obtained by the same measurements. The electrical 19 20 parameters of temperature dependent dark I-V measurements and temperature dependent light I-V measurements are 21 22 respectively collected in Table 1 and Table 2. 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 ACS Paragon Plus Environment


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Fig. 6. (a) The dark current-voltage curves of the CZTSe solar cell taken under device temperature at 60 ℃, 80 ℃, 100 ℃, 150 ℃ and 200 ℃, and (b) the light current-voltage curves of CZTSe solar cell taken under device temperature of 60 ℃, 80 ℃, 100 ℃, and 150 ℃. ACS Paragon Plus Environment

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1 2 3 4 5 6 In Figure 6(a) and Table 1, it can be seen that the I-V curves obtained in dark I-V measurement has gradually 7 8 been changed from an asymmetric I-V curve to a symmetric I-V curve, as the temperature of measurement has been 9 10 11 increased from 30 ℃ to 200 ℃. This is a consequent effect that occurs when a p-n junction is increasingly 12 13 deteriorated during temperature dependent dark I-V measurement. The I-V curve of shunt leakage current of a p-n 14 15 junction has a symmetric profile centered at zero bias voltage, and can be described as space charged limited current, 16 17 31 while the diode current of a p-n junction has an asymmetric profile presented 18 which is insensitive to temperature, 19 20 as an exponential curve and is a function of temperature. Based on these current behaviors, the I-V curves shown in 21 22 Figure 6(a) has revealed that, as the temperature rises during dark I-V measurement, irreversible heat damage to the 23 24 25 CZTSe solar cell has gradually increased the current density of the shunt leakage current of the CZTSe solar cell, and 26 27 probably has simultaneously decreased the diode current of the device, by which the exponential diode current of the 28 29 CZTSe solar cell has eventually been quenched out. The deterioration of the p-n junction due to the heat damage can 30 31 32 be quantified by a heterojunction rectifying character presented as the ratio of dark current measured at 0.5 V to dark 33 34 current measured at -0.5 V, i.e. Idark [0.5V]/Idark [-0.5V]. A higher rectifying character of a p-n junction indicates a better 35 36 quality it possesses. 32 Table 1 shows all junction rectifying character of the CZTSe solar cell obtained at all 37 38 temperatures during temperature dependent dark I-V measurements, which are 100.04, 76.93, 43.10, 5.54 and 0.95 at 39 40 41 30 ℃, 80 ℃, 100 ℃, 150 ℃ and 200 ℃, respectively. The decreasing junction rectifying character of the CZTSe 42 43 solar cell obtained as the temperature of measurement is growing shows the deterioration of the device quality. Table 44 45 1 also presents the values of shunt resistance (Rsh) and series resistance (Rs) at all temperature during temperature 46 47 48 dependent dark I-V measurements, in which decreasing values of both Rsh and Rs have been observed with the rising 49 50 ambient temperature, both indicating current loss due to the damage of the CZTSe solar cell. The decreased Rsh 51 52 indicates the formation of shunt path interconnecting each layer in the device, and the decreased Rs indicates 53 54 55 formations of defects in the device that leads to current loss. 56 57 The heat damage should be concentrated at CdS layer and AZO layer. During the fabrication of the CZTSe solar 58 59 cell, the Mo back contact layer and the CZTSe absorber layer were prepared under high temperature that is much 60 ACS Paragon Plus Environment


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1 higher than all temperatures reached during temperature dependent dark I-V measurement. Therefore, the structure or 2 3 4 composition of Mo layer and CZTSe layer should not be changed during the measurement. However, the layer 5 6 prepared by CBD was deposited at 70 ℃ - 75 ℃, while the AZO layer prepared by sputtering are deposited at room 7 8 temperature. Both CdS layer and AZO layer were prepared at temperature below the temperatures used during I-V 9 10 11 measurement, indicating that the structure or composition of these two layers could be effected by the temperature 12 13 dependent I-V measurements. It has been inferred that, the deterioration of thin film quality of CdS layer and AZO 14 15 layer is due to inter-layer metal incursion occurred as metal elements, such as Cd and Al, has self-segregated at high 16 17 18 temperatures during I-V measurement. The thickness of CdS layer and AZO layer are 50 nm and 300 nm, respectively, 19 20 which is a lot thinner than other layer that makes metal incursion more probable. The metal incursion at high 21 22 temperature can result in a metal/semiconductor/metal bridge structure that produces space charge limited current, 23 24 31 The decrease of both Rsh and Rs also indicates the 25 which has been observed in the dark I-V measurements. 26 27 formation of self-segregated metal clusters. 28 29 30 31 32 33 Table 1. Electrical parameters of CZTSe solar cell taken from temperature-dependent dark I-V measurements. 34 35 36 37 Environmental temperature Rs (Ω cm2) Rsh (Ω cm2) Junction rectifying character 38 39 30 ℃ 0.88 376.83 100.04 40 41 42 80 ℃ 0.65 43.82 76.93 43 44 45 100 ℃ 0.61 18.40 43.10 46 47 48 49 150 ℃ 0.52 1.12 5.54 50 51 52 200 ℃ 0.40 0.43 0.95 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 From the light I-V curves of the CZTSe solar cell obtained from the temperature dependent I-V measurement 7 8 shown in Figure 6(b), it can also be seen that the CZTSe thin film solar cell has been gradually damage by the rising 9 10 11 temperature. Table 2 collects the parameters of the CZTSe solar cell obtained during the light I-V measurements at all 12 13 temperature, in which it shows that, as the temperature has been increased from 30 ℃ to 80 ℃, the efficiency of the 14 15 CZTSe solar cell has been decreased from 5.47 % to 5.23 %, while Voc and FF have also been significantly decreased. 16 17 18 However, a slight increase has been observed on Jsc. The increased Jsc can be understood by the inter-layer diffusion 19 20 of Cd from the CdS buffer layer. During CBD for CdS deposition, the diffusion of Cd ion into CIGS or CZTSe 21 22 absorber layer in a CIGS/CZTSe thin film solar cell has been widely reported. 33 The ion diffusion of CdS is due to 23 24 25 the similar radius of Cd ion and Cu ion, which allows Cd ion occupy on Cu vacancy (VCu) point defect, form a new 26 27 layer of with high concentration of Cd on Cu site (CdCu) point defect at the surface of CIGS or CZTSe thin film. This 28 29 could decrease the band gap energy at the Cd diffusion layer. The increased Jsc and decreased Voc observed on the 30 31 CZTSe thin film solar cell while the ambient temperature has been raised from 30 ℃ to 80 ℃ could be due to a 32 33 34 slight shrinkage of band gap energy at the surface of CZTSe absorber layer due to the enhancement of ionic diffusion 35 36 of Cd. The CBD for CdS deposition was proceeded under temperature around 80 ℃, which allow Cd diffusion at 80 37 38 ℃. The change in Rs and Rsh of the CZTSe solar cell observed as the temperature raised from 30 ℃ to 80 ℃ also 39 40 41 indicates the diffusion of metallic ion. From Table 1, it can be seen that, Rsh has been decrease by 23.12 % due to the 42 43 temperature change from 30 ℃ to 80 ℃, indicating shunt path with low resistance that raise the shunt leakage 44 45 current, which coincides with the inference of the segregation of metal element caused by the diffusion of metal ion. 46 47 48 On the other hand, the value of Rs has been decreased by 26.14 %, also reflects the formation of metal assembly that 49 50 could form a metal bridge extending through the layers on CZTSe absorber layer. As the temperature exceeded 80 ℃ 51 52 during the temperature dependent I-V measurement, the ionic diffusion of metal ion has become more severe. By the 53 54 55 changes of measurement temperature from 80 ℃ to 100 ℃, from 100 ℃ to 150 ℃, and from 150 ℃ to 200 ℃, 56 57 the observed Rsh of the CZTSe solar cell has been decreased by 58.01 %, 93.91 %, and 61.61 %, respectively, while 58 59 the observed R of the CZTSe solar cell has been decreased by 6.15 %, 14.75 % and 23.07 %, respectively, showing s 60 ACS Paragon Plus Environment


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1 that the ionic diffusion and assembly of metal elements were becoming stronger. 2 3 4 5 6 Table 2. Device parameters of CZTSe solar cell taken from temperature-dependent I-V measurements under 7 AM 1.5 solar spectrum illumination 8 9 10 11 Environmental temperature Jsc (mA/cm2) Voc (V) FF η (%) 12 13 30 ℃ 35.04 0.30 0.52 5.47 14 15 16 80 ℃ 35.61 0.16 0.39 2.23 17 18 19 100 ℃ 34.79 0.12 0.35 1.47 20 21 22 150 ℃ 19.37 0.016 0.26 0.08 23 24 25 200 ℃ 0 0 0 0 26 27 28 It is note-worthy that, in the temperature dependent light I-V measurement, as the measurement temperature 29 30 31 becomes higher, the decreasing rates of Voc, Jsc, Rsh, Rs, and device efficiency are the lowest in measurement 32 33 temperature interval from 80 ℃ to 100 ℃, comparing to other measurement temperature interval at higher 34 35 temperatures. This shows that the threshold temperature of the CZTSe solar cell under operation at high temperature 36 37 38 is around 80 ℃ to 100 ℃, where the CdS layer and AZO layer has not yet been damaged. As the measurement 39 40 temperature becomes higher than the threshold temperature of 100 ℃, the electrical parameters obtained of the 41 42 CZTSe solar cell obtained by I-V measurement have started to rapidly decreased, due to the deterioration of CdS 43 44 45 layer and AZO layer at high temperatures. The electrical characteristics of a p-n junction diode, and the generation of 46 47 photo-current in the CZTSe solar cell have been nearly eliminated completely as the measurement temperature has 48 49 reached 200 ℃, whereat the metal incursion and segregation has reach to an equilibrium point that decreases the rate 50 51 52 of change in Rsh. At 200 ℃, it is inferred that the severely rapid diffusion and segregation of metallic elements in 53 54 CdS layer and AZO layer has led to formation of multiple inter-layer shunt paths assembled by metallic ions, which 55 56 constructed a metal/CZTSe/metal structure in the CZTSe solar cell and results in the asymmetric profile of the dark 57 58 59 I-V curve of the CZTSe solar cell obtained at 200 ℃. Additionally, beside inter-layer diffusion of metallic ion, the 60 ACS Paragon Plus Environment

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1 decrease of Rs at high temperatures could also be attribute to the enhancement of attachment between different layers 2 3 4 in the CZTSe solar cell brought by annealing effect, especially at the interface at CdS and CZTSe, and at CdS and 5 6 AZO. 7 8 Conclusions 9 10 11 In this paper, a CZTSe thin film has been successfully prepared through two step process using CuxSe/ZnxSn1-x 12 13 bi-layer precursor and non-toxic selenization process. From SEM images, TEM images and TEM-SAD pattern, it has 14 15 been shown that the CZTSe grain size of the resultant thin film has consistent diameters of around 2-3 µm with no 16 17 18 pin-holes at the surface of the CZTSe thin film and no air voids between CZTSe grains, and the CZTSe thin film is 19 20 purely single crystal. From the results of SIMS measurements, the profile of band gap energy of the CZTSe thin film 21 22 has been found to have a double-gradient band gap profile. This is due to the inter-diffusion of metal ions including 23 24 25 Cu, Zn and Sn between the CuxSe/ZnxSn1-x bi-layer precursor during the selenization process. By using the bi-layer 26 27 precursor, the process of selenization can be simplified and the homogeneosity of the CZTSe thin film can be 28 29 enhanced. The self-formation of the double-gradient band gap energy profile can be achieved without additional 30 31 32 fabrication process, which provides the preferable optical and electrical characteristics benefiting absorption of 33 34 incident light and collection of photo generated holes without raising the fabrication cost. The CZTSe thin film solar 35 36 cell prepared with the CZTSe absorber layer prepared by using the CuxSe/ZnxSn1-x bi-layer precursor has efficiency 37 38 of 5.46 %, short circuit current of 37.53 mA/cm2 and open circuit voltage of 0.31 V. Although the efficiency of the 39 40 41 CZTSe thin film solar cell prepared in this study is lower than the world record efficiency of CZTSe solar cells, the 42 43 simplicity of the fabrication process using the CuxSe/ZnxSn1-x bi-layer precursor, the high homogeneosity, the lower 44 45 cost and the non-toxic selenization process using Se gas are still providing great advantage on fabrication cost and 46 47 48 ecological issues, comparing to the method utilized to prepare the CZTSe solar cell with world record efficiency, in 49 50 which toxic H2Se gas has been used. The lower efficiency of the CZTSe thin film solar cell prepared in this study 51 52 comparing to world record efficiency could be due to the formation of a larger MoSe2 layer, which has been revealed 53 54 55 by TEM images and SIMS measurement. The weatherability test on heat damage has been conducted to the CZTSe 56 57 thin film solar cell by temperature dependent I-V measurement with and without illumination of solar light. During 58 59 the temperature dependent measurement, it has been found that, the performance of CZTSe thin film solar cell has 60 ACS Paragon Plus Environment


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1 started to be deteriorated as the temperature reached the growth temperature of CBD used to prepare the n-type CdS 2 3 4 buffer layer, which is around 80 ℃ to 100 ℃. The damage on the device performance has become more severe as 5 6 the temperature grows higher during the I-V measurement. The dark I-V curves with symmetric curve profile 7 8 centered at zero bias voltage obtained by temperature dependent dark I-V measurement exhibits characteristics of 9 10 11 space charge limited current, which has revealed that, the heat damage to the CZTSe thin film solar cell has caused 12 13 the formation of multiple inter-layer shunt paths assembled through self-segregation of metal elements and the 14 15 inter-layer diffusion of metallic ions at high temperatures, which constructed a shunt path of metal/CZTSe/metal 16 17 18 structure in the CZTSe solar cell and damage the p-n junction material. 19 20 21 22 Acknowledgments 23 24 25 This research was supported by the Green Technology Research Center of Chang Gung University, and also grant 26 27 funded by Chang Gung Memorial Hospital (BMRP 956) and Ministry of Science and Technology 28 29 (MOST105-2112-M-182-001-MY3, MOST 105-2622-E-182-002-CC3, MOST 105-2221-E-155-058). 30 31 32 33 Notes 34 35 The authors declare no competing financial interests. 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 ACS Paragon Plus Environment

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1 Reference 2 3 Tsin, F.; Vėnėrosy, A.; Hildebrandt, T.; Hariskos, D.; Naghavi, N.; Lincot, D.; Rousset, J. Electrodeposition of 4 1 5 6 ZnO-Doped Films as Window Layer for Cd-Free CIGS-Based Solar Cells. Proc. SPIE 9749, Oxide-based Materials 7 8 and Devices VII 2016, 97491H ; doi:10.1117/12.2209327. 9 10 Kim, J.; Hiroi, H; Todorov, T. K.; Gunawan, O.; Kuwahara, M.; Gokmen, T.; Nair, D.; Hopstaken, M.; Shin, B.; 11 2 12 13 Lee, Y. S.; Wang, W.; Sugimoto, H.; Mitzi, D. B. High Efficiency Cu2ZnSn(S,Se)4 Solar Cells by Applying a Double 14 15 In S /Cds Emitter. Adv. Mater. 2014, 26, 7427-7431. 2 3 16 17 Gokmen, T.; Gunawan, O.; Todorov, T.K.; Mitzi, D.B. Band Tailing and Efﬁciency Limitation in Kesterite Solar 18 3 19 20 Cells. Applied Physics Letters 2013, 103, 103506. 21 22 4 Yang, K.J.; Son, D.H.; Sung, S.J.; Sim, J.H.; Kim, Y.I.; Park, S.N.; Jeon, D.H.; Kim, J.; Hwang, D.K.; Jeon, 23 24 25 C.W.; Nam, D.; Cheong, H.; Kang, J.K.; Kim, D.H.; A Band-Gap-Graded CZTSSe Solar Cell with 12.3% Efﬁciency. 26 27 Journal of Materials Chemistry A 2016, 4, 10151-10158. 28 29 5 Gershon, T.; Lee, Y.S.; Antunez, P.; Mankad, R.; Singh, S.; Bishop, D.; Gunawan, O.; Hopstaken, M.; Haight, R. 30 31 32 Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (AgxCu1-x)2ZnSnSe4. Advanced Energy 33 34 Materials 2016, 6, 1502468. 35 36 6 Tajima, S.; Itoh, T.; Hazama, H.; Ohishi, K.; Asahi, R. Improvement of the Open-Circuit Voltage of Cu2ZnSnS4 37 38 Solar Cells Using a Two-Layer Structure. Applied Physics Express 2015, 8, 082302. 39 40 Neuschitzer, M.; Marquez, J.; Giraldo, S.; Dimitrievska, M.; Placidi, M.; Forbes, I.; Izquierdo-Roca, V.; 41 7 42 43 Pérez-Rodriguez, A.; Saucedo, E. Voc Boosting and Grain Growth Enhancing Ge-Doping Strategy for Cu2ZnSnSe4 44 45 Photovoltaic Absorbers. Journal of Physical Chemistry C 2016, 120, 9661-9670. 46 47 Sabli, N.; Talib, Z. A.; Yunus, W. M. M.; Zainal, Z.; Hilal, H. S.; Fujii, M. Effect of Argon Gas on 48 8 49 50 Photoelectrochemical Characteristics of Film Electrodes Prepared by Thermal Vacuum Evaporation from Synthesized 51 52 Copper Zinc Tin Selenide. Int. J. Electrochem. Sci. 2013, 8, 10910-10920. 53 54 Zoppi, G.; Forbes, I.; Miles, R. W.; Dale, P. J.; Scragg, J. J.; Peter, L. M. Cu2ZnSnSe4 Thin Film Solar Cells 55 9 56 57 Produced by Selenisation of Magnetron Sputtered Precursors. Prog. Photovolt: Res. Appl., 2009, 17, 315-319. 58 59 10 Vauche, L.; Risch, L.; Sánchez, Y.; Dimitrievska, M.; Pasquinelli, M.; Goislard de Monsabert T., Grand, P. P.; 60 ACS Paragon Plus Environment


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1 Selenization of Sn–Zn–Cu Sequential Films. Journal of Physics and Chemistry of Solids 2009, 70, 567-570. 2 3 4 22 Suresh Babu, G.; Kishore Kumar, Y. B.; Uday Bhaskar, P.; Sundara Raja Vanjar. Effect of Cu/(Zn+Sn) Ratio on 5 6 the Properties of Co-Evaporated Cu2ZnSnSe4 Thin Films. Solar Energy Materials and Solar Cells 2010, 94(2), 7 8 221-226. 9 10 11 23 Shohji, I.; Nakamura, T.; Mori, F.; Fujiuchi, S. Interface Reaction and Mechanical Properties of Lead-Free 12 13 Sn-Zn Alloy/Cu Joints. Materials Transactions 2002, 43, No.8, 1797-1801. 14 15 24 Gagliano, R. A.; Ghosh, G.; Fine, M. Nucleation Kinetics of Cu Sn by Reaction of Molten Tin with a Copper 6 5 16 17 18 Substrate. Journal of Electronic Materials 2002, 31, 1195-1202. 19 20 25 Takenaka, T.; Kano, S.; Kajihara, M.; Kurokawa, N.; Sakamoto, K. Growth Behavior of Compound Layers in 21 22 Sn/Cu/Sn Diffusion Couples During Annealing at 433–473 K. Materials Science and Engineering A 2005, 396, 23 24 25 115-123. 26 27 26 Ahn, S.; Jung, S.; Gwak, J.; Cho, A.; Shin, K.; Yoon, K.; Park, D.; Cheong, H.; Yun, J. H. Determination of Band 28 29 Gap Energy (Eg) of Cu ZnSnSe Thin Films: on the Discrepancies of Reported Band Gap Values. Appl. Phys. Lett., 2 4 30 31 32 2010, 97, 021905 33 34 27 Wang, M.C.; Yu, S.P.; Chang, T.C.; Hon, M.H. Kinetics of Intermetallic Compound Formation at 35 36 91Sn-8.55Zn-0.45Al Lead-Free Solder Alloy/Cu Interface. Journal of Alloys and Compounds 2004, 381, 162-167. 37 38 28 Ichitsubo, T.; Matsubara, E.; Fujiwara, K.; Yamaguchi, M.; Irie, H.; Kumamoto, S.; Anada, T. Control of 39 40 41 Compound Forming Reaction at the Interface Between SnZn Solder and Cu Substrate. Journal of Alloys and 42 43 Compounds 2005, 392, 200-205. 44 45 29 Neuschitzer, M.; Sanchez, Y.; López-Marino, S.; Xie, H.; Fairbrother, A.; Placidi, M.; Haass, S.; Izquierdo-Roca, 46 47 48 V.; Perez-Rodriguez, A.; Saucedo, E. Optimization of CdS Buffer Layer for High-Performance Cu2ZnSnSe4 Solar 49 50 Cells and the Effects of Light Soaking: Elimination of Crossover and Red Kink. Prog. Photovolt: Res. Appl. 2015, 23, 51 52 1660-1667. 53 54 55 30 Repins, I.; Beall, C.; Vora, N.; DeHart, C.; Kuciauskas, D. Dippo, P.; To, B.; Mann, J.; Hsu, W. C.; Goodrich, A.; 56 57 Noufi, R. Co-Evaporated Cu2ZnSnSe4 Films and Devices. Solar Energy Materials and Solar Cells 2012, 101, 58 59 154-159. 60 ACS Paragon Plus Environment


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1 31 Liao, Y. K.; Kuo, S. Y.; Hsieh, M. Y.; Lai, F. I; Kao, M. H.; Cheng, S. J.; Chiou, D. W.; Hsieh, T. P.; Kuo, H. C. A 2 3 4 Look into the Origin of Shunt Leakage Current of Cu(In,Ga)Se2 Solar Cells Via Experimental and Simulation 5 6 Methods. Solar Energy Materials & Solar Cells 2013, 117, 145-151. 7 8 32 Zhang, J.; Que, W.; Shen, F.; Liao, Y. CuInSe2 nanocrystals/CdS quantum dots/ZnO nanowire arrays 9 10 11 heterojunction for photovoltaic applications. Solar Energy Materials & Solar Cells 2012, 103, 30-34. 12 13 33 Le Donne, A.; Marchionna, S.; Garattini, P.; Mereu, R. A.; Acciarri, M.; Binetti, S. Effects of CdS Buffer Layers 14 15 on Photoluminescence Properties of Cu ZnSnS Solar Cells. International Journal of Photoenergy 2015, Article ID 2 4 16 17 18 583058, 8 pages; http://dx.doi.org/10.1155/2015/583058. 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 ACS Paragon Plus Environment

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Cu2 ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition Prepared by Selenization of Sputtered Novel Precursors.


Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition

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