Effects of Ge Alloying on Device Characteristics of Kesterite-Based

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Effects of Ge Alloying on Device Characteristics of Kesterite-Based CZTSSe Thin Film Solar Cells Dhruba B. Khadka, SeongYeon Kim, and Junho Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11594 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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The Journal of Physical Chemistry

Effects of Ge Alloying on Device Characteristics of Kesterite-Based CZTSSe Thin Film Solar Cells

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 The impacts of Ge alloying on crystal growth and device properties of kesterite based CZTSSe thin film solar cells fabricated by chalcogenization of sputtered stacks in S/Se ambient were investigated. It was found that Ge alloyed CZTSSe material improved the grain growth, compactness of film texture and crystallinity of absorber layers as a consequence of the device efficiencies were enhanced from ~3% to 6%. The investigations on optoelectronic characteristics of devices illustrated that the improvements in devices were mainly governed by decrease in diode ideality factor, suppression of crossover effect between white and dark JV curves, and reduction of defect level in Ge alloyed CZTSSe solar cell device. These results suggest the possibility to achieve a further improvement in the opto-electronic characteristics of the devices that could be accomplished by optimization of the technological processes with a fine tuning of the Ge content in the layers.


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1. INTRODUCTION Relying on earth abundant, non-toxic constituents and suitable optoelectronic properties, kesterite Cu2ZnSn(S,Se)4 (CZTSSe) based solar cells are being focused to develop as a potential alternative to the comparatively mature CIGS and CdTe based technologies due to constraint imposed by cost issue of scarce material constituents In, Ga, and Te coupled with toxicity issue of Cd for dominancy of thin film PV technology as clean energy source.1,2 Although the kesterite CZTSSe based solar cells have been progressed with a record device efficiency of 12.6% by adopting hydrazine based solution approach3 and that of power conversion efficiency (PCE) of 11.6% for pure CZTSe by thermal co-evaporation,4 it is still much lower to have a competitive alternative of CIGS and CdTe based solar cell devices with efficiency ~ 22%.1 A large deficit in open circuit voltage is primary limiting factor which is believed to be associated with compensating defects in CZTSSe material and non-optimized interface layer quality.2,5-8 As a consequence of high degree of cationic disorder and complex defects, there are band gap fluctuations and local electrostatic potential fluctuations which cause band tailing as well as impact on optoelectronic properties in kesterite material and these phenomena are associated with large voltage deficit in kesterite devices.7,8 Some approaches such as alkali metal doping and cations alloying in kesterite CZTSSe material have been reported together with the passivation of the deep recombination states in the bulk and recombination states at interface and grain boundary, reduction of defect densities and promotion of grain growth which lead to larger open circuit voltage (VOC) and fill factor (FF).7-10 There is still much room to improve the device performance which needs a further exploration on absorber layer processing, mitigation of defects in bulk as well as at interfaces, and their impacts on device parameters. 3 ACS Paragon Plus Environment


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As for alloying approach, Ge alloyed CZTSSe materials have attracted a strong interest due to its band gap tunability which can mainly be applied as band graded kesterite solar cell.11-13 Although substitution of cations (alloying approach) has lead the chalcopyrite CIGSe thin film solar cells to the record efficiency with rapid progress,14 there is no such exciting improvement in device efficiency in Ge-alloyed kesterite CZTSSe solar cell devices11 despite of having closely related optoelectronic properties with chalcopyrite CIGSe due to isovalency and structural similarity. Ge-alloyed CZTSSe solar cell devices have been recently reported an improved performance up to 9.4% with 30% of Ge substitution which resulted in increment in VOC by 50 mV and increased minority charge carrier lifetime compared to CZTSSe solar cell device of 8.4% efficiency fabricated by same approach.13 Similarly, Bag et al. also demonstrated small enhancement in efficiency from 9.07% for CZTSe (Ge-0%) to 9.14% for Ge-alloyed CZTSe (Ge-40%) solar cell device.15 Moreover, the device efficiencies of kesterite CZTS were reported to be increased from 4.6% to 6% by adopting the concept of graded band gap absorber layer with varying Ge content.11 Unfortunately, even if the approach adopted is supposed to be promising to enhance the device efficiency, all of reports on Ge-alloyed kesterite solar cells showed just small increment in efficiency which is much lower efficiency compared to the record efficiency of kesterite based solar cell device as well. Recently, Giraldo et al. have reported a noticeable improvement in CZTSe solar cells using small quantity of Ge from 7% (pure CZTSe) to 10.1% (with ~2% of Ge). This remarkable enhancement in device parameters reported to be governed by improvement in crystal quality and large grain, and presence of GeOx nano-inclusions.16 Thus, the reports on enhancement of device efficiency by Ge incorporation in kesterite host material as mentioned above have revealed many interesting facts and provide basis for further improvement in kesterite device. 4 ACS Paragon Plus Environment

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There is still need of more systematic explorations for optimization of film growth and optoelectronic properties of Ge- alloyed CZTSSe based solar cell device. In this article, we deal with the fabrication of Ge alloyed CZTSSe thin film solar cells in which the absorber layers were prepared by chalcogenization of stack precursors deposited by sputtering technique and In2S3 is employed as Cd-free buffer by chemical spray pyrolysis technique. We have investigated material growth properties of Ge alloyed CZTSSe absorber layers and optoelectronic properties of solar cell device. An efficiency of CZTSSe device, 3.1% is found to be enhanced to 6.09% with Ge alloyed CZTSSe solar cell device. A comparative analysis has carried out to understand the influence in solar cell parameters and optoelectronic properties of fabricated solar cell devices taking account of J-V curve and electrical properties and defect analysis. 2. EXPERIMENTAL SECTIONS 2.1. Preparation of CZTSe Thin Film: Kesterite based thin film solar cells with CZTSSe and Ge alloyed CZTSSe absorber layers were prepared by chalcogenization of stack precursor films at high temperature under selenium rich sulfoselenium vapor ambient. The precursor films having Mo/ZnS/SnS/Cu stacking order with thickness approximately of 260, 274, 161 nm for ZnS, SnS and Cu, layers respectively, were deposited at substrate temperature of 100 ℃ by Rf-magneto sputtering and the stack layers were controlled to maintain Zn-rich and Cupoor composition of absorber layer. To study the impact of Ge in CZTSSe solar cell, Ge-over layers of various thicknesses (approximately; 5, 10, 20, 30 nm), hereafter denoted by CZTSSe(Ge-x); x=1, 2, 3, 4) were deposited on Mo/ZnS/SnS/Cu stacks by Rf-sputtering of Ge metallic target. The stacked precursors were crystallized by subsequent rapid thermal



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annealing (with a heating slope of 50 oC min-1) under the selenium rich sulfoselenium vapor ambient and dwelled at ~520-540 ° C for 30 min keeping the sample inside rectangular graphite box filled with sulfur/selenium (~1:10 ratio) powder (99.99%) placed in quartz tube maintained under the flow of nitrogen gas. The schematic diagrams of annealing furnace and temperature profile for crystallization are same as our earlier reports.17,18 2.2. Solar Cell Fabrication: The solar cell devices were completed by further deposition of In2S3 as buffer layer and ZnO/ITO as window layer. A chemical etching was proceed on surface of CZTSSe/CZTSSSe(Ge-x) absorber layers for 2-3 minutes in 10% of aqueous solution of potassium cyanide (KCN) and immediately washed with de-ionized water (DIW), then dried under gentle flow of nitrogen gas. Then, indium sulfide (In2S3) (80-100 nm) thickness was deposited on absorber layer as Cd-free buffer using CSP technique. The detail of buffer deposition can be found elsewhere.19 Finally, the device structure was completed by subsequent deposition of intrinsic zinc oxide (i-ZnO) layer and indium tin oxide (ITO) layer via radio frequency (RF) magnetron sputtering having solar cell structure; Mo/CZTSSe(Gex)/In2S3/ZnO/ITO. 2.3. Thin Film and Device Characterization: The fabricated absorber layers were characterized for cross-sectional image and surface texture by field emission scanning electron microscopy (FESEM, JEOL, JSM-7001F) equipped with energy dispersive X-ray spectroscopy (EDS, Oxford, INCA) and structural and phase properties by X-ray diffraction (XRD, Rigaku, Smart Lab, with a Cu Kα source of λ=1.5412 Å, 2θ scan, rate of 3°/min) and Raman spectroscopy (spectrometer (Mmac 750) and laser, λ=532 nm, irradiation power < 1 mW) and spot size of approximately 1 μ m). The optoelectronic characterizations were performed by measuring current density-voltage (J-V) characteristics, external quantum 6 ACS Paragon Plus Environment

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efficiency (EQE) and admittance spectroscopy (AS). J-V curves were obtained 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 illumination. The quantum efficiency was measured using Xe light source; monochromator combined with light chopper and locked in amplifier system (McScience). Admittance spectroscopy (AS) measurement was carried out to study the defect in the fabricated device with an LCR meter (E4980A, Agilent) which probes from 20 Hz to 2 MHz at ac voltage 30 mV under dark condition in the temperature range of 300-90 K. Capacitance–voltage (C-V) measurements were also performed with an LCR meter to estimate the space charge width, built in potential and free carrier density. For temperature dependent J-V curve and C-f scan, 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 To assess the crystal quality and possible secondary phases, the fabricated thin films were characterized by XRD and Raman spectroscopy measurements. Figure 1a shows the XRD patterns of CZTSSe and CZTSSe(Ge-x) films with sharp and dominant characteristic peak of (112) orientation indicating high quality of thin films as earlier reports.12,15 The dominant characteristic peaks showed shifting to the higher 2θ angle with increasing the Ge alloying up to CZTSSe(Ge-3) thin film whereas that of CZTSSe(Ge-4) film shows shifting to the lower 2θ angle. It is well known that the detection of the kesterite CZTS(Se) by XRD pattern is quite difficult because of almost overlapped XRD patterns of the cubic ZnS(Se) and ternary 7 ACS Paragon Plus Environment


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Cu2SnS(Se)3 (CTSSe) with the kesterite CZTSSe.12,20,21 Therefore the crystal quality and phase purity are usually characterized by XRD and Raman spectroscopy measurements. Figure 1b depicts the Raman spectra of the fabricated films with dominant characteristic peaks of kesterite (CZGTSSe) phase. It is to be noted that although there is absence of peaks from secondary phases12 in the Raman spectra (Figure 1b), the resonant Raman scattering measurement is required to assess of the presence of possible secondary phases precisely20,21 which is beyond the scope of this study. The characteristics trend of XRD patterns and Raman spectra of fabricated films (Figure 1) were found to be in close agreement to earlier reports.12,15,22 Raman spectra of the respective CZGTSSe(Ge-x) films (Figure 1b) revealed a trend of blue shifting of the dominant Raman peak as a consequence of Ge alloying in the CZTSSe(Ge-x) crystal. But in case of CZTSSe(Ge-4), there is slight deviation in the shifting trend of dominant characteristic peaks to the lower 2θ angle in XRD patterns and similar trend is observed in case of characteristic Raman peak (Figure S1, Supporting Information). The composition data (Table S1, Supporting Information) implicate that the deviation of characteristics peak for CZTSSe(Ge-4) from trend with increasing Ge content is attributed to the strong difference in the S/(S+Se) content ratio in relation to the other ones. A significant low value of sulfur content is believed to be due to preferential formation of GeS leading Ge loss during the sulfoselenization process for the sample with the stacked precursor of thickest Ge top layer. We also observed a local variation of composition in the surface texture of CZTSSe(Ge-4) thin film which was possibly governed by the Ge-loss during chalcogenization at high temperature under sulfur/selenium ambient and as a consequence it might have a deterioration in crystal quality of the film. A better quality of CZTSSe(Ge-x) thin film can be


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grown by fine tuning of Ge content and optimized chalcogenization route to minimize the Ge loss as binary chalcogenide compound. 3.2. Compositional and Morphological Impact The fabricated thin films were characterized by SEM and EDS measurements to study the impact of Mo/ZnS/SnS/Cu/(Ge-x) stack on surface morphology and composition in the film growth. The EDS results of the fabricated films have been summarized in Table S1 (Supporting Information). A compositional analysis of fabricated films showed Cu- poor and Zn-rich composition and varying Ge and Sn contents in the kesterite CZTSSe thin film. Geloss is normally observed in Ge alloyed CZTSe thin film because GeS(Se) has higher vapor pressure which leads Ge-loss during sulfoselenization.13,15,22 Although the elemental loss of Ge as GeS(e)2/GeS(e) is expected in higher extend during crystallization with sputtered stack precursors having Ge layer on the top stack layer, the film texture is found to be better compared to stack precursor without Ge layer. It is believed that a well-controlled elemental loss during crystallization with thin Ge-over layer on the sputtered stack precursor can further improve the crystal quality. Figure 2(a-d) depicts the surface textures of sulfoselenized films having compact and well grown crystalline texture. The crystalline textures of thin films were found to be improved with slightly increase in grain size with increasing Ge alloying whereas the surface texture of CZTSSe(Ge-4) film (Figure 2e) showed comparatively less compact and slight variation in composition which is believed to be influenced of larger Ge loss as GeS(Se) compound during sulfoselenization process for the sample with the thickest Ge top layer. It indicates that a higher Ge loss in CZTSSe(Ge-x) thin film leads to non-compact surface



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texture, compositional non uniformity and hence decrease the crystallinity of the thin films20 which result in deterioration the device parameters. A detail investigation behind the deterioration of device efficiency with higher Ge alloying in stack precursors as top layer or inter layer can be another interesting matter of study. Figure 3 depicts the cross-sectional image of solar cell devices with CZTSSe absorber layer (a) and CZTSSe(Ge-3) absorber layer (b). The solar cell structure showed well buried In2S3 buffer layer in the absorber layer and thin MoS(Se)2 interface layer in between absorber and back contact, Mo layer and similar features were also observed in cross-sectional images in other devices (Figure S2, Supporting Information). The crystallized absorber layers have shown cross-sectional thickness in the range of 0.8-1.2 μm where a thin layer of MoS(Se)2 also observed to be formed as back contact interface layer during the chalcogenization. Small grains located at the bottom of the absorber layers observed in each of cross-sectional images may act as recombination centers which can be improved by optimization of crystallization condition to get further improvement in device efficiency. A stark difference in each of cross-sectional images is the crystalline feature of absorber layers which is found to be comparatively better in CZTSSe(Ge-3) solar cell device having champion efficiency. Thus, these features of surface textures and cross-sectional images corroborate the facts that the kesterite CZTSSe(Ge-x) with control Ge assisted growth improve the crystal quality, surface texture and grain growth. 3.3. Device Characteristics The photovoltaic performances of CZTSSe and CZTSSe (Ge-3) thin films solar cells are depicted in Figure 4a. The J-V curves demonstrated enhancement of device efficiency from 3.1% for CZTSSe solar cell to 6.09% for CZTSSe (Ge-3) solar cell with improvement in the device parameters VOC, JSC, and FF as listed in Table inset. The overall efficiency is found to 10 ACS Paragon Plus Environment

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be limited by low fill factor (FF) in both devices which is also correlated to the suppression of VOC. The J-V characteristics and device parameters of solar cell devices of best performances of respective absorber layers are given in the Supporting Information (Tables S2 and S3 and Figure S3). These enhancement in efficiency as depicted in Figure 4a are believed to be mainly governed by improved crystal quality of CZTSSe (Ge-3) absorber layer as discussed in growth characterization. It is point to be noted that the dark and illuminated J-V curves (Figure 4a) exhibit noticeable improvement in crossover behavior in CZTSSe (Ge-3) device which might be associated with improvement in electrical barrier nature in the buffer/absorber interface or quenching of multivalent defects.23-25 Although the device results achieved here are not high enough, a noticeable improvement in device efficiency of CZTSSe by tuning the Ge alloying in CZTSSe (Ge-x) absorber can provide a way to enhance the kesterite CZTSSe device efficiency by further optimization with Ge-over layer stack by controlling the crystallization process. Even though the device fabrication approach is different in the context of Ge-layer deposition approach and employed buffer layer, the device efficiency is in the similar trend as reported by Giraldo et al.15 It indicates that CZTSSe absorbers grown with tailoring of small Ge content improve the absorber quality and enhance the device efficiency. The spectral responses of respective solar cell devices in Figure 4b show the external quantum efficiency (EQE) which revealed noticeable enhancement in longer wavelength regime. The maximum EQE (~46% for CZTSSe) and ~67% for CZTSSe(Ge-3)) corresponds to around 570 nm and then gradually decays for visible and infrared region of the spectrum. Although EQE results support a comparative explanation to the related solar cell parameter, JSC of the device, it is quite lower than the EQE results reported for equivalent solar cell 11 ACS Paragon Plus Environment


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devices.15,16,24 It is obvious that the poor EQE response in the short wavelength regime is due to absorption losses in the window and buffer layer (ZnO, In2S3) and weak EQE response in longer wavelength regime is believed to be coupled with recombination losses in the bulk and depletion region and reflection from the solar cell surface.25,26 Moreover, the both devices showed band tailing which indicate the impact of defect complex in the performance of both devices.7,8 Even though, it is complicated to determine the band gap of absorber layers correctly due to the effect of band tailing, Eg values of absorber layer of respective solar cells were estimated to be approximately 1.118 ± 0.01 eV for CZTSSe and 1.136±0.01 eV for CZTSSe(Ge-3) from linear extrapolation of plot [E×ln(1-EQE)]2 vs. E(hν) as depicted in Figure S4 (Supporting Information). To evaluate the impact on electrical properties, we analyzed the J-V curves of two solar cell devices. It is interesting to notice that the crossover points of dark and illuminated J-V curves (at J~5 mA cm-2 for CZTSSe and J~35 mA cm-2 for CZTSSe(Ge-3)) are found to be improved in much extend for CZTSSe(Ge-3) solar cell device. The crossover phenomena are reported to be attributed to absorber/buffer interface properties associated with high density defect states which are presented either in the absorber layer close to hetero-junction interface (p+ layer).24,25,27 In another aspects, the low FF in both devices and strong crossover behavior for CZTSSe solar cell can also be concomitant to MoS(e)2 non ohmic back contact barrier existing in Mo/CZTSe interface or the high resistivity of CZTSSe absorber layer.28,29 In our case, the cross over effect is supposed to be associated with complex recombination centers as well as back contact interface which will be correlated in later discussion.


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Furthermore, the electrical parameters were extracted by fitting J-V curves according to diode model as described in reports.25,26 The plots for estimation of RS, Jo, and A are shown in Figure S5 (Supporting Information) . The deduced data showed a slight decrease of RS from 0.372 Ωcm2 (for CZTSSe) to 0.115 Ωcm2 (for CZTSSe(Ge-3)) and Jo decreases from 2.26×101

mAcm-2 (for CZTSSe) to 5.69×10-2 mAcm-2 (for CZTSSe(Ge-3)). Since the limit in VOC is

primarily due to the activity of defects which also results in increase of reverse saturation current.7,8 In this analysis, although the calculated values of Jo in our devices were much larger than the reported value ~10-6 mAcm-2,7 the decrease in the value of J0 for CZTSSe(Ge3) solar cell device commensurates the facts that the increase in device parameters especially VOC might be as a consequence of alleviation of the defect. We also observed a significant decrease in diode ideality factor (A) from ~ 4.13 (for CZTSSe) to ~1.97 (for CZTSSe(Ge-3)) which is consistent with the crossover behavior in J-V curves.23,30 It is to be noted that the diode ideality factor (A) is associated with recombination mechanism, as the value of A close to 1 indicates defect recombination in the quasi neutral region (QNR) whereas as if A is close to but ≤2, it is contributed from Shockley-Read-Hall (SRH) recombination mechanism in the space charge region (SCR). In the case of value of A more than 2, the recombination mechanism is explained by multiple steps and complex recombination process via a series of trap distributed in interface and surface, carriers tunneling phenomena and fluctuation in defect activation energy.23,27,31 By analytical study of our results, a significant decrease in the diode ideality factor and suppression of J-V crossover effect in CZTSSe(Ge-3) solar cell attributed to mitigation in complex recombination phenomena existed in CZTSSe device as consequence of improvement in crystal quality which result in increase of device efficiency in CZTSSe(Ge-3) solar cell device. 13 ACS Paragon Plus Environment


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These devices were further investigated by measuring temperature dependent J-V characteristics (J-V-T) under dark and white light illumination (Figure S6, Supporting Information). The temperature dependent VOC plot presented in Figure 5 determine the dominant recombination mechanism by extracting the activation energy in accordance of interception at T=0 K of linear fit of VOC(T) given by,25,26 =

-

× Ln " !

(1)

where J00 denote reverse saturation current prefactor, respectively. The linear extrapolation of the VOC(T) to 0 K yields approximately Ea = 0.921 eV (for CZTSSe) and 0.938 eV (for CZTSSe(Ge-3)) which is much lower than the band gap of respective absorber layers which implicates interface recombination is dominant in both devices.7,25,26,32 It is interesting to be noted that there is no noticeable difference in depreciation of Ea of both devices which implicates no much change in interface layer in CZTSSe(Ge-3) device. It also indicates that the device efficiency of CZTSSe(Ge-3) solar cell is not enhanced by improvement in interface layer quality. And other device parameters (JSC, FF,η, RS) also showed very similar trend as function of temperature. It is believed that the device efficiency can be noticeably enhanced after minimizing the depreciation of Ea by improving the absorber and buffer interface layer quality. Figure 6 depicts the charge carrier profile of CZTSSe and CZTSSe(Ge-3) devices calculated from C(V) measurements as described in references.25,33 The SCR width is estimated to be nearly equal value of 0.245 and 0.237 μm for CZTSSe and CZTSSe(Ge-3) devices at 0 V bias, respectively, whereas the built in potential (Vbi) values were estimated to


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be 0.443 and 0.542 V for CZTSSe and CZTSSe(Ge-3) device as presented in Figure S7 (Supporting Information). We extracted carrier density NCV estimated from C-V profiling at 120 K as depicted in Figure 6, which were estimated to be 12.68×1015 cm-3 for CZTSSe device and 7.24×1015 cm-3 for CZTSSe(Ge-3). The carrier density profile was found to be distributed in wide range for CZTSSe device whereas that of CZTSSe(Ge-3) device revealed comparatively confined carrier distributed in U-shaped. The distribution profile of carrier density of our devices is in similar trend to other reports in kesterite based CZTSSe devices.19,34,35 To investigate the defects in devices, we also carried out the temperature dependent capacitance–frequency (C-f-T) scans as depicted in Figure S8 (Supporting Information). The inflection point determined from C-f-T curve used for fitting of the Arrhenius plots in accordance of the equation,36 $% = 2πξ% ( exp

)*+

,- .

"

(2)

Where $% is the step frequency, and /0 is activation energy of dominant defect, and ξ0 is an emission factor comprising the product of the temperature independent terms of related parameters.25,36 Figure 7 shows the Arrhenius plot from which the activation energy of defect were calculated to be 0.192 eV for CZTSSe and 0.108 eV for CZTSSe(Ge-3) and the emission factors were found to be ~ 4×103 and ~ 2×103 s-1K-2 for the corresponding solar cell devices. This result revealed that the deep defect level (0.192 eV) for CZTSSe device was mitigated to the comparatively shallow level (0.108 eV) for CZTSSe(Ge-3) device so that the recombination phenomena is expected to be soothed in CZTSSe(Ge-3) absorber which played a supportive role to improve the device parameters which can also be correlated to the 15 ACS Paragon Plus Environment


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EQE data (Figure 4b). The enhancement of EQE results (Figure 4b) in the visible/infrared regime for CZTSSe(Ge-3) device is due to the consequence of extenuated defect activities. Although the deep defect level (0.192 eV) in CZTSSe was decreased to comparatively shallower defect level in CZTSSe(Ge-3), 0.108 eV which is close to CuZn antisite as deeper acceptor defect,5,19 the efficiency of CZTSSe(Ge-3) device is limited in some extent whereas having deeper defect level in CZTSSe device, its device performance is much suffered than CZTSSe(Ge-3) device. Our results are consistent with the previous reports on effect of defect levels in performance of solar cell devices.19,37,38 The deterioration of device parameters must be mainly associated with the defect level coupled with distribution of carriers and nonoptimized interface layer in the context of this study. It is expected to have further improvement in the device performance by addressing the issue mainly associated with absorber quality and interface optimization with well controlled device processing.

4. CONCLUSIONS We report on the impacts of Ge alloying on crystal growth and device properties of kesterite CZTSSe based thin film solar cell device. Ge alloying in CZTSSe material improved the grain growth, compactness of film texture and crystallinity of absorber layer fabricated by sulfoselenization of sputtered stack precursors. The power conversion efficiency was improved from ~3.10% with CZTSSe(Ge-0) absorber to 6.09% with CZTSSe(Ge-3) absorber employing In2S3 buffer layer as Cd free buffer. The optoelectronic characteristics of devices revealed that the enhancement in device performance is achieved by decrease in diode ideality factor, suppression of crossover effect between white and dark J-V curves, and mitigation of defect level in the Ge alloyed CZTSSe solar cell device by minimizing defect activities. The 16 ACS Paragon Plus Environment

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device efficiency can be improved by controlled processing for optimization of absorber crystallinity and interface quality to minimize the recombination dominancy.

ASSOCIATED CONTENT Supporting Information Elemental composition of CZTSSe(Ge-x) thin films, cross-sectional images of solar cell devices, device parameters of CZTSSe(Ge-x) solar cells, temperature dependent J-V curves of CZTSSe and CZTSSe(Ge-x) solar cells under dark and white light illumination, Band gap of CZTSSe and CZTSSe(Ge-3) absorber layers, Mott-Schottky plots and C-f-T spectra of devices. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected]. Phone: +82-32-835-8221. Notes The authors declare no completing financial interest

ACKNOWLEDGEMENTS This work was carried out by financial support of the National Research Foundation of Korea (NRF) funded by the Korean government (NRF-2014R1A2A1A11053109) and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government and Ministry of Trade, Industry and Energy (No. 20123010010130).



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Figure

Figure 1. X-ray diffraction (XRD) patterns (a) and Raman spectra (b) of CZTSSe and CZTSSe(Ge-x) thin films. Here, * denotes characteristic peak of kesterite phase.


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Figure 2. Surface images of CZTSSe (a) and CZTSSe(Ge-x) (b-e) thin films. Here, b-e represent CZTSe with Ge-1, 2, 3, and 4, respectively.



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Figure

3.

Cross

sectional

image

of

solar

cell

SLG/Mo/MoSe2/CZTSSe(Ge-x)/In2S3/ZnO/ITO structure. CZTSSe(Ge-3) (b) device structures have been depicted.


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devices

consisting

of

Here, CZTSSe (a) and

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Figure 4. J-V characteristic of CZTSSe and CZTSSe(Ge-3) solar cell device (a). External quantum efficiency (EQE) of corresponding devices. The data in inset table in Figure (a) summarizes the device parameters of champion solar cell device of respective type having area of ~ 0.09cm2.



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Figure 5. Open-circuit voltage (VOC) as function of temperature and its linear extrapolation line to T= 0 K of CZTSSe and CZTSSe(Ge-3) solar cell device.


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Figure 6. Charge carrier density profile determined from capacitance voltage (C-V) measurements at 120 K for CZTSSe and CZTSSe(Ge-3) solar cell device.



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Figure 7. Arrhenius plot of characteristic frequencies extracted from admittance spectra of CZTSSe and CZTSSe(Ge-3) devices (C-f-T scan) for the estimation of defect levels.


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Table of Contents Graphics


Effects of Ge Alloying on Device Characteristics of Kesterite-Based

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