Characterization of CBO and defect states of CZTSe solar cells prepared by using two-step process

We fabricated kesterite Cu₂ZnSnSe₄ (CZTSe) solar cells and studied device characteristics, where CZTSe absorbers were made by using two-step process. First, we deposited precursor CZTSe films with spin-coating or sputtering, and performed sulfurization and subsequent selenization. To complete the device, we applied In₂S₃ as a buffer layer. We obtained power conversion efficiency (PCE) of 4.18% with spin-coated CZTSe absorber and 5.60% with sputtered CZTSe absorber. Both devices showed deep defects in the bulk and strong interface recombinations near the pn junction. In addition, we observed red-kinks in the current density-voltage (J-V) curves for both devices under the filtered light illumination (>660 nm), which is attributed to large conduction band offset (CBO) between the CZTSe absorber and the buffer layer and defect states in the buffer/CZTSe absorber or in the buffer. The red-kink was also observed in CZTSe (PCE of 7.76%) solar cell with CdS buffer. Hence, to enhance the PCE with CZTSe absorber, along with suppression of deep defects which act as recombination center, optimization of CBO between absorber and buffer is also required.     

CZTS‐based materials and interfaces and their effects on the performance of thin film solar cells

Cu₂ZnSnS₄ (CZTS) and its related materials such as Cu₂ZnSnSe₄ (CZTSe) and Cu₂ZnSn(S,Se)₄ (CZTSSe) have attracted considerable attention as an absorber material for thin film solar cells due to the non‐toxicity, elemental abundance, and large production capacity of their constituents. Despite the similarities between CZTS‐based materials and Cu(In,Ga)Se₂(CIGS), the record efficiency of CZTS‐based solar cells remains significantly lower than that of CIGS solar cells. Considering that the difference between the two lies in the choice of the absorber material, the cause of the lower efficiency of CZTS‐based solar cells can be isolated to the issues associated with CZTS‐based materials and their related interfaces. Herein, these issues and the work done to understand and resolve them is reviewed. Unlike existing review papers, every unique region of CZTS‐based solar cells that contributes to its lower efficiency, namely: (1) the bulk of the absorber, (2) the grain boundaries of the absorber, (3) the absorber/buffer layer interface, and (4) the absorber/back contact interface are surveyed. This review also intends to identify the major unresolved issues and the potential improvement approaches of realizing sizable improvements in the solar cells' efficiency, thus providing a guide as to where research efforts should be focused. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Fabrication of Cu2ZnSn(SxSe1−x)4 thin film solar cell by single step sulfo-selenization of stacked metallic precursors

We have synthesized an efficient Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe) absorbers by using single-step rapid thermal sulfo-selenization process of sputtered stack metallic precursor (Zn/Sn/Cu) films. The structural and morphological studies confirm that the suitability of the rapid thermal sulfo-selenization process for the synthesis of a CZTSSe absorber without any secondary phases with large grains. The annealing atmosphere with a mixed-chalcogen source enhances the grain growth of the CZTSSe absorber as compared with pure Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) absorbers. The CZTSSe thin film solar cell shows the best conversion efficiency of ∼7%.     

一种印刷型薄膜太阳能电池p-n结调制技术

能带值为0.5~0.85 eV材料的稀缺是多结太阳能电池面临的一个主要挑战,本文使用非真空的机械化学法合成了能带值为0.83 eV的Cu₂SnS₃化合物,使用印刷技术将其制备成吸收层薄膜,并采用superstrate太阳能电池结构(Mo/Cu₂SnS₃/In₂S₃/TiO₂/FTO glass)对其光伏特性进行了研究.实验表明所制备的太阳能电池短路电流密度、开路电压、填充因子和转换效率分别为12.38 mA/cm²、320 mV、0.28和1.10%.此外,为更好地满足多结太阳能电池对电流匹配的需求,本文对所制备太阳能电池的Cu₂SnS₃/In₂S₃ p-n结进行了分析.通过在p-n结界面植入一层薄的疏松缓冲层,使调制后的太阳能电池短路电流密度从最初的12.38 mA/cm²增加到了23.15 mA/cm²,相应太阳能电池转换效率从1.1%增加到了1.92%.该p-n调制技术对印刷型薄膜太阳能电池具有重要借鉴意义.     

衬底温度对共蒸发法制备Cu2ZnSnSe4太阳电池的影响

采用共蒸发法在不同衬底温度下沉积Cu_2ZnSnSe_4(简称CZTSe)薄膜,分析了衬底温度对CZTSe材料性质及电池性能的影响。研究表明:当衬底温度较低时(380℃),CZTSe薄膜中含有SnSe_x使电池失效;随着衬底温度的升高,CZTSe薄膜的结晶质量明显提升,电池开路电压增加。但当衬底温度达到460℃时,电池的转换效率反而下降;结合CZTSe的生长机理及器件模型分析了电池效率下降可能的原因。最终在衬底温度420℃的条件下制备出效率为3.12%(有效面积0.34 cm~2)的CZTSe太阳电池。     

Application of multi-buffer layer of (Zn,Mg)O/CdS in Cu2ZnSn(S,Se)4 solar cells

(Zn,Mg)O (ZMO) buffer layer has attracted attention for having the potential to control the conduction band offset of buffer layer and large band-gap (Eg) Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber interface, where the ZMO layer is deposited by the sputtering. However, the solar cell efficiency is decreased with the ZMO layer as compared with the CdS layer. The decrease in conversion efficiency is attributed to the sputtering damage on the absorber and high light reflection from the surfaces of CZTSSe solar cells. To completely suppress the damage, a CdS layer with very thin thickness of 20 nm is inserted between the ZMO layer and the CZTSSe layer. In addition, MgF₂ layers are deposited on CZTSSe solar cells as anti-reflection coating. Ultimately, the solar cell with multi-buffer layer of ZMO/thin-CdS is almost same level as that with the CdS layer. Therefore, the multi-buffer layer can be an appropriate buffer layer of the large-Eg CZTSSe layer.     

The characteristic of Cu2ZnSnS4 thin film solar cells prepared by sputtering CuSn and CuZn alloy targets

Recent study shows that the main reason for limiting CZTS device performance lies in the low open circuit voltage, and crucial factor that could affect the V_oc is secondary phases like ZnS existing in absorber layer and its interfaces. In this work, the Cu₂ZnSnS₄ thin film solar cells were prepared by sputtering CuSn and CuZn alloy targets. Through tuning the Zn/Sn ratios of the CZTS thin films, the crystal structure, morphology, chemical composition and phase purity of CZTS thin films were characterized by X-Ray Diffraction (XRD), scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) and Raman spectroscopy. The statistics data show that the CZTS solar cell with a ratio of Zn/Sn = 1.2 have the best power convention efficiency of 5.07%. After HCl etching process, the CZTS thin film solar cell with the highest efficiency 5.41% was obtained, which demonstrated that CZTS film solar cells with high efficiency could be developed by sputtering CuSn and CuZn alloy targets.     

连续离子层吸附反应沉积后硫化法制备柔性铜锌锡硫薄膜太阳电池

在柔性钼箔衬底上采用连续离子层吸附反应法(successive ionic layer absorption and reaction)制备ZnS/Cu2SnSx叠层结构的预制层薄膜,预制层薄膜在蒸发硫气氛、550 C温度条件下进行退火得到Cu2ZnSnS4吸收层.分别采用EDS,XRD,Raman,SEM表征吸收层薄膜的成分、物相和表面形貌.结果表明,退火后薄膜结晶质量良好,表面形貌致密.用在普通钠钙玻璃上采用相同工艺制备的CZTS薄膜表征薄膜的光学和电学性能,表明退火后薄膜带隙宽度为1.49 eV,在可见光区光吸收系数大于104cm 1,载流子浓度与电阻率均满足薄膜太阳电池器件对吸收层的要求.用上述柔性衬底上的吸收层制备Mo foil/CZTS/CdS/i-ZnO/ZnO:Al/Ag结构的薄膜太阳电池得到2.42%的效率,是目前报道柔性CZTS太阳电池最高效率.     

Cu2ZnSnSe4 thin films prepared by selenization of co-electroplated Cu-Zn-Sn precursors

A novel technique for growth of high quality Cu₂ZnSnSe₄ (CZTSe) thin films is reported in our work. The CZTSe thin films were fabricated onto Mo layers by co-electroplating Cu-Zn-Sn precursors followed by annealing in the selenium vapors at the substrate temperature of 550 °C. The morphology and structure of CZTSe thin films were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and Raman scattering spectrum, respectively. The results revealed that the single phase was in the CZTSe thin films, and the other impurities such as ZnSe and Cu₂SnSe₃ were not existed though they were difficult to distinguish both from EDS and XRD.     

A PL and PLE Study of High Cu Content Cu2ZnSnSe4 Films on Mo/Glass and Solar Cells

Physics of the Solid State - Cu2ZnSnSe4 (CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and...     

Prediction and optimization of the performance characteristics of CZTS thin film solar cell using band gap grading

The results of the numerical simulation of the performance characteristics of (Cu₂ZnSnS₄) CZTS thin film solar cell due to bandgap grading is presented in this work. The investigation of the performance of this solar cell was carried out using the Analysis of Microelectronics and Photonics software (AMPS-1D). A substrate cell structure FTO/CdS/CZTS/Mo(SLG) was used as the base model. An efficiency of 8.33% was obtained from the simulation with baseline parameters while an intentional grading of the device was carried out on the device both at the front and back interface of the absorber. Front grading was observed to degrade the device performance while significant improvement of the device performance was observed with back grading. An intentional double grading of the device further enhanced the efficiency up to 12.26%.     

Synthesis and characterization of Cu2ZnSnS4 thin films by SILAR method

Semiconducting Cu₂ZnSnS₄ (CZTS) material has been receiving a great technological interest in the photovoltaic industry because of its low-cost non-toxic constituents, ideal direct band gap as a absorber layer and high absorption coefficient. CZTS thin films have been successfully deposited onto the fluorine-doped tin oxide/glass (glass/FTO) substrates coated glass substrates using successive ionic layer adsorption and reaction (SILAR) method and investigated for photoelectrochemical conversion (PEC) of light into electricity. The best solar cell sample showed an open-circuit voltage of 390 mV, a short-circuit current density of 636.9 μA/cm², a fill factor of 0.62 and an efficiency of 0.396% under irradiation of 30 mW/cm². Preliminary results obtained for solar cells fabricated with this material are promising.     

Two different effects of Na on Cu2ZnSnS4 thin-film solar cells

Among the many factors that affect the characteristics of Cu₂ZnSnS₄ thin-film solar cells, the effects of Na were examined. Because Na passivates the grain boundaries (GBs) in absorber layers, the GBs increase the minority carrier collection by providing a current pathway through which the minority carriers can reach the buffer and window layer and be collected. However, excess Na remains in the absorber layer and diffuses into the buffer and window layers, where it acts as an impurity and traps the minority carriers, thus decreasing the solar cell characteristics. By applying an annealing process after the deposition of the buffer and window layers, the Na content could be decreased. By improving the solar cell characteristics, a power conversion efficiency of 6.43% was achieved.     

Improvement of Cu2ZnSnS4 thin film morphology using Cu–Zn–Sn–O precursor fabricated by sputtering

The new precursor of Cu–Zn–Sn–O (CZTO) was proposed for Cu₂ZnSnS₄ (CZTS) thin film fabrication to improve film morphology. The CZTS thin film grown from Cu–Zn–Sn (CZT) precursors has many bumps. We deposited CZTO precursors on Mo/soda-lime glass (SLG) substrates by RF sputtering using a CZT (Cu:Zn:Sn = 2:1:1) target in Ar and O₂ atmosphere at various O₂ partial pressures (0%, 5%, 17% and 20%). Subsequently, the CZTO precursors were sulfurized in Ar and S atmosphere to fabricate CZTS thin films. The CZTO precursors were amorphous. The morphology of the CZTS thin films was improved by the CZTO precursors. All of the CZTS films fabricated in this study had the same crystal structure. Composition analysis revealed that 50% of O were detected in the CZTO precursor, but O was not detected after sulfurizing process, indicating that O was substituted by S. The CZTS thin film from the CZTO precursor fabricated at O₂ partial pressure of 20% had similar composition for solar cell absorber.     

Study on the photovoltaic property of Cu4SnS4 synthesized by mechanochemical process

Photovoltaic property of Cu₄SnS₄ (CTS) is studied by employing a superstrate solar cell structure of Mo/CTS/In₂S₃/TiO₂/fluorine-doped tin oxide (FTO) glass for the first time. The CTS absorber layer was prepared by a combination of mechanochemical and doctor blade processes. The annealing effects on the structural, optical and electronic properties of the CTS absorber layer were investigated. The novel CTS absorber layer shows conversion efficiency as high as 2.34% under the standard AM 1.5 condition.     

Interaction of ultra-short laser pulses with CIGS and CZTSe thin films

The thin-film solar cell technologies based on complex quaternary chalcopyrite and kesterite materials are becoming more attractive due to their potential for low production costs and optimal spectral performance. As in all thin-film technologies, high efficiency of small cells might be maintained with the transition to larger areas when small segments are interconnected in series to reduce photocurrent and related ohmic losses in thin films. Interconnect formation is based on the three scribing steps, and the use of a laser is here crucial for performance of the device. We present our simulation and experimental results on the ablation process investigations in complex CuIn_1?x Ga _x Se₂ (CIGS) and Cu₂ZnSn(S,Se)₄ (CZTSe) cell’s films using ultra-short pulsed infrared (~1 μm) lasers which can be applied to the damage-free front-side scribing processes. Two types of processes were investigated—direct laser ablation of ZnO:Al/CIGS films with a variable pulse duration of a femtosecond laser and the laser-induced material removal with a picosecond laser in the ZnO:Al/CZTSe structure. It has been found that the pulse energy and the number of laser pulses have a significantly stronger effect on the ablation quality in ZnO:Al/CIGS thin films rather than the laser pulse duration. For the thin-film scribing applications, it is very important to carefully select the processing parameters and use of ultra-short femtosecond pulses does not have a significant advantage compared to picosecond laser pulses. Investigations with the ZnO:Al/CZTSe thin films showed that process of the absorber layer removal was triggered by a micro-explosive effect induced by high pressure of sublimated material due to a rapid temperature increase at the molybdenum-CZTSe interface.     

Improvement of solar cell efficiency by applying Si3N4 nanopattern on glass substrate

A cylindrical Si₃N₄ nanopattern whose heights was 200 nm was fabricated on a glass substrate, and an aluminum-doped zinc oxide (AZO) layer was grown on the nanopatterned glass substrate. The nanopattern was applied to an amorphous silicon solar cell in order to increase the light-scattering effect, thus enhancing the efficiency of the solar cell. The reflectance of the solar cell on the Si₃N₄ nanopattern decreased and its absorption increased. Compared to a flat substrate, the short-circuit current density (J_sc) and conversion efficiency of a solar cell on the Si₃N₄ nanopatterned substrate were improved by 17.9% and 24.2%, respectively, as determined from solar simulator measurements.     

Enhancement of output performance of Cu2ZnSnS4 thin film solar cells—A numerical simulation approach and comparison to experiments

The formation of stable, low resistance and nonrectifying contacts to Cu₂ZnSnS₄ (CZTS) thin film photovoltaic material are the major and critical challenges associated with its effect over the output performance of fabricated solar cells. The solution of continuity equation in one dimension for a soda lime glass substrates (SLG) |Mo | CZTS | CdS | ZnO:Al cell structure is considered in the simulation of its current–voltage characteristics that is governed by the back contact material, acceptor concentration as well as thickness of the CZTS layer. Our primary simulation shows a 6.44% efficiency of the CZTS solar cell which is comparable to reported experimental data if these parameters are not optimized. However, by optimizing them a simulated conversion efficiency as high as 13.41% (V_oc=1.002 V, J_sc=19.31 mA/cm², fill factor (FF)=69.35%) could be achievable. The solar cell with a back contact metal work function of 5.5 eV, an absorber layer's thickness of 2.68 μm and an acceptor concentration of 5×10¹⁶ cm⁻³ were optimum. The presented optimization is ideal and subject to experimental verification with a precise control of the process parameters along with reduced surface as well as bulk recombination, secondary phases and thermalization losses.     

Cu(In,Ga)Se2 thin film solar cells from nanoparticle precursors

A non-vacuum process for Cu(In,Ga)Se₂ (CIGS) thin film solar cells from nanoparticle precursors was described in this work. CIGS nanoparticle precursors was prepared by a low temperature colloidal route by reacting the starting materials (CuI, InI₃, GaI₃ and Na₂Se) in organic solvents, by which fine CIGS nanoparticles of about 15 nm in diameter were obtained. The nanoparticle precursors were then deposited onto Mo/glass substrate by the doctor blade technique. After heat treating the CIGS/Mo/glass layers in Se gas atmosphere, a complete solar cell structure was fabricated by depositing the other layers including CdS buffer layer, ZnO window layer and Al electrodes by conventional methods. The resultant solar cell showed a conversion efficiency of 0.5%.     

Optimized emitter contacting on multicrystalline silicon thin film solar cells

We present an optimized contacting scheme for multicrystalline silicon thin film solar cells on glass based on epitaxially crystallized emitters with a thin Al₂O₃ layer and a silver back reflector. In a first step a 6.5 µm thick amorphous silicon absorber layer is crystallized by a diode laser. In a second step a thin silicon emitter layer is epitaxially crystallized by an excimer laser. The emitter is covered by an Al₂O₃ layer with a thickness ranging from 1.0 nm to 2.5 nm, which passivates the surface and acts as a tunnel barrier. On top of the Al₂O₃ layer a 90–100 nm thick silver back reflector is deposited. The Al₂O₃ layer was found to have an optimal thickness of 1.5 nm resulting in solar cells with back reflector that achieve a maximum open‐circuit voltage of 567 mV, a short‐circuit current density of 27.9 mA/cm², and an efficiency of 10.9%. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)