Investigation of Cu2ZnSnS4 thin-film solar cells with carrier concentration gradient

To investigate the effect of carrier concentration gradient on Cu₂ZnSnS₄ (CZTS) thin-film solar cells, the properties of CZTS solar cells were studied by numerical method. The photovoltaic performances of carrier concentration gradient CZTS solar cells were calculated by the solutions of Poisson's equation, continuity equation, and current density equation using AFors-Het v2.4 program. The carrier concentration gradient was changed to analyze its effect. Compared with CZTS solar cells without carrier concentration gradient, the photovoltaic performances of CZTS solar cells can be enhanced by using carrier concentration gradient absorber. The carrier concentration gradient can extend the distribution region of built-in electric field, which is beneficial to the drift of photo-generated carriers. However, the carrier concentration gradient also affects the recombination and series resistances of solar cells. When the defect density of CZTS layer is high, the photo-generated carriers are affected significantly by recombination, resulting in slight effect of carrier concentration gradient. Therefore, the defect density should be reduced to enhance the effect of carrier concentration gradient on improving conversion efficiency of CZTS thin-film solar cells.     

Cd掺杂的Cu_2ZnSnS_4光伏材料的发光光谱及其太阳能电池器件特性

利用溅射-硫化法制备了一系列不同Cd含量掺杂的铜锌锡硫薄膜材料,并获得了转换效率最高达10.65%的薄膜太阳能电池。利用扫描电子显微镜、变温光致发光谱、变激发密度发光光谱对材料进行了表征,分析了电池器件的电容-电压、电流-电压特性。材料的发光峰峰值显示出反常的温度依赖性,载流子表现出强烈的局域化特征。Cd的适当掺入可以抑制较深缺陷的形成并减小发光峰值和带隙值之间的能量差,从而减小了器件开路电压的损失,有利于器件效率的提升。     

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.     

New method to measure whole-wavelength transmittance of TCO substrates for thin-film silicon solar cells

High transmittance of transparent conductive oxide (TCO) substrates is one of the most important factors for achieving high efficiency in thin-film silicon solar cells. Immersion (IM) method with CH₂I₂ liquid is widely used for the evaluation of optical properties (transmittance, reflectance and absorption) for TCO substrates with textured surface in order to reduce the scattering at the TCO surface. However, in order to measure transmittance accurately, three problems have been found. (1) CH₂I₂ liquid itself absorbs the light in short wavelength region. (2) The transmittance around the absorption edge of CH₂I₂ liquids is very sensitive to its amount. (3) Scattering cannot be suppressed when the scattering surfaces are more than 2 surfaces (for example, TCO on reactive ion etching (RIE) processed glass). To overcome these problems, we proposed a new setup to measure optical properties of TCO substrates by holding the samples inside the integral sphere. As the results, we have confirmed that their absorption in all wavelength could be measured accurately and the transmittance measured by the new method was well consistent with the external quantum efficiency (EQE) of the fabricated cell while the transmittance measured with conventional IM method showed differently. Therefore, this new method could be a useful tool to evaluate TCO substrates for thin-film silicon solar cells.     

Nanoroughness control of Al-Doped ZnO for high efficiency Si thin-film solar cells

The Al-doped ZnO (ZnO:Al) front transparent conducting oxide (TCO) for high efficiency Si thin-film solar cell has been developed using RF magnetron sputtering deposition and chemical wet etching. Microscopic surface roughness of the as-deposited ZnO:Al film estimated by spectroscopic ellipsometry is closely related to the compactness of the TCO film, and shown to be a straightforward and powerful tool to optimize the deposition conditions for the proper post-etched surface morphology. Wet-etching time is adjusted to form the U-shaped craters on the surface of the ZnO:Al film without sharp etch pits that can cause the crack-like defects in the overgrown microcrystalline Si-absorbing layers, and deteriorate the V_oc and FF of the Si thin-film solar cells. That is to say, the nanoroughness control of the as-deposited TCO film with proper chemical etching is the key optimization factor for the efficiency of the solar cell. The a-Si:H/a-SiGe:H/μc-Si:H triple junction Si thin-film solar cells grown on the optimized ZnO:Al front TCO with anti-reflection coatings show higher than 14% conversion efficiency.     

Comprehensive loss modeling in Cu2ZnSnS4 solar cells

Thin film solar cells based on Cu₂ZnSnS₄ (CZTS) absorber material suffers from performance issues arising due to the presence of a non-optimal back contact barrier, low carrier lifetime, acceptor/donor point defects in bulk, interface defects at the absorber-buffer junction and grain boundaries within the absorber. We perform comprehensive simulations enumerating the impact of these performance limiting factors on CZTS solar cells. These simulations capture the experimentally observed anomalies in current-voltage (I–V) characteristics and the open-circuit voltage (V_OC) pinning in CZTS solar cells. These cause-effect relationships as elaborated in the findings are expected to be of great interest to the experimentalists working in this field.     

Implementation of Na diffusion layer at Cu2ZnSnSe4/Mo interface for flexible thin film solar cell fabricated on Ti foil by solid state selenization

A Cu₂ZnSnSe₄ (CZTSe) photovoltaic absorber thin films were prepared using a 2-step selenization process on a Ti substrate including a Na precursor layer and a Na-free Ti substrate, and the effect of Na on the solar cell performance was compared. A CZTSe flexible solar cell fabricated on a Ti foil substrate exhibited an efficiency of 3.06%, which was less than half that of a solar cell fabricated on a soda lime glass substrate. This was attributed to the absence of Na and severe Zn crowding near the back contact. By depositing a 100‐nm-thick sodalime glass thin film on a Ti substrate to supply Na, the efficiency increased up to 5.59%. In the Na-doped CZTSe absorber layer grown on the Ti substrate, the back crowding of Zn was eliminated and the upper part of the absorption layer was converted to a Zn-rich environment, which prevented the formation of Cu_Zn antisite defects.     

Microstructured design of metallic diffraction gratings for light trapping in thin-film silicon solar cells

In thin-film silicon solar cells, a flexible optical design for light collection is developed that can enhance the optical path length in absorber layer. In this paper, we demonstrate the rectangular, trapezoidal and triangular metallic diffraction gratings as back reflectors. The potential of the structures in the solar cells is investigated by means of numerical simulations, i.e., the Rigorous Coupled Wave Analysis enhanced by the Modal Transmission Line theory. The results show that the 0-order reflection can be suppressed and the energy can be transferred to high diffraction orders by metallic diffraction gratings, especially the triangular metallic diffraction gratings.     

Highly crystallized sputtered silicon with textured morphology for thin-film solar cells

A light-trapping structure with textured morphology for thin-film solar cell is demonstrated in this paper. It is fabricated through Al evaporation, and has a root-mean-roughness (Rms) of about 120 nm and lateral width of about 1 μm for single bulge. A Mo layer is introduced to be a barrier layer. Subsequently sputtered amorphous silicon film is 100% crystallized by Cu induced crystallization. Reflectivity of samples with different silicon thickness is studied to reveal the light-trapping efficiency and the reflectivity as low as 10% is obtained with only 840 nm thick silicon film. This is a low-cost structure promising for future thin-film solar cells with high efficiency.     

Investigation of Cu2ZnSnS4 solar cell buffer layer fabricated via spray pyrolysis

Copper zinc tin sulfide solar cells were fabricated by using spray pyrolysis from a window layer to an absorber layer. ZnS and In₂S₃ buffer layers were deposited on the TiO₂ layer, and the photovoltaic characteristics were investigated. The ZnS buffer demonstrated a poor photovoltaic performance because of its poor surface coverage and micro-cracks at fluorine-doped tin oxide/TiO₂ layers. The In₂S₃ buffer layer sprayed at low temperature (<360 °C) showed a large difference between photo and dark currents beyond the open-circuit voltage (V_OC). When the spraying temperature exceeded 390 °C, the devices showed high dark leakage currents at reverse biases because of the high conductivity of the buffer layer, resulting in decreased V_OC and short-circuit current density (J_SC). The optimum temperature for spraying In₂S₃ is 360 °C, and the best performing device showed 410 mV, 30.4 mA/cm², 35.3%, and 4.4% of V_OC, J_SC, fill factor, and efficiency, respectively.     

Effect of intrinsic ZnO thickness on the performance of SnS/CdS-based thin-film solar cells

Tin monosulfide (SnS) has promising properties as an absorber material for thin-film solar cells (TFSCs). SnS/CdS-based TFSCs have the following device structure: SLG/Mo/SnS/CdS/i-ZnO/AZO/Al. The optimization of thickness of intrinsic zinc oxide (i-ZnO) for SnS-absorber layers and its impact on SnS/CdS heterojunction TFSCs has been investigated at different thicknesses ranging from 39 nm to 73 nm. With the increase in thickness of i-ZnO from 39 nm to 45 nm, the overall performance improved. The highest PCE of 3.50% (with V_OC of 0.334 V, J_SC of 18.9 mA cm⁻², and FF of 55.5%) was observed for 45 nm-thick i-ZnO layers. Upon a further increase in the i-ZnO thickness to 73 nm, the device performance deteriorated, indicating that the optimum thickness of the i-ZnO is 45 nm. The device performances were analyzed comprehensively for different i-ZnO thicknesses.     

太阳电池用Cu2ZnSnS4薄膜的反应溅射原位生长及表征

通过直流反应磁控溅射技术,原位生长制备了太阳电池用Cu₂ZnSnS₄(CZTS)薄膜.采用X射线能量色散谱仪、扫描电镜、X射线衍射仪、紫外可见分光光度计和霍尔效应测试系统对薄膜进行了表征.结果表明,原位生长的CZTS薄膜具有均质、致密和平整的形貌,且由贯穿整个薄膜厚度的柱状颗粒组成.不同基底温度下生长所得薄膜的Cu/(Zn+Sn) 值均约为1,而Zn/Sn值均大于1且随着基底温度升高而减小.所得薄膜在(112)方向上择优取向明显,且结构特征受基底温度和Cu/(Zn+Sn)的共同影响.所得薄膜均具有高达10⁴cm^-1的光吸收系数,其带隙宽度随着生长温度的增加而降低,并且在500℃时为(1.51±0.01)eV.薄膜的导电类型均为p型,且具有与器件级Cu(In,Ga)Se₂(CIGS)相当的载流子浓度. 关键词： 2ZnSnS₄')" href="#">Cu₂ZnSnS₄ 直流反应磁控溅射 原位生长 太阳电池     

Advanced solar materials for thin-film photovoltaic cells

As one of the most promising solutions for the green energy, thin-film photovoltaic cell technology is still immature and far from large-scale industrialization. The major issue is getting low cost and stable module efficiency. To solve these problems, a large amount of advanced solar materials have been developed to improve all parts of solar cell modules. Here, some new solar material developments applied in different critical parts of chalcogenide thin-film photovoltaic cells are reviewed. The main efforts are focused on improving light trapping and antireflection, internal quantum efficiency and collection of photo-generated carriers.     

Nanostructured solar cell based on solution processed Cu2ZnSnS4 nanoparticles and vertically aligned ZnO nanorod array

Cu₂ZnSnS₄ (CZTS) has attracted intensive interest for application in photovoltaic technology due to its excellent semiconductor properties. We report a nanostructured CZTS solar cell which was fabricated by infiltrating of CZTS nanoparticles into CdS coated ZnO nanorod arrays. The well aligned ZnO nanorods facilitate the efficient infiltration of CZTS nanoparticles. A hole transport layer was deposited to facilitate the transport of holes. The nanostructured CZTS solar cell demonstrated a remarkably high short‐circuit current density (11.0 mA/cm²). As a result, a power conversion efficiency of 2.8% was obtained. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

铜铟硒薄膜太阳能电池的几个基础问题研究

文章简述了铜铟硒（CIS）薄膜电池的性能特点和研究现状，重点提出了影响其规模化推广的几个基础问题，包括：射频溅射法直接制备CIS薄膜、省略硒化工艺的问题；ZnO替代CdS作为CIS薄膜电池的缓冲层材料的问题；Na^＋离子对CIS薄膜及电池性能的作用及机制问题．这几个基础问题的研究成果，可以为CIS薄膜电池产业化探索新的道路。     

Photovoltaic performance of flexible Cu(In,Ga)Se2 thin-film solar cells with varying Cr impurity barrier thickness

We report the effect of Cr impurity barrier on Cu(In,Ga)Se₂ (CIGS) thin-film solar cells prepared on flexible substrates. The Cr films with varying the thickness (t_Cr) were deposited on stainless steel substrates using direct-current magnetron sputtering. The solar cell performance was improved by increasing t_Cr since the diffusion of Fe impurities from the substrate to CIGS was suppressed. Although the elemental composition, grain size, and strain of CIGS film showed little change with varying Fe content, the fill factor and the short-circuit current density increased as decreasing Fe. The Fe increased the series resistance, shunt paths, and saturation current density. The reduction of Fe caused a steeper bandgap grading in CIGS which enhances current collection due to higher electric fields in bulk CIGS. CIGS solar cells with 1000 nm-thick Cr barrier showed the best conversion efficiency of 9.05%.     

热处理对不同溶剂制备的共混体系太阳电池性能影响

本文以poly(3-hexylthiophene) (P3HT)为电子给体材料, -phenyl C₆₁-butyric acid methyl ester (PCBM)为电子受体材料, 制备出不同溶剂形成的共混体系太阳电池.从薄膜的紫外—可见吸收光谱(UV-vis)、光致发光谱(PL)、原子力表面图形(AFM)等方面,分析了热处理对不同溶剂制备的共混体系太阳电池性能的影响.结果表明较高沸点的溶剂有利于P3HT:PCBM共混体系中P3HT的有序化排列,薄膜的紫外—可见吸收和光致发光增 关键词： 热处理 不同溶剂 太阳电池 性能     

Doctor-bladed Cu2ZnSnS4 light absorption layer for low-cost solar cell application

The doctor-blade method is investigated for the preparation of Cu2ZnSnS4 films for low-cost solar cell application.Cu2ZnSnS4 precursor powder,the main raw material for the doctor-blade paste,is synthesized by a simple ball-milling process.The doctor-bladed Cu2ZnSnS4 films are annealed in N2 ambient under various conditions and characterized by X-ray diffraction,ultraviolent/vis spectrophotometry,scanning electron microscopy,and current-voltage(J-V) meansurement.Our experimental results indicate that(i) the X-ray diffraction peaks of the Cu2ZnSnS4 precursor powder each show a red shift of about 0.4°;(ii) the high-temperature annealing process can effectively improve the crystallinity of the doctor-bladed Cu 2 ZnSnS 4,whereas an overlong annealing introduces defects;(iii) the band gap value of the doctor-bladed Cu 2 ZnSnS 4 is around 1.41 eV;(iv) the short-circuit current density,the open-circuit voltage,the fill factor,and the efficiency of the best Cu2ZnSnS4 solar cell obtained with the superstrate structure of fluorine-doped tin oxide glass/TiO2/In2S3/Cu2ZnSnS4/Mo are 7.82 mA/cm2,240 mV,0.29,and 0.55%,respectively.