study of optoelectronic properties of thin film solar cell materials Cu2ZnSn(S,Se)4 using multiple correlative spatially-resolved spectroscopy techniques
Containing only earth abundant and environmental friendly elements, quaternary compounds Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are considered as promising absorber materials for thin film solar cells. The best record efficiency for this type of thin film solar cell is now 12.6%. As a promising photovoltaic (PV) material, the electrical and optical properties of CZTS(Se) have not been well studied.In this work, an effort has been made to understand the optoelectronic and structural properties, in particular the spatial variations, of CZTS(Se) materials and devices by correlating multiple spatially resolved characterization techniques with sub-micron resolution. Micro-Raman (μ-Raman) spectroscopy was used to analyze the chemistry compositions in CZTS(Se) film; Micro-Photoluminescence (μ-PL) was used to determine the band gap and possible defects. Micro-Laser-Beam-Induced-Current (μ-LBIC) was used to examine the photo-response of CZTS(Se) solar cell in different illumination conditions. Micro-reflectance was used to estimate the reflectance loss. And Micro-I-V measurement was used to compare important electrical parameters from CZTS(Se) solar cells with different device structure or absorber compositions. Scanning electron microscopy and atomic force microscopy were used to characterize the surface morphology. Successfully integrating and correlating these techniques was first demonstrated during the course of this work in our laboratory, and this level of integration and correlation has been rare in the field of PV research. This effort is significant not only for this particular project and also for a wide range of research topics. Applying this approach, in conjunction with high-temperature and high-excitation-power optical spectroscopy, we have been able to reveal the microscopic scale variations among samples and devices that appeared to be very similar from macroscopic material and device characterizations, and thus serve as a very powerful tool to understand the underlying microscopic material structures and predict the potential of improvement in device performance.First, by using an array of correlated aforementioned techniques, microscale inhomogeneity of the CdS layer thickness was found in CZTSe solar cells. Thicker CdS regions are found to cause more light-reflection loss thus yielding lower external quantum efficiencies (EQEs) than the general area. However, these regions show much less efficiency degradation at high illumination density, leading to an inversion of LBIC contrast between the CdS rich regions and general area. By improving the CdS layer uniformity, CZTSe device performance can be significantly boosted. And this finding also points out the possibility of operating thin-film photovoltaic device based on similar materials under substantially higher illumination density for concentrated photovoltaic and photo-detection. Second, Micro-Raman reveals multiple secondary phases such as ZnSe and SnSe within the CZTSe films, which are harmful for solar cell operation. In high-laser-power Raman study, CZTSe shows structural change and decomposition, which indicates poor thermal conductivity of the polycrystalline film. Different behaviors of CZTSe films prepared by different methods are observed in high-laser-power and high-temperature Raman studies, both of which offer effective approaches to examine microscopic structural variation of nominally similar CZTSe films. Because of the achieved high spatial resolution, applying μ-Raman and μ-LBIC, we are able to examine the depth variation of the thin absorber film (in the order of 1 μm) in terms of chemical composition, photo-response, and deposition method dependence. In the third part, μ-I-V curves offer direct measurements of electrical parameters reflecting the effects of the device structure, absorber thickness and elemental ratio on the CZTSe cell performance. NaF precursor, low copper and high zinc content are demonstrated necessary for high performance CZTSe devices. However, one cell with higher copper and lower zinc content experiences slower EQE droop at high power density and part of the film shows better thermal conductivity, which suggests that the CdS/CZTSe heterojunction band alignment depends on the elemental ratio of the CZTSe film and CZTSe may have a potential in concentrated PV application if the elemental ratio can be optimized.