CuGaSe₂ Semi-Transparent Thin-Film Solar Cells

Semi-transparent photovoltaic (PV) devices offer a promising route for integrating solar energy generation into buildings, known as building-integrated photovoltaics (BIPV). Among the various materials under investigation for thin-film PV devices, CuGaSe₂ (CGSe) has emerged as a potential candidate due to its band gap energy of around 1.7 eV and stability. However, it is a challenge to achieve high efficiency and high transparency in CGSe based solar cell devices at the same time.<br/><br/>In this work, 750 nm CGSe thin films were deposited via co-evaporation on different substrate configurations. The conventional Mo/SLG, SLG and semi-transparent substrates based on Mo/FTO with thin Mo layers of 7.5 and 30 nm were used. In the case of the semi-transparent back contacts, a 8 nm NaF precursor layer was evaporated before the CGSe co-evaporation process. A remarkable 70% straight-through transmittance in the near-infrared region was achieved for CGSe deposited on SLG. In previous works, CGSe semi-transparent solar cells showed a transmittance of around 60 % for the same wavelength range [1].<br/><br/>The influence of the buffer layer on the CGSe final device has been investigated. Currently, CdS, Zn(O,S) and Zn_1−xSn_xO_y are the main buffer materials used in CGSe solar cells. Larsson et al. [2] reported 1.5 µm CGSe solar cells with efficiencies of 11.9 % when using Zn_1−xSn_xO_y and an antireflective coating. Here, improvements in device performance have been achieved by using an alternative buffer layer, Zn(O_0.6S_0.4), instead of the conventional CdS. SLG/Mo/CGSe/ZnSO/AZO/Ni-Al solar cell structure yields an open-circuit voltage V_OC of 944 mV and an efficiency of 7.1%. However, the use of semi-transparent back contact resulted in a V_OC of 826 mV and an efficiency of 4.0%. The semi-transparent device performance is mainly limited by the FF, suggesting that an improved back interface is necessary. These results highlight the potential of CGSe thin films for BIPV applications, demonstrating that they can achieve both high transparency and substantial photovoltaic performance.<br/><br/>[1] R. Caballero et al, 2006 IEEE 4^th World Conference on Photovoltaic Energy conference 1, 479-482, 2006.<br/>[2] F. Larsson et al., Prog Photovolt Res Appl. 2017;25:755–763.