Studying Antimony Selenide Thin Film Solar Cells

In the field of thin-film photovoltaics, exploring and optimizing various materials and techniques is crucial for advancing device performance. Modifying different components in thin-film solar cells enhances efficiency and deepens our understanding of the underlying physical phenomena. This thesis investigates Sb2Se3 thin-film solar cells grown by the thermal evaporation technique in a superstrate configuration device. It explores both cadmium-based (Cd-based) and cadmium-free (Cd-free) window layers, as well as the influence of the absorber layer thickness on the device performance. Additionally, it evaluates the effect of air post-annealing treatment (PAT) as a passivation method. The structural properties of Sb2Se3 and the performance of CdS/Sb2Se3 devices were analyzed. Sulfur diffusion at this interface could alter the optoelectrical properties of Sb2Se3, resulting in a Voc deficit. To address this, CdSe was introduced as an alternative buffer layer, eliminating the sulfur interdiffusion. Moreover, the presence of selenium in CdSe helped mitigate selenium loss at the junction by reducing the concentration gradient. The CdSe/ Sb2Se3 solar cells showed remarkable stability, and regardless of a relatively narrow bandgap of CdSe, this replacement also increased photocurrent density (Jsc). However, the use of Cd-based materials window layers not only limits light absorption at shorter wavelength regions but also leads to Cd diffusion in the Sb2Se3 film. To overcome these limitations, a transparent, Cd-free alternative material, tin oxide (SnO2), was employed and deposited using atomic layer deposition (ALD) at EMPA laboratories. A 15 nm SnO2 film served as a suitable substrate for growing Sb2Se3 film, increasing the grain size (to approximately 1 micrometer). By applying this wide band gap material, Jsc, and carrier collection at the short wavelength regions improved. However, a higher doping density and, consequently, a higher Voc compared to CdSe-based solar cells were also observed. Also, the effects of absorber thickness were analyzed since Sb2Se3 shows a superior absorption coefficient. The structural and electrical properties of solar cells with 400 and 1200 nm thickness of Sb2Se3 were compared. We observed that an ultra-thin film of 400 nm captures photons effectively. The solar cell illustrated higher performance, particularly for the Jsc value. Applying PAT in air ambient at a low temperature on the finished devices shows passivating behavior. Oxygen diffusion throughout Sb2Se3 films differs with the film thickness. However, the oxygen exposure during the annealing always reduces the defect density. This treatment improved the overall photovoltaic parameters, especially the Voc and fill factor values.