Cadmium-free electron transport layers for hydrothermally processed semitransparent Sb2S3 solar cells

Semitransparent thin film solar cells based on wide bandgap absorber Sb2S3 have immense potential in building integrated photovoltaic (BIPV) applications. A typical thin film Sb2S3 solar cell using low-cost solution-based methods (such as hydrothermal deposition) utilizes a toxic CdS film with a narrow bandgap (2.4 eV) as the electron transport layer (ETL). Wide bandgap (3.1–3.4 eV) non-toxic TiO2 meets the optoelectronic requirements for a Cd-free ETL alternative but the hydrothermal deposition of Sb2S3 on TiO2 results in a non-uniform island-like growth, which is unsuitable for semitransparent applications (utilizing less than 100 nm Sb2S3). Therefore, in this study, using the successive ionic layer adsorption and reaction (SILAR) method, an ultrathin ZnS layer (1–3 nm) is coated on TiO2 as a surface modification layer to improve the nucleation and growth characteristics of Sb2S3 during hydrothermal deposition. The introduction of ZnS results in a pinhole-free compact mirrorlike Sb2S3 film similar to that obtained on CdS. The optimized solar cells based on CdS, TiO2, and TiO2-ZnS ETLs showed photoconversion efficiencies (PCEs) of 5.2 %, 5.1 %, and 4.3 %, respectively. A comprehensive comparative study is then reported highlighting the relationship between morphology, optoelectronic properties, and photovoltaic performance of the Sb2S3 films grown on the three ETLs. Furthermore, utilizing the excellent film morphology of Sb2S3 on TiO2-ZnS ETL, semitransparent solar cells were fabricated using an ultrathin Au (<10 nm) electrode. Semitransparent solar cells using 65 and 80 nm Sb2S3 absorber layers on TiO2-ZnS obtained PCEs (and average visible transmittances, AVTs) of 3.3 % (11.2 %) and 3.6 % (8.8 %), respectively. These results are critical to the development of the BIPV sector through environmentally friendly and non-critical materials-based solutions.