Solar photovoltaics (PV) is one of the most promising renewable sources of energy. Crystalline and multi-crystalline silicon solar cells hold most of the market share (up to 95%) of the photovoltaic industry. However, they require high-purity silicon and high production costs. Thin film technologies, including a-Si, Cu(In, Ga)(S, Se)2 (CIGS), and CdTe, have been thoroughly researched due to their minimal material consumption and scalability, yet they have struggled to achieve significant commercial success. These technologies face challenges such as low technological flexibility (e.g., use of flexible substrates), use of critical or toxic raw materials, and long-term stability. Furthermore, semitransparent photovoltaic technologies (STPVs) can harness previously unused spaces like windows and facades to produce on-site electricity. Buildings represent 40% of overall energy use and are responsible for 36% of total greenhouse gas emissions. STPVs will play a crucial role in meeting the energy requirements of a ‘zero-emission building’. The shortcomings of current PV technologies and the potential of STPVs incentivize the search for alternative PV technologies utilizing absorber materials that can effectively address these issues sustainably and at a reduced cost.

Antimony sulfide (Sb2S3) is an emerging light absorber material with favorable properties, such as a high absorption coefficient, wide bandgap (1.7–1.8 eV), earth abundance, and nontoxic constituents. Its low melting point (~550 °C) allows for obtaining high-quality crystalline thin films at low temperatures. The Sb2S3 solar cells in this thesis use a conventional planar n-i-p heterojunction with a configuration: glass/bottom contact/electron transport layer (ETL)/Sb2S3/hole transport layer (HTL)/top contact. A commercial glass/fluorine-doped tin oxide (FTO) substrate acts as the bottom contact. ETL and HTL help in the efficient and directional collection of electrons and holes. The top contact is a high-work function metal such as Au thin film (>60 nm for opaque devices). For semitransparent solar cells, the top electrode can be ultrathin Au (<15 nm) or indium tin oxide (ITO).

N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD), an organic material, has emerged as the predominant choice for HTLs; however, it is costly and requires additives that render it vulnerable to moisture and elevated temperatures. Therefore, finding suitable inorganic stable HTLs is of utmost importance. The first part of this thesis (papers I and II) utilizes copper thiocyanate (CuSCN), an inexpensive and easily processable inorganic material that is highly transparent, as the HTL to realize fully inorganic Sb2S3 solar cells. CuSCN was first evaluated in the solution-processed solar cells using Sb2S3 films deposited through chemical bath deposition (CBD) (paper I). The opaque and semitransparent planar solar cells using all-inorganic layers obtained power conversion efficiencies (PCEs) of 1.75% and 1.67%, respectively. Subsequently, a hydrothermal deposition technique was utilized to enhance the quality of the Sb2S3 thin films, resulting in all-inorganic hydrothermally deposited Sb2S3 solar cells (paper II). A direct comparison between solar cells using CuSCN as the HTL and those lacking an HTL underscored the importance of the HTL in these devices. The HTL-free solar cells achieve a modest PCE of 1.54%, which improves to 2.46% when CuSCN HTL is included. These findings were corroborated by a one-dimensional numerical simulation. A semitransparent device is fabricated with a PCE of 2.13% and an average visible transmittance (AVT) of 13.7%.

Additionally, cadmium sulfide (CdS), derived from established CdTe PV technology, has solely served as the ETL for high-efficiency Sb2S3 solar cells. Nevertheless, cadmium's toxicity raises concerns that hinder the broader acceptance of these solar cells. Additionally, it possesses a low bandgap of 2.4 eV (with its characteristic yellow color), resulting in absorption-related current loss and diminished device transparency. Therefore, in the next section of the thesis (paper III), CdS was substituted with a wide bandgap, nontoxic TiO2 as the ETL. The PCE of the cadmium-free device using TiO2 was 5.1%, which was comparable to that of the CdS-based device (5.2%). However, the hydrothermal deposition of Sb2S3 on TiO2 results in non-uniform, island-like growth, which is unsuitable for semitransparent applications that require pinhole-free thin films of less than 100 nm. This island-like growth, caused by dewetting issues, is mitigated by applying an ultrathin ZnS layer (1–3 nm) on TiO2 using the successive ionic layer adsorption and reaction (SILAR) deposition method. By utilizing the resulting excellent film morphology of Sb2S3 on TiO2-ZnS ETL, semitransparent solar cells were fabricated with an ultrathin Au (<10 nm) electrode, achieving a PCE of 3.3% and an AVT of 11.2%.

In the last part (paper IV), a highly scalable radio frequency (RF) magnetron sputtering deposition was developed to obtain high-quality, uniform, and impurity-free Sb2S3 films directly on TiO2. Unlike the island-type growth of Sb2S3 on TiO2 seen in solution-based depositions, sputtering with the binary target enables the formation of smooth and dense Sb2S3 films, even at thicknesses less than 100 nm. A thorough optimization of the post-deposition annealing parameters yielded a record PCE of 4.6%. Semitransparent solar cells with varying degrees of transparency were developed through precise thickness control via sputter deposition. The semitransparent solar cells utilizing ultrathin Au as the top contact achieved PCEs of 3.2% (AVT: 10%), 2.6% (AVT: 13.5%), and 2.0% (AVT: 16.5%) for Sb2S3 layers with thicknesses of 80 nm, 60 nm, and 40 nm, respectively. Next, ITO is employed instead of ultrathin Au as the top transparent electrode to enhance transparency (AVT: 14.9% for the 60 nm Sb2S3 layer). Finally, the highly transparent CuSCN replaces the low bandgap polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) as the HTL, further increasing the AVT to 20.5% for the same Sb2S3 thickness of 60 nm.

This thesis highlights how the thin-film deposition conditions, the ETL/HTL interfaces, and the device structure significantly influence the AVT and PCE of Sb2S3 semitransparent solar cells. In conclusion, the results of this thesis will greatly contribute to future studies on high-performance semitransparent Sb2S3 solar cells.