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.

In this study we present and discuss p-n heterostructures for photodetection. The hybrid structures consist of CeO2:ZnO-Cu2O, featuring different concentrations of CeO2, fabricated by using hydrothermal co-growth for CeO2 and ZnO, and sputtering deposition for Cu2O. As the concentration of CeO2 in the ZnO pristine nanorods was increased, the structural, optical and functional features of the materials showed relevant changes, in terms of crystalline domains and optical bandgap. After Cu2O deposition, the ternary materials were tested as UV photodectors, showing very good performance in terms of fast response and decay times. Specifically, we found that the CeO2:ZnO-Cu2O devices maintain a stable current under light irradiation, whose value was dependent on the CeO2 amount incorporated in the ZnO 1D nanostructures. Among all tested configurations, the 5.5 % hybrid CeO2:ZnO-Cu2O exhibits the highest current efficiency, accompanied by rapid rise and decay times. Our investigation suggests that the CeO2:ZnO-Cu2O configuration holds great potential for optoelectronic applications, particularly in the development of UV photodetectors.

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.

Published by American Chemical Society.An inorganic wide-bandgap hole transport layer (HTL), copper(I) thiocyanate (CuSCN), is employed in inorganic planar hydrothermally deposited Sb2S3 solar cells. With excellent hole transport properties and uniform compact morphology, the solution-processed CuSCN layer suppresses the leakage current and improves charge selectivity in an n-i-p-type solar cell structure. The device without the HTL (FTO/CdS/Sb2S3/Au) delivers a modest power conversion efficiency (PCE) of 1.54%, which increases to 2.46% with the introduction of CuSCN (FTO/CdS/Sb2S3/CuSCN/Au). This PCE is a significant improvement compared with the previous reports of planar Sb2S3 solar cells employing CuSCN. CuSCN is therefore a promising alternative to expensive and inherently unstable organic HTLs. In addition, CuSCN makes an excellent optically transparent (with average transmittance >90% in the visible region) and shunt-blocking HTL layer in pinhole-prone ultrathin(<100 nm) semitransparent absorber layers grown by green and facile hydrothermal deposition. A semitransparent device is fabricated using an ultrathin Au layer (∼10 nm) with a PCE of 2.13% and an average visible transmittance of 13.7%.

Interfacial solar desalination is an emerging technology for freshwater production, but the finding of novel solar evaporators is still challenging. In the present research, graphitic carbon foam (CF) was synthesized from the upcycling of waste plastic polyethylene terephthalate (PET) waste bottles functionalized with carrollite CuCo2S4 as a photothermal layer. Analytical characterization [X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS)] confirms the functionalization of carrollite CuCo2S4 on graphitic carbon foam. The UV-Vis spectroscopy analysis showed an enhanced optical absorption in the UV-Vis-near IR region (>96%) for functionalized CuCo2S4-CF foam compared to carbon foam (67%). The interfacial solar desalination experiment presented a significantly enhanced evaporation rate of 2.4 kg·m−2·h−1 for CuCo2S4-CF compared to that of CF (1.60 kg·m−2·h−1) and that of CuCo2S4 (1.60 kg·m−2·h−1). The obtained results proved that the newly synthesized CuCo2S4-CF from the upcycled plastic into new material for the photothermal desalination process can enhance the practice of a circular economy to produce fresh water.

Semitransparent photovoltaic (STPV) solar cells offer an immense opportunity to expand the scope of photovoltaics to special applications such as windows, facades, skylights, and so on. These new opportunities have encouraged researchers to develop STPVs using traditional thin-film solar cell technologies (amorphous-Si, CdTe, and CIGS or emerging solar cells (organic, perovskites, and dye-sensitized). There are considerable improvements in both power conversion efficiency (PCE) and semitransparency of these STPV devices. This review studies the device structure of state-of-the-art STPV devices and thereby analyzes the different approaches toward maximizing the product of PCE and average visible transmittance. The origins of PCE losses during the opaque-to-semitransparent transition in the different STPV technologies are discussed. In addition, critical practical aspects relevant to all STPV devices, such as compatibility of the top transparent electrode with the device structure, buffer layer optimization, light management engineering, scale-up, and stability, are also reported. This overview is expected to facilitate researchers across different technologies to identify and overcome the challenges toward achieving higher light utilization efficiencies in STPVs.

Sb₂S₃ is an emerging inorganic photovoltaic absorber material with attractive properties such as high absorption coefficient, stability, earth-abundance, non-toxicity, and low-temperature solution processability. Furthermore, with a bandgap of ca. 1.7 eV, it can also be used in semitransparent or tandem solar cell applications. Here, an inorganic wide-bandgap hole transport layer (HTL), copper thiocyanate (CuSCN), is used in an Sb₂S₃ solar cell employing a simple planar geometry. The compact and highly transparent CuSCN HTL was compatible with the low-cost, blade-coated carbon/Ag electrode and a semitransparent solar cell device. With Au and carbon/Ag electrodes, chemical bath deposited Sb2S3 solar cells achieved power conversion efficiencies (PCEs) of 1.75% and 1.95%, respectively. At the same time, a preliminary semitransparent Sb₂S₃ device with an ultrathin Au (similar to 15 nm) electrode showed a good average visible transmittance (AVT) of 26.7% at a PCE of 1.65%.