Photovoltaic (PV) devices can directly convert sunlight into electricity. So far, the Si-wafer-based PV module is the dominant product of the global PV market. There are still thin-film PV module productions. They occupy a small share of the total global PV market, including cadmium telluride, amorphous silicon (a-Si), and copper indium (gallium) diselenide (CIGSe). The thickness of thin film solar cells can be about 100 times thinner than Si-based PV, since they can absorb most of the illumination light owing to their high absorption coefficient. The efficiency of lab-scale CIGSe solar cells has achieved 23.6% (vacuum-based process), comparable with the Si-based technology. The alternative solution process has been applied for CIGSe solar cell fabrication, owing to low equipment cost and high material utilization. So far, the efficiency of the state-of-the-art solution-processed CIGSe has achieved 18.1% via a hydrazine precursor solution. However, hydrazine is a highly toxic and reactive solvent and, therefore, it is necessary to protect the human body and the environment by using appropriate protective equipment to prevent physical contact or exposure to either vapor or liquid.

Due to hydrazine's shortcomings, the hydrazine solution is not beneficial for mass production. Thus, developing a non-toxic solution precursor for absorber fabrication has attracted great attention. The fabrication cost of CIGSSe solar cells can be further reduced by thinning the absorber thickness. In this thesis, solution-processed submicron copper indium sulfoselenide (CISSe) and copper indium gallium sulfoselenide (CIGSSe) solar cells are fabricated on molybdenum (Mo) and tin-doped indium oxide (ITO) back contacts by a less toxic N, N-Dimethylformamide (DMF) solution.

In the first results chapter of this thesis, annealing conditions are investigated, including annealing in pure selenium and a mixture of S and Se (sulfur and selenium) atmospheres. With increasing selenium content for annealing, the corresponding CISSe devices present better PV parameters. After sulfur doping, the open circuit voltage and fill factor of CISSe solar cells are significantly improved, leading to higher efficiency than annealing in pure selenium atmosphere. The open circuit voltage of the S and Se co-annealed CISSe solar cell can reach over 600 mV, which is remarkably higher than reported in the literature.

The second results chapter of this thesis focuses on PV performance improvement by sodium chloride (NaCl) solution treatment. Three strategies for Na incorporation into solution-processed CISSe by soaking in NaCl aqueous-ethanol solution are researched, either prior to absorber deposition (pre-deposition treatment, Pre-DT), before selenization (pre-selenization treatment, Pre-ST), or after selenization (post-selenization treatment, PST). The Pre-ST CISSe solar cells achieve a better PV performance than those from the other two strategies of Na incorporation. For optimization, soaking times (5 min, 10 min, and 15 min) and NaCl concentrations (from 0.2 M to 1.2 M) of the Pre-ST are researched. When the precursor films are soaked in 1 M NaCl for 10 min, CISSe solar cells achieve an efficiency of 9.6%.

The third results chapter of this thesis is a CISSe absorber prepared on transparent conductive oxide (TCO) back contact through an environmentally benign solution, which shows great potential in the bifacial application. Ultra-thin (around 550 nm) CISSe solar cells were successfully deposited on ITO back contact via spin-coating of metal-chloride DMF solution followed by selenization. With increasing the absorber thickness to sub-micron (740 nm), the solar cells not only exhibited a higher short-circuit current density but also an improved fill factor compared to the ultra-thin devices, which results in an efficiency enhancement. Furthermore, NaCl solution pre-selenization treatment was demonstrated to improve the performance of CISSe solar cells.

In the fourth part of this thesis, the parasitic absorption in CIGSSe solar cells emerging from the Mo back contact can be significantly reduced by replacing it with ITO back contact. However, an undesirable GaO_x layer forms at the CIGSSe/ITO interface during the high-temperature selenization process, which hinders photo-carrier extraction. A thin Cu-In-TU-DMF (TU: thiourea, DMF: N, N-Dimethylformamide) intermediate layer for modification of the CIGSSe/ITO interface, which can reduce the recombination at the rear interface. Furthermore, carrier transport and collection are improved, leading to a significant improvement in efficiency.