Chalcopyrite Cu(In,Ga)(S,Se)2 (CIGS) thin-film solar cells have attracted many researchers due to the desirable material properties of CIGS absorbers such as stable crystal structure, high absorption coefficient (>105 cm-1), and widely tunable optical band gap (1.0–2.4 eV). Among various absorber preparation methods, solution-processing techniques have been considered very promising because they have great potential to be simpler, more cost-effective and desirable for industrial-scale production than conventional vacuum-based methods. However, the power-conversion efficiency (PCE) of the solution-processed devices has lagged behind that of vacuum-processed devices, requiring novel technologies to improve their device performance. Since solution-processed CIGS absorbers typically have a large grain boundary density and low minority carrier lifetime, resulting in detrimental Shockley–Read–Hall (SRH) recombination or trap-assisted recombination, a unique strategy to accelerate the grain-growth of CIGS absorbers is essential to achieve high PCEs comparable to those of state-of-the-art devices. Moreover, detrimental SRH recombination at various heterojunction interfaces should be suppressed to enhance the charge carrier separation ability and performance of solution-processed CIGS thin-film solar cells. This thesis demonstrates a series of strategic approaches to suppress deleterious SRH recombination in the CIGS thin-film solar cells of which chalcopyrite absorbers were prepared by environmentally benign alcohol-based solutions. Chapter 1 presents the scientific backgrounds helpful to understand this thesis, such as the fundamentals of photovoltaic devices, charge carrier recombination theories, and the introduction of chalcopyrite CIGS thin-film solar cells. A brief description of the research objectives and thesis organization is provided as well. Chapter 2 demonstrates a novel chalcogenization treatment that promotes the grain-growth of solution-processed CIGS absorbers. This process is designed to maximize the formation of a binary copper selenide (Cu2-xSe) phase, which becomes liquid or quasi-liquid during absorber crystallization and accelerates absorber grain-growth through liquid-phase sintering mechanisms. The CIGS absorbers prepared by this novel strategy exhibited large grain sizes and a double graded structure desirable for charge carrier separation. Moreover, the SRH recombination at the p–n junction interfaces was suppressed by an alternative n-type (Cd,Zn)S layer with a higher conduction band minimum, which could be attributable to the formation of desirable spike-type band alignment at the p–n junction interfaces. A combination of these excellent p-type absorbers and n-type layers successfully achieved the highest PCE of 14.4%, which was one of the best reported for solution-processed CIGS thin-film solar cells. Chapter 3 provides an effective strategy to improve the electrical properties of p–n junction interfaces between solution-processed CIGS absorbers and CdS layers. The thickness of n-type CdS layers often causes an undesirable trade-off between the VOC and JSC of CIGS thin-film solar cells, interrupting reliable performance improvements. In order to overcome this limitation, the formation mechanism of CdS layers during chemical bath deposition was altered using different ammonia concentrations. The heterogeneous-mechanism-induced CdS layers displayed almost perfect coverage on the polycrystalline absorbers, eliminating detrimental charge carrier losses through shunt paths. Moreover, it was revealed that the ammonia bath solution used to induce the heterogeneous mechanism reduces the Cu/(In+Ga) ratio of the absorber surfaces. These Cu depleted surfaces were expected to form a buried p–n homojunction of CIGS absorbers, which suppresses the SRH recombination at the p–n junction interfaces between CIGS absorbers and CdS layers. This high-quality heterojunction interface alleviated the trade-off between VOC and JSC successfully, allowing us to achieve a very high PCE of 15.6%, which was one of the highest efficiency reported for solution-processed CIGS thin-film solar cells. Chapter 4 introduces a novel functional layer applicable to the rear-interfaces of CIGS ultra-thin solar cells. Interestingly, it was revealed that amorphous TiO2 layers can act as a passivating contact, which not only passivates detrimental interface trap states but also provides a desirable ohmic conduction at the rear-interfaces between the solution-processed CIGS absorbers and Mo back contacts. These excellent functionalities could be obtained from thin as-deposited TiO2 layers of which thickness is equal to or less than 9 nm because the crystallization of TiO2 layers strongly depended on their thickness after annealing treatments for absorber preparation. In contrast to thick crystalline TiO2 layers, these amorphous TiO2 layers could transport the photo-generated holes in CIGS absorbers to the Mo back contacts through the band tail or defect states in the forbidden gap originating from long-range atomic disorder. These layers significantly improved the charge carrier separation ability and the VOC of the devices even increased as the absorber thickness was reduced from 750 to 300 nm. These excellent passivating contacts can be easily applied to various large-area devices without any complex contact openings, providing an unprecedented opportunity to greatly improve the performance and commercial values of CIGS ultra-thin solar cells. Chapter 5 summarizes these strategies to suppress deleterious SRH recombination in the absorber and various interfaces of solution-processed CIGS thin-film solar cells. In addition, a brief perspective on the current status and prospects of solution-processed CIGS thin-film solar cells is provided.