Strategies for Next-generation CdTe Solar Cells: from Absorber Engineering to Tandem Integration

Cadmium Telluride (CdTe) stands as the only thin-film photovoltaic technology that has achieved large-scale commercial success, offering a unique combination of high stability and low production costs. This thesis investigates several strategic pathways to further advance CdTe technology, focusing on absorber engineering, material sustainability, and next-generation tandem integration. The incorporation of selenium into the CdTe has recently enhanced the performance of the solar cells to 23.1 %. The narrower band gap of the CdSeTe/CdTe absorber boosts the short-circuit current density; also, Se introduction increases the carrier’s lifetime. Optimizing CdSexTe1-x band-grading is crucial for achieving high- performance CdTe photovoltaics. The initial study of absorber engineering revealed that CdSeTe/CdTe devices fabricated by evaporation, followed by vacuum annealing at 450 °C to mix CdSe and CdTe layers, is an important step to increase conversion efficiency. Recognizing Tellurium as a rare earth element and to further reduce costs, the second part to the research is based ultra-thin CdTe absorbers (<1µm). The absorber material CdSeTe/CdTe has a high absorption coefficient of 104 cm-1, can absorb approximately 90% of incident light having the thickness of ~1µm. By reducing the absorber thickness, the costs, the solar cells' environmental impact and material consumption are strongly limited. These ultra-thin designs not only minimize material consumption but also ensure compliance with RoHS regulations for indoor applications. we have fabricated different absorber thicknesses ranging from 0.5 mm to 0.8 mm to study ultra-thin absorbers for CdSeTe/CdTe solar cells. We analyze the impact of the thickness reduction on the electrical parameters of the devices. Currently, our best-performing ultra-thin CdSeTe/CdTe solar cell has an efficiency of 11 %. Thus, studying the effects of Se introduction in ultra-thin CdTe absorbers is essential. To investigate Se concentration’s impact on ultra-thin CdTe, we fabricated CdSeTe/CdTe devices with an absorber thickness of 0.8 μm by depositing different CdSe/CdTe ratios. Prior to this study, the group’s best-performing ultra-thin CdSeTe/CdTe solar cells had achieved an efficiency of 11%. However, by optimizing the Se introduction and the 0.8 μm absorber thickness as described in this work, an improved efficiency of 12.8% has been reached. This study shows that Se intro duction in ultra-thin CdTe results in structural properties different from those of thicker absorbers, impacting the device performance. The thesis further examines the critical CdCl2 activation treatment, a fundamental process that transforms device efficiency from <1% to >10%. We study the effect of oxygen and inert nitrogen during CdCl2 activation process on Group V (Arsenic) doping. Finally, we emphasize the potential of CdTe in tandem applications. By developing high-performance bifacial CdTe cells with transparent back contacts, this work provides a scalable route to combine CdTe with commercially established silicon solar cells by making a tandem device for breaking the single-junction efficiency limit using two of the most stable and industrially proven photovoltaic materials available today.