Crystallization modulation and dual passivation strategy for enhancing the performance of perovskite/CuIn(Ga)Se2 tandem solar cells

Two-terminal (2-T) perovskite (PVK)/CuIn(Ga)Se₂ (CIGS) tandem solar cells (TSCs) have garnered attention due to their potential for achieving high energy conversion efficiencies by combining the best bandgap matching of both materials. However, a significant challenge has been the irregular, rough morphology of commercial CIGS surfaces, which hinders the efficient integration of perovskite onto these surfaces, affecting the overall device performance. A research team from Wuhan University of Technology, led by Dr. Yong Peng, addresses this issue by introducing a novel approach involving crystallization modulation of the PVK material and holistic passivation strategies to enhance the performance of PVK/CIGS TSCs.

The study investigates the use of D-homoserine lactone hydrochloride (D-HLH) as an additive in the PVK precursor solution. D-HLH was found to significantly improve the crystallization of PVK and enhance its coverage on the rough CIGS surface. This led to a reduction in defects in the PVK films, allowing for more uniform coverage and a stronger interface between the PVK and CIGS layers. By improving the quality of the PVK layer, the efficiency of the tandem solar cells could be significantly enhanced.

In addition to improving the crystallization and coverage of the PVK layer, the study also addresses the issue of interface recombination, which is a critical factor limiting the efficiency of perovskite-based solar cells. Recombination occurs at the PVK/C60 interface, where the minority carriers are trapped due to incomplete passivation of defect sites, leading to energy loss. To address this, the researchers explore a dual passivation strategy, combining surface reconstruction and field-effect passivation techniques. The surface reconstruction involves the use of 2-thiopheneethylammonium iodide (2-TEAI) and N,N-dimethylformamide (DMF), which effectively passivate the defect sites at the PVK surface and grain boundaries. Meanwhile, lithium fluoride (LiF) is introduced as a field-effect passivation layer, which repels hole carriers away from the PVK/C60 interface, further reducing recombination losses.

Through these combined strategies, the researchers were able to achieve significant improvements in the performance of 2-T PVK/CIGS TSCs. The champion device fabricated in this study demonstrated a power conversion efficiency (PCE) of 24.6%, one of the highest reported for this type of tandem solar cell. This result not only confirms the effectiveness of crystallization modulation and passivation strategies but also opens up new avenues for achieving higher efficiency in thin-film-based solar cells.

Furthermore, the study also investigates the impact of these strategies on single-junction perovskite solar cells (PSCs), which serve as a simpler reference device. The use of D-HLH and the dual passivation strategies led to a significant improvement in the PCE of single-junction PSCs, increasing from approximately 17.9% to 19.1%. The highest average PCE of over 21.8% was achieved with the combination of 2-TEAI and LiF, showcasing the effectiveness of dual passivation in enhancing the performance of perovskite solar cells.

The underlying mechanisms behind these improvements were examined using various structural and material characterization techniques. Scanning electron microscopy (SEM) images revealed that the addition of D-HLH to the precursor solution improved the uniformity of the PVK coverage on the CIGS substrate. X-ray diffraction (XRD) patterns showed an increase in the intensity of the (100) plane peak, indicating enhanced crystallization of the PVK material. These results suggest that the crystallization modulation facilitated by D-HLH is key to achieving better coverage and reducing defects in the PVK layer.

In conclusion, this study presents a novel and effective approach for enhancing the efficiency of perovskite/CIGS tandem solar cells. By addressing the challenges of irregular CIGS surface morphology and interface recombination, the researchers were able to develop a high-performance tandem solar cell with a PCE of 24.6%. The combination of crystallization modulation with D-HLH and dual passivation strategies using 2-TEAI, DMF, and LiF represents a promising path toward achieving higher efficiency in perovskite/CIGS TSCs. These findings provide valuable insights for the development of next-generation solar cell technologies, with potential applications in commercial thin-film-based solar energy generation.