Novel inorganic hole transport materials for perovskite solar cells

Abstract

Halide perovskites are a promising candidate for future photovoltaic applications due to its high power conversion efficiency (>26%) and lower production cost. However, their poor ambient stability presents a critical challenge to its commercial viability. Multiple reports have claimed that the defect centers inside the perovskite lattice can catalyze degradation reactions in the presence of environmental stressors. Interfacial lattice defects have an outsized impact on the perovskite solar cell (PSC) stability due to their high concentrations. Owing to their low formation energy, iodine vacancy (VI) and PbI2 & Pb0 clusters are found in large numbers near the interface. The passivation of these defects is essential for the long-term stability of the perovskite lattice. Lewis base molecules, particularly chalcogenide-based ones, have been very effective in passivating these interfacial defects due to the acidic nature of these defects.

This thesis aims to improve the stability of the perovskite solar cell by developing intrinsically stable and efficient defect passivators. First, the interface defect passivation ability of oxygen, sulfur, and selenium-based organic compounds are compared in perovskite solar cells. Sulfide and selenide-passivated devices display superior stability properties compared to their oxide-passivated counterparts, owing to the robust interfacial Pb-S and Pb-Se interactions. The stronger interaction capacity of sulfur and selenium with Pb compared to oxygen can be understood using the hard and soft acid and base (HSAB) principle. Due to the higher softness of sulfur and selenium (compared to oxygen), their interaction with the soft Pb2+ (lewis acid) is markedly stronger. Despite the higher efficacy of the sulfur and selenide-based passivators, these compounds can degrade under harsh solar cell operating conditions due to their organic nature. Therefore, more stable inorganic-based defect passivators should be developed to generate higher stability in PSC devices.

In the next phase of the work, a bifunctional CuS-based film is demonstrated to serve dual functions: 1) as a hole transport layer (HTL) in p-i-n architecture PSC and 2) passivation layer for interfacial lattice defects. Al doping is incorporated in the CuS HTL to improve the interfacial valence band alignment with perovskite. Urbach energy and ideality factor measurements suggest the formation of a relatively defect-free interface in the sulfide HTL-perovskite heterojunction compared to the oxide-perovskite one. The superior quality interface is engendered by the interfacial Pb-S interaction, which led to higher stability of the sulfide HTL-based devices compared to their oxide counterparts.

Next, the stability of the perovskite film is studied on several potential binary and ternary metal sulfide HTLs, and CuInS2 (CIS) is found to offer the highest stability to the perovskite film among the studied compounds. Partial replacement of In with Ga (CIGS) in the CIS composition further enhances the stability of the perovskite solar cell. Additional studies revealed that the metal sulfides with lower electronegativity cations offer higher stability to the perovskite film. This stability enhancement is correlated to the increased electron density around the functional sulfur ion due to charge transfer from the electropositive cations. The higher electron density around the sulfur allows more robust dative bond formation with the undercoordinated Pb ions (VI). Finally, the stability of the perovskite films on CIGS HTL is compared with its oxide (CuInGaO2, CIGO) and selenide (CuInGaSe2, CIGSe) counterparts to compare the passivation effectiveness of different chalcogenide anions in inorganic systems with similar cationic configurations. The stability of the perovskite film is found to be the highest on the CIGSe film, followed by CIGS and CIGO films, which is consistent with the softness of the respective anions.

In summary, this thesis develops two novel inorganic sulfide-based HTLs and studies their interfacial defect passivation mechanism in PSC. It is observed that the anion softness and the cation electronegativity of the inorganic HTL play a critical role in its ability to interact with the interfacial Pb2+ defects. The findings of this thesis can be used as a guiding principle for developing future-generation inorganic defect passivators for perovskite solar cells.