Interfacial Engineering of Perovskite Solar Cells

Abstract

The efficiency of the lead halide perovskite solar cells has improved from 3% up to above 27% in a decade. However, the long term stability of the device still need to be improved in order to compete with traditional photovoltaic technologies, such as Silicon and GaAs. Hybrid organic and inorganic interfaces in the devices are the origin of many degradation pathways. Understanding the nature of these interfaces and chemical and physical mechanism behind their evolution under electrical, light and thermal bias is the subject of this thesis. In the following chapters, I focus on developing a electron transport layer (ETL) based on chemical bath deposition (CBD) for the synthesis of a tin dioxide (SnO₂). The conventional CBD recipe uses thioglycolic acid (TGA) to facilitate attachments of SnOx particles onto the substrate. However, nonvolatile TGA is reported to harm the operational stability of PSCs. A volatile oxalic acid (OA) is introduced as an alternative to TGA. OA, a dicarboxylic acid, functions as a chemical linker for the nucleation and attachment of particles to the substrate in the chemical bath. Moreover, OA can be readily removed through thermal annealing followed by a mild H₂O₂ treatment, as shown by FTIR measurements. Synergistically, the mild H₂O₂ treatment selectively oxidizes the surface of the SnOₓ layer, minimizing nonradiative interface carrier recombination. EELS (electron-energy-loss spectroscopy) confirms that the SnOₓ surface is dominated by Sn⁴⁺, while the bulk is a mixture of Sn²⁺ and Sn⁴⁺. This rational design of a CBD SnOₓ layer leads to devices with T₈₅ = 1500h, a significant improvement over the TGA-based device with T₈₀ = 250h. The champion device reached a power conversion effciency of 24.6%. This work offers a rationale for optimizing the complex parameter space of CBD SnOₓ to achieve efficient and stable PSCs. In addition to developing a electron transport layer (ETL) based on chemical bath deposition (CBD) for the synthesis of a tin dioxide (SnO₂), a perovskite ink additive, bis(2- oxo-3-oxazolidinyl) phosphinic chloride (BOP-Cl), was developed with the following benefits: (1) The phosphoryl and two oxo groups form six-membered intermolecular hydrogen-bonded rings with the formamidinium cation (FA), mitigating ion migrations. (2) The hydrogen bonding reduces the electrophilicity of the ammonium protons by donating electron density, therefore reducing its reactivity with the surface oxygen on the metal oxide. Furthermore, the molecule can react to form a protecting group on the nucleophilic oxygen at the tin oxide transport layer surface through the elimination of chlorine. As a result, we achieve perovskite solar cells with an efficiency of 25.0% and improved MPP stability T₉₃ = 1200h at 40–45 °C compared to a control device (T₈₆ = 550h). In addition, we show a negative correlation between the strength of hydrogen bonding of different phosphine oxide derivatives to the organic cations and the degree of metastable behavior (e.g., initial burn-in) of the device.