Improving Optoelectronic Characterisation Methods to Understand Open-Circuit Voltage and Fill Factor Losses in Wide-Bandgap Perovskite Solar Cells

• Applicant: Sandheep Ravishankar, Ph.D.
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 462572437

Perovskite solar cells have improved rapidly over the past decade, currently showing average laboratory efficiencies comparable to commercially used silicon solar cells. Such high efficiencies have generally been achieved only for perovskites with bandgaps below 1.6 eV. Currently, wide bandgap perovskites (around 1.7 eV) are being explored as a top cell in a tandem structure with silicon solar cells in order to capture a greater range of the solar spectrum and hence enhance the photovoltaic conversion efficiency.However, the performance of the tandem cell is strongly limited by the low power conversion efficiency of the perovskite cell, which shows significant losses in open-circuit voltage and fill factor. This has been ascribed to substantial non-radiative recombination at the interfaces of the perovskite solar cell and poor charge transport in the selective contacts. However, the characterization of these loss mechanisms in the perovskite solar cell has been quite difficult due to a large variability in the electronic properties of the perovskite based on materials and methods, the difficulty in interpretation of results due to the mixed ionic-electronic conduction in the perovskite and extraneous signals from the selective contacts coupled to the perovskite response. Therefore, in this project, we aim to shed light on the device physics that governs these losses in wide bandgap perovskite solar cells through novel combinations of electro-optical characterization techniques, supplemented by drift-diffusion simulations. These techniques include ultraviolet photoelectron spectroscopy (UPS), transient photoluminescence (tr-PL), space-charge-limited current (SCLC) measurements and intensity-modulated photocurrent spectroscopy (IMPS). Such combinations of characterization techniques and simulations allow overcoming the limitations provided by any given single technique, allowing to obtain consistent information and parameters regarding the physics of operation of the device.Based on the aforementioned ideas and methods, the goals of the project are:a) Quantification of energy levels and band offsets between the different layers of the perovskite solar cell that limit its open- circuit voltage.b) Calculation of charge transport losses in the selective contacts and understanding the mechanisms of charge extraction losses in the perovskite solar cell.c) Quantification of non-radiative recombination lifetimes at the perovskite/selective contact interfaces.

• DFG Programme: WBP Position