Interfaces, Stability and Energy Efficiency: Photochemical Characterisation of Perovskites for Printable Photovoltaics

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The clean generation of energy is the most important scientific and technological challenge that faces humankind in the 21st century. Concerns about climate change, energy independence, and depletion of non-renewable reserves, are pushing governments around the world to develop and implement alternative-energy policies and technologies. The development of low-cost, large area, stable photovoltaics is an absolute must to meet humankinds' current, growing, energy need. The current photovoltaic (PV) market, is dominated by crystalline silicon modules (> 85 % of the present PV market). Despite the recent substantial reductions in the manufacturing cost of mainstream silicon PV, there exists clear opportunities for PV technologies that promise either significant higher power energy conversion efficiencies (PCE) or significantly lower processing costs (both in financial and embodied energy terms). Perovskite-based solar cells, offer prospects on both fronts. Perovskite based solar cells are a relatively young technology, first reported in 2009, and have metaphorically opened the door to an exciting, new, highly efficient solid-state photovoltaic technology which could compete with silicon and thin film technologies that require vacuum deposition and/or expensive non-trivial processing. However, stability and lifetime issues must be understood and overcome to progress the field.Here we aim to look at the photochemistry and stability of printable perovskite photovoltaics with the aim to develop an understanding of materials that are suitable to manufacture at a large scale. This project will focus on developing a detailed understanding of the fundamental processes that govern the photoluminescence (PL) properties of perovskite materials. PL of perovskite thin-films is not as straightforward as initially thought highlighting the sometimes-surprising nature of these materials, here we attempt to unravel the PL data and discuss what this can tell us about these materials. We will study a series of perovskites via fluorescence microscopy (FM) coupled with an optical fibre spectrometer. This allows precise control of the measurement environment (temperature and atmosphere control) and provides information on the bulk and local photoluminescence (PL) and allows us to map the surface of the films and monitor the evolution of photoluminescence with time exposed to various controlled environments. Perovskite materials tend to be sensitive to air/moisture, light and oxygen. Molecular oxygen can be particularly problematic as photo-generated electrons at a semiconductor surface reduce oxygen to radical species and holes which are extremely reactive towards and can result in rapid degradation of devices. We aim to correlate the PL, and the changes in PL with time, with the overall PV device efficiency and stability. This is a rapid and straight forward screening method that does not need the manufacture of a complete device to evaluate and optimise the perovskite properties. This will provide much needed understanding of the stability of these devices and deliver a clear route for the optimisation and improvement of device stability.Time resolved PL provides vital information on the charge carrier kinetics and extraction (in the presence of charge selective contacts) which is a key indicator of performance. The aim is to achieve a global understanding of the charge carrier lifetime and extraction, achieved through a rigorous control of all the components of the absorber-charge selective contact interface, and an evaluation of the kinetics of charge transfer processes occurring through such interfaces. Charge contacts can then be designed/tailored to maximise conductivity, while minimising back electron transfer and recombination and thus improving device performance.