A molecular guide to efficient charge transport : Coordination materials for photovoltaic cells

Abstract: Emerging solar energy conversion and energy storage technologies play a vital role in solving the present energy crisis and achieving carbon net zero. Currently, they are limited by the use of inefficient, unstable and expensive charge transport materials. The development of new charge transport materials is still far behind the efforts that have been made to develop the light-absorbing or other components. Metalorganic coordination compounds offer unique sets of properties as hybrids between conductive metals and tunable organic molecules. The coordination of the metal centers is crucial to control in order to maximise the solar cell efficiency - or undesired electronic recombination limits the power output. Tetradentate ligands allow copper complexes to dynamically switch between dimers or monomers, pending the oxidation state of the metal ions. The high energy barrier for the reduction of CuII monomers prevents electron transfer across the TiO2|dye|electrolyte interface: Interfacial recombination is reduced and the dye-sensitised solar cells achieve greater photovoltages. Coordination complexes linked into low-dimensional coordination polymers afford charge transport with an electrical conductivity as high as 0.1 S m-1 via band-like conduction at room temperature, needless of cationic dopants. The polymers rapidly extract photoexcited charges from halide perovskite films. 14% power conversion efficiency were recorded from a perovskite solar cell based on a carbon counter electrode. The solar cell stability was much increased compared to heavily doped organic hole conductors. Emerging dye-sensitised solar cells excel especially under ambient conditions, and have been proposed as power sources for dispatched electronic devices (the Internet of things), in place of single-use and difficult-to-recycle batteries. Through tailoring of the optical response and the electrolyte composition, power conversion efficiencies of 37.5% with photovoltages of 1.00 V at 1000 lux (fluorescent lamp) are demonstrated. The increased performance is identified to stem from reduced interfacial recombination by transient photovoltage methods as well as electrochemical impedance spectroscopy. A series of prototype tests underline the feasibility of light harvesters as power sources for electronic devices, executing sophisticated computation tasks such as machine learning. The devices self-optimise their energy consumption; adaptive sleep and small supercapacitors allow to sustain device operation during periods of fluctuating energy availability.