Potential and Limitations of High Performance and Scalable Evaporated Organic Solar Cells

<br/>The bulk heterojunction (BHJ) active layer of organic photovoltaics (OPV) is typically deposited either via thermal evaporation (small molecule donors, fullerene acceptors) or by solution processing (polymer donors, non-fullerene acceptors). The commercial viability of thermal evaporation has been proven for organic light emitting diodes (OLED), including the deposition of complex multilayer stacks and co-evaporated BHJs. Evaporated blends in OLEDs and OPVs have been shown to be more stable than their solution processed counterparts. Furthermore, the solvent free deposition lowers the environmental impact compared to solution processed technologies. Looking at the power conversion efficiency (PCE), Heliatek GmbH achieved an OPV record PCE of 13.2 % with an evaporated tandem cell in 2016. Since then, novel solution processable polymers and non-fullerene acceptors paved the way for performance increases beyond 19 % that could not be matched by evaporated OPVs. As a result, the academic community is today largely focussed on lab scale performance increases and reproducible upscale of solution processed OPVs, leading to a lack of studies addressing the limitations of high performance, industrially relevant evaporated OPV materials.<br/>A key bottleneck in evaporated OPVs is the trade-off between efficient charge collection and maximising absorption by increasing the active layer thickness. This is evident from the high losses in fill factor for active layer thicknesses beyond 50 nm while solution processed cells often perform best at thicknesses &gt;100 nm. Studies on solution processed devices showed that active layer blends with efficient collection (high mobility vs. slow non-geminate recombination) exhibit a phase separated morphology. This is often achieved through post-deposition optimisation (e.g. thermal annealing, solvent vapour annealing). Similar studies on the connection between morphology, recombination kinetics and mobility in evaporated OPVs are key to identifying ways to increase their performance through improved deposition control and molecular design.<br/>Here, I will present a study on a state-of-the-art evaporated small molecule donor paired with C₆₀. The acceptor-donor-acceptor type small molecule was synthesized by Heliatek. Devices with different active layer morphologies were studied. A wide range of optical (photoluminescence, ultrafast spectroscopy) and optoelectronic device measurements (transient photovoltage, charge extraction) demonstrate that the low and imbalanced charge carrier mobility is a key bottleneck even in the optimised blendz. An in-depth characterisation of the active layer morphology (RSoXS, GIWAXS) enables us to identify the aspects of the morphology that should be addressed to further improve performance. I will contrast our findings about the collection efficiency with results from solution processed devices (PM6:Y6, BTR:PCBM) as well as giving an outlook for feasible performance improvements based on drift-diffusion simulations (OghmaNano).