Manjunatha Reddy G.N.1
University of Lille1
Efforts on molecular design, material processing and device engineering enabled new looms in the small molecule acceptor-based organic photovoltaics (OPVs) with power conversion efficiency (PCE) of over 19%, enticing further developments toward scalability and commercial viability. In doing so, it is paramount to resolve structure-stability-property interrelationships.[1-3] Here, we present a hybrid bottom-up and top-down NMR crystallography approach to correlate between morphology deterioration and performance degradation,[4] and provide an example of structure-stability-property relationship in PM6:Y6 bulk heterojunction (BHJ) solar cells.[5] The bottom-up strategy combines time-resolved magnetic resonance spectroscopy and crystallography modelling to disentangle the impact of the environment on the bulk and interfacial BHJ morphology, in order to gain insight into the molecular origins of photochemical, thermal and moisture-induced degradation reactions in PM6 and Y6 moieties.[5] A complementary top-down model examines macroscopic device properties including short circuit current density (<i>J<sub>sc</sub></i>), open-circuit voltage (<i>V<sub>oc</sub></i>), fill factor (<i>FF</i>), external quantum efficiency (EQE) and PCE values as a function of environmental parameters. The nanoscale PM6:Y6 BHJ morphology is resistant to an indoor operational condition at 22 °C, 700 lx and 45% relative humidity (RH) for over 4 years. In contrast to this, morphological degradation occurs upon photoirradiation (AM 1.5G), exposure to moisture at 85% RH and thermal annealing at 100-200 °C, leading to performance deterioration. The PM6:Y6 blend, PM6 and Y6 films exhibit different instability and degradation mechanisms, whereby the Y6 is more vulnerable to photoirradiation and (hydro)thermal treatment. Our results corroborate that PM6:Y6 morphology is acquiescent to indoor applications, calling for the design of environmentally stable molecular entities in order to formulate stable and efficient BHJ morphology for outdoor photovoltaics.<br/>[1] Advanced Materials, 2019, 31, 1903868<br/>[2] Energy & Environmental Science, 2020, 13, 3679-3692<br/>[3] Advanced Materials, 2022, 34, 2105943<br/>[4] Nature Reviews Materials, 2020, 5, 910-930<br/>[5] Advanced Energy Materials, 2023, in revision