Radiation Hardness of Organic Solar Cells

Over the past few decades, space exploration has opened up new frontiers in both scientific knowledge and technological innovation. These endeavors have created new opportunities for addressing global challenges. As we set our sights on the forthcoming era of space exploration, the demand for substantial power looms large. Electric propulsion, driven by solar energy, emerges as a pivotal technology to support these capabilities.<br/><br/>Organic photovoltaic cells (OPVs), in particular, have exhibited great promise for solar power generation in space due to their incredibly high specific power, reaching up to 40 kW/kg¹, compared, for example to the ubiquitously employed Si cells of ~ 1 kW/kg². Furthermore, OPVs have the ability to withstand tensile strains exceeding 300% when supported by elastomeric materials, making them far more flexible than their inorganic counterparts³. The continuous advancement of organic materials has enabled the power conversion efficiency (PCE) of OPVs to exceed 20%^4,5 and intrinsic operational lifetime over 30 years⁶, making them competitive with other thin-film inorganic photovoltaic devices. The only pervasive myth associated with OPVs is that the materials are intrinsically vulnerable to degradation when exposed to high energy incident radiation.<br/><br/>In this work, we found that carefully selected and designed organic materials and device structure are capable of showing high radiation hardness. Previous studies have identified proton irradiation as a prominent source of radiation-induced damage to photovoltaic systems in space, generating vacancies, interstitials, or complex defects within semiconductor materials⁷. Through simulations of proton energy loss per unit depth using SRIM/TRIM, we found that proton energies exceeding 100 keV can traverse the organic active layer without inducing collisions, consequently yielding a low total vacancy count. This phenomenon is attributed to the relatively slim nature of the organic active layer within OPV cells, measuring only a hundred nanometers in thickness, rendering them comparatively transparent to high-energy particles. To evaluate the proton tolerance of OPVs, we fabricated devices based on the two most reliable OPV configurations demonstrated by our lab. The results demonstrated that the PCE of the OPV device can be maintained even at energies as high as 1 MeV and fluences up to 10¹² cm⁻², equivalent to 20 years of operation in low Earth orbit (LEO) missions. Remarkably, this performance surpasses that of crystalline silicon (c-Si) cells, which undergo around 40% degradation, and perovskite cells, which show nearly 10% degradation at the same fluence. This compelling evidence suggests the potential of OPVs to offer extraordinarily long operational lifespans in space environments.<br/><br/>References:<br/>1. Zheng, X. <i>et al.</i> Versatile organic photovoltaics with a power density of nearly 40 W g-1. <i>Energy Environ. Sci.</i> (2023) doi:10.1039/D3EE00087G.<br/>2. Kaltenbrunner, M. <i>et al.</i> Flexible high power-per-weight perovskite solar cells with chromium oxide–metal contacts for improved stability in air. <i>Nat. Mater.</i> <b>14</b>, 1032–1039 (2015).<br/>3. Cardinaletti, I. <i>et al.</i> Organic and perovskite solar cells for space applications. <i>Sol. Energy Mater. Sol. Cells</i> <b>182</b>, 121–127 (2018).<br/>4. Zheng, Z. <i>et al.</i> Tandem Organic Solar Cell with 20.2% Efficiency. <i>Joule</i> <b>6</b>, 171–184 (2022).<br/>5. Cui, Y. <i>et al.</i> Single-Junction Organic Photovoltaic Cell with 19\% Efficiency. <i>Adv. Mater.</i> <b>33</b>, 2102420 (2021).<br/>6. Li, Y. <i>et al.</i> Non-fullerene acceptor organic photovoltaics with intrinsic operational lifetimes over 30 years. <i>Nat. Commun.</i> <b>12</b>, 1–9 (2021).<br/>7. Kirmani, A. R. <i>et al.</i> Countdown to perovskite space launch: Guidelines to performing relevant radiation-hardness experiments. <i>Joule</i> <b>6</b>, 1015–1031 (2022).