Static and Dynamic Disorder in Thermal Vacuum Evaporated Organic Solar Cells

Organic solar cells (OSCs) have attracted interest over the last decades for their ease of processing and potential to generate inexpensive electricity for a growing population [1]. However, the commercial success of OSCs has long been hindered by their low power conversion efficiencies (PCEs). The main reasons for low PCEs are radiative and non-radiative energy losses of 100s meV that usually occur via charge transfer (CT) states at the interface between donor (D) and acceptor (A) molecules.<br/>The origin of the CT state linewidth, and its influence on device performance, is still heavily debated [4]. Usually, broad spectral tails and large Stokes shifts between absorption and emission spectra are indicative of energetic disorder that negatively impacts charge transport, recombination, and the open-circuit voltage [4]. This energetic disorder can be classified into dynamic (temperature-dependent) and static (temperature-independent) contributions. While some work found negligible static disorder in OSCs [4], [6], other work observed the opposite and reported static disorder contributions of 50-150 meV [7], [8], [9].<br/><br/>In this work, we revisit the debate on dominant contributions to energetic disorder for small molecule OSCs. Using vacuum deposition techniques, we fabricate solar cells based on blends of C60 and donors like TAPC, α-Sexithiophene, DCV5T, CuPc, and Rubrene that show representative and comprehensive electronic and microstructural properties. By fabricating devices with 5 mol.% and 50 mol.% donor content, we study the impact of energetic disorder in dilute donor and intermixed D:A blends. We quantify electronic disorder by fitting temperature-dependent external quantum efficiency (EQE) spectra via semi-classical Marcus Levich Jortner theory.<br/>We find that static disorder presents a dominant contribution to the CT state linewidth across our materials systems. In addition, by coupling our device data to grazing incidence wide-angle X-ray scattering (GIWAXS) measurements, we find larger energetic disorder for aggregating than for amorphous donors. Finally, we highlight the need for reducing energetic disorder to achieve lower voltage losses in both fullerene- and well-performing non-fullerene-based blends.<br/><br/>1. Riede M, et al. Organic solar cells—the path to commercial success. Adv Energy Mater. 2021;11: 2002653.<br/>2. Jungbluth A, et al. Charge transfer state characterization and voltage losses of organic solar cells. J Phys Mater. 2021 [cited 23 Jan 2022]. doi:10.1088/2515-7639/ac44d9<br/>3. Azzouzi M, et al. Factors Controlling Open-Circuit Voltage Losses in Organic Solar Cells. Trends in Chemistry. 2019;1: 49–62.<br/>4. Tvingstedt K, et al. Temperature dependence of the spectral line-width of charge-transfer state emission in organic solar cells; static vs. dynamic disorder. Mater Horiz. 2020;7: 1888–1900.<br/>5. Vandewal K, et al. Relating the open-circuit voltage to interface molecular properties of donor:acceptor bulk heterojunction solar cells. Phys Rev B Condens Matter. 2010;81: 125204.<br/>6. Göhler C, et al. Temperature-Dependent Charge-Transfer-State Absorption and Emission Reveal the Dominant Role of Dynamic Disorder in Organic Solar Cells. Phys Rev Applied. 2021;15: 064009.<br/>7. Burke TM, et al. Beyond Langevin recombination: How equilibrium between free carriers and charge transfer states determines the open-circuit voltage of organic solar cells. Adv Energy Mater. 2015;5: 1500123.<br/>8. Kahle F-J, et al. How to interpret absorption and fluorescence spectra of charge transfer states in an organic solar cell. Mater Horiz. 2018;5: 837–848.<br/>9. Linderl T, et al. Crystalline versus Amorphous Donor-Acceptor Blends: Influence of Layer Morphology on the Charge-Transfer Density of States. Phys Rev Applied. 2020;13: 024061.