Solvent Vapor Annealing-Mediated Crystallization Directs Charge Generation, Recombination and Extraction in BHJ Solar Cells

Small-molecule (SM) donors that can be solution-processed with fullerene acceptors (e.g., PC₆₁/₇₁BM), or their “nonfullerene” counterparts, are proving particularly promising for the realization of high-efficiency bulk-heterojunction (BHJ) solar cells. In several recent studies, solvent vapor annealing (SVA) protocols have been found to yield significant BHJ device efficiency improvements via structural changes in the active layer morphologies. However, the mechanisms by which active layer morphologies evolve when subjected to SVA treatments, and the structural factors impacting charge generation, carrier transport, recombination, and extraction in BHJ solar cells with SM donors and fullerene acceptors, remain important aspects to be elucidated. In this report, we show thatin BHJ solar cells with SM donors and fullerene acceptorsselective crystallization promoted by SVA mediates the development of optimized morphologies across the active layers, setting domain sizes and boundaries. Examining BHJ solar cells subjected to various SVA exposure times, with BDT­[2F]­QdC as the SM donor and PC₇₁BM as the acceptor, we connect those morphological changes to specific carrier effects, showing that crystal growth effectively directs charge generation and recombination. We find that the SM donor-pure domains growing at the expense of a mixed donor–acceptor phase play a determining role, establishing optimum networks with 10–20 nm sized domains during the SVA treatment. Longer SVA times result in highly textured active layers with crystalline domains that can exceed the length scale of exciton diffusion, while inducing detrimental vertical morphologies and deep carrier traps. Last, we emphasize the field-dependence charge generation occurring upon SVA-mediated crystallization and link this carrier effect to the mixed phase depletion across the BHJ active layer

Triphenylamine-Based Push–Pull σ–C₆₀ Dyad As Photoactive Molecular Material for Single-Component Organic Solar Cells: Synthesis, Characterizations, and Photophysical Properties

A push–pull σ–C₆₀ molecular dyad was synthesized via Huisgen-type click chemistry and used as photoactive material for single-component organic solar cells. Steady-state photoluminescence (PL) experiments of the dyad in solution show a significant quenching of the emission of the push–pull moiety. Spin-casting of a solution of the dyad results in homogeneous and smooth thin films, which exhibit complete PL quenching in line with ultrafast photoinduced electron-transfer in the solid state. Spectroelectrochemistry reveals the optical signatures of radical cations and radical anions. Evaluation of the charge carrier mobility by space-charge limited current measurements gives an electron-mobility of μ_e = 4.3 × 10^–4 cm² V^–1 s^–1, ca. 50 times higher than the hole-mobility. Single-component organic solar cells yield an open-circuit voltage <i>V</i>_oc of 0.73 V and a short-circuit current density of 2.1 mA cm^–2; however, a poor fill factor FF (29%) is obtained, resulting in low power conversion efficiency of only 0.4%. Combined transient absorption (TA) and time-delayed collection field (TDCF) experiments show mostly ultrafast photon-to-charge conversion and a small component of diffusion-limited exciton dissociation, revealing the presence of pure fullerene domains. Furthermore, a strong field dependence of charge generation is observed, governing the device fill factor, which is further reduced by a competition between extraction and fast recombination of separated charges