Benzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophene–Pyrido[3,4‑<i>b</i>]pyrazine Small-Molecule Donors for Bulk Heterojunction Solar Cells

We report on the synthesis, material properties, and bulk heterojunction (BHJ) solar cell characteristics of a set of π-conjugated small-molecule (SM) donors composed of benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT) and pyrido­[3,4-<i>b</i>]­pyrazine (PP) units, examining the perspectives of <i>alkyl-substituted PP acceptor motifs</i> in SM designs. In these systems (<b>SM1</b>–<b>4</b>), both the type of side chains derived from the PP motifs and the presence of ring substituents on BDT critically impact (i) molecular packing, and (ii) thin-film morphologies and charge transport in BHJ solar cells. With the appropriate side-chain pattern, the ring-substituted analogue <b>SM4</b> stands out, achieving efficiencies of ca. 6.5% with PC₇₁BM, and fine-scale morphologies comparable to those obtained with some of the best-performing polymer donors in BHJ solar cells. ¹H–¹H DQ-SQ NMR analyses are used to examine the distinct self-assembly pattern of <b>SM4</b>, expected to factor into the development of the BHJ morphology

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

Hybrid Tandem Quantum Dot/Organic Solar Cells with Enhanced Photocurrent and Efficiency via Ink and Interlayer Engineering

Realization of colloidal quantum dot (CQD)/organic photovoltaic (OPV) tandem solar cells that integrate the strong infrared absorption of CQDs with large photovoltages of OPVs is an attractive option toward high-performing, low-cost thin-film solar cells. To date, monolithic hybrid tandem integration of CQD/OPV solar cells has been restricted due to the CQD ink’s catastrophic damage to the organic subcell, thus forcing the low-band-gap CQD to be used as a front cell. This suboptimal configuration limits the maximum achievable photocurrent in CQD/OPV hybrid tandem solar cells. In this work, we demonstrate hybrid tandem solar cells employing a low-band-gap CQD back cell on top of an organic front cell thanks to a modified CQD ink formulation and a robust interconnection layer (ICL), which together overcome the long-standing integration challenges for CQD and organic subcells. The resulting tandem architecture surpasses previously reported current densities by ∼20–25% and yields a state-of-the-art power conversion efficiency (PCE) of 9.4%