π-Bridge-Independent 2-(Benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile-Substituted Nonfullerene Acceptors for Efficient Bulk Heterojunction Solar Cells

Abstract

Molecular acceptors are promising alternatives to fullerenes (e.g. PC 61 / 71 BM) in the fabrication of high-efficiency bulk-heterojunction (BHJ) solar cells. While solution-processed polymer-fullerene BHJ devices have recently met the 10% efficiency threshold, molecular acceptors have yet to prove comparably efficient with polymer donors. At this point in time, it is important to forge a better understanding of the design parameters that directly impact small-molecule (SM) acceptor performance in BHJ solar cells. In this report, we show that 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM)-terminated SM acceptors can achieve efficiencies as high as 5.3% in BHJ solar cells with the polymer donor PCE10. Through systematic device optimization and characterization studies, we find that the nonfull-erene analogues (FBM, CBM and CDTBM) all perform comparably well, independent of the molecular structure and electronics of the π-bridge that links the two electron-deficient BM end groups. With estimated electron affinities within range of those of common fullerenes (4.0-4.3 eV), and a wider range of ionization potentials (6.2-5.6 eV), the SM acceptors absorb in the visible spectrum and effectively contribute to the BHJ device photocurrent. BM-substituted SM acceptors are promising alternatives to fullerenes in solution-processed BHJ solar cells. Given the outstanding headway made in the past recent years in optimizing polymer and SM donor performance using fullerenes as model systems, 5,20-22 a case could be made that further BHJ solar cell efficiency improvements

Efficient Nonfullerene Polymer Solar Cells Enabled by a Novel Wide Bandgap Small Molecular Acceptor

“…Small molecular acceptors exhibit great advantages of low production costs, easily tunable energy levels, broad absorption range, and potentially better morphological stability in the active blend film . Extensive research efforts have been dedicated in designing novel SMAs that can yield high‐efficiency PSCs . There are generally two classes of SMAs reported with either small (≈1.5 eV) or large (≈2.0 eV) optical bandgaps.…”

Recent Advances in Nonfullerene Acceptors for Organic Solar Cells

“…AFM images showed that after adding the additives, the blend could form a more uniform film and a clearer phase separation, which was beneficial for the charge transport. Beaujuge and co‐workers designed and synthesized three nonfullerene small molecule acceptors A26, A27, and A28, with BM‐based terminal group. The difference among the three acceptors was the central core varying from fluorine to carbazole and cyclopenta [2, 1‐b: 3, 4‐b′] dithiophene.…”

Benzo[1,2‐b:4,5‐b′]Dithiophene–6,7‐Difluoroquinoxaline Small Molecule Donors with >8% BHJ Solar Cell Efficiency

“…The solution and thin-film UV-vis spectra shown in Figure 2a indicate that swapping alkyls (SM1) for acrylate substituents (SM2) in [2F]Q redshift the spectral absorption by up to ≈120 nm, corresponding to a notable reduction in optical gap of 0.3 eV. Following the same trend in IP values, experimental estimates were determined via photoelectron spectroscopy in air (PESA): 5.20 eV for SM2 and 5.11 eV for SM1 (here, we note that PESA-estimated values are in general lower than DFT-calculated ones, [18,27,28] but the trends remain meaningful). [26] Examinations of the main-chain conformation and π-electron delocalization patterns by density functional theory (DFT; modeling with the gap-tuned, longrange-corrected (LRC) hybrid functional ωB97XD/6-31G(d,p), cf.…”