Enhanced Light Absorption in Fluorinated Ternary Small-Molecule Photovoltaics

Using small-molecule donor (SMD) semiconductors in organic photovoltaics (OPVs) has historically afforded lower power conversion efficiencies (PCEs) than their polymeric counterparts. The PCE difference is attributed to shorter conjugated backbones, resulting in reduced intermolecular interactions. Here, a new pair of SMDs is synthesized based on the diketopyrrolopyrrole–benzodithiophene–diketopyrrolopyrrole (BDT-DPP₂) skeleton but having fluorinated and fluorine-free aromatic side-chain substituents. Ternary OPVs having varied ratios of the two SMDs with PC₆₁BM as the acceptor exhibit tunable open-circuit voltages (<i>V</i>_ocs) between 0.833 and 0.944 V due to a fluorination-induced shift in energy levels and the electronic “alloy” formed from the miscibility of the two SMDs. A 15% increase in PCE is observed at the optimal ternary SMD ratio, with the short-circuit current density (<i>J</i>_sc) significantly increased to 9.18 mA/cm². The origin of <i>J</i>_sc enhancement is analyzed via charge generation, transport, and diffuse reflectance measurements, and is attributed to increased optical absorption arising from a maximum in film crystallinity at this SMD ratio, observed by grazing incidence wide-angle X-ray scattering

Enhanced Fill Factor through Chalcogen Side-Chain Manipulation in Small-Molecule Photovoltaics

The fill factor (FF) of organic photovoltaic (OPV) devices has proven difficult to optimize by synthetic modification of the active layer materials. In this contribution, a series of small-molecule donors (SMDs) incorporating chalcogen atoms of increasing atomic number (<i>Z</i>), namely oxygen, sulfur, and selenium, into the side chains are synthesized and the relationship between the chalcogen <i>Z</i> and the FF of OPV devices is characterized. Larger <i>Z</i> chalcogen atoms are found to consistently enhance FF in bulk-heterojunction OPVs containing PC₆₁BM as the acceptor material. A significant ∼8% FF increase is obtained on moving from O to S to Se across three series of SMDs. The FF enhancement is found to result from the combination of more ordered morphology and decreased charge recombination in blend films for the high-<i>Z</i>-chalcogen SMDs. Because this FF enhancement is found within three series of SMDs, the overall strategy is promising for new SMD materials design

Marked Consequences of Systematic Oligothiophene Catenation in Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione and Bithiopheneimide Photovoltaic Copolymers

As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress charge recombination. Here we investigate two series of in-chain donor–acceptor copolymers, <b>PTPDnT</b> and <b>PBTInT</b>, based on thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione (<b>TPD</b>) or bithiopheneimide (<b>BTI</b>) as electron acceptor units, respectively, and oligothiophenes (<b>nT</b>s) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar <b>TPD</b> polymer backbones, however backbone torsion yields greater open-circuit voltages for <b>BTI</b> polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that <b>PTPDnT</b> and <b>PBTInT</b> conjugation lengths scale with <b>nT</b> catenation up to <i>n</i> = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The <b>3T</b> with “outward” alkylation in <b>PTPD3T</b> and <b>PBTI3T</b> affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm²/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for <b>PTPD3T</b>-based PSCs with PCE = 7.7%. The results demonstrate that <b>3T</b> is the optimal donor unit among <b>nT</b>s (<i>n</i> = 1–4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based <b>PTPD3T</b> blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance