Ladder-Type Dithienonaphthalene-Based Small-Molecule Acceptors for Efficient Nonfullerene Organic Solar Cells

Two novel small molecule acceptors (DTNIC6 and DTNIC8) based on a ladder-type dithienonaphthalene (DTN) building block with linear (hexyl) or branched (2-ethylhexyl) alkyl substituents are designed and synthesized. Both acceptors exhibit strong and broad absorption in the range from 500 to 720 nm as well as appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Replacing the linear hexyl chains with the branched 2-ethylhexyl chains has a large impact on the film morphology of photoactive layers. In the blend film based on DTNIC8 bearing the branched alkyl chains, morphology with well-defined phase separation was observed. This optimal phase morphology yields efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. Benefited from these factors, organic solar cells (OSCs) based on PBDB-T:DTNIC8 deliver a highest power conversion efficiency (PCE) of 9.03% with a high fill factor (FF) of 72.84%. This unprecedented high FF of 72.84% is one of the highest FF values reported for nonfullerene OSCs. Our work not only affords a promising electron acceptor for nonfullerene solar cells but also provides a side-chain engineering strategy toward high performance OSCs

Balancing Crystal Size in Small-Molecule Nonfullerene Solar Cells through Fine-Tuning the Film-Forming Kinetics to Fabricate Interpenetrating Network

The nanoscale interpenetrating network of active layer plays a key role in determining the exciton dissociation and charge transport in all small-molecule nonfullerene solar cells (AS-NFSCs). However, fabricating interpenetrating networks in all small-molecule blends remains a critical hurdle due to the uncontrolled crystallization behavior of small molecules. In this study, we proposed that the balanced crystal size between the donor and the acceptor is an essential prerequisite to construct optimal interpenetrating networks. We also provided a solvent additive strategy to reduce the gap of crystal size between the donor and the acceptor in S-TR:ITIC all small-molecule blend system through manipulating the solution state and film-forming kinetics. As a result, the crystal size of S-TR decreased and the crystal size of ITIC increased, leading to nanoscale interpenetrating networks. This optimized morphology improved the exciton dissociation efficiency and suppressed the bimolecular recombination, achieving almost double power conversion efficiency compared to the reference device. This work demonstrates that manipulation of the balanced crystal size of donor and acceptor may be a key to further boost the efficiency of AS-NFSCs

Absorptive Behaviors and Photovoltaic Performance Enhancements of Alkoxy-Phenyl Modified Indacenodithieno[3,2‑<i>b</i>]thiophene-Based Nonfullerene Acceptors

Nonfullerene (NF) small molecular acceptors are very attractive for further improving the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) to overcome the limited absorptive region and fixed-energy-level drawbacks of fullerene-based electronic acceptors (PC₆₁BM and PC₇₁BM). The acceptor–donor–acceptor (A-D-A)-type oligomers (<b>ITIC</b>) containing an electron-rich core (four hexyl-phenyl-substituted indacenodithieno­[3,2-<i>b</i>]­thiophene) as a donor motif sealed with 2-(3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile as an acceptor motif has been intensively investigated, because of its excellent absorptive and photovoltaic properties. Side-chain modifications have been proven to be an effective approach to modulate the energy levels and absorptive behaviors of conjugated polymers, as well as conjugated small molecules. Through the introduction of various side-chain and end groups, a series of promisingly modified <b>ITIC</b>-based small molecules have been synthesized and well-studied. Herein, we reported three novel alkoxy-phenyl modified <b>ITIC</b>-type NF acceptors (namely, <b>pO-ITIC</b>, <b>mO-ITIC</b>, and <b>FpO-ITIC</b>), in which 4-hexyloxy-phenyl, 3-hexyloxy-phenyl, and 3-fluorine-4-hexyloxy-phenyl side-chains were connected on the indacenodithieno­[3,2-<i>b</i>]­thiophene core as the electron-donating segments of the A-D-A molecules. Both three small molecules exhibit good solubility in common solvents, finely tunable energy levels, and adjustable optical bandgaps. The 4-hexyloxy-phenyl and 3-hexyloxy-phenyl-substituted materials possess relatively low bandgaps (1.61 eV for <b>pO-ITIC</b> and 1.63 eV for <b>mO-ITIC</b>) and a 4.7% enhancement in the maximum extinction coefficient, compared to that of <b>ITIC</b>. As the result of the better absorption behaviors, inverted polymer solar cells based on <b>pO-ITIC</b> blended with <b>PTB7-Th</b> achieve an open-circuit voltage (<i>V</i>_oc) of 0.80 V, a short-circuit current (<i>J</i>_sc) of 14.79 mA/cm², and a fill factor (FF) of 59.1%, leading to a high-power conversion efficiency (PCE) of 7.51%, relative to the 7.31% PCE of <b>ITIC</b>-based device. By using a new thiazolothiazole-based wide-bandgap polymer (<b>PTZ-DO</b>, 1.98 eV) with deep HOMO energy level (−5.43 eV) to match the optical absorption and molecular energy levels with the three NF acceptors, excellent PCE values9.28% for <b>mO-ITIC</b> and 9.03% for <b>pO-ITIC</b>are obtained, which show increments of 15.3% and 12.2%, respectively, relative to that of <b>ITIC</b> (8.05%). This finding should offer useful guidelines for the design of novel NF acceptors for highly efficient PSCs through alkoxy-phenyl side-chains modified on the electron-donating moiety of A-D-A organic small molecules

High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π–π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices

Asymmetrical Small Molecule Acceptor Enabling Nonfullerene Polymer Solar Cell with Fill Factor Approaching 79%

Relative to the increase of open-circuit voltage and short-circuit current, promoting fill factor (FF) of the polymer solar cells (PSCs) seems to be more challenging. Here, we designed and synthesized two asymmetrical small molecule acceptors (IDT6CN-M and IDT8CN-M) with large dipole moments. We find that the strong intermolecular interaction and favorable antiparallel packing induced by the dipole moment can effectively enhance both lamellar packing and π–π stacking. The PSCs based on PBDB-T:IDT6CN-M and PBDB-T:IDT8CN-M achieved FFs of up to 76.1% and 78.9%, corresponding to PCEs of 11.23% and 12.43%, respectively. To the best of our knowledge, 78.9% FF is a new record for nonfullerene PSCs. Overall, our work provides a simple and effective molecule-designing method to promote FF of nonfullerene PSCs