New Avenues for Organic Solar Cells Using Intrinsically Charge-Generating Materials

The molecular electron acceptor material Y6 has been a key part of the most recent surge in organic solar cell sunlight-to-electricity power conversion efficiency, which is now approaching 20%. Numerous studies have sought to understand the fundamental photophysical reasons for the exceptional performance of Y6 and its growing family of structural derivatives. Though significant uncertainty about several details remains, many have concluded that initially photogenerated excited states rapidly convert into electron-hole charge pairs in the neat material. These charge pairs are characterized by location of the electron and hole on different Y6 molecules, in contrast to the Frenkel excitons that dominate the behavior of most organic semiconductor materials. Here, we summarize the current state of knowledge regarding Y6 photophysics and the key observations that have led to it. We then link this understanding to other advances, such as the role of quadrupolar fields in donor-acceptor blends, and the importance of molecular interactions and organization in providing the structural basis for Y6's properties. Finally, we turn our attention to ways of making use of the new photophysics of Y6, and suggest molecular doping, crystal structure tuning, and electric field engineering as promising avenues for future exploration.

Fused-Ring Electron-Acceptor Single Crystals with Chiral 2D Supramolecular Organization for Anisotropic Chiral Optoelectronic Devices

Supramolecular chiral organization gives π-conjugated molecules access to fascinating specific interactions with circularly polarized light (CPL). Such a feature enables the fabrication of high-performance chiral organic electronic devices that detect or emit CPL directly. Herein, it is shown that chiral fused-ring electron-acceptor BTP-4F single-crystal-based phototransistors demonstrate distinguished CPL discrimination capability with current dissymmetry factor exceeding 1.4, one of the highest values among state-of-the-art direct CPL detectors. Theoretical calculations prove that the chirality at the supramolecular level in these enantiomeric single crystals originates from chiral exciton coupling of a unique quasi-2D supramolecular organization consisting of interlaced molecules with opposite helical conformation. Impressively, such supramolecular organization produces a higher dissymmetry factor along the preferred growth direction of the chiral single crystals in comparison to that of the short axis direction. Furthermore, the amplified, inverted, and also anisotropic current dissymmetry compared to optical dissymmetry is studied by finite element simulations. Therefore, a unique chiral supramolecular organization that is responsible for the excellent chiroptical response and anisotropic electronic properties is developed, which not only enables the construction of high-performance CPL detection devices but also allows a better understanding of the structure-property relationships in chiral organic optoelectronics.

Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

Revealing the Sole Impact of Acceptor's Molecular Conformation to Energy Loss and Device Performance of Organic Solar Cells through Positional Isomers

Two new fused-ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m-BAIDIC and p-BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p-BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p-BAIDIC-based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open-circuit voltage (V_OC ) and short-current current density (J_SC ) of the devices. Therefore, the linear FREA, p-BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m-BAIDIC. When p-BAIDIC is incorporated into the binary PM6/BO-4Cl system to form a ternary system, synergistic enhancements in V_OC , J_SC , fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

The introduction of the IDIC/ITIC families of non-fullerene acceptors has boosted the photovoltaic performances of bulk-heterojunction organic solar cells. The fine tuning of the photophysical, morphological and processability properties with the aim of reaching higher and higher photocurrent efficiencies has prompted uninterrupted worldwide research on these peculiar families of organic compounds. The main strategies for the modification of IDIC/ITIC compounds, described in several contributions published in the past few years, can be summarized and classified into core modification strategies and end-capping group modification strategies. In this review, we analyze the more recent advances in this field (last two years), and we focus our attention on the molecular design proposed to increase photovoltaic performance with the aim of rationalizing the general properties of these families of non-fullerene acceptors.

High-Performance Fluorinated Fused-Ring Electron Acceptor with 3D Stacking and Exciton/Charge Transport

A new fluorinated electron acceptor (FINIC) based on 6,6,12,12-tetrakis(3-fluoro-4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-donating central core and 5,6-difluoro-3-(1,1-dicyanomethylene)-1-indanone as the electron-deficient end groups is rationally designed and synthesized. FINIC shows similar absorption profile in dilute solution to the nonfluorinated analogue INIC. However, compared with INIC, FINIC film shows red-shifted absorption, down-shifted frontier molecular orbital energy levels, enhanced crystallinity, and more ordered molecular packing. Single-crystal structure data show that FINIC molecules pack into closer 3D "network" motif through H-bonding and π-π interaction, while INIC molecules pack into incompact "honeycomb" motif through only π-π stacking. Theoretical calculations reveal that FINIC has stronger electronic coupling and more molecular interactions than INIC. FINIC has higher electron mobilities in both horizontal and vertical directions than INIC. Moreover, FINIC and INIC support efficient 3D exciton transport. PBD-SF/FINIC blend has a larger driving force for exciton splitting, more efficient charge transfer and photoinduced charge generation. Finally, the organic solar cells based on PBD-SF/FINIC blend yield power conversion efficiency of 14.0%, far exceeding that of the PBD-SF/INIC-based devices (5.1%).

Hidden Structure Ordering Along Backbone of Fused-Ring Electron Acceptors Enhanced by Ternary Bulk Heterojunction

Fused-ring electron acceptors (FREAs), as a family of non-fullerene (NF) acceptors, have achieved tremendous success in pushing the power conversion efficiency of organic solar cells. Here, the detailed molecular packing motifs of two extensively studied FREAs-ITIC and ITIC-Th are reported. It is revealed for the first time the long-range structure ordering along the backbone direction originated from favored end group π-π stacking. The backbone ordering could be significantly enhanced in the ternary film by the mutual mixing of ITIC and ITIC-Th, which gives rise to an improved in-plane electron mobility and better ternary device performance. The backbone ordering might be a common morphological feature of FREAs, providing explanations to previously observed small open circuit voltage loss and superior performance of FREA-based devices and guiding the future molecular design of high-performance NF acceptors.

Naphthodithiophene-Based Nonfullerene Acceptor for High-Performance Organic Photovoltaics: Effect of Extended Conjugation

Naphtho[1,2-b:5,6-b']dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: -3.78 eV vs IHIC2: -3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10^-3 cm² V^-1 s^-1 vs IHIC2: 5.0 × 10^-4 cm² V^-1 s^-1 ). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.

Fused Hexacyclic Nonfullerene Acceptor with Strong Near-Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency

A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 10⁵ m^-1 cm^-1 , a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10^-3 cm² V^-1 s^-1 . The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.

Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push-pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (J_SC ). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing J_SC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC₇₁ BM (PCE = 5.22%).