Unraveling the Structure-Property-Performance Relationships of Fused-Ring Nonfullerene Acceptors: Toward a C-Shaped ortho-Benzodipyrrole-Based Acceptor for Highly Efficient Organic Photovoltaics

Highly Efficient Acceptors with a Nonaromatic Thianthrene Central Core for Organic Photovoltaics

Despite the great role in determining molecular packings and organic photovoltaic outcomes, very rare candidates could be employed as central cores in current high-performance acceptors except diimide-based moieties. Herein, a new type of central core of nonaromatic thianthrene is explored firstly, affording an exotic but structurally tailorable molecular platform for acceptor design. A unique puckered rather than planar conformation of central core is adopted, caused by the 4n πe- feature, great ring strain and largely the insufficient p-π orbital overlap of lone pair on sulfur of thianthrene and coterminous benzene planes. As a result, the absorption of thianthrene-based acceptors (CS1, CS2, and CS3) shows unexpected blue shift comparing to the phenazine-based counterpart (CH20), regardless of the intrinsically strong electron-donating characteristic of low valence sulfur atoms. Even so, the desired molecular packing and fibrillary film morphology, assisted by the suitable chlorination on thianthrene, still contribute to the best device efficiency of 19.0% based on D18:CS2 blends. Such novel work renders an underdeveloped NFA platform with the potentials for achieving PCE of over 20%.

Perfluorophenyl-Incorporated Ferrocene: A Non-Volatile Solid Additive for Boosting Efficiency and Stability in Organic Solar Cells

In this study, we designed and synthesized a new non-volatile solid additive FcF10 by integrating two pentafluorophenyl (C6F5) groups into the cyclopentadienyl (CP) rings of ferrocene (Fc) through ester linkages. The FcF10 with a three-dimensional (3D) framework facilitated morphological optimization in the PM6:Y6 system through a combination of π···π, F···π, and F···F interactions between the CP and C6F5 rings in FcF10 and the C6F2 rings in Y6. The FcF10-incorporated (3.75 wt %) PM6:Y6-based solar cell device achieved a higher power conversion efficiency (PCE) of 17.00%, with a Voc of 0.85 V, a Jsc of 27.35 mA cm-2, and an FF of 73.29%, compared to the pristine PM6:Y6 device. These improvements are attributed to the formation of a favorable active layer morphology, which enhances exciton dissociation and charge transport while reducing bimolecular and trap-assisted recombination. The FcF10 additive facilitates non-covalent interactions with Y6, such as F···F, F···π, and π···π interactions between the Cp and C6F5 rings in FcF10 and the FIC end groups in Y6. These supramolecular interactions improve molecular stacking and crystallinity within the Y6 domain, as evidenced by red-shifted Y6 absorption, reduced π-π stacking d-spacing, and increased coherence lengths of Y6. Furthermore, the PM6:Y6:FcF10 device demonstrates superior thermal stability, retaining 88% of its initial PCE after prolonged thermal annealing at 85 °C. Overall, the incorporation of FcF10 achieves an optimized and stable donor-acceptor morphology, offering a promising approach for high-performance and thermally stable organic photovoltaics.

O, S, and N Bridged Atoms Screening on 2D Conjugated Central Units of High-Performance Acceptors

Almost all of central cores in high-performance acceptors are limited to the electron-withdrawing diimide structure currently, which constrains further acceptor structural innovation greatly. Herein, oxygen (O), sulfur (S), and nitrogen (N) atoms are adopted to bridge the 2D conjugated central cores, yielding three acceptor platforms of CH─O, CH─S, and CH─N that differ in structure by only two atoms. Because of the characteristic atomic outer electron configuration and hybrid orbital orientation, O-, S-, and N-bridged central cores display quite different conformations and electronic properties, namely, dibenzodioxin (planar, non-aromatic), thianthrene (puckered, non-aromatic) and phenazine (planar, aromatic), respectively. A systematic investigation discloses how the central core, especially its p-π orbital overlap between lone pair on O/S/N and coterminous benzene planes, affect the intrinsic photoelectronic properties of acceptors for the first time. Finally, CH─N-based binary device affords the highest fill factor of 83.13% in organic photovoltaics along with a first-class efficiency of 20.23%. By evaluating the strictly controlled O-, S-, and N-bridged molecular platforms comprehensively, the work reveals the potential uniqueness of diimide in determining the excellent photovoltaic outcomes of acceptors.

Unveiling the Mysterious Structure Growth of 2D and 3D All-Inorganic Perovskite Nanocrystals in Solution Phase Dynamically by Using Small-Wide Angle X-Ray Scattering Spectroscopy

Real-time analysis of the structural formation of 2D and 3D perovskites in solution is challenging due to the sensitivity of perovskite intermediates to environmental conditions and their rapid growth. Conventional techniques often require stringent sample preparation, limiting the ability to study dynamic behaviors in solution. In this study, small- and wide-angle X-ray scattering (SWAXS) is employed to analyze the morphology and dynamics of 2D and 3D perovskite nanostructures in their native colloidal state. Unlike previous studies that attribute CsPbI3 degradation to delta-phase formation, SWAXS revealed preexisting 2D Cs7Pb6I19 nanosheets in pristine CsPbI3 colloidal solutions. In situ SWAXS tracked the dynamic transformation of these structures during recrystallization in diluted solutions. Adding bis(trimethylsilyl)sulfide (TMS) disassembled the 2D nanosheets, while subsequent recrystallization in a poor solvent formed highly crystalline Cs7Pb6I19 nanosheets. The recrystallization dynamics aligned with crystal growth theory, with TMS concentration playing a critical role. Higher TMS concentrations slowed recrystallization, promoting stable lattice formation and enhanced crystallinity, resulting in bright yellow emission. Conversely, lower concentrations accelerated recrystallization, causing structural damage and limiting high-crystallinity growth. This study highlights the importance of controlling recrystallization rates through TMS concentration to optimize the crystallinity and optoelectronic properties of perovskites, offering insights into improving their performance.

Perovskite-Coupled NIR Organic Hybrid Solar Cells Achieving an 84.2% Fill Factor and a 25.2% Efficiency: A Comprehensive Mechanistic Exploration

The integration of organic dyes in perovskite solar cells (PSCs) to utilize near-infrared (NIR) photons remains a challenge. In this study, a selenium-incorporated ortho-benzodipyrrole-based NIR dye CB-2Se was developed. CB-2Se, featuring a lower bandgap of 1.35 eV, was blended with PCBM to form a bulk-heterojunction layer in PSCs for electron extraction and transport. Compared to Y6-16 acceptor, the removal of the Tz unit in CB-2Se suppresses self-aggregation, improving its compatibility with PCBM. A CB-2Se:PCBM-incorporated PSC achieved a remarkable power conversion efficiency (PCE) of 25.18% with a VOC of 1.164 V, a JSC of 25.71 mA/cm2, a Fill Factor of 84.15%, outperforming that of the PCBM-only reference device (24.35%) and the PCBM:Y6-16-based device (24.49%). The PCBM:CB-2Se layer enhanced the long-term stability of PSCs, retaining 88% of its initial efficiency after 1000 h under ambient air and thermal conditions. The photophysical interactions between NIR dyes and PSCs have been comprehensively investigated by using femtosecond transient absorption spectroscopy. Ultrafast exciton separation into free charges occurs within 200 femtoseconds at the interfaces between PCBM and CB-2Se. For the first time, a transfer of holes from CB-2Se back to the perovskite was detected, providing valuable insights into the charge dynamics of PSCs utilizing NIR dyes.

Molecular Geometry-Property Relationship of Benzodipyrrole-Based A-DA'D-A Type Acceptors for High-Performance Organic Solar Cells

Molecular geometry plays a crucial role in determining the physical and chemical properties of organic semiconductor materials. However, there is little research on the molecular geometry-property relationship of A-DA'D-A type small molecule acceptors (SMAs), which are the most representative organic semiconductor materials used in organic solar cells (OSCs). In this work, we used dichlorine-substituted benzene as the A' unit to overcome the geometry constraints imposed by the conventional A' unit that has a large size. Four isomers of benzodipyrrole-based SMAs (C-Cl46-Cl, Ɂ-Cl46-Cl, M-Cl46-Cl, and S-Cl46-Cl) with C-, Ɂ-, M-, and S-shaped molecular geometries were synthesized, and the effect of molecular geometry on their photovoltaic performance was studied. We revealed that the molecular geometry influences the physicochemical and photovoltaic properties in three aspects: 1) intrinsic physicochemical properties, including energy levels, absorption, and reorganization energy; 2) molecular stacking pattern, which governs the exciton diffusion and charge transport processes; and 3) donor-acceptor interaction and miscibility. We found that the C-shaped molecular geometry possesses a suitable energy level and absorption range, dense and ordered molecular stacking, and improved donor-acceptor interaction and miscibility. These advantages enable a record-high power conversion efficiency (PCE) of 19.94% (certified as 19.54%) for the binary OSCs based on D18:C-Cl46-Cl active layer. The other SMAs showed weaknesses in different aspects, such as limited absorption of Ɂ-shaped SMA, large reorganization energies and loose molecular stacking of M-shaped SMA, low solubility and strong aggregation of S-shaped SMA. These properties resulted in inferior photovoltaic device performance. These findings on the molecular geometry-property relationships will be instructive for future molecular design of high-performance A-DA'D-A type SMAs.

Electron-Rich Heptacyclic S,N Heteroacene Enabling C-Shaped A-D-A-type Electron Acceptors With Photoelectric Response beyond 1000 Nm for Highly Sensitive Near-Infrared Photodetectors

A highly electron-rich S,N heteroacene building block is developed and condensed with FIC and Cl-IC acceptors to furnish CT-F and CT-Cl, which exhibit near-infrared (NIR) absorption beyond 1000 nm. The C-shaped CT-F and CT-Cl self-assemble into a highly ordered 3D intermolecular packing network via multiple π-π interactions in the single crystal structures. The CT-F-based organic photovoltaic (OPV) achieved an impressive efficiency of 14.30% with a broad external quantum efficiency response extending from the UV-vis to the NIR (300-1050 nm) regions, outperforming most binary OPVs employing NIR A-D-A-type acceptors. CT-Cl possesses a higher surface energy than CT-F, promoting vertical phase segregation and resulting in its preferential accumulation near the bottom interface of the blend. This arrangement, combined with the lower HOMO energy level of CT-Cl, effectively reduces undesired hole and electron injection under reverse voltage. The PM6:CT-Cl-based organic photodetectors (OPDs) devices achieved an ultra-high shot-noise-limited specific detectivity (Dsh*) values exceeding 1014 Jones in the NIR region from 620 to 1000 nm, reaching an unprecedentedly high value of 1.3 × 1014 Jones at 950 nm. When utilizing a 780 nm light source, the PM6:CT-Cl-based OPDs show record-high rise/fall times of 0.33/0.11 µs and an exceptional cut-off frequency (f-3dB) of 590 kHz at -1 V.

Fluorinated and methylated ortho-benzodipyrrole-based acceptors suppressing charge recombination and minimizing energy loss in organic photovoltaics

The elimination of the A' unit from -type Y6-derivatives has led to the development of a new class of ortho-benzodipyrrole (o-BDP)-based A-DNBND-A-type NFAs. In this work, two new A-DNBND-A-type NFAs, denoted as CFB and CMB, are designed and synthesized, where electron-withdrawing fluorine atoms and electron-donating methyl groups are substituted on the benzene ring of the o-BDP moiety, respectively. CFB exhibits a blue-shifted absorption spectrum, stronger intermolecular interactions, shorter π-π stacking distances, and more ordered 3D intermolecular packing in the neat and blend films, enabling it to effectively suppress charge recombination in the PM6:CFB device showing a higher PCE of 16.55% with an FF of 77.45%. CMB displays a higher HOMO/LUMO energy level, a smaller optical bandgap, and a less ordered 3D packing, which contributes to its superior ability to suppress energy loss in the PM6:CMB device with a high V oc of 0.90 V and a PCE of 16.46%. To leverage the advantages of CFB and CMB, ternary PM6:Y6-16:CFB and PM6:Y6-16:CMB devices are fabricated. The PM6:Y6-16:CFB device exhibits the highest PCE of 17.83% with an increased V oc of 0.86 V and a J sc of 27.32 mA cm-2, while the PM6:Y6-16:CMB device displayed an elevated V oc of 0.87 V and an improved FF of 74.71%, leading to a PCE of 17.44%. The high PCE was achieved using the non-halogenated greener solvent o-xylene, highlighting their potential for facilitating more eco-friendly processing procedures. C-shaped disubstituted o-BDP-based A-D-A type acceptors open up new avenues for tailoring electronic properties and molecular self-assembly, achieving higher OPV performance with enhanced charge recombination suppression and reduced energy loss.

Suppressing Exciton-Vibration Coupling via Intramolecular Noncovalent Interactions for Low-Energy-Loss Organic Solar Cells

Minimizing energy loss is crucial for breaking through the efficiency bottleneck of organic solar cells (OSCs). The main mechanism of energy loss can be attributed to non-radiative recombination energy loss (ΔEnr) that occurs due to exciton-vibration coupling. To tackle this challenge, tuning intramolecular noncovalent interactions is strategically utilized to tailor novel fused ring electron acceptors (FREAs). Upon comprehensive analysis of both theoretical and experimental results, this approach can effectively enhance molecular rigidity, suppress structural relaxation, reduce exciton reorganization energy, and weakens exciton-vibration coupling strength. Consequently, the binary OSC device based on Y-SeSe, which features dual strong intramolecular Se ⋅ ⋅ ⋅ O noncovalent interactions, achieves an outstanding power conversion efficiency (PCE) of 19.49 %, accompanied by an extremely small ΔEnr of 0.184 eV, much lower than those of Y-SS and Y-SSe based devices with weaker intramolecular noncovalent interactions. These achievements not only set an efficiency record for selenium-containing OSCs, but also mark the lowest reported ΔEnr value among high-performance binary devices. Furthermore, the ternary blend device showcases a remarkable PCE of 20.51 %, one of the highest PCEs for single-junction OSCs. This work demonstrates the effectiveness of intramolecular noncovalent interactions in suppressing exciton-vibration coupling, thereby achieving low-energy-loss and high-efficiency OSCs.

Rebuilding Peripheral F, Cl, Br Footprints on Acceptors Enables Binary Organic Photovoltaic Efficiency Exceeding 19.7

Given homomorphic fluorine (F), chlorine (Cl) and bromine (Br) atoms are featured with gradually enlarged polarizability/atomic radius but decreased electronegativity, the rational screen of halogen species and locations on small molecular acceptors (SMAs) is quite essential for acquiring desirable molecular packing to boost efficiency of organic solar cells (OSCs). Herein, three isomeric SMAs (CH-F, CH-C and CH-B) are constructed by delicately rebuilding peripheral F, Cl, Br footprints on both central and end units. Such a re-permutation of peripheral halogens could not only maintain the structural symmetry of SMAs to the maximum, but also acquire extra asymmetric benefits of enhanced dipole moment and intramolecular charge transfer, etc. Moreover, central brominating enhances molecular crystallinity of CH-B without introducing undesirable steric hindrance on end groups, thus rendering a better balance between high crystallization and domain size control in PM6:CH-B blend. Further benefitting from the large dielectric constant, small exciton binding energy, optimized molecular packing and great electron transfer integral, CH-B affords the first class binary OSC efficiency of 19.78 %, moreover, the highest efficiency of 18.35 % thus far when increasing active layer thickness to ~300 nm. Our successful screening in rebuilding peripheral halogen footprints provides the valuable insight into further rational design of SMAs for record-breaking OSCs.

Platinum-Acetylide Complexes as Nonfullerene Acceptors in Organic Photovoltaic Systems

Three distinct n-type semiconductors were derived from a platinum-trialkyl phosphine complex; to lower their LUMO levels, various indene derivatives were incorporated using thiophene (PtTIC (1)), thieno[3,2-b]thiophene (PtT2IC (2)), and 4H-cyclopenta[2,1-b:3,4-b']dithiophene (PtCDTIC (3)) as the acetylide donor units. Single-crystal X-ray diffractometry analysis revealed translinear platinum-acetylide complexation in all cases. The strong (═O···S) interactions between the oxygen atoms of the indene acceptor units and the sulfur atoms of the thiophene-derived donor units induced a highly planar orientation among the heterocyclic ligands, featuring π-π interactions between the planes. Platinum complexes 1, 2, and 3 exhibited strong absorption in the 500-800 nm range, resulting from efficient intramolecular charge transfer transitions from the central platinum-containing donor unit to the terminal acceptors, as well as unique emission in the near-infrared region owing to the heavy-atom effect. The ultraviolet photoelectron spectroscopy results indicated that the LUMO levels were comparable to those of typical nonfullerene acceptors (NFAs). The n-type semiconductors comprising 1, 2, and 3 as NFAs exhibited photoelectric conversion properties in the corresponding organic photovoltaics. The highest conversion efficiency (3.0%) was attained by complex 3.

Modulating phase segregation during spin-casting of fullerene-based polymer solar-cell thin films upon minor addition of a high-boiling co-solvent

The impact of additives on the nanoscale structures of spin-cast polymer composite films, particularly in polymer solar cells, is a topic of significant interest. This study focuses on the blend film comprising poly(thieno[3,4-b]thio-phene-alt-benzodi-thio-phene) (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), exploring how additives like 1,8-di-iodo-octane (DIO) influence the film structures spin-cast from chloro-benzene solution. Combined results of specular X-ray and neutron reflectivity, grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS), and X-ray photoelectron spectroscopy indicate that DIO could significantly enhance the dispersion of PC71BM and reduce composition inhomogeneity in the film. Time-resolved GISAXS-GIWAXS with 100 ms resolution further captures a rapid spinodal decomposition of the mixture within 1 s in the constant-evaporation stage of spin-casting. Further combined with parallel analysis of time-resolved UV-Vis reflectance, these findings reveal that DIO mitigates the spinodal decomposition process by accelerating solvent evaporation, which, in turn, decelerates phase segregation, leading to a nucleation-driven process. These observations provide mechanistic insights into the role of additives in controlling the nanostructural evolution of spin-cast films by altering the kinetics of solvent evaporation and phase separation during the spin-coating process.

Nitrogen-Bridged Fused-Ring Nonacyclic and Heptacyclic A-D-A Acceptors for Organic Photovoltaics

In this work, we designed two nitrogen-bridged fluorene-based heptacyclic FNT and nonacyclic FNTT ladder-type structures, which were constructed by one-pot palladium-catalyzed Buchwald-Hartwig amination. FNT and FNTT were further end-capped by FIC acceptors to form two FNT-FIC and FNTT-FIC non-fullerene acceptors (NFAs), respectively. The two NFAs exhibit more red-shifted absorption and higher crystallinity compared to those of the corresponding carbon-bridged FCT-FIC and FCTT-FIC counterparts. Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements reveal that the 2-butyloctyl groups on the nitrogen in the convex region of FNT-FIC interdigitate with the dioctyl groups on the fluorene in the concave region of another FNT-FIC, resulting in a lamellar packing structure with a d spacing of 13.27 Å. As a consequence, the PM6:FNT-FIC (1:1 wt %) device achieved a power conversion efficiency (PCE) of only 6.60%, primarily due to the highly crystalline nature of FNT-FIC, which induced significant phase separation between PM6 and FNT-FIC in the blended film. However, FNTT-FIC, featuring 2-butyloctyl groups positioned on the nitrogen within the concave region of its curved skeleton, exhibits improved donor-acceptor miscibility, thereby promoting a more favorable morphology. As a result, the PM6:FNTT-FIC (1:1.2 wt %) device exhibited a higher PCE of 12.15% with an exceptional Voc of 0.96 V. This research demonstrates that placing alkylamino moieties within the concave region of curved A-D-A NFAs leads to a better molecular design.

Unraveling the Solution Aggregation Structures and Processing Resiliency of High-Efficiency Organic Photovoltaic Blends

The solution aggregation structure of conjugated polymers is crucial to the morphology and resultant optoelectronic properties of organic electronics and is of considerable interest in the field. Precise characterizations of the solution aggregation structures of organic photovoltaic (OPV) blends and their temperature-dependent variations remain challenging. In this work, the temperature-dependent solution aggregation structures of three representative high-efficiency OPV blends using small-angle X-ray/neutron scattering are systematically probed. Three cases of solution processing resiliency are elucidated in state-of-the-art OPV blends. The exceptional processing resiliency of high-efficiency PBQx-TF blends can be attributed to the minimal changes in the multiscale solution aggregation structure at elevated temperatures. Importantly, a new parameter, the percentage of acceptors distributed within polymer aggregates (Ф), for the first time in OPV blend solution, establishes a direct correlation between Ф and performance is quantified. The device performance is well correlated with the Kuhn length of the cylinder related to polymer aggregates L1 at the small scale and the Ф at the large scale. Optimal device performance is achieved with L1 at ≈30 nm and Ф within the range of 60 ± 5%. This study represents a significant advancement in the aggregation structure research of organic electronics.

Optimizing Molecular Packing via Steric Hindrance for Reducing Non-Radiative Recombination in Organic Solar Cells

Innovative molecule design strategy holds promise for the development of next-generation acceptor materials for efficient organic solar cells with low non-radiative energy loss (ΔEnr). In this study, we designed and prepared three novel acceptors, namely BTP-Biso, BTP-Bme and BTP-B, with sterically structured triisopropylbenzene, trimethylbenzene and benzene as side chains inserted into the shoulder of the central core. The progressively enlarged steric hindrance from BTP-B to BTP-Bme and BTP-Biso induces suppressed intramolecular rotation and altered the molecule packing mode in their aggregation states, leading to significant changes in absorption spectra and energy levels. By regulating the intermolecular π-π interactions, BTP-Bme possesses relatively reduced non-radiative recombination rate and extended exciton diffusion lengths. The binary device based on PB2 : BTP-Bme exhibits an impressive power conversion efficiency (PCE) of 18.5 % with a low ΔEnr of 0.19 eV. Furthermore, the ternary device comprising PB2 : PBDB-TF : BTP-Bme achieves an outstanding PCE of 19.3 %. The molecule design strategy in this study proposed new perspectives for developing high-performance acceptors with low ΔEnr in OSCs.

Cyclization Engineering of Electron-Deficient Maleimide Unit for Nonfused Ring Electron Acceptors Enables Efficient Organic Solar Cells

Nonfused ring electron acceptors (NFREAs) have emerged as promising materials for commercial applications in organic solar cells due to their straightforward synthesis process and cost-effectiveness. The rational design of their structural frameworks is crucial for enhancing device efficiency. In this study, we explore the use of maleimide and thiophene as key building blocks, employing cyclization engineering techniques. Additionally, cyclopentanedithiophene was chosen as the bridging unit, coupled with fluorinated terminals, to fabricate NFREAs, namely, PI-DTS and DPI-DTS. DPI-DTS demonstrated superior molecular planarity and an upshifted lowest unoccupied molecular orbital energy level. Moreover, DPI-DTS-based blend films display enhanced π-π interactions and crystallinity, alongside a predominantly face-on orientation. Consequently, DPI-DTS-based devices displayed enhanced and more balanced carrier mobility, reduced bimolecular recombination, and trap-assisted recombination, leading to improved charge transfer efficiency. Ultimately, this led to an excellent efficiency of 10.48%, with an open-circuit voltage as high as 0.914 V. These findings highlight the significant promise of aromatic imides in constructing NFREAs, and the established structure-performance relationship provides a theoretical basis for the design of high performance NFREAs.

Instantaneous Marcus theory for photoinduced charge transfer dynamics in multistate harmonic model systems

Modeling the dynamics of photoinduced charge transfer (CT) in condensed phases presents challenges due to complicated many-body interactions and the quantum nature of electronic transitions. While traditional Marcus theory is a robust method for calculating CT rate constants between electronic states, it cannot account for the nonequilibrium effects arising from the initial nuclear state preparation. In this study, we employ the instantaneous Marcus theory (IMT) to simulate photoinduced CT dynamics. IMT incorporates nonequilibrium structural relaxation following a vertical photoexcitation from the equilibrated ground state, yielding a time-dependent rate coefficient. The multistate harmonic (MSH) model Hamiltonian characterizes an organic photovoltaic carotenoid-porphyrin-fullerene triad dissolved in explicit tetrahydrofuran solvent, constructed by mapping all-atom inputs from molecular dynamics simulations. Our calculations reveal that the electronic population dynamics of the MSH models obtained with IMT agree with the more accurate quantum-mechanical nonequilibrium Fermi's golden rule. This alignment suggests that IMT provides a practical approach to understanding nonadiabatic CT dynamics in condensed-phase systems.

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.