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The photovoltaic properties and energy loss of organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) are highly dependent on their molecular structures and morphologies.

Efficient 3D NFAs through central unit connection blaze a new trail in further molecular structural optimization of state-of-the-art NFAs.

The recent progress in non-fused ring electron acceptor-based organic solar cells has been reviewed from the perspective of material design strategies.

An acceptor FEH2C8-2Cl is designed by introducing the isomerized side chains and the corresponding photovoltaic module with the active layer area 25 cm2 yields an efficiency of 11.71%.

Optical communication is a particularly compelling technology to tackle the speed and capacity bottlenecks in data communication in modern society. Currently, silicon photodetector plays a dominant role in the high-speed optical communication across the visible-near infrared spectrum. However, its intrinsic rigid structure, high working bias, and low responsivity largely limit its applications in the next-generation flexible optoelectronic devices. Herein, we report a solution-processed flexible organic photodetector (OPD) based on a narrow-bandgap nonfullerene acceptors, which exhibits a remarkable response time of 91 ns and peaking responsivity of 0.53 A W-1 (λ = 830 nm) at zero bias, exceeding the values obtained in the state-of-the-art high-speed commercial Si photodetector (326 ns; 0.26 A W-1). This exceptional performance benefits from the low parasitic capacitance and high charge mobility due to low trap states and energetic disorder in the organic photoactive film. More significantly, the flexible OPD exhibits negligible performance attenuation (

Three acceptor–donor–acceptor type fused-ring non-fullerene acceptors of FOR-IN, FOR-1F and FOR-2F, were synthesized with the same pyran-composed backbone but different terminals, affording over 15% efficiency in organic solar cells.

A low band gap acceptor F-ThBr is designed for the front cell and CH1007 is selected for the rear cell. Using the two acceptors, a tandem OSC is fabricated and offers an efficiency of 18.67%.

A series of acceptor–donor–acceptor type non-fullerene acceptors (NFAs), FCn-2Cl, with gradient substitution lengths of side chains (SCs) on molecular backbones, have been designed and synthesized for all-small-molecule organic solar cells (ASM-OSCs).

It is a great challenge to simultaneously improve the short-circuit current density (Jsc) and open-circuit voltage (Voc) of organic solar cells (OSCs) owing to the trade-off effect between the two photovoltaic parameters. Delicate chemical structure modulation of the active layer materials is always one of the effective strategies to address this issue. In this work, following a simple and efficient strategy through fine-tuning the molecular configuration in combination with the side chain modulation to address the issue, three non-fullerene acceptors (NFAs), 5T-2C8-IN, 5T-2C8-Cl and 5T-2C8-2Cl with a five-thiophene (5T) fused asymmetric molecular backbone and octyl side chains at the terminal position of the molecular backbone, have been designed and synthesized. Among them, the 5T-2C8-2Cl based photovoltaic device showed a power conversion efficiency (PCE) of 13.02% with a simultaneously improved Voc of 0.802 V and a Jsc of 24.97 mA cm−2 compared with the device of the control acceptor 6T-2C8-2Cl with a PCE of 12.43%, a Voc of 0.785 V and a Jsc of 24.40 mA cm−2.

An asymmetric acceptor NQF is designed to minimize the nonradiative loss. Using this acceptor, a binary OSC is fabricated and offers an efficiency of 17.57% with a low nonradiative energy loss (ΔEnr) of 0.177 eV.

Suppressing intramolecular vibration of non-fullerene acceptors (NFAs) by molecular rigidification has been proven to be an effective way to reduce the non-radiative recombination loss and energetic disorder of organic solar cells (OSCs). Thus far, extensive attention has been drawn on rigidifying the fused-ring backbones of NFAs, whereas the highly flexible alkyl side chains are barely concerned. Herein, an effective strategy of side chain rigidification by introducing a spiro-ring is developed for the first time and applied to construct the NFA of Spiro-F. Compared to its counterpart F-2F, the rigid spirocyclic side chain can constrain the vibrational-rotational motion and control the orientation of two highly flexible

Six dithieno[3,2-b:2′,3′-d]pyrrole (DTP)-based asymmetric non-fullerene acceptors (NFAs) were designed and synthesized to investigate the effects of DTP linked by gradient alkyl chains on the performance of organic solar cells.

Three isomeric acceptors FOM-1, FOM-2 and FOM-3 incorporating the fluorene center are designed and synthesized.

A new synthetic route to realize efficient screening of aryl side chains on the Y6 skeleton was reported, and then three acceptors with different side chains were synthesized. Among them, the BTP-OC6-based device exhibits the best PCE and stability.

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed.

A ternary strategy is recognized as a promising approach that enjoys both the simplicity of fabrication conditions and potential to improve performance in organic solar cells. Herein, a C-shaped narrow band gap non-fullerene acceptor GL1 with a C2v symmetry based on a new core was designed and synthesized. A power conversion efficiency (PCE) of 11.43% was achieved by employing PBDB-T:GL1 as an active layer to fabricate photovoltaic devices. To further promote photovoltaic performance, following a similar-structure prescreen principle, a middle band gap acceptor F-2Cl with the same backbone shape, side-chain distribution, and dipole moment orientation as GL1 was introduced as the guest acceptor into the active layer. Thus, benefiting from the collaboration of complementary absorption, cascade energy levels, and well-modified microstructure of the active layer, a 13.17% PCE was obtained with concurrently elevated Jsc, fill factor, and stability for the optimized ternary device. This work presents a successful example of prescreening the third component to simplify the workload for a high-performance ternary device.

Halogenation of terminal of acceptors has been shown to give dramatic improvements in power conversion efficiencies (PCEs) of organic solar cells (OSCs). Similar significant results could be expected from the halogenation of the central units of state-of-the-art Y-series acceptors. Herein, a pair of acceptors, termed CH6 and CH4, featuring a conjugation-extended phenazine central unit with and without fluorination, have been synthesized. The fluorinated CH6 has enhanced molecular interactions and crystallinity, superior fibrillar network morphology and improved charge generation and transport in blend films, thus affording a higher PCE of 18.33 % for CH6-based binary OSCs compared to 16.49 % for the non-fluorinated CH4. The new central site offers further opportunities for structural optimization of Y-series molecules to afford better-performed OSCs and reveals the effectiveness of fluorination on central units.

Despite much progress in organic solar cells (OSCs), higher efficiency is still the most desirable goal and can indeed be obtained through rational design of active layer materials and device optimization according to the theoretical prediction. Herein, under the guidance of a semi-empirical model, two new non-fullerene small molecule acceptors (NF-SMAs) with an acceptor-donor-acceptor (A-D-A) architecture, namely, 6T-OFIC and 5T-OFIC, have been designed and synthesized. 6T-OFIC exhibits wider absorption spectrum and a red-shifted absorption onset (λonset) of 946 nm due to its extended conjugation central unit. In contrast, 5T-OFIC with five-thiophene-fused backbone has an absorption with the λonset of 927 nm, which is closer to the predicted absorption range for the best single junction cells based on the semi-empirical model. Consequently, the device based on 5T-OFIC yields a higher power conversion efficiency (PCE) of 13.43% compared with 12.35% of the 6T-OFIC-based device. Furthermore, an impressive PCE of 15.45% was achieved for the 5T-OFIC-based device when using F-2Cl as the third component. 5T-OFIC offers one of a few acceptor cases with efficiencies over 15% other than Y6 derivatives.

Although organic solar cells (OSCs) have delivered an impressive power conversion efficiency (PCE) of over 19 %, most of them demonstrated rather limited stability. So far, there are hardly any effective and universal strategies to improve stability of state-of-the-art OSCs. Herein, we developed a hybrid electron-transport layer (ETL) in inverted OSCs using ZnO and a new modifying agent (NMA), and significantly improved the stability and PCEs for all the tested devices. In particular, when applied in the D18 : N3 system, its inverted OSC exhibits so far the highest PCE (18.20 %) among inverted single-junction OSCs, demonstrating an extrapolated T

The dominated hole transport material (HTM) used in state-of-the-art perovskite solar cells (PSCs) is Spiro-OMeTAD, which needs to be doped to improve its conductivity and mobility. The inevitable instability induced by deliquescent dopants and the necessary oxidation process in air hinders the commercialization of this technology. Here, an alloy strategy using two conjugated polymers with highly similar structures but different crystallinities for dopant-free HTM and high-performance PSCs has been demonstrated. We found that the polymeric packing and crystallinity of a polymer alloy could be regulated finely by blending the polymer PM6 and our developed polymer PMSe, which exhibits a shorter π-π stacking distance due to the improved planarity of the polymer backbone with strong C═O···Se noncovalent interactions. The structure-property relationship of the polymer alloy is investigated by theoretical and experimental analyses. The optimized PSCs using the polymer alloy HTM without any ionic dopants feature an excellent power conversion efficiency of 24.53% and a high open circuit voltage (

Ternary organic photovoltaic (OPV) strategy is an effective but facile approach to enhance the photovoltaic performance for single-junction devices. Herein, a series of ternary OPVs were fabricated by employing a wide bandgap donor (PBDB-TF) and two acceptor-donor-acceptor (A-D-A)-type nonfullerene small molecule acceptors (NF-SMAs, called F-2Cl and 3TT-OCIC). As the third component, the near-infrared SMA, 3TT-OCIC, has complementary absorption spectrum, narrow bandgap and well-compatible crystallization property to the host acceptor (F-2Cl) for efficient ternary OPVs. With these, the optimal ternary devices yield significantly enhanced power conversion efficiency of 15.23%, one of the very few examples with PCE higher than 15% other than Y6 systems. This is mainly attributed to the increased short-circuit current density of 24.92 mA cm−2 and dramatically decreased energy loss of 0.53 eV. This work presents a successful example for simultaneously improving current, minimizing energy loss and together with modifying the morphology of active layers in OPVs, which will contribute to the further construction of high performance ternary OPVs.

Two-dimensional (2D) covalent organic frameworks (COFs) with periodic functional π-electron systems are an emerging class of optoelectronic materials. However, almost all conjugated COFs so far are insoluble and hard to process, which hampers severely their optoelectronic applications. Here, a solution-processable, nanoscale and sp2 carbon-conjugated COF-like material, PDPP-C20 was successfully designed and synthesized. The solution-processed PDPP-C20 films exhibit high crystallinity and excellent charge transport properties along out-of-plane directions, combined with the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of −5.36/−3.75 eV, making PDPP-C20 suitable for electronic device applications. An efficiency as high as 21.92% has been demonstrated when it was used as a functional interfacial layer in perovskite solar cells, coupled with dramatically improved stability in comparison with the control device due to the superior hydrophobicity of PDPP-C20 layer as well as its passivation effect on perovskite surface. Furthermore, the soluble PDPP-C20 could also be used as donor in bulk-heterojunction organic solar cells and an initial efficiency of 2.46% has been achieved. These results indicate that this new class of soluble and nanoscale COF-like materials should offer a new arena of functional materials for optoelectronic devices.

A new class of polymeric hole-transport materials (HTMs) are explored by inserting a two-dimensionally conjugated fluoro-substituted pyrene into thiophene and selenophene polymeric chains. The broad conjugated plane of pyrene and "Lewis soft" selenium atoms not only enhance the π-π stacking of HTM molecules greatly but also render a strong interaction with the perovskite surface, leading to an efficient charge transport/transfer in both the HTM layer and the perovskite/HTM interface. Note that fluorine substitution adjacent to pyrene boosts the stacking of HTMs towards a more favorable face-on orientation, further facilitating the efficient charge transport. As a result, perovskite solar cells (PSCs) employing PE10 as dopant-free HTM afford an excellent efficiency of 22.3 % and the dramatically enhanced device longevity, qualifying it among the best PSCs based on dopant-free HTMs.

Molecule engineering has been demonstrated as a valid strategy to adjust the active layer morphology in all-small-molecule organic solar cells (ASM-OSCs). In this work, two non-fullerene acceptors (NFAs), FO-2Cl and FO-EH-2Cl, with different alkyl side chains are reported and applied in ASC-OSCs. Compared with FO-2Cl, FO-EH-2Cl is designed by replacing the octyl alkyl chains with branched iso-octyl alkyl chains, leading to an enhanced molecular packing, crystallinity, and redshifted absorption. With a small molecule BSFTR as donor, the device of BSFTR:FO-EH-2Cl obtains a better morphology and achieves a higher power conversion efficiency (PCE) of 15.78% with a notable fill factor (FF) of 80.44% than that of the FO-2Cl-based device with a PCE of 15.27% and FF of 78.41%. To the authors' knowledge, the FF of 80.44% is the highest value in ASM-OSCs. These results demonstrate a good example of fine-tuning the molecular structure to achieve suitable active layer morphology with promising performance for ASM-OSCs, which can provide valuable insight into material design for high-efficiency ASM-OSCs.