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Ultra-narrow bandgap (ultra-NBG) small molecule acceptors (SMAs) show great potential in organic solar cells (OSCs) due to the extend-ed near-infrared (NIR) absorption. In this work, a synergetic alkoxy side-chain and chlorine-contained end group strategy is employed to achieve A-DA’D-A type ultra-NBG SMAs by introducing alkoxy chains with oxygen atom at the second position into the thiophene β posi-tion as well as replacing the F atoms with Cl atoms in the end group. As a result, the heptacyclic BZO-4F shows a redshifted absorption onset (960 nm) than Y11 (932 nm) without oxygen atoms in the side chains. Then, the fluorinated end groups are substituted with the chlorinated ones to synthesize BZO-4Cl. The absorption onset of BZO-4Cl is further redshifted to 990 nm, corresponding to an optical ultra-NBG of 1.25 eV. When blending with the polymer donor PBDB-T, the binary devices based on PBDB-T: BZO-4F and PBDB-T: BZO-4Cl delivers power conversion efficiencies (PCEs) over 12%. Furthermore, ternary devices with the addition of BZ4F-O-1 into PBDB-T: BZO-4Cl system achieve the optimal PCE of 15.51%. This work proposes a synergetic alkoxy side-chain and chlorine-contained end group strategy to achieve A-DA’D-A type ultra-NBG SMAs, which is important for future molecular design.

Balancing charge generation and low energy loss (Eloss), especially in the wide spectral absorption region is critical to obtain high-performance organic photovoltaics (OPVs). Therefore, Y11-M and Y11-EB are designed and synthesized through modifying alkyl chains on different nitrogen aromatic rings of the reported non-fullerene acceptor Y11. Although all the molecules have almost similar low band-gap (around 1.30 eV), Y11-M and Y11-EB exhibit wider absorption in 410–870 nm region. Eventually, the conventional devices based on Y11-M and Y11-EB possess more efficient charge generation with low Eloss (around 0.44 eV). In addition, outstanding efficiencies of 16.64% and 17.15% with the fill factor of 76.15% and 74.73% are obtained in PM6:Y11-M and PM6:Y11-EB-based devices, both higher than Y11:PM6. The results highlight the importance of rational alkyl chains optimization, and a good structure-property relationship is established as well.

Developing high-performance and low-cost donor/acceptor materials is crucial for the industrialization of organic solar cells (OSCs).

Optimizing the molecular structures of organic solar cell (OSC) materials and boosting the power conversion efficiencies are the eternal theme in the solar energy region. A series of fused benzotriazole (BTA)-based A-DA'D-A structures of nonfullerene acceptors (such as Y18) were developed for application in efficient OSCs, in which high quantum efficiencies and low voltage losses could be achieved because of the optimized electron-deficient core and specific molecular geometry. Here, based on the BTA core, the bulky alkyl chain on the BTA unit was further tailored to minimize the lateral alkyl chains and enhance the crystallinity while maintaining an adequate solubility. The resulting NFAs of BTA-C1, BTA-C5, and BTA-C6 are synthesized. Compared with the well-designed molecular Y18 (BTA-C8), we found that simply replacing the 2-ethylhexyl chain with a single methyl (BTA-C1) can easily improve the fill factor up to 77%, but its poor light absorption capacity and large domain size impeded further efficiency improvement. In particular, the BTA-C5, with a shortened branch alkyl chain of 2-methylbutyl, achieves suitable solubility and enhanced crystallinity. Significantly, owing to the balanced charge carrier mobility and suitable phase separation, the BTA-C5-based binary single-junction OSCs achieve a high efficiency of 17.11%, which is one of the top values in BTA-based OSCs.

In this work, we designed and synthesized two small molecular acceptors, based on benzotriazole (BTA) as the center core with isopropyl alkyl chain, thienothiophene unit (TT) β-position connected n-hexane alkyl chain as wing, and using different end groups, 2-(3-oxygen generation-2, 3-2H − 1H-indane-1-alkyl)-malononitrile (IC) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-indane-1-alkylene) (2FIC), named BIT-IC and BIT-2FIC, respectively. The device based on PM6: BIT-IC achieved the PCE of 3.63%, which exhibited inferior performance because the device absorption is not complementary with solar spectrum, leading to the low Jsc of the device. However, the absorption band of the acceptor BIT-2FIC with 2FIC end group can reach 944 nm, and the HOMO energy level is lower than BIT-IC, therefore, the device based on PM6: BIT-2FIC can achieve more efficient exciton dissociation and charge transport, after thermal annealing treatment at 90 °C for 10 min, the device based on PM6: BIT-2FIC can achieve the PCE over 14%.

In recent years, indoor organic photovoltaics (IOPVs) have attracted increasing attention because of their ability to power microelectronic devices and sensors, especially for the internet of things (IoT). In contrast with silicon-based indoor PV, the IOPVs exhibit better performance due to their tunable bandgap via molecular design, which could achieve a better spectrum matched with the lighting sources. Based on the simulated power conversion efficiency (PCE) in theory, the maximum value can achieve over 50 % under the white LED illumination, which is much higher than the practical top PCE of 31 %, indicating there is room further to improve the performance of IOPVs by various optimization methods. Based on these benefits, the recent progress in IOPVs with different methods was summarizes, and light was shed on the remaining challenges for achieving practical applications in the future. In the end, some guidelines for the development of IOPVs were proposed.

Three polymer donors named Qx-8F, Qx-10F, and Qx-12F, with similar chemical structures, were synthesized. The energy level of these donors is manipulated by precisely controlling the fluorination sites. We demonstrate that the exciton dissociation efficiency is gradually enhanced from Qx-8F:Y6 to Qx-12F:Y6, leading to improved open circuit voltage VOC and short circuit current density JSC for donors with a higher degree of fluorination. Simultaneously, the trap-assisted recombination or bimolecular recombination intensity is reduced, while the extraction rate is enhanced because of increased charge carrier mobilities, leading to an increase in fill factor for donors with higher amount of fluorine atoms. The optimized device based on Qx-12F with the most fluorine atoms and the non-fullerene acceptor Y6 shows the highest power conversion efficiency of 15.21%, with a VOC of 0.85 V, JSC of 24.55 mA cm−2, and fill factor of 72.88%. It is further demonstrated that the optimized ternary device based on the Qx-12F:PM6:Y6 blend provides a power conversion efficiency of 16.32%.

The method to fluorinate the terminal group has achieved remarkable success and been widely used to fine-tune the intrinsic properties of organic acceptor materials. Referring to chlorination, however, it gets less attention and remains ambiguous effect on organic photovoltaic (OPV) cells. Herein, a new non-fullerene acceptor named Y19 was reported with benzotriazole as the electron-deficient core and 2Cl-ICs as the strong electron-withdrawing end groups. Y19 exhibits a wide film absorption band from 600 nm to 948 nm and low LUMO (the lowest unoccupied molecular orbital) energy level of −3.95 eV Photovoltaic devices based on PM6:Y19 show high-power conversion efficiency (PCE) of 12.76 % with high open-circuit voltage (Voc) of 0.84 V, short-circuit current density (Jsc) of 22.38 mA/cm2 and fill factor (FF) of 68.18 %. Broad external quantum efficiency (EQE) response of over 60 % in the range of 480–860 nm can be obtained. This study demonstrates that chlorination, as a low-cost molecular design strategy, has its own superiorities to improve device performance and promote the potential application in OPV.

Since the world-record power conversion efficiency of 15.7% was achieved for organic solar cells (OSCs) in 2019, the newly developed non-fullerene acceptor (NFA) Y6 with an A-DA′D-A structure (A denotes an electron-accepting moiety, D denotes an electron-donating moiety) has attracted increasing attention. Subsequently, many new A-DA′D-A NFAs have been designed and synthesized, and the A-DA′D-A NFAs have played a significant role in the development of high-performance non-fullerene organic solar cells (NF-OSCs). Compared with the classical A-D-A-type acceptors, A-DA′D-A NFAs contain an electron-deficient core (such as benzothiadiazole (BT), benzotriazole (BTA), quinoxaline (Qx), or their derivatives) in the ladder-type fused rings to fine-tune the energy levels, broaden light absorption and achieve higher electron mobility of the NFAs. This review emphasizes the recent progress on these emerging A-DA′D-A (including Y-series) NFAs. The synthetic methods of DA′D-fused rings are introduced. The relationships between the chemical structure of the A-DA′D-A NFAs and the photovoltaic performance of the corresponding OSCs are summarized and discussed. Finally, issues and prospects for further improving photovoltaic performance of the OSCs are also proposed.

In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends.

Summary The field of organic solar cells has seen rapid developments after the report of a high-efficiency (15.7%) small molecule acceptor (SMA) named Y6. In this paper, we design and synthesize a family of SMAs with an aromatic backbone identical to that of Y6 but with different alkyl chains to investigate the influence of alkyl chains on the properties and performance of the SMAs. First, we show that it is beneficial to use branched alkyl chains on the nitrogen atoms of the pyrrole motif of the Y6. In addition, the branching position of the alkyl chains also has a major influence on material and device properties. The SMA with 3rd-position branched alkyl chains (named N3) exhibits optimal solubility and electronic and morphological properties, thus yielding the best performance. Further device optimization using a ternary strategy allows us to achieve a high efficiency of 16.74% (and a certified efficiency of 16.42%).

In this work, two non-fullerene small molecule acceptors (NFSMAs), named IFIC and IFIC-F, were designed and synthesized by introducing alkoxy-phenyl as the side chain and furan as π-bridge. This method enhances the intermolecular charge transfer between HFQx-T and IFIC-F (IFIC). Besides, IFIC and IFIC-F exhibit strong and broad absorption ranging from 550 nm to 800 nm. IFIC-F with alkoxy-phenyl exhibits an absorption peak at 702 nm, which has a redshift of 17 nm compared with IFIC (685 nm). The power conversion efficiency (PCE) of organic solar cell based on HFQx-T: IFIC is 6.28%, which is higher than the PCE of device based on HFQx-T: IFIC-F (5.87%). These results indicate that inserting alkoxy–phenyl side chain and furan π-bridge are a rational strategy for the synthesis of high-performance NFSMAs.

A new non-fullerene acceptor, namely Y10, based on dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT) as the central core and 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (TC) as the electron-deficient end group, has been designed and synthesized. Y10 reveals a narrow optical energy gap (Eoptg) of 1.35 eV with a broad absorption band from 600 to 900 nm. A wide bandgap polymer, J11, as the donor material (Eoptg = 1.96 eV) is used to blend with Y10 for the construction of organic solar cell devices, which achieve an impressive power conversion efficiency (PCE) of 13.46% with an open circuit voltage (Voc) of 0.89 V, a short circuit current (Jsc) of 21.21 mA cm-2, and a fill factor (FF) of 71.55%, with thermal annealing treatment at 100 °C for 5 min and 0.8 wt% 1-chloronaphthalene (CN) as an additive. These results indicate that the incorporation of the TPBT unit as the central core and the TC unit as the electron-deficient end group provides an efficient strategy for the construction of high performance solar cells.

Organic solar cells (OSCs) have again become a hot research topic in recent years. The record power conversion efficiency (PCE) of OSCs has boosted to over 17% in 2020. Apart from the high PCE, the stability of OSCs is also critical for their future applications and commercialization. Recently, many studies have proposed that burn-in degradation can be considered as an ineluctable barrier to long-term stable OSCs. However, there is still lack of studies to explain the detailed mechanism of this burn-in process. In this work, we first investigated the mechanism of the burn-in process in the high-efficiency PM6:N3-based nonfullerene OSCs. The PM6:N3-based device achieved a profound average PCE of 14.10% but also showed a significant performance loss after the burn-in degradation. Following characterizations such as dark

Recent advances in material design for organic solar cells (OSCs) are primarily focused on developing near-infrared non-fullerene acceptors, typically A-DA D-A type acceptors (where A abbreviates an electron-withdrawing moiety and D, an electron-donor moiety), to achieve high external quantum efficiency while maintaining low voltage loss. However, the charge transport is still constrained by unfavorable molecular conformations, resulting in high energetic disorder and limiting the device performance. Here, a facile design strategy is reported by introducing the "wing" (alkyl chains) at the terminal of the DA D central core of the A-DA D-A type acceptor to achieve a favorable and ordered molecular orientation and therefore facilitate charge carrier transport. Benefitting from the reduced disorder, the electron mobilities could be significantly enhanced for the "wing"-containing molecules. By carefully changing the length of alkyl chains, the mobility of acceptor has been tuned to match with that of donor, leading to a minimized charge imbalance factor and a high fill factor (FF). We further provide useful design strategies for highly efficient OSCs with high FF. Funding Agencies|National Key Research & Development Projects of China [2017YFA0206600]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China [21875286]; Science Fund for Distinguished Young Scholars of Hunan ProvinceNational Natural Science Foundation of ChinaNational Science Fund for Distinguished Young Scholars [2017JJ1029]; Innovation-Driven Project of Central South University [2020CX001]; Research Committee of HKBU [RC-ICRS/15-16/4A-SSK, FRG/16-17/077]

Correction for ‘A new non-fullerene acceptor based on the combination of a heptacyclic benzothiadiazole unit and a thiophene-fused end group achieving over 13% efficiency’ by Yunqiang Zhang et al., Phys. Chem. Chem. Phys., 2019, DOI: 10.1039/c9cp05015a.

In recent years, the development of non-fullerene acceptor (NFA) has rapidly improved the power conversion efficiency (PCE) of organic solar cells (OSCs). However, there are still many factors restricting the further improvement of PCE, and the low fill factor (FF) value is one of the main reasons. Therefore, it is of great importance to increase FF through designing organic semiconducting materials. In this paper, Y6-eC6-HD is obtained by modifying two kinds of alkyl chains of Y6, because the regulation of the side alkyl chains as one of the effective methods can get proper film morphology which is beneficial for balanced charge transfer and improved FF. Subsequently, the device based on PM6:Y6-eC6-HD can achieve an outstanding PCE of 16.00% with a high FF of 77.32%. Such a high fill factor is higher than that of the most of Y-series molecules. These results indicate that adjusting the side alkyl chains of NFAs is a promising method to enhance the FF and therefore improving the PCE of OSCs.

Organic solar cells (OSCs) are considered to reach its second golden age with the profoundly improved power conversion efficiency (PCE) and device stability in recent years. The modification of the interface layer plays a significant role in achieving performance enhancement in OSCs. In this work, we reported for the use of the atomic layer deposited (ALD) ultrathin TiOx to modify the interface layer in OSCs. The modification with only 2 TiOx ALD cycles not only effectively passivates the interface between the ZnO electron transport layer (ETL) and the active layer, but also reduces the series resistance and improves the charge transport process in the device. We achieved an absolute 1% increase in PCE with enhanced device stability for modified OSCs. We also fabricated semitransparent OSCs by applying this interface modification strategy. The modification with 2 TiOx ALD cycles increases the electrical device performance without affecting the optical properties of the semitransparent device. An average PCE of 10.46% with an average visible transmittance (AVT) of 19.61% and color rendering index (CRI) close to 100 is demonstrated for the fabricated semitransparent device with the modification. The ALD assisted interface modification provides a straightforward way to realize high‐performance semitransparent OSCs. This article is protected by copyright. All rights reserved.