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Donor halogenation is a common molecular design strategy used to reduce voltage losses (ΔVloss) and improve the power conversion efficiency (PCE) of bulk-heterojunction (BHJ) organic solar cells. Here, the impact of donor halogenation on the performance of organic donor–acceptor (DA) solar cells based on over 30 different materials systems is investigated, and the main reason for the improved performance of solar cells after donor halogenation is ascribed to the increased energy of the charge transfer (CT) state, and the reduced reorganization energy of the CT states (λCT). Also, the impact of donor halogenation on λCT is found to be stronger for the solar cells using the Y-series acceptors (Y5, Y6, etc.) than those using the non-Y-series acceptors (fullerene, ITIC, etc.), which is conducive to achieving lower ΔVloss in organic solar cells. Finally, the impact of donor halogenation on the solar cell performance is demonstrated to be dependent on the halogen substitution position, as well as the number of halogen atoms added to the donor molecule: Halogen substitution on the side groups of the donor molecule is found to be more effective than substitution at the backbone in reducing ΔVloss. These results suggest that future molecular design strategies focusing on the reduction of materials reorganization energy will be of great importance for further improving the performance of organic solar cells.

Two fully non-fused electron acceptors based on terthiophene have been synthesized and a PCE of 11.2% is achieved.

Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2

The introduction of bulky substituents to the terminal unit is an effective means of tuning the solubility of non-fullerene acceptors and the morphology of blend films, and ultimately regulating the performance of organic solar cells.

Under the guidance of a symmetry-breaking and π-bridge-extending strategy, three asymmetric and symmetric non-fullerene acceptors (L1, L2, and L3) with DA′D core units and two 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile electron-withdrawing units (A) have been designed and synthesized via changing the alkoxy-substituted thiophene bridge unit. The asymmetric A–DA′D–π-A-type non-fullerene acceptor (NFA) L2 displays much improved solubility and a slightly larger molar absorption coefficient than its symmetric counterparts (L1 and L3). In addition, blend film based on the asymmetric NFA L2 shows suitable nanoscale phase morphology and much higher and more balanced hole and electron mobilities. Hence, compared with L1- and L3-based organic solar cells (OSCs), optimal L2-based devices can deliver a high power-conversion efficiency (PCE) of 14.06% with simultaneously enhanced current density and fill factor values. Our study indicates that a combined symmetry-breaking and π-bridge-extending strategy is powerful for the design of high-performance NFAs.

Blending the donor and acceptor with a near zero HOMO offset can realize high-performance nonfullerene organic solar cells with large open circuit voltage and fill factor.

Hybrid nanocrystals (NCs) with multiple components and junctions have attracted considerable attention due to their promising synergistic properties. In particular, great attention has been paid to the manipulation of buried multijunction heterointerfaces because they are closely related to the surface energy and carrier transfer of NCs. However, heterointerfaced NCs are usually constructed by sequential step-by-step pathways, and buried interfaces can only be formed along a given direction, resulting in the one and only spatial orientation of multiple interfaces. In this work, we demonstrate two types of Au-Ag2S-Cu2-xS NCs with disparate interfacial features. Specifically, the first type (Type I) is prepared through a routine two-step method and shows that Au domain close to the Ag2S-Cu2-xS interface; another type (Type II) is achieved by a facile one-pot synthesis procedure and contains Au domain with an interphase only with the Ag2S domain, far from the Cu2-xS domain. More importantly, type II NCs could not be formed through other traditional strategies and an underlying mechanism of formation is developed by monitoring the evolution process. Au@Ag core-shell NCs, metastable Au@Ag2S NCs and Janus Au-Ag2S NCs are formed successively before the Cu2-xS domain appears. We speculate that the Au@Ag2S intermediate plays an essential role in building the final complex nanostructure. We expect that such a simple and facile one-pot method will be used to fabricate additional asymmetric multicomponent NCs with distinctive interfacial features and promising potential applications.

Pioneering work referring to perylene diimide (PDI) acceptors has been conducted for a long period, and great efforts have been devoted to hinder the excessive self-aggregation. The main strategy adopted is constructing a three-dimensional molecular structure either with a twisted core or by forming a congestion center. In addition, it is especially important to maintain an essential intermolecular stacking affinity to endow the acceptor with a high charge-transport ability. With these considerations in mind, herein, we adopt a saddle-shaped cyclooctatetrathiophene (COTh) during the PDI acceptor design and integrate the flexibility of COTh into the final PDI acceptor molecules. The geometries for these acceptors have been verified by quantum calculation, and the main excitations have been well analyzed, which demonstrates that the linking manners between PDI and COTh would directly impact their entire configurations, conjugations, and electronic wave-function distributions. The three-dimensional skeleton guarantees the isotropic crystalline characters and effectively suppresses the severe aggregation. When the saddle feature and the large π-extended characteristic intimately combine in COTh-FPDI, balanced mobility and favorable morphology can be achieved. Finally, a power conversion efficiency (PCE) of 8.37% can be accomplished by the as-cast polymer solar cells of the COTh-FPDI:PBDB-T blend. All of the results demonstrate that this flexible saddle core could provide a new block for the novel PDI-based acceptor design in photovoltaic applications.

Perovskite solar cells (PSCs) have been brought into sharp focus in the photovoltaic field due to their excellent performance in recent years. The power conversion efficiency (PCE) has reached to be 25.2% in state-of-the-art PSCs due to the outstanding intrinsic properties of perovskite materials as well as progressive optimization of each functional layer, especially the active layer and hole transporting layer (HTL). In this review, we mainly discuss various hole transporting materials (HTMs) consisting of HTL in PSCs. The progress in PSCs is firstly introduced, then the roles of HTL playing in photovoltaic performance improvement of PSCs are emphasized. Finally, we generally categorize HTMs into organic and inorganic groups and demonstrate both their advantages and disadvantages. Specially, we introduce several typical organic HTMs such as P3HT, PTTA, PEDOT:PSS, spiro-OMeTAD, and inorganic HTMs such as copper-based materials (CuOx, CuSCN, CuI, etc.), nickel-based materials (NiOx), and two-dimensional layered materials (MoS2, WS2, etc.). On basis of reviewing the reported HTMs in recent years, we expect to provide some enlightenment for design and application of novel HTMs that can be used to further promote PSCs performance.

The side chain engineering of photoactive materials is one of the most commonly used molecular design strategies for developing high performance organic solar cells (OSCs). Herein, two alkyl (or alkoxyl) chains, corresponding to 4TIC-C8-2F and 4TIC-OC8-2F, are introduced to the two outmost thiophene rings of the A–D–A type fused ring electron acceptor (4TIC-2F). We demonstrate that the molecular orientation, crystallinity and blend film morphology can be effectively regulated by side chain engineering. Moreover, the introduction of the flexible chains can also lead to reduced energy loss of photovoltaic devices and give rise to a higher open-circuit voltage. The power conversion efficiencies (PCEs) of the PM6:4TIC-C8-2F and PM6:4TIC-OC8-2F based solar cells are 10.75% and 10.13%, respectively, higher than the PM6:4TIC-2F based devices (10.03%). Furthermore, the optimized devices based on PM6:4TIC-C8-2F give an enhanced PCE of 12.65%. Our results have revealed that side chain engineering is an efficient molecular design strategy in developing high performance small molecular acceptors.

Organic semiconductors including conjugated polymers and small molecules can be applied in many fields due to their unique advantages, such as light weight, solution processability, easy functionalization etc. During the past ten years, we mainly focused on the design and synthesis of conjugated polymer donor materials and small molecular acceptor materials for organic solar cells and hole transport materials for perovskite solar cells. To obtain planar conjugated polymers, low cost small molecular acceptors, and dopant-free hole transport polymers, we adopted intramolecular noncovalent interactions (INCIs) as the design strategy. In this brief review, we will demonstrate that the INCI strategy is very efficient in the design of high performance photovoltaic materials.

Most high-performance non-fullerene acceptors are of the acceptor-donor-acceptor (A-D-A)-type structure. Under photoexcitation, the intramolecular charge transfer effect on the A-D-A framework results in a large dipole moment change, facilitating the efficient generation of charge carriers. Achieving more efficient intramolecular charge transfer by adjusting the molecular structure is one of the current research ideas. Recently, we found that the power conversion efficiency can be improved from 4.41 to 13.13% by tuning the planarity of the non-fused ring electron acceptor backbone through steric hindrance of lateral substituents. We found that the planar backbone can effectively improve the intramolecular charge transfer, which has a great influence on the power conversion efficiency of the device. Our results demonstrate that charge transfer dynamics can be controlled by optimizing steric hindrance, which plays a crucial role in the photovoltaic performance of organic solar cells.

The rapid advance of fused-ring electron acceptors (FREAs) has made them a potential substitute to fullerene-based acceptors and offered new avenues for the construction of organic solar cells (OSCs). Nonfused-ring acceptors (NFRAs) could significantly reduce the synthetic cost while achieving reasonable power conversion efficiencies (PCEs). Widely used fullerene acceptors have been applied as a second acceptor to regulate the morphology, absorption, and electron transport. To take full advantage of both nonfullerene and fullerene acceptors at the same time, we rationally designed and synthesized two novel NFRAs with phenyl-C61-butyric acid methyl ester (PCBM) as the lateral pendent. With the incorporation of fullerene pendent in PCBM-C6 and PCBM-C10, varied UV-vis absorption and photoluminescence (PL) quenching behaviors were observed, and isotropic diffraction patterns were obtained via grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. The bulky, spherical, and electronic isotropic fullerene pendent could effectively suppress severe molecular aggregation and form the preferred blend morphology. This strategy significantly improved the efficiencies for exciton separation and charge collection relative to the control acceptor CH3COO-C6. Finally, the Voc, Jsc, and fill factor (FF) of PCBM-C10-based devices were simultaneously improved and an enhanced PCE of 13.55% was accomplished.

As a well-known electron-withdrawing group, benzo[c][1,2,5]thiadiazole (BT) has been intensively studied and adopted to construct polymer donors with tunable band gaps. However, polymer solar cells...

Fused-ring electron acceptors have made significant progress in recent years, while the development of fully non-fused ring acceptors has been unsatisfactory. Here, two fully non-fused ring acceptors, o-4TBC-2F and m-4TBC-2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o-4TBC-2F formed planar backbones, while m-4TBC-2F displayed a twisted backbone. Additionally, the o-4TBC-2F film showed a markedly red-shifted absorption after thermal annealing, which indicated the formation of J-aggregates. For fabrication of organic solar cells (OSCs), PBDB-T was used as a donor and blended with the two acceptors. The o-4TBC-2F-based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o-4TBC-2F gave a PCE of 10.26 %, which was much higher than those based on m-4TBC-2F at 2.63 %, and it is one of the highest reported PCE values for fully non-fused ring electron acceptors.

Four noncovalently fused-ring electron acceptors p-DOC6-2F, o-DOC6-2F, o-DOC8-2F and o-DOC2C6-2F have been designed and synthesized. p-DOC6-2F and o-DOC6-2F have the same molecular backbone but dif...

Herein, norbornyl (NB), a bulky annular nonconjugated spacer, is melded into π systems to construct two groups of ladder-type room-temperature phosphorescence (RTP) luminogens. The effect of the NB on π-π interactions, packing modes and RTP performance is explored systematically. The experimental and computational results demonstrate the versatility of NB in reducing π-π distances and synergistically intensifying the intermolecular interactions, which not only induces intersystem crossing from S1 to Tn but also diminishes the nonradiative decay of triplet excitons. Impressively, 1800-fold phosphorescence lifetime enhancement is achieved in comparison with the reference compounds without NB. The molecular packing and RTP performance can be further modulated by the length of the backbones and terminal end-groups. It is quite peculiar that NB-annulated phthalic acid exhibits reversible photochromism in the solid state, likely due to the formation of persistent radical pairs. Our study paves an ingenious avenue towards enhancing intermolecular interactions and provides significant implications for a better comprehensive understanding of the origin of their RTP and the inherent photophysical mechanism.

Three noncovalently fused-ring electron acceptors (FOC6-IC, FOC6-FIC, and FOC2C6-2FIC) are synthesized. Single crystals of FOC6-IC and FOC2C6-2FIC are prepared, and structure analyses reveal that the molecular backbone can be planarized via the formation of the intramolecular noncovalent interactions. These acceptor molecules can be packed closely in the solid state via π–π stacking and static interactions between the central phenylene unit and the terminal group with a distance of 3.3–3.4 A. Besides, multiple intermolecular noncovalent interactions can be observed in the single crystal structure of the fluorinated acceptor FOC2C6-2FIC, which help increase the crystallinity of acceptors and the charge mobility of the blends. Photovoltaic devices based on FOC2C6-2FIC give a power conversion efficiency of 12.36%, higher than 12.08% for FOC6-FIC and 10.80% for FOC6-IC.

In this work, we used a ternary blend strategy to improve the photovoltaic performance of organic solar cells (OSCs). PBDB-T:IDT-EDOT-based binary blend devices show a power conversion efficiency (PCE) of 9.93%, and the ternary devices with PC71BM as the third component exhibit a PCE of 12.07% with simultaneously enhanced Voc, Jsc and FF. The broadened absorption, optimized morphology and balanced charge carrier mobility of these devices are responsible for these improvements. The introduction of PC71BM can disperse the IDT-EDOT aggregates, enhance the phase purity, and increase the electroluminescence quantum efficiency (EQEEL). Furthermore, the performance of the ternary devices is not very sensitive to the weight ratio of the two acceptors. PCEs of over 11% are obtained even though the composition gradually varies from 1 : 1 : 0.2 to 1 : 0.4 : 0.8. Our results demonstrate that PC71BM is a highly promising second acceptor for the construction of high-efficiency ternary OSCs.

Molecular orientation is one of the key parameters that would greatly influence the charge transportation in sandwich-type organic photovoltaic devices. Herein, two new A–π–D–π–A type acceptor molecules Ph-DTDPo-OTE and Ph-DTDPo-TE are designed and synthesized based on a 2H-pyran derived ladder-type dipyran core (DTDPo). A branched 2-ethylhexyloxy chain is utilized to replace the linear hexyloxy one so as to improve the solubility and blend morphology. It's interesting that the dominant edge-on orientation for Ph-DTDPo-OTE could be converted to the favorite face-on packing when the alkyloxy chains are replaced by alkyl ones. The desired molecular orientations of both Ph-DTDPo-TE and PBDB-T could guarantee the efficient charge transportation. In addition, good miscibility and interpenetrated fibrils have been observed in the blend of Ph-DTDPo-TE:PBDB-T by both surficial and in-depth morphology characterization experiments. Finally, a high PCE of 12.21% (Voc = 0.82 V, Jsc = 20.83 mA cm−2 and FF = 71.14%) is achieved for Ph-DTDPo-TE, which is one of the highest values for binary OSCs based on a pyran comprising acceptor. Our research demonstrates that appropriate side chain engineering at the π-bridge is an effective strategy to regulate molecular orientations and the morphology of blend films.

In this study, a new p-type π-conjugated ladder-like polymer, poly(3,3′-(((2-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzol[1,2-b:4,5-b′]dithiophen-2-yl)-5-methyl-1,4-phenylene)bis(oxy))bis(hexane-6,1-diyl))bis(1,1,1,3,5,5,5-heptamethyltrisiloxane)) (P-Si), has been designed for SnO2-based perovskite solar cells. Our investigation reveals that this π-conjugated ladder-like polymer system decorated with hydrophobic alkyl chains and siloxane chains is likely the reason for the improvement in the surface morphology and crystallinity of the perovskite films, leading to reduced defect states and enhanced device stability. In addition, P-Si with a suitable highest occupied molecular orbital energy level (−5.41 eV) can act as a hole transport medium between the perovskite and spiro-OMeTAD to enhance the excellent hole transportation. As a result, power conversion efficiency (PCE) as high as 21.5% with 21.3% steady-state PCE was achieved. Typically, the device with P-Si can retain 98% of the original average PCE after being stored in an ambient environment with 20% relative humidity for 120 days, indicating its good long-term stability. Furthermore, after aging for 100 h under illumination at the maximum power point in a nitrogen atmosphere, the P-Si modified device retains over 98% of its initial performance.

Three new low-cost A1–D–A2–D–A1 type small molecule acceptors BT-IC4F, BT2F-IC4F and BTOR-IC4F have been designed and synthesized for high-performance organic solar cells (OSCs). In BT2F-IC4F and BTOR-IC4F, intramolecular noncovalent interactions are adopted to construct ladder-like planar molecular backbones. For BTOR-IC4F, the introduction of two alkoxyl chains at the 5,6-positions of the central benzothiadiazole unit can markedly improve the solubility, lift the lowest unoccupied molecular orbital level, and optimize the morphology of the active layer. As expected, PBDB-T:BTOR-IC4F based OSCs achieve a power conversion efficiency (PCE) of 11.48%, which is higher than those of devices based on PBDB-T:BT-IC4F (9.83%) and PBDB-T:BT2F-IC4F (8.45%). Meanwhile, the solar cells based on BTOR-IC4F demonstrate a low non-radiative energy loss of 0.28 eV.

Achieving high-efficiency organic solar cells (OSCs) with long-term thermal stability is a major challenge toward commercialization. In the present study, a ternary blend strategy was employed to improve the thermal stability of OSCs with a novel small molecular acceptor ITC6-2F serving as the second acceptor. Compared with its parent compound ITC6-IC, the introduction of fluoro substituents at the two end groups can reduce the π–π stacking distance and enhance the intermolecular interactions in films. The PBDB-T:IDT-PDOT-C6:ITC6-2F-based ternary blend film exhibits a reduced lamellar distance and an increased crystalline coherence length, compared to the corresponding binary blend films. Photo-physics and device physics analyses demonstrate that the charge-transfer state energy is dominated by PBDB-T and IDT-PDOT-C6 in the ternary OSCs. Also, ITC6-2F can facilitate the photonic energy transfer to IDT-PDOT-C6 and promote more excitons to reach the donor/acceptor (D/A) interfaces to achieve high-efficiency photoelectric conversion. More importantly, the enhanced intermolecular interactions and good miscibility between the two acceptors help to “freeze” the film morphology, leading to a significantly improved long-term thermal stability. The device efficiency remained at 80.3% of its initial value after 137 days of continuous heating at 75 °C in a nitrogen-filled glove box, which is a record result for high-efficiency OSCs reported so far.

Nonfused-ring electron acceptors have attracted much attention in recent years due to their advantages of simple synthetic routes, high yields, low costs, reasonable power conversion efficiencies (PCEs), and so on. Herein, three simple A-π-D-π-A-type acceptors (