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Alkoxy side chain engineering on the β-position of the thienothiophene units of Y6 derivatives plays a vital role in improving photovoltaic performances with simultaneously increasing open-circuit voltage ( V oc ) and fill factor (FF). In this work, we prepared a series of asymmetric non-fullerene acceptors (NFAs) by introducing alkoxy side chains and phenoxy groups on the state-of-the-art Y6-derivative BTP-BO-4F. For the comparison, 2O-BO-4F with a symmetric alkoxy side chain on the outer thiophene units and BTP-PBO-4F with an asymmetric N -attached phenoxy alkyl chain on the pyrrole ring are synthesized from BTP-BO-4F. Thereafter, we construct four asymmetric NFAs by introducing different lengths of linear/branched alkoxy chains on the β-position of the thienothiophene units of BTP-PBO-4F. The resulting NFAs, named L10-PBO, L12-PBO, B12-PBO, and B16-PBO (L = linear and B = branched alkoxy side chains), are collectively called OR-PBO-series. Unexpectedly, all OR-PBO NFAs exhibit strong edge-on molecular packing and weaker π-π interactions in the film state, which diminish the charge transfer in organic solar cell (OSC) devices. As a consequence, the optimal devices of OR-PBO-based binary blends show poor photovoltaic performances [power conversion efficiency (PCE) = 6.52-9.62%] in comparison with 2O-BO-4F (PCE = 12.42%) and BTP-PBO-4F (PCE = 15.30%) reference blends. Nevertheless, the OR-PBO-based binary devices show a higher V oc and smaller V loss . Especially, B12-PBO- and B16-PBO-based devices achieve V oc over 1.00 V, which is the highest value of Y-series OSC devices to the best of our knowledge. Therefore, by utilizing higher V oc of OR-PBO binary blends, B12-PBO and B16-PBO are incorporated into the PM6:BTP-PBO-4F-based binary blend and fabricated ternary devices. As a result, the PM6:BTP-PBO-4F:B12-PBO ternary device delivers the best PCE of 15.60% with an increasing V oc and FF concurrently.

Terminal acceptor atoms and side-chain functionalization play a vital role in the construction of efficient nonfullerene small-molecule acceptors (NF-SMAs) for AM1.5G/indoor organic photovoltaic (OPV) applications. In this work, we report three dithienosilicon-bridged carbazole-based (DTSiC) ladder-type (A-DD'D-A) NF-SMAs for AM1.5G/indoor OPVs. First, we synthesize DTSiC-4F and DTSiC-2M, which are composed of a fused DTSiC-based central core with difluorinated 1,1-dicyanomethylene-3-indanone (2F-IC) and methylated IC (M-IC) end groups, respectively. Then, alkoxy chains are introduced in the fused carbazole backbone of DTSiC-4F to form DTSiCODe-4F. From solution to film absorption, DTSiC-4F exhibits a bathochromic shift with strong π-π interactions, which improves the short-circuit current density ( J sc ) and the fill factor (FF). On the other hand, DTSiC-2M and DTSiCODe-4F display up-shifting lowest unoccupied molecular orbital (LUMO) energy levels, which enhances the open-circuit voltage ( V oc ). As a result, under both AM1.5G/indoor conditions, the devices based on PM7:DTSiC-4F, PM7:DTSiC-2M, and PM7:DTSiCOCe-4F show power conversion efficiencies (PCEs) of 13.13/21.80%, 8.62/20.02, and 9.41/20.56%, respectively. Furthermore, the addition of a third component to the active layer of binary devices is also a simple and efficient strategy to achieve higher photovoltaic efficiencies. Therefore, the conjugated polymer donor PTO2 is introduced into the PM7:DTSiC-4F active layer because of the hypsochromically shifted complementary absorption, deep highest occupied molecular orbital (HOMO) energy level, good miscibility with PM7 and DTSiC-4F, and optimal film morphology. The resulting ternary OSC device based on PTO2:PM7:DTSiC-4F can improve exciton generation, phase separation, charge transport, and charge extraction. As a consequence, the PTO2:PM7:DTSiC-4F-based ternary device achieves an outstanding PCE of 13.33/25.70% under AM1.5G/indoor conditions. As far as we know, the obtained PCE results under indoor conditions are one of the best binary/ternary-based systems processed from eco-friendly solvents.

Developing new electron transport layers has been an effective way to fabricate high-performance bulk-heterojunction organic solar cells (OSCs). Resolving the longstanding problems associated with commonly used zinc oxide (ZnO), such as electron traps and light-induced device deterioration, however, is still a great challenge. In this study, glycerol diglycidyl ether (GDE) and 1,4-butanesultone (BS) are blended with polyethyleneimine (PEI) to produce cross-linkable PEI-based materials, PEI-GDE and PEI-GDE-BS, which can function as alternative electron transport layers to replace conventional ZnO cathode-modifying layers in inverted OSCs. PEI-GDE and PEI-GDE-BS are amendable to low-temperature annealing processes to produce cross-linked films. The inverted device structure of ITO/ETL/PM6:BTP-BO-4F:PC 71 BM/MoO 3 /Ag was used to evaluate the effects of incorporating PEI-GDE and PEI-GDE-BS as electron transport materials. Compared with ZnO-based devices, the PEI-GDE- and PEI-GDE-BS-based devices exhibit significant improvements in photovoltaic performances due to smoother surface roughness, higher charge collection and exciton dissociation efficiencies, higher electron mobilities, and stronger π-π interactions. In particular, a PEI-GDE-BS-based device shows an outstanding power conversion efficiency (PCE) of 17.55% with a V OC of 0.83 V, a J SC of 27.88 mA/cm 2 , and an FF of 75.96%, which offers great possibilities in the applications of flexible solar cells.

Molecular backbone modification, alkyl-chain engineering, and end-group functionalization are promising strategies for developing efficient high-performance non-fullerene acceptors (NFAs). Herein, two new NFAs, named TPQ-eC7-4F and TPQ-eC7-4Cl, are designed and synthesized. Both molecules have linear octyl chains on fused quinoxaline-containing heterocyclics as the central backbone and difluorinated (2F)/dichlorinated (2Cl) 1,1-dicyanomethylene-3-indanone (IC) as the end-group units. The influences of alkyl-chains on fused quinoxaline backbone and different halogenated end-groups on optical, electrochemical, and photovoltaic performances of organic solar cells (OSCs) are studied. In comparison with TPQ-eC7-4Cl, TPQ-eC7-4F exhibits blue-shifted absorptions with higher molar extinction coefficients in the film state as well as in the donor/acceptor (D/A) blend film state and up-shifting lowest unoccupied molecular orbital (LUMO) energy level. As a result, the OSC devices based on the PBDB-T:TPQ-eC7-4F display an outstanding power conversion efficiency (PCE) of 15.83% with a simultaneously increased open-circuit voltage ( V oc ) of 0.85 V, a short-circuit current-density ( J sc ) of 25.89 mA cm -2 , and a fill factor (FF) of 72.20%, whereas the PBDB-T:TPQ-eC7-4Cl-based OSC device shows a decent PCE of 14.48% with a V oc of 0.84 V, a J sc of 24.56 mA/cm 2 , and an FF of 69.77%. To the best of our knowledge, this is the highest photovoltaic performance of PBDB-T-based single-junction binary-OSCs. In comparison, ascribed to the high crystallinity and low solubility of BTP-eC7-4Cl, the corresponding PBDB-T:BTP-eC7-4Cl-based OSC device shows poor photovoltaic performance (PCE of 11.87%). The experimental results demonstrate that fine-tuning the fused quinoxaline backbone with alkyl-chain and end-group functionalization are promising strategies to construct high-performance NFAs for PBDB-T-based single-junction binary-OSCs.

The molecular design of wide-bandgap conjugated polymer donors (WB-CPDs) is a promising strategy for tuning the bulk heterojunction blend film morphologies to achieve high-performance organic photovoltaic (OPV) devices. Herein, we synthesize two WB-CPDs, namely, PBQ-H and PBQ-M, with and without methyl groups on the fused-dithieno[3,2- f :2',3'- h ]quinoxaline (DTQx) moiety. We systematically investigate their structure-property relationship and OPV performances. The AFM and 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) studies reveal that the PBQ-H:BO-4Cl BHJ blend shows strengthened aggregation behavior and stronger π-π stacking on face-on orientation compared with the PBQ-M:BO-4Cl BHJ blend, enhancing the phase separation, charge transport, and fill factor (FF). Blend film absorption spectra, however, show that the PBQ-H:BO-4Cl BHJ blend exhibits a lower absorption coefficient than that of the PBQ-M:BO-4Cl BHJ blend, which decreases the short-circuit current density ( J SC ). As a consequence, the optimized PBQ-H:BO-4Cl BHJ blend delivers a higher power conversion efficiency (PCE) of 12.88% with a J SC of 23.97 mA/cm 2 , an open-circuit voltage ( V OC ) of 0.86 V, and an FF of 62.46%, compared with the PBQ-M:BO-4Cl BHJ blend (PCE of 11.81% with a J SC of 24.78 mA/cm 2 , a V OC of 0.85 V, and an FF of 56.11%). Overall, this work demonstrates that alkyl group substitution on the DTQx moiety on the basis of WB-CPDs is critical for controlling the film morphology and thus obtaining high OPV performances.

Two new small molecule donors, namely ICT4 and ICT6 with D 1 -A-D 2 -A-D 1 architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D 1 ) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (OBDT, D 2 ) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×10 5 mol -1 cm -1 ) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PC 71 BM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PC 71 BM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.

A novel, NIR absorbing organic small molecular donor material denoted as ICT3 with an A-D-D-D-A architecture having dithieno[3,2-b:2',3'-d]pyrrole (DTP) and butylrhodanine as donor and acceptor moieties, respectively, is synthesized and its thermal, photophysical, electrochemical and photovoltaic properties are explored. ICT3 has excellent stability over a broad range of temperatures with a decomposition temperature (T d corresponds to 5% weight loss) of 372 °C, soluble in most common organic solvents (solubility up to 30 mg mL -1 ) and suitable for solution processing during device fabrication. ICT3 has broad (520-820 nm) and intense visible region absorption (molar excitation coefficient is 1.69 × 10 5 mol -1 cm -1 ) and has suitable HOMO and LUMO energy levels with the [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) acceptor for efficient exciton dissociation and charge transfer. Bulk heterojunction solar cells (BHJSCs) with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ICT3:PC 71 BM/poly(9,9-bis(3'-(N,N-dimethylamino)propyl)fluorene-2,7-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl) (PFN)/aluminium (Al) structure are fabricated and the BHJSCs with the active layer as cast from chloroform solution displayed a power conversion efficiency (PCE) of 3.04% (J SC = 8.22 mA cm -2 , V OC = 0.86 V and FF = 0.43). Annealing the active layer significantly improved the PCE of these BHJSCs. While thermal annealing of the active layer improved the PCE of the BHJSCs to 4.94%, thermal followed by solvent vapour annealing enhanced the PCE to 6.53%. X-ray diffraction and atomic force microscopy analyses are carried out on the active layer and these results revealed that annealing treatment improves the crystallinity and nanoscale morphology of the active layer, enriches the device exciton generation and dissociation efficiency, charge transport and collection efficiency and reduces carrier recombination. The observed higher PCE (6.53%) of the BHJSCs having ICT3 with a DTP donor moiety broadens the scope to develop new, efficient DTP based small molecular donor materials for BHJSCs.