15.34% efficiency all-small-molecule organic solar cells with an improved fill factor enabled by a fullerene additive

Solution processed organic solar cells (OSCs) composed of all small molecules (ASM) are promising for production on an industrial scale owing to the properties of small molecules, such as well-defined chemical structures, high purity of materials, and outstanding repeatability from batch to batch synthesis. Remarkably, ASM OSCs with power conversion efficiency (PCE) beyond 13% were achieved by structure improvement of the electron donor and choosingY6as the electron acceptor. However, the fill factor (FF) is an obstacle that limits the further improvement of the PCE for these ASM OSCs. Herein, we focus on the FF improvement of recently reported ASM OSCs withBTR-Cl:Y6as the active layer by miscibility-induced active layer morphology optimization. The incorporation of fullerene derivatives, which have good miscibility with bothBTR-ClandY6, results in reduced bimolecular recombination and thus improved FF. In particular, whenca.5 wt% ofPC(71)BMwas added in the active layer, a FF of 77.11% was achieved without sacrificing the open circuit voltage (V-OC) and the short circuit current density (J(SC)), leading to a record PCE of 15.34% (certified at 14.7%) for ASM OSCs. We found that the optimized device showed comparable charge extraction, longer charge carrier lifetime, and slower bimolecular recombination rate compared with those of the control devices (w/o fullerene). Our results demonstrate that the miscibility driven regulation of active layer morphology by incorporation of a fullerene derivative delicately optimizes the active layer microstructures and improves the device performance, which brings vibrancy to OSC research

Self-assembly enables simple structure organic photovoltaics via green-solvent and open-air-printing: Closing the lab-to-fab gap

The ultimate goal of organic solar cells (OSCs) is to deliver cheap, stable, efficient, scalable, and eco-friendly solar-to-power products contributing to the global carbon neutral. However, simultaneously balancing these five critical factors of OSCs toward commercialization is extremely challenging. Herein, a green-solvent-processable and open-air-printable self-assembly strategy is demonstrated to synchronously simplify the device architecture, improve the power conversion efficiency (PCE) and enhance the shelf, thermal as well as light illumination stability of OSCs. The cathode interlayer (CIL)-free self-assembled OSCs exhibit the PCE of 15.5%, higher than that of traditional inverted OSCs of 13.0%, which is among the top values for both CIL-free self-assembled OSCs and open-air blade-coated bulk-heterojunction OSCs. The remarkable enhancements are mainly ascribed to the finely selfassembly, subtly controlled donor/acceptor aggregation rate, and delicately manipulated vertical morphology. Besides, this strategy enables 13.2% efficiency on device area of 0.98 cm(2), implying its potential for scalability. These findings demonstrate that this strategy can close the lab-to-fab gap of OSCs toward commercialized cheap, stable, efficient, scalable, and eco-friendly OSCs

Oligothiophene-based photovoltaic materials for organic solar cells:rise, plateau, and revival

202312 bcchAccepted ManuscriptRGCOthersThis work was financially supported by Youth Innovation Promotion Association CAS (No. 2019373 ), Chongqing Funds for Distinguished Young Scientists ( cstc2020jcyj-jqX0018 ), Natural Science Foundation of Chongqing ( cstc2019jcyj-msxmX0497 ), National Natural Science Foundation of China (No. 62074149 ), Research Grants Council of Hong Kong (Project nos. 15221320, C5037-18G), Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices (GDSTC no. 2019B121205001), and Hong Kong Polytechnic University (8-8480, CDA5).PublishedGreen (AAM

Efficient small-molecule donor with improved structural order and molecular aggregation enabled by side-chain modification

Side-chain modification is a proven effective approach for morphology manipulation in organic solar cells (OSCs). However, in-depth analysis and investigation involving side-chain modification towards morphology improvement, including molecular microstructure, orientating packing and aggregation are urgent for all-small-molecule (ASM) systems. Herein, employing a fluorine-modified two-dimension benzodithiophene (BDT) as central unit, we contrastively synthesized two small-molecule donors, namely BDT-F-SR and BDT-F-R, each welding alkylthio side-chains on thienyl of central BDT unit and the other grafted non-sulfuric alkyl side-chains. As predicted, the synergetic side-chain modification of fluorination and alkyl changeover triggers diverse molecular dipole moments and orientations, resulting in different molecular energy levels, thermal stabilities, molecular planarity and order. Eventually, together with the preeminent small-molecule acceptor Y6, BDT-F-R-based ASM OSCs obtain enhanced power conversion efficiency (PCE) of 13.88% compared to BDT-F-SR-based devices (PCE of 12.75%) with more suitable phase-separation and balanced carrier mobilities. The contrast results reveal that alkyl side-chains seem to be a more satisfactory partner for fluorine-modified 2D BDT-based small-molecule donors compared to alkylthio pendants, and highlight the significance of subtle side-chain modification for molecular structural order fun-tuning and morphology control, laying the foundation for efficient ASM OSCs

Design of All-Small-Molecule Organic Solar Cells Approaching 14% Efficiency via Isometric Terminal Alkyl Chain Engineering

Morphology is crucial to determining the photovoltaic performance of organic solar cells (OSCs). However, manipulating morphology involving only small-molecule donors and acceptors is extremely challenging. Herein, a simple terminal alkyl chain engineering process is introduced to fine-tune the morphology towards high-performance all-small-molecule (ASM) OSCs. We successfully chose a chlorinated two-dimension benzo[1,2-b:4,5-b′]dithiophene (BDT) central unit and two isomeric alkyl cyanoacetate as the end-capped moieties to conveniently synthesize two isomeric small-molecule donors, namely, BT-RO-Cl and BT-REH-Cl, each bearing linear n-octyl (O) as the terminal alkyl chain and another branched 2-ethylhexyl (EH) as the terminal alkyl chain. The terminal alkyl chain engineering process provided BT-RO-Cl with 13.35% efficiency and BT-REH-Cl with 13.90% efficiency ASM OSCs, both with Y6 as the electron acceptor. The successful performance resulted from uniform phase separation and the favorable combination of face-on and edge-on molecular stacking of blended small-molecule donors and acceptors, which formed a fluent 3D transport channel and thus delivered high and balanced carrier mobilities. These findings demonstrate that alkyl chain engineering can finely control the morphology of ASM OSCs, and provides an alternative for the optimal design of small-molecule materials towards high-performance ASM OSCs

Improving Molecular Planarity by Changing Alky Chain Position Enables 12.3% Efficiency All-Small-Molecule Organic Solar Cells with Enhanced Carrier Lifetime and Reduced Recombination

Molecular stacking plays an important role in defining the active layer morphology in all-small-molecule organic solar cells (ASM OSCs). However, the precise control of donor/acceptor stacking to afford optimal phase separation remains challenging. Herein, the molecular stacking of a small-molecule donor is tuned by changing the alky chain position to match a high-performance small-molecule nonfullerene acceptor (NFA), Y6. The alky chain engineering not only affects the planarity of the small-molecule donor, but also the molecular aggregation and the active layer morphology, and thus the photovoltaic performance. Notably, single-junction ASM OSCs with 12.3% power conversion efficiency (PCE) are achieved. The PCE of 12.3% is among the top efficiencies of single-junction ASM OSCs reported in the literature to date. The results highlight the importance of fine-tuning the molecular structure to achieve high-performance ASM OSCs

Molecular ordering and phase segregation induced by a volatile solid additive for highly efficient all-small-molecule organic solar cells

Morphology control remains a major challenge for all-small-molecule organic solar cells (ASM OSCs), mainly reflecting in the elusive trade-off between the molecular ordering and phase separation of the active layer. In this study, a novel volatile solid additive IC-FI, namely dihalogenated 1,1-dicyanomethylene-3-indanone, whose chemical structure is analogous to the end-group moiety of the representative non-fullerene acceptor, has been prepared and applied in BTR-Cl:N3-based ASM OSCs. The intrinsic structural feature of IC-IF enables the self-assembly of N3 along with the enhanced intermixing between BTR-Cl and N3 in the subsequent thermal annealing process. This offers nano-scale phase separation with a predominant face-on oriented molecular packing in the blend film, thereby boosting the effective charge transport and extraction process in the device. Consequently, a superior power conversion efficiency (PCE) of 14.43% with an outstanding fill factor (FF) of 73.53% was achieved in the IC-FI processed device. These results indicate that the use of the volatile solid additive is a simple and practical strategy for the optimization of the small-molecule bulk-heterojunction morphology toward highly efficient ASM OSCs

Oligothiophene electron donor and electron acceptor for all small molecule organic solar cells with efficiency over 9%

By using easily accessible oligothiophene donors and acceptor, all-oligothiophene organic solar cells (AOT-OSCs) were realized. The devices fabricated with thiazole-centered donor Tz6T and quaterthiophene-based acceptor A4T-16 exhibited a satisfactory power conversion efficiencies (PCEs) of 9.30 %. Moreover, the devices based on Tz6T/A4T-16 binary demonstrated an excellent industrial figure of merit (i-FoM) of up to 0.168 which represented one of the best cost-effectiveness in all small molecule organic solar cells, suggesting the strong potential of all-oligothiophene combinations in developing OSCs with both low cost and high efficiency

18.42% efficiency polymer solar cells enabled by terpolymer donors with optimal miscibility and energy levels

The rapid development of non-fullerene acceptors (NFAs) has placed increasing demands on polymer donors and there is still room for optimization of the currently available top-performing polymer donors to work perfectly with the state-of-the-art NFAs. To further develop and fine-tune the best-performing polymer donors, random ternary copolymerization is a simple and powerful strategy, in which the search for a suitable and easily accessible third component is the challenge. Herein, a series of terpolymer donors OPz11, OPz12, OPz13 and OPz14 are synthesized by incorporating varying fractions of the ester-substituted thiazole (E-Tz) unit into the D18 polymer. Notably, the subtle E-Tz content brings optimization in the energy levels, electrostatic potentials (ESPs), molecular aggregation, miscibility and morphology, as revealed by the open circuit voltage (V-OC) improvement from 856.1 to 865.3 mV, and fill-factor (FF) enhancement from 74.34% to 78.71%. Finally, an exciting power conversion efficiency (PCE) of 18.42% is obtained from OPz11:Y6 devices, and is among the highest values reported for PSCs. This work illustrates that the accessible E-Tz unit is a promising building block to construct terpolymer donors for high-performance organic photovoltaic cells, and random ternary copolymerization is a flexible and promising strategy for photovoltaic materials development.11Nsciescopu