Benzo[1,2-b:4,5-b']difuran Polymer-Based Non-Fullerene Organic Solar Cells: The Roles of Non-Fullerene Acceptors and Molybdenum Oxide on Their Ambient Stabilities and Processabilities

Siloxane Decorated Water-Obstructing Guest for Efficient Air-Processed OSCs

The future applications of organic solar cells (OSCs) necessitate a thorough consideration of their ambient stability and processability, particularly for large area air-processed engineering, but water-induced degradation of active layer critically restricts its development. To surmount this hurdle, a water-obstructing guest (WOG) strategy is proposed to attenuate the interaction of the active layer with water molecules, reduce defects in blend films, and enhance the devices stability under high relative humidity (RH) conditions by introducing a siloxane-containing polymer D18-SiO. In addition to suppressing trap density, the WOG with hydrophobic and low surface free energy characteristics, forms a capping layer that blocks moisture penetration while preserving ideal nano-micromorphology with high crystallinity and tight packing properties. Power conversion efficiencies (PCE) of >19% is reported for spin coating OSCs fabricated across an RH range of 20 to 90%, and PCE of >17% blade coating OSCs at 90% RH. The D18-SiO, serves as a protective barrier to enhance the device stability, and the corresponding unencapsulated OSCs retained 80.7% of its initial performance in air (≈ 40% RH) after 600-h maximum power point tracking under continuous light illumination, showcasing great potential in designing WOG strategy for large-scale production of air-processed OSCs.

The Double-Cross of Benzotriazole-Based Polymers as Donors and Acceptors in Non-Fullerene Organic Solar Cells

Organic solar cells (OSCs) are considered a very promising technology to convert solar energy to electricity and a feasible option for the energy market because of the advantages of light weight, flexibility, and roll-to-roll manufacturing. They are mainly characterized by a bulk heterojunction structure where a polymer donor is blended with an electron acceptor. Their performance is highly affected by the design of donor-acceptor conjugated polymers and the choice of suitable acceptor. In particular, benzotriazole, a typical electron-deficient penta-heterocycle, has been combined with various donors to provide wide bandgap donor polymers, which have received a great deal of attention with the development of non-fullerene acceptors (NFAs) because of their suitable matching to provide devices with relevant power conversion efficiency (PCE). Moreover, different benzotriazole-based polymers are gaining more and more interest because they are considered promising acceptors in OSCs. Since the development of a suitable method to choose generally a donor/acceptor material is a challenging issue, this review is meant to be useful especially for organic chemical scientists to understand all the progress achieved with benzotriazole-based polymers used as donors with NFAs and as acceptors with different donors in OSCs, in particular referring to the PCE.

A New Diazabenzo[k]fluoranthene-based D-A Conjugated Polymer Donor for Efficient Organic Solar Cells

The development of wide-bandgap polymer donors having complementary absorption and compatible energy levels with near-infared (NIR) absorbing nonfullerene acceptors is highly important for realizing high-performance organic solar cells (OSCs). Herein, a new thiophene-fused diazabenzo[k]fluoranthene derivative has been successfully synthesized as the electron-deficient unit to construct an efficient donor-acceptor (D-A) type alternating copolymer donor, namely PABF-Cl, using the chlorinated benzo[1,2-b:4,5-b']dithiophene as the copolymerization unit. PABF-Cl exhibits a wide optical bandgap of 1.93 eV, a deep highest occupied molecular level of -5.36 eV, and efficient hole transport. As a result, OSCs with the best power conversion efficiency of 11.8% has been successfully obtained by using PABF-Cl as the donor to blend with a NIR absorbing BTP-eC9 acceptor. Our work thus provides a new design of electron-deficient unit for constructing high performance D-A type polymer donors. This article is protected by copyright. All rights reserved.

Simple Tricyclic-Based A-π-D-π-A-Type Nonfullerene Acceptors for High-Efficiency Organic Solar Cells

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 (DTC-BO-4F, DTS-BO-4F, and DTP-BO-4F) comprising a tricyclic fused-ring core, two 2,5-bis(alkyloxy)phenylene spacers, and two difluorinated terminal groups (DF-IC) were developed. Compared with DTS-BO-4F, DTC-BO-4F and DTP-BO-4F exhibit higher molar extinction coefficients, stronger crystallinity, and more orderly stacking. The PBDB-T:DTC-BO-4F-based blend film shows suitable phase separation and higher and more balanced charge mobilities. Finally, the photovoltaic devices based on DTC-BO-4F give an outstanding PCE of 13.26% with a small nonradiative voltage loss of 0.23 eV.

Tricyclic or Pentacyclic D Units: Design of D-π-A-Type Copolymers for High VOC Organic Photovoltaic Cells

Although there are several electron-donating (D) units, only the classic benzo[1,2-b:4,5-b']dithiophenes (BDT) unit was utilized to develop D-π-A-type copolymers for high-voltage organic photovoltaic (OPV) cells. Hence, in this work, we chose two tricyclic D units, BDT and benzo[1,2-b:4,5-b']difurans (BDF), together with one pentacyclic ring, dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT), to comprehensively study the effect of different D units on the optoelectronic properties and photovoltaic performance. By copolymerized with the benzo[1,2,3]triazole (BTA) electron-accepting unit, the final copolymers J52-Cl, F11, and PE52 were combined with a nonfullerene acceptor (NFA) F-BTA3 according to the "Same-A-Strategy." As we preconceived, all the three single-junction OPV cells can obtain high open-circuit voltage (V_OC) over 1.10 V. Although the tricyclic D unit of BDF exhibits a slightly lower V_OC of 1.12 V because of its mildly larger energy loss of 0.698 eV, its higher carrier mobilities and exciton dissociation efficiency strikingly boost the short-circuit current (J_SC) and fill factor, which contribute to a comparable PCE of 10.04% with J52-Cl (10.10%). However, the DTBDT-based polymer PE52 shows the worst performance with a PCE of 6.78% and a V_OC of 1.14 V, owing to the higher bimolecular recombination and disordered molecular stacking. Our results indicate that tricyclic D units should be a better choice for constructing D-π-A-type polymers for high-voltage photovoltaic materials than the pentacyclic analogues.

Synergistic effect of carotenoid and silicone-based additives for photooxidatively stable organic solar cells with enhanced elasticity

Photochemical and mechanical stability are critical in the production and application of organic solar cells. While these factors can individually be improved using different additives, there is no example of studies on the combined effects of such additive-assisted stabilization. In this study, the properties of PTB7:[70]PCBM organic solar cells are studied upon implementation of two additives: the carotenoid astaxanthin (AX) for photochemical stability and the silicone polydimethylsiloxane (PDMS) for improved mechanical properties. A newly designed additive, AXcPDMS, based on astaxanthin covalently bonded to PDMS was also examined. Lifetime tests, produced in ISOS-L-2 conditions, reveal an improvement in the accumulated power generation (APG) of 10% with pure AX, of 90% when AX is paired with PDMS, and of 140% when AXcPDMS is added in the active layer blend, as compared to the control devices. Singlet oxygen phosphorescence measurements are utilized to study the ability of AX and AXcPDMS to quench singlet oxygen and its precursors in the films. The data are consistent with the strong stabilization effect of the carotenoids. While AX and AXcPDMS are both efficient photochemical stabilizers, the improvement in device stability observed in the presence of AXcPDMS is likely due to a more favorable localization of the stabilizer within the blend. The mechanical properties of the active layers were investigated by tensile testing and cohesive fracture measurements, showing a joint improvement of the photooxidative stability and the mechanical properties, thus yielding organic solar cell devices that are promising for flexible photovoltaic applications.