High-Efficiency Large-Bandgap Material for Polymer Solar Cells

Eco-Friendly Solvent-Processed Dithienosilicon-Bridged Carbazole-Based Small-Molecule Acceptors Achieved over 25.7% PCE in Ternary Devices under Indoor Conditions

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.

Designing high performance conjugated materials for photovoltaic cells with the aid of intramolecular noncovalent interactions

This review summarizes the recent progress in high performance photovoltaic materials with the aid of intramolecular noncovalent interactions.

Abdullah S, Brza MA, Abdulwahid RT. Influence of Fluorine Substitution on the Optical, Thermal, Electrochemical and Structural Properties of Carbazole-Benzothiadiazole Dicarboxylic Imide Alternate Copolymers

In this work four novel donor-acceptor copolymers, PCDTBTDI-DMO, PCDTBTDI-8, P2F-CDTBTDI-DMO and P2F-CDTBTDI-8, were designed and synthesised via Suzuki polymerisation. The first two copolymers consist of 2,7-carbazole flanked by thienyl moieties as the electron donor unit and benzothiadiazole dicarboxylic imide (BTDI) as electron acceptor units. In the structures of P2F-CDTBTDI-DMO and P2F-CDTBTDI-8 copolymers, two fluorine atoms were incorporated at 3,6-positions of 2,7-carbazole to investigate the impact of fluorine upon the optoelectronic, structural and thermal properties of the resulting polymers. P2F-CDTBTDI-8 possesses the highest number average molecular weight (Mn = 24,200 g mol-1) among all the polymers synthesised. PCDTBTDI-DMO and PCDTBTDI-8 show identical optical band gaps of 1.76 eV. However, the optical band gaps of fluorinated copolymers are slightly higher than non-fluorinated counterparts. All polymers have deep-lying highest occupied molecular orbital (HOMO) levels. Changing the alkyl chain substituents on BTDI moieties from linear n-octyl to branched 3,7-dimethyloctyl groups as well as substituting the two hydrogen atoms at 3,6-positions of carbazole unit by fluorine atoms has negligible impact on the HOMO levels of the polymers. Similarly, the lowest unoccupied molecular orbital (LUMO) energy levels are almost comparable for all polymers. Thermogravimetric analysis (TGA) has shown that all polymers have good thermal stability and also confirmed that the fluorinated copolymers have higher thermal stability relative to those non-fluorinated analogues. Powder X-ray diffraction (XRD) studies proved that all polymers have an amorphous nature in the solid state.

Recent Advances in Wide-Bandgap Photovoltaic Polymers

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

Soluble fluorene–benzothiadiazole polymer-grafted graphene for photovoltaic devices

Herein, a polymer based on fluorene and benzothiadazole (PFBT) was covalently grafted on brominated graphene (G-PFBT) sheets via Suzuki coupling reaction and characterized by Fourier transform infrared (FTIR), ultraviolet-visible (UV-vis), fluorescence emission (FL), and ¹H-NMR spectroscopy.

4-Alkyl-3,5-difluorophenyl-Substituted Benzodithiophene-Based Wide Band Gap Polymers for High-Efficiency Polymer Solar Cells

Two novel polymers PTFBDT-BZS and PTFBDT-BZO with 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as the donor unit, benzothiadiazole or benzooxadiazole as the acceptor unit, and thiophene as the spacer have been synthesized and used as donor materials for polymer solar cells (PSCs). These two polymers exhibited wide optical band gaps of about 1.8 eV. PSCs with the blend of PTFBDT-BZS:PC71BM (1:2, by weight) as the active layer fabricated without using any processing additive and any postannealing treatment showed power conversion efficiency (PCE) of 8.24% with an open circuit voltage (Voc) of 0.89 V, a short circuit current (Jsc) of 12.67 mA/cm(2), and a fill factor (FF) of 0.73 under AM 1.5G illumination, indicating that PTFBDT-BZS is a very promising donor polymer for PSCs. The blend of PTFBDT-BZO:PC71BM showed a lower PCE of 5.67% with a Voc of 0.96 V, a Jsc of 9.24 mA/cm(2), and an FF of 0.64. One reason for the lower PCE is probably due to that PTFBDT-BZO has a smaller LUMO offset with PC71BM, which cannot provide enough driving force for charge separation. And another reason is probably due to that PTFBDT-BZO has a lower hole mobility in comparison with PTFBDT-BZS.

Effect of fluorination pattern and extent on the properties of PCDTBT derivatives

Fluorination of PCDTBT derivatives at different positions and to different extents causes major changes in their properties.

Enhancing the Photovoltaic Performance by Tuning the Morphology of Polymer:PC₇₁BM Blends with a Commercially Available Nucleating Agent

The use of a commercially available nucleating agent as the additive for the fabrication of polymer:PC71BM-based active layers by solution-processing can greatly enhance the power conversion efficiency (PCE) of bulk heterojunction polymer solar cells (BHJ PSCs). The enhancement of device performance is mainly due to the addition of nucleating agent, which is able to regulate the drying process of the active layer and decrease the oversized domain size of conjugated polymers. Via this effective strategy to optimize the film morphology, the designed device exhibits an enhancement as great as 30.8%.