Fluorination of Benzothiadiazole–Benzobisthiazole Copolymer Leads to Additive-Free Processing with Meliorated Solar Cell Performance

Processing solvents and conditions have unique importance in the performance of bulk heterojunction organic solar cells. In the present work, we have investigated the role of a primary solvent and solvent additive in the device performance of two benzobisthiazole (BBTz)-based push–pull type polymers. In an inverted cell structure, the BBTz-<i>co</i>-fluorinated benzothiadiazole polymer (PBBTzFT) with a PC₇₁BM acceptor showed additive-free enhanced performance with a power conversion efficiency (PCE) of 6.4% from a 1,2-dichlorobenzene solvent, while the BBTz-<i>co</i>-pyridylthiadiazole polymer (PBBTzPT) showed maximum performance from a chlorobenzene (CB) solution with a 1,8-diiodooctane (DIO) additive (PCE = 2.3%). The detailed investigation by atomic force microscopy and two-dimensional grazing incidence X-ray diffraction corroborates that the fluorination of benzothiadiazole brought about optimal morphology without a solvent additive, the PCE of which is comparable with the previous nonfluorinated analogue (PCE = 6.5%) processed from CB with DIO

Exploring Alkyl Chains in Benzobisthiazole-Naphthobisthiadiazole Polymers: Impact on Solar-Cell Performance, Crystalline Structures, and Optoelectronics

The shapes and lengths of the alkyl chains of conjugated polymers greatly affect the efficiencies of organic photovoltaic devices. This often results in a trade-off between solubility and self-organizing behavior; however, each material has specific optimal chains. Here we report on the effect of alkyl side chains on the film morphologies, crystallinities, and optoelectronic properties of new benzobisthiazole-naphthobisthiadiazole (PBBT-NTz) polymers. The power conversion efficiencies (PCEs) of linear-branched and all-branched polymers range from 2.5% to 6.6%; the variations in these PCEs are investigated by atomic force microscopy, two-dimensional X-ray diffraction (2D-GIXRD), and transient photoconductivity techniques. The best-performing linear-branched polymer, bearing dodecyl and decyltetradecyl chains (C12-DT), exhibits nanometer-scale fibers along with the highest crystallinity, comprising predominant edge-on and partial face-on orientations. This morphology leads to the highest photoconductivity and the longest carrier lifetime. These results highlight the importance of long alkyl chains for inducing intermolecular stacking, which is in contrast to observations made for analogous previously reported polymers

Charge Dynamics at Heterojunction between Face-on/Edge-on PCPDTBT and PCBM Bilayer: Interplay of Donor/Acceptor Distance and Local Charge Carrier Mobility

The bulk heterojunction organic solar cell has shown much promise as a cost-effective energy harvesting device, while despite recent progress in boosted power conversion efficiency, critical photophysical process at the interface of electron donor and electron acceptor is subject of ongoing debate. Here we investigate the impact of polymer orientation of cychlopentadithiophene–benzothiadiazole copolymer (PCPDTBT) on the charge separation (CS) and recombination (CR) at the bilayer heterojunction of polymer and methanofullerene (PCBM). The charge carrier dynamics at contrasting face-on-rich or edge-on interface controlled via side-alkyl chain modification are monitored by flash-photolysis time-resolved microwave conductivity (TRMC). The data are analyzed using singlet exciton diffusion to donor–acceptor interface with quenching term at high excitation density. We show that CS is more efficient for the face-on-rich interface than edge-on, while CR is in turn retarded for the latter. Along with computational validation based on density functional theory, molecular dynamics, and the Marcus–Levich–Jortner model, our work provides a useful guide regarding interplay of polymer/fullerene interface, exciton/charge dynamics, and local charge carrier mobility

A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse and Microwave Conductivity

State-of-the-art low band gap conjugated polymers have been investigated for application in organic photovoltaic cells (OPVs) to achieve efficient conversion of the wide spectrum of sunlight into electricity. A remarkable improvement in power conversion efficiency (PCE) has been achieved through the use of innovative materials and device structures. However, a reliable technique for the rapid screening of the materials and processes is a prerequisite toward faster development in this area. Here we report the realization of such a versatile evaluation technique for bulk heterojunction OPVs by the combination of time-resolved microwave conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash lamp. Xe-flash TRMC allows examination of the OPV active layer without requiring fabrication of the actual device. The transient photoconductivity maxima, involving information on generation efficiency, mobility, and lifetime of charge carriers in four well-known low band gap polymers blended with phenyl-C₆₁-butyric acid methyl ester (PCBM), were confirmed to universally correlate with the PCE divided by the open circuit voltage (PCE/<i>V</i>_oc), offering a facile way to predict photovoltaic performance without device fabrication