Understanding the Performance of Organic Photovoltaics under Indoor and Outdoor Conditions: Effects of Chlorination of Donor Polymers

Abstract

Understanding the effects of the chemical structures of donor polymers on the photovoltaic properties of their corresponding organic photovoltaic (OPV) devices under various light-intensity conditions is important for improving the performance of these devices. We synthesized a series of copolymers based on poly­[(2,6-(4,8-bis­(5-(2-thioethylhexyl)­thiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-c:4′,5′-c′]­dithiophene-4,8-dione))] (PBDB-TS) and studied the effects of chlorine substitution of its thiophene-substituted benzodithiophene (BDT-Th) unit on its photovoltaic properties. Chlorination of the polymer resulted in a bulk heterojunction (BHJ) morphology optimized for efficient charge transport with suppressed leakage current and an increased open-circuit voltage of the OPV device; this optimization led to a remarkable enhancement of the OPV device’s power conversion efficiency (PCE) not only under the condition of 1 sun illumination but also under a low light intensity mimicking indoor light; the PCE increased from 8.7% for PBDB-TS to ∼13% for the chlorinated polymers, PBDB-TS-3Cl, and PBDB-TS-4Cl under the 1 sun illumination condition and from 5.3% for PBDB-TS to 21.7% for PBDB-TS-4Cl under 500 lx fluorescence illuminance. Interestingly, although the OPV PCEs under 1 sun illumination were independent of the position of chlorine substitution onto the polymer, PBDB-TS-4Cl exhibited better performance under simulated indoor light than its derivative PBDB-TS-3Cl. Our results demonstrate that efficient light absorption and charge-carrier generation play key roles in achieving high OPV efficiency under low-light-intensity conditions.

An Overview of High‐Performance Indoor Organic Photovoltaics

“…[35] Je et al investigated the chlorine substitution effects of thiophene-substituted benzodithiophene (BDTÀ Th) unit on its photovoltaic properties. [36] Under 1 sun illumination, the PCEs of PBDB-TS:IT-4F, PBDB-TS-3Cl:IT-4F, and PBDB-TS-4Cl:IT-4F were 8.7, 12.6, 12.7 %, respectively. On the other hand, the PCEs of PBDB-TS:IT-4F, PBDB-TS-3Cl:IT-4F, and PBDB-TS-4Cl:IT-4F were 5.3, 20.4, and 21.7 % under FL illumination of 500 lx, respectively.…”

“…This result revealed chlorination played a crucial role in optimizing BHJ morphology and increasing V oc . [36] Yin et al of a PDI-tetramer with a tetrathienylbezene (TTB) core) system in thick-film IOPVs. [37] Under 1 sun illumination, this device showed a PCE of 8.3 %.…”

Indoor application of emerging photovoltaics—progress, challenges and perspectives

“…While there are relatively less concerns over the ecotoxicity of OPV devices (e.g. the use of heavy-metal atoms in the donor and acceptor materials 17,28,108 with limited risk of chemical leach), ecotoxicity is a more serious consideration for the commercialisation of PPV and QDPV devices. A recent research study revealed that the Pb from halide perovskites is more harmful than initially expected, and is 10 times more bioavailable than other sources of lead contaminants that already exist around human life.…”

Different Morphology Dependence for Efficient Indoor Organic Photovoltaics: The Role of the Leakage Current and Recombination Losses

“…24−29 However, to the best of our knowledge, only few studies have realized that the differences in indoor OPVs may be related to the crystallinity and phase separation of the photoactive film. 30,31 Since indoor OPVs usually work under weak illumination, the charge carrier density and thus recombination mechanisms/strength may be different from the outdoor case. Therefore, we think that usual optimized film structures will not be enough in terms of driving efficient performance for indoor application as the degree of bimolecular recombination decreases and the influence of leakage current becomes prominent under low light intensity.…”

“…Recent emergence of organic photovoltaics (OPVs) under indoor light illumination has been considered bold in terms of modern electronics development. They have been considered as a major breakthrough in terms of self-sustainable indoor electronic devices such as wireless sensor nodes for the Internet of Things (IoT). − Organic semiconductors offer strong absorbance in the visible region of the solar spectrum (400 to 700 nm), matching perfectly with indoor light sources such as fluorescent lamps, light-emitting diodes (LEDs), and so forth. − Indoor OPVs present tremendous potential for large-scale manufacturing because of their negligible dependence on series resistance ( R s ) and photoactive film thickness under low light intensities. − Therefore, indoor OPVs bring a special technical advantage over Si-based PVs and copper–indium–gallium–selenide in terms of their potential of realizing more efficient devices under low fluences. , At this point, however, power conversion efficiency (PCE) of indoor OPVs has lagged behind their promised theoretical predictions. ,,, Much deliberation is required in terms of controlling the performance of indoor OPVs with photoactive film structures, for example, the performance of outdoor OPVs is more susceptible to the R s , while the performance of indoor OPVs is more sensitive to shunt resistance ( R sh ). − However, to the best of our knowledge, only few studies have realized that the differences in indoor OPVs may be related to the crystallinity and phase separation of the photoactive film. , Since indoor OPVs usually work under weak illumination, the charge carrier density and thus recombination mechanisms/strength may be different from the outdoor case. Therefore, we think that usual optimized film structures will not be enough in terms of driving efficient performance for indoor application as the degree of bimolecular recombination decreases and the influence of leakage current becomes prominent under low light intensity. − …”