Control of Nanomorphology in Fullerene-Free Organic Solar Cells by Lewis Acid Doping with Enhanced Photovoltaic Efficiency

Abstract

Generating desired efficiency enhancements in organic solar cells (OSCs) by charge-transfer doping requires to obtain modified optoelectronic properties while retaining the favorable nanomorphology. We report a thermally assisted doping based on Lewis acid tris­(pentafluorophenyl)-borane (BCF) as a p-dopant for two groups of OSCs comprising the PBDB-TF and PBDB-T donors and a nonfullerene acceptor IT-4F. We found that the face-on molecular packing in the PBDB-TF:IT-4F blend or neat PBDB-TF donor films is favorably modified with the formation of frustrated Lewis pairs (FLPs) in the donor, which is in contrast to the hampered π–π stacking in the doped PBDB-T film. The different impacts of BCF dopants on the morphology lead to contrasting photovoltaic behaviors where the PBDB-TF-based devices receive enhanced power conversion efficiencies (PCEs) in the presence of BCF, while reduction of efficiencies is observed in the PBDB-T device. In the best doping conditions with the proposed hot-film deposition, we achieve a boosted PCE of 14.1% in PBDB-TF:IT-4F solar cells at low BCF concentrations. Based on the same fluorinated donor, the described BCF doping also applies to NF-solar cells based on the NF-acceptor Y6, leading to an increase in the PCE to 16.0%. Our results suggest that controlling the degree of FLP formation in the donor component with the addition of BCF is key to obtaining desired improvements on nanomorphology and relevant photophysical properties in OSCs.

Identifying the Electrostatic and Entropy‐Related Mechanisms for Charge‐Transfer Exciton Dissociation at Doped Organic Heterojunctions

“…Recently, the bulk doping has been adopted in the OPV research. The consequent trap-filling, [36][37][38] better charge transport, [39,40] and morphology optimizations [41][42][43] are found to contribute to the enhancement of photovoltaic performances. Going a step further, we intentionally dope the organic heterojunction where the photocharge generation and recombination take place.…”

Highly Efficient Nonfullerene Organic Photovoltaic Devices with 10% Power Conversion Efficiency Enabled by a Fine‐Tuned and Solution‐Processed Hole‐Transporting Layer

“…Carefully adjusting the energy‐level matching and interface characteristics may further improve the PCE of OPV devices. [ 36 ] In this work, two donor polymers, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)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‐T) [ 37 ] and poly[(2,6‐(4,8‐bis(4‐fluoro‐5‐(2‐hexyldecyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(5‐octyl‐1,3‐di(thiophen‐2‐yl)‐4H‐thieno[3,4‐c]pyrrole‐4,6(5H)‐dione)] (TPD‐3F), [ 38 ] were systematically studied in the inverted device architecture and mixed with a typical NFA, 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IT‐4F), [ 39 ] which was used as a BHJ layer. The detailed energy‐level diagram of each material in the device is shown in Figure .…”

Enhanced dissociation of excitons and charge transfer in organic solar cells of polymer:fullerene blends with molecular BPFZn doping

“…21 In another report, TPFB-doped PBDB-TF:Y6 blends resulted in enhanced charge-transport properties and thus a higher PCE by inducing frustrated Lewis pairs in the donor. 22 These findings are encouraging to further study additional Lewis acids as a dopant with potential significance in advancing the device performance.…”

“…TPFB‐doped fullerene and norfullerene OSCs exhibited a good device performance achieved by the synergic effect of the dopant on the polymer by tuning its electronic properties and enhanced nanostructures of the acceptor 21 . In another report, TPFB‐doped PBDB‐TF:Y6 blends resulted in enhanced charge‐transport properties and thus a higher PCE by inducing frustrated Lewis pairs in the donor 22 . These findings are encouraging to further study additional Lewis acids as a dopant with potential significance in advancing the device performance.…”