Benzobisthiazole as Weak Donor for Improved Photovoltaic Performance: Microwave Conductivity Technique Assisted Molecular Engineering

Abstract

New donor-acceptor-type copolymers comprised of benzobisthiazole (BBTz) as a weak donor rather than acceptor are proposed. This approach can simultaneously lead to deepening the HOMO and LUMO of the polymers with moderate energy offset against fullerene derivatives in bulk heterojunction organic photovoltaics. As a proof-of-concept, BBTz-based random copolymers conjugated with typical electron acceptors: thienopyrroledione (TPD) and benzothiadiazole (BT) based on density functional theory calculations are synthesized. Laser-fl ash and Xe-fl ash time-resolved microwave conductivity (TRMC) evaluations of polymer:[6,6]-phenyl C 61 butyric acid methyl ester (PCBM) blends are conducted to screen the feasibility of the copolymers, leading to optimization of processing conditions for photovoltaic device application. According to the TMRC results, alternating BBTz-BT copolymers are designed, exhibiting extended photoabsorption up to ca. 750 nm, deep HOMO (-5.5 to -5.7 eV), good miscibility with PCBM, and inherent crystalline nature. Moreover, the maximized PCE of 3.8%, the top-class among BBTz-based polymers reported so far, is realized in an inverted cell using TiO x and MoO x as the buffer layers. This study opens up opportunities to create low-bandgap polymers with deep HOMO, and shows how the device-less TRMC evaluation is of help for decision-making on judicious molecular design.

Boosting photovoltaic performance of a benzobisthiazole based copolymer: a device approach using a zinc oxide electron transport layer

“…17 The underlying strategy of this polymer is that BBTz and BT are commonly recognized as electron-decient (acceptor) moieties, and thus either of them is polymerized with electron-rich (donor) units; [18][19][20][21] however, we coupled them by regarding BBTz as a weak donor, which delivered a crystalline solution-processable polymer with inherent optoelectronic property, the deep highest occupied molecular orbital (HOMO) of À5.7 eV (typically À5.0 to À5.5 eV for low bandgap polymer), and a moderate position of the lowest unoccupied molecular orbital (LUMO) at À3.9 eV. 17 The conventional normal cell of PBBTzBT-DT: [6,6]phenyl C 61 butyric acid methyl ester (PC 61 BM) ¼ 1 : 2 using a poly-(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) anode buffer layer and a Ca/Al cathode resulted in a PCE of 2.37%, the current-voltage (J-V) curve of which showed an S-kink near the open circuit condition, 22 presumably due to the large energy offset and/or interfacial resistance between PEDOT:PSS and PBBTzBT-DT. In contrast, the inverted cell (titanium oxide, TiO 2 as cathode buffer layer, molybdenum oxide, MoO x as anode buffer layer, and Au as anode) revealed a PCE of 3.84% without undesired strong electric eld dependence on charge extraction.…”

“…In addition to this optical property, we should note that the balance between the charge separation efficiency and charge carrier lifetime is maximized at a 1 : 2 blend ratio, as surveyed by laser and Xe-ash TRMC. 17 The OPV performance was measured under AM1.5G illumination (100 mW cm À2 ) as shown in Fig. 2a and the parameters are summarized in Table 1.…”

Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based on Cyclopentadithiophene as Donors in Bulk Heterojunction Organic Solar Cells: DFT-Based Modeling and Calculations

“…The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) strongly determine the optoelectronic properties of conjugated compounds, pointedly on the photovoltaic properties of donor materials. Largely, donor compound should tend to have a deep HOMO level to assure a high open circuit voltage V OC and a suitable LUMO energy level with respect to that of the acceptor unit [47][48][49].…”

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

“…5a , is a weak electron acceptor but could act as a weak electron donor against strong electron acceptors. First, the author synthesized three random copolymers of BBTz coupled with benzothiadiazole (BT) or thienopyrroledione (TPD) as the acceptor and carbazole as the donor [ 97 ]. Although the solubilities of these polymers are insufficient due to the nonoptimized alkyl chains, TRMC evaluations of their PCBM blends were successfully performed, showing the largest Δ σ max for BBTz–BT.…”