Low-Bandgap Conjugated Polymers for Bulk Heterojunction Solar Cells
Authors: Imahori Hiroshi 
Published: February 2020

Polymer solar cells (PSCs) have drawn much attention for applications in renewable energy because of their potential for low cost, lightweight, and large-area processability. On the basis of bulk heterojunction (BHJ) concept using blends of p-type semiconducting polymers and n-type semiconducting fullerenes, device performances have been improved rapidly. Low-bandgap conjugated polymers have been developed intensively in recent years with the aim of better matching between the absorption and solar spectra, reaching a power conversion efficiency (PCE) more than 7%. On the other hand, [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) and its corresponding C70 derivative ([70]PCBM) are exclusively employed as an acceptor in the highly efficient PSCs.

A facile method to control HOMO−LUMO levels of polymers by combining electron-rich (donor) and electron-deficient (acceptor) moieties in their repeating units, forming internal donor−acceptor (D−A) structures, has been intensively explored. Among the numerous building blocks, carbazole is one of the most prevalent electron-rich units owing to its fully aromatic fused ring with sufficient chemical and environmental stability. Various electron-deficient fused-heteroarenes were systematically copolymerized with 2,7-carbazole toward rational design of photovoltaic conjugated polymers by Leclerc et al. in 2008. They found that the HOMO and LUMO energy levels are mainly correlated to carbazole and acceptor units, respectively, and symmetric acceptor units allow for a higher degree of crystalline order. In their study, poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) with a relatively low HOMO level (−5.5 eV) had superior potential for applications in PSCs. The initial PCE reached 3.6% in a typical BHJ device and has been subsequently improved, exceeding 6% by thickness optimization and nanomorphology control of the BHJ layer. However, the bandgap of PCDTBT is still larger than the value (1.5 – 1.7 eV) of the ideal polymers for PCE values exceeding 10%. Recently, to lower the bandgap of PCDTBT without sacrificing the low HOMO level, further different electron-accepting monomers including phenylbenzotriazole, naphthothiadiazole, dialkoxylated benzothiadiazole, and quinoxaline-based units have been incorporated into 2,7-carbazole based copolymers. In addition, ladder-typed analogues of carbazole with forced planarity were copolymerized with benzothiadiazole unit. However, the PCE values of the PSCs with these polymers still remain at most comparable to PCDTBT-based devices.

To investigate the effect of fluorine substitution on molecular and film structures, optical, electrochemical, and photovoltaic properties of a low-bandgap polymer, PCDTBT with deep HOMO energy level, a fluorinated analogue of PCDTBT (i.e., PCDTBT-F) has been developed for the first time by replacing two hydrogen atoms on benzothiadiazole (BT) units with two fluorine atoms. An analogous polymer, PCBBBT-F with additional hexylthiophenes between the thiophene and carbazole of PCDTBT-F has also been prepared to overcome the poor solubility of PCDTBT-F. The PCBBBT-F film showed wide absorption bands in UV and visible regions with an optical bandgap of 1.82 eV that is smaller than that of PCDTBT (1.89 eV), whereas the film of PCDTBT-F exhibited blue-shifted absorption with a bandgap of 1.96 eV due to the low molecular weight arising from the deficient solubility. The HOMO energy level of PCDTBT-F is lower than that of PCDTBT owing to the electron-withdrawing fluorination of the BT unit, whereas PCBBBT-F exhibited a higher HOMO level than PCDTBT, implying that the additional incorporation of electron-donating hexylthiophenes negated the fluorination effect. A BHJ-PSC that employed PCDTBT-F or PCBBBT-F as an electron donor and a fullerene derivative [70]PCBM as an electron acceptor yielded lower PCEs of 1.29% and 1.98%, respectively, than that of PCDTBT (6.16%) due to the unfavorable film structures of PCDTBT-F:[70]PCBM resulted from the poor solubility and low molecular weight as well as low crystallinities and limited exciton lifetimes of the fluorinated polymers. These results provide valuable information on the elaborate design of PCDTBT-based polymers for the PSC applications. I will also talk about other examples of low-bandgap conjugated poymers, especially phophole-based ones for BHJ solar cells.1-6

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