Organic Solar Cells Based on the Aggregate of Synthetic Chlorophyll Derivative with over 5% Efficiency

Abstract

The power conversion efficiency (PCE) of chlorophyll (Chl)-based organic solar cells (OSCs) is generally about 2%. Herein, a Chl-a derivative (CHL) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) are successfully used to fabricate Chl-based OSCs with PCE over 5%. Two different preparation methods are used to prepare the active layer: 1) two-step spin-coating the self-aggregated CHL and PC 71 BM solutions sequentially and 2) one-step spin-coating the solution of CHL:PC 71 BM blends, forming the "bilayer" (BL) and traditional bulk heterojunction (BHJ) configurations, respectively. Based on the aforementioned two kinds of active-layer preparation methods, both inverted and regular types of OSCs are successfully investigated. All four types of devices work normally, which is likely due to the ambipolar characteristics of the CHL aggregate. Unexpectedly, the BL-based devices yield PCEs of 5.17% for the regular type and 5.19% for the inverted type, which are higher than those of the BHJ-based devices (3.96% for the regular type and 3.50% for the inverted type). The main improvement in PCEs of BL-based devices comes from the enhanced short-circuit currents, which is due to the decreased charge transfer resistance and enlarged photocurrent contribution of PC 71 BM as well as slightly enhanced electron and hole mobilities.

Semisynthetic Chlorophyll Derivatives Toward Solar Energy Applications

“…a) The molecular structures of chlorophyll‐ a derivatives investigated for OSCs and b) chemical structure of ZnChl‐45, its absorption spectra in the solid aggregate state (blue) and in a THF solution (green), the BL and BHJ device architectures using ZnChl‐45 and PC 71 BM as photoactive materials, and the energy alignments of the regular and inverted types of OSC devices. Reproduced with permission . Copyright 2019, Wiley‐VCH.…”

“…More recently, ZnChl‐45 possessing an ability to form J aggregate by self‐assembly was successfully applied to fabricate OSCs with PCE over 5% by two‐step sequencial spin‐coating method for making bilayer (BL), which is the highest PCE for Chls‐based OSCs until now (Figure b) . BHJ solar cells based on ZnChl‐45 and PC 71 BM are also prepared for comparison.…”

Synthesis of C3/C13‐Substituted Semi‐Synthetic Bacteriochlorophyll‐a Derivatives and Their Properties as Functional Dyes

“…The newly synthesized BChl‐ a derivatives 3 and 4 possessing the dicyanomethylene group(s) showed better performance than did natural type 1 . Although organic photovoltaics with higher PCE have been reported, [36] these semi‐synthetic bacteriochlorins with further modification such as conjugation with fullerene derivatives, central metal insertion, introduction of C3 1 ‐hydroxy group or C17 2 ‐long alkyl esterifying chains would lead to the application to not only single layer OSC [37] or bilayer OSCs [25b] but also to other types of photovoltaics represented by perovskite solar cells, [18,38] bio‐solar cells, [39] photocatalyst for hydrogen evolution, [6] or energy‐storage devices [40] …”

Chlorophylls in thin-film photovoltaic cells, a critical review

“…ambient humidity or matrix type, 5,14,22,31,38,39 but most importantlyon the aggregation of molecules. 4,[9][10][11][12]21,27,32,34,38,[40][41][42][43][44][45][46][47][48][49] The optical spectroscopy, due to convenience of use and availability in laboratories, serves as the most frequently applied tool for assessing the aggregation state of chlorophylls in solutions and lms.…”

“…The overwhelming majority of works on the semisynthetic chlorin or Pheo derivatives deal with the zinc complexes. [3][4][5][9][10][11][12]27,38,40,42,43,[45][46][47]49,50,52 Only Sasaki et al frequently use metal-free derivatives, 5,6,27,32,33,40,43,50,52,53 which could potentially be helpful for elucidating the role of metal ligation in the charge transport throughout the aggregate lms.…”