Higher order structures of semiconducting supramolecular polymers have a huge impact on their BHJ-OPV device performance.

Helical self-assembly of functional π-conjugated molecules offers unique photochemical and electronic properties in the spectroscopic level, but there are only a few examples that demonstrate their positive impact on the optoelectronic device level. Here, we demonstrate that hydrogen-bonded tapelike supramolecular polymers of a barbiturated oligo(alkylthiophene) show notable improvement in their photovoltaic properties upon organizing into helical nanofibers. A tapelike hydrogen-bonded supramolecular array of barbiturated oligo(butylthiophene) molecules was directly visualized by STM at a liquid-solid interface. TEM, AFM and XRD revealed that the tapelike supramolecular polymers further organize into helical nanofibers in solution and bulk states. Bulk heterojunction solar cells of the helical nanofibers and soluble fullerene showed a power conversion efficiency of 4.5%, which is markedly high compared to that of the regioisomer of butyl chains organizing into 3D lamellar agglomerates.

The design of molecular systems with high-fidelity self-assembly pathways that include several levels of hierarchy is of primary importance for the understanding of structure-function relationships, as well as for controlling the functionality of organic materials. Reported herein is a high-fidelity self-assembly system that comprises two hydrogen-bonding molecular semiconductors with regioisomerically attached short alkyl chains. Despite the availability of both discrete cyclic and polymeric linear hydrogen-bonding motifs, the two regioisomers select one of the two motifs in homogeneous solution as well as at the 2D-confined liquid-solid interface. This selectivity arises from the high directionality of the involved hydrogen-bonding interactions, which renders rerouting to other self-assembly pathways difficult. In thin films and in the bulk, the resulting hydrogen-bonded assemblies further organize into the expected columnar and lamellar higher-order architectures via solution processing. The contrasting organized structures of these regioisomers are reflected in their notably different miscibility with soluble fullerene derivatives in the solid state. Thus, electron donor-acceptor blend films deliver a distinctly different photovoltaic performance, despite their virtually identical intrinsic optoelectronic properties. Currently, we attribute this high-fidelity control via self-assembly pathways to the molecular design of these supramolecular semiconductors, which lacks structure-determining long aliphatic chains.

Hydrogen-bonding oligothiophene bearing octyl chains self-assembles into extended nanorods via the formation of cyclic hexamer. The nanorods can function as an electron-donating materials for electron-accepting PC71BM in bulk-heterojunction solar cells.

Benzodithiophene-functionalized oligothiophene with barbituric acid hydrogen-bonding unit self-assembles into nanoscopic structures via the formation of rosettes. The nanostructures show a power conversion efficiency of 3% upon mixing with PC61BM in bulk-heterojunction solar cells without thermal annealing.

はじめに:バルクヘテロ型有機太陽電池では、有 機層でのドナー及びアクセプターのナノ構造制 御が難しく、電荷分離とキャリアの取り出しの両 立が難しい。この問題を解決するために注目され ているのが、水素結合を用いた π共役オリゴマー の自己組織化である。これまでに我々は、水素結 合部位としてバルビツール酸を有するオリゴチ オフェン(BAR-T-3H4T, Fig. 1)の自己組織化に よりナノロッド構造が形成され、太陽電池性能が 向上することを報告している 。本研究では、オリゴチオフェン化合物のアルキル鎖長を変化さ せた場合の、ナノロッド形成と太陽電池性能の関係を調べた。 実験: アルキル鎖長の異なる3つの水素結合性オリゴチオフェン(Fig. 1)と PC71BM の混合溶 液をスピンコートし、CS2による solvent vapor annealing (SVA) 処理を行った。続いて Ca/Al電極 を真空蒸着し有機薄膜太陽電池を作製した。封止後、AM1.5照射下における J-V 特性と、単色光 に対する分光感度を測定した。 結果と考察:以前に報告した BAR-T-3H4Tより も、アルキル鎖長を短くした材料では性能が向 上した。特に、BAR-T-3B4Tでは室温成膜にお いてもナノロッドの形成が確認された (Fig. 2(a))。素子性能として JSC = 9.94 mA/cm、VOC = 0.74 V、FF = 62.8 %が観測され、超分子ナノロ ッド系としてはこれまでで最も高い PCE = 4.65 %を示した (Fig. 2(b))。アルキル鎖の短い BAR-T-3B4Tがナノロッドを形成し易く、かつ SVA処理によりナノロッドの伸長が起きることで、 電荷の輸送性が向上したためであると考えられる。さらに、溶液濃度が一定値以上の時に溶液攪 拌中にゲル化が観測された。興味深いことに、その状態からスピンコートした素子においては、 300 nm以上の厚い膜でも 3.64%の性能が得られ、PCEの低下が小さいことが分かった。 謝辞: 本研究は JST-CREST「戦略的創造研究推進事業」の援助のもとに行われた。 1) S. Yagai, K. Nakayama et al., Chem. Eur. J. 20, 1 – 11 (2014). Fig. 1 Chemical structure of used material, and schematic illustration of supramolecular nanorods. BAR-T-3B4T