Effects of Chain Orientation in Self-Organized Buffer Layers Based on Poly(3-alkylthiophene)s for Organic Photovoltaics

Fanji Wang; Kazuhito Hashimoto; Hiroshi Segawa; Keisuke Tajima

doi:10.1021/acsami.7b19174

Effects of Chain Orientation in Self-Organized Buffer Layers Based on Poly(3-alkylthiophene)s for Organic Photovoltaics

ACS Appl Mater Interfaces. 2018 Mar 14;10(10):8901-8908. doi: 10.1021/acsami.7b19174. Epub 2018 Mar 1.

Authors

Fanji Wang^{1

2}, Kazuhito Hashimoto², Hiroshi Segawa^{3

4}, Keisuke Tajima¹

Affiliations

¹ RIKEN Center for Emergent Matter Science (CEMS) , 2-1 Hirosawa , Wako, Saitama 351-0198 , Japan.
² Department of Applied Chemistry, Graduate School of Engineering , The University of Tokyo , 7-3-1 Hongo , Bunkyo-ku, Tokyo 113-8656 , Japan.
³ Research Center for Advanced Science and Technology , The University of Tokyo , 4-6-1 Komaba , Meguro-ku, Tokyo 153-8904 , Japan.
⁴ Graduate School of Arts and Sciences , The University of Tokyo , 3-8-1 Komaba , Meguro-ku, Tokyo 153-8902 , Japan.

PMID: 29457447
DOI: 10.1021/acsami.7b19174

Abstract

Surface-segregated monolayers (SSMs) based on two poly(3-alkylthiophene)s with semifluoroalkyl groups at either the side chains (P3DDFT) or one end of the main chain (P3BT-F₁₇) were used as self-organized buffer layers at the electrode interfaces in bulk heterojunction (BHJ) organic photovoltaic devices. Both of the SSMs greatly shifted the vacuum levels of the BHJ films at the surface due to the aligned permanent dipole moments of the semifluoroalkyl chains. Hole extraction in the BHJ of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) became more efficient in the presence of the P3DDFT buffer layer, resulting in an improved power conversion efficiency. In contrast, the SSM of P3BT-F₁₇ induced changes in the chain orientation of P3HT and the morphology of the BHJ films, resulting in decreased performance. These results indicate that the molecular design of polymer-based SSMs can affect not only the energy structure at the interface but also the morphology and the molecular orientations in the BHJs.

Keywords: bulk heterojunction; interfacial modification; molecular orientation; organic photovoltaics; poly(3-alkylthiophene); self-organized buffer layers; surface-segregated monolayers.