Hydrogen bonds in PC61BM solids

Chun-Qi Sheng,Zheng Chen,Hai-Yang Li,Guang-Hua Chen,Wen-Jie Li,Ying-Ying Du,Hong-Nian Li

doi:10.1063/1.4931791

Abstract

We have studied the hydrogen bonds in PC61BM solids. Inter-molecular interaction is analyzed theoretically for the well-defined monoclinic (P21/n) structure. The results indicate that PC61BM combines into C–H⋯Od bonded molecular chains, where Od denotes the doubly-bonded O atom of PC61BM. The molecular chains are linked together by C–H⋯Os bonds, where Os denotes the singly-bonded O atom of PC61BM. To reveal the consequences of hydrogen bond formation on the structural properties of PC61BM solids (not limited to the monoclinic structure), we design and perform some experiments for annealed samples with the monoclinic (P21/n) PC61BM as starting material. The experiments include differential scanning calorimetry, X-ray diffraction and infrared absorption measurements. Structural phase transitions are observed below the melting point. The C–H⋯Od bonds seem persisting in the altered structures. The inter-molecular hydrogen bonds can help to understand the phase separation in polymer/PC61BM blends and may be responsible for the existence of liquid PC61BM.

Highlights

The fullerene derivative PC61BM1 is one of the best and widely used electron acceptors for high-performance bulk-heterojunction (BHJ) organic solar cells (OSCs).[2,3] It is well known that the crystallinity of PC61BM in the BHJ plays crucial role in the photovoltaic conversion
The results indicate that PC61BM combines into chains through the (C–H) · · · Od bonded molecular chains, where Od denotes the doubly-bonded O atom of PC61BM
The van der Waals interaction between the C60 cages in the monoclinic PC61BM seems not greater than the interaction in fcc-C60, which rationalizes the consideration of the phenyl-butyric -acid-methyl-ester (PBM) side tails in understanding the properties such as melting of PC61BM solids

Summary

INTRODUCTION

The fullerene derivative PC61BM1 is one of the best and widely used electron acceptors for high-performance bulk-heterojunction (BHJ) organic solar cells (OSCs).[2,3] It is well known that the crystallinity of PC61BM in the BHJ (polymer/PC61BM blend) plays crucial role in the photovoltaic conversion. Considering all the efforts being empiric, we believe that fundamental understanding of the inter-molecular interaction of PC61BM pure material is desired for more effective design of OSC. Van der Waals interaction is considered as the principal factor in determining the crystal combination of PC61BM. Inter-molecular hydrogen bonds have been considered as non-existence[17] or at most an additional mechanism further stabilizing the crystal[18] the hydrogen bonds played important role in the adsorption structure of PC61BM on metal surface.[19] In our opinion, the hydrogen bonds most possibly play a crucial role in the combination of. In this work we study the hydrogen bonds in PC61BM solids. We experimentally study the hydrogen bonds in some PC61BM solids other than the monoclinic (P21/n) structure.

CALCULATION AND EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION

Significances and implications of the hydrogen bonds

CONCLUSIONS