Bulk heterojunction polymer solar cells have become one of the research hotspots in the field of photovoltaic technology due to their low production cost, light weight, simple preparation process, good flexibility and so on. Bulk heterojunction polymer solar cells have achieved energy conversion efficiency of more than 11%. The bulk heterojunction layer is the key point of the bulk heterojunction polymer solar cells and its micro-morphology has an influence on the energy conversion efficiency by affecting the open-circuit voltage, fill factor and short-circuit current of the bulk heterojunction polymer solar cells. So how to effectively control the micro-morphology of the bulk heterojunction is one of the key issues for improving the energy conversion efficiency of bulk heterojunction polymer solar cells. In this paper, the formation process of bulk heterojunction is introduced, and the micro-morphology control methods of bulk heterojunction developed in recent years are systematically summarized and discussed to provide guidance and reference for the preparation of the bulk heterojunction polymer solar cells.

Contents
1 Introduction
2 The formation process of organic bulk heterojunction
3 Regulation for micro-morphology of bulk heterojunction
3.1 Regulation for micro-morphology of bulk heterojunction by solvent-induced
3.2 Regulation for micro-morphology of bulk heterojunction by thermal annealing
3.3 Regulation for micro-morphology of bulk heterojunction by the ratio of donor and acceptor
4 Conclusion