It is known that the active layer morphology of bulk heterojunction organic solar cells has significant impact on the performance of solar cell devices. However, the widely used morphology characterization methods such as transmission electron microscopy (TEM), atomic force microscopy (AFM) have certain limitations in the characterization of organic thin film materials. By using the huge difference of the refractive index of the different materials under the soft X-ray, resonant soft X-ray scattering (R-SoXS) provides highly enhanced contrast, overcomes the drawbacks such as low contrast between/among different organic components and the lack of 3D information, which is important to obtain the phase separation information in the active layer of organic solar cells, to understand the microstructure, and to establish the relationship between the morphology and the photoelectric conversion process. This article provides an overview of the effect of active layer morphology on the performance of bulk heterojunction organic solar cells, introduces the developing process, theoretical background and the analysis method of resonant soft X-ray scattering. Based on these, the application of resonant soft X-ray scattering in the study of the morphology of organic solar cells is reviewed. The application prospects of R-SoXS are also discussed.

Contents
1 Introduction
2 Effect of active layer morphology on the performance of organic solar cell devices
3 The development process of R-SoXS
4 Theoretical background and the analysis methodology of R-SoXS
4.1 Optical constant and contrast
4.2 The experimental process of R-SoXS
4.3 Extracting morphological information from R-SoXS data
5 Research Progress on morphology characterization of organic solar cells by R-SoXS
5.1 Polymer: fullerene based organic solar cells
5.2 Polymer: non-fullerene based organic solar cells
5.3 Ternary organic solar cells
6 Conclusion