**3.1. Characterization of morphology**

Along with the research effort to prepare D-A type conjugated polymers in BHJ solar cells to achieve the world record efficiency value, studies revealing the importance of the morphology of the polymer/fullerene blend have been carried out and the experience gained on controlling the morphology has become a valuable tool to explore the full potential of a new polymer as light harvesting materials. The thin film morphology characterization tools include grazing incidence wide-angle X-ray scattering, grazing incidence small angle X-ray scattering, resonant soft X-ray scattering, small-angle neutron scattering, transmission electron microscopy, atomic force microscopy, solid-state nuclear magnetic resonance, dynamic secondary ion mass spectrometry, near-edge X-ray absorption fine structure and scanning transmission X-ray microscopy. These analysis techniques are comprehensively summarized by Huang et al. [21]. As so much work has been done to investigate the morphology of the thin film, a rational question to ask is: *what is the best morphology?* Unfortunately, so far a precise answer to this question has not been achieved. One reason is because every characterization technique only sees the film from one aspect and a thorough mapping of the material distribution in the film still remains a challenge [118-119]. Another reason may be due to the fact that the reported polymers with the highest power conversion efficiency values do not really share exactly the same morphology profile. As a result, the *optimal* morphology and the engineering method to achieve the best performance are case-by-case and mostly obtained in a trial-and-error approach. General descriptions like *homogeneous* and *interpenetrating networks* with *nanoscale phase-separated domains* are routinely used to describe the morphology in the cells with distinct performance.
