The blend composition in a bulk heterojunction BHJ polymer solar cell plays a critical role in determining its overall performance. The blend composition refers to the ratio of the electron donor usually a conjugated polymer and the electron acceptor such as fullerene derivatives materials in the active layer of the solar cell. The performance of a BHJ polymer solar cell is mainly evaluated by its power conversion efficiency PCE , which is influenced by the blend composition in the following ways:1. Morphology: The blend composition affects the morphology of the active layer, which in turn influences the charge transport and separation processes. A well-optimized blend composition can lead to a bicontinuous interpenetrating network of donor and acceptor phases, facilitating efficient charge separation and transport.2. Absorption: The blend composition can also impact the absorption of light in the active layer. A suitable blend composition ensures that both donor and acceptor materials contribute to the absorption of the solar spectrum, leading to enhanced photocurrent generation.3. Energy levels: The blend composition can affect the energy levels of the donor and acceptor materials, which in turn influence the open-circuit voltage Voc and the overall PCE. A proper blend composition ensures that the energy levels are well-aligned for efficient charge transfer and minimal energy loss.4. Stability: The blend composition can also impact the stability and lifetime of the solar cell. An optimized blend composition can lead to better phase separation and reduced recombination, resulting in improved long-term stability.In summary, the blend composition in a bulk heterojunction polymer solar cell is a crucial factor that affects its performance. An optimized blend composition can lead to improved morphology, absorption, energy level alignment, and stability, ultimately resulting in higher power conversion efficiency.