Improving Efficiency of Organic Photovoltaics by Manipulating Critical Concentration of Polymer in Bulk‐Heterojunction Solution

Abstract In the advancement of organic photovoltaics (OPV) toward scale‐up production, how to mitigate the batch instability of electron‐donating polymers originated from varied molecular weights remains a great challenge. By taking into consideration the relationship between the molecular weight of electron‐donating polymers and the relevant critical concentration ( c * ) of the solution, herein it is demonstrated that the aggregation behavior of the electron‐donating polymers can be tailored through manipulating c * of the polymer solution. It is interesting to note that the excessive aggregation in low‐molecular‐weight fractions can be circumvented while the favorable mixing ratio can be identified. By preparing the binary bulk heterojunction film under c * ‐defined conditions for mixed‐molecular‐weight polymers, a notable power conversion efficiency of 19.1% for binary devices is achieved. Of particular importance is that a linear interrelation can be established between the c * for the maximum PCE and c * for the low‐molecular‐weight fraction aggregation, validating those straightforward spectra measurements can accurately and rationally guide the molecular weight mixing of photovoltaic donor polymers, thereby fully harnessing the potent driving force and affinity inherent to the low‐molecular‐weight fractions. These findings offer a straightforward and logical framework for addressing batch variability in the large‐scale production of high‐performance OPV devices.