Improved performance of inverted-type organic solar cells by surface modification on polymer thin film

Short period plasma exposure on poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) blend film in the inverted bulk-herterojunction (BHJ) solar cells with highly transparent sol-gel derived ZnO film as electron selective layer is studied as a way to improve device performance. This study focuses on the vertical segregation of the two components in polymer thin film during the slow-drying treatment on ZnO substrates and further discovers some drawbacks on the polymer film surface that is unfavorable for carrier transport. With the use of mild plasma exposure, the surface can be modified and forms better penetrating pathways for carriers. The polymer surface adjacent to metal electrode turns into a more hydrophobic P3HT-rich surface which forms better contact, fine electrode selectivity, and lower series resistance in the device. The hole transport in P3HT interpenetrating network avoids recombination at the Ag electrode by the removal of PCBM protrusions. As a result, the current density is improved to 11.75 mA/cm² and the efficiency is increased to 3.97% with an enhanced fill factor of 62.6%. Our finding provides an efficient method for interface engineering in organic optoelectronic devices and paves a way for high power conversion efficiency for similar structures.