context: Bulk heterojunction (BHJ) polymer solar cells (PSCs) that convert absorbed sunlight into electrical energy have been in the spotlight due to their potential in large-scale and cost-effective roll-to-roll fabrication. In the past decade, massive endeavors concerning the development and interfacial engineering of novel light-harvesting materials have been made to improve the power conversion efficiency (PCE). Nowadays, the highest PCE exceeds 16% for single-junction BHJ PSCs. Interface layers, positioned between the photoactive layer and the anode/cathode electrode, play vital roles in governing the performance of PSCs. Interface layers are usually utilized to tailor the work function of the electrode for charge carrier collection maximization, modify the interface to alter the photoactive layer micromorphology, as well as minimize carrier recombination at the interfaces between the active layer and the transport layer. For a hole transport layer (HTL), pivotal parameters consist of high conductivity, high transparency, solution processability, favorable stability, etc. PEDOT:PSS, as the state-of-the-art hole transport material in organic optoelectronic devices, possesses outstanding edges of high transmittance in the visible spectrum and superior film-forming ability and thermal stability. However, PEDOT:PSS solution is strongly acidic and hygroscopic, which can etch the indium tin oxide (ITO) anode. Moreover, it has been verified that the spin-casted PEDOT:PSS thin film often exhibits inhomogeneous morphology and poor electrical conductivity, leading to poor efficiency and stability of devices. The development of a PEDOT:PSS HTL that possesses interconnected conductive domains and enhanced electrical conductivity is crucial to PSCs.
answer: indium tin oxide (ITO)