Importance of interface engineering between the hole transport layer and the indium-tin-oxide electrode for highly efficient polymer solar cells

In early studies on organic solar cells with high conductivity PEDOT:PSS, the contact between ITO and PEDOT:PSS was considered ohmic. However, because low-conductivity PEDOT:PSS (such as AI4083) is mainly utilized in contemporary solar cells, the contact between ITO and PEDOT:PSS is not ohmic anymore. Despite the high possibility that there are serious interface problems, little attention has been paid to the interface between PEDOT:PSS and ITO. Most of the previous studies of interfaces in organic solar cells have focused on the interface between the active and charge transport layers. In this work, we have employed a conjugated polyelectrolyte that uses potassium poly[9,9-bis(3′-sulfonatopropyl)fluorene-alt-(9-(2,7-diethylheptyl)-carboazole)] (WPFSCz-) between ITO and low-conductivity PEDOT:PSS to overcome complicated organic-inorganic interfacial problems. The insertion of the WPFSCz- layer provides substantial advantages in the operation of polymer solar cells. First, the inserted WPFSCz- layer modifies the work-function of ITO, thereby forming an effective cascading energy alignment, which is favorable for good hole transport. Second, the introduction of the WPFSCz- layer eliminates interfacial trap sites. The reduction in traps reduces recombination losses at the interface, resulting in an improvement in the fill factor. These effects result in a significant increase in the efficiency of non-fullerene solar cells based on PM6 and Y6, from 15.86 to 17.34%. In addition, we have found that the problem of the interface in contact with ITO occurs not only in PEDOT:PSS, but also in oxide-based charge transport layers. We have confirmed that the insertion of the WPFSCz- layer between ITO and an MoO₃(or ZnO) charge transport layer shows the same positive results.