PEDOT:PSS monolayers to enhance the hole extraction and stability of perovskite solar cellsElectronic supplementary information (ESI) available: SEM of the perovskite film and AFM of the pristine PEDOT:PSS film, water rinsed PEDOT:PSS film and ITO. See DOI: 10.1039/c8ta05234d

A hole transport layer (HTL) plays a key role in efficient hole extraction and transfer in inverted planar perovskite solar cells. A 10-20 nm thick poly (3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) layer is the most popular HTL in such a device structure. But is it essential to construct such a thick PEDOT:PSS layer? To address this question, herein self-assembled PEDOT:PSS monolayers are obtained on the indium tin oxide (ITO) surface through a facile water rinsing process. Perovskite solar cells with water rinsed PEDOT:PSS as a HTL yield improved power conversion efficiency (PCE) from 13.4% to 18.0%, compared with the control cells with as-cast PEDOT:PSS. The main contribution is from the open-circuit voltage ( V oc ) and fill factor (FF). Characterization indicates that the majority of PEDOT:PSS is washed away, but an ultra-thin layer of PEDOT:PSS can attach strongly onto ITO via In-O-S chemical bonds between the PSS chain and ITO. Subsequently, PEDOT and PSS form a bilayered structure due to Coulomb interaction. Such an arrangement induces an oriented electric field from positively charged PEDOT to negatively charged PSS, which can accelerate the process of hole extraction. Moreover, the oriented arrangement of PEDOT:PSS monolayers provides higher work function and stronger hydrophobicity, leading to the enhancement in V oc and stability in the ambient environment. This work suggests that there is still room for the efficiency improvement of perovskite solar cells by optimizing the traditional functional layers. A possible high efficiency working model of a monolayer PEDOT:PSS film and a brief fabrication process.