The inverted device structure of solar cells [1, 2] is an approach for ideal polymer solar cells (PSCs) with both competitive power conversion efficiencies (PCEs) and long-term air stability.[3, 4] In an inverted device, electrodes with high work functions (WF) need to be modified by a low-WF buffer layer as an electroncollection layer (ECL). At present, the main developed low-Wf materials are n-type metal oxides such as titanium oxide (TiO x) and zinc oxide (ZnO),[5, 6] while the use of low-WF organic materials is also being explored, such as chemisorbed small molecules/self-assembled monolayers (SAMs),[7, 8] and low-WF cross-linked conjugated polymers.[9, 10] C 60 and its derivatives exhibit unique conductivity and electron-acceptability,[11–13] thus, in principle, C 60 derivatives have potential as an effective ECL. On the other hand, C 60 derivatives feature abundant redox states with fascinating electronic structures and properties,[14] but charged C 60 is rarely applied in electronic devices. Widely used strategies for charging C 60 are physical methods such as the electrospray, collision, and scanning tunneling microscope (STM) methods.[15–17] However, the implementation of such methods is very complex, and it is difficult to collect the charged C 60 for application. Here we present a strategy to form crosslinked C 60 films with high C 60 content and controllable optical and electrical properties by in situ electrochemical deposition (ED) and doping. The ED films exhibit a high doping level of C 60, low WF, and high conductivity. We first used this kind of

C 60-based ED films as an ECL in inverted polymer solar cells (PSCs), which had a high PCE of up to 6.31%. Electropolymerization with concurrent polymer film deposition has proven to be an especially useful method for the preparation of electroactive and conducting polymer films.[18–24] In this method, the precursors are electrochemically oxidized and the coupling reaction between the monomers occurs at the electrode surface with deposition of the polymer film onto the electrode. In this research, we intentionally constructed the ED film of C 60-bisCBZ (see Scheme 1), which bears a C 60 group and two electroactive carbazole groups.[21] The carbazole is found to be a highly electroactive group with relatively low oxidation potential, very efficient coupling between oxidation species of carbazole (carbazyl radical cation), and structurally well-defined coupling products (dimeric carbazyl). These characteristics of the carbazole group benefit the control of the structures and properties of ED films, which are the essence of the present study of this novel ED film. The synthesis routes and structural characterizations of C 60-bisCBZ are included in the Supporting Information.