The efficient operation of polymer-and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-n-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C 60 -SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C 60 -SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C 60 -SB by extrinsic charges that prevent directional charge transfer from Ag to the integer charge-transfer states in C 60 -SB, pinning the Fermi level at higher energy. The reduced resistivity of the doped interlayer, as measured by impedance spectroscopy, enables efficient device operation with a broad range of interlayer thicknesses, thus simplifying the solution-based device fabrication process.

The poor environmental stability of hybrid perovskite solar cells (PSCs) remains one of the leading obstacles to their commercialization. Herein, we develop and use, for the first time, an orthogonal solvent-assisted process to transfer sheets of monolayer chemical vapor deposited (CVD) graphene onto the perovskite active layer without causing damage to the perovskite layer. We show that at this location in a standard methylammonium lead iodide PSC stack, the CVD graphene acts as a barrier layer to improve stability by (i) preventing moisture ingress into the perovskite layer and (ii) blocking the diffusion of silver ions from the electrode to the perovskite layer. Upon exposure to humidity for 1 week, unencapsulated devices with a graphene barrier retained 93% of their initial PCE, whereas those devices without a graphene barrier retained only 46%. Similarly, after heat treatment, unencapsulated devices with a graphene barrier showed no decrease in PCE, whereas those without a graphene barrier decreased to ∼75% of their initial PCE. CVD graphene is shown to be a prime candidate for improving the environmental stability of PSCs.

High quality active layers for hybrid organic-inorganic perovskite solar cells are essential for achieving maximum device performance. However, perovskite active layers in solar cells are frequently prepared with unoptimized processes that lead to layers of inferior quality. This is often the case when research focuses on other aspects of the solar cell device, such as device design and architecture, carrier transport layers, electrodes, interlayers, etc. In this study, a single-step spin-coating method was used to prepare semi-crystalline PbI2(DMSO) complex films via evaporation-induced self-assembly. These optimized intermediate films were then used to form homogeneous methylammonium lead iodide (MAPbI3) perovskite films of optimum thickness (ca. 400 nm) with uniform surface coverage, good crystallinity, high purity, and grain sizes up to one micron, by employing a sequential deposition process involving intramolecular exchange between the PbI2(DMSO) complex film and a methylammonium iodide (MAI) layer deposited on top of it. We found that for certain ranges of MAI concentration, the formation of optimal-quality perovskite active layers was independent of MAI concentration, so long as MAI deposition occurred at specific corresponding spin speeds. Planar p-i-n perovskite solar cells comprising the optimized active layers were fabricated, and they exhibited negligible hysteresis and a maximum power conversion efficiency (PCE) of 16.72%, without any additional compositional and interfacial engineering. The latter can be used in the future to further enhance the PCE. These findings demonstrate the importance of an optimized perovskite active layer for reproducibly fabricating high-efficiency planar p-i-n photovoltaic devices. Additionally, the simplicity of the PbI2(DMSO) complex film preparation and the versatility of the MAI deposition with this fabrication method further enhances the potential of this material for large-scale processing.

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