Fabrication of solution-processed perovskite solar cells (PSCs) requires the deposition of high quality films from precursor inks. Frequently, buffer layers of PSCs are formed from dispersions of metal oxide nanoparticles (NPs). Therefore, the development of trustable methods for the preparation of stable colloidal NPs dispersions is crucial. In this work, a novel approach to form very compact semiconducting buffer layers with suitable optoelectronic properties is presented through a self-functionalization process of the nanocrystalline particles by their own amorphous phase and without adding any other inorganic or organic functionalization component or surfactant. Such interconnecting amorphous phase composed by residual nitrate, hydroxide, and sodium ions, proved to be fundamental to reach stable colloidal dispersions and contribute to assemble the separate crystalline nickel oxide NPs in the final film, resulting in a very homogeneous and compact layer. A proposed mechanism behind the great stabilization of the nanoparticles is exposed. At the end, the self-functionalized nickel oxide layer exhibited high optoelectronic properties enabling perovskite p-i-n solar cells as efficient as 16.6% demonstrating the pertinence of the presented strategy to obtain high quality buffer layers processed in solution at room temperature.

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Effective control of the interface between the metal cathode and the electron transport layer (ETL) is critical for achieving high performance p-i-n planar heterojunction perovskite solar cells (PSCs). Several organic molecules have been explored as interlayers between the silver (Ag) electrode and the ETL for the improvement in the photovoltaic conversion efficiency (PCE) of p-i-n planar PSCs. However, the role of these organic molecules in the charge transfer at the metal/ETL interface and the chemical degradation processes of PSCs has not yet been fully understood. In this work, we systematically explore the effects of the interfacial modification of the Ag/ETL interface on PSCs using rhodamine 101 as a model molecule. By the insertion of rhodamine 101 as an interlayer between Ag and fullerene derivatives (PC60BM and PC70BM) ETLs improve the PCE as well as the stability of p-i-n planar PSCs. Atomic force microscopy (AFM) characterization reveals that rhodamine passivates the defects at the PCBM layer and reduces the band bending at the PCBM surface. In consequence, charge transfer from the PCBM towards the Ag electrode is enhanced leading to an increased fill factor (FF) resulting in a PCE up to 16.6%. Moreover, rhodamine acts as a permeation barrier hindering the penetration of moisture towards the perovskite layer as well as preventing the chemical interaction of perovskite with the Ag electrode. Interestingly, the work function of the metal cathode remains more stable due to the rhodamine incorporation. Consequently, a better alignment between the quasi-Fermi level of PCBM and the Ag work function is achieved minimizing the energy barrier for charge extraction. This work contributes to reveal the relevance of proper interfacial engineering at the metal-cathode/organic-semiconductor interface.

In order to get homogeneous and high-quality perovskite CH 3 NH 3 PbI 3 films, different growing methods have been developed, but most of them are unsuitable for scaling up. In this work, we studied in a systematic approach the influence of CH 3 NH 3 Cl (MACl) surface treatment in the acetonitrile (ACN) deposition route. This method does not require vacuum nor solvent quenching steps, which make it probably one of the easiest for transference from lab scale to large-area deposition techniques such as roll-to-roll printing. The properties of perovskite films grown by ACN method and the influence of the MACl on the performance of the perovskite solar cells (PSCs) are characterized in detail by different techniques. By atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) we found significant differences in the morphology and the work function of the perovskite with and without MACl treatment. Moreover, the microstructure presents very different behavior: after being exposed to radiation, the MACl treated sample presented passivated grain boundaries and higher intensity in the photoluminescence (PL) emission. The higher PL for perovskite films with MACl treatment correlates with superior photovoltaic parameters of PSCs. All these features lead to highly homogeneous perovskite layers on large areas enabling the fabrication of 10 × 10 cm 2 solar minimodules processed in air at low temperature (<100 °C). For these minimodules, we reached a PCE up to 9.2% with an aperture area of 40 cm 2 . These results demonstrate the potential of perovskite surface treatments as an effective strategy for the production of smooth, pinhole free films on large-area substrates, the gateway for further scale-up and commercialization.

Optimization of the interface between the electron transport layer (ETL) and the hybrid perovskite is crucial to achieve high-performance perovskite solar cell (PSC) devices. Fullerene-based compounds have attracted attention as modifiers on the surface properties of TiO, the archetypal ETL in regular n-i-p PSCs. However, the partial solubility of fullerenes in the aprotic solvents used for perovskite deposition hinders its application to low-temperature solution-processed PSCs. In this work, we introduce a new method for fullerene modification of TiO layers derived from nanoparticles (NPs) inks. Atomic force microscopy characterization reveals that the resulting ETL is a network of TiO-NPs interconnected by fullerenes. Interestingly, this surface modification enhances the bottom interface of the perovskite by improving the charge transfer as well as the top perovskite interface by reducing surface trap states enhancing the contact with the p-type buffer layer. As a result, rigid PSCs reached a 17.2% power conversion efficiency (PCE), while flexible PSCs exhibited a remarkable stabilized PCE of 12.2% demonstrating the potential application of this approach for further scale-up of PSC devices.

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