Perovskite solar cells based on abundant low cost materials promise to compete on performance with mainstream PV. Here we demonstrate lead-free perovskite solar cells, removing a potential barrier to widespread deployment.

Hybrid perovskites represent a new paradigm for photovoltaics, which have the potential to overcome the performance limits of current technologies and achieve low cost and high versatility. However, an efficiency drop is often observed within the first few hundred hours of device operation, which could become an important issue. Here, we demonstrate that the electrode's metal migrating through the hole transporting material (HTM) layer and eventually contacting the perovskite is in part responsible for this early device degradation. We show that depositing the HTM within an insulating mesoporous "buffer layer" comprised of Al2O3 nanoparticles prevents the metal electrode migration while allowing for precise control of the HTM thickness. This enables an improvement in the solar cell fill factor and prevents degradation of the device after 350 h of operation.

In this work we demonstrate hyperbranched nanostructures, grown by pulsed laser deposition, composed of one-dimensional anatase single crystals assembled in arrays of high aspect ratio hierarchical mesostructures. The proposed growth mechanism relies on a two-step process: self-assembly from the gas phase of amorphous TiO2 clusters in a forest of tree-shaped hierarchical mesostructures with high aspect ratio; oriented crystallization of the branches upon thermal treatment. Structural and morphological characteristics can be optimized to achieve both high specific surface area for optimal dye uptake and broadband light scattering thanks to the microscopic feature size. Solid-state dye sensitized solar cells fabricated with arrays of hyperbranched TiO2 nanostructures on FTO-glass sensitized with D102 dye showed a significant 66% increase in efficiency with respect to a reference mesoporous photoanode and reached a maximum efficiency of 3.96% (among the highest reported for this system). This result was achieved mainly thanks to an increase in photogenerated current directly resulting from improved light harvesting efficiency of the hierarchical photoanode. The proposed photoanode overcomes typical limitations of 1D TiO2 nanostructures applied to ss-DSC and emerges as a promising foundation for next-generation high-efficiency solid-state devices comprosed of dyes, polymers, or quantum dots as sensitizers.

The working mechanisms of excitonic solar cells are strongly dominated by interface processes, which influence the final device efficiency. However, it is still very challenging to clearly track the effects of inter-molecular processes at a mesoscopic level. We report on the realization of polymer-based hybrid solar cells made of prototypical materials, namely, poly(3-hexylthiophene) (P3HT) finely infiltrated in a TiO 2 scaffold, with power conversion efficiency exceeding 1%. A step-change improvement in the device performance is enabled by engineering the hybrid interface by the insertion of an appropriate molecular interlayer. An unprecedented set of characterization techniques, including time-resolved optical spectroscopy, X-ray photoemission spectroscopy, positron annihilation spectroscopy and atomistic simulations, allows us to rationalize our findings. We show that a suitable chemical structure of the interlayer molecule induces selective intermolecular interactions, and thus a preferential surface energetic landscape and morphological order at the interface which consequently drives a strong improvement in charge generation and a decrease in recombination losses.

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Photoinduced electron transfer at low-band-gap ruthenium-based dye/TiO2 has been investigated by means of ultrafast transient absorption and DFT/TDDFT calculations. We demonstrate that although the charge generation mechanism is triplet mediated upon band gap excitation, as already proven in high band gap dyes such as the well-known N3 and N719, when excess energy is provided which allows to reach high energy singlet states still in the visible spectral range, ultrafast electron transfer takes place. No intersystem crossing process is observed and charge generation happens only from the singlet excited state.

The correlation between molecular scale morphology and charge generation across hybrid photovoltaic interfaces made of metal oxides (ZnO and TiO 2 ) and a prototypical electron donor polymer, P3HT, is investigated. Device characterization and UV-NIR transient absorption spectroscopy are used to demonstrate that the local disorder of the polymer chains on the surface of the metal-oxide fi lm provides better electron injection effi ciencies than the crystalline phases, though the latter are essential for energy and charge transport. An unambiguous spectroscopic tool is also demonstrated to probe the occupation of the conduction band of ZnO following the electron injection from the polymer through the ultrafast tracking of the Burstein-Moss effect.

A quantitative method for the characterization of nanoscale 3D morphology is applied to the investigation of a hybrid solar cell based on a novel hierarchical nanostructured photoanode. A cross section of the solar cell device is prepared by focused ion beam milling in a micropillar geometry, which allows a detailed 3D reconstruction of the titania photoanode by electron tomography. It is found that the hierarchical titania nanostructure facilitates polymer infiltration, thus favoring intermixing of the two semiconducting phases, essential for charge separation. The 3D nanoparticle network is analyzed with tools from stochastic geometry to extract information related to the charge transport in the hierarchical solar cell. In particular, the experimental dataset allows direct visualization of the percolation pathways that contribute to the photocurrent.