In the context of solar energy exploitation, dye-sensitized solar cells and dye-sensitized photoelectrosynthetic cells offer the promise of low-cost sunlight conversion and storage, respectively. In this perspective we discuss the main successes and limitations of modern computational methodologies, ranging from hybrid and long-range corrected density functionals, GW approaches and multi-reference perturbation theories, in describing the electronic and optical properties of isolated components

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... nd complex interfaces relevant to these devices. While computational modelling has had a crucial role in the development of the dye-sensitized solar cells technology, the theoretical characterization of the interface structure and interfacial processes in water splitting devices is still at its infancy, especially concerning the electron and hole transfer phenomena. Quantitative analysis of interfacial charge separation and recombination reactions in multiple metal-oxide/dye/catalyst heterointerfaces, thus, undoubtedly represents the compelling challenge in the field of modern computational material science. Computation 2017, 5, 5 2 of 22 in the Earth's crust with a reduced environmental impact and possibly high long-term stability. In the context of solar energy exploitation, dye-sensitized solar cells (DSCs) [9,10] and dye-sensitized photoelectrosynthetic cells (DSPECs) [5, 11, 12] offer the promise of low-cost sunlight conversion and storage, respectively. Since the seminal paper by O' Regan and Grätzel in 1991 [9], hybrid/organic photovoltaic devices have attracted significant research interest, which has recently lead to the launch of the first commercial product. The basic functioning mechanism of a DSC is depicted in Figure 1 along with a representation of the different competing forward (solid black lines) and back (dotted red lines) electron transfer (ET) processes. The heart of the cell is represented by the photoanode, which consists of a mesoporous oxide layer (7-10 µm), usually composed of TiO 2 or ZnO nanoparticles deposited onto a transparent conducting oxide on a glass or plastic substrate. The nanocrystalline oxide is sensitized by a monolayer of dyes (D), chemically bound to the semiconductor nanoparticles. Upon solar light absorption, the adsorbed sensitizers are able to inject the photo-excited electrons in the manifold of the conduction band (CB) states of the semiconductor, typically at the femtosecond time scale. Injected electrons travel through the mesoporous film and are collected by the conductive layer of the photoanode electrode, while the oxidized dye (D + ) is rapidly reduced by the electron mediator donor in solution [13] or by the hole transporting material (HTM) in solid-state devices. The collected electrons flow in the external circuit, producing a photocurrent, and reach the counter-electrode, where the circuit is closed by the reduction of the electron mediator acceptor [14, 15] .