ZnO was one of the first metal oxides used in dye-sensitized solar cells (DSCs). It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility and probably the richest variety of nanostructures based on a very wide range of synthesis routes. However, in spite of the huge amount of literature produced in the past few years, the reported efficiencies of ZnO-based solar cells are still far from their TiO 2 counterparts. The origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnO in dye-sensitized solar cells and related devices. In this regard, a change of focus of the current research on ZnO-based DSCs (from morphology to surface control) is suggested.

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The photovoltaic performance of ZnO-based dye-sensitized solar cells (DSCs) has been studied for three different configurations involving two dyes and two types of electrolytes with the iodide/iodine as redox mediator: ZnO/N719/organic solvent electrolyte (C1), ZnO/D149/organic solvent electrolyte (C2), and ZnO/N719/ionic liquid electrolyte (C3). The DSCs were characterized by measuring current–voltage curves and photovoltage as a function of light intensity and by electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), intensity-modulated photovoltage spectroscopy (IMVS), and open circuit photovoltage decay (OCVD). The results demonstrate the good light harvesting properties of the D149 dye and highlight the photovoltage limitation of the solvent-free (ionic liquid) electrolyte. The intensity dependence of the photovoltage and the OCVD, EIS, and IMVS results provide evidence of the nonlinear character of the recombination kinetics. It has been found that by combining EIS and IMPS data it is possible to overcome the problem of transport resistance determination for cells where there is no clear transmission line feature in the impedance measurements. The resulting electron diffusion lengths (L n ) indicate good electron collection for all studied cells, pointing to poor injection as the main limitation in DSCs based on the ZnO semiconductor.

Solar cells using perovskite as semiconducting pigment have recently attracted a surge of interest owing to their remarkable solar-to-electric conversion efficiencies and ease of processing. In this direction various device architectures and materials have been employed, and attempts were made to elucidate the underlying working principles. However, factors governing the performance of perovskite devices are still obscure. For instance, the interpretation of electrochemical impedance spectroscopy (EIS) is not straightforward, and the complexity of the equivalent circuits hinders the identification of transport and recombination mechanisms in devices, especially those that determine the performance of the device. Here in we carried out a comprehensive and complementary characterization of perovskite solar cells by using an array of small-perturbation techniques: EIS and intensity-modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS). The employment of IMPS allowed us to identify two transport times separated by 2 orders of magnitude and with opposite voltage dependences. For recombination, well agreement was found between lifetimes obtained by IMVS and EIS. The feature associated with recombination and charge accumulation in an impedance spectrum through correlation to the IMVS response was experimentally identified. This correlation paves the way to reconstruct the current–voltage curve using a continuity equation model for transport and recombination in the working device. The adopted methodology demonstrates that complementary techniques facilitate the interpretation of EIS results in perovskite solar cells, allowing us for the identification of the transport-recombination mechanisms and providing new insights into the efficiency-determining steps.

A numerical model that simulates the steady-state current-voltage curve and the time-dependent response of a dye-sensitized solar cell with a single continuity equation is derived. It is shown that the inclusion of the multiple-trapping model, the quasi-static approximation and non-linear recombination kinetics leads to a continuity equation for the total electron density in the photoanode with an electron density-dependent diffusion coefficient and a density-dependent pseudo-first order recombination constant. All parameters in the model can be related to quantities accessible experimentally. The required power exponents are taken from impedance spectroscopy measurements at different voltages. The model provides new insights into the physical interpretation of the power exponents. Modeling examples involving a high-efficiency TiO(2)-based dye solar cell and a ZnO-based dye solar cell are presented. It is demonstrated that the model reproduces the transient behavior of the cell under small perturbations. The spatial dependence of the recombination rate and the influence of film thickness and of voltage dependent injection efficiency on cell performance are studied. The implications of the model are discussed in terms of efficiencies potentially attainable in dye-sensitized solar cells and other kinds of solar cells with a diffusional mechanism of charge transport.