The frequency-dependent photocurrent response of dye-sensitized TiO2 cells to modulated illumination is analyzed. Analytical expressions are derived that describe generation, collection, and recombination of electrons in a thin layer nanocrystalline solar cell under conditions of steady illumination and with a superimposed small amplitude modulation. The analysis considers illumination from the contact side and from the counter electrode side, and characteristic differences in the intensity-modulated photocurrent response are predicted for the two cases. The attenuation of the ac photocurrent by the RC time constant of the cell is also considered. The theoretical analysis shows that intensity modulated photocurrent spectroscopy (IMPS) can provide new insight into the dynamics of electron transport and collection in the dye-sensitized solar cell. Experimental IMPS data measured for high-efficiency dye-sensitized cells are fitted to the theoretical model using Bode plots in order to derive values of the lifetime (2 × 10-2 s) and diffusion coefficient (5 × 10-5 cm2 s-1) of photoinjected electrons.

The lifetime τ n and diffusion coefficient D n of photoinjected electrons have been measured in a dye-sensitized nanocrystalline TiO2 solar cell over 5 orders of magnitude of illumination intensity using intensity-modulated photovoltage and photocurrent spectroscopies. τ n was found to be inversely proportional to the square root of the steady-state light intensity, I 0, whereas D n varied with I 0 0.68. The intensity dependence of τ n is interpreted as evidence that the back reaction of electrons with I3 - may be second order in electron density. The intensity dependence of D n is attributed to an exponential trap density distribution of the form N t(E) ∝ exp[−β(E − E c)/(k B T)] with β ≈ 0.6. Since τ n and D n vary with intensity in opposite senses, the calculated electron diffusion length L n = (D n τ n )1/2 falls by less than a factor of 5 when the intensity is reduced by 5 orders of magnitude. The incident photon to current efficiency (IPCE) is predicted to decrease by less than 10% over the same range of illumination intensity, and the experimental results confirm this prediction.

The main role of a cobalt phosphate catalyst on BiVO4 is to improve the photocurrent by passivating the surface and suppressing recombination.

Recent progress toward understanding the processes taking place in dye-sensitized nanocrystalline solar cells (DSC) is reviewed, and some areas characterized by controversy or poor understanding are highlighted. The thermodynamic and kinetic criteria for successful cell design are outlined, and experimental results obtained by a range of methods for characterizing the stationary and dynamic properties of DCS are discussed. These methods include direct measurement of the quasi-Fermi level using an indicator electrode and charge extraction measurements to determine the energetic distribution of electron traps in the nanocrystalline oxide. The influence of electron trapping on dynamic measurements of electron transfer and transport is discussed within the framework of the quasistatic assumption, and a new assessment of the electron diffusion length in the DSC is given, which suggests that collection of photoinjected electrons should be considerably more efficient than previously assumed.

Thin film lead halide perovskite cells, where the perovskite layer is deposited directly onto a flat titania blocking layer, have reached AM 1.5 efficiencies of over 15%, showing that the mesoporous scaffold used in early types of perovskite solar cells is not essential. We used a variety of techniques to gain a better understanding of thin film perovskite cells prepared by a solution-based method. Twelve cells were studied, which showed AM 1.5 efficiencies of ∼11%. The properties of the cells were investigated using impedance spectroscopy, intensity-modulated photovoltage spectroscopy (IMVS), intensity-modulated photocurrent spectroscopy (IMPS), and open-circuit photovoltage decay (OCVD). Despite the fact that all 12 cells were prepared at the same time under nominally identical conditions, their behavior fell into two distinct groups. One half of the cells exhibited ideality factors of m ≈ 2.5, and the other half showed ideality factors of m ≈ 5. Impedance spectroscopy carried out under illumination at open circuit for a range of intensities showed that the cell capacitance was dominated by the geometric capacitance of the perovskite layer rather than the chemical or diffusion capacitance due to photogenerated carriers. The voltage dependence of the recombination resistance gave ideality factors similar to those derived from the intensity dependence of the open-circuit voltage. The IMVS time constant was determined by the product of the geometric capacitance and the recombination resistance. The two types of cells gave very different OCVD responses. The cells with m ≈ 2.5 showed a persistent photovoltage effect that was absent in the case of the cells with higher ideality factors. The IMPS responses provide evidence of minor efficiency losses by recombination under short-circuit conditions.

Perovskite solar cells (PSC) are shown to behave as coupled ionic-electronic conductors with strong evidence that the ionic environment moderates both the rate of electron-hole recombination and the band offsets in planar PSC. Numerous models have been presented to explain the behaviour of perovskite solar cells, but to date no single model has emerged that can explain both the frequency and time dependent response of the devices. Here we present a straightforward coupled ionic-electronic model that can be used to explain the large amplitude transient behaviour and the impedance response of PSC.

Solid-state dye-sensitized solar cells employing spiro-MeOTAD [2,2′7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene] as a hole transport phase were studied by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) over a wide range of illumination intensity. The IMPS and IMVS responses provide information about charge transport and electronhole recombination, respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO 2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO 2 was found to be 4.4 × 10 -4 cm. This value was almost independent of light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity (proportional to I 0 0.64 ).

The properties of thin blocking layers of titanium dioxide used to improve the performance of dye-sensitized nanocrystalline solar cells have been studied. TiO2 blocking layers prepared on fluorine-doped tin oxide-coated glass by spray pyrolysis have been characterized by electrochemical impedance spectroscopy, spectroscopic ellipsometry, and voltammetry. The impedance data reveal the presence of a distribution of surface states at the titanium dioxide−electrolyte interface that is similar to the one seen in the case of nanocrystalline TiO2 films. The influence of the blocking layers on the back transfer of electrons to tri-iodide ions in electrolyte-based dye-sensitized nanocrystalline cells has been studied by open circuit photovoltage decay. The results show that the ability of the blocking layer to prevent the back reaction of electrons with tri-iodide ions in the electrolyte is excellent under short circuit conditions, but is limited under open circuit conditions by electron accumulation at the surface of the titanium dioxide blocking layer.