Intensity Modulated Photocurrent Spectroscopy Response Research Articles

Intensity-Modulated Photocurrent Spectroscopy Measurements of High-Efficiency Perovskite Solar Cells.

Frequency domain characterization has long served as an important method for the examination of diverse kinetic processes that occur in solar cells. In this study, we investigated the dynamic response of high-efficiency perovskite solar cells utilizing ultra-low-intensity-modulated photocurrent spectroscopy. Distinctive intensity-modulated photocurrent spectroscopy (IMPS) attributes were detected only as a result of this low-intensity modulation, and their evolution under light and voltage bias was investigated in detail. We generally observed only two arcs in the Q-plane plots and attributed the smaller, low-frequency arc to trap-dominated charge transport in the device. Light and voltage bias-dependent measurements confirm this attribution. An equivalent circuit model was used to better understand the features and trends of these measurements and to validate our physical interpretation of the results. Additionally, we tracked the IMPS response of one of the cells over time and showed that slow degradation impacts the size and attributes of the low-frequency arc. Finally, we found that changes in the IMPS response correlate closely with the current versus voltage characteristics of the devices.

Intensity Modulated Photocurrent Microspectrosopy for Next Generation Photovoltaics.

In this report, a large-area laser beam induced current microscope that has been adapted to perform intensity modulated photocurrent spectroscopy (IMPS) in an imaging mode is described. Microscopy-based IMPS method provides a spatial resolution of the frequency domain response of the solar cell, allowing correlation of the optoelectronic response with a particular interface, bulk material, specific transport layer, or transport parameter. The system is applied to study degradation effects in back-contact perovskite cells where it is found to readily differentiate areas based on their markedly different frequency response. Using the diffusion-recombination model, the IMPS response is modeled for a sandwich structure and extended for the special case of lateral diffusion in a back-contact cell. In the low-frequency limit, the model is used to calculate spatial maps of the carrier ambipolar diffusion length. The observed frequency response of IMPS images is then discussed.

(Invited) Characterization of Charge Transfer and Recombination Processes at Metal Oxide Semiconductors for Solar Water Splitting

Metal oxide (MO) semiconductors based on abundant, low-cost materials are attractive for application in photoelectrochemical water splitting systems related to their inherent stability, specifically against oxidation. In addition, these materials provide a wide-ranging versatility in terms of composition, physical and chemical properties, and configuration of multi-component architectures. The semiconducting MO must at least comply with three requirements: (i) absorb sunlight efficiently; (ii) the valence and conduction band edges must be situated at the appropriate energies; and (iii) the electrochemical reaction must have favorable kinetics compared with charge separation, transport and recombination kinetics. Many systems have been evaluated for photo-oxidation of water in the past 10 years, with some of the more popular materials being Fe2O3, WO3, BiVO4, TiO2, etc.In order to improve the efficiency of a solar water splitting system, the specific loss mechanisms need to be elucidated; based on these insights strategies may then be developed to improve performance. However, it is not straightforward to determine the precise loss mechanisms and generally non-steady state methods need to be applied in combination with steady-state techniques. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell: the photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge separation, charge transfer and surface recombination may be deduced. The kinetic model applicable to the specific system determines the interpretation of the results. On the other hand, theoretical modeling is a very powerful method to validate the interpretation of the results, and to complement the experimental studies in order to obtain a clear picture of the dominant processes and effects.In this presentation, the results from IMPS studies on a variety of systems will be discussed and strategies to improve performance are presented and tested, both experimentally and theoretically: (i) planar, compact WO3 / BiVO4 heterojunction systems are compared with nanorod array WO3 / BiVO4 heterojunctions, with and without a hole scavenger; (ii) CuBi2O4 with and without catalysts and an alternative electron acceptor; and (iii) specific examples of other materials to highlight the proposed interpretations. The results are compared with calculations of the IMPS response using a classic kinetic model and a continuity equation approach in order to incorporate the effects of charge transport and electric field within the electrode.The authors gratefully acknowledge funding from CONACYT, SENER, and CICY through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. We also thank the Ministerio de Ciencia e Innovación, the Agencia Estatal de Investigación (AEI), and the European Regional Development Fund of the European Union through Project PID2019-110430GB-C22. AR thanks the Spanish Ministry of Education, Culture and Sports via a PhD grant (FPU2017-03684). Finally, we would like thank Dr. Antonio Zapien and the NANOMXCN initiative to foment scientific collaboration between Mexico and China for support.

Influence of light intensity on the steady-state kinetics in tungsten trioxide particulate photoanode studied by intensity-modulated photocurrent spectroscopy

Charge-carrier dynamics during photoelectrochemical (PEC) water splitting over WO3 particulate photoanode deposited on titanium sheet (Ti/WO3) was studied using intensity-modulated photocurrent spectroscopy (IMPS). The WO3 particulate film calcined at 650 °C showed high PEC performance for water oxidation to evolve O2. In linear sweep voltammetry, the photocurrent density proportionally increased with incident light intensity (I0). This means that the incident photon-to-current conversion efficiency (IPCE) of Ti/WO3 was not dependent on I0 in voltammetry. In contrast, there was no proportional relation between I0 and the photocurrent density in chronoamperometry at a constant potential, indicating that the IPCE at steady state decreased with an increase of I0. To elucidate the effect of light intensity on the steady-state IPCE, we investigated the IMPS response of Ti/WO3 photoanodes by applying a simple branching model with respect to the photogenerated holes (h+). It was found that the recombination rate constant (krec) rather than the charge transfer rate constant (kct) significantly depends on I0 at the steady-state condition. The pseudo-first-order krec increased with an increase of I0, suggesting that recombination is increased by the accumulation of photogenerated carriers during continuous PEC water oxidation. This is the reason for the decrease in the steady-state IPCE of Ti/WO3 under high-intensity light irradiation. The effects of applied potentials on the kinetic parameters (kct and krec) were also evaluated for Ti/WO3 under different I0.

Photoelectrochemical Kinetics: Hydrogen Evolution on p-Type Semiconductors

The theory of intensity-modulated photocurrent spectroscopy (IMPS) is extended to consider the two-electron two-proton hydrogen evolution reaction at p-type semiconductor photocathodes. A generalized reaction scheme is described, and two limiting cases are analyzed. The first involves photoelectrochemical desorption of hydrogen by electron/proton transfer and the second chemical desorption by reaction of adsorbed hydrogen atoms. Expressions are derived for the IMPS responses in the two cases as well as for the modulated surface electron concentration, which is related to the quasi Fermi level at the interface. The surface electron concentration expressions can be used to interpret electrical, optical, or microwave experiments that detect the excess electron concentration. The IMPS responses calculated for the two limiting cases are fitted to a simplified IMPS model that gives rate constants kt nd kr for charge transfer and recombination respectively. kt and kr are then related to the rate constants describing the two limiting schemes for the light-driven hydrogen evolution reaction.

Theory for IMPS on Rough and Finite Fractal Dye Sensitized Solar Cell

A generalized theoretical model of intensity modulated photocurrent spectroscopy (IMPS) for the random morphology in a dye sensitized solar cell (DSSC) under uniform illumination is developed. The generalized IMPS expression for the disordered semiconducting/conducting glass interface of a DSSC is obtained in term of power spectral density of roughness. Influence of surface roughness on the dynamic response of DSSC originate due to the coupling of characteristic phenomenological and morphological length scales. A detailed analysis of IMPS response is performed over finite self- affine fractals to highlight roughness induced anomalies and cause of photocurrent enhancement. The IMPS of a rough DSSC exhibit three characteristic frequency regimes: lifetime of charge carrier dependent low frequency regime, surface irregularity dependent intermediate power-law regime and diffusion controlled high frequency regime. Finally, our theory facilitates the understanding of dynamics and kinetics of charge carriers under the influence of ubiquitous surface disorder.

Unraveling the Impact of Rubidium Incorporation on the Transport-Recombination Mechanisms in Highly Efficient Perovskite Solar Cells by Small-Perturbation Techniques

We applied intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) techniques to explore the effect of rubidium (Rb) incorporation into lead halide perovskite films on the photovoltaic parameters of perovskite solar cells (PSC). IMPS responses revealed the transport mechanisms at the TiO2/perovskite interface and inside the perovskite absorber films. For recombination time constants, IMVS showed that the two perovskite solar cells differ in terms of trap densities that are responsible for recombination loss. 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. Our systematic studies revealed that Rb containing PSCs exhibit enhanced charge transport, slower charge recombination, faster photocurrent transient response, and lower capacitance than the Rb-free samples.

Spectroscopic and Intensity Modulated Photocurrent Imaging of Polymer/Fullerene Solar Cells.

Molecular spectroscopic and intensity modulated photocurrent spectroscopy (IMPS) imaging techniques are used to map morphology-dependent charge recombination in organic polymer/fullerene solar cells. IMPS uses a small (∼10%) sinusoidal modulation of an excitation light source and photocurrent responses are measured while modulation frequencies are swept over several decades (∼1 Hz-20 kHz). Solar cells consisting of either poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) blended with a soluble fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as targets. The morphologies of these polymer/fullerene systems are distinctly different due to PCBM miscibility in various polymer conformers. IMPS responses of both blend solar cells show unique morphology-dependent charge generation, transport and extraction signatures that can be spatially correlated to microscopic variations in local composition and packing by constructing IMPS images along with corresponding molecular spectroscopic imaging over the same scan area. We find that boundaries separating enriched polymer and fullerene domains promote nongeminate charge recombination appearing as positive phase shifts in the IMPS response. These zones are susceptible to degradation and we propose the approaches herein can be used to probe material and device degradation in situ under various conditions, such as oxygen content, temperature and ionizing radiation.

Kinetics of Water Oxidation at TiO2 Nanotube Arrays at Different pH Domains Investigated by Electrochemical and Light-Modulated Impedance Spectroscopy

The electronic properties of a TiO2-nanotube (NT) array used as a photoelectrode for water oxidation at neutral and basic pH were characterized by combining complementary measurement techniques: transient photocurrents, stationary photocurrent–voltage curves, photo-electrochemical impedance spectroscopy (PEIS), and intensity-modulated photocurrent spectroscopy (IMPS). Transient measurements point out the slow chemical modification of the TiO2 surface when going from dark to light, essentially around neutral pH. After this transient period, a new stationary state of the TiO2 surface is established, allowing small amplitude perturbation techniques (PEIS and IMPS) to be applied to obtain information on transfer and recombination kinetics and on surface states contribution. The relevant information was obtained via theoretical models for the PEIS and IMPS responses, involving physical parameters with values extracted by nonlinear least-squares fitting. The main conclusions taken from our experiments include t...

Characterization of Planar Lead Halide Perovskite Solar Cells by Impedance Spectroscopy, Open-Circuit Photovoltage Decay, and Intensity-Modulated Photovoltage/Photocurrent Spectroscopy

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%,1 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.

Characterization of Photocurrent Generation Dynamics in Polymer Solar Cells Based on ZnO/CdS-Core/Shell Nanoarrays by Intensity Modulated Photocurrent Spectroscopy: Theoretical Modeling

A theoretical model is developed for the dynamic characterization of hybrid polymer-based solar cells (HPSCs) based on vertically aligned ZnO/CdS-core/shell nanorod arrays (ZC-NAs) by intensity modulated photocurrent spectroscopy (IMPS). The model describes the effects of CdS shell formation on charge generation and transport dynamics. Particularly, an analytical expression for the ineffective polymer phase model in nanoarray solar cells is developed and introduced into IMPS model for the first time. The main expectations of the IMPS model are confirmed by the experimental data of the polymer/ZC-NA cells with the CdS shell thickness (L) of 3–8 nm. It is shown that the contributions from CdS absorption (f1) and polymer absorption (f2) to charge generation are determined by the core/shell nanoarray structure and the intrinsic polymer property, while the optimal CdS shell thickness (Lopt) depends on the interspacing between ZnO core nanorods and the exciton diffusion length of the polymer. The photocurrent generation is dominantly the competitive results of f1 and f2 contributions subjected to the change in L, with the polymer as a dominant absorption material. Fittings of the measured IMPS responses to the IMPS model reveal that the L-dependence of photocurrent generation dominantly originates from f1, f2, and the polymer exciton dissociation rate S at the polymer/CdS interface. Moreover, the first-order rate constants for the surface defects to trap and detrap the injected electrons in ZnO core nanorods are found to decrease with CdS shell growth and become saturated at Lopt. Furthermore, it is demonstrated that the effective electron diffusion coefficient De in the ZnO nanorods reaches a peak value at Lopt as the result of the largest photogenerated electron density in conduction band. Those results provide a guide to the design of efficient HPSCs based on the core/shell nanoarrays with complementary properties.

Examination of Niw Induced Codeposition By Intensity Modulated Photocurrent Spectroscopy (IMPS)

Introduction NiW is of interest for their outstanding catalytic property of hydrogen evolution reaction,1 , 2 robust corrosion resistance3 , 4 and improved hardness.5 NiW exhibits induced codeposition, introduced by Brenner,6 where the reduction of tungstate in aqueous electrolytes occurs by the aid of other elements being codeposited, such as nickel. Several mechanisms to describe induced codeposition have been raised,7-11 some that include an adsorbed intermediates9,11 or oxide intermediates8 that may be responsible for tungstate reduction. In the related NiMo system, adsorbed oxide intermediates have been reported using in situ Raman spectroscopy.12 , 13 Since the NiW intermediate may contain partially reduced nickel and tungsten oxide species at the working electrode surface during codeposition, a photoactive response would be expected while deposition is taking place. Intensity modulated photocurrent spectroscopy (IMPS), a technique used to characterize semiconductor electrode-electrolyte interfaces in situ,14is used here to probe the NiW intermediate during electrodeposition to help better understand the induced codeposition behavior. Experimental The NiW electrolyte contained 0.375 M sodium citrate, 1 M boric acid, 0.075 M sodium tungstate, and 0.15 M nickel sulfate at a pH was 7. The electrodeposition of pure Ni was conducted in the same electrolyte without tungstate. A three electrode cell was used with a gold working electrode, Pt counter electrode and Ag/AgCl reference. IMPS and the pretreatment before it were conducted using a Zahner IM6ex and PP210 systems. IMPS was performed in a frequency range of 500 Hz to 3000 kHz, with a UV monochromatic light source having a steady state intensity of 30 W/m2 with a 10 % amplitude of the light intensity. The working electrode potential was constant during the IMPS experiment, and a range of working electrode potentials examined in the region of NiW codeposition. Results and Conclusions In Figure 1, IMPS of NiW is presented. A typical IMPS response was generated as expected from a photoactive surface, suggesting that there are intermediates formed on the electrode surface during NiW codeposition and that they exhibit semiconducting behavior. The low frequency intercept at the origin, indicates that the electron-hole recombination process dominates the semiconducting behavior created by adsorbed intermediates. The IMPS of NiW induced codeposition was also influenced by the applied potential. The IMPS response for pure Ni electrodeposition in the same electrolyte generated a significantly smaller semicircle, consistent with the enhanced value observed when codepositing with tungstate ions and the generation of different intermediates that change the electrical properties. Conclusions A photoresponse was observed during NiW induced codeposition, suggesting that the presence of adsorbed intermediates play a role in induced codeposition. Acknowledgements We gratefully acknowledge the NSF for support of this work under grant # CHE- 0957448.

Kinetics of light-driven oxygen evolution at α-Fe2O3electrodes

The kinetics of light-driven oxygen evolution at polycrystalline alpha-Fe2O3 layers prepared by aerosol-assisted chemical vapour deposition has been studied using intensity modulated photocurrent spectroscopy (IMPS). Analysis of the frequency-dependent IMPS response gives information about the competition between the 4-electron oxidation of water by photogenerated holes and losses due to electron-hole recombination via surface states. The very slow kinetics of oxygen evolution indicates the presence of a kinetic bottleneck in the overall process. Surface treatment of the alpha-Fe2O3 with dilute cobalt nitrate solution leads to a remarkable improvement in the photocurrent response, but contrary to expectation, the results of this study show that this is not due to catalysis of hole transfer but is instead the consequence of almost complete suppression of surface recombination.

Linking optical and electrical small amplitude perturbation techniques for dynamic performance characterization of dye solar cells

This paper unifies the analytical models used widely but thus far mostly separately for electrical and optical small amplitude perturbation measurements of nanostructured electrochemical dye solar cells (DSC): electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). The models are linked by expressing the kinetic boundary condition used for solving the time-dependent continuity equation of electrons in IMPS and IMVS analysis in terms of the series and parallel impedance components found in the complete equivalent circuit impedance model of DSC. As a result, analytical expressions are derived for potentiostatic IMPS and galvanostatic IMVS transfer functions of complete DSCs that are applicable at any operating point along the solar cell current-voltage (IV) curve. In agreement with the theory, impedance spectrum calculated as a ratio of IMVS and IMPS transfer functions measured near the maximum power point matches exactly with the impedance spectrum measured directly with EIS. Consequently, both IMPS-IMVS and EIS yield equal estimates for the electron diffusion length. The role of the chemical capacitance of the nanostructured semiconductor photoelectrode in the interpretation of the so-called RC attenuation of the IMPS response is clarified, as well as the capacitive frequency dispersion in IMPS and IMVS.

Dynamic Characterization of Hybrid Solar Cells Based on Polymer and Aligned ZnO Nanorods by Intensity Modulated Photocurrent Spectroscopy

For characterization of polymer-based solar cells with vertically aligned ZnO nanorod arrays (ZnO-NAs) by intensity modulated photocurrent spectroscopy (IMPS), a dynamic IMPS model is developed, where the structure-related charge generation and transport dynamics are considered. The model describes the IMPS responses affected by the phase shift ϕn(ω) due to the exciton diffusion property (ω0) and the structure-related device ideality factor N, the electron diffusion coefficient De, the exciton dissociation rate S, and the device structure (e.g., nanorod length d and interspacing l). The main expectations of the model are confirmed by the experimental data of the polymer/ZnO-NA cells with d = 180−650 nm, offering mechanistic information on the structure-related charge generation, charge transport, and device performance. The presence of the ϕn(ω) makes IMPS response not spiral into the origin and the phase angle in its Bode plot not tend to 90°; the d-dependent direct diffusion (DD) and diffusion-reflectio...

Electric Field Effects on Charge Transport in Polymer/TiO2 Photovoltaic Cells Investigated by Intensity Modulated Photocurrent Spectroscopy

The electric field effects on the charge transport dynamics in bilayer polymer/TiO2 photovoltaic cells are studied by intensity modulated photocurrent spectroscopy (IMPS), for the first time, where a small bias voltage (Va) in the range from −0.1 to 0.2 V is applied to a cell during each IMPS measurement. The bilayer cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and nanostructured TiO2 are constructed by using a polymer layer thickness l of 350 nm but varied TiO2 layer thicknesses (d = 40 and 65 nm). All the experimental IMPS responses are satisfactorily fitted to the previously developed IMPS model for the bilayer cells at Va = 0 V, providing a detailed analysis of charge transport properties under the applied voltages. It is revealed that the external electric field almost exclusively changes the exciton dissociation rate (S) at the donor/acceptor (D/A) interface, but imposes only a very faint effect on the transport dynamics of photogenerated electrons [ϕn(ω), ...

Characterization of Polymer/TiO2 Photovoltaic Cells by Intensity Modulated Photocurrent Spectroscopy

A dynamic model for the intensity modulated photocurrent spectroscopy (IMPS) of bilayer polymer/TiO2 photovoltaic cells is developed in consideration of the exciton generation in the polymer layer, exciton diffusion to the polymer/TiO2 interface, electron injection into the TiO2 layer, and electron diffusion through the TiO2 layer; particularly, the phase shift ϕn(ω) due to the time delay between exciton generation and dissociation is included in the continuity equations for electron transport. Bilayer polymer/TiO2 cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and nanostructured TiO2 were prepared for experimental purposes, with a TiO2 layer thickness d of 120 and 65 nm. Experimental data obtained confirm all the main expectations of the model, providing important information on incident photon-to-current conversion efficiency (IPCE), exciton dissociation, and electron transport. The frequency-dependent ϕn(ω) affects the location (Phigh) where IMPS response cross...

Electron transport in solid-state dye-sensitized solar cells based on polyaniline

Solid-state dye-sensitized solar cells (DSCs) were prepared using polaniline (PANI) as hole- transport material. The charge transport and recombination in the cells were studied by means of intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS). On the basis of lifetime (τn) and mean transit time (τd) of the electrons in nanoporous TiO2 film and fitting of the IMPS response, the effective diffusion coefficient (Dn) and the diffusion length (Ln) of electrons in the TiO2 film were acquired. The electron lifetime in the PANI-based DSCs was about 10 times lower than their liquid counterparts, indicating a much more serious recombination of photogenerated electrons in the TiO2 film, very likely mainly with holes of oxidized dye molecules. With increase in the thickness of TiO2 film, both τn and τd decreased, whereas Ln and Dn increased; and the better performance of the solid-state cells was available only in a suitable thickness range of TiO2 films.

Porosity Effects on Electron Transport in TiO2 Films and Its Application to Dye-Sensitized Solar Cells

Porosity (P) of TiO2 film in dye-sensitized solar cells affects the light absorption coefficient and electron diffusion coefficient. A theoretical analytical expression of the intensity-modulated photocurrent spectroscopy (IMPS) response involving the light absorption coefficient and the electron diffusion coefficient as a function of the porosity has been proposed to investigate the influence of TiO2 film porosity on the characteristics of electron transport. The incident photon-to-current conversion efficiency (IPCE) and electron transit time depending on the porosity have been analyzed illuminating from both the electrolyte side (IE) and the substrate side (IS). The IPCE derived from the IMPS response reaches its maximum at a porosity of around 30% for IE and 41% for IS, respectively. Electron transit time increases with increasing the porosity for IE, while it declines when P < 0.41 for IS, which is attributable to the influence of the RC time constant. It has also been found that a larger RC time constant will lead to a longer transit time. The electron diffusion coefficient calculated from the transit time for IE corresponds to the results from the porosity reported in previous literature, which indicates that the dependence of the electron transit time tau(d) on the porosity is justifiable. The diffusion coefficient calculated for a larger RC time constant approaches the value from the literature when P > or = 0.41, while it is not practicable when P < 0.41 for IS.

Charge Transport and Back Reaction in Solid-State Dye-Sensitized Solar Cells: A Study Using Intensity-Modulated Photovoltage and Photocurrent Spectroscopy

Solid-state dye-sensitized solar cells employing spiro-MeOTAD [2,2‘7,7‘-tetrakis(N,N-di-p-methoxyphenyl-amine)-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 electron−hole 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 TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 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 I00.64).