On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells

The interpretation of voltage, photovoltage, and photocurrent in polymer/fullerene bulk heterojunction (BHJ) solar cells is discussed in terms of fundamental device models and results of capacitance spectroscopy. First we establish the relationship between the applied voltage (which is the difference of Fermi levels) and the variation of electrostatic potential that governs the drift field. We then show the most common distribution of carriers and Fermi levels in the blend layer, supported on experimental results of impedance spectroscopy of P3HT:PCBM solar cells. We arrive at the conclusion that charge separation and charge transportation has very little to do with a built-in electric field between metal contacts, while kinetics plays a major role in photocurrent production. Finally, we discuss the key factors relevant to understand device properties and power conversion efficiencies of the BHJ solar cells: recombination, charge generation, and the current–potential curve, based on the suggested model that emphasizes mobile electrons and holes (that we term quasifree carriers) contributing to the respective Fermi levels

Simulation of Steady-State Characteristics of Dye-Sensitized Solar Cells and the Interpretation of the Diffusion Length

Quantitative modeling of the photovoltaic response of the dye-sensitized solar cell (DSC) is an important subject for improving both the understanding of operation mechanisms and the device performance. A range of experimental techniques indicates that nonlinear recombination of the form <i>U</i>_<i>n</i> = <i>k</i>_r<i>n</i>^β, with β ≠ 1, is a property of DSCs. We show that the diffusion length <i>L</i>_<i>n</i> defined from the probability of collection is independent of the macroscopic perturbation for β≠1 only for a small perturbation, and in this case, it coincides with the value λ_<i>n</i> = (<i>D</i>_<i>n</i>τ_<i>n</i>)^1/2 that can be measured by impedance spectroscopy in homogeneous conditions. The increase of the diffusion length with the potential, usually observed experimentally, is attributed to the increase of the free carrier lifetime. We also discuss the modeling of real DSC devices under different conditions, and we conclude that the diffusion−recombination−generation equation based on <i>U</i>_<i>n</i> = <i>k</i>_r<i>n</i>^β is a fundamental ingredient of the simulation tools

Elucidating Operating Modes of Bulk-Heterojunction Solar Cells from Impedance Spectroscopy Analysis

We discuss the progress and challenges in the application of impedance spectroscopy analysis to determine key processes and parameters in organic bulk-heterojunction solar cells. When carrier transport or outer interface extraction do not severely influence the solar cell performance, a simple method to quantify the open-circuit voltage loss caused by the kinetics of charge carrier recombination is provided, based on the determination of chemical capacitance and recombination resistance. This easily allows distinguishing between energetic and kinetic effects on photovoltage, and establishes a benchmark for the performance comparison of a set of different cells. A brief discussion of impedance analysis in the much less studied case of collection-limited solar cells is introduced

Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co–Pi”-Coated Hematite Electrodes

Uniform thin films of hematite (α-Fe₂O₃) deposited by atomic layer deposition (ALD) coated with varying amounts of the cobalt phosphate catalyst, “Co–Pi,” were investigated with steady-state and transient photoelectrochemical measurements and impedance spectroscopy. Systematic studies as a function of Co–Pi thickness were performed in order to clarify the mechanism by which Co–Pi enhances the water-splitting performance of hematite electrodes. It was found that under illumination, the Co–Pi catalyst can efficiently collect and store photogenerated holes from the hematite electrode. This charge separation reduces surface state recombination which results in increased water oxidation efficiency. It was also found that thicker Co–Pi films produced increased water oxidation efficiencies which is attributed to a combination of superior charge separation and increased surface area of the porous catalytic film. These combined results provide important new understanding of the enhancement and limitations of the Co–Pi catalyst coupled with semiconductor electrodes for water-splitting applications

Interpretation of Cyclic Voltammetry Measurements of Thin Semiconductor Films for Solar Fuel Applications

A simple model is proposed that allows interpretation of the cyclic voltammetry diagrams obtained experimentally for photoactive semiconductors with surface states or catalysts used for fuel production from sunlight. When the system is limited by charge transfer from the traps/catalyst layer and by detrapping, it is shown that only one capacitive peak is observable and is not recoverable in the return voltage scan. If the system is limited only by charge transfer and not by detrapping, two symmetric capacitive peaks can be observed in the cathodic and anodic directions. The model appears as a useful tool for the swift analysis of the electronic processes that limit fuel production

Recombination in Organic Bulk Heterojunction Solar Cells: Small Dependence of Interfacial Charge Transfer Kinetics on Fullerene Affinity

We investigate the causes for obtaining higher open-circuit voltage in solar cells that use a fullerene with a smaller electron affinity. Using impedance spectroscopy technique, we show that the change of fullerene LUMO energy has very little influence on the kinetic rate of charge transfer across the interface. In terms of the Marcus theory, large reorganization energy values govern the recombination kinetic rate, which is only slightly dependent on the fullerene LUMO energy, and also depends weakly on the energy location of recombining carriers within the broad density of states. Since the recombination rate is very similar in the different devices, we conclude that the larger open-circuit voltage is due to the larger donor HOMO/acceptor LUMO offset

Role of ZnO Electron-Selective Layers in Regular and Inverted Bulk Heterojunction Solar Cells

Here the role of metal oxide (ZnO) electron-selective layers in the operating mechanisms of bulk-heterojunction polymer−fullerene solar cells is addressed. Inverted as well as regular structures containing ZnO layers at the cathode contact have been analyzed using capacitance methods in the dark and impedance spectroscopy under illumination. We systematically observed that the open-circuit voltage <i>V</i>_oc at 1 sun illumination results higher for inverted cells than that achieved by regular structures in Δ<i>V</i>_oc ≈ 30−50 mV. This shift correlates with the displacement of the flat-band potential <i>V</i>_fb extracted from Mott−Schottky capacitance analysis. A coherent picture is provided that states the hole Fermi level of the polymer highest occupied molecular orbital as an energy reference for both <i>V</i>_oc and <i>V</i>_fb. The study connects the position of the hole Fermi level to the <i>p</i>-doping character of the active layer that is influenced by the film morphology through vertical phase segregation

Toward Stable Solar Hydrogen Generation Using Organic Photoelectrochemical Cells

Organic photoactive materials are promising candidates for the generation of solar fuels in terms of efficiency and cost. However, their low stability in aqueous media constitutes a serious problem for technological deployment. Here we present organic photocathodes for the generation of hydrogen in aqueous media with outstanding stability. The device design relies on the use of water-resistant selective contacts, which protect a P3HT:PCBM photoactive layer. An insoluble cross-linked PEDOT:PSS hole-selective layer avoids delamination of the film, and an electron-selective TiO_<i>x</i> layer in contact with the aqueous solution electrically communicates the organic layer with the hydrogen-evolving catalyst (Pt). We developed a novel method for the synthesis of the TiO_<i>x</i> layer compatible with low-temperature conditions. Tuning the thickness of the TiO_<i>x</i>/Pt layer leads to a trade-off between the achievable photocurrent (∼1 mAcm^–2) and the stability of the photocathode (stable hydrogen generation of 1.5 μmol h^–1 cm^–2 for >3 h)

Molecular Electronic Coupling Controls Charge Recombination Kinetics in Organic Solar Cells of Low Bandgap Diketopyrrolopyrrole, Carbazole, and Thiophene Polymers

Low-bandgap diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction solar cells exhibit much faster charge carrier recombination kinetics than that encountered for less-recombining poly­(3-hexylthiophene). Solar cells comprising these polymers exhibit energy losses caused by carrier recombination of approximately 100 mV, expressed as reduction in open-circuit voltage, and consequently photovoltaic conversion efficiency lowers in more than 20%. The analysis presented here unravels the origin of that energy loss by connecting the limiting mechanism governing recombination dynamics to the electronic coupling occurring at the donor polymer and acceptor fullerene interfaces. Previous approaches correlate carrier transport properties and recombination kinetics by means of Langevin-like mechanisms. However, neither carrier mobility nor polymer ionization energy helps understanding the variation of the recombination coefficient among the studied polymers. In the framework of the charge transfer Marcus theory, it is proposed that recombination time scale is linked with charge transfer molecular mechanisms at the polymer/fullerene interfaces. As expected for efficient organic solar cells, small electronic coupling existing between donor polymers and acceptor fullerene (<i>V</i>_if < 1 meV) and large reorganization energy (λ ≈ 0.7 eV) are encountered. Differences in the electronic coupling among polymer/fullerene blends suffice to explain the slowest recombination exhibited by poly­(3-hexylthiophene)-based solar cells. Our approach reveals how to directly connect photovoltaic parameters as open-circuit voltage to molecular properties of blended materials

Dye versus Quantum Dots in Sensitized Solar Cells: Participation of Quantum Dot Absorber in the Recombination Process

Inorganic quantum dots (QDs) show great potential as absorbers in sensitized solar cell, but there are open questions about the role of quantum dots, where primary electron and hole generation occurs, in the recombination process in this kind of solar cells. In opposition to the conventional dye-sensitized solar cell, here we show that inorganic QDs play a direct role in the recombination process. This fact has been determined by a fingerprint of QDs in the capacitance of the device, where the QDs surface states affect the density of states (DOS) distribution. It indicates that now surface states of QD contribute to the common DOS distribution of TiO₂/QDs/ZnS, which behaves as a single entity, being impossible to distinguish between TiO₂ and QDs. This result highlights the necessity of treating (and optimizing) QD-sensitized solar cells from another perspective than dye-sensitized solar cells, considering the fundamental differences in their behavior