High Open-Circuit Voltages in Lead-Halide Perovskite Solar Cells - Experiment, Theory and Open Questions

One of the most significant features of lead-halide perovskites are their ability to have comparably slow recombination despite the fact that these materials are mostly processed from solution at room temperature. The slow recombination allows achieving high open-circuit voltages when the lead-halide perovskite layers are used in solar cells. This perspective discusses the state of the art of our understanding and of experimental data with regard to recombination and open-circuit voltages in lead-halide perovskites. A special focus is put onto open questions that the community has to tackle to design future photovoltaic and optoelectronic devices based on lead-halide perovskites and other semiconductors with similar properties

Comparison of device models for organic solar cells: Band-to-band vs. tail states recombination

The efficiency-limiting recombination mechanism in bulk-heterojunction (BHJ) solar cells is a current topic of investigation and debate in organic photovoltaics. In this work, we simulate state-of-the-art BHJ solar cells using two different models. The first model takes into account band-to-band recombination and field dependent carrier generation. The second model assumes a Shockley-Read-Hall (SRH) recombination mechanism via tail states and field independent carrier generation. Additionally, we include in both cases optical modelling and, thus, position-dependent exciton generation and non-ideal exciton collection. We explore both recombination mechanisms by fitting light and dark current-voltage (JV) characteristics of BHJ cells of five materials: P3HT, MDMO-PPV, MEH-PPV, PCDTBT and PF10TBT, all blended with fullerene derivatives. We show that although main device parameters such as short circuit current, open circuit voltage, fill factor and ideality factor are accurately reproduced by both Langevin and tail recombination, only tail recombination reproduces also the ideality factor of dark characteristics accurately. Nevertheless, the model with SRH recombination via tail states needs the inclusion of external circuitry to account for the heavy shunt present in all the blends, except P3HT:PCBM, when illuminated. Finally, we propose a means to find analytical expressions for the short circuit current by assuming a linear relation between the recombination rate and the concentration of free minority carriers. The model reproduces experimental data of P3HT cells at various thickness values using realistic parameters for this material. Dark JV measurement (circles) of a PCDTBT:PC 70BM solar cell (Park et al., Nature Photon. 3, 297 (2009) [1]), the fit with the model including recombination via tail states (solid line) and the fit with the model reported by (Koster et al., Phys. Rev. B 72, 085205 (2005) [2]) that includes bimolecular band-to-band recombination and charge transfer state (CTS) dissociation. The inset shows the JV curves under white light.Fil: Soldera, Marcos Maximiliano. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Electrotécnica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Confluencia; ArgentinaFil: Taretto, Kurt Rodolfo. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Electrotécnica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Confluencia; ArgentinaFil: Kirchartz, Thomas. Imperial College London; Reino Unid

Field-dependent exciton dissociation in organic heterojunction solar cells

In organic heterojunction solar cells, the generation of free charge carriers takes place in a multistep process which involves charge transfer (CT) states, that is, bound electron-hole pairs at the interface between donor and acceptor molecules. Past efforts to model the CT-state dissociation during solar cell operation were not able to consistently reproduce the experimentally observed field and temperature dependence. This discrepancy between model and experiment was partly due to the field-dependent free charge carrier collection process, which plays an important role in the widely used bulk heterojunction cell configuration and superimposes a possible field-dependent charge carrier generation process. In order to distinguish between generation and collection of free charge carriers, we propose the planar heterojunction cell configuration as a model system to study the field-dependent charge carrier generation process in organic heterojunction solar cells. We apply this model system to check current CT-state dissociation models against experimental data. Although the models can quantitatively account for the photocurrent's dependence on the applied voltage and the device thickness, they fail to account for the virtually negligible temperature dependence of the field-dependent charge-generation process. This discrepancy is traced back to a common feature of the models: an Arrhenius-like temperature dependence, distinctive of all processes involving a thermally activated jump over an energy barrier. As a solution to the problem, we introduce an exciton dissociation model based on a field-dependent tunnel process and demonstrate its consistency with the experimental observations. Our results indicate that the current microscopic picture of the charge-generation process in organic heterojunction solar cells being limited by the CT-state dissociation process needs to be reconsidered

Defect tolerant device geometries

The term defect tolerance is widely used in literature to describe materials such as lead-halides which exhibit long non-radiative lifetimes of carriers despite possessing a large concentration of point defects. Studies on defect tolerance of materials mostly look at the properties of the host material and/or the chemical nature of defects that affect the capture coefficients of defects. However, the recombination activity of a defect is not only a function of its capture coefficients alone but are also dependent on the electrostatics and the design of the layer stack of a photovoltaic device. Here we study the influence of device geometry on defect tolerance by combining calculations of capture coefficients with device simulations. We derive generic device design principles which can inhibit recombination inside a photovoltaic device for a given set of capture coefficients based on the idea of slowing down the slower of the two processes (electron and hole capture) even further by modifying electron and hole injection into the absorber layer. We use the material parameters and typical p-i-n device geometry representing methylammonium lead halide perovskites solar cells to illustrate the application of our generic design principles to improve specific devices .Comment: 27 pages, 9 Figure

Extracting Information about the Electronic Quality of Organic Solar-Cell Absorbers from Fill Factor and Thickness

Understanding the fill factor in organic solar cells remains challenging due to its complex dependence on a multitude of parameters. By means of drift-diffusion simulations, we thoroughly analyze the fill factor of such low-mobility systems and demonstrate its dependence on a collection coefficient defined in this work. We systematically discuss the effect of different recombination mechanisms, space-charge regions, and contact properties. Based on these findings, we are able to interpret the thickness dependence of the fill factor for different experimental studies from the literature. The presented model provides a facile method to extract the photoactive layer’s electronic quality which is of particular importance for the fill factor. We illustrate that over the past 15 years, the electronic quality has not been continuously improved, although organic solar-cell efficiencies increased steadily over the same period of time. Only recent reports show the synthesis of polymers for semiconducting films of high electronic quality that are able to produce new efficiency records

Modelado de celdas solares orgánicas de heterojuntura distribuida

Presentamos un modelo para simular el comportamiento de celdas solares orgánicas de heterojuntura distribuida. El modelo tiene en cuenta la presencia de los pares electrón-hueco enlazado como precursores de los portadores libres y el transporte de los excitones fotogenerados. La aplicación del mismo permitió ajustar curvas de tensión-corriente y predecir la dependencia de la fotocorriente en función del espesor de la capa activa para celdas preparadas y medidas por otros autores. Los parámetros relevantes para el ajuste fueron restringidos a tres para mantener la simplicidad del modelo, los cuales son la movilidad de excitones dentro del material absorbente, la densidad efectiva de estados del orbital molecular desocupado más bajo del material absorbente y el ocupado más alto del material aceptor; y la constante de recombinación de los pares electrón-hueco enlazados.This paper introduces a model to simulate the behaviour of bulk heterojunction organic solar cells. The model takes into account the presence of bound electron-hole pairs as a previous state for free charge carriers, and also the transport of photogenerated excitons. Experimental current-voltage curves measured by other authors are fitted applying this model by matching three parameters, namely the exciton mobility within the absorber polymer, the recombination constant of bound electron-hole pairs, and the effective density of states in the molecular orbitals of the absorber. In the same way, current vs. active layer thickness data from the literature are appropriately reproduced by the model.Asociación Argentina de Energías Renovables y Medio Ambiente (ASADES