Chris Groves

A microscopic model for the behavior of nanostructured organic photovoltaic devices

We present a Monte Carlo model of carrier separation and recombination in nanostructured organic photovoltaic (OPV) devices which takes into account all electrostatic interactions, energetic disorder, and polaronic effects. This permits a detailed analysis of the strong morphology dependence of carrier collection efficiency. We find that performance is determined both by the orientation of the heterojunction relative to the external electric field as well as by carrier confinement due to polymer intermixing. The model predicts that an idealized interdigitated structure could achieve overall efficiencies twice as high as blends. The model also reproduces the weakly sublinear intensity dependence of short-circuit photocurrent (ISC) seen in experiment. We show that this is not the result of space-charge effects but of bimolecular recombination. Disconnected islands of polymer in coarser blends result in bimolecular recombination even at low intensities and should therefore be minimized. By including a micros...

Monte Carlo modeling of geminate recombination in polymer-polymer photovoltaic devices.

A Monte Carlo model is used to examine geminate pair dissociation in polymer-polymer photovoltaic devices. It is found that increasing one or both carrier mobilities aids geminate separation yield eta(GS) particularly at low fields. This, in turn, leads to improved maximum power output from polymer-polymer blend photovoltaics, even when carrier mobilities are unbalanced by a factor of 10. The dynamic behaviors of geminate charges that eventually separate and recombine are examined for the first time. It is shown that geminate pairs in a bilayer become effectively free when separated by approximately 4 nm, which is far smaller than the thermal capture radius of 16 nm here. This may lead one to expect that eta(GS) would not be limited by the separation allowed by the morphology once the domain size has increased above 4 nm. In fact it is found that eta(GS) in a blend improves continuously as the average domain size increases from 4 to 16 nm. We show that although a small degree of separation may be available in a blend, the limited number of possible routes to further separation makes charge pairs in blends more susceptible to recombination than charge pairs in a bilayer.

The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics

The domain size, domain purity and interfacial width between domains for a bulk heterojunction are controllably altered through use of Cahn–Hilliard modeling and their relative effect on OPV performance is predicted using Monte Carlo modeling. It is found that locally sharp, well-connected domains of only 4 nm extent out perform morphologies with broadened interfaces and/or impure domains even when domain sizes were at the ‘optimum’ size of ∼10 nm. More generally, these data provide information on the most effective method to optimize the as-cast bulk heterojunction morphology depending upon initial domain purity and the nature of interfaces between domains. Further, it indicates why morphology optimization is more effective for some blends than others. It is shown that the quench depth of the blend can be used as a general technique to control the interfacial structure of the morphology and realize substantial increases in short circuit photocurrent.

Suppression of geminate charge recombination in organic photovoltaic devices with a cascaded energy heterojunction

The effect of cascaded energy heterojunctions on geminate charge recombination in organic photovoltaic devices is examined using a kinetic Monte Carlo model. The structure of the cascaded heterojunction, which encourages spatial separation of the geminate charge pair, is varied to recreate that found in ternary blends and tri-layers, as well as that formed by self-organization in binary blends in which one component crystallizes. It is shown that substantial reductions in charge recombination can indeed be achieved with parameters similar that reported for P3HT:PCBM solar cells. However, the efficacy of cascaded energy heterojunctions is shown to be limited for thick cascade layers (>10 nm). This provides guidance as how to design ternary organic photovoltaics, whilst also offering a possible explanation of low recombination efficiency in some semi-crystalline OPVs.

Developing understanding of organic photovoltaic devices: kinetic Monte Carlo models of geminate and non-geminate recombination, charge transport and charge extraction

This Perspective discusses the physics, implementation and findings of Kinetic Monte Carlo (KMC) models applied to organic photovoltaic devices (OPVs). It is shown that KMC models can relate morphology and energy levels on the nano-scale to geminate and non-geminate recombination, charge transport, charge injection and charge extraction measured in macro-scale OPVs. In particular, KMC investigations probing the circumstances under which geminate recombination is, and is not, a significant loss mechanism in OPVs are reviewed. Furthermore, the mechanisms which yield non-geminate (bimolecular) recombination in OPVs that is both slower than the predictions of Langevin, and charge density dependent, are discussed. It is also shown how KMC models can predict average mobility, as well as spatial heterogeneity and temporal dispersion around this average value in disordered bulk heterojunctions. Finally, it is shown how KMC can be used to quantify the effect of non-ideal electrodes, interlayers and surface wetting layers on OPV performance.

Simulation of loss mechanisms in organic solar cells: A description of the mesoscopic Monte Carlo technique and an evaluation of the first reaction method

In this letter we evaluate the accuracy of the first reaction method (FRM) as commonly used to reduce the computational complexity of mesoscale Monte Carlo simulations of geminate recombination and the performance of organic photovoltaic devices. A wide range of carrier mobilities, degrees of energetic disorder, and applied electric field are considered. For the ranges of energetic disorder relevant for most polyfluorene, polythiophene, and alkoxy poly(phenylene vinylene) materials used in organic photovoltaics, the geminate separation efficiency predicted by the FRM agrees with the exact model to better than 2%. We additionally comment on the effects of equilibration on low-field geminate separation efficiency, and in doing so emphasize the importance of the energy at which geminate carriers are created upon their subsequent behavior.

Electron transport and recombination in dye-sensitized mesoporous TiO2 probed by photoinduced charge-conductivity modulation spectroscopy with Monte Carlo modeling.

We present a combined experimental and theoretical investigation into the charge transport and recombination in dye-sensitized mesoporous TiO2. We electronically probe the photoinduced change in conductivity through in-plane devices while simultaneously optically probing signatures of the charge species. Our quasi-continuous wave technique allows us to build data sets of electron mobility and recombination versus charge density over a wide temperature range. We observe that the charge density dependence of mobility in TiO2 is strong at high temperatures and gradually reduces with reducing temperature, to an extent where at temperatures below 260 K the mobility is almost independent of charge density. The mobility first increases and then decreases with reducing temperature at any given charge density. These observed trends are surprising and consistent with the multiple-trapping model for charge transport only if the trap density-of-states (DoS) is allowed to become less deep and narrower as the temperature reduces. Our recombination measurements and simulations over a broad range of charge density and temperature are also consistent with the above-mentioned varying DoS function when the recombination rate constant is allowed to increase with temperature, itself consistent with a thermally activated charge-transfer process. Further to using the Monte Carlo simulations to model the experimental data, we use the simulations to aid our understanding of the limiting factors to charge transport and recombination. According to our model, we find that the charge recombination is mainly governed by the recombination reaction rate constant and the charge density dependence is mainly a result of the bimolecular nature of the recombination process. The implication to future material design is that if the mobility can be enhanced without increasing the charge density in the film, for instance by reducing the average trap depth, then this will not be at the sacrifice of comparably enhanced recombination and it will greatly increase the charge carrier diffusion lengths in dye-sensitized or mesoscopic solar cells.

The effect of morphology upon mobility: Implications for bulk heterojunction solar cells with nonuniform blend morphology

We use a Monte Carlo model to predict the effect of composition, domain size, and energetic disorder upon the mobility of carriers in an organic donor-acceptor blend. These simulations show that, for the changes in local morphology expected within the thickness of a typical bulk heterojunction photovoltaic device, changes in mobility of more than an order of magnitude are expected. The impact of nonuniform mobility upon space-charge-limited diode and photovoltaic (PV) device performance is examined using a drift-diffusion model. The current passing through a space-charge-limited diode is shown to depend upon the position of the layers with differing mobility. Accurate modeling of the current in such devices can only be achieved using a drift-diffusion model incorporating nonuniform mobility. Inserting a 20 nm thick layer in which the mobility is less by one order of magnitude than in the rest of the 70 nm thick PV device reduced the device efficiency by more than 20%. Therefore it seems vital to exert a high degree of control over the morphology throughout the entire blend PV device, otherwise potential PV performance may be lost.

Temperature dependence of impact ionization in GaAs

The temperature dependence of electron and hole impact ionization in gallium arsenide (GaAs) has been determined from photomultiplication measurements at temperatures between 20 K and 500 K. It is found that impact ionization is suppressed by increasing temperature because of the increase in phonon scattering. Temperature variations in avalanche multiplication are shown to decrease with decreasing avalanching region width, and the effect is interpreted in terms of the reduced phonon scattering in the correspondingly reduced ionization path length. Effective electron and hole ionization coefficients are derived and are shown to predict accurately multiplication characteristics and breakdown voltage as a function of temperature in p/sup +/in/sup +/ diodes with i-regions as thin as 0.5 /spl mu/m.

New SPM techniques for analyzing OPV materials

Organic solar cells hold promise as an economical means of harvesting solar energy due to their ease of production and processing. However, the efficiency of such organic photovoltaic (OPV) devices is currently below that required for widespread adoption. The efficiency of an OPV is inextricably linked to its nanoscale morphology. High-resolution metrology can play a key role in the discovery and optimization of new organic semiconductors in the lab, as well as assist the transition of OPVs from the lab to mass production. We review the instrumental issues associated with the application of scanning probe microscopy (SPM) techniques such as photoconductive atomic force microscopy and time-resolved electrostatic force microscopy that have been shown to be useful in the study of nanostructured organic solar cells. These techniques offer unique insight into the underlying heterogeneity of OPV devices and provide a nanoscale basis for understanding how morphology directly affects OPV operation. Finally, we discuss opportunities for further improvements in scanning probe microscopy to contribute to OPV development.