Nongeminate recombination in polymer:fullerene solar cells is frequently characterized using transient optoelectronic measurements that allow the determination of recombination rates, charge carrier ...lifetimes, and average charge carrier concentrations as a function of voltage. These data are often interpreted in terms of an empirical reaction order defining how recombination depends on measured charge density. In polymer:fullerene solar cells, the empirical reaction orders are often considerably larger than 2, which had previously been explained in terms of the nonlinear relationship between mobile and trapped charge carriers in the presence of an exponential tail of localized states. Here, we show that experimentally determined reaction orders depend not only on the shape of the density of states but also on the spatial distribution of carriers. In particular, in solar cells with small depletion regions due to small active layer thicknesses or due to large unintentional background doping of the polymers, the reaction order can assume values that are much larger than the value expected from the shape of the density of states alone.
The growing urban transport sector presents towns and cities with an escalating challenge in the reduction of their greenhouse gas emissions. Here we assess the effectiveness of several widely ...considered policy options (electrification, light-weighting, retrofitting, scrapping, regulated manufacturing standards and modal shift) in achieving the transition to sustainable urban mobility in terms of their emissions and energy impact until 2050. Our analysis investigates the severity of actions needed to comply with Paris compliant regional sub-sectoral carbon budgets. We introduce the Urban Transport Policy Model (UTPM) for passenger car fleets and use London as an urban case study to show that current policies are insufficient to meet climate targets. We conclude that, as well as implementation of emission-reducing changes in vehicle design, a rapid and large-scale reduction in car use is necessary to meet stringent carbon budgets and avoid high energy demand. Yet, without increased consensus in sub-national and sectoral carbon budgets, the scale of reduction necessary stays uncertain. Nevertheless, it is certain we need to act urgently and intensively across all policy mechanisms available as well as developing new policy options.
The performance of solar cells based on molecular electronic materials is limited by relatively high nonradiative voltage losses. The primary pathway for nonradiative recombination in organic ...donor-acceptor heterojunction devices is believed to be the decay of a charge-transfer (CT) excited state to the ground state via energy transfer to vibrational modes. Recently, nonradiative voltage losses have been related to properties of the charge-transfer state such as the Franck-Condon factor describing the overlap of the CT and ground-state vibrational states and, therefore, to the energy of the CT state. However, experimental data do not always follow the trends suggested by the simple model. Here, we extend this recombination model to include other factors that influence the nonradiative decay-rate constant, and therefore the open-circuit voltage, but have not yet been explored in detail. We use the extended model to understand the observed behavior of series of small molecules:fullerene blend devices, where open-circuit voltage appears insensitive to nonradiative loss. The trend could be explained only in terms of a microstructure-dependent CT-state oscillator strength, showing that parameters other than CT-state energy can control nonradiative recombination. We present design rules for improving open-circuit voltage via the control of material parameters and propose a realistic limit to the power-conversion efficiency of organic solar cells.
A number of recent studies have shown that the nonradiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, ...but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor/acceptor blends to determine the effect that energetic offset has on both radiative and nonradiative recombination of the charge-transfer (CT) state. We find that, for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the nonradiative voltage loss to values as low as 0.23 V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low nonradiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.
Ion migration has been proposed as a possible cause of photovoltaic current-voltage hysteresis in hybrid perovskite solar cells. A major objection to this hypothesis is that hysteresis can be reduced ...by changing the interfacial contact materials; however, this is unlikely to significantly influence the behaviour of mobile ionic charge within the perovskite phase. Here, we show that the primary effects of ion migration can be observed regardless of whether the contacts were changed to give devices with or without significant hysteresis. Transient optoelectronic measurements combined with device simulations indicate that electric-field screening, consistent with ion migration, is similar in both high and low hysteresis CH
NH
PbI
cells. Simulation of the photovoltage and photocurrent transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the perovskite-contact interfaces. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias which screen the ionic charge, reducing hysteresis.
The power conversion efficiency of the most efficient organic photovoltaic (OPV) cells has recently increased to over 10%. To enable further increases, the factors limiting the device efficiency in ...OPV must be identified. In this review, the operational mechanism of OPV cells is explained and the detailed balance limit to photovoltaic energy conversion, as developed by Shockley and Queisser, is outlined. The various approaches that have been developed to estimate the maximum practically achievable efficiency in OPV are then discussed, based on empirical knowledge of organic semiconductor materials. Subsequently, approaches made to adapt the detailed balance theory to incorporate some of the fundamentally different processes in organic solar cells that originate from using a combination of two complementary, donor and acceptor, organic semiconductors using thermodynamic and kinetic approaches are described. The more empirical formulations to the efficiency limits provide estimates of 10–12%, but the more fundamental descriptions suggest limits of 20–24% to be reachable in single junctions, similar to the highest efficiencies obtained for crystalline silicon p‐n junction solar cells. Closing this gap sets the stage for future materials research and development of OPV.
The power conversion efficiency of donor‐acceptor organic solar cells now reaches over 10% in single‐junction and tandem cells. Empirical models suggest that the maximum efficiency may be limited at 10–12%, yet recent, more refined detailed‐balance models indicate that, under favorable conditions, efficiencies in the range of 20–24% can be achieved.
Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs). Correct interpretation of CT-state spectra depends on ...knowledge of the underlying broadening mechanisms, and the relative importance of molecular vibrational broadening and variations in the CT-state energy (static disorder). Here, we present a physical model, that obeys the principle of detailed balance between photon absorption and emission, of the impact of CT-state static disorder on voltage losses in OPVs. We demonstrate that neglect of CT-state disorder in the analysis of spectra may lead to incorrect estimation of voltage losses in OPV devices. We show, using measurements of polymer:non-fullerene blends of different composition, how our model can be used to infer variations in CT-state energy distribution that result from variations in film microstructure. This work highlights the potential impact of static disorder on the characteristics of disordered organic blend devices.
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