Silver grids are utilized to exclude the expensive use of indium tin oxide (ITO) in conjugated polymer photovoltaic devices. The grids are generated by electroless deposition from elastomeric ...microfluidic channels onto transparent substrates. The organic photovoltaic devices demonstrated here, with minimized series resistance, are confirmed to have characteristics comparable to devices exploiting ITO.
Processing and patterning of electroactive materials from solvents is a hallmark of flexible organic electronics, and commercial applications based on these properties are now emerging. Printing and ...ink-jetting are today preferred technologies for patterning, but these limit the formation of nanodevices, as they give structures way above the micrometer lateral dimension. There is therefore a great need for cheap, large area patterning of nanodevices and methods for top-down registration of these. Here we demonstrate large area patterning of connected micro/nanolines and nanotransistors from the conducting polymer PEDOT, assembled from fluids. We thereby simultaneously solve problems of large area nanopatterning, and nanoregistration.
In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light ...emitted from blends of some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. We accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.
The origin of energetic disorder in organic semiconductors and its impact on opto-electronic properties remains a topic of intense controversy. Particularly the disorder at electron donor–acceptor ...interfaces for organic photovoltaics is pivotal to understand as it is expected to affect photo-carrier generation, recombination and consequently device efficiency parameters. In this work we evaluate the temperature dependence of the line-shape of the photoluminescence (PL) and electroluminescence (EL) spectra of small molecule:fullerene blend devices, with the ambition to disentangle dynamic and static disorder contributions. The EL emission spectra are dominated by charge-transfer (CT) state emission and are confirmed to be of Gaussian character and almost completely voltage independent. More importantly, a strong line-width narrowing is persistently observed upon cooling, down to a certain material specific low temperature, below which the line-width remains constant. It is consequently clear that the main portion of the line-width measured at operating conditions of room temperature or higher, is originating from thermally activated, or dynamic, disorder. The observed temperature dependence of the high-energy emission tail can be fully described by taking into account high and low frequency molecular vibrational modes, without having to rely on static disorder. The presence of low frequency molecular modes with large Huang–Rhys factors results in a Gaussian line-shape, which is additionally broadened at high temperature by thermal population of high frequency intra-molecular modes. We therefore cast strong doubts regarding the commonly used assumption that single temperature optical measurements of absorption or emission tails are able to provide meaningful information regarding the shape of a static density of states tail.
The role of ions in organo-metal halide perovskites and its impact on optoelectronic device performance remains one of the most intriguing issues in the field. The current–voltage hysteresis often ...observed in hybrid perovskite solar cells has been assigned to the presence of mobile ions inside the perovskite layer. The difficulty in studying electronic properties of solar cells results from the screening effects as well as slow dynamics of mobile ions, particularly if they are located at the interfaces. In this work, we addressed the distribution of charged species in planar-type methylammonium lead iodine (MAPI) as well as formamidinium lead iodine (FAPI) solar cells by using a modified capacitance–voltage (CV) method without illumination in combination with a Mott–Schottky (MS) analysis of experimental data. The characteristic Mott–Schottky behavior with a linear dependence of C –2(V), distinctive for a pn-junction, is not visible in pristine devices. Surprisingly, biasing the device in the forward direction results in MS behavior, which is due to the field-driven redistribution of mobile ions from the interface toward the absorber bulk. From the MS analysis, we deduced space charge concentrations of 2.5 × 1016 and 2.8 × 1016 cm–3 for the FAPI and the MAPI devices, respectively. However, the junction formation effect is not sustainable, since mobile ions relax to their initial location at ambient conditions. However, if the prebiasing is done at temperatures slightly below room temperature, the pn-junction can be stabilized for the FAPI device. In contrast, the MAPI device shows a rapid redistribution of mobile ions back to the transport layers during the measurement even at lower temperatures. This can be observed in the quite different doping profiles for the two perovskite devices.
Accurately identifying and understanding the dominant charge carrier recombination mechanism in perovskite solar cells are of crucial importance for further improvements of this already promising ...photovoltaic technology. Both optical and electrical transient methods have previously been employed to strive for this warranted goal. However, electrical techniques can be strongly influenced by the capacitive response of the device which hides the carrier recombination dynamics that are relevant under steady state conditions. To ascertain the identification of steady state relevant charge carrier dynamics, it is beneficial to evaluate thicker films to minimize the impact of device capacitance. Herein, the electrical transient response in very efficient planar co-evaporated n-i-p solar cells is studied by varying the active layer thickness from 500 nm to 820 nm and is compared to a solution-processed perovskite device with an active layer of 350 nm thickness. In the case of the n-i-p devices, the capacitance of the 500 nm solar cell leads to longer perceived decay times in the lower voltage regime, while in the higher voltage regime quite similar kinetics independent of the active layer thickness are observed, allowing us to identify the transition from capacitance-affected to the sought-after bulk charge carrier dynamics. We show that increasing the perovskite thickness by more than 50% does not affect the recombination dynamics significantly, confirming the high quality of the vacuum-processed perovskite solar cells. Finally, it is demonstrated for the first time for perovskite solar cells that the recombination order in both thicker devices ranges between 1.6 and 2, pointing towards trap-assisted and free-carrier recombination under operating conditions. We emphasize that the observed low recombination orders are in strong contrast to earlier literature as well as to the thinner solution-processed device, which suffers from both shorter carrier lifetimes and a larger device capacitance.
By evaluating perovskite solar cells up to 820 nm thick using charge extraction and transient photovoltage, first and second order recombination dynamics can for the first time be identified under operational conditions in complete devices.
We demonstrate a novel light trapping configuration based on an array of micro lenses in conjunction with a self aligned array of micro apertures located in a highly reflecting mirror. When locating ...the light trapping element, that displays strong directional asymmetric transmission, in front of thin film organic photovoltaic cells, an increase in cell absorption is obtained. By recycling reflected photons that otherwise would be lost, thinner films with more beneficial electrical properties can effectively be deployed. The light trapping element enhances the absorption rate of the solar cell and increases the photocurrent by as much as 25%.
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