Approaching the Lambertian limit in randomly textured thin-film solar cells

The Lambertian limit for solar cells is a benchmark for evaluating their efficiency. It has been shown that the performance of either extremely thick or extremely thin solar cells can be driven close to this limit by using an appropriate photon management. Here we show that this is likewise possible for realistic, practically relevant thin-film solar cells based on amorphous silicon. Most importantly, we achieve this goal by relying on random textures already incorporated into state-of-the-art superstrates; with the only subtlety that their topology has to be downscaled to typical feature sizes of about 100 nm

Exploring the validity and limitations of the Mott-Gurney law for charge-carrier mobility determination of semiconducting thin-films

Using drift-diffusion simulations, we investigate the voltage dependence of the dark current in single carrier devices, typically used to determine charge-carrier mobilities. For both low and high voltages, the current increases linearly with the applied voltage. Whereas the linear current at low voltages is mainly due to space charge in the middle of the device, the linear current at high voltage is caused by charge-carrier saturation due to a high degree of injection. As a consequence, the current density at these voltages does not follow the classical square law derived by Mott and Gurney, and we show that for trap-free devices, only for intermediate voltages, a space-charge-limited drift current can be observed with a slope that approaches two. We show that, depending on the thickness of the semiconductor layer and the size of the injection barriers, the two linear current-voltage regimes can dominate the whole voltage range, and the intermediate Mott-Gurney regime can shrink or disappear. In this case, which will especially occur for thicknesses and injection barriers typical for single-carrier devices used to probe organic semiconductors, a meaningful analysis using the Mott-Gurney law will become unachievable, because a square-law fit can no longer be achieved, resulting in the mobility being substantially underestimated. General criteria for when to expect deviations from the Mott-Gurney law when used for analysis of intrinsic semiconductors are discussed

Recombination via tail states in polythiophene:fullerene solar cells

State-of-the-art models used for drift-diffusion simulations of organic bulk heterojunction solar cells based on band transport are not capable of reproducing the voltage dependence of dark current density and carrier concentration of such devices, as determined by current-voltage and charge-extraction measurements. Here, we show how to correctly reproduce this experimental data by including an exponential tail of localized states into the density of states for both electrons and holes, and allowing recombination to occur between free charge carriers and charge carriers trapped in these states. When this recombination via tail states is included, the dependence of charge-carrier concentration on voltage is distinctly different from the case of band-to-band recombination and the dependence of recombination current on carrier concentration to a power higher than 2 can be explained

Rugate filter for light-trapping in solar cells

We suggest a design for a coating that could be applied on top of any solar cell having at least one diffusing surface. This coating acts as an angle and wavelength selective filter, which increases the average path length and absorptance at long wavelengths without altering the solar cell performance at short wavelengths. The filter design is based on a continuous variation of the refractive index in order to minimize undesired reflection losses. Numerical procedures are used to optimize the filter for a 10 μm thick monocrystalline silicon solar cell, which lifts the efficiency above the Auger limit for unconcentrated illumination. The feasibility to fabricate such filters is also discussed, considering a finite available refractive index range

Non-radiative energy losses in bulk-heterojunction organic photovoltaics

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

Transient Optoelectronic Analysis of the Impact of Material Energetics and Recombination Kinetics on the Open-Circuit Voltage of Hybrid Perovskite Solar Cells

Transient optoelectronic measurements are used to evaluate the factors determining the open-circuit voltage of a series of planar photovoltaic devices based on hybrid perovskite layers of varying iodine:bromine ratio. Employing differential charging and transient photovoltage measurements, we use a simple device model based on the charge carrier density dependence of non-geminate recombination to recreate correctly not only the measured device open-circuit voltage (VOC) as a function of light intensity, but also its dependence with bromine substitution. The 173 (±7) mV increase in device voltage observed with 20 % bromine substitution is shown to result from a 227 (±8) mV increase in effective electronic bandgap, which is offset in part by a 56 (±5) mV voltage loss due to faster carrier recombination. The faster recombination following 20% bromine substitution is avoided by ICBA (indene-C60-bisadduct) substitution into the PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) electron collection layer, resulting in a further 73 (±7) mV increase in device VOC. This is consistent with surface recombination losses at the perovskite / fullerene interface being the primary limitation on the VOC output of bromine substituted devices. This study thus presents, and experimentally validates, a simple model for the device physics underlying voltage generation in such perovskite based solar cells, and demonstrates that this approach can provide key insights into factors limiting this voltage output as function of material energetics

Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination

Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm−2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells