Julian Mattheis

Light absorption and emission in semiconductors with band gap fluctuations—A study on Cu(In,Ga)Se2 thin films

This work investigates the influence of lateral fluctuations of the fundamental band gap on the macroscopic light absorptance and emission spectra of spatially inhomogeneous semiconductors. A model assuming a Gaussian distribution for the local band gaps yields closed-form expressions for the spectral absorptance and emission. Band gap fluctuations broaden the absorption edge of the fundamental band gap, as well as the associated emission peak. The spectral position of the photoluminescence emission peak depends on the length scale of the fluctuations in relation to the characteristic charge carrier transport length. We apply the model to experimental results from Cu(In1−x,Gax)Se2 thin films routinely used as photovoltaic absorbers in thin-film ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. The films feature band gap fluctuations with standard deviations between 15 and 65 meV which would lead to losses in the range of 5–80 mV for the open circuit voltage of solar cells made from these films. The pure te...

Band gap fluctuations in Cu(In,Ga)Se 2 thin films

A simple statistical model describes measured absorption and photoluminescence data of Cu(In 1-x , Ga x )Se 2 thin films. The broadening of the transition peak in the absorption spectra stems from band gap fluctuations. The extent of the spatial inhomogeneities as expressed in the standard deviation Eg μ reaches a maximum of Eg μ = 90 meV for films with equal amounts of indium and gallium, indicating alloy disorder as one possible source of the band gap fluctuations. The fluctuations observed lead to a decrease δV OC of the maximum possible open-circuit voltage V OC of almost 150 mV. However, the experimentally measured, low V OC of solar cells with high gallium content cannot be explained by band gap fluctuations alone. Consequently, our analysis suggests that the dominant recombination process in Cu(In 1-x , Ga x )Se 2 thin film solar cells with high gallium content is not governed by the band gap energy, but is more likely due to deep levels within the forbidden gap.

Finite Mobility Effects on the Radiative Efficiency Limit of Pn-Junction Solar Cells

The maximum power conversion efficiency of a solar cell as defined by the Shockley-Queisser (SQ) radiative recombination limit relies on the assumption that the collection probability for all photogenerated electron/hole-pairs is unity. This assumption implies a virtually infinite mobility mu of the photogenerated charge carriers. In order to compute the radiative efficiency limit with finite mobility, we solve the continuity equation for minority carriers including an additional integral term that accounts for emission of photons by radiative recombination and their subsequent reabsorption. Even when assuming radiative recombination as the only recombination mechanism, the achievable efficiency is reduced drastically when mu drops below a critical value muc. This critical value depends on the absorption coefficient and the doping density of the semiconductor. Thus, we give a criterion that has to be fulfilled by any photovoltaic material in order to guarantee charge separation even in an otherwise most ideal case. Finally, we extend our theory to finite non-radiative lifetimes thereby spanning the gap between the SQ theory and the classical textbook description of a pn-junction solar cell

Reverse Bias Induced Metastable Effects in Cu(In,Ga)Se2 Photovoltaic Devices

The effects of reverse bias stressing have been studied on CuInGaSe2 thin-film solar cell devices. Finished cells were subjected to treatments of various levels of negative voltage bias at different temperatures and for different durations. Metastable changes in the defect profiles as measured by drive-level capacitance profiling (DLCP) were observed. Modeling of the effects suggests a mechanism in which a conversion of shallow acceptors to deep acceptors in the high field region near the barrier interface is largely responsible for the changes in the profiles. In addition, we observe a significant decrease in the open-circuit voltage after long exposure to bias, which may result from increased recombination through the larger number of deep defects near the interface. These effects are entirely reversible by annealing at zero bias at 370 K for one hour

Electroluminescence from Cu(In,Ga)Se 2 Thin-film Solar Cells

We compare the electroluminescence (EL) of three polycrystalline ZnO/CdS/Cu(In,Ga)Se 2 heterojunction solar cells with similar bandgaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent electroluminescence measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperatures to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. The thus broadened room temperature luminescence is one effect which reduces the open circuit voltage of the devices below the Shockley-Queisser-limit. The other effect is the existence of non-radiative recombination currents, which determine the efficiency of the device as light emitting diode. To quantify the open circuit voltage losses, we use reciprocity relations between electroluminescent and photovoltaic action of solar cells, which allow us to predict the light emitting diode efficiency. Measurements support the theory and show that Cu(In,Ga)Se 2 solar cells reach external LED efficiencies approaching.