Towards Quantitative Interpretation of Fourier-Transform Photocurrent Spectroscopy on Thin-Film Solar Cells

Abstract

The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy has gone through continuous development during the last two decades. Still, giving quantitative predictions of photovoltaic device performance is a challenging task. As new materials appear, a prediction of potentially achievable open-circuit voltage with respect to bandgap is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. First, the absolute defect density has to be calculated, taking into account optical effects, such as absorption enhancement, due to scattering. Secondly, the electrical effect of thickness variation has to be addressed. We analyzed a series of state-of-the-art hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking degradation. Based on a combination of empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage of a device with a given defect density and absorber thickness. We observed that, rather than the defect density or thickness alone, it is their product or the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives close upper bounds to the open-circuit voltage with the advantage of a much easier evaluation.

Charge transfer state characterization and voltage losses of organic solar cells

“…In addition, the emission is now damped by the e ) factor entering the equation by multiplication with the black-body spectrum was not propagated into the exponential of the absorption spectrum [82]. It is important to note that both Kahle et al and Göhler et al agree on the form of equations (34) and (35). The broken mirror symmetry is, however, only clearly visible in how Göhler et al present the equation.…”

“…Sensitive EQE measurements are arguably one of the most common experimental techniques to characterize CT states. FTPS [35][36][37] is another method to probe the EQE in the sub-gap region of a solar cell with high sensitivity. In comparison to EQE measurements, which use monochromatic light, FTPS applies white light to the device under testing and spectral information is obtained through interferometry.…”

“…Following the approach presented by Burke et al to separate the contributions of static and dynamic disorder by assuming a Gaussian distributions of CT state energies [92], equations (34) and (35) can be extended to a disordered MLJ model [82,93].…”

Below the Urbach Edge: Solar Cell Loss Analysis Based on Full External Quantum Efficiency Spectra

“…Using simple differential equations, assuming nonradiative Shockley–Read–Hall recombination, it follows that the V OC should again scale with the logarithm of the total number of recombination centers. More advanced simulations using the software AFORS-HET were also done . The variation was only in the concentration of homogeneously distributed defects.…”

“…In Figure , simulations together with experimental data on a set of a-Si:H cells of different thicknesses and different levels of defects are shown as a function of product of defect density times thickness. To link the defect absorptance with the volume density we used a scaling constant 1 ppm·cm between density and absorption coefficient . Because x -axis is the product of thickness times defect density, it makes the picture somewhat more complex.…”