Light-trapping enhanced thin-film III-V quantum dot solar cells fabricated by epitaxial lift-off

Abstract

We report thin-film InAs/GaAs quantum dot (QD) solar cells with n − i − p + deep junction structure and planar back reflector fabricated by epitaxial lift-off (ELO) of full 3-inch wafers. External quantum efficiency measurements demonstrate twofold enhancement of the QD photocurrent in the ELO QD cell compared to the wafer-based QD cell. In the GaAs wavelength range, the ELO QD cell perfectly preserves the current collection efficiency of the baseline single-junction ELO cell. We demonstrate by full-wave optical simulations that integrating a micro-patterned diffraction grating in the ELO cell rearside provides more than tenfold enhancement of the near-infrared light harvesting by QDs. Experimental results are thoroughly discussed with the help of physics-based simulations to single out the impact of QD dynamics and defects on the cell photovoltaic behavior. It is demonstrated that non radiative recombination in the QD stack is the bottleneck for the open circuit voltage (V oc ) of the reported devices. More important, our theoretical calculations demonstrate that the V oc offest of 0.3 V from the QD ground state identified by Tanabe et al., 2012, from a collection of experimental data of high quality III-V QD solar cells is a reliable -albeit conservative -metric to gauge the attainable V oc and to quantify the scope for improvement by reducing non radiative recombination. Provided that material quality issues are solved, we demonstrate -by transport and rigorous electromagnetic simulations -that light-trapping enhanced thin-film cells with twenty InAs/GaAs QD layers reach efficiency higher than 28% under unconcentrated light, ambient temperature. If photon recycling can be fully exploited, 30% efficiency is deemed to be feasible.

Electron radiation–induced degradation of GaAs solar cells with different architectures

“…In these structures, the bottom subcell consists of thin‐film GaAs, which has demonstrated the highest conversion efficiency among all types of single junction solar cells . Additionally to back contact design strategies, the position of the junction in GaAs cells has been identified as an important parameter, showing that a device with the junction closer to the bottom of n‐on‐p cells allows for operation in the radiative recombination regime . This type of cell, therefore, has a higher open circuit voltage and is preferred over the standard structure with a junction located closer to the front.…”

A facile light‐trapping approach for ultrathin GaAs solar cells using wet chemical etching

“…For III‐V solar cells however, light‐trapping schemes based on simple wet etches that directly texture the semiconductor surface have not been reported. Typical strategies to incorporate light trapping in III‐V cells employ plasmonic scatterers, nanostructured window layers, and (dielectric) nanostructures at the rear or periodic gratings for (multi)resonant absorption . Despite the success of these methods in increasing the absorptance, they can typically be experimentally challenging, difficult to upscale or introduce substantial parasitic optical or electrical losses into the cells.…”

Comparison of metal/polymer back reflectors with half-sphere, blazed, and pyramid gratings for light trapping in III-V solar cells

“…Another promising approach is represented by III-V quantum dot solar cells (QDSCs), where QDs will increase the short circuit current density (Jsc) of the GaAs solar cell [12,13]. QDSCs are promising devices both for bandgap tuning in multijunction cells and for realizing intermediate band solar cells (IBSCs) [14].…”

“…As the simplest approach for improving the absorption, highly reflective planar back reflectors could effectively double the length of the optical path, increasing the J sc . This approach has been demonstrated for GaAs solar cells [18], GaInNAs solar cells [10], and QDSCs [13]. The length of the optical path can be increased even more with a back reflector grating that induce light diffraction, of which various approaches have been published [19][20][21].…”

“…In particular, the gratings are designed for enhancing interband transitions of typical InAs/GaAs QDSCs in the wavelength range of 900 nm -1200 nm [13,24]. While this is sufficient for optimizing QDSCs working in the single-gap limit [13], the ideas presented in this work should be further developed to devise suitable strategies for the simultaneous enhancement of QD intersubband transitions, involving longer wavelengths [15,25]. In this study, different structures were simulated and fabricated for assessing diffraction efficiencies and their dependence on the design features.…”