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

Abstract

Thinning down the absorber layer of GaAs solar cells can reduce their cost and improve their radiation hardness, which is important for space applications. However, the lighttrapping schemes necessary to achieve high absorptance in these cells can be experimentally challenging or introduce various parasitic losses. In this work, a facile light‐trapping approach based on wet chemical etching is demonstrated. The rear‐side contact layer of ultrathin GaAs solar cells is wet‐chemically textured in between local Ohmic contact points using an NaOH‐based etchant. The resulting contact layer morphology is characterized using atomic force microscopy and scanning electron miscroscopy. High broadband diffuse reflectance and haze factors are measured on bare and Ag‐coated textured contact layers. The textured contact layer is successfully integrated as a diffusive rear mirror in thin‐film solar cells comprising a 300‐nm GaAs absorber and Ag rear contact. Consistent increases in short‐circuit current density (JSC) of approximately 3 mA cm−2 (15%) are achieved in the textured cells, while the open‐circuit voltages and fill factors do not suffer from the textured rear mirror. The best cell achieves a JSC of 24.8 mA cm−2 and a power conversion efficiency of 21.4%. The textured rear mirror enhances outcoupling of luminescence at open circuit, leading to a strong increase in the external luminescent efficiency.

Subcells Analysis of Thin‐Film Four‐Junction Solar Cells Using Optoelectronic Reciprocity Relation

“…These indicate that the light extraction of the bottom subcell is poor. Therefore the special back‐surface reflector, such as micro‐nanostructure texture, [ 32,33 ] can be designed to realize photon recycling and light trapping to increase the V oc and J sc . [ 28,34 ] In general, both J 01 and J 02 of subcells decrease with increasing E g , but the ratio of J 02 / J 01 increases with increasing E g .…”

Limiting mechanisms for photon recycling in thin‐film GaAs solar cells

“…Thicker devices are less dependent on the reflectances of the interfaces, as denoted by their less steep curve. This highlights the importance of an optimized optical design in the development of thin and ultra‐thin solar cells 6,9,28 …”

“…When comparing single junction GaAs solar cells in the thin‐film or wafer‐based designs, the advantages of the wafer‐based architecture are mainly the more mature and quick processing route. The thin‐film architecture presents as main advantages the reduced weight and potential of reduced costs with the removal and reuse of the wafer, 3‐5 possibility of utilizing thinner epilayers with the application of a back reflector that increases the light optical path 6‐8 ; increased resilience to particle irradiation for space application due to the reduced thickness, 9,10 bendability, 11‐14 and increased photon recycling due to reflectance of emitted photons 15‐19 …”

Interdigitated back‐contact double‐heterojunction GaInP/GaAs solar cells

“…This is justified, since the devices include an ARC, highly reflective back-contacts and ultimately also textured surfaces for redistributing the light through scattering, trapping most of the generated photons within the device. [9][10][11] Furthermore, since this assumption is made for both DDCT solar cells and the reference structure, the relative performance of the devices and therefore the conclusions of this work would remain the same, even if a more detailed optical model would be used. Even more importantly, however, the aim of the present work is to focus on charge transport and to compare different device geometries to one another, providing a detailed view of the electrical behaviour.…”

“…Consequently, they presently hold the efficiency record for single‐junction solar cells, with the the demonstration of 29.1% efficient GaAs‐based thin‐film devices, 3,4 but even they still fall relatively far behind their theoretical efficiency limit of approximately 33.5% 8 . This has also lead to the recently increased interest in ultrathin III‐As solar cells 9‐11 that can enable higher fill factors and open‐circuit voltages. Unlike the further developed IBC silicon cells, however, the efficiency of III‐As solar cells is still affected by contact‐shading, which can notably reduce the illumination of the cell due to the presence of the metallic front contact‐grid as discussed in previous studies 12‐14 .…”