Increased performance of thin-film GaAs solar cells by rear contact/mirror patterning

The effects of electron irradiation on the performance of GaAs solar cells with a range of architectures is studied. Solar cells with shallow and deep junction designs processed on the native wafer as well as into a thin‐film were irradiated by 1‐MeV electrons with fluence up to 1×1015 e−/cm2. The degradation of the cell performance due to irradiation was studied experimentally and theoretically using model simulations, and a coherent set of minority carriers' lifetime damage constants was derived. The solar cell performance degradation primarily depends on the junction depth and the thickness of the active layers, whereas the material damage shows to be insensitive to the cell architecture and fabrication steps. The modeling study has pointed out that besides the reduction of carriers lifetime, the electron irradiation strongly affects the quality of hetero‐interfaces, an effect scarcely addressed in the literature. It is demonstrated that linear increase with the electron fluence of the surface recombination velocity at the front and rear hetero‐interfaces of the solar cell accurately describes the degradation of the spectral response and of the dark current characteristic upon irradiation. A shallow junction solar cell processed into a thin‐film device has the lowest sensitivity to electron radiation, showing an efficiency at the end of life equivalent to 82% of the beginning‐of‐life efficiency.

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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

Gruginskie

Cappelluti

Bauhuis

et al. 2020

“…An approach recently adopted by several groups to increase this reflectivity even further involves patterning the rear-side contact layer to produce local Ohmic contact points. [23][24][25][26] A wet etching step is typically employed to completely remove the contact layer in between the local Ohmic contact points.…”

Section: The Light-trapping Schemementioning

confidence: 99%

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

Eerden¹,

Bauhuis²,

Mulder³

et al. 2019

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.

“…In the same study, it was shown that the rear reflectance has a direct impact on the solar cell parameters, most drastically affecting the open circuit voltage ( V oc ), due to a more efficient re‐absorption of radiatively emitted photons. Since then, multiple studies have been conducted towards increasing photon recycling in thin‐film solar cells, most of which present high quality crystal structures and efficiencies close to 25% 10,17‐24 . The main direction is to increase the reflectivity at the rear side, generally resulting in an increase of the devices' V oc .…”

Section: Introductionmentioning

confidence: 99%

“…Initially reported thin‐film III‐V solar cells typically had highly doped GaAs rear contact layers and fully metallized rear sides 2,25‐28 . However, recent studies 19,22 have demonstrated that the reflectance at the rear side of the cells can be significantly increased if the GaAs contact layers are partially removed and replaced by a suitable dielectric layer (such as ZnS) before the final rear side metallization is applied. In this approach, the locally remaining rear‐contact areas guarantee a low series resistance contact of the device.…”

Section: Introductionmentioning

confidence: 99%

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

Gruginskie

Cappelluti

Bauhuis

et al. 2020

Photon recycling mechanisms in single junction thin‐film GaAs solar cells are evaluated in this study. Modelling supported by experimentally obtained results is used in order to correlate the reflectance of the cell's rear layers, the photon recycling probability, and the solar cell performance. Solar cells with different top and bottom metallization configurations are produced, and their performance is analyzed from the optical and electrical point of view. It is shown that the photon recycling probability increases with the rear mirror reflectance and solar cell thickness, which results in the increase of the devices open circuit voltage. However, the front grid coverage, usually disregarded in rear mirror focused studies, strongly reduces the photon recycling probability. Furthermore, perimeter and interface recombination hinder the internal radiative efficiency of the solar cells, preventing further increase of the devices' open circuit voltage as a result of improvements of the rear mirror reflectivity. In order to exploit the significant benefit of increased photon recycling probability to the solar cell performance, these limiting mechanisms need to be properly addressed.