Development of back side technology for light trapping and photon recycling in GaAs solar cells

Abstract

In this work, we propose and realize three different design strategies to implement an optical cavity in GaAs thin film solar cells in order to confine its internal luminescence and hence to exploit photon recycling. The strategies are based on the definition of a highly reflective and very conductive back side, whereas front side light extraction is limited by total internal reflection. We show characterization results on the internal reflectivity of the back reflector and on the contact resistance at the rear side, important quantities for a good functioning of the final solar cell. First, a back side using only metal was optimized with a pure Ag layer leading to an internal reflectivity of 95.2% and a contact resistance of 1.0 × 10−4 Ω for a 1 cm2 device. With a metal‐dielectric stack at the back side and electrical contacts made by metals via point‐contacts, a second approach led to averaged internal reflectivity of 98.0% and contact resistance of 1.8 × 10−4 Ω for a 1 cm2 device. A third strategy in which a transparent conductive oxide in combination with a metal layer was used did not show the expected results in optical and electrical properties. We fabricated and characterized solar cells with the most promising back sides. When comparing with an ordinary reference GaAs solar cell, external radiative efficiency increased by factors of 150% and 90% for the thin film solar cells with pure Ag and with the metal‐dielectric stack at the back side, allowing enhancements of 19 and 13 mV in VOC, respectively.

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

“…While f PR and f esc can be easily calculated using the optical data of the device materials, η int is related to the overall quality of the solar cells and varies greatly depending on growth and processing conditions. Using constant values for V db = 1.145 V 17,23 and for f esc = 1.08% (which is the average of the cells from structures A and B), V oc as a function of f PR is calculated for a range of η int values, and shown in Figure 7.…”

“…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 .…”

Simple Processing and Analysis of Flexible III–V Multijunction Solar Cells Using Low‐Temperature Transfer Technology

“…The optimized design of the ARC minimizes the photon flux emitted by the solar cell to its external environment, which can be contained within the active region by limiting the front surface escape cone and reducing the flux transmitted through the rear of the cell. [ 28 ] According to the equivalent interface idea, the transfer matrix method code is used to construct an optical model of the thin‐film IMM GaInP/GaAs/InGaAs solar cell, which considers the n–k (refractive index–extinction coefficient) data of all materials used in the stack. To approximate the effect of the 3D crosshatch shape, we have calibrated the n–k of the P++ InGaAs contact layer according to the actual device.…”

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

“…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.…”