Diffractive gratings for crystalline silicon solar cells—optimum parameters and loss mechanisms

Abstract

In this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation-based optimizations of grating period Λ and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of d bulk = 40 μm and a flat front surface. For this setup, we show a maximum increase in short-circuit current density of Δj SC = 1.8 mA/cm² corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S 0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backside reflector. We analyze the trade-off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S 0,b(max) = 5.2 × 10 3 cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO 2 buffer layer should exceed d SiO2 = 120 nm.

Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography

“…Additionally, there is a shadowing effect due to the presence of the front metal contacts. Finally, the Al back reflector also contributes to parasitic absorption [17]. These effects can be discriminated 9 by the electrical measurements, where only photons absorbed in the active layer contribute to the photocurrent.…”

“…For the case of mono-crystalline silicon, periodic photonic nanostructures have been barely investigated optically but they have not been integrated in a solar cell so far [17][18][19][20][21][22].…”

Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

“…For wavelengths above the silicon band edge (-1.2 urn), the silicon is transparent and all absorption takes place in the reflector (it is assumed that the Si0 2 is transparent at all wavelengths). The measured absorption enhancement in this range demonstrates that the presence of the grating increases the reflector absorption, as has been predicted in Ref [11]. To discern how much of the measured absorption occurs in the silicon, and how much occurs in the aluminium reflector, optical simulations have been made of the fabricated structures using the simulation technique presented in Ref.…”

“…It can be seen that the desired binary profile and close to 50% duty cycle has been achieved in the silicon. A period of 1 urn was chosen for the diffraction gratings, this having been found to be optimum for 40 urn thick c-Si solar cells in previous numerical studies [10,11]. The grating depth was 300 nm for the line grating and 200 nm for the crossed grating.…”

Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

“…This concept was first proposed by Sheng et al [5] for binary linear gratings for thin-film silicon solar cells. Later this concept was investigated for other types of solar cells and other grating geometries [6][7][8]. Experimentally, considerable absorption enhancements could be achieved; however, the performance of gratings has not yet surpassed that of scattering textures.…”