Abstract
The band-to-band absorption enhancement due to various types of light trapping structures is studied experimentally with photoluminescence (PL) on monocrystalline silicon wafers. Four basic light trapping structures are examined: reactive ion etched texture (RIE), metal-assisted etched texture (MET), random pyramid texture (RAN) and plasmonic Ag nanoparticles with a diffusive reflector (Ag/DR). We also compare two novel combined structures of front side RIE/rear side RAN and front side RIE/rear side Ag/DR. The use of photoluminescence allows us to measure the absorption due to band-to-band transitions only, and excludes parasitic absorption from free carriers and other sources. The measured absorptance spectra are used to calculate the maximum generation current for each structure, and the light trapping efficiency is compared to a recently-proposed figure of merit. The results show that by combining RIE with RAN and Ag/DR, we can fabricate two structures with excellent light trapping efficiencies of 55% and 52% respectively, which is well above previously reported values for similar wafer thicknesses. A comparison of the measured band-band absorption and the EQE of back-contact silicon solar cells demonstrates that PL extracted absorption provides a very good indication of long wavelength performance for high efficiency silicon solar cells.
Highlights
Light management is crucial to solar cell design as it increases the path length of light in the absorber layer, thereby enhancing the probability of electron-hole pair generation
A red shift of 30nm of the peak intensity is seen in the PL spectra of reactive ion etched texture (RIE)/random pyramid texture (RAN) as well as in the literature [14] compared to the planar sample
Using the method mentioned in the previous section, the band-to-band absorptance spectra of a planar wafer and a wafer with RIE/rear RAN textures (RIE/RAN) structure are extracted and presented in the left Y axis of Fig. 3
Summary
Light management is crucial to solar cell design as it increases the path length of light in the absorber layer, thereby enhancing the probability of electron-hole pair generation. In the near-infrared region the probability of a band transition of a photon-excited electron reduces significantly At these wavelengths, surface textures can preferentially direct light into the solar cell at angles outside the escape cone of silicon, resulting in light trapping and increased absorption. Quantifying the absolute absorptance ABB that generates electron-hole pairs in a silicon wafer can provide a quick and accurate estimation of the maximum possible current density Jsc in a solar cell without the need for a p-n junction and current extraction. The methods of obtaining the absolute absorptance of silicon solar cells and wafers from electroluminescence spectra and photoluminescence spectra have been experimentally demonstrated by Trupke et al and Barugkin et al [7, 10] In this contribution, we extend the method of extracting ABB from PL spectra, previously applied to plasmonic structures only, to evaluate light trapping and quantify the light trapping efficiency (LTE) [16] for a range of promising structures (shown in Fig. 1) in crystalline Si wafers.
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