Understanding light harvesting in radial junction amorphous silicon thin film solar cells

Abstract

The radial junction (RJ) architecture has proven beneficial for the design of a new generation of high performance thin film photovoltaics. We herein carry out a comprehensive modeling of the light in-coupling, propagation and absorption profile within RJ thin film cells based on an accurate set of material properties extracted from spectroscopic ellipsometry measurements. This has enabled us to understand and evaluate the impact of varying several key parameters on the light harvesting in radially formed thin film solar cells. We found that the resonance mode absorption and antenna-like light in-coupling behavior in the RJ cell cavity can lead to a unique absorption distribution in the absorber that is very different from the situation expected in a planar thin film cell, and that has to be taken into account in the design of high performance RJ thin film solar cells. When compared to the experimental EQE response of real RJ solar cells, this modeling also provides an insightful and powerful tool to resolve the wavelength-dependent contributions arising from individual RJ units and/or from strong light trapping due to the presence of the RJ cell array.

Figure 1

(a) Schematic illustrating the coaxial multilayer structure of a radial junction solar cell unit; (b), (c) and (d) show respectively the SEM images of a single SiNW, a p-i-n RJ cell and an ITO coated RJ cell.

Figure 2

(a) Upper panels show the simulated electrical field distribution (E_y) through a single RJ cell at different photon wavelengths, while the lower panels (b) present their corresponding absorption profile within each material layer in the multilayer RJ structure.

Figure 3. Solar-spectrum-weighted absorption distribution within the RJ solar cell unit, over a spectrum through λ = 300 nm to 800 nm.

Figure 4

(a) Effective absorbed power within the i-layer at specific photon wavelengths and as a function of different simulation box dimension (W_sub); (b) evolution of the integrated absorption within the i-layer.

Figure 5

(a) Absorbed power broken down by material layer, at varying incident photon wavelength for a RJ cell with SiNW length of L_w = 1.7 μm and i-layer thickness of T_i = 100 nm. (b) Percentage of effective absorption in the i-layer relative to total absorption in the RJ multilayer structure. (c) Evolution of the absorption spectra in the active i-layer with different Si nanowire lengths from L_w = 0.6 um to 1.7 μm. (d) full spectrum integrated absorption in i-layer as a function of SiNW length. This trend is then fitted by Lambert-Beer law as marked by the red line.

Figure 6

(a) Absorption spectra in active i-layer while varying i-layer thickness (T_i) from 20 nm to 140 nm, (b) full spectrum integrated absorption in i-layer as a function of T_i. (c) field distribution of mode in the RJ cross section with T_i = 140 nm.

Figure 7

(a) Normalized experimental EQE spectrum (black square-line) measured for RJ cells on VLS-grown SiNWs (example SEM image in (b)), compared to the normalized calculated absorption curves for T_i = 80 nm, 100 nm and 120 nm. (c) Normalized experimental EQE and averaged simulated absorption curves weighted according to their appearance statistics. (d) Spectrum of the absorption enhancement ratio observed for an array of RJ cells (with strong light trapping effect) over that of a single RJ cell, as extracted from (c).