Analysis of optical absorption and quantum efficiency due to light trapping in a n–i–p type amorphous silicon solar cell with textured back reflector

Abstract

Textured back reflector (BR) plays an important role to achieve light trapping in amorphous silicon solar cell. We investigated n–i–p type amorphous silicon solar cell, fabricated on textured BR. The solar cells showed improvement in current density and red response of the external quantum efficiency (EQE) spectra. We used simplified ray diagram along with double reflection model to analyze the EQE because the pyramidal surface texture had apparently little or no periodicity in its surface distribution. The Beer–Lambert's exponential absorption relation was used to estimate photo‐current generation. Experimental results and theoretical analysis indicates that the trapping of unabsorbed light within the cell contributes to enhanced red response to the EQE. A further enhancement in current density and EQE was obtained by double texturing the BR. It appears that in addition to enhanced light trapping, a significant reduction in parasitic absorption of light may also contribute to the improvement in cell performance. The double texturing of the BR led to an increase in surface roughness of the cell, that further helps in the light trapping by introducing increased randomness of the light that is transmitted into the cell.

Improving Device Efficiency for n-i-p Type Solar Cells with Various Optimized Active Layers

“…The efficiency of a solar cell is a complicated combination of optical absorption [9,11] and recombination of photo generated carriers [12]. With the help of the Beer-Lambert exponential relation, the optical absorption in the active layer (I a ) is expressed as [9,11]:…”

“…With the help of the Beer-Lambert exponential relation, the optical absorption in the active layer (I a ) is expressed as [9,11]:…”

“…With the help of the Beer-Lambert exponential relation, the optical absorption in the active layer (I a ) is expressed as [9,11]: (1) where I 0 is light intensity at the front surface of the active layer, and α is the absorption coefficient. The SRH (Shockley-Read-Hall) recombination can be expressed as proportional to total trap density (N t ) as [12]: (2) where n i is the intrinsic carrier density, n & p are non-equilibrium carrier concentration in the material, v th is the thermal speed of an electron, and σ t is the capture cross section.…”

Low Reverse Saturation Current Density of Amorphous Silicon Solar Cell Due to Reduced Thickness of Active Layer

“…So we performed a further analysis to understand a possible reason behind such a high n [14]. Defects may have a role in this case as defect density (N d ) for this trace was 8×10 17 cm -3 , while that for the bottom curve with star symbol was…”

“…However, thinner cell will have lower light absorption and hence lower current density. This limitation can be overcome to a great extent by adopting to light trapping schemes [16,17].…”