Light Trapping in Thin Silicon Solar Cells

Light trapping plays a key role in solar cell to enhance light confinement and then light absorption within the cell. The reduction of the optical losses reflects in photovoltaic solar cell efficiency enhancement. To this aim different and combined strategies can be adopted. Firstly a reflectance reduction of sunlight impinging on the cell is mandatory. Then a cell surface texturing can be very helpful to reduce the reflection by increasing the chances of reflected light bouncing back onto the surface rather than surrounding air. To this purpose different approach can be followed such as substrate and/or single layer texturing. Moreover particular care should be paid to the rear side of the cell where the introduction of a reflecting mirror can produce an optical path length enhancement over a wide wavelength spectrum. Rightly combining the front side texturing and the rear side mirroring it is possible to enhance this pathlength up to 50 times the device thickness indicating that light bounces back and forth within the cell many times performing a light confinement. To this purpose Bragg reflector formed using thin film technology is one of the most promising approach to the back side reflector. Also multi-junction cell, such as tandem micromorph, can receive benefit to the light management by the introduction of inner reflector within the two cells to enhance the spectral separation between the two cells and reduce the front cell thickness where the metastability problem still remain an open request.

In this work we overview all these concepts as applied to thin film cell and to silicon based cell that actually are speedily moving toward thinner substrate making the light confinement a key point to cell efficiency enhancement and PV cost reduction.

Keywords: Amorphous silicon, antireflection coating, heterojunction, lifetime, numerical model, optical properties, reflector, scattering, solar cell, texturing, thin film, transparent conductive oxide, plasma process, light trapping, thin wafer, quantum efficiency, bragg reflector, haze, reactive ion etching, laser treatment, silicon.