Chapter 7c Thin Film Mono or Polycrystalline Silicon Solar Cells June 22, 2015

Classification of thin film solar cells

The effectiveness of light trapping can be assessed by short-circuit current. However, it will be complicated by other losses, such as bulk recombination and front surface reflection. In lab., spectral response and reflection data can be analyzed to extract detailed information about light trapping. Texturing results in oblique paths for internally confined light and maximizes total internal reflection at the illuminated device surface. It has been predicted that for weakly absorbed light, the effective optical thickness a silicon solar cell with both surfaces textured to form Lambertian diffuse reflectors can be about 50 times greater than its actual thickness.

Methods of random texturing – to enhance light trapping

Regular Structuring

The use of prismatic cover slip

Reflection analysis of light trapping

The slopes and intercepts of the linearised parts of the curve can be used to estimate the collection efficiency, the back reflectance, the back surface recombination velocity, and the effective minority carrier diffusion length. Wafer-based silicon solar cell

The slopes and intercepts of the linearised parts of the curve can be used to estimate the collection efficiency, the back reflectance, the back surface recombination velocity, and the effectiveness minority carrier diffusion length. Textured 4 um thick silicon solar cell

Voltage (V oc ) enhancements in thin silicon solar cells = J m, o due to the base layer

When the doping level increases, this can decrease the minority life time and increase the bulk recombination, reduce the efficiency of the solar cells

Problems of Polysilicon Solar Cells It was initially thought that polycrystalline silicon solar cells would suffer major short-circuit current and open- circuit voltage losses from grain boundary recombination, which would severely restrict the maximum light- generated current. However, when the grain diameter is several times larger than the intra-grain (bulk) minority diffusion length, the short-circuit current is controlled not by grain-boundary recombination, but by the intra-grain diffusion length. The presence of inclusions and tunnel junctions, both of which act as resistive shunts and degrade the open- circuit voltage and fill-factor locally.

Substrates for thin film solar cells ● Mullite, compound of alumina and silica

CVD: APCVD, LPCVD, RTCVD, PECVD Growth methods of thin film

Zone melting recrystallization (ZMR)

Thin silicon solar cells based on substrate thinning

Growth of Epitaxial layer and layer transfer by bonding with to a glass substrate