Liquid Phase Crystallization LPC

For liquid phase crystallization (LPC) nano-crystalline silicon is deposited on glass substrates and subsequently crystallized by a line-shaped energy source. A laser beam scans the sample with a velocity of 3 mm/s and locally melts the silicon. It recrystallizes polycrystalline into grains that are up to a few centimeters in length and a couple of millimeters in width. With this technique not only a morphology comparable to multi-crystalline silicon wafer solar cells are achieved but also similarly high open-circuit voltages [1,2].

In case a textured glass substrate is used for enhancing light in-coupling into the solar cell, the silicon grows conformally on the textured substrate resulting in a double-sided textured absorber layer. If the rear side texture is protected by a sacrificial capping layer during crystallization the double-side texture is preserved even after crystallization. Hence, it provides not only enhanced light in-coupling properties at the front side but also light scattering properties at rear side trapping the light within the absorber layer [3,4].

Double-sided textured liquid phase crystallized silicon thin-films on a square lattice texture with a period of 2 µm. (a) Schematic during liquid phase crystallization. (b) Cross sectional scanning electron microscopy image, here with a 6 µm thick silicon layer.

 

The challenge to face is to maintain the high electronic material quality of the silicon despite it being grown and crystallized on a rough substrate. The goal is to find an optimum balance between optical and electronic gain in order to maximize the solar cell efficiency [5–7].

Photograph of a double-sided textured silicon thin-film solar cell and scanning electron microscopy image of the underlying hexagonal sinusoidal texture with a period of 750 nm. Due to texturing the light path inside the solar cells is alternated and the samples shimmer colorfully under oblique incidence of light.