Selection of Material for the Back Electrodes of Thin-Film Solar Cells Using Polycrystalline Silicon Films Formed by Flash Lamp Annealing

Abstract

Flash lamp annealing (FLA), a rapid annealing technique with a millisecond-order duration, can form polycrystalline silicon (poly-Si) films of a few µm thickness on glass substrates by crystallizing precursor amorphous Si (a-Si) films without serious thermal damage to the substrates. We attempt to use several kinds of metal films for adhesion layers inserted between the Si films and the glass substrates to prevent Si film peeling during FLA. One of the requirements for the insertion metals is a melting point (T melt) higher than 1414 °C, to which the metal films could be heated during the crystallization induced by FLA. Of the metal films attempted, only Cr films can prevent Si film peeling from soda lime glass substrates, which have a much larger thermal expansion coefficient than quartz, indicating the necessity of sufficient adhesiveness to glass and Si as well as of a high T melt. Actual solar cell operation is demonstrated using a flash-lamp-crystallized poly-Si film as an absorber layer and a Cr film as a back electrode.

Polycrystalline silicon films fabricated by rapid thermal annealing

“…This reveals that the inserted Cr film can effectively suppress the peeling of Si films during the RTA. This may be related to the improvement of the interface property by the Cr film [15]. The liner thermal expansion coefficients of c-Si and quartz are about 4.4 9 10 -6 and 4.7 9 10 -7 /K at 1,100°C [19], respectively.…”

“…Many efforts have been done to realize the crystallization of amorphous silicon (a-Si) films for a short time. Excimer laser annealing (ELA) [12,13] and flash lamp annealing (FLA) [14][15][16] are alternative methods which can crystallize the a-Si film in several milliseconds. However, relative high cost of the equipments and nonuniformity of the crystallized poly-Si films restrict their wide use in the mass production.…”

Solar Cell Emitters Fabricated by Flash Lamp Millisecond Annealing

“…Recently, multicrystalline silicon (mc-Si) solar cells made of either bulk material or thin films have attracted considerable attention because of their high stability against light soaking. Poly-Si films are very important for silicon-gate metal oxide semiconductor (MOS) integrated circuits, charge-coupled devices, solar cells, thin-film transistors (TFTs), and various other applications [1][2][3][4].…”

“…The flash--lamp annealing (FLA) systems are successfully used for silicon recrystallization in many laboratories [1,[7][8][9]. In principle, there is no sample size limitation for the FLA systems but producing a homogeneously annealed big area wafer (at least an eight inch one) is not trivial.…”

Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films

“…Therefore, FLA can be a candidate method of forming c-Si films with high throughput and low thermal damage to substrates. We have succeeded in forming polycrystalline Si (poly-Si) films of more than 4 mm thickness by FLA of a-Si films even on conventional soda lime glass substrates [12], and demonstrated the operation of solar cells using flashlamp-crystallized (FLC) poly-Si films [13,14]. The solar cell properties obtained so far are short-circuit current density of 8.6 mA/cm 2 , open-circuit voltage of 0.30 V, fill factor of 0.51, and conversion efficiency of 1.3% [14], and they will be improved by optimizing the solar cell process.…”