Measurement of the normalized recombination strength of dislocations in Multicrystalline silicon solar cells

An improved technique is presented to measure the normalized recombination strength Gamma at dislocations in silicon solar cells that were fabricated of cast grown silicon. Gamma is the number of recombinations per unit time, length, and excess carrier density divided by the minority carrier diffusion coefficient D. The measurement is based on fitting the theoretical correlation between internal quantum efficiency IQE at a single wavelength and dislocation density rho to the measured data. The IQE is measured topographically by the light beam induced current (LBIC) method. For each point of the LBIC map a dislocation density is determined by analysing the etched sample surface with an image recognition programme. The theory for IQE(rho) combines Donolato's prediction for L(rho) with a calculation of IQE(L) made by the computer programme PC I D. L is the diffusion length of the minority carriers. The programme PC I D takes special properties of the solar cell process into account. The method was applied to solar cells made by a conventional furnace process as well as a rapid thermal process (RTP). In the latter case a correlation between Gamma and the emitter diffusion temperature was found. Finally TEM measurements were made to investigate dislocations with different values of Gamma.