26.1%-efficient POLO-IBC cells: Quantification of electrical and optical loss mechanisms

We present experimental results for interdigitated back contacted (IBC) solar cells with passivating POLO contacts for both polarities with a nominal intrinsic poly-Si region between them. We reach efficiencies of 26.1% and 24.9% on a 1.3 omega cm and 80 omega cm p-type FZ wafer and 24.6% on a 2 omega cm n-type Cz wafer, respectively. The initially measured implied efficiency potentials of the cells after passivating the surfaces are very similar, namely, 26.8%, 26.8%, and 26.4%, respectively. We attribute the difference between the efficiency potential and the final current-voltage measurement to degradation, perimeter, and series and shunt resistance losses, which we quantify by lifetime measurements. With these measurements in combination with a finite element simulation, we determine the surface recombination velocity in the nominal intrinsic poly-Si region to be in the range from 13 to 21 cm s(-1). Using the same approach, we analyze the increase of the front surface recombination velocity during cell processing from 2 to 10 cm s(-1) for the 1.3 omega cm and from 0.5 to 2.3 cm s(-1) for the 80 omega cm. This leads to the fact that cells fabricated on lowly doped bulk material are more vulnerable to a process-induced degradation of the surface passivation quality. We further determine the theoretical limits of the cells by firstly idealizing the recombination (28% for 1.3 omega cm and 28.2% for 80 omega cm) and secondly also idealizing the optics of the solar cells (29.4% and 29.5%).