Mechanism of front surface recombination velocity of solar cell with potential-induced degradation (PID)effect

Abstract: Potential-induced degradation (PID) is one of the most important and prominent module degradation mechanisms leading to significant yield losses. It was shown that Na decorated stacking faults at the SiNX/Si interface region are responsible for the mechanism of increasing non-saturation hanging states density at the front surface. In order to investigate the characteristics of recombination of non-equilibrium minority carriers at the front surface, the paper analyzes the band gap structure and electrical field at surface of solar cells material after PID test, and then deduces the model of front surface recombination velocity by measuring internal quantum efficiency (IQE) of solar cells under short wavelength monochromatic light, which is derived by Poisson equation, and continuity equation and current density equation of semiconductor physics. For the certain sample, the parameters of the materials are known, such as absorbance coefficient, and hole diffusion coefficient, and p-n junction depth, and hole diffusion length, which is modified by electrical field at front surface. And then, the front surface recombination velocity of the samples after PID test is calculated by the IQE model, which is measured by QE (quantum efficiency) equipment. In this paper, the experiment is carried out at the p-type base region with (100) p type CZ monocrystalline silicon, whose impurity density is 1.5×1016 cm-3, minority carrier lifetime is 7.2 μs, and area is 100 mm × 100 mm. The samples are prepared using constant source for the POCl3 diffusion, which is followed by plating SiNX as passivation and anti-reflection film. The last procedure is printing aluminum slurry at the back surface of samples as black electric field, where impurity density is 1×1020 cm-3. At the same time, different diffusion conditions and front surface velocity are simulated by PC1D software, in order to study the influencing mechanism of impurity and velocity at front surface of samples after PID test. And then, the front surface recombination velocity of the samples is calculated and compared with the samples which are PID-free, which shows that the values of surface recombination velocity are more reliable when using wavelengths in range of 310-360 nm than other wavelengths. Additionally, I-V characteristics of the samples before and after PID test are simulated by PC1D. It is obvious that the output characteristics of the sample 1, which has good passivation and low front surface velocity, are degraded most seriously after PID test than others. It is assumed that Na ions drift through the SiNx layer under the influence of a strong electric field and increase the impurity density at front surface, which leads to I-V characteristics degradation. For the sample 2 and the sample 4, which have poor passivation and low impurity density at front surface, electron barrier at front surface region can reduce the impacts from Na ions. Correspondingly, the open circuit voltage of these samples has few effects, however short circuit current of the samples is decreased. The result of the sample 3 shows that PID has little influence on the output electric characteristics of solar cell with higher impurity density at front surface, whether the passivation condition is good or not. In general, the experiments indicate that surface recombination velocity of PID solar cells is higher than that of PID-free, and I-V characteristics of solar cells, which have good passivation conditions and low front surface recombination velocity, are deteriorated, however, PID has few effects on the solar cells with the poor passivation conditions and high front surface recombination velocity.