太阳电池电势诱导衰减效应表面复合速度机理

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

电势诱导衰减(potential induced degradation,PID)效应是导致光伏组件输出效率下降的主要原因之一。为了研究PID太阳电池前表面非平衡载流子复合特性,该论文首先分析PID太阳电池表面能带及电场变化情况,利用连续性方程以及电流密度方程建立PID太阳电池表面复合速度S_(pPID)与短波内量子效率(internal quantum efficiency,IQE(λ))之间的数学模型。其次,通过利用太阳电池常用计算模拟软件PC1D模拟在不同工艺条件下晶体硅太阳电池IQE(λ),并且采用所构建的数学模型计算PID效应太阳电池前表面复合速度S_(pPID),与未发生PID效应时前表面复合速度S_p以及I-V特性曲线进行对比。结果表明,利用短波IQE(λ)测量PID太阳电池前表面复合速度S_(pPID)时,波长选择范围在310~360 nm之间误差较小;当太阳电池发生PID效应,前表面复合速度增大,前表面杂质浓度低、钝化效果好的太阳电池输出I-V特性下降,对钝化效果差、表面掺杂浓度高的太阳电池输出I-V特性影响较小。论文的研究结果为制备抗PID效应组件提供理论基础。

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 Si NX/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 Si NX 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 PC1 D 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 PC1 D. 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 Si Nx 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.