ENABLING TECHNOLOGY FOR p-TYPE SURFACE PASSIVATION OF Si SOLAR CELLS 27th EU PVSEC, FRANKFURT GERMANY, SEP 24-SEP29, 2012, 2DO.8.6

摘要

Effective surface passivation of p-type Si surfaces is a key efficiency improvement enabler for silicon solar cells, particularly through improvement of the back surface in PERC (Passivated Emitter Rear Cell) architecture. Low surface recombination velocity for p-type Si, due to negative fixed charge and low interface state density has attracted significant interest in Al_2O_3 film for surface passivation in solar cells. Although laboratory scale results of greater than 19% efficient PERC cells using co-fired Al approach have been reported, several challenges remain for mass implementation of passivation technology. A SiN film cap is deposited on Al_2O_3 to (a) protect Al_2O_3 from degradation during Al contact firing step (b) complement 10-20nm thick Al_2O_3 layer to improve rear reflectivity of the solar cell. The use of SiN cap layer has two manufacturing challenges: (i) Need to eliminate blistering when Al_2O_3 and SiN film stack is co-fired (ii) Need for additional handling and equipment to deposit SiN when Atomic Layer Deposition (ALD) is used to grow Al_2O_3. Although Al_2O_3 films can be grown by Plasma Enhanced Chemical Vapor Deposition (PECVD), thereby eliminating need for separate process technologies for Al_2O_3 and SiN, previous efforts by other authors have shown inferior passivation by PECVD compared to the ALD approach. We address these two challenges by using a novel PECVD based approach. Blistering failure modes due to interface delamination and AlO/SiN film interaction during firing are proposed and blister free film stacks are demonstrated. Using insight gained from blistering studies, process conditions and a process-enabling PECVD chamber are identified to demonstrate firing stable Al_2O_3/SiN films capable of ALD-like passivation performance together with the high productivity of PECVD. Surface Recombination Velocity (SRV) on realistic solar wafer surfaces with fired Al_2O_3/SiN films are around 20-50 cm/sec @ 1e15 cm-3 on p-type 1 Ω-cm substrates. Contrary to previous findings by other groups, we are able to grow Al_2O_3 films by PECVD to thicknesses >100nm without blistering issues as deposited or after firing, demonstrating further benefits of this approach over other methods for PECVD Al_2O_3 described in literature. PERC cells are fabricated using the Al_2O_3/SiNx stack for the rear passivation after POCl_3 emitter formation. Laser based approach is adopted to open contact in Al_2O_3/SiN film. Screen printed Al paste formulation and firing sequence are optimized to obtain void-free contact formation. The measured cells have measured efficiency of 19.3%. The 0.6% improved efficiency compared to a back-polished Al BSF baseline corresponds to increases of 5-8mV in open-circuit voltage, higher short circuit current and quantum efficiency gain at longer (800-1200nm) wavelengths. These results demonstrate that this novel PECVD technique provides ALD-like passivation while employing a production worthy approach for high efficiency PERC cells with high-quality, firingstable Al_2O_3/SiNx stacks.