Enabling Technology for p-Type Surface Passivation of Si Solar Cells
Creators: name: Mungekar, H.
name: Lu, X.
name: Vellaikal, M.
name: Kaur, D.
name: Zhang, L.
name: Frei, M.
name: Wijekoon, K.
name: Ponnekanti, H.
Publisher: name: WIP
Year: 2012
Subjects: Wafer-Based Silicon Solar Cells and Materials Technology
Silicon Solar Cell Improvements

description: 27th European Photovoltaic Solar Energy Conference and Exhibition; 900-903
description: 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 Al2O3 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 Al2O3 to (a)
protect Al2O3 from degradation during Al contact firing step (b) complement 10-20nm thick Al2O3 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 Al2O3 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 Al2O3. Although Al2O3 films can be grown by Plasma Enhanced Chemical Vapor Deposition (PECVD),
thereby eliminating need for separate process technologies for Al2O3 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 Al2O3/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
Al2O3/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 Al2O3 films by PECVD to thicknesses &gt;100nm without blistering issues as
deposited or after firing, demonstrating further benefits of this approach over other methods for PECVD Al2O3
described in literature. PERC cells are fabricated using the Al2O3/SiNx stack for the rear passivation after POCl3
emitter formation. Laser based approach is adopted to open contact in Al2O3/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
Al2O3/SiNx stacks.