Novel Wet Chemical Cleaning Concepts for High Efficiency Silicon Solar Cells
Creators: name: Haslinger, M.
name: Soha, M.
name: Jambaldinni, S.
name: Hajjiah, A.
name: Szlufcik, J.
name: Poortmans, J.
name: John, J.
Publisher: name: WIP
Year: 2017
Subjects: Silicon Photovoltaics
Homojunction Solar Cells

description: 33rd European Photovoltaic Solar Energy Conference and Exhibition; 628-630
description: We report on the progress of wet chemical process development for silicon surface cleaning of n-type
passivated emitter with rear totally diffused (n-PERT) solar cells. An Ozone based wet chemical cleaning process
sequence has been developed which provides both, improved effective minority carrier lifetime and reduction in cost
of ownership.
The proposed cleaning solution (i2PV: imec industrial PhotoVoltaic Clean), is a mixture of ozonated DI water
(DIW/O3) and diluted Hydrochloride and Hydrofluoric acid (HCl/HF). The silicon surface clean is performed in a
single tank and provides a hydrophobic or hydrophilic silicon surface after the clean depending on the ozone
concentration. In order to quantify the cleaning efficiency of the solution we have determined surface contamination
concentration of prominent transition metals before and after wet chemical cleaning. We also determined the impact
on solar cell performance by measuring effective minority carrier lifetime in large area 156x156 mm2 commercial ntype
cz-Silicon, 3-5 Ohm.cm, 180um thickness, after a high temperature process and passivation with thermal SiOx.
We benchmarked the cleaning efficiency of the solution by determining the spatial distribution of the effective minority
carrier lifetime using photoluminescence measurements. The impact of pre-treatment of the silicon surface has been
investigated.
The low cost of ownership is mainly determined by the low concentration of the involved chemicals (HCl and HF less
than 2%), the ozone that is switched on and off before and after the cleaning process, the application at room
temperature and the short processing time of only 2-5min. Despite the low chemical concentrations the solution shows
a highly effective impurity removal quality. Metal removal efficiency is determined and effective minority carrier
lifetime is extracted on passivated wafers. As a novel parameter a measure for the spatial distribution of the effective
minority carrier lifetime is introduced on large area Cz-Si.
The uniformity of the lifetime distribution has been defined by measuring the color distribution in the PL spatial image
using the histogram data for red in the RGB bitmap. This provides us a scale from 50 to 75. In this case 75 represents
a very good uniformity and 50 a rather poor uniformity. The minority carrier lifetime distribution uniformity is an
important parameter for potential solar cell efficiency losses. A uniformity vs effective minority carrier lifetime graph
is depicted resulting from a parameter sensitivity screening design of experiment (DoE). There is a clear trend towards
hydrophobic pre-treatment in both, minority carrier lifetime and uniformity, respectively.
The large process window, the low chemical consumption and the high metal impurity removal efficiency makes this
solution an interesting choice for silicon surface cleaning in solar cell production.