Comparing MDP, QSSPC and PL Imaging for the Production Control of Heterojunction Solar Cells

Minority carrier lifetime is a valuable parameter for quality control of the early production steps in solar cell manufacturing. It is particularly powerful for heterojunction technology as passivation of the amorphous silicon (aSi) layers is fully established directly after PECVD without further activation steps (as e.g. in the PERC process). This allows sensitive quality assessment especially if lifetime is measured injection-dependently and/or spatially resolved. Quasi-steady-state photoconductance (QSSPC), photoluminescence (PL) imaging and microwave-detected photoconductivity (MDP) are fast and contactless techniques that allow lifetime measurements inline. Each technique has its own strengths and weaknesses regarding carrier injection resolution, spatial information, quantitative accuracy, and signal understanding. At Fraunhofer ISE, each of the systems is installed in our inline wafer inspection system FWIS which allows direct comparison of the results for each wafer on a statistical scale. We present here such a comparison of QSSPC, PL and MDP on a set of 4000 silicon heterojunction precursors. We observe strong correlation between the three techniques, indicating good qualitative agreement of the measured lifetimes. Despite MDP being designed for measurement of differential lifetime over a steady-state bias injection level, even high-power MDP measurements without bias injection agree quantitatively with QSSPC measured at a carrier injection level of 10 16 cm3 , suggesting that this may be taken as the effective carrier injection density of the MDP tool for the given setup. By calibrating the instruments against each other, we are thus able to gain spatially and injection-resolved lifetime data for each individual wafer in a production environment.