Contactless Measurement of Carrier Lifetime on Silicon Ingots and Bricks
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1210-Q01-06
Contactless Measurement of Carrier Lifetime on Silicon Ingots and Bricks
James S. Swirhun, M. Keith Forsyth, Tanaya Mankad, and Ronald A. Sinton Sinton Instruments, Boulder, CO 80301 USA
ABSTRACT High efficiency silicon solar cells demand the use of high lifetime silicon wafers. Characterization of boules and bricks before wafering allows poor quality material to be rejected before expensive processing steps. This paper extends simulation techniques previously used in quasi-steady-state-photoconductance to transient photoconductance decay (PCD) measurements of high lifetime bulk samples. Simulation of transient PCD measurements on unpassivated bulk silicon yields limits on the accuracy of the measurement technique and the effects of surface recombination. INTRODUCTION High efficiency silicon solar cells promise to advance photovoltaic energy by providing more power at lower cost. In order for these cells to achieve high efficiencies, the minority carrier lifetime of the silicon wafers must be high, often greater than 1 millisecond. Measuring the minority carrier lifetime of bulk silicon ingots and bricks before wafering ensures that the wafers will meet process control limits and consistently produce high efficiency cells. Traditionally, lifetime measurements on high lifetime bulk samples are performed by photogenerating carriers with a high power laser pulse while measuring the sample photoconductivity through contacts attached to the ends of a block [1,2]. This method requires a number of expensive components and becomes more difficult for low resistivity samples of interest to solar cell manufacturers. For lower lifetime samples, a common choice is to use quasi-steady-state photoconductance (QSSPC) to measure lifetime in homogeneous bulk material. In this case, the photoconductance signal is essentially a sheet of charge with depth dependent on the diffusion length, L, and the absorption depth of the incident light, α,
(
d =2 L+ 1
α
)
(1)
The lifetime can easily be evaluated as a sheet of surface charge using the steady-state solution of the diffusion equation corrected for surface recombination. One method for measuring low lifetime samples in the quasi-steady-state is to use an eddy-current detector. In this method, an inductively couple coil is used to measure the conductivity of a sample illuminated with a slowly decaying xenon flash lamp. For high lifetime samples, a different strategy is preferred and is the focus of this paper. This strategy, known as transient photoconductance decay (PCD), consists of illuminating the sample to photogenerate carriers and then terminating the light source [3]. After the light has been terminated, the injected carrier profile moves into the bulk and the carrier density distribution decays at a rate approaching the characteristic bulk lifetime. After converting the
measured conductivity to injected carrier concentration, the lifetime at any given point in the decay curve can be calculated by, ∆n (2) τ measured = − d (∆n ) dt The most common methods use
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