Flying Spot Technique in Microcrystalline Silicon Solar Cells
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RESPONSE TIME MEASUREMENTS AND FLYING SPOT TECHNIQUE IN MICROCRYSTALLINE SILICON SOLAR CELLS R. Schwarz1,2, P. Sanguino1, S. Koynov1, M. Fernandes2, F. Maçarico2, P. Louro2, and M. Vieira2 1 Physics Department, Instituto Superior Técnico, IST, P-1096 Lisboa, Portugal 2 Electronics and Communications Department, Instituto Superior de Engenharia de Lisboa, ISEL, P-1940-014 Lisboa, Portugal Tel: +351-21-841 7775, e-mail: [email protected]
ABSTRACT Transverse charge collection in a p-i-n structure based on microcrystalline silicon and lateral transport in a single intrinsic layer were analyzed using transient photoconductivity (TPC), steady-state photocarrier grating analysis (SSPG), and the flying spot technique (FST). Photocurrents were excited either with HeNe laser light (633 nm, intensity about 20 mW/cm 2) or with 5 ns pulses from a Nd:YAG laser (532 nm, peak power of about 500 kW/cm 2 with 5 mJ pulses). The response time τR varied greatly from about 3 ms at low intensity down to a few µs under pulsed laser excitation. The minority carrier diffusion length LD measured by FST in diode structures was usually larger than values obtained from the SSPG method applied to intrinsic microcrystalline layers.
INTRODUCTION The true values for response time, carrier mobility, and ambipolar diffusion length are not easy to determine in the case of microcrystalline hydrogenated silicon films (µc-Si:H), due to the complex structure of the material and the necessity to employ different techniques to obtain a complete set of transport parameters. In addition, as a second problem, each technique when employed to obtain just one common parameter has its prerequisites and drawbacks that might render a comparison unsuccessful. In this contribution we will focus on the response time τR and the ambipolar diffusion length Lamb measured in (a) a lateral configuration using a single intrinsic µc-Si:H layer and (b) a transverse structure of an all-microcrystalline p-i-n diode. In both cases, the i-layers were deposited with exactly the same deposition parameters. The pessimism expressed above stems from the rich literature on such comparisons done in the case of hydrogenated amorphous silicon (a-Si:H). On the other hand, previous work can give us guidelines to try extrapolate the underlying physical principles from amorphous to microcrystalline silicon. The first observation concerns the intensity dependence of the (majority) carrier lifetime τmaj in a-Si:H which was theoretically discussed by A. Rose in 1963. He derived a the simple power law for photoconductivity σph in disordered semiconductor films [1]:
σ ph ∝ I α
and τ maj ∝ I α −1 (1)
A32.4.1
where I is the light intensity, and 0.5< α
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