© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-rapid.com pss Phys. Status Solidi RRL 5, No. 1, 25 – 27 (2011) / DOI 10.1002/pssr.201004426 Dynamic photoluminescence lifetime imaging for the characterisation of silicon wafers Sandra Herlufsen * , Klaus Ramspeck ** , David Hinken, Arne Schmidt, Jens Müller, Karsten Bothe, Jan Schmidt, and Rolf Brendel Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany Received 6 October 2010, revised 24 October 2010, accepted 25 October 2010 Published online 28 October 2010 Keywords lifetime, Si, wafers, photoluminescence, imaging ** Corresponding author: e-mail herlufsen@isfh.de, Phone: +00 49 5151 999 414, Fax: +00 49 5151 999 400 ** Now with Schott Solar AG, Carl-Zeiss-Straße 4, 63755 Alzenau, Germany © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The recombination lifetime of crystalline silicon (Si) is one of the most relevant parameters in solar cell production. Particularly in the case of multicrystalline silicon (mc-Si), spatially resolved information of the lifetime is of utmost importance, as this material typically has strongly inhomo- geneous recombination properties. Thus, camera-based ap- proaches are highly preferable. Up to now, camera-based photoluminescence (PL) techniques have predominantly been used under steady-state conditions [1 – 5]. For the conversion of the measured luminescence signal into the carrier lifetime, a reliable calibration procedure is required. One approach [3, 4] relates the luminescence emission to the excess carrier density determined from calibrated con- ductivity measurements carried out in the same setup. Un- fortunately, this procedure relies on the knowledge of the dopant density of the sample, the wafer thickness and the validity of the underlying carrier mobility model. Alterna- tively, a calibration procedure based on calculations of the total number of emitted photons was recently suggested [5]. However, a precise knowledge of the optical parameters, dopant concentration and thickness of the wafer is essential in this case. Thus, a calibration-free approach enabling fast carrier lifetime imaging with a high spatial resolution would be of great advantage. A calibration-free analysis can be realised by evaluating the time dependence of a physical quantity related to the excess carrier density. On the basis of measurements of the free carrier emission us- ing an infrared camera, Ramspeck et al. [6] introduced the dynamic infrared lifetime mapping (dynamic ILM) techni- que. The aim of this work is to adapt this approach for camera-based dynamic photoluminescence (dynamic PL) measurements. The main advantages of PL measurements compared to ILM measurements are: (i) PL measurements are weakly influenced by measurement artefacts such as trapping [7, 8] or depletion region modulation [9] and (ii) they can be easily performed at room temperature. The dynamic PL lifetime technique is based on two el- ementary equations. The first expresses the relation be- tween excess carrier density Δn and photoluminescence signal I PL : I PL ~ Δn(Δn + N dop ), where N dop is the dopant density [10]. The second equation describes the time de- pendence of the excess carrier density Δn in the wafer, ba- sed on the continuity equation d Δn(t)/dt = G – Δn(t)/ τ eff , where G is the generation rate and τ eff is the effective car- rier lifetime. This equation has to be solved for two cases: Δn rise (t), after a light source is switched on (constant G > 0) and Δn fall (t), after a light-source is switched off (G = 0). The carrier density Δn(t) is schematically plotted in Fig. 1 We present a fast and calibration-free carrier lifetime imaging technique based on photoluminescence (PL) measurements using an InGaAs camera for the examination of crystalline silicon wafers. The carrier lifetime is determined from the time dependent luminescence emission after optical excita- tion. A ratio, including four PL images acquired at different times during the modulated excitation, is calculated and found to depend only on the camera integration time and the effec- tive carrier lifetime. Therefore, the carrier lifetime is unambi- guously determined by this ratio without knowing any addi- tional wafer parameter. We demonstrate the applicability of the dynamic PL technique to multicrystalline silicon wafers.