Luminescence Imaging Techniques for Silicon Photovoltaics

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KILIANI, David, 2013. Luminescence Imaging Techniques for Silicon Photovoltaics [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Kiliani2013Lumin-25860, title={Luminescence Imaging Techniques for Silicon Photovoltaics}, year={2013}, author={Kiliani, David}, address={Konstanz}, school={Universität Konstanz} }

eng Kiliani, David 2014-01-15T08:05:29Z In this work, new approaches for the characterization of crystalline silicon wafers and solar cells using electroluminescence imaging (ELI) and photoluminescence imaging (PLI) were presented.<br /><br /><br /><br />Basis for the measurements was the construction of a sensitive, camera based luminescence imaging setup with a widely adjustable field of view. GaAs based optical filters were introduced for near optimal excitation light suppression. The illumination of samples for PLI is possible both with an LED panel and a homogenized solid-state laser. Both light sources have an adjustable and calibrated photon flux.<br /><br /><br /><br />A lifetime calibration method for PL images based on QSSPC measurements was implemented. To correlate the spatially resolved PLI data with integral QSSPC values, the QSSPC sensitivity distribution was characterized.<br /><br /><br /><br />In addition to the default Si CCD camera, several InGaAs cameras were evaluated and compared for the measurement of defect band luminescence. Corresponding measurements of a multicrystalline Si solar cell at different temperatures show both an anticorrelation of band-to-band PL with defect band PL and a PL increase for low temperatures.<br /><br /><br /><br />To separate the contributing factors to solar cell series resistance, a technique based on EL images was developed. A multilayered 2-D model of the solar cell is solved and fitted to p-n junction voltage maps from ELI until both voltage distributions agree. The series resistances of front metallization grid, metallization contact, emitter and base are parameters in the solar cell model and reach their respective values during fitting. The obtained results for different solar cells qualitatively respond correctly to changes, e.g. of emitter sheet resistance. However, light scattering effects lead to a systematic underestimation of the solar cell resistivity. Further refinements taking care of this issue are required to make quantitatively correct measurements.<br /><br /><br /><br />A newly developed technique for calibration-free imaging of the minority charge carrier lifetime tau_eff using time-resolved photoluminescence imaging (TR-PLI) was presented. The measurement is based on a modulated illumination of the sample, synchronized to a periodically shuttered recording of the PL emission. Three different shutter setups were described, with increasing experimental complexity and correspondingly higher time-resolution. The first setup uses a rotating shutter wheel in front of the camera lens and was able to resolve tau_eff down to ~300µs. The second setup-which was filed for patent-uses a shutter wheel in the intermediate image plane between two objective lenses, extending the measurement range down to ~5µs. The third setup is based on an electronically switched image intensifier unit, which allows reliable mapping of tau_eff from ~2.5µs to several ms. The electronic shuttering also simplifies evaluation and calibration, making this setup the preferred choice if the required image intensifier can be acquired.<br /><br /><br /><br />A comparison of the three shutter setups shows very similar results within their respective measurement range. The choice of excitation frequency has significant impact on the uncertainty of the measurement, an empirical optimum was found at f_exc = 1/tau_eff,max. Compared to the established lifetime measurement techniques QSSPC and µ-PCD, very good agreement was observed. At low excess carrier densities the measurements could not be compared as the results from QSSPC and µ-PCD were impaired by trapping and DRM artifacts, to which TR-PLI is less susceptible.<br /><br /><br /><br />To increase the dynamic range of the measurement, the TR-PLI lifetime map can be used to calibrate steady-state PL images. Also, a map of tau_eff at a laterally homogeneous excess carrier density can be obtained by the combination of several measurements.<br /><br /><br /><br /><br /><br />During the course of this work, several ideas to improve upon the presented techniques could not yet be realized. The series resistance characterization method would benefit from the implementation of a solar cell model including carrier diffusion, as well as the deconvolution of blurring effects in the luminescence images. The combined use of EL and PL might improve the robustness of the fitting procedure for this technique.<br /><br /><br /><br />For TR-PLI, a change of the evaluation method to use discrete Fourier transformation instead of fitting a model function is a promising approach to speed up the evaluation procedure, provided it does not degrade measurement quality. Alternatively, the procedure may be accelerated by the use of GPU computing for the evaluation algorithm. To extend the TR-PLI measurement range to even shorter lifetimes, a pulsed laser might be used for illumination. Extending the evaluation theory to account for rise and fall times of the illumination is another possible step to a greater measurement range. terms-of-use 2013 2014-01-15T08:05:29Z Luminescence Imaging Techniques for Silicon Photovoltaics Kiliani, David

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