Spatially resolved carrier lifetime calibrated via quasi-steadystate photoluminescence
The estimation of solar cell efficiency from minority carrier lifetime measurements requires precise and robust lifetime techniques. For multicrystalline wafers and solar cells this brings about the necessity of adequately averaged spatially resolved lifetime measurements. Materials such as e.g. multicrystalline upgraded metallurgical grade silicon frequently feature relatively low lifetimes, high trap densities, and several material parameters (charge carrier mobilities and net dopant
... net dopant concentration) that are not straightforwardly predictable or measurable. As this may substantially compromise conventional lifetime measurements, a lifetime technique which is unaffected by such restrictions is of general interest. We present a lifetime imaging technique based on a calibration of photoluminescence images via quasi-steady-state photoluminescence (QSSPL), which requires no a priori information about material parameters such as carrier mobilities and net dopant concentration. It is virtually unaffected by effects related to reabsorption of luminescence, by optical effects related to a sample's surface morphology, and by trapping. Injection dependence of lifetime is properly taken into account. We further accomplished a substantial upgrade of the hitherto existing sensitivity limit of QSSPL in terms of lifetime, to where average lifetimes down to one microsecond are now reliably measurable -yielding a sensitivity of spatially resolved lifetime well below one microsecond.