Measuring long-range carrier diffusion across multiple grains in polycrystalline semiconductors by photoluminescence imaging

Abstract

Thin-film polycrystalline semiconductors are currently at the forefront of inexpensive large-area solar cell and integrated circuit technologies because of their reduced processing and substrate selection constraints. Understanding the extent to which structural and electronic defects influence carrier transport in these materials is critical to controlling the optoelectronic properties, yet many measurement techniques are only capable of indirectly probing their effects. Here we apply a novel photoluminescence imaging technique to directly observe the low temperature diffusion of photocarriers through and across defect states in polycrystalline CdTe thin films. Our measurements show that an inhomogeneous distribution of localized defect states mediates long-range hole transport across multiple grain boundaries to locations exceeding 10 μm from the point of photogeneration. These results provide new insight into the key role deep trap states have in low temperature carrier transport in polycrystalline CdTe by revealing their propensity to act as networks for hopping conduction.

Carrier Decay and Diffusion Dynamics in Single-Crystalline CdTe as Seen via Microphotoluminescence

“…1 is comparable to those that have been previously published for polycrystalline [13] and single-crystalline CdTe [14]. This spectrum is obtained with a cw laser at 2.3 eV in order to show the full spectral region around the band edge.…”

“…These bright features have also been observed in CL at the line-shaped defects [9]. PL emission at similar energies has been observed in polycrystalline CdTe and corresponds to radiative recombination from deep impurity states that participate in carrier diffusion at low temperature [13]. The TI image of Fig.…”

“…3 shows a streak camera trace from a region of the sample selected far from any dark-line defects and excited by a focused laser beam. The PL decay of this single-crystal CdTe sample is much slower than the exciton PL lifetime measured in polycrystalline CdTe [13]. Excitonic PL is also emitted from an area that is larger than the laser spot and which grows even larger with time, clearly indicating lateral diffusion.…”

Multimode scanning near-field photoluminescence spectroscopy and its application for studies of InGaN epitaxial layers and quantum wells

“…An alternative approach is to use contactless methods that rely on an optical detection of carrier motion. A popular technique to evaluate diffusion length is to measure an increase of the luminescing area with respect to the excitation spot [37]. However, this method is limited to materials with large diffusion lengths.…”

Long Lifetime Hole Traps at Grain Boundaries in CdTe Thin-Film Photovoltaics

“…However, the method is subject to complex effects, such as high electron injection densities, which can lead to bright grain boundary contrast [6]. Spatially resolved photoluminescence at liquid nitrogen temperature has also uncovered a long-range (>10 μm) hopping transport mechanism across grain boundaries [7]. Hopping conduction takes place via impurity trap levels that give rise to the ∼1.4 eV donor-acceptor pair (DAP) peak in the CdTe luminescence spectrum.…”