Nanoscale Imaging of Photocurrent and Efficiency in CdTe Solar Cells

Abstract

The local collection characteristics of grain interiors and grain boundaries in thin-film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50-300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. Resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defects and surfaces.

Nanoscale imaging and spectroscopy of band gap and defects in polycrystalline photovoltaic devices

“…In this work, two complementary atomic force microscopy (AFM)-based optical techniques, the photothermal induced resonance (PTIR) 20, 21, 22 and a novel implementation of near-field scanning optical microscopy (NSOM), 23, 24, 25 are leveraged to characterize CdTe polycrystalline solar cells with nanoscale resolution. In PTIR, a sample placed on an optically transparent prism, is illuminated in total internal reflection geometry over a relatively large area, while an AFM tip acts as a local detector for measuring absorption spectra and maps.…”

“…The same setup used in dt-SNOM could also be used for other related NSOM experiments such as scanning photocurrent microscopy. 25 However, illumination mode NSOM has other unique advantages; for example, it can provide broadband near-field PL spectra in each pixel. 24 The dt-NSOM transmitted signal ( T ) is normalized to the background light intensity ( I bkg ) measured by positioning the NSOM tip in contact with the photodetector (see experimental for details):…”

Nanoimaging of Open‐Circuit Voltage in Photovoltaic Devices

“…[ 18,[29][30][31][32][33][34][35] The local optoelectronic properties and changes in material composition have also been mapped using near-fi eld scanning optical microscopy (NSOM) probes as local sources of excitation. [36][37][38][39][40][41][42] Very recently, photoluminescence has emerged as a promising tool to map charge recombination [43][44][45] and carriers diffusion [ 46 ] with high spatial resolution. At low temperature (70 K), photoluminescence imaging with submicrometer resolution has been implemented to map a 10 meV quasi-Fermi level splitting in CIGS solar cells, where variations in the intensity signal were attributed to changes in the material composition.…”

Aqueous Solution-Phase Selenized CuIn(S,Se)2 Thin Film Solar Cells Annealed under Inert Atmosphere

“…As shown in Figure 1a, the CIS absorber layer had a small grain size and voids, which induced rough interface contacts with the buffer/TCO layer, resulting in a decrease in FF. 30 The occasional voids in the CIS solar cell induced interfacial resistance between CdS−CIS heterojunctions. In addition, recombination loss through dangling bonds at the grain boundaries may have decreased the open circuit voltage.…”