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Title: Nanoscale imaging of photocurrent and efficiency in CdTe solar cells

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. As a result, 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 defect and surfaces.
 [1] ;  [2] ;  [1] ;  [2] ;  [3] ;  [3] ;  [3] ;  [4] ;  [3]
  1. Univ. of Maryland, College Park, MD (United States)
  2. (NIST), Gaithersburg, MD (United States)
  3. National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States)
  4. (SNL-CA), Livermore, CA (United States)
Publication Date:
Report Number(s):
Journal ID: ISSN 1936-0851; 540723
Grant/Contract Number:
Accepted Manuscript
Journal Name:
ACS Nano
Additional Journal Information:
Journal Volume: 8; Journal Issue: 11; Journal ID: ISSN 1936-0851
American Chemical Society
Research Org:
Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA)
Country of Publication:
United States
14 SOLAR ENERGY; photovoltaics; CdTe; scanning photocurrent microscopy; solar cells; NSOM
OSTI Identifier: