Direct AFM-based nanoscale mapping and tomography of open-circuit voltages for photovoltaics.

Katherine Atamanuk,Bryan D Huey,Justin Luria

doi:10.3762/bjnano.9.171

Abstract

The nanoscale optoelectronic properties of materials can be especially important for polycrystalline photovoltaics including many sensor and solar cell designs. For thin film solar cells such as CdTe, the open-circuit voltage and short-circuit current are especially critical performance indicators, often varying between and even within individual grains. A new method for directly mapping the open-circuit voltage leverages photo-conducting AFM, along with an additional proportional-integral-derivative feedback loop configured to maintain open-circuit conditions while scanning. Alternating with short-circuit current mapping efficiently provides complementary insight into the highly microstructurally sensitive local and ensemble photovoltaic performance. Furthermore, direct open-circuit voltage mapping is compatible with tomographic AFM, which additionally leverages gradual nanoscale milling by the AFM probe essentially for serial sectioning. The two-dimensional and three-dimensional results for CdTe solar cells during in situ illumination reveal local to mesoscale contributions to PV performance based on the order of magnitude variations in photovoltaic properties with distinct grains, at grain boundaries, and for sub-granular planar defects.

Highlights

Cadmium Telluride (CdTe) is an inexpensive thin-film photovoltaic with ca. 5% of the 2017 global market share for solar cells
We previously developed an efficient, method with high spatial resolution for this purpose, namely photo-conductive Atomic force microscopy (AFM) spectroscopy [1], essentially by collecting an entire array of I–V spectra in parallel via a series of consecutive pcAFM images
A conducting (B-doped) diamond-coated silicon AFM probe (Nanoworld CDT-NCHR, Soquel, CA, USA), along with a picoampere-resolution current detector (Asylum Research Orca, model 058, 5 V/nA, 1–10 kHz bandwidth), enable either the short-circuit current to be measured or the open-circuit voltage to be directly determined by engaging the secondary feedback loop as previously described

Summary

Introduction

Cadmium Telluride (CdTe) is an inexpensive thin-film photovoltaic with ca. 5% of the 2017 global market share for solar cells. In the case of thin-film solar cells, local photovoltaic (PV) properties such as the opencircuit voltage, photocurrent, and work function have been demonstrated to vary by an order of magnitude, or more, within tens of nanometers [1,2,3]. For AFM-based mapping of solar cell performance parameters that are traditionally derived from I–V measurements, such as VOC, the spatial and energetic resolution unavoidably conflicts with experimental throughput.

Results

Conclusion