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

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.

Direct nanoscale mapping of open circuit voltages at local back surface fields for PERC solar cells

“…This is based on null currents (D), using an extra custom-configured proportional-integral-derivative feedback loop during otherwise normal contact-mode AFM operation. This additional feedback system constantly updates the bias applied to the front surface contacts, in order to null the photocurrent and thereby locally maintain (and record) open circuit conditions [22]. This is an important advance, as previous efforts to map parameters such as the open circuit voltage or maximum power points generally require necessitate slow and sometimes artifact-inducing procedures.…”

Charge injection and decay of nanoscale dielectric films resolved via dynamic scanning probe microscopy

“…This maintains a fixed capacitance gradient and thus accurately maps the local surface potential. Point by point, 27 single pass, 28 and contact variations of this approach 29 have also been implemented for mapping the surface potential or related parameters such as the open‐circuit voltage of a photovoltaic 30 …”

“…Point by point, 27 single pass, 28 and contact variations of this approach 29 have also been implemented for mapping the surface potential or related parameters such as the open-circuit voltage of a photovoltaic. 30 The same conducting AFM tip that is used for KPFM measurements can also be employed to facilitate charge injection, 13 schematically demonstrated in Figure 1B for a polycrystalline barium titanate dielectric thin film similar in microstructure to those investigated herein. The biased tip induces local charge redistributions at and/or near the surface by either thermionic or Schottky emission or by tunneling.…”

Investigation of AFM-based machining of ferroelectric thin films at the nanoscale

“…8 The analysis of AFMbased machining behaviour has been utilised to characterise the hardness of materials in nanoscale sclerometry measurements, 9 and AFM-based machining has also been used to precisely remove material from regions of the sample surface in tomography investigations. [10][11][12][13] Given the simplicity of this approach, one would expect that it could be adapted to ferroic materials, which find extensive applications in data-storage, and in particular to ferroelectric oxides where there is strong interest in both the fabrication of nanostructures 14 and volumetric investigations. [15][16][17] Despite this promising utility, few AFMbased machining studies have been applied to ferroelectrics [18][19][20] (i.e., materials possessing a spontaneous polarisation that is reversible under an applied electric field).…”