KPFM surface photovoltage measurement and numerical simulation

Kelvin Probe Force Microscopy ( KPFM ) is a scanning probe microscopy technique. It is a combination of the Kelvin probe and of Atomic Force Microscopy methods. The technique consists in evaluating the difference in work function between two conducting materials, by using a nanometer scale tip ( the “KPFMtip”), and placing it close to the material to be characterised, where a difference in work function leads to an electrostatic force developing between the two, which is translated as an oscillation of the tip’s cantilever. A bia sapplied via an external circuit is varied until the force and hence the electrostatic field between sample and KPFM tip is cancelled.*

In the article “KPFM surface photovoltage measurement and numerical simulation” Clément Marchat, James P. Connolly, Jean-Paul Kleider, José Alvarez, Lejo J. Koduvelikulathu and Jean Baptiste Puel present a method for the analysis of Kelvin probe force microscopy (KPFM) characterization of semiconductor devices.
It enables evaluation of the influence of defective surface layers. The model is validated by analysing experimental KPFM measurements on crystalline silicon samples of contact potential difference (VCPD) in the dark and under illumination, and hence the surface photovoltage (SPV). It is shown that the model phenomenologically explains the observed KPFM measurements. It reproduces the magnitude of SPV characterization as a function of incident light power in terms of a defect density assuming Gaussian defect distribution in the semiconductor bandgap. This allows an estimation of defect densities in surface layers of semiconductors and therefore increased exploitation of KPFM data.*

The KPFM measurements were performed using NanoWorld ARROW-EFM conductive AFM tips with a PtIr coating.
The tip work function didn’t require calibration because only SPV measurement were performed and studied. Measurements were performed in the KPFM amplitude modulation (AM)mode rather than the frequency modulation (FM) one. The AM mode was chosen because lateral resolution was not a problem on the homogeneous bulk samples studied, allowing focus on the superior surface potential resolution that can be achieved with the AM mode.*

Fig. 1 from “KPFM surface photovoltage measurement and numerical simulation” by Clément Marchat et al:
Kelvin probe force microscopy setup schematic. The conducting cantilever carrying the KPFM tip is scanned over a surface while AC + DC potential is applied. The AC signal is a sinusoid whose frequency matches the mechanical resonance of the cantilever. The four-quadrant detector provides feedback in order to minimise cantilever oscillation by varying the DC signal thereby yielding the sample work function compared to the tip one.

*Clément Marchat, James P. Connolly, Jean-Paul Kleider, José Alvarez, Lejo J. Koduvelikulathu and Jean Baptiste Puel
KPFM surface photovoltage measurement and numerical simulation
EPJ Photovoltaics10, 3 (2019)
DOI: https://doi.org/10.1051/epjpv/2019002

Please follow this external link to read the full article: https://www.epj-pv.org/articles/epjpv/abs/2019/01/pv180014/pv180014.html

Open Access The article “KPFM surface photovoltage measurement and numerical simulation “ by Clément Marchat, James P. Connolly, Jean-Paul Kleider, José Alvarez, Lejo J. Koduvelikulathu and Jean Baptiste Puel is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

Self-assembled PCBM bilayers on graphene and HOPG examined by AFM and STM

In the article «Self-assembled PCBM bilayers on graphene and HOPG examined by AFM and STM” Yanlong Li, Chuanhui Chen, John Burton, Kyungwha Park, James R Heflin and Chenggang Tao demonstrate that PCBM molecules self-assemble into bilayer structures on graphene and HOPG substrates. They used Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM), and analyzed the observed morphology by comparison to molecular models.*

The AFM measurements were carried out in a dark environment. NanoWorld™ Pointprobe® NCST AFM probes were used in soft tapping mode and simultaneous height and phase images were acquired and reproduced across multiple samples.*

The results of this study shed light on improvement of the energy efficiency in solar cells containing graphene and organic molecules, by increasing the donor–acceptor interface area and could provide valuable insight into fabrication of new hybrid, ordered structures for applications to organic solar cells.*

Figure 5. from “Self-assembled PCBM bilayers on graphene and HOPG examined by AFM and STM” by Yanlong Li et al.: AFM images of PCBM bilayer and size distributions of holes at different conditions. (a) AFM image of a PCBM bilayer before annealing. (b) AFM image of a PCBM bilayer after annealing at 140 °C. (c) AFM image of a PCBM bilayer after annealing at 160 °C. (d) Area distribution histogram of holes (without PCBM area) obtained from measurements of the area of holes in AFM images of before (green) and after annealing at 140 °C (dark red) and 160 °C (dark blue). Monolithic silicon cantilevers (NCST, NANO WORLD) with a spring constant of 7.4 N m−1, first longitudinal resonance frequencies between 120 and 205 kHz, and nominal tip radius of 8 nm were employed in soft tapping mode. Simultaneous height and phase images were acquired and reproduced across multiple samples.
Figure 5. from “Self-assembled PCBM bilayers on graphene and HOPG examined by AFM and STM” by Yanlong Li et al.: AFM images of PCBM bilayer and size distributions of holes at different conditions. (a) AFM image of a PCBM bilayer before annealing. (b) AFM image of a PCBM bilayer after annealing at 140 °C. (c) AFM image of a PCBM bilayer after annealing at 160 °C. (d) Area distribution histogram of holes (without PCBM area) obtained from measurements of the area of holes in AFM images of before (green) and after annealing at 140 °C (dark red) and 160 °C (dark blue).

*Yanlong Li, Chuanhui Chen, John Burton, Kyungwha Park, James R Heflin, Chenggang Tao
Self-assembled PCBM bilayers on graphene and HOPG examined by AFM and STM
Nanotechnology, Volume 29, Number 18 (2018)
DOI: https://doi.org/10.1088/1361-6528/aab00a

Open Access The article “Self-assembled PCBM bilayers on graphene and HOPG examined by AFM and STM” by Yanlong Li et al. is licensed under a Creative Commons Attribution 3.0 International License. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. To view a copy of this license, visit https://creativecommons.org/licenses/by/3.0/

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

In the article cited below Katherine Atamanuk, Justin Luria and Bryan D. Huey present “a new approach for directly mapping VOC (open-circuit voltage) with nanoscale resolution, requiring a single, standard-speed AFM scan. This leverages the concept of the proportional-integral-derivative (PID) feedback loop that underpins nearly all AFM topography imaging.”*

NanoWorld™ Pointprobe® CDT-NCHR conductive diamond coated silicon AFM probes were used in the described CT-AFM experiment.

Supporting information for «Direct AFM-based nanoscale mapping and tomography of open-circuit voltages for photovoltaics”: Figure S1: Representative quasi-VOC* image from the measured photocurrent upon illumination during an applied voltage fixed at 700 mV. NANOSENSORS conductive diamond coated CDT-NCHR AFM probes were used in the described CT-AFM experiment
Supporting information for «Direct AFM-based nanoscale mapping and tomography of open-circuit voltages for photovoltaics”: Figure S1: Representative quasi-VOC* image from the measured photocurrent upon illumination during an applied voltage fixed at 700 mV.

“Cadmium Telluride (CdTe) is an inexpensive thin-film photovoltaic with ca. 5% of the 2017 global market share for solar cells. To optimize the efficiency and reliability of these, or any electronic devices, a thorough understanding of their composition, microstructure, and performance is necessary as a function of device design, processing, and in-service conditions. Atomic force microscopy (AFM) has been a valuable tool for such characterization, especially of materials properties and device performance at the nanoscale. In the case of thin-film solar cells, local photovoltaic (PV) properties such as the open-circuit voltage, photocurrent, and work function have been demonstrated to vary by an order of magnitude, or more, within tens of nanometers […] Recently, property mapping with high spatial resolution by AFM has been further combined with the ability to serially mill a surface, in order to reveal underlying surface structures and uniquely develop three-dimensional (3D) nanoscale property maps. The most notable examples are based on pure current detection with the AFM to resolve conduction pathways in filamentary semiconducting devices and interconnects […], and tomographic AFM of photocurrents in polycrystalline solar cells during in situ illumination […].”*

*Katherine Atamanuk, Justin Luria, Bryan D. Huey
Direct AFM-based nanoscale mapping and tomography of open-circuit voltages for photovoltaics
Beilstein Journal of Nanotechnology 2018, 9, 1802–1808.
doi: 10.3762/bjnano.9.171

The article cited above is part of the Thematic Series “Scanning probe microscopy for energy-related materials”.

Please follow this external link for the full article: https://www.beilstein-journals.org/bjnano/articles/9/171

The article “Direct AFM-based nanoscale mapping and tomography of open-circuit voltages for photovoltaics” by Atamanuk et. al is an Open Access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.