Photovoltaics
Photoconductivity Imaging with Super-Resolution
Reference: Mueller, T., Xia, F., Freitag, M., Tsang, J., & Avouris, P. (2009). Role of contacts in graphene transistors: A scanning photocurrent study. Physical Review B, 79(24), 245430.
The Growth of Solar Cell Energy & Photovoltaics
- Over the past decade, the solar cell energy industry has grown dramatically; photovoltaic research has emerged as a dynamic and ever-increasingly important field. Recent estimations indicate that total PV installations in the world have reached 300 GW – a 4000% increase since 2006!
(See: Kurtz, S., Haegel, N., Sinton, R., & Margolis, R. (2017). A new era for solar. Nature Photonics, 11(1), 3-5).
- The solar energy industry continues to push the bounds of the nanoscale and has been focused on increasing photovoltaic efficiency.
(See: Alharbi, F. H., & Kais, S. (2015). Theoretical limits of photovoltaics efficiency and possible improvements by intuitive approaches learned from photosynthesis and quantum coherence. Renewable and Sustainable Energy Reviews, 43, 1073-1089.)
- The innovative nanostructured solar cells that are being developed require advanced nanoscale instruments that can achieve accurate and high-resolution photoconductivity measurements.
- Now more than ever, the ability to obtain high resolution photoconductivity measurements is a critical element in the development of this growing solar cell industry. Such measurements are applicable to all photovoltaic materials, including:
| • Perovskite | • Ribbon-Si | • Si Thin-Film |
| • 2D Materials | • C-Si | • GaAs Thin-Film |
| • Graphene | • Mono-Si | • MLM |
| • MoS2 | • Poly-Si | • CdTe |
| • Expatial Si Wafers | • CIGS |
Limitations of Macroscopic Photoconductivity Techniques
Common photoconductivity techniques that employ standard Gaussian beam far-field optics are inherently limited:
1. Limited Resolution
The resolution of a lens focused beam gives at best 0.5micron resolution at 500nm illumination. Thus, such macroscopic photo-current methods study spatially averaged properties of the PV device and are ineffective for the study of nanostructured photovoltaic cells, nanoscale defects, grain boundaries, and thin film solar cells. Furthermore, the shape of a lens focused Gaussian beam makes it difficult to impossible to illuminate a sample next to an electrical contact.
2. No Structural Correlation
Most techniques do not include the capability to generate online nanometric sturctual correlation.
3. Out-of-Focus Background
Lenses focus below and above the sample plane and therefore feature an illumination background.
4. Sensitivity Reduction
Typically a reduction in sensitivity can be observed, due to regions illuminated that are not directly of interest.
5. Non-uniform Illumination
Scanning a beam over a sample with varying topography, or with some tilt in the mounting of the sample, leads to changes in light exposure from pixel to pixel thereby generating photovoltaic artifacts.
6. Partial Information
Standard macroscopic photoconductive measurements are unidimensional. They only offer photocurrent characterization, without any correlation to changes in other functional properties of a sample.
Top 5 Advantages of Nanonics Photoconductivity Near-Field Scanning Optical Microscopy
Nanonics systems for Near-field scanning photocurrent microscopy represent a fundamental paradigm shift in photoconductivity measurements, solving these limitations. Near field scanning probe microscopy (NSOM) allows for nanometric optical characterization with correlated sample morphology imaging. This innovative approach features 5 powerful advantages:
1. Super-Resolution
Imaging of light-induced current and voltage with previously unachievable; super-resolution down to 50nm
2. Photocurrent with Structural Correlation
Pixel by pixel correlation of device structure with photocurrent and photovoltage images
3. Uniform Illumination
Identical illumination at each pixel
4. Artifact-free
No optical background artifacts or noise
5. On-line Chemical Characterization
Readily correlate photoconductivity images with imaging of chemical structure
Example Case Study
Super-resolution Imaging of Photocurrent Induced in Graphene Transistor by Near-field Optical Excitation
Reference: Mueller, T., Xia, F., Freitag, M., Tsang, J., & Avouris, P. (2009). Role of contacts in graphene transistors: A scanning photocurrent study. Physical Review B, 79(24), 245430.
Customer Application:
•Super-resolution: Illuminating with an NSOM aperture down to 50nm in AFM feedback with the sample. Feedback is controlled with a tuning fork without any induced optical background.
[Read more: Review of Scientific Instruments 87, 083703 (2016)]
•Uniform Illumination: Using AFM feedback to maintain an exact distance from the surface for unvarying pixel by pixel illumination intensity with a top-hat intensity profile.
•Artifact-free: No background illumination either from the NSOM probe normal force tuning fork feedback or from variation in illumination intensity.
•Structural Correlation: Scanning the aperture with AFM feedback control to obtain simultaneously pixel by pixel structural correlation.
•On-line Chemical Characterization: A cantilevered NSOM probe that does not obscure the microscope's optical axis from above, allowing for spectral imaging on-line of Raman, fluorescence, etc.
•Multiprobe: Exclusive Nanonics multiprobe capabilities upgrades p-NSOM from one to four probes allowing for on-line Kelvin probe, electrical and thermal conductivity.
Exemplary Photoconductive Image of a Graphene Transistor Elucidating Effects of Metallic Contacts:
Additional Features
Enjoy Complete Optical Access
Nanonics SPM systems feature open optical access from above and below. This allows for the seamless integration of NSOM along with complementary macroscopic techniques for all optical geometries. With optical integration, you can obtain comprehensive characterization of PV devices. High resolution photoconductivity imaging can be readily correlated with all past measurement protocols including Raman and confocal microscopy.
Read more: Solar Energy Volume 153, 1 September 2017, Pages 134-141
Enable Advanced Applications with Mutliple SPM Probes
The exclusive Nanonics multiprobe system upgrades NSOM from one probe to two, three, or even four probes including probes for different SPM methods. Thus system with two NSOM probe provides subwavelength incoupling and outcoupling of light, which is most important for study photoconductive properties of thin films, 2D material-based photovoltaic cells and nano structural cells. Integration of NSOM probe with electrical, magnetic, KPFM and other SPM probes leads to simultaneous comprehensive characterization of photovoltaic devices, which is not possible with single probe systems. Each probe enables independent XYZ scanning together with sample scanning.
Read more: Nanoscale. 2017 May 25; 9 (20):6695-6702
