Nanodiagnostics of solar cells with vertical p-n-junctions

3.1. Contact area and scanning equipment

Contact area between two adjacent p-n-n+ junctions is most important for investigation in order to achieve better characteristics and parameters of solar cells with vertical p-n — junctions such as high efficiency and fill factor, low series resistance, high mechanical and thermal tolerance. Cross-section of the contact area is shown at Fig.7.

Nanocenter of the MPEI (TU) is participating in our research activity. Using INTEGRA Prima scanning probe microscope (SPM) we are analyzing quality of soldering processes, relief and physical properties of boundaries of vertical semiconductor structures. SPM gives an opportunity to carry out studies of spatial, physical and chemical properties of objects with the typical dimensions of less than a few nanometers (Fig.8). Owing to its multifunctionality, availability and simplicity, atomic-force microscope has become useful tool.

3.2. Experimental results

Below the several examples of taken images are shown. With help of video-microscope it is very convenient to observe quality of soldering structures at the different magnifications (Fig.9).

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Fig.9. Images of SCs with vertical p-n-junctions at the consecutive magnifications (from left to right)

Three-dimensional image taken with help of atomic force microscope is depicted at Fig. 10.

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Fig. 10. Three-dimensional image of contact area (10 p scale divisions are shown)

Topology of Al layer and adjacent area is shown at the Fig.11. At the next stage of research we are planning to use scanning capacitance microscopy, which allows mapping variations in electron carrier concentration across the sample surface with the unprecedented sensitivity, but we need to solve potential problems with samples that are too rough.

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Fig. 11. Topology of Al layer and adjacent area (scan field 170 p, resolution 670 nm); topology of the surface, marked out by green square at first scan (scan field 70 p, resolution 270 nm); topology of the surface, marked out second (scan field 11p, resolution 40 nm).

2. Acknowledgements

Author thanks Federal Agency on Science and Innovations of RF for support of this research project.

3. Conclusion

Concentrator PV/T technologies are prospective alternative of traditional solar thermal collectors and PV modules.

First results of diagnostic of solar cells with vertical p-n-junctions show high informative and useful data obtained with the help of modern methods of investigation of semiconductors. As for thickness of inter-layers connecting p-n — junction structures in developing SCs with vertical p-n junctions we have a definite reserve i. e. decreasing thickness of inter-layers it is possible to increase sensitive surface of SCs.

It is necessary to improve quality of the surface on the cutting stage of bonded (soldered) structures for improving technology and quality of solar cells and potential using scanning capacitance microscopy for detailed study of carrier’s distribution.

References

[1] ] I. Tyukhov, S. Clouston, R. Rogers, D. Strebkov, F. Vignola New solar combined concentrator technology in Oregon. Proceedings of 33rd ASES Conference "A Solar Harvest: Growing Opportunities", Ed. R. Campbell-Howe, 2004, July 11-14, Portland, Oregon, USA, p. 223-227.

[2] G. Jongerden et al. Colored solar cell unit United States Patent 7227078, June 5, 2007.

[3] I. Tyukhov, D. Strebkov, Y.. Kuzhnurov, E. Tveryanovich, S. Kivalov Integrating PV/thermal concentrator systems into buildings Proceedings of the 2005 Solar World Congress, Edited by D. Y. Goswami, S. Vijayaraghaven, R. Campbell — Howe, American Solar Energy Society, International Solar Energy Society, CD, ASES 2005.

[4] PVT ROADMAP, A European guide for the development and market introduction of PV-Thermal technology, The 6th Framework Programme, 2006.

[5] V. Simakin, D. Strebkov, I. Tyukhov Silicon multi-junctional solar cells with vertical p-n-junctions: evolution, technology, applications, and new opportunities. 14th Intern. Solar Conference “EuroSun 2004”, Proceedings, PSE GmbN, Freiburg, Germany, v. 3, p. 357-366.

[6] V. Simakin, D. Strebkov, I. Tyukhov Solar cells with vertical p-n-junctions: technology and new and new opportunities. The 2nd International Conference on Physics of Electronic Materials PHYEM’05 Kaluga, Russia May 24-27, 2005, p.149-152.

2. Experiment set up

This experiment was conducted in November 2006 in the solar energy laboratory, Kanagawa Institute of Technology, Atsugi — Japan. This PV/T experiment was a part of a water heating system with a dual-source heat pump system.

As shown in Fig. la, the dimension of PV/T to collect the solar energy is 780x770mm which gave an effective collector area of 0.606 m2. The dimension of PV/T included of frame is 820x790mm. The PV/T is layered by a glass cover, laminated to the PV cell with adhesive layer and the absorber. Fig. lb is the configuration of the water heating system. During this experiment, the heat pump was not operated in order to measure only the PV/T performance. A constant temperature layer is located closed to the PV/T to supply the constant flow rate and constant inlet water temperature. A Data logger is functioned to collect the useful information such as: collector surface temperature Tcoll, inlet water

temperature to the collector T in, outlet water temperature from the collector Tout, ambient air temperature Ta, wind speed Vwind, and solar radiation I. All data were collected from 12.000 to 14.00

for every 30 seconds. This experiment was lead under the condition of constant flow rate and constant inlet water temperature to the collector of 41/min and a 120C, respectively.