**1. Introduction**

324 Solar Cells – New Aspects and Solutions

Tung, R. T. (2001). Recent advances in Schottky barrier concepts, *Material Science and Engineering B*, Vol. 35, No. 1-3, (November 2001), pp. 1-138, ISSN 0921-5107 Wang, W. & Schiff, E. A. (2007). Polyaniline on crystalline silicon heterojunction solar cells,

Wu, C.I. & Kahn, A. (1999). Electronic states and effective negative electron affinity at

Wu, J., Walukiewicz, W., Shan, W., Yu, K. M., Ager, J. W., Li, S. X., Haller, E. E., Lu, H., &

Wu, J. (2009). When group-III nitrides go infrared: New properties and perspectives, *Journal* 

Yamaguchi, M. (2003). III–V compound multi-junction solar cells: present and future, *Solar* 

Yamaura, J., Muraoka, Y., Yamauchi, T., Muramatsu, T., & Hiroi, Z. (2003). Ultraviolet light

Zheng X., Horng, R.-H., Wuu, D.-S., Chu, M.-T., Liao, W.-Y., Wu, M.-H., Lin, R.-M., & Lu,

Zhou, Y., Ahyi, C., Tin, C.-C., Williams, J., Park, M., Kim, D.-J., Cheng, A.-J., Wang D.,

*Letters*, Vol. 83, No. 11, (July 2003), pp. 2097-2099, ISSN 0003-6951

No. 6, (September 2003), pp. 3939-3948, ISSN 0021-8979

ISSN 0003-6951

8979

0248

pp. 3209-3212, ISSN 0021-8979

pp. 2805-2807, ISSN 0003-6951

261108-3, ISSN 0003-6951

121118-3, ISSN 0003-6951

clean n-GaN (0001) surfaces and Pt, Au, and Ag, *Journal of Applied Physics*, Vol. 94,

*Applied Physics Letters*, Vol. 91, No. 13, (September 2007), pp. 133504-1-133504-3,

cesiated p-GaN surfaces, *Journal of Applied Physics*, Vol. 86, No. 6, (September 1999),

Schaff, W. J. (2003). Temperature dependence of the fundamental band gap of InN, *Journal of Applied Physics*, Vol. 94, No. 7, (July 2003), pp. 4457-1260, ISSN 1089-7550 Wu, J., Walukiewicz. W, Li., S. X., Armitage, R., Ho, J. C., Weber, E., R., Haller, E. E., Lu, H.

Schaff, W. J., Barcz, A. & Jakieka R. (2004). Effects of electron concentration on the optical absorption edge of InN, *Applied Physics Letters,* Vol. 84, No. 15, (April 2004),

*of Applied Physics*, Vol. 106, No. 1, (July 2009), pp. 011101-1-011101-28, ISSN 0021-

*Energy Materials & Solar Cells*, Vol. 75, No. 1-2, (April 2003), pp. 261-269, ISSN 0927-

selective photodiode based on an organic-inorganic heterostructure, *Applied Physics* 

Y.-C. (2008). *Applied Physics Letters*, Vol. 93, No. 26, (December 1998), pp. 261108-1-

Hanser, A., Edward, A. P., Williams, N. M., & Evans K. (2007). Fabrication and device characteristics of Schottky-type bulk GaN-based ''visible-blind'' ultraviolet photodetectors, *Applied Physics Letters*, Vol. 90, No. 12, (March 2007), pp. 121118-1–

Modern PV devices are a direct outcome of solid state devices theory and applications of the last forty years. They are devices made of crystalline structures and basically, when illuminated with solar light, they convert solar photons into electric current. In the following a quick explanation of how this happens is presented. What is a solar cell? What is the basic function behind a cell's operation? Typically, in an illuminated p-n junction, photons are absorbed and electron-hole pairs are generated. These carriers diffuse in opposite directions (separated by the existing electrostatic field at the junction), and within their respective diffusion lengths. Electrons at the p-side diffuse through the junction potential and holes (similarly) get to the opposite directions. Under open-circuit conditions, the voltage across the cell is given by the following formula:

$$V\_{oc} = kT \ln(1 + \frac{I\_L}{I\_o}) \tag{1}$$

Where k is Boltzmann's constant, T (in Kelvin) is the cell temperature, IL is the lightgenerated current, and Io is the p-n junction's reverse saturation current (see below). Cell theory and p-n junctions under a bias are briefly discussed in the next section.
