**8. References**


One of the most plausible solutions for this issue is to insert a several-tens-nanometer-thick GaN or AIN layer between TCP and n-InxGa1-x N. With this device structure, it is expected that the barrier height at the TCP/nitride semiconductor interface will be maintained at a

The cost of the sapphire substrate will become a high barrier for reducing the production cost of III-nitride based solar cells. Matsuki et al. have shown that high quality GaN can be grown on mica plates (Matsuki et al., 2005), which are inexpensive and flexible. Applying such a novel alternative to sapphire for the epitaxial growth substrate will be effective for

TCPs have a high transparency from 250 nm to the visible wavelength region, as described in section 5.1. Thus TCP/nitride semiconductor heterojunction photovoltaic devices also

We have fabricated TCP/nitride semiconductor heterojunction solar cell structures by the spin-coating method using PEDOT:PSS or PANI as the TCP layer and Si-doped GaN as the semiconductor layer. The devices exhibited high quality rectifying properties and have an approximately 1 eV barrier height. Both the PANI/epi.-GaN and PEDOT:PSS/epi.-GaN heterojunction solar cells exhibited ultraviolet-sensitive photovoltaic action. The observed open-circuit voltage was superior to previously reported values for metal/GaN Schottky photo-detectors. A characteristic frequency-dependent behaviour of the interface capacitance was found for the TCP/epi.-GaN solar cells. The *C*-*f* characteristics were analyzed based on the dielectric dispersion theory and the intrinsic capacitance and resistance were obtained. The considerable reduction of the interface capacitance in the high frequency region allowed for highly-sensitive detection of deep levels in GaN by DLOS

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**15** 

 *USA* 

AC Varonides

**High Efficiency Solar Cells via Tuned** 

**Superlattice Structures: Beyond 42.2%** 

*Physics & Electrical Engineering Dept, University of Scranton, Scranton, PA,* 

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

ln(1 ) *<sup>L</sup>*

*<sup>I</sup> V kT*

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

Photonic device (solar cells included) operation is based on a p-n junction: two regions of the semiconducting material doped p and n type respectively and brought together in contact form a p-n junction. At thermal equilibrium, the p-n dope bulk semiconducting crystal, in order to keep its equilibrium, develops an internal field and develops its own built-in potential; the latter is total due to p- and n-type carrier migration across the

Donor and acceptor atoms embedded in the lattice of the host material provide electrons and holes (as potential current carriers) that are free to wander in the crystal. In principle these carriers move randomly in the lattice, however, guiding these carriers accordingly could lead to non-zero currents coming off such semiconductors, and therefore to current producing devices. A semiconductor sample doped with donors and acceptors becomes a pn junction and therefore a device with two regions tending to overlap at their boundary.

*o*

(1)

*I*

*oc*

theory and p-n junctions under a bias are briefly discussed in the next section.

**1. Introduction** 

junction.

the cell is given by the following formula:

**2. Background theory: The p-n junction** 

clean n-GaN (0001) surfaces and Pt, Au, and Ag, *Journal of Applied Physics*, Vol. 94, No. 6, (September 2003), pp. 3939-3948, ISSN 0021-8979

