**1. Introduction**

94 Solar Cells – New Aspects and Solutions

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J. and Olson J. M. (2000b) Nitrogen-induced increase of the maximum electron concentration in group III-N-V alloys. *Phys. Rev. B*, Vol. 61, No 20, pp.R13337Novel photovoltaic (PV) technologies are currently investigated and evaluated as approaches to contribute to a more environmental friendly energy supply in many countries. One of the driving forces are the aims to reduce the emission of green house gases and the dependency on importing fossil energy resources from political unstable countries. Additionally the wish to replace nuclear power by greener and less threatening technologies will enhance the development of regenerative energy supply in many countries especially after the recent nuclear catastrophe at Fukushima nuclear power plant in Japan in March 2011. This will include the more rapid implementation of existing mature PV technologies but also the development and improvement of novel PV approaches such as organic PV (OPV) and dye-sensitized solar cells (DSSCs) together with new efficient strategies for energy storage and distribution to make electric power, deriving from PVs, available whenever and wherever it is needed.

The so-called 1st generation of solar cells based on e.g. bulk crystalline and polycrystalline silicon is still dominating the PV market. However, so-called 2nd generation solar cells mainly consisting out of thin film solar cells based on CdTe, Copper Indium Gallium Selenide (CIGS), and amorphous silicon gained distribution of ca. 25% in market share today worldwide. It is expected that this number will increase significantly within the next years. While for the 1st and 2nd generation solar cells commercial solar panels are available with decent power conversion efficiencies (PCEs) and lifetimes, the emerging 3rd generation solar cells such as OPV and DSSCs technologies are still in the development phase. Some commercially available products have recently entered the market such as e.g. solar bags representing niche products, which are so far not suitable for competing with traditional large scale applications of solar panels of the 1st and 2nd generations. In traditional solar panels the differences between best solar cell and average solar cell efficiencies are much smaller than for the emerging solar cell technologies with the consequence that modules of 3rd generation solar cells still suffer from too low performance. In Table 1 the best cell and module efficiencies of different PV technologies are compared. It has to be mentioned that especially for the emerging new PV technologies the average efficiencies are significantly lower than the results of the best cells.

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 97

interface would be in the range of the exciton diffusion length. After exciton dissociation, both holes and electrons will be transported within their pre-structured donor or acceptor phases along a direct percolation pathway to the respective electrodes. This interdigital structure can be realized by various nanostructuring approaches, which will be discussed in

Fig. 1. Schematic illustration of typical device structures for hybrid solar cells.

In hybrid solar cells, photocurrent generation is a multistep process. Briefly, when a photon is absorbed by the absorbing material, electrons are exited from the valance band (VB) to the conduction band (CB) to form excitons. The excitons diffuse to the donor/acceptor interface where charge transfer can occur leading to the dissociation of the excitons into free electrons and holes. Driven by the internal electric field, these carriers are transported through the respective donor or acceptor material domains and are finally collected at the respective electrodes. To sum up, there are four main steps: photon absorption, exciton diffusion, charge separation as well as charge carrier transport and collection. The physics of organic/hybrid solar cells is reviewed in detail elsewhere (Greenham, 2008; Saunders &

Due to the decreased size of NCs down to the nanometer scale, quantum effects occur, thus a number of physical (e.g. mechanical, electrical, optical, etc.) properties change when compared to those of bulk materials. For example, the quantum confinement effect (Brus, 1984) can be observed once the diameter of the material is in the same magnitude as the wavelength of the electron wave function. Along with the decreasing size of NCs, the energy levels of NCs turn from continuous states to discrete ones, resulting in a widening of the band gap apparent as a blue shift in the absorption and photoluminescence (PL) spectra. In general, there are two distinct routes to produce NCs: by physical approaches where they can be fabricated by lithographic methods, ion implantation, and molecular beam deposition; or by chemical approaches where they are synthesized by colloidal chemistry in

detail later in the section 6.2.2.

Turner, 2008).

**3. Donor-acceptor materials** 


Table 1. Comparison of best and average PCE values of single solar cells and modules of different PV technologies.
