Photovoltaic cells, defined

The performance of a photovoltaic cell is characterized by the power conversion efficiency r]p, which is defined by ... 

In the case of thin devices, the transmittance may not be zero and the concept of IQE becomes less well defined. Most polymer photovoltaic cells are quite thin and only the photons absorbed in the active regions of the device can contribute to the photocurrent. The useful absorptance in the cells is given by... 

The maximum conversion efficiency of a photovoltaic cell is defined as... 

In a conventional solid-state photovoltaic cell or a semiconductor/electrolyte junction, the photovoltage is related to the densities of free electrons and holes. These in turn define the quasi-Fermi levels p and pE. The photovoltage is given by... 

At this voltage (U = f/ph), the anodic photocurrent and the cathodic dark current are equal. This is called the open-circuit condition as illustrated in the energy diagram (Fig. 11.1b). The conversion efficiency tj of a photovoltaic cell is defined as... 

Assuming a quantum efficiency of unity, then /e is the photocurrent of the cell. The maximum conversion efficiency is defined by Eq. (11.5). It can be calculated for semiconductors of different bandgaps from Eqs. (11.5) and (11.12). The results are presented in Fig. 11.9. The highest efficiency is 28 % at g = 1.2 eV. This calculation is valid for solid state photovoltaic devices (p-n junction, Schottky junction) as well as for a photoelectrochemical photovoltaic cell. 

Conversion efficiencies for electrochemical photovoltaic cells have already been derived in Section 11.1.1.3, on the basis of the theory developed by Ross et al. [58], The maximum efficiencies were calculated on the basis of Eq. (11.12). If energy should be stored in a chemical fuel, further losses must be encountered, i.e. overpotentials, t)ox for the oxidation and rjred for the reduction, have to be considered. The latter are defined as the difference between the quasi-Fermi level and the redox potentials. Using Eq. (11.12), the conversion efficiency for the production of a chemical fuel is then given by... 

The rectification properties of semiconductor interfaces are the most important electrical characteristic of semiconductor contacts. Certain types of devices, such as transistors, require both ohmic and rectifying contacts on a given semiconductor surface, whereas other devices, such as Schottky barriers, are based on the inherent rectification properties of semiconductor/metal Junctions. Numerous photonic devices, such as photon detectors and photovoltaic cells, require rectification at a semiconductor Junction, and light-emitting diodes require both ohmic contacts and rectifying Junctions in a well-defined geometry. Thus, successful fabrication of a desired device structure depends entirely on the electrical properties of the specific semiconductor contacts that are formed in the process. The principles described above allow the rational fabrication of contacts with the desired properties, and also describe the operation of the resulting devices within a simple, chemically intuitive, kinetic framework. 

There are at least four major generations of photovoltaic cells whose materials define the application of plasma technology to their fabrication ... 

---