Publikationsserver der RWTH Aachen University
Generalized detailed balance theory of solar cells
Abstract
dc:descriptionFrom the detailed balance between absorbed and emitted radiation follows the efficiency limit of Shockley and Queisser (SQ-limit). In the framework of the SQ-theory, the exact working principle and the transport mechanism in the solar cell are irrelevant. The efficiency is only determined by the absorptance and emissivity of the material, which are equal according to Kirchhoff’s law. This abstraction from all internal properties of the solar cell makes the SQ-theory general, however, it also implies a fundamental difference between the SQ-theory and typical device simulators, which base on the solution of the continuity equations for charge carrier transport. The present dissertation presents a model, which is compatible with the SQ-theory and which includes the internal details of the device like e.g. charge carrier transport. The simplest version of the model solves the continuity equations for electrons and holes as well as the Poisson equation and allows thereby to simulate inorganic pn and pin-junction solar cells. This model is used for instance to calculate the mobility dependent efficiency limit of SiO2/Si superlattice absorbers. To allow simulations of organic solar cells, the model has to include the continuity equation for excitons, which are relevant in organic materials due to the higher exciton binding energy. The special geometry of organic solar cells, which usually consist of a bulk heterojunction, is also considered. The model for organic polymer-fullerene solar cells allows both the reproduction of experimental data on current/voltage and quantum efficiency measurements and the prediction of the device behaviour as a function of mobility, band offsets and the geometry of the phase separation. The final version of the model also includes the effect of multiple exciton generation, meaning the creation of multiple electron/hole pairs or excitons from a single high energy photon. The model for multiple exciton generating absorber layers allows the identification of possible bottlenecks as well as the calculation of efficiency limits for future solar cell concepts. The experimental part of this thesis uses a reciprocity theorem between electroluminescence (EL) emission and the photovoltaic quantum efficiency, which is a direct consequence of the principle of detailed balance. EL measurements of crystalline silicon, Cu(In,Ga)Se2 and GaInP/GaInAs/Ge triple cells were carried out and interpreted with the help of the reciprocity theorem. In case of crystalline silicon, the focus was on the investigation of recombination and light trapping with spectrally and spatially resolved EL. First the relation between EL and quantum efficiency is verified experimentally before the application for the extraction of diffusion lengths and back side reflectances is presented. Temperature dependent measurements of Cu(In,Ga)Se2 solar cells characterize the devices with respect to their stoichiometric inhomogeneity, which may lead to a detrimental lateral variation of the band gap. The EL measurements show that there is no significant inhomogeneity in high efficiency samples. In case of multijunction solar cells, EL measurements give access to the internal voltages of the individual subcells in the stack. By varying the injection current and detecting the spectral EL emission, this method allows determining the amount of non-radiative recombination and the diode quality factors of each individual cell.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2009
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Kirchartz, Thomas
- Contributors dc:contributor
-
- Rau, Uwe
Subjects
dc:subject × 12Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng