Research on intermediate band solar cells and development of capacitive techniques for their characterization

Resumen

This Thesis is a contribution to the development of intermediate band solar cells (IBSCs). The limiting efficiency of an IBSC is 63.2% under specific illumination conditions (under maximum solar concentration assuming the sun as a black body at 6000 K). The limiting efficiency of IBSCs surpasses the limiting efficiency of conventional single gap solar cells (40.7%) due to the presence of a third energy band, in addition to the conduction band (CB) and the valence band (VB). This third energy band is referred to as the intermediate band (IB) and is isolated from the metal contacts by means of electron or hole selective emitters. This configuration allows IBSCs to absorb photons with energies below the bandgap, generating an additional photocurrent by means of VB→IB and IB→CB transitions and providing output voltages (V) that exceed the energy of these sub-bandgap transitions divided by the electron charge (e). These characteristics are known as the operation principles of the IBSC. These principles have already been demonstrated in different PV technologies, especially in IBSC prototypes based on quantum dots (QDs). However, IBSC prototypes have not yet fulfilled these principles in real life conditions, such as operation at room temperature. In this context, our work focuses on the study of different IB technological approaches to increase the possibility of obtaining practical IBSCs. The main objective of this Thesis is to characterize IBSC prototypes that belong to the bulk approach which are perhaps the least investigated technologies. In particular we investigate IBSC prototypes based on ZnTe:O, Si:Ti and GaAs:Ti materials. In IBSC prototypes based on ZnTe:O and Si:Ti, the VB→IB and IB→CB transitions are characterized by absorption coefficients as high as 〖10〗^4 cm^(-1) and capture cross sections as low as 〖2•10〗^(-19) cm^2, respectively. These features are suitable for IBSC applications. We find the operation principles of IBSCs fulfill surprisingly in IBSC prototypes based on GaAs:Ti, where the energy levels that act as an IB arise from crystalline defects and not from the presence of Ti atoms. In this Thesis we also contribute to the demonstration of IBSC operation principles at room temperature. IBSC prototypes based on InAs/Al_0.3 Ga_0.7 As QDs, fabricated in collaboration with the University of Tokyo, demonstrate for the first time the production of photocurrent by means of the absorption of two-photons at room temperature. The absorption threshold resulting from this two-photon photocurrent corresponds to the least energetic transitions, the IB→CB transitions. This facilitates the demonstration that eV surpasses the absorption threshold at room temperature under illumination conditions similar to 0.5 suns. Finally, in order to increase the research capabilities of the IES-UPM, we implemented during this Thesis an experimental set-up dedicated to the “Deep-Level Transient Spectroscopy” (DLTS) technique. This technique provides information about recombination processes and complements the characterization obtained from other optical techniques, focused on studying electron-hole pairs generation processes.