Characterization of novel sb-nanostructures by transmission electron microscopy techniques for high efficient solar cells

Resumen

Photovoltaic (PV) cells are a major source of renewable energy so great efforts have been made to improve the efficiency and reduce costs. Multi-junction (MJ) and Intermediate Band (IB) solar cells (SC) have been proposed with the aim to exceed the Shockley-Queisser efficiency limit of a single-junction solar cell. Nevertheless, the absence of materials and nanostructures with the required properties and band structures is hindering the development of both technologies. Three different strategies have been analyzed by state-of-the-art (S)TEM techniques in this Thesis. First, GaAs(Sb)(N) superlattices (SLs) have been proposed as subcell candidates for the development of MJSCs. Its advantages are the possibility to obtain a tuneable bandgap covering the 1.0–1.15 eV region and being grown lattice-matched to GaAs substrates. In the first chapter, type-I (GaAsSbN/GaAs) and type-II (GaAsSb/GaAsN) SL structures were evaluated in terms of compositional homogeneity and uniformity. A new methodology for determining N distribution is proposed based in the suitable normalization and separation of intensity ratios by using different angle intervals in annular dark field (ADF) conditions. Measurements of texture analysis and clustering degree are used to compare the compositional inhomogeneities in both types. Our results reveal a better homogeneity of N in type-I SLs, but with a higher tendency of Sb agglomeration, and the opposite occurs in type-II SLs. After annealing treatments, both types of SLs undergo a noteworthy enhancement in the homogenization of both distributions, being the annealed type-II SLs the most balanced structure. Second, increasing sub-bandgap currents is key for implementing the IBSC concept. On one hand, type-II energy band alignment as the obtained in GaAs1 xSbx/InAs QDs (x>0.16) is an effective measure, since it leads to longer carrier lifetimes. Additionally, denser QD structures must be required to achieve the sufficient photo-absorption. In this chapter, capping with GaAs0.8Sb0.2 to obtain type-II vertical aligned (VA) InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, the use of very fine GaAs spacers induces the formation of agglomerations that disables the development of the expected columns of VAQDs. Two types of agglomerations have been modelled closely related to different QD configurations of the first layer. Second, the addition of Sb inhibits the formation of agglomerations promoting VAQD columns. Each column consists of a sequence of alternating pyramidal QDs of In(Ga)As and GaAsSb blocks surrounded the QD apex like a collar. Thirdly, the use of thin capping layers (CL) of AlAs on InAs QDs has been introduced as a way to improve the efficiency of QDSCs, with the elimination of the wetting layer (WL) being the proposed cause for this improvement. In this chapter, a complete analysis of the AlAs/InAs QD system, varying the thickness of the CLs has been made. Firstly, a strong reduction of the WL has been found as the thickness of the AlAs CL increases but not a complete elimination. Surprisingly, there is no spatial separation between WL and CL with a defined interface, but only a graded layer with a variable content of both elements. Secondly, the heights and contents of the capped QDs show a noticeable increase as the thickness of the CL increases, indicating a shielding effect against QD decomposition. Third, although Al atoms are spread evenly on the surface, their distribution on QDs varies depending on the CL thickness. Initially, the Al content at the apex of the QD is lower than in the WL regions and a significant accumulation occurs on the lower flanks of the QD.