Improving the production of photocurrent in the middle junction of a InGaP/Ga(In)As/Ge triple-junction solar cells (TJSC) can improve the overall conversion efficiency of cell. One possible method to improve the middle junction photocurrent is inserting a quantum dot (QD) superlattice (SL) into the stack. It has been predicted that QD-SL enhanced TJSCs have an efficiency ceiling of 47% under a one-sun AM0 illumination spectrum. Additionally, QD array enhanced GaAs cells have the added benefit of possible intermediate band effects, anisotropic absorption and enhanced radiation tolerance. Embedding InAs quantum dots (QDs) in a single junction GaAs solar cell can increase sub-GaAs bandgap photocurrent generation. This method has been shown to improve the short circuit current density (Jsc) of single junction cells under simulated 1 sun air mass zero (AM0) illumination. However, the increase in strain due to the InAs QD self-assembly may cause defects that reduce the minority carrier lifetime resulting in losses in the cell open circuit voltage (Voc) on the order of 300-500 mV. The introduction of strain compensation (SC) layers into the superlattice (SL) structure of a QD solar cell has previously been shown to improve the device performance, including the partial recovery of Voc. Strain compensation can be used effectively to balance the residual strain, impede dislocation formation, and improve the solar cell characteristics. The effect of GaP strain compensation on the solar cell electrical and material properties was investigated. High resolution X-ray diffraction (HRXRD) scans along the symmetric (004) Bragg peak show weak intensity and wide FWHM at the zero order SL peak (SL0) for non-SC samples. Optimum SC thickness was theoretically determined using a zero in plane stress method and experimentally verified using HRXRD. Optimal strain compensation was then used to increase the QD SL stacking from 5x to 10x and 20x. Use of SC resulted in shifting of the SL0 peak toward the substrate peak as well as reduced FWHM and improved SL peak definition. Kinematical diffraction modeling of the QD structures using numerical simulation indicated this peak shift resulted from reduced overall strain in the SL stack up to 5ML of SC. The material quality improvement in the SC QD solar cells was manifested in an improved spectral response and Jsc. The optoelectronic results for GaAs solar cells with QD SL’s demonstrate a strong dependence on GaP SC layer thickness. In addition, comparison of multi junction (MJ) solar cells which incorporate the SC QD SL’s demonstrate the utility of additional sub-GaAs bandgap current contribution as a tool for additional current-matching optimization in MJ solar cells.

A tandem approach is proposed using Silicon nanostructures to increase the efficiency of so-called third generation photovoltaic solar cells.Si quantum dot nanostructures (or silicon nanocrystals)are synthesized by depositing silicon-rich nitride (SRN) layers using plasma-enhanced chemical vapour deposition (PECVD). We have shown the intrinsic formation of silicon nanocrystals (nc-Si) in non-stoechiometric amorphous hydrogenated silicon nitride (a-SiNx:H) layers using pure silane (SiH4) and ammonia (NH3) as reactants. The NH3 would provide more hydrogen in the silicon nitride film leading to an improvement of the crystallinity of Si quantum dots (QD) by favouring the disorder-to-order transition. Furthermore, hydrogen dissociated from the NH3 would passivate the surface of a Si QD more effectively.Transmission Electron Microscopy (TEM) was employed to explore the microstructure of the as-deposited Si-in-SiNx composite films. The chemical bonds of these films were examined by using Fourier Transform Infrared (FTIR) spectroscopy in the wavenumber range from 400 to 4000 cm-1 with a resolution of 4 cm-1. The photoluminescence (PL) property of silicon nanocrystals in silicon-rich nitride (SRN) layers are also investigated. The peak position of PL could be controlled by adjusting the flow rates of ammonia and silane . Two types of luminescent mechanisms, such as radiative defects in the film and the quantum confinement effect (QCE) in silicon nanocrystals, have been proposed to explain the origin of light emission from these structures. These two mechanisms are inherently coexisting in our samples and the photoluminescence spectrum depends on the contribution of each other. The optical absorption properties of the deposited films are obtained and analyzed from light transmittance measurements. Spectroscopique ellipsometry have been performed in order to analyse the refractive index and the extension coefficient. All these measurements were carried out at room temperature. These techniques have given good correlation in the extraction of the absorption coefficient induced by the Si nanocrystal in the visible /UV energy range. Measurements of photocurrent have shown a great increase of the induced currrent in the visible/UV energy range for an optimum of deposition conditions. These results will be discussed in order to reach a better knowledge of the physical properties of this third generation photovoltaic all silicon included material for the tandem solar cell application approach.