Page - 192 - in Photovoltaic Materials and Electronic Devices

lattice mismatch between GaAs and Si, the polar/nonpolar character and of the strong tensile stress, since the thermal expansion coefficient of GaAs is about twice that of the silicon value. The low intensity of these peaks is directly linked to the subsistence of non-radiative recombination channels due to the defects in the structure. Figure 5. PL spectra recorded at 11 k from the pin-GaAs/Si structure (red line) and from pin-GaAs/Si with InAs/InGaAs multilayer QDs (blue line). For the structure containing multiple layer QDs, the PL measurement reveals a broad band centered at 1100 nm. This band is likely to arise from the luminescence of the InAs QDs. Although this result confirms the formation of InAs/GaAs QDs, the broadening of the PL band with relatively weak intensity confirms that the QDs structural properties are altered. Figure5. PL spect a recor ed at 11 k from the pin-GaAs/Si structure (red line) and frompin-GaAs/Siwith InAs/InGaAsmultilay rQDs(blue line). TofurtherassesstheimpactofintroducingtheInAsQDswithinthepin-GaAs/Si we have performed the spectral response measurements from samples with and without QDs. The results are shown by Figure 6. The photo-response obtained frompin-GaAs/n+-SiwithoutQDsfor thehighenergyphotons (beyondtheGaAs band gap) produces the same range of photo-response obtained by the reference cell grown on GaAs substrate. This assures that the photocarriers collected by the structure are mainly created by the pin-GaAs prepared on the Si substrate. However, for lowerenergyphotons, thespectral responseof thereferencecelldropsabruptly at 868 nm corresponding to the band gap energy of GaAs (1.42 eV). In the meantime, thephoto-response frompin-GaAs/n+-Si recoversaproportionof thebelowGaAs bandgapphotons toanextentofupto1200nm. Theobservedenhancement isdue tophotocarriersgeneratedbysiliconsubstrate. A more pronounced improvement in the photo-response at long wavelengths is observedforthestructurecontainingQDs. Thisimprovementisduetotheabsorption ofphotonsbelowthebandgapenergyofGaAsbyInAsQDs. Although the structural properties of the multiple QDs appear rather to be degradedprincipallyasaconsequenceof the initial surfaceroughness, theoptical andelectricpropertiesofpin-GaAs/n+-SiwithInAsQDsshowthat the InAsQDs have been formed and successively contribute to the electron–hole pair creations in thebelowbandgapenergyrangewhichincreasedthephotocarriercollections[30,31]. This initialassessmentprovidesevidenceof thepotentialofourproposedyielding structures for the fabricationof futurenovel, lowcost,highperformancesolarcells. At this time, no contact grid coatings were applied and the electrical contact was basically made with indium-zinc alloys pads on the front surface. In-situand ex-situoptimization of the solar cell fabrication is in progress, a necessary step to obtainsignificantvalues fromtheactivesolarcellparameters. 192