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Controlled doping of quaternary alloys of In x Ga 1−x As y Sb 1−y with tellurium is fundamental to obtain the n-type layers needed for the development of optoelectronic devices based on p-n heterojunctions. InGaAsSb epitaxial layers were grown by liquid phase epitaxy and Te doping was obtained by incorporating small Sb 3 Te 2 pellets in the growth melt. The tellurium doping levels were in the range 10 16 -10 17 cm −3 . We have used lowtemperature photoluminescence (PL) spectroscopy to study the influence of the Te donor levels on the radiative transitions shown in the PL spectra. The PL measurements were done by exciting the samples with the 448 nm line of an Ar ion laser with varying excitation powers in the range from 10 to 200 mW. For the low-doped sample the PL spectrum showed a narrow exciton-related peak centred at around 610 meV with a full width at half maximum (FWHM) of about 7 meV which is evidence of the good crystalline quality of the layers. For higher Te doping, the PL spectra show the presence of band-to-band and donor-to-acceptor transitions which overlap as the Te concentration increases. The peak of the PL band shifts to higher energies as Te doping increases due to a band-filling effect as the Fermi level enters into the conduction band. From the peak energy of the PL spectra, and using a model that includes the band-filling and band-shrinkage effects due to the carriers, we have estimated the effective carrier concentration due to doping with Te in the epilayers.

We have studied GaxIn1−xAsySb1−y/GaSb heterostructures for x=0.84 and y=0.14 using the photoacoustic technique with the heat transmission configuration. A theoretical model, which includes all the possible nonradiative recombination mechanisms that contribute to heat generation, was developed to calculate the photoacoustic signal for this type of heterostructure. The Auger recombination lifetime τAuger was determined by fitting our experimental results to the calculated frequency dependence of the theoretical photoacoustic signal. The obtained value for τAuger is compatible with those reported in the literature for semiconductors with band-gap energies below and above 0.5 eV, the energy region where there is a lack of experimental τAuger values.

Graphical abstract

Using the liquid phase epitaxy technique we have grown quaternary p-type Ga 0.84 In 0.16 As 0.14 Sb 0.86 layers on n-type GaSb substrates (p-N), as well as n-type layers on p-type substrates (n-P). It has been known that doping with tellurium to obtain n-type InGaAsSb layers, has important effects on the quality of the interface at the heterojunction epitaxial layer/substrate, because of the high diffusion coefficient of the Te atoms. In this work, we have applied the photoacoustic technique to measure the interface recombination velocity in both types of samples: p-N and n-P heterojunctions, and we have compared these results with measurements of atomic force microscopy made along the cross section of the interface substrate-epitaxial layer. Results from both techniques show that there is a better interface quality for the p-N heterojunction as compared to those for the N-p interfaces. The discussion of this behaviour is presented in terms of the behaviour of Te diffusion during the growth of the InGaAsSb epitaxial layers.

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Inverted organic photovoltaic solar cells were fabricated with the configuration of FTO/TiO 2 / P3HT:PC 61 BM/MoO 3 /Ag. Besides, the influence of transport layers, titanium dioxide and molybdenum trioxide, on the performance of solar cells were investigated. These compounds showed excellent optical (around 80% for molybdenum trioxide and 95% for titanium dioxide), electrical (like charge carrier density of 3.3 x10 15 cm -3 and 2.5 x10 14 cm -3 for titanium and molybdenum, respectively) and structural (anatase and amorphous hexagonal phase for titanium and molybdenum, respectively) properties to be used as transport layers. Also the influence of the thickness of the electron transport layer is studied, as well as the thickness, temperature and heat treatment time of the active layer. The correct selection of TiO 2 's thickness (70 nm) and active layer's thickness (250 nm) and annealing (at 100 degrees for 8 minutes) can increase the power conversion efficiency. Moreover, the cell fabricated with transport layers and the best conditions found showed a maximum efficiency of 3.3%, which indicates that the titanium dioxide and molybdenum trioxide played a determining role in the solar cell performance.