The thin film solar cells based on Cu2ZnSnS4 (CZTS) compounds have been considered the potentially substitute to Cu(In,Ga)Se2 and CdTe absorbers due to its low cost, earth-abundant and non-toxic components. The present convention efficiency based on Mo/p-CZTS/n-CdS solar cell is far away from the theoritical value, which originated from the interfacial recombination at the Mo/CZTS heterojunction. In our study, we sputtered a 25nm thickness temporary Ge layer between the Mo electrode layer and the CZTS absorbed layer fabricated by spin coating, which could reduce the recombination centers and the Sn-loss. The partial Ge element would diffuse to CZTS which resulted in the formation of kesterite Cu2ZnSnGeS4 (CZTGS) thin films in which the (112) diffraction peaks shifted towards higher angle. The reason is the decreased lattice constant due to the replacement of large Sn atoms by smaller Ge atoms. The field emission scanning electron microscopy (FESEM) showed that the regularity and uniformity have been improved and the voids decreased after inserting Ge layer. The photovoltaic device (ITO/i-ZnO/CdS/CZTGS/Mo) was fabricated. The illuminated J–V characteristics showed that the open circuit voltage (Voc) increased from 513.42 to 620.71 mV, the short circuit current density (Jsc) increased from 11.57 to 12.44 mA, and the efficiency increased from 2.41 to 3.88% after inserting the Ge layer, which attribute to the decreased recombination at the Mo/CZTS contact and the broaden absorption spectra.
The deep etching basin on silicon wafer was fabricated by the etchant of 52% NaOH (saturated) solution and 10 mL sopropyl alcohol (IPA) using SiO2, CrAu and SiO2/CrAu thin films as the mask. The corrosion-resistance properties of the three masks were compared at the different etching time. The three masks maintained the compact surface in one minute. After 3 minutes, the SiO2 mask completely fell off. After 5 minutes, the CrAu mask began to fall off and the small holes appeared. After 10 minutes, the pores in the SiO2 and CrAu masks increased, which seriously affected the structure of the etching basin. The mask of the SiO2/CrAu composite layer did not detach after 15 mins etching which showed the strong corrosion-resistance. In addition, strong alkaliin etching using the SiO2/CrAu and SiO2/CrPt masks heated at 90°C in the above ethant was compared. It showed that the lateral/depth etching ratio was 0.82:1 and 1.17: 1 under two kinds of masks respectively. SiO2/CrAu mask after 15 minutes etching showed smooth and firm surface, while some pits appeared at the surface of SiO2/CrPt due to the larger stress and inferior adhesion. The etching rate of silicon under the different concentration and temperature was studied. The rate was up to 15 μm/ min in 52% saturated NaOH etchant at 100°C. The experimental results provide the effective mask for wafer level packaging of uncooled infrared focal plane detectors.
High indium composition InxGa1-xAs/GaAs quantum wells (x˃0.4) in which the well width reached to 7 nm without relaxing were grown on (100) GaAs substrates by MBE. The good crystal quality and optical properties of the high strained InGaAs/GaAs QW were obtained by controlling quasi-2D growth model and optimizing the growth condition including the growth temperature, growth rate, and V/III BEP ratio. Photoluminescence (PL) showed that the cutoff wavelength was about 1.3μm at room temperature with narrow full width at half maximum below 30meV. Dilute nitrogen and high In composition InGaAsN/GaAs QW extended wavelength infrared photodetectors at 1.3 and 1.55 μm were also realized.
Recently excellent infrared detectors have been demonstrated using InAs/GaSb superlattice materials sensitive at wavelength from 3um to greater than 32um. Using empirical tight binding method (ETBM), different structures as InAs(xML)/GaSb(8ML), (x=2, 4, 6, 8) and InAs(14ML)/GaSb(7ML) were designed for various cut-off wavelengths from short to long IR wavelength. These materials were grown by MBE with valved cracker cells for arsenic and antimony on p-type GaSb(001) substrates. The microstructure and the bandgap Eg were verified by high resolution X-ray diffraction and photoresponse spectra. The temperature dependence of Eg and photoresponse responsivity Rv were studied. The differential resistance under zero bias R0 in MWIR photodiode was measured up to 106 ohms. The ideality factor in the range of 1.5 to 2.1 indicates the generation-recombination current and surface leakage current are the dominant leakage in the depletion region. These results will promote InAs/GaSb superlattices infrared detectors research in multi-color from short to long wave IR application.
The short- and mid-wavelength infrared detectors based on short period type II superlattices (SLs) InAs
(2ML) / GaSb (8ML) and InAs (8ML) / GaSb (8ML) were grown by molecular-beam epitaxy on
semi-insulating GaAs substrates. An interfacial misfit mode AlSb quantum dot layer and a thick GaSb layer
were grown as buffer layers. Room-temperature optical transmittance spectra showed clear absorption edge at
~2μm and ~5μm. The 50% cutoff wavelength of the two photoconductors was 2.1μm and 5.05μm in
photoresponse at 77K respectively. The blackbody detectivity was beyond 2×108 cmHz1/2/W at 77K and 1×108
cmHz1/2/W at room temperature with 8 V/cm bias.
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