The electrical and optical properties of zinc selenide

Zinc selenide is a II - VI compound semiconductor with a vide band gap of 2.67 eV at room temperature, and is therefore capable of emitting visible luminescence. The main purpose of the research reported in this thesis was to study zinc selenide crystals grown in the department with the aim of developing a suitable material for the manufacture of red- emitting electroluminescent diodes. It is hoped that these will eventually be cheaper and easier to produce than gallium arsenide-phosphide devices. The most satisfactory means of reducing the resistivity of zinc selenide to values consistent with its use as a device (approx. 1 ohm cm.) was to heat nominally undoped crystals In molten zinc. Measurements of the Hall voltage and conductivity over the range 15ºK to 400ºK revealed a shallow donor level (Ed = 0.012 eV) thought to be associated with unremoved trace impurities of chlorine. Manganese was Introduced into several crystals to provide an efficient luminescent centre. The characteristic manganese emission band at 85ºK was found to lie at 5870 Å with a half width of 0.13 eV. Under 3650 Å excitation, however, the manganese emission was swamped by a band at 6400 Å attributed to copper contamination, and a broad band at 6150 Å which appeared to be the self-activated emission of zinc selenide. The manganese emission could be isolated when crystals were excited in one of the two characteristic excitation bands (5040 Å and 5370 Å). A smaller excitation band was also observed at 4660 Å, All samples, Including those containing manganese, heated in zinc to reduce their resistivity, were found to emit the self-activated band only, thought to be the result of chlorine impurity. An Anger process was assumed to account for the disappearance in semiconducting samples of the manganese emission under photo excitation and yet explain its appearance in the emission from electroluminescent diodes containing manganese

Improved efficiency of microcrystalline silicon thin film solar cells with wide band-gap CdS buffer layer

In this paper, we have reported a new structure based upon an optical simulation of maximum light trapping and management in microcrystalline silicon thin film solar cells by using multi texture schemes and introducing an n-type cadmium sulphide (CdS) buffer layer with the goal of extreme light coupling and absorption in silicon absorber layer. Photon absorption was improved by optimising the front and back texturing of transparent conductive oxide (TCO) layers and variation in buffer layer thickness. We have demonstrated that light trapping can be improved with proposed geometry of 1μm thick crystalline silicon absorber layer below a thin layer of wide band gap material. We have improved the short circuit current densities by 1.35mA/cm2 resulting in a total short circuit current of 25 mA/cm2 and conversion efficiency of 9% with the addition of CdS buffer layer and multi textures, under global AM1.5 conditions. In this study, we have used 2 Dimensional Full Vectorial Finite Element (2DFVFEM) to design and optimize the proposed light propagation in solar cell structure configuration. Our simulation results show that interface morphology of CdS layer thickness and textures with different aspect and ratios have the most prominent influence on solar cell performance in terms of both short circuit current and quantum efficiency

Some electrical and optical properties of cadmium sulphide

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Formation And Characterization Of Pbxcd1-Xs Interlayer For PbSCdSZnS Quantum Dot Sensitized Solar Cells

Sel suria sensitif titik kuantum (QDSSCs) mempunyai kecekapan yang rendah disebabkan oleh penggabungan semula antara muka elektrolit-elektrod. Titik kuantum plumbum sulfida (PbS), titik kuantum plumbum cadmium sulfida (PbxCd1-xS), titik kuantum kadmium sulfida (CdS) dan diikuti oleh salutan zink sulfida (ZnS) telah berjaya dimendapkan ke atas elektrod TiO2 melalui kaedah penjerapan dan tindakbalas lapisan berturut-ion (SILAR) sebagai fotoanod bagi QDSSCs. Pemendapan PbxCd1-xS di antara lapisan teras PbS dan lapisan luar CdS akan mengurangkan penggabungan semula dan meningkatkan kecekapan bagi QDSSCs. Elektrod TiO2 akan dibentukan dengan memendapkan TiO2 berliang meso di atas kaca oksida timah berdopkan florin (FTO) setelah pengkalsinan pada suhu 450 °C. Sel suria disediakan dengan mengapitkan fotoanod TiO2 berliang meso dengan fotokatod Cu2S. Enam kitaran SILAR PbS, CdS, ZnS dan PbxCd1-xS serta lapisan Bagi lapisan pelbagai PbS/PbxCd1-xS/CdS/ZnS sampel, kesan kitaran SILAR bagi PbxCd1-xS dikaji dengan empat jenis pecahan molar, x iaitu 0.05, 0.1, 0.15 and 0.2. Pengukuran ketumpatan arus-voltan (J-V) mengesahkan kecekapan sel suria untuk empat kitaran SILAR lapisan PbxCd1-xS dengan pecahan molar, x dalam 0.05 bagi lapisan pelbagai PbS/PbxCd1-xS/CdS/ZnS sampel akan meningkatkan sebanyak 38.2 % apabila berbanding dengan lapisan pelbagai PbS/CdS/ZnS sampel. In adalah kerana jurang jalur diperolehi bagi empat kitaran SILAR lapisan PbxCd1-xS dengan pecahan molar, x dalam 0.05adalah antara jurang jalur diperolehi bagi lapisan teras PbS dan lapisan luar CdS dengan pengukuran UV-Vis spektra penyerapan. Selain itu, perangkap keadaan di antara lapisan teras PbS dan lapisan luar CdS dapat diturunkan dengan pemendapan lapisan PbxCd1-xS dalam sampel lapisan pelbagai PbS/PbxCd1-xS/CdS/ZnS. Antara sampel lapisan pelbagai PbS/PbxCd1-xS/CdS/ZnS, empat kitaran SILAR bagi lapisan PbxCd1-xS dengan pecahan molar, x dalam 0.05 menunjukkan kecekapan sel suria yang paling tinggi iaitu 0.34 % apabila berbanding dengan empat kitaran SILAR bagi lapisan PbxCd1-xS dengan pecahan molar, x dalam 0.1, 0.15 and 0.2. Ini adalah disebabkan oleh empat kitaran SILAR bagi lapisan PbxCd1-xS dengan x dalam 0.05 mempunyai jalur konduksi yang tinggi akan membawa kepada suntikan electron yang lebih cepat dari PbS/PbxCd1-xS jalur konduksi ke elektrod TiO2. Oleh itu, penggabungan semula yang rendah akan diperolehi dan kecekapan sel suria akan meningkatkan. Di samping itu, empat kitaran SILAR bagi lapisan PbxCd1-xS memberikan kecekapan sel suria yang tinggi daripada enam kitaran SILAR bagi lapisan PbxCd1-xS dalam lapisan pelbagai PbS/PbxCd1-xS/CdS/ZnS sampel. Ini adalah disebabkan oleh pemendapan titk kuantum yang tinggi bagi PbxCd1-xS dengan enam kitaran SILAR lapisan PbxCd1-xS mengelakkan penusukan elektrolit dan menurunkan kecekapan sel suria dalam pengukuran J-V. _______________________________________________________________________________________________________ Quantum dot sensitized solar cells (QDSSCs) have low efficiency due to the recombinations at electrolyte-electrode interfaces. Lead sulphide (PbS) quantum dots (QDs), lead cadmium sulphide (PbxCd1-xS) QDs, cadmium sulphide (CdS) QDs and followed by coating with zinc sulphide (ZnS) were deposited on TiO2 electrode as TiO2 mesoporous photoanode using successive ionic layer adsorption and reaction (SILAR) method for QDSSCs. PbxCd1-xS QDs deposited between PbS core and CdS shell layer could reduce the recombination and improve the efficiency. TiO2 electrode was formed with the deposition of TiO2 mesoporous film on fluorine doped tin oxide glass (FTO) after calcination at 450 °C. The PbS QDs, PbxCd1-xS QDs, CdS QDs and coating with ZnS were formed on TiO2 electrode with SILAR method. Solar cells were prepared by sandwiching the TiO2 mesoporous photoanode with Cu2S counter electrode. Six SILAR cycles of PbS, CdS, ZnS and PbxCd1-xS as well as multilayer of PbS/CdS/ZnS and PbS/PbxCd1-xS/CdS/ZnS were prepared for characterizations. In multilayer of PbS/PbxCd1-xS/CdS/ZnS, the effects of number of SILAR cycles of PbxCd1-xS were studied with four different molar fraction, x of 0.05, 0.1, 0.15 and 0.2. From current-density voltage (J-V) measurement, four SILAR cycles of PbxCd1-xS interlayer with molar fraction, x of 0.05 in multilayer PbS/PbxCd1-xS/CdS/ZnS samples showed 38.2 % improvement in the efficiency when compared to the multilayer PbS/CdS/ZnS sample. This was because the band gap value obtained for four SILAR cycles of PbxCd1-xS interlayer with molar fraction, x of 0.05 were between band gap value of PbS core and CdS shell layer during UV-Vis spectrometer analysis. Moreover, the traps states were reduced between the PbS core and CdS shell layers with the deposition of PbxCd1-xS interlayer in multilayer of PbS/PbxCd1-xS/CdS/ZnS sample. Among the multilayer PbS/PbxCd1-xS/CdS/ZnS samples, four SILAR cycles of PbxCd1-xS interlayer with molar fraction, x of 0.05 provided the highest efficiency of 0.34 % when compared with four SILAR cycles of PbxCd1-xS interlayer with molar fraction, x of 0.1, 0.15 and 0.2. This is due to the conduction band of four SILAR cycles of PbxCd1-xS interlayer with molar fraction, x of 0.05 was higher and lead to faster electron injection from the conduction band of PbS/PbxCd1-xS to the TiO2 electrode. Thus, lower recombination was obtained and the efficiency was improved. Besides that, four SILAR cycles PbxCd1-xS interlayer showed higher efficiency than six SILAR cycles of PbxCd1-xS interlayer in samples with multilayer PbS/PbxCd1-xS/CdS/ZnS. This was owing to the high loading of PbxCd1-xS QDs with six SILAR cycles of PbxCd1-xS interlayer would prevent the penetration of electrolyte and decreased the efficiency in J-V measurement

The electrical and optical properties of zinc suphide platelets

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Electroluminescence in zinc selenide

The main purpose of the research reported was to study zinc selenide crystals with the object of developing red and yellow light emitting devices. Zinc sulphide and zinc sulpho-selenide mixed crystals were also studied to extend the colour range to green and blue. Most of the electroluminescent devices were of the Schottky barrier type and were prepared on chemically etched crystal surfaces. Electroluminescence (EL) was always observed when such devices were reverse biased. ‘Forward bias EL was only observed in diodes which contained a relatively thick (~200 A) semi-insulating layer under the Schottky contact. Almost all ZnSe diodes free of intentionally added luminescent centres emitted a yellow-orange band (self-activated) when biased in the reverse or forward directions. The optimum brightness (e.g. 800 Ft-L with a conversion power efficiency of 4 x 10 (^-3)) in the yellow region of the spectrum was obtained with reverse biased ZnSe: Mn diodes. The characteristic manganese emission in EL occurred at 5785 A, but was usually found to be swamped and broadened by the onset of self-activated emission which lies in the same region of the spectrum. A good red emission at 6400 A was obtained from reverse biased ZnSe: Mn, Cu, Cl diodes with a brightness of 200 Ft-L and a conversion power efficiency of 1.5 x 10 (^-3) Free exciton and pair emission in the blue have also been observed in undoped forward biased ZnSe diodes. These emissions have been studied in the temperature range from 20 - 360 K. Excitons became bound to neutral donors, or acceptors at temperatures below 65 K. The pair emission observed at low temperatures was associated with donor and acceptor levels with ionization energies around 26 meV and 122 meV respectively

Edge emission and exciton recombination in cadmium sulphide

The Stokes (U.V.) and anti-Stokes (A.S.) photo-excited emissions of large, doped and undoped CdS single crystals, grown under controlled partial pressures of cadmium and sulphur, at liquid helium temperatures were examined to establish a correlation between the crystal growth conditions and the spectral distribution of the green edge and exciton emissions. Anti-Stokes excitation spectra were also obtained. Two longitudinal optical phonon assisted series constituted the green emission. The “high energy series" (H.E.S.) was attributed to the recombination of free electrons with holes bound to acceptors some 0.17eV above the valence band, the "low energy series" (L.E.S.) to a distant-pair recombination process involving electrons bound to donors some 0.03eV below the conduction band and holes bound to the same acceptor. The mean separation between the donors and acceptors was about 100 Å. Only the L.E.S. was observed in A.S. excited green emission. The I(_1)and I(_2) bound excitons which dominated the blue emissions are associated with exciton recombination at neutral acceptors and neutral donors respectively. I*(_2) emission, associated with excitons bound to neutral donors loosing some of their recombination energy in raising the donor electron to an excited state of the donor, was observed and used to evaluate a donor ionisation energy of 0.026eV. Blue emission was excited by A.S. radiation in several crystals and ascribed tentatively to I(_1) emission. A model is developed to explain the variation of the emission characteristics with crystal growth conditions. A cadmium vacancy- donor impurity complex is suggested as the acceptor involved in the green and exciton emissions, with the hole in an excited state of the complex, and as the centre through which the two-step A.S. excitation process proceeds. Sulphur vacancy-acceptor impurity complexes and donor impurities are suggested to explain the donors associated with the L.E.S. and 1(_2) emissions