Photophysical investigation of the formation of defect levels in P doped ZnO thin films

Zinc oxide (ZnO) offers a major disadvantage of asymmetry doping in terms of reliability, stability, and reproducibility of p-type doping, which is the main hindrance in realization of optoelectronic devices. The problem is even more complicated due to formation of various native defects in unintentionally doped n-type ZnO. The realization of p-type conductivity in doped ZnO requires an in-depth understanding of the formation of an effective shallow acceptor, as well as donor-acceptor compensation. Photophysical properties such as photoconductivity along with photoluminescence (PL) studies have unprecedentedly and effectively been utilized in this work to monitor the evolution of various in-gap defects. Phosphorus (P) doped ZnO thin films have been grown by RF magnetron sputtering under various Ar to O₂ gas ratios to investigate the effect of O₂ on the donor-acceptor compensation by comprehensive photoconductivity measurements supported by the PL studies. Initial elemental analyses indicate presence of abundant zinc vacancies (V_Zn) in O-rich ambience. The results predict that P sits in the zinc (Zn) site rather than the oxygen (O) site causing the formation of P_Zn–2V_Zn acceptor-like defects, which compensates the donor defects in P doped ZnO films. Photocurrent spectra uniquely reveal presence of more oxygen vacancies (V_O) defects states in lower O₂ flow, which gets compensated with an increase in the O₂ flow. Successive photocurrent transients indicate probable presence of more V_O in the films grown with lower O₂ flow and more V_Zn in higher O₂ flow. Overall the photosensitivity measurements clearly present that O-rich ambience expedites the formation of acceptor defects which are compensated, thereby lowering the dark current and enhancing the ultraviolet photosensitivity.     

Perylene derivative sensitized multi-walled carbon nanotube thin film

Perylene-sensitized multi-walled carbon nanotubes (PV-MWNT) have been prepared by a π-stacking between nanotubes and perylene derivatives, N,N′-diphenyl glyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide (PV). The resultant nanocomposites have been characterized by transmission electron microscope (TEM), UV-vis absorption, photoluminescence (PL) and photocurrent spectra. Long range ordering can be observed in the form of PV-MWNT via π-stacking by TEM. Red-shift in the optical spectra consisting of the UV-vis absorption and PL spectra with the attraction of PV on the surface of the MWNTs were observed. The evident quenching in PL spectra of PV-MWNT was ascribed to the absorption and transfer of recombination energy by MWNT. Photosensitivity spectra demonstrated an increasing photocurrent in the ultraviolet region and a broadening photosensitivity in the red spectral region for solar cells based on PV-MWNT.     

Diamond-like carbon thin films by microwave surface-wave plasma CVD aimed for the application of photovoltaic solar cells

The nitrogen doped diamond-like carbon (DLC) thin films were deposited on quartz and silicon substrates by a newly developed microwave surface-wave plasma chemical vapor deposition, aiming the application of the films for photovoltaic solar cells. For film deposition, we used argon as carrier gas, nitrogen as dopant and hydrocarbon source gases, such as camphor (C₁₀H₁₆O) dissolved with ethyl alcohol (C₂H₅OH), methane (CH₄), ethylene (C₂H₄) and acetylene (C₂H₂). The optical and electrical properties of the films were studied using X-ray photoelectron spectroscopy, Nanopics 2100/NPX200 surface profiler, UV/VIS/NIR spectroscopy, atomic force microscope, electrical conductivity and solar simulator measurements. The optical band gap of the films has been lowered from 3.1 to 2.4 eV by nitrogen doping, and from 2.65 to 1.9 eV by experimenting with different hydrocarbon source gases. The nitrogen doped (flow rate: 5 sccm; atomic fraction: 5.16%) film shows semiconducting properties in dark (i.e. 8.1 × 10^{− 4} Ω^{− 1} cm^{− 1}) and under the light illumination (i.e. 9.9 × 10^{− 4} Ω^{− 1} cm^{− 1}). The surface morphology of the both undoped and nitrogen doped films are found to be very smooth (RMS roughness ≤ 0.5 nm). The preliminary investigation on photovoltaic properties of DLC (nitrogen doped)/p-Si structure show that open-circuit voltage of 223 mV and short-circuit current density of 8.3 × 10^{− 3} mA/cm². The power conversion efficiency and fill factor of this structure were found to be 3.6 × 10^{− 4}% and 17.9%, respectively. The use of DLC in photovoltaic solar cells is still in its infancy due to the complicated microstructure of carbon bondings, high defect density, low photoconductivity and difficulties in controlling conduction type. Our research work is in progress to realize cheap, reasonably high efficiency and environmental friendly DLC-based photovoltaic solar cells in the future.     

基于微波光电导衰退法的新型少子寿命测量仪

本文在传统微波光电导衰退法的基础上,提出了一种可用于测量半导体少数载流子寿命的新型少子寿命测量仪。通过增加频移器和混频器,对高频微波探测信号进行调制和解调,得到包含了样品少子浓度信息的低频信号。同时介绍了测量仪的基本原理和测量装置的结构,并分别对仪器内部的微波源、激光器、频移器和混频器的原理和参数选择进行了探讨。实验表明:测量结果波形与理论预期波形相吻合,证实了测量仪的可行性;新的测量仪保持了传统方法非接触、无损伤的优点,并降低了系统检测设备的性能要求,提高了测量信噪比。     

半导体材料少子寿命测试仪的研制开发

少数载流子寿命(简称少子寿命)是半导体材料的一项重要参数，它对半导体器件的性能、太阳能电池的效率都有重要的影响。我们采用微波反射光电导衰减法研制了一台半导体材料少子寿命测试仪，本文将对测试仪的实验装置、测试原理及程序计算进行了较详细的介绍，并与国外同类产品的测试进行比较，结果表明本测试仪测试结果准确、重复性高，适合少子寿命的实验室研究和工业在线测试。     

Formation and properties of AgInSe2 thin films by co-evaporation

The electrical and optical properties of silver indium selenide thin films prepared by co-evaporation have been studied. X-ray diffraction indicates that the as prepared films were polycrystalline in nature. The lattice parameters were calculated to be a=0.6137 and b=1.1816 nm. Composition was determined from energy dispersive analysis of X-ray. Silver indium selenide thin films were also prepared by bulk evaporation of powdered sample for comparative study. They have an optical band gap (E_g) of 1.25 eV and it is a direct allowed transition. Refractive index (n) and extinction coefficient (k) were calculated from absorption and reflection spectra. Steady-state photoconductivity was measured from 300 to 400 K. Carrier lifetime was calculated from transient photoconductivity measurements at room temperature at different intensities of illumination.     

Synthesis based structural and optical behavior of anatase TiO2 nanoparticles

The effect of synthesis method on optical and photoconducting properties of titanium dioxide (TiO₂) nanoparticles has been investigated. Sol–gel and co-precipitation methods have been employed to prepare pure anatase phased TiO₂ nanoparticles calcinated at different temperatures below 500 °C. The optimized value of average crystallite size is within the range of 19−21 nm for a common calcination temperature of 400 °C for both the methods. The redshift in optical band gap of 0.9 eV has been observed for the sample synthesized by co-precipitation method with respect to the sol–gel method. The photoluminescence spectrum exhibits broad visible emission in both routes of synthesis while photoconductivity shows fast growth and decay of photocurrent in TiO₂ prepared by co-precipitation method as compared to TiO₂ prepared by the sol–gel method under visible illumination. Crystal structure based Rietveld refinement of X-ray diffraction data, scanning electron microscopy as well as photoluminescence and photoconductivity measurements were performed to characterize nanocrystalline anatase TiO₂.     

给/吸电子基团取代对三苯胺基二苯乙烯衍生物光导性能的影响

分别以甲氧基作为给电子基团、溴作为吸电子基团,设计并合成了一系列三苯胺基二苯乙烯衍生物.用红外光谱、核磁共振谱及元素分析对其化学结构进行了表征.通过紫外-可见吸收光谱和循环伏安法研究了其能带结构.以三苯胺基二苯乙烯衍生物作为电荷传输层、TiOPc作为光生层,制备了双层光电导体,研究了给/吸电子基团取代对光导性能的影响.     

The effect of sputtering power on the structure and photocatalytic activity of TiO2 films prepared by magnetron sputtering

TiO₂ films were fabricated by direct current reactive magnetron sputtering. The effect of the sputtering power on the film structures, morphologies, and properties was investigated in detail. It is found that the concentration of oxygen impurities increased with increasing sputtering power accompanied by the bandgap (E_g) narrowing and broadening of photoluminescence (PL) peaks. The oxygen impurities were found to mainly play the role of recombination centers, leading to the decrease of photocatalytic activity. Furthermore, the photoconductivity to dark conductivity ratio could be used to evaluate and even predict photocatalytic activity to some extent.