Preparation Of Zn1-xMgxO Thin Films And ZnO:Al/Si Heterojunction And Its Photovoltaic Properties

ZnO is a II-VI wide direct band gap semiconductor. ZnO doped with Mg can form ternary alloy Zn1-xMgxO. With different Mg content, the band gap of Zn1-xMgxO alloy will change from3.3eV to4.0eV, which is used to adjust luminescence properties of ZnO and improve ultraviolet light emitting efficiency. So Zn1-xMgxO has a large application potential in semiconductor lasers, optical bandgap engineering and thin-film solar cells. Al-doped ZnO thin films have excellent optical and electrical properties, rich sources, non-toxic property, low price, and high thermal and chemical stability. This strongly suggests Al-doped ZnO thin films to be an ideal replacement for ITO as a new type of transparent conductive films which are widely used in gas sensors, flat panel displays and solar cells. The main application of ZnO:Al thin films in solar cells is used as the window transparent electrodes. But the research on preparation ZnO:Al/Si heterojunction solar cells by directly depositing ZnO:Al thin film on silicon substrates is quite less. Preparation of ZnO:Al/Si heterojunction solar cells have potential applications because of its advantages such as excellent photovoltaic effect, simple fabrication process and a lower preparation temperature.In this thesis, Zn1-xMgxO thin films with different Mg contents were prapared by radio frequency (RF) magnetron sputtering method, while Zn0.88Mg0.12O films were annealed in air for an hour. The structure, morphology and optical properties of thin-film samples were studied. In addition, ZnO:Al/Si heterojunction solar cell were prapared by directly depositing ZnO:Al thin film on texturing silicon substrates using RF magnetron sputting. The effect of substract temperature, oxygen argon ratio and sputting power on the structure and morphology of ZnO:Al thin films, and on the performance of ZnO:Al/Si heterojunction solar cells were studied. The major results as follows:1). High purity Mg pieces were placed on the ZnO target, Zn1-xMgxO(x==0.1,0.16,0.18,0.24)films on ordinary glass were obtained using RF magnetron sputtering method. All samples showed ZnO hexagonal wurtzite structure and Mg2+effectively replaced Zn2+. The sample surface grains were uniform and compact and the films exhibited an excellent light transmittance property, such as, visible light transmission was about90%. With the increase of Mg content, the absorption edge and UV emission peak of Zn1-xMgxO film got blue shift, by which the band gap was adjusted.2). Zn0.88Mg0.12O films were annealed in air for an hour, when annealing temperature increased, the sample crystal plane spacing decreaced, grains grew and crystalline quality was improved. The absorption edge slightly moved towards shorter wavelenghth direction, so the band gap increased. UV emission peak enhanced and visible emission peak reduced relatively.3). In mixed solution with NaOH and ethanol at mole ratio of1:7, ethanol and deionized water at volume ratio of1:4, when the reaction temperature was maintained90℃, and the reaction time was30min, we got an ideal Si texured structure. The pyramids were uniform and compact with a single pyramid of size in between2and10μ m, and there was no gap between adjacent pyramids.4). When sputtering gas was pure argon, sputting power was120W and substrate temperatures were chosen at room temperature,200℃,300℃,400℃and500℃respectively, ZnO:Al thin films had the best C-axis preferred orientation at400℃, the diffraction peak of crystal plane (002) had the minimum FWHM, the surface particles were uniform and dense, of the size about100nm, and the best crystalline quality was achieved. The Voc of ZnO:Al/Si heterojunction solar cell increased with the increase of substrate temperature, a maximum was reached at400℃, and remained unchanged at500℃; the Isc first increased and then decreased, a rapid decreased occur at500℃. In short, the battery performance was better at400℃.5). When the substrate temperature was400℃, sputting power was120W, oxygen/argon ratio were0:20,3:17,5:15,7:13, respectively, it was found that the circumstance of pure argon, ZnO:Al films had the best C-axis preferred orientation, the diffraction peak of crystal plane (002) had the minimum FWHM, the surface particles were uniform and dense, with a relatively large size and the best crystalline quality. The Voc of ZnO:Al/Si heterojunction solar cell was insensitive to the change of oxygen/argon ratio, while the Isc rapidly decreased with the increase of the oxygen/argon ratio. To sum up, the solar cell had a better performance in the circumstance of pure argon.6). When the substrate temperature was400℃, the sputtering gas was pure argon, sputtering power were80W,120W,150W and180W, respectively, the ZnO:Al films had the best C-axis preferred orientation at120W, the surface particles were uniform and dense, with the best crystalline quality. With the increase of sputtering power, the Voc of ZnO:Al/Si heterojunction solar cell had little change; the Isc first increased and then decreased, with a maximum at120W. To sum up, the solar cell performance was better at120W.7). When the substrate temperature was400℃, the sputtering gas was pure argon, and sputtering power was120w, we could get the best crystalline quality of ZnO:Al thin films. Under this condition, the best performance of ZnO:Al/Si heterojunction solar cells was:open circuit voltage Voc was0.152V. short circuit current Isc was2.155mA, maximum output power was108.8uW, fill factor of the FF was0.331and conversion efficiency of about1.1%.