Investigation Of ZnO Transparent Conductive Oxide Thin Films For Efficient Utilization Of The Solar Spectrum

Doped ZnO, as transparent electrodes, has been widely used in thin film solar cells, flat panel display devices, owing to its intrinsic advantages, such as low resistivtiy, high visible transmittance, low cost and nontoxic. Textured Al doped ZnO, prepared by post-etched method has been one of the main TCO materials for silicon based thin film solar cells. However, all commonly used AZO films possess the following drawbacks, low transmittance and weak light scattering properties in the long-wavelength region, which seriously restricted the long-wavelength response of broad spectral high conversation efficiency Si based thin film solar cells. Therefore, fabrication of ZnO-TCO with wide broad spectral range light trapping ability was great importance for improving the long-wavelength response and enhancing the conversion efficiency of Si based thin film solar cells. In this thesis, wide broad spectral range light trapping ZnO-TCO films were achieved using two methods by magnetron sputtering technique. The first one was based on the theory of high valence difference dopants introducing lower doping content and lower ion scattering for improving carrier mobility and long-wagelength transmittance. The other was based on broad surface feature distributions can improve light trapping effect in different region. All the fabricated ZnO-TCO films were used as front contact in p-i-n type Si based thin film solar cells. Detailed description of the research was listed as following.Firstly, the properties of high valence difference W doped ZnO films (WZO) are investigated by means of Castep software based on the density-functional theory (DFT) and pulsed magnetron sputtering technique. The influence of deposition condition on the optical, electrical properties as well as morphologies characteristics of WZO films was systematically investigated. The results were listed below:①The theoretical result shows that after incorporation of W into ZnO, the WZO film shows a typical n-type metallic characteristic and high transmittance in the long-wavelength region.②The experimental result shows that WZO film possesses good broad spectral range transmittance characteristics with an average transmittance over85%at the wavelength region from400nm to1800nm (substrate temperature over200℃).③The conductivity was improved after co-doped with H2. With the optimal H2/Ar doping ratio of7.84%, low resistivity and broad spectral range transmittance HWZO film was achieved at room temperature (Rt), with resistivity of8.33×10-4Ω·cm, carrier mobility of40.8cm2/Vs and average transmittance between400nm to1800nm over80%.④Good light trapping HWZO film was fabricated at Rt by optimizing the deposition paramaters, sputtering pressure and sputtering power. Higher EQE compared to FTO/AZO film used as front contact in μc-Si:H solar cell, showing that the WZO thin film has a great potential application as front contact for high efficiency broad spectral range Si based tandem solar cells.Secondly, both the theoretical and experimental investigation were conducted on Mo doped ZnO (MZO) film.①The theoretical results showing that after introducing Mo into ZnO, the MZO film shows high transmittance in broad spectral range and n type conductivity. Low resistivity (7.68×10-4Ω·cm) and high transmittance (average transmittance between400nm to1100nm over75%, air referenced) MZO film was obtained after deposition paramaters optimized. The resistivity was lower than that of the reported MZO films.②After incorporation of H2, the optical and electrical properties were further improved, the resistivity was reduced to4.71×10-4Ω·cm and average transmittance between400nm to1100nm increased to80%.③Higher conversation efficiency was obtained compared to the referenced FTO/AZO and industrial AZO film used as front.contact in μc-Si:H and μc-SiGe:H solar cell, respectively.Thirdly, high light trapping AZO, AZO/HAZO and AZO/ZnO films in long-wavelength region were fabricated based on the theory that with broad surface feature distributions for improving light trapping effect in different region.①Compared to the traditional post-etched AZO film, lower resistivity, higher light trapping effect (400-1100nm) was obtained. Using this novel AZO film as front contact, there is an enhancement of9.05%,2.17%,3.03%and14.64%for short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and conversation efficiency (Eff.) was achieved compared to the traditional post-etched AZO film for μc-Si:H solar cell.②The light trapping effect and Eff. were further enhanced after optimization.③Higher long-wavelength transmittance AZO/HAZO and AZO/ZnO films were fabricated using lower doped AZO and intrinsic ZnO film as the small feature size feature topography, respectively. The application of AZO/ZnO film as front contact for a-Si:H/μc-Si:H solar cell was investigated.