The Preparation And Properties Of The Ge_1-xC_x Films Used For Absorber Layer Of Tandem Solar Cell

As the fossil fuels approaching vanishing point and environmental pollutiongetting worse, the development and utilization of solar energy as a clean and renewablesource attracts great attention to nowadays. It is the main approach of using solar energythrough solar cell, in which thin film solar cells have developed greatly due to theadvantages of saving raw materials and lower cost. However, a proper sunlightabsorption layer material with all performance requirements met is still in research.Ge1-xCxthin film by incorporating C into Ge can achieve smaller lattice constant andlarger band gap compared with Ge keeping the same high absorption efficiency, and hasthe properties of low stress and variable band gap. Thus, developing Ge1-xCxthin film asa kind of absorber layer material for tandem solar cells has a bright future.Ge1-xCxthin film have been prepared by reactive radio frequency magnetronsputtering of a single crystal Ge target in the discharge gasses of CH4and co-sputteringof mixed Ge/Graphite target on quartz substrates and single crystal Si, and its depositionrate, component, structure, band gap and electronic property are investigated by thegeneral test methods. Besides, the characteristics of the GeC surface and B doping havebeen calculated using the first principles density functional theory. The main results areas follows:Ge1-xCxthin films have been fabricated by reactive sputtering and the influence ofthe main process parameters on deposition rate, optical properties and chemical bondingproperties of the films are discussed. Within our experiments, the deposition rateincreases as increasing the radio frequency (RF) power and CH4flow rate, whiledecreases as the increase of H2flow rate and substrate temperature. The optical gapmainly depends on the composition and chemical bonding of the films. It is concludedthat the optical gap decreases with increasing the RF power and substrate temperature,first decreases and then increases with the increase of H2flow rate and increases asincreasing the CH4flow rate. Through the measurements of x-ray photoelectronspectroscopy (XPS), it is found that high RF power and H2flow rate, low substratetemperature and CH4flow rate are beneficial for the forming of Ge-C bond.Moreover, all the films are mainly amorphous through the XRD patterns. Throughthe investigation of the annealing process, it is found that the Ge1-xCxfilms are still mainly amorphous after annealing. The optical gap decreases as increasing temperatureat low C content, while keeps constant for high C content. When the Ge1-xCxfilms areannealed at400℃and300℃, C-H and Ge-H bonds in the film begin breaking,respectively. At last, we evaluate the performance of amorphous Ge1-xCxfilms preparedby reactive magnetron sputtering and find that amorphous Ge1-xCxfilms have a highabsorption coefficient, wide band gap and better surface electrode contactcharacteristics, which indicates a promising material as an absorber layer material oftandem solar cell.Microcristalline Ge1-xCxthin films have been fabricated by co-sputtering and theinfluence of the substrate temperature and H2flow rate on deposition rate, opticalproperties, chemical bonding properties and crystal structure of the films are discussed.Within our experiments, the deposition rate nearly has no change as increasing thesubstrate temperature and H2flow rate. The optical gap decreases as increasing thesubstrate temperature while keeps constant with increasing H2flow rate. Through themeasurements of XRD, it is found that though high substrate temperature is beneficialfor the crystallization of Ge1-xCx, the content of Ge-C bond in the films is low whichindicates a separation of Ge and C phases. The atomic hydrogen and H+ion have anetching effect on weak bonds which promotes the formation of Ge-C bond and isbeneficial for fabricating microcrystalline Ge1-xCxthin film.Besides, comparing with reactive magnetron sputtering technique, the amorphousGe1-xCxfilms deposited by co-sputtering are more comprehensive and have high opticalgap, refractive index and Ge-C bond content with the same C content, but an extremelylow deposition rate.Structural parameters, mechanical properties and electronic structure ofzinc-blende GeC have been calculated using the first principles density functionaltheory. The calculated structural parameters are in good agreement with the theoreticaland experimental data which indicates that the models and methods used here arecredible. Surface relaxation, stability and electronic structure of the GeC(001) surfacehave been investigated. Both Ge-and C-terminated surface have been analyzed and theresults show that the C atoms relax inward while Ge atoms relax outward, showingrelaxation oscillation. The band structures and density of states of two surfaces arediscussed, showing metallic surface properties for Ge-terminated surface and P-typesemiconductor property with a direct band gap of0.6eV for C-terminated surface.The average surface energy of zinc-blende GeC(001) plane is3.91J/m2and theGe-terminated surface is more stable than C-terminated surface.The B-doped GeC at different concentrations has been studied to investigate Bdoping on the electronic structure and optical peroperties. It can be concluded that thedoping of B substituting of C atoms in GeC inducing the band gap of GeC transformingfrom indirect band gap to direct band gap. The static dielectric constant of B-doped GeCis found to be remarkably large. Furthermore, after B doping, a new absorption peakappears in the infrared band (0-2eV) and the calculated optical properties are all showna red shift property.