An Exploratory Study On Preparation And Modification Of Two-dimensional Carbon Materials For SIS Structure Photovoltaic Devices

Benefited from its excellent electrical and optical properties(The transmittance in visible region was about 98%, ultra-high carrier mobility of 2×105 cm2/(V.S), high electrical conductivity of 106 S/m), stable chemical properties, graphene has shown a huge temptation to improve the performance, as well as to achieve lower costs of solar cells. This paper starts with the present problems in prepration and photoelectric-photovoltaic application of carbon materials(complex production process, harsh deposition conditions, and poor quality of films). Aim for achiving tow-dimensional carbon materials with excellent optical and electrical properties controllably, as well as modifing the SIS structure photovoltaic device, this text launched the research at following works. Firstly, tow-dimensional carbon materials were prepared by Radio-Frequency Plasma Enhanced Chemical Vapor Deposition(RF-PECVD). The corresponding growth thermodynamics and kinetics mechanism were systematically investigated. Meanwhile, we prepared devices with C/Si heterojunction and researched their characteristics of photoelectric properties. Then, monolayer graphene films of high quality were deposited by CVD and then they were transferred to silicon crystals to farbricated devices with different structures. On this basis, we succeed in doping modification of graphene film. The electronic structure and properties of the films, as well as its roles in device performance, were anlysied. The details are as below:(1) Carbon nanomaterials with different structure and properties were synthesized on different substrates by PECVD. The results showed that it is feasible to obtain carbon materials under the substrate temperature of 300~1000℃. A lower temperature is even more defects prone to the obtained carbon materials. Low temperature and pressure is unfavorable to growth low-dimensional carbon materials. High temperature and pressure can realize deposition of high quality graphene film more easily. While, carbon materials grown under the medium temperature and pressure is more likely to present as isolated and discontinuous forms.(2) A variety of variables were modulated in carbon materials growth with PECVD. It manifested that appropriate temperature can not only promote the decomposition of methane, provide sufficient power to create nucleation sites, but also can prompt carbon materials grown in the vertical direction, which is contribute to growth of graphene. Although the layer of graphene films can incresse with deposition time, which is conducive to the uniformity and continuity of the film. The overlong time would induce the carbon materials transfer themselves from the quantitative to qualitative changes. A moderate substrate can introduce nucleation centers, as well as suppress excess nucleation and facilitate planar growth.(3) The principal chemical mechanisms relevant to the growth of graphene by PECVD are presented. We demonstrated that the differences in growth conditions reveal different mechanisms. Nevertheless, the realizability of thermodynamic is identical. Thermodynamic temperature is the most important factors in the pyrolysis of methane. Homogeneous gas and gas-solid reaction in deposition of graphene are controlled by the rules of chemical kinetics. The nucleation and growth of graphene is a competition between the thermodynamics of adsorption and the kinetics of film growth. By modulating the kinetic factors which can affect the path and rate of the reaction, carbon films of various feature and structures for different purposes can be achieved controllably.(4) Large area and high-quality graphene films were synthesized using methane as source gases by chemical vapor deposition technique. The films exhibit high transmittance and a relatively good electrical conductivity. By inserting graphene films into SIS structure solar cells, the advantage of carbon materials and silicon-based solar cells were combined. The Jsc and Voc improved significantly. Thus, the higher energy conversion efficiency can be obtained. The improved solar cell performance was attributed to the multiple roles of graphene in SIS structures, such as improvement of light transmission and current flow, reduction of interfacial states and removal of Fermi pinning effect at the hetero-junctions between graphene films and oxides.(5) Doping modification of graphene film was achived by ways of lattice replacement. A two-step growth process was developed to synthesis N-doped graphene films by using pentachloropyridine as nitrogen source. The obtained N-doped graphene films exhibit n-type semiconductor property with the electron mobility of 521.8 cm2/V.s and density of 2.38×1013 cm-3, respectively. It was found that the graphene/Si-based heterojunction solar cells with N-doped graphene layer posess much more excellent characteristics of photovoltaic in illumination. The maximum achieved energy conversion efficiency was 6.24%. The inserted nitrogen atoms have both positive and negative effects on graphene film. It can effectively reduce the resistivity of graphene films, but also affect the carrier mobility of graphene film.