| Research on environmental friendly renewable energy sources is important forsustainable development of human society. Recently, we have seen a growing interest indeveloping and commercializing photovoltaics, which convert solar energy to electricityin efficient way. Currently, more than85%of the market products are based oncrystalline silicon solar cells, that have mature manufacturing technology and stabledevice performance. Yet Si solar cells are rather expensive, which limits their wideapplications especially for residential use. Therefore, it is imperative and significant forresearchers to adopt new materials and new structures in order to develop low-cost,high-efficiency next generation solar cells. To this end, nanomaterials are promisingcandidates, and have been studied intensively during past years. However,nanomaterial-based solar cells in previous literature reports generally show relativelylow power conversion efficiencies, poor stability, and it remains challenging to producelarge-area, thin film nanomaterial solar cells. This dissertation explores a completelynew idea combining nanomaterials with crystalline silicon and proposes a"carbon nanotube film-Si heterojunction solar cell" model. This model overcomes thelow efficiency of nanomaterials, and compared with silicon, can simplify the fabricationprocess and lower production cost.The author used chemical vapor deposition method to directly synthesize carbonnanotube (CNT) films, and obtained high-quality, uniform films by precisely controllingthe carbon source feeding rate during reaction period. After purification and filmexpansion, the author produced large-area freestanding (100cm2), stable, highlytransparent and conductive CNT films, that could meet the basic requirements formaking solar cell components.Based on controlled synthesis of CNT films, the author built the model of CNTfilm-silicon heterojunction solar cells. In this model, the crystalline Si substrate isresponsible for the absorption of incident photons and production of excited chargecarriers (free electrons and holes), which diffuse to the interface with CNTs. Chargeseparation occurs at the CNT-Si heterojunction where electrons and holes are separatedby the built-in electric field and accelerated toward opposite directions. The CNT film also serves as the top transparent electrode of the cell, collects and transports holes tooutside circuit. In this cell, the CNT film displays metallic behavior as a macroscopicfilm, therefore it forms the Schottky junction with the underlying n-type Si wafer.The author found that the oxide layer formed at the interface between the CNT filmand Si surface plays an important role in solar cell performance. Due to the oxideformation, the structure of solar cells changes from "CNT film-Si" to "CNTfilm-oxide-Si", where electron tunneling (versus thermal emission) becomes thedominating mode in charge transportation. At the same time, recombination ofelectron-hole pairs is strongly inhibited, and cell efficiency can be improved. The authorused dilute HNO3to treat the CNT film-Si cell and produced a thin oxide layer at theCNT-Si interface, and simultaneously improved the CNT film conductivity by HNO3doping, resulting in substantially enhanced power conversion efficiencies of about10%under standard illumination condition (AM1.5,100mW/cm2).The author used cured polydimethylsiloxane (PDMS) sheets to encapsulateHNO3-treated solar cells, which further enhanced light absorption by the cell, leading toa high short-circuit current density of about30mA/cm2. By controlled HNO3oxidation,doping and PDMS encapsulation, the solar cell power conversion efficiency reached10.9%with good environmental stability in long-time storage.The author further added acid solution onto the CNT film-Si solar cell, to fullyinfiltrate the nanoscale pores between the CNT network and Si substrate, thereforeimproved the interfacial contact between CNTs and Si, resulting in more effectivecollection of charge carriers. The final solar cell efficiency reached13.8%.
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