臺灣博碩士論文加值系統

本實驗將石墨烯應用於兩種太陽能電池:第一部分，石墨烯與超薄太陽能電池的結合;第二部分，石墨烯運用於蕭特基太陽能電池。
超薄太陽能電池具備便宜與高效能源的潛力，但在實際的議題裡，當其厚度低於100奈米以下，會產生裂痕、空洞與不均勻的分布，這些缺陷將造成漏電流通道使其無法運作。本文將石墨烯作為上層電極，製作出厚度在5~100奈米的超薄透明太陽能電池。石墨烯的延展性使其能服貼於粗糙的表面且高穿透度的特性更適用於當作太陽能電池的電極。利用石墨烯的優勢將其延伸於染料敏化太陽能電池，並探討二氧化鈦厚度與效率的關係探討 :第一，載子的傳輸;第二，利用光學檢測發現元件能短時間完成染料吸附;第三，載子的再結合隨著二氧化鈦厚度的增加限制轉換效率;第四，染料電荷補償的極限在二氧化鈦厚度10奈米。這項研究開啟超薄太陽能電池在透明與可撓性的應用，且發現其元件限制。
第二部分探討石墨烯應用於蕭特基太陽能電池。石墨烯具有高載子遷移率與雙極性的特性，使其有改善電子元件的潛力。在蕭特基元件中，石墨烯與半導體接面代表著簡單的異質結構，此方面的研究還未被深入了解且元件擁有較低的載子注入效率。本文探討石墨烯與金屬在蕭特基原件上的交互作用，發現石墨烯與白金的載子注入效率為石墨烯與黃金的3倍。由於能態密度的改變，造成瑞查生常數的變化，改善電流注入，這個現象使轉換效率提升13倍。

In this thesis, graphene were applied to two kinds of solar cells. In the first part of thesis, graphene were used as electrode for ultrathin dye solar cell while in the second part of this thesis, graphene is applied as the metal of metal/semiconductor Schottky-junction solar cell.
Ultrathin solar cells have the potential to form ubiquitous, cheap, and efficient sources of energy. Practical issues, however, prevent the formation of solar cells below 100 nm. Cracks, voids, and inhomogeneities result in leakage pathways that render ultrathin solar cells unusable. We here demonstrate the use of graphene as the top electrode for ultrathin solar cells with thicknesses between 5 nm and 100 nm. Graphene’s mechanical strength allows suspension over rough surfaces which enable ultrathin solar cell possible and its featureless absorption curve enables characterization of the solar cell’s constituents. These advantages were applied to study the effect of TiO2 thickness on solid state dye sensitized solar cells (DSSCs) performance in the limit of ultra-thin films: investigation of the carrier transport even for 10 nm thin films. Second, optical measurement of dye loading revealed a fast saturating behavior that was attributed to the quasi two-dimensional nature of the thin TiO2 film. Third, carrier recombination was found to be significantly enhanced in these thin films which limit the performance for increasing TiO2 thicknesses. Finally, charge compensation of the photoexcited dye poses limitations on the minimum thickness of the TiO2 which was found to be 10 nm. This work opens up a new route to produce ultrathin solar cells for transparent and flexible applications and also highlight the limits of their scaling.
In the second part of this thesis, Graphene/Si Schottky-junction solar was studied. Graphene’s high carrier mobility and ambipolar nature has the potential to improve electronic devices. The absence of a band-gap necessitates heterostructure devices. Schottky-barrier devices consisting of an interface between graphene and a semiconductor represent the simplest heterostructure. Despite its simplicity, graphene-based Schottky barrier devices are not well understood and exhibit low injection efficiencies. We here investigate the impact of graphene/metal interaction on the properties of the Schottky-barrier. Besides the commonly employed Au/graphene we use Pt/graphene contacts. We find that the injection efficiency for Pt is 3 times higher than for Au and systematically study the origin of this behavior. We identify a large difference in the Richardson’s constant due to changes in the density of surface states. The demonstrated ability to increase the injection current was applied to improve the efficiency of graphene-based Schottky solar cells by 13x.

中文摘要 2
Abstract 4
Table of contents 6
Chapter 1 Introduction 10
1.1 Motivation 10
1.2 Ultrathin graphene-based solar cells 12
1.3 Increasing the efficiency of graphene-based schottky barrier device 15
Chapter 2 Process and Equipment 17
2.1 Experimental process 17
2.2 Graphene electrode synthesis 17
2.2.1 Pretreatment of copper foil 17
2.2.2 Graphene growth 18
2.2.3 Transfer 20
2.3 Fabrication of solar cells 23
2.3.1 Ultrathin graphene-based solar cells 23
2.3.1.1 Fluorine-doped tin oxide (FTO) etching 23
2.3.1.2 Deposition of Titanium dioxide (TiO2) 24
2.3.1.3 Dye sensitizer adsorption 25
2.3.1.4 Final device 25
2.3.2 Graphene-based schottky solar cells 27
2.3.2.1 Insulator window 27
2.3.2.2 Graphene electrode transfer 27
2.3.2.3 Metal deposition 28
2.4 Experimental set-up 29
2.4.1 Chemical vapor deposition system 29
2.4.2 Electrochemical polishing system 29
2.4.3 E-beam evaporator 30
2.4.4 Sputter 31
2.4.5 Atomic force microscope 32
2.4.6 Electrical measurement system 34
2.4.7 Transmittance measurement 34
Chapter 3 Results and Discussions 37
3.1 Ultrathin graphene-based solar cells 37
3.1.1 Effect of the graphene top-electrode 37
3.1.2 Optical properties and sensitizer loading 38
3.1.3 Influence of TiO2 thickness 40
3.2 Graphene-based schottky barrier device 44
3.2.1 I-V characteristic 44
3.2.2 The difference of behavior between Au and Pt 46
3.2.3 Graphene-based Schottky barrier solar cells 50
Chapter 4 Conclusion and Outlook 52

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