隨著能源危機的到來和環保意識的抬頭，太陽能已引起廣泛的關注。然而，光伏發電元件的性價比相較於石化能源較低，以至於還無法被大量地使用。為提高性價比，發展低成本或高效率的太陽能裝置是必要的。此研究的目的是要結合矽奈米結構和高分子層，以製備具潛力的異質接面太陽能電池。
在此實驗中，將分為三個部分來討論矽奈米結構/聚3,4-乙烯基二氧噻吩-聚苯乙烯磺酸(PEDOT:PSS)異質接面太陽能電池。首先，利用金屬輔助化學蝕刻(MaCE)以形成不同長度的矽奈米線(SiNW)。長的奈米線有較佳的光吸收能力，但同時表面缺陷的數目也增加，大幅提高載子復合的機率，因此存在一個適當的長度去平衡光吸收與表面缺陷。以兩百奈米長的矽奈米線為基板的電池擁有較低的反射和較少的缺陷，因而有較佳的性能。
然而，PEDOT:PSS卻不能完全滲入如此長度的矽奈米線內，導致在矽奈米線的底部區域，仍有許多表面缺陷沒有被PEDOT:PSS所覆蓋。因此，在MaCE蝕刻之後，利用後KOH蝕刻(post-KOH dipping)來擴大矽奈米線的間距。使用不同長度的矽奈米線當作起始的基板，改變不同KOH蝕刻時間，我們發現若起始基板為兩百奈米和三百奈米長且KOH蝕刻後為150奈米長的矽奈米結構都有最佳的效率。接著，我們討論不同間距對150奈米長的奈米結構的影響。我們相信較佳效率來自PEDOT:PSS在矽表面有較佳的覆蓋率而大幅降低表面復合的機率，同時也因增加接觸面積而降低接觸電阻。
另一部分，則是討論電洞傳輸層PEDOT:PSS的性質。矽的載子遷移率大約在103 cm2V-1S-1數量級，遠高於PEDOT:PSS的載子遷移率。也就是，在矽和PEDOT:PSS的界面會因電子和電洞傳輸的不平衡，而產生一個載子複合的區域。因此，為了提高PEDOT:PSS中電洞的遷移率，二甲基亞碸(DMSO)和氧化石墨烯(GO)被當作二次摻雜物(secondary dopant)加入PEDOT:PSS中，而以0.2 wt% GO添加後的PEDOT:PSS有最佳的電性，且其電池亦有最佳的效率。
此矽奈米結構/PEDOT:PSS異質接面太陽能電池可以使用簡易且低溫的濕式蝕刻和溶液旋塗法來製作，因而降低了製造成本。在最佳化矽奈米結構和二次摻雜物的添加量後，可以達到高於13%的光電轉換效率，相信會是具潛力的光伏元件。

Recently, with further intensified energy crisis and consciousness of environmental protection, solar energy has caught much attention due to its non-polluting and highly abundant properties. However, an efficiency-to-cost ratio of photovoltaic techniques is still too low to substitute for fossil energy. Therefore, developing a cost-effective or high-efficient solar device has aroused many interests and become a must in the field of energy harvesting. Here, a potential candidate that combines a silicon substrate with a polymer layer as a heterojunction solar cell will be elaborated.
In this study, a Si nanostructure/poly(3,4-ethylenedioxythiophene): poly(stylenesulfonate) (PEDOT:PSS) heterojunction solar cell is proposed and examined. First, different lengths of Si nanowire (SiNW) structures formed by metal-assisted chemical etching (MaCE) were fabricated to discuss a trade-off between light absorption efficiency and amounts of e--h+ recombination centers at surface defects. A solar device with 200 nm SiNWs possesses relatively low reflectance and less trapping defects, resulting in the best performance among the designed lengths of the NWs.
Nonetheless, PEDOT:PSS cannot fully infiltrate into 200-nm-length SiNWs. Moreover, without passivation of PEDOT:PSS, there appear lots of surface defects at the bottom region of SiNWs. Hence, a step of post-KOH dipping is executed after the MaCE process to widen the spacing among SiNWs. Several lengths of SiNWs were used as starting substrates and devices with 150-nm-long nanostructures owned the best performance for both the starting substrates with the 200- and 300-nm-long SiNWs that might stem from suppressed surface recombination and also reduced contact resistance, benefiting from a better coverage of PEDOT:PSS on the surface of SiNWs.
Moreover, influences from PEDOT:PSS as a hole transport layer are also discussed. In principle, the mobility of PEDOT:PSS (~10-2 cm2V-1S-1 for pristine film) is much smaller than Si’s (~103 cm2V-1S-1); thus, there exists a recombination region at the interface between Si and PEDOT:PSS due to the unbalanced mobility between an electron and a hole. Hence, in order to escalate the mobility of PEDOT:PSS, secondary dopants such as dimethyl sulfoxide (DMSO) and graphene oxide (GO) were mixed into PEDOT:PSS and a solar device based on the modified PEDOT:PSS of 0.2 wt% GO addition provided the best mobility and thus the best efficiency.
Such Si nanostructure/PEDOT:PSS heterojunction solar cell could be simply fabricated via low temperature wet etching and spin-coating methods that can dramatically reduce fabrication cost. After optimizing the Si nanostructures and the amount of the secondary dopants, power conversion efficiency above 13% can be achieved and is believed to be ready for applications of energy harvesting.

摘要 I
Abstract III
Acknowledgements V
Content VI
List of Figures VIII
List of Tables XIV
Chapter 1 Introduction 1
Chapter 2 Literature Review 3
2.1 Light-absorbing layer based on n-type Si nanostructures 3
2.1.1 Radial junction structures 3
2.1.2 Comparison of Si nanowire (SiNW) and Si nanocone (SiNC) 4
2.2 PEDOT:PSS hole transport layer 10
2.2.1 Chemical structures of PEDOT:PSS 10
2.2.2 Mechanisms for increasing conductivity 11
2.2.2.1 Resonance structure change 11
2.2.2.2 Conformation change 13
2.2.2.3 Screening effect 14
2.2.2.4 Morphology change 15
2.2.3 Methods for increasing conductivity 16
2.2.3.1 Solvent Additive Method 16
2.2.3.2 Polar Solvent Post-Treatment Method 18
2.2.3.3 Polar-Solvent Vapor Annealing Method (PSVA) 19
2.3 Si nanostructure/PEDOT:PSS heterojunction solar cell 20
2.3.1 Combination of a Si nanostructure and a suitable organic layer 21
2.3.2 Ways to improve performance of the heterojunction solar cell 24
2.3.2.1 An organic solvent treatment for a high hole transport rate 25
2.3.2.2 Structure modification for a larger contact area 27
2.3.2.3 A passivation layer for deactivation of Si/PEDOT:PSS interfaces 31
2.3.2.4 Back surface modification for recombination suppression 34
2.4 Motivation 38
Chapter 3 Design of Experiment 39
3.1 Fabricate nanowires on a Si substrate 39
3.2 PEDOT:PSS with secondary dopant additive 40
3.3 Fabrication of a Si nanostructure/PEDO:PSS heterojunction solar cell 41
3.4 Analysis measurement 42
3.4.1 Scanning Electron Microscope (SEM) 42
3.4.2 Optical properties measurement 43
3.4.3 Atomic Force Microscope (AFM) 44
3.4.4 Four-point probe measurement 45
3.4.5 Photovoltaic properties measurement 45
Chapter 4 Results and discussions 46
4.1 Characteristics of Si nanostructures 46
4.1.1 Different lengths of Si nanowires (SiNWs) 46
4.1.2 Tapered 200- and 300-nm-long SiNWs 48
4.1.3 Different spacing of 150-nm-long tapered SiNWs 50
4.2 Characteristics of graphene oxide (GO) and GO-modified PEDOT:PSS films 52
4.3 Solar cell performance measurement 58
4.3.1 Device based on different lengths of SiNW structures 59
4.3.2 Device based on tapered 200- and 300-nm-long SiNWs 61
4.3.3 Device based on different spacing of 150-nm-long tapered SiNWs 63
4.3.4 Device based on GO-modified PEDOT:PSS films 65
Chapter 5 Conclusions 68
References 70

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