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ABSTRACT Squaraine Donor Based Organic Solar Cells By Guodan Wei Chair: Stephen R. Forrest There are three main ongoing avenues to improve the power conversion efficiency of organic photovoltaics (OPV): the development of new organic materials, improved process control and novel device architecture design. In this thesis, through molecular design with chemical modification of functional organic molecules, a family of new highly absorptive solution processable squaraine (SQ) materials have been systematically synthesized and explored to improve the sunlight harvesting and charge transport. The spin-cast SQ donors are then coated with fullerene acceptors to form a unique nanocrystalline heterojunction (NcHJ) OPV device. This combination of a novel and efficient family of SQ donors, a unique NcHJ device architecture and optimized fabrication processes leads to high efficiency solar cells. For example, solar cells with efficiencies of ~5.7 % and a fill factor ~0.74 are achieved. We find a correlation between solar cell fill factor with the SQ thin film density, providing support for the molecular design concept that planar end groups result in close intermolecular stacking, and hence improved charge transport and exciton diffusion. Finally, thermal annealing of the films results in the formation of nanocrystalline morphologies that lead to further improvements in device performance. The microcrystal growth of SQ donors have been characterized by XRD, AFM and TEM. xix
Chapter 1 Introduction to solution processed organic solar cells Organic photovoltaics (OPV) have been considered as a potential low-cost solar energy conversion solution for an affordable and sustainable energy future. They have attracted tremendous academic interest in recent years, which stems from the potential for cost effectiveness, mechanical flexibility and easy processing [1]. As the demand for cost-competitive renewable energy sources grows, considerable advances are being placed on the synthesis [2], processing [3], theory [4] and application of new organic semiconductor materials [5]. Up to now, there has a significant improvement in device efficiency for organic solar cells with the introduction of novel organic materials [6] that tune molecular electronic properties and new device architectures [7]. This thesis is focused on solution processed small molecule squaraine donors, device processing and the relationship between molecular structure and device properties. Several efficiency improvement strategies are demonstrated to control crystallization of donor materials through thermal and solvent annealing processes in different device architectures. This chapter aims to summarize the development and characteristics of organic solar cells and highlight recent research progress in the area of solution-processed small molecular solar cells. First, pertinent definitions and device operationof organic solar cells are presented. Characteristics of organic solar cells follow, along with the difference between a planar and a bulk heterojunction (HJ). Then, recent efforts to improve solution-processed small molecular organic solar cells are highlighted. 1
Page 1 and 2: SQUARAINE DONOR BASED ORGANIC SOLAR
Page 3 and 4: DEDICATION To my two sons Anderson
Page 5 and 6: characterization of crystallized sq
Page 7 and 8: Chapter 3 Efficient and ordered squ
Page 9 and 10: List of Publications, Conference Pr
Page 11 and 12: Figure 1.10: Molecular structure of
Page 13 and 14: shown in (d), which correspond to t
Page 15 and 16: Figure 6.3: Absorption spectra of A
Page 17 and 18: Figure 7.7: Schematic diagram on st
Page 19: List of Appendices Appendix 1 .....
Page 23 and 24: The optical-to-electrical conversio
Page 25 and 26: photogenerated excitons cannot reac
Page 27 and 28: Because the L D is typically limite
Page 29 and 30: single D/A planar heterojunction wa
Page 31 and 32: and aids dissociation. Thus, k PPd
Page 33 and 34: 1.4 Recent research progress of sol
Page 35 and 36: etween these conjugated small molec
Page 37 and 38: in the long-wavelength region, good
Page 39 and 40: (squaraine 1) or n-hexenyl (squarai
Page 41 and 42: and hole mobility. In Chapter 3, we
Page 43 and 44: References [1] L. M. Chen, Z. R. Ho
Page 45 and 46: [25] M. S. Kim, B. G. Kim, and J. K
Page 47 and 48: Chapter 2 Solution processed squara
Page 49 and 50: field. L d must be longer than the
Page 51 and 52: Figure 2.1:(a) X-ray-diffraction (X
Page 53 and 54: fraction of PC 70 BM increases, the
Page 55 and 56: pure, nanocrystalline SQ film of a
Page 57 and 58: Table 2.1: Summary of solar cell ch
Page 59 and 60: Figure 2.6: Current density-Voltage
Page 61 and 62: The highest efficiencies (Fig. 2.8)
Page 63 and 64: Here, J s is the reverse saturation
Page 65 and 66: PC 70 BM and SQ across the waveleng
Page 67 and 68: lended films have a more uniform an
Page 69 and 70: Tantiwiwat, and T. Q. Nguyen, Adv.
Page 71 and 72:
Chapter 3 Efficient and ordered squ
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determined using a variable angle a
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dichloromethane (DCM) solvent on in
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local organization of SQ thin films
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Figure 3.4: (a) External quantum ef
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the range of power intensities. In
Page 83 and 84:
interdigitated BHJ characteristics
Page 85 and 86:
References [1] D. Bagnis, L. Beveri
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Chapter 4 Solvent annealed nanocrys
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Figure 4.1:The XRD peaks for squara
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Figure 4.2: The effect of as-cast a
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TEM image shown in Fig. 4.3c is har
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Figure 4.5:The power conversion eff
Page 97 and 98:
References [1] P. Peumans, S. Uchid
Page 99 and 100:
1814(2005). [22] V. Shrotriya, G. L
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exciton dissociation at these inter
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Figure 5.1:(a) the squaraine/C 60 d
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A significant increase in V oc is o
Page 107 and 108:
Figure 5.3: External quantum effici
Page 109 and 110:
Figure 5.4: 3D atomic force microsc
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nanocrystalline growth as inferred
Page 113 and 114:
References [1] T. Rousseau, A. Crav
Page 115 and 116:
Chapter 6 Functionalized Squaraine
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6.2 Experiment of six squaraine/C 6
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6.3 Physical properties of six func
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Absorption coefficient (cm -1 ) 6.3
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π-stacked molecules (DPSQ) would b
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Figure 6.5: (a) Measured absorption
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Figure 6.6: Perspective atomic forc
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Current density (mA/cm 2 ) 20 15 On
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The FF versus incident power intens
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show defined spots, their intensity
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Current density (A/cm 2 ) phenyl gr
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Table 6.3: Performance of annealed
Page 139 and 140:
References [1] C. V. Hoven, X. D. D
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Chapter 7 Conclusions and outlooks
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Figure 7.1: (a) Comparison of Curre
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size. This precise structural contr
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Figure 7.2: Absorption spectrum of
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Figure 7.3: Fill factor versus powe
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Figure 7.5: TEM image and selected
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7.2.4 Two squaraine donor blending
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7.2.5 Solvent annealing of function
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References [1] F. Yang, K. Sun, and
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second photon (~ 10 3 s -1 ), is ty
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e( x) Ec( x) n( x) Nc( x)exp kT
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Since the reverse saturation curren
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Here, is the quasi-Fermi-level spl
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then once again increases rapidly.
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Photovoltaic Energy Conversion, 266
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L&M model, the limiting efficiency
Page 173 and 174:
Appendix 3 List of Publications, Co
Page 175 and 176:
1. Guodan Wei, Xin Xiao, Siyi Wang,
Page 177:
circuit voltage using electron/hole
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