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(Fig.1.2(a)), the junction between the donor and acceptor materials is flat and the organic films are sandwiched between two metal electrodes with different work functions. Typically, these organic layers are vacuum deposited and their crystalline order is maintained during the growth process. In addition, their layer structures can be precisely controlled during the growth process. In this case, the photogenerated electrons and holes are spatially located in the acceptor and donor, respectively, making the recombination between these charges unlikely. Transport of photogenerated charges toward their respective electrodes is driven by the built-in electric field as well as by the concentration gradient at the heterojunction. ! "#$#%&' ( )( %*+, $-. *$& / , "0 &' ( )( %*+, $-. *$& Figure 1.2: Schematic illustration of two organic solar cell architectures: (a) planar and (b) bulk heterojunction The exciton diffusion length, L D , in an organic material is limited due to weak intermolecular interactions. Typically, L D is between 20 Å and 200 Å [10], which is considerably shorter than the optical absorption length (~1000 Å). This situation forms an exciton diffusion bottleneck in a planar HJ solar cell, whereby most of the 4
photogenerated excitons cannot reach a D/A interface prior to dissociation into free carriers, ultimately limiting device performance. In addition to the exciton diffusion limitation, planar PV cells suffer from two other problems that significantly limit their device performance. First, the deposition of the cathode metal onto the acceptor layer introduces damage, thereby reducing the exciton diffusion in that layer. Second, the incident optical electric field intensity is less than its maximum value at the HJ interface where photogenerated charge transfer is most efficient [8]. To solve these problems, an exciton blocking layer (EBL) is inserted between the acceptor layer and the metal cathode [11]. It is composed of wide-band gap material that effectively prevents the photogenerated excitons from diffusing to, and quenching at the acceptor/cathode interface. Meanwhile, this EBL material is transparent across the solar spectrum and acts as an optical spacer between the photoactive region and the metal interface. It should be thick enough to place the region of the highest incident optical intensity at the D/A interface, which is located at a distance of approximately an integer multiple of quarter wavelengths (λ/4n, where n is the index of refraction of the organic material) from the metal cathode. Thus, the optical intensity can be maximized at the D/A interface [9]. Furthermore, the EBL also functions as a buffer layer, preventing damage to the photoactive layer during metal deposition. Typically, the EBL is about 10 nm thick to avoid the introduction of resistance to the cell and provide efficient electron transport to the cathode. The most commonly used EBLs are wide energy gap (and hence transparent) materials, such as bathocuproine 5
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 and 20: List of Appendices Appendix 1 .....
Page 21 and 22: Chapter 1 Introduction to solution
Page 23: The optical-to-electrical conversio
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
Page 73 and 74: determined using a variable angle a
Page 75 and 76:
dichloromethane (DCM) solvent on in
Page 77 and 78:
local organization of SQ thin films
Page 79 and 80:
Figure 3.4: (a) External quantum ef
Page 81 and 82:
the range of power intensities. In
Page 83 and 84:
interdigitated BHJ characteristics
Page 85 and 86:
References [1] D. Bagnis, L. Beveri
Page 87 and 88:
Chapter 4 Solvent annealed nanocrys
Page 89 and 90:
Figure 4.1:The XRD peaks for squara
Page 91 and 92:
Figure 4.2: The effect of as-cast a
Page 93 and 94:
TEM image shown in Fig. 4.3c is har
Page 95 and 96:
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
Page 101 and 102:
exciton dissociation at these inter
Page 103 and 104:
Figure 5.1:(a) the squaraine/C 60 d
Page 105 and 106:
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
Page 111 and 112:
nanocrystalline growth as inferred
Page 113 and 114:
References [1] T. Rousseau, A. Crav
Page 115 and 116:
Chapter 6 Functionalized Squaraine
Page 117 and 118:
6.2 Experiment of six squaraine/C 6
Page 119 and 120:
6.3 Physical properties of six func
Page 121 and 122:
Absorption coefficient (cm -1 ) 6.3
Page 123 and 124:
π-stacked molecules (DPSQ) would b
Page 125 and 126:
Figure 6.5: (a) Measured absorption
Page 127 and 128:
Figure 6.6: Perspective atomic forc
Page 129 and 130:
Current density (mA/cm 2 ) 20 15 On
Page 131 and 132:
The FF versus incident power intens
Page 133 and 134:
show defined spots, their intensity
Page 135 and 136:
Current density (A/cm 2 ) phenyl gr
Page 137 and 138:
Table 6.3: Performance of annealed
Page 139 and 140:
References [1] C. V. Hoven, X. D. D
Page 141 and 142:
Chapter 7 Conclusions and outlooks
Page 143 and 144:
Figure 7.1: (a) Comparison of Curre
Page 145 and 146:
size. This precise structural contr
Page 147 and 148:
Figure 7.2: Absorption spectrum of
Page 149 and 150:
Figure 7.3: Fill factor versus powe
Page 151 and 152:
Figure 7.5: TEM image and selected
Page 153 and 154:
7.2.4 Two squaraine donor blending
Page 155 and 156:
7.2.5 Solvent annealing of function
Page 157 and 158:
References [1] F. Yang, K. Sun, and
Page 159 and 160:
second photon (~ 10 3 s -1 ), is ty
Page 161 and 162:
e( x) Ec( x) n( x) Nc( x)exp kT
Page 163 and 164:
Since the reverse saturation curren
Page 165 and 166:
Here, is the quasi-Fermi-level spl
Page 167 and 168:
then once again increases rapidly.
Page 169 and 170:
Photovoltaic Energy Conversion, 266
Page 171 and 172:
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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