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characteristics. Hence, the power conversion efficiency can be expressed as, P J V FF m sc oc p P 0 P (1.6) 0 Figure 1.5: (a) increasing series and (b) reducing parallel resistance. In each case the effect of the resistances is to reduce the area of the maximum power rectangle compared to J sc × V oc [25]. The power generated by a solar cell is dissipated through the resistance of the contacts and through leakage currents around the sides of the device. These effects are electrically equivalent to two resistances: R s and R p . The series resistance, R s , is a function of cell material, particularly through the front surface to the contacts, and from resistive contacts. R s is a particular problem for bulk HJ solar cells, especially under high power intensity illuminations. The parallel resistance, R p , arises from leakage of current and recombination current through the junction. Both the R s and R p reduce the fill factor as shown in Fig. 1.5. For an efficient solar cell, R s should be keptsmall and the R p maximized.[25]. 12
1.4 Recent research progress of solution processed bulk HJ solar cells 1.4.1 Recent research progress of polymer bulk HJ solar cells The design of a bulk HJ structure allows for extraction of charges from a much larger volume of active material, increasing both light absorption and photocurrent generation [17, 26]. Recent efforts focus on controlling donor/acceptor phase separation [27], increasing and balancing charge-carrier mobilities [28], and searching for new materials that maximize absorption of the solar spectrum [16]. Research on polymer:fullerene blend solar cells started in 1995 with an accepted power conversion efficiency of 2% [26]. Solution processed mixtures of polyphenylene vinylenes (PPV) and [6,6]-phenyl-C 61 -butyric acid methylester (PCBM) demonstrated a charge transfer rate at the PPV and PCBM interface on a picosecond time scale, leading to high efficiency. With appropriate organic solvents and spin-cast conditions, wellcontrolled phase separation improved the polymer bulk HJ cell efficiencies to 2.5% [19], indicating the intimate correlation between nanomorphology of active layers and device performance. However, a bottleneck in the PPV-related bulk cells is that the lower hole mobility of PPV phase in the blend films, compared with the electron mobility of PCBM, leads to space-charge-limited photocurrent and carrier recombination. Regioregular poly(3-hexylthiophene) (P3HT), a polymer with higher hole mobility, was introduced and started to replace PPV, achieving an efficiency surpassing 5% with optimized phase separation [29]. The main disadvantage of P3HT is the poor matching of its photon absorbance with the solar illumination spectrum. The bandgap of P3HT is around 1.9 eV, 13
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 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 aids dissociation. Thus, k PPd
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
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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
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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
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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
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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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