improving efficiency of organic photovoltaics 11/08/12 nanjia zhou northwestern university
TRANSCRIPT
IMPROVING EFFICIENCY OF ORGANIC PHOTOVOLTAICS
11/08/12
Nanjia ZhouNorthwestern University
http://econews.com.au/news-to-sustain-our-world/un-takes-hard-look-at-20-years-since-rio-earth-summit/http://planetark.org/enviro-news/item/53111http://thecityfix.com/blog/choking-on-smog/
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International Energy Outlook 2011Nanjia ZhouChang/Marks Group
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International Energy Outlook 201103/23/2012
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Department of Materials Science03/23/2012
Photo courtesy: NREL
Clean, renewable
Reduced dependence on Fossil Fuels
Distributed generation
Proven, reliable
Modularity and Scalability
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http://www.heliatek.com/technologie/organische-photovoltaik/?lang=en
Photo courtesy: NREL
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National Renewable Energy Laboratory (NREL)
Photo courtesy: NREL
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Photo courtesy: NREL
Mitsubishi Chemical announced a certified 10.0% cell
National Renewable Energy Laboratory (NREL) Nanjia ZhouChang/Marks Group
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Photo courtesy: NREL
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Department of Materials Science03/23/2012
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http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2000/press.html
Photo courtesy: NREL
Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa "for the discovery and development of conductive polymers".
The Nobel Prize in Chemistry 2000
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Common conducting polymers
Soliton
Polaron
Bipolaron
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Gate
Insulating layer
Organic layer
Source Drain
+ ++ +++ + + ++ + + + + + + + + +
-30V
- - - - - - - - - - - - - - - - - - -
++++++++++++++
1. Organic Field Effect Transistor (OFET)
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1. OFET
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Photo courtesy: NREL
1. OFET
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2. Organic Light Emitting Diode (OLED)07 08 09 10
2.0, 2.2, 2.4, 2.6’’ QVGA2.5’’ QVGA(Landscape)2.6, 2.8, 3.0’’ LQVGA3.1’’ WVGA
4.1’’WQVGA
3.5’’ WQVGA*3D4.8’’ WVGA7.0’’ WSVGA
14-15.4’’WXGA
21-23’’UXGA 40’’/42’’ Full HD
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2. OLED
1. Brighter, thinner, lighter, faster 2. Bright from all viewing angles 3. Need less power to run 4. A lot cheaper to produce
5. Expanding memory capability - coating new layer on top of existing one
6. Wider temperature range
OLED is a display device that sandwiches carbon based films between the two electrodes and when voltage is applied creates light.
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2. OLED
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3. OPV
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3. OPV
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Holger Spanggaard, Frederik C. Krebs, Solar Energy Materials and Solar Cells, Volume 83, Issues 2–3, 15 June 2004, Pages 125-146
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Harvard Science in the News: https://sitn.hms.harvard.edu/sitnflash_wp/2012/03/issue113/
Photo courtesy: NREL
OPV vs. Silicon Cell
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Narrower absorption range than silicon cellLower current output
Donor: Electron rich materialAcceptor: Electron poor material
Bulk Heterojunction: Blend of D/A materials forming a heterogeneous mixture
Domain size
OPV vs. Silicon Cell
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Li et al. Nature Photonics 6, 153–161, (2012)
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Kietzke, Advances in OptoElectronicsVolume 2007 (2007), Article ID 40285
a. Short circuitb. Open circuitc. Forward biasd. Reverse bias
Characterization
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• Class A Solar Cell Analyzer– AM 1.5G simulated light– Scan area: 5 x 5 cm– Light and dark evaluation– Series resistance evaluation– Xenon lamp with an adjustable
intensity of 850 - 1150 W/m2
– Calibrated with KG3-filtered Si standard from NREL. Spectral mismatch = 1.00
• Spectral Response– Examines wavelengths from 400 - 1100
nm– Includes 15 filters in this range
• Circulating Bath– Cell test temperature variable from -
15C to +75C
NU Characterization
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ORGANIC PHOTOVOLTAICSCathode – Conventional cellLow work function metal: Ca/Al(Anode – Inverted CellHigh work function metal: Ag/Au)Cathode IFL – LiF, n-type semiconductors: ZnO, TiO2Active Layer: blend of donor and acceptor materials
Transparent conductive substrate – ITO, Carbon based material
Anode IFL: PEDOT:PSSP-type semiconductor: NiO, WO3, MoO3, V2O5
graphene oxide
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Nanjia ZhouChang/Marks Group
ORGANIC PHOTOVOLTAICSμED μCT
μCC
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Nanjia ZhouChang/Marks Group
ORGANIC PHOTOVOLTAICS
Band Gap Tuning:• Lower band gap to raise Jsc.• Higher IP polymers and lower EA
acceptors to reduce energy loss during charge separation.
STRATEGIES FOR EFFICIENCY ENHANCEMENT
Decrease LUMO-LUMO offset
Morphology Control:• Domain sizes comparable to exciton diffusion
length• Efficient carrier transport• Experimentally: solvent, additive, processing
conditions, and self organizing properties of materials
Photon Management:• Alter device architecture to focus photo
absorption in active layer• Potentially reduce active layer thickness for
higher IQE
Ordered Heterojunction:• Donor/acceptor form interdigitated
morphology within nanometer scale• Direct pathways of donor/acceptor to
electrodes
Nelson, J. Materials Today 2011, 14, 462–470.Li, G.; Zhu, R.; Yang, Y. 2012, 1–9.Kang, M.-G.; Xu, T.; Park, H. J.; Luo, X.; Guo, L. J. Adv. Mater. 2010, 22, 4378.
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RESEARCH OUTLINE
Novel high efficiency donor-type polymers, study of device characteristics
Materials Development
Nanoscale patterning of polymer active layer surface.Photonic and plasmonic enhancement of photocurrent
Plasmonic Structure
Tandem device to enhance light harvesting;Optimize electron/hole transport
Nanowire, Tandem, Hybrid, etc.
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Band Gap Tuning
Photon Management
Novel Device Architecture
Processing conditions;Solvent choice;Polymer stacking;Characterization
Morphology-Carrier Transport Relationship
Morphology Control
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Control of nanoscale morphology: Domain size variation
Morphology Carrier Transport Relationship
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BILAYER VS. BHJ28
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EFFECT OF PROCESSING CONDITIONS29
Annealing
Treat, N. D., Brady, M. A., Smith, G., Toney, M. F., Kramer, E. J., Hawker, C. J. and Chabinyc, M. L. (2011), Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater., 1: 82–89. doi: 10.1002/aenm.201000023
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EFFECT OF PROCESSING CONDITIONS30
Solvent choice
Lee et al. J. Am. Chem. Soc. 130, 3619 (2008)
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CRYSTALLINITY AND PACKING31
Huang et al. J. Phys. Chem. C, 2012, 116 (18), pp 10238–10244
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CHARACTERIZATION32
Lu et al. Nature Communications 3, Article number: 795
Atomic Force Microscope (AFM),Transmission Electron Microscope (TEM)
Grazing Incidence Wide Angle X-ray Scattering (GIWAXS)
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Increase DIO concentration
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PC71BM BTIBDT
High sensitivity of DIO
0 0.5% 1.0% 3.0% 10.0%
N. Zhou, A. Guerrero, H. Heitzer, S. Lou et. al.
PRECISE MANIPULATION OF DOMAIN SIZES
S
N
S
O OR1
S
S
OR2
OR2
n
High Performance Donor Materials
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Li et al. Nature Photonics 6, 153–161, (2012)
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Zhou et al. Adv. Mater., 24: 2242–2248
1) R1=2-hexyldecyl, R2=2-ethylhexyl; PCE=3.4%2) R1=2-butyloctyl, R2=2-ethylhexyl; PCE=4.8%3) R1=2-hexyldecyl, R2=n-dodecyl; PCE=5.5%4) R1=2-butyloctyl, R2=n-decyl; PCE=3.3%5) R1=n-octyl, R2=2-butyloctyl; PCE=1.6%
BTIBDT BASED COPOLYMERS35
BTIBDT BASED COPOLYMERSBithiophene Imide (BTI) and Benzodithiophene (BDT) Copolymers Inverted OPVs
S
S
OR2
OR2
Sn Sn
S
N
S
O OR1
Br BrS
N
S
O OR1
S
SOR2
OR2
+
n
P1: R1 = 2-butyloctyl, R2 = 2-ethylhexylP2: R1 = 2-hexyldodecyl, R2 = n-dodecyl
i
The electron deficiency of the BTI units leads to polymers with a low-lying HOMOs (~-5.6 eV). Inverted solar cells are fabricated to investigate the OPV performance of the BTI-based polymers and achieve power conversion efficiencies up to 5.5%, with substantial Vocs above 0.9 V which are among the highest Vocs reported to date for polymer/PCBM solar cells.
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Zhou et al. Adv. Mater., 24: 2242–2248
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Bithiophene Imide (BTI) and Benzodithiophene (BDT) Copolymers Inverted OPVs
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BTIBDT BASED COPOLYMERS• Inverted architectures:
ITO/ZnO/polymer:PC71BM/MoOx/Ag
• Inverted structures avoid oxidation of low work-function cathodes, such as Al or Ca, and acidic PEDOT:PSS etching of ITO, both of which limit conventional devices.
• ZnO is an excellent cathode interfacial layer due to its high electron mobility, excellent thermal stability, and hole-blocking properties.
• MoOx conduction band (-2.60 eV) lies sufficiently above the donor LUMOs (-3.7 eV, estimated from the HOMOs and optical band gaps), to block electrons, while the low work function (-5.6 eV) forms a good anode contact with the donor polymers
Zhou et al. Adv. Mater., 24: 2242–2248
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Polymer:PC71BM Solvent Thickness (nm) Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
P1 1:1.5 DCB 95 0.992 4.16 47.9 1.98
P2 1:1 DCB 100 0.940 4.76 56.3 2.52
P1 1:1.5 DCB+DIOa 95 0.876 9.69 51.8 4.39
P2 1:1 DCB+DIOa 100 0.922 9.62 62.0 5.50
J-V Curve EQE and UV-vis
Stability
Device Performance
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BTIBDT BASED COPOLYMERS
Stability
• Best device achieved without thermal annealing
• Initial drop due to microstructural reorganization
Zhou et al. Adv. Mater., 24: 2242–2248
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BTIBDT BASED COPOLYMERSConventional Device and SCLC measurement
Polymer:PC71BM Voc [V] Jsc [mA/cm2] FF (%) PCE (%)
P1 1:1.5 0.872 7.87 51.1 3.51
P2 1:1 0.905 8.06 55.1 4.02
Space charged limited current (SCLC) measurement
ITO/PEDOT:PSS/polymer/Au
Hole mobilities OTFT: 1.2 × 10-4 and 2.8 × 10-4 cm2V-1s-1 SCLC: 5.5 × 10-5 and 1.9 × 10-4 cm2V-1s-1
for P1 and P2-based OTFTs and hole only devices, respectively
Conventional Device Performance
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• Without DIO, predominantly phase-segregated morphologies for the P1/PC71BM and P2/PC71BM blends.
• The dark regions in the TEM images confirm large PC71BM domain sizes -- far larger than typical exciton diffusion lengths (~10 nm). Consequently, poor exciton dissociation and low current density are expected.
• Significantly more homogeneous morphologies are found for P1/PC71BM and P2/PC71BM films processed with DIO.
• nanosacle phase separation and bicontinuous interpenetrating networks result in more efficient charge separation and transport, leading to more than doubled Jsc.
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BTIBDT BASED COPOLYMERSMorphology Optimization
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Zhou et al. Adv. Mater., 24: 2242–2248
BTI and Dithienosilole (DTS) Copolymers Inverted OPVs
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BTIDTS BASED COPOLYMERS
S
N
S
O O
Br Br
+
P1: R = 2-ethylhexyl, R' = 2-ethylhexylP2: R = n-dodecyl, R' = 2-ethylhexyl
i
S SMe3Sn SnMe3
Si
S
N
S
O O
SS
SiR'R'
R' R'R
R
BTIR-Br2 DTSR'-Sn2
n
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Polymer DCB:DIO
(%:%)
Voc
(V)
Jsc
(mA/cm2)
FF
(%)
PCE (%)
P1 100:0 0.838 6.06 52.6 2.67
P1 98:2 0.834 12.51 53.1 5.54
P2 100:0 0.847 6.46 48.4 2.65
P2 98:2 0.803 12.81 62.3 6.41
X. Guo, N. Zhou, et al. JACS (2012)
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BTI BASED COPOLYMERS
X. Guo, N. Zhou, et al. JACS (2012)
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BTI BASED COPOLYMERS
X. Guo, N. Zhou, et al. JACS (2012)
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BTI BASED COPOLYMERS
X. Guo, N. Zhou, et al. JACS (2012)
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BTIDTG COPOLYMERS
Polymer % of DIO additives Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
PBTISi-C6 2.0 0.824 12.59 60.6 6.29PBTISi-C8 2.0 0.803 12.81 62.3 6.41PBTISi-EH 2.0 0.791 12.49 56.4 5.57PBTIGe-C6 3.0 0.770 11.76 47.2 4.27
X. Guo, N. Zhou, et al. JACS (2012)
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BTI/TPD-DTS COPOLYMERS
X. Guo, N. Zhou, et al. JACS (2012)
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S
N
S
O O
SS
SiEHHE
S
N
S
O O
SS
SiEHHE
neat polymerpolymer/PC71BMDCB:DIO=100:0
polymer/PC71BMDCB:DIO=98:2
BTIDTS BASED COPOLYMERSBTI and Dithienosilole (DTS) Copolymers Inverted OPVs
P1
P2π-π stacking distances are 3.6 Å and 3.5 Å for P1 and P2, respectively
X. Guo, N. Zhou, et al. JACS (2012)
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BTIDTS BASED COPOLYMERS
X. Guo, N. Zhou, et al. JACS (2012)
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BTIDTS MORPHOLOGY
TEM images of PBTISi-C8 with different D/A ratio: (a) P:PC71BM (1:1), (b) P:PC71BM (1:1.5), and (c) P:PC71BM (1:2) blend films processed in DCB as solvent with 2% DIO.
X. Guo, N. Zhou, et al. JACS (2012)
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BTIDTG COPOLYMERS
X. Guo, N. Zhou, et al. JACS (2012)
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HiGH FF OPV AND MORPHOLOGY STUDY
X. Guo, N. Zhou, et al. Nature Photonics (2012)
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HiGH FF OPV AND MORPHOLOGY STUDY
X. Guo, N. Zhou, et al. Nature Photonics (2012)
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HiGH FF OPV AND MORPHOLOGY STUDY
X. Guo, N. Zhou, et al. Nature Photonics (2012)
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HiGH FF OPV AND MORPHOLOGY STUDY
X. Guo, N. Zhou, et al. Nature Photonics (2012)
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Nanoscale patterning of polymer active layer surface.Photonic and plasmonic enhancement of photocurrent
Photon Management
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PLASMONIC OPV
Enhancement atFront electrode
M. Heo, H. Cho, J. Jung, J. Jeong, S. Park, J. Kim, Adv. Mater. 2011, 23, 5689M. Kang , T. Xu , H. Park , X. Luo , L.J. GuoAdv. Mater. 2010, 22, 4378J. Yang, J. You, C. Chen, W. Hsu, H. Tan, X. Zhang, Z. Hong, Y. Yang ACS Nano 2011 5 (8), 6210
Device Fabrication
Ag nanowire grating as electrode
blending Au nanoparticle into interfacial layer
metallic nanoparticles at front electrode
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PLASMONIC OPVDevice Fabrication
• Lower transmission at TCO
• Reducing conductivity of TCO
• Disrupting organic/inorganic interface at TCO
• Affecting film quality and possibly polymer packing
• Introducing extra step that cannot be easily incorporated into fabrication, thus affect reproducibility.
Modifying Front Electrode
Modifying Rear Electrode
• No effect on device fabrication prior to metal evaporation
• Easily convert the metallic electrode into a plasmonic grating.
• The grating can increase the amount of light trapped in the active layer through three potential pathways.
• Trap the light in the form of surface plasmon polaritons (SPP).
• Increase the surface area of the interface.
• Increase the diffraction of light.
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PLASMONIC OPV
Collaboration with S. Lubin (Odom group), A. Hryn (Odom group)
Device Fabrication
N. Zhou, S. Lubin, A. Hryn, X. Guo et. al. Manuscript in preparation
ITOZnOActive LayerMoOx
Ag
ITO ZnO solution processing
Active layer Hot embossing
Thermal evaporation of rear electrode
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Nanjia ZhouChang/Marks Group
PLASMONIC OPV
Collaboration with S. Lubin (Odom group), A. Hryn (Odom group)
Device Voc [V] Jsc [mA/cm2] FF (%) PCE (%)
Plasmonic 0.580 11.5 64.0 4.28
No plasmonic 0.582 9.38 68.5 3.74
Device Performance
N. Zhou, S. Lubin, A. Hryn, X. Guo et. al. Manuscript in preparation
Plasmonic 0.565 12.8 56.2 4.07
Sq 600 nm
Sq 400 nm
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Tandem device to enhance light harvesting;Optimize electron/hole transport
Novel Device Architectures
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Yang Yang et al. Nature Photonics, 6, 2012, 180
TANDEM SOLAR CELLS
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TANDEM SOLAR CELLS• The maximum efficiency for a two junction tandem under the
AM1.5G spectrum and without concentration is 47 %. At the peak efficiency the top cell has a bandgap of 1.63 eV and the bottom cell has a bandgap of 0.96 eV.
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PCE greater than 40% is the highest efficiency obtained to date for triple junction inorganic photovoltaic cells.
TANDEM SOLAR CELLS
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P3HT PSBTBT
TANDEM ORGANICS SOLAR CELLS
Jin Young Kim, Kwanghee Lee, Nelson E. Coates, Daniel Moses, Thuc-Quyen Nguyen, Mark Dante, and Alan J. HeegerScience 13 July 2007: 317 (5835), 222-225.
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• Dresden, Germany-based OPV thin-film developer Heliatek has achieved a new organic tandem solar cell world record with 10.7% cell efficiency on a 1.1cm2 substrate.
• “Deposition of small organic molecules in a low temperature, roll-to-roll vacuum process”
TANDEM ORGANICS SOLAR CELLS
• Yang Yang group, UCLA• reaches 10.6% efficiency a world’s first for
polymer organic photovoltaic devices• “Their tandem structure consists of a front
cell with a larger (or high) band gap material and a rear cell with a smaller (or low) band gap polymer, connected by a designed interlayer.”
• Sumitomo's low–band gap polymer
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INTERLAYERMetal ohmic contacts with sub-cells
Ag, Au: Yakimov et al. (2002)
Colsmann et al. (2006) used n-doped and p-doped organic layers on either side of a thin Au layer as hole blocking and electron blocking layer, respectively.
first time reaches nearly additive Voc =1 V
p-type layer: high work function metal oxides such as molybdenum oxide (MoO3) or tungsten oxide (WO3), PEDOT:PSS
n-type layer: solution-processed ZnO and TiO2
A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 2002, 80, 1667.A. Colsmann, J. Junge, C. Kayser and U. Lemmer, Appl. Phys. Lett., 2006, 89, 203506.
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PROCESSING• Sustain the solution process of the top polymer layer and protect the bottom
polymer layer• deposited ITO or thermally evaporated semitransparent metal layers
such as Au• An n-type layer using ZnO or TiO2 is coated from an alcohol based solvent• aqueous based PEDOT:PSS
• Difficult to find a solvent for the top polymer layer that does not dissolve the bottom polymer layer. (high boiling point solvents for the top polymer layer may be detrimental to the integrity of underlying layers)
• Wettability of interlayer and top cells• Modified PEDOT:PSS layer (dilute by alcohol, add surfactant)• Thin layer of metal
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Yang Yang et al. Nature Photonics, 6, 2012, 180
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N. Zhou, et al. Manuscript in preparation
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ITO NANOWIRE ELECTRODE69
N. Zhou, et al. Manuscript in preparation
SiO OO
ITO
IFL by ALD
Vertically grown ITO nanowire on ITO substrate
Filled with active layer
Vertically grown ITO nanowire on ITO substrate
Surface modification
SAM growth
Active layer fillingElectrode deposition
• Inefficiency of hole transport due to relatively poor hole mobility of polymer semiconductor comparing to the electron mobility of PCBM
• Reduce hole transport distance• Greatly enhance electrode (anode) surface area• Potentially increase hole transport and hole collection efficiencies
Li, S. Q.; Guo, P.; Zhang, L.; Zhou, W.; Odom, T. W.; Seideman, T.; Ketterson, J. B.; Chang, R. P. H. ACS Nano 2011, 5, 9161–9170.
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ITO NANOWIRE ELECTRODE70
N. Zhou, P. Guo, S. Li, et. al.
Facilitate hole transport
Current fabricated device structure
Proposed device structure
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SUMMARY AND OUTLOOKMorphology control• Manipulation of domain (PCBM cluster) size• Processing conditions• Characterization of domain size • Relate to J-V parameters
Photon management• A simple method through hot embossing• Convert the standard Ag electrode • >20% increase in Jsc
Novel Device Architectures• Tandem OPVs• Facilitate carrier transport/reduce bimolecular recombination probability• Active layer filtration through solution processing/surface functionalization• Further optimize processing conditions to create interdigitated structure
Materials development• BTI-BDT demonstrated one of the highest Voc• BTI-DTS achieved performance of 6.4%• Highest performance in literatures: 8.7%
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ACKNOWLEDGEMENTAdvisers: Prof. Tobin Marks Prof. RPH Chang
Prof. Antonio Facchetti
Marks groupDr. Xugang GuoMimi LouStephen Loser
Chang group:Dr. D. B. BuchholzShiqiang LiPeijun GuoMarks B team members
Committee Members: Prof. Lin Chen Prof. Mark Hersam
Collaborators:Dr. Antonio Guerrero, Prof. Juan Bisquert group, SpainSteven Lubin and Alex Hryn, Prof. Teri Odom group
Thank you!
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