Molecular Thin Films and SmallMolecule Organic Photovoltaics Sanggyu

Molecular Thin Films and Small-Molecule Organic Photovoltaics Sanggyu Yim Kookmin University 진공학회 2011하계학술대회 Tutorial 1

Organic? OLED Flexible Display 진공학회 2011하계학술대회 Tutorial vs. Inorganic? Organic Semiconductors Nano(Bio) Sensor Solar Cell OTFT 3

Why Solar Cells? Humanity’s Top 10 Problems for Next 50 year 1. Energy 2. Water 3. Food 4. Environment 150, 000 km 1. 5 x 1022 J/day 5. Poverty 6. Terrorism & War 7. Disease 8. Education 9. Democracy 10. Population 진공학회 2011하계학술대회 Tutorial 4

Classification of Solar Cells 진공학회 2011하계학술대회 Tutorial 5

Why Organic Thin-Film Solar Cells? 1. Low Cost • Room temp. processing • Cheap materials • Mass production : roll-to-roll, ink-jet printing, etc. 2. Flexibility • Flexible substrate • Unbreakable devices 3. Variety of Property • Various materials : fine tuning available • Unlimited possibility 진공학회 2011하계학술대회 Tutorial 6

Solar Cell Efficiencies 진공학회 2011하계학술대회 Tutorial 7

Energy Conversion Process of Si Solar Cell 진공학회 2011하계학술대회 Tutorial 8

Small Molecule Organic Photovoltaic (OPV) Cell Energy Conversion Process Basic Device Structure (Exciton Blocking Layer) 진공학회 2011하계학술대회 Tutorial 9

Materials Electron Donor Materials Sub. Pc Phthalocyanine (Pc) Electron Acceptor Materials EBL Materials PTCDA C 60 진공학회 2011하계학술대회 Tutorial PTCBI BCP BPhen 10

History of Small Molecule OPV Cells v First Organic Solar Cell ITO/Cu. Pc(30 nm)/PV(50 nm)/Ag hp = 0. 95% @ AM 2. 0 C. W. Tang, Appl. Phys. Lett, 48(1986)183 진공학회 2011하계학술대회 Tutorial v Bulk Heterojunction ITO/Cu. Pc(15 nm)/Cu. Pc: C 60 (1: 1, 10 nm)/C 60(35 nm)/BCP/Ag hp = 5. 0% @ AM 1. 5 J. Xue et al, Adv. Mater. , 17(2005)66 v Tandem Cell hp = 5. 7% @ AM 1. 5 J. Xue et al, Appl. Phys. Lett, 85 (2004)5757 11

Timeline of Power Conversion Efficiency (PCE) of Small Molecule OPV cells B. P. Rand et al, Prog. Photovolt. : Res. Appl. , 15 (2007) 659. 진공학회 2011하계학술대회 Tutorial 12

Efficiency Measurement h. EQE(l) = Af(l) · h. CG(l) · h. CC(g, m) h. IQE(l) = h. CG(l) · h. CC(g, m) Jmax · Vmax JSC · VOC · FF h. PCE (%) = ----- X 100 = ------ X 100 Pin (l) A · IAM 1. 5 G 진공학회 2011하계학술대회 Tutorial 13

Org. vs. Inorg. : Fundamental Difference S. E. Gledhill et al, J. Mater. Res. , 20 (2005) 3167 진공학회 2011하계학술대회 Tutorial 14

Excitons (a) Frenkel, (b) Wannier and (c) Charge-Transfer exciton 진공학회 2011하계학술대회 Tutorial 15

Energy Level Diagram of Organic Heterojunction between a Donor(D) and an Acceptor(A) Layer 0 vacuum level Energy / e. V 3. 5 Cu. Pc 4. 8 ITO 5. 2 5. 3 ITO/PEDOT: PSS 3. 5 4. 2 4. 5 C 60 BCP Al 6. 2 7. 0 B. P. Rand et al, Prog. Photovolt. : Res. Appl. , 15 (2007) 659. 진공학회 2011하계학술대회 Tutorial 16

HOMO-LUMO Energy Levels of Representative Organic Materials 진공학회 2011하계학술대회 Tutorial 17

Possible Reasons for Low PCE of Small Molecule OPV cells 1. Narrow absorption band 2. Low charge carrier mobility 3. Lack of ways of controlling morphology § Material selection § Material growth technique § Device architecture 진공학회 2011하계학술대회 Tutorial 18

Device Fabrication 진공학회 2011하계학술대회 Tutorial 19

Schematic of Device Fabrication System 진공학회 2011하계학술대회 Tutorial 20

Transparent Electrode & HTL v Indium tin oxide (ITO) v Hole transport layer (HTL) § E-beam evaporation § PEDOT: PSS § Thickness ~ 147 nm § Spin coating § Sheet resistance ~18Ω/cm 2 진공학회 2011하계학술대회 Tutorial 21

Purification of Materials v Temperature gradient sublimation S. R. Forrest, Chem. Rev. , 97 (1997) 1793. 진공학회 2011하계학술대회 Tutorial 22

Organic Molecular Beam Deposition (OMBD) Substrate s lid So l so ce ur o u rce Knudsen Effusion Cell 진공학회 2011하계학술대회 Tutorial 23

Film Thickness Estimation v Quartz crystal microbalance (QCM) v Ex-situ calibrations SEM 진공학회 2011하계학술대회 Tutorial AFM Calibration 24

Molecular Thin Film Growth Techniques 1. Organic molecular beam deposition (OMBD) 2. Vacuum thermal evaporation (VTE) 3. Organic vapor phase deposition (OVPD) F. Yang et al, Nat. Mater. , 4 (2005) 37. 진공학회 2011하계학술대회 Tutorial 25

EBL, Metal Electrode & Encapsulation v Exciton blocking layer (EBL) v Metal electrode v Encapsulation § BCP § Al, Ag § UV-curable polymer § OMBD § Thermal evaporation § in glove box 진공학회 2011하계학술대회 Tutorial 26

Challenge for Improving PCE v Materials selection § Electron donor materials Zn. Pc Al. Cl. Pc Sub. Pc Zn. Pc Cu. Pc Jsc 5. 27 7. 76 4. 54 Voc 0. 81 0. 45 FF 0. 49 0. 42 0. 53 PCE 2. 08 1. 31 1. 09 Cu. Pc Sub. Pc Sn. Pc D. Y. Kim et al, Sol. Energy Mater. Sol. Cells, 93 (2009) 1452. 진공학회 2011하계학술대회 Tutorial 27

Challenge for Improving PCE v Materials selection § Electron acceptor materials F 16 Cu. Pc J. L. Yang et al, Org. Electron. , 11 (2010) 1399. 진공학회 2011하계학술대회 Tutorial DBP D. Fujishima et al, Sol. Energy Mater. Sol. Cells, 93 (2009) 1029. C 70 S. Pfuetzner et al, Appl. Phys. Lett. , 94 (2009) 223307. 28

Challenge for Improving PCE v Tandem structure Narrow absorption band M. Riede et al, Nanotechnology. , 19 (2008) 424001. J. Xue et al, Appl. Phys. Lett. , 85 (2004) 5757. 진공학회 2011하계학술대회 Tutorial 29

Challenge for Improving PCE v Tandem structure J. Drechsel et al, Appl. Phys. Lett. , 86 (2005) 244102. 진공학회 2011하계학술대회 Tutorial 30

Challenge for Improving PCE v Bulk heterojunction (BHJ) § OVPD (organic vapor phase deposition) F. Yang et al Nature Mater. , 4 (2005) 37. 진공학회 2011하계학술대회 Tutorial 31

Challenge for Improving PCE v Bulk heterojunction (BHJ) § Gradient cell 진공학회 2011하계학술대회 Tutorial Mixed Gradient Jsc 5. 06 5. 00 Voc 0. 47 0. 50 FF 0. 44 0. 54 PCE 1. 05 1. 36 32

Challenge for Improving PCE v Bulk heterojunction (BHJ) B, P. Rand et al, J. Appl. Phys. , 98 (2005) 124902. Low hole mobility 진공학회 2011하계학술대회 Tutorial 33

Challenge for Improving PCE v Templating effect P. Sullivan et al, Appl. Phys. Lett. , 91 (2007) 233114. 진공학회 2011하계학술대회 Tutorial 34

Challenge for Improving PCE v Substrate temperature 30ºC 90ºC 150ºC Jsc 7. 6 8. 8 10. 0 Voc 0. 56 0. 52 0. 50 FF 0. 38 0. 45 0. 49 PCE 1. 61 2. 05 2. 44 M. Deisenroth et al, J. Appl. Phys. , 101 (2008) 진공학회 2011하계학술대회 Tutorial 35

Theoretical Maximum of PCE M. Riede et al, Nanotechnology. , 19 (2008) 424001. 진공학회 2011하계학술대회 Tutorial 36

Long Term Stability Zn. Pc: C 60 tandem solar cell R. Franke et al, Sol. Energy Mater. Sol. Cells, 92 (2008) 732. pentacene: C 60 solar cell W. J. Potscavage et al, Appl. Phys. Lett. , 90 (2007) 253511. § Only few data on OSCs based on small molecules § Need to standardize measurements for OSCs. 진공학회 2011하계학술대회 Tutorial 37

Current Challenge v Surface nanostructuring – solvent vapor treatment 진공학회 2011하계학술대회 Tutorial 38

Current Challenge v Surface nanostructuring – NIL (nanoimprint lithography) 진공학회 2011하계학술대회 Tutorial 39

Current Challenge v Organic photonic crystal (PC) layer 진공학회 2011하계학술대회 Tutorial 40

Summary § § There has been large progress in small-molecular weight based organic solar cells. Yet, the power conversion efficiency is relatively low, but various efforts to improve the device performance have been made. To improve the device performance, exploit of new materials having broad absorption band high charge carrier mobility, and optimization of interfacial structure are needed. Investigation on the better device architecture such as highefficiency tandem cells and optimization of open-circuit voltage and short-circuit current are also necessary. 진공학회 2011하계학술대회 Tutorial 41