The document discusses the application of titanium oxide as an electron injection layer in organic optoelectronic devices through solution-based processes like sol-gel. It describes how amorphous and nano-crystalline titanium oxide synthesized by sol-gel and deposited via spin-coating can improve device performance when used as the electron injection layer in polymer LEDs and organic solar cells. Experimental results show that devices using the titanium oxide layer have higher efficiencies compared to those using other solution-processed layers like cesium carbonate. The document concludes that solution-processed titanium oxide is a promising material that can be employed as an electron injection layer for applications in printed electronics.

1. The Application of Titanium Oxide on Organic Optoelectronic Devices by Sol-Gel Process Juo-hao Li, MiHyae Park and Yang Yang Department of Materials Science and Engineering University of California – Los Angeles, California MRS. 2008, Fall. Boston

2. Outline Introduction / Motivation Printed electronics & solution process Amorphous titanium oxide Solution processed electron injection layer PLEDs using TiO X as the ETL Nano-crystalline titanium oxide Sol-gel processed Nano-crystalline TiO 2 The application on PLEDs & Organic Solar Cell Summary MRS. 2008, Fall. Boston

3. Organic & Polymer Light Emitting Diodes Source:http://electronics.howstuffworks.com/oled1.htm Reflective Metal: Aluminum, Silver tris(quinolinolate) Al (Alq 3 ) poly( p -phenylene vinylene) (PPV) PSS: poly(styrenesulfonic acid) PEDOT: poly(3,4 ethyenedioxythiophene) Transparent Conductive Oxide (ITO) MRS. 2008, Fall. Boston

4. Polymer Solar Cell Structure & Operation Al 4.3 eV RR-P3HT PCBM MRS. 2008, Fall. Boston

5. Roll to Roll process Printing Process Inkjej printing Ink-transfer Printed electronics & solution process Franky So, Junji Kido, and Paul Burrows, MRS BULLETIN • VOLUME 33 • JULY 2008 Low cost High throughput Flexible M. BERGGREN, D. NILSSON AND N. D. ROBINSON, nature materials, 6, p1-5, 2007 MRS. 2008, Fall. Boston

6. Solution processed interfacial layer OLEDs architecture & charge injection layer Cathode HIL/HTL Anode/ Substrate Emissive layer - + EIL/ETL - + - - + + Increase injection and luminance efficiency Reduce the barrier energy Help charge transport Avoid the quenching effect Quantum confinement Charge injection & Charge balance MRS. 2008, Fall. Boston + -

7. Challenges of solution processed interfacial layer Thin film deposition Easy processing & comparability with other organic layer Energy level alignment Quantum confinement Thickness control Charge balance Interface morphology Stability Thermal stability & degradation MRS. 2008, Fall. Boston

8. Spin-coated Cs 2 CO 3 as EIL ITO/PEDOT/LEP/ Cs 2 CO 3 /Al ITO PEDOT LEP Al Cs 2 CO 3 + - The solvent used to dissolve Cs 2 CO 3 would not wash out the underneath LEP layer all solution process J. Huang et al. Adv. Funct. Mater. 2007, 17, 1966–1973 Cs 2 CO 3 (Sol.) Cs 2 CO 3 (Evap.) Cs/Al Solution Processed Cs 2 CO 3 layer MRS. 2008, Fall. Boston

9. Spin-coated TiO x as EIL ITO/PEDOT/LEP/ TiO x /Al Sol-gel metal-oxide as an EIL ITO PEDOT LEP Al TiO x + - Current Density and Luminance increase compared with solution processed TiO x EIL A clear solution containing TiO X is synthesized by sol-gel process and spin coated on top of LEP surface J. Li and Yang Yang et al. SPIE Optics & Photonics, San Diego 2008 MRS. 2008, Fall. Boston

10. Charge injection ability Photovoltaic measurement – Built-in potential Devices with TiO x EIL have similar built-in potential compared with the devices with Ca & Cs 2 CO 3 layer Low injection barrier High current density MRS. 2008, Fall. Boston

11. Concentration dependence 2.97 6.04 17.3 6.4 1.0 mg/ml * Measured at the brightness of 1000 cd/m 2 13.52 18.98 3.67 Power Efficiency* [lm/W] 14.21 7.76 3.3 0.5 mg/ml 16.92 6.27 2.8 0.2 mg/ml 5.84 16.67 5.0 0.1 mg/ml Current* efficiency [cd/A] J* [mA/cm 2 ] Volt* [V] TiO X Conc.

12. Interface electronic structure analysis- UPS TiO X interface layer provide a good coverage and low work function surface 0 mg/ml 1.0 mg/ml TiOx feature peak Polymer peak MRS. 2008, Fall. Boston

13. Solution processed amorphous Metal-Oxide as EIL GPF TiOx J(h) J(e) PEDOT Al ● ○ ○ ○ ● ● Current flow For Green PLEDs, Efficiency increases 27% (15  19 lm/W), 1000 cd/m 2 @ 2.8 V Hole blocking Charge balance Quenching prevention J. Li and Yang Yang et al. SPIE Optics & Photonics, San Diego 2008 Energy alignment & Hole blocking MRS. 2008, Fall. Boston Device performances are better than the ones with Cs 2 CO 3 EIL

14. Nano - crystalline TiO 2 J. Wang et al, J. Phys. Chem. C (2007) 111, 14925 Synthsis: Nonhydrolytic sol-gel route Low temperature Alcohol solvent Anatase Nano-crystalline TiO 2 suspension solution MRS. 2008, Fall. Boston

15. Structure : ITO / PEDOT / LEP / EIL / Al LEP : 1% Green PF in p-xylene EIL : Modified TiO 2 Application of nano-crystalline TiO 2 on PLEDs Device efficiency are comparable with devices with Cs 2 CO 3 EIL Power efficiency 14 lm/W @ 1000 cd/m 2, ,2.8V MRS. 2008, Fall. Boston

16. Nanocrystalline TiO 2 is a good candidate as the cathode structure Application of nano - crystalline TiO 2 on OPV V oc (V) = 0.58 J sc (mA/cm 2 ) = 10.76 PCE (%) = 4.2 FF(%) = 67 ITO / PEDOT / P3HT:PCBM/n doped TiO 2 / Al MRS. 2008, Fall. Boston Adv. Funt. Mat. submitted

17. Energy alignment & Hole blocking - - - + + + MRS. 2008, Fall. Boston Energy, eV PCBM 4.9 3.7 6.1 PEDOT ITO 5.2 4.7 n doped TiO 2 3.9 7.6 4.2 Al 4.0 2.0 7.0 6.0 5.0 3.0 P3HT 3.0

18. Characterization - XRD, TEM n-doped TiO 2 Proper energy level Stabilizing the TiO 2 nanostructure Interface resistance Hole blocking TiO 2 n doped TiO 2 50nm 50nm

19. Summary Solution processed interfacial layer can be applied in the process of printed electronics and act as charge injection layer to improve the interface and device performance Sol-gel processed amorphous titanium oxide is demonstrated to be a good electron injection layer used for efficient PLEDs. Sol-gel processed nano-crystalline titanium oxide can be employed as the EIL in both OPV and PLEDs MRS. 2008, Fall. Boston

20. Prof. Yang Yang’s gourp members Dr. Jianhui Hou , Solarmer Inc. Funding Agencies, CDA, Air Fource, ONR, NSF Acknowledgement Thank you for your attention MRS. 2008, Fall. Boston