Solution deposition of transparent conductive oxides. Marlies K. Van Bael

Transcription

1 Solution deposition of transparent conductive oxides Marlies K. Van Bael

2 Intro: Transparent Conductive Oxide TCO in display industry pass electrical signal to switch pixel on/off allow light to go out TCO in transparent thin film transistor (a-ingazno) High mobility for high speed switching TCO in photovoltaics collect photocurrent and pass to external circuit allow light to go in TCO in functional and architectural glass (SnO 2 :F) High transparency in VIS

3 Transparent Conductive Oxide Mostly used are ITO SnO 2 :In 2 O 3 Photovoltaics and Displays a-igzo InGaZnO x TFT s due to high mobility FTO SnO 2 :F Smart windows Prepared by Vacuum based processes Resistivity around <10-4 Ohm cm Minami et al. Semicond. Sci Technol 2005

4 Aim of SBO2 wetcoat Objective Obtain TCO (Al:ZnO) thin films by wet processing No Indium No vacuum Understand Synthesis conditions TCO formation TCO performance

5 Solution based routes Objective TCO (Al:ZnO) thin films by wet processing No Indium No vacuum From precursor solutions Organic solvent based route Water based route Deposition by spin coating High temperature crystallisation High temperature anneal in reducing conditions Target conductivity for solution deposition achieved 1 m K. Schellens et al. (2012) thin solid films

6 Solution based routes Objective TCO (Al:ZnO) thin films by wet processing No Indium No vacuum From precursor solutions State of the art resistivity obtained Still order of magnitude above vacuum processed layers Mainly because of lower carrier concentration Why do solution based systems underperform? Can we learn from in depth characterization? Can we improve by synthesis conditions? From dispersions of nanoparticles Aim at intrinsic optimal building blocks In depth study Al doping

7 Nanoparticle building blocks in depth doping study Conductivity = 1/resistivity = Carrier conc * carrier charge * mobility ZnO + Al tertrahedral octahedral substitutional interstitial segregation ZnAl 2 O 4 Al 2 O 3 free charge carriers electron traps isolators It is believed that Al 3+ in substitutional Zn 2+ positions (tetrahedral!) increases number of carriers HIGH CONDUCTIVITY POOR CONDUCTIVITY

8 Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor Various routes Solvothermal synthesis Hydrothermal synthesis Synthesis under reflux conditions Precipitation synthesis Library of various particles Fine, but which ones are the best building blocks?

9 Intensity (a.u.) Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor Questions Is ZnO:Al formed? How much Al is in the particles? Is the Al where we want it (substitutional)? Does this lead to free charge carriers? What is effect of synthesis conditions, i.o.w. can we optimize? nanorod XRD morphology No secondary phases Effect on crystal structure ( ) 0 % Al 0,5 % Al 1 % Al 1,5 % Al 2 % Al 2,5 % Al 3 % Al 5 % Al Fine, but which ones are the best building blocks?

10 Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor e-diffraction Effect on lattice parameters Questions Is ZnO:Al formed? How much Al is in the particles? Is the Al where we want it (substitutional)? Does this lead to free charge carriers? What is effect of synthesis conditions, i.o.w. can we optimize Au (internal? standard) ZnO

11 Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor Questions Is ZnO:Al formed? How much Al is in the particles? Is the Al where we want it (substitutional)? Does this lead to free charge carriers? In interstitial position Oh environment Conductivity What is effect of synthesis conditions, i.o.w. can we optimize? In substitutional Zn 2+ position Td environment Conductivity

12 Nanoparticle building blocks in depth doping study 100-x mol Zn Salt Benzyl alcohol x mol Al Salt 100-x mol Zn Salt Benzyl amine x mol Al Salt High pressure 200 C 6h Atm pressure 100 C 1h Centrifuge & Wash Centrifuge & Wash ZnO:Al 50 nm powder 50 nm ZnO:Al powder Drastic effect of synthesis conditions Al uptake (ICP-AES) on particle morphology Varies linearly with input 80% of Al input in ZnO:Al A. Kelchtermans et al. (2013) RSC Advances (in press)

13 Crystallographic position (%) Crystallographic position (%) Nanoparticle building blocks in depth doping study 100-x mol Zn Salt Benzyl alcohol x mol Al Salt 100-x mol Zn Salt Benzyl amine x mol Al Salt 4,0 [6] Al 3,5 [4] Al 3,0 2,5 2,0 1,5 1,0 0,5 High pressure 200 C 6h Only 0,05% Al Td All the rest Oh Centrifuge & Wash 2,5 50 nm 50 nm ZnO:Al powder intercept = 0,05 slope = 0,0 Always 0,05% Al Td 0,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 Actual Al content (%) 27 Al-NMR Discriminate between and Al doping A. Kelchtermans et al. (2013) RSC Advances (in press) 3,5 O3,0 T h d 2,0 1,5 1,0 0,5 0,0 NMR by Prof. dr. P. Adriaensens [6] Al [4] Al Atm pressure 100 C 1h More! 0,25% Al Td All the rest Oh Centrifuge & Wash ZnO:Al powder intercept = 0,26 Always 0,25% slope Al = 0,0Td 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 Actual Al content (%)

14 transmittance (a.u.) transmittance (a.u.) Nanoparticle building blocks in depth doping study 100-x mol Zn Salt Benzyl alcohol x mol Al Salt 100-x mol Zn Salt Benzyl amine x mol Al Salt High pressure Atm pressure 200 C 6h 100 C 1h Only Centrifuge 0,05% Al Td No free All carriers & the Wash rest seen Oh in FTIR 0% 0,5% 1% 1,5% 2% 2,5% 3% 5% ZnO:Al powder 50 nm 2.5% 50 nm More Centrifuge! 0,25% Al Td FTIR band All & the due Wash rest to free Oh carriers 0% 0.5% 1% 1.5% 2% 3% 5% ZnO:Al powder Hammarberg 2009 J. Colloid And Interface Sci wavenumber (cm -1 ) Charge carriers wavenumber (cm -1 ) A. Kelchtermans et al. (2013) RSC Advances (in press)

15 Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor Questions Is ZnO:Al formed? How much Al is in the particles? Is the Al where we want it (substitutional)? Does this lead to free charge carriers? What is effect of synthesis conditions, i.o.w. can we optimize?

16 Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor Questions Is ZnO:Al formed? How much Al is in the particles? Is the Al where we want it (substitutional)? Does this lead to free charge carriers? What is effect of synthesis conditions, i.o.w. can we optimize? Increase reaction time stirring 27 Al NMR Yes we can! A. Kelchtermans et al. (2013) submitted

17 Nanoparticle building blocks in depth doping study Strategy Start from known recipes for ZnO Introduce Al precursor Questions Is ZnO:Al formed? How much Al is in the particles? Is the Al where we want it (substitutional)? Does this lead to free charge carriers? What is effect of synthesis conditions, i.o.w. can we optimize? Increase reaction time stirring 27 Al NMR Does more Td Al increase conductivity? A. Kelchtermans et al. (2013) submitted

18 Nanoparticle building blocks in depth doping study Increase reaction time stirring 27 Al NMR Additional tools show that longer reaction time leads to higher conductivity Coll. V. Univ. Oxford Longer reaction increases plasmon resonance signal due to free charge carriers A. Kelchtermans et al. (2013) submitted Longer reaction increases microwave resonant bandwith due to increased conductivity

19 Nanoparticle building blocks in depth doping study Increase reaction time stirring?? Al in interstitial Td positions Other detrimental mechanisms?? 27 Al NMR But stirring doesn t!? NO increased plasmon resonance intensity upon stirring A. Kelchtermans et al. (2013) submitted NO increased microwave resonance bandwith Upon stirring

20 Nanoparticle building blocks in depth doping study NMR T 1 relaxometry shows different tetrahedral environments for Al, consistent with different donor behavior Coll. P. UHasselt In depth insight in the synthesis mechanism should help us further to understand how we can increase carrier concentration Damm H. et al. (2013) RSC advances synthesis platform of SBO4 may be the ideal tool to explore effect of synthesis parameters and optimize synthesis See lecture by G. Huyberechts TCO layers from optimal nanoparticle building blocks require further study on ink formulation, deposition and thermal treatment Ongoing work

21 Conclusion From precursor solutions State of the art resistivity obtained Still order of magnitude above vacuum processed layers Mainly because of lower carrier concentration Why do solution based systems underperform? We CAN learn from in depth characterization! We CAN improve by synthesis conditions! From dispersions of nanoparticles Aim at intrinsic optimal building blocks In depth study Al doping

22 SIM SoPPoM Users meeting SBO2 wetcoat Prof.dr. M.K. Van Bael 21 october 2013 wetcoat J. Hadermann C. Detavernier, B. Capon J. Martins, F. Vandenbroeck A. Hardy, M.K. Van Bael, H. Damm K. Elen, J. Manca, J. Drijkoningen Thank you for your attention!