N-Type Materials for Organic Thermoelectrics: High electron mobility polymers and fullerenes

Transcription

1 N-Type Materials for Organic Thermoelectrics: High electron mobility polymers and fullerenes Dr. Ruth A. Schlitz Materials Research Laboratory University of California, Santa Barbara

2 Compelling properties of organic materials Inexpensive Solution processable Mechanically flexible Earth-abundant elements Low thermal conductivity Typical κ < 1 W/m-K Fundamental challenge: maximize the power factor

3 Challenge: stable extrinsic doping n-type PANI (unstable) Why so few prior results for n-type organics?

4 High LUMO makes n-doping unstable ev < LUMO < 4 ev 5 ev < HOMO < 6 ev Ca Stability is a challenge for n-type charge transfer doping 4.0 Al, Ag Dopant? Activation 5.0 Dopant! 6.0 P3HT PBTTT P(NDIOD-T2) Can we choose a stable polymer-donor pair that will donate upon activation? PCBM PDI

5 Reactive Dopants for Electron Conduction N-DMBI dihydro-1h-benzoimidzol-2-yl + P(NDIOD-T2) P. Wei et al., JACS 2010 Z. Chen et al., JACS 2008 H. Yan et al., Nature Energy levels preclude charge transfer doping of P(NDIOD-T2) N-DMBI HOMO -5.3 Hydride transfer reactions A. Colter et al., JACS, 1976

6 N-DMBI dopes P(NDIOD-T2) n-type

7 N-DMBI dopes P(NDIOD-T2) n-type Carrier concentration increases to 6 x cm -3 Consistent with ESR data

8 N-doping of P(NDIOD-T2) is stable Dopant still active after one month under N 2 PANI reported to p-type in minutes!

9 Thermoelectric Power Factor P(NDIOD-T2) + N-DMBI Prior Studies (PANI) N-type P(NDIOD-T2) follows same trend as p-type

10 Effects of increasing dopant load Conductivity saturates at a moderate doping load Estimate < 1% of N-DMBI active 12

11 Why so few dopants active? Neat P(NDIOD-T2) N-DMBI + P(NDIOD-T2) (10:1) Solubility issue: dopants segregate out

12 Spin density persists after washing aggregates N-DMBI + P(NDIOD-T2) (10:1) After Wash with H 2 O No detectable change in spin density after washing with H 2 O

13 A new DMBI derivative: FB8 N-DMBI Fulvio Brunetti (Hawker) FB 8 N-DMBI + P(NDIOD-T2) (10:1) FB8 + P(NDIOD-T2) (10:1) Is morphology change indicative of doping efficiency?

14 Comparison of N-DMBI & FB8 N-DMBI FB 8 FB8 is comparable to N-DMBI Synthetic expertise to increase solid-solid solubility

15 P(NDIOD-T2) tracks same trend as p-type Result of similar electronic structure?

16 What about other candidate organics? Can fullerenes be doped with fullerenes? Fullerenes P3HT PBTTT PCBM Thermal conductivity (W m 1 K 1 ) C 60 PCBM-SC PCBM-EVP PCBNB P(NDIOD-T2) PDI

17 Self Doping of Fullerenes High mobility (1-10 cm2/vs) n-type semiconductors Wide selection of solution processable structures Charge-transfer compound positive charge negative charge PCBM 1,3-bis(diisopropylphenyl)imidazol-2-ylene-C 60 H. Li, G. Bazan, JACS 2011 Dopants with similar structures offer potential to harness exceptional thermal properties of fullerenes

18 Novel Fullerene Dopant! PCBM + IDipp P3HT PCBM:IDIPP power factor is about 10x that of P3HT at a similar conductivity!

19 Conclusions N-DMBI dopes P(NDIOD-T2) n-type Maximum σ achieved: ~ 10-3 S/cm Solubility limits doping efficiency Fullerene charge transfer compound dopes PCBM n-type Power factor 10x P3HT at a similar conductivity! N-doped and p-doped polymer follow similar trend Cold N P Hot

20 Acknowledgments Synthetic Collaborators (MURI) Fulvio Brunetti, Craig Hawker Chabinyc Group at UCSB Andreas Lorbach, Gui Bazan Chabinyc Group Anne Glaudell Gregory Su

21 N-Type Materials for Organic Thermoelectrics: Water Soluble Dicationic Perylene Diimides Boris Russ University of California, Berkeley

22 N-type organic materials are scare and have limited performance

23 N-type organic materials are scare and have limited performance Strategies Perylene diimides for doping (PDI s) P(NDIOD- also have T2) potential and PCBM as n-type show materials promise for improving n-type behavior Are stable electron doping strategies possible with PDI s? 6.0 PCBM P(NDIOD-T2) PDI

24 Self-doping PDI material system Recent report of self-doping dicationic perylene diimide salt (B. Gregg et al., Adv. Materials, 2012) O N N O OH Water soluble Conductivity tunable by orders of magnitude (as high as 10-3 S/cm) O N O Increased conductivity with baking N OH Provides an intriguing system for investigation N-type Electronic conductivity can be broadly tuned Long-term potential for investigating structure-property relationships through chemical design

25 Probing the impact of charge separation on TE properties through synthetic design Charged perylene diimide electron density distribution Fulvio Brunetti (Hawker) Comparing cationic perylene diimide variants with ethyl, butyl, and hexyl chain charge spacers

26 Remarkably high n-type thermoelectric performance observed Electrical Conductivity (S/cm) Spacer Length (n) Seebeck Coefficient (μv/k) Modification of charge separation leads to 100X increase in conductivity without significant change in thermopower.

27 Comparison with other organic TE s DIHEXYL-PDI DIBUTYL-PDI DIETHYL-PDI Dihexyl perylene diimide is best-in-class n-type small molecule thermoelectric material

28 Both electronic and structural modifications can impact electrical transport Tuning charge concentration Tuning morphology Are these changes electronic or Electrical conductivity structural in can nature? change by 2-3 orders of magnitude Che et al. JACS, 2007 Rivnay et al. Nature Mat., 2009

29 Optical insight into electronic properties Controlling properties through annealing Tunable electrical conductivity Tunable concentration of PDI radical anions 4 1 Electrical Conductivity (S/cm x 10-3 ) Abs Intensity C 100C 120C 140C 0 80C 100C 120C 140C Annealing Temperature ( o C) Increased Annealing Temp Wavelength (nm) Ongoing studies are probing the effect of annealing temperature on thermoelectric properties

30 Optical insight into modifications of electronic properties - annealing Spectra as temperature ramped from RT to 120 o C under N 2 Abs Intensity (normalized) Polaron peaks grow in with increasing temperature Wavelength (nm)

31 Optical insight into modifications of electronic properties air exposure Wavelength (nm) Abs Intensity (normalized) Exposure to air at RT over 15 hours Polaron peaks decrease with increased air exposure

32 All variants display similar optical features Abs Intensity (Normalized) Π- Π* Vibronic Shoulder Polaron Peaks Electron spin resonance studies Wavelength are underway (nm) to probe role of charge How carrier does concentration the morphological in observed packing modulation of these variants of TE properties compare?

33 Increased side chain length alters the packing morphology Diethyl-PDI Dibutyl-PDI Dihexyl-PDI Q z (A -1 ) 1 Q z (A -1 ) 1 Q z (A -1 ) Q xy (A -1 ) Q xy (A -1 ) Q xy (A -1 ) Synthetic design and molecular simulation efforts are underway to probe the impact of structure on TE properties

34 Developing design principles to push organic n-types to a new frontier Strategic Vision Tuning Electronics Properties Synthetic Design Molecular Doping DIHEXYL-PDI Tuning Morphological Assembly Synthetic Design DIETHYL-PDI DIBUTYL-PDI

35 Acknowledgments Collaborators (MURI) Fulvio Brunetti, Max Robb, Craig Hawker Segalman Group at UC Berkeley Levi Miller, Shrayesh Patel, Michael Chabinyc Funding Support MURI-AFOSR NDSEG Graduate Research Fellowship

36 Summary and Vision Forward Present novel design strategies to radically tune thermoelectric properties in multiple n-type systems Demonstrate best-in-class n-type performance Dicationic Doped Doped P(NDIOD-T2) Perylene PCPM Diimides + N N N N N N Great opportunity to make further advances through strategic synthetic design and in-depth characterization Opens pathway for highly tunable all solution-processed green thermoelectrics