ptimization of Conjugated-Polymer-Based Bulk Heterojunctions J.C. (Kees) Hummelen Molecular Electronics Materials Science Centre Plus University of Groningen, The Netherlands GCEP Solar Energy Workshop ct. 18 18, 2004 Stanford University
~ 20 nm ~ 500 nm SUBSTRATE Al CPC BLEND TC Introduction, Morphology, Mobilities, Improving V oc, Improving I sc, - N.S.Sariciftci, A.J. Heeger, USPatents 1992/1993 - G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270, 1789-91 (1995) - S. E. Shaheen, C. J. Brabec, F. Padinger, T. Fromherz, J. C. Hummelen, N. S. Sariciftci, Appl. Phys.Lett. 78, 841 (2001).
pening the box of Pandora: from bilayer to mixture p/n p:n metal top electrode transparent bottom electrode glass - D A Light G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270, 1789 (1995)
Basic processes in a PV cell h+ eexc Electrode 1 ito n Electrode 2 Light
Commonly used PPVs EH MEH-PPV M DM MDM-PPV (C 1 C 10 PPV) M
The standard fullerene acceptor Microsoft owerpoint Presentatio PCBM A highly processable methanofullerene (50-80 weight % of the D:A blend!) Available from Nano-C Inc. (Westwood, Mass) nano-c.com
PCBM (PhCl) crystal structure Fullerene moieties at < 10 Å in THREE dimensions Spheres: orientation always K! M.T. Rispens, A. Meetsma, R. Rittberger, C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Chem. Commun. 2116-2118 (2003)
SCLC: Experimental and calculated (solid lines) J-V characteristics of IT/PEDT:PSS/PCBM/LiF/Al devices with thicknesses L=90 nm and 170 nm (symbols),using V bi =1.4 ev and V Rs =30 Ω The device band diagram is indicated in the inset. The electron transport is described by SCLC, with an electron mobility µ e=2.0 10-7 m 2 /Vs and a dielectric constant ε r =3.9. Charge carrier mobility in PCBM (MDM-PPV: holes µ h = 5 10-7 cm 2 /Vs) PCBM: electrons µ e = 2 10-3 cm 2 /Vs (C 60 films: 8.10-2 cm 2 /Vs ; C 60 single crystals: 0.5 cm 2 /Vs) Mobility measurements on PCBM films i.e. PCBM does 4000 times better than MDM-PPV! Average hopping distance in PCBM ~ 3 nm! Mihailetchi, van Duren, Blom, Hummelen, Janssen, Kroon, Rispens, Verhees, Wienk, Adv. Funct. Mat. 13, 43-46 (2003)
Charge carrier mobility in blend µ [m 2 /Vs] 10-6 10-7 10-8 10-9 10-10 10-11 T=295K µ e µ h µ h pure MDM-PPV 20 30 40 50 60 70 80 90 100 weight percentage PCBM [wt.-%] enhanced intermolecular interaction by adding PCBM. MDM-PPV MDM-PPV:PCBM C. Melzer et al., Adv. Funct. Mat., (accepted) M. Kemerink et al., Nano Lett., 2003,3, 1191 electron mobility increases due to the increase number of percolated pathways. more balanced transport: µ e 10 µ h.
Morphology: influence of spin cast solvent: a from toluene MDM-PPV:PCBM from chlorobenzene b 0.5 µm 0.5 µm Surface Height (nm) 8 4 0-4 0.0 0.5 1.0 1.5 2.0 2.5 Distance (µm) Surface Height (nm) 8 4 0-4 0.0 0.5 1.0 1.5 2.0 2.5 Distance (µm)
Morphology: influence of spin cast solvent: 0 Active layer PEDT LiF Aluminum Glass IT - SMU + Current Density (ma/cm 2 ) -1-2 -3-4 -5 from toluene from chlorobenzene -6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Voltage (V) S. E. Shaheen, C. J. Brabec, F. Padinger, T. Fromherz, J. C. Hummelen, N. S. Sariciftci, Appl. Phys.Lett. 78, 841-3 (2001).
Morphology control: compatibilizing with the PCBX series PCBM PCBE PCBPr PCBEH PCBDM H
Dropcasting using different acceptors Gilch-PPV/PCBM Toluene 1:4 Gilch-PPV/PCBDM Toluene 1:4 2001 01096 2002 01505 TEM Unfiltered T. Martens, Z. Beelen, J. D Haen, J. Manca, IMEC
Dropcasting using different acceptors Gilch-PPV/PCBM Chlorobenzene 1:4 2001 01078 Gilch-PPV/PCBDM Chlorobenzene 1:4 2002 01521 T. Martens, Z. Beelen, J. D Haen, J. Manca, IMEC TEM Unfiltered
The photoinduced LUM-LUM electron transfer process (from excited donor) Light E ref LUM HM V oc(max) D A Anode Cathode
Improving V oc E ref V oc(max) D A Anode Cathode is there some truth in this picture?
Soluble fullerene derivatives with varying acceptor strength N N PCBM-[6,6] EH-Azafulleroid-[5,6] EH-Ketolactam
PCBM E red tuning CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 Joop Knol, Floris Kooistra
V oc versus E red acceptor in MDM-PPV/acceptor cells (CB) 900 800 2,3,4-(Me) 3 PCBM 3,4-(Me). 2 PCBM, TCBM PCBM PCBM V oc 700 AF C 60 600 KL -1.15-1.10-1.05-1.00-0.95 E red vs Fc/Fc +
Higher fullerenes: [70]PCBM M.M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen, P. A. van Hal, and R. A. J. Janssen, Angew. Chemie 42, 3371-5 (2003)
The photoinduced HM-HM electron transfer process (to excited acceptor) E vac LUM Light V oc(max) HM D A Anode ~ hole transfer Cathode
IPR Fullerenes C 60 -C 80 (calculated)
UV-Vis spectrum of [60]PCBM 50 extinction coefficient / L -1 g cm -1 40 30 20 10 0 300 400 500 600 700 800 wavelength / nm
Synthesis of [70]PCBM Me NNHTs base, C 70 DCB Me + + Me Me 85 % 15 % (chiral)
UV-Vis spectra of [60]PCBM and [70]PCBM 50 extinction coefficient / L -1 g cm -1 40 30 20 10 0 300 400 500 600 700 800 wavelength / nm [60]PCBM:MDM-PPV (4:1, w/w) and [70]PCBM:MDM-PPV (4:1, w/w) (normalized) All in toluene
PL decay of the fullerene emission at 720 nm of [70]PCBM:MDM-PPV (4:1 w/w) films Counts 10000 1000 PL quenching: 0, 30, 60, 95 % 100 10 0 1 2 3 4 Time / ns Spun from chlorobenzene, o-xylene, and DCB. Yellow: pristine [70]PCBM film
AFM tapping mode height images [70]PCBM:MDM-PPV (4:1 w/w) films on glass CB z-range = 86 nm rms roughness = 12 nm o-xylene z-range = 37 nm rms roughness = 7 nm DCB z-range = 8.2 nm rms roughness = 1.0 nm 10000 Counts 1000 100 10 0 1 2 3 4 Time / ns
Spectral response (EQE) of IT/PEDT-PSS/fullerene:MDM-PPV/LiF/Al cells 0.7 0.6 0.5 0.4 EQE 0.3 0.2 0.1 0.0 400 500 600 700 800 900 wavelength / nm [70]PCBM:MDM-PPV cells, spun from CB and DCB [60]PCBM: MDM-PPV cell spun from CB; active areas = 0.1 cm 2
I / V characteristics of [70]PCBM:MDM-PPV devices Current Density / macm -1 20 15 10 5 0-5 1000 10 0.1 1E-3 1E-5-2 -1 0 1 2 V C = 0.77 V I SC = 7.6 ma/cm 2 FF = 0.51 η = 3.0 % M.M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen, P. A. van Hal, and R. A. J. Janssen, Angew. Chemie 42, 3371-5 (2003) -10-1.0-0.5 0.0 0.5 1.0 Voltage / V [70]PCBM Available from Nano-C Inc. (Westwood, Mass) nano-c.com
Morphology of the bulk heterojunction Formation phases, domain sizes, percolation, wetting, processing, annealing, thickness Efficiency exciton diffusion, charge separation, charge recombination, (balanced) mobilities Stability phase separation (crystallization), (photo-)chemistry
Some provoking remarks No theoretical grounds for inferior efficieny of molecular devices Higher efficiency => improved stability Molecular materials offer infinite architectural opportunities (even for 3 rd generation tricks) General: 225M$ is very attractive! Beware of groups that not have a real focus on sustainable energy? (PV in this case)
Joop Knol Minze T. Rispens Floris Kooistra Luis Sanchez Jan Alma Patrick van t Hof Valentin Mihailetchi Paul Blom (RuG) Paul van Hal René Janssen (TU/e) Acknowledgments Acknowledgments Jan M. Kroon Wiljan Verhees Martijn Wienk (ECN) Christoph Brabec (now Siemens/Konarka) Serdar Sariciftci (Uni Linz) David Kronholm Henning Richter (Nano-C) Novem (The Netherlands rganization for Energy and the Environment) E.E.T. ( Economy, Ecology, Technology by Dutch EZ, C&W, VRM depts)