q8/13/98 12: 35 ~0.5~11 082 UCSB POLYMER INST -f 518 637 2884

Finai Report

Institute IbrPdpners and Organic Solids University dCaHbmia at Santa Barbara

Sanh Barbara, CA 93106

Submitted to: Department of Energy Advanced Energy Projects Walter Pohnsky, R o w Director

Principal Investigator: Alan J. Heeger

DepartmRnt of p$gTsics an8 Mataials DepartIlleat (joint) Telephone: (809 899-3184 FAX: (805) t ~ u - 4 7 5 ~

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DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus. product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, m m - mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect thosc of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be iIlegible electronic image products. Images are produced from the best available original document.

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me rrcognized need to develop inexpensive and renewable energy sources continues to stimulate new approaches directed toward efficient, law c a t photovoltaic devices. Due b the potential advantages which wolild be realized with polymer-based photovoltaic materials (e-g. low cost fabncatim in hrge sites and in desired shapes), efficient "plastic" solar celIs would have major impact.

For photovoltak cells made with pure conjugated pokymers, energy conversion efficiencies were typically 10-3- W 2 % , I too low to be used m practical applications. The recent discovery of photoinduced'electron transfer in composites of conducting poiymers (as donors) and buckminseerfullerene, Ca, and its derivatives (as ac~eptors)~-6 provided a molecular approach to high efficiency photovoltaic conversion. Since the time scale for photoinduced charge transfer is subpicosecond, more than 103 tirnes faster than the radiative or nonradiative decay of photo-excitations? the quantum efficiency for charge msfer and charge separarIon froin d m r to acceptor is dose to unity. Thus, photoinduced charge transfer across a donor/ac$eptsa @/A) interface provides an effective method to overcome early time carrier recombination in organic syscems and thus to enhance the opwlecmnic response of these @.ate-. For example. with the addition of only 1% & the photoconductivity of MEH-PPV:a increases by an order af magnitude over that of pure MEH-ppV.36

Although the quantum efficiency for photoinduced charge separation is near unity for a D/A pair, the conversion efEcienc$ in a bilayer heternjunction device is limited?

(i) Due to the moiecuiar R a r e of the charge separhon process, efficient charge separation occurs ody at &e Di(A interface; thus. photoexcitations created far from the D/Ajunction recombine prier EO diffusing to the haemjunction.

(ii) Even if charges are sepped at the D/A interface, the photovoltaic conversion efficiency is limited by rhr: Cdcr collection efficiency; Le. &e separated charges must be coIlected with hi@ ef@iency.

Consequently, inferpenetra&g pAase separrrttd D/A network composites would appear to be S e d pkotohziau muter ia l s .~~~ Through control of the morphology of the phase sqw&on into an kntcrpenetrcrfing nehuork, one cam uchieve a high interfa- arda within a bulk mater id Since any point in the composite is within a yew nanometers of Q D/A inteeace, such a composite i s a "bulk D/A! hetemjunction" malerial. Because of the interfrzciul potential bam'er @s denonstruted by the buiWn potential in the bilayer DI'A heterojune lion b d & , ultrafast photoinduced charge transfer anti charge separation w i l t occur with qrranftrm efficiency appraaciiing unity, leaving holes in the &$nodphase and electrons in the acteptot phase. Xhis process i s illustrated in l+tke ,upper portion of Fig. In If the network ir bicontinuous, (as shown s c ~ m z + a i l y in Fig. Ib, the coilection efficiency cam be equally effiient.8

Such a bicontinuous D/A r~empk material is prom is in^ for use in thin film solar cells. In addition tu the high quantum etliciency of charge separation, the electronic scsucture of such a bicontinuous D/A netwo& a w s one to choose contact electrodes with work- functions which optimize the c-t cQ"kction efficiencies of hoks from the donor phase and electrons from the acceptor p+se. Thus, thin film sandwich &vices with bicontinuous D/A composites as the active -4 promise to function as efficient solar cells with high q c and Tie-

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During the period of this DOE funding, we made sisnificant progress toward creating “kuZk D/A heterojlurctian” materials. This progress is summarized briefly in the following paragraphs. Reprints of publicaticp which fesulfed from this research are attached.

I. “Plastic’’ Photovoltaic Celfs Made with Donor-Acceptor Composites: Enbanced Carrier Cotkctian Efficiency via a Network of Internal Heterojunctions 9

The carrier colIection efficiency (Q) and energy conversion efficiency (qL) of polymer photovoltaic ceIls has been imprwed by blending the semiconducting polymer with & or its functionalized derivatives. Usmg composite films of MEH-PPV, poly(2-meth01cy-5-(2’- ethyl-hexyIoxy)- 1,4phenylene vihylene), and fullerenes, qc-7.4 % electrondphoton and qe-1 .2 9% (20 mW/cm2 at 430 q, 100 times better than in devices made with pure MEH- PPV. At low illumination @W/caP2), the efficiencies are cvea higher, % -1S% and qe-1.7 %. A two-step process is involve&

(i) The efficient charge sep@on results from photoinduced electron transfer from the

(ii) The high collection effii$ency results from a bicoaunuous network of internal

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MEH-PPV (as donor) 10 % (as acceptor):

donodacceptor hetemjunc&ns.

E. Cbarge Separation and ~ ~ o l t a i c Coaversioa in Polymer Composites with loternad Donor/Acc$pew Heterojunctions 10

The phorosensitiviry of semimudwing polymers can be enhanced by blending donor and acceptor polymers to opthi& pbtojnduced charge separation. We have Srudied the photovoltaic response of devices bbrkaoed from a novel phase-separated polymer blend (composite) made with poly( 2-qlmy-5- ( 2-ethyl-hexyloxy)- 1 +phenylene vinylene), MEH-PPV, as donor and cyano-PBv, 43-PPV. as acceptor. me photoluminescence and elecnoluminescence of both mers are quenched in the blend, indicative of rapid and efficient separati electron-hole pairs with electrons on the acceptor and hales on the with such a composite semiconducting polymer as the photosensitive &um show promising photovoltaic charactaistics with canier collection efficiency of 5 % elec&ons/photon and energy conversion efficiency of 0-9 %, - 20 times larger than in 4- made with pm MEH-PPV and -100 times larger than in diodes made with CN-Py. The phorosensiavity and the quantum yield increase with reverse bias voltage. to 0.3 NWat and 80% eiecaons/photon respectively at -10 V, comparable to d t s obtained fra& phmdhdes made with iaorgimk semiconductors.

1. For a review of photovoi devices made with plyacetylene, see: J. KanickL in Handbook of Condwting P%wnen. ed. T. A. Skotheim (New York. Marcel Dekker, 1986) p.543; S. GIenis, G. Tourillon, F. Gamier, Thin Solid Films 111,93 (1984); S. K a y , W. Riess, V. Dyz+pmxv, M. Schwoeret, Synth. Merals SJ, 427 ( 1993); H. Antoniadis, B.R. Hsie4, W. Abkowitz, SA. Jenckhe, M. Stolka Synth. Metals 64,265 (1994); M&s, J.J.M. Halls, D.D.D.C. Bradley, R.H. Frield, A.B. Holmes, J. Phys.: dens. Matter 6, 1379 (1994); G. Yu, C. Zhang, A.J. Heeger. Appl. Phys. Le N.S. Sariciftci, L. Smilowiti, A4. Heeger and F. Wudl, Science 258, 1474 (1992); L. Smilowitz. N-S. SariciQci. R. Wu, C. Gettinger, AJ. Heeger and F. Wudl, Phys. Rev. 5 47, 13835 (1993): B. Kraakl. C.H. La, D. McBranch, D. Moses, N.S. Sariciftci and A.J. Hex!&. Chem. Phys. Lea. 213,389 (1993); N.S. Sariciftci

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and AJ. Heeger, Intern. 3, Mod Phys. B 8, U7 (1994); B. Kmbel, D..McBm& N.S. Sariciftci, 0. Moses artd AJ. Heeger, Phys. Rev. B 50, 18543 (1994). C.H. be, G. Yu, D. M o s ~ , Pakbaz, C. zhang, N.S. Sariciftci, AJ. Heeger and

K. Lee, R.AJ. Jmssen, R.S. Sariciftci and AI. Heeger, Phys. Rev. B 49,5781 (1994). R.A.J. Janssen. N.S. SaEiciftci and AJ. H ~ g e r , I. Chem. Phys., 100, 8641 (1994). G. Yu, K Pakbaz and kJ. k g + r , Appl. Phys. Lcn, 64,3422 (1994). N.S. SatiCihci, D. Braun,:C "raaag, V. Srdaaov, AJ. Hecgcr, G. S- and E WudL AppL Phys. Lea. 6% S@ (1993). N.S. Sariciftsi and AJ.

F. Wudl, p h y ~ . Rev, B. 418,45425 (1993).

W- S. Patent 5,331,183. .

9. G. Yo and A. J. Heegcr, f. Ao$. B.. 78,45 10 (1995). 10. G. Yu, J. Gao, J.C. H d F. W d l sad AJ. Hag=, Science, 270,1789

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