Bakulin, A.A.

Beljonne, D.

Cornil, J.

Friend, R.H.

Gelinas, S.

Loosdrecht, P.H.M. van

Niedzialek, D.

Pavelyev, V.G.

Pshenichnikov, M.S.

Rao, A.

Vaynzof, Y.

English

Novel optical pump-push – photocurrent probe ultrafast spectroscopy experiments on organic photoconversion systems show that excessive excitation energy in such systems is not lost but used to reach delocalised states that act as the gateway for long-range charge separation. We also show that the developed experimental approach can be generalised to inorganic and hybrid photoconversion systems

Proceedings - University of Groningen

   University of GroningenUltrafast Pump-Push Photocurrent Spectroscopy of Organic Photoconversion SystemsBakulin, A.A.; Rao, A.; Vaynzof, Y.; Gelinas, S.; Pavelyev, V.G.; Loosdrecht, P.H.M. van;Pshenichnikov, M.S.; Niedzialek, D.; Cornil, J.; Beljonne, D.Published in:Proceedings of the XVIIIth International Conference on Ultrafast PhenomenaIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Bakulin, A. A., Rao, A., Vaynzof, Y., Gelinas, S., Pavelyev, V. G., Loosdrecht, P. H. M. V., Pshenichnikov,M. S., Niedzialek, D., Cornil, J., Beljonne, D., & Friend, R. H. (2013). Ultrafast Pump-Push PhotocurrentSpectroscopy of Organic Photoconversion Systems. In M. Chergui, S. Cundiff, A. Taylor, R. D. Vivie-Riedle, & K. Yamanouchi (Eds.), Proceedings of the XVIIIth International Conference on UltrafastPhenomena (41 ed., Vol. 41, pp. 05020-p.1-05020-p.3). University of Groningen, The Zernike Institute forAdvanced Materials.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022 Ultrafast Pump-Push Photocurrent Spectroscopy of Organic Photoconversion Systems A. A. Bakulin1,4, A. Rao1, Y. Vaynzof1, S. Gelinas1, V. G. Pavelyev2, P. H. M. van Loosdrecht2, M. S. Pshenichnikov2, D. Niedzialek3, J. Cornil3, D. Beljonne3, and R. H. Friend1 1Cavendish laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB30HE, UK 2Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 3Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium 4AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands Abstract: Novel optical pump-push – photocurrent probe ultrafast spectroscopy experiments on organic photoconversion systems show that excessive excitation energy in such systems is not lost but used to reach delocalised states that act as the gateway for long-range charge separation. We also show that the developed experimental approach can be generalised to inorganic and hybrid photoconversion systems. In organic light-to-charge conversion systems, photon absorption leads to the formation of a tightly bound electron-hole pair (Fig. 1a). In photosynthesis, separation of charges is achieved with the aid of cascaded energy levels and ion screening. Organic photovoltaic (OPV) devices in contrast possess none of these elements, yet surprisingly, some OPV systems work very efficiently. Hence, the fundamental mechanism of how the organic heterojunction enables efficient long-range charge separation remains unclear. Here we address this issue by applying novel electro-optical pump-push-probe technique, performed on working OPV diodes (Fig. 1a,b) [1]. An ultrafast visible ‘pump’ pulse illuminates the OPV cell and creates a population of charged states. The charge carrier causes a geometrical rearrangement of organic molecules, known as polaron formation. For the hole polaron, the optical transition P1 between it and the π valence band is strongly allowed, forming an absorption band typically between 0.2 and 0.6eV [2]. Here we selectively excited the P1 transition with an IR ‘push’ pulse, therefore donating excess energy to the system in different moments of photoconversion process. The perturbation of the charge dynamics on the molecular level affected the OPV device performance and the corresponding change of the photocurrent (PC) induced by the IR-‘push’ pulse is detected as a function of the delay between ‘pump’ and push [3]. In a complementary all-optical experiment, the effect of the ‘push’ pulse on charge dynamics is observed using an additional IR ‘probe’ pulse [4]. Figure 1c shows pump-push photocurrent data for a set of organic solar cells indicating a change in photocurrent when the push pulse arrives after the pump. The relative amount of additional photocurrent is very dependent on the material. Polymer-polymer blends (PFB/F8BT and P3HT/F8TBT) displayed a large push-induced increase of photocurrent (10-50%). In contrast, highly efficient polymer-fullerene cells demonstrated a modest photocurrent increase (<6%). The             This  is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.     EPJ Web of ConferencesDOI: 10.1051/C© Owned by the authors, published by EDP Sciences, 2013,epjconf 201/05020 (2013)4134105020Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20134105020EPJ Web of Conferences magnitude of the signal decays slowly (0.2-10 ns) as the delay time increases, which we attribute to the bound-state recombination dynamics. 				 			Fig.1. Free-energy diagram of photoconversion in organic systems (a) and layout of pump-push photocurrent (b) experiment. (c) Increase in photocurrent due to interaction with an IR push pulse for a set of OPV cells. Solid lines are exponential fits convoluted with the Gaussian ‘charge-generation’ function. We	monitor	 the	 effect	 of	 the	 ‘push’	pulse	on	 the	 charges	directly	by	 an	all‐optical	 pump‐push‐probe	 technique	 (Fig.	 1b).	 Pump	 pulses	 create	 charges	 in	 the	 OPV	material	 and	 their	evolution	is	manipulated	/monitored	with	the	IR	‘push’/‘probe’	pulse.			Fig.	2.	(a) Transient-absorption and (b) transient-anisotropy kinetics for MDMO-PPV:PC70BM film excited at 580 nm and probed 3 m. Green line in figure (a) shows dynamics with push pulse arriving at ~2 ps delay. The effect of push pulse on the isotropic (c) and anisotropic (d,e) transients, calculated from the difference between ‘with-push’ and ‘no-push’ measurements, for MDMO-PPV/PC70BM, PCPDTBT/PC70BM films. In all figures, solid lines are (multi)exponential fits convoluted with the Gaussian function at zero delay. Figures	 2a,b	 show	 isotropic	 and	 anisotropy	 pump‐probe	 transients	 for	 a	 typical	 OPV	sample.	 The	 green	 circles	 present	 analogous	 dynamics	 when	 a	 push	 pulse,	 ~2ps	 after	 the	‘pump’	deposits	a	0.5eV	excess	energy	to	the	charges,	bringing	them	to	the	 ‘hot’	state.	This	is	observed	as	a	sudden	drop	of	signal	amplitude	because	fewer	charges	absorb	the	probe	beam.	Figure	2c	presents	the	difference	in	response	with	and	without	the	‘push’	pulse	showing	that	deposited	excess	energy	is	lost	on	sub‐ps	time	scale.	Figures	2d,e	show	the	relative	difference	(a) (c) (b) (a) (b) (c)(d) (e) 05020-p.2XVIIIth International Conference on Ultrafast Phenomenabetween	‘with‐push’	and	‘no‐push’	anisotropic	transients.	As	a	general	trend,	we	observe	that	anisotropy	is	reduced	by	the	 ‘push’.	The	loss	 in	anisotropy	after	the	push	corresponds	to	the	re‐excited	‘hot’	charge	re‐localisation	to	the	neighbouring	molecule	with	a	different	orientation	of	 the	charge‐associated	dipole.	The	continued	 loss	of	anisotropy	provides	 further	evidences	that	charge	carriers	become	more	mobile	after	the	‘push’	pulse.			Fig. 3. Microelectrostatic simulations of the charge distribution at the P3HT/F8TBT heterojunction, with electron and hole densities shown in blue and red respectively. (a) the charge distribution in the lowest charged state configuration, and (b) the excited charged state configuration, created upon absorption of an IR-‘push’ photon. The	 fast	 relaxation	 and	 anisotropy	 loss	 observed	 in	 3‐pulse	 experiments	 cannot	 be	described	by	the	currently	dominant	models	of	charge	separation	through	relaxation‐assisted	intermolecular	 hopping.	 However,	 the	 observed	 results	 can	 be	 explained	 by	 charge	delocalisation	 achieved	 via	 band	 states.	 The	 quantum	 chemistry	 calculations	 show	 that	 the	absorption	of	 the	 IR	 ‘push’	photon	promotes	charges	 from	the	 localised	polaronic	 state	 (Fig.	3a)	 to	 the	de‐localised	band	state	 (Fig.	3b),	 increasing	 the	effective	radius	of	 the	charge	pair	and,	therefore,	reducing	the	binding	energy.	In	 conclusion,	 by	 applying	 new	 types	 of	 ultrafast	 spectroscopes	 to	 range	 of	 organic	photoconversion	systems,	we	have	demonstrated	that	delocalised	band	states	are	critical	 for	long‐range	 charge	 separation	 in	 OPV	 devices.	 Although	 the	 delocalised	 states	 are	 extremely	short‐lived	 (<1ps),	 they	 enable	 charges	 to	 override	 the	 otherwise	 dominant	 Coulomb	interaction.	Our	results	provide	a	new	framework	to	understand	charge	generation	in	organic	systems	and	outline	the	basis	for	the	design	of	improved	OPVs.	In	particular,	we	have	already	observed	similar	effects	in	different	organic‐base	systems	including	hybrid	and	dye‐sensitised	light	converters.[5]	References 1. A. A. Bakulin, A. Rao, V. G. Pavelyev, P. H. M. v. Loosdrecht, M. S. Pshenichnikov, D. Niedzialek, J. Cornil, D. Beljonne and R. H. Friend, Science 335,  1340 (2012) 2. J. Holt, S. Singh, T. Drori, Y. Zheng and Z. V. Vardeny, Physical Review B 79 (19) (2009) 3. J. G. Müller, J. M. Lupton, J. Feldmann, U. Lemmer, M. C. Scharber, N. S. Sariciftci, C. J. Brabec and U. Scherf, Phys. Rev. B 72, 195208 (2005) 4. K. C. Vishnubhatla, J. Clark, G. Lanzani, R. Ramponi, R. Osellame and T. Virgili, Applied Physics Letters 94 (4), 041123 (2009) 5. Y. Vaynzof, A.A. Bakulin, S. Gelinas, and R.H. Friend, Phys. Rev. Lett. 108, 246605  (2012)            (a) (b) 05020-p.3

Ultrafast Pump-Push Photocurrent Spectroscopy of Organic Photoconversion Systems

https://pure.rug.nl/ws/files/2366954/2013EPJWebConfBakulin.pdf

Ultrafast Pump-Push Photocurrent Spectroscopy of Organic Photoconversion Systems

Abstract

Similar works

Full text

Available Versions

Proceedings - University of Groningen