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Solution-processed p-dopant as interlayer in polymer solar cells Frederic Guillain, James Endres, Lydie Bourgeois, Antoine Kahn, Laurence Vignau, and Guillaume Wantz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016
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ACS Applied Materials & Interfaces
Solution-processed p-dopant as interlayer in polymer solar cells
F. Guillain †, J. Endres ‡, L. Bourgeois §, A. Kahn ‡, L. Vignau †*, G. Wantz †*
†
Bordeaux INP, IMS, CNRS, UMR 5218, F-33400, Talence, France Univ. Bordeaux, IMS, UMR 5218, F-33400, Talence, France
‡
§
Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA
Univ. Bordeaux, ISM, Groupe de Spectroscopie Moléculaire, 351 cours de la Libération, 33405 Talence, France
Abstract: We report here an original approach to dope the semiconducting polymer-metal interface in an inverted bulk-heterojunction (BHJ) organic solar cell. Solution-processed 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), is deposited on top of a P3HT:PC61BM layer before deposition of the top electrode. Doping of P3HT by F4-TCNQ occurs after thermally induced diffusion at 100°C of the latter into the BHJ. Diffusion and doping are evidenced by XPS and UV-Vis-NIR absorption. XPS highlights the decrease in Fluorine concentration on top of the BHJ after annealing. In the same time, a charge transfer band attributed to doping is observed in the UV-Vis-NIR absorption spectrum. Inverted 1 ACS Paragon Plus Environment
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polymer solar cells using solution-processed F4-TCNQ exhibit power conversion efficiency of nearly 3.5% after annealing. This simple and efficient approach, together with the low annealing temperature required to allow diffusion and doping, leads to standard efficiency P3HT:PC61BM polymer solar cells, which are suitable for printing on plastic flexible substrate.
Keywords: polymer solar cells, doping, solution-processing, hole transport layer, F4-TCNQ
INTRODUCTION Polymers solar cells have shown significant improvement in performance in the last 10 years. The development of new materials, such as low band gap polymers, has allowed cells to reach more than 10% power conversion efficiency (PCE)1. Increasing efficiency and stability of solar cells also involves the careful design of device architectures, including new interlayer materials. To achieve cost-effective in-line manufacturing of polymer solar cells, it is desirable to print all the device layers. A considerable amount of work has been already done on the development of solution processable donor/acceptor bulk heterojunctions (BHJ). To enhance charge extraction, interlayers are
commonly
inserted
between
the
electrodes
and
the
BHJ.
PEDOT:PSS
(poly((ethylenedioxythiophene):poly(styrene) sulfonate) is the material commonly used as hole transport layer (HTL). With a work function of about 5.2 eV and a conductivity of 10-1 to 103 S.cm1
, depending on supplier and treatment, 2 it allows efficient hole extraction. However, PEDOT-PSS
has been shown to limit the lifetime of solar cells due to its acidic
3
and hygroscopic
4
nature.
Moreover, PEDOT:PSS is commercially available in aqueous solution, which is, in terms of surface energy issues, a limiting factor for its deposition on top of the BHJ in inverted devices. The use of high work function transition metal oxides (TMOs) such as MoO3 5, WO36,7, V2O5 8 or NiO 9 is an alternative method. However, almost all these oxides are toxic for both human and environment. 2 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
Toxicity can even be a bigger issue when using nanoparticles (NPs). Most of the “non NPs” solution-processed oxides need high temperature treatment (from 150°C to 450°C) to achieve the best performance
10
. When deposited on top of the active layer, in an inverted architecture, this
thermal annealing can lead to critical failure of solar cells, mainly due to diffusion and crystallization of PCBM
11
. When the active layer is annealed at 150°C for 15h under inert
atmosphere, power conversion efficiency is reduced to less than 30 % of its initial value using different donor/acceptor materials
12
. Thus, it is critical to develop new solution processable
interlayer materials, which need no or mild thermal annealing. Doping of semiconducting polymers improves conductivity and charge injection. In 2008 researchers from Cambridge (UK) demonstrated that the conductivity of poly(3-hexylthiophene) (P3HT) can be increased from 10-5 to 10-1 S.cm-1 by doping with 10% w/w of F4-TCNQ.13 Polymer doping in organic solar cells has attracted some interest in the last 5 years. Solution doping of BHJ has been studied and has shown improvement of devices performance14–17. Recently, a layer of doped P3HT laminated as HTL on top of a BHJ has been introduced18. The dopant, molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)-ethane-1,2-dithiolene] (Mo(tfd)3), was solubilized in chlorobenzene. Effective doping of P3HT was observed and a power conversion efficiency of about 3% was achieved using a P3HT:PC61BM active layer, in the range of what is usually obtained using this BHJ
19
. Another example of interface doping as hole extraction layer was also developed by
Kim et al. in 2014 20. In this publication, Vanadium (V) triisopropoxide was infiltrated into a BHJ to dope the semiconducting polymer. The formation of a vanadium oxide (VOX) layer as well as charge transport channels in the BHJ led to enhanced charge extraction. Power conversion efficiencies above 3% were achieved with P3HT:PC61BM. In our work, the F4-TCNQ dopant is inserted from the interface into the BHJ of inverted polymer solar cells (iPSCs). F4-TCNQ is a well-known dopant commonly used in PIN OLEDs to create the p-doped transport layer via co-sublimation under vacuum. F4-TCNQ is known for its tendency to
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diffuse 21–23 and in some cases it causes issues, such as recombination when F4-TCNQ diffuses into the emitting layer through the hole transport layer 24 Here we use this property as an advantage. F4TCNQ is dissolved in isopropyl alcohol (
