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ITO modification for efficient inverted organic solar cells Diana K. Susarova, Alexander V Akkuratov, Andrey I. Kukharenko, Seif O Cholakh, Ernst Z Kurmaev, and Pavel A Troshin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01106 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017
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Langmuir
ITO modification for efficient inverted organic solar cells Diana K. Susarova1, Alexander V. Akkuratov1, Andrey I. Kukharenko2, 3 , Seif O. Cholakh3, Ernst Z. Kurmaev2,3, and Pavel A. Troshin* 4, 1 1
Institute for Problems of Chemical Physics of Russian Academy of Sciences, Semenov ave 1,
Chernogolovka 142432, Moscow region, Russia 2
M. N. Mikheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences,
Yekaterinburg 620990, Russia 3
Institute of Physics and Technology, Ural Federal University, Mira street 19, Yekaterinburg 620002,
Russia 4
Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, 143026, Nobel st. 3,
Moscow, Russia
* Corresponding author: Prof. Pavel A. Troshin; e-mail:
[email protected] Abstract
We demonstrate a facile approach to designing transparent electron-collecting electrodes by depositing thin layers of medium and low work function metals on top of transparent conductive metal oxides (TCOs) such as ITO and FTO. The modified electrodes were fairly stable for months under ambient conditions and maintained their electrical characteristics. XPS spectroscopy data strongly suggested integration of the deposited metal in the TCO structure resulting in additional doping of the conducting oxide at the interface. Kelvin probe microscopy measurements revealed significant decrease in the ITO work function after modification. Organic solar cells based on three different conjugated polymers have demonstrated state of the art performances in inverted device geometry using Mg- or Yb-modified ITO as electron collecting electrode. The simplicity of the proposed approach and the excellent ambient stability of the modified ITO electrodes allows one to expect their wide utilization in research laboratories and electronic industry.
1. Introduction Organic solar cells represent a rapidly developing research field with a good promise for practical application in bulk renewable energy production. The development of new photoactive materials based on conjugated polymers, fullerene derivatives and small molecules [1, 2, 3, 4], optimization of the device architectures [5, 6] and investigation of the long term stability of the ACS Paragon Plus Environment
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photoactive materials [7] and devices [8-9] represent the mainstream research directions in the field of organic photovoltaics. Semitransparent metal oxide (TCO) electrodes based on ITO and FTO are commonly used in the production of organic and hybrid solar cells. An appropriate modification of TCO by using various buffer layers materials allows to extract efficiently either holes (standard device architecture) or electrons (inverted configuration). Inverted organic solar cells are known to be more stable as compared to the standard devices since they do not comprise acidic PEDOT-PSS and the top electrodes composed of active low work function metals [10]. Reducing the ITO work function for efficient electron collection was achieved using metal oxides (TiOx, ZnO) [11, 12, 13, 14], alkali metal salts (e.g. Cs2CO3) [15] and organic materials, such as fullerene derivatives [16, 17, 18], conjugated polymers [19, 20], zwitterions [21], selfassembled monolayers of organic conjugated molecules [22, 23, 24] or peptides [25], amines [26, 27], ethylene glycols [28], poly(4-vinylpyridine) [29] and etc. However, all these modifiers have certain disadvantages related to the complexity of their processing, insufficient electronic performance or poor stability. In particular, self-assembled monomolecular layers require long soaking times to form continuous molecular coverage, which is incompatible with typical roll-to-roll OPV production technology [30]. Additionally, monolayers and thin ionic interlayers generally provide high device efficiencies, while the operation stability is strongly impaired due to the possible diffusion of the modifier molecules to the photoactive bulk heterojunction layer. Metal oxide interlayers are much more robust compared to the thin organic coatings, but their use in devices is still associated with a number of challenges. For instance, titanium oxide is known as a strong oxidation photocatalyst, which can induce degradation of active layer materials and ruin the photovoltaic performance of the devices even in the presence of trace amounts of oxygen, which are practically unavoidable in plastic solar cells [31-32]]. Titanium in the high valence state might directly oxidize adjacent organic layers thus leading to a strong interfacial degradation. It has been also recognized that the performance of the solar cells incorporating TiOx interlayer depends
strongly
on
light
soaking,
which
hampers
significantly
their
practical
application [33-34]. This problem is known to be even more severe in case of using ZnO as electron transport layer (ETL) material [35-37]. One of the most direct approaches to use ITO as electron-collecting electrode is based on its modification by depositing thin layers of low work function metals such as calcium [38, 39, 40, 41]. Following these reports, however, one can conclude that thin calcium layer deposited on top of ITO enables efficient electron collection. However, these devices become very air-sensitive
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and, therefore, have to be processed under strictly anoxic and humidity-free conditions inside the glove box. In the present work, we show for the first time that the deposition of low work function metals on top of ITO or FTO does not produce any metallic coatings. On the contrary, metal penetrates to the conductive oxide, gets integrated in its structure leading to an additional doping at the surface and significant decrease in the electrode work function. Systematic screening have shown that many metals can be used for effective TCO modification, while the best results were obtained with Yb and Mg. The modified TCOs were shown to be fully air stable, thus allowing for fabrication of high-efficiency inverted solar cells under ambient conditions. The simplicity of the proposed approach allows one to expect its industrial utilization making the modified ITO and FTO readily available commercial products for advanced optoelectronic applications.
2. Experimental 2.1. Device fabrication The patterned ITO-coated (25×25×1.1 mm) or FTO-coated (25×25×2 mm) glass substrates were cleaned by successive sonication in deionized water, acetone and, finally, in isopropanol for 10 min and dried in air. Afterwards, the TCO slides were additionally cleaned by oxygen plasma treatment for 5 min (40 kHz, 200 W). The thin metal layers with 1-3 nm thickness were deposited by thermal evaporation under high vacuum (~10–6 mbar) on the clean ITO or FTO surface. ZnO and TiOx layers for reference devices were deposited following the previously reported procedures [42]-43. Polymer P3HT (12 mg) and fullerene derivative [60]PCBM (6.7 mg) were dissolved together in 1 ml of the chlorobenzene (PhCl). The conjugated polymers PCDTBT or P1 and fullerene derivative [70]PCBM were dissolved together in odichlorobenzene (o-DCB) in 1:2.5 and 1:2 weight ratios to achieve the total material concentrations of 21 mg/ml and 18 mg/ml, respectively. The resulting polymer/fullerene solutions were filtered through a 0.45 µm PTFE syringe filter and spin-coated at 900-1000 rpm on the modified ITO or FTO substrates in air and inside glove box under argon (O2, H2O
