Inverted semi-transparent organic solar cells with spray coated, surfactant free polymer top-electrodes

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Abstract

Depositing a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) buffer layer or top-electrode from aqueous solution on a non-polar active layer in inverted organic solar cells is commonly considered a very challenging task. In this work we utilize spray deposited PEDOT:PSS seeds to effectively reduce the surface free energy atop the active layer in poly-hexylthiophene polymer solar cells before applying a PEDOT:PSS top-electrode. Though aqueous PEDOT:PSS is repelled from non-polar surfaces, very small droplets can adhere to the surface. The distribution of the sprayed PEDOT:PSS droplets can be controlled via the substrate temperature and the material flow rate. The less time the droplets have to contract along the surface before drying, the better is the surface seed coverage and the more homogenous is the formation of a subsequently deposited PEDOT:PSS electrode. The respective solar cells are semi-transparent and exhibit an overall power conversion efficiency η≈2%.

Highlights

► Surfactant free polymer electrode for inverted organic solar cells. ► Surface free energy modification by sprayed polymer seeds. ► Semi-transparent, inverted device.

Introduction

Polymer solar cells are a gradually maturing technology with power conversion efficiencies now exceeding 9% [1]. While the efficiencies of such solar cells will continue to improve, the true economic viability of these devices will only be realized through the concurrent advancement of inexpensive fabrication technologies. Accordingly, several in-line fabrication-compatible printing and coating technologies have been investigated, including gravure printing, doctor blading, inkjet printing, slot die coating and spray coating [2], [3], [4]. The latter has drawn a lot of attention as spray coating combines the advantages of an easily scalable and mature process with low investment costs. There have been reports of bulk heterojunction organic solar cells based on spray coated poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) with power conversion efficiencies exceeding 4% [5], [6], [7] and active areas of more than 12 cm2 [8]. Spray coated charge carrier transport layers of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have also been examined [9]. As PEDOT:PSS can exhibit conductivities of up to 1400 S/cm, it is suitable for standalone transparent anodes [10], [11], [12] and cathodes [13]. It can also act as a top electrode or as part of this electrode in the industrially more relevant inverted device architecture [14]. The conductivity of PEDOT:PSS electrodes can be improved further by supporting metal grids [15]. Unfortunately, as PEDOT:PSS usually comes in aqueous solution, it exhibits very poor wetting properties on a non-polar P3HT:PCBM surface or any other non-polar organic layer. Normally, PEDOT:PSS wetting is improved by additives such as fluorinated surfactants and/or solvents with surface energies comparable to the surface energy of the underlying surface [16], [17]. However, the surfactants remain in the PEDOT:PSS layer and so must be chosen carefully. Alternatively, poly(allylamine hydrochloride) and dextran (PAH-D) can be used to modify the P3HT:PCBM surface polarity and to allow PEDOT:PSS wetting [18]. A similar approach is to wet the surface with isopropanol while at the same time some isopropanol is added to the PEDOT:PSS solution [19]. Without giving process details, Weickert et al. proposed the use of spray coated PEDOT:PSS seeds on a P3HT:PCBM surface to allow a subsequently spincast (low-conductive) PEDOT:PSS layer to wet the originally non-polar surface [20].

In this work we present a fully spray coated and highly conductive polymer top-electrode in an inverted polymer solar cell. By comprehensively investigating PEDOT:PSS seeding, i.e. systematically changing the P3HT:PCBM surface free energy, we avoid the utilization of any surfactants when depositing a spray cast PEDOT:PSS top electrode.

Section snippets

Experimental details

For the fabrication of inverted polymer solar cells as depicted in Fig. 1 we utilized structured ITO substrates that were cleaned in isopropanol for 15 min. Before spincasting the zinc oxide buffer layer we spin-rinsed the sample with ethanol. As an advancement of the process described in Ref. [21], the zinc oxide layer was deposited via spincoating from a filtered, 100 °C, 20 mg/mL zinc acetylacetonate hydrate/ethanol solution. The hot solution was applied on a rotating sample that was

Deposition of PEDOT:PSS seeds

Deposition of PEDOT:PSS onto non-polar surfaces is problematic due to a mismatch of their surface free energies. In order to circumvent this problem, here we systematically changed the surface free energy by spraying a PEDOT:PSS/DMSO/isopropanol formulation at a low flow rate onto the ITO/ZnO/P3HT:PCBM sample before applying the wet film, that later forms the conductive electrode. If a wet film forms on the P3HT:PCBM surface at this stage of the process, PEDOT:PSS will not adhere to the

Conclusion

In this work we have shown how to build an inverted polymer solar cell comprising a highly conductive PEDOT:PSS top electrode without adding any surfactants to the PEDOT:PSS formulation. This was accomplished by utilizing spray coated PEDOT:PSS seeds to modify the surface free energy of P3HT:PCBM layers. By controlling the density of the seeds on the non-polar P3HT:PCBM surface through material flow rate, spray time and substrate temperature, the surface free energy could be systematically

Acknowledgements

The authors would like to thank Shaun Howard (CSIRO) for providing the contact angle measurement setup. PEDOT:PSS Clevios PH1000 was supplied by Heraeus Clevios GmbH, Germany. We acknowledge the Federal Ministry of Education and Research (BMBF) and the AAS/ISL for funding of travel expenses under contracts AUS 10/019 and Mobility Call 2010-11, respectively. The ultrasonic spray deposition equipment was purchased within the project ARC LIEF (LE100100147). We further thank the Victorian

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    (2010)
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