Influence of moisture out-gassing from encapsulant materials on the lifetime of organic solar cells

doi:10.1016/j.solmat.2014.09.030

Solar Energy Materials and Solar Cells

Volume 132, January 2015, Pages 485-491

https://doi.org/10.1016/j.solmat.2014.09.030 Get rights and content

Highlights

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High moisture levels were found in polymeric encapsulants used for OPV devices.
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The effect of entrained moisture levels on OPV device durability was evaluated.
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Even <20 ppm retained moisture in encapsulant was found to limit device durability.
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Results reveal corrosion at metal-electrode surface as a major degradation factor.

Abstract

The results of a study on the effects of encapsulant pre-conditioning (drying) on the durability of “conventional” and “inverted” bulk-heterojunction organic solar cells based on P3HT:PCBM are presented. The architectures of the conventional and inverted devices were ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/ZnO/P3HT:PCBM/MoO₃/Ag, respectively. Quantitative analysis of the moisture content, moisture out-gassing, and re-absorbing properties of flexible barrier encapsulant films under ambient conditions was conducted. The effect of moisture out-gassing from the encapsulation materials on device performance was studied and the lifetime of conventional and inverted devices was found to decline significantly for the devices encapsulated using materials without pre-conditioning. This study reveals the essential role of pre-conditioning of materials in the encapsulation of organic photovoltaic devices. Current-voltage characteristics exhibit a clear increase in the series resistance of the devices during storage under ambient conditions, and Spectroscopic Impedance analysis indicates that charge-transport resistance within the devices increases significantly during the degradation process. The results are consistent with the primary degradation mechanism being corrosion at the metal-electrode surface.

Introduction

The field of Organic Photovoltaics (OPVs) has attracted worldwide attention over the past two decades due to potential advantages such as flexibility, large-scale printability, light weight and potential utility in a wide variety of products. Through gradual developments in new active polymers and electrode materials, together with improvements in interfacial properties, proven energy conversion efficiencies of more than 10% have been reported for laboratory-scale OPV devices [1], [2], [3]. At present, this emerging technology is at a pre-commercial stage of development and there are several challenges that must be addressed before it can be realized commercially.

Foremost among these challenges are improving efficiency, scalability, and the durability of the devices. It is well known that OPV device performance is highly sensitive to oxygen and moisture which can cause interfacial instabilities as well as permanent physical or chemical changes to the active polymer and other layers resulting in reduced device lifetime [4], [5]. Irradiation of OPV devices can lead to even more complex degradation mechanisms involving factors such as oxidation and de-lamination of metal electrodes and other deposited layers, [6] photo-oxidation of the active polymer material, [7], [8] photo-oxidation of inorganic oxide nanocomposite films, [9] degradation of the PEDOT:PSS layer, [10], [11] and chemical degradation of metal electrodes and the ITO electrode [12]. To reach sufficiently long lifetimes for commercial applications OPV devices require encapsulation using barrier materials having low permeability towards atmospheric oxygen and moisture. As such, developing improved flexible and transparent barriers and new encapsulation methodologies are critical steps for improving the stability of such devices.

Flexible barrier materials displaying water vapour transmission rates (WVTR) of less than 10⁻⁴ g m⁻² day⁻¹ are considered essential for durable printed flexible organic solar cells [13]. Materials displaying such ultra-high barrier properties have been reported and are now available commercially [14], [15], [16], [17]. Furthermore, sealing materials such as pressure-sensitive adhesives, [18] UV-curing cements [19], and hot-melt adhesives are often used to bond the barrier films onto the devices. Among these, the use of pressure-sensitive transfer adhesives has attracted much attention as they can be used readily in roll-to-roll lamination systems to encapsulate solar-cell modules fabricated on flexible substrates [20]. Although the barrier films applied to OPV devices are designed to protect against the ingress of atmospheric moisture into the device, ‘out-gassing’ of residual dissolved moisture and oxygen from these materials after device encapsulation can also lead to device degradation. For example, in preliminary work, we found that a number of polymer-based encapsulation materials readily absorbed moisture when exposed to ambient conditions, and that even trace levels of moisture absorbed and retained within the barrier films and adhesive materials appeared to cause significant reduction in the stability of OPV devices.

In the present work, we discuss the results of a detailed study of the effects of retained moisture in encapsulation materials comprising commercially-available flexible barrier films and transfer-adhesives on the shelf-life of OPV devices having so-called “conventional” or “inverted” architectures. The results of this work highlight the importance of thorough and systematic drying of encapsulation materials prior to use in encapsulation of OPV devices.

Section snippets

Device fabrication and encapsulation

The OPV device structures discussed in this work are shown schematically in Fig. 1(a) and (b). The pre-patterned indium tin oxide (ITO)-coated glass substrates (Kintec Company) were cleaned by successive ultrasonic treatments in acetone and isopropyl alcohol for 20 min followed by UV–ozone treatment for 10 min. For the preparation of conventional devices, cleaned substrates were spin-coated with an aqueous solution of poly-3,4-ethylenedioxythiophene–poly(styrenesulphonate) (PEDOT:PSS) in air at

Moisture adsorption and desorption properties of the encapsulant materials

It is useful to characterise the moisture absorption and desorption behaviour of the encapsulant materials before considering the impact of this moisture on the operational lifetime of the OPV devices. Fig. 3 shows the moisture desorption behaviour of the encapsulant film integrated with the adhesive layer on drying at 100 °C and ~10⁻³ mbar vacuum. These results indicate that samples stored under ambient conditions (20 °C and 50% Relative Humidity) have a saturated moisture content of about 1000

Conclusion

The stability of conventional and inverted OPV devices is strongly influenced by moisture and oxygen and thus robust encapsulation is required before they are exposed to ambient conditions. Encapsulation using high-quality barrier materials can significantly extend the lifetime of devices. Through a systematic analysis, it has been found that barrier films integrated with adhesives exposed to ambient conditions contain significant amounts of moisture. The dependence of device parameters on

Acknowledgements

This research was supported by the CSIRO Manufacturing Flagship and the Victorian Organic Solar Cell Consortium through funding from a Sustainable Energy Research and Development Grant from the Victorian Government Department of Primary Industries, a Victoria Science Agenda Grant from the Victorian Government Department of Business and Innovation, the Australian Renewable Energy Agency (ARENA), and the Australian Centre for Advanced Photovoltaics (ACAP). HW acknowledges a Postdoctoral

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View full text

Influence of moisture out-gassing from encapsulant materials on the lifetime of organic solar cells

Highlights

Abstract

Introduction

Section snippets

Device fabrication and encapsulation

Moisture adsorption and desorption properties of the encapsulant materials

Conclusion

Acknowledgements

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Synth. Met.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Mater. Today

Organic Electron.

Organic Electron.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

10.2% Power conversion efficiency polymer tandem solar cells consisting of two identical sub-cells

Adv. Mater.

A polymer tandem solar cell with 10.6% power conversion efficiency

Nat. Commun.

Solar cell efficiency tables (version 39)

Prog. Photovolt.: Res. Appl.

Stability of polymer solar cells

Adv. Mater.

Photooxidative stability of substituted poly(phenylene vinylene) (PPV) and poly(phenylene acetylene) (PPA)

J. Appl. Polymer Sci.

Reversible degradation of inverted organic solar cells by concentrated sunlight

Nanotechnology