Scalability and stability of very thin, roll-to-roll processed, large area, indium-tin-oxide free polymer solar cell modules

Abstract

Polymer solar cell modules were prepared directly on thin flexible barrier polyethylene terephthalate foil. The performance of the modules was found to be scalable from a single cell with an area of 6 cm² to modules with a total area of up to 186 cm². The substrate thickness was also explored and the performance was found to be independent of thickness in the range of 20–130 μm. The thinner substrates were found to present some challenge regarding handling but were not limited in performance. Large area modules on a substrate thickness of 45 μm were finally prepared by full roll-to-roll processing employing P3HT:PCBM as the active material and were found to exhibit a total area efficiency of >1% (1000 W/m⁻²; AM1.5G) with a typical active-area efficiency in the 1.5–1.6% for total module area of >110 cm² due to high fill factors in excess of 50%. The modules were also found to have an active-area efficiency of >1% under low light levels (∼100 W m⁻²). The modules were then subjected to extensive stability testing for a minimum of 1000 h employing several ISOS protocols. The modules presented higher than 80% of the initial performance (T80) in the dark (ISOS-D-1), in dark under elevated temperature of 65 °C (ISOS-D-2), under low light (ISOS-LL), under full sunlight (ISOS-L-2), and under outdoor testing (ISOS-O), which was conducted in two locations in India and Denmark. We estimate maximum T80 for those tests to be 2800, 5000, 1300, 1000, and 3500 h respectively. The modules showed significant sensitivity to high humidity and had low values for T80 for dark storage tests at 50 °C/85%RH (ISOS-D-3) and accelerated operation conditions with 0.7 sun/65 oC/50%RH (ISOS-L-3). We found the modules to be particularly suited for information and communications technology (ICT) and mobile applications where low humidity (<50%) and lower temperatures (<65 °C) can be anticipated and we estimate operational lifetimes in excess of 1 year.

Introduction

The possibility for application of polymer solar cells (PSCs) as a means for harvesting light and converting it into electricity has been subject to intense research with aims varying from bulk energy production to niche products and gadgets to academic studies. The vision of PSCs has however remained the same no matter what the purpose or intentions have been, and PSCs currently remain as the only photovoltaic technology that potentially presents flexible substrates, no abundance problems, no environmental concerns and full scalability through printing and coating technology.

The state-of-the-art of the laboratory PSCs is however still far removed from the vision of the widely disseminated low-cost solar cells as the concern for laboratory solar cells is focused on increasing the power conversion efficiency through materials design with little emphasis on operational stability and large-scale processing. It is for this reason that the few attempts at translating the record power conversion efficiencies of laboratory cells (now reaching 10% as claimed by Mitsubishi and others) to low-cost processing of PSCs through coating and printing remains largely futile. It is imperative for PSCs, if they are to be seen in real world applications, to demonstrate scalability of any new device concept, materials choices, and processing methods when brought forward while simultaneously maintaining the low cost objective of PSCs.

It has been repeatedly shown that the use of indium tin oxide (ITO) as a transparent conductor in PSCs is the major cost driving factor both from a material and processing perspective. ITO may account for as much as ∼90% of the embedded energy in a PSC module [1]. Not only is indium expensive due to the scarcity, but the associated cost volatility due to its high demand from display industries is feared to create a bottleneck in the supply chain of indium in the future. Furthermore, the brittle nature of ITO limits the mechanical stability of flexible PSCs [2]. As a result, the future of low cost PSCs relies on a transparent conductor that is free of ITO and that preferably involves only solution processing under ambient conditions.

The operational stability of PSCs is yet another challenge that must be tackled in tandem with low-cost scalability and efficiency. Unlike inorganic solar cells, PSCs are far more sensitive to ambient conditions such as humidity, temperature, and radiation. Water, O₂, and UV radiation are the three well-known key factors that accelerate degradation of PSCs [3], [4]. Apart from the choice of photoactive polymer and other materials and their processing, the choice of barrier material and encapsulation method is very critical for the stability of PSCs. An encapsulation with good barrier properties to water, oxygen and UV-light can significantly prolong the stability of PSCs.

In this paper, we comprehensively report ITO-free, large-area, polymer solar cell modules fabricated completely through an all-solution process using a combination of roll-to-roll (R2R) printing and coating methods. We further report on the operational stability of these modules under different simulated and real conditions following several ISOS protocols [5].