PEN as substrate for new solar cell technologies

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Abstract

The possible use of polyethylene naphthalate as substrate for low-temperature deposited solar cells has been studied in this paper. The transparency of this polymer makes it a candidate to be used in both substrate and superstrate configurations. ZnO:Al has been deposited at room temperature on top of PEN. The resulting structure PEN/ZnO:Al presented good optical and electrical properties. PEN has been successfully textured (nanometer and micrometer random roughness) using hot-embossing lithography. Reflector structures have been built depositing Ag and ZnO:Al on top of the stamped polymer. The deposition of these layers did not affect the final roughness of the whole. The reflector structure has been morphologically and optically analysed to verify its suitability to be used in solar cells.

Introduction

The use of flexible plastic substrates is becoming an issue of great interest in thin film silicon solar cells technology, as they can contribute to cost reduction in the production process being compatible with the use of roll-to-roll deposition systems and with large area deposition [1]. If we compare their properties with those of flexible metallic substrates commonly used in roll-to-roll processes, polymer substrates turn to be cheaper and make the serial interconnection between modules simpler. A further step deals with the choice of the most suitable substrate. It is a general statement that such substrate must be cheap, lightweight, abundant and provide sufficient humidity barrier to ensure long-term solar cell operation. Additionally, different solar cell technologies present different requirements that might limit the choice, especially those concerning thermal stability. In the case of CdTe-based solar cells, most of the fabrication technologies require temperatures in the range of 450–500 °C, which most commercially available transparent polymers cannot withstand. Dark yellow Kapton and Upilex™ are the polymide films used in this case [2]. Similar requirements and solutions are found when dealing with Cu (In,Ga)Se2 (CIGS) solar cells [3]. Less strict temperature limitations exist in amorphous silicon (a-Si:H) solar cell production, though best-quality material is obtained at temperatures between 150 and 300 °C [4].

Microcrystalline silicon (μc-Si:H) technology is now also focusing on low-temperature deposition, and material with good transport and optoelectronic properties has been reported using deposition techniques like plasma-enhanced chemical vapour deposition (PECVD) [5] and hot-wire CVD (HWCVD) [6]. This technology is compatible with the use of transparent and low-cost thermoplastic polymer substrates like polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or ethylene-tetracyclododecene co-polymer (E/TD). μc-Si:H solar cells deposited at 100 °C on E/TD [7] and below 80 °C on PET [8] as well as microelectromechanical systems (MEMS) [6], have already been reported. Additionally, another emerging technology compatible with the use of the above-mentioned plastic substrates is that of organic semiconductor devices. In this field, solar cells have already been deposited on PET [9] and other devices like thin film transistors (TFT) on PEN [10].

In μc-Si:H technology, like in many thin film technologies, the need to reduce time and costs—thus, to reduce the thickness of the active layer–has highlighted the role of light trapping strategies, like the use of textured front contacts and/or back reflectors, that allow enhanced light absorption in thinner devices [11]. Additionally, a reduction in the active layer of the devices also enhances the open circuit voltage and the fill factor due to enhanced carrier collection. Light trapping can be achieved by using either a textured transparent conductive oxide (TCO) or a textured substrate (glass or plastic). Furthermore, these benefits arising from the use of textured substrates to increase light confinement are also promising for the solar cell technology based on organic semiconductors.

Our main goal is the future use of PEN as substrate in low-temperature deposited μc-Si:H solar cells. The transparency of this polymer, despite not being as good as that of glass or ethylene tetrafluoroethylene (ETFE), allows its use both as transparent front substrate or as metal coated back substrate. This work is a preliminary technological step in such development focusing, on one hand, on the deposition of TCO layers (ZnO:Al) on top of PEN. On the other hand, our approach to light trapping using polymer substrates will be to transfer controlled roughness to the PEN substrate by hot-embossing lithography (HEL) aiming to a future use in PEN/metal/TCO/μc-Si:H(n–i–p)/TCO structured solar cells. This imprinting technique allows the reproduction of a surface on a plastic substrate heated above its glass transition temperature (Tglass). Besides, both the capability to produce repeatable features over a large area and the fact that a given master can be used several times make HEL a very interesting technique [12], [13].

The choice of the most suitable plastic substrate was a crucial aspect in this work, as it must be “texturable” by HEL and compatible with our state-of-the-art device-quality silicon thin film deposition. PEN is a semi-crystalline, thermoplastic polyester material, with a Tglass ∼125 °C, but a working temperature up to 155 °C [14]. It has good optical clarity— which makes it a feasible candidate for both substrate and superstrate structures—and is chemically resistant to most diluted acids and organic solvents [15].

In this paper we present results concerning the optimisation of the structural, optical and electrical properties of ZnO:Al layers deposited on PEN, comparing the results with those achieved on glass. Additionally, results regarding random roughness transference on PEN using HEL are also presented. Morphological analysis has been carried out to verify the reproducibility of the master in the stamping process and the conservation of the texture with the subsequent depositions. Reflector structures (PEN/Ag/ZnO:Al) on textured polymer have also been studied optically.

Section snippets

Experimental

The ZnO:Al samples have been deposited by RF magnetron sputtering at room temperature, i.e. with no intentional heating, on PEN (0.125 mm-thick) and Corning glass on the same run. The target used was ZnO:Al2O3 (98:2 wt%, 99.999% purity). Different RF power values have been considered using an Ar gas pressure of 3×10−3 mbar. Structure of the samples was analysed by X-ray Diffraction (XRD) in a Bragg–Brentano θ/2θ configuration Siemens D-500 diffractometer. Dark conductivity at room temperature (σRT

ZnO:Al on PEN at low temperature

A series of samples has been deposited simultaneously on glass and PEN without any intentional heating of the substrate at P=3×10-3mbar, and changing the RF power used between 60 and 120 W, which in our case caused a variation in the power density between 3 and 6 W cm−2. Good adherence to the substrate was observed, regardless the substrate considered. Nevertheless, bending of the PEN substrates, not possible in the case of the glass ones, indicated compressive stress. This difference between the

Conclusions

The viability to use PEN as substrate for low-temperature solar cells has been studied in this paper. ZnO:Al layers have been deposited on PEN and glass by RF magnetron sputtering at room temperature. Optical transmittance measurements indicated that the PEN/ZnO:Al structure is suitable to be used in solar cells with light entering the device through the substrate. Moreover, the electrical values obtained also pointed in the same direction.

Hot-embossing lithography has been proven as a viable

Acknowledgements

This work has been financed by the Spanish Government (MAT2001-3541-C03-01). The authors also want to thank the Scientific–Technical Services of the Universitat de Barcelona for the AFM, XRD and SEM measurements, and Dr. Chris Mills from the Laboratori de Recerca en Nanobioenginyeria del Parc Científic de Barcelona for the stamping of the polymers and the interferomentry measurements.

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