Elsevier

Synthetic Metals

Volume 161, Issues 19–20, October 2011, Pages 2174-2178
Synthetic Metals

Efficiency enhancement of inverted organic photovoltaic devices with ZnO nanopillars fabricated on FTO glass substrates

https://doi.org/10.1016/j.synthmet.2011.08.025Get rights and content

Abstract

ZnO nanopillars are grown on a ZnO seed layer on fluorine-doped tin oxide (FTO)-coated glass substrates for the fabrication of inverted organic photovoltaic devices based on poly(3-hexylthiophene) and (6,6)-phenyl C61-butyric acid methyl ester. It is found that the oriented ZnO nanopillars play an important role in collecting photogenerated electrons and act as an electron-transport path to the cathode. OPV devices with a ZnO nanopillar layer grown on a ZnO seed layer exhibit a threefold increase in power conversion efficiency compared with that of devices with a ZnO seed layer only.

Introduction

Organic photovoltaics (OPVs) have attracted much attention during the last decade due to their numerous potential advantages including relatively inexpensive and light-weight materials, compatibility with flexible plastic substrates, and ease of fabrication [1], [2]. The most commonly used strategy is to create a so-called bulk heterojunction (BHJ) structure with two separated phases on a nanometer scale through mixing an electron donor material such as poly(3-hexylthiophene) (P3HT) and an electron acceptor such as (6,6)-phenyl C61-butyric acid methyl ester (PCBM) [3], [4]. This BHJ nanophase separation increases the interfacial area where the photogenerated excitons are efficiently dissociated into free charge carriers (holes and electrons), which transport to and are collected at respective electrodes. For BHJ structure based organic semiconductor materials, the optimal thickness of the active layer is typically ∼100 to 200 nm, which is determined by a tradeoff between light absorption and charge carrier transport in the active layer [5], [6]. Increasing the active layer thickness can enhance the absorption and thus the photogeneration of charge carriers. However, when the thickness exceeds the optimal value, the overall power conversion efficiency (PCE) begins to fall [7], [8]. This could be attributed to the increased charge recombination and/or space charge effect, which are derived from the low charge carrier mobilities of the organic semiconductors and the increased series resistance of the thick organic photoactive layer [9]. The optimum active layer thickness could be increased by introducing pre-established inorganic nanostructures to improve charge carrier transport and collection. One type of promising nanostructures is the vertically oriented inorganic semiconductor nanopillars [10], [11]. Crystalline semiconductor nanopillars are advantageous for highly efficient charge transport once the charges are captured by the nanopillar array “antennas” [12]. Accordingly, it might be possible to achieve both enhanced absorption and improved charge transport for OPV devices with a thicker active layer using crystalline nanopillars. ZnO is viewed suitable for such nanostructured photovoltaic devices due to its high electron mobility and good transparency in the visible wavelength range [13], [14], [15], [16]. Moreover, ZnO nanocrystals can be grown vertically on substrates, enabling the fabrication of ZnO nanopillar OPVs with efficient electron transport and collection [17], [18].

Previous studies on ZnO nanopillar-embedded inverted OPVs used exclusively indium–tin-oxide (ITO) coated glass substrates as cathode [12], [13], [17], [18], [19], [20]. It is known that the use of ITO substrates for mass production of solar cells will become problematic due to the limited resource of indium. On the other hand, fluorine-doped tin oxide (FTO) is inexpensive as well as chemically and thermally stable compared with ITO. Although FTO glass substrates have been widely used for dye-sensitized solar cells, their application in BHJ OPVs is not well established. In this study, we use FTO glass substrates to grow ZnO nanopillar arrays to fabricate inverted OPV devices. It was found that uniform and vertically aligned ZnO nanopillars could be readily formed on FTO glass substrates. The influence of ZnO nanopillar arrays on the device performance was investigated. Compared with OPVs with only a ZnO seed layer, the PCE of the OPVs with vertically aligned ZnO nanopillars on a ZnO seed layer was increased by approximately threefold. The improvements are considered due to the increased interfacial area of ZnO/organic semiconductors for effective exciton dissociation, as well as the improved electron collection and electron transport achieved by incorporating ZnO nanopillars. The highest power conversion efficiency (PCE) of 1.02% achieved for the ZnO nanopillar based P3HT:PCBM OPV devices on FTO glass substrates is comparable to the PCE values reported for similar OPV devices fabricated on ITO glass substrates [17], [18], [19].

Section snippets

Experimental details

The aligned ZnO nanopillars were prepared via an established two-step solution approach [21], [22]. A ZnO precursor dispersion was obtained by adding LiOH to a Zn(Ac)2 solution in ethanol. A ZnO seed layer was then deposited on a fluorine-doped tin oxide (FTO)-coated glass substrate by spin coating the ZnO precursor dispersion at 5000 rpm for 30 s, followed by heating at 300 °C for 3 h in air. ZnO in the seed layer adopted a preferential orientation with the c-axis perpendicular to the substrate,

Results and discussion

Fig. 1(a)–(c) shows the FE-SEM images of the aligned ZnO nanopillars grown a ZnO seed layer on an FTO glass substrate. The cross section of the sample in Fig. 1(a) shows clearly the ZnO nanopillars on the ZnO nanoparticle seed layer. The length of the nanopillars is about 1 μm. The ZnO nanopillars in Fig. 1(a) are less oriented, probably due to the damage caused by sample cutting. Areas away from the edges as revealed in Fig. 1(b) and (c) are comprised very well aligned nanopillars with the long

Conclusions

Uniform and vertically aligned ZnO nanopillars on a ZnO seed layer were prepared based on an aqueous solution method on FTO glass substrates, which were used to fabricate P3HT:PCBM OPV devices. The overall photovoltaic performance of the devices with a ZnO nanopillar layer was greatly enhanced compared to OPV devices with a ZnO seed layer only. It is considered that the vertically aligned ZnO nanopillar array layer in the OPV devices could increase the interfacial area between ZnO and P3HT:PCBM

Acknowledgements

This work has been financially supported by the National High-tech R&D Program (Grant No. 2009AA05Z422), the National High-tech R&D Program (Grant No. 2009AA050602), the National Basic Research Program of China (Grant Nos. 2011CBA00705, 2011CBA00706, 2011CBA00707).

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