Spontaneous formation of nanoripples on the surface of ZnO thin films as hole-blocking layer of inverted organic solar cells

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Abstract

A simple method for spontaneous formation of nanoripples on ZnO thin films was developed, and these nanostructured ZnO films were used as hole-blocking layer in inverted organic solar cells. Moreover, the size (height) of nanoripples on ZnO surface could be controlled in the range of several tens of nanometers. Among various ZnO films, surface structures with ∼70 nm-high nanoripples resulted in the best photovoltaic performance of the organic solar cell consisting of a stack of indium tin oxide/ZnO/ regioregular poly (3-hexyl thiophene), phenyl-C61-butyric acid methyl ester/Ag. The power conversion efficiency of inverted organic solar cells consisting of with 70 nm-high ZnO nanoripples (∼3.2%) was higher than that of a relatively flat ZnO surface by a factor of ∼2. Existence of nanoripples on ZnO results in a higher contact area between ZnO and active layer, leading to an enhanced photovoltaic performance.

Highlights

► Nanoripples with variable sizes were created on ZnO thin films. ► ZnO was used as hole-blocking layer of inverted organic solar cells. ► Existence of nanoripples on ZnO enhanced photovoltaic performances.

Introduction

Use of solar light as energy source has been regarded as one of the environmental-friendly ways of producing energy. Among various energy devices using light, organic solar cell (OSC) has been attracting particular attention for the last decades due to its cost-effectiveness and potential application in flexible devices [1], [2], [3], [4], [5]. One of disadvantages of OSCs is its short life-time, which is due to the fast (photo)-degradation of various interfaces existing in OSCs [6], [7], [8]. In order to increase stability of OSC, an inverted structure of OSC (IOSC) has been developed: in a regular device structure, electron–hole pairs are created in polymer active layers, holes and electrons are injected into transparent conducting oxide and counter-electrode (e.g. Al), respectively. In contrast, in IOSC, electrons are injected into the transparent conducting oxide, i.e. the electrode alignment is reversed [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].

Various methods have been used for improving power conversion efficiency of OSCs. Different polymers were used as light-harvesting layer for enhancing power conversion efficiency of OSCs [21], [22]. Carrier mobility has been increased by doping highly-conducting nanostructures such as carbon nanotubes and metal nanoparticles in active and buffer layers existing in OSCs [23], [24]. Methods for increasing light absorption and more efficiently separating electron–hole pairs have also been developed using various intermediate layers and modifiers [25], [26], [27], [28], [29].

Among various nanomaterials used as electrodes and buffer layers in OSCs, nanorods and wires with a high conductivity and increased contact area between electrodes and active layers have attracted much attention [5], [30], [31], [32], [33], [34], [35], [36], [37]. Synthesis of nanowires can be achieved using electrochemical or hydrothermal methods [34], [35], [36], [37], [38], [39]. In the present work, a simpler strategy for the formation of ZnO nanoripple structure, whose dimension is comparable to that of nanowires, is presented. Surface structures comparable to the nanowire-incorporated thin films can spontaneously form by properly heating spin-coated ZnO films. For spin-coating of ZnO, various mixtures of ZnO nanoparticles and zinc acetate solution were used. It is demonstrated that by adjusting the amount of ZnO naonaprticles added in zinc acetate solution nanoripple size can be controlled for the best device performance of IOSC consisting of these ZnO thin films on transparent conducting oxide as hole-blocking and electron-colleting layer.

Section snippets

Experimental

The IOSCs studied in the present work consisted of a stack of 150 nm thick-indium tin oxide film on glass/ZnO/active layer/Ag as shown in Fig. 1. For the preparation of diverse ZnO thin films, ZnO sol–gel solutions with various amounts of ZnO nanoparticles were spin-coated and heated differently (Fig. 2). The mean thickness of ZnO film was about 70 nm, and the detailed structure of each ZnO film is shown in Fig. 2. For the preparation of ZnO sol–gel solutions zinc acetate [Zn(CH3COO)2.2H2O] was

Results and discussion

As shown in Fig. 1, the IOSCs fabricated and studied in the present work consisted of a stack of indium tin oxide-coated glass, ZnO thin films, RR-P3HT:PCBM layer and Ag electrode. As it has been already demonstrated by Sekine et al. surface of spin-coated ZnO thin films using zinc acetate solution on indium tin oxide could be converted into a ridge structure by the following procedures: the sample was heated from room temperature to 350 °C with a heating rate of 11 °C/min, and the heating was

Conclusion

A simple route for formation of various ZnO nanoripples on ZnO thin films was developed. Structure of ZnO surfaces with nanoripples can be controlled by various amounts of ZnO naoparticles added to zinc acetate solution for spin-coating of ZnO thin films. Photovoltaic performances of the IOSC consisting of these ZnO thin films depended on the surface structure of ZnO, i.e. the photovoltaic performance could be optimized by adjusting the amount of ZnO nanoparticles used for the fabrication of

Acknowledgements

This study was supported by the Korea Institute of Materials Science (KIMS) and the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant (No. 20103020010050) funded by the Ministry of the Knowledge Economy, Republic of Korea.

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