Effects of thermal annealing on the efficiency of bulk-heterojunction organic photovoltaic devices

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Abstract

This paper studies the effect of the annealing process on the performance of P3HT/PCBM photovoltaic devices, in terms of their efficiencies. The basic photovoltaic devices are annealed on a hot-plate, at a temperature of 150 °C for 10 min in air. For comparison, the thermal annealing of photovoltaic devices is carried out using rapid thermal annealing (RTA) equipment, at a temperature of 150 °C for 10 min in different environments such as in vacuum, in nitrogen and in argon, individually. The light conversion efficiency (Eff) of the resulting photovoltaic devices increases from 2.29% (hot-plate annealing) to 2.77% after annealing in a vacuum environment. As a result, the organic photovoltaic devices, annealed in a vacuum show enhanced efficiencies compared with those annealed in different gas environments.

Introduction

Conjugated polymers have been widely used in organic photodiodes and organic photovoltaic devices based on organic semiconductor materials, because of their advantages, which include the following: they require a simplified fabrication process which is also at low-temperatures, they are low-cost, have good flexibility, high-quantum efficiencies, and high-absorption coefficients [1], [2], [3]. The high efficiency photovoltaic devices in particular, based on conjugated polymer and fullerene composites, are key in many important applications such as wearable displays, radio frequency (RF) identification tags and smart cards [4], [5]. Flexible batteries and photovoltaic devices might also be possible candidates for this type of application. Thus, because of these practical advantages, previous research topics have focused on high efficiency [6], [7], wide band gap [8], [9], large area processing [10], [11], stability [12], [13], free transparent electrodes [14], [15] and others [16], [17]. However, organic photovoltaic device performances are generally poor in comparison with photovoltaic devices based on III–V compounds or silicon [18], but recent studies have reported remarkably improved performances of the these photovoltaic devices, by up to 5% in a single device structure, through studies such as varying the ratio of donor–acceptor materials [19], [20], optimizing the annealing time and the annealing temperature of the active layer [21], [22], the type of electrical treatment used [23], [24], the type of adhesive layer on or under the anode or cathode layer [25], [26], and using various active materials having a wide band gap [6], [27]. The performances of the photovoltaic devices in particular changes significantly at different annealing temperatures, such as the phase separation for the bulk-heterojunction formation, the evolution of the film morphology and the device efficiency.

This paper investigates the effect of the annealing process on the efficiency of P3HT/PCBM photovoltaic devices due to variations in the annealing conditions in different environments (vacuum, N2, Ar are studied here) by RTA equipment, and comparing with the traditional annealing in air by hot-plates.

Section snippets

Experimental

Fig. 1 shows the schematic diagram of the bulk-heterojunction photovoltaic devices, and their energy band structure. The fabrication procedures, and the device characteristics, are measured using the following method. The glass substrate with patterned indium thin oxide (ITO) film is used for the substrate of the photovoltaic devices. The sheet resistance is 30 Ω/square, and the film thickness is 150 nm. The glass substrate is chemically cleaned according to the method based on successive baths

Results and discussion

Fig. 2 shows the AFM image of the surface morphology of the photovoltaic devices annealed by hot-plate in air, and the devices annealed using RTA equipment at a temperature of 150 °C for 10 min in environments of a vacuum, nitrogen, and argon, individually. The root mean square (RMS) roughnesses are 2.45 nm, 2.86 nm, 3.11 nm and 3.50 nm for annealing in vacuum, nitrogen, argon, and hot-plate for the P3HT:PCBM thin-films, respectively. The surface of the spin-coated organic film is both homogeneous

Conclusions

The effects of the annealing process on the performance of P3HT/PCBM photovoltaic devices have been studied in terms of the resulting efficiencies. In order to analyze the effects of the annealing processes for producing organic photovoltaic devices, the devices have been annealed using thermal RTA equipment, at a temperature of 150 °C for 10 min in different environments of a vacuum, nitrogen, and argon, individually. The basic photovoltaic devices have been annealed on a hot-plate, at a

Acknowledgements

This work was supported by the IT R&D Program of MKE/KEIT (No. 2009-F-016-01, Development of Eco-Emotional OLED Flat-Panel Lighting), the IT R&D Program (No. 2008-F-024-02, Development of Mobile Flexible IOP Platform) of MKE in Korea, the ERC program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST), (No. R11-2007-045-01003-0), and the National Research Laboratory (NRL, No. R0A-2007-000-20111-0) Program.

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