High-performance polymer photovoltaic cells with thick P3HT:PCBM films prepared by a quick drying process

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Abstract

In this manuscript, we report high-performance polymer photovoltaic cells with P3HT:PCBM film prepared from polymer solution added with 1-dodecanethiol (P3HT and PCBM are poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, respectively). The addition of 1-dodecanethiol into polymer solution resulted in remarkable enhancement in the energy conversion efficiency of polymer photovoltaic cells. The efficiency enhancement for the device with a P3HT:PCBM (1:2 w/w) film is more significant than for the device with a P3HT:PCBM (1:1 w/w) film. Devices with energy conversion efficiency as high as 4% were demonstrated with a thick P3HT:PCBM film prepared by a quick drying process.

Introduction

Polymer photovoltaic cells, which have architecture of a polymer film sandwiched between two electrodes, have gained great attention, since they provide a unique solution for cheap, clean and renewable energy, arising from the fabrication of the active polymer layer through a simple solution processing, such as printing or coating [1], [2], [3], [4], [5], [6], [7], [8], [9]. They can even be portable renewable energy sources due to the high mechanical flexibility of materials [10], [11], [12], [13]. Though Indium-tin oxide (ITO) is usually used as the transparent electrode for polymer photovoltaic cells, it has problems of poor flexibility and the shortage of indium in earth. Replacement of ITO by other flexible and transparent materials, such as conducting polymers and nanowires, had been demonstrated in polymer optoelectronic devices [14], [15], [16], [17], [18], [19], [20], [21].

To significantly enhance the photovoltaic efficiency is still one of the most important tasks for the polymer photovoltaic cells. The light-to-electricity conversion process involves four steps: (1) light absorption and exciton formation, (2) exciton dissociation, (3) charge transport, and (4) charge collection. These four steps are affected by the chemical structure of active materials, that is, the donor and the acceptor, and the morphology of the active polymer film [4], [5], [22], [23], [24]. P3HT and PCBM become popular polymer donor and organic acceptor for polymer photovoltaic cells due to their easy availability, excellent solubility in common organic solvents, and high mobility. Great progress has recently been made on P3HT:PCBM photovoltaic cells. The early polymer photovoltaic cells using a P3HT:PCBM film fabricated through a quick drying process usually exhibit low energy conversion efficiency of less than 2.5%. Two approaches have been developed to enhance the energy conversion efficiency up to 4–5%. One approach is the heat treatment of the P3HT:PCBM film [25], [26], and the other approach is the preparation of a thick P3HT:PCBM film through a slow drying process [27]. However, these two approaches are not feasible for the fabrication of polymer photovoltaic cells in practical application. An efficient way to fabricate high-performance polymer photovoltaic cells requires the preparation of the active polymer film through an easily controllable and quick drying process.

A recent finding by Peet et al. [28] that the introduction of alkyl thiol into polymer solution enhanced the efficiency of polymer photovoltaic cells provides a solution to fabricate high-performance polymer photovoltaic cells with the active polymer film prepared through a quick drying process. Though efficiency increase was observed for P3HT:PCBM photovoltaic cells by introducing alky thiol or other additives into the P3HT:PCBM solution, the energy efficiencies reported were below 3% [29], [30]. This is probably due to a thin P3HT:PCBM film used in the devices, while a thick polymer film can enhance the light absorption [31]. Here, we report the efficiency enhancement of various P3HT:PCBM photovoltaic cells by addition of 1-dodecanethiol into the P3HT:PCBM solution and high-efficiency polymer photovoltaic cells with a thick P3HT:PCBM film fabricated through a quick drying process. P3HT:PCBM photovoltaic cells with energy conversion efficiency of 4% were demonstrated.

Section snippets

Experimental details

P3HT, which was synthesized by Rieke Metals, Inc., and PCBM were purchased from Sigma-Aldrich and were used without further purification. P3HT:PCBM solutions were prepared by dissolving P3HT and PCBM in 1,2-dichlorobenzene under vigorous stirring overnight. 1-dodencanethiol was added into the P3HT:PCBM solution 2–3 h before the preparation of P3HT:PCBM films.

The devices were fabricated through the following process. At first, a PEDOT:PSS

Effect of 1-dodecanethiol on optical properties and photovoltaic performance of thin P3HT:PCBM films

At first, we investigated the effect of 1-dodecanethiol on the optical properties and photovoltaic performance of thin P3HT:PCBM films. Fig. 1 shows the UV–vis absorption spectra of P3HT:PCBM films, which had a thickness of 100 nm. The absorption of the P3HT:PCBM film increased, and the vibronic features became more remarkable after the addition of 1-dodecanethiol into the P3HT:PCBM solution. This effect is similar to the observation by Pivrikas et al. [30]. The P3HT:PCBM (1:2 w/w) film had

Conclusions

In conclusion, the addition of 1-dodecanethiol could enhance the absorption and photovoltaic performance of P3HT:PCBM films with different P3HT to PCBM weight ratios. The enhancement is more salient for the P3HT:PCBM film with more PCBM. High-performance P3HT:PCBM photovoltaic cells with a thick P3HT:PCBM film prepared from P3HT:PCBM solution added with 1-dodecanethiol through a quick drying process were demonstrated. The devices with 4% energy conversion efficiency were obtained. The presence

Acknowledgement

This research work was supported by the National University of Singapore (Project no.: RG-284-001-136).

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