Influence of Thickness and Annealing Temperature on the Optical Properties of Spin-Coated Photoactive P3HT:PCBM Blend

Md. Shahinul Islam, Md. Earul Islam, Abu Bakar Md. Ismail, Hartmut Baerwolff

Department of Analog & Optoelectronics, Cologne University of Applied Sciences, Cologne, Germany.
Department of Applied Physics & Electronic Engineering, Rajshahi University, Rajshahi, Bangladesh.
Department of Material Science & Engineering, Rajshahi University, Rajshahi, Bangladesh.

DOI: 10.4236/opj.2013.38A004                

Abstract

Influence of annealing temperature and thickness on the optical characteristics of the blend of poly (3-hexylthiophene) (P3HT) and Phenyl C61 butyric acid methylester (PCBM) layer has been investigated in this report. Photoactive polymer material (P3HT:PCBM) was deposited on indium tin oxide (ITO) substrate by spin-coating. The morphology of P3HT:PCBM composite layer was investigated by Atomic Force Microscope (AFM). The surface roughness was found to reduce after heat treatment. The absorption of the composite layer was found to increase with its number of layer (thickness). On the other hand, the photoluminescence (PL) quenching, which indicates efficient charge separation in the bulk heterojunction, was found higher for the thinner layer. Absorption was also found to increase with the annealing temperature. Therefore, to optimize the thickness of the P3HT:PCBM photoactive layer that will provide best absorption while providing efficient charge separation, annealing at optimized temperature might be an effective tool.

Keywords

Organic Semiconductor; Photoactive Polymer; Bulk Heterojunction; Solar Energy Materials

1. Introduction

Recently superb performance has been demonstrated from the bulk heterojunction photovoltaic devices using P3HT as donor and PCBM as acceptor [1-3]. P3HT and PCBM have several advantages such as low-cost fabrication, large-area devices, lightweight cells with flexibility, and material diversity [4,5]. In a blend, the performance of P3HT and PCBM as a thin-film heterojunction depends on various parameters, such as film growth methods [6], processing conditions [7], morphology of the deposited thin-film [8], heat treatment [9] etc. It has useful optoelectronic properties such as, a good chemical stability, high mobility, and low optical band gap [10,11]. Especially, with higher hole-mobility of 0.1 cm²/V.s, P3HT offers the possibility of realizing high-efficiency BHJ PV cells [12,13]. P3HT:PCBM-based device performance is highly influenced by the molecular weight of P3HT, the dissolving solvent, and thermal annealing [14-16], and researchers have focused on the optical/morphological properties of P3HT:PCBM films. Based on the above background in this report, the change in optical properties of the blend of P3HT:PCBM with film layer thickness and annealing has been studied. We found that the annealing temperature might be an effective tool for optimizing the thickness of the P3HT:PCBM layer that might provide best absorption and efficient charge separation.

2. Experimental

Readymade blend of P3HT and PCBM at 1:1 ratio was bought from Sigma-Aldrich, Germany, and was used without further purification. ITO thin films were deposited on a glass substrate at room temperature by electron beam evaporation technique. The commercially available ITO powder (99.99% pure) obtained from Inframat Advanced Materials, USA, and was used as the evaporation source material. The ITO films were annealed at 600˚C in air to obtain a transparency of 90%. The blend of P3HT:PCBM was warmed at 45˚C for 20 minutes to make it homogenous prior to spin coating. This blend was then deposited by spin coating on ITO at speed of 200 rpm in air. After deposition whole structure was annealed in air at 130˚C for 2 minutes.

3. Results and Discussion

The surface morphology of as-deposited and annealed P3HT:PCBM composite layer was studied by AFM. The rough surface represents the self-organization of P3HT that also indicate ordered structure formation in the thin film [17]. But it can also lead to poor contact with the electrode. The roughness of our as-deposited layer (0.5 nm) reduced significantly (0.04 nm) after annealing as evident from Figure 1.

The absorption characteristics of P3HT:PCBM composite film was investigated in the wavelength 300 - 700 nm. The P3HT:PCBM film showed a significant change in absorption when the layer thickness was increased. From Figure 2 we observe that absorption increased with the increasing layer thickness. For the as spun films all the films with various thicknesses showed a main peak around 510 nm. But when the layer thickness was increased to 3-layers (~150 nm) a shoulder peak appeared around 445 nm. This peak is believed to originate from the highly crystalline P3HT domains with improved stacking [17,18].

Figure 3 shows the optical absorption spectra after heat treatment (annealing) for single layer of P3HT:

Conflicts of Interest

The authors declare no conflicts of interest.

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