Effects of long-term UV–visible light irradiation in the absence of oxygen on P3HT and P3HT:PCBM blend

Abstract

We report on the photochemical behaviour of the active layer of polymer solar cells when exposed to UV–visible light and temperature in the absence of oxygen. The paper focuses on the effects of UV–visible light irradiations on pristine poly(3-hexylthiophene) (P3HT) and subsequent blend with [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), both deposited on an inert substrate (KBr). The modifications of the chemical structure of the materials were monitored by UV–visible and infrared spectroscopies. We first observed a slow photochemical evolution of P3HT upon exposure to artificial accelerated photoageing. We observed that the process was considerably slowed down when P3HT was blended with PCBM. The comparison with the behaviour of MDMO-PPV-based active layers irradiated in similar conditions demonstrated that P3HT is much more stable. Finally, the extrapolation of the data obtained in conditions of artificial accelerated ageing to natural ageing suggested that the P3HT:PCBM blend would be chemically stable for at least three years under use conditions if well protected from oxygen.

Introduction

As the world energy demand continues growing and the cost of natural resources is rising, new solutions for cheaper and cleaner energy production are required. Among the various options, solar cells are expected to be a major contributor to fulfil the future needs. However, alternatives to silicon-based devices must be developed to reduce the power production costs. Due to the combination of several interesting properties (low-cost and fast manufacturing methods, large surface processability, lightness, and flexibility) polymer:fullerene bulk heterojunction organic solar cells (OSCs) are a promising solution to provide cheap electricity. Therefore, huge efforts have already been taken to improve their photovoltaic performances. The reported efficiencies have thus risen sharply during the past years, the highest efficiency measured being closed to 7.9% [1], [2], [3]. In contrast, less attention has been paid to another critical aspect of OSCs, such as their rather poor stability. OSCs are known to be oxygen and moisture sensitive [4], [5], [6], [7]. As a direct consequence, they must be encapsulated to overcome this drawback [8]. The second problem is that OSCs exhibit only weak resistance to elevated temperatures [9], [10], and paradoxically also degrade under illumination [5], [7], [11]. Until now, most of the published studies have focused on the operational lifetime of the devices [8], [12], [13], [14], [15], [16], [17], [18], [19], while the mechanisms that cause OSC degradation remain largely misunderstood. In particular, very few investigations have focused on the various routes by which the active layer evolves, such as degradation at the electrode interfaces, morphological evolutions in the bulk, or chemical degradation of the materials [7].

In this paper, the evolution of the chemical structure of the active layer components of OSCs under irradiation in the absence of oxygen is analysed. We focused on the most prominent system in OSCs: the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). P3HT and P3HT:PCBM thin layers were deposited on an inert substrate (KBr) and subjected to artificial accelerated ageing. A comparison with results obtained using MDMO-PPV and extrapolation of the data to natural ageing are also discussed.