A new method to improve the lifetime stability of small molecule bilayer heterojunction organic solar cells

https://doi.org/10.1016/j.solmat.2012.11.006Get rights and content

Abstract

Unencapsulated small molecular solar cells with a layer of carboxylated copper phthalocyanine (CuPc⁎) introduced between the donor copper phthalocyanine (CuPc) and acceptor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDI), and using ITO and Ag as the cathode and anode were investigated for 35 days. The performance and stability were compared between traditional ITO/PEDOT:PSS/CuPc/PTCDI/Ag cells and the new ITO/PEDOT:PSS/CuPc/CuPc*/PTCDI/Ag cells. The comparison shows that the modified cell has an enhanced stability compared to traditional cells. The improvement of the lifetime is likely due to the CuPc* layer working as buffer layer to prevent humidity and oxygen from diffusing into the active layer. Different thicknesses of CuPc were investigated to study the effects of CuPc layer thickness on the electrical properties of these devices. It was found that the properties of the cell with 20 nm thickness of CuPc were the best.

Highlights

► We added an additional layer to small molecule organic solar cells to improve stability. ► Stability was monitored over 35 days. ► Carboxylated copper phthalocyanine (CuPc*) improved device stability in air. ► Improvement was attributed to hydrophilic functional groups of the CuPc*.

Introduction

Many organic semiconducting materials have extremely interesting properties in terms of photocurrent generation, and exhibit very high absorption coefficients in the visible region of the solar spectrum for even <100 nm films, making them promising compounds for photovoltaic devices [1], [2]. Organic photovoltaic (OPV) cells have attracted research attention in recent years due to their potential advantages compared with conventional inorganic solar cells. These advantages include low fabrication cost, semi-transparency, light weight, and mechanical flexibility, which could expand the range of applications [3], [4], [5], [6], [7], [8], [9], [10], [11]. The low power conversion efficiency and the poor stability of OPV devices are the most critical factors hampering their application. In the last few years, much effort has been made on the device structure optimization to improve the power conversion efficiency and the lifetime of OPV devices. The stability of organic solar cells has been tested by several groups for devices using indium tin oxide (ITO) as an anode with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as a transparent anode buffer layer, which allows light to pass through and collects the holes generated in the active layer. Active donor and acceptor layers are used, with a low work function metal (e.g., Ag, Ca, Al) as the cathode on top of the cell for electron collection [12], [13], [14], [15], [16]. It was found that water and oxygen cause degradation upon exposure to air for OPV cells without encapsulation. Water absorption increases the series resistance of the OPV cell, leading to degradation of the cell performance [15]. OPV cells with small molecules as active layers such as phthalocyanines (Pcs) are highly sensitive to oxygen [17], [18]. Oxygen can easily diffuse into the active layer. The presence of oxygen leads to additional holes and acts as an electron acceptor, which lowers the mobility in the OPV cell. Electrons from the extended π-system or from a d-orbital of the central metal are transferred to the oxygen molecule. The radicals formed were able to reduce the singlet excitons that are formed in the Pc layer [19]. To achieve stability of small molecule bilayer solar cells, either the development of improved barrier materials for packing, or development of devices with less air sensitivity is required. In this paper, we describe a new method to improve the lifetime stability of small molecule bilayer OPV cells using the functionalized organic semiconductor material carboxylated copper phthalocyanine (CuPc*) as an active third layer. The CuPc* layer was introduced between the CuPc and PTCDI active layers which provided improved cell stability and enhanced performance.

Section snippets

Carboxylated CuPc (CuPc*) preparation

For the synthesis of CuPc*, the method described by Osburn et al [20] was employed, with slight modifications. CuSO4·5H2O was used as copper source instead of CuCl2. In the final steps of purification (water washings), it was found that the product partially dissolves in water when the pH becomes higher than ∼4. Therefore, the final washings were carried out with a 0.1 M HCl solution instead of pure water. The wet product was first dried in air, and then in high vacuum. Fig. 1 shows the chemical

Results and discussion

Fig. 4, Fig. 5 show current and voltage results over time for devices with and without CuPc*. The photovoltaic characteristics VOC and JSC are normalized to their maximum values. Rapid degradation and significant decreases in VOC and JSC were observed over 35 days for the standard device without CuPc*. The VOC falls by 50% in less than 25 days, and the JSC falls by 50% in less than 15 days. However, the VOC and JSC behavior of the device with the CuPc* layer showed a greater stability over the

Conclusion

A layer of carboxylated copper phthalocyanine (CuPc*) was investigated as an encapsulating layer deposited between the donor layer (CuPc) and acceptor layer (PTCDI). The traditional OPV bilayer device without CuPc* showed a rapid degradation in JSC and VOC. However, the bilayer solar cell with a layer of CuPc* showed significant improvement in the stability and performance. An OPV device with 20 nm of CuPc covered with CuPc* had the best electrical characteristics compared to other thicknesses

References (23)

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