A new method to improve the lifetime stability of small molecule bilayer heterojunction organic solar cells
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)
Organic photovoltaics: technology and market
Solar Energy Materials & Solar Cells
(2004)- et al.
Flexible, long-lived, large-area, organic solar cells
Solar Energy Materials & Solar Cells
(2007) - et al.
Organic solar cell modules for specific applications—from energy autonomous systems to large area photovoltaics
Thin Solid Films
(2008) - et al.
Large area plastic solar cell modules
Materials Science and Engineering B
(2007) - et al.
Thermo-cleavable solvents for printing conjugated polymers: application in polymer solar cells
Solar Energy Materials & Solar Cells
(2009) Polymer solar cell modules prepared using roll-to-roll methods: knife-over-edge coating, slot-die coating and screen printing
Solar Energy Materials & Solar Cells
(2009)- et al.
A complete process for production of flexible large area polymer solar cells entirely using screen printing—first public demonstration
Solar Energy Materials & Solar Cells
(2009) - et al.
Reel-to-reel wet coating as an efficient up-scaling technique for the production of bulk-heterojunction polymer solar cells
Solar Energy Materials & Solar Cells
(2009) - et al.
Pathways for the degradation of organic photovoltaic P3HT:PCBM based devices
Solar Energy Materials & Solar Cells
(2008) - et al.
Low work function metal modified ITO as cathode for inverted polymer solar cells
Solar Energy Materials & Solar Cells
(2010)
Copper phthalocyanine films deposited by liquid–liquid interface recrystallization technique (LLIRCT)
Journal of Colloid and Interface Science
Cited by (17)
Highly enhanced long time stability of perovskite solar cells by involving a hydrophobic hole modification layer
2017, Nano EnergyCitation Excerpt :In contrast, the ZnPc based device slowly degraded to 81% of its original PCE after 50 h illumination. It is possible that ZnPc absorbs UV light, leading to better stability under illumination [5,38]. For further characterization of humidity sensitivity, two kinds of devices were kept in RH 75% atmosphere provided by a NaCl saturated solution in a sealed vessel (Fig. 4b).
Stability and degradation of unencapsulated CuPc bilayer heterojunction cells under different atmospheric conditions
2014, Solar Energy Materials and Solar CellsCitation Excerpt :The advantages of organic photovoltaics (OPV) compared with conventional inorganic solar cells are due to low cost fabrication, light weight, semi-transparency, and mechanical flexibility which could expand the range of applications for which solar cells could be used [3–8]. Organic polymeric [9–11] and monomeric (small molecular) [12–14] materials are under extensive investigation for these devices. Copper phthalocyanine (CuPc) is a well-known small molecule organic semiconductor with high chemical stability and high optical absorption.
Composite interfacial modification in TiO<inf>2</inf> nanorod array/poly (3-hexylthiophene) hybrid photovoltaic devices
2013, Journal of Alloys and CompoundsCitation Excerpt :Over the past decades, organic bulk hetero-junction (BHJ) solar cells have attracted considerable attention because of their potential application in the low-cost, large-area and flexible energy generation devices [1–5].
Recent progress in organic-metal complexes for organic photovoltaic applications
2023, Materials Chemistry FrontiersEFFECT of SUBSTRATE TEMPERATURE on the STRUCTURAL and OPTICAL PROPERTIES of NiTsPc THIN FILMS
2020, Surface Review and Letters