ReviewStability and degradation of unencapsulated CuPc bilayer heterojunction cells under different atmospheric conditions
Introduction
There has been a great deal of interest in the study of organic semiconductor materials for photovoltaic (PV) applications in recent years. Organic semiconductors have interesting chemical, physical, and optical properties for thin films (<100 nm) including high absorption coefficients in the visible region of the solar spectrum and photocurrent generation, making them promising compounds for photovoltaic devices [1], [2]. 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], [4], [5], [6], [7], [8]. Organic polymeric [9], [10], [11] and monomeric (small molecular) [12], [13], [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. CuPc is a p-type semiconductor and hole conducting material that serves as an electron donor [13], [15], [16], [17], [18]. Perylene derivatives (PV) show n-type semiconductor behavior and can serve as electron acceptor materials [13], [19], [20].
Several methods have been discussed in order to improve the power conversion efficiency for small molecule OPV devices. Recently, Heliatek GmbH, technology reported a high efficiency small molecule OPV with 12.0% cell efficiency [21]. A critical and problematic issue for organic solar cells is lifetime. Improving the stability of OPV devices under continuous illumination is one of the most crucial issues to be addressed. Stabilities reaching up to 4500 h under continuous illumination have been reported for the devices protected from oxygen and humidity [22]. Although significant work was carried out on CuPc based solar cells, the influence of oxygen and humidity on their photovoltaic performance have not been addressed. In the present investigation, we have made attempts to probe the influence of air and humidity in their OPV performance of CuPc based solar cells and to understand the underlying mechanism. We studied the degradation of electrical performance of CuPc solar cells in air and under different conditions as a function of time for 70 days with illumination only during testing. This separates the effects of reactions with the atmosphere from effects due to illumination and addresses the environmental factors on their stabilities. Small molecule OPV devices with the structure of ITO/PEDOT:PSS/CuPc/PTCDI/Ag (described in detail below) were fabricated and studied under different conditions: vacuum, dry air, standard air atmosphere, and pure oxygen.
Section snippets
Experiment
The structure of our OPV bilayer solar cell is shown in Fig. 1. The substrates were 25 mm×25 mm×1.1 mm glass, pre-coated with 1200–1600 Å indium tin oxide (ITO), purchased from Sigma-Aldrich. The substrates have a sheet resistance of 8–12 Ω/sq and 84% transmittance. 25% of the ITO area was etched to avoid short circuit formation while connecting to the silver cathode. The etching procedure was as follows: 75% of the ITO was masked by adhesive tape and the unmasked part was etched by dipping (for 20
Results and discussion
Fig. 2 shows all the photovoltaic characteristics, normalized to their maximum value as a function of time for Device A (regular atmosphere). The performance is shown in Table 2. The performance of the cell (except FF) shows rapid degradation during the measurement period. The half-life, where the efficiency falls by 50%, was reached in less than 10 days. Loss of efficiency results from decreases in both Jsc and the Voc.
H2O and oxygen are believed to be two of the most important factors for the
Conclusion
The stability of ITO/PEDOT:PSS/CuPc/PTCDA/Ag cells were studied under standard air, dry air, pure oxygen, and vacuum conditions. The cells were studied with minimum illumination to determine the role of air and humidity on their stability and performance. Oxygen exposed cells show rapid degradation during the measurement period even in the absence of humidity. Cells kept under vacuum did not show much degradation. Our structured OPV device is sensitive to an oxygen atmosphere because oxygen can
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