Stability and degradation of unencapsulated CuPc bilayer heterojunction cells under different atmospheric conditions

Abstract

The stability of small molecule copper phthalocyanine bilayer organic solar cells (ITO/PEDOT:PSS/CuPc/PTCDI/Ag) was investigated without encapsulation for 70 days. Photovoltaic performance characteristics were measured as a function of the age of solar cells without illumination (except during testing) under vacuum, dry air (0% humidity), and standard air conditions. The solar cells exposed to dry air and standard air atmosphere showed a rapid degradation in their efficiencies during the measured period. It was found that oxygen has a strong negative influence on the efficiency even in the absence of humidity. In vacuum, we observed a small increase in the performances of organic solar cells for the first days, possibly due to removal of oxygen from the cells, followed by a minor degradation over the measurement period. Moving the cell from vacuum into a dry oxygen atmosphere led to rapid degradation in their efficiencies. A film of CuPc layer exposed to air results in a steady increase in the resistivity of the device.

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.