Degradation of electrical properties of small molecule organic solar cells under oxygen and moisture
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Degradation of electrical properties of small molecule organic solar cells under oxygen and moisture Laura Ciammaruchi1,2, Aldo Di Carlo1 and Ching W. Tang2 1
CHOSE – Center for Hybrid and Organic Solar Energy, University of Rome Tor Vergata, Italy
2
Chemical Engineering department, University of Rochester, NY, USA
ABSTRACT We investigated the photovoltaic (PV) parameters of a planar hetero-junction solar cell with di-[4-(N,Nditolyl-amino)-phenyl] cyclohexane (TAPC) as donor (D) and C60 as acceptor (A), upon exposing the acceptor side to oxygen and moisture. We found that for the same time of exposure, even minor oxygen amounts lead to more detrimental results compared to moisture. We argue that the photo-conversion efficiency (PCE) decreases due to creation of recombination centers at the interface, which induce losses in exciton diffusion and charge generation. Under the same conditions, we also registered a direct connection between the cell PV parameters’ decay and a C60 thin-film conductivity loss. INTRODUCTION Beside the very promising outlook organic electronics seems to have ahead, owing to their potential of providing quite inexpensive photovoltaic cells, some criticalities like lifetime and degradation phenomena are still to be addressed towards commercialization of such products. Organic solar cells (OSC) undergo many degradation pathways during their lifetime: efficiency losses due to light [1], oxygen [2,3] and water [4] have already been reported, but a deep and unambiguous understanding of such degradation processes is far from been reached. In its most general definition degradation refers to the process of impairing certain properties of a given material or device. In organic solar cells, degradation brings to a decrease in the performance, which is manifested in a change in the Current density-Voltage (J-V) characteristics. Understanding the physical and chemical reasons behind device degradation is crucial in advancing the cell performance and stability. In this work we investigated how a deliberate introduction of an ambient gas (such as oxygen and water vapor) at the D/A interface affected the cell performances, for up to 14 days. We also correlated the electrical results obtained for the OSC with variations in the conductivity of aC60 thin-film. EXPERIMENT The organic solar cells were fabricated by thermal evaporation following the same bilayer structure realized by Zhang et al. [5]: ITO|MoOx(3nm)|TAPC(3nm)|C60(40nm)|Bathophenanthroline(8nm)|LiF(1nm)|Al(100nm), with an active area of 0.1 cm2.The intent of the experiments was to investigate the cell behavior after exposing the acceptor side to ambience gases (oxygen and moisture) over time, 2 nm distant
from the D/A interface. The experiment set-up was the following: ITO|MoOx(3nm)|TAPC(3nm)|C60(2nm)‖C60(38nm)|Bphen(8nm)|LiF(1nm)|Al(100nm).The symbol ‖ delimits where the cell fabrication stopped and the degradation over time occurred for the partial cell structure, while to the right of such symbol the remaining layers were all pristine and
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