ITO-Based Cathode for Application in Semitransparent Organic Photovoltaic Cells
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1013-Z02-08
ITO-Based Cathode for Application in Semitransparent Organic Photovoltaic Cells Ging-Meng Ng, Elizabeth Lekha Kietzke, Thomas Kietzke, Li-Wei Tan, Pooi-Kwan Liew, and Furong Zhu Institute of Materials Research and Engineering, 3 Research Link, Singapore, 117602, Singapore ABSTRACT A high performance semitransparent cathode is one of the major impediments to the high performance semitransparent and tandem organic photovoltaic (OPV) cells. In this work, we discuss the possible designs of semitransparent cathode to improve the performance of semitransparent OPV cells. The optical properties of the poly(3-hexylthiophene) (P3HT): 1-(3methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)-based OPV cells were studied by the optical admittance analysis. The performance of the OPV cells made with an opaque Ca(10nm)/Ag(100nm) cathode and a semitransparent Ca(10nm)/Ag(10nm)/ITO cathode are discussed. The interfacial properties at the cathode/organic interface were analyzed using the time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The TOF-SIMS depth profile revealed that calcium oxide formed at the Ca/organic interface in semitransparent OPV cells, which was induced by the ITO sputtering process. It shows that the presence of calcium oxide at the organic/cathode interface could be the main reason resulting in a poor fill factor of a semitransparent OPV cell (~23%) as compared to that of a reference cell (~43%). INTRODUCTION Thin film organic photovoltaic (OPV) cells offer an attractive option for achieving low-cost power generation. Organic semiconductors as the active components in the OPV devices have many advantages in terms of large area, cost effectiveness, chemical tenability and mechanical flexibility. This field is still in its growing stage, particularly regarding aspects like design and optimization of device structures and the performance improvements [1,2,3,4]. Two approaches are competing: whereas vapour deposited small molecules come with the advantage of easy multilayer preparation, solution processable polymers and molecules promise low cost large area production by technologies such as reel to reel, screen or ink jet printing. An excellent review about polymer solar cells can be found in [5]. OPV cells with 5% energy conversion efficiency under simulated AM1.5 solar light illumination have been demonstrated using both vacuum sublimation and solution process approaches [3,6,7]. However, limited absorption of the sun spectrum and a relatively low open circuit voltage are the two main factors limiting the efficiency of current OPV cells. In addition to search for new low band gap organic semiconducting materials for photovoltaic application, high performance semitransparent cathode for semitransparent and tandem OPV cells is highly desired. The basic idea for tandem OPV cell is each single solar cell can be made very thin (20-40 nm), which has advantages for charge transport. The open circuit voltage will be added up, as demonstrated by [8,9]. This aside, tandem OPV cells can fully utilize the s
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