The effects of electrode modifications on performance and stability of organic photovoltaic devices

Abstract

While recent developments of organic photovoltaic (OPV) device have aroused much research interest, low power conversion efficiency (PCE) and poor stability of OPV devices are still key obstacles for commercial applications. PCE of OPV devices depend crucially on photogenerated current and voltage, i.e. the short circuit current (Isc) and open circuit voltage (Voc). The Voc has been shown to be the main limitation for further PCE improvement. In this work, the effects of electrode modifications and interface optimizations to Voc were studied. In addition, various surface engineering techniques were explored for enhancing stability of OPV devices. In a classical metal-insulator-metal (MIM) model, Voc is defined by a work function difference between two electrodes (∆Φelectrodes). In recent year, it was found that the Voc is much more depends on an energy offset between highest occupied molecular orbital of the donor (HOMOD) and the lowest unoccupied molecular orbital of the acceptor (LUMOA), i.e. HOMOD - LUMOA energy offset, irrespective to the work function of the electrodes. To elucidate a relation between Voc and the two models, anode and cathode buffer layers were used to modify the work functions of the electrodes in copper phthalocyanine (CuPc) / fullerene (C60) OPV device. We showed that introduction of different electron buffer layers can modified the effective work functions of two electrodes, thus the ∆Φelectrodes. The Voc was found to change linearly with the ∆Φelectrodes, following the classical MIM model. However, when the ∆Φelectrodes reaches a certain threshold value, the Voc will become saturated, showing negligible change with further ∆Φelectrodes increase. The result suggests that the changes in Voc can be consistently considered using coherent picture incorporating the MIM and the HOMOD - LUMOA energy offset models. While Voc changes linearly with the ∆Φelectrodes, the HOMOD - LUMOA energy offset sets a limit on the maximum possible Voc. Apart from controlling the ∆Φelectrodes, different interlayers have been inserted in between a donor and acceptor layer. It was observed that the Voc can be greatly influenced by the energy level of the interlayer. These results suggest a possible pathway to increase the HOMOD - LUMOA energy offset and thus to maximize the Voc. The exciton dissociation conditions for the above donor / acceptor multi-junction devices were also discussed. The effects of cathode and anode buffer layers on device reliability including operation and storage stability were studied. Operation stability of unpackaged CuPc / C60 OPV device was significantly improved by introducing a metal-doped organic exciton blocking layer (EBL). While devices with non-doped EBL, such as bathocuproine (BCP) and tris(8-hydroxy-quinolinato) Aluminium (Alq3), showed > 20% decrease in PCE for first 150 minutes operation; the device with Mg-doped Alq3 (Mg:Alq3) EBL showed less than ~ 5% variation in PCE over the same operation period. After 120 hours further storage in air, non-doped devices degraded and showed less than 1% of their initial efficiencies; while device with a Mg:Alq3 EBL could maintain 15% of its initial PCE. Important factors contributing to the stability improvement will be discussed. While it is commonly accepted that a proper encapsulation can effectively protect the OPV device from degradation, obvious reduction in PCE in an encapsulated CuPc / C60 OPV device was observed after ~ 2000-hour measurement. With a polymerized fluorocarbon film (CFx) coated on ITO, a negligible change in PCE over the same period was demonstrated. A similar approach was also applied on an encapsulated rubrene / C60 OPV device. Interestingly, it was observed that the introduction of CFx layer could effectively protect the encapsulated device from degradation. Stability enhancement mechanisms of the CFx layer were explored via dark current-voltage (I - V) characteristics; X-ray photoemission studies (XPS) and ultra-violet photoemission studies (UPS).

Date of Award	3 Oct 2011
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)