Characterizations on Organic Photovoltaics via Impedance Spectroscopy

Abstract

This dissertation utilizes impedance spectroscopy (IS) to analyze organic photovoltaics (OPVs) The OPV devices are simply simulated to a circuit model and the value of each element in the model is defined by fitting the impedance spectra With the value of each element the OPV device can be studied separately in terms of active layer part and interlayer part We can derive more electrical information from the value of the separated elements which mainly corresponds to the physical difference from the active layer or the interlayer 1 8-Diiodooctane (DIO) has been known for its role of improving the polymer morphology and enhancing performance of polymer bulk heterojunction (BHJ) solar cell First the impedance spectroscopy was used to investigate the interface of poly(4 8-bis-alkyloxybenzo(1 2-b:4 5-b’)dithiophene-2 6-diyl-alt- (alkyl thieno(3 4-b) thiophene-2-carboxylate)-2 6-diyl) (PBDTTT-C):PC70BM in BHJ with DIO as additive Based on our results we were able to simulate the device into an equivalent circuit model which allows us to conveniently analyze the organic/organic interfacial contact in the OPV device Thus we demonstrate that the impedance spectroscopy can an effective approach in characterizing the donor/acceptor interfaces such that a direct correlation can be established between the morphology and the device performance of BHJ devices Besides most photovoltaics operate at high temperature under sunlight In this work the thermal instability of DIO-based high-efficiency BHJ OPVs is studied The BHJ layers were heated to various temperatures to investigate the changes in their physical properties using atomic force microscopy phase images The mobilities of the carriers were characterized at various temperatures using the space-charge-limited current method and the carrier lifetime was calculated by applying impedance spectroscopy to the simulated equivalent circuit of the OPV devices We also propose an approach for improving the performance of poly(3-hexylthiophen) (P3HT)-based OPVs P3HT-based BHJ film can absorb the energy from 532-nm laser light and be transformed into favorable morphology A combination of traditional thermal annealing and laser annealing improved device performance with a slight increase in fill factor and a significant improvement in short-circuit current density Better crystallization and a higher degree of molecular order in the thermal/laser co-annealed P3HT-based BHJ film were observed through X-ray diffraction and Raman spectroscopy In this work the BHJ layer was also analyzed via IS to investigate the interfacial property of donor and accepter materials The simulated elements represent to the BHJ layer demonstrate different result from the DIO-based OPVs under annealing process To improve the OPV device performance we also doped phosphorescent material tris(phenylpyrazole)iridium (Ir(ppz)3) into the BHJ layer of a P3HT and indene-C60 bisadduct (ICBA) blend to form more excitons at the triplet state Triplet-state excitons have longer lifetimes than those of singlet-state excitons Surface phase separation was determined via atomic force microscopy and the vertical distribution of various molecules was analyzed via secondary ion mass spectroscopy Several annealing processes were applied to the BHJ layer doped with Ir(ppz)3 to investigate the thermal stability of the film The exciton lifetime in the BHJ film was characterized using femtosecond time-reserved photoluminescence The BHJ layer doping with Ir(ppz)3 was analyzed through IS as well The variation on the elements which represent to the BHJ layer is different from the one as analyzed in previous works The physical meanings on the Ir(ppz)3-doped BHJ film was well studied At last we investigate the interface between the active layer and contacts in OPVs since the contact materials strongly affect the energy barrier at the interfaces The interfacial characteristics are simply defined as a resistance-capacitance (R-C) shunt pair and extracted by fitting the impedance spectra to the equivalent circuit model A change in the energy barrier is found to affect the values of R and C at the interface and the carrier transition time In addition the effect of electron buffer layer (TiO2) thickness on the interfacial characteristics is analyzed using impedance spectroscopy The interfacial area between the hole buffer layer (MoO3) and the active layer affects the values of R and C at the interface We deliver a fast and simple way to investigate the OPV devices not only on BHJ layer but also on interfaces between BHJ layer and electrodes via IS It is a method to combine the physical property and current density-voltage (J-V) characteristics of OPV devices together By analyzing the OPV devices through IS more unclear physical meanings in BHJ or interface between contact and BHJ layer are explainable

Date of Award	2015 May 20
Original language	English
Supervisor	Wei-Chou Hsu (Supervisor)