Optically Tunable Magneto‐Capacitance Phenomenon in Organic Semiconducting Materials Developed by Electrical Polarization of Intermolecular Charge‐Transfer States

Abstract

DOI: 10.1002/adma.201305965 formed in organic donor:acceptor composites under photoexcitation through charge transfer in organic materials. [ 15,16 ] In particular, changing photoexcitation intensity can largely modify the density of intermolecular CT states and thus forms an effective tuning method on magneto-capacitance generated from intermolecular CT states. In this paper, we demonstrate the optically-tunable magneto-capacitance by using intermolecular CT states in organic donor:acceptor composites. Essentially, our magneto-capacitance from optically generated CT states can be also named as magneto-photocapacitance. Furthermore, we fi nd that changing the density of intermolecular CT states can lead to a tuning on both magnitude and line-shape of magnetocapacitance. In particular, the line-shape tuning suggests that the interactions between intermolecular CT states can affect the spin-exchange interaction within individual intermolecular CT states in the development of magneto-capacitance. We use the N,N ′-diphenylN,N ′-bis(3-methylphenyl)-1,1′biphenyl-4,4′-diamine (TPD) as donor and the 2,5-bis(5tert butyl-2-benzoxazolyl)-thiophene (BBOT) as acceptor for the generation of intermolecular CT states under photoexcitation. To prepare high-quality thin fi lms for device fabrication, the TPD and BBOT were dispersed into an inert poly(methyl methacrylate) (PMMA) matrix. The devices for capacitance measurements were made by sandwiching TPD:BBOT:PMMA composite fi lms between two electrodes (see Experimental section). The good quality of the fabricated devices can be shown by no detectable leaking current or injection current under driving voltages between −1 V and +1 V (Figure S1 in the supporting information). Figure 1 a depicts the magneto-capacitance from the ITO/TPD:BBOT:PMMA/Al device under photoexcitation. It can be seen that a photoexcitation can lead to a magnetocapacitance in the device when an alternating bias of 50 mV is applied without a DC bias during the capacitance measurements. Specifi cally, the capacitance increases quickly and then saturates around 80 mT with applied magnetic fi eld under photoexcitation, while no magneto-capacitance is observed under dark condition. It is also noticed that the pure TPD and BBOT do not exhibit such magneto-capacitance phenomenon under either dark or photoexcitation. Therefore, the magnetocapacitance phenomenon observed from the photo-excited TPD:BBOT:PMMA composite can be attributed to the intermolecular CT states formed between TPD and BBOT molecules. In general, magneto-capacitance can be developed through two different channels: magnetic fi eld-dependent electrical polarization and magnetic fi eld-dependent electrical current, namely polarization-based and transport-based magneto-capacitance. [ 1,17,18 ] Here, when a magnetic fi eld changes the singlet/ triplet ratio in intermolecular CT states, the polarization-based Magneto-capacitance, which can be produced by magneticallydependent electrical polarization in functional materials, [ 1,2 ]