Photocharge-Modulated Photoemission Spectroscopy and its Applications for Characterizing Organic Donor-Acceptor Junction

Description

Performance of many organic optoelectronic devices including organic photovoltaic(OPV) devices depends critically on properties of junctions formed between donors (D) andacceptors (A) in the devices. In many D/A junctions, charge transfer (CT) state would beformed by electron transfer from a D molecule to an A molecule. Before excitation, theelectron-hole pair in the CT state is bound by Columbic attraction. Upon excitation, they canseparate as free carriers. During this process, electron-hole pair in the CT state needs toovercome an energy barrier, namely a binding energy (EBCT), to achieve full dissociation.Although the EBCTvalue has been known to have important implication on the open circuitvoltage (Voc) of all OPV devices, understanding on it is limited. For example, while therehave been some reports on measurement of singlet exciton binding energy in single organicsemiconductor, there is few studies on the estimation of the EBCTof the CT exciton in a D/Ajunction. The published EBCTvalues in the literatures either rely on measuring exciplexemission properties of the CT states or combinations of calculation and complicatedmeasurements (e.g. using two-photon photoemission spectroscopy). It is thus highly desirableto develop a general (i.e. work for all D/A junctions with or without exciplex emission) anddirect experimental (i.e without need of additional calculations) technique for measuring EBCTof CT states.The PI and his team have recently been exploring a new measurement technique,named as the photocharge-modulated photoemission spectroscopy (PMPES) for measuringthe EBCT. This is a conventional UPS system modified with an additional xenon lamp as anexcitation source of low-intensity and broad spectral window, extending from the visible toinfrared region. With this additional xenon lamp excitation, the bound CT state can absorb aphoton with suitable energy for charge separation and generate an unbound electron-hole pair.It has to be noted that this process cannot be achieved with the original UV source, as thehigh energy UV photon would directly eject the electrons out of the sample instead. Thissuggests that the properties of bounded and unbounded CT states at a D/A junction can beprobed by UPS respectively without and with the additional xenon lamp excitation. Bycomparing the obtained UPS results (with and without the additional xenon lamp excitation),we propose a method for directly measuring the EBCTof the CT exciton in a D/A junction. Apreliminary experiment has been carried out to measure EBCTvalues of four D/A junctions.The measured EBCTvalues match well with the Voc loss (ionization potential of D – electronaffinity of A – measured Voc; i.e. IPD-EAA-Voc) in the corresponding OPV devices. Thisresult suggests that the PI has successfully developed a new technique for the directmeasurement of the EBCTof various D/A junctions (including both emissive and non-emissivejunctions).While the preliminary result is encouraging, more experiments have to be carried outon a much wider scope of organic semiconductors to define the limitation and to explore thefull potential of this new technique. All the experimental results will be further confirmedwith theoretical calculations/ modelling. Another interesting observation from the preliminaryresults is that the measured EBCTvalues range widely from 0.2 to 0.6 eV (which isconventionally estimated to be ~ 0.3 eV). Factors leading to such a wide-ranged EBCTare stillunclear. In this project, possible factors influencing the EBCT, including the interfacial energylevel alignment and device structural properties will be examined. Upon successfulimplementation, the project will not only develop a new experimental technique forcharactering CT states at D/A junctions, but also provide new insight on CT properties.