Efficient exciton management is a key issue to improve the power conversion efficiency of organic photovoltaics (OPVs). It is well known that the insertion of an exciton blocking layer (ExBL) having a large band gap promotes the efficient dissociation of photogenerated excitons at the donor-acceptor interface. However, the large band gap induces an energy barrier which disrupts the charge transport. Therefore, building an adequate strategy based on the knowledge of the true charge transport mechanism is necessary. In this study, the true electron transport mechanism of a bathocuproine (BCP) ExBL in OPVs is comprehensively investigated by in situ ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, density functional theory calculation, and impedance spectroscopy. The chemical interaction between deposited Al and BCP induces new states within the band gap of BCP, so that electrons can transport through these new energy levels. Localized trap states are also formed upon the Al-BCP interaction. The activation energy of these traps is estimated with temperature-dependent conductance measurements to be 0.20 eV. The Al-BCP interaction induces both transport and trap levels in the energy gap of BCP and their interplay results in the electron transport observed.
Cesium azide (CsN3) is confirmed to be decomposed during thermal evaporation. Only Cs could be deposited on tris(8-hydroxyquinolinato)aluminum (Alq3) and n-type doping is easily achieved. Organic light-emitting devices with CsN3 show highly improved current density-luminance-voltage characteristics compared to the control device without CsN3. To understand the origin of the improvements, in situ x-ray and UV photoemission spectroscopy measurements were carried out and a remarkable reduction in electron injection barrier is verified with successive deposition of Al on CsN3 on Alq3. CsN3 has a potential as alternative to doping the electron transport layer by replacing the direct deposition of alkali metals.
1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) is known to improve hole injection when inserted between the hole transport layer and the indium tin oxide (ITO) anode in organic light emitting devices. To clarify the origin of the improvement, the interfacial electronic structures between N,N′-diphenyl-N, N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′ diamine (NPB, typical hole transport layer) and ITO with a NTCDA insertion layer were explored. The NTCDA layer generates an interface state when it interacts with ITO and also induces large interface dipole. The interface state assists hole transport and the interface dipole pulls entire energy levels of NPB up, reducing the hole injection barrier.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.