Overcoming Electrode‐Induced Losses in Organic Solar Cells by Tailoring a Quasi‐Ohmic Contact to Fullerenes via Solution‐Processed Alkali Hydroxide Layers

Abstract

It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi‐ohmic contact via solution‐processed alkali hydroxide (AOH) interlayers on top of n‐type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C60) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C60 highest occupied molecular orbital for AOH‐modified AZO contacts. These novel interfacial gap states are a result of ground‐state electron transfer from the metal hydroxide‐functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n‐doped (metal‐like) interfacial molecular layer. Furthermore, the strong “light‐soaking” effect is no longer observed in devices with a AOH interface.

Modified SnO2 with Alkali Carbonates as Robust Electron-Transport Layers for Inverted Organic Solar Cells

“…To our knowledge, the reduction of the WF with alkali carbonates–modified SnO 2 might originate from the formation of an interface dipole between the alkali atoms and the metal oxides (M–O–A) in SnO 2 /alkali carbonate samples, where M, O, and A stand for metal, oxygen, and alkali metal species, respectively. 23…”

“…To our knowledge, this might originate from the increased dipole moments at the interface between the active layers and the ETLs because of the increasing electronegativity of the alkali metals, from Li to Rb. 23 It has been reported that the counter anions of electron-transport materials showed different behaviors in OSCs. 64,65 Because these alkali carbonates possess different metal ions, especially the sizes of these metal ions are different, the different behaviors in our iOSCs are expected.…”

Improving the Performance of PbS Quantum Dot Solar Cells by Optimizing ZnO Window Layer

“…However, the quality of solution-based ZnO film suffers from the surface defects or dangling bonds, which may act as charge trap sites or recombination centers [ 21 – 23 ]. To solve the above-mentioned problems, several strategies such as surface passivation or doping were reported to control the interfacial properties of heterojunction [ 24 – 28 ]. All of them have made promising progresses in the improvement of interface quality.…”

Finely Tuned Cores in Star‐Shaped Zwitterionic Molecules for Interface Engineering of High‐Performance Polymer Solar Cells

“…In the original development of ETLs, a number of n‐type metal oxides (e.g., ZnO, TiO 2 , and CsCO 3 ) have been used as cathode interlayers to replace the low work‐function metals (e.g., Ca, Ba, and Mg) in the standard device geometry of PSCs for alleviating the air‐sensitive problem associated with such cathodes . However, these n‐type metal oxides could have poor interfacial contact with the organic active layer, which will reduce the electron extraction and increase the series resistance ( R S ) . Thus, in the last few years, finding solution‐processed organic interlayers become an important topic in the field of OSCs.…”