Reduced energy loss at the cathode - active layer interface in laminated OPVs for indoor applications
Gulzada Beket a, Qilun Zhang c, Anton Zubayer b, Thomas Österberg a, Mats Fahlman c, Feng Gao b, Jonas Bergqvist a
a Epishine AB, Westmansgatan 47b, Linköping, Sweden
b Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, SE-58183, Sweden
c Laboratory of Organic Electronics, ITN, Linköping University, Sweden
Poster, Gulzada Beket, 077
Publication date: 21st November 2022

It has been known from calculations and morphology simulations that in Organic Solar Cells highly ordered arrangement of morphology (Fig.1) would lead to an ideal performance where electrons and holes always have pathways to their respective electrodes [1]. One would assume that placing same electron accepting transport layer close to its acceptor phase in bulk heterojuction (BHJ) would result in a maximum voltage output. There are some studies which show that this phenomenon is partly irrelevant. Ding et al. and Kotadiya have investigated counterintuitive yet effective strategy to reduce energy loss at the interface by inserting tunnel thick fullerene electron accepting layer between BHJ active layer and anode. Ding et al. have showed that energy loss is attributed to a static dipole at the interface of active layer and anode interface [2]. Kotadia et al. have concluded that insertion of tunnel layer at the interface reduces the injection barrier and creates Ohmic contact on organic semiconductors [3].

Although these studies come in handy in understanding the interfacial energy losses and enhancing voltage output, these findings are limited due to the thickness of the interlayer. When interlayer exceeds the threshold of tunneling, charge transport will be hindered.

 Here we explore interface of bulk thick fullerene interlayer on cathode and polymer:fullerene based BHJ layer on laminated OPV device structure for indoor applications. Utilization of pure electron accepting layer between active layer and cathode would seem ideal, as morphology as per Fig 1. hence energetics could be beneficial for electron extraction at the cathode. Surprisingly we find that placing “right” contacts close to its electrodes does not necessarily give maximum voltage output.

By modifying the fullerene electron accepting layer in two different ways, we have eliminated energy loss at the active layer – cathode interface. Firstly, using Polyethyleneimine (PEI) as n-dopant to PCBM, we modify the work function of the interlayer so electron would have an aligned energetics to hop to the respective electrode. Secondly, by creating “spacer” effect by utilizing Polystyrene (PS), i.e weakening the electrostatic coupling between cathode electrode and organic semiconductor, which can be speculated from UPS data. We show that by modifying the morphology of PCBM with PS we can change energy level alignment. Neutron Reflectivity measurements display that PEI and PS phase segregate from PCBM, arranging themselves at the surface. Our findings show that pure phase close to its contacts might lead to energy losses which can be eliminated by changing its morphology hence energetics.

Given the fact that indoor organic photovoltaics are one of the least explored parts of OPV, and the increasingly popular discussion on interfaces in organic optoelectronic devices, these findings would be relevant to address different branches of organic electronics, providing insights for future morphology and device design.

 

Swedish Foundation for Strategic Research is greatly acknowledged for funding the project. 

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