Proceedings of nanoGe Fall Meeting19 (NFM19)
Publication date: 18th July 2019
With conventional c-Si solar cells approaching their theoretical efficiency limit, large research efforts are currently devoted to addressing their fundamental loss mechanisms. One appealing strategy to reduce losses due to carrier thermalization is the implementation of organic layers that can split the energy of blue photons into two triplet excitons via singlet fission. [1] However, efficiently harvesting these excitons relies on favorable excited state dynamics at the hybrid organic/c-Si interface. [2] The introduction of suitable ultrathin organic interlayers can assist this process two ways: (i) they can be used to tune the interfacial energy-level alignment and thus promote exciton dissociation at the hybrid interface and (ii) they can act as charge acceptor, thereby shifting the exciton dissociation process to the organic/organic interface.
In this work, we employed tetracene (Tc) as singlet fission sensitizer and tested two different interlayer materials, Buckminsterfullerene (C60) and 2,2´-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ). C60 has already been shown to facilitate efficient harvesting of singlet fission-generated excitons [1, 3, 4], whereas its effectivity to tune the interfacial energy-level alignment is only moderate. In contrast, F6TCNNQ has been found to be extremely potent in increasing the work functions of inorganic semiconductors [5]. F6TCNNQ and C60 can therefore expected to preferably induce schemes (i) and (ii), respectively.
We performed photoelectron spectroscopy to monitor the formation of the inorganic/organic and organic/organic interfaces and their effect on the energy-level alignment. By means of time-resolved photoluminescence and external quantum efficiency measurements we then evaluated the interlayers’ effect on the exciton dynamics at the hybrid interfaces. Together with supplemental structural characterizations we were able to identify some pitfalls that thus far prevented us to successfully employ organic interlayers to improve the triplet exciton harvest yield.
RWM acknowledges funding support in the form of a postdoctoral fellowship from the Helmholtz Association. The work at AMOLF is part of the Netherlands Organization for Scientific Research (NWO). Financial support was provided by the German Federal Ministry for Research and Education (BMBF) through the project ‘‘Silicon In-situ Spectroscopy at the Synchrotron’’ (SISSY), Grant No. BMBF-03SF0403 and the BMBF network project “EPRoC” Grant No. 03SF0565A. We thank the Norbert Koch group of the Humboldt University of Berlin for providing the F6TCNNQ.
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