Discerning Bulk and Interfacial Polarons in a Dual Electron Donor/Acceptor Polymer.

Jose M Marin-Beloqui,Tracey M Clarke,Kealan J Fallon,Hugo Bronstein

doi:10.1021/acs.jpclett.9b01264

Jose M Marin-Beloqui, Tracey M Clarke + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpclett.9b01264

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Abstract

The active layer of organic solar cells typically possesses a complex morphology, with amorphous donor/acceptor mixed domains present in addition to purer, more crystalline domains. These crystalline domains may represent an energy sink for free charges that aids charge separation and suppresses bimolecular recombination. The first step in exploiting this behavior is the identification and characterization of charges located in these different domains. Herein, the generation and recombination of both bulk and interfacial polarons are demonstrated in the dual electron donor/acceptor polymer XIND using transient absorption spectroscopy. The absorption spectra of XIND bulk polarons, present in pristine polymer domains, are clearly distinguishable from those of polarons present at the donor/acceptor interface. Furthermore, it is shown that photogenerated polarons are transferred from the interface to the bulk. These findings support the energy sink hypothesis and offer a way to maximize morphology relationships to enhance charge generation and suppress recombination.

Highlights

Despite organic photovoltaic (OPV) devices recently exhibiting high power conversion efficiencies (>15%1−4), they are still quite far from the theoretical limit efficiency of 23−24%.5,6 This disparity depicts the need for more fundamental studies of the different processes that occur at the donor/acceptor interface
Bimolecular recombination of free charges before they reach their respective electrodes has been established as one of the main loss mechanisms in OPV devices.[7−10] It has been proposed that bimolecular recombination can be inhibited in active layers with a morphology comprised of both pure, crystalline domains and amorphous/
Transient absorption spectroscopy (TAS) is a useful technique for measuring bimolecular recombination, as the polaron signal amplitude is directly proportional to the number of charges generated in the blend.[14]

Summary

Introduction

Despite organic photovoltaic (OPV) devices recently exhibiting high power conversion efficiencies (>15%1−4), they are still quite far from the theoretical limit efficiency of 23−24%.5,6 This disparity depicts the need for more fundamental studies of the different processes that occur at the donor/acceptor interface. The P3HT:XIND transient spectrum at 1 μs (Figure 2a), where XIND acts as electron acceptor, showed the expected peak around 1000 nm assigned to the P3HT positive polaron.[25] The additional band at 1400 nm must correspond to XIND, consistent with the similarity of this feature to that seen in the pristine XIND polymer sample.

Results

Conclusion