Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers

Abstract

Organic semiconductors including conjugated polymers and fullerenes continue to demonstrate promising potential for application in low-cost, printable solar cells. Power conversion efficiencies are now approaching 10 %, and major attention is increasingly turning towards identifying and overcoming the degradation mechanisms that limit the device lifetime. Recent advances in the performance of organic photovoltaics have largely arisen through the development of novel molecular structures using a donor-acceptor copolymer motif. Copolymers incorporating diketopyrrolopyrrole (DPP) units have attracted strong widespread interest, but the main role of this unit in the optoelectronic properties and device performance is not yet clear. This work investigates the natures of the main optical absorption transitions of DPP-based copolymers and chal-lenges the assumption that the DPP-unit behaves as a traditional acceptor unit. This insight leads to a clearer understanding of the excitation energy dependent photodegradation mechanism of the materials, providing ways to improve the operational stability of DPP-based solar cells. ABSTRACTDonor-acceptor copolymers are an important class of conjugated polymer on account of their chemically tunable energy levels and ambipolar charge transport properties. These materials typically exhibit two strong absorption bands in the UV-visible range, whose natures have previously been explored using theoretical analyses. In this work, we experimentally elucidate the electronic origins of these transitions and consider their effects on photostability using resonant Raman spectroscopy and transient absorption spectroscopy. In particular, we identify two dominant electronic transitions for a material comprising diketopyrrolopyrrole (DPP) acceptor and selenophene donor units: a strong transition at low energy (520-1150 nm) that is localised within the DPP unit, and a much weaker transition at higher energy (320-520 nm) that is delocalised along the conjugated backbone of both the DPP and selenophene units. The polymer is found to have good photostability under prolonged excitation in the low energy absorption band, but is much less stable to higher energy excitation. In this latter case, the selenophene ring is identified as the photodegradation site. These effects are correlated with ultrafast transient absorption spectroscopy results, which associate the photodegradation with a higher yield of polaron pairs arising from excitation of the higher energy transition. Our findings provide insight into the design of photostable low energy gap conjugated copolymers for application in organic photovoltaic devices and demonstrate a specific vulnerability of the selenophene donor unit.

Exploring the origin of high optical absorption in conjugated polymers

“…We also find little charge transfer between the thiophene block and the DPP unit upon excitation to the first excited state of DPP-TT-T, consistent with a study by Wood et al 20 , in contrast to the other copolymers studied. We suggest that the low excitation energy in DPP based copolymers is due to coupling of the polymer excitation to the relatively low excitation energy and high oscillator strength of the DPP unit alone (See Fig S6.9 and S6.10).…”

“…This polymer is interesting on account of the high field-effect transistor mobilities, very promising performance achieved as the donor in solar cells, 19 and high photostability 20 . Moreover the solar cell performance using this polymer has been correlated with the position of the branching point on the polymer side chains 21 and with the molecular weight of the polymer 22 but without any convincing mechanism for the trends.…”

Twist and Degrade—Impact of Molecular Structure on the Photostability of Nonfullerene Acceptors and Their Photovoltaic Blends

“…Apparent absorption below the onset is due to scattering, induced by rough thin films. [38] Figure 1d shows the normalized Raman spectra of IDTBR and IDFBR at 457 nm excitation. This planar structure enables effective π-stacking in the solid state, resulting in a more crystalline morphology than IDFBR.…”

Ultrastrong Coupling of Electrically Pumped Near‐Infrared Exciton‐Polaritons in High Mobility Polymers

“…Such phonon‐assisted scattering was previously observed and modeled for a J‐aggregate dye molecule in a polymer matrix . The nonradiative decay mechanism in DPPT‐BT might be enhanced by further negative detuning, as more vibrational modes of the thiophene, DPP, and BT units are located at just slightly higher energies of 169–187 meV (1366–1510 cm −1 ) . So far there are only a handful examples of polymers showing strong coupling, and further investigation into the particular LP population mechanisms of these materials is highly desirable.…”