π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells

Abstract

An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond-nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low.

Effect of Oligomer Length on Photophysical Properties of Platinum Acetylide Donor–Acceptor–Donor Oligomers

“…15 Our group has an ongoing interest in studying the triplet excited state properties of platinum incorporated π-conjugated structures. [16][17][18] Very recently, we investigated the photophysical properties of a series of platinum acetylide containing π-conjugated donoracceptor-donor (DAD) type small molecules 19 and showed that increasing acceptor strength results in localization of the excited state electrons on the acceptor unit which is located distal from the platinum center. As a result, the rate of intersystem crossing decreases with increased extent of charge transfer due to the increased average distance between the acceptor and the platinum.…”

Heavy metal complex containing organic/polymer materials for bulk-heterojunction photovoltaic devices

“…Besides on the linear platinum acetylides comprising photovoltaic materials, heavy metal organometallic polymers containing cyclometalated Pt or Ir complexes which was attached via the C^N or O^O diketonate ligands have been attracted gradually increasing attention due to better involvement from the orbitals of Pt or Ir center and the cyclometalated organic ligand to polymer backbone in both the ground and the excited states of the material [71][72][73][74][75]. Strong spin-orbit coupling on the heave metal center allows for the mixing of singlet metal-to-ligand charge transfer state ( 1 MLCT) with the formally forbidden triplet metal-to-ligand charge transfer ( 3 MCLT) and 3 π-π* states.…”

“…Schanze group also presented a π-conjugated organometallic isoindigo-based polymer 39 (Iin-C18-Poly) that contains cyclometalated platinum unit was developed and applied as electron donor into BHJ solar cells with PCBM [74]. Isoindigo is an excellent accepter in D-A type photovoltaic materials, attribute to its intrinsic electron-deficient nature [88][89][90].…”

“…In this review, recent progress on the development and application of heavy-metal complex containing photovoltaic donor materials for the bulk-heterojunction solar cells are presented. Low-bandgap platinum-acetylide and the cyclometalated Pt or Ir containing polymers, oligomersor small molecules [71][72][73][74][75][76][77] as donor materials for the active layer of BHJ solar cells are successively demonstrated, in which the heavy metal complex was chemically bonded to the molecular backbone. Finally, physically doping of heavy-metal complexes as additives into the BHJ solar cells to enhance the photovoltaic performance are also discussed [78][79][80][81].…”

C–N cross coupling approach synthesis and electronic properties of donor–acceptor oligomer

“…Conjugated donor–acceptor oligomers (–D–A–D–A–) or polymers are an emerging class of materials that have attracted considerable attention in organic electronics because of their urgent demand in organic photovoltaic cells (OPVCs), organic light‐emitting diodes (OLEDs), organic field effect transistors (OFETs) as well as a hole‐transporting material (HTM) . Donor–acceptor (D–A) interaction in conjugated –D–A–D–A– molecules leads to a narrow band gap and thus, the molecule absorbs the light in the deep‐red or NIR spectral range .…”