Ordering of PCDTBT Revealed by Time‐Resolved Electron Paramagnetic Resonance Spectroscopy of Its Triplet Excitons

Abstract

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy is shown to be a powerful tool to characterize triplet excitons of conjugated polymers. The resulting spectra are highly sensitive to the orientation of the molecule. In thin films cast on PET film, the molecules' orientation with respect to the surface plane can be determined, providing access to sample morphology on a microscopic scale. Surprisingly, the conjugated polymer investigated here, a promising material for organic photovoltaics, exhibits ordering even in bulk samples. Orientation effects may significantly influence the efficiency of solar cells, thus rendering proper control of sample morphology highly important.

Electronic Structure Trumps Planarity: Unexpected Narrow Exciton Delocalization in PNDIT2 Revealed by Time‐Resolved Electron Paramagnetic Resonance (EPR) Spectroscopy

“…[20] Our assignment of the unusually large line widths observed in the TREPR data to a high intrinsic flexibility of PNDIT2 and its building blocks retained even in the polymer if dissolved in 1-chloronaphthalene is consistent with prior results. All other molecules investigated here show much smaller line widths, although still pretty large as compared with other conjugated polymers such as poly(3-hexylthiophene) (P3HT) or poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT).…”

“…[20,21] Optical excitation at the respective wavelengths in the visible range was carried out using an optical parametric oscillator (OPO) pumped by an Nd:YAG laser. [20,21] Optical excitation at the respective wavelengths in the visible range was carried out using an optical parametric oscillator (OPO) pumped by an Nd:YAG laser.…”

“…[5,6] Using organic molecules as a replacement for conventional inorganic semiconductors comes with a number of advantages, such as mechanical flexibility, low cost, and the nearly infinite possibility of tailoring molecules for each special application. Whereas for conventional inorganic semiconductors, it is rather straightforward to create either p-type or n-type materials using well-established protocols, only very few n-type conjugated polymers are known to date, with poly{[N,N′bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDIT2), also known as P(NDI2OD-T2) or Polyera ActivInk N2200, [7] being a notable between 0.5 and 5 nm in systems that lack long-range order [16] only TREPR spectroscopy [17] with a time resolution of up to 10 ns allows for real-time observation, e.g., of short-lived radical-pair [18,19] and triplet states [20,21] generated by pulsed laser excitation. Furthermore, as EPR is exclusively sensitive to paramagnetic states, but polymers are normally diamagnetic, applying conventional EPR spectroscopy requires to insert spin probes into the material of interest.…”

Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

“…It has also been shown that films of the polymer PCDTBT fabricated using similar methods to those employed here exhibited partial orientation effects in the triplet exciton TR-EPR spectrum. 72 …”

PCDTBT: From Polymer Photovoltaics to Light-Emitting Diodes by Side-Chain-Controlled Luminescence

“…18,19 The low-bandgap and thermally stable copolymer poly[N-9′-heptadecanyl-2,7-carbazole-alt- 21 Also beneficial for charge injection is a face on orientation of the PCDTBT backbone in as-cast films. 22 Due to the low-lying HOMO level of 5.5 eV, PCDTBT exhibits ambipolar character and can also be used as acceptor material in combination with strong donors such as P3HT. 23 Recently, we have shown that PCDTBT is prone to carbazole (Cbz) homocoupling defects which add to the two most important parameters molecular weight and dispersity.…”