Influence of Copolymer Interface Orientation on the Optical Emission of Polymeric Semiconductor Heterojunctions

Abstract

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ( approximately 65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Theory of interfacial charge-transfer complex photophysics inπ-conjugated polymer-fullerene blends

“…Recent research by many groups have, however, detected an interfacial excited-state charge transfer complex (hereafter exciplex) below the optical gaps of both D and A upon photoexcitation. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] It is now recognized that exciplex formation reduces the yield of mobile charge carriers. 20,21,25 Understanding the details of PICT is therefore clearly important for reaching enhanced device performance.…”

Femtosecond hot-exciton emission in a ladder-typeπ-conjugated rigid-polymer nanowire

“…[22][23][24][25][26][27][28][29][30][31][32][33] Obviously, there is a clear gap between (i) and (ii): in the frequency domain, this gap appears as a large shift from the excitation λ ex to the onset of λ em , as shown in Fig. 1, which corresponds to initial hot-exciton cooling.…”

“…6 If excitation occurs yielding a large E K , any resultant emission from the hot excitons will lie underneath the strong linear absorption band; therefore, its intensity will be very weak, and it is usually not expected to be observed in experiment. [1][2][3][4][5][6][7][8][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In the time domain, this gap represents a period between T * 2 and ∼3 ps. To date, no one has explored the physical processes of hot excitons in π -conjugated polymers because of this.…”

“…To date, no one has explored the physical processes of hot excitons in π -conjugated polymers because of this. [1][2][3][4][5][6][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Femtosecond pump-probe techniques are of little use here due to the superposition of stimulated emission from the hot exciton with photoinduced absorption to high-lying excited states; [2][3][4][5]23,[34][35][36][37][38][39] there are also limitations on the availability of the deep-blue spectral component from the white-light supercontinuum needed as a probe. [34][35][36][37][38][39] 045308-1 1098-0121/2013/87(4)/045308 (9) ©2013 American Physical Society In pristine thin films, π -conjugated polymers are typically amorphous glasses, having a statistical distribution of chain lengths and random orientations, and each chain usually is composed of multiple segments due to chain folding, twisting, kinks, or chain defects, etc., and all of these yield poorly resolved absorption and emission spectra and provide a large heat bath to enable excitons to migrate nondispersively over relatively large distance during their lifetimes.…”

“…[34][35][36][37][38][39] 045308-1 1098-0121/2013/87(4)/045308 (9) ©2013 American Physical Society In pristine thin films, π -conjugated polymers are typically amorphous glasses, having a statistical distribution of chain lengths and random orientations, and each chain usually is composed of multiple segments due to chain folding, twisting, kinks, or chain defects, etc., and all of these yield poorly resolved absorption and emission spectra and provide a large heat bath to enable excitons to migrate nondispersively over relatively large distance during their lifetimes. [1][2][3][4][5][6][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] Moreover, the rich vibronic modes and the strong electron-phonon coupling on each single chain greatly broaden the overall optical transition cross sections in absorption and emission. [1][2][3][4][5][6][20][21][22][23][24]…”