Conjugated Thiophene Dendrimer with an Electron-Withdrawing Core and Electron-Rich Dendrons: How the Molecular Structure Affects the Morphology and Performance of Dendrimer:Fullerene Photovoltaic Devices

Abstract

The combination of electron-rich and electron-poor moieties in conjugated molecules is frequently utilized in order to red shift the absorption spectrum and improve photon harvesting in bulk heterojunction photovoltaic devices. In this study we characterize a conjugated thiophene dendrimer that has an electron-withdrawing core and electron-rich dendrons in order to investigate the effects of this design approach on the salient properties that influence the performance of photovoltaic devices with this dendrimer donor. Beside the absorption onset, these properties are the morphology of dendrimer:fullerene films and the dynamics of photoinduced carrier generation and loss. For comparison we also characterize a control dendrimer with the same structure but without the electron-withdrawing core. In addition to lowering the band gap by ca. 0.5 eV, the electron-withdrawing core also planarizes the dendrimer resulting in enhanced order in bulk heterojunction films. We observe longer photocarrier lifetimes in this ordered structure compared to the films of the predominantly amorphous control. The characterization of dendrimer:fullerene bulk heterojunction photovoltaic devices shows no voltage loss despite the decreased absorption onset. The properties of the device are consistent with the improved photocarrier lifetimes, but they are limited by a low short-circuit photocurrent density. We attribute this to electron confinement in the core that hinders transfer to the fullerene acceptor.

The influence of solid‐state microstructure on the origin and yield of long‐lived photogenerated charge in neat semiconducting polymers

“…For this reason, we expect changes in the microstructure of the polymer to have little influence on the mobility measured via TRMC. An example of this insensitivity to long‐range order can be found in our previous work on dendrimers having oligothiophene branches: the relatively disordered dendrimers exhibited TRMC hole mobilities identical to neat regioregular P3HT films of a higher degree of crystallinity 57. Similarly, Dicker et al previously observed, in combined pulse‐radiolysis and flash‐photolysis TRMC experiments, that there was no discernable difference in the GHz hole mobility between regioregular and regiorandom P3HT, which had field‐effect transistor hole mobilities differing by up to two orders of magnitude 58.…”

Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology

“…prepared star‐shaped molecules 155 and 156 to show, how a systematic modification of the core unit can change the properties from molecular to device level 197. 198 Oligomer 155 exhibited an absorption maximum at 424 nm, which was assigned to the transition from individual dendrons because of the meta ‐linkage. In contrast, owing to the insertion of electron‐poor cyano groups (→ 156 ), an additional charge transfer band appeared at 511 nm.…”

Organic Semiconductor Photovoltaic Materials

“…Dendrimer 5d showed a broad absorption in the range 300-700 nm and a reduced bandgap of 1.7 eV, giving an efficiency of 1.3 %. Kopidakis and coworkers have developed such an oligothiophene (5e) with electron-deficient tricyanobenzene as core and electron-rich dendrons [62]. Compared to the control dendrimer with the same structure but without the electron-deficient core (5e-1), the appending electrondeficient core in 5e-2 planarizes the structure and lowers the bandgap, thus leading to enhanced structure order in bulk heterojunction films.…”

“…Notably, despite its lower bandgap, an expected larger J SC for 5e-2 as comparison to 5e-1 was not recorded. The electron trapping effect in the cyanobenzene core can account for its lower J SC , impeding electron transfer to the acceptor [62]. Another approach for broadening the absorption is introducing acceptors into the terminal of oligothiophenes.…”