Influence of Intramolecular Charge-Transfer Characteristics of Excitons on Polaron Generation at the Donor/Acceptor Interface in Polymer Solar Cells

Abstract

To understand the importance of excess energy in the charge separation mechanism in polymer solar cells (PSCs), we focused on the dissociation of the Coulombically bound electron and hole pair at the donor (D) and acceptor (A) interface. A push-pull-type copolymer poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and a homopolymer poly(3-hexylthiophene) (P3HT) were used as model compounds to correlate the chemical structure of the donor materials with the mechanism of photoinduced charge separation at the D/A interface in PSCs. In the case of PTB7, excitons with intramolecular charge-transfer (ICT) characteristics are initially generated due to the push-pull actions between the electron-donating and electron-accepting building units, resulting in electron density displacement to facilitate interfacial charge separation. On the other hand, the delocalized exciton state of P3HT is known to be favorable for the hot exciton dissociation and charge generation while lacking the ICT characteristics. Therefore, understanding the effect of ICT characteristics and delocalization of the exciton state of polymers is important for enhancing the charge separation at the D/A interface and thus the photocurrent in PSCs. Both PTB7:NIDCSEO3 and P3HT:NIDCSEO3 blends exhibited strongly pi-pi stacked D and A regions with highly crystalline dicyanodistyrylbenzene-naphthalimide-based acceptor, NIDCSEO3, which is beneficial for the exciton delocalization and reduces the effect of blend morphology when comparing the charge separation at the D/A interface. Through the temperature- and pump-dependent femtosecond transient absorption experiments in this work, it was found that the charge separation via the hot CT state is dominant in PTB7:NIDCSEO3 owing to the delocalized ICT-type excitons of PTB7. On the other hand, P3HT:NIDCSEO3 exhibited a complex charge generation mechanism comprising both hot and relaxed states including a polaron pair state within P3HT while preserving the delocalized exciton state based on the highly crystalline homopolymer structure.