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 π−π stacked D and A regions with highly crystalline dicyanodistyrylbenzenenaphthalimide-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.

Application of indacenodiselenophene central core and modulation of terminal group interaction for high-efficient P3HT-based organic solar cells

“…According to the previous study on an analogue acceptor, the peak at 1000 nm is possibly assigned to the polaron absorption band, and the region below 900 nm is possibly attributed to the intramolecular charge-transfer. 39 The co-occurrence of polaron with singlet exciton immediately upon excitation may be due to the high crystallinity property and strong π–π stacking of BTA46 . After 1 ns, the singlet exciton at 1150 nm is completely converted to polarons in the range of 950–1100 nm.…”

“…As the previous study, the positive signals at around 650 nm belong to the polaron pairs of P3HT. 39 And, P3HT also exhibits broad excited state absorption (ESA) bands over 750 nm, where the absorption band for the singlet exciton is centered at 1250 nm. 40,41 P3HT can also form polaron even upon initial excitation, 42 therefore, here the characteristic absorption band centered at 1000 nm observed after 1 ns is attributed to polaron absorption.…”

Copolymers of 3-arylthieno[3,2-b]thiophenes bearing different substituents: Synthesis, electronic, optical, sensor and memory properties

“…Among the conjugated polymers that have been used in OSCs, OFETs and memory devices, electron-donor and electron-acceptor moieties in the polymer backbones have played significant roles [34][35][36]. Donor-acceptor (DA) architecture could regulate electronic transition, intramolecular interactions, pull-push effect and bandgap which are required for highperformance devices [37,38].…”

Intrachain Delocalization Effect of Charge Carriers on the Charge-Transfer State Dynamics in Organic Solar Cells

“…1,2 The dissociation mechanism of singlet (S 1 ) excitons into free carriers at donor (D) CP/acceptor (A) interfaces in the BHJ OSCs is well understood by two concepts: (1) the hot-exciton dissociation and (2) the dissociation through a cold (relaxed) charge-transfer (CT) state, the Onsager-like process. 3,4 The charge-dissociation and recombination dynamics at the interfaces, which have a significant impact on the PCEs, are closely related to the D/A energetics, blend morphologies, and charge-delocalization; thus, their complex correlations are being extensively investigated. 5−8 Various CP nanostructures have been introduced into the OSCs as a strategy to increase the PCEs.…”

“…In the field of conjugated polymer (CP)-based organic solar cells (OSCs), the excited-state dynamics study of bulk heterojunction (BHJ) blends has received great attention over the past decade because it can provide important information to further improve power conversion efficiencies (PCEs). , The dissociation mechanism of singlet (S 1 ) excitons into free carriers at donor (D) CP/acceptor (A) interfaces in the BHJ OSCs is well understood by two concepts: (1) the hot-exciton dissociation and (2) the dissociation through a cold (relaxed) charge-transfer (CT) state, the Onsager-like process. , The charge-dissociation and recombination dynamics at the interfaces, which have a significant impact on the PCEs, are closely related to the D/A energetics, blend morphologies, and charge-delocalization; thus, their complex correlations are being extensively investigated. − …”