Charge Recycling Mechanism Through a Triplet Charge-Transfer State in Ternary-Blend Organic Solar Cells Containing a Nonfullerene Acceptor

Abstract

We investigated the side-group and backbone-extension effects of a push−pull conjugated polymer (CP) on the charge-generation mechanism in ternary-blend organic solar cells (OSCs) containing PC 71 BM and a nonfullerene acceptor (NFA, ITIC). Transient absorption spectroscopy results revealed that the CP extension increased the hole-transfer efficiency from ITIC to the CPs, inducing the formation of triplet (T 1 ) excitons by nongeminate recombination (via triplet charge-transfer states, 3 CT) because of the finely mixed morphologies. Particularly, the 3 CT state relaxation to T 1 excitons was efficiently suppressed by PC 71 BM in the ternary-blend films based on the side-group and backbone extended CPs. This caused the recycling of the free carriers generated by the hole transfer from ITIC to the CPs, thereby increasing the efficiency of the OSC having the optimized ternaryblend morphology. Our findings provide new insights on the mechanism of charge generation through the photoinduced hole transfer from NFAs to CPs.

Elucidating the Chain-Extension Effect on the Exciton-Dissociation Mechanism through an Intra- or Interchain Polaron-Pair State in Push–Pull Conjugated Polymers

“…The formation dynamics of the LPs at the interface within the BHJ film (disordered domains) is faster than that of the DPs in the ordered domains that are away from the interfaces . We confirmed that the formation dynamics of the PIA peak at approximately 960 nm were faster than those of the PIA peak at approximately 700 nm in the TAS of CP/PC 71 BM BHJ blend films . The SE signal quenching dynamics at approximately 700 nm in the TAS of CP thin films was clearly observed due to the inter-PP formation (Figures S8–S10).…”

“…We assigned the PIA peaks in the dotted–dashed circles at approximately 1200 nm in Figure to the S 1 excitons of the CP corresponding to each panel. In our previous report, the OD value of the PIA peak at approximately 1200 nm significantly decreased (the peak shape totally collapsed) in the TAS of the PBT-OTT/PC 71 BM bulk-heterojunction (BHJ) blend film compared to that in the TAS of the PBT-OTT neat film owing to PBT-OTT S 1 quenching by PC 71 BM . In contrast, the PIA peaks at approximately 700 and 900 nm in the TAS of the PBT-OTT/PC 71 BM blend film were considerably larger and more distinct than those for the PBT-OTT neat film; thus, we assigned these peaks to the positive polaron states in the PBT-OTT domains.…”

“…Thus, an in-depth analysis of the effects of chain disordering/stiffness and intra-/interchain-aggregate patterns on the photophysics of CPs is one of the critical factors to keep up with the PCEs of inorganic and perovskite SCs. Additionally, a clear understanding of the photophysics of nanostructure-dependent CPs can provide valuable information to reduce the nongeminate recombination losses of free carriers in OSCs based on nonfullerene acceptors. ,, However, studies on polaron-mediated exciton-dissociation mechanisms in push–pull CPs dependent on chain-aggregate (excitonic-coupling) patterns are rare.…”

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

“…In the case of the 680 nm pump (the interchain packing-dominant domains are more excited than the intrachain ordering-dominant domains), the formation dynamics of CT states from P4TNTz-2F to PC 71 BM (the k 1 pathway at the top of Figure ) is slightly faster (3.25 ps) in blend #2 than in blend #1 (3.32 ps) (Table ). Generally, the CT states form well more in the amorphous BHJ blends than in the highly crystalline BHJ blends because of the more efficient formation of interfaces between the donors and acceptors in the amorphous BHJ blends than in the highly crystalline BHJ blends. , However, the target analysis results of Figure a and Table show the opposite tendency. This implies that the highly crystalline P4TNTz-2F NWs with the 6 nm widths in blend #2 would promote more efficiently the CT from P4TNTz-2F to PC 71 BM than the relatively low crystalline P4TNTz-2F NWs with the 14 nm widths in blend #1; the large surface area of the narrow P4TNTz-2F NW structure allows the efficient formation of the interfaces within the BHJ blend.…”

Tuning Excited-State Energy Transfer for Light Energy Conversion: A Virtual Issue

“…A trade-off between radiative energy loss, energy loss associated with CT state formation, and non-radiative energy loss has been carefully analyzed to design efficient NFA-based OPVs. The extensions on the donor-conjugated polymer have enhanced the efficiency of hole transfer from NFA to the polymer in a ternary blend organic solar cell . Transient absorption spectroscopy revealed the free charge carrier recycling as a result of suppression of triplet exciton formation from 3 CT states by PC 71 BM.…”