On the absence of triplet exciton loss pathways in non-fullerene acceptor based organic solar cells

Abstract

We investigate the viability of highly efficient organic solar cells (OSCs) based on nonfullerene acceptors (NFA) by taking into consideration efficiency loss channels and stability issues caused by triplet excitons (TE) formation. OSCs based on a blend of the conjugated donor polymer PBDB-T and ITIC as acceptor were fabricated and investigated with electrical, optical and spin-sensitive methods. The spin-Hamiltonian parameters of molecular TEs and charge transfer TEs in ITIC e.g., zero-field splitting and charge distribution, were calculated by Density Functional Theory (DFT) modelling. In addition, the energetic model describing the photophysical processes in the donor-acceptor blend was derived. Spin-sensitive photoluminescence measurements prove the formation of charge transfer (CT) states in the blend and the formation of TEs in the pure materials and the blend. However, no molecular TE signal is observed in the completed devices under working conditions by spin-sensitive electrical measurements. The absence of a molecular triplet state population allows to eliminate a charge carrier loss channel and irreversible photooxidation facilitated by long-lived triplet states. These results correlate well with the high power conversion efficiency of the PBDB-T:ITIC-based OSCs and their high stability.

The Path to 20% Power Conversion Efficiencies in Nonfullerene Acceptor Organic Solar Cells

“…Furthermore, other studies have proposed that the formation of T 1 states has little impact on the performance of OSC devices. [ 170–172 ] Thus, it is clear from these divergent reports that additional investigations are required on the impact of T 1 formation on device performance to better understand its role in the losses of OSCs; in‐line with the contrasting findings discussed here, it is entirely possible that T 1 formation may affect some blends more severely than others.…”

Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics

“…Curved red arrow depicts the ultrafast vibrational thermalization process after photoexcitation in each electronic CT state. We neglect triplet CT states (CT 3 ) since they have been shown to have a negligible effect in some OPV systems 72 , 73 . …”

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

“…In the first two materials, we observe a strong negative signal at ~333 mT in both the neat and dilute (10 wt%) films, with spin-locking also present in the neat films (Supplementary Figs. [21][22][23][24][25]. This finding suggests that the dissociation of excitons is present in well-studied DF emitters in the solid state, with HFI-ISC between inter-CT states contributing to the triplet to singlet conversion in these OLED devices.…”

“…3d, full field range data in Supplementary Fig. 19), we observe a narrow negative signal at ~333 mT, which is attributed to inter-CT states 23 . To understand the relevance of this feature, we note that under an external magnetic field (B 0 ~333 mT), the Zeeman interaction decouples HFI-induced transitions between the inter-1 CT 0 and either inter- 3 CT + or inter-3 CT - 24 .…”