Triplet exciton formation for non-radiative voltage loss in high-efficiency nonfullerene organic solar cells

Mechanism study on organic ternary photovoltaics with 18.3% certified efficiency: from molecule to device

“…3f and Table S3 (ESI†) display the rising kinetics of PBDB-TF GSB in binary and ternary films, directly reflecting the hole transfer process with a biexponential function: i = a 1 exp(− t / τ 1 ) + a 2 exp(− t / τ 2 ). As demonstrated in previous reports, 47–49 a 1 is related to the proportion of the ultrafast hole transfer process at the interface characterized by τ 1 , while a 2 is attributed to a diffusion mediated process controlled by a pure domain and characterized by τ 2 . Clearly, the ternary blends show a higher proportion of the fast process, especially in PBDB-TF:BTP-eC9:BTP-S2 ( a 1 = 56.8%), which is larger than that of PBDB-TF:BTP-eC9 ( a 1 = 51.6%), quantifying the higher hole transfer rate in ternary films compared to the binary blend.…”

Geminate and Nongeminate Pathways for Triplet Exciton Formation in Organic Solar Cells

“…TA has been widely used to explore the photophysical processes occurring in OSCs, [ 20,26,50,51,54,55 ] as it is able to provide insights into the evolution of both optically bright and dark states on timescales spanning femtoseconds to milliseconds. Thus, TA is well suited to probing optically dark T 1 states in OSCs as the distinct T 1 photoinduced absorption (PIA) signatures, typically located in the near‐infrared (NIR) spectral region, [ 20–22,25,26,56 ] are a clear fingerprint for the presence and molecular location of these states. Furthermore, TA can, in theory, distinguish between the presence of a monomolecular (direct ISC or geminate BCT) or bimolecular (nongeminate BCT) T 1 formation pathways through the fluence dependence of T 1 generation; monomolecular pathways show no fluence dependence, [ 56 ] while bimolecular events exhibit a strong fluence dependence.…”

“…[ 20,25,26 ] However, in the case where significant bimolecular pathways are present, the fluence dependent behavior of T 1 formation will dominate, masking any underlying monomolecular processes. Thus, as many fullerene and NFA OSCs blends demonstrate nongeminate BCT T 1 formation, [ 20–22,25,26 ] TA can, in general, only be reliably used to detect the nongeminate pathway.…”

“…Recently, the recombination of charge carriers via spin‐triplet excitons has been identified as another significant nonradiative voltage loss pathway in both fullerene and NFA OSCs. [ 20–23 ] For example, in the benchmark PM6:Y6 system, [ 24 ] the fraction of charge carriers that recombine via the triplet exciton (T 1 ) of the low E g component, Y6, is ≈90%; [ 20 ] comparable T 1 recombination fractions have also been reported in fullerene acceptor OSCs. [ 25,26 ] As a result, χ in Equation 2 is limited to 0.1 and the EQE EL of the PM6:Y6 blend is reduced by a factor of 10, lowering the V OC by ≈60 mV.…”

All‐Green Solvent‐Processed Planar Heterojunction Organic Solar Cells with Outstanding Power Conversion Efficiency of 16%

“…In addition, we fitted this dynamic process by a biexponential function: i = A 1 exp(− t /τ 1 ) + A 2 exp(− t /τ 2 ), in which τ 1 represents the ultrafast charge transfer time at the interfaces and τ 2 represents the exciton diffusion time toward interfaces before hole transfer, as well as A 1 and A 2 represent their prefactors, respectively. [ 42,43 ] As displayed in Figure 5h and Table S2 in the Supporting Information, fast charge separation rates were observed in all binary films.…”