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

Abstract

Organic solar cells (OSCs) have recently shown a rapid improvement in their performance, bringing power conversion efficiencies to above 18%. However, the open‐circuit voltage of OSCs remains low relative to their optical gap and this currently limits efficiency. Recombination to spin‐triplet excitons is a key contributing factor, and is widely, but not universally, observed in donor–acceptor blends using both fullerene and nonfullerenes as electron acceptors. Here, an experimental framework that combines time‐resolved optical and magnetic resonance spectroscopies to detect triplet excitons and identify their formation mechanisms, is reported. The methodology is applied to two well‐studied polymer:fullerene systems, PM6:PC60BM and PTB7‐Th:PC60BM. In contrast to the more efficient nonfullerene acceptor systems that show only triplet states formed via nongeminate recombination, the fullerene systems also show significant triplet formation via geminate processes. This requires that geminate electron–hole pairs be trapped long enough to allow intersystem crossing. It is proposed that this is a general feature of fullerene acceptor systems, where isolated fullerenes are known to intercalate within the alkyl sidechains of the donor polymers. Thus, the study demonstrates that engineering good donor and acceptor domain purity is key for suppressing losses via triplet excitons in OSCs.

Controlling the Spin Exchange Energy through Charge Transfer for Triplet State Management in Organic Semiconductors

“…In state-of-the-art NFA OPVs, Δ V nr is ∼200 mV . This contrasts with inorganic technologies, where Δ V nr can reach <100 mV. , To address this deficit, the field has recently been focused on improving Φ PL of the low band gap component in a neat film, which is considered to provide the upper bound for Φ PL in a low-offset blend where recombination can proceed via the lowest energy S 1 state in the system. , In contrast to OLEDs, it had been noted that triplet management was a relatively understudied topic in OPVs. ,,− However, there have now been several recent reports on the critical role of recombination via T 1 in NFA OPVs. − As with OLEDs, it is generally expected that the T 1 state on the low band gap component will be the lowest energy excitation in the OPV system and could act as a trap state for nonradiative recombination . Indeed, the nongeminate recombination of free charge carriers in an OPV is also expected to produce, to a first approximation, 25% spin-singlet and 75% spin-triplet states following spin statistics .…”

“…Prior to the adoption and extensive optimization of nonfullerene electron acceptor (NFA) materials, the power conversion efficiencies (PCEs) of OPVs using fullerene electron acceptors (FAs) were relatively low (∼11%) . Thus, despite the widespread observation of triplet recombination in fullerene OPVs, − the presence of other more substantial loss pathways drew attention away. However, with the PCEs of single junction NFA OPVs, now around 19%, , attention has turned to identifying those aspects of device operation where significant loss processes remain, including recombination via T 1 states. − …”

Direct detection of spin polarization in photoinduced charge transfer through a chiral bridge

“…20 The generation and time evolution of spin polarization is very informative about the spin dynamics of paramagnetic states. 56,57 We performed trEPR measurements at 40 K on our model system CdSe QD-χ-C 60 (7.8 µM in 1,2,4-trichlorobenzene, blue line). For comparison, we also investigated a solution containing both CdSe QD functionalized with 2 (CdSe QD-χ) (7.8 M) and 1 mM of 3.…”

Exciton Dynamics and Charge Carrier Generation in Organic Semiconductors

“…The existence of interfacial CT state has been evidenced from both absorption [54][55][56] and luminescence [57][58][59] measurements. In some donor-acceptor materials, the charge pair at interfacial CT state has been demonstrated to be Coulombically bound [60][61][62] , hence, it is highly susceptible to recombine in the process known as geminate recombination [63][64][65][66][67][68] (Figure 1.4).…”

“…Following charge-pair separation, the free-charge carriers are transported to the respective electrodes. During the transport, the free-charge carriers may recombine in process called non-geminate recombination [63][64][65][66][67]79,80 (Figure 1.4). In BHJ OSCs, if the grain sizes of donor and acceptor materials are small, the probability of nongeminate recombination increases.…”