Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

Abstract

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.

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

“…[20] Conversely, fullerene blends often show geminate BCT T 1 formation. [23,43,52,53] Thus, fullerene acceptor blends are the ideal model systems to demonstrate how it is possible to probe the three main T 1 formation mechanisms, clarifying the strength of our approach and the complementarity of optical and magnetic resonance techniques.…”

Electron spin resonance resolves intermediate triplet states in delayed fluorescence

“…While optical spectroscopy has proved a powerful tool to examine the photophysics and spinconversion rates in TADF 30,31,35,36 , trESR provides a complementary window into the underlying spin physics. TrESR is sensitive to paramagnetic states and can therefore probe the formation dynamics and coupling mechanisms of triplet excitons 37 , as highlighted by broad applications in studying triplet excitons in organic photovoltaics [38][39][40] , singlet fission [41][42][43][44] , photosensitizers 37,[45][46][47] , and molecular electronics 48,49 . Because trESR is not sensitive to singlet excitons (due to their diamagnetism) it can only explicitly probe forward ISC.…”

Structure–Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

“…Additionally, certain pathways lead to very distinct populations of the triplet sublevels reflected in characteristic polarization patterns that can unequivocally be assigned, such as triplet states formed via back electron transfer (Thurnauer et al, 1975; Budil and Thurnauer, 1991). Those states have been observed and assigned in OPV materials as well (Niklas et al, 2015), allowing even to disentangle the contributions due to several different and concurrent triplet formation pathways (Thomson et al, 2017).…”