Triplet Exciton Generation in Bulk-Heterojunction Solar Cells Based on Endohedral Fullerenes

Abstract

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu(3)N@C(80) (Lu(3)N@C(80)-PCBEH) show an open circuit voltage (V(OC)) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC(61)BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (J(SC)) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the J(SC) in blends containing the high voltage absorber Lu(3)N@C(80)-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu(3)N@C(80)-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC(61)BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu(3)N@C(80)-PCBEH are responsible for the reduced photocurrent.

Triplet transport in thin films: fundamentals and applications

“…However, an ongoing challenge in this field is to translate these encouraging results from solution into commercially relevant thin films, ideally made by low-cost printing methods. Therefore, it would be interesting to fabricate thin films that can harness triplet excitons created by singlet fission, chargetransfer (CT) states at the polymer:fullerene interface, [23][24][25][26][27][28][29] or nanocrystal photosensitization, [30][31][32][33][34][35][36] and extract the energy contained in these excitons either through TTA via photon upconversion or improved PCEs in solar cells. While singlet fission enhanced organic solar cells have been demonstrated, any improvement in the PCE of the solar cell by the enhanced external quantum efficiency (EQE) from the down-converted photons was offset by the poor exciton or charge transport in existing thin films.…”

Fullerene derivatives as nano-additives in polymer composites

“…132 Later, an organic solar cell based on Lu 3 N@C 80 was investigated in terms of the influence of the metallofullerene on the simultaneous reduction of photocurrent J SC . 141 The reasons responsible for the decrease in J SC were analyzed using photoluminescence and photoinduced absorption techniques and it was found that the formation of triplet molecular excitons and corresponding intramolecular electron transfer may decrease the efficiency of the solar cells. The existence of triplet excitons was also proved using photoluminescencedetected magnetic resonance where the triplet in the resonance spectra quenched by charge transfer in the blends with PCBM appears in the case of a blend based on Lu 3 N@C 80 .…”

Light-induced electron spin resonance as a probe for charge separation and recombination in hybrid donor:acceptor systems

“…For [51] used photoluminescence detected magnetic resonance (PLDMR) to detect the generation of triplet excitons in endohedral fullerenes. Using pulsed laser systems, it is also possible to study charge carrier dynamics in the nanosecond time domain.…”