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.
ABSTRACT
A new family of fullerene-based compounds, namely, soluble [60]- and [70]fullerene homodimers and the [60]/[70]heterodimer linked through 2-pyrazolino-pyrrolidino bridges, has been synthesised by simple procedures and in high yield. Electrochemical studies confirm their suitability to act as electron acceptors in combination with poly(3-hexylthiophene-2,5-diyl) (P3HT). Their optical properties in solution and in the solid state were studied. A significantly stronger absorption in [70]fullerene-containing dimers relative to [60]homodimer in solution in the visible range was observed. Furthermore, in all donor-acceptor blends studied an efficient charge transfer was observed by means of photoluminescence (PL), photoinduced absorption and light-induced electron spin resonance spectroscopy. The [70]homodimer was found to be a distinctive species, being the strongest PL quencher and most efficient acceptor with the longest lifetime of the charge-separated (polaron) states. As a consequence, bulk-heterojunction solar cells based on this novel [70]homodimer blended with P3HT demonstrated the highest quantum and power conversion efficiencies of 37 and 1 %, respectively, compared to those of [60]fullerene dimers.
