Regioisomer effects of [70]fullerene mono-adduct acceptors in bulk heterojunction polymer solar cells.

Despite numerous organic semiconductors being developed during the past decade, C derivatives are predominantly used as electron acceptors in efficient polymer solar cells (PSCs). However, as-prepared C mono-adducts intrinsically comprise regioisomers that would mask individual device performances depending on the substituent position on C. Herein, we separate the regioisomers of C mono-adducts for PSC applications for the first time.

Identification of a triplet pair intermediate in singlet exciton fission in solution.

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed.

How disorder controls the kinetics of triplet charge recombination in semiconducting organic polymer photovoltaics.

Recent experiments by Rao et al. (Nature, 2013, 500, 435-439) indicate that recombination of triplet charge-separated states is suppressed in organic polymer-fullerene based bulk-heterojunction (BHJ) photovoltaic cells exhibiting a high degree of crystallinity in the fullerene phase relative to systems with more disorder. In this paper, we use a series of Frenkel-exciton lattice models to rationalize these results in terms of wave-function localization, interface geometry, and density of states.

Nanosecond intersystem crossing times in fullerene acceptors: implications for organic photovoltaic diodes.

Triplet-exciton formation through intersystem crossing of photogenerated singlet excitons in fullerene acceptors can compete with charge generation in organic photovoltaic diodes. This article reports the intersystem crossing timescale (τISC ) of the most commonly used fullerene acceptors, PC60 BM and PC70 BM, in solutions and in spin-coated films. These times are on the nanosecond timescale, and are longer than the characteristic times for charge generation (τd ).

Unequal partnership: asymmetric roles of polymeric donor and fullerene acceptor in generating free charge.

Natural photosynthetic complexes accomplish the rapid conversion of photoexcitations into spatially separated electrons and holes through precise hierarchical ordering of chromophores and redox centers. In contrast, organic photovoltaic (OPV) cells are poorly ordered, utilize only two different chemical potentials, and the same materials that absorb light must also transport charge; yet, some OPV blends achieve near-perfect quantum efficiency. Here we perform electronic structure calculations on large clusters of functionalized fullerenes of different size and ordering, predicting several features of the charge generation process, outside the framework of conventional theories but clearly observed in ultrafast electro-optical experiments described herein.

Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes.

Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here we report the time dependence of the separation of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the electric field generated between the charges, we measure ~200 millielectron volts of electrostatic energy arising from electron-hole separation within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers.

Temperature-independent singlet exciton fission in tetracene.

We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10–270 K). Low-intensity, broad-band measurements allow the identification of spectral features while minimizing bimolecular recombination. Hence, by directly observing both species, we find that the time constant for the conversion of singlets to triplet pairs is ~90 ps.

The role of spin in the kinetic control of recombination in organic photovoltaics.

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes.

Recombination dynamics of charge pairs in a push-pull polyfluorene-derivative.

We investigate the properties of long-lived species in F8BT films through time-resolved photoluminescence (PL) measurements at room temperature and 10 K. The kinetics consist of an initial exponential decay (τ = 2.26 ns) followed by a weak power-law decay (I(t) [proportionality] t(-1)) up to at least 1 ms, both of which depend weakly on temperature.

Direct observation of photoinduced bound charge-pair states at an organic-inorganic semiconductor interface.

It is generally considered that photoinduced charge transfer at the organic-inorganic interfaces in hybrid photovoltaic devices immediately results in a pair of free charge carriers. We extend a novel interface-selective ultrafast "optical pump-push photocurrent probe" technique to study hybrid photovoltaic systems and observe bound electron-hole pair states at the organic-inorganic interface formed between electron-accepting zinc oxide and electron-donating conjugated polymers. We estimate that ∼50% of photogenerated charges stay bound and later recombine, thus hindering the photovoltaic performance of polymer/ZnO cells.