19 research outputs found
the path to silicon-singlet fission heterojunction devices
Singlet exciton fission is an exciton multiplication process that occurs in certain organic materials, converting the energy of single highly-energetic photons into pairs of triplet excitons. This could be used to boost the conversion efficiency of crystalline silicon solar cells by creating photocurrent from energy that is usually lost to thermalisation. An appealing method of implementing singlet fission with crystalline silicon is to incorporate singlet fission media directly into a crystalline silicon device. To this end, we developed a solar cell that pairs the electron-selective contact of a high-efficiency silicon heterojunction cell with an organic singlet fission material, tetracene, and a PEDOT:PSS hole extraction layer. Tetracene and n-type crystalline silicon meet in a direct organic–inorganic heterojunction. In this concept the tetracene layer selectively absorbs blue-green light, generating triplet pairs that can dissociate or resonantly transfer at the organo-silicon interface, while lower-energy light is transmitted to the silicon absorber. UV photoemission measurements of the organic–inorganic interface showed an energy level alignment conducive to selective hole extraction from silicon by the organic layer. This was borne out by current–voltage measurements of devices subsequently produced. In these devices, the silicon substrate remained well-passivated beneath the tetracene thin film. Light absorption in the tetracene layer created a net reduction in current for the solar cell, but optical modelling of the external quantum efficiency spectrum suggested a small photocurrent contribution from the layer. This is a promising first result for the direct heterojunction approach to singlet fission on crystalline silicon
Improving the light-harvesting of second generation solar cells with photochemical upconversion
Photovoltaics (PV) offer a solution for the development of sustainable energy
sources, relying on the sheer abundance of sunlight: More sunlight falls on
the Earth’s surface in one hour than is required by its inhabitants in a year.
However, it is imperative to manage the wide distribution of photon energies
available in order to generate more cost efficient PV devices because single
threshold PV devices are fundamentally limited to a maximum conversion
efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the
production of c-Si cells with efficiencies as high as 25%,1 close to the
limiting efficiency of ∼30%. But these cells are rather expensive, and
ultimately the cost of energy is determined by the ratio of system cost and
efficiency of the PV device. A strategy to radically decrease this ratio is to
circumvent the SQ limit in cheaper, second generation PV devices. One
promising approach is the use of hydrogenated amorphous silicon (a-Si:H),
where film thicknesses on the order of several 100nm are sufficient.
Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and
the material suffers from light-induced degradation. Thinner absorber layers
in a-Si:H devices are generally more stable than thicker films due to the
better charge carrier extraction, but at the expense of reduced conversion
efficiencies, especially in the red part of the solar spectrum (absorption
losses). Hence for higher bandgap materials, which includes a-Si as well as
organic and dye-sensitized cells, the major loss mechanism is the inability to
harvest low energy photons
Effect of a back reflector
Photochemical upconversion is applied to a hydrogenated amorphous silicon
solar cell in the presence of a back-scattering layer. A custom-synthesized
porphyrin was utilized as the sensitizer species, with rubrene as the emitter.
Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement
of ~2 % was observed at a wavelength of 720 nm. Without the scattering layer,
the EQE enhancement was half this value, indicating that the effect of the
back-scatterer is to double the efficacy of the upconverting device. The
results represent an upconversion figure of merit of 3.5 × 10–4 mA cm–2 sun–2,
which is the highest reported to date
Beyond Shockley–Queisser: Molecular Approaches to High-Efficiency Photovoltaics
Molecular
materials afford abundant flexibility in the tunability
of physical and electronic properties. As such, they are ideally suited
to engineering low-cost, flexible, light-harvesting materials that
break away from the single-threshold paradigm. Single-threshold solar
cells are capable of harvesting a maximum of 33.7% of incident sunlight,
whereas two-threshold cells are capable of energy harvesting efficiencies
exceeding 45%. In this Perspective, we provide the theoretical background
with which upper efficiency limits for various multiple-threshold
solar cell architectures may be calculated and review and discuss
various reports that employ processes such as triplet–triplet
annihilation and singlet fission in multiple-threshold devices comprised
of molecular materials
Quintet formation, exchange fluctuations, and the role of stochastic resonance in singlet fission
Quintet formation is a part of a photochemical energy conversion process, which could be exploited to help achieve higher efficiency values for photovoltaics. Here, the authors propose a stochastic and coherent resonance mechanism for formation of the quintet spin state in singlet fission materials, that is still viable in the strong-exchange regime
Singlet and Triplet Exciton Dynamics of Violanthrone
The exciton dynamics of violanthrone-79 are investigated in solution and in the solidstate. In solution, the photo-prepared singlet is found to exhibit a strong ground-state bleachand stimulated emission feature, but when sensitized in its triplet state, exhibits only a narrowand weak ground-state bleach. As supported by density functional theory calculations,this is explained by the triplet state having absorptions in the same region, with a similaroscillator strength, as the ground state molecule. In solid films, the excited singlet isfound to survive only 100 ps, giving way to a long-lived transient absorption spectrum withcharacteristics reminiscent of the triplet in solution. This is interpreted in terms of singletfission in the solid film.</div
Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System
Singlet fission is
the spin-allowed conversion of a photogenerated
singlet exciton into two triplet excitons in organic semiconductors,
which could enable single-junction photovoltaic cells to break the
Shockley–Queisser limit. The conversion of singlets to free
triplets is mediated by an intermediate correlated triplet pair (TT)
state, but an understanding of how the formation and dissociation
of these states depend on energetics and morphology is lacking. In
this study, we probe the dynamics of TT states in a model endothermic
fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline
and disordered regions. We observe the formation of different TT states,
with varying yield and different rates of singlet decay, depending
on the excitation energy. An emissive TT state is observed to grow
in over 1 ns when excited at 480 nm, in contrast to excitation at
lower energies where this emissive TT state is not observed. This
suggests that the pathway for singlet fission in these nanoparticles
is strongly influenced by the initial sub-100 fs relaxation of the
photoexcited state away from the Franck–Condon point, with
multiple possible TT states. On nanosecond time scales, the TT states
are converted to free triplets, which suggests that TT states might
diffuse into the disordered regions of the nanoparticles where their
breakup to free triplets is favored. The free triplets then decay
on μs time scales, despite the confined nature of the system.
Our results provide important insights into the mechanism of endothermic
singlet fission and the design of nanostructures to harness singlet
fission