We describe the development of solar water-splitting cells comprising earth-abundant elements that operate in near-neutral pH conditions, both with and without connecting wires. The cells consist of a triple junction, amorphous silicon photovoltaic interfaced to hydrogen- and oxygen-evolving catalysts made from an alloy of earth-abundant metals and a cobalt|borate catalyst, respectively. The devices described here carry out the solar-driven water-splitting reaction at efficiencies of 4.7% for a wired configuration and 2.5% for a wireless configuration when illuminated with 1 sun (100 milliwatts per square centimeter) of air mass 1.5 simulated sunlight. Fuel-forming catalysts interfaced with light-harvesting semiconductors afford a pathway to direct solar-to-fuels conversion that captures many of the basic functional elements of a leaf.
One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells 1 . Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions 1-3 . Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carriermultiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.Carrier multiplication is the process in which the absorption of a single, high-energy photon results in the generation of two or more electron-hole pairs. The excess energy of the initially excited electron is used to excite a second electron over the bandgap, rather than being converted into heat through sequential phonon emission. Carrier multiplication is important for the operation for high-speed electronic devices 4 , but is especially relevant for solar cells 1 , because relaxation of hot carriers through phonon emission is a common loss mechanism in bulk semiconductor solar cells. In this context, semiconductor quantum dots are promising building blocks for future solar cells 1 . In addition to the size-tunability of the quantum-dot optical properties, the carrier-multiplication efficiency in quantum dots was reported to be much higher than in bulk materials, where the process is generally referred to as impact ionization. It has been argued that carrier multiplication is more efficient in nanostructured semiconductors owing to quantum-confinement effects causing (1) a slowing of the phonon-mediated relaxation channel 1 and (2) enhanced Coulomb interactions 2 , resulting from forced overlap between wavefunctions and reduced dielectric screening at the quantum-dot surface 3 . In recent years, several femtosecond spectroscopy studies have revealed highly efficient carrier multiplication in PbSe and PbS (refs 2, 5-9) (refs 18, 19) quantum dots. In initial studies, carrier-multiplication efficiencies may have been overestimated owing to several experimental complications, including too high excitation fluences (generating multiple carriers by sequential absorption of multiple photons), lack of stirring of quantum-dot suspensions (causing photo-induced charging) and sample-to-sample variability 19 . Furthermore, recent tight-binding calculations 20 suggest carrier multiplication in quantum dots is not only not enhanced relative to bulk, but is actually lower. Answering the...
Integrating a silicon solar cell with a recently developed cobaltbased water-splitting catalyst (Co-Pi) yields a robust, monolithic, photo-assisted anode for the solar fuels process of water splitting to O 2 at neutral pH. Deposition of the Co-Pi catalyst on the Indium Tin Oxide (ITO)-passivated p-side of a np-Si junction enables the majority of the voltage generated by the solar cell to be utilized for driving the water-splitting reaction. Operation under neutral pH conditions fosters enhanced stability of the anode as compared to operation under alkaline conditions (pH 14) for which long-term stability is much more problematic. This demonstration of a simple, robust construct for photo-assisted water splitting is an important step towards the development of inexpensive direct solar-to-fuel energy conversion technologies.photoelectrochemical | hydrogen | solar energy | storage P hotosynthetic organisms convert the energy of sunlight into chemical energy by splitting water, producing molecular oxygen and hydrogen equivalents in the highly conserved enzyme complex photosystem II (PSII) (1). Absorbed photons are transferred to the reaction center of PSII, where a single electron/hole charge separation occurs. The oxidative power of the photo-produced hole in PSII is transferred to the oxygen evolving complex (OEC) where water splitting occurs. The electron is transferred to the adjacent photosystem I (PSI), where it participates in the reduction reaction of NAD þ into NADH, which is ultimately used to fix CO 2 . Crucial in the above configuration is the separation of the functions of light collection and conversion from catalysis. Whereas light collection/conversion generates electron/ hole pairs one at a time, water splitting is a four-electron/hole process (2, 3). Hence, the multielectron catalysts of PSII and PSI, positioned at the terminus of the photosynthetic charge-separating network, are compulsory so that the one photon-one-electron/ hole "wireless current" can be bridged to the four-electron/hole chemistry of water splitting.An artificial photosynthesis can be designed if the one-electron/hole wireless current of a semiconductor can be integrated directly with catalysts to perform the four-electron-four proton catalysis of water splitting. To this end, an important recent advance has been the creation of a cobalt-phosphate (Co-Pi) catalyst (4, 5) that captures the functional elements of the OEC of PSII (6). As in PSII OEC, the Co-Pi catalyst self-assembles upon oxidation of an earth-abundant metal [Co 2þ for Co-Pi vs. Mn 2þ for OEC (7-9)] in phosphate-buffered solutions at neutral pH (4, 10), exhibits high activity in natural water and sea water at room temperature (11), activates water by proton-coupled electron transfer (3) [as does the OEC of PSII (12, 13)], and is self-healing (14) [as is PSII (15-18)]. Moreover, X-ray Absorption Spectroscopy (XAS) studies (19,20) have established that the Co-Pi catalyst is a structural relative of PSII OEC. PSII OEC is a Mn 3 CaO 4 -Mn cubane (21) where the fourth M...
Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core-shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (∼2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to ∼1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to ∼1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.
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