We have developed a simple spin coating method to make high-quality nanoporous photoelectrodes of monoclinic BiVO4 and studied the ability of these electrodes to transport photogenerated carriers to oxidize sulfite and water. Samples containing molybdenum and featuring [001] out-of-plane crystallographic texture show a photocurrent and external quantum efficiency (EQE) for sulfite oxidation as high as 3.1 mA cm−2 and 60%, respectively, at 1.23 V versus reversible hydrogen electrode. By using an optical model of the electrode stack to accurately determine the fraction of electrode absorptance due to the BiVO4 active layer, we estimate that on average 70 ± 5% of all photogenerated carriers escape recombination. A comparison of internal quantum efficiency as a function of film processing, illumination direction, and film thickness shows that electron transport is efficient and hole transport limits the photocurrent (hole diffusion length <40 nm). We find that Mo addition primarily improves electron transport and texturing mostly improves hole transport. Mo enhances electron transport by thinning the surface depletion layer or passivating traps and recombination centers at grain boundaries and interfaces, while improved hole transport in textured films may result from more efficient lateral hole extraction due to the texturing itself or the reduced density of deep gap states observed in photoemission measurements. Photoemission data also reveal that the films have bismuth-rich, vanadium- and oxygen-deficient surface layers, while ion scattering spectroscopy indicates a Bi–V–O surface termination. Without added catalysts, the plain BiVO4 electrodes oxidized water with an initial photocurrent and peak EQE of 1.7 mA cm−2 and 30%, respectively, which equates to a hole transfer efficiency to water of >64% at 1.23 V. The electrodes quickly photocorrode during water oxidation but show good stability during sulfite oxidation and indefinite stability in the dark. By improving the hole transport efficiency and coating these nanoporous BiVO4 films with an appropriate protective layer and oxygen evolution catalyst, it should be possible to achieve highly efficient and stable water oxidation at a practical pH.

Numerical modeling of Hall effect data is used to demonstrate the existence of a conductive inversion layer at the surface of high-quality n-type single crystals of iron pyrite (cubic FeS2) grown by a flux technique. The presence of the inversion layer is corroborated by Hall measurements as a function of crystal thickness and photoemission spectroscopy. This hole-rich surface layer may explain both the low photovoltage of pyrite solar cells and the widely-observed high p-type conductivity of polycrystalline pyrite thin films that have together perplexed researchers for the past thirty years. We find that the thickness and conductivity of the inversion layer can be modified by mechanical and chemical treatments of the pyrite surface, suggesting that it may be possible to eliminate this hole-rich layer by passivating surface states and subsurface defects. Furthermore, modeling of the high-temperature electrical conductivity shows that the electronic band gap is 0.80 ± 0.05 eV at room temperature (compared to 0.94 eV according to optical transmission data), confirming that photovoltages of ∼500 mV should be attainable from pyrite under solar illumination.

PbSe quantum dot (QD) field effect transistors (FETs) with air-stable electron mobilities above 7 cm2 V–1 s–1 are made by infilling sulfide-capped QD films with amorphous alumina using low-temperature atomic layer deposition (ALD). This high mobility is achieved by combining strong electronic coupling (from the ultrasmall sulfide ligands) with passivation of surface states by the ALD coating. A series of control experiments rule out alternative explanations. Partial infilling tunes the electrical characteristics of the FETs.

Iron pyrite (cubic FeS2) is a promising candidate absorber material for earth-abundant thin-film solar cells. Here, we report on phase-pure, large-grain, and uniform polycrystalline pyrite films that are fabricated by solution-phase deposition of an iron(III) acetylacetonate molecular ink followed by sequential annealing in air, H2S, and sulfur gas at temperatures up to 550 °C. Phase and elemental compositions of the films are characterized by conventional and synchrotron X-ray diffraction, Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy (XPS). These solution-deposited films have more oxygen and alkalis, less carbon and hydrogen, and smaller optical band gaps (Eg = 0.87 ± 0.05 eV) than similar films made by chemical vapor deposition. XPS is used to assess the chemical composition of the film surface before and after exposure to air and immersion in water to remove surface contaminants. Optical measurements of films rich in marcasite (orthorhombic FeS2) show that marcasite has a band gap at least as large as pyrite and that the two polymorphs share similar absorptivity spectra, in excellent agreement with density functional theory models. Regardless of the marcasite and elemental impurity contents, all films show p-type, weakly activated transport with curved Arrhenius plots, a room-temperature resistivity of ∼1 Ω cm, and a hole mobility that is too small to measure by Hall effect. This universal electrical behavior strongly suggests that a common defect or a hole-rich surface layer governs the electrical properties of most FeS2 thin films.

Thin films of colloidal semiconductor nanocrystals (NCs) are inherently metatstable materials prone to oxidative and photothermal degradation driven by their large surface-to-volume ratios and high surface energies.(1) The fabrication of practical electronic devices based on NC solids hinges on preventing oxidation, surface diffusion, ripening, sintering, and other unwanted physicochemical changes that can plague these materials. Here we use low-temperature atomic layer deposition (ALD) to infill conductive PbSe NC solids with metal oxides to produce inorganic nanocomposites in which the NCs are locked in place and protected against oxidative and photothermal damage. Infilling NC field-effect transistors and solar cells with amorphous alumina yields devices that operate with enhanced and stable performance for at least months in air. Furthermore, ALD infilling with ZnO lowers the height of the inter-NC tunnel barrier for electron transport, yielding PbSe NC films with electron mobilities of 1 cm2 V–1 s–1. Our ALD technique is a versatile means to fabricate robust NC solids for optoelectronic devices.

We combine optical absorption spectroscopy, ex situ transmission electron microscopy (TEM) imaging, and variable-temperature measurements to study the effect of ultraviolet (UV) light and heat treatments on ethanedithiol-treated PbS quantum dot (QD) films as a function of ambient atmosphere, temperature, and QD size. Film aging occurs mainly by oxidation or ripening and sintering depending on QD size and the presence of oxygen. We can stop QD oxidation and greatly suppress ripening by infilling the films with amorphous Al2O3 using room-temperature atomic layer deposition (ALD).Keywords: nanocrystals; PbS; photothermal stability; quantum dots; solar cells

We measure the room-temperature electron and hole field-effect mobilities (μFE) of a series of alkanedithiol-treated PbSe nanocrystal (NC) films as a function of NC size and the length of the alkane chain. We find that carrier mobilities decrease exponentially with increasing ligand length according to the scaling parameter β = 1.08−1.10 Å−1, as expected for hopping transport in granular conductors with alkane tunnel barriers. An electronic coupling energy as large as 8 meV is calculated from the mobility data. Mobilities increase by 1−2 orders of magnitude with increasing NC diameter (up to 0.07 and 0.03 cm2 V−1 s−1 for electrons and holes, respectively); the electron mobility peaks at a NC size of ∼6 nm and then decreases for larger NCs, whereas the hole mobility shows a monotonic increase. The size-mobility trends seem to be driven primarily by the smaller number of hops required for transport through arrays of larger NCs but may also reflect a systematic decrease in the depth of trap states with decreasing NC band gap. We find that carrier mobility is independent of the polydispersity of the NC samples, which can be understood if percolation networks of the larger-diameter, smaller-band-gap NCs carry most of the current in these NC solids. Our results establish a baseline for mobility trends in PbSe NC solids, with implications for fabricating high-mobility NC-based optoelectronic devices.

Iron pyrite (FeS2) is a promising earth-abundant semiconductor for thin-film solar cells. In this work, phase-pure, single-crystalline, and well-dispersed colloidal FeS2 nanocrystals (NCs) were synthesized in high yield by a simple hot-injection route in octadecylamine and then were subjected to partial ligand exchange with octadecylxanthate to yield stable pyrite NC inks. Polycrystalline pyrite thin films were fabricated by sintering layers of these NCs at 500−600 °C under a sulfur atmosphere.

We show that ligand exchange with short-chain carboxylic acids (formic, acetic, and oxalic acid) can quantitatively remove oleic acid from the surface of PbSe and PbS quantum dot (QD) films to yield p-type, carboxylate-capped QD solids with field-effect hole mobilities in the range of 10−4−10−1 cm2 V−1 s−1. For a given chemical treatment, PbSe devices have 10-fold higher mobilities than PbS devices because of stronger electronic coupling among the PbSe QDs and possibly a lower density of surface traps. Long-term optical and electrical measurements (i) show that carboxylate-capped PbSe QD films oxidize much more gradually in air than do thiol-capped PbSe films and (ii) quantify the slower and less extensive oxidation of PbS relative to PbSe QDs. We find that whereas the hole mobility of thiol-capped samples decreases continuously with time in air, the mobility of carboxylate-capped films first increases by an order of magnitude over several days before slowly decreasing over weeks. This behavior is a consequence of the more robust binding of carboxylate ligands to the QD surface, such that adsorbed oxygen and water initially boost the hole mobility by passivating surface states and only slowly degrade the ligand passivation to establish an oxide shell around each QD in the film. The superior hole mobilities and oxidation resistance of formic- and acetic-treated QD solids may prove useful in constructing efficient, stable QD photovoltaic devices.Keywords: field-effect transistors; PbS; PbSe; quantum dots; solar cells

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1 ± 0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60 ± 5% and 80 ± 7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE < 100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.