Investment into photovoltaic (PV) research has accelerated over the past decade as concerns over energy security and carbon emissions have increased. The types of PV technology in which the research community is actively engaged are expanding as well. This review focuses on the burgeoning field of atomic layer deposition (ALD) for photovoltaics. ALD is a self-limiting thin film deposition technique that has demonstrated usefulness in virtually every sector of PV technology including silicon, thin film, tandem, organic, dye-sensitized, and next generation solar cells. Further, the specific applications are not limited. ALD films have been deposited on planar and nanostructured substrates and on inorganic and organic devices, and vary in thickness from a couple of angstroms to over 100 nm. The uses encompass absorber materials, buffer layers, passivating films, anti-recombination shells, and electrode modifiers. Within the last few years, the interest in ALD as a PV manufacturing technique has increased and the functions of ALD have expanded. ALD applications have yielded fundamental understanding of how devices operate and have led to increased efficiencies or to unique architectures for some technologies. This review also highlights new developments in high throughput ALD, which is necessary for commercialization. As the demands placed on materials for the next generation of PV become increasingly stringent, ALD will evolve into an even more important method for research and fabrication of solar cell devices.

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Quantum dot sensitized solar cells (QDSSCs) are of interest for solar energy conversion because of their tunable band gap and promise of stable, low-cost performance. We have investigated the effects of self-assembled monolayers (SAMs) with phosphonic acid headgroups on the bonding and performance of cadmium sulfide (CdS) solid-state QDSSCs. CdS quantum dots ∼2 to ∼6 nm in diameter were grown on SAM-passivated planar or nanostructured TiO(2) surfaces by successive ionic layer adsorption and reaction (SILAR), and photovoltaic devices were fabricated with spiro-OMeTAD as the solid-state hole conductor. X-ray photoelectron spectroscopy, Auger electron spectroscopy, ultraviolet-visible spectroscopy, scanning electron microscopy, transmission electron microscopy, water contact angle measurements, ellipsometry, and electrical measurements were employed to characterize the materials and the resulting device performance. The data indicate that the nature of the SAM tailgroup does not significantly affect the uptake of CdS quantum dots on TiO(2) nor their optical properties, but the presence of the SAM does have a significant effect on the photovoltaic device performance. Interestingly, we observe up to ∼3 times higher power conversion efficiencies in devices with a SAM compared to those without the SAM.

Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial region's occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure.

Pure, polycrystalline CdS deposited by atomic layer deposition (ALD) on Si(100) or glass using dimethyl cadmium and in situ generated H 2 S is investigated in detail. This ALD system follows saturation behavior typical of ALD systems, and the growth rate monotonically decreases with temperature from 100 °C-300 °C; by 400 °C linear growth rate behavior is no longer seen. The crystal structure as determined by X-ray diffraction and transmission electron microscopy gradually transitions from zincblende to wurtzite with increasing temperature until the film is primarily wurtzite by 400 °C. Further, the average grain size increases with temperature. Transmission electron microscopy images and selected area diffraction patterns confirm the presence of zincblende and wurtzite crystals because of stacking faults and demonstrate that {111} crystal planes are more oriented parallel to the substrate at lower temperatures. Ultraviolet-visible spectroscopy shows that the bandgap is 2.3-2.42 eV in the 100 °C-400 °C range with a slight increase occurring with temperature. The roughness of the films is found to increase both with temperature and cycle number as observed with atomic force microscopy and scanning electron microscopy. Density functional theory calculations were used to understand observations concerning the growth rate and the bandgap of the films deposited at different temperatures.