Elijah Thimsen

Plasmonic solar water splitting

The study of the optoelectronic effects of plasmonic metal nanoparticles on semiconductors has led to compelling evidence for plasmon-enhanced water splitting. We review the relevant physics, device geometries, and research progress in this area. We focus on localized surface plasmons and their effects on semiconductors, particularly in terms of energy transfer, scattering, and hot electron transfer.

Alkali-metal-enhanced grain growth in Cu2ZnSnS4 thin films

The highest efficiency solar cells based on copper zinc tin sulfide (CZTS), a promising photovoltaic material comprised of earth abundant elements, are built on soda lime glass (SLG), a substrate which contains many impurities, including Na and K. These impurities may diffuse into CZTS films during processing and affect film structure and properties. We have investigated the effects of these impurities on the microstructure of CZTS films synthesized by ex situ sulfidation of Cu–Zn–Sn alloy films co-sputtered on SLG, Pyrex, and quartz. CZTS films synthesized on SLG were found to have significantly larger grains than films grown on the other substrates. Furthermore, we show that by including a bare additional piece of SLG in the sulfidation ampoule, the grain size of films grown on nominally impurity-free quartz increases from 100s of nm to greater than 1 μm. This demonstrates conclusively that impurities in SLG volatilize in S-containing atmospheres and incorporate into nearby CZTS films synthesized on other substrates. Impurity concentrations in these CZTS films were examined using depth profiling with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Of all the impurities present in SLG, the TOF-SIMS experiments implicated Na, K, and Ca as possible elements responsible for the enhanced grain growth. To investigate the effects of these impurities individually, we introduced very small and controllable amounts of Na, K, or Ca into the sulfidation ampoule during CZTS synthesis. Impurity amounts as low as 10−6 moles of Na or 10−7 moles of K resulted in a dramatic increase in grain size, from 100s of nm to several microns, for films deposited on quartz, while Ca loading had no visible effect on the final microstructure. Based on this vapor transport mechanism, we thus demonstrate an approach for delivering precisely controlled amounts of specific impurities into CZTS films on arbitrary substrates to facilitate large-grain growth.

Structural, optical, and electronic stability of copper sulfide thin films grown by atomic layer deposition

Copper sulfide films of nanometer thickness are grown by atomic layer deposition (ALD) and their structural and optoelectronic properties investigated as a function of time and storage environment. At temperatures as low as 80 °C polycrystalline thin films are synthesized, which index to the stoichiometric (Cu2S) chalcocite phase. As-prepared and prior to exposure to room ambient, conductive films are obtained as a result of a high mobility (4 cm2 V−1 s−1) and a relatively moderate p-type doping of 1018 cm−3. However, exposure to air results in a rapid rise in conductivity due to heavy p-type doping (>1020 cm−3). The evolving electronic properties in air are correlated with a change in both crystalline phase and optical constants. Surface analysis corroborates a copper deficiency induced by room temperature oxidation in air. Surprisingly, storage in a <0.1 ppm oxygen and water atmosphere significantly slows but does not halt the rise in conductivity with time. However, an Al2O3 overlayer—also grown by ALD—results in significantly lower carrier concentrations as well as dramatically slower carrier addition with time, even under ambient conditions. The implications for future use of Cu2S in more efficient (p/n+) and stable thin film photovoltaics are discussed.

Electrospray-assisted characterization and deposition of chlorosomes to fabricate a biomimetic light-harvesting device

Photosynthesis is an efficient process by which solar energy is converted into chemical energy. Green photosynthetic bacteria such as Chloroflexus aurantiacus have supramolecular antenna complexes called chlorosomes attached to their cytoplasmic membrane that increase the cross section for light absorption even in low-light conditions. Self-assembled bacteriochlorophyll pigments in the chlorosome interior play a key role in the efficient transfer and funneling of the harvested energy. In this work it was demonstrated that chlorosomes can be rapidly and precisely size-characterized online in real time using an electrospray-assisted mobility-based technique. Chlorosomes were electrospray-deposited onto TiO2 nanostructured films with columnar morphology to fabricate a novel biomimetic device to overcome the solvent compatibility issues associated with biological particles and synthetic dyes. The assembled unit retained the viability of the chlorosomes, and the harvesting of sunlight over a broader range of wavelengths was demonstrated. It was shown that the presence of chlorosomes in the biomimetic device had a 30-fold increase in photocurrent.

Stabilizing Cu2S for photovoltaics one atomic layer at a time.

Stabilizing Cu2S in its ideal stoichiometric form, chalcocite, is a long-standing challenge that must be met prior to its practical use in thin-film photovoltaic (PV) devices. Significant copper deficiency, which results in degenerate p-type doping, might be avoided by limiting Cu diffusion into a readily formed surface oxide and other adjacent layers. Here, we examine the extent to which PV-relevant metal-oxide over- and underlayers may stabilize Cu2S thin films with desirable semiconducting properties. After only 15 nm of TiO2 coating, Hall measurements and UV-vis-NIR spectroscopy reveal a significant suppression of free charge-carrier addition that depends strongly on the choice of deposition chemistry. Remarkably, the insertion of a single atomic layer of Al2O3 between Cu2S and TiO2 further stabilizes the active layer for at least 2 weeks, even under ambient conditions. The mechanism of this remarkable enhancement is explored by in situ microbalance and conductivity measurements. Finally, photoluminescence quenching measurements point to the potential utility of these nanolaminate stacks in solar-energy harvesting applications.

High electron mobility in thin films formed via supersonic impact deposition of nanocrystals synthesized in nonthermal plasmas

Thin films comprising semiconductor nanocrystals are emerging for applications in electronic and optoelectronic devices including light emitting diodes and solar cells. Achieving high charge carrier mobility in these films requires the identification and elimination of electronic traps on the nanocrystal surfaces. Herein, we show that in films comprising ZnO nanocrystals, an electron acceptor trap related to the presence of OH on the surface limits the conductivity. ZnO nanocrystal films were synthesized using a nonthermal plasma from diethyl zinc and oxygen and deposited by inertial impaction onto a variety of substrates. Surprisingly, coating the ZnO nanocrystals with a few nanometres of Al2O3 using atomic layer deposition decreased the film resistivity by seven orders of magnitude to values as low as 0.12 Ω cm. Electron mobility as high as 3 cm(2) V(-1) s(-1) was observed in films comprising annealed ZnO nanocrystals coated with Al2O3.

Impact of Different Electrolytes on Photocatalytic Water Splitting

Despite extensive research in photocatalytic water splitting, electrolyte usage varies greatly across different photocells. Photocatalytic water splitting continues to be performed in a wide range of electrolytes, from very acidic to very basic, with incomplete understanding of how the electrolyte composition affects performance. This study provides guidelines for electrolyte selection in water splitting applications. To determine properties that comprise an ideal electrolyte for photocatalytic electrolysis, the effects of several parameters were studied: pH, dissolved oxygen, conductivity, and composition. The photoactive anode was a nanostructured thin TiO 2 film synthesized by a flame aerosol process. The photocatalytic conversion efficiency increased with both pH and conductivity, but changes in dissolved oxygen levels had no discernible effect. The electrolyte composition was adjusted using selected salts and bases. Although the effect of the cation was negligible, anions were found to reduce efficiencies if their oxidation potential makes them thermodynamically favored over water molecules for oxidation. The results of these studies were applied in an analysis of the prospects for splitting seawater to produce hydrogen.

Plasma synthesis of stoichiometric Cu2S nanocrystals stabilized by oleylamine.

Nonthermal plasmas can produce high quality nanocrystals in a continuous process without requiring solvents. A nonthermal plasma process is demonstrated to synthesize stoichiometric Cu2S nanocrystals, which show no signs of oxidation by spectrophotometry after 2 months in the ambient when stabilized with oleylamine and dispersed in toluene.

Nonthermal plasma synthesis of metal sulfide nanocrystals from metalorganic vapor and elemental sulfur

Nanocrystal synthesis in nonthermal plasmas has been focused on elemental group IV semiconductors such as Si and Ge. In contrast, very little is known about plasma synthesis of compound nanocrystals and the time is ripe to extend this synthesis approach to nanocrystals comprised of two or more elements such as metal sulfides, oxides and nitrides. Towards this end, we studied, in an argon–sulfur plasma, the synthesis of ZnS, Cu2S and SnS nanocrystals from metalorganic precursors diethyl Zn(II), hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane, and tetrakis(dimethylamido) Sn(IV), respectively. In situ optical emission spectroscopy was used to observe changes in relative concentrations of various plasma species during synthesis, while ex situ material characterization was used to examine the crystal structure, elemental composition and optical absorption of these nanocrystals. For a constant metalorganic vapor feed rate, the elemental composition of the nanocrystals was found to be independent of the sulfur flow rate into the plasma, above a small threshold value. At constant sulfur flow rate, the nanocrystal composition depended on the metalorganic vapor feed rate. Specifically, the ensemble metal atomic fraction in the nanocrystals was found to increase with increasing metalorganic vapor flow rates, resulting in more metal-rich crystal phases. The metalorganic feed rate can be used to control the composition and crystal phase of the metal-sulfide nanocrystals synthesized using this plasma process.

Contact Radius and the Insulator–Metal Transition in Films Comprised of Touching Semiconductor Nanocrystals

Nanocrystal assemblies are being explored for a number of optoelectronic applications such as transparent conductors, photovoltaic solar cells, and electrochromic windows. Majority carrier transport is important for these applications, yet it remains relatively poorly understood in films comprised of touching nanocrystals. Specifically, the underlying structural parameters expected to determine the transport mechanism have not been fully elucidated. In this report, we demonstrate experimentally that the contact radius, between touching heavily doped ZnO nanocrystals, controls the electron transport mechanism. Spherical nanocrystals are considered, which are connected by a circular area. The radius of this circular area is the contact radius. For nanocrystals that have local majority carrier concentration above the Mott transition, there is a critical contact radius. If the contact radius between nanocrystals is less than the critical value, then the transport mechanism is variable range hopping. If the contact radius is greater than the critical value, the films display behavior consistent with metallic electron transport.