earned a B.S. degree in chemistry from the University of Dayton in 2001 and as an undergraduate worked on conductive polymer syntheses at the Air Force Research Laboratory at Wright Patterson Air Force Base. He completed an M.S. degree in 2004 and Ph.D. degree in 2008 at Portland State University and joined the Lewis group at Caltech in 2008. He is currently an NSF-ACCF postdoctoral fellow (2009) and has been studying the electrical characteristics of inorganic semiconductors in contact with conductive polymers. His research interests include molecular semiconductors for solar energy conversion, porphyrin macrocycles for optoelectronic applications, and catalyst materials for photoelectrolysis. Emily L. Warren received a B.S. in chemical engineering at Cornell University in 2005. She received an M.Phil in Engineering for Sustainable Development from Cambridge University in 2006. She is currently a graduate student in Chemical Engineering at the California Institute of Technology. Her research interests include semiconductor photoelectrochemistry, solar energy conversion, and semiconductor nanowires. She is currently a graduate student in Chemical Engineering at the California Institute of Technology, working under Nathan S. Lewis. James R. McKone is in his third year of graduate studies in the Division of Chemistry and Chemical Engineering at the California Institute of Technology, working under Nathan S. Lewis and Harry B. Gray. In 2008 he graduated from Saint Olaf College with a Bachelor of Arts degree, double-majoring in music and chemistry. His current research focuses on semiconductor-coupled heterogeneous catalysis of the hydrogen evolution reaction using mixtures of earth-abundant transition metals. Shannon W. Boettcher earned his B.A. degree in chemistry from the University of Oregon, Eugene (2003), and, working with Galen Stucky, his Ph.D. in Inorganic Chemistry from the University of California, Santa Barbara (2008). Following postdoctoral work with Nate Lewis and Harry Atwater at the California Institute of Technology (2008-2010), he returned to the University of Oregon to join the faculty as an Assistant Professor. His research interests span synthesis and physical measurement with the goal of designing and understanding solid-state inorganic material architectures for use in solar-energy conversion and storage.

The low toxicity and a near-ideal choice of bandgap make tin perovskite an attractive alternative to lead perovskite in low cost solar cells. However, the development of Sn perovskite solar cells has been impeded by their extremely poor stability when exposed to oxygen. We report low-dimensional Sn perovskites that exhibit markedly enhanced air stability in comparison with their 3D counterparts. The reduced degradation under air exposure is attributed to the improved thermodynamic stability after dimensional reduction, the encapsulating organic ligands, and the compact perovskite film preventing oxygen ingress. We then explore these highly oriented low-dimensional Sn perovskite films in solar cells. The perpendicular growth of the perovskite domains between electrodes allows efficient charge carrier transport, leading to power conversion efficiencies of 5.94% without the requirement of further device structure engineering. We tracked the performance of unencapsulated devices over 100 h and found no appreciable decay in efficiency. These findings raise the prospects of pure Sn perovskites for solar cells application.

The faradaic efficiency for O 2 (g) evolution at thin-film WO 3 photoanodes has been evaluated in a series of acidic aqueous electrolytes. In 1.0 M H 2 SO 4 , persulfate was the predominant photoelectrochemical oxidation product, and no O 2 was detected unless catalytic quantities of Ag + (aq) were added to the electrolyte. In contact with 1.0 M HClO 4 , dissolved O 2 was observed with nearly unity faradaic efficiency, but addition of a hole scavenger, 4-cyanopyridine N-oxide, completely suppressed O 2 formation. In 1.0 M HCl, Cl 2 (g) was the primary oxidation product. These results indicate that at WO 3 photoanodes, water oxidation is dominated by oxidation of the acid anions in 1.0 M HCl, H 2 SO 4 , and HClO 4 , respectively.

Photoexcitation of a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Phn), n = 1-5, and A= perylene-3,4:9,10-bis(dicarboximide) (PDI) results in rapid electron transfer to produce 1(PTZ+*-Phn-PDI-*). Time-resolved EPR (TREPR) studies of the photogenerated radical pairs (RPs) show that above 150 K, when n = 2-5, the radical pair-intersystem crossing mechanism (RP-ISC) produces spin-correlated radical ion pairs having electron spin polarization patterns indicating that the spin-spin exchange interaction in the radical ion pair is positive, 2J > 0, and is temperature dependent. This temperature dependence is most likely due to structural changes of the p-phenylene bridge. Charge recombination in the RPs generates PTZ-Phn-3*PDI, which exhibits a spin-polarized signal similar to that observed in photosynthetic reaction-center proteins and some biomimetic systems. At temperatures below 150 K and/or at shorter donor-acceptor distances, e.g., when n = 1, PTZ-Phn-3*PDI is also formed from a competitive spin-orbit-intersystem crossing (SO-ISC) mechanism that is a result of direct charge recombination: 1(PTZ+*-Phn-PDI-*) --> PTZ-Phn-3*PDI. This SO-ISC mechanism requires the initial RP intermediate and depends strongly on the orientation of the molecular orbitals involved in the charge recombination as well as the magnitude of 2J.

Time-resolved transient optical absorption and EPR (TREPR) spectroscopies are used to probe the interaction of the lowest excited singlet state of perylene-3,4:9,10-bis(dicarboximide) ((1*)PDI) with a stable tert-butylphenylnitroxide radical ((2)BPNO(*)) at specific distances and orientations. The (2)BPNO(*) radical is connected to the PDI with the nitroxide and imide nitrogen atoms either para (1) or meta (3) to one another, as well as through a second intervening p-phenylene spacer (2). Transient absorption experiments on 1-3 reveal that (1*)PDI undergoes ultrafast enhanced intersystem crossing and internal conversion with tau approximately = 2 ps to give structurally dependent 8-31% yields of (3*)PDI. Energy- and electron-transfer quenching of (1*)PDI by (2)BPNO(*) are excluded on energetic and spectroscopic grounds. TREPR experiments at high magnetic fields (3.4 T, 94 GHz) show that the photogenerated three-spin system consists of the strongly coupled unpaired electrons confined to (3*)PDI, which are each weakly coupled to the unpaired electron on (2)BPNO(*) to form excited doublet (D(1)) and quartet (Q) states, which are both spectrally resolved from the (2)BPNO(*) (D(0)) ground state. The initial spin polarizations of D(1) and Q are emissive for 1 and 2 and absorptive for 3, which evolve over time to the opposite spin polarization. The subsequent decays of D(1) and Q to ground-state spin polarize D(0). The rates of polarization transfer depend on the molecular connectivity between PDI and (2)BPNO(*) and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.