Our most recent papers...
471. Surface Oxidation as a Cause of High Open-Circuit Voltage in CdSe ETA Solar Cells
Kirmayer, S; Edri, E.; Hines, D.; Klein-Kedem, N.; Cohen, H.; Niitsoo, O.; Pinkas, I.; Kamat, P. V.; Hodes, G. Adv. Mater. Interf. 2015, ASAP.
TiO2/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open-circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 ºC). Annealing in N2 produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N2-annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo-induced charge trapping, in particular at absorber grain boundaries of the air versus N2-annealed systems, correlated with the formation of Cd–O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface-oxidation, which both impedes electron injection from the CdSe to the TiO2, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers.
470. Size-Dependent Photovoltaic Performance of CuInS2 Quantum Dots Sensitized Solar Cells
Jara, D. H.;Yoon, S.; Stamplecoskie, K. G.; Kamat, P. V. Chem. Mater. 2015, ASAP.
The optical and electronic properties of quantum dots (QDs) which are drastically affected by their size have major impact on their performance in devices like solar cells. We now report the size dependent solar cell performance for CuInS2 QDs capped with 1-dodecanethiol. Pyramidal shaped CuInS2 QDs with diameter between 2.9 nm and 5.3 nm have been synthesized and assembled on mesoscopic TiO2 films by electrophoretic deposition. Time resolved emission and transient absorption spectroscopy measurements have ascertained the role of internal and surface defects in determining the solar cell performance. An increase in power conversion efficiency (PCE) was observed with increasing size of QDs, with maximum values of 2.14 and 2.51% for 3.9 and 4.3 nm size particles, respectively. The drop in PCE observed for larger QDs (5.3 nm) is attributed to decreased charge separation following bandgap excitation. Since the origin of photocurrent generation in CuInS2 QDSC arises from the defect dominated charge carriers it offers the opportunity to further improve the efficiency by controlling these defect concentrations.
469. Dual Nature of the Excited State in Organic-Inorganic Lead Halide Perovskites
Stamplecoskie, K. G.; Manser, J. S.; Kamat, P. V. Energy Environ. Sci. 2015, 8 (1), 208 - 215.
The rapid increase in efficiency of methylammonium lead halide perovskite solar cells necessitates further investigation into the nature of perovskite absorption features and optical properties. Films obtained from the deposition of solutions containing lead halides and the CH3NH3+ organic cation is known to yield the CH3NH3PbI3 perovskite structure upon annealing. In examining the precursor solution used in the processing of CH3NH3PbI3 solar cells, we find that Pb2+ readily forms plumbate complexes in the presence of excess iodide ions and exhibits characteristic absorption bands at 370 (PbI3-) and 425 nm (PbI42-). Through comparative spectral analysis of the absorption features of charge transfer complexes in the solution phase and the final solid-state perovskite films, we are able to fully classify the absorption features in the excited state of CH3NH3PbI3 across the transient absorption spectrum recorded following laser pulse excitation. In particular, we attribute the broad photoinduced absorption to a charge-transfer excited state, and show correlation between the photoinduced absorption and 480 nm bleach signals. These observations lead us to propose a band structure composed of two distinct transitions that is consistent with the various spectral features and kinetic behavior of the CH3NH3PbI3 excited state. Characterization of this unique dual excited state nature provides further insight into the optoelectronic behavior of hybrid lead halide perovskite films and thus aids in elucidating their exceptional photovoltaic properties.
468. The Origin of Catalytic Effect in the Reduction of CO2 at Nanostructured TiO2 Films
Ramesha, G. K.; Brennecke, J. F.; Kamat, P. V. ACS Catal. 2014, 4 (9), 3249–3254.
Electrocatalytic activity of mesoscopic TiO2 films towards the reduction of CO2 is probed by depositing a nanostructured film on a glassy carbon electrode. The one-electron reduction of CO2 in acetonitrile seen at an onset potential of -1.1 V (vs. NHE) is ~0.5 V lower than the one observed with a glassy carbon electrode. The electrocatalytic role of TiO2 is elucidated through spectroelectrochemistry and product analysis. Ti3+ species formed when TiO2 film is subjected to negative potentials have been identified as active reduction sites. Binding of CO2 to catalytically active Ti3+ followed by the electron transfer facilitates the initial one-electron reduction process. Methanol was the primary product when the reduction was carried out in wet acetonitrile.
467. Band Filling with Free Charge Carriers in Organometal Halide Perovskites
Manser, J. S.; Kamat, P. V. Nat. Photon. 2014, 8, 737–743.
The unique and promising properties of semiconducting organometal halide perovskites have brought these materials to the forefront of solar energy research. Here, we present new insights into the excited-state properties of CH3>NH3PbI3 thin films through femtosecond transient absorption spectroscopy measurements. The photoinduced bleach recovery at 760 nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via recombination of free electrons and holes. Additionally, charge accumulation in the perovskite films leads to an increase in the intrinsic bandgap that follows the Burstein–Moss band filling model. Both the recombination mechanism and the band-edge shift are studied as a function of the photogenerated carrier density and serve to elucidate the behaviour of charge carriers in hybrid perovskites. These results offer insights into the intrinsic photophysics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications.