Our most recent papers...
445. CdS Nanowire Solar Cells: Dual Role of Squaraine Dye as a Sensitizer and a Hole Transporter
Choi, H.; Kamat, P. V. J. Phys. Chem. Lett. 2013, 4, 3983–3991.
The squaraine dye (SQ) anchored onto CdS nanowires serves as a photosensitizing dye and a hole acceptor. This dual role of the squaraine dye has been successfully exploited in a nanowire solar cell to improve the photoconversion efficiency. Electrophoretic deposition of CdS NWs and CdS NWs+SQ composite onto conducting glass electrodes was performed to obtain robust photoanodes and evaluate the photovoltaic performance of nanowire solar cells (NWSCs). Whereas the sensitization property of the SQ extends the response of CdS NWSCs into the near-IR (NIR) region, its redox property facilitates shuttling of holes to the electrolyte and suppressing the charge recombination process. Transient absorption measurements confirm the formation of cation radical of the dye arising from these two processes. The dual role of the squaraine dye has enabled us to improve the power conversion efficiency of NWSCs by a factor of ~20. Photoelectrochemical, spectroelectrochemical, and spectroscopic measurements provide insight into the multifaceted role of squaraine dye in improving the performance of NWSCs.
444. Nickel-Doped MnO2 Nanowires Anchored onto Reduced Graphene Oxide for Rapid Cycling Cathode in Lithium Ion Batteries
Radich, J. G.; Chen, Y.-C.; Kamat, P. V. ECS Journal of Solid State Science and Technology 2013, 2 (10), M3178-M3181.
Nickel-doped MnO2 nanowires were synthesized directly onto reduced graphene oxide (RGO) to generate a composite cathode material with improved high-rate cycling characteristics. The presence of RGO improves the electrochemical characteristics of the cathode in Li-ion half-cell architecture. Cyclic voltammetry, electrochemical impedance spectroscopy, and electrode cycling are conﬁrm that RGO plays a major role in enhancing the ability of the NixMn(1-x)O2 to reversibly intercalate lithium ions at 1C rate. The chronocoulometric response of the RGO-based electrode shows the improvements originate from faster reaction kinetics and transport of Li+ coupled with increased speciﬁc capacitance and Li+ adsorption.
443. Trap and Transfer. Two-Step Hole Injection Across the Sb2S3/CuSCN Interface in Solid State Solar Cells.
Christians, J. A.; Kamat, P. V. ACS Nano 2013, 7 (9), 7967–7974.
In solid-state semiconductor-sensitized solar cells, commonly known as extremely thin absorber (ETA) or solid-state quantum dot sensitized solar cells (QDSCs), transfer of photogenerated holes from the absorber species to the p-type hole conductor plays a critical role in the charge separation process. Using Sb2S3 (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. The hole transfer from excited Sb2S3 into CuSCN, which limits the overall power conversion efficiency of these solar cells, is now independently studied using transient absorption spectroscopy. In the Sb2S3 absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S−•) species. This trapped hole is transferred from the Sb2S3 absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. This process was monitored through the spectroscopic signal seen for the S−• species in Sb2S3, providing direct evidence for the hole transfer dynamics in ETA solar cells. Elucidation of the hole transfer mechanism from Sb2S3 to CuSCN represents a significant step toward understanding charge separation in Sb2S3 solar cells, and provides insight into the design of new architectures for higher efficiency devices.
442. Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions.
Hines, D. A.; Kamat, P. V. J. Phys. Chem. C 2013, 117 (27), 14418–14426.
With the increased interest in quantum dot sensitized solar cells (QDSCs) there comes a need to better understand how surface modification of quantum dots (QDs) can affect the excited state dynamics of QDs, electron transfer at the QD-metal oxide (MO) interface, and overall photoconversion efficiency of QDSCs. We have monitored the surface modification of solution based QDs via the steady-state absorption and emission characteristics of colloidal CdSe passivated with β-Alanine (β-Ala). The trap-remediating nature of the β-Ala molecule, arising from the Lewis-basicity of the amine group, is realized from the hypsochromic shifts seen in excitonic absorption and emission bands as well as an increase in fluorescence quantum yield. Transient absorption measurements of CdSe-TiO2 films prepared with and without β-Ala as a linker molecule further reveal the role of the surface modifier in influencing excited state electron transfer processes. Electron transfer at this interface was dependent on the method of QD deposition: CdSe-TiO2 (direct deposition, ket = 1.5 × 10^10 s-1), CdSe-linker-TiO2 (attaching linker molecule first to TiO2 so that β-Ala interaction is minimal, ket = 2.4 × 10^9 s-1) or linker-CdSe-linker-TiO2 (linkage via full β-Ala encapsulation in solution prior to deposition, ket = 6.4 × 10^8 s-1). These results imply that the surface chemistry of colloidal CdSe plays an important role in mediating electron transfer reactions.
441. Metal-Cluster-Sensitized Solar Cells. A New Class of Thiolated Gold Sensitizers Delivering Efficiency Greater Than 2%.
Chen, Y.-S.; Choi, H.; Kamat, P. V. J. Am. Chem. Soc. 2013, 135 (24), pp 8822–8825.
A new class of metal-cluster sensitizers has been explored for designing high-efficiency solar cells. Thiol-protected gold clusters which exhibit molecular-like properties have been found to inject electrons into TiO2 nanostructures under visible excitation. Mesoscopic TiO2 films modified with gold clusters deliver stable photocurrent of 3.96 mA/cm2 with power conversion efficiencies of 2.3% under AM 1.5 illumination. The overall absorption features and cell performance of metal-cluster-sensitized solar cells (MCSCs) are comparable to those of CdS quantum-dot-based solar cells (QDSCs). The relatively high open-circuit voltage of 832 mV and fill factor of 0.7 for MCSCs as compared to QDSCs show the viability of these new sensitizers as alternatives to semiconductor QDs and sensitizing dyes in the next generation of solar cells. The superior performance of MCSCs discussed in this maiden study lays the foundation to explore other metal clusters with broader visible absorption.