Semiconductors group

Abstract:

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

Abstract:

We observe a strong "light-soaking" effect in SnO 2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the device's short-circuit photocurrent and efficiency increase significantly over 20-30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing optical-pump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of free-electrons in the SnO 2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer occurs. To verify this hypothesis, we perform capacitance and charge extraction measurements which indicate a shift in the surface potential of SnO 2 of up to 70 mV with light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate and ensuing photocurrent. The cause for the shift in surface potential is not clear, but we postulate that it is due to the photoinduced charging of the SnO 2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. Understanding temporally evolving processes in DSCs is of critical importance for enabling this technology to operate optimally over a prolonged period of time. This work specifically highlights important changes that can occur at the dye-sensitized heterojunction, even without direct light absorption in the metal oxide. © 2012 The Royal Society of Chemistry.

Abstract:

The coating of n-type mesoporous metal oxides with nanometer thick dielectric shells is a route that has proven to be successful at enhancing the efficiency of some families of dye-sensitized solar cells. The primary intention is to introduce a "surface passivation layer" to inhibit recombination between photoinduced electrons and holes across the dye-sensitized interface. However, the precise function of these dielectric interlayers is often ambiguous. Here, the role of a thin MgO interlayer conformally deposited over mesoporous SnO 2 in liquid electrolyte and solid-state dye-sensitized solar cells is investigated. For both families of devices the open-circuit voltage is increased by over 200 mV; however, the short-circuit photocurrent is increased for the solid-state cells, but reduced for the electrolyte based devices. Through electronic and spectroscopic characterization we deduce that there are four distinct influences of the MgO interlayer: It increases dye-loading, slows down recombination, slows down photoinduced electron transfer, and results in a greater than 200 mV shift in the conduction band edge, with respect to the electrolyte redox potential. The compilation of these four factors have differing effects and magnitudes in the solid-state and electrolyte DSCs but quantitatively account for the difference in device performances observed for both systems with and without the MgO shells. To the best of our knowledge, this is the most comprehensive account of the role of dielectric shells in dye-sensitized solar cells and will enable much better interfacial design of photoelectrodes for DSCs. © 2012 American Chemical Society.

Abstract:

Using transient terahertz photoconductivity measurements, we have made noncontact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1 ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170 cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.