Vertical arrays of ZnO nanowires can decouple light absorption from carrier collection in PbS quantum dot solar cells and increase power conversion efficiencies by 35%. The resulting ordered bulk heterojunction devices achieve short-circuit current densities in excess of 20 mA cm(-2) and efficiencies of up to 4.9%.
The ability to produce small scale, crystalline silicon spheres is of significant technological and scientific importance, yet scalable methods for doing so have remained elusive. Here we demonstrate a silicon nanosphere fabrication process based on an optical fibre drawing technique. A silica-cladded silicon-core fibre with diameters down to 340 nm is continuously fed into a flame defining an axial thermal gradient and the continuous formation of spheres whose size is controlled by the feed speed is demonstrated. In particular, spheres of diameter o500 nm smaller than those produced under isothermal heating conditions are shown and analysed. A fibre with dual cores, p-type and n-type silicon, is drawn and processed into spheres. Spatially coherent break-up leads to the joining of the spheres into a bispherical silicon 'p-n molecule'. The resulting device is measured to reveal a rectifying I-V curve consistent with the formation of a p-n junction.
Simultaneous positive and negative photocurrent response in asymmetric quantum dot infrared photodetectors J. Appl. Phys. 113, 043721 (2013); 10.1063/1.4789963 Modeling and analysis of intraband absorption in quantum-dot-in-well mid-infrared photodetectors J. Appl. Phys. 111, 033713 (2012); 10.1063/1.3684603 Effect of overgrowth temperature on the mid-infrared response of Ge/Si(001) quantum dots Appl. Phys. Lett. 100, 053507 (2012); 10.1063/1.3682304Bias and temperature dependence of the escape processes in quantum dots-in-a-well infrared photodetectors HgTe colloidal quantum dot films are studied for photodetection over the 3-5 lm atmospheric transparency window. The temperature dependence of the conductivity indicates that the material behaves approximately as an intrinsic semiconductor. In photoconduction, the responsivity can be as high as several hundred mA W À1 at room temperature. The dark current presents 1/f noise which is larger than that for homogeneous conductors, and this noise decreases with temperature. A specific detectivity of 2 Â 10 9 Jones is obtained for a sample with a 6 lm cut-off wavelength at 130 K. These values are obtained for the thickest films studied ($400 nm) and whose thicknesses are still much less than the optical absorption length. The time response can be faster than 100 ns.
is sufficiently thick to harvest all near-infrared photons. This limitation can be relaxed by employing an ordered bulk heterojunction (OBHJ) architecture in which the n-type metal oxide acceptor is composed of 1D nanostructures such that the directions of photon absorption and charge collection are decoupled, allowing for absorption in an optically dense film while maintaining efficient charge collection. PbS QD PVs employing OBHJs of TiO 2 nanopillars [24] and ZnO nanowire arrays [25] have previously been shown to enhance short-circuit current density (J SC ) by more than 20% over planar counterparts. J SC above 30 mA cm −2 has been achieved in PbS QD PVs by employing relatively long ZnO nanowire arrays (length greater than 1 μm) and QDs with smaller band gap (first exciton absorption at approximately 1030 nm), but PCE was limited to 6.1% due to relatively low V OC and poor fill factor (FF). [26] In this work, we demonstrate a device architecture that combines a ZnO nanowire array OBHJ architecture with band alignment engineering of the PbS QD film in an effort to maximize J SC while preserving the V OC and FF achieved by optimized planar devices (Figure 1a). Previous PbS QD OBHJ devices have employed QD layers with uniform energy levels using a single ligand exchange treatment. However, a multistep ligand exchange process can be used to alter the conduction and valence band energy levels of the QD layers [27] (Figure 1b) and consequently promote charge extraction and reduce carrier recombination at the anode. [8,28] We confirm that this multi-step ligand exchange process produces discrete functional layers through nanoscale cross-sectional elemental analysis. The resulting combination of band alignment engineering and nanostructured heterojunction approaches yields devices with improved J SC and PCE compared to planar devices that utilize band alignment engineering alone. Champion nanowire devices achieve J SC above 30 mA cm −2 and 9.6% PCE under 100 mW cm −2 AM1.5G illumination. Our photocurrent density is the record for a PbS QD PV device with an excitonic absorption peak of at least 1.3 eV, [23,26] which is significant because a band gap of 1.3 eV maximizes the efficiency potential of a PV device under terrestrial illumination. [29,30] Finally, we demonstrate that this enhanced performance is a result of both improved light harvesting due to the presence of the ZnO nanowire array and improved carrier collection due to the 3D junction formed by the OBHJ; these effects are generalizable to other thin film PV absorber materials with transport lengths that are incommensurate with their absorption coefficients.ZnO nanowire arrays were synthesized on indium tin oxide (ITO) via a hydrothermal method, as follows. A 40 nm thick textured polycrystalline seed film of ZnO was first deposited directly on ITO by a sol-gel process. Next, ZnO nanowires were Colloidal quantum dots (QDs) have gained attention for a range of optoelectronic device applications, including photodetectors, [1,2] light-emitting diodes, [3] lasers,...
Hyperdoped black silicon fabricated with femtosecond laser irradiation has attracted interest for applications in infrared photodetectors and intermediate band photovoltaics due to its sub-bandgap optical absorptance and light-trapping surface. However, hyperdoped black silicon typically has an amorphous and polyphasic polycrystalline surface that can interfere with carrier transport, electrical rectification, and intermediate band formation. Past studies have used thermal annealing to obtain high crystallinity in hyperdoped black silicon, but thermal annealing causes a deactivation of the sub-bandgap optical absorptance. In this study, nanosecond laser annealing is used to obtain high crystallinity and remove pressure-induced phases in hyperdoped black silicon while maintaining high sub-bandgap optical absorptance and a light-trapping surface morphology. Furthermore, it is shown that nanosecond laser annealing reactivates the sub-bandgap optical absorptance of hyperdoped black silicon after deactivation by thermal annealing. Thermal annealing and nanosecond laser annealing can be combined in sequence to fabricate hyperdoped black silicon that simultaneously shows high crystallinity, high above-bandgap and sub-bandgap absorptance, and a rectifying electrical homojunction. Such nanosecond laser annealing could potentially be applied to non-equilibrium material systems beyond hyperdoped black silicon.
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