1 . This raises the possibility of a silicon-based optoelectronic technology. The luminescence properties may be understood on the basis of the injection or creation of electrons and holes in the interconnected network of wires which recombine radiatively with high efficiency 1,2 . Elucidating the electron-transport mechanisms has been held back by several difficulties, particularly that of making stable, high-quality contacts to the porous material. Here we report experiments that probe the conduction process using photoemission stimulated by hard-ultraviolet/X-ray synchrotron radiation, obviating the need for good electrical contacts. We find that the conductivity of porous silicon films is temperature-dependent, and that the films become insulating at low temperatures. We suggest that these results may be understood in terms of a percolation process occurring through sites in the porous network in which conductivity is thermally activated, and we postulate that this activation may be the consequence of a Coulomb blockade effect 3,4 in the nanoscale channels of the film. This is consistent with our observation of optical 'unblocking' of conducting pathways. These results imply that the size distribution of the nanowires in the silicon backbone plays a key role in determining the conduction properties, and that porous-silicon light-emitting diodes may use only a small (and the least efficient) fraction of the material. Improvements in electroluminescence efficiency may be possible by taking into account the percolative nature of the conduction process.These studies probe photoelectron emission in the hard-ultraviolet to X-ray (X-UV) region, with photon energies between 90 eV and 120 eV. Such energies probe the Si L 23 edge transitions. This ensures that the primary excitation probes material characterized by Si-Si bonds: that is, the silicon backbone of the porous film. The sensitivity of the L 23 edge to local chemistry has been noted previously 5,6 . The X-ray attenuation length above the L edge for bulk silicon is ϳ500 Å , and the effects reported here were observed for films that are thicker than the attenuation depth, so that some fraction less than the whole thickness was directly excited by X-ray photons. We measured the total electron yield (TEY), which includes photoelectrons, primary Auger electrons, cascaded Auger electrons and secondary electrons. The TEY was measured by sensing the current flowing into the back of the Si wafer, which normally compensates exactly for the photoemitted electrons escaping from the surface. Such rapid charge compensation is always observed even for weakly conducting systems, because the small photoexcited current (typically 10 −10 A) is easily supplied. But we have found that porous silicon films do not behave in this way; instead the TEY steadily decreases as temperature is reduced, and may fall below background noise levels at low temperatures.TEY spectra obtained during a cooling cycle in which a conducting/insulating transition occurred are shown in Fig. 1. At the hi...

Closed-ampoule Zn diffusion in InP results in a net acceptor concentration that is much smaller than the Zn concentration. After subsequent annealing of InP in an atmosphere without Zn, the Zn and net acceptor concentrations have become almost identical, due to a decreased Zn concentration and an increased net acceptor concentration. The annealing treatment gives rise to a decreased contact resistivity. The diffusion depth has not changed. Annealing with a SiN cap on the InP surface does not have this effect on the concentrations. These annealing effects also take place in InGaAsP on InP layers. The results can be explained quantitatively by assuming that Zn is incorporated as both substitutional acceptors and interstitial donors and that only the interstitial Zn is driven out by the annealing, owing to its large diffusion coefficient. Profiles calculated with this interstitial-substitutional model can be fitted to experimental profiles assuming Zn to diffuse as singly ionized interstitial donors. This model can also describe earlier reported results on Zn diffusion in n-type InP for which a profile cutoff is found at a depth where the acceptor concentration equals the background donor concentration and in which the acceptor solubility is higher than in undoped InP.

The mesoporous (meso)-TiO2 layer is a key component of high-efficiency perovskite solar cells (PSCs). Herein, pore size controllable meso-TiO2 layers are prepared using spin coating of commercial TiO2 nanoparticle (NP) paste with added soft polymer templates (SPT) followed by removal of the SPT at 500 °C. The SPTs consist of swollen crosslinked polymer colloids (microgels, MGs) or a commercial linear polymer (denoted as LIN). The MGs and LIN were comprised of the same polymer, which was poly­(N-isopropylacrylamide) (PNIPAm). Large (L-MG) and small (S-MG) MG SPTs were employed to study the effect of the template size. The SPT approach enabled pore size engineering in one deposition step. The SPT/TiO2 nanoparticle films had pore sizes > 100 nm, whereas the average pore size was 37 nm for the control meso-TiO2 scaffold. The largest pore sizes were obtained using L-MG. SPT engineering increased the perovskite grain size in the same order as the SPT sizes: LIN < S-MG < L-MG and these grain sizes were larger than those obtained using the control. The power conversion efficiencies (PCEs) of the SPT/TiO2 devices were ∼20% higher than that for the control meso-TiO2 device and the PCE of the champion S-MG device was 18.8%. The PCE improvement is due to the increased grain size and more effective light harvesting of the SPT devices. The increased grain size was also responsible for the improved stability of the SPT/TiO2 devices. The SPT method used here is simple, scalable, and versatile and should also apply to other PSCs.

TiO 2 has been recognized as a promising material for a wide range of emerging applications, including hydrogen generation, [1] CO 2 reduction, [2] degradation of organic pollutants, [3] self-cleaning coating, [4] quantum-dot-sensitized solar cells, [5] dyesensitized solar cells (DSSCs), [6] and more recently perovskite solar cells (PSCs). [7,8] Thermal annealing is a critical process involved in the fabrication of TiO 2 films for PSCs. For instance, a compact TiO 2 film that acts as an electron transport layer (ETL) for PSCs usually requires an annealing temperature of over 400 C to induce the crystallization from its amorphous precursor to anatase. [9,10] The fabrication of the mesoporous TiO 2 film for mesoscopic PSCs also needs high-temperature sintering of around 450-550 C to remove organic binders from TiO 2 paste and promote the interconnection between the TiO 2 nanoparticles. [11,12] A typical conventional annealing method is a time-consuming batch process involving the uses of a hotplate, furnace, or oven, for 1-3 h to fabricate these layers, including a long cooling period. [12-14] Such a process makes it challenging to develop high throughput or in-line production of PSCs. Therefore, there is a need to develop an alternative annealing method, enabling the rapid and scalable production of high-quality metal oxide films for PSCs for future commercialization. In addition, conventional annealing methods are commonly limited to below 550 C to avoid glass substrate bending or breakage due to a glass transition temperature of 564 C for substrate based on soda-lime glass. [15] Previous studies have suggested that an annealing temperature beyond 600 C enhances the crystallinity of the TiO 2 films and interconnection between the nanoparticles, which improves the performance of the PSCs and other devices. [13,16-18] To date, several alternative annealing methods have been developed to fabricate TiO 2 ETL for PSCs and DSSCs. For instance, Watson et al. demonstrated the use of an ultrafast near-IR (NIR) heating process to sinter mesoporous TiO 2 film on metal substrates for DSSCs. [19] Sánchez et al. developed a rapid flash IR method to anneal mesoporous TiO 2 film for PSCs with a peak temperature of %640 C and achieved a production rate of 15 cm 2 min À1 (1 cm 2 in 4 s). [11] Kim et al. demonstrated a flame annealing process to anneal the TiO 2 film for PSCs and DSSCs with a peak temperature up to 1000 C and