Girija Sahasrabudhe

Titanium dioxide/silicon hole-blocking selective contact to enable double-heterojunction crystalline silicon-based solar cell

In this work, we use an electron-selective titanium dioxide (TiO2) heterojunction contact to silicon to block minority carrier holes in the silicon from recombining at the cathode contact of a silicon-based photovoltaic device. We present four pieces of evidence demonstrating the beneficial effect of adding the TiO2 hole-blocking layer: reduced dark current, increased open circuit voltage (VOC), increased quantum efficiency at longer wavelengths, and increased stored minority carrier charge under forward bias. The importance of a low rate of recombination of minority carriers at the Si/TiO2 interface for effective blocking of minority carriers is quantitatively described. The anode is made of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction to silicon which forms a hole selective contact, so that the entire device is made at a maximum temperature of 100 °C, with no doping gradients or junctions in the silicon. A low rate of recombination of minority carriers at the Si/TiO2 interface is crucial for effective blocking of minority carriers. Such a pair of complementary carrier-selective heterojunctions offers a path towards high-efficiency silicon solar cells using relatively simple and near-room temperature fabrication techniques.

Low-Temperature Synthesis of a TiO2/Si Heterojunction

The classical SiO2/Si interface, which is the basis of integrated circuit technology, is prepared by thermal oxidation followed by high temperature (>800 °C) annealing. Here we show that an interface synthesized between titanium dioxide (TiO2) and hydrogen-terminated silicon (H:Si) is a highly efficient solar cell heterojunction that can be prepared under typical laboratory conditions from a simple organometallic precursor. A thin film of TiO2 is grown on the surface of H:Si through a sequence of vapor deposition of titanium tetra(tert-butoxide) (1) and heating to 100 °C. The TiO2 film serves as a hole-blocking layer in a TiO2/Si heterojunction solar cell. Further heating to 250 °C and then treating with a dilute solution of 1 yields a hole surface recombination velocity of 16 cm/s, which is comparable to the best values reported for the classical SiO2/Si interface. The outstanding performance of this heterojunction is attributed to Si-O-Ti bonding at the TiO2/Si interface, which was probed by angle-resolved X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that Si-H bonds remain even after annealing at 250 °C. The ease and scalability of the synthetic route employed and the quality of the interface it provides suggest that this surface chemistry has the potential to enable fundamentally new, efficient silicon solar cell devices.

Rhombohedral Polytypes of the Layered Honeycomb Delafossites with Optical Brilliance in the Visible

We report the synthesis of the Delafossite honeycomb compounds Cu3Ni2SbO6 and Cu3Co2SbO6 via a copper topotactic reaction from the layered α-NaFeO2-like precursors Na3Ni2SbO6 and Na3Co2SbO6. The low-temperature exchange reaction exclusively produces the rhombahedral 3R polytype subcell, whereas only the hexagonal 2H polytype subcell has been made by conventional synthesis. The thus-synthesized 3R variants are visually striking; they are bright lime-green (Ni variant) and terracotta-orange (Co variant), while both of the conventionally synthesized 2H variants have a burnt-red color. The new structures are characterized by powder X-ray diffraction and Rietveld analysis as well as magnetic susceptibility, X-ray photoelectron spectroscopy (XPS), and diffuse-reflectance optical spectroscopy. Using thermogravimetric analysis, we identify a second order 3R → 2H phase transition as well as a first-order structural transition associated with rearrangement of the honeycomb stacking layers. The optical absorbance spectra of the samples show discrete edges that correlate well to their visual colors. Exposing Cu3Ni2SbO6 to O2 and heat causes the sample to change color. XPS confirms the presence of Cu(2+) in these samples, which implies that the difference in color between the polytypes is due to oxygen intercalation resulting from their different synthetic routes.

Strong topological metal material with multiple Dirac cones

We report a new, cleavable, strong topological metal, Zr2Te2P, which has the same tetradymite-type crystal structure as the topological insulator Bi2Te2Se. Instead of being a semiconductor, however, Zr2Te2P is metallic with a pseudogap between 0.2 and 0.7 eV above the Fermi energy (EF). Inside this pseudogap, two Dirac dispersions are predicted: one is a surface-originated Dirac cone protected by time-reversal symmetry (TRS), while the other is a bulk-originated and slightly gapped Dirac cone with a largely linear dispersion over a 2 eV energy range. A third surface TRS-protected Dirac cone is predicted, and observed using angle-resolved photoemission spectroscopy, making Zr2Te2P the first system, to our knowledge, to realize TRS-protected Dirac cones at M¯ points. The high anisotropy of this Dirac cone is similar to the one in the hypothetical Dirac semimetal BiO2. As a result, we propose that if EF can be tuned into the pseudogap where the Dirac dispersions exist, it may be possible to observe ultrahigh carrier mobility and large magnetoresistance in this material.

Double-heterojunction crystalline silicon solar cell with electron-selective TiO2 cathode contact fabricated at 100°C with open-circuit voltage of 640 mV

A double-heterojunction c-Si solar cell was fabricated at maximum process temperature of 100°C. We demonstrate an electron-selective passivated contact to Si using TiO2, which increased the open-circuit voltage by 45 mV compared to a device with a direct metal to n-type substrate contact. In the fabricated structure, PEDOT/Si replaced the front-side p-n junction of conventional Si-based solar cells while the Si/TiO2 interface is formed on the back-side. Compared to previous work [1], the Voc has increased from 620 to 640 mV while maintaining a maximum process temperature of 100°C. Critical to the improved performance is better passivation of the Si/TiO2 interface. The increase in Voc can be attributed to an interface recombination velocity of ~75 cm/s, which is consistent with photoconductance decay measurements.

Electron-blocking NiO/crystalline n-Si heterojunction formed by ALD at 175°C

Silicon heterojunction solar cells have been the subject of growing research interest. Such cells replace the typical p+nn+ or n+pp+ structure of standard devices with selective heterojunction contacts, which block one type of carrier while allowing the other to pass freely (Fig. 1) [1-3]. Previously [4], we demonstrated a PEDOT/n-Si/TiO2 heterojunction cell fabricated below 100°C with no p-n junctions in the Si. However, the organic polymer PEDOT is known to be unstable over long periods of time; furthermore, recent data indicates that the PEDOT/n-Si interface might be a non-ideal minority carrier emitter, leading to a high J0 and low upper limit to VOC. Therefore, we are currently investigating inorganic electron-blockers on crystalline silicon. Nickel oxide (NiO), because of its large conduction band offset and small valence band offset with silicon (Fig. 2) [5], is a potential candidate for electron-blocking on n-Si. Here, we report atomic layer deposited (ALD) metal/15nm-i-NiO/Si diodes. We find that the NiO film leads to a heterojunction which blocks electrons compared to diodes with the NiO omitted. The characteristics depend on the top metal, indicating that the NiO passivates the Si surface so that the Fermi level is depinned and diodes with a higher Schottky barrier height can be fabricated. Devices with Ag have electron-blocking and hole-transmitting behavior.