Gabriel Man

Hole-blocking titanium-oxide/silicon heterojunction and its application to photovoltaics

In contrast to the numerous reports on narrow-bandgap heterojunctions on silicon, such as strained Si1−xGex on silicon, there have been very few accounts of wide-bandgap semiconducting heterojunctions on silicon. Here, we present a wide-bandgap heterojunction—between titanium oxide and crystalline silicon—where the titanium oxide is deposited via a metal-organic chemical vapor deposition process at substrate temperatures of only 80–100 °C. The deposited films are conformal and smooth at the nanometer scale. Electrically, the TiO2/Si heterojunction prevents transport of holes while allowing transport of electrons. This selective carrier blocking is used to demonstrate a low-temperature processed silicon solar cell.

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.

Fabrication of Reproducible, Integration‐Compatible Hybrid Molecular/Si Electronics

Reproducible molecular junctions can be integrated within standard CMOS technology. Metal-molecule-semiconductor junctions are fabricated by direct Si-C binding of hexadecane or methyl-styrene onto oxide-free H-Si(111) surfaces, with the lateral size of the junctions defined by an etched SiO2 well and with evaporated Pb as the top contact. The current density, J, is highly reproducible with a standard deviation in log(J) of 0.2 over a junction diameter change from 3 to 100 μm. Reproducibility over such a large range indicates that transport is truly across the molecules and does not result from artifacts like edge effects or defects in the molecular monolayer. Device fabrication is tested for two n-Si doping levels. With highly doped Si, transport is dominated by tunneling and reveals sharp conductance onsets at room temperature. Using the temperature dependence of current across medium-doped n-Si, the molecular tunneling barrier can be separated from the Si-Schottky one, which is a 0.47 eV, in agreement with the molecular-modified surface dipole and quite different from the bare Si-H junction. This indicates that Pb evaporation does not cause significant chemical changes to the molecules. The ability to manufacture reliable devices constitutes important progress toward possible future hybrid Si-based molecular electronics.

Double-heterojunction crystalline silicon solar cell fabricated at 250°C with 12.9 % efficiency

Double-heterojunction crystalline silicon solar cells were fabricated at temperatures of <;250°C using Si/organic and Si/metal-oxide heterojunctions, but no p-n junction in silicon. The first heterojunction, formed by spin-coating organic PEDOT:PSS on n-type silicon, functions as a front surface field that separates the photogenerated carriers and blocks electron dark-current while allowing hole photo-current to pass though. The second heterojunction, formed via metal-organic chemical vapor deposition of titanium dioxide on n-type silicon, functions as a back surface field that reduces hole dark-current while allowing electron photocurrent to pass through. Compared to a single heterojunction solar cell with only a Si/PEDOT heterojunction, the double-heterojunction device is more efficient with a power conversion efficiency of 12.9% under AM1.5.

Hole-blocking crystalline-silicon/titanium-oxide heterojunction with very low interface recombination velocity

We demonstrate a hole-blocking crystalline-silicon/titanium-oxide heterojunction that can be fabricated by a modified MOCVD process at only 100 oC substrate temperature. Ultra thin layers of only 1-4 nm TiO2 can be reliably deposited on silicon with no pinholes. Band alignment at the Si/TiO2, experimentally determined using surface spectroscopy, confirms that Si/TiO2 interface has a large barrier at the valence band, which blocks holes. The hole-blocking characteristics allow the Si/TiO2 heterojunction solar cells to achieve power conversion efficiencies > 7%. Finally, the electrical quality of the Si/TiO2 interface was characterized in terms of interface recombination velocity. We show that annealed Si/TiO2 interfaces can achieve recombination velocities of ~ 200 cm/s.

The effect of structural order on solar cell parameters, as illustrated in a SiC-organic junction model

To understand the title topic a model system of single crystal SiC, modified with an interfacial molecular monolayer of alkyl siloxane molecules, with polycrystalline pentacene deposited on it, was fabricated. In this way a change in the length of the alkyl chain could change the structural order of the pentacene film by changing the surfaces hydrophobicity, while no significant variation was found in the surface potential, and, thus, in the surface dipole. The pentacene film grown on top of the monolayers showed, with increasing alkyl chain length, increased lateral order and decreased band gap state density, as observed by X-ray diffraction and surface photovoltage spectroscopy. The Voc, Jsc and fill factor of solar cells, made with these material combinations, improved with increasing alkyl chain length. We explain this as a result of increased 2D film growth with increasing alkyl chain length of the monolayer, as the surface becomes more hydrophobic, which increases ordering of the pentacene film. Thus, this model system illustrates the role of ordering in charge separation and recombination.

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.