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31 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Photoelectrochemical Hydride Generation with Oxide-Coated Silicon

Photoelectrochemical generation of a potent organic hydride donor at silicon is demonstrated. Two different oxide-coated p-type silicon photoelectrodes reduced 1,2,3,5,6-pentamethyl-1H-benzo[d]imidazol-3-ium hexafluorophosphate, [PMBI][PF 6 ], to its corresponding imidazole hydride, PMBIH, in the presence of a proton source. Under 1 sun illumination, in acetonitrile with 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) buffer, the p-Si photoelectrodes convert PMBI + to PMBIH with good Faradaic efficiencies (FEs): 78% FE at −2.3 V vs Fc +/0 for Si|TiO 2 and 83% FE at −2.6 V vs Fc +/0 for Si|SiO 2 (where Si|SiO 2 represents silicon coated with an oxide layer). Generally, the Si|TiO 2 catalyzed the reaction at milder potentials than Si|SiO 2 , but the Si|SiO 2 had better selectivity for PMBIH generation over H 2 evolution than Si|TiO 2 . In light of prior studies of these photoelectrodes, the differences in selectivity and onset potential suggest a difference in mechanism, likely an outer-sphere electron transfer (ET) mechanism at the SiO 2 surface and potentially a proton-coupled ET process at the TiO 2 surface. To help understand reaction efficiency and identify areas of improvement, a thermochemical model for understanding net hydride transfer from the semiconductor to an acceptor in solution was developed. We find that the reactions in the present system are quite downhill. The high overpotentials (even while maintaining selectivity over H 2 evolution) emphasize the need for improved catalysts. Furthermore, this approach to evaluate the thermodynamics of net hydride transfer should be broadly valuable for electrochemical and photoelectrochemical processes.

14 SOLAR ENERGY↗
DOE OSTI2021

Optical and electrochemical effects of H 2 and O 2 bubbles at upward-facing Si photoelectrodes

The effects of the size, contact-angle, and coverage of gas bubbles on solar fuels devices were characterized at cm-scale, upward-facing planar and microwire-array Si photoelectrodes in stagnant electrolytes. Experimental measurements were supported by ray-tracing simulations of surface attached gas bubble films. A dilute, redox-active tracer allowed for the quantification of the mass-transport effects of bubble coverage during photoanodic O 2 (g) evolution at upward-facing photoanodes in 1.0 M KOH(aq.). Measurements of the gas coverage at upward-facing p-Si photocathodes in 0.50 M H 2 SO 4 (aq.) allowed for the nucleation rate and contact angle of H 2 (g) bubbles to be evaluated for systems having various surface free energies. Under simulated solar illumination, the rapid departure of small O 2 (g) bubbles produced stable photocurrents at upward-facing oxygen-evolving Si photoanodes and yielded increased mass-transport velocities relative to a stagnant electrolyte, indicating that bubbles can provide a net benefit to the photoelectrochemical performance of an upward-facing photoanode in solar fuels devices.

14 SOLAR ENERGY↗
DOE OSTI2022

Novel and effective surface passivation for high efficiency n- and p-type Silicon solar cell

The project objective was to develop a novel Si surface passivation method using chalcogens, sulfur (S) and/or selenium (Se), as passivating elements, to withstand industry-standard high temperature contacting and metallization schemes for p-type Si based passivated emitter and rear contact (p-PERC) solar cells. The back surface passivation of PERC cells has been improved drastically with the invention and successful application of an Al 2 O 3 passivation layer. However, the front n + diffused junction surface is still poorly passivated by the standard amorphous silicon nitride (SiNx) anti-reflection coating (ARC) layer. This project sought to address the passivation challenges of both front n+ emitter and undiffused p-Si back surface. Improved p-PERC solar cell performance with open circuit voltage (V OC ) > 680 mV and efficiency of 22% were targeted to validate superior defect passivation properties as compared to standard SiO 2 / Al 2 O 3 passivation. During this project, we systematically investigated process-structure-properties-performance relationships of this novel advanced defect passivation approach. The S/Se passivation was carried out by reacting industrial Czochralski (Cz) Si wafers in H 2 S and H 2 Se gases in a chemical vapor deposition (CVD) reactor at temperatures up to 700°C. After an exhaustive optimization of the process parameters (temperature, time, and gas concentration), we established an optimized process and demonstrated extremely low surface recombination velocities (SRVs) of 1.5 cm/s and 8 cm/s on n-type and p-type Si, respectively, by S-passivation. In-depth surface and interface characterization were performed using soft x-ray and photoelectron spectroscopies (XPS, UPS, XES), combined with capacitance-voltage-frequency (C-V-f) measurements, to decipher the surface chemical/electronic structure and interface defect state densities. These measurements provided critical understanding of the defect passivation mechanism and elucidated the presence of surface S-Si bonds, a reduction of surface dipoles, and low interface state densities (D it ) < 10 11 cm -2 ev -1 . We also found that the Se-passivation is inferior to the S-passivation (by at least one order of magnitude in SRV). Application of the optimized S-passivation to the n+ diffused emitter surface led to a low surface recombination current density, J0 ≈ 40 fA/cm 2 (~ 1/4 of the industry-standard SiNx-passivation), and high implied V OC (686 mV) in p-PERC solar cell structures. The S-passivation process also was found to improve the bulk quality of the p-type Si, better than the SiO 2 or Al 2 O 3 passivation processes. After successful demonstration of efficient passivation of Si surface defects by S, we extensively studied the air, thermal, and illumination stability of the passivation structure. S-passivation itself degrades in air due to competing reactions with moisture and oxygen to form oxides, which can be eliminated by a SiNx capping layer (also acting as a anti-reflective coating). After SiNx process optimization, we demonstrated illumination and thermally stable S-passivation with SRV < 5 cm/s and J 0 < 80 fA/cm 2 . These enhancements in Si passivation, incorporated into p-PERC cells, achieved an efficiency of 19.93% with V OC = 649 mV, using manufacturing metallization and contacting schemes. The low cell performance (cell V OC is much less than the implied V OC = 686 mV, anticipated from surface passivation) was identified due to degradation of S-passivation during the metal firing step (out-diffusion of S from the Si interface to the SiNx surface). The S-passivation of Si surfaces shows significant promise with excellent passivation quality, essential for high performance (high V OC , high efficiency) solar cells. Integration of this innovative defect passivation into devices, however, demands further development of the capping layer, low temperature (<700°C) metallization process, and/or engineering of advanced device structures. Surface passivation-dominated advanced Si solar cells, such as tunnel oxide passivated contacts and Si heterojunctions, are increasingly of interest due to their high-performance potential and will have a growing photovoltaic market share in the near future.

14 SOLAR ENERGY↗
DOE OSTI2023

Exploiting Fixed Charge at Selective Contacts for Silicon Photovoltaics

The goal of this work was to examine and exploit fixed interface charges in ultra-thin (0-3 nm thickness) tunnel dielectric layers sandwiched between silicon and selective contact metal oxides (molybdenum oxide, MoO x , titanium oxide, TiO 2 ). Specifically, the interface fixed charge was varied by changing processing conditions and materials over a wide range and the effect on the current-voltage behavior and the Schottky barrier height was quantified. In budget period 1 (year 1), the objective was to grow aluminum oxide tunnel layers with quantified fixed charge over a range of ~1E12 to ~-2E12 cm -2 and combine these with MoO x and TiO 2 layers on Czochralsi (CZ) mono p- and n-Si, respectively. We sought conditions that maximize fixed charge and minimize Schottky barrier height to MoO x contacts to p-Si. This objective, along with investigating the effects of silicon surface terminations, occupied the majority of the project timeline (Q1-Q8) due to complications with high specific contact resistivity values. In the last two quarters of the project period, the objective was to insert HfO2 layers between Si and aluminum oxide layers to quench the fixed charge for layers with otherwise identical thermal history. Another objective was to lower the barrier height at TiO 2 -based contacts on n-type Si, but that was never investigated completely.

14 SOLAR ENERGY↗
NASA NTRS1987

Electrochemical studies of thin films of conducting polymers and conducting polymer composites deposited on metal and semiconductor electrodes

Electrochemical studies indicate that poly(Isothianaphthene) or PITN, can be p-doped only. Electrochemical properties of PITN and Nafion-PITN in acetonitrile solutions containing tetra-phenyl Phosphonium chloride as supporting electrolyte are compared. In both cases, the electrochemical behavior of thin films are different from that of thick films. In addition, Nafion does not seem to alter the electrochemical properties of PITN. Cyclic voltammetric and chronocoulometric measurements were made to compute the diffusion coefficient of the counter ions. Electrochemical behavior of both PITN and Nafion-PITN in acetonitrile solution containing different counter ions are described. PITN, when electrochemically deposited, apparently neither passivates surface states present nor forms ohmic contacts with p-Si or p(+)Si single-crystal electrodes.

Nagasubramanian, G.↗
NASA NTRS1990

Novel Si(1-x)Ge(x)/Si heterojunction internal photoemission long-wavelength infrared detectors

The feasibility of a novel p(+)-Si(1-x)Ge(x)-p-Si heterojunction internal photoemission (HIP) IR detector is demonstrated. A degenerately doped p(x)-Si(1-x)Ge(x) layer is required for strong IR absorption to generate photoexcited holes. The Si(1-x)Ge(x) layers are grown by molecular beam epitaxy, with boron concentrations up to 10 to the 20th/cu cm achieved by using an HBO2 source. Photoresponse at wavelengths ranging from 2 to 10 microns has been obtained with quantum efficiencies above 1 percent. The tailorable cutoff wavelength of the HIP detector has been demonstrated by varying the Ge composition ratio in the Si(1-x)Ge(x) layers.

Lin, T. L.↗
NASA NTRS2000

p-n Junction Diodes Fabricated on Si-Si/Ge Heteroepitaxial Films

A set of photolithographic masks was designed for the fabrication of diodes in the Si-Si/Ge material system. Fabrication was performed on samples obtained from two different wafers: (1) a complete HBT structure with an n (Si emitter), p (Si/Ge base), and an n/n+ (Si collector/sub-collector) deposited epitaxially (MBE) on a high resistivity p-Si substrate, (2) an HBT structure where epitaxial growth was terminated after the p-type base (Si/Ge) layer deposition. Two different process runs were attempted for the fabrication of Si-Si/Ge (n-p) and Si/Ge-Si (p-n) junction diodes formed between the emitter-base and base-collector layers, respectively, of the Si-Si/Ge-Si HBT structure. One of the processes employed a plasma etching step to expose the p-layer in the structure (1) and to expose the e-layer in structure (2). The Contact metallization used for these diodes was a Cu-based metallization scheme that was developed during the first year of the grant. The plasma-etched base-collector diodes on structure (2) exhibited well-behaved diode-like characteristics. However, the plasma-etched emitter-base diodes demonstrated back-to-back diode characteristics. These back-to back characteristics were probably due to complete etching of the base-layer, yielding a p-n-p diode. The deep implantation process yielded rectifying diodes with asymmetric forward and reverse characteristics. The ideality factor of these diodes were between 1.6 -2.1, indicating that the quality of the MBE grown epitaxial films was not sufficiently high, and also incomplete annealing of the implantation damage. Further study will be conducted on CVD grown films, which are expected to have higher epitaxial quality.

Das, K.↗
DOE OSTI2020

Materials Data on SiP2 by Materials Project

SiP2 is Pyrite structured and crystallizes in the cubic Pa-3 space group. The structure is three-dimensional. Si4- is bonded to six equivalent P2+ atoms to form corner-sharing SiP6 octahedra. The corner-sharing octahedral tilt angles are 65°. All Si–P bond lengths are 2.41 Å. P2+ is bonded in a trigonal non-coplanar geometry to three equivalent Si4- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiP by Materials Project

PSi crystallizes in the orthorhombic Cmc2_1 space group. The structure is two-dimensional and consists of two PSi sheets oriented in the (0, 0, 1) direction. there are six inequivalent Si4- sites. In the first Si4- site, Si4- is bonded to one Si4- and three P4+ atoms to form corner-sharing SiSiP3 tetrahedra. The Si–Si bond length is 2.36 Å. There are two shorter (2.29 Å) and one longer (2.31 Å) Si–P bond lengths. In the second Si4- site, Si4- is bonded to one Si4- and three P4+ atoms to form corner-sharing SiSiP3 tetrahedra. There are two shorter (2.29 Å) and one longer (2.31 Å) Si–P bond lengths. In the third Si4- site, Si4- is bonded to one Si4- and three P4+ atoms to form distorted corner-sharing SiSiP3 tetrahedra. The Si–Si bond length is 2.35 Å. There are one shorter (2.27 Å) and two longer (2.28 Å) Si–P bond lengths. In the fourth Si4- site, Si4- is bonded to one Si4- and three P4+ atoms to form distorted corner-sharing SiSiP3 tetrahedra. There are one shorter (2.27 Å) and two longer (2.28 Å) Si–P bond lengths. In the fifth Si4- site, Si4- is bonded to one Si4- and three P4+ atoms to form distorted corner-sharing SiSiP3 tetrahedra. The Si–Si bond length is 2.35 Å. There are one shorter (2.27 Å) and two longer (2.28 Å) Si–P bond lengths. In the sixth Si4- site, Si4- is bonded to one Si4- and three P4+ atoms to form distorted corner-sharing SiSiP3 tetrahedra. There are one shorter (2.27 Å) and two longer (2.28 Å) Si–P bond lengths. There are six inequivalent P4+ sites. In the first P4+ site, P4+ is bonded in a distorted T-shaped geometry to three Si4- atoms. In the second P4+ site, P4+ is bonded in a distorted T-shaped geometry to three Si4- atoms. In the third P4+ site, P4+ is bonded in a distorted T-shaped geometry to three Si4- atoms. In the fourth P4+ site, P4+ is bonded in a distorted T-shaped geometry to three Si4- atoms. In the fifth P4+ site, P4+ is bonded in a distorted T-shaped geometry to three Si4- atoms. In the sixth P4+ site, P4+ is bonded in a distorted T-shaped geometry to three Si4- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiP by Materials Project

PSi is Zincblende, Sphalerite structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. Si4- is bonded to four equivalent P4+ atoms to form corner-sharing SiP4 tetrahedra. All Si–P bond lengths are 2.31 Å. P4+ is bonded to four equivalent Si4- atoms to form corner-sharing PSi4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si3P by Materials Project

Si3P is alpha bismuth trifluoride structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. there are two inequivalent Si+1.33- sites. In the first Si+1.33- site, Si+1.33- is bonded in a distorted body-centered cubic geometry to four equivalent Si+1.33- and four equivalent P4+ atoms. All Si–Si bond lengths are 2.66 Å. All Si–P bond lengths are 2.66 Å. In the second Si+1.33- site, Si+1.33- is bonded in a body-centered cubic geometry to eight equivalent Si+1.33- atoms. P4+ is bonded in a body-centered cubic geometry to eight equivalent Si+1.33- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiP2 by Materials Project

SiP2 crystallizes in the orthorhombic Pbam space group. The structure is two-dimensional and consists of two SiP2 sheets oriented in the (0, 0, 1) direction. there are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded to four P2+ atoms to form corner-sharing SiP4 tetrahedra. There are a spread of Si–P bond distances ranging from 2.25–2.28 Å. In the second Si4- site, Si4- is bonded to four P2+ atoms to form corner-sharing SiP4 tetrahedra. There are a spread of Si–P bond distances ranging from 2.26–2.34 Å. There are four inequivalent P2+ sites. In the first P2+ site, P2+ is bonded in a distorted T-shaped geometry to three Si4- atoms. In the second P2+ site, P2+ is bonded in a single-bond geometry to one Si4- atom. In the third P2+ site, P2+ is bonded in a single-bond geometry to one Si4- atom. In the fourth P2+ site, P2+ is bonded in a trigonal non-coplanar geometry to three Si4- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Damp Heat Degradation of Polycrystalline Silicon Passivated Contacts

After 1000 hours of damp heat exposure at 85% humidity and 85 C, LPCVD n-poly and p-poly are stable for both metallized and non-metallized regions whereas for PECVD, only n-poly is stable; PECVD p-poly suffers significant degradation and defects became more pronounced with exposure time; and PECVD p-poly degrades more than LPCVD p-poly with or without additional dielectric layers (i.e., SiNx + Al2O3 stack).

damp heat degradation↗