In this paper, investigations toward understanding the bulk and surface components of light-induced degradation (LID) in low-iron crystalline silicon (Si) solar cells are explored. The bulk effects, arising from boron–oxygen defects, are determined by comparing degradation of cell parameters and their thermal recovery, with that of the minority-carrier lifetime (τ) in sister wafers. It is found that the lifetime of wafer, τ, is recovered fully after annealing, but cell efficiency is recovered partially. It is also shown that cells having SiN:H coating experience a surface degradation (ascribed to surface recombination). The surface LID is seen as an increase in the q/2kT component of the dark saturation current (J02). The surface LID does not recover fully upon annealing and is attributed to degradation linked to the SiN:H–Si interface. This behavior is also exhibited by multi-crystalline silicon cells that have very low oxygen content and do not show any bulk degradation.

An ever increasing demand on energy has fostered many new generation technologies, which include photovoltaics. In recent years, photovoltaic industry has grown very rapidly. The installed capacity of PV for 2013 was about 37 GW and 2014 sales are expected to be around 45 GW. However, there has been excess production for last several years, which is responsible in part for the low prices (about 60 c/W). To lower the PV energy costs further, a major strategy appears to be going to high efficiency solar cells. This approach is favored (over lower cost/lower efficiency) because cell efficiency has a very large influence on the acceptable manufacturing cost of a PV module. Hence, the PV industry is moving toward developing processes and equipment to manufacture solar cells that can yield efficiencies >20%. Therefore, further research is needed within existing technologies to accomplish these objectives. Likewise, research will continue to seek new materials and devices.

The ZnO and mixed (ZnO:GaN) thin films are synthesized by (RF) magnetron sputtering in Ar and mixed O-2 and N-2 gas ambient at 100 degrees C, followed by post-annealing at 500 degrees C in ammonia for 4 h. The mixed (ZnO:GaN) thin films deposited under Ar gas ambient failed to reduce the bandgap, whereas (ZnO:GaN) thin films grown under mixed O-2 and N-2 gas ambient showed bandgap reduction. The (ZnO:GaN) films deposited under mixed O-2 and N-2 gas exhibited enhanced crystallinity, with shifting the optical absorption into the visible light regions. The bandgap reduction in mixed (ZnO:GaN) thin films is realized by varying the RF power. As a result, mixed (ZnO:GaN) films grown under mixed O-2 and N-2 showed higher photocurrents than the mixed (ZnO:GaN) thin films deposited under Ar gas ambient. Our results indicate that reduced bandgap with enhanced PEC response can be attained using the appropriate gas ambient and by varying the RF power using mixed (ZnO:GaN) films. (C) 2013 Elsevier B.V. All rights reserved.

We have measured and compared surface roughness and the degree of damage for wafers cut by three different sawing techniques - slurry, Ni-based diamond wire, and resin-based diamond wire sawing. The local damage was determined by angle polishing followed by defect etching, TEM, SEM/EBSD imaging and Raman imaging. It showed that each of the cutting processes produces a thin layer of amorphous Si at the surface and dislocation loops that can go about 1 μm deep below the surface. A new approach was used to quantify the average damage over a large area. We determined the effective surface recombination (SRV) as a function of depth. Because the effective SRV is a function of the carrier loss close to the surface, it is well-suited to define damage distribution at and below the surface. Wafers with surface damage were step etched in (HF:HNO3:CH3COOH::1:1:5), and the effective lifetime was measured with a Sinton system after each etching step, with iodine-ethanol passivation. The SRV plots as a function of depth, representing depth distribution of the damage, were compared for large groups of wafers cut by each technique. Our results show that for optimized cutting, all three cutting methods produce damage depth of about 5μm (each surface). However, the degree of damage is higher for slurry cut wafers.

Mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with significantly reduced bandgaps were synthesized by using zinc oxide and gallium nitride target at 100 degrees C followed by post-deposition annealing at 500 degrees C in ammonia for 4 h. All the films were synthesized by RF magnetron sputtering on Fluorine-doped tin oxide-coated glass. We found that mixed zinc oxide and gallium nitride (ZnO:GaN) thin films exhibited significantly reduced bandgap, as a result showed improved PEC response, compared to ZnO thin film. Furthermore, mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with various bandgaps were realized by varying the O-2 mass flow rate in mixed O-2 and N-2 chamber ambient. (c) 2013 Elsevier B. V. All rights reserved.

Co-doped ZnO:(Al,N) and ZnO:(Ga,N) films were deposited by co-sputtering using radio-frequency magnetron sputtering on F-doped tin-oxide-coated glass. We found that the ZnO:(Al, N) and ZnO:(Ga, N) films exhibited greatly enhanced crystallinity compared to ZnO: N films doped by pure N and deposited under similar conditions. Furthermore, the ZnO:(Al, N) and ZnO:(Ga, N) films showed much higher N-incorporation than ZnO: N films deposited with pure N doping. As a result, the ZnO:(Ga, N) films showed significantly higher photocurrents than ZnO: N doped only by N. The ZnO:(Cu,Ga) films were synthesized by RF magnetron sputtering in O-2 gas ambient at room temperature and then annealed at 500 degrees C in air for 2 hours. We found that the carrier concentration tuning does not significantly change the bandgap and crystallinity of the ZnO: Cu films. However, it can optimize the carrier concentration and thus dramatically enhance PEC response for the bandgap-reducedp-type ZnO thin films.

A major drawback for CuO as an efficient photocathode in photoelectrochemical (PEC) water splitting is its instability in aqueous solution. In this paper, we report that Ti alloying can enhance the stability of CuO in PEC water splitting but at the cost of reduced crystallinity and optical absorption, and therefore reduced photocurrent. We further report that a balance between the stability and photocurrent can be realized by a bilayer configuration—a thin Ti-alloyed CuO layer on a pure CuO thin film. Our results indicate that the thickness of the top Ti-alloyed CuO layer should be optimized to realize the best stability and photocurrent.

ZnO thin films with significantly reduced bandgaps were synthesized by doping N and co-doping Al and N at 100oC. All the films were synthesized by radio-frequency magnetron sputtering on F-doped tin-oxide-coated glass. We found that co-doped ZnO:(Al,N) thin films exhibited significantly enhanced crystallinity as compared to ZnO doped solely with N, ZnO:N, at the same growth conditions. Furthermore, annealed ZnO:(Al,N) thin films exhibited enhanced N incorporation over ZnO:N films. As a result, ZnO:(Al,N) films exhibited improved photocurrents than ZnO:N films grown with pure N doping, suggesting that charge-compensated donor-acceptor co-doping could be a potential method for bandgap reduction of wide-bandgap oxide materials to improve their photoelectrochemical performance.

We discuss results of our investigations toward understanding bulk and surface components of light-induced degradation (LID) in low-Fe c-Si solar cells. The bulk effects, arising from boron-oxygen defects, are determined by comparing degradation of cell parameters and their thermal recovery, with that of the minority-carrier lifetime (τ) in sister wafers. We found that the recovery of t in wafers takes a much longer annealing time compared to that of the cell. We also show that cells having SiN:H coating experience a surface degradation (ascribed to surface recombination). The surface LID is seen as an increase in the q/2kT component of the dark saturation current (J 02 ). The surface LID does not recover fully upon annealing and is attributed to degradation of the SiN:H-Si interface. This behavior is also exhibited by mc-Si cells that have very low oxygen content and do not show any bulk degradation.

ZnO and mixed ZnO:GaN thin films were synthesized on fluorine-doped tin oxide substrates by reactive radio frequency magnetron sputtering in mixed N-2 and O-2 ambient. Mixed ZnO:GaN films exhibited better crystallinity compared to ZnO film and bandgap narrowing was observed for mixed ZnO:GaN thin films. The n-type conductivity is revealed for both ZnO and ZnO:GaN thin films by Mott-Schottky plots as well as photocurrent polarity in current-voltage (I-V) analysis. ZnO:GaN thin films exhibited improved photocurrents than ZnO films. Our results suggest a potential method for synthesizing heterogeneous photocatalysts with both high crystallinity and bandgap reduction, which should help to improve their photoelectrochemical performance.

ZnO thin films have been deposited in mixed Ar/N 2 gas ambient at substrate temperature of 500°C by radiofrequency sputtering of ZnO targets. We find that an optimum N 2 -to-Ar ratio in the deposition ambient promotes the formation of well-aligned nanorods. ZnO thin films grown in ambient with 25% N 2 gas flow rate promoted nanorods aligned along c -axis and exhibit significantly enhanced photoelectrochemical (PEC) response, compared with ZnO thin films grown in an ambient with different N 2 -to-Ar gas flow ratios. Our results suggest that chamber ambient is critical for the formation of aligned nanostructures, which offer potential advantages for improving the efficiency of PEC water splitting for H 2 production.

In this study, report on the synthesis of ZnO:N thin films by reactive RF magnetron sputtering using a Zn metal target & ZnO target in mixed N-2 and O-2 ambient. We found that the N concentration in ZnO:N thin films can be effectively controlled by varying the RF power. ZnO:N films with narrowed bandgaps were synthesized. The photoelectrochemical properties of nitrogen-incorporated ZnO (ZnO:N) films were measured and compared with those of pure ZnO films. We find that nitrogen incorporation narrows the bandgap of ZnO and shifts the optical absorption into the visible-light regions. We further find that the ZnO:N films provide considerable photoresponse in the long-wavelength regions. As a result, the ZnO:N films exhibit higher photocurrents than pure ZnO films.

This chapter contains sections titled: Introduction Experimental Results and Discussion Conclusions

To optimize materials and devices for solar photoelectrochemical hydrogen production, a detailed understanding of the chemical and electronic properties, in particular at the reactive surfaces and interfaces, is needed. In this review article we will show how electron and soft X-ray spectroscopies can provide such information. We will present exemplary studies using X-ray photoelectron spectroscopy, soft X-ray emission spectroscopy, UV photoelectron spectroscopy, and inverse photoemission. While the first two techniques mainly give insight into the chemical properties at and near the surface, the latter two methods allow us to derive the electronic levels relevant for photoelectrochemical water splitting at the surface of the investigated material. Ultimately, the ideal experiment would be performed in situ, in which the device is studied under working conditions, i.e., in a liquid environment and under illumination. We will give a short outlook on how this can be achieved experimentally under the strict requirements of the measurement environment. (C) 2012 Elsevier B.V. All rights reserved.

ZnO:N films were deposited by reactive RF magnetron sputtering on F-doped tin oxide coated glass substrates in mixed N-2 and O-2 gas ambient. Their PEC properties were measured and compared with those of as-deposited and annealed ZnO films. The ZnO: N films exhibit photoresponse in the visible-light region, yielding higher total photocurrents than ZnO thin films. ZnO: N thin films with reduced bandgaps were synthesized by reactive RF magnetron sputtering using ZnO target at 100 degrees C followed by post deposition annealing at 500 degrees C in air for 2 h. ZnO: N thin films showed enhanced N incorporation and shift of the optical absorption into the visible light regions. As a result, ZnO: N films showed improved PEC response, compared to ZnO thin films