Karine van der Werf

Excellent crystalline silicon surface passivation by amorphous silicon irrespective of the technique used for chemical vapor deposition

Crystalline silicon surface passivation by amorphous silicon deposited by three different chemical vapor deposition (CVD) techniques at low (T∼130 °C) temperatures is compared. For all three techniques, surface recombination velocities (SRVs) are reduced by two orders of magnitude after prolonged thermal annealing at 200 °C. This reduction correlates with a decreased dangling bond density at the amorphous-crystalline interface, indicating that dangling bond saturation is the predominant mechanism. All three deposition methods yield excellent surface passivation. For a-Si:H layers deposited by radio frequency plasma enhanced CVD, we obtain outstanding carrier lifetimes of 10.3 ms, corresponding to SRVs below 1.32 cm/s.

High quality crystalline silicon surface passivation by combined intrinsic and n-type hydrogenated amorphous silicon

We investigate the influence of thermal annealing on the passivation quality of crystalline silicon (c-Si) surfaces by intrinsic and n-type hydrogenated amorphous silicon (a-Si:H) films. For temperatures up to 255 °C, we find an increase in surface passivation quality, corresponding to a decreased dangling bond density. Due to the combined chemical and field effect passivation of the intrinsic/n-type a-Si:H layer stack, we obtained minority carrier lifetimes with a value as high as 13.3 ms at an injection level of 1015 cm−3. For higher annealing temperatures, a decreased passivation quality is observed, which is attributed to hydrogen effusion.

Triple Junction n-i-p Solar Cells with Hot-Wire Deposited Protocrystalline and Microcrystalline Silicon

We have implemented a number of methods to improve the performance of proto-Si/proto-SiGe/μc-Si:H triple junction n-i-p solar cells in which the top and bottom cell i-layers are deposited by Hot-Wire CVD. Firstly, a significant current enhancement is obtained by using textured Ag/ZnO back contacts developed in house instead of plain stainless steel. We studied the correlation between the integrated current density in the long wavelength range (650-1000 nm) with the back reflector surface roughness and clarified that the rms roughness from 2D AFM images correlates well with the long wavelength response of the cell when weighted with a Power Spectral Density function. For single junction 2-μm thick μc-Si:H n-i-p cells we improved the short circuit current density from the value of 15.2 mA/cm 2 for plain stainless steel to 23.4 mA/cm 2 for stainless steel coated with a textured Ag/ZnO back reflector. Secondly, we optimized the μc-Si:H n-type doped layer on this rough back reflector, the n/i interface, and in addition used a profiling scheme for the H 2 /SiH 4 ratio during i-layer deposition. The H 2 dilution during growth was stepwise increased in order to prevent a transition to amorphous growth. The efficiency that was reached for a single junction μc-Si:H n-i-p cell was 8.5%, which is the highest reported value for hot-wire deposited cells of this kind, whereas the deposition rate of 2.1 A/s is about twice as high as in record cells of this type so far. Moreover, these cells show to be totally stable under light-soaking tests. Combining the above techniques, a rather thin triple junction cell (total silicon thickness 2.5 μm) has been obtained with an efficiency of 10.9%. Preliminary light-soaking tests show that this type of triple cells degrades by less than 4%.

Reaction Mechanism for Deposition of Silicon Nitride by Hot-Wire CVD with Ultra High Deposition Rate(>7 nm/s)

The deposition process of silicon nitride (SiN x ) by hot-wire chemical vapor deposition (HW CVD) is investigated by exploring the effects of process pressure and gas-flow ratio on the composition of the deposited SiNx films. Furthermore, experiments with D 2 and deuterated silane were performed to gain further insight in the deposition reactions taking place. It appeared that the N/Si ratio in the layers determines the structural properties of the deposited films and since the volume concentration of Si-atoms in the deposited films is constant with N/Si ratio, the structure of the films are largely determined by the quantity of incorporated nitrogen. Because the decomposition rate of the ammonia source gas is much smaller than that of silane, the properties of the SiN x layers are largely determined by the ability to decompose the ammonia and to incorporate nitrogen into the growing material. It appeared that the process pressure greatly enhances the efficiency of the ammonia decomposition, presumably caused by the higher partial pressure of atomic hydrogen. With this knowledge we increased the deposition rate to a very high value of 7 nm/s for dense transparent SiN x films, much faster than conventional deposition techniques for SiN x can offer. Despite this high deposition rate good control over the composition is achieved by varying the flow ratio of the source gasses. Depositions performed with deuterated silane as a source gas reveal that almost all hydrogen in N-rich films originates from ammonia, probably caused by SiN x matrix formation by cross linking reactions

High Quality Hot-wire Microcrystalline Silicon for Efficient Single and Multijunction N-i-p Solar Cells

In this paper, the potential of hot-wire chemical vapor-deposited (HWCVD) microcrystalline silicon (µc-Si) for use in solar cells is explored. Incorporation of the material in the currentlimiting bottom cell of two tandem cells on plain stainless steel resulted in FF values as high as 0.77, which is much higher than the highest single junction FF. A combination of experiments, calculations and computer simulations was employed to identify causes for the observed high tandem cell FF values. Both the light intensity and the spectral composition of the bottom cell illumination in a tandem were found to contribute to an increase of the bottom cell FF. The fact that the operational voltage of a tandem cell is higher than that of the current-limiting subcell, was calculated to lead to a tandem FF that can be far higher than that of the limiting cell. Computer simulations with the AMPS computer code show that the current mismatch in a tandem cell reduces the recombination in the current-limiting cell, possibly by slightly enhancing the internal field of that cell. Use of a 1.5 µm µc-Si:H hot-wire deposited absorber layer in a single junction cell on a textured back reflector yielded a Voc, FF and Jsc of 0.543 V, 0.656 and 23.60 mA/cm 2 , respectively, which combine to an 8.4 % record efficiency for µc-Si single junction n-i-p cells with a hot-wire intrinsic layer.

Oxygenated Protocrystalline Silicon Thin Films for Wide Bandgap Solar Cells

Protocrystalline silicon, which is a material that has enhanced medium range order (MRO), can be prepared by using high hydrogen dilution in PECVD, or, alternatively, using high atomic H production from pure silane in HWCVD. We show that this material can accommodate percentage-level concentrations of oxygen without deleterious effects. The advantage of protocrystalline SiO:H for application in multijunction solar cells is not only that it has an increased band gap, providing a better match with the solar spectrum, but also that the solar cells incorporating this material have a reduced temperature coefficient. Further, protocrystalline materials have a reduced susceptibility to light-induced defect creation. We present the unique result in the PV field that these oxygenated protocrystalline silicon solar cells have an efficiency temperature coefficient (TCE) that is virtually zero (TCE is between -0.08%/°C and 0.0/°C). It is thus beneficial to make this cell the current limiting cell in multibandgap cells, which will lead to improved annual energy yield.

Silicon Nitride as Dielectric Medium Deposited at Ultra High Deposition Rate (>7 nm/s) using Hot-Wire Chemical Vapor Deposition

The deposition process of silicon nitride (SiNx) by hot-wire chemical vapor deposition (HWCVD) is investigated by exploring the effects of process pressure and gas-flow ratio on the composition of the SiNx films. It appeared that the N/Si ratio in the layers determines the structural properties of the deposited films. The volume concentration of Si-atoms in the deposited films appeared to be independent of N/Si ratio. Because in a silane/ammonia mixture the decomposition rate of ammonia is smaller than that of silane, the properties of the SiNx layers are largely determined by the ability to incorporate nitrogen into the growing material. An increase in the process pressure greatly enhances the efficiency of the ammonia decomposition, which is ascribed to the higher partial pressure of atomic hydrogen originating from the decomposition of silane molecules. With this knowledge we were able to increase the deposition rate of high-density SiNx films to a very high value of 7 nm/s, much faster than any commercial plasma deposition technique can offer. Despite this high deposition rate, the SiNx layers still posses a high mass density of 2.6 g/cm3 and good thermal stability. Current–voltage (I–V) and capacitance–voltage (C–V) measurements show that silicon nitride deposited at high deposition rate has good potential for application as the dielectric layer in various applications.

Design and photovoltaic performance of nanorod solar cells with amorphous silicon absorber layer thickness of only 25 nm

We report on the design and photovoltaic performance of nanostructured three dimensional (nano-3D) solar cells with ultrathin amorphous hydrogenated silicon (a-Si:H) absorber layers. Zinc oxide (ZnO) nanorods are employed as the building blocks for the nano-3D solar cells. The ZnO nanorods with controlled morphology are prepared by aqueous solution deposition at 80°C. The nanorod a-Si:H solar cells are realized by depositing n-i-p a-Si:H layers over Ag-coated ZnO nanorods. The photovoltaic performance of the nano-3D solar cells is experimentally demonstrated. With an ultrathin absorber layer of only 25 nm, an efficiency of 3.6% and a short-circuit current density of 8.3 mA/cm2 are obtained, significantly higher than values achieved for the planar or even the textured counterparts with a three times thicker (∼75 nm) a-Si:H absorber layer. By increasing the absorber layer thickness in the nano-3D solar cells from 25 nm to 75 nm, the efficiency improved from 3.6% to 4.1% and the short-circuit current density increased from 8.3 mA/cm2 to 13.3 mA/cm2. The orthogonalization of the light path and the carrier transport path plays an important role in these nano-3D devices.