Thin-film silicon solar cells offer the advantages of low material and manufacturing costs. In order to enhance the absorptance of incident light in the optically thin silicon absorber layer, this technology requires advanced light-trapping concepts. Conventional devices apply randomly textured transparent conductive oxide substrates serving as light-scattering front contacts as well as reflective light-scattering metal back contacts. In recent years, several novel light-trapping concepts based on periodic nanostructures and periodically nanotextured interfaces, such as grating couplers, photonic crystals or plasmonic reflection gratings, have been suggested and prototyped. For these concepts the absorption of incident light in the solar cells is enhanced by light-coupling to waveguide modes which are supported by the silicon absorber layer of the solar cells but can be excited at the same time by incident light.
In this contribution, our recent progress on light-trapping in periodically structured prototype thin-film silicon solar cells made of hydrogenated amorphous silicon and hydrogenated microcrystalline silicon is presented. The prototype solar cells show a superior light-trapping effect compared to solar cells applying the conventional random texture for light-trapping. To better understand this improved light-trapping effect, the coupling of incident light to waveguide modes in periodically nanostructured thin-film silicon solar cells is analysed in-depth. Therefore, the shape of the grating structure and the geometry of the unit cell of the two-dimensional periodic grating structure of the thin-film silicon solar cells are varied systematically and the excitation of the waveguide modes is studied. To characterize the coupling of incident light to individual waveguide modes, advanced characterization techniques, i.e. angular and polarization dependent spectral response measurements of resolution below 3 nm as well as near-field scanning optical microscopy, are developed and employed. Finally, based on our study new routes for improved designs of the periodic nanostructure of thin-film silicon solar cells will be outlined.

In the past few years a large amount of work has been performed on thin film a-Si:H and nc-Si solar cells with periodically textured back reflectors, composed of photonic and plasmonic crystals. These periodic back reflectors were predicted to i) introduce strong diffraction of light resulting in wave-guiding modes and ii) enhance light concentration through generation of plasmons. This light trapping is particularly needed at long wavelengths (lambda;> 600 nm) where photon absorption is low. Rigorous simulations predicted very large gains (>40%) in absorption and photo-current, with the absorption predicted to approach the Lambertian limit. It is an open question whether periodic back reflectors can exceed the performance of randomly textured back reflectors. However experimental solar cells fabricated with periodic back reflectors exhibit photo-currents considerably lower than rigorous predictions. Our measured external quantum efficiency (EQE) for a 1000 nm nc-Si cell with a periodic array of nano-cones is more than a factor of 1.5 to 2 lower than the simulated values in the long wavelength region (above 600 nm). Similar results are found for solar cells fabricated in other laboratories.
We analyze systematically the sources of these differences between experiment and prediction. We evaluate the parasitic losses in the ITO anti-reflection coating, and find only small losses (<5%) that cannot account for this discrepancy. There are considerable parasitic losses in the metal back reflector that partly contribute to the lower values measured in experiment. We evaluate this loss by simulations with periodic dielectric back reflectors. Another factor that has previously been overlooked is the loss of phase coherence of incoming waves within the solar cell, caused by random variations of thickness and structural inhomogeneity. This incoherence can severely deplete the wave-guided modes and can account for the much lower EQE and photo-currents observed experimentally. The loss of this phase coherence is apparent in the difference between measured and simulated absorption even for flat solar cells. We evaluate the loss of phase coherence in flat cells from experimental measurements, and will discuss how this can be a limiting factor for light trapping in periodic solar cells.

Black silicon (b-Si) is a material with an optically graded boundary which exhibits a reflectivity as low as 1 - 5 % in the whole range of Si absorption. A cone-like nano-texture, with a depth of less than 1 mu;m, leads to a density graded surface with a smooth variation of the refractive index. Nano-texturing of thin silicon wafers and films also results in an additional enhancement of the optical absorption due to extremely efficient light trapping [1]. The unique optical properties of b-Si, as well as the opportunity to produce nano-textures on different silicon substrates (mono- or polycrystalline wafers, a-Si:H and µc-Si:H films and others) are of significant interest for photovoltaic applications, in particular for thin-film solar cells [2].
Heterojunction solar cells manufactured from large grained polycrystalline silicon (poly-Si) films, prepared by electron-beam-crystallization, exhibit promisingly high open-circuit voltages (V_oc), but suffer from a low short-circuit current (J_sc) due to the lack of an appropriate light-trapping scheme [3]. We applied a b-Si nano-texture in order to increase the absorption and, thus, to improve the J_sc. The nano-texturing of the 10 mu;m thick poly-Si absorber was achieved using metal-catalyzed wet-chemical etching. The process parameters such as metal particle size, etch time and etch solution were optimized in order to maximize the light trapping effect in the poly-Si film. With an additional wet-chemical post-treatment we control the final morphology and improve the electronic properties of the nano-texture [4]. UV-Vis spectroscopy reveals a reflectivity below 5 % in the spectral range important for Si solar cell operation and excellent light trapping in the film also for wavelengths above 700 nm. The passivation of the nano-textured Si surface with hydrogenated amorphous silicon (a-Si:H) was optimized using monocrystalline silicon (c-Si) wafers as a model system. Injection-level dependent lifetime measurements reveal excellent passivation after the (i)a-Si:H deposition, showing effective carrier lifetimes above 1 ms.
The good optical properties of the nano-textured thin-film poly-Si solar cells result in a significant increase of J_sc. However, due to some reduction in the V_oc the overall energy conversion efficiency did not improve in the same way. Further optimization of the doped a-Si:H layer stack on the nano-textured Si surface is needed in order to benefit from the excellent optical properties and to transform them into higher efficiencies.
[1] S. Koynov, M. S. Brandt, M. Stutzmann, J. Appl. Phys. 110, 043537 (2011).
[2] S. Koynov, M. S. Brandt, M. Stutzmann, Appl. Phys. Lett. 88, 203107 (2006).
[3] D. Amkreutz, J. Müller, M. Schmidt, T. Hänel, T. F. Schulze, Prog. Photovolt: Res. Appl.19, 937 (2011).
[4] M. Algasinger, J. Paye, F. Werner, J. Schmidt, M. S. Brandt, M. Stutzmann, S. Koynov, Adv. Energy Mater. 3, 1068 (2013).

Hydrogenated nanocrystalline silicon (nc-Si:H) is widely used as absorber layer in the bottom cell of multijunction thin-film silicon solar cells. The nano-textured substrates with morphology features in the order of 1 mu;m are commonly used to achieve efficient light trapping in the state-of-the-art devices. However, the nano-textured substrates deteriorate (by the incorporation of defect rich filaments) the electrical performance of solar cells by decreasing the open-circuit voltage (Voc) and fill factor (FF) compared to flat substrates. In addition, the Voc and FF of nc-Si:H solar cells show considerable drop as the cell thickness (and defect rich filaments) increases. As a result the performance of solar cell with absorber layers with typically thicknesses of 2-4 mu;m do not benefit from the higher generated current densities. Therefore, textured substrates which can provide efficient light trapping and maintain high Voc and FF for thick absorber layers under high deposition rates are highly desirable.
Recently, Sai et al. [AIST,Japan] have shown that periodically textured substrates with a large period resulted in higher Jsc and improved Voc and FF in 3-mu;m-thick nc-Si:H solar cells compared to substrates with a small period. Their work suggests that substrates with an even larger period, which we here refer to as micro-textures (feature size >5 mu;m), might have the potential to maintain high Voc and FF in thick cells by growing nc-Si:H material free from defective filaments. In this contribution, we explore the behavior of nc-Si:H solar cells deposited on the microtextured glass substrates. The micro-textures on glass were generated by wet-etching with In2O3:Sn as catalyst in a solution composing of HF and H2O2 for 30 min. The 1.5 microns AZO layers were fabricated by RF magnetron sputtering. Nano-textured AZO was obtained by wet-etching. This procedure allowed to obtain micro-textures with large opening angles and smooth U-shape. The advantages of the micro-textures for nc-Si:H solar cells were systematic studied by using three different textured substrates: (i) nano-textured ZnO:Al (AZO) on flat glass, (ii) micro-textured glass coated with as-deposited ZnO:Al, and (iii) modulated surface textured substrate by superposing nano-textured ZnO:Al on micro-textured glass.
The micro-textured substrates result in higher Voc and FF than nano-textured substrates. For thick solar cells, high Voc and FF are maintained. Particularly, the Voc only drops from 564 to 541mV as solar cell thickness increases from 1 to 5 mu;m. The improvement in electrical performance of solar cells is ascribed to the growth of dense nc-Si:H layers free from defective filaments on micro-textured substrates. Thereby, micromorph tandem solar cells with an initial efficiency of 13.3%, Voc = 1.464 mV and FF = 0.759 are obtained, indicating the high potential of micro-textures for high-efficiency multi-junction thin-film silicon solar cells.

Light trapping is an important issue for hydrogenated amorphous silicon (a-Si:H) thin-film solar cells. Texturing the back reflector by a periodic structure has emerged as an effective method to improve the cell performance, because the periodic back reflector can couple the incident light into guided modes and Bloch-wave surface plasmon polaritons (BW-SPPs) which can propagate along the absorber layer. In this study, an anti-ring array, a novel nanostructure, was fabricated by monolayer of polymer/nanosphere hybrid and applied as a back reflector in substrate-type a-Si:H thin-film solar cells.
The anti-ring structure comprises a nanodome centered inside a nanohole. Several dips were observed in the specular reflectance spectra of Ag-coated anti-ring array. These dips were caused by BW-SPPs and the corresponding wavelengths were related to the period of the nanostructure. When a period of 1000 nm was used, the cell exhibits a Voc of 0.81 V, a Jsc of 11.11 mA/cm², a FF of 66.01% and a power conversion efficiency of 5.94%, which is 39% improvement in comparison with that of the cell fabricated on a flat back reflector. Compared to the flat counterpart, 17.9% enhancement in Jsc and 39% enhancement in power conversion efficiency are obtained without degradation of the FF. The enhancement can be attributed to the BW-SPPs effect and the diffuse scattering of light.
We also found that the cell fabricated on an anti-ring back reflector outperform that fabricated on a nanohole back reflector (i.e. without center nanodome) when a period of 1000 nm was used. The angle-resolved reflectance spectrum shows that the Ag-coated anti-ring array can scatter more light into large angle than the Ag-coated nanohole array, which is consistent with the simulation result based on finite-difference time-domain (FDTD) method.

Nano-textured “black” silicon (b-Si) exhibits a reflectivity of only a few percent in the visible range as well as excellent light scattering properties [1]. It is therefore especially well-suited to enhance the short circuit current (jsc) of silicon solar cell concepts like silicon hetero-junction (SHJ) solar cells on thin wafers [2] or polycrystalline (poly-Si) thin film solar cells [3]. However the nano-textured surface is prone to high defect densities and thus a major challenge is the development of suitable passivation layers. Up to now solar cell concepts comprising b-Si rely on surface passivation with Al2O3 [4], or thermal silicon oxides [5]. Such dielectric passivation layers must be opened locally for contact formation. An alternative but up to now unexplored route is the passivation with hydrogenated amorphous silicon. This approach is advantageous since passivation and contacting of the nanostructured surface can be achieved with a single layer and furthermore the amorphous-crystalline SHJs excellent passivation enables the highest open circuit voltages of any silicon wafer-based technology [2].
In this communication we present nano-textured amorphous-crystalline silicon hetero-junctions which exhibit excellent passivation quality and low reflection values. Minority carrier lifetimes above 1.3 ms, and implied open circuit voltage above 700 mV on nano-textured silicon surfaces with reflectivity below 5 % are reached using plasma-enhanced chemical vapor deposition of approximately 5 to 6 nm thick intrinsic amorphous silicon layers for the passivation of b-Si.
Nano-textured silicon has been implemented in wafer-based and thin film amorphous-crystalline SHJ solar cells. For both cell types the reflectivity was strongly reduced and an effective reflectivity below 5% was obtained. For the thin film-based solar cell an increase in jsc by 30% has been obtained. For wafer-based solar cell jsc was increased as compared to a planar reference, however due to inferior blue response jsc is lower than for a conventional textured reference. Numerical simulations indicate that this loss is due to a photo inactive region inside the nanotexture. Furthermore these early prototype cells experience a drop of the implied voltage during cell processing, which leads to a final open circuit voltage of 614 mV. Options to preserve the excellent initial passivation of the intrinsic amorphous silicon layers during the whole process and to improve the poor blue response of these solar cells will be discussed. Utilizing these improvements may open a viable route towards high-efficiency b-Si solar cells with thin absorbers.
[1] S. Koynov et al., Appl. Phys. Lett. 88 (2006) 203107
[2] M. Taguchi et al., IEEE J. of Photovoltaics (2013) in press
[3] D. Amkreutz et al., Prog. Photovolt: Res. Appl 19 (2011) 937
[4] P. Repo et al., Energy Procedia 38 (2013) 866-871
[5] J. Oh et al., Nature Nanotechn. 7 (2012) 743-748

Poor internal reflection of IR light at the lossy metal rear reflector of crystalline silicon solar cells limits the spectral response of the cells near the silicon bandgap, even for highly reflective metals. Here, we present a thorough experimental and theoretical investigation of the optical properties of Si/dielectric/metal structures that are representative of the rear of rear-passivated crystalline silicon solar cells, and extend the analysis to “absorbing dielectrics” that represent the rear transparent conductive oxide (TCO) layer in silicon heterojunction solar cells. By calculating the electric field intensity at the surface of the metal, we find that for thin dielectric layers, p-polarized light arriving at the back surface above the Si/dielectric critical angle is strongly absorbed in plasmonic modes that cause significant internal reflection losses. We employ a ray tracer to calculate the total reflectance of a solar cell with random pyramids, using as input the internal reflectance values as a function of incident angle determined from the field intensities. The results reveal that—consistent with our measurements—dielectric layers at least 150 nm thick minimize absorption in the metal reflector; the rear passivation layers in solar cells can thus serve an optical as well as electrical role if properly designed. Finally, we demonstrate this in silicon heterojunction solar cells by measuring record IR internal quantum efficiency with a thick, low-refractive index dielectric buffer layer sandwiched between the rear of the cell and the metal reflector.

Improving the conversion efficiency of thin-film silicon solar cells is a delicate interplay between spectral utilization, materials processing and light management. For enhancing the photo-current density generated by the solar cell, the spectral absorptance of the absorber layer must be enlarged. This means that the spectral losses due to reflectance and supporting layers have to be minimized. In this respect, light management techniques play an important role. In particular, combining light scattering at textured interfaces with efficient rear reflector enables enhanced light coupling in the absorber layers.
In this contribution we report our recent studies on light management applied to nano-crystalline silicon solar cells. We analysed the effect of decoupled texturization between front and rear side on the absorptance of the absorber layer. At the front side, we considered high aspect ratio pyramidal features for anti-reflective effect and enhanced light in-coupling, while at the rear side we took into account shallower pyramidal features for effective light scattering. The novelty of our study lies in the optical modelling of the complete p-i-n solar cell structure based on nc-Si:H and in the analysis of its spectral performance by means of the excited wave-guided modes. A 3-D Maxwell solver based on finite element method was used for the simulation of absorptance and reflectance spectra, while an iterative method based on the calculation of poles of the Fresnel coefficients was deployed for sampling the excited wave-guided modes.
Our study comprised three simulation phases, in which the thickness of the intrinsic nc-Si:H layer was kept constant to 2 µm. Firstly, an ample parameters space was investigated. Varying period, height and duty cycle of both front and back side textures, we could rank the three most promising structures to be used later on. In the second phase, we especially focussed on the metallo-oxide interface at the back side. Regardless the simulated geometrical structure, we found that the insertion of doped layers based on nc-SiO_x:H with the concurrent usage of an appropriate back transparent conductive oxide (TCO) resulted in the minimization of plasmonic losses in the silver rear reflector. In the third phase we finally optimized the thickness of the front TCO.
Our best simulated solar cell structure showed an implied photo-generated current density equal to 35.65 mA/cm², which is +3.40% higher than the value predicted by Tiedje-Yablonovitch limit calculated for the same thickness and in the wavelength range between 300 nm and 1200 nm. This enhancement was ascribed to the optimized decoupled front and back side texturization, which increased the light in-coupling at long wavelengths. In fact, by sampling the resonance peaks of the nc-Si:H absorptance on a dispersion-relation diagram, we found that all of them were related to wave-guided modes concurrently excited by the front and the back textures.

Widespread implementation of silicon-based photovoltaic (PV) technology is still limited in no small measure by high materials costs. One interesting approach to reduce materials consumption is shown in recent reports from our group by utilizing thin, ribbon-like, micro-scale Si solar cells, devices generated using precise methods of micromachining Si(111) wafers in conjunction with transfer-printing techniques that allow their deterministic assembly on secondary substrates. The design of our earliest generations of microcells, however, requires crucial improvements to address fundamental criteria for optimal carrier collection efficiency, as a passivation layer (e.g., thermal oxide, silicon nitride) is not incorporated, leading to substantial losses from surface recombination.
In this presentation, we describe a simple, yet robust approach for fabrication of silicon solar microcells that integrates passivation. By incorporating a thermal oxide as etching/diffusion mask, which also doubles as an effective electrical passivation and anti-reflection layer, we demonstrate significant advance in microcell design that enhances process reliability, energy conversion efficiencies, and modes of module assembly. We report a best cell efficiency of 11.7% under an AM1.5D solar spectrum for an optically thin (30 µm thick) device measured on a non-reflective substrate, which is a substantial improvement over previously reported results. External quantum efficiency measurements specifically show a marked improvement in the blue response that results from mitigating losses due to surface recombination in these high-surface-area, micro-scale devices.
Light management strategies explored for these microcells include integrating backside reflectors (BSR) and luminescent materials, which redirect light to the otherwise unilluminated sidewall and bottom surfaces of these thin devices to compensate the inherent low optical absorption. Utilizing a diffuse BSR for a device embedded in a transparent waveguiding polymer matrix would double its power output (The efficiency in this scenario reaches around 20%). Additionally, doping the polymer matrix with fluorescent inorganic nanorods (CdSe/CdS) or organic dye molecules (DCM), which absorbs incident light and emits into total internal reflection (TIR) modes inside the waveguide, boosts concentration ratio to around 3 and 4 (both with a diffuse BSR), respectively, furthering lowering down the power-referenced materials consumption of these lightweight devices.

Utilizing thin (~50 mu;m) crystalline Si (c-Si) has recently been a promising candidate for reducing material costs in solar cell applications. Insufficient light absorption by employing a thin c-Si wafer requires a nanostructured surface for reducing optical reflectance as low as 5 % in solar spectrum. However, surface evolution of higher-index crystalline planes as well as a higher surface-to-volume ratio of c-Si seriously degrade the internal quantum efficiency (IQE) especially for a blue region (wavelengths of 400~600 nm) due to the increased surface recombination.
We present that the OH-terminated surface of nanostructured (nanoholes) Si improved the passivation performance of atomic-layer-deposited (ALD) Al2O3 in comparison to a conventional H-terminated surface, which then resulted in the improvement in blue response. The OH-termination of nanostructured surface prior to ALD Al2O3 process has been carried out by inserting O2 plasma followed by HF treatment. A decreased amount of dangling bonds as well as the improved quality of interfacial SiOx between Al2O3 and Si were confirmed using Raman spectroscopy. Surface recombination velocity (SRV, cm/s) extracted by the effective carrier lifetime (mu;s) also revealed a superior performance of the OH-terminated surfaces on the nanostructured silicon. The improvement in SRV results was developed more evidently with increasing the depths of nanoholes from 300 to 800 nm. As a result, our new approach improved the IQE in wavelengths of 300~470 nm so that the short circuit current density of 21.5 mA/cm2 was obtained, which was 11.4 % higher than that of conventional H-terminated surface.

Enhancing light absorption in ultrathin film silicon solar cells is important for reducing costs and improving performance. In this work, we integrate a genetic algorithm with finite difference time domain simulations to determine the optimum silicon nanostructure with maximum solar absorption for some fixed equivalent thickness. Different single-sided (top or bottom) and double-sided gratings structures are evaluated with building blocks of nanowires, nanoholes, nanocones, and tapered nanohole structures. We compare the performance of these structures to that of a thin film with an optical path length of twice the thickness as well as the Lambertian limit. We find that double-sided grating structures improve light absorption over the entire solar spectrum compared to single-sided structures. This work demonstrates the ability of the genetic algorithm optimization technique to quickly search through a large parameter space to determine nanostructures with maximum solar absorption.

Although high-efficiency (~14-15%) a-Si:H/mu;c-Si:H tandem solar cells have been demonstrated in the initial state, the stabilized efficiencies of such devices after long-term illumination are limited to ~12% due to the light-induced degradation of a-Si:H, known as the Staebler-Wronski effect [1]. Therefore, the suppression of the light-induced degradation is crucial in order to make significant progress of thin-film silicon solar cells.
In our previous studies [2, 3], we have demonstrated that high-efficiency and low-degradation a-Si:H solar cells can be obtained when the a-Si:H absorber layer is deposited by a remote plasma process using a triode PECVD technique. Although the deposition rate is relatively low (0.01-0.03 nm/s) compared to the conventional diode-type PECVD process (~0.2 nm/s), the light-induced degradation in conversion efficiency (Δeta;/eta;_ini) of single-junction solar cell is substantially reduced (e.g., Δeta;/eta;_ini~11% and ~19% for the cells deposited by triode and diode PECVD, respectively, at an absorber thickness of 250 nm). As a result, a stabilized efficiency of 9.6% has been attained using a commercially-available TCO substrate.
In this contribution, we report on the further progress of a-Si:H single-junction solar cells realized by optimizing whole device design. We show that stabilized efficiencies of as high as 10% can be attained even when the solar cell is thickened up to ~400 nm. Results of the material characterization such as microstructure parameters and light-induced metastable defects in the a-Si:H layers and devices are also presented.
[1] D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977) . [2] S. Shimizu et al., J. Appl. Phys. 97, 033522 (2005). [3] T. Matsui et al., Prog. Photovolt: Res. Appl. 21, 1363 (2013).

Light-induced degradation (LID) of amorphous silicon (a-Si:H) solar cells due to the Staebler-Wronski effect has been widely discussed in literature. However, it has been most often discussed with respect to the degradation of the intrinsic absorber layer.
In the present study, LID of a-Si:H solar cells is studied with respect to the amorphous silicon carbide (p-(a-SiC:H)) layer that is part of the window layer of high efficiency solar cells. We have deposited solar cell series varying the p-(a-SiC:H) thickness and the substrate roughness of single junction solar cells in superstrate configuration. The solar cell design is state-of-the-art using low-pressure chemical vapor deposition zinc-oxide for front and back contacts that are in contact with p- and n-doped silicon oxide layers. Plasma-enhanced chemical vapor deposition (PECVD, 40 and 13 MHz) has been used for all silicon layers, using a cluster tool with dedicated chambers for p-doped, intrinsic, and n-doped layers.
During light soaking, a systematic open-circuit voltage (Voc) increase could be observed for thin p-layers, while Voc decreases for thick p-layers. This effect is more pronounced for rough than for smooth substrates: The critical p-(a-SiC:H) thickness, at which light soaking has no effect on the Voc, increases with increasing substrate roughness. These Voc changes have a strong impact on the conversion efficiency of the solar cells. First, the optimum p-(a-SiC:H) thickness depends on the substrate roughness. Second, highest stabilized cell efficiencies are obtained using thinner p-(a-SiC:H) layers than what is optimum in initial state. Different contributions of short-circuit current, fill factor, and Voc to LID of the conversion efficiency are discussed. All trends could be reproduced using different cell designs in three different PECVD systems.
To discriminate the effect of effective p-layer thickness on rough substrates, the nominal thicknesses are corrected by the effective surface as determined from AFM measurements.
Different mechanisms could lead to the observed Voc changes. These are investigated by bias light and bias voltage dependent EQE measurements and by analyzing the degradation /annealing kinetics of the solar cells. The changes are related to layer properties as measured by ellipsometry, photothermal deflection spectroscopy, and conductivity.
Finally, we will briefly discuss our latest tandem and triple junction solar cells where we incorporated these a-Si:H cells as top cells.

The a-Si:H solar cells having high open-circuit voltage (Voc>950 mV) are highly desirable for the top junction in thin-film silicon based tandem and triple-junction solar cells. Except for the high Voc, the top cell should also have high spectral response (between 350-600 nm wavelength range) to allow thinner absorber layer in order to reduce the light-induced degradation. Therefore, doped layers with high transparency are required. Commonly the a-Si:H solar cells deploy a-SiC:H as p-layer and nc-Si:H or a-Si:H as n-layer. Those doped layers will inevitably lead to high parasitic absorption losses, and thus make it difficult to achieve sufficient photocurrent with a thin absorber layer. In this contribution, we will first discuss how to achieve high Voc by processing a-Si:H at high-pressure and high-power regime. Then deployment of highly transparent silicon oxide (SiOx) doped layers will be discussed to obtain better spectral response (or external quantum efficiency) and higher Voc than the conventional doped layers. Specifically, following key points will be presented in the conference.
1. The a-Si:H is deposited at high-pressure (>5 mbar ) and high-power (>0.1 W/cm2) regime, which results in larger bandgap than materials commonly processed at low-pressure and low-power regime. The bandgap of a-Si:H can be tuned by H2/SiH4 dilution, power and pressure. High performance device-grade a-Si:H can be obtained over wide deposition window, in contrast to the narrow window at low-pressure regime.
2. Highly transparent p-SiOx:H with sufficient conduction is investigated. Firsly, it should have good ohmic contact with front ZnO TCO. Second, the control of crystallinity of p-SiOx:H is a critical point to obtain high Voc for a particular absorber layer. Finally, a very thin layer of i-SiOx:H inserted in the p/i interface can significantly reduce the boron diffusion during the deposition of i-layer, and thus considerably improve the blue spectral response. Consequently, EQE higher than 70% at lambda;=400 nm is achievable.
3. Low absorption n-SiOx:H to replace absorptive n-aSi:H is necessary to achieve high spectral response over 500-700 nm wavelength range. Furthermore, the low refractive-index n-SiOx:H layer can also function as intermediate reflection layer in multijunction devices. Control of the i/n interface is crucial to achieve high FF. The insertion of an ultra-thin (<3 nm) n-aSi:H or n-aSiOx layer can significantly increase the FF of solar cells and result in FF comparable to cells with a-Si:H n-layer, without reduction of spectral response compared to single n-SiOx layer.
After optimization of the p-SiOx:H and n-SiOx:H doped layers, a-Si:H solar cells with high Voc, high FF and excellent spectral response is obtained (Voc>960 mV, FF>74%, and efficiency>10%). The light-induced degradation of solar cells with SiOx:H doped layers are investigated, and will be compared to the solar cells with conventional doped layers.

The manufacturing cost of thin film Si based tandem and triple junction cells and modules is at present too high to meet current module market prices. Conventionally, microcrystalline silicon is used as the low-bandgap absorber in micromorph solar cells (a-Si/µc-Si tandem cells). However, due to the considerable thickness needed for the µc-Si:H absorber, it takes three to four times as many deposition reactors compared to single junction cells to produce tandem cells, leading to high cost of ownership. One of the approaches to reduce processing time of the low-bandgap layer(s) in multijunction silicon-based solar cells is the use of hydrogenated amorphous silicon germanium (a-SiGe:H). In general however, a-SiGe:H has not been considered a viable option because of (i) the high defect density for PECVD a-SiGe:H, at band gaps < 1.4 eV, and (ii) the cost of GeH4. On the other hand, due to its direct gap nature, the thickness of an a-SiGe:H absorber layer can be kept 10 times smaller than that of µc Si:H.
We are investigating whether a-SiGe:H can be reconsidered for inexpensive production of multijunction thin film Si based solar cells if HWCVD is used as the deposition method. HWCVD is a simple and low cost deposition technique allowing high deposition rates while maintaining good defect passivation. Early results reported by NREL include the achievement of material with a band gap close to that of µc Si:H (1.2 eV) with an equivalent photoresponse (in excess of two orders of magnitude). Their work has led to 8.64% single junction cells without any band-gap profiling in the absorber layer.
We now continued this development to provide a novel thin film alloy for the struggling micromorph technology. We have produced a-SiGe:H materials with Tauc band gaps ranging from 1.6 eV down to 1.2 eV. Due to the efficient dissociation of silane and germane gases at the hot filament, a high deposition rate is achieved. Moreover, the dissociation rate of germane is three times faster than that of silane. The deposition rate for the lowest-gap material is 0.7 nm/s and is always higher than 0.5 nm/s. With this deposition rate, an active absorber layer (i-layer) of 150 nm is readily deposited within 5 minutes. This should be compared to the roughly one-hour long deposition time needed to deposit a 2-µm thick µc Si:H film at the commonly used deposition rate of 0.5 nm/s. Using a GeH4/SiH4 ratio of 1, we deduce from Raman spectroscopy that the films already contain 60-70% Ge, showing that Ge is preferentially incorporated in the film. We will report an extensive microstructure analysis and will present our first cells.

In thin-film silicon solar cells, the transport of photoexcited carriers in the active layer plays an essential role in device operation. Thus, improving the carrier transport directly leads to higher device performance. So far, the carrier transport was extensively studied for as-grown films at room temperature. Nevertheless, it is not well studied during growth. Here, we characterize the transport of photoexcited carriers during growth for further understanding the growth process and improving the device performance.
We performed in-situ measurement of the photocurrent during a-Si:H growth by plasma-enhanced chemical vapor deposition [1]. The device-grade intrinsic a-Si:H film growing on a glass substrate was illuminated with the intensity modulated probe laser. The photocurrent excited by this illumination was collected by dc biased interdigit contacts on the glass substrate. Because the photocurrent was superimposed on various other currents such as plasma associated currents and a leakage current, we used a lock-in technique to distinguish it. The measurement setup was placed in 60 MHz very-high-frequency discharge in a parallel plate configuration [2].
We found that during the growth, the photocurrent gradually increased as the film grew with time. Such a time evolution of the photocurrent indicates the existence of a defect-rich surface layer in the early stage of growth and the formation and growth of a bulk layer at a later stage. We also found a significant improvement of the photoconductivity after the growth, i.e., during postgrowth annealing. The photoconductivity was increased by one order of magnitude at the temperatures of 433 - 513 K. The characteristic time for the postgrowth annealing was obtained to be of the order of 100 - 10000 s, depending on the temperature.
[1] S. Nunomura, I. Sakata and M. Kondo, Appl. Phys. Express. 6, 126201 (2013). [2] S. Nunomura, I. Yoshida and M. Kondo, Appl. Phys. Lett. 94, 071502 (2009).

Silicon dangling bonds (db's) are the dominant defects in hydrogenated amorphous silicon (a-Si:H). Their creation by light is of great importance for the performance of a-Si:H based solar cells. Yet, the underlying processes are not understood in detail. Not even the structural and energetic characteristics of the defect - a prerequisite for modeling - are well established due to the variety of possible realizations of a db defect the amorphous network. To make any progress, theoretical models must be developed that reproduce the statistical distribution of the experimental findings.
Within the German EPR Solar project, we have calculated electron paramagnetic resonance (EPR) parameters of dangling bonds obtained by removing a hydrogen atom from a random Si-H bond. These calculations agree well with experiment on the g-tensor distribution. However, the experimentally observed red-shift in the Si hyperfine couplings compared to dangling bonds in a crystalline environment cannot be fully reproduced [1]. This indicates that the theoretical defect ensemble deviates from the experimental one, either due to an unknown selection mechanism in experiment, or due to an unintended bias in theory. One such bias might be the dangling-bond creation by H abstraction.
In my talk, I will compare the H-abstraction ensemble to an alternative set of defects created by inserting H into a Si-Si bond. The obtained models fall into two classes. In the first one, H indeed breaks the bond by attaching itself to one of the Si atoms, thereby creating a dangling bond at the other atom or -- after network reconstruction -- in a different place in the network. These dangling-bond realizations confirm our previous conclusion that the amorphous network has an intrinsic propensity to form dangling-bond defects upon perturbation of the network [2]. In the other cases, H assumes a bond-center position in a +1 charge state. The additional spin-polarized electron is then trapped at a suitable site nearby, without breaking any bonds. The calculated EPR parameters for the latter are distinctly different.
[1] Phys. Rev. B 84, 193304 (2011).
[2] Phys. Rev. B 87, 125308 (2013).

Hydrogenated amorphous silicon (a Si:H) is a semiconductor with important applications in a host of large-area electronic devices, including solar cells. Characteristically, a-Si:H bulk material suffers from the so-called Staebler-Wronski effect (SWE). This is manifested by (self-limiting) light-induced generation of electronically-active defects, most likely in the form of Si dangling bonds. The increase in Si dangling bond density is detrimental for device performance. The microscopic origin of this defect is still under debate, however. In recent years, the interface between a Si:H and crystalline silicon (c-Si) has received increased attention because a Si:H films passivate c Si surfaces remarkably well. For atomically-sharp interfaces, lowered recombination occurs by passivation of c-Si surface states, and near-surface a-Si:H bulk defects. In recent work, we confirmed that light-induced degradation (LID) also occurs in a Si:H/c-Si structures by dangling-bond creation.
Here, we report on repeated low-temperature annealing and visible-light soaking of thin hydrogenated amorphous silicon films deposited on crystalline silicon surfaces. We observe that during annealing the electronic improvement of the interfaces follows stretched exponentials as long as hydrogen evolution in the films can be detected. Once such evolution is no longer measured, the electronic improvement occurs much faster. Based on these findings, we propose a model linking native and light-induced defects in hydrogenated amorphous silicon and discuss the reversibility of light-induced defect creation in this material.

We present time-resolved terahertz and femtosecond near-infrared transient absorption studies of photocarrier dynamics in PECVD thin film a-Si:H and a-Si1-xGex:H, revealing carrier trapping and dispersive transport characteristics on ultrafast time scales. Carriers were photogenerated in the extended electronic states using optical pulses 35 fs in duration generated by an amplified 1 kHz Ti:S laser system, and the induced absorbance associated with the carrier distribution was measured at pump-probe time delays extending to 300 ps. Far-infrared measurements were carried out with single-cycle THz probe pulses synchronously generated by optical rectification in ZnTe and detected by electro-optic sampling, and measurements at sub-gap near-infrared wavelengths were carried out using a compressed white-light continuum. The THz measurements, which effectively provide a non-contact probe of the photoconductivity through the relation between the photocarrier absorption and the conductivity, are highly sensitive to carrier localization, allowing us to observe the carrier trapping dynamics.
We observe significant differences in the response probed in the two spectral regions. Following a fast subpicosecond transient associated with the initial carrier relaxation, measurements at near-infrared wavelengths show highly nonexponential decay dynamics that are strongly dependent on initial carrier density. Time-resolved THz measurements yield a fast relaxation response on a time scale of picoseconds, with a much weaker dependence of the overall decay transient on initial carrier density. We present modeling of these results in terms of contributions from time-dependent populations of free (high-mobility) carriers and trapped (low-mobility) carriers. The effects of carrier localization are modeled as multiple trapping into low-mobility band tail states with an exponential distribution of binding energies, a mechanism that yields an effective time-dependent mobility that decays as a power law, reflecting the free carrier population. Bimolecular recombination is assumed to occur with a diffusion-limited rate that includes the effective time-dependent mobility associated with the multiple trapping process. We find that this model successfully fits the observed time and excitation density dependences of both the THz and the near-infrared responses, allowing us to determine both the carrier trapping dynamics and the carrier recombination dynamics.
This work was supported by the National Science Foundation under grants DMR-1106379 and DMR-0706407. We thank Brent Nelson (NREL) for preparing the thin film samples used in these studies.

Amorphous hydrogenated silicon carbide (a-SiC:H) and silicon-carbon alloys are of significant interest for a number of interesting applications including microelectronic, optoelectronic, MEM/NEM, and biomedical devices due to it&’s large bandgap (2-3 eV), high oxidation resistance, high Young&’s modulus and hardness, and biocompatibility. Most recently, plasma deposited a-SiC:H has garnered additional interest as a potential low dielectric constant (low-k) material due to the ability to dramatically reduce k through the introduction of significant amounts of nano-porosity through careful control of hydrogen and terminal methyl group content in the as deposited films. As we will demonstrate in this report, the ability to precisely tune the hydrogen/terminal methyl group content in a-SiC:H allows a remarkable range of material properties to be observed that can be concisely explained using the Phillips-Thorpe Bond Constraint-Percolation theory originally developed for oxide and chalcogenide glasses. We will specifically demonstrate that a remarkable range in dielectric constant (< 3 - > 7), Young&’s Modulus (< 5 - > 200 GPa), and thermal conductivity (0.09 - 4 W/mK) can be achieved in plasma deposited a-SiC:H films and that the range of observed properties is directly related to the average network and bond coordination of the films. We will additionally demonstrate how critical inflections points in the observed structure-property relationships can be easily explained using the Phillips-Thorpe Constraint Theory.

Rapid thermal processing is widely used manufacturing process in the photovoltaic (PV) industry. However, the processing parameters have evolved empirically over time, mainly due to lack of understanding about the actual phase formation mechanisms during the processing which occurs in short time scales of few seconds to minutes. For example, the Ag-Si contact formation begins with printing a mixture of an Ag powder, glass frit (mixture of metal oxide such as PbO, B₂O₃, ZnO and Bi₂O₃) and an organic binder over the antireflection coating which is subsequently fired up to about 800 °C. It is known that the frit allows the paste to react with and burn through the anti-reflective coating such that the metal can react with underlying c-Si during firing. However, the precise phase transformations between Ag, Si, SiNx, and frit constituents, which happens within few seconds (typically <10 s) during RTP, giving rise to optimal Ag-Si contacts are not well understood. While there are several proposed mechanisms for Ag-Si cell contact formation during rapid thermal processing, there is no in-situ characterization in the actual processing conditions.
We have established a rapid thermal processing/X-ray diffraction/fluorescence (RTP/XRD-XRF) facility, where we are able to monitor and characterize the Ag-Si cell contact formation with a time resolution of a fraction of a second. The facility utilizes the intense synchrotron X-ray source to gather structural and chemical information while material is being processed. We utilize a large fast area detector with few ms time resolution to gather a large solid angle diffracted beams, while an energy dispersive vortex detector for in-situ chemical analysis.

Crystalline Silicon based photovoltaics continues to be the dominant technology for large scale deployment of solar energy. While impressive cost gains in Silicon based PV have come with scale, there remains a strong push for increased efficiencies and further lowering of manufacturing costs. However, so far there has not been a production proven approach that reduces the cost while maintaining or increasing the efficiency. Attempts to reduce the amount of silicon used, for example, have let to development of various kerfless wafer manufacturing approaches. While some of these approaches have shown the potential for reduced costs, they have also compromised on the efficiency mainly because of the inferior quality of the material.
Epitaxy based kerfless silicon wafers has shown the potential to reverse this trend offering lower manufacturing costs while maintaining or even enhancing the efficiency. We will present key aspects of Crystal Solar&’s patented high througput production silicon epitaxial reactor and demonstrate how such a reactor has enabled high efficiency n and p type cells with standard thickness wafers. We also will show results on thin silicon epitaxy (less than 50 microns) and discuss means for handling such thin wafers through the cell and module manufacturing. We further demonstrate how both the thick and the thin wafers grown by epitaxy enable manufacturing costs significantly below $0.50/Wp for high efficiency single crystal photovoltaic modules.

Thin mono-Si, grown by vapor phase epitaxy, offers numerous advantages making it a potentially high-efficiency, low-cost, manufacturing technology. In this method, a crystalline Si film is grown on a monocrystalline reusable temporary-substrate that has a porous surface of suitably tailored characteristics. The epitaxial film (of appropriate thickness) is separated to become a free-standing wafer for solar cell fabrication. Alternately, cell fabrication is completed while the Si film is attached to the temporary substrate, followed by separation of the cell from the substrate. The advantages of this technology include: (i) high quality mono-Si is obtained directly from the gas phase, (ii) no kerf/cutting losses, (iii) low oxygen (no light induced degradation in P type due to B-O defects and lower manufacturing cost. However, with this approach there are certain crystal defects and impurities that could impact the final performance of the solar cell product, which are not yet completely understood. Although cell efficiencies comparable to mainstream silicon PV are possible, it is expected that understanding the mechanisms of the specific defects, impurities, and impurity-defect interactions can lead to their mitigation with a concomitant increase in the cell performance. Hence, we have begun a study to determine the mechanism(s) that limit the efficiency of current cells and establish approaches to overcome these limitations. In this paper, we will describe the nature of defects and impurities in the epitaxial Si layer and their influence on the cell efficiency. Our study has shown that defect generation mechanisms are of two types: (i) Type A- interface defects that originate/nucleate from factors such as surface cleanliness, quality of the porous Si at the surface, and factors related to the nucleation kinetics at the initial growth, and (ii)Type B- propagation of the interface defects and generation of bulk defects through thermal stress. Type A defects are predominantly stacking faults (SF), while type B are primarily dislocations. Analyses of the solar cells have revealed an interesting behavior — in spite of the fact that SF density is low, they can have a controlling effect on the solar cell performance. We will discuss mechanisms of defect generation/propagation, and discuss approaches of mitigation.

Silicon has been driving the great success of semiconductor industry, and emerging forms of silicon have generated new opportunities in electronics, biotechnology, and energy applications. Here we demonstrate large-area free-standing ultrathin single-crystalline Si at the wafer scale as new Si materials with processability. We fabricated them by KOH etching of the Si wafer and show their uniform thickness from 10 to sub-2 mu;m. These ultrathin Si exhibits excellent mechanical flexibility and bendability more than those with 20-30 mu;m thickness in previous study. Unexpectedly, these ultrathin Si materials can be cut with scissors like a piece of paper, and they are robust during various regular fabrication processings including tweezer handling, spin coating, patterning, doping, wet and dry etching, annealing, and metal deposition. We demonstrate the fabrication of planar and double-sided nanocone solar cells and highlight that the processability on both sides of surface together with the interesting property of these free-standing ultrathin Si materials opens up exciting opportunities to generate novel functional devices different from the existing approaches.

Silicon CMOS technology is approaching the end of scaling due to unavoidable physical limitations. Still, we have to find out a new way to utilize its huge intellectual, human, and production resources. Large-area electronics have been evolving on the basis of amorphous silicon, organic, and oxide semiconductor materials and low-temperature fabrication technologies such as solution-based process and printing technique. However, these devices have serious problems originated from their low electrical performance, high operating voltage, and poor reliability under operation. In order to implement silicon technology on foreign substrates, transfer of single-crystalline silicon to glass has been attempted on the basis of conventional wafer bonding approaches. However, these techniques require high process temperature and cannot solve high cost of production issues when applied to large area. In this work, we propose a novel low-temperature local layer transfer technique using meniscus force, and n-channel MOSFET fabrication on glass will be demonstrated.
In order to form “midair cavity”, a 100-nm-thick (100)-oriented SOI layer was patterned to a channel shape and BOX layer was etched by HF solution until fine SiO2 columns were left underneath the source and drain regions. The size of residual SiO2 columns was controlled to less than 2 mu;m by etching time. The wafer and a counter substrate were placed in close face-to-face contact with filling water, and the samples were heated at 80 oC on a hot plate for 15 min. As the water evaporates through the midair cavity, capillary bridges are formed in between the SOI layer and the counter substrate and the meniscus force (F = 2πR^2γcostheta;E/H (H << R)) generates in capillary bridges. The meniscus force rapidly increases with decreasing the height of capillary bridge (H). Eventually, the SOI layer is transferred to the counter substrate when removing the wafer. The key process of MOSFET fabrication is the thermal oxidation of SOI layer on midair cavity. The thermal oxidation and adjacent H2 anneal ensure good MOS interface and the SiO2 layer work as the blocking layer of contamination from glass. When the channel length (L) was as short as 5 mu;m, no significant bending of SOI layer due to oxidation strain was observed and the layer transfer to glass was carried out without any problem. After meniscus force-mediated layer transfer (MLT) of SOI, additional gate SiO2 deposition and metallization were performed at the maximum temperature of 300 oC. From Id-Vg characteristic of n-channel MOSFET fabricated on glass, the transistor showed very high mobility of 742 cm^2V-1s-1, low threshold voltage of 1.5 V. These values are much better compared to those of the transistors fabricated by conventional materials. These results suggest that the proposed MLT technique and MOSFET fabrication process opens up a new field of silicon applications that is independent of scaling.

The talk focuses on synthesis and device applications of crystalline Ge_1-ySn_y and Ge_1-x-ySi_xSn_y alloys grown on Si and Ge via designer CVD routes. The Ge_1-ySn_y alloys are found to produce strong electroluminescence and tunable photoluminescence near the Sn composition threshold of the direct-indirect band gap crossover. The IR coverage provided by Ge_1-ySn_y photodiodes extends well beyond that of elemental Ge into the broader telecom, range thereby offering an attractive alternative to current Ge technologies at 1.55 µm. The Ge_1-x-ySi_xSn_y alloys represent the first viable ternary semiconductor among group IV elements with independently tunable lattice parameter and electronic structure. Photodiodes of these materials are shown to exhibit precisely adjustable absorption edges (0.80-1.1 eV) and state-of-the-art dark current densities for Ge-based group IV systems. It is shown that the optical emission properties in both systems are dominated by direct transitions exhibiting a non-linear compositional dependence. The relative position of the direct and indirect edges suggests a variety of applications in Si-based photonics, including photovoltaics.
The talk also introduces a novel synthetic approach to a related class of semiconductors described by the general formula (IV)_5-2y(III-V)_y comprising of specifically designed tetrahedral structures based on a Si or Ge parent lattice incorporating III-V components. These materials offer an alternative approach for enhancing the optical capabilities of Si and Ge, potentially leading to the design of new optical devices including solar cells.

A new class of Si-(III-V) alloys that are single-phase and lattice matched to silicon have been recently demonstrated. [1] These materials are projected to have a larger bandgap than silicon making them uniquely suited for a top cell absorber in a tandem solar cell with silicon forming the bottom cell. These alloys can also be used as a bridge between silicon and III-V absorbers that have a large lattice mismatch. Here we report the optoelectronic characteristics of Si3AlP thin films grown lattice matched on Si and GaP. In agreement with the value predicted by density functional theory, [2] we find that this material has an indirect bandgap of 1.3 eV. In its current form Si3AlP has an extremely high concentration of n-type carriers ~1021 cm-3 and a mobility of ~6 cm2/V-s.
Si3AlP was epitaxially grown on Si (100) and GaP (100) substrates in an ultra-high vacuum chamber with a base pressure of ~10-11 Torr. Following organic-cleaning of the surfaces, the substrates were then mounted on a sample holder and loaded into the growth chamber via a load lock. The samples were outgassed (and native oxide of Si removed via heating) until the pressure in the system returned to the base pressure. The temperature of the substrate was then set to ~550°C, and the Al Knudsen cell was heated to provide the desired flux of atomic Al. Gaseous P(SiH3)3 precursor was introduced into the chamber through a manual leak valve to a pressure of 1x10-5 Torr resulting in epitaxial growth of Si3AlP. Structural and compositional analysis was carried out to ensure the epitaxial layers are lattice matched to substrates and have the correct stoichiometric ratios.
Spectroscopic ellipsometry (SE) was used to obtain complex dielectric function ε = ε1 + iε2 spectra. Initial attempts to model the SE data of Si3AlP on Si were fraught with uncertainty in properties of a native oxide layer. X-ray photoemission spectroscopy identified that this surface overlayer is less than 1-nm thick and consists of Si, Al, and O. The overlayer was removed by etching the surface with 4% HF, which led to more reliable SE model. The optical characteristics obtained from Si3AlP grown on Si match those obtained from Si3AlP grown on optically transparent GaP, and indicate an indirect bandgap of 1.3 eV. The absorption coefficient of Si3AlP is not as high as theoretically predicted [2] and is only marginally higher than that of Si. Sub-bandgap absorption is found to increase with decreasing energy indicating free-carrier absorption (FCA) that agrees with the Drude model. The high carrier concentration and low mobility deduced from FCA is also confirmed by Hall measurements that show an n-type characteristic. The low mobility is likely an outcome of scattering due to a high carrier concentration.
Funded by the U.S. DOE SETP, DE-AC36-08GO28308.
[1]Watkins, J. Am. Chem. Soc., vol. 133, p. 16212, 2011
[2] Yang, J. Am. Chem. Soc., vol. 134, p. 12653, 2012

Ge compounds (SiGe, GeSn) have interesting electrical and optical properties, which make them interesting for a wide variety of applications: high performance CMOS circuits, photovoltaics, photo detectors, MEMS, etc. Ge is relatively scarce in respect with Si, and therefore thin film deposition is preferred for large-scale applications of Ge containing compounds. GeSn has been predicted to exhibit carrier mobilities exceeding both that of Ge and Si. In addition, GeSn exhibits a direct band gap for Sn concentration of ±6.5%, and is therefore promising for optical applications [1].
However, epitaxial growth of GeSn on Si substrates poses several challenges: the limited solubility of Sn in Ge (0.5%), compositional fluctuations, Sn segregation and large lattice mismatch (>4%). It is critical to suppress these effects for obtaining high performance devices with GeSn layers.
Recently, we demonstrated single crystalline GeSn layers on Si(111) substrates by solid phase epitaxy (SPE) of amorphous GeSn layers with excellent structural quality [2]. This technique has an advantage in the realization of thin GeSn layers directly on Si. The layers show excellent physical properties as demonstrated by the fabrication of depletion-mode GeSn pMOSFETs on Si(111) using SPE of amorphous GeSn layers, TaN/Al2O3 metal-gate/high-k gate stacks, and Ni-based metal S/D contacts. The GeSn MOSFET devices show +100% improvement in hole mobility with respect to bulk Si and good transfer characteristics with On/Off ratio of ~100 for ultrathin (<10 nm) GeSn layers on Si.
Structural investigation by XRD and TEM showed the presence of twin defects in the GeSn layers after crystallization. These defects can have significant impact on the carrier mobility and it is therefore important to suppress the formation of twin defects during crystallization. The formation of twin defects is possible related to the presence of contaminants, in particular oxygen, at the interface of the crystalline substrate and amorphous layer.
In this work we present a method to effectively suppress the formation of twin defects in GeSn layers fabricated by SPE. Prior to amorphous GeSn deposition, we grow 2-3 monolayers of epitaxial Ge by molecular beam epitaxy on the Si substrate. This Ge buffer layer effectively cleans the surface of the Si substrate. Limiting the Ge buffer thickness below the critical thickness of relaxation (3 monolayers) prevents Ge island formation. Hence, deposition of amorphous GeSn on this Ge buffer layer still produces a smooth layer. It is confirmed by XRD and TEM that subsequent solid phase epitaxy at 500 degrees C leads to single crystalline GeSn without the abundance of twin defects. This result is expected to have significant impact on the physical properties of GeSn layers made by SPE and additionally on electronic and optoelectronic applications.
[1] S. Gupta et al., J. Appl. Phys. 113, 073707 (2013)
[2] R. R. Lieten et al., Appl. Phys. Lett. 102, 052106 (2013)

Even with the success of the III-V materials for single-junction and multi-junction solar cells, the photovoltaic industry remains dominated by silicon-based technology. However, silicon-based technology is rapidly maturing, and systematic improvements are being met with diminishing returns. Large changes in efficiency will likely be achieved by innovative approaches; however, the integration of any new technology must also account for the pervasive influence of silicon in existing industrial facilities. One solution is to integrate silicon technology into a tandem cell configuration. However, the most promising materials for tandem cell development (e.g. the III-V's) suffer from significant lattice mismatch with silicon. Additionally, the III-V's tend to rely on rare, expensive, and toxic elements, almost prohibitively so. To this end, we have focused on the development of a structurally analogous class of materials to the III-V's, the II-IV-V₂'s, with specific emphasis on ZnSiP₂.
Existing research on ZnSiP₂ and other II-IV-V₂ compounds is fragmented and rarely performed with photovoltaic applications in mind. Our initial focus is to confirm computational results and characterize the opto-electronic properties of ZnSiP₂. Theory indicates that ZnSiP₂ should have a direct, albeit symmetry forbidden, band gap of 2.1 eV. We have succeeded in synthesizing phase-pure single crystals of ZnSiP₂ in a zinc flux with yields consistently above 90%. Crystal growth is highly tolerant of initial stoichiometry, and there is preliminary evidence that some opto-electronic properties can be tuned by flux synthesis conditions. Crystals are needle-like, deep red, and range up to 1.5 cm in length. XRD confirms phase purity and indicates less than 1% lattice mismatch with silicon. PL and UV-Vis spectroscopy confirm a band gap of 1.8-2.0 eV. Preliminary results indicate that absorption is low, due in part to the symmetry forbidden transition at the direct gap. However, we are actively investigating methods to "unlock" the forbidden transition in pursuit of improved absorption.

Thermoelectric materials are used to convert waste heat into electrical energy. The efficiency of such materials is determined by the thermoelectric figure of merit ZT. While conventional bulk materials like Bi2Te3 or Sb2Te3 are suitable for room temperature applications, SiGe mixed crystals have shown a great potential for high temperature applications. According to theoretical predictions, nanostructured materials such as superlattices, nanowires or 0-3 nanocomposites should exhibit a thermal conductivity by orders of magnitude lower than bulk silicon, which leads to a higher figure of merit. An enhancement of ZT by nanostructuring may offer a new field of application for Si-based thin-film technology as a replacement for the conventional thermoelectric materials, which are problematic in respects of ecology and processing.
Periodic and aperiodic SiGe multilayers with stacks of m Si and n Si1 - xGex layers of different thicknesses and doping levels were grown by molecular beam epitaxy on (001) or (111) Si substrates. The influence of the layer thickness, the composition x and the stacking sequence on phonon propagation is investigated and related to recent theoretical models. A high ZT can be related to a decrease in the thermal conductivity as a result of the phonon scattering at interfaces. It has been found that an aperiodic stacking sequence may further reduce the thermal conductivity. Theoretical models such as the Anderson localization of phonons are discussed.
In another approach, we investigate nanocrystalline silicon particles embedded in an amorphous film or an oxide matrix of SiO2 as effective thermoelectric hybrid materials. These quantum dots are formed in a phase-separation process in thin films deposited by chemical or physical vapor deposition. Alternatively, a solid-state transformation is used in a quartz-aluminum system to form Si nanoparticles in an Al2O3 layer. The formation of the nanocrystals can be tuned by rapid thermal annealing with respect to the uniformity in size, distribution, and surface structure. In order to maximize the thermoelectric power factor, a high doping level of the particles is required. With the low thermal conductivity of the amorphous matrix, a figure of merit close to 1 may be achieved at room temperature.

The design, fabrication and characterization of heterojunction solar cells and thin film transistors comprised of crystalline silicon (c-Si) substrates will be presented. In particular, the application of hydrogenated amorphous silicon (a-Si:H) and epitaxially grown hydrogenated crystalline silicon (c-Si:H) layers to form high-quality heterojunctions on c-Si substrates will be discussed. The a-Si:H and c-Si:H layers may be grown in the same plasma-enhanced chemical vapor deposition (PECVD) reactor at temperatures close to 200°C. The impact of amorphous-to-crystalline and crystalline-to-amorphous phase transitions during the growth of the hydrogenated Si layers will be also discussed. The demonstrated devices include heterojunction solar cells with conversion efficiencies exceeding 21%, heterojunction bipolar transistors with current gains exceeding 500 and heterojunction field-effect transistors with subthreshold slopes close to 70mV/dec, ON/OFF ratios larger than 10^7 and operation voltages below 1V.
The authors are grateful to Dr. Devendra K. Sadana, Dr. Ghavam G. Shahidi and Dr. T-C. Chen of IBM Research for technical discussion and managerial support, and Prof. Sigurd Wagner of Princeton University for allowing the usage of his PECVD facility for this work.

Silicon based heterojunction (SHJ) solar cells consisting of a crystalline (c Si) absorber sandwiched between two doped, hydrogenated amorphous (a Si:H) layers exhibit high energy conversion efficiencies up to 24.7% [1]. The doping of the a Si:H layers necessary to form the p/n and n/n+ junctions leads to relatively high defect density in the a-Si:H bulk and at the interfaces. Therefore a thin nominally intrinsic a-Si:H buffer layer with a lower defect density is inserted to suppress recombination and improve open circuit voltage (Voc). The mechanisms that govern the Voc are well understood and values up to 750mV have been reached in devices [1]. Hence SHJ are superior to homojunction solar cells in terms of Voc, however they suffer from a lower fill factor (FF) [2]. The key issue at present is therefore the improvement of the FF which is a more complex challenge since transport phenomena compete against recombination processes at the a-Si:H/c-Si as well as at the TCO/a-Si heterojunction.
We report numerical simulations to elucidate the transport and recombination processes that govern the FF in SHJ solar cells. To this end the device simulator AFORS-HET has been extended to describe tunneling currents at hetero and Schottky junctions. Furthermore the defect densities of the dangling bond defects in the a-Si:H layers are calculated according to the defect pool model [3]. Transport and recombination paths are analyzed as function of the doping and defect densities in the a-Si:H layers as well as the band offsets and defect densities at the hetrojunctions in a range which is experimentally accessible. It is concluded that the transport is dominated by thermionic emission and by tunneling at the a-Si:H/c-Si as well as at the TCO/a-Si:H interface while the transport via interface defects has only minor influence. Therefore the width of the valence band spike which is a function of the a-Si:H defect density and doping has a major influence on the fill factor. The major loss mechanisms are recombination via dangling bond defects at the heterointerface and in the a-Si:H bulk while the recombination at the Schottky contact is negligible. Therefore maintaining a high band bending at the heterointerface up to high injection levels is crucial to reduce the electron concentration and thus suppress recombination. The band bending is not only governed by the a-Si:H properties but also by the TCO work function which is confirmed by analysis of charge carrier lifetime measurements. Based on these simulations we finally propose optimization strategies for improving the FF and thus to overcome one major drawback of SHJ solar cells.
[1] M. Taguchi et al., IEEE J. of Photovoltaics, in press, (2013)
[2] S. De Wolf, et al., Green, 7-24, 2, (2012)
[3] M. J. Powell, et al., Phys. Rev. B, 53, 10121-32 (1996)

Silicon heterojunction (SHJ) solar cells have been proven to show conversion efficiencies as high as 24.7 %. For this type of photovoltaic cell, plasma enhanced chemical vapor deposition is typically used to deposit hydrogenated intrinsic amorphous silicon (a-Si:H) layers, for the passivation of the wafer surface, and doped a-Si:H layers to form both electron and hole collecting layers at the front and rear. The current generation in such a device is limited mainly by optical losses in the front layers, namely the transparent conductive oxide layer—needed for lateral conduction to the screen printed metallic grid—and the a-Si:H layers in particular. Electron-hole pairs generated within these layers can neither be separated nor collected efficiently before they recombine. This gives rise to parasitic absorption which reduces the short-circuit current density (Jsc). In this paper we present a possible approach to mitigate these optical losses and thereby increase the current.
As it exhibits a wider bandgap than a-Si:H, amorphous silicon oxide (a-SiOx:H) can help to reduce the absorption of light and thus the generation of carriers which are subject to premature recombination. In our study, we use a stack of intrinsic a-Si:H and a-SiOx:H as front intrinsic passivation layer. With this structure we obtain an increase in Jsc of up to 0.43 mA/cm2, which is due to reduced reflection and a higher transparency at short wavelengths. Despite the fact that excellent interface passivation is maintained (surface recombination velocities below 3 cm/s) this gain in Jsc is strongly offset by losses in fill factor (FF) which we link to impeded carrier collection across the amorphous/crystalline interface. Indeed, aided by device simulations, we can relate this effect to an increased valence band offset introduced by the opening of the bandgap when introducing a-SiOx:H. However, the carrier extraction can be improved when the cell temperature is increased. In fact, for our cells, we find that an a-SiOx:H window layer can help to reduce the temperature coefficient to a value as low as -0.1 %/°C (relative drop in efficiency) which is even lower than what we obtain for our reference heterojunction solar cells (-0.3 %/°C) for the same temperature range. Hence, even though at room temperature the proposed structure leads to lower FF values compared to the reference, at higher temperatures—those closer to the real working conditions of a photovoltaic module—they show superior performance in both experiment and simulation. Therefore, a-SiOx:H layers could help to increase the energy yield of SHJ solar cells, especially in warmer climates.

Silicon heterojunction solar cells enable high efficiencies up to 24.7% with industrial processing. Intrinsic hydrogenated amorphous silicon (a-Si:H(i)) layers deposited on both sides of a crystalline silicon (c-Si) wafer provide an excellent surface passivation. Electron and hole collectors are created by subsequent depositions of doped-layers. a-Si:H(i) material grown close to the amorphous-to-crystalline transition are known to produce an efficient passivation. To come closer to this regime, the film can be treated by a H2 plasma after its deposition. However, H2 plasma treatments prior the film growth may introduce severe surface damage reducing strongly the passivation quality. Hence, the effect of prolonged H2 plasma treatments remains uncertain: On one hand, the H2 plasma may improve the film properties; on the other hand, defects may be created at the crystalline-amorphous interface strongly reducing the surface passivation. Furthermore, it is known that prolonged H2 plasma treatments etch a-Si:H films. In the present study we investigated the potential of using this etching effect for a-Si:H layer patterning.
Intrinsic and phosphorous-doped a-Si:H stacks are deposited by PECVD and H2 plasma treatments are applied to selectively etch the doped layer while keeping the intrinsic one pristine. A highly reproducible etch-rate of 2.3 nm/min is obtained. The minority carrier lifetime is measured before and after etching to monitor the surface passivation quality. A sharp drop in lifetime is observed once the intrinsic layer starts to be etched. HR-TEM shows a significant defect creation in the c-Si lattice when this surface is directly exposed to the H2 plasma. In the case of a selective etching of the doped layer, it is observed that the intrinsic layer provided an efficient shielding and the c-Si surface remains pristine. Etching of the entire doped-intrinsic stack immediately followed by a new passivation deposition has been done. Despite the new passivation, a low carrier lifetime is measured. Therefore, the carrier lifetime drop previously observed can be attributed to c-Si surface defects rather than a modification of the a-Si:H bulk properties. However, the a-Si:H material is strongly modified during the H2 plasma treatment. Evidences of nanometric voids and an increase of hydrogen content have been found using ATR-FTIR. D2 plasma treatments combined with thermal desorption spectroscopy show that plasma treatments produce a strong exchange between the hydrogen atoms from the a-Si:H bulk with the hydrogen atoms from the plasma.
As a conclusion, we demonstrated a nanometric-accurate H2 plasma etching capable of selectively patterning a doped a-Si:H layer while preserving a good surface passivation quality.

Efficient carrier selective contacts and excellent surface passivation are key to solar cells with high power conversion efficiency. We explore substoichiometric molybdenum trioxide (MoOx, x<3) as a dopant-free, hole-selective contact for silicon solar cells. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, we demonstrate that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap at 2 eV above the valence band maximum. Using a hydrogenated amorphous silicon passivation layer between the oxide and the silicon absorber, we fabricate a silicon heterojunction solar cell with a high open-circuit voltage of 711 mV and a power conversion efficiency of 18.8%. Due to the wide band gap of MoOx, we observe a substantial gain in photocurrent of 1.9 mA/cm2 in the ultraviolet and visible part of the solar spectrum, when compared to a p-type hydrogenated amorphous silicon emitter of a traditional silicon heterojunction cell. Our results emphasize the strong potential for oxides as carrier selective heterojunction partners to inorganic semiconductors.

High efficiency (>20%) silicon solar cells require high lifetime wafers, well-passivated surfaces, and contacts with low recombination current densities. The last is achieved by either minimizing the metal-to-silicon contact area (e.g. PERT or PERL cell architectures) or by forming contacts with lower recombination than metal directly on silicon (e.g. a-Si/c-Si heterojunctions). This contribution explores using n-type and p-type transparent conducting oxides (TCO) for carrier selective, passivated heterojunction contacts to silicon solar cells.
One route to passivated contacts is to separate the metal from the silicon surface by inserting a passivating dielectric (e.g. SiO2, Al2O3) that is sufficiently thin to allow carrier tunneling between the silicon and metal. In particular, SiO2 can provide good passivation of Si surfaces even at thicknesses <20Å where tunneling occurs. However, at these thicknesses the states in the metal still influence and increase recombination. One solution is to further separate the silicon from the metal by placing a semiconducting layer between the dielectric and the metal. Preferably, this semiconductor is an energy-selective contact having favorable band alignments with silicon to transport either electrons or holes, while blocking the opposite carrier.
We formed passivated contacts to phosphorus diffused n-type silicon wafers with a thermally grown SiO2 passivation layer, a tin doped indium tin oxide (ITO) layer followed by metal. The final stack is Si/SiO2/ITO/metal. The SiO2 was grown at 1000 C and chemically thinned to <20 Angstroms. ITO was sputter deposited onto the SiO2 followed by a Ti/Ag metal contact. The Fermi-level of the n-type ITO is nearly aligned with that of n-type silicon, while its valence band edge is energetically much deeper than that of Si. This band alignment allows electrons to tunnel from the silicon, through the thin SiO2, into conduction band states of ITO and out to the metal contact. On the other hand, holes are blocked from tunneling due to a lack of corresponding transport states in the ITO. In a similar configuration, we also form passivated contacts to boron diffused n-type silicon wafers with SiO2 and a Cu-based p-type TCO to transport holes while blocking electrons at the emitter surface.
We characterized each of these contacts by measuring the contact resistivity, and the recombination current density. We present an analysis of tradeoffs between these two quantities and obtain a figure-of-merit for passivated contacts. We predict the maximum efficiency for a cell having optimized area coverage for each contact scheme.
Funding for this project was provided by DOE contract # DE-AC36-08GO28308

For a path towards wide-bandgap heterojunctions on crystalline silicon, with applications ranging from photovoltaics (PV) to heterojunction bipolar transistors (HBTs), we have recently demonstrated that TiO₂ deposited on crystalline Si(100) by a novel chemical vapor deposition process (le; 100°C) forms a very well-behaved heterojunction [1]. With a maximum processing temperature of 100°C, this interface has been used to make 7% efficient PV cell on Si without any p-n junction, with results limited by the semi-transparent cathode [2]. Electrons can pass freely from the silicon to and through the TiO₂, but the very low valence band of TiO₂ prevents holes in the silicon from entering the TiO₂. Ultraviolet Photoelectron Spectroscopy (UPS) shows the valence band edge of our CVD-deposited amorphous TiO₂ to be 3.39 eV below the valence band of the Si,, resulting in a large valence band offset., while IPES demonstrated that the conduction band offset was less than 200 meV [3].
The electrical quality of the TiO₂/p-Si interface is crucial for minority carrier applications such as HBTs and PV, where a reduced interface recombination is desired. In this work, we measure the interface recombination velocity (SRV) of minority carriers at the TiO₂/p-type Si interface, which reflects the number of interface defects, using photoconductance decay on high lifetime substrates. As-grown, the SRV ranges from 200 to 600 cm/s, depending on the deposition conditions. For comparison, the SRV of high-quality thermally-grown SiO₂ on silicon is typically ~50-100 cm/s, and “poor” interfaces can have SRV on the order of 10⁶ cm/s.
However, the as-grown interface degrades rapidly in air, with SRV increasing from ~ 200 cm/s to 10⁶ cm/s within a day when left in air. Annealing at 250°C has little effect initially. However in exposure to air, the SRV slowly rises to ~400 cm/s, and then plateaus and remains stable for at least 70 days after annealing. X-ray Photoelectron Spectroscopy (XPS) data focusing on the bonding at the Si/TiO₂ interface show distinct changes with annealing, suggesting that a “bridging” oxygen structure at the interface may be responsible for the stability.
In summary, we demonstrate that annealing leads to a TiO₂/Si interface which has a low rate of minority carrier recombination that is stable in air.
1. S. Avasthi et al., Appl. Phys. Lett. 102, 203901 (2013)
2. J. Jhaveri et al., presented at 38th IEEE Photovoltaic Specialists Conference, Tampa, FL, June 2013
3. G. Man et al., presented at 2013 Fall MRS Meeting, Boston, MA, December 2013

Traditional silicon heterojunction (SHJ) solar cells are known to achieve excellent open circuit voltages due to their high level of surface passivation, yet the amorphous silicon (a-Si:H) layers at the front contact limit the short circuit current due to parasitic absorption in the a-Si:H. Therefore, we have proposed stacks of Al₂O₃ and ZnO as an alternative to the
a-Si:H(i)/a-Si:H(p)/TCO stack that is currently being used in SHJ solar cells [1].
Just like a-Si:H, Al₂O₃ is known for its excellent ability to passivate c-Si surfaces. It is also known that there exists a high density of negative fixed charge at the Al₂O₃/c-Si interface. Therefore, when Al₂O₃ is deposited on n-type c-Si, a hole inversion layer is formed at the interface, forming a junction. This junction is used to form the hole-selective contact.
To extract the carriers and to provide lateral conductivity, ZnO is deposited on a thin (~1 nm) film of Al₂O₃ using atomic layer deposition (ALD). The Al₂O₃ film should be thin enough to allow the holes from the c-Si to tunnel through and recombine with electrons from the ZnO, analogous to the tunneling recombination mechanism in traditional SHJ solar cells [2].
To assess the potential of Al₂O₃/ZnO stacks as hole-selective contacts on c-Si, the surface passivation was investigated as well as the tunneling recombination current. It was found that for an Al₂O₃ thickness of ~1 nm the tunneling recombination is efficient enough for application in solar cells. It was also found that for Al₂O₃ thicknesses of < 3 nm, the surface passivation is significantly reduced by the presence of the ZnO. To improve the surface passivation of stacks with 1 nm of Al₂O₃, a thin film of 3 nm a-Si:H was inserted between the c-Si and the Al₂O₃ to reduce the interface defect density, significantly reducing the surface recombination velocity [3]. Furthermore, results of theoretical modeling are provided to indicate directions for further improvement of selectivity of the stacks and the efficiency of the tunneling recombination.
References:
[1] D. Garcia-Alonso, S. Smit, S. Bordihn, W.M.M. Kessels, "Silicon passivation and tunneling contact formation by atomic layer deposited Al₂O₃/ZnO stacks", Semicond. Sci. Technol. 28 (2013) 082002.
[2] S. De Wolf, A. Descoeudres, Z.C. Holman and C. Ballif, "High-efficiency Silicon Heterojunction Solar Cells: A Review", Green 2(1), 7-24 (2012).
[3] S. Smit, D. Garcia-Alonso, S. Bordihn, M.S. Hanssen, W.M.M. Kessels, Metal-oxide-based hole-selective tunneling contacts for crystalline silicon solar cells, Sol. Energy Mater. Sol. Cells. (2013).

We present an in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy study of the role of hydrogen migration in the surface passivation of Si solar cells by Al₂O₃ thin films deposited via atomic layer deposition (ALD). The ATR-FTIR measurements are complemented by the minority carrier lifetime, interface defect density, and built-in charge density measurements to elucidate the role of hydrogen in the chemical passivation of c-Si. The quality of Si surface passivation plays an integral role in the performance of c-Si-based solar cells. Recently, Al₂O₃ has been shown to be an effective passivant for a c-Si surface, with surface recombination velocities (S_eff) <5 cm/s reported. The passivation of the Si surface via Al₂O₃ is achieved by a reduction in the defect density at the interface (D_it) (chemical passivation) and an increase in the fixed negative charge (Q_f) associated with the Al₂O₃ films (field effect passivation). Several reports in literature attribute the chemical passivation of the Si surface by Al₂O₃ to the migration of hydrogen to the c-Si/Al₂O₃ interface during the annealing process thereby passivating the Si dangling bond defect states. We have studied this chemical passivation using an in situ ATR-FTIR spectroscopy setup. Al₂O₃ is deposited via a TMA-H₂O process onto high-lifetime c-Si internal reflection crystals (IRCs) followed by thermal annealing at 400 °C. The evolution of the the Si-H_x stretching vibrations is monitored during the entire process allowing us to estimate the loss/gain in interfacial hydrogen in each step. We compare the chemical information obtained through infrared spectroscopy with minority carrier lifetime measurements in order to correlate the surface passivation with the changes in the Si/Al₂O₃ interface observed via infrared spectroscopy. We have used deuterated precursors which enable us to differentiate between various sources of hydrogen present in the ALD process. Thus, the evolution of the Si-H_x stretching modes can be correlated to the source of the interfacial hydrogen. In addition, we have performed effusion studies during the annealing process to better understand the changes in the Si/Al₂O₃ interface observed using infrared spectroscopy.
We gratefully acknowledge the support from the NCPV Fellowship Program and U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory.

We present a study of Plasma enhanced chemical vapor deposited (PECVD) silicon nitride (SiNx:H) layers to passivate wafer Si cell surfaces to achieve very low surface recombination. As a result of this study, we demonstrate cell diode prefactor Jo values, a measure of recombination in the cell, as low as 22 fA / cm2 on textured and 15 fA / cm2 on planarized P-diffused n-type Cz wafer Si surfaces. Silicon nitride films have long been ubiquitous in the semiconductor industry for optical antireflection, electrical insulation, and chemical passivation properties in various devices geometries and applications. Silicon solar cells have utilized these properties due to the additional positive charge passivation characteristics appropriate for n-type surfaces created via diffusion or implantation methods. We confirm that the passivation quality of n-type Si surface typically increases with Si content in the SiNx:H towards Si-rich nitride stoichiometries. However, Si-rich nitride is resistive to etching by HF and has too large refractive index (>2.5) for optimum antireflection coating. By switching to high-power PECVD with H-dilution, we have identified another deposition regime for depositing SiNx:H close to stoichiometric composition with refractive index near 2.0, with surface passivation quality close to the Si-rich nitride. In addition, creating a ~ 1-2 nm thin interface oxide between the wafer and the nitride leads to a significant passivation improvement. We present our effort to understand and control of these critical variables by quantifying film characteristics as well as lifetime performance on both nascent and diffused n-type surfaces, with and without various oxide interlayers. Additional post growth annealing effects will also be addressed. This work was supported by the U.S. Department of Energy under contract DE-AC36-08-GO28308.

Recent progress in understanding electronic wave functions in condensed matter nanostructures has led to an ability to synthesize isolated, quantum confined building blocks with a variety of tailored optical properties. No matter what optical gap is engineered and how cleverly exciton energy is redistributed, though, novel materials composed of such nanostructures need to also exhibit efficient carrier dynamics and energy transport-now the central issue in harnessing the true power of quantum dot materials for solar cells, light emission and many other uses. This has led to the consideration of quantum dots encapsulated within amorphous matrices, but such environments fundamentally change the nature of quantum confinement and so the optoelectronic properties of the dots. The relationship between amorphous matrix and the character of quantum confinement is computationally elucidated here with particular emphasis paid to the location and shape of electronic states near the effective valence and conduction band edges. For instance, valence band edge states tend to be localized within nanocrystals while conduction band edge states tend to reside at the interface between nanocrystals and the surrounding amorphous matrix. In addition, confined states within nanocrystals exhibit a ribbon-like electronic structure that can be explained in terms of crystalline symmetry and interface curvature. Finally, there exists a critical nanocrystal size below which quantum confinement is not possible. Understanding and designing to such properties is critical for optimizing device performance with respect to carrier injection, internal conversion and carrier transport. These key aspects of carrier dynamics are explored using an incoherent (Fermi Golden Rule) hopping model. As part of this analysis, hole and electron mobilities are estimated in the absence of phonon assistance, showing the significant role of the amorphous matrix in improving both.

Sunlight absorption in semiconducting materials generates out-of-equilibrium electron populations - also known as hot carriers - relaxing towards equilibrium through a host of scattering processes at the subpicosecond time scale. While such dissipation processes typically result in the loss of more than half of the energy associated with the absorbed sunlight, a microscopic understanding of this ultrafast regime is still missing.
In this talk, we provide a detailed picture of the first picosecond after sunlight absorption in semiconductors of wide use in photovoltaics (PV) such as Si, GaAs, and CdTe. Our results are based on ab initio calculations combining density functional theory and the GW plus Bethe-Salpeter Equation (GW-BSE) approach together with electron-phonon interactions. We computed the lifetimes and k-space dependence of electron-electron and electron-phonon scattering events responsible for ultrafast solar energy dissipation. Using this information, we simulated the ultrafast dynamics of hot carriers using an empirical-parameter-free formulation of the Boltzmann equation.
Despite the well-known hurdles for extracting hot carriers in practice, a clear understanding of hot carrier dynamics emerges for several materials of interest in PV, and novel engineering paradigms are suggested. In closing, we briefly discuss how our formalism can be applied to other problems of interest in solar energy - notably, photocatalysis employing hot carriers in transition metal oxides, and hot electron generation from plasmon excitation in metals.

Silicon quantum dots (SiQDs) with diameters less than 5 nm are promising for next-generation photovoltaics with attractive features that include gap tunability, an optimum stability against oxidation [1], multi-exciton generation [2], and environmental neutral footprint. However, their optical gaps are simply too large and current SiQD-based solar cells perform poorly due to low carrier mobilities [3,4]. These drawbacks have motivated a multi-functionalization scheme in which the synergism between the dot, ligand, bridge and matrix lead to a substantial improvement of absorption as well as a desirable charge dynamic for free carrier extraction. As a proof of concept, we have carried out ab initio calculations on 2.6 nm SiQDs embedded in P3HT with the triphenylamine (TPA) and the C8H8N2S molecules serving as the terminating ligand and dot-to-dot bridge, respectively. By using a conjugating vinyl bond connection and establishing a type-II energy level alignment at the dot-ligand interface, low-energy photons can be absorbed via direct generation of spatially separated excitons. Both local and spatially separated excitons will subsequently evolve to charge transfer states with the electron and hole localized on the dot and within the matrix, respectively. While the electrons can efficiently hop between neighboring dots connected by the bridge molecules via a superexchange mechanism, hole mobility is also high due to the delocalized wave function in the matrix. Although efficient polaron dissociation in such systems is a challenge, we show that this can be solved through the introduction of an external electric field.
References
[1] Li, H. et al., ACS Nano 6, 9690, 2012.
[2] Nozik, A. J., Chem. Rev. 110, 6873, 2010
[3] Niesar, S. et al., Green 1, 339, 2011.
[4] Liu, C.-Y. et al., Nano Lett. 9, 449, 2009.

Raman microscopy has proven to be a very useful technique for inferring stress distributions in materials, since the positions of vibrational peaks are sensitive to local stress. This method has been applied extensively for crystalline silicon, and would be useful for amorphous silicon as well, particularly for studying local stress and composition of nanostructured amorphous/crystalline devices. Toward that goal, we have simulated the Raman spectrum of hydrogenated amorphous silicon with density-functional perturbation theory, using atomistic structures from the WWW algorithm with different stress states. We calculate the change in peak positions and intensities as a function of stress, and compare with experimental measurements. The results can be used to map out stress distributions in amorphous materials and the relative amounts of crystalline and amorphous material in a hybrid structure.

Silicon is a proven material and technology for application in solar photovoltaics, with over 50 years of precedents. However, with increasing market penetration of Si comes heightened requirements for load-leveling and storage of electricity for use during off-peak hours. One option for storage is in chemical bonds. My talk will highlight recent advances from our research into using micron-scale Si structures for conversion of the energy in sunlight directly into chemical bonds. This will include device design and performance characteristics of three tandem architectures that have been of recent interest. One is based entirely on crystalline Si (c-Si) microwire arrays, the second employs a tandem amorphous-Si on c-Si microwire array architecture, and the third uses a WO3 semiconducting layer on c-Si microwire arrays. All devices demonstrated at least 900 mV open-circuit potential and efficient H2 evolution when illuminated with simulated sunlight conditions. The benefits and challenges for robust implementation of each will be discussed.

We report on the optimization of thin film silicon tandem junction solar cells for applications in photoelectrochemical water splitting devices. Thin film silicon technology stands out as an attractive choice for water splitting applications, because it combines low-cost production, earth-abundance and versatility. The requirement to generate a photovoltage above 1.23 V, the electrochemical potential needed to electrolyze water, gives great importance to the latter characteristic, as thin film silicon solar cells can be adjusted to satisfy the specific thermodynamic requirement of different photoelectrochemical systems, i.e. provide an extended range of achievable voltages, without impairing device efficiency.
Tandem junction solar cells consist of two sub-cells connected in series. In this work, we investigate two types of p-i-n tandem solar cells: two amorphous (a-Si:H/a-Si:H) sub-cells and amorphous connected to microcrystalline (a-Si:H/µc-Si:H) sub-cells.
a-Si:H and µc-Si:H layers were deposited by plasma enhanced chemical vapor deposition, using a mixture of SiH4, H2, CH4, B(CH3)3 and PH3 gases. The optical band gap E04 was evaluated using photothermal deflection spectroscopy measurements and the crystallinity ICRS of µc-Si:H was determined by means of Raman spectroscopy. Solar cells were investigated by current-voltage under AM 1.5 illumination and quantum efficiency measurements. The photoelectrochemical performance of the solar cells was evaluated in an aqueous 0.1 M H2SO4 solution, using a three-electrode configuration under halogen lamp irradiation.
By varying the substrate temperature and the SiH4 to total gas-flow concentration (SC) during deposition, we show that in the case of a-Si:H/a-Si:H tandem cells, the VOC can be significantly improved. An optimum is found, when the substrate temperature for the intrinsic a-Si:H layers with 4% SC is maintained at 120°C, which results in a VOC of 1.87 V with 10.0% efficiency. The increase in VOC is attributed to wider E04 of the individual a-Si:H sub-cells. For µc-Si:H solar cells, the VOC is strongly related to the intrinsic layer crystallinity, which can be controlled by varying the SC during deposition. µc-Si:H single junction devices with SC of 4.8% promote 540 mV and provide a crystallinity of up to 60%. By increasing SC up to 6%, photovoltages of 655 mV were achieved to the detriment of eta; and ICRS. Here, further improvement in the control of the bulk and interface properties is needed. When connecting a 60%-crystalline µc-Si:H bottom-cell with the optimized a-Si:H top-cell we could achieve a VOC of 1.42 V and an efficiency of 10.8%.
Based on photoelectrochemical experiments, we show the performance of the developed tandem solar cells, used as photocathodes, in contact to an electrolyte. a-Si:H/µc-Si:H photocathodes with a Pt back contact exhibit a photocurrent onset potential of 1.3 V vs. the reversible hydrogen electrode (RHE) and a high photocurrent of 9.0 mA/cm2 at 0 V vs. RHE.

Microchannel plates (MCP) are attractive for amplification purposes for very fast detectors and are commonly used for image intensifying device [1]. Amplification of the signal is obtained by the multiplication of electrons in the microchannels upon avalanche mechanisms. In order to overcome some critical limitations of current MCP technology, we recently proposed to use hydrogenated amorphous silicon (a-Si:H) instead of lead silicate glass [2]. The use of a-Si:H associated with a new device structure should allow for better performance, simplify the construction of the device and permit a possible vertical integration of the MCP on top of the readout electronics. These advantages should broaden the range of application including particle detection and imaging.
First a-Si:H based MCP prototypes were fabricated with a simple two-terminal designed [3]. Avalanches mechanisms could be demonstrated by electron beam induced current imaging, but analysis of the gain appeared very problematic. The main drawback of the simple 2-terminal device structure is the fact that separation of the signal (resulting from the avalanche process) from the leakage is almost impossible.
To overcome these limitations, MCPs with a 3-terminal monolithic configuration were designed and successfully fabricated on oxidized Si wafer. These new devices comprise channels with a diameter of 3-5 µm drilled by deep reactive ion etching (DRIE) into 80-100 µm thick a-Si:H based layer stacks. The latter requires a careful control and optimization of the deposition to minimize the stress build up and integrity of MCP. Gain characterization was performed in continuous mode operation and gain values in excess of 30 could be recorded for an aspect ratio of only 12.5.
In this paper, we will describe and discuss the processes and issues related to the fabrication of such 3 terminal a-Si:H based MCPs. Gain analysis in relation with device geometry will be presented. Present performances, limitations and possible improvements will be discussed.
[1] J. L. Wiza, Nucl. Instr. and Meth. 162, 1979, 587-601.
[2] A. Franco et al., Nucl. Instr. and Meth. in Phys. Res. A 695 (2012) 74.
[3] N. Wyrsch et al., MRS Proc. Vol. Vol. 1245, (2010) 193.

The world-wide total installed PV power is growing so fast that within the end of this decade the electricity generated by solar energy is in the same order as hydro- and nuclear electricity. In combination with the seasonable fluctuations of solar power, this poses enormous technological challenges on the electricity grid and its storage capacity. Finding a cheap technology to store solar energy becomes, much faster than everybody is realizing, the crucial issue for a further successful introduction of solar energy technologies in to our energy infrastructure. In this contribution, we look at the conventional silicon based PV technologies and their opportunities in solar fuel technologies.
Current state-of-the-art system components, like silicon PV modules and electrolyzers, can achieve a solar-to-hdyrogen (STH) conversion efficiency of 15%. Alternative approaches, based on photoelectrochemical(PEC)-photovoltaic(PV)water splitting, have demonstrated higher STH efficiencies of 15 up to 18% based on III-V semiconductor materials and Platinum electrodes. The problems of these approaches are that they are not cost-effective, the materials are not abundantly available and not resistant against aqueous environment. We present the important role of silicon processing devices and processing technology to tackle these problems.
We present promising results on silicon based water splitting PEC/PV devices. The light excited charge carriers in photoelectrodes and electrolytes induce oxidation (photoanode) and redox reduction reactions (photocathode) at the electrode/electrolyte interfaces. The overpotential of the photoelectrodes are suppressed by the supplied voltage of the PV devices integrated in to the water splitting device. The cost effective, abundantly available and water resistant silicon can be integrated in both the PEC and PV part of the devices. We give two examples. First a BiVO4 based photoanode combined with single and double junction thin film silicon based PV devices. Integration of an a-Si:H/nc-Si:H double junction results in an impressive 5.2% STH conversion efficiency. Secondly, we show that the amorphous silicon carbide (a-SiC:H) material is a highly stable and efficient photocathode material. We demonstrate the first results on fully silicon based PEC/PV devices with the stable and highly efficient hydrogen evolution. It will be discussed that these results open cost-effective routes to achieve STH conversion efficiencies based on silicon technology well above 10%.

Thanks to its high refractive index and high transparency at telecommunication wavelength range, hydrogenated amorphous Si (a-Si:H) could constitute waveguide core for large scale photonic integrated circuits. Low temperature deposition below 350°C of device-grade a-Si:H flims allows for vertical stacking optical layers just by repeating a-Si:H core fabrication and SiO₂ cladding layer, thus, constructing 3D photonic integrated circuits without thermal damage of the underlying large scale electrical circuit (LSI). The 3D photonic integrated circuit could provide larger integration density as well as low crosstalk at waveguide crossings due to excellent optical isolation with SiO₂ layer in between. Overall, 3D a-Si:H photonic integrated circuit would be very promising as next generation Si photonics. Therefore, it would be imperative to continuously improve performance of each device.
In particular, low-loss optical waveguides are very important, because it is the most fundamental device and the propagation loss is relevant to the power dissipation of photonic integrated circuits. Optical excitation of a-Si:H with telecommunication wavelength range at 1550 nm happens to be associated with the transition involving a Si dangling bond defect state, hereafter called defect absorption, so that it is imperative to minimize defect density in order to obtain high transparency at 1550 nm. Recently indeed our low defect a-Si:H wire waveguide exhibited a very low propagation loss of 1.2 dB/cm. Other research groups have also reported the a-Si:H submicron-scale waveguides. The losses, however, are not low enough for a few centimeters-scale "optical global wirings".
In this paper, we demonstrate a submicron-scale a-Si:H ridge waveguide with a record-low propagation loss of 0.59 dB/cm owing to the low absorption of the a-Si:H film, smaller overlap of the optical field with top and sidewall surface of the waveguide by increasing the thickness of a-Si:H core as well as smoother top surface processed with CMP. The waveguide has a core with thickness and width of 440 and 780 nm, respectively, that underlies a 100 nm thick ridge structure, and is fabricated with the low cost i-line stepper photolithography and low temperature process below 350°C, making it compatible with backend process of complementary metal oxide semiconductor (CMOS) fabrication.

We report crystallization of silicon films using a heating method of 2.45 GHz microwave irradiation. 50-nm thick amorphous silicon (a-Si) films were formed on 20-cm2 sized glass substrates by plasma enhanced chemical vapor deposition. An a-Si sample were placed in a quartz container. Black carbon powers were also introduced in the container to completely cover the silicon sample. The container including the sample with carbon was put in a commercial microwave oven. Microwave at a power of 1000 W was irradiated for 2 and 3 min. Carbon powders effectively absorbed microwave and heated itself to a high temperature. Blight light emission with yellow-orange color was observed from carbon. Using the blackbody radiation theory, the temperature was estimated as 1000 and 1200oC for micro-wave irradiation for 2 and 3 min. After termination of microwave irradiation, the sample was taken out the container and carbon powders were removed. The silicon films was uniformly changed to be golden colored with maintaining mirror surfaces. Raman scattering spectra of the samples were measured. Sharp crystalline silicon phonon peaks were observed for the samples irradiated with microwave for 2 and 3 min. No broad amorphous silicon peak was observed for the samples irradiated with microwave, while there was only a broad amorphous silicon peak for the as-fabricated sample. The peak wave number and full width at half maximum (FWHM) were 520.1 and 14.0 cm-1 for the sample irradiated with microwave for 2 min. They were 520.2 and 8.0 cm-1 for the sample irradiated with microwave for 3 min. The peak wave numbers of the silicon films were close to that of bulk crystalline silicon. It indicated that microwave heating crystallized silicon films with no serious lattice strain. FWHM of the sample irradiated with microwave for 3 min was shorter than that of the sample irradiated with microwave for 2 min. Large crystalline grain formation was probably achieved. This paper will also discuss application of the present microwave heating to fabrication of thin film transistors and solar cells.

Reflection loss of silicon solar cells can be reduced by texturing the surfaces. We have investigated the plasmaless texturing process for crystalline Si solar cells using chlorine trifluoride (ClF ₃ ) gas treatments. The ClF ₃ gas can etch silicon without plasma near room temperature. Additionally, plasmaless etching has an isotopic property independent of crystal orientation without damaging the substrates. Plasmaless etching using ClF ₃ gas is known to form random and microscopic textured structures on Si substrates.
Our research group has demonstrated improved electrical performance of mono-crystalline Si solar cells textured by the dry etching of the phosphorus-diffused layers in the previous report (MRS 2011). This process has an advantage to form a selective emitter structure self-alignedly during the texturing step. In this study, we tried to improve performance of multi-crystalline Si solar cells by modifying the fabrication procedure and optimizing the texturing conditions.
The reflectance of the random-textured surfaces using ClF ₃ gas was reduced to below 10% at a wavelength of 600 nm. We have fabricated solar cells by phosphorus diffusion onto the textured surfaces. Multi-crystalline Si solar cells were fabricated with textured surfaces and a mirrored surface. The short-circuit current density was improved about 35%, and the efficiency of the textured cell is much larger than the mirror cell.

We have developed a completely new type of Silicon we call it as Silicon Colloids. They are polydisperse [1] or monodisperse [2,3] micro and nanoparticles, ranging from 400 nm to 3 micrometres with a perfect spherical shape. Silicon Colloids constitute the new material platform that could bring together several areas of research as semiconductor and colloidal sciences and metamaterials. Several factors support Silicon Colloids as a new paradigm:
1. The synthesis method based on CVD techniques is very similar to that used in epitaxial growth of silicon [1].
2. Silicon Colloids benefit from the advantage of colloidal science for processing particle ensembles, using different self-assembling methods as used in soft matter.
3. Silicon Colloids have huge refractive index value (3.5). Therefore, they block efficiently Infrared radiation. We have developed coatings and pigments for, A) thermal insulating paints, B) UV and IR resistant plastics and polymers, and C) sun screen filters for cosmetics [4].
4. Silicon nanocavitites show well-defined Mie resonances with a magnetic response in the optical region [5]. We have developed photonic crystals made of monodisperse silicon colloids with a magnetic response in the near IR region [3].
5. Silicon Colloids can further be processed, in a similar manner as performed for silicon technology, to process electronic devices in a single particle.
[1] R. Fenollosa, F. Meseguer, M. Tymczenko, Adv. Mater. 20, 95 (2008).
[2] J. T Harris, J. L Hueso, and B. A Korgel, Chem. Mater., 22, 6378-6383, (2010)
[3] Lei Shi, J. T. Harris, R. Fenollosa, I. Rodriguez, X. Lu, B. A. Korgel, and F. Meseguer Nature Communications, 4, 1904 (2013)
[4] I. Rodriguez, R. Fenollosa, A. Perez-Roldan, and F. Meseguer, 2011 Patent Ref. Ref. WO2011-109236841.
[5] L. Shi, T. U. Tuzer, R. Fenollosa, and F. Meseguer Adv. Materials, 24, 5934, (2012).

The efficiency of silicon based thin-film solar cells is strongly influenced by the light trapping properties within the whole multi-layer stack. To increase light trapping rough interfaces are introduced leading to improved light in-coupling, scattering and multiple internal reflections. Suitable rough interfaces can be realized by sputtering of ZnO:Al and a subsequent etching step or LPCVD deposited ZnO on the front glass substrate. Instead of these techniques, we consider a sol-gel derived scattering layer, based on agglomerated silica particles coated on the front glass by dip-coating. Afterwards a sputtered ZnO front contact, a thin-film silicon a-Si:H/µc-Si:H tandem cell structure and a back contact are produced. To analyze the light trapping properties of these thin-film silicon solar cells with incorporated particles, numerical optical simulations of the external quantum efficiency (EQE) are compared with experimental data. The optical properties of the solar cell are described by means of the Maxwell&’s equations, which are solved using Finite Integration Technique (FIT). The topology of the different layers is characterized by AFM-scans defining the interfaces properties of the simulated multi-layer stack. To meet the computational amount of the simulations, high performance computers are used.
The simulation results of the cell structure with a sol-gel derived scattering layer are in good agreement with the experimental data. Optical simulations of cells with and without textured glass/ZnO interface show an enhanced quantum efficiency for devices with a textured glass/ZnO interface. However, the observed gain in EQE is above the expected 2% range which originate from the reflection at a flat glass/ZnO interface. This result demonstrates that a textured glass/ZnO interface leads to an altered light trapping within the multi-layer stack. Based on the simulation performed the potential of solar cells with textured interface at the glass/ZnO interface will be presented und the losses within the within the device structure e.g. internal n/p-contact will be discussed.

High efficiency multi-junction thin-film solar cells have been reported by using hydrogenated microcrystalline silicon (mu;c-Si:H) material due to high JSC and low light-induced degradation of mu;c-Si:H [1]. To achieve high JSC, the mu;c-Si:H absorber should be relatively thick to obtain sufficient long-wavelength absorption while the thick absorber weakens the built-in electrical field, decreases the fill factor and increases production cost. To avoid using thick absorber, a good light management plays a critical role to improve the cell performance when using thinner absorber. One approach is to reduce the parasitic absorption losses occurring at the back contact by inserting a transparent conducting oxide (TCO) using an ex-situ sputtering step. Veneri et al. have presented hydrogenated microcrystalline silicon oxide (mu;c-SiOx:H) can be a replacement for n-type and back TCO layers in amorphous silicon thin-film cells [2]. In this work, we employed mu;c-SiOx:H as a dual-function layer for both n-type and back reflecting layer in the mu;c-Si:H single-junction cells and multi-junction solar cells.
The silicon-based thin films were prepared by a 27.12 MHz PECVD system equipped with in-situ NF3 plasma cleaning. Oxygen incorporation in mu;c-SiOx:H films was achieved by the dissociation of CO2 with highly H2-diluted SiH4. Electrical and optical measurements were carried out to investigate the conductivity, bandgap, reflectivity and crystallization of the mu;c-SiOx:H films. The solar cells with patterned area of 0.25cm2 were characterized by an AM1.5G illuminated J-V measurement system and a quantum efficiency instrument.
The mu;c-SiOx:H films with different oxygen content were prepared by adjusting rf power, pressure and CO2-to-SiH4 flow ratio, resulting in conductivity from 14 to 3×10-9 S/cm and O-content from 0 to 41.8 at.%. Considering the trade-off between bandgap, refractive index and conductivity of mu;c-SiOx:H(n), the mu;c-Si:H single-junction cell using mu;c-SiOx:H(n) as both n-type and back reflecting layer exhibited a significantly increase in JSC by 21 % as compared to the cell with mu;c-Si:H(n)/Ag. This major improvement of JSC originated from the increased absorption, which was confirmed by the quantum efficiency instrument showing the increased response at wavelength from 580 to 1110 nm. Without ex-situ sputtering for back TCO, all PECVD process simplified the fabrication process and could have a better interface quality, which enhanced the fill factor. In addition, the conversion efficiency of a-Si:H/mu;c-Si:H tandem cells using mu;c-SiOx:H(n)/Ag was enhanced to 10.63%, with JSC=10.36 mA/cm2, VOC=1.36 V and FF=75.2 %. The light management in tandem- and triple- junction cells was improved by the utilization of mu;c-SiOx:H(n), resulting from more optical reflection at long wavelength back to the bottom absorber, which was confirmed by the spectral response measurement.
1. Meier et al., Appl. Phys. Lett. 65, 860 (1994)
2. Veneri et al., Appl. Phys. Lett. 97, 023512 (2010)

As the cornerstone of modern semiconductor industry, Silicon (Si) has been widely and intensively studied for decades. Recently researchers tried to integrate Si into optical fibers to explore its applications in photonics. Different methods of producing Si-core quartz fiber have been demonstrated. One is to put high purity Silicon inside a quartz preform and draw quartz fiber with molten Si core, and the other is to use high pressure Si precursor gas (SiH4) to deposit Si inside a fiber channel. Both of the above methods use expensive ingredients and produce Si-core quartz fiber at high cost. Herein we report a novel, low-cost method to produce silicon-core, quartz-cladding fiber. The inputs of the production process are quartz tube and Aluminum wire, and output of the production process is fiber with crystalline Si core. The Silicon core forming process is studied and in-fiber Si core is analyzed by a multiplicity of materials characterization tools, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, etc. Mechanism of the produce method is discussed and applied to other materials system, e.g. iron and nickel. The ability to produce pure crystalline Silicon core fiber out of inexpensive metal and quartz combination provides a simple, low-cost way to produce high-quality Si-core quartz fiber.

We report on the effects of the frequency dispersion in light sensitive materials used in photoimpedance wireless sensors. An example of such a sensor is a gated semiconductor connecting two or more fixed capacitances. The impedance of the device under illumination is changed by the change in the photoresistance of the semiconductor layer and the change in the gate-semiconductor capacitance.We report on the modeling, simulation of the frequency dispersion of the impedance of this device and compare the modeling results with the device characterization and parameter extraction data. The model links the sensitivity and the dynamic range of the sensor and the frequency dispersion of the impedance. It is applicable to the materials with localized states in the gap, such as a:Si-H, where the position of the quasi-Fermi level in the energy gap determines the characteristic response time. We evaluate the dynamic range and sensitivity of the wireless photoimpedance sensors and show their advantages for wireless sensing applications compared to more conventional light sensors.

We report the study of the influence of indium as an impurity on ultra-thin film (2.5mu;m c-Si thick) silicon solar cells. The design of the cell reported here is such that it should elucidate the impact of indium dopant which is concentrated in the thin film. Indium, a deep level in silicon (0.15eV above the valence band), acts as a p-type dopant and as a sensitizer. Absorption through sub-bandgap transitions is expected based on the previously proposed Impurity Photovoltaic (IPV) Effect [M. J. Keevers, et al., J.Appl.Phys. 75(8):4022-4031, 1994]. It is assumed that the implementation of a novel vertical PN junction configuration and the IPV effect enhances the efficiency of ultra-thin solar cells. The most efficient indium doped cell fabricated to date has a conversion efficiency of 4.76 %, a short-circuit current density of 18.3 mA/cm² and an open-circuit voltage of 0.53 V under 1 sun illumination. A comparable boron doped (as a p-type) solar cell was fabricated yielding a maximum conversion efficiency of 4.16 %, a short-circuit current density of 17.3 mA/cm² and an open circuit voltage of 0.50 V. The cells have not been optimized with any type of light trapping technique and 11.24 % is covered by the metal contacts. We will present results which will quantify the IPV effect for indium and show the impact of the variation of inter-digitated electrodes in thin film geometry. We will also discuss how this design may be optimized and used in conjunction with a deposited poly-silicon technology.

Hydrogenated amorphous silicon (a-Si:H) p-i-n photodiodes are commonly used as pixel sensors in digital radiographic flat-panel imaging detectors. Photodiode performance is one of the factors limiting the signal-to-noise ratio and image quality. In particular, a high sensor sensitivity in the visible spectral range is required to provide an efficient optical coupling with conventional phosphors such as CsI:Tl or Gd₂O₂S:Tb. One of the approaches to minimize the absorption losses in the p-layer is to use an a-Si1-xCx:H alloy having a wider band gap than a Si:H. However, the use of this technology in industry is limited because the most of production lines are for a-Si TFT backplanes, and the cost of their upgrade for additional doping gases may be unacceptably high. This work reports a carbon-free, blue-enhanced a-Si:H n-i-p photodiode with an optimized protocrystalline p layer. Our study also includes electrical and optical characterization of thin (~20 nm) B-doped films grown on a-Si:H substrates at hydrogen dilution ratios ranging from 50 to 200. Although the used deposition conditions correspond to the microcrystalline phase region, thin layers are mostly protocrystalline due to the amorphous substrate. This conclusion is supported by Raman spectroscopy measurements. We have also found that the optical band gap can be varied by adjusting the RF power. By widening the band gap and tuning the impurity concentration in the p layer, absorption and recombination losses at the p-i interface were reduced. The current-voltage, capacitance-voltage, and spectral-response characteristics of fabricated photodiodes are correlated with the doping level, optical band gap, and deposition conditions for p-layers. The optimized device exhibits a leakage current of about ~100 pA/cm² at 5 V reverse bias. The external quantum efficiency reaches a peak value of 92% at a wavelength of 510 nm, and, at shorter wavelengths, decreases down to 66%@400nm.

Expanding far beyond traditional applications at infrared telecommunications wavelengths, SiC nanophotonic devices have recently proven its merits for working with visible range optical signals. Reconfigurable wavelength selectors are essential sub-systems for implementing reconfigurable WDM networks and optical signal processing.
Visible range to telecom band spectral translation in SiC/Si can be accomplished using a SiC wavelength selector under appropriate near ultraviolet optical bias (near-UV: 300nm-400nm), acting as reconfigurable active filters in the visible and near infrared ranges (400nm-850nm). In this paper we present a monolithically integrated wavelength selector based on a multilayer SiC/Si integrated optical filters that requires appropriate steady states optical switches to select the desired wavelengths in the VIS-NIR ranges. The selector filter is realized by using double pi&’n/pin a-SiC:H photodetector sandwiched between two ITO contacts acting as biased optical gating elements. Visible communication channels (400nm-650nm) are transmitted together, each one with a specific bit sequence. The combined optical signal (MUX signal) is analyzed by reading out the generated photocurrent, under near-UV steady state background and different intensities.
Results show that background intensity and wavelength work as selectors in the infrared region, shifting the sensor sensitivity. The background intensity balances the electric internal fields across the device leading to diverse charge accumulation at the different layer interfaces, which filter one or more input channels depending on the light penetration depth of each optical signal wavelength. Low intensities of the background in the NUV range select the NIR range while high intensities select the visible part accordingly to its wavelength. Here, the optical gain depends on the wavelength penetration depth of the input channels across both front and back photodiodes. It is very high in the red range, decreases in the green range, and stays near the unity in the blue region and strongly decreases in the near-UV range. The transfer characteristics effects due to changes in steady state light intensity and wavelength backgrounds are presented. The relationship between the optical inputs and the output signal is established. A capacitive optoelectronic model will be presented to explain the physical influence the background intensity and wavelength tested using the experimental results. A numerical simulation supports the model. Some applications with the multilayer SiC/Si optical technology are also pointed out and can provide a smart solution to communication problem by providing a possibility of optical bypass for the internet transit traffic by dropping the fractional traffic that is needed at a particular point.

In this paper we demonstrate the use of UV steady state illumination to increase the spectral sensitivity of a double pi&’n/pin photodiode beyond the visible spectrum. Increased sensitivity in the range of 400 nm-850 nm is experimentally demonstrated. The concept is extended to implement a 1 by 4 wavelength division multiplexer with channel separation in the visible/near infrared ranges.
Optoelectronic characterization of the devices is presented and shows the feasibility of tailoring the wavelength and bandwidth of a polychromatic mixture of different wavelengths. The device consists of a p-i'(a-SiC:H)-n/p-i(a-Si:H)-n heterostructure with low conductivity doped layers, sandwiched between two transparent contacts. Spectral response and I-V characteristics, with and without background illumination are presented.
Results show that the spectral current under UV front light irradiation (350 nm) increases with the background intensity in the 470nm-800nm range and decreases for low power wavelengths in the visible range. Under back irradiation the spectral current decreases for wavelengths higher than 550nm and strongly increases beneath them. The optical gains have opposite behaviors under front and back irradiations. Under front irradiation and low power intensity the gain is high and presents a well defined peak at 750 nm and strongly quenches in the visible range. As the power irradiation increases the peak shifts to the visible range and can be deconvoluted into two peaks, one in the red range that slightly increases with the power density of the background and another in the green range that strongly increases with the intensity of the UV radiation. In the blue range the gain is much lower showing the filtering properties of the device at different UV background intensities. Under back irradiation the gain is high in the violet/blue ranges and strongly quenches for higher wavelengths whatever the intensity of the background.
Several monochromatic pulsed lights in the UV/VIS range, separately or in a polychromatic mixture illuminated the device. Independent tuning of the wavelengths is performed by steady state low ultraviolet optical bias superimposed from both sides. Results show that, front background enhances the light-to-dark sensitivity of the medium, long and infrared wavelength channels and quench strongly the low wavelength, depending optical amplification on the background intensity. Back UV background has the opposite behavior; it enhances only channel magnitude in short wavelength range and strongly reduces it in the long ones. This nonlinearity provides the possibility for selective tuning of a specific wavelength. It provides a low-cost solution to many aspects of optical and optoelectronic interconnection technologies and makes the bridge between the visible light and the infrared communications. A capacitive optoelectronic model supports the experimental results. A numerical simulation will be presented.

We report on the defect properties including tail state densities and deep level defects in nanocrystalline (Si,Ge) films and devices. The materials and devices were grown using VHF-PECVD techniques. Grain size was systematically varied by changing the pressure and H2 dilution during growth. It was found that the grain size increased as the pressure increased from 100 mTorr to 1 Torr range. The grain size also increased as the hydrogen dilution decreased. Electron mobility increased as the grain size increased. The deep defect densities were found to be approximately in the middle of the gap and could be reduced by counter-doping the n type as grown material with ppm levels of B. The open-circuit voltage and fill factor of the p-i-n devices fabricated on stainless steel could be controlled by controlling the grain size and by controlling the amorphous/crystalline ratio at various stages of the growth. It was found that the highest voltage and fill factor were obtained when the initial nucleating layer on the n-back contact layer had a deliberately designed higher ratio of amorphous to crystalline phase. We attribute this increase to an effective induced back-surface field to drive the holes away from the back surface when such layers are used.

In the race for Si-based highly efficient solar cells the interdigitated back contacted solar cell and the heterojunction cell with an intrinsic thin amorphous layer (HIT) stand out [1]. The combination of these two approaches in one structure will allow for an enhancement of the cell performance as it combines the advantages of moving all metal contacts to the rear side on the one hand and using the exceptional passivating properties of a-Si thin films to the crystalline Si base material [2]. Notwithstanding one has to address the challenges pertaining to implementing the final structure. So far the best efficiency obtained was 15.7% on n-type c-Si using screen printing process for metallization [3]. Extensive research has been carried out on solar cells with p-type absorber wafer, however more work is needed for promising solar cells on n-type wafers exhibiting higher lifetimes [4].
We are studying this structure using a 3D TCAD simulation package from Synopsis. The cell structure studied is on n-type crystalline Si (c-Si) absorber. The structure layout is the following CO/doped-aSi (p and n)/i-aSi/c-Si (n)/i-aSi/doped a-Si (n+)/Si3N4. i-aSi stands for intrinsic amorphous Silicon and CO for conductive oxide. We will investigate the effect of the following parameters on the cell performance: doping level of c-Si substrate, the coverage of p-type a-Si emitter layer as well as the gap introduced between the two differently doped regions (emitter and back surface field) which can strongly reduce the cell performance, in addition to the effect of metal coverage on the rear side passivation. In addition, we will explore other parameters such doping levels of the emitter and BSF layers as well as the interface states density at the various interfaces of the above mentioned structure.
References:
1. Diouf, D., et al., Study of interdigitated back contact silicon heterojunctions solar cells by two-dimensional numerical simulations. Materials Science and Engineering: B, 2009. 159-160(0): p. 291-294.
2. Diouf, D., J.-P. Kleider, and C. Longeaud, Two-Dimensional Simulations of Interdigitated Back Contact Silicon Heterojunctions Solar Cells, in Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells, W.J.H.M. Sark, L. Korte, and F. Roca, Editors. 2011, Springer Berlin Heidelberg. p. 483-519.
3. Haschke, J., et al., Interdigitated Back-Contacted Silicon Heterojunction Solar Cells With Improved Fill Factor and Efficiency. Photovoltaics, IEEE Journal of, 2011. 1(2): p. 130-134.
4. Sark, W., L. Korte, and F. Roca, Introduction - Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells, in Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells, W.J.H.M. Sark, L. Korte, and F. Roca, Editors. 2011, Springer Berlin Heidelberg. p. 1-12.

We present studies of the temperature dependence of bimolecular recombination in hydrogenated silicon-germanium alloys (a-Si_1-xGe_x:H) measured using femtosecond transient absorption. Time-resolved measurements were carried out on HWCVD thin films of varying Ge content. Carriers were excited into the extended electronic states using optical pulses 35 fs in duration generated by an amplified 1 kHz Ti:S laser system, and recombination of the resulting carrier population was monitored over four decades in time via the decay of the induced absorbance measured at sub-gap near-infrared wavelengths using a compressed white-light continuum. Measurements were carried out over a range of initial carrier densities from approximately 10¹⁸ to 10¹⁹ cm^-3 and at temperatures ranging from approximately 100 to 300 K. For all alloy compositions, the observed decay of the induced absorbance is highly nonexponential and strongly dependent on initial carrier density. The carrier response also shows a strong temperature dependence, with significantly slower decay of the carrier population observed at lower temperature. We find that the carrier dynamics for a wide range of alloy compositions are well-represented by a model for dispersive bimolecular recombination in which recombination is assumed to occur with a diffusion-limited rate reflecting an effective time-dependent mobility that decays as a power law that is associated with dispersive transport, and we find that the observed temperature dependence is in accord with a simple physical model involving multiple trapping and thermally activated detrapping processes within an exponential distribution of band tail states. We relate the results to band tail distributions measured by photothermal deflection spectroscopy.
This work was supported by the National Science Foundation under grants DMR-1106379 and DMR-0706407. We thank Brent Nelson (NREL) for preparing the thin film samples used in these studies, and John Viner (University of Utah) for carrying out the PDS measurements.

For 3D applications, some processes involve the deposition of thin layers materials on a substrate. During the following manufacturing process, the total stack can as well incur some thermal treatments, and mechanical properties change over time. It is essential to determine changing internal stress; these controls must ensure the functionality and reliability of the device.
We have developed methodologies for the determination of wafers stresses from experimental measurements and simulations. In the case of multilayers, the use of our models first requires to have a clear understanding of the materials&’ mechanical properties involved in the stack.
We have proposed a methodology to measure thin films elastic properties from the measurement of the evolution of a test structure deflection under a mechanical loading. But another critical aspect is the determination of the thermal expansion coefficient for the thin layers constituting the stack.
Another critical aspect is to study the thermo-mechanical evolution of polymers. These materials are often used in 3D stacks as passivation layers or bonding materials.
In this work, we propose to address these two ways with the same experimental technique (KLA Flexus). The Flexus is a contact-less characterization equipment based on a laser profilometer technique. The equipment is equipped with an in-situ furnace, which allows us to heat the wafer with a controlled temperature ramp. The mean stress in the deposited layer is obtained by using the bow variation of the plate before and after the deposition step and during thermal loads.
For polymers, the material crosslinking should be optimized to achieve a reproducible change in the stress during the various thermal treatments.
In the case of metals, the technology is used to study the stability of the microstructure and allows extracting the thermal expansion coefficient of materials.

We report photo-induced minority carrier annihilation properties at the silicon interface in MOS structure using a 9.35 GHz microwave transmittance measurement system. 7-ohmcm n-type 500-micrometer-thick crystalline silicon substrates coated with 100-nm-thick ther-mally grown SiO2 layers were prepared. The initial samples had the minority carrier effective lifetime and recombination defect density of 1.1x10-3 s, and 2.8x1010 cm-2, respectively. A 100-nm-thick, 2-mm long and 4-mm-wide Al electrodes were formed on the top and bottom surfaces, respectively to form a vertical structure of Al/SiO2/Si/SiO2/Al. The bias voltage was applied from 4.0 and -4.0 V at the top surface with keeping the rear surfcace Al at 0 V. 635-nm light illumination to the left side silicon region of the Al electrode at the top surface caused photo-induced carriers. Microwave transmittance was measured in the right side silicon region of the Al electrode. Measurement and analyses of microwave absorption by photo-induced carriers laterally diffusing across the silicon region coated with Al electrodes from its left to right region revealed that the density of carrier recombination defect states at the silicon surfaces was increased by the bias voltage applied to the top Al electrode. The bias voltage application at 2.0 and -2.2 V gave peaks of the densities of surface recombination defects of 5.2x1010 and 5.4x1010 cm-2, respectively, while it was 2.8x1010 cm-2 in the case of the bias free. Those results shows that the minority carrier recombination probability increased when the deplection region was formed with the Fermi level positioning at the mid gap at the rear and top surfaces by 2.0 and -2.2 V voltages application, respectively.

We propose a simplified numerical model for hot-wire deposition of silicon using silane gas. The model solves a set of coupled diffusion-reaction-flow equations in a cylindrical chamber with a co-axial filament. The spatial distributions for the radical species Si, Si2H2, SiH3, and H and the stable gases silane, disilane, and hydrogen, as well as film growth rates and thickness uniformity, are calculated as a function of initial silane flow rate, filament temperature, and pumping speed. The results are compared with experimental radical and film growth measurements in the literature. In particular, the effects of high silane depletion conditions and the implications for high rate silicon film deposition will be discussed.

Laser heating and annealing of amorphous silicon films is of interest for improved material properties. However, due to the variety of possible laser treatments with regard to wavelength, pulse duration, repetition rate etc, a defined characterization of laser treated material is a challenge which we try to approach by comparing oven and laser treated material. Here we report on results of oven heat treatments (up to T = 500 - 600°C) on microstructure and hydrogen content of hydrogenated amorphous silicon films to be used as reference for the impact of laser treatment. Microstructure is characterized by infrared absorption measurements which yield the concentration of bonded hydrogen and the infrared microstructure parameter R(IR) (1) as well as by effusion measurements of (low dose) implanted helium (2). Since He does not bind to the silicon material, He effusion spectra are sensitive to microstructure effects and detect in particular isolated voids / cavities (2). Predominantly plasma-deposited films were investigated which were undoped as well as phosphorus or boron doped. By using different substrate temperatures Ts ge; 200°C the hydrogen content C(H) was varied. The results show for films with C(H) ge; 5 at. % a strongly increasing concentration of isolated voids for undoped and P doped material with rising annealing temperature Ta. E.g. for undoped a-Si:H deposited at Ts = 200°C the estimated concentration of isolated voids increases from about 10¹⁸ cm^-3 in the as-deposited state to >10²⁰ cm^-3 by annealing. In contrast, for B-doped material the isolated void concentration is high in the as-deposited state and decreases with rising Ta. No isolated voids are detected (estimated detection limit 10¹⁷ cm^-3) for material with C(H) < 4at. %, both in the as-deposited and annealed states. The results of the effusion measurements of implanted helium are discussed in relation to infrared microstructure parameter and hydrogen content. The data suggest void (cavity) formation by hydrogen (H₂) precipitation.
1. A.H. Mahan, P. Raboisson, D.I. Williamson, R. Tsu, Solar Cells 21, 117 (1987)
2. W. Beyer, Physica Status Solidi (c) 1, 1144 (2004).

As a material for thin-film photovoltaic solar cell device applications, hydrogenated nanocrystalline silicon (nc-Si:H) offers a number of inherent advantages when contrasted with the case of hydrogenated amorphous silicon (a-Si:H). In addition to possessing a higher short-circuit current density and an improved long wavelength response, nc-Si:H is recognized as being relatively insensitive to light-induced degradation when contrasted with the case of a-Si:H; light induced degradation is widely recognized as a fundamental impediment to the success of a-Si:H thin-film based photovoltaic solar cell device technologies. In recent years, a number of record solar cell and module efficiencies have been achieved through the use of nc-Si:H based photovoltaic solar cells. As a result, interest in the material properties of nc-Si:H has been quite intense as of late. This appears likely to remain to be the case for the foreseeable future.
The crystalline volume fraction within nc-Si:H is usually determined through the use of Raman spectroscopy. That is, through a process of peak decomposition and baseline correction, by identifying which peaks are associated with the crystalline component of this material, the crystalline volume fraction within a sample of nc-Si:H may be ascertained. The resultant crystalline volume fraction is known to be sensitive to the wavelength of the Raman source, different crystalline volume fractions being obtained depending upon which Raman source is employed. We suspect that these variations that are observed in the crystalline volume fraction are related to the inhomogeneities that are present within the nc-Si:H itself. While short wavelengths, such as 442 nm, attenuate very quickly within the nc-Si:H, longer wavelengths, such as 735 nm, will attenuate very gradually. Accordingly, the region probed by Raman source will be strongly dependent upon the source wavelength. In this study, we examine how the obtained crystalline volume fraction changes in response to the Raman source. Four different Raman sources are considered for this experiment, the wavelengths varying between 442 and 735 nm. The experiments are performed on eleven nc-Si:H-based photovoltaic solar cells. Correlations between the variations in the crystalline volume fraction, the attenuation depth, and the impurity content profile, as determined through a Secondary Ion Mass Spectroscopy analysis, are explored. The device implications of these results are investigated.

In a recent publication [1], we reported on how the crystalline volume fraction is determined by the mean crystallite size for the case of hydrogenated nanocrystalline silicon (nc-Si:H). Unfortunately, only three samples of nc-Si:H were considered for the purposes of this study and the magnitude of the error was such that the observed trend, that the crystalline volume fraction increases as the mean crystallite size decreases, while suggestive, was not convincingly demonstrated. In this study, we consider an additional eight nc-Si:H samples, these samples being produced using a variety of hydrogen dilution ratios. For each sample considered, the x-ray diffraction and Raman spectra are measured. Through the application of Scherrer&’s equation, the x-ray diffraction results are used in order to determine the corresponding mean crystallite size. We have performed these x-ray diffraction scans over much greater periods of time compared to our previous analysis, and thus, the amount error has been considerably reduced. Through peak decomposition and baseline correction, the Raman results are used in order to estimate the corresponding crystalline volume fraction; while we previously used a 442 nm Raman source, we now use a 514 nm Raman source for this study. Plotting the crystalline volume fraction as a function of the mean crystallite size, it is convincingly demonstrated that larger mean crystallite sizes tend to favor reduced crystalline volume fractions. The ability to randomly pack smaller crystallites with a greater packing fraction than their larger counterparts is suggested as a possible explanation for this observation. How these results are shaped by the hydrogen dilution that is used in the preparation of these samples is also considered. The device implications of these results are considered.
[1] K. J. Schmidt, Y. Lin, M. Beaudoin, G. Xia, S. K. O&’Leary, G. Yue, and B. Yan, Mater. Res. Soc. Symp. Proc. 1536, DOI: 10.1557/opl.2013.599, 2013.

Hydrogenated polymorphous silicon (pm-Si:H) is one of the most promising candidates for a stable thin film silicon solar cells. We report on the light-induced degradation kinetics of pm-Si:H solar cells having either substrate or superstrate device configuration. pm-Si:H solar cells with superstrate structure show very interesting degradation kinetics when compared to hydrogenated amorphous silicon (a-Si:H), summarized by macroscopic structural changes and irreversible changes in solar cell characteristics, while nevertheless preserving a higher stabilized efficiency. Such results suggest a device degradation mechanism including structural changes, active hydrogen motion, and interface delamination mediated by fast hydrogen diffusion and accumulation at the interface. Interestingly, we have found that pm-Si:H solar cells having substrate structure are remarkably stable under same light-soaking condition. The difference between the two types of devices is shown to be related to interface delamination which only occurs in superstrate devices. We further demonstrate a strong correlation between short-circuit current density decrease upon light-soaking and solar cell area loss.

Relationships between crystallinity, adhesion, and deposition rate were investigated for nanocrystalline silicon films deposited at 100°C by the catalytic chemical vapor deposition (Cat-CVD) technique for applications to solar cells and thin films transistors (TFTs) on flexible substrates. Hydrogen dilution of the source gas was proven to be the key factor governing all these characteristics in the low temperature deposition. As the hydrogen-to-silane dilution ratio, R_H = [H₂] / [SiH₄], increased, the crystallinity increased but the deposition rate and the adhesion were degraded. When R_H increased from 34 to 72, the crystalline volume fraction increased from 57% to 67%, linearly proportional to R_H. However, the deposition rate decreased from 35 nm/min to 8 nm/min at the same time. When R_H exceeded 64, it was impossible to grow films that are thicker than 100 nm due to the poor adhesion. When R_H was higher than 74, no sign of film deposition was observed after 40 minutes of the process time. Lowering the chamber pressure was found to improve adhesion at the expense of substrate self-heating. Based on these results, gradation of process variables were attempted to grow thick (>600 nm) films. Initially, the films were deposited at high R_H and low pressure for a short period of time, and then the pressure was raised slightly and R_H was reduced gradually. The resulting films exhibited a high crystallinity, a reduced incubation layer thickness, a fast growth rate, and good adhesion, while the substrate temperature was maintained successfully at 100°C.

As the semiconductor devices shrink, spin-on dielectrics (SOD) has been widely adopted to form insulating layers due to its excellent gap-filling property, high throughput, and competitive cost of ownership. Polysilazanes, Si-N containing polymers, are suitable for SOD applications since a high quality, thin silicon dioxide layer can be obtained through curing a polysilazane film. However, the curing process usually requires very high temperature, which is not preferable by semiconductor manufactures. To resolve the problem, several additives were tested to reduce the curing temperature. It was found out that amine compounds were effective to convert polysilazanes into silica, therefore the curing could happen at lower temperature. We also observed that the SOD containing amine additives had less shrinkage during conversion, and showed higher etch resistance during a wet etch process. These results, as well as the lower curing temperature, are benefit to make better insulating layers. Further investigation is on the way to optimize the formulation conditions and curing processes.

Recently, considerable attention has been directed towards aSi/cSi HIT solar cells because of their potential for further efficiency improvement and cost reduction [1, 2]. The efficiency of this type of cell is highly dependent on the quality of the absorber layer and the interface properties. One of the approaches to achieve a high quality absorber layer is to epitaxially grow it. Using epitaxial thin film technology, high quality c-Si can be grown [3]. Since the cost of growing c-Si layers epitaxially is high, reducing the thickness is paramount. However, reducing the thickness will reduce the amount of sunlight the layer can absorb.
In this work the effect of epitaxially grown p-type c-Si thickness on thin film a-Si(n+)/i-aSi /c-Si(p)/c-Si(p+) HIT solar cell is studied experimentally. Cells with 2, 4 and 6 mu;m thick p-type c-Si layers are fabricated and compared to study the effect of the absorber layer thickness. The c-Si layer with nominal boron concentration of 1e16 cm-3 was grown on the p+ substrate by LPCVD at 900°C in an Applied Materials ‘Epi Centura&’ reactor using SiH4. The layers were then capped with 5 nm undoped a-Si:H followed by 15 nm n+ a-Si:H emitter, using PECVD. The intrinsic aSi:H is used to improve the passivation at the interface between aSi(n+)/cSi(p) [4, 5]. Finally 80nm of ITO was sputtered.
The 6mu;m cells provide Voc: 0.579 V, Jsc: 9.34 mA/cm2, FF: 51.48% and eta;: 7.1% whereas 4mu;m cells gives Voc: 0.575 V, Jsc: 20.18 mA/cm2, FF: 60.24% and eta;: 6.99%. 2mu;m cells achieve Voc: 0.570 V, Jsc: 17.81 mA/cm2, FF: 64.66% and eta;: 6.57%. The results show that the Jsc drops from 21.2 mA/cm2 to 17.28 mA/cm2 when the c-Si thickness is decreased from 6 mu;m to 2 mu;m. The peak EQE decreases from 75% to 60%. The Voc doesn&’t change significantly as expected. In addition the efficiency drops from 7.1% to 6.57%. The rather small drop in Jsc and efficiency indicates a trade-off between cost of growing the thick cells vs. the performance achieved. It should be noted that cheap, novel light management schemes including plasmonic nanoparticles can be used to improve the Jsc even further.
Acknowledgement: Authors would like to thank Professor Judy Hoyt from MIT for growth of the epitaxial c-Si
[1].Alberi et al. “Material quality requirements for efficient epitaxial film Si solar cells”, Appl. Phys. Lett. 96, 073502 (2010)
[2].Taguchi, M et al., “HIT Cells - High-Efficiency Crystalline Silicon Cells with Novel Structures”, Progress in Photovoltaics: Research and Applications, vol.8, pp. 503-513, 2000.
[3].Beaucarne et al. “Epitaxial thin-film Si solar cells”, Thin Solid Films Volumes 511-512 (2006)
[4].Alnuaimi , A., et al. “Effect of interface states (Dit) at the a-Si/c-Si interface on the performance of thin film a-Si/c-Si/c-Si heterojunction solar cells." PVSC, 38th IEEE, 2012.
[5].Alnuaimi , A., et al. “Reduction of interface traps at the aSi/cSi interface by Hydrogen and Nitrogen annealing” Accepted for Journal of Solar Energy, 2013

Much of the energy conversion efficiency of a Si-based solar cell is affected by physical processes that occur within a few molecular layers of a surface or interface of the device. Consequently, research on the interfacial chemistry at the Si surface has a tremendous role in this research area.
One of the strategies to modify the interfacial chemistry at Si surfaces is the organic functionalization. It consists in a process of direct, covalent attachment of organic layers (i.e. those that contain carbon) to the Si surface. This attachment provides for the incorporation of many new properties to the semiconductor substrate, including surface passivation and luminescence stabilization of Si nanostructures. However, the design of the functionalization step to be introduced in a solar cells manufacturing process requires judicious compromises and trade-offs between such factors as process time, temperature, pressure, solubility, crystallinity, and molecular orbital energy levels of the materials applied to obtain it.
In this work we analyze different routes to obtain an optimal functionalization of silicon (100) and (111) surfaces, as the (100) surface is the most used as substrate for silicon-based solar cells, and the (111) is the one obtained after anisotropic texturization of monocrystalline silicon, a usual procedure for saw damage etching and for enhancing the antireflection properties in the initial steps of a standard solar cells production process.
Our results show that the best option is to produce hydridation of the Si surface dangling bonds and, subsequently, alkylation. Six different alkylation processes are discussed: (i) radical initiation; (ii) thermal breaking; (iii) acidic catalysis; (iv) organometallic functionalization; (v) photo-initiation; and (vi) electrochemical functionalization. The introduction of a chlorination process between the hydridation and the alkylation process is also discussed.

We report an effective method to fabricate high-quality thin film with composite quantum dots (CQDs) as a building block for thermoelectric materials. More than 3 times the usual Ge deposition can be incorporated within a CQD. Such thin-film-like CQD materials are shown to exhibit reduced thermal conductivity and enhanced electrical conductivity compared to the conventional QDs. Moreover, the dot size and morphology, Si/Ge interface density, and composition distribution in CQDs can be manipulated by tuning the inserted Si thickness as well as increasing the stack number of CQDs, or through an annealing process. Selective chemical etching experiments further show that a thin inserted Si layer in CQDs modifies the growth mechanism through surface-mediated diffusion and SiGe alloying effects. The reduction of thermal conductivity and enhanced electrical conductivity make such thin-film-like CQD materials promising candidates for future TE applications in nano- or micro-electronics.

Silicon nanowires (SiNWs) were uniformly decorated with ultra nanocrystalline diamond (UNCD) by a novel route using a cheap seeding source ‘paraffin wax&’. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicate that this one-dimensional ultra-nano crystalline diamond decorated SiNWs (UNCD/SiNWs) have almost uniform diameters ranging from 30 to 150 nm with a bulbous tip of diameter 250 nm. Electron energy loss spectroscopy (EELS) confirm the presence of diamond on SiNWs, similarly High resolution transmission electron microscopic (STEM) images ratify the presence of SiNW and UNCD by measuring the lattice spacing. Energy dispersive spectroscopic (EDS) elemental mapping reveals a strong signal of C indicating a high density, uniform coating of UNCD throughout the wire as well as the tip. Additionally they exhibit enhanced electron field-emission (EFE) properties with a turn-on field of about 3.74 Vµm-1 at an anode-cathode separation of 100 mu;m. The threshold field for attaining a current density of J > 1mAcm-2 was around 25 Vµm-1. The enriched EFE properties of UNCD/SiNWs render them advantageous over many other electron field emitters.

Interest in porous silicon (por-Si) substrates remains high due to their potential use in applications ranging from optoelectronics to chemical sensors. Previous work in our lab, and others, has explored the bonding of organic monolayers to p-type por-Si surfaces, via the hydrosilylation of olefins and aldehydes. In this project, we have adapted simple benchtop methodologies to functionalize photoluminescent samples of n-type por-Si using aldehydes. Light-promoted, thermal, carbocation-mediated, and Lewis acid catalyzed hydrosilylation reactions were all successfully employed to bind aldehydes to hydride-terminated, photoluminescent samples of n-type por-Si surfaces. The resulting organic monolayers, bound to the por-Si via covalent linkages, greatly decreased the rate of substrate oxidation and degradation when exposed to alkaline solutions (pH 14, NaOH). Monolayers prepared from aliphatic aldehydes provided greater substrate protection when compared to phenyl-terminated coatings. Additionally, we observed that monolayers prepared via thermal-facilitated hydrosilylation pathways resulted in the highest reaction yields and exhibited the most effective substrate protection. Evidence of this initial progress is provided primarily through characterization of surfaces via transmission mode Fourier transform infrared spectroscopy (FTIR).

Porous silicon (por-Si), a substrate that exhibits vast surface areas on the order of 500 m^2/cm^3, is an attractive candidate for use in many applications. These include microelectronics, medical diagnostics, and drug delivery, among others. However, hydride-terminated por-Si is easily oxidized to silicon dioxide under aqueous and ambient environments. One method of chemical passivation involves the use of a selection of hydrosilylation reactions. These reactions functionalize organic groups onto hydride-terminated porous silicon. The resulting monolayers, bound through direct silicon-carbon (via primary olefins) and silicon-oxygen (via aldehydes) bonds, are produced via thermal, carbocation, and Lewis acid mediated pathways. All of these wet, benchtop methods result in the formation of stable monolayers which protect the underlying silicon surface from ambient oxidation and chemical attack. This project compared the functionalization and stability of p-type and n-type porous silicon through surface characterization via transmission mode Fourier transform infrared (FTIR) spectroscopy. Surface degradation tests were performed by exposing por-Si to alkaline solutions (pH 14, NaOH). The functionalized samples oxidized and dissolved at a greatly reduced rate when compared to the deterioration rate of the silicon-hydride control samples. Bare p-type por-Si demonstrated greater stability than n-type por-Si, showing a slower rate of surface oxidation in air and pH 14 solution. Similarly, functionalized p-type por-Si proved more resistant to oxidation and organic monolayer degradation than the analogous n-type samples. Based on these initial results, the thermal reaction appears to yield the greatest efficiency and stability.

Silicon nanowires (SiNWs) may impact future photovoltaic applications. SiNWs studied in this work were prepared by Electroless Metal Deposition technique on p doped polished Si (111) and Si (100) wafer chips. The cleaned Si wafer chips were immersed into an aqueous HF/AgNO₃ solution for 15-30 min at room temperature. The length of SiNWs could be effectively controlled through tuning the treatment time. SiNWs arrays were dipped in 30 wt.% HNO₃ aqueous solution for 60 s and repeated for several times to remove all residual Ag from the SiNWs surfaces. Samples prepared on Si (100) substrates have a black aspect to the naked eye, while samples prepared on Si (111) substrates have brown-yellow aspect. Total reflectance (Rt) and total diffuse reflectance (Rdt) for polished Si and SiNWs samples were measured in an ISP-REF Ocean Optics (OO) integrating sphere. Rt of polished Si shows a peak which correspond to Γ_25'-Γ₁₅ direct transition and Rdt shows an step which correspond to Γ_25'-L₁ indirect transition. For SiNWs the general shapes of the spectra changes drastically: Rt of the SiNWs samples is smaller than Rdt of polished Si and the peak corresponding to transition Γ_25' -Γ₁₅ disappears. Besides, the step is less abrupt for samples prepared on Si (100) and disappears for samples prepared on Si (111). Studies of diffuse reflectance (Rd) of non polarized or polarized incident light allow knowing the different factors which affects the very small reflectance of SiNWs arrays. The experimental setup consists mainly of an OO HLS2000 halogen lamp and an OO S2000 spectrophotometer. Light is coupled from and to these instruments with optic fibers using collimating lenses at the other extreme of the fiber. The sample is arranged on a goniometer. In the case of measurements with polarized light, a Glam Thompson polarizer was used. The collecting optical fiber was arranged very near the sample to capture Rd in the Littrow configuration. Dependence of integrated Rd (IRd) spectra with incident angle (theta;) was studied. For samples prepared on (100) Si substrates, the IRd increases with theta;. IRd for p polarized incident light is greater than for s polarized light. Also, Rd spectra show structures which correspond to interference fringes. For these reasons SiNWs prepared on (100) Si substrates can be considered as a thin film whose refraction index depends on polarization light. For samples prepared on (111) Si substrates, Rd does not depend on polarization. IRd shows maxima at certain angles. In these angles, the direction of incident light is normal to [100] and [010] oriented SiNWs. This was verified by X-Ray diffraction measurements and is in agreement with SiNWs growing along these directions. Moreover, Rd of SiNWs prepared on (111) substrates can be modeled by an ensemble of diffuse reflectors.

Abstract
Design and development of highly mesoporous organic-inorganic functional materials has gained much attention in the recent past. The structural diversity and enhanced physiochemical properties of organic-inorganic hybrid materials make them suitable candidates for various applications in areas ranging from host-guest chemistry, catalysis, opto electronics and targeted drug delivery.[1] Organically functionalized mesoporous silica has received wide interest in this regard as they can incorporate small molecules inside the silica framework.[2] Development of nanotechnology has brought revolutionary changes especially in bio-medical sector with engineered nano devices for mankind.[3] Apart from the advantages, severe health impacts and environmental pollution are considered to be the adverse side effects of nanotechnology. In this context, removal and extraction of nanomaterials from environment is of great interest to protect the living organism. We have synthesized a hyperbranched silicon based organic-inorganic hybrid material via a one pot synthesis of tetraethyl orthosilicate (TEOS) with diols. The talk will be focused on the synthesis, characterization and applications of the silicon based hybrid materials for the removal of nanoparticles from nanowaste materials.
References
1. A. Comotti, S. Bracco, P. Valsesia, M. Reretta and P. Sozzani, Angew. Chemie. Int., 2010, 10, 1760-1764.
2. M. R. Sohrabi, Z. Matbouie, A. A. Asgharinezhad and A. Dehghani, Microchim. Acta.,2013, 1-9.
3. A. M. Thayer, Chem. Eng. News., 2003, 81, 15-22.
Acknowledgement
The authors thank the Agency for Science, Technology and Research (ASTAR) for the financial support of the work. The authors also thank Department of Chemistry and NUS for technical support.

Effective absorption of sunlight is a critical factor to improve efficiency of solar cells. We have been developing 4-terminal spectrum splitting solar cells consisted of a-Si:H top cell and CuInGaSe₂ bottom cell with designed optical filter to split solar spectrum into each cells. Recently we reported 22% efficiency for the spectrum splitting cells, which is highly efficient compared to mechanically-stacked multi-junction solar cells. To achieve the target of 25% (@1 Sun), higher V_oc and current gain in short wavelengths are required for a-Si:H top cell. For these properties, wide bandgap (E_g) i-layer is needed in addition to p-type window layer. Thus, we have focused on intrinsic a-SiO_x:H instead of the a-Si:H. In literature, a-SiO_x:H films were deposited at 180-200 ^oC which are preferred substrate temperatures for silicon thin film solar cells. However, those films had lower photosensitivity about ~10⁴ than the a-Si:H (10⁵-10⁶). This may limit the V_oc of the devices in spite of using the alloys to increase E_g.
In this work, we aimed to fabricate the i-a-SiO_x:H films at extremely low temperature of 100 ^oC with better photosensitivity and wider E_g than reported properties for high V_oc top cell of the spectrum splitting cells. By depositing them at 100 ^oC, we expect that we can add more hydrogen into the films and increase the film E_g and sensitivity. Up to now, we have succeeded fabricating i-a-SiO_x:H films with the photosensitivity more than 10⁵ and wide E_g (>1.85 eV) at 100 ^oC. In addition, a-SiO_x:H solar cells has been successfully fabricated with developed p- and n- doped SiO_x:H layers at 100 ^oC.
The a-SiO_x:H films were made by VHF PECVD at 100 ^oC. The effects of R ratio (=H₂/SiH₄) and CO₂/SiH₄ ratio on the quality of the i-a-SiO_x:H films were investigated. The various films were made under the wide ranges of the R (16-32) and the CO₂/SiH₄(0.1-0.5).
As for E_g, it increased from 1.84 to 1.95 eV with high H₂ and CO₂ addition during deposition. However, strong H₂ dilution combined with low CO₂ addition led to microcrystalline Si phase. Also, high CO₂ addition (CO₂/SiH₄ > 0.3) made the deposited film with low photosensitivity (~10³ ) due to considerable incorporation of oxygen. Under the region of the low CO₂ (CO₂/SiH₄ = 0.1-0.2) combined with the R of (20-22) just before microcrystallization (R>24) for sufficient H₂ incorporation, promising a-SiO_x:H film with high photosensitivity (~2×10⁵) and suitable Eg of 1.85 eV was obtained at 100 ^oC. By using this film, a solar cell with a simple structure of {Asahi VU/p-a-SiO_x:H/i-a-SiO_x:H/n-mu;c-SiO_x:H/Ag/Al} was firstly demonstrated and an efficiency of 6.05% (V_oc=0.966V, J_sc=9.22 mA/cm², FF=0.683) was attained. Higher performance is expected by optimizing quality control of i-layer and p/i interface. The experimental results suggest that the i-a-SiO_x:H film fabricated at low temperature is promising for high V_oc top cell of the spectrum splitting solar cells.

We report on the effects of post-synthesis treatments to improve the optoelectronic properties of electrochemically etched silicon nanocrystals (SiNCs). One of the major challenges in the utilization of SiNCs produced by electrochemical etching is represented by the interfaces between nanocrystallites as nanocrystal interfaces often present defects and are not easy to reach for surface passivation. Here we have investigated a range of approaches such as ultrasonication, filtration and surface engineering via atmospheric pressure microplasma to provide effective means for the fragmentation, size selection and for the surface functionalization of the SiNCs leading to improved properties with long-term stability. However, each process needs to be tuned to prevent detrimental surface degradation. Ultrasonication, filtration and microplasma processing of SiNCs are investigated by photoluminescence, ultraviolet-visible absorption, Fourier transform infrared spectroscopy and transmission electron microscopy. Films of post-treated SiNCs fabricated on glass are also investigated as above and including Kelvin probe and electrical measurements.

The ability to print Group IV semiconductor devices from quantum dot inks opens up a pathway towards lower cost, lower energy, and larger area semiconducting devices that retain the benefits of quantum confinement - potentially leading to a paradigm shift for the semiconductor industry. Achieving this goal in nanoparticles is complicated by the same properties that make Group IV materials so desirable in bulk form, namely the long mean free path, the ability to form stable robust oxides, and control over surface passivation. In this overview talk, I will discuss the challenges in preparing semiconducting thin films (~100 nm to 1 micron) from printed solutions of colloidal silicon and germanium quantum dots with focus on the role of the surface passivation and the ligand exchange. The electrical, optical and structural properties of the quantum dot thin films will be compared to the colloidal solution properties and bulk system properties. I will describe the properties of the resulting photoconductors and photovoltaic devices prepared using bulk-heterojunction (i.e. TiO2) device structures that retain the properties of the quantum dots, such as high absorption coefficient and band-gap tuning. I conclude by discussing the importance of doping in quantum dot Si and Ge inks in the context of pn junctions and also compare the properties of thin films printed from colloidal-solution versus plasma-fabricated quantum dots.

Crystalline silicon wafer (c-Si) solar cells dominate the global photovoltaics (PV) market but are reaching the limit of what is possible: 24.7% conversion efficiency has been reached on wafer-sized cells, closely approaching the theoretical limit of 29.4%. A new approach is required, and one that can exceed the limit for c-Si both in theory and practice is the tandem cell, an approach where two or more subcells with different band gaps are combined into one cell with reduced thermalization and transmission loses. Silicon nanocrystals (Si NCs), or quantum dots, are a promising candidate for the top cell of an all-Si tandem solar cell, with a band gap from 1.3-1.7 eV, tunable by adjusting NC size. They are readily produced within a Si-based dielectric matrix by precipitation from the Si excess in multilayers of alternating stoichiometric and silicon-rich layers. In this work, we select SiC as a host matrix as it has a band gap (2.4 eV) quite close to that of Si NCs, which improves transport and absorption. We study majority carrier transport using test structures with coplanar contacts, and minority carrier transport using solar cell devices with Si NC/SiC as the absorber layer.
All films exhibit ohmic behavior, rather than the exponential dependence on electric field usually observed for tunneling between quantum dots. We also find that conductivity is somewhat dependent on the size and distribution of Si NCs in poly-SiC, and that a remote plasma hydrogen passivation (RPHP) has a much more pronounced effect, increasing the conductivity of some films from 1E-4 S/cm to 0.1 S/cm. The effectiveness of the RPHP however is strongly dependent on the Si NCs in the film. Transport is thermally activated and data can be fitted with two activation energies (Ea1asymp;0.2 eV and Ea2asymp;0.1 eV) for a wide range of samples. The activation energies change by some tens of meV with RPHP, indicating that they cannot be the activation energy of a given doping level.
We explain these observations in terms of amorphous semiconductor theory, where Ea1 is the activation energy for extended state transport, and Ea2 represents hopping. Ea1 (asymp;0.2 eV) is the difference between the extended states and the Fermi level, which together with the high conductivity of some samples can only be explained by background doping. A comparison with SIMS data reveals that background doping occurs during film deposition. Nitrogen is the main dopant in one chamber, oxygen in another. This makes the films n type. The effect of RPHP can be explained with the passivation of scattering centers, increasing mobility, or the passivation of traps, releasing more carriers into conducting states.
P-i-n cells consisting of a Si NC/SiC membrane sandwiched between p- and n-a-Si:H were used to study minority carriers (holes). Holes are shown to travel by drift in the inbuilt field, rather than diffusion. Effective mobility-lifetime products of 1E-10 cm2/V were deduced.

Quantum confined (QC) semiconductor systems have drawn much attention in photovoltaics research due to their promising, tunable, optoelectronic properties and potential for efficiency improvements through quantum-mechanical effects. Here, we report a study of a nanocomposite material, called nanocrystalline silicon (nc-Si:H), consisting of silicon nano-particles (SiNPs) embedded in a hydrogenated amorphous silicon (a-Si:H) matrix. We demonstrate the tuning of the optical band gap of SiNP/a-Si:H hybrid films in which the SiNP size has been reduced into the quantum confined regime. This new result advances the earlier studies[1] and opens up new device possibilities such as an all nanocomposite silicon-based multi-junction thin film solar cell with high hole mobility.
Films are prepared by depositing the SiNPs and a-Si:H sequentially from separate plasma reactors in a common deposition chamber to form a hybrid material layer-by-layer. Since the depositions of the two phases are decoupled, we can independently optimize the SiNPs and a-Si:H properties. Namely, the a-Si:H shows a low density of dangling bond defects in electron spin resonance (ESR), while the SiNPs exhibit high crystallinity as determined by Raman spectroscopy. In addition, photoluminescence spectroscopy (PL) shows that the diameter of the SiNPs included in the hybrid films can be tuned within 3-7 nm, well within the quantum-confined regime.
Optical properties of hybrid SiNP/a-Si:H films were explored using visible to near infrared PL, optical absorption, and photothermal deflection spectroscopy (PDS). Low temperature PL measurements reveal two primary emission features, one from conventional a-Si:H near 1.3 eV and a second peak which can be attributed to recombination in SiNPs. The energy of this peak is higher than the bulk c-Si bandgap (~1.2 eV), and with decreasing SiNP size, it increases to ~1.7 eV. This quantum confinement effect agrees with our DFT theory presented separately in this symposium. In addition, we also see that the PL peak for SiNPs surrounded by a-Si:H shifts to lower energy relative to the isolated SiNPs. This shift is also consistent with our modeling results which show that surrounding SiNPs with a-Si:H leads to a softening of the confinement barrier, confinement only of the valance band electronic states, and a redshift in the optical gap.
Support of the DOE SunShot(DE-EE0005326) and NSF MRSEC programs(DMR 0820518) are gratefully acknowledged.
[1] James Kakalios, U. Kortshagen et al., Mater. Res. Soc. Symp. Proc. Vol. 1321, 2012, Pages 337-348

Silicon nano-crystals are a novel material that have many potential applications as a tunable high band gap layer in thin film solar cells, a gain medium suitable for silicon based opto-electronics and light emission, and very recently as a thin film electronic ink. The quantum confinement of carriers in silicon nanocrystals are well known to exhibit light emission and enhanced photo-luminescence.
We develop here a novel method to generate silicon nanocrystals, and relate experimental measurements to simulations. In our procedure, we grow super-lattices of a-Si:H and silicon oxide (SiO2) by plasma enhanced chemical vapor deposition (PECVD), with individual layer thicknesses varying between 5 and 20 nm. We have developed a novel step-wise annealing procedure, where these films were annealed to in temperature steps to 1100 C, that results in crystallization of silicon nano-crystals within an amorphous silicon matrix. X-Ray diffraction peaks confirms nano-crystallite formation through the appearance of diffraction peaks. Raman measurements indicate a crystalline volume fraction exceeding 50%. Transmission electron microscopy (TEM) demonstrate the appearance of silicon nano-crystals that are 5-15 nm in size. Photo-luminescence exhibits peaks between 700-800 nm associated with the nanocrystal formation. We will describe the nano-crystal dimension and photo-luminescence as a function of the superlattice layer thickness, and annealing conditions, to understand the crystallization process. To understand the morphology of these nano-crystals we have performed classical molecular dynamics simulations that generate nano crystals of varying sizes by an annealing procedure. The simulated nanocrystals have many structural similarities to the experimentally synthesized. We will discuss the interfaces between the nano-crystals with the amorphous matrix, both theoretically and experimentally.

Nano structured Si absorber layers are candidates to improve efficiencies of thin film Si solar cells without increasing costs. Si-SiO₂ nano sponge-like nanocomposites are promising materials as they exhibit a widened band gap due to quantum confinement and electrical interconnectivity due to percolation of the nanostructured Si. The sponge-like structures can be formed upon annealing of substoichiometric SiO_x films (x<1), which leads to spinodal phase separation into a perlocated network of Si nanowires embedded in SiO₂, tentatively accompanied by crystallization of the Si.
Here the influence of the precursor composition on the evolving sponge-like nanostructure and on the optical properties is investigated. SiO_x layers have been grown by reactive sputter deposition where the composition of SiO_x films was controlled by varying the oxygen flow during the deposition and subsequently measured by Rutherford backscattering spectroscopy (RBS). SiO_x layers with compositions between x=0 and x=1.2 have been addressed. The Si-SiO₂ nanocomposites are fabricated using a very rapid thermal processing by scanning a diode laser line source. Dwell times in the ms range and power densities of the red laser light of about 10³ W/cm² have been investigated.
Laser treatment of the precursor SiO_x layers leads to decomposition into Si and SiO₂ thereby forming Si-SiO₂ sponge-like structures as observed by energy filtered transmission electron microscopy (EFTEM). While thin a-Si films show crystallization, oxygen rich films with Si structures smaller than 2nm do not show crystallization. The widening of the band gap due to quantum confinement has been confirmed by optical measurements.
Our results demonstrate that the composition of the precursor material is of crucial importance to obtain a Si-SiO₂ nano sponge-like material suitable as PV absorber.

There is an increasing interest in finding novel concepts for increasing the efficiency-to-cost ratio in solar cell devices. Among the different possibilities, semiconductor nanowires have shown to provide various paths towards this goal. In this talk, I will discuss different aspects in considering semiconductor nanowires for next generation photovoltaics: 1) increased device freedom for optimizing the carrier extraction and light absorption, 2) a self-concentrating effect which provides the potential to surpass the Shockley-Queisser limit [1], and 3) three-dimensional-heterostructure design in up-conversion strategies [2].
References:
[1] P. Krogstrup et al, Nature Photon, 7, 306 ( 2013)
[2] E. Alarcon-Lladoacute; et al. In review (2013)

Nanocrystalline silicon (nc-Si) is traditionally grown using a high dilution of SiH4 in H2 which leads to the formation of single crystal silicon crystallites within an amorphous silicon (a-Si) matrix. This approach typically leads to crystallites that are too large to be quantum confined. If nc-Si could be grown with quantum confined silicon nanoparticles (SiNPs) the band gap could be tuned, which might allow for the fabrication of all silicon multiple junction photovoltaic cells or the realization of hot carrier collection. To produce nc-Si with quantum confined SiNPs we have developed a system to decouple the two growth processes. For the SiNP growth, a highly dilute SiH4 in Ar precursor is introduced to a quartz tube where a plasma decomposes the precursor to nucleate and grow SiNPs. These are injected into a standard parallel plate plasma enhanced chemical vapor deposition reactor where the a-Si is grown. For the growth of the nc-Si the SiNPs and a-Si are sequentially deposited to give a multiple layer structure.
To create highly compact SiNPs and produce smooth layers, a slit aperture is placed on the exit of the quartz tube. The slit width controls the pressure in the quartz tube, which controls both the size of the SiNPs and the velocity that the SiNPs have on exiting the SiNP reactor. Computational fluid dynamics modeling of the process shows that higher reactor pressures lead to longer residence times in the plasma (larger SiNPs) and high gas velocities exiting the SiNP reactor (high density films). This is confirmed experimentally showing there is a trade off between smaller SiNPs and the density of the SiNP layer. To overcome this we have also mixed SF6 into the gas stream to reduce the SiNP diameter while maintaining a high SiNP reactor pressure.
Films of nc-Si have been grown with both SF6 treated and untreated SiNPs. By controlling the a-Si thickness we are able to control the crystal fraction of the nc-Si films, as observed by a change in the ratio of the a-Si peak to the c-Si peak from Raman spectroscopy. From scanning electron microscopy images of focused ion beam prepared cross sections of the center of nc-Si films, a-Si and SiNP layers can be clearly observed. However, since the thickness of SiNP layer has a Gaussian spatial profile, film morphology varies from alternating SiNP/a-Si layers at the middle to regions where the SiNP layer is thin enough that the nanoparticles are fully encapsulated in a-Si, similar to traditional nc-Si. This allows a continuous variation from one extreme to the other to be studied. Electron spin resonance measurements show a low density of defect states indicating very high quality nc-Si. Temperature dependent photoluminescence and photothermal deflection spectroscopy have also been used to characterize the optical properties of this unique material. In additional we are using THz spectroscopy to understand the carrier lifetime of the nc-Si to explore carrier transfer from a-Si to the SiNPs.

One of the promising technologies for third generation solar cells is represented by devices that use nanocrystals (quantum dots). In particular, solar cells with nanocrystals that exhibit new physical phenomena such as carrier multiplication (CM), present great potential for efficiency improvement and have attracted vast attention in the photovoltaic scientific community. The principle of exploiting the benefits of CM has been shown by integrating PbSe or PbS nanocrystals in quantum dot solar cells. Nevertheless, the full assessment of the life-cycle, environmental aspects of PbSe/PbS nanocrystals is required. Indeed, other materials can instantaneously offer better opportunities that can be in principle more economically viable and with lower environmental impact. Since silicon-based technology is mature and dominates the solar cell market, a third generation photovoltaic technology based on elemental silicon (Si) nanocrystals represents a sensible approach. While Si nanocrystals offer a range of opportunities that still need to be explored, CM effects are in this case triggered only for relatively higher photon energies. Therefore, alloying Si with another element, which decreases the band gap, might offer the possibility of activating CM at lower photons energies. It has to be noted that nanocrystals made of Si and germanium (Ge) have been already produced, however the Si-Ge system is not expected to show the benefits of a direct-bandgap semiconductor. On the other hand, the silicon-tin (Si1-xSnx) system is an interesting candidate as an optically active material where the concentration of Sn can be effectively used to extend the range of achievable bandgaps below the energy gap of silicon (1.15 eV) down to 0.45 eV; furthermore, a transition from an indirect semiconductor behavior to a direct one is expected to occur with increasing Sn concentration in the nanocrystals. However, the Si-Sn system presents some synthetic challenges due to thermodynamic instability. Very recently we have demonstrated the synthesis of semiconducting SiSn nanocrystals via a highly non-equilibrium spatially confined short-pulsed laser process. In this contribution, the potential of using spatially-confined plasma to induce the growth of SiSn nanocrystals via kinetic pathways will be discussed in details. Our investigations suggest that alloying between Si and Sn can occur at relatively high Sn concentrations (<50%) resulting in the synthesis of semiconducting SiSn nanocrysatls with quantum confinement effects. Due to the successful alloying, quantum confinement and band gap narrowing is observed in the red shift of the room temperature photoluminescence (PL) maximum with respect to elemental Si nanocrystals. Furthermore we will show that the integration of surfactant-free surface-engineered SiSn nanocrystals into thin films allows for adequate absorption and carrier transport as required for their implementation in successful quantum dot solar cell technology.

Porous nanocrystalline silicon (pnc-Si) membranes were first reported and used for protein separation by Striemer et al. in 2007 [1]. These nanopores are formed during rapid thermal annealing of ultrathin (~ 15nm) amorphous Si films sandwiched between nm-thick SiO2 layers. Subsequent research has been carried out to study and optimize this novel material for applications in separation and cell culture [2-4]. The pore shape is one of the key parameters for separation applications. We report that the temperature ramp up rate during annealing greatly affects the final pore shape. A fast ramp up rate produces pores with very circular shapes whereas a low ramp up rate results in elongated, irregular pores. The interpretation for this phenomenon is related to the crystallization speed since pore formation is associated with Si nanocrystal formation. At low ramp up rates the amorphous silicon film undergoes slow solid state crystallization. Voids are formed at the interface of silicon nanocrystals and the amorphous silicon matrix. These voids move through the amorphous silicon film, and coalesce with other voids, which produces elongated pores. In contrast, at high ramp up rates, amorphous silicon crystallizes very quickly which prevents the circular pores from moving, coalescing and forming irregular pores. TEM movies of pore creation and evolution taken in-situ during annealing confirm our interpretation.
[1] C. C. Striemer, T. R. Gaborski, J. L. McGrath, P. M. Fauchet, Nature 2007, 445, 749.
[2] D. Z. Fang, C. C. Striemer, T. R. Gaborski, J. L. McGrath, P. M. Fauchet, J Phys-Condens Mat 2010, 22, 454134.
[3] T. R. Gaborski, J. L. Snyder, C. C. Striemer, D. Z. Fang, M. Hoffman, P. M. Fauchet, J. L. McGrath, Acs. Nano. 2010, 4, 6973.
[4] A. A. Agrawal, B. J. Nehilla, K. V. Reisig, T. R. Gaborski, D. Z. Fang, C. C. Striemer, P. M. Fauchet, J. L. McGrath, Biomaterials 2010, 31, 5408.

Two crucial points of photovoltaic technologies are performance represented by photoconversion efficiency and economical production. Si radial p-(i)-n junction nanowires have provided opportunities of concurrent fulfilling the two points because Si radial p-n junction can demonstrate reasonable photoconversion efficiency with marginal or poor quality silicon and growth of Si radial p-n junction nanowires has been achieved on cheap substrates such as stainless steel foil. However, studies on Si nanowire-based photovoltaic cells on stainless steel have been limited to simple demonstration of fabrication without thorough investigation of physical properties of Si radial p-i-n junction. Almost reported cases have employed sputtering of amorphous Si radial shell to form radial p-n junction. Thus, the Si nanowire photovoltaic cells on stainless steel have shown very low cell efficiency much less than 1%, and a few reports have shown efficiency of 3%. Here we present fabrication and characterization of photovoltaic devices based on Si radial p-i-n junction nanowires prepared by catalyzed growth and epitaxial regrowth of doped Si radial shell on stainless steel.
Si radial p-i-n junctions consisted of core Si nanowires and Si shells. Core Si nanowiress were prepared by Au-catalyzed vapor-liquid-solid growth via chemical vapor deposition (CVD). Dimensions and electrical doping profiles of epitaxial Si shells were precisely controlled by low-pressure CVD growth. Unintentional incorporation of transition metal impurities in Si nanowires grown on stainless steel substrate was observed. To minimize the unintentional incorporation, growth condition for Si nanowires on stainless steel was studied. Temperature study of Si radial shell growth revealed the optimal growth temperature at which autodoping and hemispherical growth on planar region can be avoided. The physical properties of Si radial p-i-n junction nanowires were investigated by light absorption measurement, current-voltage characterization, quantum efficiency measurement, and photovoltaic response measurement. Optical characterization revealed that randomly oriented Si nanowires on stainless steel substrate act as excellent light absorber in the almost visible wavelengths. Additionally, the photoconversion efficiency of Si radial p-i-n junction nanowires cell can be significantly higher than 3% with proper contact geometry.

Laser processing of thin-film silicon is a promising approach for the realization of polycrystalline silicon for large area electronics and solar cell applications. Here we investigate the modification of a-Si:H with different hydrogen content (30, 13 and <1 at. %) by femtosecond (fs) laser materials processing. The different hydrogen content of the intrinsic a-Si:H layers with thicknesses of either 50 nm or 300 nm was achieved by varying the temperature during PECVD growth (25°C, 200°C and 520°C). Single 30 fs light pulses of an amplified Titanium Sapphire laser (1 kHz repetition rate, 790 nm centre wavelength) are attenuated and focussed to an e-2-spot width of 420 µm leading to peak fluences between 30 mJ cm-2 and 120 mJ cm-2. The modified spots were characterized by optical microscopy, imaging ellipsometry at 658 nm, Raman micro-spectroscopy and scanning electron microscopy (SEM). The intensity profile of the laser beam in combination with microscopy allows analyzing the fluence dependence of the material modification across individual spots. Qualitative depth information of the material modification is obtained from Raman micro-spectroscopy using different excitation wavelengths (473 nm and 633 nm).
The general findings are the following: i) despite the low absorption coefficient of a-Si:H at 790 nm a high local energy deposition close to the surface of the a-Si layer is achieved via nonlinear absorption. Consequently, a distinct material modification (hydrogen effusion, recrystallization, ablation, etc.) in a thin layer near the surface can be achieved enabling the realization of electronic circuits for large area electronics and modification of contact and intermediate layers for solar cell application. In addition, pulse properties (duration, energy, shape, etc.) allow to control the laser modification depth profile. ii) The material ablation threshold depends strongly on the hydrogen content. For low hydrogen (< 1%) content a 100% higher fluence has to be applied before material ablation sets in. Surprisingly, below ablation threshold irradiation of this sample even increases the Si-H related Raman signal (2000-2100 cm 1 Stokes shift) by more than a factor of two in comparison to an untreated area. A detailed analysis of thin-film silicon modified by fs-laser pulses will be presented.

Thin film silicon and silicon materials used in solar cell and thin film transistors have evolved from amorphous silicon (a-Si:H) to nano-crystalline silicon (nc-Si:H) and large grain polycrystalline silicon (poly-Si). nc-Si:H films have provided significant improvements in the efficiency of thin film silicon solar cells. However, thin film silicon solar modules have lower efficiency than c-Si modules and as a result struggle to compete in the market. One method for improving the efficiency of thin film Si solar cells is to make large grain poly-Si films. Large grain polysilicon thin films (LTPS) with high mobility are also needed for the thin film transistors used in high resolution LCD and OLED displays. Currently LTPS TFTs are made using laser induced crystallization or metal induced solid phase crystallization of a-Si:H. These are expensive methods which are difficult to scale to large size displays. A low cost process for producing thin film poly-Si or possibly poly-SiGe is highly desirable for both photovoltaic and display applications.
We present our progresses in the fabrication of poly-Si, poly-SiGe and poly-Ge thin films using a pulsed-light induced crystallization method. We used sputter deposited a-Si, a-SiGe, and a-Ge as the starting materials. Sputtered Si, SiGe and Ge thin films are a lower cost and less complex process than the industry standard PECVD process. Sputtered films also do not require a de-hydrogenation process.
We used a pulsed-Xenon-lamp system with multiple-lamps to illuminate large-area samples. This process is much less expensive than the typical excimer laser process used in the display industry and is suitable for large area scale-up. Using this process we have demonstrated that we can uniformly crystallize a-Si, a-SiGe, and a-Ge with a single-pulse or multi-pulse process on 10×5 cm2 glass substrates. We found that the required crystallization power for a-Ge is much lower than a-Si. The power needed to crystallize a-SiGe is between the power required a-Ge and a-Si with power increasing with increasing Si fractions.
We did significant material characterizations using SEM, AFM, Raman and Spectroscopic Ellipsometry (SE). The SEM and AFM images showed a significant increase of the surface roughness, which corresponds to the formation of crystallites. Two kinds of features are observed in some poly-SiGe films with large features of a few µm, which could be the large grains; and small features of 0.5 µm, which could be a phase separations of Ge and Si. No Raman signal was measurable in the as-deposited films. Distinct Ge-Ge, Si-Ge, and Si-Si vibration modes were observed at 285 cm-1, 390-1, and 470 cm-1, respectively, in the poly-SiGe films formed after the pulsed-light treatments. Their intensity ratios depend on the Ge/Si ratio and the light intensity used for the crystallization. We are in the process of making TFTs and solar cells. The device characteristics will be presented at the conference.

Ultrashort pulse-length lasers are being employed in academic research on fundamental aspects of light interaction with matter already for many years. But now that compact, robust and stable fs lasers have become available, this type of lasers has also become of interest for industrial applications, e.g. in the PV industry. In Solliance we investigate the application of fs lasers for back-end series interconnection of thin film PV modules, but also their potential for depth selective laser crystallization of silicon. The latter application is described in this contribution.
In the heating process of solid matter by pulsed (laser) light, there are two crucial length scales which determine the borders of the heat propagation: the optical absorption depth do = 1/α and the thermal diffusion length dth = 2radic;lambda;T. Using a fs UV laser with a pulse length of 250 fs and a wavelength of 343 nm to heat silicon, these length scales are approximately the same and amount to about 10 nm. Through this virtue it is possible to achieve depth selective laser crystallization of amorphous silicon, in which only a top layer of 10-20 nm is crystallized where the underlying material is unaffected by the heat of the laser. This depth selectiveness of the crystallization process opens a wealth of new opportunities for the usage of thin crystalline silicon layers. In our contribution we will present recent results on the growth of epitaxial silicon and on the application of laser-crystallized p-layers as a novel highly transmissive window layer in n-i-p thin film silicon solar cells by this method.

Improvement of material properties by annealing processes is well known in silicon semiconductor technology and has been widely studied. However, in hydrogenated amorphous silicon (a-Si:H) materials the out-diffusion of hydrogen during the annealing process can cause the formation of blisters or even holes at the interface to the substrate [1,2]. This reduces the reliability of devices and even prevents the use of post-deposition annealing.
A promising post-deposition approach is annealing by absorption of intense laser irradiation since it offers a high selectivity combined with short processing times. Laser-annealing of a-Si:H bears an elevated risk of blister formation since this high speed process involves high rate and high temperature treatment. To allow for such experiments the interface between layer and substrate had to be modified.
To enhance the accessible range of thermal treatment towards higher annealing rates and final temperatures first silicon oxide based interlayers were introduced. Silicon dioxide layers are known for improved H2 solubility and diffusivity [3] and hence should prevent ablation. Within this study additional interface modifications were applied since it turned out that even some SiO2 layers were insufficient.
The impact of the interface modification on the blister formation was studied microscopically and the microstructure of the a-Si:H, i.e. the quality of the film, was monitored by IR-spectroscopy. The results show that only specially prepared oxides reduce the ablation of the films. Moreover it is shown that a proper surface texture allows for further improved adhesion of the film. Therefore the accessible temperature range could be extended up to a temperature where crystallisation starts.
1. H.R. Shanks, J. Appl. Phys. 52, 811 (1981).
2. Serényi, M. et al. Nanoscale Research Letters 8, 84 (2013).
3. W. Beyer and F. Einsele, in Advanced Characterization Techniques for Thin Film Solar Cells, edited by D. Abou-Ras, T. Kirchartz, and U. Rau (Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2011), pp. 449-475.

While low temperature poly-silicon (LTPS) using Excimer Laser Annealing (ELA) has enabled higher levels of integration and device performance, the technology is not compatible with backplane manufacturing for large-format displays made on Gen8 - Gen12 glass panels. This provides the motivation to investigate alternative process techniques for silicon-based technology. Alternative strategies for LTPS include solid-phase crystallization (SPC), metallization-induced crystallization (MIC), and flash-lamp annealing (FLA).
The FLA system used in this work is a NovaCentrix PulseForge 3300, which anneals the material using a series of short but intense bursts of broad spectrum light from xenon flash lamps. High peak power (e.g. 20kW) over microseconds time scale can provide control over the depth of heating to avoid damage when processing on substrates such as glass or plastic. The unit&’s modular design allows multiple lamps to be ganged together to accommodate arbitrarily large substrates.
The FLA process was investigated for the crystallization of a-Si deposited on display glass. Input factors to the FLA system included lamp intensity, pulse width, number of pulses and repetition rate. Additional factors included use of an anti-reflective SiO₂ layer, and substrate heating (steady-state). The degree of crystallization was quantified by Raman Spectroscopy and spectroscopic ellipsometry using an effective medium approximation. High-resolution SEM imaging was used to determine the average grain size. Different combinations of furnace annealing and FLA were studied for crystallization and activation of samples implanted with boron and phosphorus. Use of FLA for crystallization of MIC samples will also be presented.

Poly-Si TFTs with large crystal grains have been developed for high-performance. However variation of device characteristics become larger and control of crystal orientation is key issue for reducing device variation. We have developed highly bi-axially oriented (110)-(111)-(211) poly-Si thin films with very long over-100 µm grains by continuous wave laser crystallization with parallel double-line beams (DLB) without seed crystals. In this work, high mobility and low variation characteristics of poly-Si TFTs with the bi-axially oriented crystal grains were reported. A buffer SiO2 film with a thickness of 1 µm was deposited on a quartz substrate by PECVD. An a-Si thin film with a thickness of 150 nm was deposited by PECVD using SiH4 gas at 430°C. The samples were then annealed in N2 ambient at 490°C for 20 min for reducing hydrogen content in the a-Si film. A cap SiO2 thin film was deposited with a thickness of 100 nm by PECVD. After the deposition of the thin films, double-line laser beam was irradiated on a-Si film. A DPSS CW green laser with a wavelength of 532 nm was used for the laser crystallization. The laser power is 8.5 W and scan speed is 0.25 cm/s. After the laser crystallization, the cap SiO2 was etched by BHF solution, and the laser-crystallized poly-Si thin film was patterned as TFT active layer by using digital micro-mirror lithography and dry etching (Cl2: 40 sccm, HBr: 40 sccm). Gate oxide was deposited at a thickness of 50 nm by ICP CVD and Mo metal gate electrode was deposited at a thickness of 200 nm by sputtering. Gate Mo was etched by phosphoric-nitric acid solution. Self-aligned source and drain were formed by ion implantation with As (ion dose: 2×10^15 cm-2, accelerating voltage: 66 keV) using the Mo gate as a mask. The impurity activation was carried out by furnace annealing with the temperature of 550°C for 30 minutes in N2 ambient. The sacrificial oxide on S/D region was removed by BHF and APCVD SiO2 was deposited at a thickness of 150 nm. Mo metal pad was deposited by sputtering, and finally the sample was annealed in H2 ambient at a temperature of 400°C for 30 minutes. The crystallinity of the laser-crystallized poly-Si thin films was characterized by in-plane X-ray diffraction and electron backscattering diffraction (EBSD) measurements. The crystal orientations were (110), (111) and (211) for laser lateral crystallized plane, the transverse side plane and the surface plane, respectively. And CSL Sigma 3 twin boundaries were dominant and its fraction was 51.2%. The poly-Si TFTs were fabricated at conditions of parallel and perpendicular channel direction to the laser scanning direction and different channel widths of W=10, 20, 30, 50 mu;m (channel length is fixed at 10 mu;m). At the parallel TFT, high field effect electron mobility of 560 cm2/Vs was achieved and the values of the electron mobility show a low variation of below 10%.

The high efficiency of the HIT solar cell derives from a Type II heterojunction between wide-bandgap a-Si and c-Si [Tanaka, et. al., Jpn. J. Appl. Phys., 31, 3518 (1992)]. However, this heterojunction has both a conduction and a valence-band offset, which can lead to undesirable ‘S-shaped&’ current-voltage characteristics [A. Kanevce, et. al., J. Appl. Phys. 105, 094507 (2009)]. We present a new class of heterojunctions formed between c-Si and the wide bandgap transition metal-oxides TiO₂ and ZnO, which block the flow of holes but allow the flow of electrons. TiO₂/Si interfaces have reached a carrier recombination velocity of < 100 cm/s.
TiO₂ layers were deposited by CVD from titanium(IV) tetra-(tert-butoxide) [S. Avasthi, et al., APL 102, 203901 (2013)], while ZnO layers were deposited by plasma-enhanced ALD using diethyl-zinc and CO₂ [D.A. Mourey, et al., Elec. Dev., IEEE Trans., 57, 530 (2010 )]. The fabrication temperature is only 100 °C for TiO₂/Si and 200 °C for ZnO/Si heterojunction, making them especially well-suited for low-cost crystalline silicon solar cells.
Photoelectron spectroscopy shows a large valence-band barrier (ΔE_V) of 3.4 eV at the TiO₂/Si interface, but only a small conduction-band barrier (ΔE_C) of 0.1 eV [J. Jhaveri, et al., 39th IEEE PVSC (2013)]. Therefore the heterojunction blocks the flow of holes from silicon to TiO₂ but allows transport of electrons from Si to TiO₂. Because the ZnO band offsets are similar [V. Srikant, et al., J. Appl. Phys., 83, 5447 (1998)], the ZnO/Si interface is also expected to selectively block the flow of only holes.
The hole-blocking property of the metal-oxide/Si interfaces was tested on Al/metal-oxide/c-Si/Ag devices made on p-type and n-type Si (100) with < 10 nm undoped metal-oxide films. Without the metal-oxide device characteristics were ohmic. With the metal-oxides, the Al/ZnO/p-Si and Al/TiO₂/p-Si devices, show diode-like characteristics with a J₀ of 10^--6- and 10^--9- A/cm², respectively. Since the current from p-Si to Al is dominated by holes, the change in characteristics, from ohmic in Al/p-Si/Ag to diode-like in Al/metal-oxide/p-Si/Ag, shows that the holes are being blocked by the large ΔE_V at the metal-oxide/Si interface. I-V characteristics were also measured under light, by shining light through the semi-transparent Al electrode. Due to the Al-induced electric field in Si, the photogenerated holes in Si are swept towards the Ag electrode while electrons in the Si are swept towards the TiO₂ (or ZnO). The separated electrons do not see any electron barrier (since ΔE_C asymp;0), yielding a well-behaved solar cell I-V curve devoid of any ‘S-shaped&’ character.
We also measured the interface recombination velocity (IRV) of the TiO₂/Si junction by photoconductance decay. IRV at as-deposited interfaces on n- and p-Si vary over a wide range, from 500 to 10^-5- cm/s. However, annealing at only 250 °C yields IRVs of <100 cm/s, which corresponds to an interfacial defect densitiy of only ~10^-11- cm^-2-.

Silicon (Si) is established as a necessity in the electronics industry. Among the three states of matter, only the solid and gaseous states of Si have been utilized: in the solid state, Si as a single-crystal Si wafer is used for semiconductor devices, and in the gaseous state, Si as a vacuum deposition source material is used for making Si thin films. However, the liquid state of Si has never been applied industrially.
We have been researching liquid Si as a semiconducting liquid for use in so-called “printed electronics,” in which all elements of an electronic device are printed[1-3]. We chose cyclopentasilane (CPS) as the raw material. CPS can be converted to polydihydrosilane by photoinduced polymerization. Polydihydrosilane is mixed with an organic solvent to form Si ink. We fabricated not only intrinsic Si ink but also doped-Si ink, both n- and p-types. In the solution process, coating and pyrolysis processes are essential for device development. Process parameters in these processes and the quality of the resultant solid film strictly depend on the properties and behavior of liquid Si, including those of CPS, polydihydrosilane, and Si ink.
Here we clarified the structure and properties of CPS, the photopolymerization process of CPS, the structure of the polymer (polydihydrosilane) in solution, the criteria for forming a uniform polymer film on a substrate, and the pyrolysis process from polymer film to an amorphous Si one. We also evaluated properties of the resultant amorphous films. The quality of a solution-processed film was inferior to that of a vacuum-processed one just after the pyrolysis process; however, its quality can be improved by hydrogen radical treatment, elevating it to a device-grade film. So far, devices that we have developed with liquid Si are a poly-Si thin film transistor (TFT) [1], single-grained Si-TFT[2], and thin-film solar cells[3]. As for TFTs, their excellent properties have been demonstrated.
In addition to the coating and pyrolysis process described above, we recently developed a novel deposition method to fabricate amorphous silicon (a-Si) films. This method, named liquid source vapor deposition (LVD), is one in which a-Si films can be deposited by using vaporized CPS molecules in atmospheric pressure. We fabricated not only intrinsic Si film but also doped ones, both n- and p-types. On the contrary of a-Si films through the pyrolysis process, LVD films have higher quality. These films are very suitable for applications using a-Si films.
References
[1] T. Shimoda, Y. Matsuki, M. Furusawa, T.Aoki, I. Yudasaka, H.tanaka, H. Iwasawa, D. Wang, Y.Takeuchi, Nature 440 (2006) 783.
[2] J. Zhang, M. Trifunovic, M. van der Zwan, H. Takagishi, R. Kawajiri, T. Shimoda,C. I. M. Beenakker, and R. Ishihara,, Applied Physics Letters 102, 243502 (2013).
[3] T. Masuda, N. Sotani, H. Hamada, Y. Matsuki, T. Shimoda, Applied Physics Letters 100, 253908 (2012).

The aluminum-induced crystallization (AIC) of amorphous silicon is an efficient low-temperature method to fabricate silicon thin films. The Al/amorphous Si (Al/aSi) interface plays a key role. It must be in a proper oxidation state in order to form monocrystalline Si grains having their [111] axis orthogonal to the substrate surface, upon annealing.
When this method is implemented on the whole area of a macroscopic wafer, several crystalline Si grains nucleate and expand laterally by a few µm, until neighboring grains meet and form grain boundaries. The resulting layer is polycrystalline with a strong fiber texture. We have recently succeeded to grow epitaxial GaAs nanowires on such a platform (1). To go beyond this self-assembling method, we have patterned the initial Al-aSi bilayer into small patches. When these patches are sufficiently small (typically, their lateral size must be smaller than the average size of the self-assembled grains of a continuous layer), each of them transform into an individual single-crystal wafer. The ensemble constitutes an array of independent platelets sharing a common [111] normal orientation. It can be used as an ensemble of micro- or nano-substrates for the epitaxial growth of semiconductors. This new technology would eliminate the use of thick mono-crystalline wafers for epitaxy and can be implemented on various supports such as glass, metal foils or ceramics.
We will present a detailed study of this localized AIC of Si. Nucleation and growth kinetics are investigated as a function of the dimensions of the initial Al/aSi platelets and of the patterning process. Nucleation rates and activation energies are deduced and compared to the case of a continuous layer.
(1) Y. Cohin, O. Mauguin, L. Largeau, G. Patriarche, F. Glas, E. Soslash;ndergaring;rd, et J.-C. Harmand. Nano Lett. 2013, 13, 2743-2747.

Compared to amorphous silicon (a-Si) polycrystalline silicon (poly-Si) is an attractive alternative material for many electronic applications because of its larger carrier mobility and superior long term stability. Moreover, poly-Si can be fabricated on low-cost glass and plastic substrates. Depending on the desired substrate the processing temperatures are limited to values well below 600 °C. This, however, can have a significant impact on the structural and electronic properties of the poly-Si thin-films.
This paper provides a comprehensive study of the influence of post-hydrogenation on the electrical and optical properties of solid phase crystallized polycrystalline silicon (poly-Si) thin films. The kinetics of grain-boundary defect passivation were measured. With increasing passivation time the concentration of silicon dangling-bonds decreases. According to Raman backscattering measurements this is caused by the formation of Si-H complexes. However, defect passivation is accompanied by the formatkion of large H-stabilized platelet-like clusters. The influence of post hydrogenation on the electrical properties was investigated using temperature dependent conductivity and Hall-effect measurements. For poly-Si on Corning glass, the dark conductivity decreases upon hydrogenation, while it increases when the samples are fabricated on silicon-nitride covered Borofloat glass. Hall-effect measurements reveal that for poly-Si on Corning glass the hole concentration and the mobility decrease upon post-hydrogenation, while a pronounced increase is observed for poly-Si on silicon-nitride covered Borofloat glass. This indicates the formation of localized states in the band gap, which is supported by sub band-gap absorption measurments. The results will be discussed in terms of hydrogen-induced defect passivation and generation mechanisms.