We report an experimental study of photocarrier lifetime, transport, and excitation spectra in silicon-on-insulator doped with sulfur far above thermodynamic saturation. The spectral dependence of photocurrent in coplanar structures is consistent with photocarrier generation throughout the hyperdoped and undoped sub-layers, limited by collection of holes transported along the undoped layer. Holes photoexcited in the hyperdoped layer are able to diffuse to the undoped layer, implying (μτ)h ∼ 5 × 10−9 cm2/V. Although high absorptance of hyperdoped silicon is observed from 1200 to 2000 nm in transmission experiments, the number of collected electrons per absorbed photon is 10−4 of the above-bandgap response of the device, consistent with (μτ)e < 1 × 10−7cm2/V.

The transducer consists of a p-i’(a-SiC:H)-n/p-i(a-Si:H)-n heterostructures produced by PECVD and optimized for the detection of the fluorescence resonance energy transfer between fluorophores with excitation in the violet(400 nm) and emissions in the cyan (470 nm) and yellow (588 nm) range of the spectrum. The thickness and the absorption coefficient of the i’- and i- layers were tailored for cyan and yellow optical confinement, respectively in the front and back photodiodes acting both as optical filters. The devices were characterized through transmittance and spectral response measurements and under different electrical.

To simulate the FRET pairs and the excitation light a chromatic time dependent combination of violet, cyan and yellow wavelengths was applied to the device. The generated photocurrent was measured under negative and positive bias to readout the combined spectra. The independent test signals were chosen in order to sample all the possible chromatic. Different wavelength backgrounds were also superimposed.

Results show that under negative bias the phorocurrent signal presents eight separate levels each one assigned to the different polychromatic mixtures. If a blue background is superimposed the yellow channel is enhanced and the cyan suppressed while under red irradiation the opposite behavior occurs. So under appropriated steady state optical bias the sensor will detect separately the cyan and yellow fluorescence pairs. An electrical model, supported by a numerical simulation, gives insight into the transduction mechanism.

The study focuses on the influence of the hydrogenated amorphous silicon carbide (a-SiC:H) buffer layer in hydrogenated amorphous silicon (a-Si:H) single-junction and tandem thin-film solar cells. By increasing the undoped a-SiC:H buffer layer thickness from 6nm to 12nm, the JSC in single-junction cell was significantly improved, and the efficiency was increased by 4.5%. The buffer layer also effectively improves the efficiency of the a-Si:H/a-Si:H tandem cells by 7% as a result of the increase in open-circuit voltage (VOC) and short-circuit current (JSC). Although the bottom cell absorbs less short-wavelength photons, the wider-bandgap doped and buffer layers were still necessary for improving the cell efficiency. Presumably, this is because these wider-bandgap layers allow more photons to reach the bottom cell. Also, they can reduce interface recombination.

Studies of the electronic transport properties of n-type doped hydrogenated amorphous/nanocrystalline silicon (a/nc-Si:H) films deposited in a dual-plasma co-deposition reactor are described. For these doped a/nc-Si:H, the conductivity increases monotonically for increasing crystal fractions up to 60% and displays marked deviations from a simple thermally activated temperature dependence. Analysis of the temperature dependence of the activation energy for these films finds that the dark conductivity is best described by a power-law temperature dependence, σ = σo (T/To)n where n = 1 – 4, suggesting multiphonon hopping as the main transport mechanism. These results suggest that electronic transport in mixed-phase films occurs through the a-Si:H matrix at lower nanocrystal concentrations and shifts to hopping conduction between clusters of nanocrystals at higher nanocrystal densities.

In this work, we report on investigation of p-type semiconducting polymer, {poly(3,4 polyethylenedioxythiophene)-poly(styrenesulfonate)} (PEDOT:PSS) as the p-layer in NIP and PIN hydrogenated amorphous silicon (a-Si:H) solar cells. The rectification ratio of solution-casted diode is ∼ 10, it increases to 3×104 when PEDOT:PSS is deposited by Spin Coating technique. We observed additional photovoltaic effect when light is illuminated through polymer side. So far, best solar cells characteristics observed for PEDOT:PSS/a-Si:H hybrid solar cells are Voc ≈ 720 mV and Jsc ≈ 1 - 2 mA/cm2.

A new plasma deposition system was built with the capability of varying the electrode spacing in the DC Saddle Field plasma enhanced chemical vapor deposition system. An ion mass spectrometer was installed just below the substrate holder to sample the ion species travelling towards the substrate. Silane plasma and amorphous silicon film studies were conducted to shed light on the impinging ion species, ion energy distributions, and film properties with varying electrode spacing. The results indicate that decreasing the distance between the substrate and cathode leads to a reduction in the high energy ion bombardment.

Lower absorption, lower refractive index and tunable resistance are three advantages of doped silicon oxide containing nanocrystalline silicon grains (nc-SiOx) compared to doped microcrystalline silicon, for the use as p- and n-type layers in thin-film silicon solar cells. In this study we show how optical, electrical and microstructural properties of nc-SiOx layers depend on precursor gas ratios and we propose a growth model to explain the phase separation in such films into Si-rich and O-rich regions as visualized by energy-filtered transmission electron microscopy.

A novel setup for Raman measurements under small angles of incidence during the parallel plate plasma enhanced chemical vapor deposition of μc-Si:H films is described. The possible influence of disturbances introduced by the setup on growing films is studied. The substrate heating by the probe beam is investigated and reduced as far as possible. It is shown that with optimized experimental parameters the influence of the in-situ measurements on a growing film can be neglected. With optimized settings, in-situ Raman measurements on the intrinsic layer of a microcrystalline silicon solar cell are carried out with a time resolution of about 40 s corresponding to 20 nm of deposited material during each measurement.

In this work, the effect of bandgap grading of hydrogenated amorphous silicon germanium (a-Si1-xGex:H) absorber near the p/i and the i/n interfaces was investigated. The a-Si1-xGex:H single-junction solar cells were improved by applying both p/i grading and i/n grading. Our results showed that both the p/i and the i/n grading can increase the open-circuit voltage (VOC) as compared to the cell without grading. The i/n grading can further improve the FF. Presumably the potential gradient created by the i/n grading can facilitate the hole transport thus it can improve the FF. However, the JSC decreased as the i/n grading width increased. The reduction of JSC was due to the loss in the red response, which can be attributed to the replacement of lower bandgap material by the larger ones. Combining the effects of VOC, JSC and FF, a suitable thickness of the p/i and the i/n grading was 20 nm and 45 nm, respectively. Finally, the grading structures accompanied with further optimization of doped layers were integrated to achieve a cell efficiency of 8.59 %.

We have investigated the stability of short channel (1.5μm) p-Type polycrystalline silicon (poly-Si) Thin Film Transistors (TFTs) on the glass substrate under AC bias stress. The variation of threshold voltage in short channel poly-Si TFT was considerably higher than that of long channel poly-Si TFT. Threshold voltage of the short channel TFT was considerably moved to the positive direction during AC bias stress, whereas the threshold voltage of a long channel was rarely moved. The variation of threshold voltage in the short channel p-type TFT under AC bias stess was more compared to that under DC bias stress. The threshold voltage of short channel (L=1.5μm) poly-Si TFT was increased about -7.44V from -0.305V to -7.745V when VGS = 5 (base value) ~ -15V (peak value), VDS = -15V was applied for 3,000 seconds. This positive shift of threshold voltage and significantly degraded s-swing value in the short channel TFT under dynamic stress (AC) may be due to the increase of the stress-induced trap state density at gate insulator / channel interface region.

Low temperature deposited Interdigitated All Back Contact a-Si:H/c-Si Heterojunction (IBC-SHJ) devices are a promising approach for high efficiency, low cost solar cells on thin wafers. Thin intrinsic a-Si:H films (i-a-Si:H) deposited below 300°C provide excellent surface passivation and high Voc. However, the optical properties of a-Si:H layers and electronic band alignment at the heterointerface are critical to reduce optical losses and transport barriers in IBC-SHJ solar cells. At the front illumination surface, a wide band gap (Eg) i-a-Si:H layer with good passivation is desirable for high Voc and Jsc while at the rear surface a narrower Eg i-a-Si:H layer with good passivation is required for higher FF and Voc as seen in 2D numerical simulation. Various substrate temperature, H2/SiH4 dilution ratio and plasma power conditions were explored to obtain i-a-Si:H with good passivation and desired Eg. All the deposited films are characterized by Variable Angle Spectroscopic Ellipsometry (VASE) to determine Eg and thickness and by Fourier Transform Infrared spectroscopy (FTIR) to estimate hydrogen content and microstructure factor. Passivation qualities are examined by quasi-steady state photoconductance (QSS-PC) measurement. The i-layer Eg, was varied in the range from ~1.65eV to 1.91eV with lifetime >1 ms. Lowest Eg is obtained just prior to the structure transition from amorphous to epitaxial-like growth. The FF of IBC-SHJ devices improved from 20% to 70% as Eg of the a-Si:H rear passivation layer decreased from 1.78 to 1.65 eV.

There are only very few reports on the effects of concentration in thin film silicon-based solar cells. Due to the presence of midgap states, a fast decline in fill factor was observed in earlier work. However, with the advent of more stable and lower defect density protocrystalline silicon materials as well as high quality micro-/nanocrystalline silicon materials, it is worth revisiting the performance of cells with these absorber layers under moderately concentrated sunlight. We determined the behavior of the external J-V parameters of pre-stabilized substrate-type (n-i-p) amorphous and microcrystalline solar cells under moderate concentrations, between 1 sun and 21 suns, while maintaining the cell temperature at 25oC. It was found that the cell efficiency of both the amorphous and the microcrystalline cells increased with moderate concentration, showing an optimum at approximately 2 suns. Furthermore, the enhancement in efficiency for the microcrystalline cells was larger than for the amorphous cells. We show that the Voc’s up to 0.63 V can be reached in microcrystalline cells while FF’s only decrease by 9%. The effects have also been computed using the device simulator ASA, showing qualitative agreement.

We present a first-principles lattice dynamics for the assembly of the transition-metal (M)-encapsulated Sin clusters in amorphous phase (a-MSin), which has been proposed as a potential candidate for the channel material of the next-generation thin-film transistors (TFTs) [N. Uchida et al., Appl. Phys. Express1, 121502 (2008)]. The shape of calculated vibrational density of states (VDOS) curve of a-MoSi10 is similar to the counterpart of the high pressure phase of a-Si (HPA-Si) although the present systems are obtained as a result of pressure relaxation. Its radial distribution function (RDF) among Si themselves is characterized by the absence of a gap between the first and second shells, which is also the case in . We further present the VDOS of a-WSi10, whose curve shape is again similar to that of HPA-Si. A difference between a-MoSi10 and a-WSi10 is that the W-atom displacement components extracted from the vibration eigenvectors are mainly distributed over a lower frequency range (< ~ 150 cm-1) than the Mo counterpart (~ 150 cm-1 to ~ 300 cm-1). This may be attributed to a larger atomic mass of W than Mo.

High quality polycrystalline silicon (poly-Si) thin film solar cell was successfully fabricated on soda-lime glass substrates by electron beam (Ebeam) evaporation at low processing temperature. The initial poly-Si seed layer (p+-type 0.5 μm thick) was grown via the aluminum induced crystallization (AIC) method at 450 °C. Prominent interdiffusion and Si crystallization have been observed. X-ray diffraction (XRD) shows that (111) is the dominating crystalline orientation. Post annealing at 450 °C for six hours has produced densely packed Si grains with dimension of more than 10 μm in the plane of the film. Non-destructive Raman spectroscopy reveals the remarkable crystalline improvement for samples after thermal treatment. After removing the top diffused Al by chemical means, an absorber layer (p-type) of 0.9 μm thick was subsequently deposited onto the seed layer by Ebeam evaporation at 500 °C. Transmission electron microscopy (TEM) confirmed good homo-epitaxial growth. Without breaking the high vacuum, an n-type amorphous Si (a-Si) layer (0.7 μm thick) was coated onto the absorber layer to form p-n junction. The corresponding I-V characteristics suggest that our low temperature processing technique is applicable for production of poly-Si thin film solar cell on low cost substrates.

The tip of a Si rod was melted by laser diode (LD) irradiation and we succeeded in dropping small Si droplets by vibration of the Si rod using a piezo actuator. We confirmed multiple small Si droplets under a condition of the resonance frequency of 5.8 kHz for the rod length of 6.0 mm. We observed ejection of droplets from a cone edge of molten Si and the minimum width of the solidified Si was ∼ 1 μ m in diameter. The solidified Si show high crystallinity with the Raman scattering TO phonon band of 515.6 cm-1.

An amorphous silicon (a-Si:H) photoconductor array with two distinct integrated amorphous silicon carbon alloy (a-SiC:H) high pass filters is used to detect two of the cell intrinsic fluorophores. The cutoff wavelength of the filters is tuned by the carbon content in the film. The fluorophores of interest – reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) are indicative of the redox state of the cells. Concentrations down to 1 μM for NADH and 50 μM for FAD were detected.

We report on the investigation of large area a-Si:H/a-SiGe:H double-junction and a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells prepared by our proprietary High Frequency (HF) glow discharge technique. For investigative purposes, we initially used the simpler double-junction structure. We studied the effect of: (1) Ge content, (2) cell thickness, and (3) SiH4 and GeH4 gas flow on the light-induced degradation of the solar cells. Our results show that the double-junction cells with different Ge concentration have open-circuit voltage (Voc) in the range of 1.62-1.75 V. Voc exhibits a flat plateau in the range of 1.65-1.72 V for both initial and stabilized states. The light-induced degradation for cells in this range of Voc is insensitive to the Ge content. In terms of thickness dependence of the intrinsic layers, we found that the initial efficiency increases with cell thickness in the thickness range 2000-4000 Å. However, light-induced degradation increases with increasing thickness. Consequently, the stabilized efficiency is invariant with cell thickness in the thickness range studied. The results of SiH4 and GeH4 gas flow on cell characteristics demonstrate that the deposition rate decreases by only 20% when the active gas flow is reduced to 0.25 times standard flow. The initial and stabilized efficiencies are similar. The information gleaned from the study was used to fabricate high efficiency, large area (~464 cm2) double- and triple-junction solar cells. The highest stable efficiency, as measured by NREL, was 9.8% and 11.0% for the double- and triple-junction structures, respectively.

Amorphous and microcrystalline silicon are currently used for electronic devices such as solar cells and thin-film transistors. This paper shows that silicon nanoparticle dispersion has the potential to be used as source material for polycrystalline silicon thin-film thus opening a route to solution processed silicon devices. After deposition, a classical thermal or microwave annealing step is used to induce the coalescence of the silicon nanoparticles. Both sintering techniques are studied in terms of morphology, electrical and optical properties.

Modulated surface-textured substrates for thin-film silicon solar cells exhibiting high haze in a broad range of wavelengths were fabricated. Glass substrates coated with different thicknesses of a sacrificial layer were wet-etched allowing the manipulation of the surface morphology with surface roughness ranging from 200 nm up to 1000 nm. Subsequently, zinc-oxide layers were sputtered and then wet-etched constituting the final modulated textures. The morphological analysis of the substrates demonstrated the surface modulation, and the optical analysis revealed broad angle intensity distributions and high hazes. A small anti-reflective effect with respect to untreated glass was found for etched glass samples. The performance of solar cells on high-haze substrates was evaluated. The solar cells outperformed the reference cell fabricated on a randomly-textured zinc-oxide-coated flat glass. The trend in the efficiency resembled the increased surface roughness and the anti-reflective effect was confirmed also in solar cell devices.

We have fabricated large-area, thin-film multijunction solar cells based on hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) made in a large area batch reactor. The device structure consisted of an a-Si:H/nc-Si:H/nc-Si:H stack on Ag/ZnO back reflector coated stainless steel substrate, deposited using our proprietary High Frequency (HF) glow discharge technique. For the nc-Si:H films, we investigated two deposition rate regimes: (i) low rate <1 nm/s and (ii) high rate >1 nm/s. We optimized the deposition parameters, such as pressure, gas flow, dilution, and power. We did SIMS analysis on the optimized films, and found the impurity concentrations were one order of magnitude lower than the films made with the conventional RF process. In particular, the oxygen concentration is reduced to ~1018 cm-3. This value is among the lowest oxygen concentration reported in literature. The low impurity content is attributed to proprietary cathode hardware and the optimized deposition process. During the initial optimization and investigative phase, we fabricated small-area (0.25 cm2 and 1.1 cm2) cells. The information obtained from the initial phase was used to fabricate large-area (aperture area 400 cm2) cells, and encapsulated the cells using the same flexible encapsulants that are used in our commercial product. We have light soaked the low-rate and high-rate encapsulated modules. The highest initial efficiency of the low-rate modules is 12.0% as confirmed by NREL. The highest corresponding stable efficiency attained for the low-rate samples cells is 11.35%. For the high-rate small-area (1.1 cm2) cells, the highest initial active-area efficiency and corresponding stable efficiency attained are 13.97% and 12.9%, respectively. We present the details of the research conducted to develop the low- and high-rate cells and modules.