Changes of infrared absorption by light soaking and thermal quenching in a-Si : H

The light- and thermal-induced changes of Si---H bonds in undoped a-Si : H have been studied on the infrared absorption of Si---H stretching mode, using the infrared photothermal deflection spectroscopy and the photothermal bending spectroscopy. The results show that the IR absorption of Si---H bonds increases by light soaking of the high power 500 mW/cm². The change of IR absorption of Si---H bonds reoccurs by thermal annealing at 200°C. This change is related to the increase of the dangling bonds under light soaking. Furthermore, we observe the change of the elasticity modulus by light soaking, using the photothermal bending spectroscopy. The structural change in a-Si : H is discussed based on these results.     

Titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) for durable solar absorber coatings

By the incorporation of silicon into titanium-containing amorphous hydrogenated carbon films (a-C:H/Ti), the lifetime stability at 250°C in air can be strongly enhanced. A combined PVD/PECVD process for the vacuum deposition of these titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) is described. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy (XPS). Optical constants for these films have been determined in the wavelength range from 400 to 2500 nm by means of spectrophotometry. Single layers of a-Si:C:H/Ti and a-C:H/Ti deposited on aluminum and copper substrates have been subjected to comparative aging tests. At 250°C in air, the stability of the a-Si:C:H/Ti films is significantly higher than that of the a-C:H/Ti films. If the silicon content is not too high, the aging properties under humid conditions do not suffer a lot from the incorporation of silicon. However, if the silicon content is clearly higher than the carbon content, the humidity resistance will decrease. For an absorber coating for flat plate solar collectors, the optimized silicon content is expected to be in the range where the high-temperature stability in air is already improved, and where the humidity resistance is still good. For vacuum collectors, a higher silicon content might be advantageous.     

Excellent thermal stability of remote plasma-enhanced chemical vapour deposited silicon nitride films for the rear of screen-printed bifacial silicon solar cells

In this work the thermal stability of the electronic surface passivation of remote plasma-enhanced chemical vapour deposited (RPECVD) silicon nitride (SiN) films is investigated with the aim to establish a cost-effective screen-printing and firing-through-the-SiN process for bifacial silicon (Si) solar cells. As a key result, RPECVD SiN films provide an excellently thermally stable surface passivation quality if they feature a refractive index in the range between 2.0 and 2.2. After a short anneal above 850°C the surface recombination velocity on 1.5 Ωcm p-type float-zone (FZ) Si remains at a very low level of about 20 cm/s. First bifacial silicon solar cells with screen-printed rear contacts on 1.5 Ωcm p-type FZ Si yield a very promising rear efficiency of 13.4%.     

Crystallographic analysis of high quality poly-Si thin films deposited by atmospheric pressure chemical vapor deposition

Crystallinity of thin film polycrystalline silicon (poly-Si) grown by atmospheric pressure chemical vapor deposition has been investigated by X-ray diffraction measurement and Raman spectroscopy. Poly-Si films deposited at high temperatures of 850–1050°C preferred to 2 2 0 direction. By Raman spectroscopy, the broad peak of around 480–500 cm⁻¹ belonged to microcrystalline Si (μc-Si) phase was observed even for the poly-Si deposited at 950°C. After high-temperature annealing (1050°C) 3 3 1 direction of poly-Si increased. This result indicates that the μc-Si phase at grain boundary became poly-Si phase preferred to 3 3 1 direction by high-temperature annealing. Effective diffusion length of poly-Si films deposited at 1000°C was estimated to be 11.9–13.5 μm and 10.2–12.9 μm before and after annealing, respectively.     

Low-temperature growth of thick intrinsic and ultrathin phosphorous or boron-doped microcrystalline silicon films: Optimum crystalline fractions for solar cell applications

The growth kinetics and optoelectronic properties of intrinsic and doped microcrystalline silicon (μc-Si:H) films deposited at low temperature have been studied combining in situ and ex situ techniques. High deposition rates and preferential crystallographic orientation for undoped films are obtained at high pressure. X-ray and Raman measurements indicate that for fixed plasma conditions the size of the crystallites decreases with the deposition temperature. Kinetic ellipsometry measurements performed during the growth of p-(μc-Si:H) on transparent conducting oxide substrates display a remarkable stability of zinc oxide, while tin oxide is reduced at 200°C but stable at 150°C. In situ ellipsometry, conductivity and Kelvin probe measurements show that there is an optimum crystalline fraction for both phosphorous- and boron-doped layers. Moreover, the incorporation of p-(μc-Si:H) layers produced at 150°C in μc-Si:H solar cells shows that the higher the crystalline fraction of the p-layer the better the performance of the solar cell. On the contrary, the optimum crystalline fraction of the p-layer is around 30% when hydrogenated amorphous silicon (a-Si:H) is used as the intrinsic layer of p–i–n solar cells. This is supported by in situ Kelvin probe measurements which show a saturation in the contact potential of the doped layers just above the percolation threshold. In situ Kelvin probe measurements also reveal that the screening length in μc-Si:H is much higher than in a-Si:H, in good agreement with the good collection of microcrystalline solar cells     

Control of μc-Si/c-Si interface layer structure for surface passivation of Si solar cells

Outstanding passivation properties for p-type crystalline silicon surfaces were obtained by using very thin n-type microcrystalline silicon (μc-Si) layers with a controlled interface structure. The n-type μc-Si layers were deposited by the RF PE-CVD method with an insertion of an ultra-thin oxide (UTO) layer or an n-type amorphous silicon (a-Si : H) interface layer. The effective surface recombination velocity (SRV) obtained was very small and comparable to that obtained using thermal oxides prepared at 1000°C. The structural studies by HRTEM and Raman measurements suggest that the presence of UTO produces a very thin a-Si : H layer under the μc-Si. A crystal lattice discontinuity caused by these interface layers is the key to a small SRV.     

Microcrystalline-phase p-type a-Si:O:H windows prepared by Cat-CVD

Different amounts of oxygen, boron-doped hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) deposition were carried out using catalytic chemical-vapor deposition (Cat-CVD) process. Pure silane (SiH₄), hydrogen (H₂), oxygen (O₂), and diluted diborane (B₂H₆) gases were used at the deposition pressure of 0.1–0.5 Torr. The tungsten catalyst temperature (T_fil) was varied from 1700 to 2100 °C. Sample transmittance measurement shows an optical-band gap (E_gopt) variation from 1.45 to 2.1 eV X-ray diffraction (XRD) spectra have revealed silicon microcrystalline phases for the samples prepared at the temperature greater than T_fil1900 °C. For the used silicon oxide deposition conditions, no strong tungsten filament degradation was observed after a number of sample preparations.     

More stable low gap a-Si:H layers deposited by PE-CVD at moderately high temperature with hydrogen dilution

In the present work, several series with variation of deposition parameters such as hydrogen dilution ratio, VHF-power and plasma excitation frequency f_exc have been extensively analyzed. Compared with “conventional” more-stable layers obtained at 200–250°C and high H₂ dilution ratios of about 10, it was observed that electrical transport properties after light-induced degradation of layers deposited at “moderately high” temperatures (300–350°C) are equivalent but required lower H₂ dilution ratios (between 2 and 4). As a consequence, the deposition rate of more stable layers obtained at moderately high temperatures is increased by a factor of 2. Moreover, optical gaps of a-Si:H deposited at 300–350°C are significantly lower (by approx. 10 meV); furthermore, they decrease with f_exc.     

Single-crystalline silicon solar cell with pp heterojunction of c-Si substrate and μc-Si : H film

Annealing effects of the single-crystalline silicon solar cells with hydrogenated microcrystaline silicon (μc-Si : H) film were studied to improve the conversion efficiency. Boron-doped (p⁺) μc-Si : H film was deposited in a RF plasma enhanced chemical vapor deposition system (RF plasma CVD) on the rear surface of the cell. With the optimized annealing conditions for the substrate, the conversion efficiency of 21.4% (AM1.5, 25°C, 100 mW/cm²) was obtained for 5 × 5 cm² area single crystalline-solar cell.     

Optimization of PECVD SiN:H films for silicon solar cells

We have grown silicon nitride (SiN:H) thin films on silicon and glass by the Plasma Enhanced Chemical Vapor Deposition (PECVD) Method at low temperature in order to study their electro-optical properties and correlate these properties to the chemical composition of the layers, so that optimum films may be achieved for silicon solar cells. By varying the silane to ammonia ratio in the plasma gas we have been able to modify the index of refraction, the optical band gap and the silicon surface state passivation properties of the films. From this information we have determined that the optimum silane to ammonia ratio, with other constant parameters in our system, should be 20/65. Our results indicate that the mid-gap surface state density in silicon can be reduced down to 10¹⁰ cm⁻² eV⁻¹ when this optimum (silane to ammonia) ratio is used for depositing SiN:H layers. We have confirmed this optimal ratio by making quantum efficiency measurements on silicon solar cells having their emitter passivated with SiN:H layers deposited with different silane to ammonia ratios. A great reduction of the surface recombination velocity was achieved, as observed from the internal quantum efficiency measurements, for cells with optimal SiN:H layers as compared to those with non-optimum SiN:H layers.     

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H₂ to SiH₄ that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.     

Application of titanium containing amorphous hydrogenated carbon films (a-C:H/Ti) as optical selective solar absorber coatings

A combined PVD/PECVD process for the vacuum deposition of titanium containing amorphous hydrogenated carbon films is described. Elemental compositions of the deposited films have been determined by in situ core level photoelectron spectroscopy (XPS). The long-term stability of the plasma process has been demonstrated. Target poisening has not been observed. We have fabricated optical selective surfaces by the deposition of a-C:H/Ti multilayers onto aluminum substrates. Eventhough we have not optimized layer thicknesses and stoichiometries so far, the experimental results are promising: solar absorptance α_S of 0.876 and thermal emittance _100°C of 0.061 have been achieved yielding an optical selectivity sα_S/_100°C of 14.4. Accelerated aging tests of these coatings have demonstrated their aging stability: the service lifetime is predicted to amount to more than 25 years. Raman spectroscopy has been used to monitor changes in the structure of the aged coatings. Degradation mechanisms are being discussed.     

IR, ESCA and Auger analysis of thermally hydrogenated a-Si:H thin films

Hydrogenated amorphous silicon thin films have been prepared by thermal evaporation. IR investigations showed the existence of all expected Si---H. The electron spectroscopy for chemical analysis exhibited the existence of oxygen and carbon atoms on the silicon surface which led to a shift in the Si2p core level. Auger spectroscopy also exhibited a peak shift in the kinetic energy. Both shifts are interpreted on the bases of the environmental change in the amorphous structure.     

Suppression of photo-induced dilation in cyanide treated hydrogenated amorphous silicon films

Effects of cyanide (CN) treatment with hydrogenated amorphous silicon (a-Si:H) films have been investigated. The decrease of ΔV/V was observed in cyanide treated a-Si:H films and the successive thermal annealing at 200°C after CN treatment induced the further reduction of the ΔV/V. XPS spectra show the indirect evidence that the cyanide species is present within 10 nm from the hydrogenated amorphous silicon surface. The results of CN treatment with a-Si:H solar cells are demonstrated.     

The effect of substrate temperature on P-CVD deposited a-SiGe : H films

Undoped a-SiGe : H films were deposited by the RF plasma chemical vapor deposition method. Films deposited at different substrate temperatures ranging between 100°C and 300°C were studied for their optoelectronic and structural properties. Structural defects like vacancies and microvoids were studied by positron lifetime spectroscopy (PLTS) at room temperature. Optoelectronic properties of the films were correlated with the PLTS measurements. The observations show a decrease in the deposition rate with substrate temperature. Good optoelectronic properties and proper structural relaxation have been obtained with a decrease in microvoid concentration.     

The influence of operation temperature on the output properties of amorphous silicon-related solar cells

The influence of the operation temperature on the output properties of solar cells with hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) photovoltaic layers was investigated. The output power after longtime operation of an a-Si:H single junction, an a-Si:H/a-Si:H tandem, and an a-Si:H/a-SiGe:H tandem solar cell was calculated based on the experimental results of two types of temperature dependence for both conversion efficiency and light-induced degradation. It was found that the a-Si:H/a-SiGe:H tandem solar cell maintained a higher output power than the others even after longtime operation during which a temperature range of 25°C to 80°C. These results confirm the advantages of the a-Si:H/a-SiGe:H tandem solar cell for practical use, especially in high-temperature regions.     

Narrow-bandgap a-Ge:H/a-Si:H multilayers for amorphous silicon-based solar cells

Precisely defined multilayers consisting of a-Ge:H wells and a-Si:H barrier layers have been prepared and characterized as a new type of narrow-bandgap materials for amorphous silicon-based solar cells. It is found that the optical and electrical properties of the layered structures are dramatically improved compared to bulk a-Si_1−ξGe_x:H alloy, being explained by the quantum confinement effects in the ultra-thin a-Ge:H wells. The light-induced changes in the photoconductivity is also remarkably reduced for the multilayers.     

Substrate temperature and hydrogen dilution: parameters for amorphous to microcrystalline phase transition in silicon thin films

Amorphous to microcrystalline phase transition in hydrogenated silicon (Si:H) is realized separately with the variations of substrate temperature and hydrogen dilution. The Raman spectroscopy reveals structural transformations and marks the transition. It occurs at 450°C with 10% silane concentration, whereas that is noted at 250°C with a silane concentration of 4.5%. The material evolved in the transition region is a well-developed amorphous matrix containing a small fraction (12%) of crystallites. A uniform distribution of small (100 Å) crystallites in the films is observed by transmission electron microscopy. The transition material is photosensitive.     

Methods of observation and elimination of semiconductor defect states

A method of observation of interface states for ultrathin insulating layer/semiconductor interfaces is developed by use of X-ray photoelectron spectroscopy (XPS) measurements under bias. The analysis of the energy shift of the semiconductor core level as a function of the bias voltage gives energy distribution of interface states. When the atomic density of SiO₂ layers is low (e.g., SiO₂ layers formed at 350 °C), only one interface state peak is observed near the midgap, and it is attributed to isolated Si dangling bonds at the interface. For SiO₂ layers with a high atomic density (e.g., SiO₂ layers formed at 700 °C), on the other hand, two interface state peaks, one above and the other below the midgap, are observed, and they are attributed to Si dangling bonds interacting weakly with a Si or oxygen atom in SiO₂. Interface states can be passivated by cyanide treatment which simply involves the immersion in cyanide solutions such as KCN and HCN solutions. When the cyanide treatment is applied to indium tin oxide/SiO₂/mat-textured single crystalline Si metal-oxide-semiconductor (MOS) solar cells, the photovoltage is greatly increased, leading to a high conversion efficiency of 16.2%. When the cyanide treatment is performed on polycrystalline Si (poly-Si)-based MOS diodes, a greater effect in comparison to that for single crystalline Si-based MOS diodes is observed due to the elimination of defect states in poly-Si as well as Si/SiO₂ interface states. The cyanide treatment can also increase the conversion efficiency of pn-junction single crystalline and poly-Si solar cells.     

Angle-dependent XPS analysis of silicon nitride film deposited on screen-printed crystalline silicon solar cell

In this work silicon nitride (Si₃N₄) film was deposited as an antireflection coating (ARC) on crystalline silicon solar cell (cell?A) using plasma-enhanced chemical vapor deposition (PECVD). Two solar cells XA and XB of approximately equal area were diced from cell#A and characterized by angle-dependent X-ray photoelectron spectroscopy (XPS). The XPS profiling shows the presence of silicon (Si), nitrogen (N), carbon (C) and oxygen (O) in the Si₃N₄ film. The presence of C and O indicates that organic substances, involved in processing steps were not released completely from the surface and may diffuse in Si₃N₄ ARC during deposition. The XPS spectra corresponding to Si2p, N1s, C1s and O1s were recorded at angles 0° (normal to the surface), 30° and 45°, as angle increases spectra becomes more surface sensitive. Peak positions in Si2p and N1s spectra explain the oxygen contamination in the Si₃N₄ film. The shift in the peak positions of C1s and O1s as angle increases from 0° to 45° explains the surface contamination of carbon and oxygen. The atomic composition of elements Si, N, C and O show more carbon, oxygen concentration and smaller N/Si ratio than stoichiometry, i.e. Si₃N₄ in cell XB. However, cell XA not only show better photovoltaic performance in terms of parameters open-circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF) and efficiency (η) but also have more uniform texturization and regular pyramids on the surface as revealed by scanning electron microscopy (SEM). The presence of higher concentration of impurities (carbon and oxygen), non-uniformity in texturization and in the Si₃N₄ film as well could be responsible for less satisfactory photovoltaic performance of cell XB.