Optoelectronic properties of doped layers for a-Si solar cells prepared using helium dilution

This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH₄) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360Å, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm)⁻¹ and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

A new perspective on the characterization of materials for a-Si:H solar cells

A study has been carried out on a-Si:H solar cell materials fabricated under a wide range of deposition conditions in different laboratories. The results on both thin films and corresponding Schottky barrier cell structures demonstrate that analysis and characterization based solely on the neutral dangling bonds are clearly inadequate. Contributions of charged defects to the properties of a-Si:H, their effect on light-induced changes are identified together with the limitations of methods commonly used to characterize the solar cell properties and stability of a-Si:H materials. Self-consistent fitting of a wide range of results on films and Schottky barrier cell structures is obtained with a gap state distribution in which charged defects are included.     

Toward stabilized 10% efficiency of large-area (>5000 cm) a-Si/a-SiGe tandem solar cells using high-rate deposition

This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10⁶ for a-Si:H films has been maintained up to the deposition rate of 15 Å/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm² a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.     

Development of low cost production technologies for polycrystalline silicon solar cells

To develop a technology of forming grooves for low cost cell production, a multi-blade wheel grinding method was investigated. The process time of groove formation on the surface of 10 × 10 cm² polycrystalline silicon substrate was reduced to 30 s by a newly developed high-speed groove formation machine. Simultaneous formation of junction and anti-reflection coating by atmospheric pressure chemical vapor deposition (APCVD) technique was also investigated. For electrodes formation process, single firing method for both side electrodes made possible to simplify the firing process and to speed up from a conventional speed of 400 mm/min to 5000 mm/min.     

Advanced cost-effective crystalline silicon solar cell technologies

An overview is given concerning current industrial technologies, near future improvements and medium-term developments in the field of industrially viable crystalline silicon terrestrial solar cell fabrication (without concentration).     

Engineering of a-Si:H device stability by suitable design of interfaces

Where a-Si:H pin devices are concerned, one of the main obstacles regarding improved performance is device stability, usually attributed to adverse behaviour at various interfaces within the device. Several attempts have been made to overcome this problem, such as the use of blocking layers at the interfaces. Although these have led to some improvements in device performance, most of the problems associated with device stability remain. This is mainly due to the defects at the interfaces, since the blocking layers (silicon alloys with carbon, nitrogen or oxygen) usually have a high density of bulk states, in comparison to intrinsic a-Si:H films.In this paper, we present a method that seems to be capable of improving device stability. It consists of performing a controlled removal of oxide interlayers at the interfaces, by an appropriate etching process. This enables the production of highly smoothed interfaces, and reduces possible cross-contamination of the i-layer from the adjacent doped layers. This amounts to a new design of typical pin devices, in which thin absorber layers are placed at the p/i and i/n interfaces. Their purpose is to trap most of the impurity atoms diffused from the doped layers, after which they are removed by appropriate etching.The fabrication of the absorbers (sacrificial layers), the nature of the etching and the tailoring of the defect profile at the interfaces will be discussed, including the performance exhibited by the resulting devices.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

A simple fabrication process for 20% efficient silicon solar cells

Recently, a substantially simplified PERC silicon solar cell has been developed at ISFH with independently confirmed 1-sun efficiencies of up to 20.0%. This paper describes the details of the relatively simple cell fabrication process and experimentally characterizes the new cells. The simplified design involves reflection control by means of random pyramids, the direct evaporation of the front metal grid onto the random pyramids, elimination of the need for nontextured areas underneath the contact grid, and the use of a single phosphorous diffusion (1-step emitter).     

Activation of dopant in the p-layer of amorphous silicon solar cells under illumination

We have investigated the effect of light-soaking on the p-doped layer of amorphous silicon (a-Si:H) solar cells by low temperature (50–300 K) AC conductance measurements. The experimental results are interpreted on the basis of an equilibration model of the doped material. The model takes into account the finite dimension of the layer and its presence inside a complex structure. It is shown that the Fermi level shifts after light soaking, which can result in activation of the doping impurities.     

Amorphous silicon solar cells deposited with non-constant silane concentration

The performance and light-soaking behavior of hydrogenated amorphous silicon (a-Si:H) solar cells with absorber layers deposited under non-constant silane concentration (SC) - a measure of silane dilution in hydrogen - using plasma enhanced chemical vapor deposition (PECVD) are investigated. Constant SC values during deposition close to the amorphous to microcrystalline phase transition lead to the formation of crystallites after a certain thickness. To prevent this transition, SC is adjusted during growth to produce an amorphous material that is close to the microcrystalline phase transition without the inclusion of a detectable microcrystalline phase. By adjusting SC during deposition it was possible to achieve an increased open-circuit voltage that is up to 40 mV higher than that for a conventional amorphous silicon solar cell at initial efficiencies above 9%. The best solar cells produced with non-constant SC show improved stability against light induced degradation, which leads to a relative loss in fill factor of only 11.4%, resulting in a stabilized fill factor of 62.5%.     

The application of grating couplers in thin-film silicon solar cells

As an alternative to randomly textured transparent conductive oxides as front contact for thin-film silicon solar cells, the application of periodic light grating couplers was studied. The periods and groove depths of transparent gratings made of zinc oxide were tuned independently from each other and varied between 1 and 4 μm and 100 and 600 nm, respectively. The one-dimensional grating couplers were realized using photolithography. We have analysed the optical properties of the gratings and the properties of amorphous and microcrystalline silicon solar cells incorporating these grating couplers. The achieved results are discussed with respect to the performance of cells deposited on flat and randomly textured substrates.     

Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells

Excimer laser-crystallized silicon solar cells fabricated show a steady increment of the current densities with exposure to simulated sunlight, over a 30 min period. The current density of the amorphous silicon cell under identical conditions remains steady, with no significant change. The process was observed to be reversible upon cooling, and the performance increase is attributed to the energy barrier introduced by the enhanced bandgap of a nanocrystalline silicon middle layer, created as a result of the crystallization. It is suggested that the thermal energy due to prolonged illumination allows carriers to cross the barrier increasing output currents.     

Optical modelling of a single-junction p-i-n type and tandem structure amorphous silicon solar cells with perfect current matching

Using the admittance analysis method, the optimal design of a single junction a-Si : H solar cell is suggested and its photovoltaic parameters are calculated. The technique is then extended to design a tandem structure of two cells stacked one on the top of the other and connected in series. The top cell is considered of a-Si : H and bottom of a-SiGe : H and the condition of current matching is applied to determine the tandem's optimal design. The efficiency of the single-junction cell with the optimal design is predicted to be 13.1% and that of the tandem cell with the perfect current matching is 20.8%. The results of our calculations are discussed in the light of the recent experimental results.     

Analysis of light scattering in a-Si:H-based solar cells with rough interfaces

The main features of a recently developed semi-coherent optical model for a-Si:H thin film solar cells with rough interfaces are presented. In contrast to the previous optical models, the model takes into account also the interference fringes observed in measured wavelength-dependent characteristics of a-Si:H solar cells. The simulations of the quantum efficiencies of the cells with different intrinsic a-Si:H layer thicknesses and interface root mean square (rms) roughness of 40 nm are shown and compared with the measured data.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Degradation and annealing of amorphous silicon solar cells by current injection: experiment and modeling

In this paper we report in detail on the effect of current injection in amorphous silicon solar cells. A set of devices has been degraded and then annealed at different current intensities. Device performances during the whole experiment have been monitored by current–voltage characteristics and quantum efficiency curves. It has been found that annealing rate increases with current intensity, while stabilized photovoltaic parameters decrease. Time evolution of efficiency and short-circuit current during degradation has been reproduced by a numerical device modeling, resulting in a pronounced increase of defects near the p–i interface. The model also demonstrated that annealing results are not well reproduced if current-induced annealing is not energy selective.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

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.     

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.