Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

International audienceWe report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates

Thin crystalline silicon solar cells based on epitaxial films grown at 165 °C by RF-PECVD

International audienceWe report on heterojunction solar cells whose thin intrinsic crystalline absorber layer has been obtained by plasma enhanced chemical vapor deposition at 165 °C on highly doped p-type (1 0 0) crystalline silicon substrates. We have studied the effect of the epitaxial intrinsic layer thickness in the range from 1 to 2.5 μm. This absorber is responsible for photo-generated current whereas highly doped wafer behave like electric contact, as confirmed by external quantum efficiency measurements and simulations. A best conversion efficiency of 7% is obtained for a 2.4 μm thick cell with an area of 4 cm2, without any light trapping features. Moreover, the achievement of a fill factor as high as 78.6% is a proof that excellent quality of the epitaxial layers can be produced at such low temperatures

The role of hydrogen in the formation of microcrystalline silicon

The growth mechanisms of microcrystalline silicon thin films at low temperatures (100-250°C) by plasma CVD are still a matter of debate. We have shown that ue-Si:H formation proceeds through four phases (incubation, nucleation, growth and steady state) and that hydrogen plays a key role in this process, particularly during the incubation phase in which hydrogen modifies the amorphous silicon network and forms a highly porous phase where nucleation takes place. In this study we combine in-situ ellipsometry and dark conductivity measurements with ex-situ high resolution transmission electron microscopy to improve our understanding of microcrystalline silicon formation

Silicon epitaxy below 200°C: Towards thin crystalline solar cells

International audienceLow temperature plasma processes provide a toolbox for etching, texturing and deposition of a wide range of materials. Here we present a bottom up approach to grow epitaxial crystalline silicon films (epi-Si) by standard RF-PECVD at temperatures below 200°C. Booth structural and electronic properties of the epitaxial layers are investigated. Proof of high crystalline quality is deduced from spectroscopic ellipsometry and HRTEM measurements. Moreover, we build heterojunction solar cells with intrinsic epitaxial absorber thickness in the range of a few microns, grown at 175 °C on highly doped (100) substrates, in the wafer equivalent approach. Achievement of a fill factor as high as 80 % is a proof that excellent quality of epitaxial layers can be produced at such low temperatures. While 8.5 % conversion efficiency has already been achieved for a 3.4 µm epitaxial silicon absorber, the possibility of reaching 15 % conversion efficiency with few microns epi-Si is discussed based on a detailed opto-electrical modeling of current devices

Hydrogen evolution during deposition of microcrystalline silicon by chemical transport

International audienceWe have exposed a freshly deposited boron-doped hydrogenated amorphous silicon (a-Si:H) layer to a hydrogen plasma under conditions of chemical transport. In situ spectroscopic ellipsometry measurements revealed that atomic hydrogen impinging on the film surface behaves differently before and after crystallization. First, the plasma exposure increases the hydrogen solubility in the a-Si:H network leading to the formation of a hydrogen-rich subsurface layer. Then, once the crystallization process engages, the exceeding hydrogen starts to leave the sample. We have attributed this unusual evolution of the exceeding hydrogen to the role of the grown hydrogenated microcrystalline (Μc-Si:H) layer that gradually prevents the atomic hydrogen coming from the plasma to reach the Μc-Si:H/a-Si:H interface. Consequently, the hydrogen solubility, initially increased by the hydrogen plasma, recovers its initial value of an untreated a-Si:H material. To support the idea that the out-diffusion is a consequence and not the cause of the growth of the Μc-Si:H layer, we have solved the combined diffusion and trapping equations that govern the hydrogen diffused into the sample, using appropriate approximations and a specific boundary condition traducing the lack of hydrogen injection during the Μc-Si:H layer growth

In situ generation of indium catalysts to grow crystalline silicon nanowires at low temperature on ITO

International audienceIn situ generation of indium catalyst droplets and subsequent growth of crystalline silicon nanowires on ITO by plasma-enhanced CVD are reported, and the wurtzite (Si-IV) phase is clearly evidenced in some wire

Stable microcrystalline silicon thin-film transistors produced by the layer-by-layer technique

International audienceMicrocrystalline siliconthin films prepared by the layer-by-layer technique in a standard radio-frequency glow discharge reactor were used as the active layer of top-gate thin-film transistors(TFTs). Crystalline fractions above 90% were achieved for silicon films as thin as 40 nm and resulted in TFTs with smaller threshold voltages than amorphous siliconTFTs, but similar field effect mobilities of around 0.6 cm2/V s. The most striking property of these microcrystalline silicontransistors was their high electrical stability when submitted to bias-stress tests. We suggest that the excellent stability of these TFTs, prepared in a conventional plasma reactor, is due to the stability of the μc-Si:H films. These TFTs can be used in applications that require high stability for which a-Si:HTFTs cannot be used, such as multiplexed row and column drivers in flat-panel display applications, and active matrix addressing of polymer light-emitting diodes

Multi-resonant absorption in ultra-thin silicon solar cells with metallic nanowires

International audienceWe propose a design to confine light absorption in flat and ultra-thin amorphous silicon solar cells with a one-dimensional silver grating embedded in the front window of the cell.We show numerically that multi-resonant light trapping is achieved in both TE and TM polarizations. Each resonance is analyzed in detail and modeled by Fabry-Perot resonances or guided modes via grating coupling. This approach is generalized to a complete amorphous silicon solar cell, with the additional degrees of freedom provided by the buffer layers. These results could guide the design of resonant structures for optimized ultra-thin solar cells

The configurational energy gap between amorphous and crystalline silicon

The crystallization enthalpy of pure amorphous silicon (a-Si) and hydrogenated a-Si was measured by differential scanning calorimetry (DSC) for a large set of materials deposited from the vapour phase by different techniques. Although the values cover a wide range (200-480 J/g), the minimum value is common to all the deposition techniques used and close to the predicted minimum strain energy of relaxed a-Si (240 ± 25 J/g). This result gives a reliable value for the configurational energy gap between a-Si and crystalline silicon. An excess of enthalpy above this minimum value can be ascribed to coordination defects

Hybrid solar cells based on thin-film silicon and P3HT

International audienceHybrid concepts based on a nanoscale combination of organic and inorganic semiconductors are a promising way to enhance the cost efficiency of solar cells through a better use of the solar spectrum, a higher aspect ratio of the interface, and the good processability of polymers. A new type of solar cells has been investigated. It is based on a heterojunction between regio-regular poly(3-hexylthiophene) as an organic electron donor and silicon as an inorganic electron acceptor. In a first step towards nanostructured devices, cells made of flat thin films of these materials have been studied as a model case of the heterojunction. The materials were characterized through ellipsometry and absorption spectroscopy. The devices were studied by means of their spectral response and their I-V characteristics. By combining these results, the contribution of each layer and the mechanisms of photocurrent generation are explained. The best cells to-date show a power conversion efficiency of 1.6% under AM 1.5 illumination, with a Voc of 0.704 V and a Jsc of 4.22 mA/cm2