Synthesis and photoluminescence characteristics of AlN nanocrystalline solids

A new condensed form of AlN nanocrystalline solids was obtained directly from reactions of metal Al and (NH₄Cl+NH₄I) in liquid ammonia at 550 °C, without the subsequent consolidation process as in the conventional method. The synthesized product is a transparent bulk solid, while the constituted nanocrystals have an average size of about 18 nm and possess the same wurtzite structure as bulk AlN. (NH₄Cl+NH₄I), which plays a role of a catalyst in the present synthetic route, is indispensable. The photoluminescence spectrum of the AlN nanocrystalline solids shows a broad blue band centered at 400 nm. Received: 20 June 2000 / Accepted: 22 June 2000 / Published online: 9 August 2000     

XPS surface analysis of monocrystalline silicon solar cells for manufacturing control

X-ray photoelectron spectroscopy (XPS) has been applied to surfaces of silicon wafers in the different stages of the assembly line for large-scale monocrystalline silicon solar cell manufacturing (ISOFOTON, Malaga, Spain). XPS results have shown that a considerable amount of carbon is present on the pyramidal-textured monocrystalline silicon surface. This amount decreases slightly but is still present after the process of phosphor diffusion (p-n junction), as well as after subsequent calcination in humid air for SiO₂ film formation (passivation). This amount of carbon may be buried during the process of CVD coating an anti-reflection TiO₂ film. After calcination of the film in order to obtain the TiO₂ rutile phase, an even higher amount of carbon is detected on the TiO₂ anti-reflection coating surface. This indicates that not all organics from the tetra-isopropile ortho-titanate (TPT) precursor were released from the film. Furthermore, in this case phosphor is found in excess on the SiO₂ wafer surface (dead layer) and also on the rutile TiO₂ surface, indicating that an extra phosphor diffusion from the bulk silicon through the TiO₂ film has taken place during calcination. These results demonstrate how thermal treatments applied in the solar cell manufacturing assembly line can influence and may change the intended compositional distribution. These treatments may also introduce defects that act as recombination centres for charge carriers in the solar cell device. Received: 13 September 2000 / Accepted: 10 January 2001 / Published online: 3 May 2001     

The Epilift technique for Si solar cells

We present an overview of the Epilift technique, which allows the fabrication of single-crystal silicon films, suitable for photovoltaic purposes. Epitaxial layers are grown by liquid phase epitaxy on partially masked, single-crystal silicon substrates. The layers are detached from the substrate by selective chemical or electrochemical etching, allowing the substrate to be re-used. Epilayers grown on (100) substrates display highly textured surfaces as well as narrow overgrowth widths of the epitaxial layer over the oxide, making them particularly suitable for photovoltaic devices. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

High-temperature processing of crystalline silicon thin-film solar cells

The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the “high-temperature approach” thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 °C. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Black multi‐crystalline silicon solar cells

A simple method for nano‐scale texturing of silicon surfaces based on local metal‐catalyzed wet chemical etching, which results in an almost complete suppression of reflectivity in a broad spectral range, has been successfully applied to produce black multi‐crystalline silicon solar cells. The performance of the cells is compared to that of reference cells without surface nano‐texturing. A considerable increase of the short circuit current (by 36–42% with respect to the reference cells) without deterioration of other performance parameters is observed under natural sun illumination. Means of further optimization of such black solar cells are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inversion layer resistance in silicon inversion layer solar cells

The inversion layer resistance is very important for metal-insulator-semiconductor inversion layer (MIS/IL) solar cells, and usually it is the main part of the series resistance. It is found that the inversion layer resistance and the junction depth are determined by the operating voltage for an MIS/IL solar cell. On the basis of MIS theory, a general relationship between the operating voltage and the inversion layer resistance (and the junction depth) has been investigated. Practical computations have been done for MIS/IL solar cells with a silicon nitride insulator layer. It is found that the inversion layer resistance has a minimum value for operating voltage near 0.4 V, and the junction depth decreases monotonically with the increase of the operating voltage.     

Transparent,flexible and low‐resistive precision fabric electrode for organic solar cells

We report the use of conducting precision fabrics as transparent and flexible electrode for organic semiconductor‐based thin film devices. Precision fabrics have well‐defined mesh openings, excellent flexibility and are fabricated by high‐throughput roll‐to‐roll manufacturing. Optimized fabrics reached light transmittance over 95% throughout the visible and near infrared spectra. A significant part of the transmitted light is scattered, which is particularly advantageous for solar cell applications. Surface resistivity is as low as ～3 Ohms/square, which decreases Ohmic losses when scaling up to large area devices. We demonstrate that solar cells fabricated onto these electrodes show very similar characteristics to those prepared on ITO. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface passivation schemes for high‐efficiency n‐type Si solar cells

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J_0e) of 49 fA/cm² have been reached with this approach on SiO₂ passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (V_oc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures

Physical vapor deposition techniques such as sputtering and laser ablation – which are very commonly used in thin film technology – appear to hold much promise for the synthesis of nanocrystalline thin films as well as loosely aggregated nanoparticles. We present a systematic study of the process parameters that facilitate the growth of nanocrystalline metals and oxides. The systems studied include TiO₂, ZnO, γ-Al₂O₃, Cu₂O, Ag and Cu. The mean particle size and crystallographic orientation are influenced mainly by the sputtering power, the substrate temperature and the nature, pressure and flow rate of the sputtering gas. In general, nanocrystalline thin films were formed at or close to 300 K, while loosely adhering nanoparticles were deposited at lower temperatures. Received: 31 October 2000 / Accepted: 9 January 2001 / Published online: 26 April 2001     

Decoupling crystalline volume fraction and VOC in microcrystalline silicon pin solar cells by using a µc‐Si:F:H intrinsic layer

Microcrystalline silicon thin film pin solar cells with a highly crystallized intrinsic μc‐Si:F:H absorber were prepared by RF‐plasma enhanced chemical vapour deposition using SiF₄ as the gas precursor. The cells were produced with a vacuum break between the doped layer and intrinsic layer depositions, and the effect of different subsequent interface treatment processes was studied. The use of an intrinsic μc‐Si:H p/i buffer layer before the first air break increased the short circuit current density from 22.3 mA/cm² to 24.7 mA/cm². However, the use of a hydrogen‐plasma treatment after both air breaks without an interface buffer layer improved both the open circuit voltage and the fill factor. Although the material used for the absorber layer showed a very high crystalline fraction and thus an increased spectral response at long wavelengths, an open‐circuit voltage (V_OC) of 0.523 V was nevertheless observed. Such a value of V_OC is higher than is typically obtained in devices that employ a highly crystallized absorber as reported in the literature (see abstract figure). Using a hydrogen‐plasma treatment, a single junction μc‐Si:F:H pin solar cell with an efficiency of 8.3% was achieved.

     

Fabrication and magnetic properties of highly ordered Co16Ag84 alloy nanowire array

Highly ordered Co-Ag alloy nanowire arrays embedded in the nanochannels of anodic alumina membranes (AAMs) were successfully fabricated using electrodeposition. Scanning electron microscopy and transmission electron microscopy observations revealed that the ordered Co-Ag alloy nanowires were uniformly assembled into the hexagonally ordered nanochannels of the AAMs. Magnetic measurements showed that the perpendicular coercivity (H_c⊥) of the ordered nanowire arrays increased dramatically as the annealing temperature (T_a) rose from 300 °C, reached its maximum (183 Oe) at 400 °C and then decreased sharply as T_a further increased beyond 400 °C. However, there was little change in the parallel coercivity (H_c∥) of the nanowire arrays during the annealing process. The mechanism of this phenomenon was attributed to the special structure of the AAMs and nanowires. Received: 27 November 2000 / Accepted: 3 May 2001 / Published online: 25 July 2001     

A fullerene silirane derivative to improve the open circuit voltage in a polymer–fullerene solar cell: a theoretical study

In this letter quantum chemical calculations are performed on fullerene derivatives with varying reduction potentials, successfully used as electron acceptor in bulk heterojunction solar cells with the aim to investigate the energy levels of the frontier orbitals. We have successfully correlated the theoretical lowest unoccupied molecular orbital (LUMO) levels of different fullerenes with the open circuit voltage of the photovoltaic device based on the polymer–fullerene blend. We have also proposed a new fullerene silirane derivative with a raised LUMO level useful to increase the open circuit voltage of a polymer solar cell. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the current linearity at low illumination of high‐efficiency back‐junction back‐contact silicon solar cells

The relation between current and illumination intensity of three structures of high‐efficiency back‐junction back‐contact silicon solar cells was analyzed. Both, n‐type cells with non‐diffused front surface and p‐type cell with floating n‐emitter show a pronounced non‐linearity due to strong illumination dependence of the passivation quality of the non‐diffused surface and the floating junction respectively. Quantum efficiency (QE) of this cell type drops significantly for the illumination lower than 0.5 suns. In contrast the QE of n‐type cells with n⁺‐front surface field (FSF) is linear. Low illumination current characteristics of all three of the analyzed structures could be well described by physical models. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monocrystalline Si waffles for thin solar cells fabricated by the novel perforated-silicon process

2 for this waffle-shaped film when it is attached to glass substrates. Received: 15 May 1998/Accepted: 28 May 1998     

Crystalline Si thin-film solar cells: a review

The present review summarizes the results of research efforts in the field of crystalline silicon thin-film solar cells on foreign substrates. The large number of competing approaches can be broadly classified according to the grain size of the crystalline Si films and the doping of the crystalline absorber. Currently, solar cells based on microcrystalline Si films on glass with an intrinsic or moderately doped absorber film achieve efficiencies around 10%, whereas thin-film cells fabricated from large-grained polycrystalline Si on high-temperature-resistant substrates have efficiencies in the range of 15%. The paper discusses the limitations of various approaches and describes recent developments in the area of thin, monocrystalline Si films that may open the way towards 20% efficient thin-film Si solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchatel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Electronic behaviour of thin-film CdTe solar cells

New support is given for one of the controversial models about the electronic consequences of the CdCl₂ treatment of a thin-film CdTe solar cell: the assumption that deep acceptor states are introduced in the bulk of the CdTe layer as a result of the CdCl₂ treatment. A detailed study of the doping profile using capacitance–voltage (C-V) measurements is performed as a first step. The above assumption is numerically simulated with our simulation programme SCAPS. In this way, anomalous features of the C-V measurements are fully explained, and further correspondence between calculated and measurable quantities is found. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Electroacoustics in a silicon solar cell

We present electroacoustic measurements of a p/n⁺ silicon solar cell as a function of the stimulating potential modulation frequency, gaining information on the different power dissipation sources within the device by means of thermal depth profiling. Theoretical calculations of the electroacoustic signal based on a reinterpretation of the Rosencwaig and Gersho model have also been performed, and the results of these calculations have been compared to the experimental data.     

Modeling light trapping and electronic transport of waffle-shaped crystalline thin-film Si solar cells

Monocrystalline Si films from the novel perforated-Si process are candidates for the fabrication of thin-film solar cells because their waffle shape enhances the optical absorption and hence permits the use of films with a thickness of only a few microns. We study the optics of waffle cells by three-dimensional Monte Carlo ray-tracing. A high photogeneration of 38 mA/cm² from a film of thickness W_f=4 μm is possible due to a detached Al-back surface reflector that has an effective reflectance of 99.7% at 1250 nm. Our analytical model for light trapping in thin films explains this high reflectance. Two-dimensional numerical transport modeling reveals the existence of an optimum texture period p≈2W_f that originates from a carrier collection efficiency that increases with texture period while the photogeneration decreases with period. For well-passivated cells the optimum thickness W_f is at least one fifth of the diffusion length L. Efficiencies of 17% to 18% are feasible with waffle films of 1 to 3 μm in thickness. We introduce an analytic model for the minority carrier transport that agrees with two-dimensional numerical modeling to within 10% and reduces the computation time by orders of magnitude. This analytic model is also applicable to conformal thin-film geometries differing from the waffle geometry. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Electronic properties of Cu(In,Ga)Se2 heterojunction solar cells–recent achievements, current understanding, and future challenges

The recent achievements of high-efficiency Cu(In,Ga)Se₂ heterojunction solar cells are reviewed with a special focus on the understanding of the electronic transport properties of the devices. We discuss the basic limitations of the device performance, the present understanding of electronic device analysis, as well as the role of intrinsic defects and of the interfaces for the performance of the solar cells. Received: 12 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999