Experimental Study of Energy Exchange Attending Electron Emission from Carbon Nanotubes

Phenomena based on nanoscale transport processes offer new possibilities for direct refrigeration by electron emission between opposing electrodes across a vacuum region. The average energy of emitted electrons depends upon the magnitude and shape of the potential energy barrier in the vacuum region, which is affected by the emission gap, emitter work function (potential barrier height), and emitter tip geometry. Emitted electrons are replaced by other electrons to maintain charge continuity, and the difference in energy between the emitted and replacement electrons produces a heating or cooling effect, known as the Nottingham effect, at the emitter surface. Theoretical studies indicate the possibility of very large (> 100 W/cm²) cooling rates, but experimental confirmation is lacking due to challenging material and experimental requirements. To obtain the results discussed in this paper, the energy exchange attending electron emission from multi-walled carbon nanotube (MWNT) array samples is measured with an uncertainty of approximately 1 μ W. The results are found to depend strongly on the adhesive used to bind the MWNT arrays to the substrate, and this effect is explored by using both silver and carbon paints as the adhesive material. An attempt to determine the effect of the emitter work function by intercalating the MWNT arrays with potassium was unsuccessful. Heating curves as a function of the emission current are presented for various sample groups, and these curves provide insight into the mechanisms involved in the energy exchange associated with field emission from MWNT arrays, including the Nottingham effect and Joule heating.     

Self-aligned locally diffused emitter (SALDE) silicon solar cell

This paper presents, for the first time, a low-cost, high-throughput manufacturing approach for fabricating n-base dendritic web silicon solar cells with selectively doped emitters and self-aligned aluminum contacts using rapid thermal processing (RTP) and screen printing. The self-aligned locally diffused emitter (SALDE) structure is p⁺ nn⁺⁺ where aluminum is screen-printed on a boron-doped emitter and fired in a belt furnace to form a deep self-doped p⁺-layer and a self-aligned positive contact to the emitter according to the well-known aluminum-silicon (Al---Si) alloying process. The SALDE structure preserves the shallow emitter (20.2 μm) everywhere except directly beneath the emitter contact. There the junction depth is greater than 5 μm, as desired, in order to shield carriers in the bulk silicon from that part of the silicon surface covered by metal where the recombination rate is high. This structure is realized by using n-base (rather than p-base) substrates and by utilizing screen-printed aluminum (rather than silver) emitter contacts. Prototype dendritic web silicon (web) cells (25 cm² area) with efficiencies up to 13.2% have been produced.     

Simple analytical solution and efficiency improvement of polysilicon emitter solar cells

A simple analytical model has been developed to simulate the performance of solar cells with polysilicon contact on the front surface. The polysilicon layer with a columnar grain structure is modeled by an effective recombination velocity using a two-dimensional transport equation. A one-dimensional transport equation in the single-crystal emitter is solved, taking into account bulk recombination and non-uniformly doped emitter. Then, simple analytical expressions for the emitter reverse saturation current and light-generated current densities are obtained. The collection of the light-generated carriers in polysilicon layer has been discussed and an analytical solution of the light-generated current is derived. The results show that the polysilicon layer can result in a decrease in emitter reverse saturation current density and an increase in solar cell photovoltaic parameters. In fact, the emitter region should not be treated as a ‘dead layer’ because thin polysilicon layer front surface contact gives an improvement of about 60 mV for the open-circuit voltage, 3.6 mA/cm² for the photocurrent, and 3.9% for the cell efficiency.     

Phosphorous emitter etch back and bulk hydrogenation by means of an ECR-hydrogen plasma applied to form a selective emitter structure on mc-Si

In this paper, we study the effect of hydrogen-electron cyclotron resonance plasma (ECR plasma) on the phosphorous-doped emitter of a solar cell based on multicrystalline silicon (POLIX^®). The purpose of this experiment is to realise a selective emitter structure, using the front metal contacts as a mask. We show that hydrogen plasma etches the surface of the emitter away, and simultaneously diffuses into the silicon and increase the bulk lifetime. Both minority carrier lifetime and etch rate depend on the grain orientation. Hydrogen diffusion is hindered by the high phosphorous concentration of the emitter, as shown on the SIMS profiles. Besides, SIMS profiles are revealing an anomalous behaviour of phosphorous, which diffuses into the silicon at temperatures as low as 350°C on (1 0 0) oriented grains.     

Another approach to form p+ emitter for rear junction n-type solar cells: Above 17.0% efficiency cells with CVD boron-doped epitaxial emitter

Epitaxial boron-doped emitters by CVD can provide a valuable alternative to the use of aluminum or diffused boron for the creation of p-type emitters. Compared to the traditional boron-diffused emitters for high-efficiency cells, epitaxial emitters need shorter process time, no boron skin removal step and have more opportunities to optimize the emitter doping profile. Our work proves that epitaxial emitters can be a good alternative for the p-type emitter. Very promising cell results with the highest cell efficiency of 17.0% on FZ material with LPCVD emitter and 16.7% on CZ material with APCVD emitter under 1 Sun have been achieved. Good fill factors have been obtained, which indicate good metal contacts are obtained on the epitaxial emitter. Cell results on n-type material are very encouraging and indicate a high potential of such epitaxial emitters for rear junction n-type solar cells.     

Selective emitter formation with a single screen-printed p-doped paste deposition using out-diffusion in an RTP-step

We present a new way to realise a selective emitter structure using a single screen-printed phosphorous paste deposition as dopant source to obtain a doping differential, with rapid thermal diffusion. The heavily doped part of the emitter is situated underneath the deposited phosphorous lines, and the lightly doped emitter in between is obtained via the gas phase, because phosphorous out-diffuses from the paste during the high-temperature step. SIMS profiles show a difference of a factor 4 in magnitude for the surface concentration between the regions underneath and beside the deposited paste. Photovoltaic results show an improvement in efficiency of 0.7% in comparison with the reference cell (homogeneous emitter), due to a 2 mA/cm² short-circuit current increase.     

Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

Effect of Evanescent Waves on the Dark Current of Thermophotovoltaic Cells

ABSTRACT

The output power of thermophotovoltaic (TPV) cells may be greatly increased when the gap between the emitter and cell is reduced to submicron distances (near-field regime), at which photon tunneling due to evanescent waves becomes important. Accurate modeling of TPV cells in these conditions is crucial for the design and optimization of near-field TPV systems. The conventional or standard modeling method uses the summation of the dark current and the short-circuit current, while the direct method applies the photon chemical potential. It has been shown that the two methods are linked through a modification of the direct method using Wien’s approximation. By contrasting different modeling approaches, we quantitatively analyze the effects of evanescent waves on the TPV cell performance parameters, especially the dark current, for different emitter and cell materials in the near-field regime. Our results show that the saturation current by radiative recombination is strongly affected by evanescent waves and the bandgap energy. The current-voltage characteristics calculated by different modeling methods are displayed to demonstrate that a constant saturation current typically used in the standard method could cause substantial error in the near-field regime. For a TPV system with an emitter operating at relatively low temperatures, we show that it is necessary to include the photon chemical potential in the computation of the net radiative heat transfer between the emitter and receiver.     

Thin semiconducting layers as active and passive emitters for thermophotonics and thermophotovoltaics

Thermophotovoltaics involves the photovoltaic conversion by a receiver cell of radiation from an emitter, which could be heated by various sources including sunlight. A prime difference from normal solar photovoltaics is that emitted energy unable to be used by the receiver can, in principle, be recycled allowing high conversion efficiency. Thermophotonics is a recent development of this concept where the emitter is “active”, namely a heated diode, increasing the rate of energy transfer for a given emitter temperature and concentrating emission in an energy range more suited for conversion by the receiver. This paper evaluates thin semiconducting layers as emitters for thermophotovoltaics and thermophotonics. It is shown that thermophotonics avoids a major challenge for thermophotovoltaics: the sensitive dependence of system efficiency on the recycling of below bandgap radiation. Possible ways of achieving the high external quantum efficiency light-emitting diode required for thermophotonics are discussed.     

Lifetime extension of in‐core self‐powered neutron detector using new emitter materials

In this paper, new in‐core self‐powered neutron detector emitter candidate materials, ie, vanadium, cobalt, and silver, have been examined in their lifetimes compared with the commonly used rhodium emitter. Using a new quantitative lifetime evaluation model, the lifetimes of vanadium and cobalt were determined to be longer than that of rhodium, but these materials were also shown to have the disadvantage of low signal intensities. Under normal operating conditions, we showed that rhodium emitter can be used for 2 cycles of pressurized water reactors (PWRs) with lifetime of 4.35 years, whereas silver can be used for 5 cycles of PWRs with lifetime of 8.04 years. Three sensitivity tests were performed for rhodium and silver about (1) the emitter size, (2) the fuel assembly burnup, and (3) the emitter temperature variations. From the test results, we observed that the lifetimes of rhodium and silver emitters remained 2 and 5 cycles long, respectively. We concluded that silver can significantly extend the in‐core detector's lifetime in PWR operation.     

A contactless photoconductance technique to evaluate the quantum efficiency of solar cell emitters

A new technique, the spectral response of the steady-state photoconductance, is proposed for solar cell characterisation in research and development. The emphasis of this paper is on the evaluation of the carrier collection efficiency of the emitter region based on a simple, two-wavelength approach. We show that in addition to the well-established determination of the wafer recombination properties that results from a long-wavelength photoconductance measurement, detailed emitter quantum-efficiency information can be obtained by performing a second measurement with short-wavelength light. The method is experimentally demonstrated with silicon solar cell precursors having emitters with markedly different levels of surface and bulk recombination losses. The main advantages of the spectral photoconductance technique are that it is fast, contactless, and can be used immediately after junction formation before metallisation; these properties make it very appropriate for routine monitoring of the emitter region, including in-line process control.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Analytical modelling and minority current measurements for the determination of the emitter surface recombination velocity in silicon solar cells

A new analytical model is used to describe the emitter of silicon solar cells in order to gain information on the surface recombination velocity S. The procedure takes advantage of the combined use of experimental measurements, done to determine the emitter saturation current J_oe, and analytical modelling to relate J_oe to S. Several experiments have been carried out on silicon solar cells having different emitter profiles subjected to various surface treatments. The influence of the surface on significant cell parameters has been analysed.     

An animation tool for demonstrating the importance of edge filters in thermophotovoltaic applications

In order to achieve high efficiency in a thermophotovoltaic (TPV) generator, it is important that a high fraction of emitted photons with energies below the TPV cell bandgap are reflected back to the emitter. This can be accomplished in several ways, and one suggestion is to place an edge filter between the emitter and the TPV cell array in an elliptical optic design. An animation tool, developed in the Excel® program, for determining the efficiency of an optical system is presented. The animation components are a black body emitter, an edge filter, and an array of TPV cells. The tool has been used to demonstrate the importance of an efficient filter and the usefulness of optics that makes the edge of the filter as sharp as possible. It is available at <www.du.se/tpv>     

Effect of the front surface field on crystalline silicon solar cell efficiency

The present paper reports on a simulation study carried out to determine and optimize the effect of the high–low junction emitter (n⁺-n) on thin silicon solar cell performance. The optimum conditions for the thickness and doping level of the front surface layer with a Gaussian profile were optimized using analytical solutions for a one dimensional model that takes on the theory relevant for highly doped regions into account. The photovoltaic parameters of silicon solar cells with front surface field layer (n⁺-n-p structure) and those of the conventional one (n-p structure) are compared. The results indicate that the most important role played by the front surface field layer is to enhance the collection of light-generated free carriers, which improves the efficiency of the short wavelength quantum. This is achieved by a drastic reduction in the effective recombination at the emitter upper boundary, a property primarily responsible for the decrease in the emitter dark current density. The findings also indicate that the solar cell maximum efficiency increase by about 2.38% when the surface doping level of the n⁺-region and its thickness are equal to 2.10²⁰ cm^?3 and 0.07 μm, respectively.     

Determination of front surface recombination velocity of silicon solar cells using the short-wavelength spectral response

A method of determination of recombination velocity S_f of minority carriers at the front surface of an n⁺–p–p⁺(p⁺–n–n⁺) silicon solar cell in which the n⁺(p⁺) front emitter is made by diffusion of dopant impurity in the p(n) region is presented. This method uses the short-wavelength spectral response of the cell to determine S_f and is applicable if the front emitter of the cell has a linearly varying built-in field. It was applied to a p⁺–n–n⁺ solar cell that had a Gaussian distribution of the dopant impurity in the p⁺ front emitter up to a depth of 0.078 μm from the surface. Using the spectral response data of cell in 380 nm<λ< 500 nm range S_f was found to have a nearly constant value 6×10⁵ cm s⁻¹ in 400 nm<λ<460 nm range. Below and above this wavelength range the value of S_f was found to be slightly smaller. For comparison the value of S_f was also determined assuming the p⁺ region to be uniformly doped, and this value was found to be significantly smaller than based on the diffused emitter model. The analysis showed that for a diffused junction cell, the assumption that the front emitter is uniformly doped, ignores the presence of the built-in field in the emitter region and leads to overestimation of minority carrier recombination in the emitter. Consequently for a given contribution of the front emitter region to the spectral response of the cell, this assumption underestimates the front surface recombination and determines a smaller value of S_f. On the other hand, the present method can be expected to determine a realistic value of S_f independent of λ for most diffused junction silicon solar cells using the spectral response data in a suitable short-wavelength range since each such cell indeed has a built-in electric field in the emitter region.     

High-efficiency low-cost integral screen-printing multicrystalline silicon solar cells

This paper describes how the efficiency and throughput of industrial screen-printed multi-Si solar cells can be increased far beyond the state-of-the-art production cells. Implementation of novel processes of isotropic texturing, shallow emitter or single diffusion selective emitter, combined with screen-printed metallization fired through a PECVD SiN_x ARC layer, have been described. Novel dedicated fabrication equipment for emitter diffusion and a PECVD SiN_x deposition system are developed and implemented thereby removing the processing bottlenecks linked to the diffusion and bulk passivation processes. Several types of back-contacted solar cells with improved visual appeal required for building integrated photovoltaic (BIPV) application have been developed.     

Characterisation of rare earth selective emitters for thermophotovoltaic applications

We investigate selective radiation emitters made from rare earth oxides suitable for thermophotovoltaic (TPV) systems. Yb₂O₃ and Er₂O₃ emitters were fabricated and their radiation power, temperature and emissivity were measured in the entire relevant spectral range. We found temperatures of 1735 K for the Yb₂O₃ emitter and of 1680 K for the Er₂O₃ emitter both heated with a 1.35 kW butane burner. The maximum emissivities of the selective peaks were 0.85 at 1.27 eV, and 0.82 at 0.80 eV for the Yb₂O₃ and the Er₂O₃ emitter, respectively. The emission spectra show gas emission lines originating from the combustion process in addition to the selective emission bands. An estimation based on a simplified combustion model show that a TPV system with a system efficiency of about 10% can be realised using an Yb₂O₃ emitter, silicon photocells and a perfect selective filter.     

POCl3 diffusion for industrial Si solar cell emitter formation

POCl₃ diffusion is currently the de facto standard method for industrial n-type emitter fabrication. In this study, we present the impact of the following processing parameters on emitter formation and electrical performance: deposition gas flow ratio, drive-in temperature and duration, drive-in O₂ flow rate, and thermal oxidation temperature. By showing their influence on the emitter doping profile and recombination activity, we provide an overall strategy for improving industrial POCl₃ tube diffused emitters.