Polycrystalline silicon p-n junctions

Silicon films deposited on recrystallized metallurgical silicon substrates have been used for the fabrication of low cost solar cells. The substrate is polycrystalline, and the active region of the solar cell is epitaxial with respect to the substrate. Since the dark current-voltage characteristics of a solar cell are important factors affecting its conversion efficiency, the characteristics of a number of epitaxial mesa diodes of the configuration n⁺-silicon/p-silicon/p⁺-metallurgical silicon/graphite have been measured over a wide temperature range to study the effects of grain boundaries. The results were analyzed on the basis of the two-exponential model.     

A study of efficiency in low resistivity silicon solar cells

A general device analysis program has been utilized to study the efficiency of silicon solar cells. The analysis is applied to specific geometries of both n⁺-p and n⁺-p-p⁺ solar cells and involves a numerical solution of the basic transport and continuity equations. This approach allows solutions free of typical limiting assumptions involved in solving the device equations apart from those relating to lifetime, mobility variations and diffused region profiles. The analysis includes available empirical information of diffusion length, mobility, and lifetime as a function of doping as well as a Gaussian profile for the diffused region. Results are presented which illustrate the limitations of efficiency as a function of doping. It was found that the maximum efficiencies for both types of cell converge at lower resistivities to around 16% with AMO radiation and a single layer absorbing SiO antireflecting film. It was also found that the minority carrier lifetime, both in the n⁺ surface and p-type bulkk regions, presents serious limitations to conversion efficiency particularly in the low resistivity cells.     

Recent progress in research and development of crystalline silicon solar cells in Japan

Recently, the research and development of crystalline silicon solar cells in Japan has greatly advanced. Fundamental research has been conducted on the recombination and passivation of minority carriers at Si/SiO₂ interfaces and in those bulk regions containing grain boundaries. Qualities of silicon feedstock and substrates have been improved and large-area cell efficiencies using Czochralski single and cast polycrystalline substrates have reached 20% and 17%, respectively, by using low-cost cell fabrication processes. Such high efficiency values are realized by tenacious improvement of substrate quality and the development of new processes for fabricating solar cells.     

High-efficiency,low-cost solar cells on 3-inch silicon wafers

The present work describes practical ways of attenuating the severe limitations imposed by different kinds of areal inhomogeneities on the performance of large-area p⁺?n?n⁺ single-crystal silicon solar cells. The test devices were processed on 3-inch, lower-grade n-wafers and the minimization of the cells' cost was one of the desired objectives. A combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, high-efficiency (> 17%) 3-inch silicon solar cells with improved overall electro-optical performance. These devices possess short-circuit current densities similar to those of sophisticated laboratory samples prepared on high-quality, small-area silicon.     

Schottky solar cells on thin epitaxial silicon

Schottky solar cells fabricated on 10, 20 and 30 μm epitaxial silicon produce a current density ranging from about 10–22 mA/cm², depending on Si thickness and orientation, in close agreement with theoretically predicted data. These results are also in close agreement with recent data on p-n solar cells, using thin epitaxial silicon. Data reported herein predict that 10% efficient Schottky solar cells could be produced using about 20 μ of silicon on a suitable substrate. A 7.6% efficient Schottky solar cell on epitaxial silicon has been recently fabricated and tested using AM1 sunlight (100 mW/cm²).     

Minority carrier lifetimes in silicon solar cells determined from spectral and transient measurements

Measurements of the spectral collection efficiency and short circuit current decay rate following an X-ray pulse have been made on three types of single crystal silicon solar cells. The cell types were n⁺ - p, p⁺ - n, and p⁺ - n - n⁺ with base resistivities of 0.3, 10 and 10 Ω-cm, respectively. Minority carrier lifetimes were determined from both experiments using analytical or device code calculations, as required. For the n⁺ - p and p⁺ - n cells, nominal lifetimes of 2 and 5 ωsec, respectively, were obtained. A lifetime greater than 100 ωsec was inferred for the p⁺ - n - n⁺ device. This value represents a minimum estimate since our analysis is inaccurate when the diffusion length exceeds the cell thickness, as is the case here. The difference in base lifetime for the p⁺ - n and p⁺ - n - n⁺ structures is attributed to gettering during the phosphorus diffusion to form the back surface field layer.     

19% efficient n‐type Czochralski silicon solar cells with screen‐printed aluminium‐alloyed rear emitter

High and stable lifetimes recently reported for n‐type silicon materials are an important and promising prerequisite for innovative solar cells. To exploit the advantages of the excellent electrical properties of n‐type Si wafers for manufacturing simple and industrially feasible high‐efficiency solar cells, we focus on back junction n⁺np⁺ solar cells featuring an easy‐to‐fabricate full‐area screen‐printed aluminium‐alloyed rear p⁺ emitter. Independently confirmed record‐high efficiencies have been achieved on n‐type phosphorus‐doped Czochralski‐grown silicon material: 18·9% for laboratory‐type n⁺np⁺ solar cells (4 cm²) with shadow‐mask evaporated front contact grid and 17·0% for front and rear screen‐printed industrial‐type cells (100 cm²). The electrical cell parameters were found to be perfectly stable under illumination. Copyright © 2006 John Wiley & Sons, Ltd.     

Electronic properties of undoped polycrystalline silicon

Standard semiconductor measurements and techniques have been applied to undoped, high-purity polycrystalline silicon to determine its electronic properties. Resistivity and Hall mobility were determined as function of temperature, and the ability of polycrystalline silicon to form Schottky barriers and p?i?n junctions has been evaluated experimentally. Present results show Si grain boundaries behaving as p-type layers separating high-resistivity grains. An effective mobility, half the monocrystalline value, and an average carrier density in the 10¹⁵ cm^?3 range are deduced. A qualitative model is discussed to describe present results.     

Influence of the dopant density profile on minority-carrier current in shallow,heavily doped emitters of silicon bipolar devices

Experimental determination of the dependence of recombination current in p⁺ and n⁺ regions on the dopant profile for shallow emitters of ion-implanted silicon solar cells is described. The results are analyzed by extending a previous analytical model for the transport of minority carriers in heavily doped regions. The extension accounts for an effective electric field, defined by heavy-doping effects at the surface, and suggests that the energy-gap narrowing for p⁺ silicon is slightly smaller than that for n⁺ silicon and/or that minority-carrier diffusivities are substantially lower than the majority-carrier ones at comparable dopant densities. The very high dopant densities achieved with the ion implantation/laser annealing technique provide an in situ surface passivation that supresses surface recombination and minimizes the emitter recombination current.     

Injection enhancement of photocurrent in polycrystalline silicon p +-n-n + structures

Injection enhancement of photocurrent in structures of the p ⁺-n-n ⁺ type fabricated on the basis of films of polycrystalline silicon grown on commercial silicon substrates has been investigated. The questions of obtaining injection photodetectors and other bistable elements with the aid of α-irradiation and heat treatment are discussed. Fiz. Tekh. Poluprovodn. 31, 425–426 (April 1997)     

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

We have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n⁺ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 John Wiley & Sons, Ltd.     

Analysis of the p+/p window layer of thin film solar cells by simulation

The application of a p⁺/p configuration in the window layer of hydrogenated amorphous silicon thin film solar cells is simulated and analyzed utilizing an AMPS-1D program. The differences between p⁺-p-i-n configuration solar cells and p-i-n configuration solar cells are pointed out. The effects of dopant concentration, thickness of p⁺-layer, contact barrier height and defect density on solar cells are analyzed. Our results indicate that solar cells with a p⁺-p-i-n configuration have a better performance. The open circuit voltage and short circuit current were improved by increasing the dopant concentration of the p⁺ layer and lowering the front contact barrier height. The defect density at the p/i interface which exceeds two orders of magnitude in the intrinsic layer will deteriorate the cell property.     

Grain boundary states and the characteristics of lateral polysilicon diodes

In lateral n⁺p^?p⁺ diodes made in LPCVD polycrystalline silicon films, the energy distribution of the traps at the grain boundaries is found to be U shaped. They have a density of about 10¹² cm^?2 and a capture cross section of about 10^?16 cm². The forward current of the diodes is ascribed to recombination, the reverse current to field-enhanced generation via these traps.     

Some prospective applications of silicon electrodeposition from molten fluorides to solar cell fabrication

Electrodeposition of both epitaxial and polycrystalline continuous films of dense, coherent, and well-adherent silicon coatings was achieved from molten fluorides. A dissolving Si anode and an operating temperature of about 750 C were utilized. Silicon electrocrystallization epitaxy (ECE) produced films with the (111) orientation on Si substrates of the same orientation. The unintentionally doped films were of p-type character with a resistivity in the range 0.05 - 0.10 Ω-cm. Polycrystalline Si films were similarly electroplated onto various polycrystalline metal substrates. Uniform coherent, and well-adherent coatings with grain diameters as large as 40 – 50 μm were obtained. The useful rate of electrodeposition of Si could be significantly increased by the application of an alternating square wave pulse (ASWP)₂technique. Cathodic current pulses as high as 300 mA/cm² (growth rate of about 5 μm/min) were demonstrated. The cathodic current efficiencies, for all modes of growth, were about 70 – 100%. The effects of the various operating parameters, and some prospective applications to the fabrication of solar cells, are discussed. This was paper A-6 in the 18th Annual Electronic Materials Conference, Salt Lake City, June 23–25, 1976.     

Passivation of defects in polycrystalline silicon solar cells by molecular hydrogen annealing

The paper presents a new technique for passivating grain boundaries and intragrain defects in the polycrystalline silicon material of the solar cells. Enhancement in the minority-carrier diffusion length has been achieved by thermal annealing in molecular hydrogen in previously formed n⁺ p junctions. The annealing temperature dependence of the diffusion length has been studied in the temperature range of 400 to 700° C. The study shows a significant increase in the diffusion length from 21-8/ im to 67-8/ jm by this process. The diffusion length increases with annealing in molecular hydrogen and shows a peak value at 600° C.     

17.6% efficient multilayer thin-film silicon solar cells deposited on heavily doped silicon substrates

We report new results for multilayer thin-film silicon solar cells deposited onto electronically inert, heavily doped crystalline silicon substrates. The n-p-n-p-n active layers of a total thickness of 17 μm combined with a 15-μm thick p⁺-type buffer layer were deposited by chemical vapour deposition epitaxially onto a 10¹⁹ cm⁻³ doped Czochralski-grown silicon substrate. The cells fabricated using these layers exhibit an energy conversion efficiency of up to 17.6%, as measured by Sandia National Laboratories, which is the highest efficiency ever achieved for a thin-film silicon cell deposited onto such an electronically inert crystallographic template. An open-circuit voltage of 664.2 mV is also reported, the highest ever for a cell on such substrates.     

The work function difference of the MOS-system with aluminium field plates and polycrystalline silicon field plates

The work function difference of the AlSiO₂Si-system was measured by the MOS-capacitance-voltage technique for n- and p-type silicon as substrate and was compared to the results obtained by different authors applying the photoemission technique. It could be seen that the work function differences measured in this work differ largely from the values measured by the photoemission technique.On the basis of the results obtained the work function differences of the p⁺polySiSiO₂nSi- and n⁺polySiSiO₂pSi-system were defined by comparative measurements. From this it was evident that the location of the Fermi level in heavily doped polycrystalline silicon is identical to the location of the Fermi level in monocrystalline silicon of the same impurity concentration.     

Study of the morphological growth features and optical characteristics of multilayer porous silicon samples grown on n-type substrates with an epitaxially deposited p +-layer

This study is concerned with the growth features of multilayer porous silicon with layers of different porosity, obtained by electrochemical etching on an n-type single-crystal silicon (111) wafer with a p ⁺-layer epitaxially deposited onto the surface. The possibility of obtaining a multilayer system of ordered pores of various sizes within a single technological cycle is demonstrated. The differences in the optical characteristics of separate layers of the grown structure are shown.     

A new type of high-efficiency bifacial silicon solar cell with external busbars and a current-collecting wire grid

Results regarding bifacial silicon solar cells with external busbars are presented. The cells consist of [n⁺p(n)p⁺] Cz-Si structures with a current-collecting system of new design: a laminated grid of wire external busbars (LGWEB). A LGWEB consists of a transparent conducting oxide film deposited onto a Si structure, busbars adjacent to the Si structure, and a contact wire grid attached simultaneously to the oxide and busbars using the low-temperature lamination method. Bifacial LGWEB solar cells demonstrate record high efficiency for similar devices: 17.7%(n-Si)/17.3%(p-Si) with 74–82% bifaciality for the smooth back surface and 16.3%(n-Si)/16.4%(p-Si) with 89% bifaciality for the textured back surface. It is shown that the LGWEB technology can provide an efficiency exceeding 21%.     

An analytical model for the edge-illuminated silicon solar cell under concentrated sunlight

An analytical model for the edge-illuminated p⁺nn⁺ solar cell is derived. The model employs the Fletcher boundary conditions for the p⁺n and nn⁺ space-charge regions and the ambipolar approach for the low region, the lightly-doped n-type base region. For high-level condition, the ambipolar approach yields complete information about the low region, including the ohmic drop, the Dember voltage, and the hole concentration profile.