DEVELOPMENT OF HIGHLY EFFECTIVE AND RESOURCE-SAVING SILICON HIGH-VOLTAGE PHOTOVOLTAIC DEVICES

ABSTRACT

Increasing the efficiency of solar cells and reducing the volume of material consumed during its manufacture is one of the most important tasks of today. For this purpose, a 3-sided sensitive solar cell was developed. Its efficiency increased by 2.81 times when illuminating three surfaces and 1.72 times when illuminating two surfaces compared to illuminating one surface. One of the main environmental parameters is temperature. Daily temperatures vary depending on the time of year. Therefore, it is important to study the effect of temperature on solar cells. This research paper examined the effect of temperature on the photovoltaic performance of a triple-sensitivity silicon solar cell. It has been established that the temperature coefficients of the photoelectric parameters of a three-sided sensitive solar cell do not change when its various areas are illuminated. The industrial application of the concept of creating multilaterally sensitive silicon photovoltaic structures due to the effective focusing of light incident on a crystal in a wide spectral range is experimentally considered, which allows the development of photovoltaic energy-generating structures to triple their energy or reduce the consumption of expensive silicon material by three times.

АННОТАЦИЯ

Повышение коэффициента полезного действия солнечных элементов и уменьшение объема расходуемого материала при его изготовлении-одна из важнейших задач сегодняшнего дня. Для этого был разработан 3-х сторонний чувствительный солнечный элемент. Его коэффициент полезного действия увеличился в 2,81 раза при освещении трех поверхностей и в 1,72 раза при освещении двух поверхностей по сравнению с освещением одной поверхности. Одним из основных параметров среды является температура. Суточная температура меняется в зависимости от времени года. Поэтому важно изучить влияние температуры на солнечные элементы. В этой научной работе изучалось влияние температуры на фотоэлектрические параметры солнечного элемента на основе кремния с тройной чувствительностью. Установлено, что температурные коэффициенты фотоэлектрических параметров трехстороннего чувствительного солнечного элемента не изменяются при освещении различных его областей. Экспериментально рассмотрено промышленное применение концепции создания многосторонне чувствительных кремниевых фотоэлектрических структур за счет эффективного фокусирования света, падающего на кристалл в широком спектральном диапазоне, что позволяет при разработке фотоэлектрических энергогенерирующих конструкций утроить их энергию или в три раза снизить расход дорогостоящего кремниевого материала.

Keywords: three-way sensitivity, solar cell, silicon, photovoltaic generator, lighting.

Ключевые слова: трехсторонняя чувствительность, солнечный элемент, кремний, фотоэлектрический генератор, освещения.

INTRODUCTION

In the world, the gradual increase in humanity's demand for electrical energy and global environmental pollution due to carbonization of the atmosphere as a result of the combustion of traditional fuels contribute to the active introduction of environmentally friendly renewable energy sources (RES) and work on their rational use [1]. In this regard, the development of photovoltaic converters for concentrated sunlight as well as high-voltage micro-energy devices on a small surface area plays an important role. It is important to find ways to improve the efficiency of silicon-based solar cells and micro-energy devices, reduce solar energy consumption, especially for the development of high-voltage array solar cells that convert concentrated sunlight into electricity, and improve operating efficiency [2].

The world is conducting large-scale research into the possibilities of developing and improving the efficiency of high-voltage and efficient silicon-based photovoltaic energy exchangers, as well as scientific research aimed at creating and improving models of single- and multi-sided sensitive silicon photovoltaic structures with horizontal and vertical p-n junction, which allow for comprehensive research basic photoelectric properties. Important tasks are considered to be increasing the efficiency of targeted scientific research in this area, including single- and multi-sided sensitive silicon photovoltaic structures with horizontal and vertical p-n junctions and high-voltage microenergy devices, comprehensive determination of basic photovoltaic properties, modeling of light-receiving three-way sensitive vertical p-n junctions of transition silicon solar cells, optimization of high voltage capture on a small surface, reduction of silicon costs when using vertical p-n junctions of solar cells. Therefore, in this research work, new high-voltage photovoltaic generators based on silicon solar cells with 3-way sensitivity are investigated.

A semiconductor photovoltaic generator consisting of a photovoltaic panel with p-n junctions located parallel to the incident radiation, in which the photovoltaic converters are made in the form of microminiature parallelepipeds and are switched into a solid-state matrix, in contrast to a solar cell of planar design, due to the parallel arrangement of p-n junctions relative to incident radiation, it has increased sensitivity in the red part of the spectrum and radiation resistance. Due to the series connection of photovoltaic converters, the converter generates high voltage and low current at the same power, and has small light losses per contact area [3].

However, this photovoltaic generator has low efficiency due to significant recombination losses of light-generated charge carriers, while the possibility of a vertical p-n junction to effectively collect charge carriers is not fully used, which leads to high consumption of expensive semiconductor material per unit of converted energy.

A solar generator with concentration is known, containing a panel of a honeycomb structure having a front front sheet, a rear front sheet and a honeycomb grid located between them. The front sheet contains alternating rows of solar cells and wedge-shaped reflectors that reflect additional radiation onto the solar cell. A design variant of this solar cell comprises a preformed sheet of polymer having flat sections on which said rows of solar cells are mounted, alternating with wedge-shaped sections covered with a reflective layer to form wedge-shaped reflectors [6]. The disadvantage of this solar generator is the increased thermal load on the solar cell due to the absorption of solar energy, and therefore there is a need to ensure effective heat removal, which in this device is provided through the use of a honeycomb cellular structure of the solar panel. In addition, due to the acute angle of incidence of reflected radiation on the surface of the solar cell, the conversion efficiency of additional reflected radiation is lower than in the case of directly incident radiation [8]. This solar generator is intended for use in spacecraft and has a fairly complex design of considerable size [10].

The task ahead was to develop a solar generator of a miniature design with improved photovoltaic and energy-economic indicators [4][5]. The solution to the problem was achieved by the fact that in a solar generator containing rows of solar cells mounted on a base made of polymer material and alternating with wedge-shaped reflectors located along said rows of solar cells, fixed to said base or made integral with the latter in the form of wedge-shaped sections covered reflective layer, the indicated rows of solar cells are made in the form of rectangular blocks with switched micro-photoconverters with p-n junctions located parallel to the incident radiation, having a three-sided working surface along the front and two side faces, and the indicated wedge-shaped reflectors are made in the form of a straight triangular prism having cross section in the shape of an isosceles right triangle with the hypotenuse belonging to the installation plane of the solar cell [9]. In addition, to solve the problem, the height of the specified wedge-shaped reflectors is equal to the height of the specified block of micro-photoconverters, and the alternation step of the specified rows of solar cells and wedge-shaped reflectors d is determined by the relation d = a + 2h, where a and h are, respectively, the width and height of the specified micro block - photoconverters.

In the proposed design of the solar generator, by making rows of solar cells in the form of rectangular blocks with switched micro-photoconverters with p-n junctions located parallel to the incident radiation, the micro-miniature nature of the proposed design is ensured. Each block is a photoelectric generator with high sensitivity and can serve as a source of high voltage, while making it with a three-sided working surface along the front - upper edge and two side edges provides a significant increase in the photocurrent value compared to an analogue with a one-sided working surface on front - top edge. This occurs due to an increase in the amount of solar radiation incident on the block due to an increase in the working surface, as well as due to a more complete use of the ability of vertical p-n junctions to effectively collect charge carriers throughout the entire volume of the semiconductor material when the side faces are irradiated. Making wedge-shaped reflectors in the form of a straight triangular prism, having a cross section in the shape of an isosceles right triangle with a hypotenuse belonging to the solar cell installation plane, ensures that the reflected radiation falls on the side faces of the blocks at a right angle, corresponding to the greatest efficiency of conversion of the incident energy. Thus, the proposed design provides a significant reduction in the consumption of expensive silicon per unit of converted energy, while the irradiation of the working surface on all faces of the block of micro-photoconverters, in contrast to the closest analogue, does not exceed a single factor, i.e. there is no concentration of incident radiation, and there is no the need to use additional heat sink. The height of the wedge-shaped reflectors does not exceed the height of the blocks, and the pitch of alternating blocks and reflectors is determined by their placement close together, which contributes to the compactness of the design.

Figure 1. Particular designs of matrix PT, double-sided illuminated photovoltaic energy sources with a vertical p-n junction

Figure 2. Double-sided illuminated photovoltaic energy sources are located at 45 degrees

In Fig. 1. shows a general view of the solar generator; in Fig. 2 – cross section of the solar generator and diagram of the passage of solar radiation; in Fig. 3a and 3b – electrical circuits for parallel and serial connection of micro-photoconverter units. The solar generator (Fig. 3) contains a base 1 made of polymer material, on which rows of solar cells 2 are installed, alternating with wedge-shaped reflectors 3, made in the form of wedge-shaped sections of the base 1, covered with a reflective layer 4. Each row 2 is made in the form of a rectangular block, consisting of micro-photoconverters containing layers of silicon p-type 5 and n-type 6 with a flat p-n junction 7, sequentially switched through contact layers 8. Blocks 2 are made with a three-sided working surface along the front face 9 and two side faces 10, perpendicular to the plane of the p-n junction 7. Wedge-shaped reflectors 3 are made in the form of a straight triangular prism, having a cross section in the shape of an isosceles right triangle with a hypotenuse c belonging to the solar cell installation plane (Fig. 3). The height of the reflectors 3 is equal to the height h of the micro-photoconverter blocks 2, i.e. The hypotenuse c of an isosceles right triangle is equal to twice its height h. Wedge-shaped reflectors 3 are located close to blocks 2, thus, the alternation step d of blocks 2 and wedge-shaped reflectors 3 is determined by the relation d = a + 2h, where a is the width of blocks 2.

A block of switched micro-photoconverters 2 can be made from a conventional solar cell in the form of silicon wafers with a p-n structure, in which the p- and n-sides are covered with a continuous contact layer, tinned, a stack of a large number of wafers is made, and soldering is performed with medium-melting solder in furnace, then cutting is carried out with a diamond disc into rectangular blocks normal to the planes of the p-n junction. The outer layer with the crystalline structure damaged as a result of cutting is removed by successive grinding, mechanical polishing and acid etching, then an antireflective coating is applied to the front face 9 and side faces 10 of the blocks. The base 1 can be molded from a polymer material, for example, polyvinyl chloride, with a reflective layer of aluminum applied to the surface of the wedge-shaped sections. Blocks 2 are fixed to base 1 by gluing [4]. Switching of blocks 2 into an electrical circuit is carried out by connecting contact layers 8 at the ends of the blocks to surface-mounted conductors or printed wiring elements 11, which can be made on base 1. In Fig. 3a and 3b show electrical diagrams of parallel and serial connections of blocks 2, respectively.

The solar generator works as follows. The front surface of the solar generator is irradiated by solar radiation hv at right angles to the plane of the front faces 9 of blocks 2, i.e. parallel to the plane of p-n junctions 7. Part of the solar radiation flux is reflected from the wedge-shaped reflectors 3 and hits the side faces 10 of blocks 2 also at a right angle, corresponding to the greatest efficiency of conversion of the incident energy. Blocks of micro-photoconverters 2 convert solar radiation into electric current, while the electrical power removed from each block with a three-sided working surface N_block is determined by the sum of the electrical power from the conversion of solar energy incident on the front face N_fr and two side faces N_side:

N_block = N_fr. + 2N_side

Examples of the proposed solar generator: Example 1. The solar generator contains three blocks measuring 10×1×1 mm², including 26 in series with switched micro-photoconverters made of mono-Si type KDV-10 with p-n junctions formed by the diffusion of phosphorus at temperature 1000 ºС. The blocks are installed on a polyvinyl chloride base and connected in parallel (4.a). Example 2. In the solar generator according to example 1, the blocks have dimensions of 5×1×1 mm² and include 12 micro-photoconverters. Example 3. In a solar generator according to example 2, blocks of micro-photoconverters are connected in series (Fig. 5. b).

Figure 3. General view of the solar generator

1-base, polymer material, solar cell rows, 3-wedge reflectors, 4-reflective layer, 5-p-type and 6-n-type silicon, 7- p-n junction, 8-pin layers, 9-front face, 10-side faces, 11-element printed circuit

Figure 4. Side of solar generator

Figure 5. Electrical diagrams for parallel and serial connection of blocks, respectively

Results and its discussion

The parameters of the solar generator samples according to examples 1-3, as well as the results of measurements of photoelectric parameters when irradiated with white light from a solar radiation simulator with a density of 100 mW/cm² are given in the table, lines 1÷3. For comparison, line 4 shows the parameters of one block of micro-photoconverters included in the sample of example 2, made with a one-sided working surface on the front edge, line 5 shows the parameters of a solar generator consisting of one block of micro-photoconverters in the form of a plate with a one-sided working surface, equal in area to the sample of example 2. 

Table 1.

As can be seen from Table 1, the values of the photoelectric parameters of the samples of the proposed solar generator are significantly higher than in analogue designs with a one-sided working surface of the PС, with a significant reduction in silicon consumption per unit area or output power.

The proposed miniature solar generator design can be used as a power source for low-power electronic and optoelectronic devices for various purposes.

In order to assess the economic efficiency of introducing a silicon high-voltage solar generator with a vertical p-n junction, a comparison of experimentally determined technical parameters and economic parameters known from open Internet data was carried out. For this purpose, a prototype of a silicon high-voltage solar generator with a vertical p-n junction is considered, a photograph of which is shown in Fig. 6.

Figure 6. Photo from above of a silicon high-voltage solar generator with a vertical p-n junction and parallel connection of individual blocks

In Figure 6 shows a photograph of a sample for the case of parallel connection of individual blocks of silicon structures with a vertical pn junction. But the study also examined structures when they were connected in series. The technical parameters of the experimental elements were adopted for the case of one- and three-sided lighting. The comparison results are given in table 2. 

Technical and economic parameters of various designs of photovoltaic devices made from industrial solar cells with geometric dimensions (156×156×1) mm.

Table 2.

From the data in table 2 it follows that if we accept for comparison a silicon photovoltaic converter with a horizontal p-n junction and dimensions of 156 × 156 × 1 mm (row 1, table 2) for the case of one-way lighting and photoelectric converters with a vertical p-n junction, consisting of a series-connected 156 pcs micro photovoltaic converters with dimensions of each 156×1×1 mm, for the case of three-way lighting (row 6, table 2), you can see the following condition:

- the cost of the received power for the first case is 0.14 $/W (penultimate column) or the specific consumption of silicon is 12.4×10^-3 kg/W (last column);

- the cost of the received power for the first case is 0.05 $/W (penultimate column) or the specific consumption of silicon is 4.66×10^-3 kg/W (last column).    

 It follows that the introduction of photoelectric converters with a vertical p-n junction, consisting of series-connected microelements, in their three-sided illuminated case, achieves a 2.7-fold increase in economic efficiency. Or, in relation to the consumption of material for obtaining such energy devices, it is possible to achieve a reduction in consumption by the same factor.

The most important thing is that this approach allows you to obtain high voltages from a small area. This result is unattainable when using industrial photovoltaic converters with a horizontal p-n junction.

Conclusions

The industrial applicability of the concept of creating multilaterally sensitive silicon photovoltaic structures by effectively redirecting light incident on a crystal over a wide spectral range has been experimentally demonstrated, allowing up to a threefold increase in photovoltaic energy output or up to a threefold reduction in material consumption for the fabrication of structures.

Ways have been proposed for using poly- and multicrystalline silicon wafers as a base material for the manufacture of high-voltage structures with a vertical p-n junction, and it has been established that when the grain boundaries are perpendicular to the front of the p-n junction, a significant expansion of the spectral sensitivity is achieved, hence an increase in efficiency 1.07 ÷ 1.19 times.

For the first time, a new design of a tripartite-sensitive high-voltage silicon photoelectric energy converter with horizontal and vertical p-n junction has been scientifically substantiated and developed, and their main characteristics have been determined.

The features of the photoelectric characteristics of bi- and multi-sidedly sensitive silicon p-n structures, as well as the influence of variations in temperature and illumination on them, are determined, which make it possible to recommend conditions for the wide-functional implementation of the developed structures for converting concentrated radiation.

The amount of electricity produced by silicon-based solar cells increased as their illumination surface increased. Due to this, the concentration of photogenerated electrons and holes in it increases. When the lighting surface increases, it will mainly affect the short-circuit current. The open circuit voltage remains virtually unchanged. It is desirable to manufacture and use in industry high-voltage photovoltaic generators based on three-sided sensitive solar cells.

References:

1. A. Mardani, A. Jusoh, E. K. Zavadskas, F. Cavallaro, and Z. Khalifah, “Sustainable and Renewable Energy: An Overview of the Application of Multiple Criteria Decision Making Techniques and Approaches,” Sustainability 2015, Vol. 7, Pages 13947-13984, vol. 7, no. 10, pp. 13947–13984, Oct. 2015, doi: 10.3390/SU71013947.
2. P. Fath, S. Keller, P. Winter, W. Jooß, and W. Herbst, “Status and perspective of crystalline silicon solar cell production,” Conference Record of the IEEE Photovoltaic Specialists Conference, pp. 002471–002476, 2009, doi: 10.1109/PVSC.2009.5411274.
3. Р.Алиев “Разработка и характеристики кремниевых фотоэлектрических модулей с вертикальными p-n-структурами”. // Фундаментальные и прикладные вопросы физики. Труды международной конференции посвященной 70-летию физико-технического института нпо «физика-солнце», 2013, 73-75 стр.
4. Алиев Р., Гуломов Ж., Мирзаалимов Н., Абдувохидов М., Урмонов Б., Мирзаалимов А., Матбобоева С., Абдуазимов В. Полупроводниковый фотоэлектрический преобразователь. Патент РУз № FAP 01806 от 19.11.2020.
5. Алиев Р., Мухтаров Э., Мирзаалимов А. Солнечный генератор. Патент РУз № FAP 00623 от 23.05.2011.
6. Алиев Р., Мирзаалимов А., Алиев С. Разработка и некоторые фотоэлектрические параметры кремниевого солнечного генератора с вертикальными p-n-переходами // Applied Solar Energy - USA. 2013- Vol.49, №2. - pp. 59-61.
7. Полупроводниковый фотоэлектрический генератор. Авторское свидетельство СССР №288163, МПК H 01 L. 1968.
8. J. Gulomov, R. Aliev, N. Mirzaalimov, B. Rashidov, J. Alieva Study of Mono- and Polycrystalline Silicon Solar Cells with Various Shapes for Photovoltaic Device with 3D Format: Experiment and Simulation // Journal of Nano- and Electronic Physics. Volume 14 (2022 Year), No.5, pp. 05012-1 - 05012-8.
9. Mirzaalimov A., Aliev R., Mirzaalimov N., Gulomov J. Simulation photoelectric parameters of vertical junction solar cells // International Journal of Advanced Trends in Computer Science and Engineering. Volume 10, №.2, March - April 2021. 543-548. (№14).
10. Мирзаалимов А.А., Гуломов Ж.Ж., Абдувохидов М.К. Создание фотоэлектроэнергетических устройств нового поколения с высокоэффективной кремниевой базой // Universum: технические науки: электрон. научн. журн. 2020. 6(75), 3-часть. С. 87-89. (№35).