29 R&D on Photovoltaic or Solar cell

S.S. islam

Introduction

A solar or photovoltaic cell (also known as ‘solar battery’) is electrical device used to convert energy from light into electrical energy. The working of solar cell is based on photovoltaic effect, which is a physicochemical process. It is a type of photoelectric cell, which can be described as the device whose electrical properties (e.g., resistance, current, etc.) change as light is exposed onto it. Large scale photovoltaic modules are termed as solar panels, and are made up of numerous solar cells.

Figure 1 A traditional solar panel made up of silicon. Electrical contacts (silver-colored strips) are printed on the wafer.

Regardless of the source, whether sunlight or artificial light, solar cells are termed as photovoltaic. They can be used as photodetectors to detect light or any electromagnetic wave in the visible range, examples include infrared detectors. Besides, solar cells can also be used to measure the intensity of the light incident on them.

A photovoltaic cell operates on the following three attributes:

•   Excitons or electron-hole pairs are generated on absorption of light.

•   The charge carriers are separated as electrons and holes.

•   The separated charge carriers are collected at their respective electrodes, i.e., electrons at the positive electrode, and hole at the negative electrode.

In addition to this, a solar thermal collector can be used for direct heating or indirect generation of electrical power from the heat generated. In these devices, heat is supplied when sunlight is absorbed. Further, a photoelectrolytic or photoelectrochemical cell is ether a type of photovoltaic cell (similar to dye-sensitized solar cells developed by Edmond Becquerel) or a device which directly splits water into its constituent hydrogen and oxygen by using solar irradiation.

Theory

The working of a solar cell can be summarised as (Figure 2):

•  Photons present in sunlight hitting the solar panel are absorbed by the material of the panel, e.g., Silicon.

•  Absorption of photons leads to the excitation of electrons from their atomic/molecular orbital. Upon excitation, the electron either dissipates energy as heat and return to its previous orbital, or travels through the cell to be collected at the electrode. A counter current flows through the material to cancel the potential developed by this extra electron. This current is captured as electricity. The nature of the chemical bonds of the material are crucial in this process. Most common used material is silicon, which is used in two layers: one layer is doped with boron, and other by phosphorous. The chemical electric charges in the two layers are different and subsequently both drive and direct the current of electrons.

•  An array of solar cells is used to convert solar energy into a usable amount of electricity (DC).

•  DC electricity produced can be converted to AC by using inverters.

Figure 2 Schematics showing the working of a solar cell.

Most common solar cell comprises a silicon based large area p-n junction. Other variants of solar cells include organic, dye-sensitised, perovskite, quantum dot solar cells, etc. To allow light into the active material and for collecting generated carriers, a transparent conductive film is usually coated on the illuminated side of the solar cell. Films deposited usually have high transmittance as well as high electronic conductivity, e.g., ITO (indium tin oxide), conducting polymers, conductive networks of nanowires, etc.

Efficiency

 

Figure 3 Shockley-Queisser limit for theoretical maximum efficiency of a solar cell. Semiconductors having bandgap from 1 to 1.15 eV (NIR light) have highest efficiencies among single-junction solar cells. Multi-junction cells may have higher efficiencies.

The solar cell efficiency constitutes various efficiencies: (a) reflectance efficiency, (b) thermodynamic efficiency, (c) separation (of charge carriers) efficiency, and (d) conductive efficiency. The product of these components gives the overall efficiency of the cell.

The solar cell efficiency depends upon voltage, temperature coefficients, and allowed shadow angles. However, the measurement of these factors is complex. To alleviate this problem, various other parameters are calculated and these parameters are used to determine the above mentioned factors. These parameters include:

•   Thermodynamic efficiency

•   Quantum efficiency (QE)

•   Integrated quantum efficiency

•   Open circuit voltage (VOC) ratio

•   Fill factor

QE accounts for the reflectance and recombination losses,VOC ratio, as well as fill factor. Resistive losses affect the fill factor, QE, as well as VOC ratio. Fill factor, a crucial parameter in performance of the cell, can be obtained as the ratio of actual maximum power obtained to the product of open circuit voltage and closed circuit current.

The efficiency of single junction (p-n junction) silicon cells has now reached ~33.16% (Shockley-Queisser limit). For infinite layers of single junction cells, the efficiency can theoretically reach ~86% by concentrated sunlight.

Figure 3 Predicted timeline for solar cell efficiencies (National Renewable Energy Laboratory).

Timeline of the Development of Material

Usually, the solar cells are named after the semiconductor material used for their production. The material used for creating solar cell must be able to absorb sunlight. Solar cells can be configured to either capture sunlight reaching the surface of Earth, or work in space. Additionally both single layer (single-junction) or multiple layers (multi-junctions) of the material can be used to produce solar cells.

Figure 4 Global market share of photovoltaic technology.

There are three classifications of solar cells:

a. First generation cells: These are the conventional or traditional wafer based cells which are produced form crystalline silicon. These are the dominant commercially available photovoltaic cells, and uses polysilicon, and single crystal silicon.

b. Second generation cells: These include thin film solar cells made up of amorphous silicon, CdTe, etc. These are commercially used in photovoltaic power plants, small standalone power systems, etc.

c. Third generation cells: These include various thin film technologies are are the future generation photovoltaics. Majority of them are at experimental stage and are not commercially available. These cells use both organic (e.g., organometallic) as well as inorganic materials. Although at this stage, they have poor absorption and low efficiencies, they are investigated for their potential of producing highly efficient and economic solar cells.

We will now briefly discuss various materials used for solar cell production:

Crystalline Silicon (c-Si)

Crystalline silicon or c-Si is the most commonly used bulk material for producing solar cells. It is also termed as ‘solar grade silicon’. Depending upon the crystallinity, and crystal size, bulk silicon is separated in multiple categories. The cells use the concept of p-n junction. Typical wafers used in solar cell production are 160-240 mm thick.

Monocrystalline Silicon (mono-Si)

These are more efficient and more costly than other types of cells. The cell corners appear clipped, like an octagon, as the wafer material is cut from cylindrical ingots, which are usually grown via Czochralski method. Solar panels using mono-Si cells are characterised by a distinctive pattern of small white diamonds.

Epitaxial Silicon

Epitaxial wafers of crystalline silicon can be grown over monocrystalline silicon ‘seed’ wafer via CVD process. After growth, these wafers are detachable as self-standing wafers of controlled thickness. The efficiencies of solar cells produced form these wafers are comparable to that of wafer-cut cells, however, they are much cheaper when CVD is performed at atmospheric pressures. The light absorption can be enhanced by tailoring the surface of epitaxial wafers.

Polycrystalline or Multicrystalline Silicon (multi-Si)

These are made from cast square ingots which are large blocks of molten silicon, cautiously cooled and solidified. Due to the presence of multiple small crystals, they are characterised by metal flake effects. These are low cost and most common type of cells. However, they have lower efficiency than the monocrystalline type.

Ribbon Silicon

It is a type of polycrystalline silicon produced by drawing flat thin films from molten silicon. It is cheaper than polycrystalline silicon. Additionally, silicon waste is greatly reduced since sawing from ingots is not required. Their efficiency is usually poor.

Mono-Like-Multi Silicon (MLM-Si)

This is also known as cast-mono. Small mono material is placed as seeds in polycrystalline casting reactors. The grown crystal is similar to mono material, however, the edges are polycrystalline. After slicing, inner portions are high efficiency mono-like materials (however, these are square and not clipped), while the edges are polycrystalline. This processing produces mono-like cells at the cost of poly.

Thin Films

The amount of active material is greatly reduced in thin-film cells. Most common configurations involve sandwiching the active material between two glass panes. For comparison, silicon solar panels use single glass pane. This makes thin film cells around two times heavier than c-Si panels. However, the ecological impact (estimated by analysing life cycle) of thin film based cells is smaller.

Cadmium Telluride (CdTe)

The only thin film material comparable to silicon in terms of cost per watt is CdTe. Nonetheless, Cd is highly toxic and Te is rare. One square meter of CdTe contains approximately the same amount of Cd as a single C cell nickel-cadmium battery, in a more stable and less soluble form.

Copper Indium Gallium Selenide (CIGS)

It is direct bandgap material with highest efficiency (about 20%) among commercially important thin film materials. Typical synthesis technique involves vacuum processing such as co-evaporation and sputtering. This makes them costly. Researchers at IBM and Nanosolar are trying to decrease the synthesis costs by using non-vacuum solution based techniques.

Silicon Thin Film

Thin films of silicon are typically deposited via chemical vapour deposition (e.g., PECVD) by using silane and hydrogen gases as precursors. Various types of thin films can be grown by controlling the process parameters. These include amorphous silicon (a-Si or a-Si:H), proto- or nano-crystalline silicon (nc-Si or nc-Si:H) also termed as microcrystalline silicon.

Among these, a-Si is a well developed thin film technology. a-Si solar cell is produced from noncrystalline or microcrystalline silicon. The bandgap of a-Si (1.7 eV) is higher than that of c-Si (1.1 eV). Therefore, visible portion of solar radiation is more strongly absorbed by a-Si than infrared portion (which has higher power density). Thin film a-Si solar cells are produced by depositing a thin Si layer on a glass substrate via PECVD.

Proto-crystalline silicon having percentage of nanocrystalline silicon is most suitable for high open circuit voltage. The bandgap of nc-Si is similar to c-Si. Therefore, both nc-Si and a-Si can be joined to form multi-junction or ‘tandem’ cells, wherein top a-Si layer absorbs light in visible range and IR is absorbed by the second or bottom nc-Si layer.

Gallium Arsenide (GaAs) Thin Films

When used as thin film solar cells, single-crystal GaAs cells possess the highest efficiency (28.8%) among thin film single-junction cells. However, they are very expensive and are generally used in multi-junction cells for concentrated photovoltaics (CPV, HCPV) and in solar panels for spacecrafts.

Perovskite Solar Cells

These cells contain perovskites-structured materials as active layer. Most often, it is a solution-processed hybrid organic-inorganic tin or lead halide based material. Originally, they had very low efficiencies (<5% in 2009), which have now been enhanced upto 20% (as of 2014). They represent a very rapidly advancing technology among solar cells. They are extremely attractive for commercialisation owing to the lower costs involved in scaling up their production.

Light Absorbing Dyes

Dye-sensitized solar cells (DSSCs) are produced from low cost materials and no elaborate production equipment is needed. They can be formed as flexible sheets. Even though they do not have the highest efficiencies among thin film solar cells, their price per performance ratio is substantially high allowing them to rival the electricity generation via fossil fuels.

Quantum Dots (QDs)

QD solar cells (QDSCs) are based on Gratzel cell (DSSC architecture), however, they use low bandgap semiconductor quantum dots (e.g., CdS, CdSe, Sb2S3, etc.) as light absorbers, rather than organic or organometallic dyes. The size of the QDs strongly influences their bandgap. Additionally, QDs possess high extinction coefficients and are also able to generate multiple excitons per photon absorption.

A QDSC includes a mesoporous TiO2 layer as backbone of the cell, similar to DSSC. This layer is made photoactive by depositing QDs over it via chemical bath depositions, electrophoretic deposition or successive ionic layer adsorption and reaction. Their efficiency is greater than 5% for liquid-junction as well as solid-state cells. By depositing both TiO2 and QD layers in a single step, the cost can be reduced. This has been achieved by depositing a conducting solar paint made up of TiO2 and CdSe onto any conducting surface. Nonetheless, the QDs have poor room temperature absorptions in QDSC. To overcome this issue, plasmonic nanoparticles (such as nanostars) can be used. In addition to this, IR pumping sources can be used to excite interband as well as intraband transitions in QDs.

Organic/Polymer Solar Cells

Organic and polymer solar cells are made from thin films (~100nm) of semiconductors including polymers, e.g., polyphenylene vinylene and compounds of small molecules such as copper phthalocyanine (a blue-green organic pigment) and carbon fullerenes and their derivatives like PCBM.

They can be processed from liquid solution, providing the benefits of a simple roll-to-roll printing process, resulting in low-cost and mass production. Further, these cells could be beneficial for such applications relying on mechanical flexibility and disposability. Presently, these cells have poor efficiencies and no commercial device is available.

Cells, Module, Panels and Systems

Solar modules are assemblies of solar cells, and produce electrical power from solar light. They are different from a ‘solar thermal module’ or ‘solar hot water panel’. A solar array produces solar power using solar energy.

Figure 5 Schematics of the components of a photovoltaic system.

Solar photovoltaic panel or module constitutes a group of several solar cells oriented in one plane. These modules have a glass sheet on the illumination side, which protects the active material and also allows light to pass through it and reach the active material. Solar cells are connected in such manner than the overall voltage is the sum total of individual voltages.

Table-1 Prices of most popular photovoltaic systems in USD.

Source: IEA:Technology Roadmap: Solar Photovoltaic Energy report, 2014 edition.

Timeline of Solar Cell Development

• Edmond Becquerel was to first to demonstrate photovoltaic effect in 1839, when he made a photovoltaic cell. In 1873, Willoughby Smith explained the impact of light on selenium while electric current was passed through it. First solid state photovoltaic cell was developed by Charles Fritts in 1883, in this, he deposited thin gold layer on selenium to create junctions. This device has an efficiency of ~1%.
• In 1887, outer photoelectric effect was described by Heinrich Hertz and first cell based on this effect was proposed by Aleksandr Stoletov in 1888.
• In 1905, Albert Einstein proposed quantum theory of light and explained photoelectric effect. For this work, he was awarded Physics Nobel Prize in 1921.
• In 1941, Vadim Lashkaryov invented p-n junctions in CuO and AgS photocells.
• In 1946, Russell Ohl discovered modern junction semiconductor solar cell, and patented it.
• Calvin Souther Fuller and Gerald Pearson, in 1954, demonstrated first photovoltaic cell at Bell Laboratories.
• In 1958, solar cells were incorporated into Vanguard I satellite, after which they become popular.

    Applications

First application of solar cells was in the Vanguard satellite in 1954, where they were used as alternate power source to the primary battery power source. The addition of solar cells on the outer body of the spacecraft lead to extending the mission without any major changes in the satellite or its power sources. After this, large wing shaped solar arrays were used on Explorer 6, and following this, solar cells became a common feature in spacecrafts. There were 9600 Hoffman solar cells integrated in these arrays.

By 1960s, solar cells became major power source for the majority of the Earth orbital satellites and in other space probes, because of their ability to provide best power to weight ratio. The early success of solar cells in spacecrafts can be attributed to-(a) expensive power sources could be used, (b) the availability of few alternative power options, (c) since cost was not an issue, best possible cells could be used. Spacecraft market led the development of highly efficient solar cells. Later, terrestrial applications of solar cells began to gain attention. Space applications included solar cells build from gallium arsenide based III-V semiconductor materials, and later involved III-V multi-junction cells. Whereas, terrestrial applications mainly used silicon.

Research and industrial production- Some facts & Figs.

Terrestrial applications of solar cells gained attention with US National Science Foundation’s Advanced Solar Energy Research and Development Division driven ‘Research Applied to National Needs’ programme. This gained impetus owing to the oil crisis in 1973, and during this time, several solar cell producing firms were established (e.g., Exxon, ARCO, Shell, etc.). These were soon joined by technology industries such as GE, IBM, Tyco, etc.

Declining Costs and Exponential Growth

 

Figure 6 Historical price per watt for c-Si cell.

Figure 7 Learning curve of photovoltaics: Swanson’s Law.

Figure 8 Installed photovoltaic capacity worldwide.

In 1990s, polysilicon cells gained much popularity. Their efficiency is lower than that of monosilicon, nevertheless, their low cost drives their research activities. By 2000, polysilicon cells became prominent in low cost panel market. However, owing to their low efficiency, monosilicon cells have gained importance.

Because of high silicon prices in 2004–2008, silicon consumption was lowered. Crystalline silicon panels dominate worldwide markets and are mostly manufactured in China and Taiwan. Solar photovoltaics is growing fastest in Asia, where China and Japan presently account for almost 50% of worldwide installation. Globally installed photovoltaic capacity reached nearly 301 GW in 2016, which supplied 1.3% of the global power by 2016.

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