The series of articles dedicated to the topics of energy obtained from renewable sources continues with an article on the efficiency of solar cells and types of existing solar cells. These articles have the role of shedding light on the ways to obtain clean energies and to bring the public as much information as possible, so as to raise awareness of the importance of switching to ways of obtaining energy through non-polluting technologies and with the lowest possible consumption of resources.

Solar Cell Efficiency

Solar panel efficiency refers to how much electricity we get from the solar energy that falls on the panel.

The surface area of photovoltaic cells is usually very small and the current generated by a single cell is small. That is why several such photovoltaic cells are linked to each other, in series or in parallel, so that they can produce enough current to be used in practice. These cells are integrated into the panels to increase their mechanical strength and weather resistance.

The electrical efficiency of a photovoltaic cell is a physical property that represents how much electricity a cell can produce for a given solar irradiation. The basic expression for the maximum efficiency of a photovoltaic cell is given by the ratio of the output power to the incident solar energy (radiation flux area). The efficiency is measured under ideal laboratory conditions and represents the maximum achievable efficiency of the photovoltaic cell or module.

The real efficiency of a solar panel depends on several factors: the material from which it is produced, the type of mounting chosen (fixed or mobile), the angle from which the light falls on the panel, partial shading, aging of the panels, temperature.

To better understand how a solar cell works and what its efficiency means, you should know that there are several types of materials from which they can be manufactured, each with different characteristics.

Types of Solar Cells

The most well-known types of solar cells are: CIS (Cooper Indium Diselenide), CIGS, CdTe (Cadmium Telluride), monocrystalline, polycrystalline, amorphous and thin film.

Of this crowd, monocrystalline and polycrystalline solar cells stood out as the most efficient, but we are talking about the commercial cells on the market at the moment.

The most widely used are silicon-based solar cells.

Silicon-based solar cells

The most widely used material for the manufacture of semiconductor-based solar cells is silicon.

Silicon is the almost ideal material. It is inexpensive, can be produced in a single crystal at a high degree of purity, and can be impurified into an “n” or “p” type semiconductor. By simple oxidation, thin insulating layers can be created.

Solar cells based on crystalline silicon require a layer thickness of at least 100 μm or more to be able to absorb sunlight efficiently. In thin-film cells of the direct semiconductor type such as GaAs or even silicon with a highly disturbed crystal structure, 10 μm is sufficient. (source: Wikipedia.org)

Depending on the crystalline state, the following types of silicon are distinguished:

Monocrystalline

The cells result from silicon plates from a crystal (Wafer)). These crystals are the basic material for the semiconductor industry and are quite expensive.

Polycrystalline

The cells are made of plates that contain areas with crystals with different orientations. They can be manufactured, for example by the molding process, are cheaper and as such the most widespread in the production of photovoltaic devices. They are often also called polycrystalline solar cells.

Amorphous

Solar cells consist of a thin layer of amorphous silicon (without crystallization) and because of this they are called thin-film cells. They are very cheap, but they have a low yield in the sunlight spectrum, however they have advantages in low light. That is why they are used in pocket computers and watches.

Microcrystalline

These are thin-layer cells with a microcrystalline structure. They have a better yield than amorphous cells and do not have as thick a layer as polycrystalline ones. They are partly used in the manufacture of photovoltaic panels, but they are not as widespread.

Tandem solar cells are layers of superimposed solar cells, usually a combination of polycrystalline and amorphous layers. The layers are made of different materials and thus tuned to different ranges of light wavelengths. By using a wider spectrum of sunlight, these cells have a higher yield than simple solar cells. They are partly used in the manufacture of solar panels but are relatively expensive. An appreciable cheapening will be achieved by using them in combination with lens systems, the so-called concentrating systems.

Aging of solar cells

Over time, the operating parameters of the semiconductor elements of solar cells change. In this case, the efficiency decreases over their lifetime.

Over a period of 20 years, under terrestrial conditions, the efficiency decreases by about 10%, while in space this percentage is reached in a much shorter time due to the much stronger radiation fields.

Often, the loss of yield happens due to causes independent of the solar cells, such as soiling of the surfaces of the protective glass of the modules, mold starting from the frame of the module, shading of the modules by the surrounding vegetation grown in the meantime, yellowing of the polymers that constitute the contact material between the cell and the glass.

Yield of crystalline solar cells

With current solar cells, the efficiency is about 12 – 17%. Usually, the manufacturer grants a warranty on efficiency of 80 – 85% (at peak power) after 20 years. This means very low losses over a long period of time, which justifies the use of photovoltaic systems.

For the actual aging of the solar cells, defects from recombination are responsible, which reduces the lifespan of the load carriers by about 10% compared to the initial value.

Yield of amorphous solar cells

These cells achieve an advanced degree of aging of up to 25% in the first year of operation, which is why for this type of solar panels, the technical characteristics in the accompanying documents do not give the power achieved during manufacturing, but the power after the aging process. As a result, this type of panels have better characteristics when purchased than those in the documents. Aging occurs under the action of light and is the result of the so-called Staebler-Wronski effect (SWE). After about 1,000 hours of exposure to the sun, the amorphous silicon cells reach a stable degree of saturation.

Technical characteristics of solar cells

The technical parameters of the solar cells are given for standard conditions (STC, Standard Test Conditions): light intensity of 1000 W^/m2 in the panel area, constant solar cell temperature 25 °C, AM light spectrum 1.5 global; DIN EN 61215, IEC 1215, DIN EN 60904, IEC 904.

The characteristics of a solar cell are: no-load voltage, short-circuit current, voltage at the optimal operating point, current at the maximum power point, estimated maximum power, filling factor, coefficient of change of power with cell temperature, solar cell efficiency η at an illuminated surface A and light intensity P.

Yield is the ratio of the power delivered by the panel to the power contained in the total incident light. Semiconductors with a stable no-zone use only part of the sunlight. The maximum theoretical efficiency that can be achieved in this case is 33%, while the maximum theoretical efficiency in multi-band no-band systems that react to all wavelengths of sunlight is 85%.

Table 1 – Yield and service life depending on PV cell 
 material Source: Wikipedia.org

The efficiency of commercial solar cells is about 20%, and the modules built with them reach an efficiency of about 17%.

The most efficient solar cells

The most efficient type of solar cell to date is a multi-junction concentrator solar cell with an efficiency of 46.0% produced by Fraunhofer ISE in December 2014.

The highest efficiency achieved without concentration includes a material obtained by Sharp Corporation at 35.8% using a triple-junction manufacturing technology in 2009, and Boeing Spectrolab (40.7% using a three-layer design).

U.S.-made specialty gallium arsenide (GaAs), Alta Devices, produces commercial cells with an efficiency of 26%, claiming to have the world’s “most efficient solar” single cell dedicated to flexible and lightweight applications.

For Silicon solar cells, the American company SunPower remains the leader with a certified module efficiency of 22.8%, well above the market average of 15-18%.

There is a continuous effort to increase the conversion efficiency of photovoltaic cells and modules, mainly for a competitive advantage. There are several groups of materials that are being developed. Ultra-high efficiency devices (η> 30%) are made by using GaAs and GaInP2 semiconductors with multifunctional tandem cells. High-quality, single-crystal materials are used to obtain high-efficiency cells with low cost (η> 20%).

So far, the highest reported power conversion efficiency ranges from 6.7% to 8.94% for small molecule, 8.4%–10.6% for polymer OPVs, and 7% to 21% for perovskite OPVs. OPVs are expected to play a major role in the photovoltaic market. Recent improvements have increased efficiency and reduced costs while remaining benign and renewable for the environment.

Several companies have begun incorporating power optimizers into PV modules called smart modules. These modules perform maximum power point tracking (MPPT) for each module, measure performance data for monitoring, and provide additional safety features. Such modules can also compensate for shading effects, in which a shadow falling over a section of a module causes the electrical output of one or more cell strings in the module to decrease.

One of the major causes of decreased cell performance is overheating. The efficiency of a solar cell decreases by about 0.5% for every temperature increase of 1 degree Celsius. This means that a 100-degree increase in surface temperature could decrease the efficiency of a solar cell by about half. Self-cooling solar cells are a solution to this problem. Instead of using energy to cool the surface, pyramids and cone shapes can be formed from silica and attached to the surface of a solar panel. Thus, visible light can reach solar cells, but reflect infrared (heat-carrying rays).

Source: Wikipedia.org