Nowadays, 85% of photovoltaic (PV) cells are made from silicon. Additionally, these cells can be created in volume by assuming wafer creation methods founded by the Integrated Circuit (IC) business. Nevertheless, silicon has some disadvantages, including the full efficiency of approximately 33%, energy-demanding processing, and brittleness.
Substitute PV technologies using novel resources, designs, and assembly techniques have been advanced to aid silicon’s drawbacks. Novel components include the compound semiconductors gallium arsenide (GaAs) and gallium phosphide (GaP), in addition to the mineral perovskite (CaTiO). The novel energy-focusing Concentrated PV (CPV) construction techniques use multi-junction, thin-film, and large crystals for enhanced energy effectiveness and sturdiness.
Although silicon PV cells are expected to control large-scale power generation owing to mass manufacture and dwindling prices, substitute technologies will be used in rare applications — for example, solar energy.
Though a profound understanding of the photovoltaic procedure necessitates knowledge with quantum mechanics, the elementary values of PV cell process are moderately straightforward: they use the semiconductor p-n junctions.
These PV cells are composed of many of these p-n junctions, meaning that the generated current is maximized. In commercial products, the formed cells are attached to create modules and ultimately panels. The DC voltage can then be turned into AC by a converter.
Figure 1: Single-junction PV cell operation: Photons of appropriate energy liberate electrons, which cross the semiconductor junction and generate a potential difference. (Source: Cyferz at English Wikipedia)
Figure 2: PV cells are combined into modules then into panels to form end products. (Source: Wikipedia)
First Generation PV Cells: Single-Junction Silicon
The first PV panels which were developed were mainly created from crystalline silicon (“c-Si”). The main drivers for silicon’s great interest are its performance and convenience. The wholesale material is very available (28% of the Earth’s crust), and the methods and amenities for production have been adopted from the chip business. However, the creation of wide-scale silicon wafers for PV panels is very energy demanding, complicated, and costly.
However, silicon PV panels have become 30% cheaper in the past 12 months. However, the technology is still too expensive for many niche uses.
Silicon Advantages: Efficiency and Band Gap
There are several advantages to silicon applications in PV technology. It has high efficiency, generating 110W per 1 meter at 10 percent efficiency. However, the efficiency is limited by its band gap, which is the volume of energy needed to release an electron from an atom into the “conduction band.”
Photon energy is determined by wavelength (those with shorter wavelength have more energy). Numerous sunlight photons incoming a c-Si lattice will carry inadequate energy to release an electron and will, consequently, only heat up the material.
In 1961, William Shockley and Hans-Joachim Queisser considered the hypothetical maximum PV effectiveness for cells made of just one semiconductor across a variety of bandgaps, revealing the optimal bandgap as 1.13 eV, yielding maximum efficiency of 33 %. This is very similar to Silicon's bandgap.
Figure 3: Shockley and Queisser's calculation of maximum efficiency against band gap for single-junction PV cell semiconductors. Silicon has a band gap of 1.1 eV. (Source: Wikipedia)
Silicon Drawbacks: Crystal Size, Energy, Efficiency, And Fragility
There are also many other issues with Silicon applications in PV cells: Maximum theoretical efficiency is only 33%, fragility, energy-intensive, high-temperature, complex processing, and expensive applications.
New Developments in PV Technology
Novel PV products have recently been commercialized, such as isolators, meters, controllers, and inverters.
Second-Generation PV Technology
Second-generation PV panels are mainly nanometer layers of PV upon on glass, plastic, or metal. These cells are termed “thin-film” PV (TFPV) cells and are less costly, less energy exhaustive and lighter. However, they are also less efficient as they are composed of tiny crystals. These TFPV panels, therefore, provide 20 percent efficiency compared to today’s 10 percent efficiency. Furthermore, they are also degraded quickly.
Figure 4: Multi-junction TFPV cell internal structure. (Source: NREL)
Third-Generation PV Technologies
Novel technology has been improved in various categories:
- Materials: Improving the amount of energy production
- Structure: Reductions in energy demand of production
- Processing: Improving the size of molecules to maximize efficiency
- Mechanical: Increasing the number of photons that fall on a unit area
The amount of energy produced can be increased by using materials with lower or higher bandgaps. Most of the energy from the sun is carried by photons with energy below the optimal bandgap of silicon. Indium arsenide (InAs), for example, can be utilized to complement silicon as it has a band gap of 0.36 eV.
Furthermore, semiconductors with higher bandgaps enable additional electricity generation due to the energy of shorter wavelength photons. For example, gallium arsenide (GaAs) has a bandgap of 1.43 eV, whereas gallium phosphide (GaP) has a band gap of 2.25 eV. Numerous lines of investigation have caused further compounding. For example, using indium gallium arsenide (InGaAs) and indium gallium phosphide (InGaP).
Substitute bandgap semiconductors have an inferior maximal efficiency. However, when used together in a multilayer structure can boost efficiency. Although silicon has an extreme efficiency of 33%, this can also be improved by multilayer PV panels.
Investigators are identifying novel collections of resources which associate the maximal efficiencies, involving the benefits of the first generation along with the more basic and less expensive manufacturing techniques of the second.
For example, a collection of resources which has led to lots of excitement is resultant from the mineral perovskite (CaTiO). This cluster of materials has band gaps from around 1.4- 2.5 eV. The hypothetical extreme efficiency of the perovskite group does not match silicons, however, current efficiency gains from approximately 4 to 20% have elevated confidence that commercial products will finally be as, or even more, efficient as TFPV panels.
The main benefit of the perovskite group is the relative ease and minimal processing temperatures with which large crystals can be grown. These crystals are ideal for enhancing electron mobility and enhance efficiency whilst reducing creation costs.
Furthermore, other researchers have found that varying facets of the perovskite crystals have a range of efficiency. Research now aims to process the material so that the majority of the efficient facets border with the PV cell electrodes. However, erosion rates still need to be reduced.
An additional development aim for third generation PV panels is Concentrated PV (CPV) technology. This technology is intended to emphasize sunlight with lenses and mirrors, enhancing the number of photons which reach a unit area of PV panel. The method characteristically uses high-efficiency, multi-junction PV cells.
Concentrating the beans enhances efficiency, allowing dramatic decreases in size, dropping the price and heaviness of the creation, and enhancing the number of installation locations.
“Low” CPV concentrates the equivalent of 2 to 100x sunlight, whereas “high” CPV can increases the light to approximately 1000x sunlight. CPV systems frequently utilize solar trackers and occasionally cooling systems.
Table 1: Efficiency of c-Si, TFPV, and CPV technologies (Source: IRENA)
Case Study: Energy-Harvesting Wireless IoT Sensors
The main use for PV technology is within the renewable energy industry. However, 3rd generation technologies (which are cheaper, durable, and smaller) have the potential to introduce energy-harvesting niche uses.
Wireless IoT Sensors
Creators of IoT sensors are keen to utilize energy harvesting. It is envisioned that the IoT will encompass many thousands of sensors, located remotely and secluded, away from power sources. These products are also likely to run Bluetooth low energy and ZigBee, which do not require excessive power. Additionally, PV cell recharging expands battery life to several years.
Small Li-ion cells can be charged by Energy harvesting technology, such as MikroElektronika’s energy-harvesting module. This silicon PV cell is able to create 0.4W at 4V. However, the voltage and current from a PV cell vary a lot. Therefore, the output must be controlled throughout Li-ion battery charging.
Maxim’s MAX17710 power management IC can monitor and control unregulated sources (such as PV cells) with production levels reaching 1 µW to 100 mW. The equipment also comprises an enhancement regulator circuit for boosting the battery, whilst an interior regulator defends the cell from overcharging. A power management IC is also offered by Texas Instruments called the bq25504.
This device is explicitly intended to proficiently obtain and control power produced from PV cells. The chip assimilates a DC-DC boost converter/charger which necessitates only microwatts of power and a voltage as low 330 mV to begin energy gathering.
Figure 5: Application circuit for energy-harvested battery charging using TI power management IC. (Source: Texas Instruments)
Third-Generation PV Technology Applied
Although present PV cell energy-harvesting resolutions work reasonably well, there are some disadvantages. For example, MikroElektronika’s energy-gathering module is 7 x 6.5 x 0.3 cm and is reasonably heavy and fragile. Nevertheless, products like this one are the sole practical choice due to their efficiency.
Third generation cells assimilate technologies to enhance efficiency, with those currently in development expected to double in efficiency in the next few years.
A third-generation 4 cm2 TFPV cell in direct sunlight would receive 0.22 W power, generating around 44 mW at 20% efficiency, fully recharging a 300 mAh Li-ion battery in approximately 25 hours. Although this would take a few days of sunshine, this Li-ion battery will only use a few mAh a day, therefore only needing the PV cell to top-up the battery.
Dense 3rd-generation PV cells are not yet commercialized. However, when mass manufacture commences, charges will probably be too excessive for wireless IoT sensor uses. As technology develops and demand upsurges, however, TFPV cells are likely to become less expensive.
Concurrently, the efficacy of TFPV PV cells will increase further, conveying future advantages, such as:
- Energy harvesting from artificial light sources
- Decreases in panel dimensions
- More power availability
- Amplified wireless sensor range
- Numerous sensors power-driven from a single PV panel
Niche uses (such as energy-harvesting wireless IoT sensors) which need effective, solid, durable, and cheap PV technology, will benefit from 3rd-generation PV cells. This technology would allow wireless sensors to function dependably with minimal maintenance. As third-generation PV technologies evolve, we can expect to see additional wireless sensor designs, such as harvesting energy from indoor lighting and other applications that require compact, efficient, powerful, and robust designs.
This information has been sourced, reviewed and adapted from materials provided by Mouser Electronics.
For more information on this source, please visit Mouser Electronics.