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Why OPV is one of the most suitable technology for indoor IoT applications ?

Why OPV is one of the most suitable technology for indoor IoT applications ?

Sadok Ben Dkhil

Sadok Ben Dkhil

Billions of IoT devices are expected to be installed over the coming years with almost half of them connected inside buildings. Currently, the use of batteries to power these devices places significant constraints on this development. That’s why the range and frequency of data transmission are curtailed to achieve sufficient battery life. Additional operation and maintenance costs are also incurred by providing replacement batteries.

Energy harvesting has the potential to solve these hardware issues, providing greater reliability and operational lifetimes in wireless sensor networks. Different energy harvesting technologies exist such as photovoltaic, piezoelectrics, thermoelectric generators and RF harvesters [2].

Photovoltaic energy harvesting, which is the conversion of light into electrical energy, is the most common method of harvesting energy and is now a well-estabilished technology. The amount of energy harvested depends on the intensity and spectral content of the light falling on the surface of the solar cell, the incident angle of the light, and the size, sensitivity, temperature, and type of solar cells used. There are two categories of solar cells available in the market : solar cells specfically designed to harvest electrical energy from indoor light and those designed for harvesting electrical energy from the outdoor environment. In particular, organic photovoltaic (OPV) cells have received increasing attention in the past few decades due to their potential as one type of next generation solar cell with several advantages like color, transparency, and light weight. Recenly, OPV cells have achieved rapid progress with the development of new organic photovoltaic materials, device structure engineering, and understanding of device physics.

The power conversion efficiencies (PCEs) of OPV cells under 1 sun illumination have recently surpassed 18%. In comparison with the competing photovoltaic technologies, such as traditional c-Si and newly emerging perovskite solar cells, OPV devices are the most compatible for indoor application. In fact, the OPV materials have great tunability of their absorption spectra, it is very easy to match them with varied indoor light sources, offering unique integration prospects as energy sources for off-grid indoor electronic devices.

1. What differences with traditional PV cells ?

As I mentioned in my previous newsletter, most of the traditional solar cells present on the market are based on silicon technology. In particular, c-Si, a-Si, and CIGS PVs shown excellent device performance under 1 Sun conditions and dominate the market for outdoor applications. However they exhibit low photo-voltages and a significant drop in performance when used under low-light intensities or indoor conditions. In indoor application, photovoltaic devices need to operate under very different conditions than those experienced outdoors, e.g., light illumination intensities that are typically 10-1000 times lower than direct sunlight.

The majority of light sources currently used for indoor lighting are fluorescent lamps, incandescent lamps, and white light-emitting diode (LED) lamps. Both fluorescent and white LED lamps are used for indoor applications and they emit light in wavelength ranges from 350 nm to 750 nm. As indoor light sources emit radiation in the UV-vis spectral range, OPV devices with good spectral response in the visible region are the more suitable candidates for indoor light harvesting applications. In addition to that, OPV technology has the unique advantages of solution processibility, flexibility, lightweight components, and the ability to customize the design and geometry.

That’s why it’s efficient to use a cell with strong absorption in this range.

Recent work using a new high bandgap organic semiconductor called (IO-4Cl), demonstrates an organic solar cells with a PCE of 26.1% measured nder the indoor illumination conditions, simulated by a 2,700K LED lamp at 1,000 lux. This  OPV cell has an absorption band that ranges from 400 to 700nm which corresponds well to the emission of indoor sources. The same OPV cells gives a PCE of 9.80% under the illumination of AM1.5G (100mWcm−2 ) which shows the real imapct of the compatibility between the emission spectra of the light source and the absorption of the cell on the final performances of the device.

2. Our solution for IoT applications

Currently at Dracula Technologies, we are able to produce solar cells with PCE up o 12% at the lab scale under standard illumination conditions (AM1.5). Under the indoor illumination conditions, a PCEs that ranged from 20 to 25 % were obtained at our laboratory with some specific organic modules. More importanly, these high performances are accompanied by a long-term stability even under extreme aging conditions.

Recently our team has successfully published in colaboration with our colleagues from different  laboratory in the world a power conversion efficiency of 9.74% (AM1.5) due to our development of a new organic−inorganic hybrid high band gap material. This new material contributed to the significant improvement of the stability of organic solar cells in air and light illumination.

As mentioned before with Dracula Technologies, we generate energy from ambient light because we are using specific efficient materials which harvest both natural and artificial light. The high efficiency in low-light conditions makes our OPV modules suitable candidates for indoor applications. We are producing our complete devices using inkjet printing technique (sheet to sheet and not Roll to Roll) which has a great potential for specific shape and production at an affordable price compared to the other technologies as the inkjet printing sheet to sheet requires small start-up volumes of inks and is highly economical in materials use, as there are no startup or trail ends.

By the way, I have the pleasure of inviting you to order our LAYERs modules to test and validate them with your use case and tell us the degree of your satisfaction. We will be really happy to receive your comments and develop specific modules that are compatible with your energy needs.

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A short overview of the third generation solar cells: concept, materials and performance.

A short overview of the third generation solar cells: concept, materials and performance.

Sadok Ben Dkhil

Sadok Ben Dkhil

Most solar cells present on the market are based on silicon wafers, they are called first-generation technology. Second-generation cells are thin-film technologies that are often commercially available, such as copper indium gallium selenide (CIGS), cadmium telluride (CdTe), gallium arsenide (GaAs) and amorphous silicon (a-Si:H). Currently, inorganics like crystalline silicon, polycrystalline silicon (poly-Si), cadmium telluride, and copper indium germanium selenide with power conversion efficiency (PCE) of 15–20% are dominant in solar energy production.

Third-generation cells are less commercially advanced ‘emerging’ technologies. This includes organic photovoltaics (OPVs), copper zinc tin sulphide (CZTS), perovskite solar cells, dye-sensitized solar cells (DSSCs), and quantum dot solar cells. 

During the past five years, organic and perovskite solar cells have reached points in their technological evolutions where large scale deployment is possible, and this is the reason why I decided to focus on these two technologies which constitute two emerging technologies with more promising potential.

In a general photovoltaic device, the conversion starts with light induced charge generation, followed by transport of the generated charges and collection of the charges by the electrodes. In this context organic and perovskite solar cells differ in the mechanism of charge generation due to the significantly different nature of the active layer materials (layer that will absorb light and convert it into electricity), namely organic semiconductors and hybrid organic-inorganic perovskite. I will summarize the main operation principles of organic and perovskite solar cells, the materials, and discuss the advantages as well the limitations of each technology.

It is however important to mention that both organic and perovskite solar cells, have the advantage to be processed by a great versatility of methodologies, including solution or vacuum processing techniques. Besides, both types of devices are built in similar architectures and there is a growing trend material from organic solar cells to be used in perovskite solar cells.

1. Organic solar cells

The first solar cell family that will be discussed is organic solar cells. These solar cells have photoactive layers comprising in general a semiconducting polymer and a fullerene derivative (or NFA for non-fullerene acceptor derivative) and have been the subject of several scientific publications. This family contain bulk heterojunctions (BHJs). “ A heterojunction is an interface between two different semiconducting materials”.

!! The current improvement from ≈2.5% in 2013 to 18 % in 2020 in organic solar cells performance is primarily credited to these novel non-fullerene acceptors (NFA).

The organic photovoltaic devices hold some significant advantages over inorganic devices, including low cost of materials, lightweight, strong and tunable absorption characteristics, flexibility, and the potential to be fabricated using roll-to-roll or sheet to sheet printing techniques. Unfortunately, some organic materials suffer from stability issues arising from photochemical degradation. Organic solar cells can be made transparent or different colors, but this will lower the overall efficiency.

The ability to make them flexible have some advantages in several markets but it means they are more susceptible to moisture and oxygen ingress that can cause degradation than if they had been laminated to glass.

What is an organic solar cell structure ?

OPV cell structure

Figure 1 : Schematic diagram of typical BHJ-OSC (Regular OSC, inverted OSC and tandem OSC)

Often an organic solar cell is built on a transparent conductive oxide (TCO) electrode material like indium tin oxide (ITO) substrate. In literature, commonly three types of organic solar cells can be found, and schematic representation is shown in the figure. In the so-called “Regular” or “standard”  BHJ configuration, ITO is coated with a thin layer of hole transporting layer (HTL); on top of this HTL, a blend of donor and acceptor is coated, and then a thin electron transporting layer (ETL) and a low work-function metal cathode are usually vapor deposited.

In the so-called “inverted” configuration due to the inverted polarity of charge collection, ITO is coated with thin layer ETL, before a blend of donor/acceptor. A thin layer of HTL and high work-function metal anode are deposited on top of BHJ blend.

Nevertheless, both of abovementioned single junction configurations can only achieve efficiency maximum ≈14–16% in the most favourable case due to either the lack of a broad absorption range of the donor/acceptor blend or limited charge carrier mobility, which require a thin (80–120 nm) blend for efficient charge collection. To avoid these limitations, two or more sub-cells with complementary and/or congruent photo absorption profiles are linked by a thin interconnecting layer (ICL) to construct tandem configuration.

How do oganic solar cells work ?

OPV cell

Figure 2 : Simplified working principle of an organic solar cell

The process of photo-generation and charge transport to the external circuit within a polymer based photoactive layer consisting of a donor and acceptor is well known in literature and can be separated into five steps (see figure 2). The first step is light absorption and photogeneration of an exciton (An exciton is an electrostatically bound electron–hole pair) within the active layer (donor and acceptor phases). It is important to note that exciton in conjugated polymers are not locally defined; they can diffuse along a polymer chain or between adjacent polymer chains.

The diffusion length characterizes the distance an exciton can travel before recombination takes place. These excitons must dissociate prior any recombination in the BHJ. Since charge separation takes place at the donor/acceptor interface, the internal structure on the nanoscale of the active layer film is utmost importance for the efficiency of the charge separation. If this nanomorphology of the active layer is not yet optimized, only a fraction of the excitons is able to find the donor/acceptor heterojunction interface at which exciton dissociation can take place which will limit the performance of the cell.

Once the exciton has dissociated the free electron and hole migrate to the cathode and anode, respectively. The migration may occur due to the driving force from the gradient in chemical potentials of the electrons and holes at the donor–acceptor junction. Furthermore, charge concentration gradients can produce diffusion currents. Electron- and hole-blocking layers are often included in BHJ solar cells to direct the charge migration to the desired electrodes in order to decrease recombination. The photo-generated holes and electrons within BHJ photoactive layers migrate through interconnected donor and acceptor phases, respectively. In order to reach the respective electrode holes and electrons must avoid coming into contact at an interface and undergoing recombination.

Finally, the separated charges must cross the photoactive layer/electrode interfaces to reach the external circuit.

Organic photovoltaic technology is unlikely to challenge silicon’s dominance for large-scale electricity generation but offer promise in applications where lower cost, flexibility, weight, low energy requirement, durability and low light conditions can be traded-off against efficiency. For example, the light-absorbing layer of organic solar cells provides flexibility so the cells can be tuned and optimized for different light spectra. This new approach to powering indoor applications could provide the Internet of Things with an inexpensive source of renewable energy, potentially replacing batteries. This particular role of organic cells for indoor application will be discussed more in detail in the next newsletter.

2. Perovskite solar cells

Perovskite solar cells are new 3rd-generation solar cells that appear to have a very good chance of contributing to large scale solar energy production based on their high PCE and compatibility with scalable processes and are therefore included in this newsletter. Perovskite solar cells warrant discussion because never before in the history of solar cells research has such rapid progress in increasing the PCE been witnessed as that which has occurred for these solar cells.

Organic–inorganic lead halide perovskite is regarded as one of ideal materials for photovoltaics ( At present there are different perovskitetype compounds which are not all very efficient), which demonstrated a certified power conversion efficiency (PCE) of 25.2% in 2019 surpassing the PCEs of the well-known high efficiency thin-film solar cells based on copper-indium-gallium-arsenide or cadmium iodide (CdTe).

At present, the most challenging issue in perovskite solar cells is the long-term stability, which must be cleared up before putting it into practical applications. As we know that the stability of perovskite solar cells upon the severe environment, e.g., thermal treatment, light illumination, humidity, etc., appears to be the bottleneck that impedes their further commercialized. Among them, the humidity is demonstrated to be one of the possible causes for the degradation of perovskites. The commercialization of perovskite solar cells requires extensive research and development of new perovskite materials that are not only very effective in photoelectric conversion but also non-toxic and stable. Thus, in the next few years, more efforts are required for the development of inorganic lead-free perovskite solar cells.

What is a perovskite solar cell structure ?

Perovskite solar cell

Figure 3 :  Four device configurations of PSCs: planar and mesoscopic structures.

A perovskite solar cell is a multi-layered device composed of a transparent conducting oxide (TCO) as a front electrode (in general based on FTO for Fluorine Tin Oxide) a light-absorbing perovskite layer, sandwiched between the N-type electron transport layer (ETL) and the P-type hole transport layer (HTL), and a metal back electrode. As shown in this figure, the perovskite solar cells can be fabricated in either a regular n–i–p or inverted p–i–n configuration, and the former can be further divided into mesoporous and planar structures while the latter is usually planar. The selection of ETL and HTL is based on the workability of the energy-band alignment of the structure and the processing solvents.

How do Perovskite solar cells work ?

Contrary to that observed for organic solar cells, the absorption of photons in perovskite materials within perovskite solar cells does not lead to the formation of a large lifetime exciton. Numerous studies reported a small exciton binding energy for perovskite absorber in the range of a few milli-electronvolts, which indicates that practically the photon absorption leads to free carrier generation, unlike organic solar cells. This non-excitonic nature of the charge generation is crucial for the development of high-performance devices. The efficient generation of free electrons and holes in one step is one of the main advantages of perovskite solar cells since in excitonic solar cells significant losses in energy occur through exciton migration and exciton dissociation as explained in the operating mode of organic solar cells. 
In perovskite solar cells charge separation can occur either by injection of photo generated electrons into ETL or injection of holes in HTL. Furthermore, free electrons created near the perovskite/HTL interface have to diffuse through the entire width of the absorber layer before being extracted at the ETL/perovskite interface, with increased chances of recombination. Similar considerations apply to the holes near the ETL/perovskite interface.

To gain market share from crystal silicon solar cells, alternative technologies have to provide a desirable combination of high efficiency, low manufacturing costs and excellent stability. Recent research suggests that perovskite solar cells have the potential to meet some of these conditions (particularly high efficiency and low manufacturing costs) and then become competitive in the marketplace. For that, an intensive research effort must be carried out in order improve the stability of these new materials and avoid toxic derivatives.

 

Some example of companies working in the OPV and Perovskite devices manufacturing     


So far, there are a few companies focusing on development of perovskite solar panel: Microquanta Semiconductor, Solar-Tectic, Oxford Photovoltaics, Saule Technologies, and GreatCell Solar previously known as Dyesol.
 
The most key market participants involved in the manufacture of organic photovoltaic include Heliatek, InfinityPV ARMOR, Epishine, Sunew and Dracula Technologies.

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What is Energy Harvesting ?

What is Energy Harvesting ?

Enora-arche-200x200

Enora Arche

With growing numbers of IoT devices in activity, IoT electronics’ manufacturers have to find ways to replace batteries or extend battery life. In fact, depending on the application, batteries used for IoT devices have an estimated lifespan of 1 to 3 years. Replacing all those dead batteries causes higher maintenance costs and raises controversial debates regarding battery production and end of life.

One viable solution is to replace batteries or ensuring their floating with Energy Harvesting (EH) solutions.

« Energy harvesting is the capture and conversion of ambient (free) energy into electric energy. » *

This electric energy can be accumulated and stored for future consumption. The storage is advised due to the intermittent nature of ambient energy. Usually, IoT makers use a battery or supercapacitor to enable supply in the absence of the ambient energy and during peak conditions, such as during wireless transmission.

The challenge is to make devices self-powered by harvesting energy which can come from different type of sources :

1. Solar energy through photovoltaic materials

Harvesting solar energy is based on the photovoltaic effect discovered in 1839 by Edmond Becquerel. For short, it is the ability of a material to absorb light and convert it into electricity.
Most solar cells presently on the market are based on silicon wafers, the so-called first generation technology. Second-generation cells are thin-film technologies that are often commercially available, such as copper indium gallium selenide (CIGS), cadmium telluride (CdTe), gallium arsenide (GaAs) and amorphous silicon (a-Si:H).

Third-generation cells are less commercially-advanced ‘emerging’ technologies. This includes organic photovoltaics (OPVs), copper zinc tin sulphide (CZTS), perovskite solar cells, dye-sensitised solar cells (DSSCs), and quantum dot solar cells. The two last generations are unlikely to challenge silicon’s dominance for large-scale electricity generation but offer promise in applications where lower cost, weight, low energy requirement and durability can be traded-off against efficiency. 

The third generation of photovoltaic cells, so-called thin film, is revolutionizing the energy harvesting market by offering a technology efficient in low light condition, lightweight and greener than previous generation.

2. Thermal energy through thermoelectric materials

Thermal energy is the kinetic energy of microscopic agitation of an object, which is due to a disordered agitation of its molecules and atoms. Thermal energy is a part of the internal energy of a body. Thermoelectric materials, such as crystals convert a constant temperature into electrical energy. It is for instance particularly useful for harvesting wasted heat from cars. Thermoelectric generators are also used to convert heat from the human body into a voltage to power medical sensors.

3. Mechanical energy through piezoelectric materials

The concept of piezoelectric was born in 1880 by the Curie brothers. Piezoelectricity meaning literally “electricity resulting from pressure”, piezoelectric materials accumulate a charge in response to a mechanical stimulation. They are used for instance to harvest the energy generated by footsteps or crowd to power display system.
Piezoelectric materials are still at the development stage so far even if some devices are commercially available. It is considered as one of the most promising technology for harvesting energy.

4. Vibrational energy through electrodynamic transducers

Electrodynamic harvesters transform kinetic energy (vibrations) into electricity. Energy is created when a magnetic presence passes by a coil, this energy is then captured and converted into a usable current. It knows staggering but still theoretical applications within the body sensor networking and smart car markets as well as the IoT.

5. RF energy

It’s about creating electric tension by collecting wave propagation. This kind of energy harvesting solution gained an increasingly popularity during recent years since the introduction of RFID (Radio Frequency Identification).
RFID tags can be sticked or incorporated in objects, animals or humans. In fact. RFID tags contains antennas connected to an electronic chip which allow them to answer queries from the transceiver.

*Emerging Technology Analysis, Gartner 2017

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