The New Era of PV Technology: Organic Solar cells

Introduction

Typically, traditional crystalline solar cells are made of silicon. Instead of silicon, an organic solar cell uses carbon-based materials and organic electronics to produce electricity from the sun as a semiconductor. “Plastic solar cells” or “polymer solar cells” are also sometimes referred to as organic cells. In their physical structure, organic cells are made of compounds that are typically dissolved in ink and printed on thin plastics, which means that OPVs can be flexible and incorporated in more places or structures than crystalline photovoltaics. One of the biggest differences between silicon photovoltaics and organic photovoltaics (OPV) They may even be used to create windows for solar power.

While organic photovoltaics is an exciting new technology, there’s a long way to go before the efficiencies already achieved in solar cells based on silicon can be matched. Given the wide range of potential applications for OPVs, however, it may not be long before they are a commonly used solar energy generation technology. In addition, organic cells are cheap to produce and physically versatile, meaning organic solar products may be able to compete with traditional crystalline cells once performance can be refined. Organic solar cells generate electricity via the photovoltaic effect, just like monocrystalline and polycrystalline silicon solar cells. In three simplified steps, a photovoltaic cell turns sunlight into usable electricity:

1. Light is absorbed from a semiconducting material and knocks electrons loose.
2. Flowing loose electrons and generating an electric current
3. The current is collected and transmitted to cables

The photovoltaic process is the same in an organic solar cell, but carbon-based compounds are used as the semiconducting material instead of silicon. Overall, organic cells are structured very similarly to the solar cells of crystalline silicon. The most notable difference between the two types of cells is the semiconducting layer; organic cells use carbon-based compounds (organic molecules) instead of crystalline silicon, which are printed onto a plastic backing in an extremely thin layer.

Tandem organic cells

The use of an organic solar cell as the top cell in a hybrid tandem solar cell stack has been researched and studied recently. Because organic solar cells have a greater gap in the band than traditional inorganic photovoltaics such as silicon or CIGS, higher energy photons can be absorbed without losing much of the energy due to thermalization and therefore operate at higher voltages. The unabsorbed lower energy photons and higher energy photons pass through the top organic solar cell and are then absorbed by the inorganic cell at the bottom. Organic solar cells are also a low temperature processable solution with a low cost of $10 per square metre, resulting in a printable top cell that improves the overall efficiency of existing technologies for inorganic solar cells. Much research has been done to allow such a hybrid tandem solar cell stack to be established, including research into the deposition of semi-transparent electrodes that maintain low contact resistance while having high transposition resistance. Tandem organic solar cells (OSCs), consisting of organic semiconductors sandwiched between two electrodes, have recently attracted considerable attention due to their lightweight, low cost, and ability to print large-area flexible printed solar panels.

Using non-fullerene acceptors, single-junction OSCs with power conversion efficiency (PCE) above 14% were recorded. However, the optical absorption of organic semiconductors is quite limited in contrast to traditional silicon solar cells, which have very wide and strong absorption with a solar spectrum coverage below 1100 nm. Almost all reported high-performance OSCs have optical bandgaps between 1.45 and 1.55 eV absorption spectra, corresponding to a spectral response edge between 800 and 855 nm. Moreover, there are relatively narrow absorption peaks in organic semiconductors, which hinder the conversion of solar energy. Given the restrictive absorption spectrum of the bulk heterojunction in single-junction cells, it is difficult to achieve full solar spectrum coverage in OSCs. Several low-bandgap (LBG) donor and acceptor materials with near-infrared (NIR) absorption spectra have been successfully designed and synthesised to harvest more sunlight. Although these materials exhibit very impressive photocurrents, due to the severe thermalization loss of high energy photons, the overall PCEs remain moderate. One way to minimise the loss of thermalization of these LBG-OSCs and improve the efficiency of the device is to insert wide-bandgap (WBG) OSCs to build tandem architecture devices in front of LBG-OSCs. Tandem OSCs usually have two or more sub cells linked in series with a complementary absorption spectrum. The thermalization loss and transmission loss can be suppressed in this type of solar cell because the WBG active layer in the front cell and the LBG active layer in the back cell are harvested by high-energy photons and low-energy photons, respectively. Therefore, photons with different energies can be used for photocurrent production in tandem OSCs.

Dye-Sensitized Solar Cells (DSSC)

Dye-sensitized solar cells (DSSCs) belong to the group of thin-film solar cells which, due to their low cost, simple preparation methodology, low toxicity and ease of production, have been under extensive research for more than two decades. However, due to their high cost, reduced abundance, and long-term stability, there is a lot of scope for the replacement of current DSSC materials. By optimising material and structural properties that are still less than the efficiency offered by first and second-generation solar cells, i.e. other thin film solar cells and Si-based solar cells that offer ~ 20-30% efficiency, the efficiency of existing DSSCs reaches up to 12%, using Ru(II) dyes.

As shown in above figure, four key parameters for a DSSC include the working electrode, sensitizer (dye), redox-mediator (electrolyte), and counter electrode. DSSC is a working electrode assembly soaked with a sensitizer or dye and sealed with a thin layer of electrolyte to the counter electrode soaked with the help of a hot melt tape to prevent electrolyte leakage. Typically, DSSCs are constructed with two sheets of conductive transparent materials that assist a semiconductor and catalyst deposition substrate, also acting as current collectors. There are two main characteristics of the substrate used in the DSSC – first, the substrate requires more than 80% transparency to allow optimum sunlight to pass to the DSSC – Secondly, it should have high electrical conductivity for efficient charge transfer and decreased energy loss in DSSCs. In DSSCs, fluorine-doped tin oxides (FTO, SnO2: F) and indium-doped tin oxides (ITO, In2O3: Sn) are typically used as a conductive substrate. These substrates consist of layers of indium-doped tin oxide and fluorine-doped tin oxide-coated soda lime glass. ITO films have a sheet resistance transmittance of > 80% and 18 Ω/sq, while FTO films show a lower transmittance of ~ 75% in the visible region and 8.5 Ω/sq sheet resistance. As far as their commercial application is concerned, a DSSC in building-integrated modules needs to be sustainable for > 25 years to avoid disturbance of the building environment for repair or replacement, and a life span of 5 years is sufficient for portable electronic chargers integrated with apparel and accessories. However, due to their sandwiched glass structure, DSSCs are quite bulky, but the flexible DSSCs that can be processed using roll-to-roll techniques may come as an alternative, but then have to compromise with the shorter life span.

Single-layer organic cells

An organic layer sandwiched between two electrodes consists of a single layer of organic solar cells:

1. A thin transparent metal or transparent conducting oxide (TCO) layer such as Indium Tin Oxide (ITO) is the front electrode; this layer should have a high working function.
2. A metal with a relatively low working function is the back electrode. Commonly used are Al, Mg, Ca.

Once an external circuit is made by connecting the two electrodes with a conductor, an electric field in the organic layer is created by the difference in the working functions. If we remember that the electrons are excited to the LUMO after light absorption, leaving a hole in the HOMO, creating excitons, the electrical field in the organic layer will therefore help to break up the pairs of excitons, pulling electrons to the positive electrode and holes to the negative electrode. Single layer organic solar cells are also referred to as Schottky diodes as only one interface (organic layer/metal electrode) is rectified, the other electrode is formed, and the organic semiconductor is ohmically contacted. Though simple to manufacture, this class of solar cell has low quantum efficiency and conversion efficiency. The drawback is due to the fact that the electrical field (in the organic layer) resulting from the difference between the two electrodes’ working functions is generally not sufficient to achieve an effective exciton pair separation. Instead, the organic layer has an electron-hole recombination process. In addition, both the electrons and the holes travel in the same material in the single layer device and recombination losses are generally high. The efficiency of the OSC device is reduced by the slow transport of charges, but the likelihood of charge recombination in the device also increases. Exciton formation, which are strongly bound dipole charges of photoexcited semiconducting organics, causes the low PCE of the OSC device. To create free carriers, the bound exciton needs an additional exciton dissociation step, because the free electrons and holes are desired as an efficient charge carrier that can reduce the efficiency of carrier generation. There is only one place in a single layer OSC device that is the interface between the photo-active layer and a cathode to dissociate excitons into free carriers. It was later known that the excitons are more effectively dissociated at the donor-acceptor interface and that it is possible to develop a bilayer OSC device by inserting an acceptor layer between a donor organic semiconductor and a cathode.

Bilayer organic cells

By including a separate organic donor layer and a separate acceptor layer between the two previous electrodes, the double-layer organic solar cell improves the functionality of the single layer OPV. There are differences in electron affinity and ionisation energy between these two material layers, so electrostatic forces are generated at the interface between the two layers. In order to make the differences large enough, the materials are properly chosen, so these local electric fields are strong, which can break up the charged particles much more efficiently than do the single layer photovoltaic cells. The electron acceptor is the layer with greater electron affinity and ionisation potential, and the electron donor is the other layer. The Bi-layer structure is also called a solar cell device for planar donor-acceptor heterojunctions. The downside is the small interface that allows it to be reached and dissociated only by charge carriers from a thin layer. To absorb enough light, a typical polymer layer needs a thickness of at least 100 nm. Only a small fraction of the excitons can reach the heterojunction interface at such a large thickness, bearing in mind that the diffusion length of charged particles is on the order of 4-10 nm. However, compared with inorganic devices, the bilayer OSC device PCE is still reported to be lower. This is due to the shortage in organic semiconductors of intrinsic exciton diffusion length, which is between 10-20 nm.

Bulk heterojunction organic cells

Instead of having distinct donor and distinct acceptor layers, the electron donors and acceptors are mixed together in the blend layer solar cell device, forming a polymer blend. If the mixture length scale is similar to the length of the exciton diffusion, most of the excitons produced in either material can reach the interface where excitons break effectively. Then electrons were transported through the device to the acceptor domains and collected by one electrode, and holes were pulled in the opposite direction and gathered on the other side. If molecular mixing occurs on a scale that allows good contact between similar molecules (charge percolation) and most excitons to reach the D/A interface, the strong point of this type is the large interface area. A dispersed heterojunction organic solar cell device is also called this type of configuration. The most efficient device structure of organic solar cells is currently the bulk heterojunction. Compared to the classical (bilayer) junction, the interface between two different components is randomly distributed over the active layer in this structure. Morphological control is to ensure that the collecting electrodes are transported by electrons and holes, but not necessarily for large interface charge generation and to put an end to exciton loss. Incomplete use of light incident due to a poor match of the absorption spectrum active layer with the solar spectrum and also low charge carrier mobility of organic semiconductors limits electrical current densities.

Conclusion

Because of low material costs, ease of manufacturing, high throughput and flexibility, etc., organic solar cells are gaining importance. Furthermore, as used in other solar cell technologies, organic solar cells (polymer and dye sensitised solar cells) do not require materials and/or toxic materials of very high purity. However, their lower efficiencies (0.01-15%) arising due to the different recombination losses are the key limitation of organic solar cells as of now. Organic solar cells have certain drawbacks, including their low efficiency and short lifetime (only 5% efficiency compared to 15% of silicon cells). Nevertheless, their numerous advantages can be justified by current international investment and research in the development of new polymer materials, new combinations and structures to improve efficiency and, in the coming years, achieve low-cost and large-scale production. The objective of the next decade is to produce commercially viable organic solar cells and so much is still going on in this aspect.