Introduction: Solar cells are employed to utilize the sun’s spectrumutilizeransforms solar energy into electrical energy suitable for battery storage. The solar cell gets its name from the fact that it functions as an electrical energy source when light strikes it. First-generation solar cells have a very long energy payback period and are based on crystalline silicon. The energy needed to turn sand into silicon, also known as the processing cost, is what is used to calculate the energy payback time.
The energy required to turn sand into silicon crystal is greater than the energy obtained from a silicon-based solar cell. Because of the processing costs, first-generation solar cells are quite expensive. The reduction of processing costs and management of e-waste (damage solar cell) is main concern in the upcoming generations of solar cells. Perovskite-based solar cells, which have a far shorter energy payback period than first-generation solar cells, have been proposed as fourth generation solar cells in recent years. The use of perovskite-based fourth–generation solar cells is not without its difficulties. This article describes the problems and advantages of existing technology as well as the development of solar cell technology.
Solar cell Materials
In general, the sun’s spectrum is made up of 52% NIR (700–2500 nm), 43% visible (400–700 nm), and 5% UV (300–400 nm) light. The process of a material used in a solar cell absorbing light from the sun is crucial. The following requirements must be met for a material to absorb solar energy:
Eg≤1240λ(μm) (phonton energy)1240𝜆(𝜇m) (phonton energy) Where, Eg is the bandgap of the semiconductor material, and λ𝜆 is the wavelength of solar energy. Consequently, the energy band gap of the material used in solar cells needs to be between 1 and 2 eV to maximize the usage of solar energy.
Because germanium crystals are more expensive than silicon ones, silicon was initially chosen for first-generation solar cells rather than germanium ones. Additionally, amorphous silicon is used in first-generation solar cells to lower the cost of manufacture; nevertheless, this decreases the efficiency of the solar cell.
Where, Eg is the bandgap of the semiconductor material, and
is the wavelength of solar energy. Consequently, the energy band gap of the material used in solar cells needs to be between 1 and 2 eV to maximize the usage of solar energy.
Because germanium crystals are more expensive than silicon ones, silicon was initially chosen for first-generation solar cells rather than germanium ones. Additionally, amorphous silicon is used in first-generation solar cells to lower the cost of manufacture; nevertheless, this decreases the efficiency of the solar cell.
The classification of Solar cell
The structure, deposition methods, and materials utilized (inorganic, organic, or hybrid) determine how solar cells are classified, as shown in Fig. 1.
From Ist generation to 2nd generation benefits and challenges
- Cost was minimized
- Large area fabrication not possible due to material cost
- E-waste: Destroying the damaged solar cell was not possible so this pollutes the environment with E-waste.
From 2nd generation to 3rd generation benefits and challenges
- Organic materials: biodegradable and not make the soil polluted
- Very thin flexible
- Non toxic
- Low-cost large area solar cell was possible
- Efficiency was less than 5%
From 3rd generation to 4th generation benefits and challenges
- Based on Hybrid (organic and inorganic) material
- Employ benefits of both organic (flexibility) and inorganic material(stability)
- Low cost
- Efficiency is high crosses the crystalline silicon
Figure 2 displays the solar cell’s efficiency for different generation
The comparison table of the various generation of solar cells (SCs)
Table 1
Comparison among various generation of SC
Solar cell working principle and parameters:
The production, segregation, and accumulation of photo-induced charge carriers are the fundamental processes that drive solar cell operation. The operation of a conventional p-n junction solar cell is depicted in Fig. 3. When solar light first hits the p-n junction, photoelectronselectrons and photo holes are produced.
Following generation, the p-n junction’s internal electric field sweeps the photohole and photoelectron to their corresponding regions. At the electrodes, charge carriers are eventually collected, allowing light to travel from the n-region to the p-region.
Figure 4 shows the I-V characteristic of a traditional p-n junction solar cell as follows:
The solar cell’s fourth quadrant I–V characteristics verify that it can function as a cell or as an electrical source.
Solar cell parameters:
- The open circuit voltage (Voc): The voltage present at the device terminals when there is no current (I=0) flowing in the solar cell is known as the open circuit voltage (Voc), as seen in Fig. 4.
- The short circuit current (Isc): When a solar cell’s terminals are shorted (V=0), the current flowing through them is known as the short circuit current
- Fill factor (FF): It is defined as following:
maximum power is transferred by the solar cell. Therefore, these terms are generally current and voltage corresponds to maximum power transfer.
4. Incident power (Plight): The incident solar spectrum power is termed as incident power and can be expressed by the symbol Plight.
5. Efficiency (η𝜼) : The efficiency of a solar cell is defined as the ratio of output electric power to incident optical power, and it is represented by the symbol η.
6. Quantum efficiency: Photons make up light. Quantum efficiency is defined as the ratio of photons impinge on the solar cell to photons absorbed by the solar cell.
Organic Solar cell (3rd Gen. Solar cell)
The key features of organic solar cell are as following:
- Low material cost
- Low fabrication cost
- Flexible in nature
- Low energy payback time
- Easy to integrate
- Light weight
- Environmental friendly
Structure of organic SC (OSC):
Figure 5 depicts the standard device structure for the organic solar cell. The following is a description of each section of Fig. 5:
- Transparent electrode: This electrode serves as an anode by allowing light to enter the solar cell. A glass or PET substrate coated with indium tin oxide is an example of a transparent electrode.
- Electron transport layer (ETL): ETL is a component of OSCs that is utilized to efficiently gather photogenerated electrons. It makes it easier for the produced electron to travel toward the anode. Large band gap materials are typically among them. ZnO, TiO2, SnO2, LiF, and other materials are frequently utilized in ETL.
- Photoactive material: It is the primary layer of the solar cell where the solar spectrum is absorbed and where charge carriers are produced in response to the absorption. To fabricate this layer, an organic substance is used.
- Hole transport layer: It is employed to gather photogenerated holes in photoactive materials efficiently. It makes it easier for the holes created to travel toward the cathode and obstruct the electrons. Materials like PEDOT:PSS, MoO3, V2O5, PQT-12, etc. are frequently utilized for HTL.
- Front electrode: It serves as the solar cells’ cathode. Usually, a metal electrode like Ag, Au, or Al is used.
Other 3rd generation structures based on the OSC
Fig.6 organic solar cell structures based on (a) single layer organic material (b) Bi- layer organic material (c) bulk heterojunction of organic material
Donor polymers are those of the n type, whereas acceptor polymers are those of the p type. For the creation of OSCs, common n-type polymers include C60, PC61BM, DPM12, ICMA, C70, PC71BM, etc. Common p type polymers for organic SCs include PCDTBT, PTBT-Th, MEHPPV, and P3HT.
Working of organic solar cell:
The operation of the bilayer organic SCs is shown by the band diagram of the SC in Fig. 7. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) make form the band diagram of organic material. Three phases are involved in how organic SC functions:
- Exciton generation: Exciton creation is the process by which light impacting on a material produces electrons and holes. Exciton refers to the created pair of electron and hole.
- Exciton diffusion: Following creation, the excitons diffuse into the organic substance in the direction of the appropriate electrode.
- Exciton dissociation: At the intersection of the donor and acceptor materials, electrons and holes dissociate during diffusion.
- Exciton collection: Finally, the electron and hole collection that causes the SC’s current to occur occurs.
Limitation of Organic SC:
- The efficiency of OSCs is very less (maximum 6 %).
- The stability issue is also the concern of OSCs.
4th Gen. Solar cells (Perovskite based Solar cells)
The mineralist L. A. Perovski made the discovery of perovskite material, which bears his name. Perovskite has the chemical formula ABX3. X is an anion, whereas A, B are cations. The molecules A are bigger than the molecules B. The A and B cations have cuboctahedra and octahedra geometry, respectively, and coordinate with 12 and 6 X anion numbers. These materials are used in LEDs, LASERs, phototransistors, photodetectors, and other devices in addition to solar cells.
Band diagram of Perovskite based solar cell:
Fig. 8 displays the band diagram for a solar cell based on perovskites. The structure of the perovskite-based solar cell is also shown in Fig. 8. The perovskite layer is positioned between the HTL and ETL. Perovskite substance is where light absorption occurs. Within the perovskite material, electrons and holes are created by the absorbed radiation. ETL and HTL are then used to move these produced electrons and holes to the appropriate electrode.
Why Perovskite solar cell (PSC) Technology
- Bandgap tuning of the perovskite is possible to enable their ultrathin film of around 500 nm to absorb the complete visible solar spectrum.
- It has increased the efficiency of the OSC from 3.8 (2009) to 26.1 (2024).
- The raw material used and the possible fabrication method are both low cost
- Lightweight and flexible
- Perovskite solar cells are suitable for internet of thing application
Issue and Challenges with PSC
- Stability issue in PSC due to
- Continuous illumination
- Moisture
- High temperature (150 ~150oC)
- UV radiations
- Commercialization of PSC is still challenging
- Toxicity issues: Fabrication, deployment, and disposal
- Long term stability
Hybrid Perovskite:
For solar cell applications, researchers are currently investigating hybrid (organic-inorganic) perovskite rather than pure perovskite because it combines the advantages of both inorganic (strong light absorption, high efficiency) and organic (flexibility, low temperature processing) materials. The bandgap of the hybrid perovskite is appropriate for solar devices. The most popular perovskite substance for producing highly efficient PSCs is CH3NH3PbI3. In order to provide effective bandgap tuning and stability, the CH3NH3PbI3 material is changed in recent study with BA2MA-1BnX3n+1 (BA: Butyl ammonium, MA: Methyl ammonium, B: Metal cation (Pb, Sn), X: Halide).
References:
- Wojciechowski, K., Stranks, S. D., Abate, A., Sadoughi, G., Sadhanala, A., Kopidakis, N., … & Snaith, H. J. (2014). Heterojunction modification for highly efficient organic–inorganic perovskite solar cells. ACS nano, 8(12), 12701-12709.
- Upadhyay, R. K., Singh, A. P., Upadhyay, D., Ratan, S., Kumar, C., & Jit, S. (2019). High-performance photodetector based on organic–inorganic perovskite CH 3 NH 3 PbI 3/ZnO heterostructure. IEEE Photonics Technology Letters, 31(14), 1151-1154.
- Yang, Z., Lai, J., Zhu, R., Tan, J., Luo, Y., & Ye, S. (2022). Electronic Disorder Dominates the Charge-Carrier Dynamics in Two-Dimensional/Three-Dimensional Organic–Inorganic Perovskite Heterostructure. The Journal of Physical Chemistry C, 126(30), 12689-12695.
