Emerging Solar Cells
Emerging solar cells consisting of organic materials
Organic Solar Cell
Organic solar cells are made up of electron donor and electron acceptor materials. In the electron donor region, exciton electron-hole pairs are generated. These donor materials are generally conjugated polymers with delocalized π electrons. Upon light exposure, these π electrons are excited from the molecule's highest occupied molecular orbital (HOMO) to the molecule's lowest unoccupied molecular orbital (LUMO). For the successful transition, the energy of incident light must be higher than the energy gap between HOMO and LUMO. The generated exciton dissociates in an electron and a hole. The electron acceptor takes the holes to the hole selective contact while the electrons are transported by the electron donor material itself. From the contact, electrons are collected in the external circuit. (Picture credit: Kashif Hussain)
Dye Solar Cell
Dye-sensitized solar cells also known as dye solar cells are a class of hybrid solar cells. A typical structure consists of a mesoporous TiO2 scaffold on compact TiO2 /transparent conductive contact. A monolayer of dye molecules is deposited on top of mesoporous TiO2. In conventional dye solar cells, a redox electrolyte is filled and sealed with a platinum electrode and a transparent conductor. In solid-state dye solar cells, the redox electrolyte is replaced with a solid hole transporting material (organic Spiro-OMeTAD).
Incident light excites the dye molecules from their ground state to an excited state (energy) level. The excited dye injects electrons into the conduction band of TiO2, retaining positive charge (or transfers to the hole transport material in case of solid-state dye solar cell). The redox electrolyte supplies electron to the ground state dye leading to the dye regeneration. The oxidized redox electrolyte diffuses towards the platinum electrode (holes towards Au cathode in case of solid-state dye cell) and then reduces by getting electrons from the external circuit.
Dye solar cells work even at very low light levels. The cells are solution-processable, easy to fabricate, flexible, cheap, and semitransparent, making them a suitable candidate for building integration, indoor application, and tandem solar cell technology. More information...
Perovskite Solar Cell
Inorganic-organic hybrid perovskite (IOHPs) solar cells are one of the fastest-growing solar cell technologies ever. A simple IOHP has a chemical formula of ABX3 with a perovskite structure. Here, A is an organic cation and B is an inorganic cation. X represents a halide anion. A and B cations have very different sizes. B cations make a crystal structure consisting of X anions. The A cation is filled within a void in BX3 unit cell, making ABX3 perovskite unit cell. As an example, CH3NH3PbI3 structure is shown here. In the crystal structure, the methylamine cation is surrounded by PbI3 octahedra. Bandgap (and other electronic and physical properties) of a perovskite material can be changed by replacing A and/or B and/or X with another similar atom. The bandgap of CH3NH3PbI3 perovskite is around 1.55eV which is very much suitable for a solar cell. Perovskite materials exhibit wide optical absorption, high photon to electron conversion, and high trap tolerance. Moreover, the solution-processability of such perovskites makes them an ideal candidate for high-efficiency and cheaper solar cells. Since these materials have a direct bandgap, they are also useful for light-emitting diodes, and semiconductor lasers.
In a typical perovskite solar cell, a perovskite absorber is sandwiched between a hole transport layer (HTL) and an electron transport layer (ETL). One of the sides is covered by a metal contact while the other side is covered by a transparent conductor. When the light is absorbed by the perovskite material, it generates electron-hole pairs. These pairs dissociate into electrons and holes by the thermal energy. Electrons and holes start diffusing due to the concentration gradient (later by diffusion and photo-generated induced electric drift) towards respective contacts (electrons --> ETL and holes --> HTL). Electron and HTL energy levels are aligned such that the conduction band of ETL is below the conduction band edge of the perovskite, and the valence band of HTL is above the balance band of the perovskite. This energy alignment helps electrons and holes to flow towards ETL and HTL respectively. Using the contacts, the electrons are collected in the external circuit leading to the electricity generation.
Typically used organic HTL materials are Spiro-OMeTAD, PolyTPD, and PEDOT:PSS. For the ETL, typically PCBM (organic) and TiO2 are employed. More information...
CH3NH3PbI3 crystal structure
Perovskite Solar Cell