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Hybrid solar cell
Hybrid solar cells combine advantages of both organic and inorganic semiconductors. Hybrid photovoltaics have organic materials that consist of conjugated polymers that absorb light as the donor and transport holes. Inorganic materials are used as the acceptor and electron transport. These devices have a potential for low-cost by roll-to-roll processing and scalable solar power conversion.
Photovoltaics convert sunlight into electricity by the photovoltaic effect. Electrons absorb photon energy that excites them to the conduction band from the valence band. This generates a hole-electron pair separated by a potential barrier (such as a p-n junction), and induces a current.
Hybrid solar cells mix an organic material with a high electron transport material to form the photoactive layer. The two materials are assembled in a heterojunction-type photoactive layer, which can have greater power conversion efficiency than a single material. One of the materials acts as the photon absorber and exciton donor. The other facilitates exciton dissociation at the junction. Charge is transferred and then separated after an exciton created in the donor is delocalized on a donor-acceptor complex.
The acceptor material needs a suitable energy offset to the binding energy of the exciton to the absorber. Charge transfer is favorable if the following condition is satisfied:
where A and D refer to the acceptor and donor, respectively, EA is the electron affinity, and U the exciton's coulombic binding energy on the donor. An energy diagram of the interface is shown in figure 1. Commonly used photovoltaic polymers such as MEH-PPV feature exciton binding energy from 0.3-1.4 eV.
The energy required to separate the exciton is provided by the energy offset between the LUMOs or conduction bands of the donor and acceptor. After dissociation, the carriers are transported to the respective electrodes through a percolation network.
The average distance an exciton can diffuse through a material before annihilation by recombination is the exciton diffusion length. FOr polymers, this is on the order of 5–10 nanometers. The time scale for radiative and non-radiative decay is from 1 picosecond to 1 nanosecond. Excitons generated within this length close to an acceptor would contribute to the photocurrent.
To deal with this short exciton diffusion length, a bulk heterojunction structure is used rather than a phase-separated bilayer. Dispersing the particles throughout the polymer matrix creates a larger interfacial area for charge transfer to occur. Figure 2 displays the difference between a bilayer and a bulk heterojunction.
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Hybrid solar cell
Hybrid solar cells combine advantages of both organic and inorganic semiconductors. Hybrid photovoltaics have organic materials that consist of conjugated polymers that absorb light as the donor and transport holes. Inorganic materials are used as the acceptor and electron transport. These devices have a potential for low-cost by roll-to-roll processing and scalable solar power conversion.
Photovoltaics convert sunlight into electricity by the photovoltaic effect. Electrons absorb photon energy that excites them to the conduction band from the valence band. This generates a hole-electron pair separated by a potential barrier (such as a p-n junction), and induces a current.
Hybrid solar cells mix an organic material with a high electron transport material to form the photoactive layer. The two materials are assembled in a heterojunction-type photoactive layer, which can have greater power conversion efficiency than a single material. One of the materials acts as the photon absorber and exciton donor. The other facilitates exciton dissociation at the junction. Charge is transferred and then separated after an exciton created in the donor is delocalized on a donor-acceptor complex.
The acceptor material needs a suitable energy offset to the binding energy of the exciton to the absorber. Charge transfer is favorable if the following condition is satisfied:
where A and D refer to the acceptor and donor, respectively, EA is the electron affinity, and U the exciton's coulombic binding energy on the donor. An energy diagram of the interface is shown in figure 1. Commonly used photovoltaic polymers such as MEH-PPV feature exciton binding energy from 0.3-1.4 eV.
The energy required to separate the exciton is provided by the energy offset between the LUMOs or conduction bands of the donor and acceptor. After dissociation, the carriers are transported to the respective electrodes through a percolation network.
The average distance an exciton can diffuse through a material before annihilation by recombination is the exciton diffusion length. FOr polymers, this is on the order of 5–10 nanometers. The time scale for radiative and non-radiative decay is from 1 picosecond to 1 nanosecond. Excitons generated within this length close to an acceptor would contribute to the photocurrent.
To deal with this short exciton diffusion length, a bulk heterojunction structure is used rather than a phase-separated bilayer. Dispersing the particles throughout the polymer matrix creates a larger interfacial area for charge transfer to occur. Figure 2 displays the difference between a bilayer and a bulk heterojunction.