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.
Theory 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 cell
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:
E A A − E A D > U D {\displaystyle E_{A}^{A}-E_{A}^{D}>U_{D}}
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.
Types of interfaces and structures Controlling the interface of inorganic-organic hybrid solar cells can increase the efficiency of the cells. This increased efficiency can be achieved by increasing the interfacial surface area between the organic and the inorganic materials to facilitate charge separation and by controlling the nanoscale lengths and periodicity of each structure so that charges separate and move toward the appropriate electrode without recombining. The three main nanoscale structures used are:
mesoporous inorganic films infused with electron-donating organic, alternating inorganic-organic lamellar structures, and nanowire structures.
Mesoporous films Mesoporous films have been used for hybrid solar cells. The structure of mesoporous thin film solar cells typically includes a porous inorganic that is saturated with organic surfactant. The organic absorbs light, and transfers electrons to the inorganic semiconductor (usually a transparent conducting oxide), which then transfers the electron to the electrode. Problems with these cells include their random ordering and the difficulty of controlling their nanoscale structure to promote charge conduction.
Ordered lamellar films Alternating layers of organic and inorganic compounds have been controlled through electrodeposition-based self-assembly. This is of interest because lamellar structure and periodicity of the alternating organic-inorganic layers can be controlled through solution chemistry. To produce this type of cell with practical efficiencies, larger organic surfactants that absorb more of the visible spectrum must be deposited between the layers of electron-accepting inorganic.
Films of ordered nanostructures Researchers grew nanostructure-based solar cells that use ordered nanostructures like nanowires or nanotubes of inorganic surrounding by electron-donating organics utilizing self-organization processes. Ordered nanostructures offer the advantage of directed charge transport and controlled phase separation between donor and acceptor materials. The nanowire-based morphology offers reduced internal reflection, facile strain relaxation and increased defect tolerance. The ability to make single-crystalline nanowires on low-cost substrates such as aluminum foil and to relax strain in subsequent layers removes two more major cost hurdles associated with high-efficiency cells. There have been rapid increases in efficiencies of nanowire-based solar cells and they seem to be one of the most promising nanoscale solar hybrid technologies.
Fundamental challenge factors Hybrid cell efficiency must be increased to start large-scale manufacturing. Three factors affect efficiency. First, the bandgap should be reduced to absorb red photons, which contain a significant fraction of the energy in the solar spectrum. Current organic photovoltaics have shown 70% of quantum efficiency for blue photons. Second, contact resistance between each layer in the device should be minimized to offer higher fill factor and power conversion efficiency. Third, charge-carrier mobility should be increased to allow the photovoltaics to have thicker active layers while minimizing carrier recombination and keeping the series resistance of the device low.
Types of hybrid solar cells
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