Hybrid solar cells, made of a blend of inorganic materials and natural polymers, turn sunlight into electricity – and their manufacturing cost is cheaper than standard photovoltaic cells. Although they may not beat silicon solar cells’ efficiency, there is potential for improvement.
Dr. Feng Gao, from Linköping University (Sweden), explains the potential of these systems, recent developments in this field, and the challenges for the future.
Hybrid Solar Cells
Solar cells are devices which can convert the light of the sun into electricity. For the solar cell to be effective, both materials should have appropriate electronic characteristics; they should both have a band gap structure and behave as semiconductors. For these materials, exposure to sunlight generates electric charges (positive and negative) which the device then collects in the form of an electric current.
Inorganic Material: Semiconductors Nanocrystals
Hybrid solar cells use nanocrystals of semiconductor materials such as CdTe, CdS, PbS, PbSe, ZnO and TiO2. These crystals can be shaped as either particles or rods (nanodots and nanorods respectively). Each of these materials will have different value of band gap; this will affect the amount of sun light captured and converted into electricity.
The morphology, or form and structure, of the materials can also affect the efficiency in the energy conversion; in particular, the presence of so-called “traps”. Traps are sites which absorb the charges generated by the sun light, which prevents the charge from converting into a current, causing a decrease in the efficiency.
Organic Polymers: Conjugated Systems
The polymers researchers use for hybrid solar cells are organic molecules with a conjugated electronic structure.
In conjugated systems, single and double carbon-carbon bonds alternate; in this way, the electrons are not localized between just two atoms but they are delocalized, or spread out, over all the atoms, which affects the electronic properties of the system.
In conjugated polymers, the conjugated structure is present in the repeating unit. The picture on the side shows the structure of MEH-PPV, one of the polymers used for hybrid solar cells; the alternate single and double bonds are highlighted. Examples of other polymers used for these systems are MDMO-PPV, P3HT, etc., all showing the same characteristics.
Hybrid Solar Cells: Interface Between the Different Materials
When light irradiation forms charges in the solar cell, the cell separates and collects them according to the nature of the charge (i.e. positive and negative). Generally, in hybrid systems, the polymer phase collects the positive charges, and the inorganic nanocrystals collects the negative charges.
For the separation and collection to be effective, it is important to mix the two phases well, without making the mixture overly homogenous, to avoid the charges “meeting” and recombining, before collection. The interface between the two phases is, therefore, a key point in obtaining high efficiency.
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