Often called PV panels, these convert the energy of light (photons) into electrical energy as DC current. Panels come in variety of sizes but it is common to find them with 60 cells each with a power output of about 240 W. At this time, other variations include concentrated PV and thin-film solar. For PV panels, efficiency reaches to about 14%.
Generally, monocrystalline cells have higher efficiencies than polycrystalline cells, but the monocrystalline cells are more expensive to manufacture. Drawbacks of conventional photovoltaic systems are the high initial cost and limited electrical output when compared to the solar input. Other developments are coming fast.
Crystalline silicon technology currently makes up about 80% of the market. It uses silicon wafers as its main raw material. Wafers are cut from cast ingots in thin slices with a thickness in the range of 200 to 300 μm.
In crystalline silicon manufacturing, the wafers are processed, interconnected and laminated into a substrate, which is typically made of glass. Commercial-grade solar modules using crystalline silicon technology can convert sunlight to useable electrical energy at a relatively high efficiency ranging from 14% to 19%. Although efficiency is good, this technology can be expensive due to the high cost of the silicon wafers, which make up 40% to 50% of the price of the finished solar module.
Thin-film photovoltaics account for the other 20% of the solar module market. Thin films use no silicon wafers. Instead, photovoltaic material is deposited in a thin layer — typically with a thickness of 1 μm or less — to a glass substrate or a flexible thin metal or plastic substrate.
After deposition, the substrate is processed and separated into individual cells, which are connected in series. Thin-film solar modules only achieve conversion efficiencies in the range of 8% to 10%, but are much less expensive to manufacture because silicon wafers are not required as a raw material. The goal with both technology types is to drive efficiency up and costs down.
The 19% efficiency for PV panels mentioned earlier leave a lot of room for improvement. Researchers at a national lab and one university may be on to the next big step. For example, teams from NREL and the University of Colorado, Boulder (UCB), have reported the first designed molecular system that produces two triplet states from an excited singlet state of a molecule, with what they say is essentially perfect efficiency. The development could lead to a 35% increase in lightharvesting yield in cells for photovoltaics systems.
The experiments, using a process called singlet fission, demonstrated a 200% quantum yield for the creation of two triplets of the molecule 1,3-diphenylisobenzofuran (DPIBF) at low temperatures. In singlet fission, a light-absorbing molecular chromophore shares its energy with a nearby nonexcited neighboring molecule to yield a triplet excited state of each. If the two triplets behave independently, two electron-hole pairs can be generated for each photon absorbed in a solar cell. This process could subsequently increase by one-third the conversion efficiency of solar photons into electricity or solar fuels. The researchers identified DPIBF as a promising candidate while searching for molecular chromophores that have the required ratio of singlet and triplet energy states.
NREL and Los Alamos National Labs had previously demonstrated an analgous two-electrons-from-one photon bonus using semiconductor quantum dots in a process NREL termed Multiple Exciton Generation. The latest advance is the first to demonstrate the electron multiplication phenomenon via the singlet-fission process in molecules.