A solar cell that is 9.1% efficient doesn't sound like much, but it is impressive when you are talking about a polymer/fullerene organic solar cell.
Organic cells on plastic substrates have been able to demonstrate about 6% efficiency at best. But that changed recently when Polyera Corp. managed to get 9.1% efficiency out of its inverted bulk heterojunction solar cell architecture using its newest proprietary ActivInk PV2000 semiconductor material. The significance of this development is that it brings organic solar cells close to the 10% efficiency mark that manufacturers like Polyera say is necessary to realize any kind of a commercial product.
Polyera claims the high efficiency of this material represents a substantial breakthrough in the development of organic solar cell technology for large-scale manufacturing of low-cost, lightweight, flexible, and optically semi-transparent solar modules.
Organic solar cells have the potential to be manufactured on large areas at high speeds on lightweight substrates like plastic. But besides better efficiency, they need to exhibit a longer operational lifetime to be truly cost-competitive with traditional energy sources, Polyera says. However, in the short term, other properties such as the ability to be light, flexible, and optically semi-transparent make them useful for use as windows that also act as solar panels.
Polyera also says its active layer materials can also be deposited using a broad range of film thicknesses without lowering cell efficiency; this broader process window improves yields and simplifies manufacturing. Polyera claims its materials can be processed at temperatures low enough to be compatible with a wide range of simple printing processes and common, inexpensive plastic substrates like PET or PEN.
Aside from the substrate (usually PET foil) and conductive electrodes (usually Al and ITO), Polyera's OPV cell comprises two main layers: The active Layer comprised of both n- and p-type semiconductors, and an interlayer usually made from the conducting polymer PDOT:PSS. It serves to smooth the electrode surface and increase its work function.
To further increase the efficiency of its cells, Polyera says it needs to work on the percentage of light absorbed by the active layer (primarily affected by the band-gap and thickness of the layer, but is also affected by absorption in other layers, as well as reflection and scattering), charge separation (keeping electrons and holes away from each other so that they do not recombine and lose energy as heat or light), and charge transport (how well charge carriers are transported out of the active layer to the circuit contacts).
Polyera is trying to improve the short-circuit current, the open-circuit voltage, and the cell fill-factor (the actual power relative to the theoretical power produced by the cell). The short circuit current is primarily affected by band-gap, carrier mobility, and film formation properties of the active layer. The open-circuit voltage is primarily affected by the material band-gap and the device structure. Polyear says fill factor is particularly difficult to predict and design, but seems related to the relative mobilities of the electrons and holes.
Another common indicator of performance is external quantum efficiency (EQE), the ratio of the number of charge carriers generated in a cell relative to the number of photons of a given energy hitting it. It measures the response of a cell to a given wavelength (i.e. energy) of light. The ideal shape for the EQE would be a square, says Polyera, where nearly all the energy within a given wavelength range would be converted. In practice, however, a number of factors make this difficult to achieve.