The drive for alternative energy has increased development of photovoltaic (“PV”) solar cells, which may have numerous applications such as for powering stand-alone small scale devices up to power plants that may be connected to the electrical grid. The challenge for solar cells is to efficiently convert light into electrical energy. Traditional photovoltaic cells are commonly composed by doped semiconductor material such as silicon or gallium arsenide (GaAs) with depositing metallic contacts. Doped semiconductor material such as silicon forms a thin layer on the top of the cell, producing a p-n junction having a specific, band gap energy. Photons from a light source hit the top of the solar cell and are transmitted to the doped semiconductor material. Transmitted photons have the potential to impart their energy to an electron, generating an electron-hole pair. In a depletion region created around the interface of the n-doped and p-doped regions, a drift electric field accelerates both electrons and holes towards respective n-doped and p-doped regions in the cell. The resulting current is termed the photocurrent. Consequently, due to the accumulation of charges, a potential voltage and a photocurrent appears to generate electricity from the solar energy spectrum. Certain semiconductor materials such as GaAs provide optimal band gaps for solar energy conversion and are therefore more efficient in conversion than silicon.
Traditional solar cells based on the p-n junction are limited to certain wavelengths of transmitted light. Thus, only photons within those wavelengths are efficiently converted to electrical energy. There is no semiconductor that can be deployed for making a solar cell that converts the entire solar spectrum to electrical energy. The Shockley-Queisser limit for conversion efficiency of solar cells is calculated in the framework of the principle of detailed balance assuming ideal conditions of only radiative inter-band electron transitions in the solar cell. The model assumes that photoelectrons generated by the above-band gap photons quickly relax to the conduction band edge by transferring their excess energy to the semiconductor lattice due to the intra-band scattering on optical phonons. This relaxation absorbs about 30% of solar energy. The sub-band gap photons compose another 30% of solar energy that the Shockley-Queisser model of an ideal p-n junction misses.
One solution that has been suggested is multi junction solar cells containing several p-n junctions. Each junction is tuned to a different wavelength of light, reducing one of the largest inherent sources of losses, and thereby increasing efficiency. Traditional single junction cells have a maximum theoretical efficiency of 34%, a theoretical “infinite-junction” cell would improve this to 87% under highly concentrated sunlight. However this efficiency is gained at the cost of increased complexity and manufacturing price.
Another solution is an intermediate band (IB) concept that makes use of the energy of sub-band gap photons based on the non-linear effect of two-photon absorption. If the total energy of two sub-band-gap photons exceeds the energy of band-gap, consecutive absorption of two photons may transfer a valence band electron into the conduction band resulting in an additional photocurrent. The IB concept exploits a band of intermediate electronic states located in the semiconductor band gap for resonant absorption. If IB states did not result in electron-hole recombination (like impurity defect states), IB solar cells would convert up to 63% of concentrated sunlight into electricity.
Quantum dot solar cells have also been investigated to form IB solar cells. Quantum dots are particles of semiconductor material that have been reduced below the size of the Exciton Bohr-radius, and have band gaps that are tunable across a wide range of energy levels by changing the quantum dot size. The ability to tune the band gap allows for solar cells that can convert photons in regions, such as infrared, that were previously not captured. Type-I quantum dots facilitate two-photon absorption of sub-band gap photons, however, they also lead to generation of additional dark current reducing both open circuit voltage and conversion efficiency of the cells. Since the depletion region is the most sensitive part of solar cells where electronic states easily facilitate recombination, the location of type-I quantum dots inside the depletion region boosts the dark current of quantum dot IB solar cells. Thus, quantum dots suffer from recombination of holes and electrons, which serves as a major limiting factor for conversion efficiency of quantum dot solar cells
Thus, there is a need for a solar cell that can increase efficiency by capturing photons at a wide range of wavelengths using quantum dots. There is a further need for the incorporation of the traits of Type II quantum dots in a solar cell. There is a further need for the spatial separation of a quantum dot absorber layer from the depletion region for taking advantage of suppression of addition dark current associated with electron-hole recombination through quantum dots located in the depletion region.