Technical Field
The present invention relates to solar cells and more particularly to a tandem solar cell device and method to achieve greater collection efficiency.
Description of the Related Art
Solar cells employ photovoltaic cells to generate current flow. Photons in sunlight hit a solar cell or panel and are absorbed by semiconducting materials, such as silicon. Electrons gain energy allowing them to flow through the material to produce electricity.
When a photon hits silicon, the photon may be transmitted through the silicon, the photon can reflect off the surface, or the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure. When a photon is absorbed, its energy is given to an electron in a crystal lattice. Electrons in the valence band may be excited into the conduction band, where they are free to move within the semiconductor. The bond that the electron(s) were a part of form a hole. These holes can move through the lattice creating mobile electron-hole pairs.
A photon need only have greater energy than that of a band gap to excite an electron from the valence band into the conduction band. Since solar radiation is composed of photons with energies greater than the band gap of silicon, the higher energy photons will be absorbed by the solar cell, with some of the energy (above the band gap) being turned into heat rather than into usable electrical energy.
A solar cell may be described in terms of a fill factor (FF). FF is a ratio of the maximum power point (Pm) divided by open circuit voltage (Voc) and short circuit current
            (              J        sc            )        ⁢          :        ⁢    FF    =                    P        m                              V          oc                ⁢                  J          sc                      .  The fill factor is directly affected by the values of a cell's series and shunt resistance. Increasing the shunt resistance (Rsh) and decreasing the series resistance (Rs) will lead to a higher fill factor, thus resulting in greater efficiency, and pushing the cells output power closer towards its theoretical maximum. The increased efficiency of photovoltaic devices is of utmost importance in the current energy environment.
To increase efficiency, tandem cells have been employed where a first solar cell is integrated with a second solar cell. Such cells typically employ a microcrystalline Silicon based bottom cell, which includes microcrystalline Silicon for a p-doped layer, an intrinsic layer and an n-doped layer for the cell. These types of cells suffer from slow growth rates during manufacture. The growth rate for high quality microcrystalline cells may be about 2-3 Angstroms/sec for a 1.5 micron thickness, and can take 1-2 hours to grow the microcrystalline Silicon. This results in higher cost. In addition, since a top cell in such device also includes a form of Silicon, current sharing between the top and bottom cells is excessive, e.g., Jsc for the tandem device is often less than Jsc for an individual cell.
Thin-film materials of the type Cu(In,Ga)(S,Se)2 (CIGS), while efficient, include rare indium metal, which is expected to be of high cost and short supply in future large-scale photovoltaic device production—an issue which is further exacerbated by the growing indium consumption for thin film display production. Other materials such as Cu2S and CdTe have also been proposed as absorbers but while Cu2S suffers from low stability in devices, rare tellurium and toxic cadmium limits CdTe usage.