Submitted by Valerie Ding on Sat, 12/28/2013 - 11:27am
Solar cell efficiency is subject to multiple types of loss.
- Thermodynamic losses are intrinsic. The Shockley-Quiesser Limit explains the thermodynamic losses in a solar cell, namely those of blackbody radiation and radiative recombination (Polman & Atwater, 2012). Blackbody radiation refers to the situation that, at any temperature above absolute zero, the solar cell itself will be emitting electromagnetic blackbody radiation which it cannot capture and thus loses. Radiative recombination is the loss of energy when an electron and a hole combine and emit a photon (Henry, 1980). Thermodynamic losses are widely established and heavily researched (Markvart, 2007). Thus, this work does not focus on thermodynamic losses.
- Problems of mobility and non-radiative recombination. Electron and hole (charge-carrier) mobility in semiconductor materials are measures of the speed at which electron or holes are transported through the semiconductor (Ishiwata, et al., 2013). Low mobility introduces efficiency loss when electrons reach the p-n junction and recombine with remaining holes from a previously excited electron, thus recombining and losing the opportunity to be captured. Non-radiative recombination refers to the process by which electrons and holes recombine, as described previously, and do not emit a photon, but rather a phonon, which is a lattice vibration (Wan, 2003). Primary examples of non-radiative recombination include the Shockley-Read-Hall effect (Shockley & Read, 1952), which describes recombination in impure materials (Hall, 1952), and the Auger effect, which describes how an electron may recombine with a hole but transfer the emitted energy to another electron in the conduction band (Auger, 1923). Because there is a considerable amount of work being done on reducing recombination and improving charge-carrier mobility in solar cells, and because it is mostly experimental and most directly related to matters of solar cell design, this work does not focus on these two problems in solar cell efficiency.
- Spectrum losses. Traditional solar cell materials such as silicon, with a bandgap of 1.1 eV which is greater than the energy range of infrared light (Siklitsky, 2001), cannot harness or fully harness highly-represented wavelengths of light from the Sun; if the incoming photon energy is greater than the bandgap, the cell will only harness the bandgap energy at most, and if the incoming energy is less than the bandgap, the cell will not harness any energy from that interaction. This work focuses on reducing spectrum losses by using tunable-bandgap quantum dots to address the infrared light lack and by using multijunction design to cover more portions of the incoming solar spectrum, thus allowing for higher representation of all energies in the solar spectrum and thereby increasing solar cell efficiency.