The derivation of the simple diode equation uses certain assumption about the cell. The ideality factor of a diode is a measure of how closely the diode follows the ideal diode equation. However, lifetime and carrier diffusivity vary with carrier concentration and so does I0Ī solar cell is a large diode. J0 will be a constant if the recombination parameters are constant. J0 is an expression of the recombination in the device.
The ideality factor is closely tied to the diode prefactor I0, or J0 when current densities are used. Given the confusing nature of the term an alternative term of "recombination parameter" was proposed but it does not seem to have caught on.ĭespite its somwhat counterintuitive name, the reverse saturation current is central to the operation of photovoltaic devices. The term "reverse saturation current" is even more confusing in photovoltaics since solar cells almost never operate in reverse bias and rarely in the dark. In practice, p-n junctions have imperfections so the current in reverse bias, while small, is larger than I0. It is termed a "saturation current" since the ideal diode equation quickly converges to -I0 for negative voltages. The second term in the ideal diode equation is I0, which is described by slighly different terms including: "saturation reverse current", 1 "reverse saturation current", "saturation current" Green1982, Schroder2006 or "dark saturation current." It is the current that flows in reverse bias due to thermally generated carriers. The plot gives the IV curve of a diode in the darkalong with the ideality factor of a diode in the dark as a function of voltage J0 a) current voltage curve b) ideality factor useing procedure outline on Simulation of a diode in the dark using PC1D with standard silicon parameters. However, Auger injection will dominate above 1e16 where the ideality factor is 2/3. For example, a silicon solar cell might be expected to have an ideality factor of two at high-level injection. In practice, there is usually more than one recombination mechanism so it is unusual for the ideality factor to precisely match one of the values above. Two majority and one minority carriers required for recombination. Recombination limited by minority carrier. Recombination limited by both carrier types.
Radiative (band to band) high-level injection Recombination limited by minority carriers. Radiative (band to band) in low-level injection The table below gives a simplified view of the recombination mechanisms and the resulting ideality factor. Other recombination mechanisms give a different ideality factor depending on the number of carriers that limit the recombination. The ideal diode derivation makes the assumption of low-level injection so that only one carrier limits the recombination. Some are parasitic and some are fundamental to the recombination and diode operation. The ideality factor is also known as the "quality factor" 2 and denoted with A or m giving rise to the terms A-factor etc.ĭepartures from ideal happen for a variety of reasons. Where n is the ideality factor and is one for an ideal diode. At its simplest, the ideal diode derivation 1 results in the equation: