If some of a Si solar cell's generated carriers recombine at defects, impurities, surface damage, etc. before reaching the contacts, the current output is diminished.  Because it is a quantitative measure of such phenomena, minority-carrier lifetime (τ) characterization (ASTM, 1993) is frequently used to qualify the crystalline Si material before it is used in device processing. Quality in a silicon PV material is nearly synonymous with τ. The parameters used in crystal growth have a direct bearing on τ, because they determine impurity levels and defect structures that give rise to carrier recombination sites. Impurity incorporation, segregation, and evaporation during the crystal-growth process can be altered via ambient choice, growth rate, number of solidification steps, choice of container, heat-source characteristics, selection of source material, and other factors that vary from one process to another.

Heavy doping imposes an upper limit on lifetime according to τ_A = 1/C_AN_A², where τ_A is the Auger-limit lifetime.  C_A is the Auger coefficient, and Nτ < 2 μs is unlikely to be useful in most PV processes due to balance-of-systems costs. Poor-quality material cannot generate enough PV energy to justify the costs of the total PV system.  Thus, the τ-N space available for PV applications is the non-hatched region in the figure below-left, and the four labeled curves are "quality" contours.  In addition to the two limits, curves representing moderate τ (typical of Czochralski-grown CZ silicon) and high τ (typical of the best commercially available FZ silicon) are included.  Note that there is a vast discrepancy between τ_A and the lifetime of the best available silicon. So there is potential for higher lifetimes and new device designs to take advantage of it.  Transition-metal impurity effects on τ and solar cell efficiency as a function of their concentration levels are reasonably well understood from quantitative and detailed experimental studies (Davis et al., 1980).  Some metals such as titanium have a significant effect on τ even in concentrations as low as a few 100 ppta (parts per trillion, atomic).  Others, such as copper, can be tolerated at a few ppma (parts per million, atomic).  Fortunately, most of the detrimental impurities have small effective segregation coefficients, and their concentrations can be reduced during directional solidification (DS).