Assessing possibilities & limits for thin film solar cells

Some simple criteria can serve to estimate to what extent further developments are possible for the various types of photovoltaic cells and to which degree basic limitations may affect such developments. To do so, we use several yardsticks, namely 1. the fraction that the efficiency of the best commercially available module represents of the most efficient, reported laboratory cell; 2. the fractions of the maximal possible current for this cell that are represented by the short circuit current and the current at maximum power of this best laboratory cell; and 3. the fractions of the optical bandgap or the energy of the effective optical absorption edge of the cell, that are represented by the open circuit voltage and the voltage at maximum power of this best laboratory cell. An additional yardstick is the fraction of the efficiency of the best laboratory cell of the Shockley-Queisser (SQ) efficiency. This last yardstick reflects mostly academic ability rather than industrial promise. To some extent the first yardstick expresses the manufacturing maturity of the cell type, shown by the higher value for CdTe than for CIGS, or for the single-, compared to the multi-junction a-Si modules, with the ratios for the dye and organic systems much lower still. The ratio for the highly developed Si cells does show what can be expected. While the second and the third yardsticks express a limit, related to the SQ one, the rather low molecular cell values appear to indicate basic limitations for such cells, which we note. The significant differences between a-Si on the one hand and CdTe and CIGS cells on the other hand genuinely reflect the different physics of these polycrystalline systems. The need to identify and define limitations and distinguish then from immaturity is becoming critical for future thin film cell development and this type of analysis can, therefore help us making our choices for the future.