Illumination level independence in relation to emitter profiles on industrial high efficiency local Al-BSF cells

If the world´s answer to alternative energy production is to be silicon photovoltaics, the fabricated devices need to be robust and highly efficient in varying operating conditions. Rear oxide passivated local Al-BSF cells have a prevalent issue that hinders their performance in particular operating conditions, this issue being bias light dependence (reduced response at low illumination levels). It has been demonstrated by many [1,2,3] that to obtain high quantum efficiency at longer wavelengths, the cells need to be illuminated with a sufficient high level of bias light. If quantum efficiency is shown to be bias light dependant, at low light conditions the cell will simply underperform. Although most cells suffer from this type of degradation, in practice some high efficiency cells reach maximum spectral responsivity already at 0.3 suns and are considered to be bias independent. Regardless if in practice, cells can be named bias independent, the mechanisms for bias dependency is a very relevant characteristic of high efficiency solar cells of the present and future. In this paper we present a phenomenon that has been observed and repeated in separate experiments. We compare differences in rear passivation, specifically between a fresh deposited silicon oxide versus one that has been used also as a diffusion mask. We observe a relationship between bias dependence and the process flows as well as a relationship with the densification recipe. As expected the 90 ohm/sq emitter outperforms the higher doped emitters in the UV wavelength range. What is not expected is that when measuring without a bias light, the higher sheet resistance emitters outperform the lower sheet resistance emitters in wavelengths above 700 nm. Another observation is that the cells that have been passivated with fresh silicon oxide indicate a large bias dependence at ultra low bias levels below .01 sun, but saturate performance above .05 sun. The diffused silicon oxide cells increase their quan- um efficiency as the bias light is increased. The cells studied in this paper are fabricated using 148.25 cm², 160μm p-type Cz-Si wafers with screen printed Ag front contacts and are rear-side passivated with a deposited rear silicon oxide/silicon nitride stack. The highest efficiency of the cells studied is 19.2 % with a V_oc of 637mV, J_sc of 38.2 mA/cm² and a fill factor of 79.1%.