Influence of Na doping on tunnelling rear contact passivation in Cu(In,Ga)Se2 solar cells

Sammanfattning: In this thesis Cu(In,Ga)Se2 (CIGS) solar cells with different sodiumdoping of the CIGS absorber and varying Al2O3 rear surface passivationlayer thickness have been manufactured and electrically characterised. Baseline samples and samples without passivation were used asreferences for the passivated samples. For the passivated samplesbetween 1 and 7 nm of Al2O3 were deposited by ALD. The electricalcharacterisation included current-voltage (IV, JV), quantum efficiency (QE, EQE), capacitance-voltage (CV) and temperature dependent currentvoltage (IVT, JVT) measurements. The results show that it is indeed possible to use a tunnel current toconstruct an electrical contact, but that the electrical contact isvery sensitive to sodium doping. The samples with post-depositiontreatment and without Na start to block the tunnel current when thepassivation layer reaches a thickness of about 2 nm, while no blocking of the tunnel current could be observed for the samples with Na pre-deposition. The samples with pre-deposition treatment showed acontinued increase in efficiency all the way to a passivation layerthickness of about 7 nm. When trying to construct samples with eventhicker passivation layer the CIGS started to peel off. For thisreason the optimal thickness for the pre-deposition treated samplescould not be found. The samples with the highest efficiency was thesamples with pre-deposition treatment and thick passivation layer. Those samples showed an increase of 3 percent (absolute) compared tothe unpassivated sample with the same deposition treatment and 1.6 percent higher efficiency compared to the baseline sample. Concluding that tunnelling passivation layer is comparable to the passivationlayer with point contact methods.