Surface recombination in silicon solar cells decreases the collection probability of charge carriers and can hence lower the cell efficiency significantly. The reduction of this detrimental effect is addressed in this work by passivating the surface with different atomic layer deposited (ALD) thin film coatings. It is shown that surface passivation with aluminum oxide (Al2O3) or hafnium oxide (HfO2) is dependent on the oxidant used in the ALD process. Higher deposition temperatures lead to better surface passivation with aluminum nitride (AlN) films where carrier lifetimes of more than 1 ms can be reached with additional corona charge. In general, all of the thin films studied here provide good surface passivation with low values for interface defect density. Surface passivation becomes even more important in the case of black silicon (b-Si), i.e. nanostructured silicon surface, where the large surface area results in high surface recombination velocity. Black silicon itself has gained much interested especially for photovoltaic applications due to its low surface reflectance on a wide spectral range and acceptance angle. However, the increase in the surface recombination has always hindered the application of b-Si e.g. in solar cells. This work presents that ALD Al2O3 can solve this issue by providing completely conformal coating and excellent passivation also on nanostructured surfaces. In addition to surface passivation, Al2O3 coating further reduces the reflectance leading to values less than 1 % on the whole spectral range relevant for solar cell operation. As good surface passivation is reached on b-Si, its applicability on different photovoltaic devices including two types of solar cells and a photodiode is demonstrated. First, it is shown that different emitter diffusion processes do not hinder the optical properties of black silicon and that excellent surface passivation is also reached on boron doped emitters using ALD Al2O3. This discovery is then applied in the fabrication of n-type front contact solar cells with b-Si and Al2O3 passivation on the front surface and an efficiency of 18.7 % is reached. Even higher efficiencies of 22.1 % are gained with back contact solar cells. This is so far the highest efficiency reached with black silicon solar cells. Finally, Al2O3 surface passivation and black silicon are applied on induced junction photodiodes and external quantum efficiency of 96 % is reached on the measured wavelength range of 250-950 nm. This proves that the use of b-Si and Al2O3 surface passivation is not limited only to solar cells but also other optoelectronic applications can benefit from it.
|Publication status||Published - 2016|
|MoE publication type||G5 Doctoral dissertation (article)|
- black silicon
- atomic layer deposition
- surface passivation
- photovoltaic devices