Black silicon (b-Si), i.e. a nanostructured silicon surface, is currently a subject of great interest within the photovoltaics (PV) community due to its excellent optics. While PV-related b-Si research has mainly focused on the reduction of reflectance and surface recombination, other possible effects of nanostructured surfaces have received less attention. This thesis investigates b-Si from a wider perspective and concentrates on engineering of surface and bulk defects in b-Si solar cells. The thesis first focuses on b-Si surfaces. It is demonstrated that no trade-offs are required between the optical and electrical properties of wet-chemically fabricated b-Si by the application of atomic-layer-deposited (ALD) aluminum oxide (Al2O3) surface passivation. The current mainstream solar cell architectures, however, have a phosphorus-doped emitter on the front, and thus, the negatively-charged Al2O3 is non-optimal. This work addresses the issue by using positively-charged ALD SiO2/Al2O3 stacks, which result in reduced recombination at diffused b-Si phosphorus emitter surfaces. In addition to affecting surface passivation, heavy phosphorus doping is shown to accelerate the consumption of silicon in standard cleaning solution, which strongly impacts both electrical and optical properties of b-Si emitters. All these results need to be considered in the design of high-efficiency b-Si solar cells. The second main theme of the work is engineering of bulk-related phenomena by b-Si. The nanostructures are shown to enhance gettering of detrimental metal impurities. Indeed, intentionally iron-contaminated b-Si wafers have more than three times higher minority carrier lifetime than polished samples after gettering (720 µs vs. 200 µs). In addition to impurity gettering, the impact of b-Si on another important bulk phenomenon, i.e., light-induced degradation (LID), is demonstrated. Black multicrystalline (mc-Si) passivated emitter and rear cells (PERC) show no or only slight degradation under illumination at elevated temperature, while standard acidic-textured equivalents suffer from severe LID. The increased gettering efficiency and reduced LID clearly demonstrate that benefits provided by b-Si are not limited only to the excellent optics. Finally, dry-etched b-Si is applied to industrial mc-Si PERC solar cells and modules. The fragile nanostructure is demonstrated to remain intact through cell and module fabrication at industrial production lines. Indeed, the prototype modules appear uniformly black after processing without anti-reflection coatings. Furthermore, the b-Si modules are shown to retain their performance until an incidence angle of 60°, whereas the modules with standard acidic-textured cells start to lose their performance already after a 30° tilt. Hence, the results demonstrate that the optical and electrical properties of b-Si can be maintained also at module level.
|Translated title of the contribution||Kidevirheiden kontrollointi mustassa piissä|
|Publication status||Published - 2019|
|MoE publication type||G5 Doctoral dissertation (article)|
- black silicon
- solar cells
- surface passivation
- light-induced degradation