Nanostructuring of Solar Cell Surfaces

Abstract

Solar energy is by far the most abundant renewable energy source available, but the levelized cost of solar energy is still not competitive with that of fossil fuels. Therefore there is a need to improve the power conversion effciency of solar cells without adding to the production cost. The main objective of this PhD thesis is to develop nanostructured silicon (Si) solar cells with higher power conversion efficiency using only scalable and cost-efficient production methods. The nanostructures, known as 'black silicon', are fabricated by single-step, maskless reactive ion etching and used as front texturing of different Si solar cells. Theoretically the nanostructure topology may be described as a graded refractive index in a mean-field approximation between air and Si. The optical properties of the developed black Si were simulated and experimentally measured. Total AM1.5G-weighted average reflectance well below 1% was measured for different crystalline grades of Si. Furthermore, the reflectance of RIE-textured Si remains below that of KOH-textured Si at all incident angles below 70°. RIE- and conventionally textured, screen-printed Si solar cells were fabricated on 156x156 mm2 CZ Si wafers and characterized for comparison. Power conversion eciency of 16.5% was obtained for this batch of RIE-textured Si solar cells. The eciency of the KOH-textured reference cell was 17.8%. Quantum Efficiency measurements and carrier loss analysis show that the lower eciency of the RIE-textured cells is primarily due to increased emitter and surface recombination. The large-area screen-printed solar cells were furthermore characterized at varying incident angles. The angle-dependent analysis shows that RIE-textured cells have a higher normalized power output averaged over the range of incident angles between 0 and 90. This result indicates the potential of improved cell performance and higher output power at diffuse light conditions and during daily and yearly operation. A second batch of RIEtextured solar cells with laser-doped selective emitters (LDSE) was fabricated. A power conversion eciency of 18.1% and a ll factor of 80.1% were obtained by laser doping and subsequent Ni/Cu plating in combination with RIE-texturing. This result shows the potential of improved efficiency of RIE-textured compared to conventionally textured cells, especially when laser doping on black Si is combined with improved surface passivation schemes such as atomic layer deposition (ALD) of Al2O3. ALD Al2O3 passivation on black Si yields surface recombination velocity (SRV) below 80 cm/s and implied open-circuit voltage (iVOC) of 680 mV. Surface recombination velocity of 20 cm/s and implied open-circuit voltage of 695 mV is obtained for black Si passivated by doped poly-Si and a tunnel oxide.