T. Sonsalla and S. Zivanovic
Louisiana Tech University,
Keywords: solar cells, surface plasmons, metal nanoparticles, amorphous Si
Summary:Hydrogenated amorphous silicon (a-Si:H) can absorb the visible part of the solar spectrum better than crystalline silicon. For this reason, a-Si:H solar cells can be much thinner than crystalline silicon solar cells. Thinner cells reduce fabrication costs by utilizing less silicon. However, due to the atomic structure of amorphous silicon, power conversion efficiencies are much lower than in crystalline silicon counterparts. To increase the power conversion efficiencies of a-Si:H solar cells, this study focused on incorporating metal nanoparticles onto the front transparent conductive electrode to induce increased forward light scattering and near-field light coupling. Silver, gold, and aluminum nanoparticles were materialized on the surface of an ITO coated glass substrate by electron beam evaporation and thermal annealing. Different deposition thicknesses and annealing temperatures were used to analyze the effects on metal nanoparticle size and coverage. It was found that a metal deposition thickness of 10 nm created a mesh-like structure across the ITO surface and a reduction in sheet resistance occurred. Additionally, it was discovered that initial deposition thickness is linearly related to metal nanoparticle size (i.e. larger initial deposition thickness equals larger nanoparticles). Furthermore, annealing the 5-15 nm silver samples at 300°C led to a metal nanoparticle coverage of 30% regardless of initial metal deposition thickness. The optical transmittance, reflectance, and absorbance characteristics of the samples with different metal thicknesses were obtained as well. It was found that the optical transmittance was higher with a decrease in metal deposition thickness in non-annealed samples. Finally, p-i-n junction amorphous silicon solar cell structures were fabricated utilizing substrates with different thicknesses of different metals deposited via electron-beam evaporation. Samples deposited with gold and aluminum did not provide an efficiency enhancement over the reference samples. However, it was found that electron-beam evaporation of 7.4 and 10 nm of silver onto the ITO coated glass substrates led to the best power conversion efficiencies. Thus, future investigation into silver deposition thicknesses between 7.4-10 nm should be investigated.