PhD thesis supervisor: dr. Jonas Gradauskas (apply for recommendation)
Development and investigation of efficiency‑enhancing layers in solar cells
The efficiency of single‑junction solar cells currently remains significantly below the theoretical Shockley–Queisser limit of about 30%. The efficiency of multijunction solar cells can reach up to 68 % in the limit of an infinite number of junctions. However, fabrication of multijunction cells faces technological challenges and still suffers from inefficient light absorption, since only photons whose energies match the bandgap Eg of individual semiconductor are effectively utilized. Another reason for low efficiency is the presence of hot carriers generated by photons with energies both higher and lower than Eg. Hot carriers generate a photovoltage whose direction is opposite to the classical photovoltage of a p-n junction solar cell.
To address these issues, it is proposed to develop and optimize new, higher efficiency solar cell designs incorporating:
1) a graded gap (varying Eg) layer formed on top of the p-n junction. Its advantages:
- efficient continuous absorption – excess energy of high‑energy photons (h > Eg) is not wasted on harmful carrier heating,
- electric field of the graded gap layer generates an additional photovoltage that increases the total cell voltage,
- no need to match electrical interconnection technologies between different semiconductor materials.
2) an additional conduction‑band (or valence‑band) offset layer near the p-n junction. Its advantages:
- extended spectral sensitivity into the longer wavelength region (h < Eg),
- the action of hot carriers becomes beneficial rather than detrimental, because the additional potential barrier generates a photovoltage whose direction coincides with the classical photovoltage of a solar cell.