120
S. Leu and D. Sontag
➂ (a) Pinholes in the silicon nitride layer, which affect negatively the quality of
the surface passivation and lead locally to increased recombination
(b) Ohmic resistance in the pn-junction
➃ Auger recombination
➄ (a) Scattering absorbs light in impurities.
(b) Impurities in the silicon such as iron and defects in crystal lattice cause
recombination.
➅ (a) Reduced charge carrier lifetimes due to a high content of impurities (oxygen,
carbon, metals)
(b) Series resistance in silicon material (low doping)
(c) Shockley-Read-Hall recombination in the bulk material because of foreign
atoms, dislocations, interstitial deposits of foreign atoms, precipitates by
oxygen, vacancies
(d) photons with energy E ph = hν > E g. will most likely release their
excess energy by thermalisation (see Chap. 3)
➆ Recombination on the back side in spite of passivation and back surface field.
The losses can be also classified according to their nature:
(a) Optical losses: ➀, ➁c, ➄a, ➅d
(b) Recombination losses: ➁a, ➁d, ➂a, ➃, ➄b, ➅a, ➅c
(c) Ohmic losses: ➁b, ➂b, ➅b.
They can be split up as follows: recombination losses: 2.5%; Ohmic and optical
losses 2.0% each. Overall, the losses amount to about 6–7%. At a thermodynamic
efficiency limit of 29.4% for silicon single junction solar cells with sunlight without
light concentration,
25 the maximum cell efficiency achievable in mass production is
approximately ~23.5% (=29.4–6%).
The development of standard solar cell was focused on improving passivation
and reducing losses on the front side. Sophisticated coating processes and optimization in phosphorus diffusion led to substantial improvements, reducing thereby the
recombination losses on the front side of the solar cell to one third of the recombination losses on the back side. The development of the standard solar cell with
the Al-BSF is a big step in cell manufacturing. Nevertheless, the cell shows limitations. The back contributes only moderately to light trapping, for light with long
wavelengths. The reflectance of the back is about 60–70%,
26 and not over 90%, as
it should be. Also, the surface recombination rate of 200–600 cm s
−1 can be further
improved. This leads us to the concept of the Passivated Emitter Rear Cell (PERC),
whose motivation was to improve the back side, especially its passivation and light
trapping properties. The standard Al-BSF solar cell achieves today, at best, 20% cell
efficiency. So, there is still considerable room for improvement to reach the 23.5%
efficiency limit mentioned above.
25 This is valid for all homojunction cells.
26 Long-wave light penetrates the solar cell and can escape from the cell on the back. To prevent
this, the light-trapping on the back can be improved so that the light on the back is reflected and
directed back into the cell.
S. Leu and D. Sontag
➂ (a) Pinholes in the silicon nitride layer, which affect negatively the quality of
the surface passivation and lead locally to increased recombination
(b) Ohmic resistance in the pn-junction
➃ Auger recombination
➄ (a) Scattering absorbs light in impurities.
(b) Impurities in the silicon such as iron and defects in crystal lattice cause
recombination.
➅ (a) Reduced charge carrier lifetimes due to a high content of impurities (oxygen,
carbon, metals)
(b) Series resistance in silicon material (low doping)
(c) Shockley-Read-Hall recombination in the bulk material because of foreign
atoms, dislocations, interstitial deposits of foreign atoms, precipitates by
oxygen, vacancies
(d) photons with energy E ph = hν > E g. will most likely release their
excess energy by thermalisation (see Chap. 3)
➆ Recombination on the back side in spite of passivation and back surface field.
The losses can be also classified according to their nature:
(a) Optical losses: ➀, ➁c, ➄a, ➅d
(b) Recombination losses: ➁a, ➁d, ➂a, ➃, ➄b, ➅a, ➅c
(c) Ohmic losses: ➁b, ➂b, ➅b.
They can be split up as follows: recombination losses: 2.5%; Ohmic and optical
losses 2.0% each. Overall, the losses amount to about 6–7%. At a thermodynamic
efficiency limit of 29.4% for silicon single junction solar cells with sunlight without
light concentration,
25 the maximum cell efficiency achievable in mass production is
approximately ~23.5% (=29.4–6%).
The development of standard solar cell was focused on improving passivation
and reducing losses on the front side. Sophisticated coating processes and optimization in phosphorus diffusion led to substantial improvements, reducing thereby the
recombination losses on the front side of the solar cell to one third of the recombination losses on the back side. The development of the standard solar cell with
the Al-BSF is a big step in cell manufacturing. Nevertheless, the cell shows limitations. The back contributes only moderately to light trapping, for light with long
wavelengths. The reflectance of the back is about 60–70%,
26 and not over 90%, as
it should be. Also, the surface recombination rate of 200–600 cm s
−1 can be further
improved. This leads us to the concept of the Passivated Emitter Rear Cell (PERC),
whose motivation was to improve the back side, especially its passivation and light
trapping properties. The standard Al-BSF solar cell achieves today, at best, 20% cell
efficiency. So, there is still considerable room for improvement to reach the 23.5%
efficiency limit mentioned above.
25 This is valid for all homojunction cells.
26 Long-wave light penetrates the solar cell and can escape from the cell on the back. To prevent
this, the light-trapping on the back can be improved so that the light on the back is reflected and
directed back into the cell.