Empowering Photovoltaics Turnkey Services Technologies Empowering Photovoltaics Architecture of solar cells

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Content 1 Simple solar cells 2 Losses

Simple solar cell pn-diode with contacts contacts emitter base EF p-doped n-doped EF

Absorption – usable spectrum h < EGAP: cannot be absorbed h > EGAP: excess energy is lost to heat

Absorption – usable spectrum What can we do?

Absorption – usable spectrum What can we do? Nothing h < EGAP: losses: 24% h > EGAP: losses: 33% 100 %

Absorption – usable spectrum What can we do? Nothing h < EGAP: losses: 24% h > EGAP: losses: 33% h < EGAP 24% h > EGAP: 33% 43%

Open-circuit voltage Band gap of silicon: 1.1 eV But open-circuit voltage at „1 sun“ illumination is much less What can we do? h < EGAP 24% h > EGAP: 33% 43%

Open-circuit voltage Band gap of silicon: 1.1 eV But open-circuit voltage at „1 sun“ illumination is much less What can we do? Not much Voltage is about 30 % less than the band gap: losses about 13 % h < EGAP 24% h > EGAP: 33% VOC< EGAP: 13% 30%

Open circuit-voltage Band gap of silicon: 1.1 eV But open-circuit voltage at „1 sun“ illumination is much less What can we do? Not much Voltage is about 30 % less than the band gap: losses about 13 % World record monocrystalline solar cell: jsc=42.2 mA/cm², Voc=706 mV, FF=82,8% Eta=24,7% h < EGAP 24% h > EGAP: 33% VOC< EGAP: 13% FF: 5% 25%

Content 1 Simple solar cells 2 Losses

Loss analysis Losses optical electrical Reflection Shading Transmission ohmic recombinatoric Semiconductor: emitter base Metal: finger busbars contact back contact Emitter: semiconductor surface Base: Space charge region

Loss analysis Losses optical electrical Reflection Shading Transmission ohmic recombinatoric Semiconductor: emitter base Metal: finger busbars contact back contact Emitter: semiconductor surface Base: Space charge region

Reducing transmission Simple solar cell: pn-diode with contacts contacts emitter base

Reducing transmission Absorption length is too high But we do not want thicker wafers (180 – 240 µm) Diffusion length must be 2 times the wafer thickness We want less material and cheap material

Development of wafer thickness and silicon usage

Reducing transmission Engineering solution: Full area aluminium contact to reflect transmitted light contacts emitter base full area aluminium contact silver solder pads

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Reducing reflection Reflection of light is big loss mechanism „old“ untextured solar cells look shiny, metal-like base emitter contacts full area aluminium contact

Reducing reflection Engineering solution I: Front surface texture Improved absorption & light trapping base emitter contacts full area aluminium contact texture

antireflection coating Reducing reflection Engineering solution II: Antireflection layer Single PECVD SiNx layer with thickness d=85 nm and refractive index n=2.05 base emitter contacts full area aluminium contact texture antireflection coating

Antireflection layer

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Optimizing shading vs ohmic losses Minimizing shading losses High aspect ratio Buried contacts Contacts on backside Minimizing ohmic losses High aspect ratio (90 µm / 20 µm) Highly doped emitter, but this means high Auger recombination Screen printing Light induced plating Evaporation of contacts Different technology n + Al p R base emitter contact finger busbar

Optimizing shading vs ohmic losses Engineering solution: Screen printing silver contacts Optimization of pastes: high conductivity, low contact resistance Optimization of contact grid layout For fixed finger width and emitter sheet resistance an optimum exists for the finger spacing Emitter should not be highly doped, because of Auger recombination But emitter must be highly doped for minimizing emitter sheet resistance and contact resistance R emitter 49% contact 12% base 2% finger 37%

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Radiative recombination Shockley-Read-Hall recombination Recombination Losses Electrons and holes can recombine before they reach the contacts 3 Recombination mechanisms light ECB EVB trap states surface states Radiative recombination Auger recombination Shockley-Read-Hall recombination

Emitter recombination Emitter recombination occurs via Auger mechanism High doping increases Auger recombination But we need high doping for conductivity and low contact resistance to the silver (ohmic losses) Engineering solution: Standard homogenous emitter: High doping to reduce ohmic losses, but keep emitter shallow (< 0.5 µm), so diffusion length of holes is still enough to reach the space charge region  50 – 65 Ohm/sq Good news: Only short wavelength are absorbed in the first µm of the solar cell Selective emitter: see next chapter Auger recombination

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Reducing rear surface recombination Surface passivation p+-doped layer passivates by field effect, e.g. by additional Boron doping Surface states can be reduced by saturation of dangling bonds, e.g. by SiO2 surface states Shockley - Read Hall recombination n+ p n+-doped p-doped EF surface states

Reducing rear surface recombination Engineering solution: Back surface field (BSF) by aluminium doping Good news: Aluminium which is already there, forms an eutectic at 577°C with Si during the fast firing and forms an alloy p+-Al BSF n+ p+ p p-doped n+-doped EF p+-doped

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Reducing front side recombination Surface passivation Passivation by field effect Passivation by reduction of surface states by saturation of dangling bonds. Best surface passivation: SiO2 SiNx layer acts as front surface passivation n+ p+ p

Reducing front side recombination Engineering solution: SiNx is a quite good surface passivation Good news: SiNx which is already there passivates mainly by field effect. Positve surface charges form at the Si / SiNx interface which repel holes Hydrogen in the SiNx saturates surface states to some extent SiNx layer acts as front surface passivation n+ p+ p

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Reducing the recombination in the base Bulk recombination in the base is dominated by traps: Shockley-Read-Hall mechanism Impurities and crystal defects form trap states But we want to use cheap multicrystalline material! Engineering solution I: Move impurities to regions where they do not harm: Getter effect Engineering solution II: Passivate dangling bonds of crystal defects: Hydrogen passivation trap states Shockley - Read Hall recombination

Reducing the recombination in the base Perfect crystal Float zone silicon Too expensive . . . Si

Reducing the recombination in the base Real crystal with impurities and defects Impurities form trap states Defects cause dangling bonds, which form trap states Si Fe Foreign substitutional interstitial Grain boundary Dislocation Si

Reducing the recombination in the base Getter material: Phosphorus glass from the POCl3 diffusion, getters and is removed Aluminium layer at the rear side getters during fast firing Si Fe material Si Getter

Reducing the recombination in the base Getter effect At high temperatures impurities are released from original state Impurities diffuse through the crystal Impurities are captured in the getter material Getter material Si Fe

Reducing the recombination in the base Hydrogen source Amorphous SiNx layer contains several percent of atomic hydrogen H H H H H H H During the fast firing hydrogen diffuses in less than 1 second through the whole cell Getter material Si Fe

Reducing the recombinaton in the base Hydrogen passivation Saturation of dangling bonds by atomic hydrogen Getter material Si Fe H

Specific resistivity of base Saturation current of base is inversely proportional to the doping level Doping should be as high as possible Recombination of minority charge carriers increases with higher doping Doping should be not be too high Theoretical optimal doping for a 200 µm thick solar cell is 4 x 1016 cm-³  0.4 Ohm cm Technologically the specific resistivity should be 0.5-2.5 Ohm cm

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric

Loss analysis Emitter: semiconductor surface Base: Space charge region Metal: finger busbars contact back contact ohmic optical Losses electrical Reflection Shading Transmission recombinatoric will be neglected

Loss analysis Summary World record monocrystalline solar cell: h < EGAP 24% Summary World record monocrystalline solar cell: jsc=42.2 mA/cm², Voc=706 mV, FF=82,8% Eta=24,7% FF loss: 5 % h > EGAP: 33% VOC< EGAP: 13% 5 % 25%

Loss analysis Summary World record monocrystalline solar cell: h < EGAP 24% Summary World record monocrystalline solar cell: jsc=42.2 mA/cm², Voc=706 mV, FF=82,8% Eta=24,7% FF loss: 5 % Industrial screen printed multicrystalline solar cell: jsc=34,5 mA/cm², Voc=623 mV, FF=78 % Eta=16,8% Optical, ohmic, recominatoric, FF losses: 14 % h > EGAP: 33% VOC< EGAP: 13% 14% 16%

Loss analysis Losses in short-circuit current obtained jsc 32,1 mA/cm² 71% contact reflection 3,2 mA/cm² 7% front reflection 4,1 mA/cm² 9% emitter recombination 1,5 mA/cm² 3% free carrier and rear contact absorption 2 mA/cm² 4% recombination in bulk and at rear 2,5 mA/cm² 6%

Summary Architecture of the solar cell: selective emitter

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