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Photovoltaic effect and cell principles

Photovoltaic effect and cell principles. 1. Light absorption in materials and excess carrier generation. Photon energy h  = hc/  (h is the Planck constant) photon momentum  0. Light is absorbed in the material.  (x) is the light intensity.

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Photovoltaic effect and cell principles

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  1. Photovoltaic effect and cell principles

  2. 1. Light absorption in materials and excess carrier generation Photon energy h = hc/ (h is the Planck constant) photon momentum  0 Light is absorbed in the material. (x) is the light intensity  = () is the absorption coefficient Absorption is due to interactions with material particles (electrons and nucleus). If particle energy before interaction was W1, after photon absorption is W1+ h • interactions with the lattice – low energy photons, results in an • increase of temperature • interactions with free electrons - important when the carrier • concentration is high, results also in temperature increase • interactions with bonded electrons- the incident light may generate • some excess carriers (electron/hole pairs)

  3. Light intensity decreases with the distance x form the surface Φ0 = Φin (1 – R) R is the surface reflexivity is the so-called absorption length x=xLΦ(xL) = 0.38Φ0

  4. Photovoltaic Quantum generator This process can be realised in different materials

  5. Semiconductors Before interaction with photon (in thermodynamic equilibrium) After interaction with photons h > Wg n = n0 + Δn , p = p0 + Δp np > ni2 Δn, Δp excess carrier concentration (no thermodynamic equilibrium) (Δn = Δp, because electron-hole pairs are generated )

  6. Excess carrier generation conductionband thermalisation Wc bandgap Wg Wv valence band

  7. Silicon crystalline amorphous

  8. hn (eV) l (nm) Carrier generation with respect to solar spectrum Total generation

  9. Suitable materials Silicon GaAs CuInSe2 amorphous SiGe CdTe/CdS Efficiency of excess carrier generation by solar energy depens on the semiconductor band gap

  10. Carrier recombination τ is carrier lifetime irradiative recombination Auger recombination recombination via loca1 centres Resulting carrier lifetime

  11. Excess carrier concentration Diffusion current is connected with carrier concentration gradient Dn= kTμn/q Dp = kTμp/q Continuity equations usually τn = τp = τ In the dynamic equilibrium electron diffusion length hole diffusion length Excess carrier concentration can be found solving continuity equations under proper boundary conditions

  12. Electrical neutrality is in homogeneous semiconductor no potencial difference To separate excess carrier generated, an inhomogeneity with a strong internal electric field must be created WFn W WFp

  13. Photovoltaic effect and basic solar cell parameters To obtain a potential difference that may be used as a source of electrical energy, an inhomogeneous structure with internal electric field is necessary. • Suitable structures may be: • PN junction • heterojunction (contact of different materials).

  14. Principles of solar cell function In the illuminated area generated excess carriers diffuse towards the PN junction. The density JFV is created by carriers collected by the junction space charge region • in the N-type region • in the P-type region • in the PN junction space charge region

  15. Illuminated PN junction: A supperposition of photo-generated current andPN junction (dark) I-V characteristic I I in dark VOC irradiation V V IPV illuminated ISC Solar cell I-V chacteristic and its importan points Vmp VOC

  16. Modelling I-V characteristics of a solar cell PN junction I-V characteristics Parallel resistance Rp Series resistance RS Aill – illuminated cell area A - total cell area Output cell voltage V = Vj- RsI

  17. Influence of parasitic resistances (Rs and Rp) If Rp is high If V V

  18. Influence of temperature I (A) I01 ~ Consequently V(mV) For silicon cells the decrease of VOC is about 0.4%/K Pm (W) Rs increases with increasing temperature Rp decreases with increasing temperature Both fill factor and efficiency decrease with temperature At silicon cells K-1 temperature (°C)

  19. Organic semiconductors Hoping mechanism : A1- + A2 ->  A1 + e- + A2  ->  A1 + A2- 

  20. V hn - + - + - + - + Reflecting Electrode (Al) - + P type organic semiconductor N type organic semiconductor Transparent Conducting Oxyde Transparent Substrate P & N materials and cells Perylen pigment (n) Cu Phtalocyanin (p) Technological advantages of OSCs : • Wet processing (Ink pad printing) • Soft cells • Large surfaces • Low cost • Molecular materials

  21. Photochemical cells glass TCO coating Pt electrolyte dye on TiO2 nanocrystals TCO coating glass light Iodine/iodide redox system 3I- I3- + 2e-

  22. To maximise current density JPV • it is necessary • maximise generation rate G • minimise losses losses electrical optical recombination • reflection • shadowing • not absorbed radiation • emitter region • base region • surface • series resistance • parallel resistance

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