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 Absorption is due to interactions with material particles (electrons and nucleus). If particle energy before interaction was W 1, after photon absorption is W 1 + 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)  (x) is the light intensity  =  ( ) is the absorption coefficient Light is absorbed in the material.

Φ 0 = Φ in (1 – R ) is the so-called absorption length x=x L Φ(x L ) = 0.38 Φ 0 R is the surface reflexivity Light intensity decreases with the distance x form the surface

Photovoltaic Quantum generator This process can be realised in different materials

Before interaction with photon (in thermodynamic equilibrium) After interaction with photons h > W g n = n 0 + Δn, p = p 0 + Δp ( Δn = Δp, because electron-hole pairs are generated ) np > n i 2 Δn, Δp excess carrier concentration (no thermodynamic equilibrium) Semiconductors

conduction band valence band bandgap thermalisation WgWg WcWc WvWv Excess carrier generation

Silicon crystalline amorphous

Carrier generation with respect to solar spectrum Total generation h (eV) (nm)

Efficiency of excess carrier generation by solar energy depens on the semiconductor band gap Suitable materials Silicon GaAs CuInSe 2 amorphous SiGe CdTe/CdS

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

Excess carrier concentration Diffusion current is connected with carrier concentration gradient D n = kTμ n /q D p = kTμ p /q Continuity equations In the dynamic equilibrium electron diffusion lengthhole diffusion length Excess carrier concentration can be found solving continuity equations under proper boundary conditions usually τ n = τ p = τ

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 W Fn W Fp W

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).

Principles of solar cell function In the illuminated area generated excess carriers diffuse towards the PN junction. The density J FV 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

I PV illuminated in dark V I I SC V OC irradiation I V Illuminated PN junction: A supperposition of photo-generated current andPN junction (dark) I-V characteristic Solar cell I-V chacteristic and its importan points V OC V mp

Modelling I-V characteristics of a solar cell Series resistance R S Parallel resistance R p PN junction I-V characteristics A ill – illuminated cell area A - total cell area Output cell voltage V = V j - R s I

VV If R p is high If Influence of parasitic resistances (R s and R p )

Influence of temperature I (A) V(mV) P m (W) temperature (°C) I 01 ~ Consequently For silicon cells the decrease of V OC is about 0.4%/K R s increases with increasing temperature R p decreases with increasing temperature Both fill factor and efficiency decrease with temperature K -1 At silicon cells

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

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

electrolyte Pt TCO coating glass light dye on TiO 2 nanocrystals Photochemical cells Iodine/iodide redox system 3I - I e -

To maximise current density J PV it is necessary maximise generation rate G minimise losses losses opticalrecombination electrical reflection shadowing not absorbed radiation emitter region base region surface series resistance parallel resistance