Carriers.If an external load is connected across the terminals thecharges flow (and can do work!)? Longer wavelength light can penetrate into the p-region →where the photons generate EHPsThese charge carriers diffuse randomly as there is no E-field.In this p-region the mean diffusion distanceL.=/D.teD。= diffusivity; t,= recombination lifetimeElectrons within a distanceL,fromthe depletion region canreach the depletion region and be drifted across by E。11
Carriers • If an external load is connected across the terminals the charges flow (and can do work!) • Longer wavelength light can penetrate into the p-region à where the photons generate EHPs • These charge carriers diffuse randomly as there is no E-field • In this p-region the mean diffusion distance De= diffusivity; τe = recombination lifetime • Electrons within a distance Le from the depletion region can reach the depletion region and be drifted across by Eo 11 Le De e = τ
Photogenerated Carriers. EHPs generated by L。are eventually annihilatedby recombination - so make Le as long aspossible for optimum conversion: Volume that contributes to the photovoltaiceffect is given by Lh + W + Le: The current that flows due to the photogeneratedcarriers is called the photocurrent12
Photogenerated Carriers • EHPs generated by Le are eventually annihilated by recombination – so make Le as long as possible for optimum conversion • Volume that contributes to the photovoltaic effect is given by Lh + W + Le • The current that flows due to the photogenerated carriers is called the photocurrent 12
Inefficiencies: Sihas a bandgap of 1.1 eV → corresponds to a thresholdwavelength of 1.1μm → energy greater than 1.1 μm is wasted(typically about 25%)High energy photons are lost at the surface - crystal surfaceshave a high density concentration of recombination sites whichfacilitate recombination of photogenerated EHP's (typically40% lost)· Reflections from the surface-typically anti-reflection-coatings (ARC) are used but these are not perfect (typically 10- 20% lost):Other electrical losses due to loadingOptimal efficiency of a PV solar cell is circa 25% at roomtemperature13
Inefficiencies • Si has a bandgap of 1.1 eV à corresponds to a threshold wavelength of 1.1µm à energy greater than 1.1 µm is wasted (typically about 25%) • High energy photons are lost at the surface – crystal surfaces have a high density concentration of recombination sites which facilitate recombination of photogenerated EHP’s (typically 40% lost) • Reflections from the surface – typically anti-reflectioncoatings (ARC) are used but these are not perfect (typically 10 – 20% lost) • Other electrical losses due to loading • Optimal efficiency of a PV solar cell is circa 25% at room temperature 13
Photocurrent - SCWiththePV cell short-circuited,the onlycurrent flowing is that generated by theincident light (ph) = photocurrentLightIph depends onThenumberof EHPsgeneratedwithdepletionlayervolumeThediffusionlengthsThe greater the light intensity (I,), thegreater the photogeneration rateV=0Isc = -Iph = -kln-In=light intensity-k=constantthatdependsondevice14
Photocurrent - SC • With the PV cell short-circuited, the only current flowing is that generated by the incident light (Iph) = photocurrent • Iph depends on – The number of EHPs generated with depletion layer volume – The diffusion lengths • The greater the light intensity (In), the greater the photogeneration rate • Isc = -Iph = -kIn – In = light intensity – k = constant that depends on device Light Isc V = 0 Iph 14
Photocurrent - loaded.If R is not a short circuit (a realresistance),avoltageappearsacrossLightthe p-n junction, which reduces thebuilt-in potential Φ, and hence leadsto minority carrier injection anddiffusion (as in a normal diode)So, in addition to Iph, there is aVforward diode current IdI=-Iph + IdRBythe Shockleydiode equation,the current diverted throughthe diode isIa = I,expl/-n=idealityfactor15
Photocurrent - loaded • If R is not a short circuit (a real resistance), a voltage appears across the p-n junction, which reduces the built-in potential Φ, and hence leads to minority carrier injection and diffusion (as in a normal diode) • So, in addition to Iph, there is a forward diode current Id I=-Iph+ Id Light R I V η = ideality factor 15 ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ −⎥ ⎦ ⎤ ⎢ ⎣ ⎡ = exp 1 kT eV I I d o η By the Shockley diode equation, the current diverted through the diode is