P N Junction at Forward Bias P N

P- N Junction at Forward Bias P N Va The electro static potential barrier is lowered by forward bias Potential Barrier = Vb - Va In Ec EFn Ei EFp Ev Direction of current Hole Diffusion Hole Drift Electron Diffusion Electron Drift • Forward current flows mainly because of the diffusion of carriers E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 1

Part-III P- N Junction at Reverse Bias The electro static potential barrier is increased by forward bias Potential Barrier = Vb + Va In Direction of current Hole Diffusion EFp Ec EFn Ei Ip Ev Hole Drift Electron Diffusion Electron Drift current flows mainly because of the drift of carriers E for • Reverse Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 2

Diode Current: Qualitative Solution • Total current crossing the diode is sum of the diffusion and drift current • Under equilibrium, V=0, there is no current flow • In reverse bias both electron and hole diffusion components are negligible due to large barrier. Current, Igen, independent of voltage. • In forward bias current flow mainly due to diffusion of carriers, which increases exponentially. I = I (diffusion) – I (generation) = 0 when V = 0 • In forward bias, the probability that a carrier can diffuse across junction proportional exp(q. Vf / k. T) Therefore diffusion current in forward bias is Idiff * exp(q. Vf / k. T) I I (diffu. ) EI (gen. ) for Energy 12/19/2021 © IIT Bombay, C. S. Solanki V A CEP course for Applied Materials 3

Lecture-19 Junction under illumination Time t=0 Ec EF Ec Ei Ev Current flow Hole Diffusion Ln W Lp Hole Drift Electron Diffusion Drift E Electron for Energy 12/19/2021 © IIT Bombay, C. S. Solanki • Shining of light will generate electron-hole pair throughout the semiconductor A CEP course for Applied Materials 4

Junction under illumination Lecture-19 Carrier will die P-type N-type Time t>0 + Ec Ei - Ev Ln Lp W Pile up of carriers, Carrier will die, ξ responsible for generation of will not participate forward voltage in current Direction of the light (Photovoltaic effect) generated current E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 5

P-N junction under illumination • Generation of voltage in P-N junction radiation + Ln(n), ln(p) - Carrier conc. profile npo Ln W Lp pn 0 -xpo xn 0 Direction of current flow under illumination x I I (diffu. ) • P-N behaves like a forward bias P-N junction under illumination V I (gen. ) E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 6

P-N junction under illumination • Solar cell I-V Equation E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki Where IL is photo current A CEP course for Applied Materials 7

Junction depth Generation probability, GP Collection probability, CP Junction depth Light generated current = E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki • Design criteria for junction ? : Where should the junction be located in a solar cell ? • Design criteria for junction : Junction should be close the surface from where light enters A CEP course for Applied Materials 8

Short-Circuit Current, Isc I Pm X Im • The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i. e. , when the solar cell is short circuited). Vm Voc At V=0 Itotal = -IL= Isc = q A W (Lp + Ln) E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki • The short-circuit current is due to the generation and collection of light-generated carriers. • The short-circuit current is the largest current which may be drawn from the solar cell. A CEP course for Applied Materials 9

Open Circuit Voltage: Voc I Isc Pm Im Vm X Voc • The open-circuit voltage, Voc, is the maximum voltage available from a solar cell, and this occurs at zero current. • The open-circuit voltage corresponds to the amount of forward bias on the solar cell junction due to illumination. by setting Itotal = 0 E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 10

Maximum power: Pm I Im Isc Pm X r e w Po Vm Voc • Power out of a solar cell increases with voltage, reaches a maximum (Pm) and then decreases again. Pm = I m x V m • Remember we get DC power from a solar cell E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 11

Fill Factor: FF I Ideal diode curve Isc Pm Im Vm Voc • The FF is defined as the ratio of the maximum power from the actual solar cell to the maximum power from a ideal solar cell • Graphically, the FF is a measure of the "squareness" of the solar cell E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 12

Efficiency: η I Im Isc Pm X • Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. r e w Po Vm Voc • The efficiency is the most commonly used parameter to compare the performance of one solar cell to another. • Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell. E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 13

PV market growth with cost reduction E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 14

Si-current status • Poly-si contributed to 94% of PV market • Si prices are increasing causing solar cell prices to go high • The shortage of Si is already there since last two years Cost of solar PV modules E for Energy 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 15

Production and Cost trend of solar PV E for • Most Energy of PV modules are produced from Si 12/19/2021 © IIT Bombay, C. S. Solanki A CEP course for Applied Materials 16