EE 5611 Introduction to Microelectronic Technologies Fall 2014 Tuesday, September 23, 2014 Lecture 07 1
Introduction to Solar Cells Topics to be covered: Solar cells and sun light Review on semiconductor p-n junction Operation principle of semiconductor p-n junction solar cells Efficiency limits and loss mechanisms in solar cells Circuit model of solar cells Design of high efficiency semiconductor solar cells 2
I-V Relationship for Solar Cells (1) total dark s dark light qv kt ( e 1) Same as I-V relationship for PN diode ( + L W ) light qg Lp n + With the assumption of 100% collection I The IV relationship for a solar cell: IV for PN diode shifted down by the amount of Light By convention, we flip the IV graph to the first quadrant with positive current and voltage 3
I-V Relationship for Solar Cells (2) Cell Light Dark sc : short-circuit current density, when V0 sc Light qg( Lp + Ln + W ) V oc : open-circuit voltage, when I0 V oc qv kt ( e 1) qg( L + L + W ) n p s kt sc ln + 1 q s 4
Fill Factor and Efficiency Maximum power point: point corresponding to the maximum IV product Fill factor (FF): FF I I mp sc V V mp oc Efficiency: η I mp P V in mp I sc VocFF P in 5
Open-Circuit Voltage V oc The open-circuit voltage, V OC, is the maximum voltage available from a solar cell, and this occurs at zero current. It corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated carriers. Cell Light s qv kt ( e 1) When 0, V V OC, Cell V oc kt light ln + 1 q s Commercial devices Si single-junction solar cell devices typically have open-circuit voltages around 600 mv. 6
Short-Circuit Current I sc The short-circuit current I SC, is the maximum current that can be drawn from a solar cell, and this occurs when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). Cell Light s qv kt ( e 1) When V 0,, So SC Light, Cell SC and I I A, SC Light light A is cross-section area of the cell Commercial single-junction Si solar cell devices have short-circuit currents of 28~35 ma/cm 2 7
Light Generation Current Light current with the assumption of full collection and uniform generation ( + L W ) light qg Lp n + Light current considering the non-ideal collection probability [ ( ) ] g x, λ dλ C ( x dx light q G( x) C p ( x) dx q p ) (x) : C p x Collection probability x Collection probability determines how likely a carrier is to be collected. It depends on where it is generated in the material, the surface recombination mechanisms and the diffusion length. λ 8
Factors Affecting the Short-Circuit Current The surface area of solar cell The number of photons (i.e., the power of the incident light source) The spectrum of the incident light The optical properties (absorption and reflection) of the solar cell The collection probability of the solar cell, which depends on the surface passivation (to reduce the surface recombination) and the minority carrier diffusion length. I SC A light Aq x G( x) C p ( x) dx q x [ ] g( x, λ) dλ C ( x) dx λ p A: Surface area of solar cell G(x): generation rate at location x into the cell, depends on incident light intensity, reflection and absorption C p (x): collection probability at location x into the cell, depends on recombination and minority carrier diffusion length or time 9
Quantum Efficiency External quantum efficiency (EQE): number of electrons collected per incident photon at each wavelength under the short-circuit condition. Internal quantum efficiency (IQE): number of electrons collected per photon entering the device at each wavelength under the short-circuit condition. ( λ) [ 1 ( λ) ] IQE( λ) EQE R 10
Theoretical Single-unction Solar Cell Efficiency Limit Definition: percentage of power converted from sunlight to electrical energy under the standard testing condition (STC) The modern SQ (William Shockley and Hans Queisser) limit calculation is a maximum efficiency of 33% for any type of single junction solar cell. The SQ limit for single junction silicon solar cell is 30%. Current solar cell production efficiencies vary by the band gap of the semiconductor material. 11
Loss Mechanisms of Semiconductor Solar cells The calculated 67% (70% for silicon) energy loss include: Energy converts to heat (hv >E G ); Photons pass through the cell (hv < E G ) ; Energy is lost from local recombination of newly created holes and electrons (less than100% collection probability). The highest efficiency for single-junction Si solar cell is 24%. Compared to theoretical limit of 30%, 6% loss is due to the following: Reflection loss even with the anti-reflection coating; Photon loss due to contact shading; Loss due to incomplete absorption Loss from the electrical contacts which carry current to the load; Manufacture impurities in the Si; 12
Summary of Loss Mechanisms of Semiconductor Solar cells Loss Optical loss Electrical loss Heat hv >E G No absorption hv <E G Reflection Shadowing Incomplete absorption Ohmic SC material Contact material Metal-SC Recombination SC surface SC body Depletion region Manufacture defects proving alternate current path 13
Design Considerations on Loss Reduction Minimize the top contact coverage of the cell surface, but this might increase the series resistance Anti-reflection coating or surface texturing can be used on the top surface of the cell to reduce reflection. Make the cell thicker to increase absorption, although this may decrease the collection probability due to recombination. The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping. Passivation layer on the top surface to reduce surface recombination 14