Solar Photovoltaic Technologies

Transcription

1 Solar Photovoltaic Techologies ecture-17 Prof. C.S. Solaki Eergy Systems Egieerig T Bombay

2 ecture-17 Cotets Brief summary of the revious lecture Total curret i diode: Quatitative aalysis Carrier flow uder bias i exteral circuit Carrier cocetratio rofile i forward ad reverse bias o-idealities i -V curve 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-2

3 Recombiatio of excess miority (x ) (x ) carriers x Carrier will diffuse ad recombie with the electros x x ( x e ( x ) e ) 0 -x 0 =0 0 x 0 =0 x 0 qv kt ( x ) ( e 1) e x variatio i miority carrier coc. With bias ad deth from the juctio 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-3

4 ecture-17 iode Curret: Quatitative Solutio Miority hole diffusio curret The hole diffusio curret at ay oit i -side ca be calculated as (electric field is zero i quasi eutral regio): d ( x ) ( x) qa qa ( x) dx The total hole curret ijected ito side at the juctio ca be obtaied by uttig x = 0 (.e. at x 0, juctio edge) i above equatio ( at x 0 ) qa P qa 0 ( e qv / kt 1) 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-4

5 iode Curret: Quatitative Solutio Miority electro diffusio curret With similar aalogy, we ca calculate the curret at the juctio at P-side (at x =0, juctio edge) due to ijectio of electros from -Side ( at x 0 ) qa qa 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies ( e qv / kt Total curret is the sum of the two diffusio curret, There is o recombiatio of carrier i the deletio regio, therefore curret remais costat there ( x 0) ( x The mius sig for (x ) has take because x is defied i the egative x directio 0) 1)

6 ecture-17 Flow of carrier i reverse bias, Excess of electros 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-6

7 Flow of carriers i forward bias E c E i lack of holes E v 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-7

8 J, Miority ecture-17 Total iode Curret P There is o recombiatio or geeratio i the deletio regio, which meas, dj/dx = 0 or J is costat (from cotiuity equatio). -x o x 0 x Total curret is costat iside the device at all oits J total Majority carrier curret Miority carrier curret -x o x 0 x Hole curret Electro curret 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-8

9 ecture-17 Observatio-1 iode -V curve Observatio-2 0 e qv / kt V 0 V 0 V few kt q l total l 0 q kt V 0 qa( 2 i A 2 i ) Observatio-3: Saturatio curret is strog fuctio of itrisic carrier coc. 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-9

10 ecture-17 Reverse saturatio curret Observatio-4: i asymmetric juctio oly lightly doed side is imortat. (Also true for eetratio of sace charge regio) 0 0 qa( qa( 2 ) i A 2 i for juctio ) for juctio V iode corresodig to blue curve will have higher reverse saturatio curret, could be due to differet material or lower doig level 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-10

11 Carrier cocetratio i P- diode log scale i2 / A A P i2 / Miority ad majority Carrier cocetratio uder forward bias coditio (ote log y- axis) -x x x i2 / A A iear scale P i2 / the reverse bias the deletio regio acts as a sik for the miority carriers (ote liear scale) -x x x 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-11

12 ecture-17 eviatio from deal Curret i excess of (that is redicated by theory) is foud i diode i small forward bias ad all reverse bias voltages the forward bias there are additio charges i the deletio regio results i the recombiatio the reverse bias there are less charges i the deletio regio (high electric field) results i the geeratio of carriers Real -V curve i the forward bias Real -V curve i the Reverse bias The reaso for the deviatio i the -V curve from the ideal is the egligece of the recombiatio-geeratio i the deletio regio the extra curret arises from the geeratio-recombiatio i the sace charge regio 8/1/2008 T Bombay, C.S. Solaki Solar Photovoltaic Techologies 17-12