Photovoltaic cell and module physics and technology

Transcription

1 Photovoltaic cell and module physics and technology Vitezslav Benda, Prof Czech Technical University in Prague 6/21/2012 1

2 Outlines Photovoltaic Effect Photovoltaic cell structure and characteristics Photovoltaic cell construction and technology PV modules construction and technology Summary 6/21/2012 2

3 Solar energy TW Photovoltaics Direct transformation energy of solar irradiation into electricity

4 1. Light absorption in materials and excess carrier generation Photon energy h = hc/ (h is the Planck constant) photon momentum 0 Light is absorbed in the material. x (x) is the light intensity ( x) 0 exp( x) 0 exp xl = () is the absorption coefficient x L 1 x L is the so-called absorption length ( x) dx 0.68 ( x) dx 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 0 0 interactions with the lattice results in an increase of temperature interactions with free electrons - results also in temperature increase interactions with bonded electrons- the incident light may generate some excess carriers (electron/hole pairs) 4

5 At interaction with photons of energy h W g are generated and carrier generation increases electron-hole pairs dn G( ; x) ( ) ( ) 0( )exp ( ) x dt gen n = n 0 + Δn, p = p 0 + Δp Excess carriers recombine with the recombination rate is so called carrier lifetime In dynamic equilibrium n = p = G dn R dt rec n

6 Efficiency of excess carrier generation by solar energy depens on the semiconductor band gap Suitable materials Silicon GaAs CuInSe 2 amorphous SiGe CdTe/CdS To obtain a potential difference that may be used as a source of electrical energy, an inhomogeneous structure with internal electric field is necessary. 6

7 Suitable structures with built-in electric field: p-type Junction Radiation n-type PN junction W c W g W F W v L n SCL L p heterojunction (contact of different materials). PIN structures

8 Principles of solar cell function In the illuminated area generated excess carriers diffuse towards the PN junction. The density J PV is created by carriers collected by the built-in electric field region J PV Total generated current density H H n ( ) qg( ; x) dx q dx J sr (0) J sr ( H ) 0 0 J sr is surface recombination JPV JPV ( ) d

9 Illuminated PN junction: supperposition of photo-generated current and PN junction (dark) I-V characteristic A I I in dark V OC irradiation V V I PV illuminated I SC Solar cell I-V chacteristic and its importan points V mp V OC PhotoVoltaic Power System Course May 2010, Aalborg University 9

10 Important solar cell electrical parameters open circuit voltage V OC, short circuit current I SC maximum output power V mp I mp fill factor V FF V mp OC I I mp SC V mp V OC efficiency Vmp I P in mp V OC I P SC in FF All parameters V OC, I SC, V mp, I mp, FF and η are usually given for standard testing conditions (STC): spectrum AM 1.5 radiation power 1000 W/m 2 cell temperature 25 C. PhotoVoltaic Power System Course May 2010, Aalborg University 10

11 Modelling I-V characteristics of a solar cell R s I Parallel resistance R p I PV D R p V R L Series resistance R S PN junction I-V characteristics J 01 n 2 i D q L n n 1 p p0 J qvj J 01 exp 1 1kT Dp 1 qnd i J L n 02 p n0 sc J 02 qvj exp 2kT Output cell voltage V = V j - R s I A - total cell area A ill illuminated cell area I V R I s s AillJ PV AJ q AJ q kt kt 01 exp 1 02 exp V R I V RsI R p

12 I (A) Influence of temperature For a high R p I 01 ~ n 2 i Consequently BT 3 V OC kt q W exp kt V OC T 0 For silicon cells the decrease of V OC is about 0.4%/K g ln I I PV V (V) 25 C 35 C 45 C 55 C 65 C 75 C 85 C 95 C R s increases with increasing temperature R p decreases with increasing temperature Both fill factor and efficiency decrease with temperature FF 0 0 T T At silicon cells 0.5% K T 1 1

13 The series resistance R s influences the cell efficiency At a constant irradiance

14 PV cell (module) with a low R s the efficiency increases with irradiance PV cell (module) with a high R s The efficiency decreases with increasing irradiance

15 To maximise current density J PV it is necessary maximise generation rate G minimise losses losses optical recombination electrical reflection shadowing not absorbed radiation emitter region base region surface series resistance parallel resistance 15

16 Two types of band structure -direct (GaAs like) -undirect (Si like) Basic types of solar cells: Crystalline silicon cells Thin film cells

17 PV cells and modules from crystalline silicon (c-si) PV cells are realised from crystalline silicon wafers of thickness 0,15 0,25 mm and sides of mm c-si mono (37 %) Kerfs losses about 40% c-si multi (45 %)

18 Standard mass production (c-si cells) starting P-type wafers chemical surface texturing phosphorous diffusion SiN(H) antireflection surface coating and passivation contact grid realised by the screen print technique contact firing edge grinding cell measuring and sorting mono-crystalline 17% multi-crystalline 16%

19 Increasing cell efficiency Selective emitter Back contact cells Hetero junction cells (HIT)

20 A single solar cell ~0.5 V, about 30 ma/cm 2 For practical use it is necessary connect cells in series to obtain a source of higher voltage and in parallel to obtain a higher current Solar cell PV module PV field PhotoVoltaic Power System Course May 2010, Aalborg University 20

21 I (A) Cell connection in parallel R s R s R s R s R p R p R p R p Optimum situation: all cells have the same V MP If characteristics of individual cells in parallel differ, efficiency decreases ,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 V (V) 21

22 Cells in series.. the same current flows through all cells voltage does sums R s R s R s R s R p R p R p R p Simplified module model Optimum situation: all cells have the same I MP If characteristics of individual cells in series differ, efficiency decreases I I PV I V R I V R I ' ' s s 01exp q 1 I02 exp q 1 m 1 kt m 2kT V ' RsI R sh 22

23 PV c-si module technology soldering back covering foil (tedlar) EVA hardened glass 19

24 Module parameters open circuit voltage V OC, short circuit current I SC maximum output power V mp I mp fill factor V FF V mp OC I I mp SC Vmp I efficiency P in mp V OC STC (25 C, 1kW/m 2, AM 1,5) I P SC in FF NOCT (Nominal Operating Conditions Temperature) Ambient temperature 20 C, 800 W/m 2, wind 1 m/s

25 Basic types of solar cells: Crystalline silicon cells Thin film cells c on ta c t a ntire fle c tio n c o a tin g N -ty p e P -ty p e Suitable materials Basic problem: cost... CuInSe 2 amorphous silicon amorphous SiGe CdTe/CdS 25

26 Thin film solar cells CIS CdTe/CdS Amorphous Si Market share: 1.5% 5.7% 4.7% 26

27 Amorphous silicon solar cells TCO: SnO 2 ITO (indium-tin oxide) ZnO 600nm 1% <12% substrate TCO Light trapping TCO Ag or Al contact 27

28 Plasma enhanced CVD (PECVD) RF electrode and substrate create the capacitor structure. In this space the plasma and incorporated deposition of material on substrate takes place deposition of silicon nitride 3SiH 4 + 3NH 3 Si 3 N H 2 deposition polysilicon layers SiH 4 Si + 2H 2. 30

29 Thin film solar cell technology Amorphous (microcrystalline) silicon solar cells transparent substrate (glass) TCO a-si:h p+ layer (20-30 nm) a-si:h intrinsic ( 250 nm) a-si:h n+ layer (20 nm) TCO (diffusion barrier) Ag or Al SiH 4, H 2, B 2 H 6, PH 3, GeH 4, etc 31

30 Tandem cells W g1 > W g2 irradiation 30

31 Thin film modules on glass substrates The module area is limited by the reaction chamber volume Very expensive equipment 35

32 Market share development 2011

33 PV module cost development Reduction of silicon cost USD/kg USD/kg USD/kg Reduction of C-Si module cost Thin-film modules are not cheaper than modules from crystalline silicon 33