Solar Photovoltaics & Energy Systems

Transcription

1 Solar Photovoltaics & Energy Systems Lecture 5. III-V, Thin Film, and Organic Solar Cells ChE-600 Wolfgang Tress, May

2 Summary Crystalline Silicon Solar Cells 35 efficiency [%] J sc = 41.8 ma cm -2, V oc = 0.74 V, FF = bandgap E g [ev] 2

3 Alternative Photovoltaic Technologies 3

4 Can we do better than Silicon? Can we do cheaper than Silicon? Can we do more than Silicon? 4

5 The Open Circuit Voltage Ideally: V oc,rad = k BT e ln J ph J em,0 + 1 Alternative expression: CB E C n E F V oc = E F n E F p band gap n = N C exp E n C E F k B T e = E g k BT ln N CN V e e np temperature recombination light intensity E V E g VB ev oc p E F p = N V exp E p F E V k B T 5

6 Increasing the Open Circuit Voltage V oc V oc = E F n E F p e = E g k BT ln N CN V e e np E g e light intensity Cooling, concentration, high absorption, light trapping (thinner layers) T 6

7 Recombination radiative electron-hole CB - - Φ em VB + via defects (SRH type) trap - + at surfaces (wrong electrode) - Au Auger recombination CB VB enters thermodynamic limit pure, defectfree material passivation, reduction of contact area material property (indirect semiconductor) unavoidable in absorber to be avoided Decrease of minority carrier lifetime 7

8 Si is an Indirect Semiconductor E g Si 1.1 ev a ideal E = e E = 0 ; E < E g 1 ; E E g a real E = 1 r E exp αd Conservation of energy and momentum Auger recombination Indirect transition low absorption coefficient at E g 8

9 III-V Semiconductors: GaAs 9

10 III-V Semiconductors: GaAs 35 efficiency [%] Si GaAs bandgap E g [ev] E g GaAs 1.42 ev η = 28 % reached Michael Rohlfing, Peter Krüger, and Johannes Pollmann: Quasiparticle band-structure calculations for C, Si, Ge, GaAs, and SiC using Gaussianorbital basis sets, Phys. Rev. B48 (1993) (doi: /PhysRevB ), Fig

11 III-V SC: Band Gap Engineering 11

12 III-V SC: Multi Junction Solar Cells Epitaxial growth Lattice matched ISE, Freiburg Presented at the 29th European PV Solar Energy Conference and Exhibition, September 2014, Amsterdam, The Netherlands Epitaxial growth Lattice matched 12

13 III-V SC: Multi Junction Solar Cells World record four-junction solar cell with η = 46 % 13

14 Multi Junction Concentrator Solar Cells 500 x concentration Cooling Direct light Sun tracking 14

15 Generations of Solar Cells 15

16 Thin Film Solar Cells: CdTe heterojunction thin International Journal of Photoenergy Volume 2013 (2013), Article ID , 6 pages

17 Thin Film Solar Cells: CIGS Alloying of CIS (CuInSe 2 ) with CGS (CuGaSe 2 ) to tune band gap ev chalcopyrite 17

18 Thin Film Solar Cells: Module Glass substrate thin film deposition laser scribing series connection of stripes 18

19 Thin Film Solar Cells: Market Share 19

20 Thin Film Solar Cells: Production 20

21 Energy Payback Time (EPBT) 21

22 Record Efficiencies 22

23 NREL Chart 23

24 Organic Solar Cells 24

25 Pi-Conjugated Molecules: LCAO Frontier orbitals: Highest Occupied Molecular Orbital (HOMO) Lowest Unoccupied Molecular Orbital (LUMO) HOMO LUMO transitions can be in visible range 25

26 Frontier Orbitals of Oligoacetenes 26

27 Charge Transport and Disorder Amorphous film distribution of sites low mobility 27

28 Excitonic Semiconductors S. E. Gledhill et al. J. Mat Res. 20, 3167 (2005) P. Würfel, CHIMIA 61, 770 (2007) Molecular film: low interactions between Van-der-Waals bound molecules molecular properties remain Low dielectric constant high exciton binding energy 28

29 Donor-Acceptor Concept energy electrode Donor Acceptor electrode

30 Steps of energy conversion nm [1] Tang, Appl. Phys. Lett. 48, 183 (1986) [2] Hiramoto et al. Appl. Phys. Lett. 58, 1062 (1991) [3] B. Maennig et al., Appl. Phys. A 79, 1 (2004) Steps of energy conversion Exciton generation via light absorption Exciton migration (diffusion) Exciton separation and dissociation Charge carrier transport Charge carrier collection/injection [3] Challenges Disorder, poor charge transport Excited states hard to split Low fill factor High recombination, low voltage Narrow absorption, but broad onset Low long-term stability 30

31 Bulk Heterojunction: Morphology Tune amount of mixing/demixing Energy vs. entropy Annealing Solvents TEM tomography Nano Lett., 2009, 9 (2), pp DOI: /nl803676e 31

32 Absorption and EQE Narrow absorption bands 32

33 EQE and JV-curve p-htl n-etl J ph depends on voltage Charge collection problem Reduces fill factor 33

34 ETL D:A HTL Metal ITO ETL D:A HTL Metal Light Management 50 nm p-htl n-etl Microcavity and Plasmonics: Marc Schiffler Thin film optics: interference Beer-Lambert law does not apply 34

35 Tandem Solar Cells Current matching Triple Junction: Florent Sahli 35

36 How to Increase Current and Voltage E Donor Acceptor E F p - + E g DA - ev oc LUMO E F n Voltage: Reduce offset energies Combination of donor and acceptor HOMO + Current: Low band gap materials Complementary absorption of donor and acceptor push-pull molecules 36

37 Tuning Absorption in Polymers 37

38 Working Principle and Role of Contacts Energy diagrams at short circuit: Modelsystem HTL = hole transport layer E E Vac HTL Donor Acceptor Al HTL Donor Acceptor Al WF - WF - E F E F + + Provide built-in field due to difference in work functions 38

39 Flat vs. Bulk Heterojunction Energy diagrams at short circuit: HTL = hole transport layer HTL Donor Acceptor Al E E Vac HTL Donor Acceptor Al blocked - diffusion Donor 8 nm Acceptor 40 nm - + no changes of V oc + HTL Donor:Acceptor Al Missaligned contacts S-Shape Build selective device! recombination + - D:A 30 nm decrease of V oc 39

40 Geminate Recombination electrode Donor Acceptor electrode Geminate recombination Non-geminate recombination

41 Charge Transfer States: hot, (de)localized? energy Vandewal et al. Nature Materials 2014 electrode Donor Acceptor - hot electrode Coulombically bound? 41

42 Charge Transfer State: Delocalized? Compare to TD DFT simulations Nature of CT states: Heewon Bahng In collaboration with U. Roethlisberger, EPFL 42

43 Charge Transfer State and V oc Donor absorption V oc,rad = 1.0 V ZnF 4 Pc:ZnPc:C 60 EQE FTPS scaled V oc,rad = 1.2 V 0.0 ZnPc : 2.0 : 3 ZnPc V oc,rad = k BT e ln J ph J em,0 + 1 J ph = e EQE E Φ AM1,5g E de 10-4 Charge-transfer absorption energy / ev J em,0 = e EQE E φ BB T 0 de 43

44 State-of-the-Art Efficiencies J sc = 16 ma/cm 2 Fullerene free Single junction η > 11 % Triple η > 13 % 44

45 Fabrication Low-Cost: Roll-to-roll Printing Slot die coating Evaporation penscetriaru.5hark.net popupcity.net 45

46 Organic Photovoltaics State of the Art Building integrated, Flexible, Colorful, Semitransparent Record η = 13.2 % o Stability - In module? % - Not really in market yet - Efficiency 46

47 Future applications Pictures: heliatek and Konarka 47

48 Summary: Organic semiconductors... have a huge potential low cost roll to roll and solution processability large area and ultra-thin devices variety of different hydrocarbons are well suited for solar cells low energy input different dyes abundant are different film consists of weakly bound molecules mostly amorphous and even intermixed films optical excitation is localized charge transport occurs mainly via hopping optical and electrical gap decoupled 48

49 Dye-Sensitized Solar Cells 49

50 Dye-Sensitized Solar Cell: Ingredients Hardin, Slideshare 50

51 Dye-Sensitized Solar Cell: Schematics Absorption on monolayer of dye, decoupled from transport Nanostructure surface area 51

52 Dye-Sensitized Solar Cell: Energetics Maçaira, J., Andrade, L. & Mendes, A. Modeling, simulation and design of dye sensitized solar cells. RSC Adv. 4, (2013). Redox potential of electrolyte fixed 52

53 History and State of the Art 1980s sensitization of TiO Nature paper: η = 7 % Solid state DSC Now: η = 13 % Dye with designated (broad or IR) spectrum Electrolyte for high voltage Yasemin Saygili 53

54 Applications Consumer: indoor? Building integrated Challenges: Efficiency, Stability 54