Chapter 9: Photovoltaic Devices

Transcription

1 Chapter 9: Photovoltaic Devices Solar energy spectrum Photovoltaic device principles pn junction photovoltaic I-V characteristics Series and shunt resistance Temperature effects Solar cell materials, devices, and efficiencies Current solar cell research PHYS5320 Chapter Nine 1

2 Photovoltaic devices or solar cells convert incident solar radiation energy into electrical energy. Incident photons are absorbed to photogenerate charge carriers that pass through an external load to do electrical work. Photovoltaic device applications range from calculators (~mw) to power plants (~MW). PHYS5320 Chapter Nine 2

3 Why Solar Energy? Clean Nearly unlimited Sunlight striking the earth in 1 hour: J All energy consumed on the earth recently per year: J PHYS5320 Chapter Nine 3

4 Solar cell plant PHYS5320 Chapter Nine 4

5 Cars and airplanes powered by photovoltaic devices PHYS5320 Chapter Nine 5

6 Solar Energy Spectrum Solar energy spectrum is typically represented by power per unit area per unit wavelength. The integrated intensity above Earth s atmosphere gives the total power flow through a unit area perpendicular to the direction of the sun. This quantity is called the air-mass zero (AM0) radiation and it is approximately constant at a value of kw m 2. PHYS5320 Chapter Nine 6

7 Solar Energy Spectrum The actual intensity spectrum on Earth s surface depends on the absorption and scattering effects of the atmosphere, which increase with the sun beam s path through the atmosphere. The shortest path through the atmosphere is when the sun is directly above the location and the received spectrum is called air mass one (AM1). Air mass m is defined as the ratio of the actual radiation path to the shortest path. AMm = AM(1/cos). PV devices can be tilted to face the sun and maximize the collection efficiency. The terrestrial light has a diffuse component (due to scattering) in addition to the direct component. PHYS5320 Chapter Nine 7

8 Photovoltaic Device Principles short λ The heavily doped n-region is very narrow and most photons are absorbed in the depletion region and within the neutral p-side. EHPs generated in the depletion region are immediately separated by the built-in field. The electrons drift to the n-side and the holes drift to the p-side. PHYS5320 Chapter Nine 8

9 Photovoltaic Device Principles Finger electrodes on the n-side to allow illumination to enter the device. Anti-reflection coating. An open circuit voltage is established with the p-side positive with respect to the n-side. The photogenerated electrons in the neutral p-region will diffuse into the depletion region and drift to the n-side only if they are within the minority carrier diffusion length L e to the depletion region. Otherwise, they will recombine with the holes and get lost. The photogenerated holes in the neutral n-region within the minority carrier diffusion length L h to the depletion region will also probably contribute the photovoltaic effect. PHYS5320 Chapter Nine 9

10 Photovoltaic Device Principles Losses for PV devices made of silicon: (1) Bandgap energy ~1.1 m. Incident photons with wavelengths larger than 1.1 m are wasted (~25%). (2) High-energy photons are absorbed near the crystal surface and get lost by the recombination in the surface region (~40%). (3) Reflection loss (~1020%). The upper limit to the efficiency of single-crystalline silicon PV cells is 2426% at room temperature. PHYS5320 Chapter Nine 10

11 Chapter 9: Photovoltaic Devices Solar energy spectrum Photovoltaic device principles pn junction photovoltaic I-V characteristics Series and shunt resistance Temperature effects Solar cell materials, devices, and efficiencies Current solar cell research PHYS5320 Chapter Nine 11

12 pn Junction Photovoltaic I-V Characteristics Consider a pn junction PV cell. The current and voltage in the figure follow the conventional definition of positive current and positive voltage for pn junctions. The photocurrent I ph is proportional to the light intensity I. The current in the case of a short circuit is I sc : I sc I ph KI PHYS5320 Chapter Nine 12

13 pn Junction Photovoltaic I-V Characteristics A current passing through a load resistor will generate a forward bias voltage on the pn junction. There will be a forward diode current. I d ev I 0 exp 1 nkbt I ev I ph Io exp 1 nkbt I V R PHYS5320 Chapter Nine 13

14 pn Junction Photovoltaic I-V Characteristics The power delivered to the load is P out = IV, which is the rectangle shown by the dashed lines in the figure. Maximum power is delivered to the load when the rectangular area is maximized (by changing R or the intensity of illumination). The fill factor is defined as FF IV I sc V max oc PHYS5320 Chapter Nine 14

15 Limiting Factors on V oc ev OC E g 1 T T sun kt ln emit sun ln 4n I 2 ln QE The first term arises from fundamental thermodynamic losses based on Carnot s theorem. The terms in the square brackets account for three entropyrelated contributions: 1st: entropy increase caused by photon absorption and reradiation; 2nd: the effect of incomplete light trapping. 3rd: the loss owing to non-radiative exciton recombination. A. Polman, H. A. Atwater, Nat. Mater. 2012, 11, 174. PHYS5320 Chapter Nine 15

16 Chapter 9: Photovoltaic Devices Solar energy spectrum Photovoltaic device principles pn junction photovoltaic I-V characteristics Series and shunt resistance Temperature effects Solar cell materials, devices, and efficiencies Current solar cell research PHYS5320 Chapter Nine 16

17 Series and Shunt Resistance Effective series resistance R s : carriers must travel through the neutral n- or p-regions to the electrodes. Shunt or parallel resistance R p : a fraction of the photogenerated carriers can flow through the edges of the device or through grain boundaries in polycrystalline devices instead of flowing through the external load R L. PHYS5320 Chapter Nine 17

18 Series and Shunt Resistance Both series and shunt resistances can significantly deteriorate the solar cell performance. The available maximum output power decreases with the series resistance. In order to improve the cell efficiency, we need to reduce the series resistance and maximize the shunt resistance. PHYS5320 Chapter Nine 18

19 Temperature Effects When the circuit is open, evoc I ph Id I0 exp 1 nkbt I ph I V 0 evoc exp nkbt oc I 0 nk BT ln I e I ed p p A Lp ph 0 n0 ed n n L n p0 An 2 i ed Lp N PHYS5320 Chapter Nine 19 p d ed I ph = KI, where K is a constant and I is the light intensity. L n n N a

20 PHYS5320 Chapter Nine 20 Temperature Effects Assume n =1 at two different temperatures T 1 and T 2, but at the same illumination level: 2 i2 2 i B oc1 2 B oc2 ln ln n n I I T k ev T k ev 0 B oc ln I KI T nk ev T k E N N n B g v c 2 i exp If we neglect the temperature dependences of N c and N v, we obtain: B g 1 2 B g 1 B oc1 2 B oc T T T k E T T k E T k ev T k ev 1 2 g 1 2 oc1 2 oc 1 T T e E T T V V

21 Temperature Effects A silicon solar cell has V oc1 = 0.55 V at 20 C (T 1 = 293 K). V oc2 at 60 C (T 2 = 333 K) will be given by V oc V The output voltage and the efficiency of a solar cell increase with decreasing temperatures. PHYS5320 Chapter Nine 21

22 Chapter 9: Photovoltaic Devices Solar energy spectrum Photovoltaic device principles pn junction photovoltaic I-V characteristics Series and shunt resistance Temperature effects Solar cell materials, devices, and efficiencies Current solar cell research PHYS5320 Chapter Nine 22

23 Solar cell materials PV Module Production by Region PHYS5320 Chapter Nine 23

24 PV Production by Technology Solar cell production is still dominated by multicrystalline Si (multi Si, 55.2% in 2014), monocrystalline Si (mono Si, 35.6% in 2014). PHYS5320 Chapter Nine 24

25 Market Share of Thin-Film Technologies Thin film solar cells are getting increasing attention. They are mainly made of CdTe, amorphous Si (a Si) and copper indium( gallium) selenide [CI(G)S]. PHYS5320 Chapter Nine 25

26 Solar Cell Efficiencies The efficiency of a solar cell is one of its most important characteristics because it allows the device to be assessed economically in comparison to other energy conversion devices. The cost of a solar cell is also important and it is often expressed as the cost per unit electrical power generation. A low-cost goal is established at US$ /kWh for solar power. This goal requires a cell conversion efficiency of about 50% and with a total cost of US$125/m 2. PHYS5320 Chapter Nine 26

27 Solar Cell Efficiencies PHYS5320 Chapter Nine 27

28 Solar Cell Devices Inverted pyramid textured surface substantially reduces reflection losses and increases absorption probability in the solar cell device. PHYS5320 Chapter Nine 28

29 Solar Cell Devices Use AlGaAs window layer on GaAs to passivate surface states and thereby increase the short-wavelength photogeneration efficiency. AlGaAs has a wider bandgap than GaAs, allowing photons to pass through it. The lattices of AlGaAs and GaAs are matched to reduce defects. PHYS5320 Chapter Nine 29

30 Solar Cell Devices Heterojunction solar cells: energetic photons (h > 2 ev) are absorbed in AlGaAs. Less energetic photons (1.4 ev < h < 2 ev) are absorbed in GaAs. PHYS5320 Chapter Nine 30

31 Solar Cell Devices Tandem or cascaded cells: GaAs/GaSb, a-si:h/a-si:ge:h (amorphous and hydrogenated) PHYS5320 Chapter Nine 31

32 Chapter 9: Photovoltaic Devices Solar energy spectrum Photovoltaic device principles pn junction photovoltaic I-V characteristics Series and shunt resistance Temperature effects Solar cell materials, devices, and efficiencies Current solar cell research PHYS5320 Chapter Nine 32

33 The theoretical efficiency limits for solar cells can be estimated in a number of ways, ranging from the constraint placed by the second law of thermodynamics to realistic models. In 1961, Shockley and Queisser described the detailed-balance limit of efficiency for pn junction solar cells, balancing the radiative transfer between the sun and the solar cell modeled as blackbodies. They calculated an efficiency limit of 31% for a 1.3-eV material under an AM 1-sun configuration and 41% for a 1.1-eV material when the light is maximally concentrated. In the Shockley-Queisser analysis, two major factors limit the conversion efficiency to 31%: (1) the excess kinetic energy of hot photogenerated carriers created by the absorption of photons above the band gap is lost as heat through phonon emission (thermalization losses), and (2) photons less than the band gap are not absorbed. PHYS5320 Chapter Nine 33

34 Multijunction solar cells Photon utilization is significantly improved. GaInP: Ga 0.35 In 0.65 P GaInAs: Ga 0.83 In 0.17 As PHYS5320 Chapter Nine 34

35 Sunlight concentration Concentrators are a strategy for reducing the semiconductor material in photovoltaic cells. Typical concentration ratios are PHYS5320 Chapter Nine 35

36 Sunlight concentration Achieved efficiencies (symbols) and theoretical efficiency limits (solid lines) estimated by the detailed-balance method as a function of the number of junctions and the structure of the material for AM1-sun and concentrated illumination. Higher material quality is correlated with higher efficiencies. PHYS5320 Chapter Nine 36

37 Semiconductors with tunable band gaps The lines indicate the data for ternary alloys. The solid lines correspond to direct-bandgap semiconductors, and the broken lines to indirect-bandgap semiconductors. The black, gray, and red dots indicate binary III V, II VI, and a quaternary III V alloy, respectively. PHYS5320 Chapter Nine 37

38 High-efficiency multijunction solar cells Advanced high-efficiency multijunction solar cell concepts and their thermodynamic efficiency limits under 500 suns using the AM1.5 direct spectrum. The black bar indicates a grading in the lattice constant in these metamorphic approaches. Each subcell consists of a pn junction, front and back surface passivation layers, with as many as 50 layers total. PHYS5320 Chapter Nine 38

39 Goals of solar cell research: (1) Reduce production and materials cost (2) Improve power conversion efficiency Four types of unconventional solar cells: (1) Plastic solar cells (2) Dye-sensitized solar cells (3) Semiconductor nanostructure-based solar cells (4) Perovskite solar cells PHYS5320 Chapter Nine 39

40 Plastic Solar Cells The use of organic semiconductors, including conjugated polymers and small organic molecules, combine the photoelectrical properties of inorganic semiconductors with the large-scale, lowcost technology of plastic materials. PHYS5320 Chapter Nine 40

41 Plastic Solar Cells Schematic representation of a bulk heterojunction (BHJ) solar cell, showing the phase separation between donor (red) and acceptor (blue) materials. The use of the BHJ structure greatly increases the interface area between the donor and acceptor materials. PHYS5320 Chapter Nine 41

42 (a) Schematic of the device architecture of a polymer bulk heterojunction solar cell. (b) TEM image of a thin slab of an actual device, showing the individual layers: glass, SiO 2, indium tin oxide (ITO), PEDOT:PSS, MDMO-PPV:PCBM (1:4 by weight), LiF, and Al layers. (c) AFM phase and TEM images (1 1 m 2 ) of an MDMO-PPV/PCBM (1:4 by weight) composite film, showing phase separation into a PCBM- (red in AFM, dark gray in TEM) and polymer-rich phase (green in AFM, light gray in TEM). PHYS5320 Chapter Nine 42

43 Plastic Solar Cells PHYS5320 Chapter Nine 43

44 Donor and acceptor materials used in plastic bulk heterojunction solar cells. Donors: MDMO-PPV = poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene]; P3HT = poly(3-hexylthiophene); PFDTBT = poly[2,7-[9-(2-ethylhexyl)-9- hexylfluorene]-alt-5,5-(4,7-di-2-thienyl-2,11,3-benzothiadiazole)]. Acceptors: PCBM = 3-phenyl-3H-cyclopropa[1,9][5,6]fullerene-C 60 -I h -3-butanoic acid methyl ester; [70]PCBM: 3-phenyl-3H-cyclopropa[8,25][5,6]fullerene-C 70 -D 5h (6)-3-butanoic acid methyl ester. PHYS5320 Chapter Nine 44

45 Many applications will benefit from rollable/foldable solar cells. Cars, aircraft and various electric appliances can cover part of their power demand from ambient illumination of their free form cases. The integration of solar cells with textiles is not only interesting for powering portable devices, but also opens a wealth of opportunities for the integration of electronic features with architectural fabrics. PHYS5320 Chapter Nine 45

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47 One recent example of polymer solar cells (tandem): Y. Yang et al., Nat. Commun. 2013, 4, PHYS5320 Chapter Nine 47

48 Y. Yang et al., Nat. Commun. 2013, 4, PHYS5320 Chapter Nine 48

49 Dye-Sensitized Solar Cells (DSSCs) Light absorption and charge separation separated Efficiency up to ~12% Use of liquid electrolyte Brian O Regan, Michael Grätzel, Nature 1991, 353, 737. A. Yella et al., Science 2011, 334, 629. PHYS5320 Chapter Nine 49

50 Solar cells based on quantum dots In the Shockley-Queisser analysis, two major factors limit the conversion efficiency: (1) The excess kinetic energy of hot photogenerated carriers created by the absorption of photons above the bandgap is lost as heat through phonon emission (thermalization losses); (2) Photons less than the bandgap are not absorbed. Multiple exciton generation (MEG) in quantum dots can lead to enhanced solar photon conversion efficiency in QD solar cells. PHYS5320 Chapter Nine 50

51 Quantum efficiency for exciton formation from a single photon versus photon energy expressed as the ratio of the photon energy to the quantum dot bandgap for three PbSe QD sizes (3.9, 4.7, 5.4 nm in diameter), one PbS and PbTe size (5.5-nm diameter for both), and bandgap energy E g (0.91 ev, 0.82 ev, 0.73 ev, 0.85 ev, and 0.91 ev, respectively). PHYS5320 Chapter Nine 51

52 To capture and use photons less than the bandgap energy, intermediate band (IB) materials can be used. The figure shows the IB in the bandgap formed from an array of QDs and the possible optical transitions. CB is conduction band, and VB is valence band. PHYS5320 Chapter Nine 52

53 Variously Shaped QDs PHYS5320 Chapter Nine 53

54 Configurations for quantum dot solar cells. Right: QDs used to sensitize a nanocrystalline TiO 2 film to visible light. This configuration is analogous to the dyesensitized solar cell where the dye is replaced by QDs. Left: QDs dispersed in a blend of electronand hole-conducting polymers; SC stands for semiconductor. The occurrence of MEG can produce higher photocurrents and higher conversion efficiency. PHYS5320 Chapter Nine 54

55 CdSe x Te 1 x quantum dots are used. ZnS/SiO 2 strongly suppresses back recombination. PHYS5320 Chapter Nine 55

56 TBAI: tetrabutylammonium iodide EDT: 1,2-ethanedithiol PHYS5320 Chapter Nine 56

57 Perovskite solar cells (CH 3 NH 3 PbI 3-x Cl x ) Y. Yang et al., Science 2014, 345, 542. PHYS5320 Chapter Nine 57

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59 W. Y. Nie et al., Science 2015, 347, 522. PHYS5320 Chapter Nine 59

60 Perovskite solar cells (CH 3 NH 3 PbI 3-x Cl x ) Large absorption coefficient High carrier mobilities Very small trap density Long carrier lifetime High carrier diffusion length CH 3 NH 3 PbI 3 Electron mobility: 24 cm 2 V 1 s 1 Hole mobility: 164 cm 2 V 1 s 1 Carrier recombination lifetimes: ~100 s (1 sun); ~200 s (0.1 sun) Carrier diffusion lengths: 175 m (1 sun); 3 mm (0.1 sun) Q. F. Dong, Y. J. Fang, Y. C. Shao, P. Mulligan, J. Qiu, L. Cao, J. S. Huang, Science 2015, 347, 967. PHYS5320 Chapter Nine 60

61 Transparent conducting oxides (TCOs) for solar cells TCOs are an increasingly important component of solar cell devices, where they act as electrode elements, structural templates, and diffusion barriers, and their work function controls the open-circuit device voltage. They are employed in applications that range from crystalline-si heterojunctions to organic polymer solar cells. The desirable characteristics of TCO materials include high optical transmissivity across a wide spectrum and low resistivity. Additionally, TCOs for terrestrial solar cell applications must use low-cost materials. Materials that combine optical transparency over much of the solar spectrum with reasonable electrical conductivity generally fall into three classes: very thin pure metals, highly doped conjugated organic polymers, and degenerately doped wide-bandgap oxide or nitride semiconductors. PHYS5320 Chapter Nine 61

62 PHYS5320 Chapter Nine 62

63 This solar cell has achieved record efficiencies (19.2%) for a simple cell geometry. PHYS5320 Chapter Nine 63

64 Copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells have demonstrated efficiencies near 20% in the laboratory and have already been in commercial production. PHYS5320 Chapter Nine 64

65 New Materials: (1) Transparent, Conductive Carbon Nanotube Films (A) Transparent SWCNT films of the indicated thickness on quartz substrates. (B) A large, 80-nm-thick film on a sapphire substrate 10 cm in diameter. (C) Flexed film on a Mylar sheet. (D) AFM image of a 150- nm-thick SWCNT film surface. Z. C. Wu et al. Science 2004, 305, PHYS5320 Chapter Nine 65

66 New Materials: (1) Transparent, Conductive Carbon Nanotube Films PHYS5320 Chapter Nine 66

67 New Materials: (1) Transparent, Conductive Carbon Nanotube Films MWCNT forest conversion into sheets. (A) Photograph of a self-supporting 3.4-cm-wide, meter-long MWCNT sheet that has been hand drawn from a nanotube forest at an average rate of 1 m/min. (B) SEM image of a MWCNT forest being drawn into a sheet. (C) SEM micrograph showing the cooperative 90-degree-rotation of MWCNTs in a forest to form a sheet. (D) SEM micrograph of a 2D rereinforced structure fabricated by overlaying four nanotube sheets with a 45-degree-shift in orientation between successive sheets. PHYS5320 Chapter Nine 67

68 New Materials: (1) Transparent, Conductive Carbon Nanotube Films Optical transmittance versus wavelength for a single MWCNT sheet, before and after densification, for light polarized perpendicular to () and parallel to ( ) the draw direction and for unpolarized light (the two curves on the right), where the arrow points from the data for the undensified sample to those for the densified sample. M. Zhang et al. Science 2005, 309, PHYS5320 Chapter Nine 68

69 New Materials: (2) Transparent, Conductive Metal Nanowire Films (a) Schematic of the materials method. CuAc 2 /PVA composite fibers were prepared by electro-spinning. The fibers were calcined in air to get CuO nanofibers. The CuO nanofibers were reduced to Cu nanowires by annealing in a H 2 atmosphere. (b) SEM image of Cu nanowires. Scale bar is 10 m. (f) Schematic of a modified electro-spinning setup. (g,h) SEM images of Cu nanowires with controlled orientations. The scale bars are 20 m. PHYS5320 Chapter Nine 69

70 New Materials: (2) Transparent, Conductive Metal Nanowire Films H. Wu et al. Nano Lett. 2010, 10, PHYS5320 Chapter Nine 70

71 New Materials: (3) Transparent Graphene as Electrodes Schematic of the roll-based production of graphene films grown on a copper foil. The process includes adhesion of polymer supports, copper etching (rinsing) and dry transfer-printing on a target substrate. A wet-chemical doping can be carried out using a setup similar to that used for etching. S. Bae et al. Nat. Nanotechnol. 2010, 5, 574. PHYS5320 Chapter Nine 71

72 New Materials: (3) Transparent Graphene as Electrodes Photographs of the roll-based production of graphene films. (a) Copper foil wrapping around a 7.5-inch quartz tube to be inserted into an 8-inch quartz reactor. The lower image shows the stage in which the copper foil reacts with CH 4 and H 2 gases at high temperatures. (b) Roll-to-roll transfer of graphene films from a thermal release tape to a PET film at 120 C. (c) A transparent ultra-large-area graphene film transferred on a 35-inch PET sheet. (d) Screen printing process of silver paste electrodes on graphene/pet film. The inset shows 3.1-inch graphene/pet panels patterned with silver electrodes before assembly. (e) An assembled graphene/pet touch panel showing outstanding flexibility. (f) A graphene-based touchscreen panel connected to a computer with control software. PHYS5320 Chapter Nine 72

73 New Materials: (3) Transparent Graphene as Electrodes PHYS5320 Chapter Nine 73

74 Reading Materials S. O. Kasap, Optoelectronics and Photonics: Principles and Practices, Prentice Hall, Upper Saddle River, NJ 07458, 2001, Chapter 6, Photovoltaic devices. PHYS5320 Chapter Nine 74