BASIC CONCEPTS OF Photovoltaics

BASIC CONCEPTS OF Photovoltaics
Presentations are at BASIC CONCEPTS OF Photovoltaics parts based on tutorials of Isaac Balberg Hebrew Un. Jerusalem and Gary Hodes Weizmann Inst. 1st practical PV cell: Bell Labs, 1954: 6% efficiency Basic Physics and Materials Science of Solar Cells J. M. Pearce,

From atomic levels to bands
Band Gap EG E : distance between atoms where system’s energy is minimized

Energy Levels Fermi Level Vacuum An Atom Energy E = 0 Filled States
Chemistry is controlled by the states around the filled/empty transition, i.e., around the …… Fermi Level Fermi Level Vacuum An Atom Energy E = 0 Filled States Empty A Small Molecule A Large Molecule HOMO LUMO Bulk Material The chemical behaviour of any substance is dictated by its energy levels: their energy, momentum, and density. Even the smallest system, an atom, has an infinite number of energy levels, though only afew are occupied by electrons in its lowest energy condition. As atoms combine to form molecules, the number of states is still infinite but the density of states is spread over a wider range of energy levels. A large molecule is even more complex and finally bulk material has such a high density of states that there seems to be a continuum of states at all energies. Chemistry is controlled by the exchange of electrons around the transition from filled to empty states. In molecules, these are called the LUMO and HOMO states. In a solid they are called the conduction and valence bands. The Fermi level in a solid metal is like the HOMO in a molecule. Dan Thomas, Univ. Guelph, Canada

Band Structure Semiconductor Insulator Metal Large Small, but non-zero
ΔE between filled and empty states Small, but non-zero ΔE between filled and empty states Infinitesimal energy difference (ΔE) between filled and empty states Semiconductor Conduction Band Insulator Conduction Band Valence Band Band Gap ΔE = EG Core Bands Vertical direction corresponds to increasing energy. Horizontal direction suggests relative number of states at a given energy. Simplified picture is that the state density is constant. Not true by a long shot, but we use it in simple picture because there are so many states (order of Avogadro’s number) that it is not a bad first scheme. Metal Dan Thomas, Univ. Guelph, Canada

From energy levels to bands
HOMO LUMO 1 e- energy 4 (8) If EG < ~ x kTB  semiconductor EG EV EC CB VB

Fermi Level • focus on the electrons near the filled/empty boundary.
• each material’s energy state distribution is unique; different EF. E=0 (vacuum level) EF (Fermi level) Minimum energy to remove electron from sample, work function Metal 1 Metal 2 EF (Fermi level) Molecules have chemical bond strengths of a few electron-volts. Bonds are formed by some atoms giving up their electrons and other atoms accepting those electrons. Hence, the important energy states, as far as chemical bond formation is concerned, lie within a few eV of the Homo-Lumo gap. In molecules, that may very well be just the HOMO and the LUMO, but in the solid state, there are many (almost an infinitude) of states near the Fermi level. It is the details of the state density that makes one solid’s chemical behaviour different from another’s. • the closer an electron is to the vacuum level, the weaker it is bound to the solid • or, the more energetic is the electron Dan Thomas, Univ. Guelph, Canada

Two Conductors in Contact
electron flow – + leads to charge separation Contact potential difference Note that the vacuum level shifts for the two materials. This is because the charge that is transferred establishes a potential that produces the shift in all the states of the sample. Fermi level equal throughout sample Dan Thomas, Univ. Guelph, Canada

Metal in an Electrolyte Solution
Charge is transferred to equilibrate Fermi level with redox potential, producing a charge separation and a contact potential difference. Redox potential = Electrochemical potential of the electron ≈ Fermi level Fermi levels are aligned + – Charge distribution shown does not have any significance with respect to the energy axis. It is only meant to show that a charge distribution appears at the interface. (More on this tomorrow.) Dan Thomas, Univ. Guelph, Canada

Two Electrolyte Solutions
A charge separation arises to align the “Fermi” level (= redox potential) and produces a potential at the interface. “Fermi” level There is even a junction potential produced when the two solutions are of the same substance, but at a different concentration. + – Dan Thomas, Univ. Guelph, Canada

An Ion in Solution • ion’s electronic structure: HOMO, LUMO, HOMO-LUMO gap. Lowest Unoccupied Molecular Orbital HOMO-LUMO Gap “Fermi” level Highest Occupied Molecular Orbital EF Energy levels have breadth because of intramolecular energy states (rotation, vibration) and interactions with the solvent. The Fermi level is in quotes because a Fermi level is actually ionly present for a metal, but the concept is useful here so we use it loosely. Dan Thomas, Univ. Guelph, Canada

Electrochemical Thermodynamics
Every substance has a unique propensity to contribute to a system’s energy. We call this property Chemical Potential. m When the substance is a charged particle (such as an electron or an ion) we must include the response of the particle to an electrical field in addition to its Chemical Potential. We call this Electrochemical Potential. These are perhaps the most fundamental measures of thermodynamics. Z is the charge on the particle, F is Faraday’s constant, phi is the field under consideration. EF = Fermi level (~electrochemical potential of electrons = Dan Thomas, Univ. Guelph, Canada

Semiconductor doping “Doping” – deliberate introduction of impurities into a high-purity, low-defect semiconductor crystal Ofer Sinai,

Semiconductor doping “Doping” – deliberate introduction of impurities into a high-purity, low-defect semiconductor crystal Impurity content is low  host chemical/crystalline properties preserved Nevertheless, impurities completely dominate the electrical behavior Ofer Sinai,

Semiconductor doping Intrinsic semiconductor  very low room temp., Si intrinsic carrier concentration ≈ 1010 cm-3 (Cu: ~1023 cm-3) Ofer Sinai,

Impurities introduce free charge carriers
Semiconductor doping Impurities introduce free charge carriers P B

Impurities introduce free charge carriers
Semiconductor doping Impurities introduce free charge carriers P Donor impurities Negative charge carriers n-type semiconductor Ofer Sinai,

Semiconductor doping Impurities introduce free charge carriers P B
Acceptor impurities Positive charge carriers (holes) p-type semiconductor Ofer Sinai,

Semiconductor Materials

Electrons and holes in semiconductors
Formation of p-type semiconductor B3+ ---> 3e- - e- acceptors (NA) 1 e- energy B C N Al Si P Ga Ge As

Formation of n-type semiconductor
Electrons and holes in semiconductors Formation of n-type semiconductor P5+ ---> 4e-+ e- donors (ND) 1 e- energy B C N Al Si P Ga Ge As

Impurities determine conduction
Si intrinsic carriers: ~ cm-3 Si atom density: ~5∙1022 cm-3 E.g., a ppm impurity can increase the amount of carriers a million-fold! Between doping rates of 1013 – 1020 cm-3, doping determines Carrier concentration Carrier polarity Ofer Sinai,

What is the effect of doping?
The Fermi level, EF, is a key parameter Intrinsic  EF is near the center of the forbidden gap E E Conduction band (CB) EGap EFermi Valence band (VB) Fermi-Dirac distribution Ofer Sinai,

The concept of the Fermi level

Donor impurities add occupied levels near the CB edge Added free electrons  Fermi level is raised E E Conduction band (CB) EFermi Valence band (VB) Ofer Sinai,

Acceptor impurities add unoccupied levels near VB edge Added free holes  Fermi level is lowered E E Conduction band (CB) EFermi Valence band (VB) Ofer Sinai,

Basic component in electronics
The p-n junction Basic component in electronics E ≈ Conduction band Valence band Local vacuum level n-type side EFermi Conduction band Valence band Local vacuum level EFermi p-type side Ofer Sinai,

Basic component in electronics
The p-n junction Basic component in electronics E ≈ Conduction band Valence band Local vacuum level Conduction band Valence band Local vacuum level n-type side p-type side Ofer Sinai,

Charge carriers diffuse in both directions
The p-n junction Charge carriers diffuse in both directions + – E ≈ Conduction band Valence band Local vacuum level Conduction band Valence band Local vacuum level Conduction band Valence band Local vacuum level n-type side p-type side Ofer Sinai,

A space-charge region (SCR) is formed
The p-n junction A space-charge region (SCR) is formed E ≈ Local vacuum level Conduction band EFermi Valence band n-type side p-type side Ofer Sinai,

The junction is rectifying:
The p-n junction The junction is rectifying: n-type side p-type side Ofer Sinai,

The p-n junction – + Forward bias: n-type side p-type side
Ofer Sinai,

The p-n junction + – Reverse bias: n-type side p-type side
Ofer Sinai,

The energy diagram of the p-n junction
space charge region

The p-n (homo)junction
I = 0 actually means Idiffusion = -Idrift (= -I0)

The p-n junction under forward bias
# of carriers that can diffuse over the potential barrier increases by Boltzmann factor exp(qV/kT) : Idiffusion = I0 [exp(qV/kT)] (I0 ~ same as in equilibrium) As V  , the current  ;  diffusion current dominates in the circuit

The concept of the quasi-Fermi level

Forward & reverse bias in p-n junction
@ large |Vreverse|  “breakdown”

Reverse current saturates @ I0 Large |V|  “breakdown”.
The p-n junction under reverse bias Reverse current I0 For small |V|, I0 is maintained; Large |V|  “breakdown”.

The photoelectric effect

Photon-induced electron-hole pair generation in a semiconductor
Photon enters, is absorbed, and excites electron from VB to CB  hole left in VB,  electron-hole pair. incident photon

Photoexcitation in the p-n junction
In illuminated junction, IL is drift current, due to e--h+ hole generation, which adds to I0, the diffusion current. Illumination yields a drift photocurrent, IL

Back to Space Charge layers Light absorption I = I0e-ad 2ee0V W =
qND(A) 1/2 W = Width of space charge layer inversely proportional to [doping density]1/2 Typical widths of space charge layer: N = 1022/cc (metallic) ~1-2 atomic layers, tenths of nm N = 1018/cc (heavily doped semiconductor) 10s of nm N = 1016/cc (medium doped semiconductor) 100s of nm N = 1014/cc (low doped semiconductor) few µm space charge layer 2 (21) Light absorption I = I0e-ad In a PV cell, the width of the space charge layer should be wide enough to absorb most of the light in the E-field region –a few 100 nm in a typical cell.

@Voc, Idiff is so large that it cancels (I0+IL)
The p-n junction under illumination I = Idark - IL Voc = (kT/q) . ln(IL/I0+1) @Voc, Idiff is so large that it cancels (I0+IL)  Voc is determined by the light-induced drift current

What a Semiconductor is good at: p-n junction = photocurrent ‘slide’
Light Long-lived excess electrons and holes Dad Internal field creates (minority) carrier drift

Solar Cell Summary Conventional p-n junction Absorb light
Absorbed light creates carriers Carrier collection, by diffusion, drift space 1 e- energy after textbooks & R. Collins, CSM

Solar Cell Summary Conventional p-n junction + I-V characteristic
Voc = V Jsc = 26.7 mA/cm2 FF= 73.3%  = 11.8% Absorb light Absorbed light creates carriers Carrier collection, by diffusion, drift space 1 e- energy after textbooks & R. Collins, CSM

Inside a p/n junction Solar Cell

1954 2014 Chapin Fuller Pearson

Solar cell generations
Si (crystalline) cells : 1st generation cells (thin film) CdTe, CIGS, α-Si : 2nd generation cells Dye cells, organic cells and related ones : 3rd generation cells 1 (44) There are newer ones and ‘generation number’ becomes fuzzy at this stage

Solar Cell (r)evolutions
3 d generation nano crystalline ~ 20 nm TiO2 Organic (polymer/ small molecule) 1st generation Si 2nd generation CdTe, CIGS poly-crystalline m amorphous (a-Si:H; polymers) Single- crystalline I added a question mark and animation cm 51

Current Types of PV Cells
Primarily based on solid-state electronic material systems Elemental Semiconductors Single or multi-crystal Polycrystalline films Amorphous thin film Inorganic Compound Semiconductors Single crystal Polycrystalline thin film Organic, Excitonic (molecules, polymer) Interpenetrating network Nanocrystalline; dye-sensitized thin films ……………………………………………………… Si,Ge (non) concentrator; single-& multi- junction homo- & hetero-junction; photoelectrochem; MIS-inversion (Ga,In)(As,P) Cu(In,Ga)Se2 CdTe P3HT/PCBM; porphyrins ++ perovskites* dye+TiO2 (ZnO)

Definition of efficiency:
Single Junction PV Lab Cells: Present Status (1-4 cm2 ; most tandems are much smaller) ~ [74%] 26% single crystal Si; ([71%] ~29 % GaAs) ~ [79%] ~21% single jctn. PX thin films (CIGS,CdTe,Si) ~ [88%] ~12% dye sensitized solar cell (DSSC) ~ [89%] ~11% organic molecule, polymer, perovskite) ~ [62%] 38% “big Mac” tandem 3-junction (~ [54%] 46% “bigger Mac” tandem 4-junction +concentration) Definition of efficiency: Data from Solar Cell Eff #41, Progr in PV 2013 and other sources

The Photovoltaic (PV) effect:
Generalized picture High energy state Low Absorber one electron energy space e- p+ contact Metastable high and low energy states Absorber transfers charges into high and low energy state Driving force brings charges to contacts Selective contacts (1) cf. e.g., Green, M.A., Photovoltaic principles. Physica E, 14 (2002) 11-17

Basics of photovoltaic cells
Charge separation in space e- hn h+ EC EV EF e- h+ hn Charge separation in energy 1 e- energy 1 e- energy space coordinate

Basics of photovoltaic cells
hn h+ Amps @ short circuit VOC Volts @ open-circuit V load @maximum power

Dark- and Photo- I-V (current-voltage) characteristics of a PV cell
VOC Voltage ISC max power 8 (29) fill factor = (I mp . Vmp) / (I SC . VOC) mp : max power

Other ways of creating a built-in field to separate charges
p-n heterojunction CdTe/CdS CdS CdTe back contact (Cu/Cu2Te) TCO front contact e- h+ Silicon homojunction

Other ways of creating a built-in field to separate charges -2
n-type semiconductor E0 metal EF work function electron affinity Space charge layer Metal-semiconductor junction with semiconductor/ liquid electrolyte junction  photoelectrochemical cell [PEC], where EF ≅ ERedox

Other ways of creating a built-in field to separate charges -3
p-i-n (I = insulator) cell EO EC EV 6 (35) N = 1018/cc (heavily doped semiconductor) 10s of nm N = 1016/cc (medium doped semiconductor) 100s of nm N = 1014/cc (low doped semiconductor) few µm Reminder of typical space charge layer widths

The Photovoltaic (PV) effect:
Generalized picture High energy state Low Absorber one electron energy space e- p+ contact Metastable high and low energy states Absorber transfers charges into high and low energy state Driving force brings charges to contacts Selective contacts (1) cf. e.g., Green, M.A., Photovoltaic principles. Physica E, 14 (2002) 11-17

Why are efficiencies so low?
Consider the Solar Spectrum

Optical Problems for Quantum conversion of solar energy
In Solar Cells Most Solar Energy is “Wasted” as Heat In any system with concentration, most of the diffuse radiation is “lost” why is this so?

Photovoltaic Conversion is a Quantum (threshold) Conversion Process
in Solar Cells Most Solar Energy is “Lost” as Heat ! Photovoltaic Conversion is a Quantum (threshold) Conversion Process Infra- visible ultra- -Red violet (IR) (UV) Solar Energy Spectrum Why does our solar cell absorb only about 15 percents of the sunlight's energy? Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic -- it is made up of a range of different wavelengths, and therefore energy levels. (See How Special Relativity Works for a good discussion of the electromagnetic spectrum.) Light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits our cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts (eV) and defined by our cell material (about 1.1 eV for crystalline silicon), is required to knock an electron loose. We call this the band gap energy of a material. If a photon has more energy than the required amount, then the extra energy is lost (unless a photon has twice the required energy, and can create more than one electron-hole pair, but this effect is not significant). These two effects alone account for the loss of around 70 percent of the radiation energy incident on our cell. Why can't we choose a material with a really low band gap, so we can use more of the photons? Unfortunately, our band gap also determines the strength (voltage) of our electric field, and if it's too low, then what we make up in extra current (by absorbing more photons), we lose by having a small voltage. Remember that power is voltage times current. The optimal band gap, balancing these two effects, is around 1.4 eV for a cell made from a single material.

Single p-n junction solar cell
high energy photon - partial loss Energy Energy e e - - h n h n p p - - type type useable photo - - voltage ( voltage ( qV qV ) ) n n - - type type low energy photon - total loss h h + + space O. Niitsoo

External Quantum Efficiency (EQE ~ IPCE)
η() ≡ Ip()/qN() N() = # of impinging photons

External quantum efficiency of different types of cells
Wavelength (nm) In organic based solar cells EQE does not have sharp edge. This limits current efficiency. Solar Cell Eff #35, Progr. in PV, 2010

Internal Quantum (or collection) Efficiency (IQE)
Y() ≡ η() / T() ≡ Ip() / qN()T() N()T() = absorbed photons

Power Losses in Solar Cells

Losses in PV cell Etendu; Photon entropy –TD < Eg not absorbed
lack of concentration Eg Carnot factor –TD Emission loss- (current) Electrical power out >Eg thermalized < Eg not absorbed Current – Voltage Characteristics After Hirst & Ekins-Daukes Prog.Photovolt:Res:Appl. (2010) Nayak, ……, Cahen., Energy Environ. Sci., 2012

“Etendue”  VOC loss (“photon entropy”)
ΔG = ΔH – TΔS ΔqVOC = EG – kT ln W = EG – kTln 46,200 = EG – 10.7 kT ≅ K

“Etendue”  VOC loss (“photon entropy”)
Richard King

Improve performance using concentrated sunlight
but … diffuse (scattered) radiation lost upon concentration

Optical Problems for Quantum conversion of solar energy
In Solar Cells Most Solar Energy is “Wasted” as Heat in any system with concentration, most of the diffuse radiation is “lost”

What else can we do about this?
Better utilization of sunlight: Photon management: Multi-bandgap, multi-junction photovoltaics Bandgap (eV) 5 6 7 8 9 1 2 Four-junction device with bandgaps 1.8 eV/1.4 eV/1.0 eV/0.7 eV Theoretical efficiency > 52%

Photon Management  multi-junction device structures
Thermodynamic Efficiency Limits non-concentrated Sunlight (AM 1.5) # of Junctions Efficiency Optimum EG (eV) 1 30% 1.3 2 42% 3 49% 4 53% infinite 68%

Optical Frequency Shifting
BASIC RESEARCH NEEDS FOR SOLAR ENERGY UTILIZATION Report on the Basic Energy Sciences Workshop on Solar Energy Utilization

Up-conversion for a single junction
2 photons of energy 0.5 Eg< hν< Eg are converted to 1 photon of hν> Eg

Other ways to beat, IN PRINCIPLE… the Shockley- Queisser Limit
e.g., Baluschev et al., PRL 97 (2006): (T-T annihilation) Photon Up-Conversion Photon up-converter Solar Cell

Down-conversion for a single junction
1 photon of energy hn > 2Eg is converted into 2 photons of hn > Eg

Other ways to beat, IN PRINCIPLE… the Shockley- Queisser Limit
Photon down converter Solar Cell Photon Down-Conversion Singh et al., J.Chem. Phys.42 (1965) singlet fission after R. Collins, CSM

Shockley-Queisser* (SQ) Limit
* detailed balance, photons-in = electrons-out + photons-out; curve is on earth RT, for single absorber / junction Photosynthesis Prince, JAP 26 (1955) Loferski, JAP 27 (1956) Shockley & Queisser JAP (1961) cf. also Duysens (1958) “The path of light in photosynthesis”; Brookhaven Symp. Biol.

Up- and down-conversion together, based on AM1.5
Normalized efficiency Eg Ge CdS GaAs Cu2S Si a-Si:H a-Si:H:F

IN PRINCIPLE … efficiency  possible in quantized systems
Why ? Quantization changes relative rates of carrier relaxation channels: Slows carrier cooling - Phonon bottleneck Can break selection rules Increases carrier confinement Allows non-equilibrium carrier populations Hot carrier distributions Energy selective contacts Carriers must equilibrate and be collected before relaxing to band edges Hot carrier collection from / after R. Collins, CSM

Other ways to beat the SQ limit
Multiple exciton generation Hot electrons Intermediate bandgap EG EV EC EC* e- h+ e- h+

Multiple exciton generation Hot electrons Intermediate bandgap EG EV EC EC* e- EF e- EF h+

Multiple exciton generation Hot electrons Intermediate bandgap e- EG EV Ei EC e- 4 (for 3 slides) (71.5) h+

Types of junction for solar cells
1. Homojunctions

Types of junction for solar cells
2. Heterojunctions

amorphous Si & Micromorph Tandem Solar Cell
Thin Film PV Three major approaches amorphous Si & Micromorph Tandem Solar Cell CdTe Cu(InGa)Se2

Polycrystalline Thin Film PV 2nd Generation
© Materials Research Society 2012 From Ginley & Cahen, Ch, 18 (Ginley, Collins, Cahen) Fundamentals of Materials for Energy and Environmental Sustainability< Cambridge Un. Press, 2011

The principle of nanostructured cells
light absorption depth e- h+ light-absorbing semiconductor contact electron conductor hole conductor absorber e- h+ hole selective contact electron EC EV e- h+ 4 (48) electron (hole) selective contact; conductor; transport medium Advantage of high surface area: Allows the use of locally thin absorber and therefore poor quality (wider range of) absorbers

Organic photovoltaic cells OPV
Two problems of OPV: Low diffusion lengths of electron/hole 2. Low dielectric constant – high binding energy e- h+

Two problems of OPV: Low diffusion lengths of electron/hole
2. Low dielectric constant and high effective mass – high binding energy e- h+ effective mass of electrons and holes dielectric constant of material e- h+ e- Binding energy of H atom = me4 2h2ε2 = 13.6 eV e- h+ e- Binding energy of exciton ? h+ Wannier-Mott excitons – extended; low BE few/10s meV Frenkel excitons – localized; high BE s meV

Organic vs. Inorganic PV cells
Electron-hole pair: Organic vs. Inorganic PV cells Inorganic semiconductor Organic semiconductor MOLECULAR PICTURE Exciton binding energy < kT → dissociation by space charge region E-field Exciton binding energy >> kT → requires donor/ acceptor, (D/A) type structure from A. Kahn, Princeton U

Exciton dissociation at Donor/Acceptor interface.
Organic Photovoltaics: Basics OPV in a nutshell Cathode Anode glass Layer of Donor and layer of Acceptor between electrodes Exciton dissociation at Donor/Acceptor interface. Limited efficiency due to: Small [dissociation sites] (only D/A interface). High recombination possibility if D or/and A layer is thicker than ~10 nm. Low light absorption, because films are thin.

Stability still not good enough for practical use, but improving
Notwithstanding these problems, OPV is now at ~ 11% conversion efficiency Stability still not good enough for practical use, but improving Advantages: Cheap (in capital and in energy) Roll-to-roll manufacturing (large scale possible) 8 (56)

Dye sensitized solar cell (DSC or DSSC)
h+ light HOMO LUMO e- semiconductor dye TiO2 e- EC EV TiO2 I- + h+ ---> I 2I + I- ---> I3- (I is soluble in I-) Need single monolayer dye on TiO2, but …. then low absorption At counter electrode, I is reduced back to I- Important difference between this cell and “standard’ photovoltaic cells or previous nanocrystalline cell: Charge generation and charge separation occur in different phases: recombination is inherently low.

Solution - use high surface area semiconductor
Early attempts increased surface area by roughening electrode - several times increase Breakthrough: porous, nanocrystalline TiO2 Made by sintering a colloid or suspension of TiO2 O’Regan, B.; Grätzel, M. Nature 1991, 353, 737. Dye molecule bonded to TiO2 Only monolayer of dye (at most) on each TiO2

DSSC STABILITY RESULTS
3G Solar Jerusalem, Israel

The most common dye: Ru(dcbpyH2)2(NCS)2 or RuL2(NCS)2
cis-bis(4,4’-dicarboxy-2,2’-bipyridine)-bis(isothiocyanato)ruthenium(II) e- N Ru C -O O N=C=S h+ Ti N=C=S Excitation of dye is a metal-to-ligand charge transfer Ru d-orbitals ligand p* orbital Ti4+/3+ ca. 1.7 eV DSC 5 (61)

Change dye in DSC to semiconductor
Semiconductor-sensitized solar cells (quantum dot cells) ETA (extremely thin absorber) solar cells 2.5 (63.5) Semiconductor does not have to be a single monolayer – typically few nm to few tens nm Variations Hole conductor – liquid or solid (if solid, commonly called ETA cell) Semiconductor may be in form of quantum dots – increase in EG

p/n vs. excitonic solar cells
INORGANIC high dielectric constant minority carrier device ORGANIC low dielectric constant excitonic device includes jiggling & wiggling  dielectric constant Exciton from B. Kippelen, Georgia Tech

Semi-transparent “Plastic” OPV: Light for greenhouse plants and
Power the fans, Pump the water courtesy David Ginley, NREL

Classical PV Absorbers (Inorganic Semiconductors) are
threshold absorbers Need other material type as PV absorber, to take advantage of molecular absorption of plants

Semi-transparent OPV cells
Uli Lemmer et al. PV cell Transmission vs. Solar spectra

(hybrid) PV Cells: decreasing STATIC disorder
- Cathode Anode Substrate Light A Bulk heterojunction cell Dye-sensitized / ETA OM Perovskite

Energy Payback Times a few views

Different process steps through production of c-Si solar cells
+ their relative part of the gross energy requirement (GER) Stoppato Energy 33, 224 (2008)

Estimated Energy Payback Times for Solar Cells
2010 Organic PV (OPV) Proc. IEEE PVSC 2010 A. Anctil et al., RIT, NREL courtesy David Ginley, NREL Cahen group WIS 2013

Estimated Solar Cell Energy Payback Times 2013
from First Solar website… Peng, Lu, Yang, Renew. Sustain. Energy Rev. 19 (2013) 255–274

PV production history and forecast
Quo vadis ? PV production history and forecast Cumulative PV GW 10 (81.5) Wikipedia

Solar Cell Power Stations TODAY
World’s Largest Solar-Electric Plant 0.55 GWp ( ~100 MWc) Topaz Solar farm (CA, USA) In 12/2014 Global Cumulative Installed PV Power ~ 0.15 TWp PRC goal >2012 ≥ 0.01 TWp/yr PRC GWp solar (PV) 30 TWp (~ 6 TWC) requires 1 such plant, every HOUR, for ~ 12 years (+ storage…) David Cahen 05/2010

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