Transcript What is Nanoscience?

Next Generation Solar Power
In earlier lectures I covered present day power technologies
Including recent developments and new directions
However, two of those technologies may actually undergo discontinuous change:
Solar might be transformed by different materials and designs
Nuclear might become much safer & less expensive
Either development could be an absolute game changer!
Thus: Next Gen Solar is the topic of this lecture, with Next Gen Nuclear to follow
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
A caution about today's Next Gen Solar discussion
Experience tells me that many of you are particularly interested in solar power
But I know that understanding its many forms can be REALLY FRUSTRATING (!)
However, my field IS "Optoelectronics" (= light  electronics conversion)
And I have taught that field for all of my academic half-career
So today I will attempt to explain almost ALL leading-edge photovoltaics research
But via words and drawings I think you CAN absorb
Which will hopefully supply you with a GATEWAY into this field
That field is the subject of the following "Best Research Cell Efficiencies" chart
Released annually by the U.S. National Renewable Energy Lab (NREL)
Around which I will organize this lecture
Exploring each of the solar cells types it reports
The full (high resolution version) of the NREL chart:
This file is HUGE, starting out more than four times the size of a PowerPoint slide
So I will only use the hi-res version here (and you may have come back for details)
Its efficiency vs. year data fall into groups by cell type:
Multi-junction / Tandem
Single crystal GaAs
Single crystal Si
Polycrystal thin films
Amorphous Si thin film
"Emerging PV"
I'll explore all of these groups in the course of this lecture
But first we need to recall, and extend, our photovoltaic basics:
(link to my earlier Solar Power lecture)1
Photovoltaic's ambitious goal is to efficiently convert light of ALL different colors
Wavelength
Energy
5 eV
UV
2 eV
1 eV
0.5
eV
Infrared
1) http://www.virlab.virginia.edu/Energy_class/Lecture_notes/Solar%20Power.pptx
But different colors interact very differently with matter:
Infrared
UV
Liberates electrons from bonds
Vibrates a few atoms
But most light just
passes right through
IFF the material has an electron
liberation energy ≤ light energy
Liberates electrons from bonds
but gives them so much
excess energy that they
ricochet all around
Excess energy is lost as struck
atoms start to vibrate (=heat)
But liberation of electrons & creation of holes is not enough!
Liberated & created, they have no reason to move in ANY particular direction
Term for what they will do is diffuse = Randomly wander
Diffusion is analogous to "the drunkard's walk"
Drunk randomly bouncing off light posts:
But we've got two types of wandering drunks: electrons and holes
And these unique types of drunks can "annihilate" one another
That is, the electron can just fill the hole (in the bond) => zip!
To create a net flow (=> electricity) a photovoltaic cell MUST include a cliff
Over which the "electron drunks" will fall (and the "hole drunks" ascend)
Or the way I was first taught it:
Electrons ~ Ball bearings (which fall DOWN)
Holes ~ bubbles (which FLOAT up)
Figure: http://www.sos.siena.edu/~jcummings/teaching/astronomy/lectures/reveal.js-master/ch10.html#/
Solar cell's selective "cliff" is provided by an electric field
Which WE do NOT have to create by applying an external battery or power supply
Instead, electric fields are naturally generated at the junction between two materials
But only if electrons choose to shift from one material to the another
OCCURS ONLY WHEN: Material at right has electrons in energy levels higher than
empty levels in material at left => Electron finds new lower energy home!
This negative charge displacement ALSO creates an electric field
Which, via careful materials selection, gets us to this point:
Sunlight in
Electric Field
Light-liberated electron
(and bond hole) created
Electric field sorts things out, pushing holes left and electrons right => "electricity"
But only if drunkard's walk of light-liberated holes & electrons GETS them to the
electric field BEFORE they have recombined with one another!
In they recombine first, their energy is instead lost as heat (a.k.a. vibrating atoms)
So why use a thick layer that puts the field (cliff) so far away? (!)
Because light is not all absorbed (and converted to electrons + holes) at the surface
Strong light absorption requires tenths to tens of microns of material
So with necessarily thick layer, situation is more like this :
Sunlight in
Electric Field
Light-liberated electrons
(and bond holes) created
It is then REALLY IMPORTANT that freed electrons & holes do NOT recombine
before reaching the sorting electric field at the junction between the layers
Their average random walking survival distance is called their diffusion length
Diffusion length depends on the structure and purity of the material because:
Electrons & holes are drawn to impurities and crystalline flaws
Once together at such TRAPS, electrons & holes can easily recombine
So the purity and perfection of PV crystals can be extremely important!
Returning to the NREL figure and its charter members: Si and GaAs
Si single crystal ~ 28% efficiency
GaAs single crystal ~ 35% efficiency
Multi-junction / Tandem
Single crystal GaAs
Single crystal Si
Polycrystal thin films
Amorphous Si thin film
"Emerging PV"
The crystallography of these most basic semiconductors:
Silicon has same diamond structure as carbon (also column IV in periodic table):
Which is based on every atom
having 4 symmetrically oriented
bonding electrons (and neighbors):
GaAs has a zincblende crystal structure which is really almost identical, except:
Here column III atoms bond to column V atoms
Each PAIR has 8 bond electrons = Average of 4 per atom
So it forms the SAME BONDING STRUCTURE as Si
Trick ALSO works with paired column II and VI atoms!
From my own UVA Virtual Lab website, here: http://www.virlab.virginia.edu/VL/Semiconductor_crystals.htm
That makes these close cousin, plain vanilla, solar cells
They are the oldest solar cells types, with strong further improvement unlikely
GaAs is slightly closer to the ideal Shockley-Quiesser single material bandgap
Which is why GaAs single crystal solar cells are ~5% more efficient
Si raw material is more common/cheaper, and non-toxic
Nevertheless, GaAs is a strongly bonded (very stable) chemical compound
and thus significantly less toxic then pure arsenic
Both require energy intensive, high temperature single crystal growth
Further, the best Si solar cells require particularly low concentrations of "traps"
Si raw material thus requires extra, energy intensive, purification
These ARE the current photovoltaic gold standards!
(with silicon's lower cost outweighing its slightly lower efficiencies)
But solar cells can also be made out of non-single crystals
Polycrystalline cells of various materials (including Si): ~ 20% efficiency
Amorphous (non-crystalline) Si cells ~ 13%
Multi-junction / Tandem
Single crystal GaAs
Single crystal Si
Polycrystal thin films
Amorphous Si thin film
"Emerging PV"
Why do thin film efficiencies (of same materials) drop to 15-20%?
Multi-crystalline:
Poly/microcrystalline:
Amorphous:
Nature will generally find SOME way to complete depicted incomplete bonds
But the result will often be distorted or incomplete bonds
Excellent for attracting, or even binding, electrons or holes
Producing a dense gauntlet of traps for our wandering electrons/holes to get past!
Sunlight in
Electric Field
Light-liberated electrons
(and bond holes) created
The irony of strong vs. mushy semiconductors:
Silicon is a VERY strong, stable, high melting point, semiconductor
Surpassed only its Column IV cousin diamond carbon, and their hybrid SiC
However, this means that its atoms are NOT going to easily move around
And it's very unlikely that faults in silicon crystals will self-heal
So, from the outset, Si PV crystals must be extremely pure and perfect!
In less stable, low melting point semiconductors, atoms CAN move around
Thus (at least to some extent) these materials CAN self-heal
And much poorer starting crystals (or polycrystals) often work well for PV
But there is also a dark side: Low T semiconductors ARE less stable
So their PV cells often readily degrade (accelerated by UV sunlight)
Further, breaking down, they can release their often toxic components
Examples of mushy, surprisingly successful, thin film materials:
For which energy intensive crystal growth can often be replaced by:
Just spraying them onto metal backing sheets
Or even printing them on as liquid or inkjet inks
- Polycrystalline silicon (the old hand)
- CdTe = column II + column VI compound
Mimicking GaAs zincblende crystal, mimicking Si/C diamond bonding
- CIGS = Copper Indium Gallium Selenide = CuInxGa1-xSe2
Cu takes one site, either In or Ga take the next, and Se the last:
http://en.wikipedia.org/wiki/Copper_indium_g
allium_selenide
CIGS crystal retains tetrahedra of diamond & zincblende crystals
That simple strong geometry likely improves its general quality
But especially useful: Purple sites can be either In or Ga
Like earlier example of AlGaAs, it is an alloy of two compounds
The two (fixed stoichiometry) compounds are:
CuInSe2 with an electron liberation energy (bandgap) of 1.0 eV, and
CuGaSe2 with an electron liberation energy (bandgap) of 1.7 eV
Thus for CuInxGa(1-x)Se2, as x changes from 0 to 1, CIGS bandgap goes 1 to 1.7 eV
So can tune one CIGS cell to maximum Shockley-Quiesser single material efficiency
OR build multi-junction cell out of of different composition CIGS layers!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
There is also a variant on CIGS called CZTS
Which stands for copper zinc tin sulfide = Cu2ZnSnS4 (vs. CIGS: CuInxGa1-xSe2)
Having crystal structure:
Versus CIGS crystal structure:
With color coding of Cu, Zn, Sn, S
Versus Cu, In or Ga, Se
While not identical, they are very, very similar, as are their solar cell properties
The big difference is that CZTS uses more abundant, less toxic elements
And thus should be more environmentally friendly
And could end up being cheaper
But at this point CIGS cells are still ~ 2X more efficient (21.7% vs. 11.9%)
CZTS figure: http://en.wikipedia.org/wiki/CZTS
But a particularly enigmatic thin film is amorphous Si:
I depicted its amorphous structure as this:
Those many screwed up or incomplete bonds can act as electron/hole traps:
So how would ANY electron/holes EVER make it to the junction E-field?
Solution (co-invented by RCA Labs friend) = It's not really amorphous Si!
It's amorphous silicon hydride = Amorphous Si stuffed with hydrogen
Tiny hydrogen atoms easily migrate into the structure
And THEY end up attaching to any of the broken bonds above
Effectively nullifying site's effectiveness as a trap
Although what goes in can come back out => Possible lifetime/stability problems
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Which prepares us to take a look at the top of the chart:
Multi-junction / Tandem cells of various materials with efficiencies up to ~ 45%
Multi-junction / Tandem
Single crystal GaAs
Single crystal Si
Polycrystal thin films
Amorphous Si thin film
"Emerging PV"
These "multi-junction/tandem" cells look like the champs:
They have the highest efficiencies!
And their efficiencies are still climbing rapidly!
But caution is in order:
This chart does NOT deal with practicality
This chart does NOT deal with cost
It reports best, one of a kind, not yet reproduced, laboratory efficiencies ONLY
AND, at present, it only extends up to 50% efficiency
With multi-Junction / tandem cells still only producing 35-45% efficiencies
AND these are THE most complex cells making practicality & cost BIG issues!
Recall that they are designed to overcome the Shockley-Quiesser Limit:
Multi-Junction / Tandem cells = Stacking solar cells of different materials
Top: Material with large bond energies ~ purple/blue light:
High energy photons use ALMOST ALL of their energy liberating electrons
While less energetic photons pass right through!
Middle: Material with medium bond energies ~ green light:
Medium energy photons use ALMOST ALL of their energy liberating electrons
While less energetic photons pass right through!
Bottom: Material with low bond energies ~ red light:
Low energy photons use ALMOST ALL of their energy liberating electrons
~ ALL photon energy => electron liberation => ~ 100% energy capture!!
But as noted in my first lecture on solar lecture energy:
Multi-junction solar cells have not yet had any real impact. Why?
Shortcoming #1) Efficiency
Single-material / single-junction designs have 20-35% efficiency
Multi-junction cells ought to be able to double or triple that efficiency
But to date, at best, they increase efficiency ~ 1.5X
(even if they ARE still heading upward!)
Shortcoming #2) Complexity - which will have large impact on cost:
Efficiency MUST increase MORE than the number of junctions:
3X efficiency + 3 junctions likely => MORE cost per watt out
Instead, we need a way of making 2-3 junctions at ≤ 1.5X the cost
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Ways of easily making MANY layers of cells/junctions?
Create all cell layers using ~ one process and/or one apparatus
Method #1) Use cells having almost identical crystal structures
Materials above one another in this plot
=> Tightly combined cells
of lattice constant vs. bandgap:
Created in same or similar apparatus
Silicon Based Semiconductor Heterostructures – Column IV Bandgap Engineering, John C. Bean, Proceedings of the IEEE,
pp. 571-587 (April 1992)
Classic example:
AlAs and GaAs:
Each is a chemical compound
= Having a strict stochiometery (here 1:1)
But you can make alloy mixture of the two:
AlxGa1-xAs = (fraction x of AlAs) + (fraction 1-x of GaAs)
Atomistically: As atoms go on yellow sites, either Al or Ga atoms go on blue sites
You can then build a tandem cell with say:
Top cell of Al0.6Ga0.4As (targeting blue / UV light)
Bottom cell of GaAs (targeting red / IR light)
Or other alloys, or even alloys of alloys!
From my own UVA Virtual Lab website, here: http://www.virlab.virginia.edu/VL/Semiconductor_crystals.htm
Non or partially crystalline thin films might also be used
But a newer possibility is Quantum Dot multi-junction/tandem solar cells
Quantum Mechanics concluded that electrons act like waves
But all types of wave act ~ the same
Imagine making big water waves in a bowl: Only certain size waves persist
Because wave of a given wavelength will bounce back and forth
And to ADD to one another, the wave AND its reflections must be "in phase"
For that to occur: The wave must be sized so that it "fits the box"
To fit, distance out and back across bowl must = multiple of wavelength (l)
Or, for a given size of "bowl" (L): Strong wavelengths = 2L / integer
Acceptable wavelengths => Acceptable energies
With energy of waves changing with SIZE of enclosure!
Because electrons ARE waves, same holds for them:
Smaller "bowl" (quantum dot) => Smaller fitting wavelengths
Smaller wavelength = More frequent wave oscillation => Higher Energy
Hence smaller QUANTUM DOTs => Higher electron energy levels
=> Higher differences between those energy levels
Difference between two of those levels = electron liberation energy (bandgap)
So different quantum dot sizes absorb light at different energies
(for energies we are interested in, dots must be nanometers in size)
Different sized dots in different layers => Wavelength selective layers
Offering a new type of Shockley-Quiesser beating multi-junction solar cell
What are "quantum dots" made of? All sorts of different atoms & compounds!
Let's explore quantum dot possibility in smaller steps:
Building from example of conventional semiconductor photovoltaic cell:
Swap of charge across junction => ELECTRIC FIELD => Sorting of electrons/holes
OUT
Electrical Current Pump
IN
An analogous one Quantum Dot ("QD") photovoltaic cell:
For instance, when semiconductor or polymer quantum dot meets metal:
OUT
IN
But nano dots put out nano power, so we need LOTS of them to work together:
A quantum dot (but not yet "multi-junction") solar cell:
Sticking, for the moment, with dots of the same size:
Transparent front conductor
Quantum dots
Charge separating Electric fields
Back Conductor
What is going on here? You choose special quantum dot and back conductor materials
So that interface between them swaps charge, setting up electric field
Which propels ONLY photo-generated holes into back conductor
Leaving photo-generated electrons to be collected by front conductor
Result is MANY nano electron pumps working together ("in parallel"):
But this STILL uses only ONE thin light-absorbing QD layer => Very little current
An improved multi quantum dot solar cell:
THIS would be much better = MORE DOTS!
With materials chosen so that:
- Blue metal collects only charge from green
- Green sucks positive charge from dots
- Yellow sucks negative charge from dots
- Gray metal collects only current from yellow
- Green and yellow self-segregate into such a pillared structure
- Dots go to interface
Sounds incredibly complex, doesn't it? But such designs ARE being researched
Green and Yellow = Immiscible conducting polymers (e.g. "Block co-polymers")
But additional quantum dots STILL achieve light to electrical energy conversion of only ~ 9%
Putting it in low "Emerging PV" corner of the NREL chart:
To REALLY improve things we need multi-junction quantum dot solar cells:
Using Quantum Size Effect AND flexibility of quantum dots to give multi-junction design:
First capture Blue Light with small quantum dots => few higher voltage electrons
Then capture Red Light with deeper large quantum dots => many lower voltage electrons
I1, V1
I2, V2
Done right (probably with more layer/sizes), you might efficiently capture light of ALL colors
But this would require incredible control of internal arrangement and electrical current paths
Making the creation of quantum dots the almost trivial part of task
REAL CHALLENGE here is the required complex 3D SELF-ASSEMBLY
I'd be LOT easier if quantum dots could be randomly distributed in layers!
More like this:
But we'd still need electric fields to separate light-liberated electrons from holes!
And HERE we would NOT want fields between the dots and their surrounding layer:
For instance, field below would drive positive holes out of dot, trapping electrons
Until dot got so negative that it started pulling holes back
Then NOTHING (electrons nor holes) would escape to deliver power!
+
+
+
+
--- --+
+
+
+
You'd instead want electric fields at layer boundaries
Which COULD be accomplished by:
1) Choosing dot and layer materials that have similar energy levels
So they don't naturally swap charge / build up interface electric fields
2) But choosing layer materials with differing energy levels
So that they do transfer charge across their interfaces
Thereby adding properly directed charge-pumping electric fields
Producing this Quantum dot multi-junction / tandem solar cell:
3D self-assembly => A LOT easier
Layer material selection => More difficult
I1, V1
I2, V2
But full multi-junction quantum dot cells have NOT yet been realized!
But before leaving multi-junctions, what's NREL talking about here?
Higher efficiencies for "concentrator"cells - what's this all about?
It refers to concentrating sunlight on SMALL cells via LARGE lenses or mirrors:
http://www.greenrhinoenergy.com/solar/technologies/pv_concentration.php
Why would concentration be desirable?
Reason #1) Photovoltaic cells get more efficient as light gets more intense
Revising earlier figure to explicitly depict "traps" created by impurities/flaws:
Sunlight in
Electric Field
Light-liberated electrons
(and bond holes) created
Even the most crystalline / highest purity PV materials have some "traps" (
)
These reduce the "diffusion length" of wandering holes and electrons
Reducing number reaching to and sorted/pumped by electric field
But there are limited number of "traps" interfering with wandering electrons/holes
So overwhelm them by sending in more intense light
=> More electrons + holes
=> These quickly saturate (fill up) the traps
=> Allowing other electrons/holes to pass by!
Reason #2 for concentration is easier to understand:
If the cost of a square meter of solar cell >> Cost of same size lens/mirror:
(which is particularly likely for complex multi-junction solar cells)
It's cheaper to buy large lens/mirror and combine with small solar cells!
You still use ALL of the light captured by the full size lens/mirror
AND
You get further benefit of Reason #1 improvement in basic cell operation
Thus "concentrator cells" at right can deliver same or higher power out!
vs.
or
Lens
Mirror
This finally gets us to NREL's "Emerging PV" category:
These are new, long-shot possibilities, some of which are improving rapidly:
Multi-junction / Tandem
Single crystal GaAs
Single crystal Si
Polycrystal thin films
Amorphous Si thin film
"Emerging PV"
To make this busy corner clearer, let me enlarge it (and its key):
I have already described:
- Basic Quantum Dot cells
- CZTS variant of CZTSSe
Leaving us only:
THESE "emerging PVs" which can be sorted two different ways:
By years of research (more mature 1st):
- Dye sensitized cells
- Organic cells
- Perovskite cells
Or efficiency (most efficient 1st):
- Perovskite cells
- Dye sensitized cells
- Organic cells
To those I want to add cells using Carbon Nanotubes (CNTs) and Graphene
Because these are getting so much recent press attention!
Despite the fact that they are "no-shows" on the NREL chart
Onward: Newer designs/materials tend to build on the older
And research papers assume you know the earlier research
(and are very hard to comprehend if you don't!)
So let's tackle these final types in ~ historical order:
Dye Sensitized Solar Cells (DSSC's):
All of the preceding cells come from the physics & electrical engineering world
Known as the "Device Physics Community"
Of which, yes, I am a card-carrying member:
DSSC's were instead developed by chemists (or even the occasional biologist)
The subject matter is fundamentally the same
But the terminology is fiercely different, as is the approach:
- Physicists focus on atomic scale mechanisms (getting vague at larger scales)
- Chemists focus on macroscopic properties (getting vague at the atomic scale)
So what follows is my attempt at both bridge-building AND translation
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Chem speak (vs. Physics speak):
Papers on DSSC's (and the organic cells to follow) talk about three things:
1) Dyes or photo-sensitizers where light produces unbonded electrons & holes
Using my 1st solar lecture key of
= atomic cores,
= electrons:
So this part is easy to translate: It's a semiconductor
2) Hole transport materials (or layers) where only holes can move
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Only holes? But weren't electrons really moving?
Yes, but the subtle distinction is as follows:
A single hole move = valence electron slithering from one bond to an adjacent one:
And it really doesn't have to jump as it's actually a quantum mechanical cloud
Which can sort of just ooze from one bond to another:
This requires very little additional energy
- Physics speak: This electron remains in the "valence band"
- Chem speak: It's in the "HOMO" = Highest (normally) Occupied Molecular Orbital
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
As opposed to the movement of a truly unbonded electron:
This electron cannot just ooze from one side of an atomic core to the other:
Because it's outside the atomic core (which is already packed w/ electrons)
So it's at a much higher energy - Physicist: it's in the "conduction band"
Chemist: It's in the "LUMO" = Lowest (normally) Unoccupied Molecular Orbital
Which, also in Chem-speak, makes the material above an:
3) Electron Transport Material (or layer)
In their preferred liquid environment, Chemists can simplify things further:
Electron Transport Material (wet) => Solution with negative ions (anions)
Hole Transport Material (wet) => Solution with positive ions (cations)
All three things happen in the classic solar cell semiconductor
Why wouldn't all three things happen in a single material?
1) If bonds are too strong (the "bandgap" is too large), light won't be absorbed,
=> Electrons will not be liberated from bonds
2) If bonds are too strong, any loose electrons won't stay liberated,
=> Killing off (unbonded) electron transport
3) If material's bonds are oriented or separated differently, result might be that
Quantum mechanical "oozing" of electrons between bonds is far less likely
=> Killing off valence band hole transport
Chemists like to divide the three functions between different materials
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Chemist's DSSC (or organic) solar cell:
Which divides three functions between different layers
Sunlight in
Thick Electron
Transport layer
Thin Dye layer
Thick Hole
Transport layer
Actual geometry is more complex:
With light coming in through either Electron or Hole transport layer
And instead of flat planes, they use much more convoluted 3D geometry
This seems to do the required job of sorting electrons from holes
Sending them in opposite directions to form an "electrical current"
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
But hold it: There is no "push" => No power!
Remember? Electrical Power = Flow x Pressure = Current (I) x Voltage (V)
Above scheme shows no way of producing the pressure/voltage:
It's not yet a pump!
It's just bucket leaking electrons to left / holes to right:
To be a bit more technical:
While it's "short circuit current" (unopposed flow) might be finite
It's "open circuit voltage" (electrical force) would be ~ zero
Thus, while papers on DSSC's and Organic Solar Cells seldom mention it,
There has got to be something more!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
There must be at least one pushing Electric Field:
Sunlight in
Thick Electron
Transport layer
Thin Dye layer
Thick Hole
Transport layer
Interfacial Electric Fields
THUS material of "hole transport layer" CANNOT ONLY transport holes
Line up of its energy levels vs. those of the the Dye layer
must promote electron transfer across that interface (=> E field)
AND/OR Material of "hole transport layer" CANNOT ONLY transport electrons
Line up of its energy levels vs. those of the Dye layer
must promote electron transfer across that interface (=> E field)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Now (finally) moving on to actual DSSC configurations:
Geometry:
Corresponding energy levels:
Dye/Sensitizer coats surfaces of TiO2 particles
Electrons photo-generated in the dye move into the TiO2 electron transport layer
Holes in dye layer are then filled by electrons from I- ions arriving in the electrolyte
By absorbing holes, this serves the role of a hole transport layer
(with I- ions then regenerated at far right Pt electrode)
Energy levels are indeed carefully chosen to promote interfacial electron transfer
Thereby supplying (even if not mentioned) the pushing electric fields
Highly Efficient Dye-Sensitized Solar Cells: Progress and Future Challenges, Y. Zhang et al.,
Energy and Environmental Science 6, pp. 1443-1464 (2013)
Or getting rid of liquids and moving to all solid state:
Perovskite Dye on Al2O3 particles (taking electron transport place of previous TiO2)
(HTM = hole transport material, TCO = top contact transparent oxide)
THICK Perovskite Dye (= better absorbing) on TiO2 particles (electron transporter)
The Light and Shade of Perovskite Solar Cells, Michael Grätzel, Nature Materials 13, pp. 838-842 (November 2014)
Where I just snuck in perovskite solar cells
Which have been rocketing up the NREL chart:
Attributed to BOTH electrons and holes moving REALLY well in perovskites:
LONG electron/hole diffusion lengths = very few and/or ineffective traps
Perovskite = "Mushy" less stable but (remarkably) self-healing semiconductor
Perovskite family structure and formula:
Take as the basic repeating crystal building block ("unit cell"):
The CUBE with fat gray B's in its corners
A = Organic or inorganic cations (+) = green: 1 per cell
B = Metal cations (+) = gray: 1 per cell
8 corner atoms x (1/8 of each inside our cube) = 1 inside each unit cell
X = Anion (-), frequently halide = purple: 3 per cell
Don't count any atoms outside of the gray atom bounded cube
Every gray cube edge (12 of them) has a purple X atom at its center
But only ¼ of each of those purple X atoms is inside our cube (i.e.
)
Thus X count = 12 edge atoms x (1/4 of each inside or cube) = 1
So general formula of repeating perovskite unit cell is: ABX3
Figure: The Light and Shade of Perovskite Solar Cells, Michael Grätzel, Nature Materials 13, pp. 838-842 (November 2014)
For perovskites used in solar cells:
ABX3 is made up of:
A organic cation = Methylammonium (CH3NH3+)
B metal cation = Lead (Pb+)
X anion = Iodine (I-) and/or Bromine (Br-) and/or Chlorine (Cl-)
Synthesis DOES NOT REQUIRE energy intensive high temperature crystal growth!
Perovskites can instead be evaporated onto surfaces OR
Deposited from liquid using PbX2 and CH3NH3X dissolved in solvents
Offering:1) Simple (almost trivial) fabrication technology
2) Excellent performance due to long free electron / hole diffusion lengths
3) Combined or separated absorption + hole & electron transport by layer
Which makes perovskites sound like the PV silver bullet
But the "fly in the ointment" is:
- It is built around toxic lead
- Which, because things go together so easily, also come back apart!
Specifically: In contact with water, lead iodide perovskite releases PbI
Which is a known carcinogen
Use of which is banned in many countries
1
This has fueled intensive research on alternate perovskites
Including those which replace Pb with Sn
But these have not yet produced comparable performance
Nor do they achieve comparable cell stability/lifetimes
So perovskite PV is a "Stay tuned for further developments!" sort of story
1The
Light and Shade of Perovskite Solar Cells, Michael Grätzel, Nature Materials 13, pp. 838-842 (November 2014)
Bringing us to organic solar cells
Similar to cells above but instead made from layers of organic chemicals
They work, they can be very cheap to build, they can literally be flexible
But you know what happens if organics are left in sunlight (e.g. rubbers & plastics)
They fade, crack, and eventually crumble - Why? - Answer:
UV not only liberates electrons from C-C bonds, breaks those bonds
But some TOUGH ORGANICS have been getting a lot of press attention:
Graphene:
Carbon Nanotube (CNT) = Rolled up Graphene
But they are NOWHERE on NREL's Best Research Cell Efficiencies chart
So why all of the hype? What is really going on?
To explain I need to discuss the complete structure of a solar cell:
Which would have to include at least these parts:
Sun
Metallic
Wire
Metallic
Contact
Layer
Electric Field
Light-liberated
electrons & holes
Transparent
Metallic
Contact
Layer
Metallic
Wire
But likely with these final proportions and arrangement:
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
In the full structure, where could Graphene/Nanotubes be used ?
Well, what are they really good at / good for?
1) They are really, really strong – but that's largely irrelevant to solar cells
2) Electrons can move through them really, really easily and quickly
Which is to say that they are superb conductors
So they COULD be used in: Top wire, bottom metallic contact, bottom wire
And this IS (frequently) suggested in the press and by researchers
But in those places standard metals already work pretty well
And substitution of graphene/CNTs => Minimal impact on efficiency
Top contact offers a somewhat bigger opportunity for improvement:
IT must be both a good conductor and largely transparent
THAT is a rare and scientifically challenging combination:
A "good conductor" must pass a lot of electrical current easily
Electrical current = (#number of free electrons) x (average electron speed)
Graphene/CNT electrons can travel at exceptionally high speeds
So graphene is a good conductor despite having fewer electrons than metals
A "transparent material" has to allow light (here sunlight) to pass through
Light = Oscillating electric (and magnetic) fields
When light's electric field strikes conductor, field shifts electrons:
Electron shift => polarization => counter electric field
If there are enough electrons, counter electric field cancels light field
And such free-electron rich materials (metals) end up acting as mirrors
But graphene has fewer electrons => Poor mirror (i.e. it's more transparent!)
So graphene/CNT's could help out in transparent front conductor
This could trim the cost a little
Because alternatives such as Indium Tin Oxide (ITO) are costly
And/or it might slightly goose up efficiencies (say, a few percent)
Which would certainly be nice, but which would not yet be a huge deal
To be a huge deal, graphene/CNT's would have get out of the boonies of the cell!
AND instead enable big improvement in the function of the:
Semiconductor/dye, electron or hole transport layers
But graphene is NOT a natural semiconductor, it's effectively a metal meaning that:
Electron liberation energy ~ 0 => No pushing force (Voltage) from layer(s)
However, SOME forms of CNT are semiconductors so there is a possibility here
But ONLY possibility seeming to justify current level of excitement is:
If, in the heart of the cell, CNT's (or modified form of graphene)
could use excess photon energy to liberate additional electrons
Instead, now, one photon gives all of its energy to only one electron
IF that photon's energy is MORE than enough to liberate that electron
excess energy goes into electron's kinetic energy and it just ricochets around,
causing atoms to vibrate, sucking up that energy as waste heat
= Fundamental reason ALL solar cell efficiencies are << 100%
In graphene, some believe they've seen 1 energetic photon liberating 2 electrons
HOWEVER, reports have been rare and that result very hard to achieve
Leading to my personal conclusion that:
Current press on graphene/CNT solar PV seems very premature and/or naive
Credits / Acknowledgements
Some materials used in this class were developed under a National Science Foundation "Research
Initiation Grant in Engineering Education" (RIGEE).
Other materials, including the "UVA Virtual Lab" science education website, were developed under even
earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate
Education" (NUE) awards.
This set of notes was authored by John C. Bean who also created all figures not explicitly credited above.
Copyright John C. Bean (2017)
(However, permission is granted for use by individual instructors in non-profit academic institutions)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm