Organic Energy Materials Flashcards
What occurs as a result of pi-conjugation?
Thermal and Chemical Stability
What is Huckel’s approximation?
We only concern ourselves with the MO energies of the conjugated system
What is the number of pi-bonded MOs equivalent to?
The number of pi electrons
How do we lower the energy of molecular orbitals?
We increase the number of bonding interactions and decrease the number of anti-bonding interactions
How do we determine the net bonding interaction for a conjugated system?
(1) Draw all MO wavefunctions including any nodes
(2) For each wavefunction, count the number of bonding interactions (B) and the number of anti-bonding interactions (A)
(3) The net bonding of each wavefunction is equal to B - A
(4) The net bonding of the pi-system is equal to the sum of the electronically occupied interactions
How does increasing the pi-conjugation impact the HOMO-LUMO energy gap?
It generally leads to a smaller HOMO-LUMO gap
What do we call the band of delocalised electrons?
The valance band
What do we call the band of empty orbitals?
The conduction band
What is an exciton?
An electron-hole pair
How does the exciton binding energy relate to distance?
It is inversely proportional to distance
What are the values of the exciton binding energy of organic semiconductors compared to inorganic semiconductors?
They are much higher values; the electron-hole pair are tightly bound
How can we decrease the rate of charge recombination?
We can use a donor-acceptor strategy
This is where the electron is accepted by an acceptor at lower energy than the donor’s LUMO
How can we maximise the donor-acceptor interface in the photoactive layer?
We can implement bilayer structures, interdigitated structures or bulk heterojunction structures
What is the ionisation potential of a material?
It is the ability to lose electron(s) - oxidation
It is directly related to the energy of the HOMO
What is the electron affinity of a material?
It is the ability of a material to gain electron(s) - reduction
It is directly related to the energy of the LUMO
What is the equation to calculate the band gap (Eg)?
The modulus of E of LUMO - E of HOMO
How can we determine the optical band gap?
By absorption spectroscopy
What is a bathochromic shift?
A red shift (shift to increasing wavelengths)
What is a hypsochromic shift?
A blue shift (shift to decreasing wavelengths)
What is a hyperchromic shift?
An increase in intensity
What is a hypochromic shift?
A decrease in intensity
How do we graphically calculate the onset wavelength?
We draw a line of best-fit of the absorption line at the highest intensity
How can we calculate the optical band gap energy?
hc / onset wavelength
What shift occurs with increasing pi-conjugation?
A bathochromic shift occurs; this is consistent with the narrowing HOMO-LUMO band gap
What shift does increasing planarity lead to?
A bathochromic shift occurs; this is due to increased orbital overlap in planar systems
How does having a more regioregular backbone affect the band gap energy?
It leads to a smaller band gap (narrower HOMO-LUMO energies)
How does having a regioirregular backbone affect the band gap energy?
It leads to a larger bandgap and hence the weakest conductivity
What is an auxochrome?
A non-photoactive substituent that modifies the light-absorbing property of its chromophore (pi-system)
What effect do +M groups have on the band gap energy?
They lower the band gap energy, hence resulting in a bathochromic (red) shift
What effect do -M groups have on the band gap energy?
They increase the band gap energy, resulting in a hypsochromic (blue) shift
How can we increase the favourability of storing charge?
We are generally interested in stabilising the LUMO of the electron acceptor, enhancing the electron affinity
What happens when the applied voltage is approximately equal to the energy of the LUMO?
There is no driving force for the reaction to take place
What happens when the applied voltage is greater than the energy of the LUMO?
A reduction reaction can occur
How do we perform linear voltammetry?
We sweep our applied voltage in a single direction between two potentials
What can we learn from linear voltammetry?
We can determine the half potential for a given electron transfer process
It is useful for comparing the relative number of electrons transferred (n) for a redox-active molecule capable of multiple electron transfer processes
If we sweep our voltage to more negative values, what process is occurring?
We are reducing our molecule
If we sweep our voltage to more positive values, what process is occurring?
We are oxidising our molecule
What is the cyclic voltammetry?
We sweep our voltage back and forth between two potentials, typically at the same start and end point
Why is cyclic voltammetry more useful than linear voltammetry?
We can determine values for reduction and oxidation potentials
How can we calculate half potentials from a cyclic voltammogram?
It is the midpoint between the reduction potential and the oxidation potential
How can we calculate energy change from a cyclic voltammogram?
The modulus of the reduction potential - the oxidation potential (essentially the peak separation distance)
How do we know if a reaction is electrochemically reversible?
If the energy change is approximately equal to 59 mV / n in a polar solvent, our process is electrochemically reversible
What is the equation for the energy of the HOMO, determined by the cyclic voltammogram?
-n x ((Eₒₓ - E₁/₂ of Fc/Fc₊) + IP Fc, vac)
What is the equation for the energy of the LUMO, determined by the cyclic voltammogram?
-n x ((E red - E₁/₂ of Fc/Fc₊) + IP Fc, vac)
What is the height of a cyclic voltammogram representative of?
It is representative of the number of electrons transferred
What effect does having an electron donor with a higher (destabilised) HOMO level have on the ionisation potential?
It will decrease the ionisation potential as it is easier to oxidise
What effect does having an electron acceptor with a lower (stabilised) LUMO level have on the electron affinity?
The electron affinity will be higher, making it easier to reduce
What is the Theoretical Energy (Charge) capacity (Q)?
The maximum amount of charge that can be accumulated (stored) per gram of active material
How can we calculate the theoretical charge capacity (Q)?
Qthero = n x F / FW
How can we increase Q?
We need to maximise the number of redox-active sites and decrease the FW
What is a battery?
They convert chemical energy into electrical energy to power a device
In which direction do electrons flow during battery discharge?
Electrons flow towards the cathode
In what direction does the current flow?
It is conventionally opposite to the electron flow
What does a cathode contain?
A polymeric binder
Our ‘active’ material
A liquid electrolyte
What does an anode contain?
Graphite or Li metal
A liquid electrolyte
What separates our anode and cathode?
A permeable membrane
What process occurs at our anode?
Oxidation
What process occurs at our cathode?
Reduction
In which direction do anions flow?
Anions flow towards the anode
In which direction do cations flow?
Cations flow towards the cathode
What does the overall cell potential have to equal for a spontaneous reaction?
The overall cell potential has to be positive as the Gibbs free energy change is equal to -nFE cell
How can we work out the equilibrium constant from the Gibbs free energy change?
Gibbs free energy change = -RTxln(keq)
What is the energy capacity (Wh)?
How much energy a battery contains (the amount of electrical work that can be done)
What is the power capacity (W)?
How quickly a battery can discharge its energy (the rate at which energy is used to do electrical work)
What does the diagonal line in a Ragone plot equivalent to?
The discharge time (how long a device is expected to last when consumed at a given rate)
What is the C-rate?
It is related to the discharge of a battery
What is the equation to calculate C-rate?
C -rate is equal to 1/t
What is charge (Q) equal to?
Current x Time
What are battery discharge curves?
They show the power/voltage over time
What information can we obtain from a battery discharge curve?
The nominal voltage
What are disadvantages of lead-acid batteries?
Heavy and large
Brittle
Prone to leakage, gassing and sulfonation
What are the advantages and disadvantages of NiCad batteries?
Good cycle life and physical durability
High cost and low power generation requires several in series
What is the ‘memory effect’?
A loss in nominal capacity due to the repeated partial charging and discharging
The cell experiences a potential drop at the last point of charging
What causes the ‘memory effect’?
Caused by changes in the physical characteristics of undesired active materials in the cell
What do intercalation materials in the anode and cathode do in a Lithium ion battery?
They exchange Li+ ions between them during discharging (towards the cathode) and charging (to the anode)
LiCoO₂ is used as the intercalation material in a lithium-ion battery
What is intercalation?
The reversible insertion of molecules or ions into materials with layered structures
What does a coin cell battery contain?
Stainless steel casings
Springs
Anode
Separator
Cathode
What is the equation for capacity retention?
Capacity retention = (Experimental Discharge Capacity / Theoretical Capacity) x 100
What are the benefits of inorganic energy materials?
Porous materials that allow for excellent Li+ diffusion
Thermally robust and chemically resistant
Insoluble in electrolyte solvents
What are the disadvantages of inorganic energy materials?
Limited and less precise capacity tuning
Expensive
Relatively heavy, toxic, brittle, non flexible
Poor solution processability
What are the benefits of organic energy materials?
Precise properties tuning
More sustainably sourced
Cheaper
Solution processable
Lightweight and flexible
What are the disadvantages of organic energy materials?
Poor C-rates
Low porosity
Lower thermal and chemical resistance
Soluble in organic electrolytes
Poor mechanical properties
How can we enhance the C-rate performance?
We can increase molecular weight by using polymers; this increases the thermal stability
We can introduce porosity by making metal-organic hybrids
What issues can occur in the cathode?
Phase changes
Dissolution
What issues can occur in the anode?
SEI (solid electrode interface) and dendrite growth
Cracks and pulversiations
