Organic Energy Materials Flashcards
Limitations of current energy storage solutions?
Use inorganic TM which are expensive, relatively heavy, brittle, limited supplies
Limitations of renewable energy (RE) sources?
Intermittent and sensitive to external factors - which limits how much we can benefit from RE
What electronic properties do conjugated π - systems have influence on?
Conductivity
Light harvesting
Light emission
Electrochemical reversibility
What 2 ways can π conjugation occur?
Through bond
Through space
Huckels rule for aromaticity
4n + 2 π electrons in a cyclic planar array
How does π - conjugation effect thermal and chemical stability?
Compared to non-conjugated dienes heat of hydrogenation of a conjugated diene is significantly lower. Suggesting π - conjugation has a stabilising effect
What is the band gap, Eg ?
Energy gap between the π orbital (valence band) and π* orbital (conduction band) of a π - conjugated system
What effect does extended π - conjugation have on the band gap?
Results in a smaller band gap
How to promote conductivity in conjugated system
In ground state e are not able to move freely/ conduct so must promote e into conduction band (hv, heat)
What is an exciton
electron-hole pair
Exciton binding energy, Eb?
Eb = Eg - Ex
Eg = Energy of band gap
Ex = Energy of phonon emitted upon charge recombination
Issue with excitons in organic semiconductors? and how to fix
Excitons are tightly bound ( electron tightly bound to hole)
So donor-acceptor strategies are typically employed to achieve greater and longer lived charge separation
What are the different donor-acceptor interfaces of photovoltaic devices?
Monolayer
Bilayer
Interdigitated
Bulk heterojunction
UV visible electronic transitions
of possible allowed transitions only π -> π* and n -> π* transitions are accessible in UV vis absorption range
Therefore need to maximise solar harvesting efficiencies by designing conj organic molecules that can absorb more of solar spectrum
What does a smaller band gap mean in terms of wavelengths of light?
A smaller band gap means longer wavelengths of light can promote the e into conduction band, taking advantage of all wavelengths of light
What is ionisation potential, IP?
Ability to lose electrons (become oxidised)
What is electron affinity, EA?
Ability to gain electrons (become reduced)
How to determine the band gap?
Absorption spectroscopy
The optical energy band gap is the energy required to photoexcite an electron from the HOMO to the LUMO (units eV)
obtained experimentally from πonset
Equation for Eg,opt
Eg,opt = hc/πonset
What effect does increasing conjugation have on the absorption profile of a molecule?
This shifts the absorption profile to a higher πonset (red shift) (bathochromic shift) and decreases the band gap
What effect does heteroatom have on absorption profile?
Inclusion of heteroatom joining benzene rings means the molecule is more planar and enhances conjugation resulting in bathochromic shift with higher πonset and therefore a smaller Eg
What effect does a regioregular backbone have on absorption profile?
Will polarise conjugated systems leading to bathochromically shifted, higher πonset and smaller Eg
What effect do substituents have on absorption profile?
+M groups increase conjugation efficiency across the chromophore leading to bathochromic shift with smaller Eg opt
-M groups limit conjugation efficiency across molecule leading to hypochromic shift with a larger Eg opt
Add OH to benzene ring
1) SO3/H2SO4
2) NaOH
3)H+
In energy storage applications how can we make it more energetically favourable to store charge
By stabilising the LUMO of the electron acceptors to enhance their electron affinity
What is linear voltammetry?
Sweep applied voltage in single direction between 2 potentials
Useful for determining half potential
What is cyclic voltammetry?
Sweep back and forth between 2 potentials, typically same start and end point
More useful technique for determining reduction and oxidation potential as well as half potentials
Also useful for determining if electron transfer process is electrochemically reversible
Equation to determine if an electrochemical redox reaction is reversible
ΞE = |Ered - Eox| ~ 59/n mV
where n = number of e- transfered
What are the effects of electron donor with destabilised HOMO?
Higher HOMO level easy to oxidise because of lower ionisation potential
What are the effects of electron acceptors with stabilised LUMO
Lower level LUMO easier to reduce as it has a higher/greater electron affinity
What is a battery?
Electrochemical cells that convert chemical energy into electrical energy
During battery discharge electrons flow towardsβ¦
the cathode
in a battery oxidation occurs atβ¦
The anode
In a battery reduction occurs atβ¦
The cathode
Primary batteries
Irreversible disposable
Secondary batteries
Rechargeable, electrolytic cells
1V =
1J / 1C
The voltage of multiple batteries in series isβ¦
sum of individual cell voltages
The current of
