Topic 2: Molecular/organic semiconductors

Question 1

• Organic semiconductors allow us to engineer E_g, I_p, and A_g/s/c for a given application. What are E_g, I_p, and A_g/s/c

Answer 1

• E_g - band gap
• I_p – ionisation potential
• A_g/s/c – electron affinity in gas, solid, crystal
• We can tune the barrier to electron/hole injection at the frontier molecular orbitals

Question 2

• How can parent structure affect electrical properties, using the following molecules and their corresponding energy levels for guidance?

Answer 2

• Addition of a ring (1T à BT) extends degree of conjugation, decreasing band gap (resonance stabilisation effect
• Electronegative hetero atom reduces HOMO-LUMO equally is case of Nitrogens
• Others will increase I_p/A_s­

Question 3

• What impact would addition of fluorine’s across a double bond have on electronic structure?

Answer 3

• Substitution effects: electronegative induction greatly increase I_p (more E needed to eject) as electrons in HOMO/LUMO more strongly bound to positive carbon nuclei
• Screening reduced by σ-bonded electrons as electron associated with C-bonds withdrawn
  
  • reduction in screening of electrons from σ-bond as electrons are being pulled out of C-C by fluorine
• No change in E_g

Question 4

• How does conjugation size affect electronic structure?

Answer 4

• As more units added less energy needed to remove an electron, I_p decreases
• ?A_s increases, as an additional electron is increasingly stabilised by increasingly large conjugated molecule.

Question 5

Describe the effect of conjugation length on electronic structure in 1D carbon wires

Answer 5

• Pierls distortion result in an alternation in bond length on 1D wires of carbon, preventing their formation.
• Huckels theory predicts that metallic behavious as n –>∞
• But, instead of C_2p band being half filled (metallic behaviour) with longer chain length, a band semiconductor band gap forms
• Unfavourable energy of long bond is outweighed by stabilising effect of short bond
  *

Question 6

carbon quantum dots?

Question 7

Describe the effect of molecular conformation on electronic structure with reference to conjugated systems

Answer 7

• Ideally, as you increase chain length of a straight wire, E_g will continue to decrease
• However, twists/bends disrupt conjugation

Question 8

• How can distortion be induced in conjugated sytems?

Answer 8

• By adding a charge to the molecule

Question 9

• Outline a method for estimating conjugation chain length

Answer 9

• E.g in a system of polymers of increasing length, an increase in conjugation will cause an onset in absorption, shifting to longer λ.
• The point where increasing chain length, resulting in the same energy of absorption, is an indication that a twist occurs after this chain conjugation length.

Question 10

• Describe two methods of deliberately disrupting conjugation

Answer 10

• Introduce a CH₂ as linker between two chains
• Create steric hindrance across a bond to form a twist, causing energy levels to split and conjugation to be disrupted