PV materials

PV materials

1. Organic

   • Earth abundant
   • Non toxic
   • cheap
   • tuneable properties
   • Tuneable properties
   • Processed at low temp (under 150C)
     • low energy outlay
     • compatible with flexible substrate
   • Thin film- less material
   • roll-to-roll manufacture
   • Low mobility (hopping transport)
   • Flexible: weak intermolecular interactions
   • Fullerenes
     • Rapid electron transfer
     • Long lived charge separated state
     • Don't absorb light very strongly

2. Silicon

   • Indirect semiconductor for long wavelengths
   • High absorption by thick cell
   • efficient charge separation requires build in electric potential (doping)
   • Few defects for efficient transport (series resistance)
   • Improvements
     • Thin film
       • lower purity requirement
       • faster deposition
       • lower raw materials cost
       • lower energy pay back time
       • solution processed at low temps.
       • roll-to-roll manufacturing

3. Perovskites

   • High charge carrier mobility
   • Small exciton binding energy
     • thermal energy enough
   • Narrow tuneable band gap
   • Direct band gap (no momentum change)
     • electron + photon -> large absorption coefficient
   • Processability
     • Low temp.
     • Column solvent e.g. DMF
   • Salt like minerals that readily dissolve in moisture
   • Dimerise to form weakly absorbing semiconductor

4. Coilloidal nano-crystals

   • Solution processable
   • Low temps (under 200C)
   • Tuneable band gap
     • Alloying
       • low Eg -> low Voc
     • Quantum size effect
   • Si nanocrystals have direct transitions
   • ligand must be short to be transparent to charge

5. DSSCs

   • Absorption and transport carried out by different materials
   • Exciton has large binding energy which must be overcome
   • Fast kinetics
     • photoelectron transfer
     • reduction of oxidised dye
     • Electron diffusion through TiO2
     • Reduction of redox couple
   • Slow kinetics
     • Reduction of redox couple by electron from TiO2 CB
     • Reduction of oxidised dye by electron from TiO2 CB
     • Relaxation of excited dye
   • I-/I3- corrosive to metal electrodes
   • Homo energy of dye must be deep enough for redox couple to regenerate it spontaneously
   • LUMO must be shallow enough for spontaneous electron transfer to TiO2 CB (>0.1eV)
   • Dye must bind strongly to TiO2 to maximise mechanical strength and electronic coupling between dye LUMO and TiO2 CB
   • Dye design
     • Light harvesting portion
     • Anchor group
       • Bidentate offers improved mechanical and electronic coupling (faster transfer)
       • Conjugated link -> faster tranfer
     • Bulky side group to increase solubility and reduce aggregation and physical barrier between holes in electrolyte and electron in CB
     • Ru bipyridal complexes common as broad absorption but expensive
     • Organics
       • high molar extinction coefficients
       • Stable
       • Photophysical properties can be tailored
       • Narrow spectra band width
       • Push-pull group keep electron and hole separated and electron away from redox couple
       • Increasing conjugation reduced band gap