Step 1: Light Absorption => Exciton Generation
- light is absorbed in the donor material, e.g., a conjugated polymer
- excitons are thus created, strongly bound electron-hole pairs on the polymer chain
- very high absorption coefficient, device thickness on ~100nm scale, as compared to the inorganic polycrystalline semiconductor CuInSe2 (~1 micron) and crystalline Silicon (~100 micron)
- but: only narrow absorption bands, as shown for two conjugated polymers P3HT and PCPDTBT in comparison to CuInSe2. This drawback could be circumvented by synthesis of novel materials, or multijunction concepts (tandem solar cells).
Step 2: Exciton Diffusion => to Acceptor Interface
- the photogenerated excitons are strongly Coulomb bound due to the low dielectric constant in organic materials, and the correspondingly low screening length: charges can ‘see’ each other very well
- electrically neutral excitons can only move by diffusion
- in order to disociate into an electron-hoe pair, it has to find an acceptor site (e.g., fullerene molecule)
- short exciton diffusion length of only a few nanometres
- therefore, no bilayer concept, instead bulk heterojunction solar cells of intermixed donor and acceptor materials (shown in the figure), such as conjugated polymers blended with fullerene derivatives
Step 3: Exciton Dissociation => Polaron Pair Generation
- excitons dissociate only at energetically favourable acceptor molecules such as the fullerenes, when the energy gain is larger than the exciton binding energy
- then, an electron transfer (or charge transfer) takes place, dissociating the exciton into an electron on the fullerene acceptor, and a hole remaining on the polymer
- this electron-hole pair is still Coulomb bound, and is called geminate pair or polaron pair
Step 4: Polaron Pair Dissociation => Free Electron–Hole Pairs!
- the polaron pairs are Coulomb bound
- they also need to be dissociated, this time by an electric field ( = built-in voltage + applied voltage)
- therefore, the photocurrent in organic solar cells depends strongly on the applied voltage
- this is a major loss mechanism in organic solar cells
Step 5: Charge Transport => Photocurrent!
- the electrons and holes are transported to the respective electrodes, driven by the electric field, and moved by a hopping transport process
- hopping: very slow charge transport, low carrier mobility, at least a factor of 1000 smaller than for crystalline Silicon… while the power conversion efficiency of organic solar cells is only factor 4 worse;-)
- indeed, our current research indicates that a loss of free charge carriers by nongeminate recombination during the charge transport to the contacts is only marginal
- and, higher mobility does not improve the power conversion efficiency significantly. Will be covered in a later post;-)
So much for now, see you later.