Notes on Disordered Matter

After the introductory posts about organic solar cells – split in parts zero, one and two, – I would like to present a somewhat more intuitive picture today… well, picture indeed says it all;-)

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.

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