Does Conjugation Help Exciton Dissociation? A Study on Poly(p‐phenylene)s in Planar Heterojunctions with C₆₀ or TNF

Abstract

Internal photocurrent quantum yields near 100% can be obtained from the separation of loosely bound geminate pairs when sufficiently large electric fields are applied to organic heterojunctions. The fields needed for complete electron-hole dissociation decrease to those prevailing in organic solar cells under operating conditions when well-conjugated polymers are employed.

The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

“…[ 24,25 ] Large and ordered domains can be expected to result in enhanced delocalization of the electron and hole wave functions, which would in turn reduce the effective binding energy of the CT pairs. [ 48 ] Moreover, Si-PCPDTBT:PCBM blends exhibit less geminate recombination to the ground state and to the triplet state of the polymer compared to C-PCPDTBT:PCBM, [ 49 ] supporting the idea of enhanced CT dissociation effi ciency. The independence of charge generation on excess energy in Si-PCPDTBT:PCBM can therefore be explained in terms of the enhanced CT dissociation effi ciency: in this blend all the CT states, including the ones created at the bottom of the CT band dissociate with the same (high) effi ciency in presence of moderate electric fi elds.…”

Local Intermolecular Order Controls Photoinduced Charge Separation at Donor/Acceptor Interfaces in Organic Semiconductors

“…This observation is complimentary to the results that other authors have presented, as discussed in Section 1. [3][4][5][6][7][8][9][10][11][13][14][15][16] The hypothesis is that intermolecular delocalization across molecular aggregates creates a large effective separation distance between the electron and hole after the initial electron transfer event and enables them to escape their mutual coulomb attraction.…”

“…For example, it appears that fullerene aggregates enhance the production of free charges versus bound geminate pairs in donor/fullerene blends. [3][4][5][6][7][8][9] Similarly, donor polymers with longer conjugation lengths can enhance charge separation, [10][11][12] and How free charge is generated at organic donor-acceptor interfaces is an important question, as the binding energy of the lowest energy (localized) charge transfer states should be too high for the electron and hole to escape each other. Recently, it has been proposed that delocalization of the electronic states participating in charge transfer is crucial, and aggregated or otherwise locally ordered structures of the donor or the acceptor are the precondition for this electronic characteristic.…”

Visualizing charge separation in bulk heterojunction organic solar cells

“…A point charge description of the electron and hole of the charge pair state appears to result in a high binding energy that would prevent efficient charge separation. Ultrafast initial long-range carrier separation during thermalization of the charge pair states 13,16 or delocalization of CT states 9,17,18 have been proposed to explain the separation mechanism, whereas other researches argue that only the closely separated charge pair states are initially created [19][20][21] and favourable structural organization is more important than excess energy 13,22,23 . Recently, ultrafast spectroscopy methods have been employed to follow dynamics of Frenkel-type or CT exciton splitting into charge pairs 7,9,[24][25][26] .…”