The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors

Abstract

The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (<1 picosecond) via infrared (IR) optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modeling showed that the IR photons promote bound charge pairs to delocalized band states, similar to those formed just after singlet exciton dissociation, which indicates that such states act as the gateway for charge separation. Our results suggest that charge separation in efficient organic photoconversion systems occurs through hot-state charge delocalization rather than energy-gradient-driven intermolecular hopping.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…122 In the PPP technique, a working device is first pumped by visible light, creating separated charges and/or localised CT states. The pump excitation is followed by an infrared push pulse, which is supposed to re-excite the localised CT states into separated charges.…”

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

“…C harge separation has a key role in determining solar energy conversion efficiency of semiconductor-based systems for producing solar electricity and solar fuels through solar cells [1][2][3][4] , photoelectrocatalysis [5][6][7][8] and photocatalysis [9][10][11][12][13] . As a key step in energy conversion, electron-hole pairs generated by light absorption need to be separated and transferred to the surface of the semiconductors 1,9,[14][15][16][17] .…”

“…Thus, certain facets of a semiconductor prefer reduction while others favour oxidation [18][19][20] . However, reports on the reduction and oxidation reaction facets on the same semiconductor crystal (such as TiO 2 ) were often contradictory in the literature [21][22][23][24][25][26] . For example, it was reported that rutile {011} and anatase {001} faces provided the sites for oxidation, while the rutile {110} and anatase {101} faces offered the sites for reduction 24 .…”

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

“…Light absorption by the polymer has been largely improved towards the near infrared (NIR) spectral region by the introduction of low-bandgap co-polymers 4,5 . Exciton dissociation, however, remains to be understood and has recently been the subject of several investigations [6][7][8][9][10] .…”

“…There, the large energy offset (  0.5 eV) between the lowest unoccupied molecular orbitals (LUMOs) of polymer and fullerene, possibly gated by the formation of hot delocalized states 9 , drives exciton dissociation and charge separation. This is supposed to occur only after diffusion of the excitons in the polymer, which is usually forming an interpenetrating network of domains with the fullerene 12 .…”