Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation

Abstract

Charge carriers are photogenerated with very different spatial distributions in conventional inorganic photovoltaic (IPV) cells and in organic photovoltaic (OPV or excitonic) cells. This leads to a fundamental, and often overlooked, mechanistic difference between them. Carriers are generated primarily at the exciton-dissociating heterointerface in OPV cells, resulting in the production of electrons in one phase and holes in the other—the two carrier types are thus already separated across the interface upon photogeneration in OPV cells, giving rise to a powerful chemical potential energy gradient ∇μhv that promotes the photovoltaic effect. This occurs also in high-surface-area OPV cells, although their description is more complex. In contrast, both carrier types are photogenerated together throughout the bulk in IPV cells: ∇μhv then drives both electrons and holes in the same direction through the same phase; efficient carrier separation therefore requires a built-in equilibrium electrical potential energy difference ∅bi across the cell. The open-circuit photovoltage Voc is thus limited to ∅bi in IPV cells, but it is often greater than ∅bi in OPVs. The basic theory necessary to compare IPVs to OPVs is reviewed. Relevant experiments are described, and numerical simulations that compare semiconductor devices differing only in the spatial distribution of photogenerated carriers are presented to demonstrate this fundamental distinction between the photoconversion mechanisms of IPV and OPV devices.

Progress on a Photovoltaic Cell Design Consisting of Silicon Nanowires and Poly (3‐ hexylthiophene)

“…The exciton nd ag junction fo is formed by the absorption of a photon by the organic p-type polymer, and then diffuses to the heterojunction where it dissociates into charge carriers by the electric field created by the p-n junction. The h moves back into the HOMO state of the p-type polymer while the e is transferred to the E C of the n-type inorganic semiconductor [17,38,41]. Therefore, there are two different transport mechanisms taking place simultaneously within the polymer semiconductor: the transport of electrically neutral excitons to the p-n junction, and the transport of the charge carrier to the electrode.…”

“…There it is dissociated into charge carriers by the energy band offset of the p-n junction. The e is transferred to the E C of the n-type inorganic semiconductor and the h moves back into the HOMO state of the organic polymer [38,41]. Optimally, the photon would be absorbed within the depletion region of the p-n junction where the exciton would be immediately split into charge carriers.…”

“…Once at the p-n junction, the energy band offset between the organic semiconductor's LUMO state and the inorganic semiconductor's E C must be greater than the E B of the Immediately upon charge carrier generation, the carriers may B exciton [41]. recombine via the potential drop betw orks its way back through the harge carrier forms a olaron with the localized band-like state by coupling with e -/h + exciton Energy een the E C of the inorganic phase and the HOMO state of the organic phase or by Coulombic forces [38].…”

“…This is due to the fact that charge carriers are not generated upon absorption of a photon by the polymer lattice. Instead, an excited but electrically neutral state, called and exciton 4 , is created which requires additional energy (E B ) to separate into charge carriers [38,41]. This energy is gained form the offset of energy states at the heterojunction [41][42][43][44][45][46].…”

High-Efficiency Photochemical Water Splitting of CdZnS/CdZnSe Nanostructures

“…Although PECs featuring QDs-sensitized TiO 2 and ZnO electrodes provided greater values for water splitting than corresponding bare electrodes, their values are still less than 4% [11][12][13]. Moreover, an external potential is mandatory to separate the charges efficiently to prevent the recombination of electron-hole pair under illumination [20].…”