Probing the pathways of free charge generation in organic bulk heterojunction solar cells

Abstract

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

“…First, free charge generation does not benefit from a larger driving force (channel I vs channel II) nor from excess photon energy. These findings substantiate the conclusions from earlier studies on fullerene‐based solar cells, which were interpreted in terms of a cold generation process, involving a low energy CT state manifold as a precursor to free charges . Second, the lack of a field dependence of generation, independent of the excitation energy, suggests a low energetic barrier for the dissociation of such low energy precursor states.…”

“…Notably, the activation energy for CT dissociation is unaffected by entropic effects . Temperature‐dependent studies on fullerene‐based blends revealed activation energies for CT dissociation from few tens to ≈100 meV, with some important exemptions as discussed next. We are aware of only one publication reporting temperature‐dependent measurements on the CT binding in a NFA blend .…”

“…In order to elucidate the role of the pathways on the free carrier generation, we performed TDCF experiments as a function of the electric field and temperature with excitation at different photon energies. The experimental details on TDCF have been described elsewhere . In short, the device is excited with a short laser pulse (≈5 ns) while being held at a given pre‐bias ( V pre ).…”

A Dual Threat: Redox‐Activity and Electronic Structures of Well‐Defined Donor–Acceptor Fulleretic Covalent‐Organic Materials

“…Merging the properties of three‐dimensional buckyballs (or buckybowls) and covalent‐organic frameworks (COFs) provides access to a novel class of materials combining ultrafast energy/electron transport characteristic of three‐dimensional (3D) fulleretic linkers with high modularity, crystallinity, and surface area, which are intrinsic properties of COFs . The precise donor–acceptor alignment achieved in the materials is imposed by the rigid COF scaffold and is crucial for efficient energy or charge transfer as it can influence the distance of exciton diffusion, π–π stacking, or Förster radius, and as a result, can enhance device performance …”

Spin‐Dependent Electron–Hole Recombination and Dissociation in Nonfullerene Acceptor ITIC‐Based Organic Photovoltaic Systems

“…These processes highly depend on optoelectronic properties of a given donor (D), acceptor (A), and D:A interface. At the interface, build‐in intermolecular charge transfer (CT) states that act as intermediate states between the photo‐generated Frenkel exciton (FE) and charge separation (CS) states play a dominant role in the electron–hole dissociation . Nevertheless, little is known from spin‐dependent dissociation and recombination channels in the NFA system …”