Recovering lost excitons in organic photovoltaics using a transparent dissociation layer

Abstract

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

Effect of axial halogen substitution on the performance of subphthalocyanine based organic photovoltaic cells

“…Assuming the post-dissociation carrier collection efficiency for thin bilayer OPVs to be unity, exciton diffusion lengths fitted to the EQE suggest exciton dissociation efficiency to be comparable between the devices (L D = 10.2 nm for SubPcCl, and 10.3 nm for SubPc-F). The interface between MoO 3 and SubPc has been recognized as not fully quenching, as understood from previously reported EQE fits [20], thus leading to overestimation of the exciton diffusion length in these samples [9,11,12]. The pointwise product of the EQE and AM1.5D spectra results in similar short-circuit current density (J SC ) between the two devices.…”

Decreased Recombination Through the Use of a Non‐Fullerene Acceptor in a 6.4% Efficient Organic Planar Heterojunction Solar Cell

“…We therefore investigate the effect of the anode confi guration on solar cell performance, by comparing two solar cell architectures (thickness in nm) with or without the presence of a These layers may either be exciton blocking HTLs, including LiF [ 11 ] and tris[4-(5-phenyl thiophen-2-yl)phenyl]amine, [ 12 ] or exciton dissociating HTLs, such as N , N ′-bis(naphthalen-1-yl)-N , N ′-bis(phenyl)-2,2′-dimethylbenzidine for the donor material boron subphthalocyanine chloride (SubPc). [ 13 ] Here, we examine the promising donor material boron subnaphthalocyanine chloride (SubNc, Figure 1 a). Previously used as a donor material, SubNc combines strong red absorption (extinction coeffi cient k peaks at 1.4 for a wavelength λ = 686 nm, absorption edge at λ = 730 nm) with an open-circuit voltage of V oc ≈ 0.8 V when combined with C 60 .…”

8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer

“…In planar heterojunction devices, several mechanisms have been proposed to extract photocurrent from an additional active layer. In a three-layer stack with cascade-energy-level-alignment, a centered ambipolar layer can enable double exciton dissociation at both interfaces with the donor and acceptor, increasing the photocurrent generation in the cell 21,26 . However, this is at the expense of the voltage generated by the solar cell, as the V OC is ultimately limited by the energy levels of the outer layers in such a cascade structure.…”