Visualizing excitations at buried heterojunctions in organic semiconductor blends

Interfaces play a crucial role in semiconductor devices, but in many device architectures they are nanostructured, disordered and buried away from the surface of the sample. Conventional optical, X-ray and photoelectron probes often fail to provide interface-specific information in such systems. Here we develop an all-optical time-resolved method to probe the local energetic landscape and electronic dynamics at such interfaces, based on the Stark effect caused by electron–hole pairs photo-generated across the interface. Using this method, we found that the electronically active sites at the polymer/fullerene interfaces in model bulk-heterojunction blends fall within the low-energy tail of the absorption spectrum. This suggests that these sites are highly ordered compared with the bulk of the polymer film, leading to large wavefunction delocalization and low site energies. We also detected a 100 fs migration of holes from higher- to lower-energy sites, consistent with these charges moving ballistically into more ordered polymer regions. This ultrafast charge motion may be key to separating electron–hole pairs into free charges against the Coulomb interaction.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability. A.C.J. thanks the University of Cambridge for funding (CHESS). Synchrotron measurements were undertaken on the SAXS beamline at the Australian Synchrotron, Victoria, Australia and we acknowledge the help of N. Lal with the measurements. S.H. thanks the framework project Soltech for funding

On the energetics of bound charge-transfer states in organic photovoltaics

Using temperature-dependent optical-control spectroscopy, we show that the binding energy of localised charge-transfer state is about 90 meV in a range of organic photovoltaic systems.China Scholarship Council; Winton Programme for the Physics of Sustainability; the Engineering and Physical Sciences Research Council; the University of Cambridge (CHESS); Royal Society University Research Fellow; St John’s College, Cambridg

Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination

Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm−2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells

High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes

Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL half-life of 46 hours under continuous operation. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer. Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From PL quantum efficiency and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that perovskite-based LEDs have significant potential for light-emission applications

Research data supporting "Visualising Excitations at Buried Heterojunctions in Organic Semiconductor Blends"

Original data resulting in the publication "Visualising Excitations at Buried Heterojunctions in Organic Semiconductor Blends". It contains raw files of pump-probe (transient absorption), pump-push-probe, small-/wide-angle X-ray scattering, photothermal deflection spectroscopy, UV-Vis absorption, and photoluminescence measurements