In ZnO quantum dot/graphene heterojunction photodetectors, fabricated by printing quantum dots (QDs) directly on the graphene field-effect transistor (GFET) channel, the combination of the strong quantum confinement in ZnO QDs and the high charge mobility in graphene allows extraordinary quantum efficiency (or photoconductive gain) in visible-blind ultraviolet (UV) detection. Key to the high performance is a clean van der Waals interface to facilitate an efficient charge transfer from ZnO QDs to graphene upon UV illumination. Here, we report a robust ZnO QD surface activation process and demonstrate that a transition from zero to extraordinarily high photoresponsivity of 9.9 × 10 A/W and a photoconductive gain of 3.6 × 10 can be obtained in ZnO QDs/GFET heterojunction photodetectors, as the ZnO QDs surface is systematically engineered using this process. The high figure-of-merit UV detectivity D* in exceeding 1 × 10 Jones represents more than 1 order of magnitude improvement over the best reported previously on ZnO nanostructure-based UV detectors. This result not only sheds light on the critical role of the van der Waals interface in affecting the optoelectronic process in ZnO QDs/GFET heterojunction photodetectors but also demonstrates the viability of printing quantum devices of high performance and low cost.

Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)-molybdenum disulfide (MoS) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1-100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet-triplet transition is feasible because of the large singlet-triplet splitting in organic materials and the strong spin-orbit coupling in TMD crystals. The back electron transfer would reduce the yield of free carrier generation at the heterojunction if it is not avoided. On the other hand, the spin-selective back electron transfer could be used to manipulate electron spin in hybrid electronic devices.

Monolayer transition-metal dichalcogenide crystals (TMDC) can be combined with other functional materials, such as organic molecules, to from a wide range of heterostructures with tailorable properties. Although a number of works have shown that ultrafast charge transfer (CT) can occur at organic-TMDC interfaces, conditions that would facilitate the separation of interfacial CT excitons into free carriers remain unclear. Here, time-resolved and steady-state photoemission spectroscopy are used to study the potential energy landscape, charge transfer and exciton dynamics at the zinc phthalocyanine (ZnPc)/monolayer (ML) MoS 2 and ZnPc/bulk MoS 2 interfaces. Surprisingly, although both interfaces have a type-II band alignment and exhibit sub-100 femtosecond CT, the CT excitons formed at the two interfaces show drastically different evolution dynamics. The ZnPc/ML-MoS 2 behaves like typical donor-acceptor interfaces in which CT excitons dissociate into electron-hole pairs. On the contrary, back electron transfer occur at ZnPc/bulk-MoS 2 , which results in the formation of triplet excitons in ZnPc. The difference can be explained by the different amount of band bending found in the ZnPc film deposited on ML-MoS 2 and bulk-MoS 2. Our work illustrates that the potential energy landscape near the interface plays an important role in the charge separation behavior. Therefore, considering the energy level alignment at the interface alone is not enough for predicting whether free charges can be generated effectively from an interface.

Graphene field effect transistor sensitized by a layer of semiconductor (sensitizer/GFET) is a device structure that is investigated extensively for ultrasensitive photodetection. Among others, organometallic perovskite semiconductor sensitizer has the advantages of long carrier lifetime and solution processable. A further step to improve the responsivity is to design a structure that can promote electron-hole separation and selective carrier trapping in the sensitizer. Here, the use of a hybrid perovskite-organic bulk heterojunction (BHJ) as the light sensitizer to achieve this goal is demonstrated. Our spectroscopy and device measurements show that the CH 3 NH 3 PbI 3 -PCBM BHJ/GFET device has improved charge separation yield and carrier lifetime as compared to a reference device with a CH 3 NH 3 PbI 3 sensitizer only. The key to these enhancement is the presence of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), which acts as charge separation and electron trapping sites, resulting in a 30-fold increase in the photoresponsivity. This work shows that the use of a small amount of electron or hole acceptors in the sensitizer layer can be an effective strategy for improving and tuning the photoresponsivity of sensitizer/GFET photodetectors.

How tightly bound charge transfer (CT) excitons dissociate at organic donor-acceptor interfaces has been a long-standing question in the organic photovoltaics community. Recently, it has been proposed that exciton delocalization reduces the exciton binding energy and promotes exciton dissociation. In order to understand this mechanism, it is critical to resolve the evolution of the exciton's binding energy and coherent size with femtosecond time resolution. However, because the coherent size is just a few nanometers, it presents a major experimental challenge to capture the CT process simultaneously in the energy, spatial, and temporal domains. In this work, the challenge is overcome by using time-resolved photoemission spectroscopy. The spatial size and electronic energy of a manifold of CT states are resolved at the zinc phthalocyanine (ZnPc)-fullerene (C) donor-acceptor interface. It is found that CT at the interface first populates delocalized CT excitons with a coherent size of 4 nm. Then, this delocalized CT exciton relaxes in energy to produce CT states with delocalization sizes in the range of 1-3 nm. While the CT process from ZnPc to C occurs in about 150 fs after photoexcitation, the localization and energy relaxation occur in 2 ps. The multidimensional view on how CT excitons evolve in time, space, and energy provides key information to understand the exciton dissociation mechanism and to design nanostructures for effective charge separation.

In organic semiconductors, optical excitation does not necessarily produce free carriers. Very often, electron and hole are bound together to form an exciton. Releasing free carriers from the exciton is essential for the functioning of photovoltaics and optoelectronic devices, but it is a bottleneck process because of the high exciton binding energy. Inefficient exciton dissociation can limit the efficiency of organic photovoltaics. Here, nanoscale features that can allow the free carrier generation to occur spontaneously despite being an energy uphill process are determined. Specifically, by comparing the dissociation dynamics of the charge transfer (CT) exciton at two donor–acceptor interfaces, it is found that the relative orientation of the electron and hole wavefunction within a CT exciton plays an important role in determining whether the CT exciton will decompose into the higher energy free electron–hole pair or relax to the lower energy tightly‐bound CT exciton. The concept of the entropic driving force is combined with the structural anisotropy of typical organic crystals to devise a framework that can describe how the orientation of the delocalized electronic wavefunction can be manipulated to favor the energy‐uphill spontaneous dissociation of CT excitons over the energy‐downhill CT exciton cooling.

The electronic properties of small molecule organic crystals depend heavily on the molecular orientation. For multi-layer organic photovoltaics, it is desirable for the molecules to have a face-on orientation in order to enhance the out-of-plane transport properties. However, it is challenging to grow well-ordered and smooth films with a face-on stacking on conventional substrates such as metals and oxides. In this work, metal-phthalocyanine molecules is used as a model system to demonstrate that two-dimensional crystals such as graphene can serve as a template for growing high quality, ultra-flat organic films with a face-on orientation. Furthermore, the molecule-substrate interaction is varied systematically from strong to weak interaction regime with the interaction strength characterized by ultrafast electron transfer measurements. We find that in order to achieve the optimum orientation and morphology, the molecule-substrate interaction needs to be strong enough to ensure a face-on stacking while it needs to be weak enough to avoid film roughening.

Plasmonic metal nanostructures provide a promising strategy for light trapping and therefore can dramatically enhance photocurrent in optoelectronics only if the trapped light can be coupled effectively from plasmons to excitons, whereas the reverse transfer of energy, charge, and heat from excitons to plasmons can be suppressed. Motivated by this, this work develops a scheme to implement a metafilm with Ag nanoparticles (NPs) embedded in 10 nm thick silica (Ag NPs–silica metafilm) to the active device channel of a hybrid perovskite film/graphene photodetector. Remarkably, an enhancement factor of 7.45 in photoresponsivity, the highest so far among all the reports adopting plasmonic metal NPs in perovskite photodetectors, has been achieved on the photodetectors with the Ag NPs–silica metafilms. Considering that the synthesis of the Ag NPs–silica metafilms can be readily scaled up to coat both rigid and flexible substrates, this result provides a low-cost metaplatform for a variety of high-performance optoelectronic device applications.