Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50 meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.

/pdf/excitons-versus-free-charges-in-organo-lead-tri-halide-1xlzucygw2.pdf

Excitons versus free charges in organo-lead tri-halide perovskites

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

Photodetectors based on graphene/WSe2/graphene heterostructures combine an ultrafast photoresponse with high quantum efficiency. Two-dimensional crystals such as graphene and transition-metal dichalcogenides1 demonstrate a range of unique and complementary optoelectronic properties2,3. Assembling different two-dimensional materials in vertical heterostructures4 enables the combination of these properties in one device, thus creating multifunctional optoelectronic systems with superior performance. Here, we demonstrate that graphene/WSe2/graphene heterostructures ally the high photodetection efficiency of transition-metal dichalcogenides5,6 with a picosecond photoresponse comparable to that of graphene7,8,9, thereby optimizing both speed and efficiency in a single photodetector. We follow the extraction of photoexcited carriers in these devices using time-resolved photocurrent measurements and demonstrate a photoresponse time as short as 5.5 ps, which we tune by applying a bias and by varying the transition-metal dichalcogenide layer thickness. Our study provides direct insight into the physical processes governing the detection speed and quantum efficiency of these van der Waals heterostuctures, such as out-of-plane carrier drift and recombination. The observation and understanding of ultrafast and efficient photodetection demonstrate the potential of hybrid transition-metal dichalcogenide-based heterostructures as a platform for future optoelectronic devices.

Picosecond photoresponse in van der Waals heterostructures

Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance.

/pdf/correlated-electron-hole-plasma-in-organometal-perovskites-1lbilck9w5.pdf

Correlated electron–hole plasma in organometal perovskites

Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2D) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics, of pivotal importance for the understanding and optimization of device operation, is still lacking. Here, we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photoexcited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact ...

Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2

The 2D semiconductor MoS2 in its mono- and few-layer form is expected to have a significant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration. Here, modulation spectroscopy in the sub-ps and ms time scales is used to study the photoexcitation dynamics in few-layer MoS2. The results suggest that the primary photoexcitations are excitons that efficiently dissociate into charges with a characteristic time of 700 fs. Based on these findings, simple suggestions for the design of efficient MoS2 photovoltaic and photodetector devices are made.

Charge Photogeneration in Few-Layer MoS2

1 Introduction 2 Radiation-Matter-Interaction in the Two-Level-System 3 Molecular Exciton 4 Excited States in Solids 5 Photoexcitation Dynamics 6 Photphysics Tool Box 7 Vibrational Spectroscopy 8 Charge Transfer and Transport 9 Pump-probe and Other Modulation Techniques 10 Conclusions and Future Perspectives

The Photophysics behind Photovoltaics and Photonics

The two-dimensional semiconductor MoS2 in its mono- and few-layer form is expected to have a significant exciton binding energy of several 100 meV, leading to the consensus that excitons are the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration (CPG). Here we use continuous wave photomodulation spectroscopy to identify the optical signature of long-lived charge carriers and femtosecond pump-probe spectroscopy to follow the CPG dynamics. We find that intitial photoexcitation yields a branching between excitons and charge carriers, followed by excitation energy dependent hot exciton dissociation as an additional CPG mechanism. Based on these findings, we make simple suggestions for the design of more efficient MoS2 photovoltaic and photodetector devices.

Charge photogeneration in few-layer MoS2

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.

Guglielmo Lanzani

Papers

Charge Photogeneration in Few-Layer MoS2

The Photophysics behind Photovoltaics and Photonics

Charge photogeneration in few-layer MoS2

Long-Lived Charged States in Single-Walled Carbon Nanotubes

Pump Probe and Other Modulation Techniques