Photo-physics and optoelectronics of 2D layerd perovskites

In 2016 our team at Los Alamos National Lab have successfully made efficient solar cells using Ruddlesden-Popper perovskites (RPPs), a class of organic-inorganic hybrid perovskite with 2D layered structure where nanometer-thick perovskite slab are stacked (as in graphite) and each layer is sandwiched between organic material layers (e.g. buthylamine) [see top-left schematic in Figure below]. Such RRPs are natural quantum well systems where both the perovskite well thickness and the organic spacing layer can be tailored chemically, thus providing unique direct control over the quantum and dielectric confinement of charge carrier in this 2D structure. From a device perspective, the main advantage presented by RPPs with respect to 3D bulk perovskites is their stability during operation and in different environment; our team demonstrated >2000 hrs constant solar cell operation in 65% humidity without significant degradation (see Tsai/Nie et al.Nature 536, 312–316, 2016) with efficiency >12%.
In 2017, we have published a study (Blancon et al. Science, 2017) which explained one of the main microscopic physical origin of the performances observed in 2D layered perovskite solar cells observed only for perovskite layer thickness >1.3 nm. Precisely, at room temperature, we found that the presence of edge-state of lower energy at the edges of the perovskite layers leads to dissociation of strongly bound electron-hole pairs (excitons) to free charge carriers that are longer lived, thus facilitating both extraction of charges during solar cell operation and efficient photoluminescence promising for LEDs. Both processes are illustrated below. We emphasize that intrinsic dissociation of excitons in 2D system is unique to 2D layer perovskites and address a long-standing bottleneck of 2D nano-structures where device performances had been limited by strong exciton binding energy at room temperature.

Photo-physics of organometallic perovskite thin films

Although the interest of using organometallic perovskite materials as active materials in solar cells have motivated intense research during the past few years, the photo-physical properties of these materials is still little known. However understanding the fundamental properties of perovskite - more particularly the optical properties and kinetics of carriers - will be beneficial for improving the solar cell performances targeting the Shockley-Queisser limit.
Recently our team has developed a new method to otain thin films composed of ultra-large perovskite grains (100s of um to mm) which provide reproducible and stable solar cell performance around 15% (see Nie et al. Science 347, 6221, pp. 522-525, 2015). In this work, we also demonstrated in depth how large grain perovskite is beneficial for photovoltaics. Specifically we showed that large grain perovskite have very low defect density as compared to smaller grains (<10 um) using time-resolved photoluminescence (time-correlated single-photon counting).

Investigation of layered transition metal di-chalcogenides for (opto-)electronics applications

In 2D transition metal di-chalcogenides (TMDs) field-effect transistors, the contacts between TMDs and different metals seem to play a major role in limiting the performances of these devices. Recently, it has been show that phase-engineering the TMD material directly under the metal contacts (see figure below for device fabrication via lift-off approach) strongly reduces the contact resistance in TMD-based field-effect transistors and thus improve greatly the performances of these devices (Kappera et al. Nature Materials 13, 1128–1134, 2014).

Phase-engineering method allows for transforming the TMD phase from 2H (semiconducting) to 1T-phase (metallic) under the contact thus reducing strongly the interface TMD/metal contact resistance while improving the field-effect transistor performances.

To further understand the improved characteristics of these devices, we investigated their local opto-electonic response by means of scanning photocurrent microscopy. We recently demonstrated that the Schottky barriers at the interface TMD/metal contact is reduced by more than one order of magnitude which enable the phase-engineered devices to operated with an enhanced responsivity by more than 30x under bias (Yamaguchi-Blancon et al. ACS nano 9, 840-849, 2015).

Scanning photocurrent microscopy technique

Please find here a brief overview of the scanning photocurrent microscopy technique (SPCM) ad how to reconstruct the photocurrent map of a standard planar FET device. This system is used in combination with photoluminescence and/or reflection imaging system to locate the position of both the sample channel and the metal contacts.
Please see details in Yamaguchi-Blancon et al. ACS nano 9, 840-849, 2015.


Measurement of the absolute cross-section of individual single-wall carbon nanotubes

We obtained the first direct measurement of the absorption cross-section of an individual single-wall carbon nanotube (SWNT) deposited on silicon wafer. The chirality of the SWNT was obtained using Raman spectroscopy: this semiconducting CNT is a (18,5) with corresponding diameter around 1.64 nm. The absorption cross-section close the its resonance S33 is 0.35 nm2/nm (1.8 x 10-17cm2/ Carbon atom). This value drops to 0.12 nm2/nm (0.6 x 10-17cm2/ Carbon atom) away from any resonances. 
See details Journal of Physical Chemistry Letters 3(9), pp 1176-1181 (2012)

We investigated the absolute absorption cross-section of individual single-wall carbon nanotubes freely suspended across a trench. Combining absorption spectroscopy and Raman scattering techniques we identified the structure and chirality of each nanotube. In this work we provide absolute values for the absorption cross-section and the oscillator strengths of semiconducting nanotubes with diameters between 1.8 and 2.5 nm (see figure below). Moreover, we investigated the effect of substrate interation on the absorption properties of an individual single-wall carbon nanotubes and demonstrated in this case reduced absorption as well as resonance broadening.
see details Nature Communications 4:2542 (2013)

(c) Experimental oscillator strength per Carbon-atom obtained from the values reported in Blancon et al. Nature Commun. 4:2542 (2013).
(d) Theoretical values computed from Choi et al. Nano Lett., 13(1):54–58, 2013.

Spatial modulation spectroscopy: imaging and measurement of the absolute absorption cross-section of individual nano-objects on transparent and opaque substrates

Spatial modulation spectroscopy (SMS) is a far-field spectroscopy technique providing direct access to the absolute absorption cross-section of individual nano-objects (size < 50 nm). The following movie illustrates the principle of the SMS method aiming at measuring the absolute absorption cross-section spectrum of an individual elongated nano-object (e.g. a carbon nanotube).

The video is composed of:

  • (Left-hand side) the two high numerical aperture objectives (100X) which focalize/collect the laser beam light. The position of the object is modulated (frequency kHz, amplitude few 100 nm) with respect to laser spot in the focal plane.
  • (Right-hand side – top panel) laser spot intensity profile along the modulation direction. The black dot sketches the nanotube position.
  • (Right-hand side – middle panel) sketch of the time evolution of the optical signal (at the fundamental frequency) detected by a photodiode placed after the collection objective.
  • (Right-hand side – bottom panel) reconstructed absolute absorption cross-section spectrum. The relative transmission ΔT/T is directly proportional to the absorption cross-section of the nano-object.

We imaged individual carbon nanotubes (CNTs) deposited on opaque substrates via their absorption. In the figure below, we present  three images of the same individualized CNT deposited on a commercial silicon wafer with a 300 nm silicon dioxyde top layer (device typically encounter in CNT-based transistors). The top and bottom views were obtained with scanning electron microscopy and atomic force microscopy, respectively. The middle image was obtained using our reflective spatial modulation spectroscopy technique.