143 research outputs found
Photo-FETs: phototransistors enabled by 2D and 0D nanomaterials
The large diversity of applications in our daily lives that rely on photodetection technology requires photodetectors with distinct properties. The choice of an adequate photodetecting system depends on its application, where aspects such as spectral selectivity, speed, and sensitivity play a critical role. High-sensitivity photodetection covering a large spectral range from the UV to IR is dominated by photodiodes. To overcome existing limitations in sensitivity and cost of state-of-the-art systems, new device architectures and material systems are needed with low-cost fabrication and high performance. Low-dimensional nanomaterials (0D, 1D, 2D) are promising candidates with many unique electrical and optical properties and additional functionalities such as flexibility and transparency. In this Perspective, the physical mechanism of photo-FETs (field-effect transistors) is described and recent advances in the field of low-dimensional photo-FETs and hybrids thereof are discussed. Several requirements for the channel material are addressed in view of the photon absorption and carrier transport process, and a fundamental trade-off between them is pointed out for single-material-based devices. We further clarify how hybrid devices, consisting of an ultrathin channel sensitized with strongly absorbing semiconductors, can circumvent these limitations and lead to a new generation of highly sensitive photodetectors. Recent advances in the development of sensitized low-dimensional photo-FETs are discussed, and several promising future directions for their application in high-sensitivity photodetection are proposed.Peer ReviewedPostprint (author's final draft
Strategies for controlled electronic doping of colloidal quantum dots
Over the last several years tremendous progressed progress has been made in incorporating
Colloidal Quantum Dots (CQDs) as photoactive components in optoelectronic devices. A
significant part of that progress is associated with significant advancements made in
achievingon controlled electronic doping of the CQDs and thus improving the electronic
properties of CQDs solids. Today, a variety of strategies exists towards that purpose and this
minireview aims at surveying major published works in this subject. Additional attention is
given to the many challenges associated with the task of doping CQDs as well as to the
optoelectronic functionalities and applications being realized when successfully achieving light
and heavy electronic doping of CQDs.Peer ReviewedPostprint (author's final draft
Ag2ZnSnS4 Nanocrystals Expand the Availability of RoHS Compliant Colloidal Quantum Dots
The demonstration of the quantum confinement effect in colloidal quantum dots (QDs) has been extensively studied and exploited mainly in Pb and Cd chalcogenide systems. There has been an urgent need recently for the development of non(less)-toxic colloidal QDs to warrant compliance with current safety regulations (Restriction of Hazardous Substances (RoHS) Directive 2002/95/EC). Herein, we report Pb/Cd-free, solution processed luminescent Ag2ZnSnS4 (AZTS) colloidal QDs. We present a selective and controlled amine and thiol-free synthesis of air stable luminescent AZTS QDs by the hot injection technique. By controlling the reaction conditions we obtain controlled size variation and demonstrate the quantum confinement effect that is in good agreement with the theoretically calculated values. The band gap of the AZTS QDs is size-tunable in the near-infrared from 740 to 850 nm. Finally, we passivate the surface with Zn-oleate, which yields higher quantum yield (QY), longer lifetime, and better colloidal stability.Peer ReviewedPostprint (published version
The Role of Surface Passivation for Efficient and Photostable PbS Quantum Dot Solar Cells
Peer ReviewedPostprint (author's final draft
Matildite Contact with Media: First-Principles Study of AgBiS2 Surfaces and Nanoparticle Morphology
Motivated by the interest in AgBiS2 material for solar light harvesting applications, a detailed bulk first-principles quantum mechanical study of its surface properties is presented. Density functional theory based calculations with the Perdew–Burke–Ernzerhof functional have been carried out for different surface orientations and terminations of the matildite polymorph. From the results, two particularly stable facets are predicted to dominate Wulff shaped AgBiS2 nanoparticles. These are the (001) type nonpolar surface and the (111) polar terminations where facets are exposed containing solely Ag or S atoms. The Wulff equilibrium shape is predicted to be a cube with only two edges capped. This particular shape explains a previously reported surface enrichment of Ag with respect to Bi of ∼1.5. The (001) surfaces display an ionic character similar to bulk AgBiS2, with a low work function of 4.31 eV, although the inspection of the density of states (DOS) reveals a bandgap increased by 0.3 eV compared to bulk. This surface effect could explain the bulk wavelength overestimation of the absorption coefficient decay, as previously determined. Last but not least, the DOS of the (111) polar termination reveals a metallic character, where Fermi level is located below that on the (001) surfaces. Possible implications of the different electronic structure of these surfaces in the reported photoactivity are discussed.Peer ReviewedPostprint (author's final draft
Highly efficient, ultrathin, Cd-free kesterite solar cells in superstrate configuration enabled by band level tuning via Ag incorporation
Kesterite, or Cu2ZnSn(S,Se)4 (CZTS) is promising in developing sustainable PV technology due to its earth-abundant, non-toxic composition. However, issues including instability of interface, high density of defects that fails to allow the short charge-collection length to meet its light absorption needs, use of Cd that fails to comply with the restriction of hazardous substances (RoSH), are promoting the development of alternative, eco-friendly device structure. Here, this study reports an important progress on this subject by adopting the superstrate configuration to kesterite, thus to realize advantageous light management and high defect tolerance in an ultrathin device. By incorporating Ag in kesterite to overcome the detrimental alignment at the pristine interface, a solar cell with PCE of 8.1% has been fabricated with ~ 200 nm absorber and ~ 15 nm TiO2 buffer, representing a PCE improvement of nearly three-fold from the baseline Cu2ZnSn(S,Se)4 device and breaking the 5% PCE limit for superstrate kesterite cells to date. Moreover, this enables the sole use of TiO2 as novel buffer material free of toxic Cd. Further analysis reveals the critical role of Ag in synergistically tailoring band offset and bandgap, along with largely reduced density of defects, leading to this substantial performance improvement.Peer ReviewedPostprint (published version
Plasmonic light trapping leads to responsivity increase in colloidal quantum dot photodetectors
We report broadband responsivity enhancement in PbS colloidal quantum dot (CQDs)
photoconductive photodetectors due to absorption increase offered by a plasmonic scattering layer
of Ag metal nanoparticles. Responsivity enhancements are observed in the near infrared with a
maximum 2.4-fold increase near the absorption band edge of 1 lm for 400 nm thick devices.
Additionally, we study the effect of the mode structure on the efficiency of light trapping provided
by random nanoparticle scattering in CQD films and provide insights for plasmonic scattering
enhancement in CQD thin films.This research has been partially supported by Fundacio´
Privada Cellex Barcelona. We also acknowledge support from
European Commission’s Seventh Framework Programme for
Research under contract PIRG06-GA-2009-256355
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