• Energy Research

Bi2S3 nanocrystals are employed as an n-type, non-toxic, inorganic, solution-processed semiconductor in thin film solar cells. The first solution processed-inorganic p-n junction based on p-type PbS QDs and n-type Bi2S3 nanocrystals with both phases contributing to photocarrier generation is demonstrated. The reported devices show a power conversion efficiency of 1.6% for 860 nm PbS QDs and over 1% for 1300 nm PbS QDs.

More-efficient charge collection and suppressed trap recombination in colloidal quantum dot (CQD) solar cells is achieved by means of a bulk nano-heterojunction (BNH) structure, in which p-type and n-type materials are blended on the nanometer scale. The improved performance of the BNH devices, compared with that of bilayer devices, is displayed in higher photocurrents and higher open-circuit voltages (resulting from a trap passivation mechanism).

A post-deposition in situ passivation strategy using a multi-functional molecular agent is reported with enhanced colloidal dispersibility of an environmentally-friendly AgBiS2 nanocrystal ink, achieving a PCE over 10% in a solar cell.

Article Link Computational and experimental data and analyses for the Nature Photonics article: Kavanagh, S.R. & Wang, Y. et al. Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells. Nat. Photon. (2022). https://doi.org/10.1038/s41566-021-00950-4 Abstract: Strong optical absorption by a semiconductor is a highly desirable property for many optoelectronic and photovoltaic applications. The optimal thickness of a semiconductor absorber is primarily determined by its absorption coefficient. To date, this parameter has been considered as a fundamental material property, and efforts to realize thinner photovoltaics have relied on light-trapping structures that add complexity and cost. Here we demonstrate that engineering cation disorder in a ternary chalcogenide semiconductor leads to considerable absorption increase due to enhancement of the optical transition matrix elements. We show that cation-disorder-engineered AgBiS2 colloidal nanocrystals offer an absorption coefficient that is higher than other photovoltaic materials, enabling highly efficient extremely thin absorber photovoltaic devices. We report solution-processed, environmentally friendly, 30-nm-thick solar cells with short-circuit current density of 27 mA cm���2, a power conversion efficiency of 9.17% (8.85% certified) and high stability under ambient conditions.

Schottky nanojunctions are presented, fabricated from solution‐processed PbS quantum dot films and self‐assembled Ag nanoparticle arrays. If the Ag nanoparticle arrays are sufficiently dense to ensure the overlap of the individual depletion regions set up around the discrete Ag nanoparticles, open circuit voltages and shunt resistances similar to reference Schottky junctions with bulk Ag contacts can be achieved. Light is incident from the Ag nanoparticle side of the device and can excite localized surface plasmon resonances in the metal. These plasmonic resonances strongly concentrate the electromagnetic field within tens of nanometers of the nanoparticles. The resulting absorption profile is modified such that carriers are generated close to the metal–semiconductor interface, within the depletion region of the Schottky diodes. In this way, the generation profile is tailored across the device to achieve more efficient carrier collection. External quantum efficiency spectra exhibit clear spectral features characteristic of near‐field assisted absorption, as predicted by full field optical simulations. By comparing two different active layer thicknesses—resulting in one depleted reference device and one partially depleted—it is possible to differentiate between the optical (due to changes in absorption) and electrical (due to changes in carrier collection efficiency) effects of modifying the generation profile in Schottky diodes.

This research has been supported by Fundacio0 Privada Cellex Barcelona and the European Commission’s Seventh Framework Programme for Research under contract PIRG06-GA-2009-256355 and the Ministerio de Ciencia e Innovacion under Contract No. TEC2011-24744.

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced. The first-the Schottky device-was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type CQD film. The second-the excitonic device-employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate CQD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety.

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 μm 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.