Paul W. Cyr

Enhanced infrared photovoltaic efficiency in PbS nanocrystal/semiconducting polymer composites: 600-fold increase in maximum power output via control of the ligand barrier

We report a comparison of photoconductive performance of PbS nanocrystal/polymer composite devices containing either oleic acid-capped or octylamine capped nanocrystals (NCs). The octylamine-capped NCs allow over two orders of magnitude more photocurrent under −1V bias; they also show an infrared photovoltaic response, while devices using oleic acid-capped NCs do not. Further improvement in the photovoltaic performance of films made with octylamine-capped NCs occurs upon thermally annealing the composite layer at 220 °C for 1 h. The procedure leads to a 200-fold increase in short circuit current, a 600-fold increase in maximum power output, and an order of magnitude faster response time.

Photoconductivity from PbS-nanocrystal∕semiconducting polymer composites for solution-processible, quantum-size tunableinfrared photodetectors

We report photoconductivity at infrared wavelengths, 975–1300nm, from a polymer∕nanocrystal quantum dot composite. Biased films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) sensitized with PbS nanocrystals (∼5nm diameter) demonstrate photocurrent at wavelengths beyond the response of the polymer and corresponding to the absorption of the nanocrystals. The photocurrent is attributed to absorption in the nanocrystals with subsequent hole transfer to the polymer and had an internal quantum efficiency of ∼5×10−6to∼10−5charges∕photon at 5V bias.

Efficient solution-processed infrared photovoltaic cells: Planarized all-inorganic bulk heterojunction devices via inter-quantum-dot bridging during growth from solution

Solution-processed thin-film organic, inorganic, and hybrid photovoltaic devices have achieved power conversion efficiencies as high as 5%. However, these devices remain limited by their capture of visible energy; more than a half of the sun’s power lies in the infrared. Herein the authors demonstrate photovoltaic devices effective across the visible and all the way out to 1700nm. Only through the use of ethanedithiol as a bridging molecule to affect interparticle linking were they able to achieve fabrication of smooth, continuous quantum dot films on rough, high-surface area transparent metal oxides. This allowed them to increase light absorption while maintaining efficient charge separation and extraction and at the same time avoiding electrical short circuits. They obtained monochromatic infrared power conversion efficiencies of 1.3%, a 50-fold gain over the previous published record of 0.025% in IR solution-processed photovoltaics. The authors demonstrate quantum size-effect tuning of device band gaps...

Efficient excitation transfer from polymer to nanocrystals

We quantify experimentally the efficiency of excitation transfer from a semiconducting polymer matrix to quantum dot nanocrystals. We study 5±0.5 nm PbS nanocrystals embedded in MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)]) polymer. We determine the excitation transfer efficiency from normalized photoluminescence excitation measurements. When the composites are made using as-synthesized PbS nanocrystals capped by oleate ligands, the excitation transfer efficiency is about 20%. Replacing these ligands with shorter chains results in a factor-of-3 enhancement in the excitation transfer efficiency. Our findings provide guidance to the realization of efficient electroluminescent devices.

Solution-processed infrared photovoltaic devices with >10% monochromatic internal quantum efficiency

Large-area, physically flexible, solution-cast photovoltaics are of urgent interest to realize low-cost solar cells. Polymer, polymer-fullerene, and polymer-nanocrystal photovoltaics absorb light only to wavelengths as long as 750 nm, with the exception of one recent report out to 1000 nm. Half of the sun’s power spectrum lies beyond 700 nm; one third beyond 1000 nm; and infrared emitters of growing interest in thermal photovoltaics emit predominantly in the 1–3μm range. We report herein a processible infrared photovoltaic device active beyond 1μm. Our best devices exhibit external quantum efficiencies exceeding 1% and estimated monochromatic internal quantum efficiencies greater than 10%. This represents an improvement by more than 1000 compared to the best previously reported processible >1μm infrared photovoltaics. We employ a novel device architecture in which the infrared-absorbing active layer is based purely on semiconductor nanoparticles with no semiconducting polymer matrix. The replacement of a ...

Quantum dots in a metallopolymer host: studies of composites of polyferrocenes and CdSe nanocrystals

The photoluminescence (PL) of composite materials based on ferrocene-based organometallic polymers and CdSe nanocrystals (nc-CdSe) has been investigated in solution and in thin films. The polymers studied, poly(ferrocenylmethylphenylsilane) (PFMPS), poly(ferrocenylphenylphosphine) (PFP), and poly(ferrocenylphenylphosphine sulfide) (PFP-S), all quench the PL of the nc-CdSe. In solution, the relative quenching effects are solvent dependent, and are in the order PFMPS < PFP in toluene but in the order PFP < PFP-S < PFMPS in THF. Stern–Volmer analysis is consistent with coordination of PFP to the nc-CdSe. In the thin films, the relative quenching strength of the polymers is PFMPS ∼ PFP-S < PFP. Photoinduced absorption spectroscopy was employed to study the nature of the PL quenching.

Optical control over photoconductivity in polyferrocenylsilane films

We report the study and elucidate the origin of the photoconductivity of polyferrocenylsilanes achieved through photooxidation performed by ultraviolet irradiation in the presence of chloroform. The persistence over months of the changes in the optoelectronic properties allowed more detailed studies of the charge photogeneration process. The photocurrent spectrum mimics that of the absorption indicating that the photooxidized material is not a mechanical mixture of oxidized and unoxidized polymer units. Photomodulation spectroscopy revealed the existence of long-lived photoexcited states with a lifetime in the millisecond range. They have been interpreted as trapped excitons at the oxidized sites where the polymer is deformed due to the presence of the chloroform derived counter ions. Because of the relatively long lifetime of the trapped excitons they can dissociate and the formed charge carriers can be separated in an externally applied electric field. The effect of the polymer chain deformation around the oxidized unit extends over the neighboring polymer units. In light of the potential applications of this class of polymers in various electronic and photonic devices, the clarification of such a basic process as the photocurrent generation will be a key factor for further technological development.

Control over exciton confinement versus separation in composite films of polyfluorene and CdSe nanocrystals

Composite films of polyfluorene derivative poly(9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl) and cadmium selenide nanocrystals were investigated using photomodulation spectroscopy exciting only the nanocrystal phase. Efficient charge separation resulting in hole injection into the polymer was observed in films in which the nanocrystals had been stripped of surface trioctylphosphine oxide passivating groups. The resulting induced absorption band centered at 1.2 eV was assigned to bipolarons or π-dimers formed on the polymer in the near vicinity of the polymer/nanocrystal interface. The intensity dependence of this band suggests bimolecular recombination, supporting the interpretation of the band as due to charge separation. The measured wide distribution of lifetimes for the photogenerated states confirms the glassy nature of the polymer.

Quenching Platinum Octaethylporphine Phosphorescence in Solution by Poly(ferrocenylsilane)

Abstract We describe experiments that determine the quenching kinetics by poly(ferrocenylsilane) (PFS) for platinum octaethylporphine (PtOEP) phosphorescence in toluene solution. The phosphorescence quenching process was interpreted in terms of diffusion-controlled kinetics. Pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR) and dynamic light scattering (DLS) were used to characterize the diffusion behavior of PFS and PtOEP in toluene solution. We found that the ferrocene group present in the repeat unit of polymer backbone is a good quencher for PtOEP phosphorescence. Quenching by the polymer involves the entire PFS polymer chain instead of individual ferrocene groups. The intrinsic quenching ability of PFS was found to be higher than that of a model compound, Bu-FS, that contains a single ferrocene group.

Polyferrocenes: metallopolymers with tunable and high refractive indices

The refractive index, molar refraction and Abbe number of polyferrocene derivatives are reported and the values indicate that these materials are very promising for a range of photonics applications.