Efficient Photodetection at IR Wavelengths by Incorporation of PbSe–Carbon‐Nanotube Conjugates in a Polymeric Nanocomposite

Polymeric nanocomposites have demonstrated great potential for the construction of physically flexible, large-area optoelectronic devices that can remain photoactive over a wide spectral bandwidth, depending on the customizable constituents. They bolster the prospect of developing ultrafast, sensitive, and superior optical counterparts of the corresponding electronic devices. With the successful integration of inorganic quantum dots (QDs) into conjugated polymeric matrices, efficient photodetection in different ranges of the electromagnetic spectrum has already been realized. This is enabled by the tunable optical absorption and emission of the QDs (by virtue of quantum size effects), and also by the fact that the QDs can often preserve their optoelectronic integrity within a host matrix. To generate excitons at a good quantum efficiency, the QDs should have an appreciable absorption cross section at the excitation wavelength. The photogenerated excitons should disintegrate into free charge carriers at higher rate than competitive exciton recombination. Subsequently, these charge carriers should be extracted from the photoconverter before relaxation. To realize efficient photon conversion, the rates of photogenerated-carrier separation, interfacial transfer across the different contacts, transport through the matrix, and subsequent collection at the electrodes must all be fast enough compared to exciton recombination. Therefore, it is imperative to provide an efficient transport of charges within the nanocomposite by incorporating compatible constituents that facilitate the transport process. In research on photoconducting devices that use conjugated polymers, electron-accepting materials such as C60 and singlewalled carbon nanotubes (SWNTs) have been utilized in some of the past studies. SWNTs are fascinating materials because of several peculiar physical properties. Envisioned as a rolledup graphene sheet capped with fullerene-like structures, a SWNT can behave as a metal or semiconductor as a function of the wrapping angle of the sheet and the diameter of the nanotube. In the “metallic” state, SWNTs are good ballistic conductors with a reported current density at least one order of magnitude higher than that of copper wires with the same diameter. Because of their good mechanical properties (high elastic modulus and high optical transparency), SWNTs have been used for constructing large-area transmissive films, making them an ideal component for electrical coupling in futuristic photonic devices. The aim of the current study is to investigate the photodetection efficiency of a polymeric nanocomposite containing IR-active PbSe QDs and conducting SWNTs that are chemically attached to each other by a novel procedure. PbSe QDs, endowed with a large Bohr radius, exhibit excellent quantum size effects and can be tuned to a precisely targeted absorption wavelength in the IR region. Therefore, a stable SWNT– PbSe conjugate (by chemical bonding of both components) is a promising candidate for the realization of efficient optoelectronic behavior. The large interfaces in the SWNT–PbSe conjugate offer a tremendous opportunity for efficient exciton dissociation after photogeneration in the PbSe QDs. Efficient dissociation and charge transfer depend on the differences in potential energy and electron affinity between the photoactive species (PbSe QDs) and the other components. As can be seen in Scheme 1, the band alignments with a higher electron affinity than SWNTs allow a nonactivated electron transfer from PbSe QDs to SWNTs. Because the ionization potential of the polymer (PVK: poly(vinyl carbazole)) lies closer to the value for a vacuum than that of the QDs, the transfer of photogenerated holes to the polymer also proceeds without C O M M U N IC A TI O N