Quantum Dot Sensitization of Organic−Inorganic Hybrid Solar Cells

Abstract

A high surface area pn-heterojunction between TiO 2 and an organic p-type charge transport material (spiro-OMeTAD) was sensitized to visible light using lead sulfide (PbS) quantum dots. PbS quantum dots were formed in situ on a nanocrystalline TiO 2 electrode using chemical bath deposition techniques. 1 The organic hole conductor was applied from solution to form the sensitized heterojunction. The structure of the quantum dots was analyzed using HRTEM technique. Ultrafast laser photolysis experiments suggested the initial charge separation to proceed in the subpicosecond time range. Transient absorption laser spectroscopy revealed that interfacial charge recombination of the initially formed charge carriers is much faster than in comparable dye-sensitized systems. 2,3 The sensitized heterojunction showed incident photon-to-electron conversion efficiencies (IPCE) of up to 45% and energy conversion efficiencies under simulated sunlight AM1.5 (10 mW/cm2) of 0.49%.

The advent of mesoscopic injection solar cells

“…23,24 Light absorption produces excitons or electron-hole pairs in the QD. The electron is subsequently injected within femtoseconds into the semiconducting oxide support while the hole is transferred to a hole conductor or an electrolyte present in the pores of the nanocrystalline oxide film.…”

“…Efficient and rapid hole injection from PbS quantum dots into triarylamine hole conductors has already been demonstrated and IPCE values exceeding 50% have been reached without attempting to optimize the collector structure nor to retard interfacial electron hole recombination. 23 QDs have much higher optical cross sections than molecular sensitizers, depending on the their size. However, they also occupy a larger area on the surface of the mesoporous electrode decreasing the QD-concentration in the film.…”

“…As recombination occurs on a picosecond time scale, the use of mesoporous oxide collector electrodes presents a promising strategy, because the electron transfer from the quantum dot to the conduction band of the oxide collector electrode occurs within femtoseconds. 23 This opens up research avenues that ultimately may lead to photo-converters reaching external quntum efficiencies values of several hundred percent. A calculation based on Henry's model 26 shows that the maximum conversion efficiency of a single junction cell could be increased from 34 to 44% by exploiting IMI effects.…”

Photovoltaic and photoelectrochemical conversion of solar energy

“…The challenge is now to find ways to collect the excitons before they recombine. As recombination occurs on a picosecond time-scale, the use of mesoporous oxide electrodes to support the quantum dot presents a promising strategy, because transfer of the electron from the quantum dot to the conduction band of the oxide collector electrode can occur within femtoseconds (Plass et al 2002). This opens up research avenues that ultimately may lead to photo-converters reaching external quantum efficiencies of several hundred per cent.…”

Quantum dot containing nanocomposite thin films for photoluminescent solar concentrators

“…By introducing zero dimensional structures into photovoltaic cells, the creation of multiple electron-hole pairs per photon through impact ionization can be achieved, suggesting a potential capability of quantum yields >1 (Nozik, 2002). In quantum dot sensitized nanocrystalline TiO 2 solar cells, semiconductor nanocrystals substitute for organic molecules (Plass et al, 2002;Wijayantha et al, 2004). Hereby the possible advantages of quantum dots over dye molecules include the tunability of optical properties with size and better heterojunction formation with solid hole conductors.…”