Mn-doping in semiconductors like ZnS, ZnSe, CdS, and CdSe shows dramatic increase in lifetime due to the spin forbidden - transition of the Mn. Particularly, Mn-doping was recently used in CdS/CdSe sensitized cells as a strategy to boost solar cell efficiency due to very long lifetime of Mn d−d transitions ( - ). Metal ion doping as a band gap engineering tool has also been employed for improvement of type II-based solar cell performance because metal ions could make changes in the Fermi level, band gap, and conductance. have investigated the effect of shell thickness and surface passivation as another strategy to improve the efficiency of type II PbS/CdS-based solar cells. It has been shown that use of CdTe/CdSe core/shell nanocrystals prepared by the one-pot synthesis method without core seed purification could make structural and optical properties of nanocrystals comparable to the nanocrystals synthesized using purified core seed, which can give higher absorption and better crystallinity. Since the effective mass of the electron is lower than the hole in ZnSe, one can expect efficient conductivity in the ZnSe/CdS interface particularly where the electron accumulation is made by doping a paramagnetic material like Mn with semifilled d orbital. As an example, theoretical calculations from density-functional and many-body perturbation theory show the conduction and valence band offsets of 0.66 and 0.32 eV for ZnSe/CdS, respectively. It follows that much recent research efforts have been devoted to the synthesis of different type II core/shell structured QDs, like TiO 2/CdS, CdSe/ZnSe, CdTe/CdSe, CdTe/CdS, CdSe/ZnTe, CdSe/ZnTe, ZnTe/CdSe, CdS/CdSe, CdS/ZnSe, and ZnSe/CdS, as well as to the use of such QDs in emerging technology for solar cell applications. Apart from the wide photon absorption range for type II QDs, the improvement also refers to an effective charge separation of electron-hole pairs in the type II nanostructures that facilitates electron abstraction from QDs, suppresses recombination, and therefore leads to better electron transportation. Moreover, the use of type II nanocrystals in solar cell applications leads to better power conversion efficiency compared to the corresponding nanocrystals made up entirely from the core or shell materials. This offers charge carrier localization in two separate materials so that electrons and holes are confined in the shell and core, respectively. Schematic illustration of charge transfer mechanism in Mn-doped ZnSe/CdS. In this heterostructure, the core and shell are made up of two different semiconductors, with a higher conduction (valence) band of the core than the conduction (valence) band of the shell (Scheme 1). These nanoscale crystals are capable of integrating multistructures with different functionalization into a single nanoscale particle with controllable electronic structure for development of photovoltaic cells. In particular, type II core/shell quantum dots are promising for efficient sensitization due to their long-time charge separation and possibility for electron confinement in the conduction band of the shell when their band structure is carefully designed. Among various kinds of sensitizers employed in sensitized solar cells, quantum dots (QDs) are regarded as promising candidates by virtue of their size-dependent optical and electronic properties, high light-absorption ability, photostability, and multiple exciton generation. The world energy demands for renewable and cheap resources of solar energy have generated a large interest in sensitized solar cell technology due to high power conversion efficiency with low cost of production. Nanotechnology has led to huge progress in the use of semiconductor nanocrystals for applications in diverse areas like organic light emitting diodes, biosensing, biolabeling, solar cells, and imaging and detection, to mention a few examples. It is demonstrated that a device constructed with 0.25% Mn-doped ZnSe/CdS leads to an enhancement of the electron injection rate and power conversion efficiency by 4 times and 1.3, respectively. The mid-states generated by a proper Mn content alleviate carrier separation and enhance the electron injection rate, thus facilitating electron transport to the TiO 2 substrate. By using Mn-doping as a band gap engineering tool for core/shell QDs an effective improvement of absorption spectra could be obtained. Colloidal Mn-doped ZnSe/CdS core/shell quantum dots (QDs) are synthesized for the first time and employed as a strategy to boost the power conversion efficiency of quantum dot sensitized solar cells.
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