Semiconductor quantum dots (QDs) are extremely interesting materials for the development of photovoltaic devices, but currently the present the drawback is that the most efficient devices have been prepared with toxic heavy metals of Cd or Pb. Solar cells based on "green" QDs totally free of Cd or Pbpresent a modest efficiency of 2.52%. Herein we achieve effective surface passivation of the ternary CuInS 2 (CIS) QDs that provides high photovoltaic quality core/shell CIS/ZnS (CIS-Z) QDs, leading to the development of high-efficiency green QD solar cells that surpass the performance of those based on the toxic cadmium and lead chalcogenides QDs. Using wide absorption range QDs, CIS-Zbased quantum dot sensitized solar cell (QDSC) configuration with high QD loading and with the benefit of the recombination reduction with type-I core/shell structure, we boost the power conversion efficiency of Cd-and Pb-free QDSC to a record of 7.04% (with certified efficiency of 6.66%) under AM 1.5G one sun irradiation. This efficiency is the best performance to date for QDSCs and also demonstrates that it is possible to obtain comparable or even better photovoltaic performance from green CIS QDs to the toxic cadmium and lead chalcogenides QDs.

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CdSe0.45Te0.55 alloyed quantum dots (QDs) with excitonic absorption onset at 800 nm and particle size of 5.2 nm were prepared via a noninjection high-temperature pyrolysis route and used as a sensitizer in solar cells. A postsynthesis assembly approach with use of bifunctional linker molecule mercaptopropionic acid (MPA) capped water-soluble QDs, obtained via ex situ ligand exchange from the initial oil-dispersible QDs, was adopted for tethering QDs onto mesoporous TiO2 film. With the combination of high loading of the QD sensitizer and intrinsic superior optoelectronic properties (wide absorption range, high conduction band edge, high chemical stability, etc., relative to their constituents CdSe and CdTe) of the adopted CdSe0.45Te0.55 QD sensitizer, the resulting CdSexTe1-x alloyed QD-based solar cells exhibit a record conversion efficiency of 6.36% (Jsc = 19.35 mA/cm(2), Voc = 0.571 V, FF = 0.575) under full 1 sun illumination, which is remarkably better than that of the reference CdSe and CdTe QD based ones. Furthermore, the solar cells with Cu2S counter electrodes based on eletrodeposition of Cu on conductive glass show long-term (more than 500 h) stability.

Searching suitable panchromatic QD sensitizers for expanding the light-harvesting range, accelerating charge separation, and retarding charge recombination is an effective way to improve power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs). One possible way to obtain a wide absorption range is to use the exciplex state of a type-II core/shell-structured QDs. In addition, this system could also provide a fast charge separation and low charge-recombination rate. Herein, we report on using a CdTe/CdSe type-II core/shell QD sensitizer with an absorption range extending into the infrared region because of its exciplex state, which is covalently linked to TiO 2 mesoporous electrodes by dropping a bifunctional linker molecule mercaptopropionic acid (MPA)-capped QD aqueous solution onto the film electrode. High loading and a uniform distribution of QD sensitizer throughout the film electrode thickness have been confirmed by energy dispersive X-ray (EDX) elemental mapping. The accelerated electron injection and retarded charge-recombination pathway in the built CdTe/CdSe QD cells in comparison with reference CdSe QD-based cells have been confirmed by impedance spectroscopy, fluorescence decay, and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) analysis. With the combination of the high QD loading and intrinsically superior optoelectronic properties of type-II core/shell QD (wide absorption range, fast charge separation, and slow charge recombination), the resulting CdTe/CdSe QD-based regenerative sandwich solar cells exhibit a record PCE of 6.76% (J sc = 19.59 mA cm −2 , V oc = 0.606 V, and FF = 0.569) with a mask around the active film under a full 1 sun illumination (simulated AM 1.5), which is the highest reported to date for liquidjunction QDSCs.

Even though previously reported CdTe/CdSe type-II core/shell QD sensitizers possess intrinsic superior optoelectronic properties (such as wide absorption range, fast charge separation, and slow charge recombination) in serving as light absorbers, the efficiency of the resultant solar cell is still limited by the relatively low photovoltage. To further enhance photovoltage and cell efficiency accordingly, ZnTe/CdSe type-II core/shell QDs with much larger conduction band (CB) offset in comparison with that of CdTe/CdSe (1.22 eV vs 0.27 eV) are adopted as sensitizers in the construction of quantum dot sensitized solar cells (QDSCs). The augment of band offset produces an increase of the charge accumulation across the QD/TiO2 interface under illumination and induces stronger dipole effects, therefore bringing forward an upward shift of the TiO2 CB edge after sensitization and resulting in enhancement of the photovoltage of the resultant cell devices. The variation of relative chemical capacitance, C μ, between ZnTe/CdSe and reference CdTe/CdSe cells extracted from impedance spectroscopy (IS) characterization under dark and illumination conditions clearly demonstrates that, under light irradiation conditions, the sensitization of ZnTe/CdSe QDs upshifts the CB edge of TiO2 by the level of ∼50 mV related to that in the reference cell and results in the enhancement of V oc of the corresponding cell devices. In addition, charge extraction measurements have also confirmed the photovoltage enhancement in the ZnTe/CdSe cell related to reference CdTe/CdSe cell. Furthermore, transient grating (TG) measurements have revealed a faster electron injection rate for the ZnTe/CdSe-based QDSCs in comparison with the CdSe cells. The resultant ZnTe/CdSe QD-based QDSCs exhibit a champion power conversion efficiency of 7.17% and a certified efficiency of 6.82% under AM 1.5G full one sun illumination, which is, as far as we know, one of the highest efficiencies for liquid-junction QDSCs.