To boost the power conversion efficiency (PCE) of quantum dot solar cell (QDSC) by long-lived charge separated state, we are introducing CdSe{Au} nanohybrid material (NHM) which acts as a better light harvester than CdSe quantum dot (QD) alone. Steady state absorption studies show broadening of the absorption band of CdSe{Au} NHM up to 800 nm. The steady state and time-resolved luminescence studies reveal ultrafast electron transfer from CdSe QD to Au nanoparticles (NPs), forming a charge separated state. The measured PCE of the CdSe{Au} NHM is 4.39% which is significantly higher than pure CdSe QDs (3.37%). The enhancement of PCE has been explained by femtosecond transient absorption (TA) and electrochemical impedance spectroscopy (EIS). The ultrafast TA studies suggest subpicosecond electron transfer from CdSe QDs to Au NP and slower charge recombination in NHM. Interestingly 3 times higher recombination resistance at the interface of TiO 2 −CdSe{Au} as compared to TiO 2 −CdSe is shown by EIS measurements, which has also explained the enhancement of PCE for the NHM. To the best of our knowledge this is the first report of PCE for any kind of metal−semiconductor NHM.

In search of a viable way to enhance the power conversion efficiency (PCE) of quantum dot-sensitized solar cells, we have designed a method by introducing a hole transporting layer (HTL) of p-type CuS through partial cation exchange process in a postsynthetic ligand-assisted assembly of nanocrystals (NCs). High-quality CdSe and CdSSe gradient alloy NCs were synthesized through colloidal method, and the charge carrier dynamics was monitored through ultrafast transient absorption measurements. A notable increase in the short-circuit current concomitant with the increase in open-circuit voltage and the fill factor led to 45% increment in PCE for CdSe-based solar cells upon formation of the CuS HTL. Electrochemical impedance spectroscopy further revealed that the CuS layer formation increases recombination resistance at the TiO/NC/electrolyte interface, implying that interfacial recombination gets drastically reduced because of smooth hole transfer to the redox electrolyte. Utilizing the same approach for CdSSe alloy NCs, the highest PCE (4.03%) was obtained upon CuS layer formation compared to 3.26% PCE for the untreated one and 3.61% PCE with the conventional ZnS coating. Therefore, such strategies will help to overcome the kinetic barriers of hole transfer to electrolytes, which is one of the major obstacles of high-performance devices.

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CdTe/ZnTe type-II core/shell nanocrystals were synthesized and characterized by the red-shift in the UV-Vis absorption and emission spectra along with the increase in both emission quantum yield and lifetime. The charge carrier dynamics was investigated through ultrafast transient absorption spectroscopy revealing the excited state carrier distribution and the dynamics through which the carriers decay. Upon laser pulse excitation the transient absorption spectrum was characterized by a broad ground state bleach signal in the core/shell nanocrystals in accord with the absorption spectra. Slower electron cooling was found in the core/shell nanocrystals compared to the CdTe core due to the type-II band-alignment that decouples the electron from hole preventing Auger-assisted electron cooling process. The recombination was found to be much slower in the core/shell nanocrystals due to the better surface passivation provided by the ZnTe shell eliminating the surface trapping process. The low band-gap CdTe based nanocrystals with a charge separated state are a viable candidate for photovoltaic applications and finally, we have investigated the potentiality of the synthesized nanocrystals as sensitizers in quantum dot solar cells.