Core/Shell Colloidal Quantum Dot Exciplex States for the Development of Highly Efficient Quantum-Dot-Sensitized Solar Cells

Abstract

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.

Boosting the efficiency of quantum dot sensitized solar cells up to 7.11% through simultaneous engineering of photocathode and photoanode

“…For comparison, Cu 2 S CE film was fabricated and treated in a similar way as those of the Cu 2-x Se CE films. The wide band-gap ZnS layer plays an important role in reducing the internal recombination at QDs as well as the charge recombination at the QD/electrolyte and TiO 2 /electrolyte interfaces before charge injection and thus improving the efficiency [37,69,70]. Figure 4a shows the photocurrent density-voltage (J-V) characteristics of these QDSSCs under the standard simulated AM 1.5 illumination with an intensity of 100 mW cm -2 in the presence of a mask.…”

“…To date, the potential of QDSSCs has not been well demonstrated and the reported best PCEs for solution stable CdS/CdSe QDSSCs are at the level of ~6% [25][26][27][28]. Previous studies have been concentrated on developing better techniques for the deposition of QD sensitizers [29][30][31][32][33][34][35][36] and the broadening their absorption profiles [28,[37][38][39][40][41] over the past decades. Although the photocurrent density (J sc ) obtained from QDSSCs is comparable to that of DSSCs, the PCE remains much lower than their analogues.…”

Fabrication of CdSe/CdTe Quantum Dots Co-Sensitized TiO2Nanorods by Electrochemical Atomic Layer Deposition Method

“…35 Hence, the driving force for electron injection from QDs to TiO 2 is enhanced thus a higher J sc and a lower recombination possibility are obtained and the V oc of CdSe/CdTe co-sensitized solar cell is also increased due to the improved J sc , which can be explained by the equation V oc = (k B T/qβ)(Inj sc /j 0 + 1), where k B is the Boltzmann constant, T is the temperature, q is the electron charge, β is a parameter related with the nonlinear recombination, and j 0 the diode dark current. 36 Consequently, the CdSe/CdTe co-sensitized solar cells take the advantages of broader light absorption of combined CdSe and CdTe and the improved charge injection and separation efficiency, manifesting better photovoltaic performance than the sole CdSe or CdTe based solar cell. …”

Advancing colloidal quantum dot photovoltaic technology

“…PCE improvement for these architectures focuses on three main schemes: (1) FF enhancement, by integrating counter electrodes that are compatible with polysulfide electrolytes [173][174][175][176], (2) short-circuit current enhancement, by improving absorption in the CQD materials or harvesting the near-IR portion of the spectrum [156,177] and (3) open-circuit voltage enhancement, through the use of alternative redox systems or by suppressing interfacial recombination [178,179]. The highest PCE to date achieved in a CQD-SSC device is 8.21% [180].…”