Zhenxiao Pan

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

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 TiO2 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 liquid-junction QDSCs.

Near Infrared Absorption of CdSexTe1–x Alloyed Quantum Dot Sensitized Solar Cells with More than 6% Efficiency and High Stability

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.

Zn–Cu–In–Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%

The enhancement of power conversion efficiency (PCE) and the development of toxic Cd-, Pb-free quantum dots (QDs) are critical for the prosperity of QD-based solar cells. It is known that the properties (such as light harvesting range, band gap alignment, density of trap state defects, etc.) of QD light harvesters play a crucial effect on the photovoltaic performance of QD based solar cells. Herein, high quality ∼4 nm Cd-, Pb-free Zn-Cu-In-Se alloyed QDs with an absorption onset extending to ∼1000 nm were developed as effective light harvesters to construct quantum dot sensitized solar cells (QDSCs). Due to the small particle size, the developed QD sensitizer can be efficiently immobilized on TiO2 film electrode in less than 0.5 h. An average PCE of 11.66% and a certified PCE of 11.61% have been demonstrated in the QDSCs based on these Zn-Cu-In-Se QDs. The remarkably improved photovoltaic performance for Zn-Cu-In-Se QDSCs vs Cu-In-Se QDSCs (11.66% vs 9.54% in PCE) is mainly derived from the higher conduction band edge, which favors the photogenerated electron extraction and results in higher photocurrent, and the alloyed structure of Zn-Cu-In-Se QD light harvester, which benefits the suppression of charge recombination at photoanode/electrolyte interfaces and thus improves the photovoltage.

Boosting power conversion efficiencies of quantum-dot-sensitized solar cells beyond 8% by recombination control.

At present, quantum-dot-sensitized solar cells (QDSCs) still exhibit moderate power conversion efficiency (with record efficiency of 6-7%), limited primarily by charge recombination. Therefore, suppressing recombination processes is a mandatory requirement to boost the performance of QDSCs. Herein, we demonstrate the ability of a novel sequential inorganic ZnS/SiO2 double layer treatment onto the QD-sensitized photoanode for strongly inhibiting interfacial recombination processes in QDSCs while providing improved cell stability. Theoretical modeling and impedance spectroscopy reveal that the combined ZnS/SiO2 treatment reduces interfacial recombination and increases charge collection efficiency when compared with conventional ZnS treatment alone. In line with those results, subpicosecond THz spectroscopy demonstrates that while QD to TiO2 electron-transfer rates and yields are insensitive to inorganic photoanode overcoating, back recombination at the oxide surface is strongly suppressed by subsequent inorganic treatments. By exploiting this approach, CdSe(x)Te(1-x) QDSCs exhibit a certified record efficiency of 8.21% (8.55% for a champion cell), an improvement of 20% over the previous record high efficiency of 6.8%, together with an additional beneficial effect of improved cell stability.

Highly Efficient Inverted Type-I CdS/CdSe Core/Shell Structure QD-Sensitized Solar Cells

Presynthesized high-quality CdS/CdSe inverted type-I core/shell structure QDs have been deposited onto TiO(2) electrodes after first coating with bifunctional linker molecules, mercaptopropionic acid (MPA), and the resulting quantum dot sensitized solar cells (QDSCs) exhibited record conversion efficiency of 5.32% (V(oc) = 0.527 V, J(sc) = 18.02 mA/cm(2), FF = 0.56) under simulated AM 1.5, 100 mW cm(-2) illumination. CdS/CdSe QDs with different CdSe shell thicknesses and different corresponding absorption onsets were prepared via the well-developed organometallic high-temperature injection method. MPA-capped water-dispersible QDs were then obtained via ligand exchange from the initial organic ligand capped oil-dispersible QDs. The QD-sensitized TiO(2) electrodes were facilely prepared by pipetting the MPA-capped CdS/CdSe QD aqueous solution onto the TiO(2) film, followed by a covering process with a ZnS layer and a postsintering process at 300 °C. Polysulfide electrolyte and Cu(2)S counterelectrode were used to provide higher photocurrents and fill factors of the constructed cell devices. The characteristics of these QDSCs were studied in more detail by optical measurements, incidental photo-to-current efficiency measurements, and impedance spectroscopy. With the combination of the modified deposition technique with use of linker molecule MPA-capped water-soluble QDs and well-developed inverted type-I core/shell structure of the sensitizer together with the sintering treatment of QD-bound TiO(2) electrodes, the resulting CdS/CdSe-sensitized solar cells show a record photovoltaic performance with a conversion efficiency of 5.32%.

Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach

A postsynthesis assembly approach, an ex situ ligand exchange route, was developed for fast (within 2 h) and high loading (34% coverage) deposition of CdSe QDs on TiO(2) films. With the combination of high-quality QD sensitizers and the effective deposition technique, a record photovoltaic performance with an efficiency of 5.4% was observed for the resulting cell device.

Band Engineering in Core/Shell ZnTe/CdSe for Photovoltage and Efficiency Enhancement in Exciplex Quantum Dot Sensitized Solar Cells

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, Cμ, 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 1.5 G full one sun illumination, which is, as far as we know, one of the highest efficiencies for liquid-junction QDSCs.

Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%

The mean power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of the counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with submillimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm(2), Voc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs.

Mn doped quantum dot sensitized solar cells with power conversion efficiency exceeding 9

Transition metal ion (especially Mn2+) doping has been proven to be an effective approach to modify the intrinsic photo-electronic properties of semiconductor quantum dots (QDs). However, previous works to directly grow Mn doped QDs on TiO2 film electrodes at room temperature resulted in the potential of the Mn dopant not being fully demonstrated in quantum dot sensitized solar cells (QDSCs). Herein, Mn doped CdSe0.65Te0.35 QDs (simplified as Mn : QD) were pre-synthesized via a “growth doping” strategy at high temperature. A QD-sensitized photoanode with the configuration TiO2/Mn : QD/Mn : ZnS/SiO2 was prepared and corresponding cell devices were constructed using Cu2S/brass counter electrodes and polysulfide electrolyte, together with reference cells with the photoanode configurations TiO2/Mn : QD/ZnS/SiO2, TiO2/QD/Mn : ZnS/SiO2, and TiO2/QD/ZnS/SiO2. The photovoltaic performance results indicate that TiO2/Mn : QD/Mn : ZnS/SiO2 cells exhibit the best photovoltaic performance among all the studied cell devices with a power conversion efficiency (PCE) for the champion cell of 9.40% (Jsc = 20.87 mA cm−2, Voc = 0.688 V, FF = 0.655) under AM 1.5 G one full sun illumination, which is among the best results for QDSCs. The open circuit voltage decay (OCVD), impedance spectroscopy (IS) and transient absorption (TA) measurements confirm that the Mn2+ dopant can suppress charge recombination and improve the photovoltage and PCE of the resulting cells.

Charge Recombination Control for High Efficiency Quantum Dot Sensitized Solar Cells

Benefiting from the unique excellent optoelectronic properties of quantum dot light absorbers, quantum dot sensitized solar cell (QDSCs) are a promising candidate for the low-cost third-generation solar cells. Over the past few years, the power conversion efficiency (PCE) of QDSCs presents a rapid evolution from less than 1% to beyond 8%. Charge recombination is regarded as one of the most significant factors in limiting the photovoltaic performance of QDSCs. A significant improvement in the PCE of QDSCs has been obtained by charge recombination control. Some effective routes to suppress charge recombination processes, such as adopting preprepared high-quality QD sensitizers, tailoring the electronic properties of QDs, and interface engineering with the use of organic or inorganic thin layer overcoating the sensitized photoanode have been overviewed in this perspective. Also, the possible accesses to better performance (higher efficiency and stability) of the QDSCs have been proposed on the basis of achievements obtained previously.