| Summary: | 碩士 === 國立成功大學 === 化學工程學系 === 102 === SUMMARY
Quantum dot (QD) sensitized photoelectrodes have attracted great interest in the past few years. They are greatly applied not only in quantum dot sensitized solar cells but also in colloidal quantum dot thin film solar cells and photoelectrochemical (PEC) cells for water splitting. The properties such as multiple exciton generation (MEG), hot electron injection and tunable band gap are gaining momentum to overcome the efficiency limitations. Although a starting point of relatively low performing optoelectronic device, the breakthrough of QDs-based device has been witnessed for boosting conversion efficiency in just a few years. In this review, we highlight the recent evolvements achieved in surmounting the obstacles such as lower open circuit voltage (Voc) or charge recombination. With the specific investigation and innovation for main structures constructing the QDs-based devices, we anticipate the future direction with an aim to highly light to electric power conversion efficiency. And finally, we used the Cd2+ seed pre-treatment to enhance the loading of CdSxSe1-x to increase the overall power conversion efficiency to 5.3%.
Key words: review of QD-based optoelectronic devices, CdSxSe1-x QDs, sensitized solar cell
INTRODUCTION
In this brief review and article, we aim to focus on the utilization of QD-photoelectrodes in PV and PEC devices, QDSSCs, CQDs thin film solar cells and water splitting. The basic opto-electronic characteristics and mechanisms of QDs-based devices will be discussed at first. After the explicit definition and consideration, we will be absorbed in the progress on the study of these interesting and promising photoelectrodes, further to more discussion
about each part of the device structures. The literature for optimizing and engineering for QDs-based device is quite large, so we do not to cover the researches exhaustively. Finally, the pivotal path to improve the performance of QDs-based devices and the stability of QDs itself will be discussed in detail.
PRINCIPLES AND MECHANISMS
In this section, we focus on the working principle for PV and PEC devices, including QDSSCs, CQDSCs and QDs-based photoelectrode for PEC water splitting. With the promisingly optoelectronic properties in QD such as ionization impact, Auger recombination and mini-band transfer, QD materials show the potential to apply in the sensitizer in many QDs-based devices.
PROGRESS OF THE COMPONENT STRUCTURES IN QDS-BASED DEVICES
Much attention has been drawn to the development for the QDs-based PV or PEC devices. The intensive researches are concentrated on each formation of the optoelectronic devices to improve the sun-light conversion efficiency, lying primarily to the wide band gap metal oxides, QDs sensitizers, electrolytes and counter electrodes. The following investigation mainly about QDSSCs, CQDSCs and QDs-based water splitting highlights the composed structure with an aim toward highly efficient sunlight energy conversion.
At the heart of the photoelectrode is always a nanocrystalline metal oxide film. It plays a significant role to load QDs sensitizers and conduct electrons. As a wide band gap semiconductor for the QDs scaffold, the large surface area is available for QDs adsorption. Therefore, nanosized porous structure has been commonly used in QDs-based PV or PEC devices, primarily consisting of TiO2 or ZnO. Nanoparticle films offer very high surface area to extend the amount of sensitizer loading. However, unlike dye adsorption in DSSCs, larger QDs have some difficulty entering the inner pores the film. The direct exposure of the oxide film in electrolyte leads to a series degree of interfacial recombination, deteriorating the Voc. Thus, selecting a proper candidate to adsorb QDs is an important issue.
QDs-Sensitizer
There are many kinds of QDs semiconductors applying to the sensitizers on the wide band gap metal oxides. They play important roles for light absorption, charge excitation and separation. QDs semiconductors especially such as CdS, CdSe, PbS, CuInS2, Sb2S3 and their alloys with other elements have been commonly investigated for pursuit of higher power conversion efficiency. Besides the stability when immersing in electrolyte, QDs sensitizers should completely cover on all the surface of the metal oxide avoiding direct contact between metal oxide and electrolyte. Thus, the most used methods are based on increasing the coverage of QDs. All methods are mainly classified into in-situ and ex-situ ones from QDs synthesis and their incorporation into the photoactive electrode. Recently, some improvements for the doping QDs, surface treatment and combination of organic dye absorbers have become the latest trend in QD-based optoelectronic devices.
Electrolyte
In QDSSCs, the electrolyte plays a pivotal role in the rejuvenation of QDs materials by capturing the photo-generated holes. The rapid electron-transfer from electrolyte to the oxidized QDs sensitizer must be indispensable in the whole charge transfer process while having excellent long-term stability. Further, the important process includes the self-redox in the couple to deliver the hole to counter electrode. What must be significantly addressed is the Voc influenced by the potential of the redox couple. This value is corresponded to the difference between the quasi-Fermi level of metal oxide and the redox potential of the electrolyte.
Counter Electrode
In addition to the investigation of photoanodes, the performance of the QDSSCs is determined by the electrocatalytic properties and tolerance of the CEs toward the redox couples. The CE is responsible for catalysis and reduction of the oxidized redox species with the electrons transporting from the external circuit. For the polysulfide used widely in liquid state QDSSCs, besides discharging the electrons quickly, the CE must sustain in the sulfur/sulfide aqueous surrounding for a long time in pursuit of the long-term stability. Finally, the amount of the CE catalytic activity and surface area is also a crucial parameter that affects the overall performance.
STRATEGIES TO OPTIMIZE THE QD-BASED DEVICES
Although a volume of work has been conducted on the analysis of each element assembling QDs-based PV or PEC cells to enhance the sun-light conversion efficiency, there is still an obvious gap between QDSSCs and DSSCs. It is imperative that the efficiency of QDSSCs should be ~10% to make them competitive. In recent years, the efficiency has been gradually achieved to 5~7% in average for high efficiency device no matter for QDSSCs or for CQDs thin film solar cells. Accordingly, strategies for improve the performance are discussed as follow to bring the great potential of fabricating a highly efficient QD-based devices.
RESULT AND DISCUSSION
For the result and discussion, we apply the Cd2+ seed pre-treatment before SILAR process. And we observed that the cells with Cd2+ pre-treatment showed higher photocurrent, which leads the improved PCE of 5.3%. And the electrolyte condition was based on the based one with the larger alkali compound to increase the conductivity. However, the performance in solid state with CdSxSe1-x QDSSC was not as well as expected, just only 0.4%.
CONCLUSION
QDs-based photo-electrodes have emerged as the representative not only for the third generation PV devices but also for the water splitting in PEC systems. Owing to the optoelectronic properties such as multiple exciton generation (MEG), size-dependent band gap and high extinction coefficient, QDs have been the appropriate substitution for dye molecules as the photo-induced sensitizer. The QDSSCs are composed of electron acceptor, QD-sensitizer and hole acceptor which approaches to the assembly of p-i-n junction. With optimization for each element, the overall conversion efficiency has closed to 7% and have the potential to achieve 10%.
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