Quantum dot-sensitized solar cells (QDSCs), as promising candidates for cost-effective photoelectrochemical solar cells, have attracted much attention due to their characteristic properties such as processability at low cost, feasibility to control light absorption spectrum in a wide region, and possibility of multiple electron generation with a theoretical conversion efficiency up to 44%. QDSCs have analogous structures to dye-sensitized solar cells typically consisting of semiconductor photoanodes sensitized with quantum dots (QDs), redox electrolytes, and counter electrodes (CEs). Much effort has been dedicated to optimizing each component of QDSCs for higher device performance. In this review, recent advances of photoanodes with various architectures, QDs with tunable band gaps, electrolytes in liquid, quasi-solid or solid state, and CEs with great electrocatalytic activity for QDSCs will be highlighted. We aim to elaborate the rational strategies in material design for QDSC applications. Finally, the conclusion and future prospects emphasize the key developments and remaining challenges for QDSCs.

Recent advances in quantum dot-sensitized solar cells: insights into photoanodes, sensitizers, electrolytes and counter electrodes

The increasing energy demands and global population together with concerns over global warming are driving the search and development of clean and renewable energy sources such as solar cells, fuel cells, batteries, and supercapacitors; in the last few decades, quantum dot-sensitized solar cells (QDSCs) have attracted significant interest because of their perceived benefits over some alternative solar cells in terms of high light, thermal, and moisture stability; tunable absorption range; high absorption coefficient; multiple exciton generation possibility; solution processability, as well as their facile fabrication and low-cost availability. Compared to molecular dyes, quantum dots (QDs) have several advantages, which have exhibited higher molar extinction coefficients and a tunable photoresponse, resulting in a dramatic increase in the power conversion efficiency (PCE) from 5% to 13%. This review article presents a comprehensive overview of the development of QDSCs, including the photoanodes, sensitizers, electrolytes, and counter electrodes (CEs), and discusses future prospects for the development of highly efficient and stable QDSCs.

Recent progress in quantum dot sensitized solar cells: an inclusive review of photoanode, sensitizer, electrolyte, and the counter electrode

Development of new low-cost technologies for high-efficiency quantum dot-sensitized solar cells (QDSCs) is an effective way to solve current energy and environmental problems. Over the past few years QDSCs employing inorganic semiconductor nanocrystals as the light harvesters, introduced in 2010 with a power conversion efficiency (PCE) of 5%, have attracted much attention since the recent breakthrough PCE reports of over 12%. In this review, the strategies to obtain highly efficient QDSCs are discussed in detail from the viewpoints of QD sensitizers, properties of photoanode films, counter electrodes, redox electrolytes, and the interfacial passivation method. Furthermore, the limitations and further prospects are also discussed of how to fabricate QDSC devices with high efficiency and excellent stability. It is expected that these goals can be realized by virtue of the above effective strategies. We believe that this review will not only offer a theoretical basis and technical support for further industrial application of QDSCs, but also provide inspiration and references for performance improvement of other types of solar cells.

Performance improvement strategies for quantum dot-sensitized solar cells: a review

A ∼70% improvement in power conversion efficiency (PCE, η) is observed for the devices fabricated with a binary hybrid composite of graphitic carbon nitride and zinc oxide nanorods, i.e., (g-C3N4–ZnO NR) [η ≈ 2.43%, for an optimized weight ratio (0.5 : 1)] as compared to the pristine ZnO NR device (η ≈ 0.65%). Systematic investigations reveal that g-C3N4 boosts the light harvesting ability of the photovoltaic devices primarily by impeding photo-induced electron interception to the redox couple and injecting electrons into the conduction band of the semiconductor. Electrochemical impedance spectroscopy (EIS) analysis shows a reduced tunneling of photo-induced electrons to the sulfide–polysulfide (S2−/Sn2−) redox shuttle in the case of (g-C3N4–ZnO NR) composite devices. Higher recombination resistance (Rk) indicates that the g-C3N4 sheet acts as a barrier for photo-induced electron interception at the working electrode/electrolyte interface. Preliminary investigation using steady state and dynamic photoluminescence analyses suggest a similar fact about the photo-induced electron injection from g-C3N4 sheets to ZnO, contributing to the enhanced light harvesting ability of (g-C3N4–ZnO NR) composite devices.

Graphitic carbon nitride as a photovoltaic booster in quantum dot sensitized solar cells: a synergistic approach for enhanced charge separation and injection

Hydrothermally regulating phase composition of TiO2 nanocrystals toward high photocatalytic activity

TiO2 polydisperse mesoporous spheres (PMSs) and nanocrystals were selectively prepared via slightly altering the precursor dosage in a solvothermal reaction. Afterwards, the submicrometer-sized PMSs and nanocrystals were adopted as sample materials to shed light on the effect of the photoanode structure over the performance of CdS/CdSe-cosensitized solar cells by comparing four types of photoanode structures: nanocrystal film, blend film, bilayer film and PMSs film. It was found that the bilayer photoanode could promote the harvesting of incident light by increasing both the CdS/CdSe QDs loading amount and the scattering effect. According to the transient photovoltage measurements, the carrier recombination at the TiO2/electrolyte and FTO/electrolyte interfaces were substantially passivated in the bilayer cell. Moreover, a high electron diffusion rate of 142.0 × 10−9 m2 s−1 was also obtained in the bilayer cell, indicating high photoelectrons collection efficiency in the film structure. Therefore, the bilayer cell can well integrate the structural advantages of the PMSs and nanocrystals, which contributes to a high conversion efficiency of 4.70%, demonstrating a ∼54% and ∼12% improvement compared with the cells derived from nanocrystal and PMSs, respectively.

The effect of photoanode structure on the performances of quantum-dot-sensitized solar cells: a case study of the anatase TiO2 nanocrystals and polydisperse mesoporous spheres hybrid photoanodes

Two different kinds of quantum dot sensitized solar cells (QDSSCs) were fabricated based on the photoanode materials P25 (Cell-P25) and hierarchical TiO2 microspheres (HMS, Cell-HMS). The trap state distributions and charge transport/recombination dynamics are comparatively investigated by means of time-resolved charge extraction and transient photovoltage measurements. The dynamics results prove that, compared to Cell-P25, Cell-HMS possesses higher density and lower trap state energy characteristics, as well as a 30% lower charge transport rate. In addition, the boundary level dividing lower-voltage iso-energetic charge recombination and higher-voltage multiple-trap limited charge recombination is experimentally determined. Furthermore, Cell-HMS is found to exhibit a slightly higher boundary voltage and a 96% lower recombination rate with respect to Cell-P25. A physical model is proposed to describe the influence of trap state distribution on charge transport/recombination, highlighting the importance of photoanode morphology in optimizing the photovoltaic performance of QDSSCs.

The influence of hierarchical TiO2 microspheres on the trap state distribution and charge transport/recombination dynamics in quantum dot sensitized solar cells

All-inorganic perovskite materials (CsPbX3) have attracted increasing attention due to their excellent photoelectric properties and stable physical and chemical properties. The dynamics of charge carriers affect the photoelectric conversion efficiencies of perovskite materials. Regulating carrier dynamics by changing pressure is interesting with respect to revealing the key microphysical processes involved. Here, ultrafast spectroscopy combined with high-pressure diamond anvil cell technology was used to study the generation and transfer of photoinduced carriers of a Mn-doped inorganic perovskite CsPbBr3 material under pressure. Three components were obtained and assigned to thermal carrier relaxation, optical phonon–acoustic phonon scattering and Auger recombination. The time constants of the three components changed under the applied pressures. Our experimental results show that pressure can affect the crystal structure of Mn-doped CsPbBr3 to regulate carrier dynamics. The use of metal doping not only reduces the content of toxic substances but also improves the photoelectric properties of perovskite materials. We hope that our study can provide dynamic experimental support for the exploration of new photoelectric materials.

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Pressure-Induced Tunable Charge Carrier Dynamics in Mn-Doped CsPbBr3 Perovskite

Effects of hydrogen atoms and UV radiation (λ > 210 nm) on nanoporous organosilicate glass (OSG) low-k films are studied in the temperature range from 20 °C to 300 °C. The purpose of the study is to understand the mechanisms of low-k films modification that can happen during the cleaning from carbon containing residues formed from sacrificial porogen and accumulated during the air storage. It is shown that exposure of low-k films to hydrogen atoms at low temperature leads to slight modification of hydrocarbon bonds in hydrocarbon residues not bonded to Si. At high temperature (T ⩾ 300 °C), the relative concentration of –CH x bonds changes in a complex way and depends on the amount and structure of the carbon-containing compounds. The general trend is relatively rapid decrease of –CH2 bonds concentration, while the terminal –CH3 groups are more stable. Temperature also initiates the reaction of hydrogen atoms with low-k with partial modification of low-k matrix breaking Si–O bonds. The destruction of Si–O and Si–CH2 groups leads to the formation of oxygen-deficient centers, followed by the formation of Si–(CH3)2 groups due to their interaction with methyl groups. At 300 °C, the total number of Si–CH3 + Si–(CH3)2 groups starts to decrease indicating on partial removal of the methyl groups bonded to silicon. Besides with increasing temperature a slight modification of the structure of matrix under exposure to H atoms is also observed. UV radiation has almost no effect on these processes in the studied conditions. Thus, there exist the ‘optimal’ conditions for H atom impact on OSG low-k films which allows improving film performance by removing porogen residue without damage.

Xiao-Juan Shi

Papers

The effect of photoanode structure on the performances of quantum-dot-sensitized solar cells: a case study of the anatase TiO2 nanocrystals and polydisperse mesoporous spheres hybrid photoanodes

The influence of hierarchical TiO2 microspheres on the trap state distribution and charge transport/recombination dynamics in quantum dot sensitized solar cells

Pressure-Induced Tunable Charge Carrier Dynamics in Mn-Doped CsPbBr3 Perovskite

Effect of H atoms and UV wideband radiation on cured low-k OSG films