Review of roll-to-roll fabrication techniques for colloidal quantum dot solar cells

Colloidal quantum dots(CQDs)are of great interest to photovoltaic(PV)technologies as they possess the benefits of solution-processability,size-tunability,and roll-to-roll manufacturability,as well as unique capabilities to harvest near-infrared(NIR)radiation.During the last decade,lab-scale CQD solar cells have achieved rapid improvement in the power conversion efficiency(PCE)from~1%to 18%,which will potentially exceed 20%in the next few years and approach the performance of other PV technologies,such as perovskite solar cells and organic solar cells.In the meanwhile,CQD solar cells exhibit long lifetimes either under shelf storage or continuous operation,making them highly attractive to industry.However,in order to meet the industrial requirements,mass production techniques are necessary to scale up the fabrication of those lab devices into large-area PV modules,such as roll-to-toll coating.This paper reviews the recent developments of large-area CQD solar cells with a focus on various fabrication methods and their principles.It covers the progress of typical large-area coating techniques,including spray coating,blade coating,dip coating,and slot-die coating.It also discusses next steps and new strategies to accomplish the ultimate goal of the low-cost large-area fabrication of CQD solar cells and emphasizes how artificial intelligence or machine learning could facilitate the developments of CQD solar cell research.

Surface-Modified Graphene Oxide/Lead Sulfide Hybrid Film-Forming Ink for High-Efficiency Bulk Nano-Heterojunction Colloidal Quantum Dot Solar Cells

Solution-processed colloidal quantum dot solar cells(CQDSCs) is a promising candidate for new generation solar cells.To obtain stable and high performance lead sulfide(PbS)-based CQDSCs,high carrier mobility and low non-radiative recombination center density in the PbS CQDs active layer are required.In order to effectively improve the carrier mobility in PbS CQDs layer of CQDSCs,butylamine(BTA)-modified graphene oxide(BTA@GO) is first utilized in PbS-PbX2(X=I-,Br-) CQDs ink to deposit the active layer of CQDSCs through one-step spin-coating method.Such surface treatment of GO dramatically upholds the intrinsic superior hole transfer peculiarity of GO and attenuates the hydrophilicity of GO in order to allow for its good dispersibility in ink solvent.The introduction of B TA@GO in CQDs layer can build up a bulk nano-heterojunction architecture,which provides a smooth charge carrier transport channel in turn improves the carrier mobility and conductivity,extends the carriers lifetime and reduces the trap density of PbS-PbX2 CQDs film.Finally,the BTA@GO/PbS-PbX2 hybrid CQDs film-based relatively large-area(0.35 cm2) CQDSCs shows a champion power conversion efficiency of 11.7% which is increased by 23.1% compared with the control device.

Recent progress of colloidal quantum dot based solar cells

Colloidal quantum dot(CQD) solar cells have attracted great interest due to their low cost and superior photo-electric properties. Remarkable improvements in cell performances of both quantum dot sensitized solar cells(QDSCs) and PbX(X = S, Se) based CQD solar cells have been achieved in recent years, and the power conversion efficiencies(PCEs) exceeding 12% were reported so far. In this review, we will focus on the recent progress in CQD solar cells. We firstly summarize the advance of CQD sensitizer materials and the strategies for enhancing carrier collection efficiency in QDSCs, including developing multi-component alloyed CQDs and core-shell structured CQDs, as well as various methods to suppress interfacial carrier recombination. Then, we discuss the device architecture development of PbX CQD based solar cells and surface/interface passivation methods to increase light absorption and carrier extraction efficiencies. Finally, a short summary, challenge, and perspective are given.

Novel Hybrid Ligands for Passivating Pb S Colloidal Quantum Dots to Enhance the Performance of Solar Cells

We developed novel hybrid ligands to passivate Pb S colloidal quantum dots(CQDs),and two kinds of solar cells based on as-synthesized CQDs were fabricated to verify the passivation effects of the ligands.It was found that the ligands strongly affected the optical and electrical properties of CQDs,and the performances of solar cells were enhanced strongly.The optimized hybrid ligands,oleic amine/octyl-phosphine acid/Cd Cl2improved power conversion efficiency(PCE)to much higher of 3.72%for Schottky diode cell and 5.04%for p–n junction cell.These results may be beneficial to design passivation strategy for low-cost and high-performance CQDs solar cells.

Double-ended passivator enables dark-current-suppressed colloidal quantum dot photodiodes for CMOS-integrated infrared imagers

Lead sulfide(PbS)colloidal quantum dot(CQD)photodiodes integrated with silicon-based readout integrated circuits(ROICs)offer a promising solution for the next-generation short-wave infrared(SWIR)imaging technology.Despite their potential,large-size CQD photodiodes pose a challenge due to high dark currents resulting from surface states on nonpassivated(100)facets and trap states generated by CQD fusion.In this work,we present a novel approach to address this issue by introducing double-ended ligands that supplementally passivate(100)facets of halidecapped large-size CQDs,leading to suppressed bandtail states and reduced defect concentration.Our results demonstrate that the dark current density is highly suppressed by about an order of magnitude to 9.6 nA cm^(2) at -10 mV,which is among the lowest reported for PbS CQD photodiodes.Furthermore,the performance of the photodiodes is exemplary,yielding an external quantum efficiency of 50.8%(which corresponds to a responsivity of 0.532 A W^(-1))and a specific detectivity of 2.5×10^(12) Jones at 1300 nm.By integrating CQD photodiodes with CMOS ROICs,the CQD imager provides high-resolution(640×512)SWIR imaging for infrared penetration and material discrimination.

Colloidal quantum dot for infrared-absorbing solar cells:State-oftheart and prospects

Colloidal quantum dot(CQD)shows great potential for application in infrared solar cells due to the simple synthesis techniques,tunable infrared absorption spectrum,and high stability and solution-processability.Thanks to significant efforts made on the surface chemistry of CQDs,device structure optimization,and device physics of CQD solar cells(CQDSCs),remarkable breakthroughs are achieved to boost the infrared photovoltaic performance and stability of CQDSCs.In particular,the CQDSC with a high power conversion efficiency of~14%and good stability is reported,which is very promising for infrared-absorbing solar cells.In this review,we highlight the unique optoelectronic properties of CQDs for the development of infrared-absorbing solar cells.Meanwhile,the latest advances in finely controlling surface properties of CQDs are comprehensively summarized and discussed.Moreover,the device operation of CQDSCs is discussed in-depth to highlight the impact of the device structure optimization of CQDSCs on their photovoltaic performance,and the emerging novel types of CQDSCs,such as semitransparent,flexible,and lightweight CQDSCs,are also demonstrated.The device stability of CQDSCs is also highlighted from the viewpoint of practical applications.Finally,the conclusions and possible challenges and opportunities are presented to promote the development steps of the CQDSCs with higher infrared photovoltaic performance and robust stability.

Electrochemical biosensor employing PbS colloidal quantum dots/Au nanospheres-modified electrode for ultrasensitive glucose detection

Rapid and accurate detection of glucose is of great significance for diabetic management.Highly sensitive glucose sensors promise to achieve noninvasive detection technology,enabling more convenient and efficient means for large-scale screening and long-term dynamic monitoring of diabetes patients.In this work,we demonstrate a sensitive glucose electrochemical biosensor through the synergetic labelling strategy utilizing PbS colloidal quantum dots(CQDs)and Au nanospheres(AuNSs).The PbS CQDs/AuNSs/glucose oxidase(GOx)mixture could be stably immobilized on the carbon electrode surface via the onestep dip-coating method.The electrochemical biosensor employing PbS CQDs/AuNSs/GOx-modified electrode integrates the functions of specific molecule recognition,signal transduction as well as signal amplification.The sensor is capable of transducing the glucose enzyme-catalyzed reaction into significant current signals,exhibiting a good linear response in the glucose concentration range of 0.1μM-10 mM with the limit of detection being 1.432 nM.

Broadband, sensitive and spectrally distinctive SnS_(2) nanosheet/PbS colloidal quantum dot hybrid photodetector

Photodetectors convert photons into current or voltage outputs and are thus widely used for spectroscopy,imaging and sensing.Traditional photodetectors generally show a consistent-polarity response to incident photons within their broadband responsive spectrum.Here we introduced a new type of photodetector employing SnS_(2) nanosheets sensitized with PbS colloidal quantum dots(CQDs)that are not only sensitive(~105AW−1)and broadband(300–1000 nm)but also spectrally distinctive,that is,show distinctive(positive or negative)photoresponse toward incident photons of different wavelengths.A careful mechanism study revealed illumination-modulated Schottky contacts between SnS_(2) nanosheets and Au electrodes,altering the photoresponse polarity toward incident photons of different wavelengths.Finally,we applied our SnS_(2) nanosheet/PbS CQDs hybrid photodetector to differentiate the color temperature of emission from a series of white light-emitting diodes(LEDs),showcasing the unique application of our novel photodetectors.

Highly-efficient thermoelectric-driven light-emitting diodes based on colloidal quantum dots

Driven by sub-bandgap electric work and Peltier heat,thermoelectric-driven light-emitting diodes(TED-LEDs)not only offer much enhanced power-conversion-efficiency but also eliminate the waste heat generated during the operation of LEDs.However,costeffective and high-efficiency TED-LEDs are not readily accessible for the epitaxially grown III-V LEDs due to the high chip cost and efficiency droop at low-medium brightness(current densities).Here we show that electroluminescence of colloidal quantum dots(QDs)LEDs(QLEDs)circumvents the deficiencies faced by conventional LEDs.The optimal red-emitting device fabricated by cost-effective solution processing technics exhibits external-and internal-power-conversion-efficiency of 21.5%and 93.5%at 100 cd/m^(2),suited for high-efficiency solid-state lighting and high-resolution display.At this brightness,the electric driving voltage(V)of 1.89 V is lower than the photon voltage(Vp=hv/q=1.96 V,q being the elemental charge).With typical Vp=1.96 V,electroluminescence can be detected with the driving voltage as low as 1.0-1.2 V.Luminance of the thermoelectric-driven QLEDs(TED-QLEDs)remains ideally diffusion-dominated with the driving voltage lower than~1.5 V,and further improvement on charge transport is expected to extend the linear ideality to all practical driving voltages.

Photostability of colloidal single photon emitter in near-infrared regime at room temperature

The photostability of a colloidal single photon emitter in near-infrared regime at room temperature is investigated.The fluorescence lifetime,blinking phenomenon,and anti-bunching effect of a single CdTeSe/ZnS quantum dot with an emission wavelength of 800 nm at room temperature are studied.The second-order correlation function at zero delay time is much smaller than 0.1,which proves that the emission from single quantum dots at 800 nm is a highly pure single-photon source.The effects of the irradiation duration on the fluorescence from single quantum dots are analyzed.The experimental results can be explained by a recombination model including a multi-nonradiative recombination center model and a multi-charged model.

Ligand engineering of colloid quantum dots and their application in all-inorganic tandem solar cells

How to effectively utilize the energy of the broad spectrum of sunlight is one of the basic problems in the research of tandem solar cells. Due to their size effect, quantum confinement effect and coupling effect, colloidal quantum dots(QDs) exhibit new physical properties that bulk materials don’t possess.CdX(X = Se, S, etc.) and Pb X(X = Se, S, etc.) QDs prepared by hot-injection methods have been widely studied in the areas of photovolitaic devices. However, the surfactants surrounding QDs seriously hinder the charge transport of QDs based solar cells. Therefore, how to fabricate high-performance tandem solar cells via ligands engineering has become a major challenge. In this paper, the latest progress of colloidal QDs in the research of all-inorganic tandem solar cells was summarized. Firstly, the improvement of QDs surface ligands and the optimization of ligands engineering were discussed, and the control of the physical properties of QDs films were realized. From the aspects of colloidal QDs, ligand engineering, and solar cell preparation, the future development direction of colloidal QDs solar cells was proposed, providing technical guidances for the preparation of low-cost and high-efficiency nanocrystalline solar cells.

Tuning energy transfer efficiency in quantum dots mixture by controling donor/acceptor ratio

Improving the emission performance of colloidal quantum dots(QDs)is of paramount importance for their applications on light-emitting diodes(LEDs),displays and lasers.A highly promising approach is to tune the carrier recombination channels and lifetime by exploiting the energy transfer process.However,to achieve this precise emission optimization,quantitative modulation on energy transfer efficiency is highly desirable but still challenging.Here,we demonstrate a convenient approach to realize tunable energy transfer efficiency by forming QDs mixture with controllable donor/acceptor(D/A)ratio.With the mixing ratio ranging from 16/1 to 1/16,the energy transfer efficiency could be effectively tuned from near zero to~70%.For the high mixing ratio of 16/1,acceptors obtain adequate energy supplied by closely surrounding donors,leading to~2.4-fold PL enhancement.While for the low mixing ratio,the ultrafast and efficient energy extraction process directly suppresses the multi-exciton and Auger recombination in the donor,bringing about a higher threshold.The facile modulation of emission performance by controllably designed mixing ratio and quantitatively tunable energy transfer efficiency will facilitate QD-based optoelectronic and photovoltaic applications.

How to get high-efficiency lead chalcogenide quantum dot solar cells?

Lead chalcogenide colloidal quantum dots(CQDs)are regarded as attractive absorption materials for novel solar cells(SCs).The cost of lead chalcogenide CQD has been decreased to a commercialization target of$5/g due to the direct production of CQD inks.However,the photoelectric conversion efficiency(PCE)of lead chalcogenide CQDSCs is presently close to 14%,well below the commercialization target(20%),which is only 41%of the theoretical Shockley-Queisser limit efficiency.In this study,the different losses of open-circuit voltage(V_(oc)),fill factor(FF),and short circuit current density(J_(sc))for current CQDSCs are systematically discussed,as well as the percentage and likely causes of each loss.Then the primary reasons for the CQDSCs’performance constraints are highlighted.Following that,we focus on the CQDSCs interfaces(i.e.,CQD/CQD,CQD/HTL,and ETL/CQD)and explore viable ways to reduce device performance loss.Finally,based on the discussion above,we propose many enhancements to significantly solve numerous major obstacles impeding device performance to boost the PCE of CQDSCs for future commercialization significantly.

Strong circularly polarized luminescence from quantum dots/2D chiral perovskites composites

Chiral perovskites(CPs)have attracted enormous attentions since they have combined chirality and optoelectrical properties well which is promising in circularly polarized luminescence(CPL)application and of great importance for future spin-optoelectronics.However,there is a key contradiction that in chiral perovskites chirality distorts the crystal structure,leading to poor photoluminescence(PL)properties.Achieving the balance between chirality and PL is a major challenge for strong CPL from chiral perovskites.Differently,two-dimensional(2D)chiral perovskite has shown fascinating chiral induced spin selectivity(CISS)effect which can act as spin injector under ambient conditions.Here,we propose an effective strategy to achieve high CPL activity generated from quantum dots(QDs)by introducing 2D chiral perovskite as a chiral source,providing spin polarized carriers through the CISS effect.The as-synthesized QDs/CP composites exhibit dissymmetry factors(glum)up to 9.06×10^(−3).For the first time,we performed grazing incident wide angle X-ray scattering(GIWAXS)measurements,showing the chirality originates from the distorted lattices caused by the large chiral organic cations.Besides,time-resolved PL(TR-PL)measurements verify the enhanced CPL activity should be attributed to the charge transport between two components.These findings provide a useful method to achieve CPL in QDs/2D chiral perovskite heterojunctions which could be promising in spinoptoelectronics application.

Rational design of eco-friendly Mn-doped nonstoichiometric CuInSe/ZnSe core/shell quantum dots for boosted photoelectrochemical efficiency

Colloidal core/shell quantum dots(QDs)with environment-friendly feature and controllable optoelectronic properties are promising building blocks in emerging solar technologies.In this work,we rationally design and tailor the eco-friendly CuInSe(CISe)/ZnSe core/shell QDs by Mn doping and stoichiometric optimization(i.e.,molar ratios of Cu/In).It is demonstrated that Mn doping in In-rich CISe/ZnSe core/shell QDs can effectively engineer the charge kinetics inside the QDs,enabling efficient photogenerated electrons transfer into the shell for retarded charge recombination.As a result,a solar-driven photoelectrochemical(PEC)device fabricated using the optimized Mn-doped In-rich CISe/ZnSe core/shell QDs(Cu/In ratio of 1/2)exhibits improved charge extraction and injection,showing a~3.5-fold higher photocurrent density than that of the pristine CISe/ZnSe core/shell QDs under 1 sun AM 1.5G illumination.The findings indicate that transition metal doping in“green”nonstoichiometric core/shell QDs may offer a new strategy for achieving high-efficiency solar energy conversion applications.

Significant Lifetime Enhancement in QLEDs by Reducing Interfacial Charge Accumulation via Fluorine Incorporation in the ZnO Electron Transport Layer

ZnO nanoparticles are widely used for the electron transport layers(ETLs)of quantum dots light emitting devices(QLEDs).In this work we show that incorporating fluorine(F)into the ZnO ETL results in significant enhancement in device electroluminescence stability,leading to LT50 at 100 cd m^(−2) of 2,370,000 h in red QLED,47X longer than the control devices.X-ray photo-electron spectroscopy,time-of-flight secondary ion mass spectroscopy,photoluminescence and electrical measurements show that the F passivates oxygen vacancies and reduces electron traps in ZnO.Transient photoluminescence versus bias measurements and capacitance-voltage-luminance measurements reveal that the CF4 plasma-treated ETLs lead to increased electron concentration in the QD and the QD/hole transport layer interface,subsequently decreasing hole accumulation,and hence the higher stability.The findings provide new insights into the critical roles that optimizing charge distribution across the layers play in influencing stability and present a novel and simple approach for extending QLED lifetimes.

用于PPG信号测试的大面积柔性胶体量子点红外光电二极管

以铟镓砷、碲镉汞等外延生长的单晶材料作为吸光层的光电二极管是红外探测技术的主流方案.然而,它们的刚性结构、极高成本以及有限尺寸使其无法满足诸如人体内外环境监测,便携式医疗等智能穿戴应用的需求.基于低温液相工艺的胶体量子点(CQDs)红外光电二极管是实现大面积柔性红外光电探测器极具竞争力的候选方案,但由于其发展年限较短,仍存在暗电流密度大、均匀性差以及工作稳定性差等问题.本文通过在CQDs墨水中引入聚酰亚胺来制备CQDs光电二极管.这种聚合物配体既实现了对CQDs表面缺陷态的钝化,又能够保持胶体墨水的单分散性和持久性.这两点使得光电探测器的吸光层形貌得以改善,同时暗电流密度显著降低,极大地优化了CQDs光电二极管的均匀性和工作稳定性.采用该方案制备的大面积柔性CQDs光电二极管能够实现高于10^(13)Jones的比探测率,可在环境光下实现可穿戴式光电体积描记(PPG)心率信号监测,极大地降低了成本和功耗.

Photovoltaic devices employing ternary PbSxTe1-x nanocrystals

Colloidal quantum dots(CQDs), especially lead chalcogenide CQDs, are regarded as promising materials for the next generation solar cells, due to their large absorption coefficient, excellent charge transport,and multiple exciton generation effect. We successfully synthesized highly-crystalline, monodispersed,well-alloyed PbS_xTe_(1-x) nanocrystals via a one-pot, hot injection reaction method. Energy-filtered transmission electron microscopy suggested that the S and Te anions were uniformly distributed in the alloy nanoparticles. The photovoltaic performance of CQD solar cells based on ternary PbS_xTe_(1-x) was reported for the first time. The photovoltaic devices using PbS_xTe_(1-x) were more efficient than either the pure PbS or the PbTe based devices. In addition, the PbS_xTe_(1-x) based devices showed a significantly improved stability than that of the PbTe based devices.