The interparticle distance limit for multiple exciton dissociation in PbS quantum dot solid films.

Understanding the behaviour of multiple exciton dissociation in quantum dot (QD) solid films is of fundamental interest and paramount importance for improving the performance of quantum dot solar cells (QDSCs). Unfortunately, the charge transfer behaviour of photogenerated multiple exciton in QD solid films is not clear to date. Herein, we systematically investigate the multiple exciton charge transfer behaviour in PbS QD solid films by using ultrafast transient absorption spectroscopy. We observe that the multiple exciton charge transfer rate within QD ensembles is exponentially enhanced as the interparticle distance between the QDs decreases. Biexciton and triexciton dissociation between adjacent QDs occurs via a charge transfer tunneling effect just like single exciton, and the charge tunneling constants of the single exciton (ß1: 0.67 ± 0.02 nm-1), biexciton (ß2: 0.68 ± 0.05 nm-1) and triexciton (ß3: 0.71 ± 0.01 nm-1) are obtained. More importantly, for the first time, the interparticle distance limit (≤4.3 nm) for multiple exciton charge transfer between adjacent QDs is found for the extraction of multiple excitons rapidly before the occurrence of Auger recombination. This result points out a vital and necessary condition for the use of multiple excitons produced in PbS QD films, especially for their applications in QDSCs.

Interface Passivation Effects on the Photovoltaic Performance of Quantum Dot Sensitized Inverse Opal TiO2 Solar Cells.

Quantum dot (QD)-sensitized solar cells (QDSSCs) are expected to achieve higher energy conversion efficiency than traditional single-junction silicon solar cells due to the unique properties of QDs. An inverse opal (IO)-TiO2 (IO-TiO2) electrode is useful for QDSSCs because of its three-dimensional (3D) periodic nanostructures and better electrolyte penetration compared to the normal nanoparticles (NPs)-TiO2 (NPs-TiO2) electrode. We find that the open-circuit voltages Voc of the QDSSCs with IO-TiO2 electrodes are higher than those of QDSSCs with NPs-TiO2 electrodes. One important strategy for enhancing photovoltaic conversion efficiency of QDSSCs with IO-TiO2 electrodes is surface passivation of photoanodes using wide-bandgap semiconducting materials. In this study, we have proposed surface passivation on IO-TiO2 with ZnS coating before QD deposition. The efficiency of QDSSCs with IO-TiO2 electrodes is largely improved (from 0.74% to 1.33%) because of the enhancements of Voc (from 0.65 V to 0.74 V) and fill factor (FF) (from 0.37 to 0.63). This result indicates that ZnS passivation can reduce the interfacial recombination at the IO-TiO2/QDs and IO-TiO2/electrolyte interfaces, for which two possible explanations can be considered. One is the decrease of recombination at IO-TiO2/electrolyte interfaces, and the other one is the reduction of the back-electron injection from the TiO2 electrode to QDs. All of the above results are effective for improving the photovoltaic properties of QDSSCs.

Lead Selenide Colloidal Quantum Dot Solar Cells Achieving High Open-Circuit Voltage with One-Step Deposition Strategy.

Lead selenide (PbSe) colloidal quantum dots (CQDs) are considered to be a strong candidate for high-efficiency colloidal quantum dot solar cells (CQDSCs) due to its efficient multiple exciton generation. However, currently, even the best PbSe CQDSCs can only display open-circuit voltage ( Voc) about 0.530 V. Here, we introduce a solution-phase ligand exchange method to prepare PbI2-capped PbSe (PbSe-PbI2) CQD inks, and for the first time, the absorber layer of PbSe CQDSCs was deposited in one step by using this PbSe-PbI2 CQD inks. One-step-deposited PbSe CQDs absorber layer exhibits fast charge transfer rate, reduced energy funneling, and low trap assisted recombination. The champion large-area (active area is 0.35 cm2) PbSe CQDSCs fabricated with one-step PbSe CQDs achieve a power conversion efficiency (PCE) of 6.0% and a Voc of 0.616 V, which is the highest Voc among PbSe CQDSCs reported to date.

Recombination Suppression in PbS Quantum Dot Heterojunction Solar Cells by Energy-Level Alignment in the Quantum Dot Active Layers.

Using spatial energy-level gradient engineering with quantum dots (QDs) of different sizes to increase the generated carrier collection at the junction of a QD heterojunction solar cell (QDHSC) is a hopeful route for improving the energy-conversion efficiency. However, the results of current related research have shown that a variable band-gap structure in a QDHSC will create an appreciable increase, not in the illumination current density, but rather in the fill factor. In addition, there are a lack of studies on the mechanism of the effect of these graded structures on the photovoltaic performance of QDHSCs. This study presents the development of air atmosphere solution-processed TiO2/PbS QDs/Au QDHSCs by engineering the energy-level alignment (ELA) of the active layer via the use of a sorted order of differently sized QD layers (four QD sizes). In comparison to the ungraded device (without the ELA), the optimized graded architecture (containing the ELA) solar cells exhibited a great increase (21.4%) in short-circuit current density ( Jsc). As a result, a Jsc value greater than 30 mA/cm2 has been realized in planar, thinner absorption layer (∼300 nm) PbS QDHSCs, and the open-circuit voltage ( Voc) and power-conversion efficiency (PCE) were also improved. Through characterization by the light intensity dependences of the Jsc and Voc and transient photovoltage decay, we find that (i) the ELA structure, serving as an electron-blocking layer, reduces the interfacial recombination at the PbS/anode interface, and (ii) the ELA structure can drive more carriers toward the desirable collection electrode, and the additional carriers can fill the trap states, reducing the trap-assisted recombination in the PbS QDHSCs. This work has clearly elucidated the mechanism of the recombination suppression in the graded QDHSCs and demonstrated the effects of ELA structure on the improvement of Jsc. The charge recombination mechanisms characterized in this work would be able to shed light on further improvements of QDHSCs, which could even benefit other types of solar cells.

Highly Luminescent Phase-Stable CsPbI3 Perovskite Quantum Dots Achieving Near 100% Absolute Photoluminescence Quantum Yield.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems such as CdSe and PbS, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental nonradiative recombination centers and thus quenching luminescence. In this paper, we show that a high room-temperature photoluminescence quantum yield of up to 100% can be obtained in CsPbI3 perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized with our improved synthetic protocol that involves introducing organolead compound trioctylphosphine-PbI2 (TOP-PbI2) as the reactive precursor, which also leads to a significantly improved stability for the resulting CsPbI3 QD solutions. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidence the negligible electron or hole-trapping pathways in our QDs, which explains such a high quantum efficiency. We expect the successful synthesis of the "ideal" perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices.

High Efficiency Quantum Dot Sensitized Solar Cells Based on Direct Adsorption of Quantum Dots on Photoanodes.

Unambiguously direct adsorption (DA) of initial oil-soluble quantum dots (QDs) on TiO2 film electrode is a convenient and simple approach in the construction of quantum dot sensitized solar cells (QDSCs). Regrettably, low QD loading amount and poor reproducibility shadow the advantages of DA route and constrain its practical application. Herein, the influence of experimental variables in DA process on QD loading amount as well as on the photovoltaic performance of the resultant QDSCs was investigated and optimized systematically, including the choice of solvent, purification of QDs, and sensitization time, as well as QD concentration. Experimental results demonstrated that it is essential to choose appropriate solvent as well as control purification cycles of original QD suspensions so as to realize satisfactory QD loading amount and ensure the high reproducibility. In addition, DA mode renders efficient electron injection from QD to TiO2, yet low QD loading amount and adverse QD agglomeration in comparison with the well-developed capping ligand induced self-assembly (CLIS) deposition approach. Mg2+ treatment on TiO2 photoanodes can promote the QD loading amount in DA mode. The optimized QDSCs based on DA mode exhibited efficiencies of 6.90% and 9.02% for CdSe and Zn-Cu-In-Se QDSCs, respectively, which were comparable to the best results based on CLIS mode (6.88% and 9.56%, respectively).

Alloying Strategy in Cu-In-Ga-Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells.

I-III-VI2 group "green" quantum dots (QDs) are attracting increasing attention in photoelectronic conversion applications. Herein, on the basis of the "simultaneous nucleation and growth" approach, Cu-In-Ga-Se (CIGSe) QDs with light harvesting range of about 1000 nm were synthesized and used as sensitizer to construct quantum dot sensitized solar cells (QDSCs). Inductively coupled plasma atomic emission spectrometry (ICP-AES), wild-angle X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses demonstrate that the Ga element was alloyed in the Cu-In-Se (CISe) host. Ultraviolet photoelectron spectroscopy (UPS) and femtosecond (fs) resolution transient absorption (TA) measurement results indicate that the alloying strategy could optimize the electronic structure in the obtained CIGSe QD material, thus matching well with TiO2 substrate and favoring the photogenerated electron extraction. Open circuit voltage decay (OCVD) and impedance spectroscopy (IS) tests indicate that the intrinsic recombination in CIGSe QDSCs was well suppressed relative to that in CISe QDSCs. As a result, CIGSe based QDSCs with use of titanium mesh supported mesoporous carbon counter electrode exhibited a champion efficiency of 11.49% (Jsc = 25.01 mA/cm2, Voc = 0.740 V, FF = 0.621) under the irradiation of full one sun in comparison with 9.46% for CISe QDSCs.

Photoreceptor images of normal eyes and of eyes with macular dystrophy obtained in vivo with an adaptive optics fundus camera.

PURPOSE: To report on images of the human photoreceptor mosaic acquired in vivo with a newly developed, compact adaptive optics (AO) fundus camera. METHODS: The photoreceptors of two normal subjects and a patient with macular dystrophy were examined by using an AO fundus camera equipped with a liquid crystal phase modulator. In the eye with macular dystrophy, the fixation point in the AO images was identified using scanning laser ophthalmoscope (SLO) microperimetric image superimposed on a color fundus photograph. RESULTS: Photoreceptor cells were detected as bright dots approximately 4 microm in diameter in normal subjects. In the eye with macular dystrophy, the fixation point was located within the bull's eye lesion and uniform small whitish spots with irregular patchiness were observed in the AO images of this area. The distance between the small spots was 3-4 microm. In other parts of the bull's eye retinal lesion, the whitish spots were larger and of different sizes. CONCLUSIONS: The photoreceptor mosaic could be identified in photographs of eyes of normal subjects and an eye with macular dystrophy in vivo by an AO fundus camera. In the eye with macular dystrophy, a relatively uniform photoreceptor mosaic was observed around the fixation point, whereas presumed debris of photoreceptor degradation was observed in the other bull's eye retinal lesion.

Binocular open-view Shack-Hartmann wavefront sensor with consecutive measurements of near triad and spherical aberration.

We have developed a binocular open-view Shack-Hartmann wavefront sensor for measuring time variation of binocular accommodation, vergence, pupil sizes (i.e., the binocular near triad), and monochromatic aberrations. The device measures these values16 times per second for up to 1 min. Our purpose is to introduce the new instrument. We have confirmed the accuracy of the device. Refractions for a 4 mm pupil were accurate across the range of measurements of model eyes and normal human eyes. We measured binocular dynamics of accommodation, vergence, and spherical aberrations.

Adaptive optics fundus camera to examine localized changes in the photoreceptor layer of the fovea.

PURPOSE: To examine highly localized photoreceptor disruptions in the fovea by a high-resolution adaptive optics (AO) fundus camera combined with Fourier-domain optical coherence tomography (FD OCT). DESIGN: Observational case series. PARTICIPANTS: Three eyes of 3 patients who showed dark foveal spots by slit-lamp biomicroscopy. METHODS: Three patients who reported metamorphopsia but showed no changes in the retina in conventional fundus photographs were examined. High-resolution retinal images were obtained with the AO fundus camera and by FD OCT. The images were compared with the findings obtained by standard clinical tests, including Amsler charts and fluorescein angiography (FA). MAIN OUTCOME MEASURES: Quantitative measurements of the area of photoreceptor disruption. RESULTS: Slit-lamp biomicroscopy revealed an irregularly shaped dark spot in the fovea centralis but no changes in FA in the 3 cases. The photoreceptor mosaic was absent in a highly localized area of the fovea in the images obtained by the AO fundus camera, and the photoreceptor outer segment was absent or disturbed at the corresponding area by FD OCT in all 3 cases. The horizontal and vertical sizes of the area of disturbance of the photoreceptor mosaic in the AO images in the 3 eyes were 400x200 microm, 300x120 microm, and 300x200 microm. These sizes were comparable to the photoreceptor outer segment disturbances in the OCT images which were 330x150 microm, 280x100 microm, 200x150 microm, respectively. CONCLUSIONS: Localized OS disturbances were able to be detected in eyes with a dark foveal spot by AO fundus camera 2-dimensionally and by FD OCT axially. The good correspondence of the sizes of the area of photoreceptor disturbances obtained by AO images to those by FD OCT images indicate that the AO images can be used to evaluate and follow the 2-dimensional area of focal changes of the photoreceptors in the fovea quantitatively.

In vivo measurements of cone photoreceptor spacing in myopic eyes from images obtained by an adaptive optics fundus camera.

PURPOSE: To determine the cone spacing in normal and myopic eyes from the images obtained by an adaptive optics (AO) fundus camera. METHODS: Nineteen eyes of 19 healthy volunteers with a mean +/- SD spherical equivalent refractive error of -3.7 +/- 3.3 diopters (D) (range, -0.3 to -11.1 D) and a mean axial length of 25.4 +/- 1.61 mm (range, 23.4-28.0 mm) were investigated in a prospective cross-sectional study. An AO fundus camera equipped with a liquid crystal phase modulator was used to obtain the images of the photoreceptor mosaic. The spacing between the cones was calculated manually at a retinal locus 2 degrees temporal from the center of the fovea. The magnification of the image was calculated by the axial length measured with an IOL Master. RESULTS: The axial length was correlated with the refractive error (Pearson, r = -0.869; P < 0.001). The average cone spacing in the moderate- to high-myopia group (-6.5 +/- 2.3 D, n = 9) was 4.71 +/- 0.44 microm, which was significantly greater (P = 0.002) than the 3.90 +/- 0.47 microm in the normal and low-myopia groups (-1.1 +/- 0.9 D, n = 10). The cone spacing was significantly correlated with the axial length (r = 0.77, P < 0.001). CONCLUSIONS: The AO fundus camera is capable of acquiring images of the photoreceptors in normal and myopic eyes. The greater spacing between cones in the myopia group is consistent with histological findings. These results suggest that retinal expansion should be considered in addition to Knapp's law when aniseikonia is evaluated in axial myopia.