Solution-processed intermediate-band solar cells with lead sulfide quantum dots and lead halide perovskites.

The intermediate-band solar cell (IBSC) with quantum dots and a bulk semiconductor matrix has potential for high power conversion efficiency, exceeding the Shockley-Queisser limit. However, the IBSCs reported to date have been fabricated only by dry process and their efficiencies are limited, because their photo-absorption layers have low particle density of quantum dots, defects due to lattice strain, and low bandgap energy of bulk semiconductors. Here we present solution-processed IBSCs containing photo-absorption layers where lead sulfide quantum dots are densely dispersed in methylammonium lead bromide perovskite matrices with a high bandgap energy of 2.3 eV under undistorted conditions. We confirm that the present IBSCs exhibit two-step photon absorption via intermediate-band at room temperature by inter-subband photocurrent spectroscopy.

Highly Efficient 17.6% Tin-Lead Mixed Perovskite Solar Cells Realized through Spike Structure.

Frequently observed high Voc loss in tin-lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the Voc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu(In,Ga)Se2 solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved Voc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements.

Mixed Sn-Ge Perovskite for Enhanced Perovskite Solar Cell Performance in Air.

Lead-based perovskite solar cells have gained ground in recent years, showing efficiency as high as 20%, which is on par with that of silicon solar cells. However, the toxicity of lead makes it a nonideal candidate for use in solar cells. Alternatively, tin-based perovskites have been proposed because of their nontoxic nature and abundance. Unfortunately, these solar cells suffer from low efficiency and stability. Here, we propose a new type of perovskite material based on mixed tin and germanium. The material showed a band gap around 1.4-1.5 eV as measured from photoacoustic spectroscopy, which is ideal from the perspective of solar cells. In a solar cell device with inverted planar structure, pure tin perovskite solar cell showed a moderate efficiency of 3.31%. With 5% doping of germanium into the perovskite, the efficiency improved up to 4.48% (6.90% after 72 h) when measured in air without encapsulation.

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.

Ultrafast Electron Injection from Photoexcited Perovskite CsPbI3 QDs into TiO2 Nanoparticles with Injection Efficiency near 99.

Photoexcited electron injection dynamics from CsPbI3 quantum dots (QDs) to wide gap metal oxides are studied by transient absorption spectroscopy. Experimental results show under a low excitation intensity that ∼99% of the photoexcited electrons in CsPbI3 QDs can be injected into TiO2 with a size-dependent rate ranging from 1.30 × 1010 to 2.10 × 1010 s-1, which is also ∼2.5 times faster than that in the case of ZnO. A demonstration QD-sensitized solar cell based on a CsPbI3/TiO2 electrode is fabricated that delivers a power conversion efficiency of 5%.

Improvement of Photovoltaic Performance of Colloidal Quantum Dot Solar Cells Using Organic Small Molecule as Hole-Selective Layer.

A novel organic small molecule bis-triphenylamine with spiro(fluorene-9,9'-xanthene) as the conjugated system, named BTPA-4, is successfully synthesized and employed as the hole-selective layer (HSL) in colloidal quantum dots solar cells (CQDSCs). The introduction of BTPA-4 layer can significantly prolong effective carrier lifetime (τeff), increase charge recombination resistance (Rrec), and thus diminish the interfacial charge recombination at the PbS-QDs/Au electrode interface. The effect of BTPA-4 as HSL in the device performance is especially significant for the open-circuit voltage (Voc) and power conversion efficiency (PCE), with a ∼ 10% and 15% enhancement respectively, comparing with those of device without the HSL. Furthermore, the PbS CQDSCs with BTPA-4 possessed a noticeably stable property for over 100 days of storage under ambient atmosphere.

Ligand-dependent exciton dynamics and photovoltaic properties of PbS quantum dot heterojunction solar cells.

The surface chemistry of colloidal quantum dots (QDs) plays an important role in determining the photoelectric properties of QD films and the corresponding quantum dot heterojunction solar cells (QDHSCs). To investigate the effects of the ligand structure on the photovoltaic performance and exciton dynamics of QDHSCs, PbS QDHSCs were fabricated by the solid state ligand exchange method with mercaptoalkanoic acid as the cross-linking ligand. Temperature-dependent photoluminescence and ultrafast transient absorption spectra show that the electronic coupling and charge transfer rate within QD ensembles were monotonically enhanced as the ligand length decreased. However, in practical QDHSCs, the second shortest ligand 3-mercaptopropionic acid (MPA) showed higher power conversion efficiency than the shortest ligand thioglycolic acid (TGA). This could be attributed to the difference in their surface trap states, supported by thermally stimulated current measurements. Moreover, compared with the non-conjugated ligand MPA, the conjugated ligand 4-mercaptobenzoic acid (MBA) introduces less trap states and has a similar charge transfer rate in QD ensembles, but has poor photovoltaic properties. This unexpected result could be contributed by the QD-ligand orbital mixing, leading to the charge transfer from QDs to ligands instead of charge transfer between adjacent QDs. This work highlights the significant effects of ligand structures on the photovoltaic properties and exciton dynamics of QDHSCs, which would shed light on the further development of QD-based photoelectric devices.

Architecture of the Interface between the Perovskite and Hole-Transport Layers in Perovskite Solar Cells.

The interface between the perovskite (PVK, CH3 NH3 PbI3 ) and hole-transport layers in perovskite solar cells is discussed. The device architecture studied is as follows: F-doped tin oxide (FTO)-coated glass/compact TiO2 /mesoporous TiO2 /PVK/2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-MeOTAD)/Au. After a thin layer of 4,4,4-trifluorobutylammonium iodide (TFBA) was inserted at the interface between PVK and Spiro-MeOTAD, the photovoltaic efficiency increased from 11.6-14.5 % to 15.1-17.6 %. TFBA (10 ppm) was added in the PVK solution before coating. Owing to the low surface tension of TFBA, TFBA rose to the surface of the PVK layer spontaneously during spin-coating to make a thin organic layer. The PVK grain boundaries also seemed to be passivated with the addition of TFBA. However, large differences in Urbach energies and valence band energy level were not observed for the PVK layer with and without the addition of TFBA. The charge recombination time constant between the PVK and the Spiro-MeOTAD became slower (from 8.4 to 280 µsec) after 10 ppm of TFBA was added in the PVK. The experimental results using TFBA conclude that insertion of a very thin layer at the interface between PVK and Spiro-MeOTAD is effective for suppressing charge recombination and increasing photovoltaic performances.

Indoor Light Performance of Coil Type Cylindrical Dye Sensitized Solar Cells.

A very good performance under low/diffused light intensities is one of the application areas in which dye-sensitized solar cells (DSSCs) can be utilized effectively compared to their inorganic silicon solar cell counterparts. In this article, we have investigated the 1 SUN and low intensity fluorescent light performance of Titanium (Ti)-coil based cylindrical DSSC (C-DSSC) using ruthenium based N719 dye and organic dyes such as D205 and Y123. Electrochemical impedance spectroscopic results were analyzed for variable solar cell performances. Reflecting mirror with parabolic geometry as concentrator was also utilized to tap diffused light for indoor applications. Fluorescent light at relatively lower illumination intensities (0.2 mW/cm2 to 0.5 mW/cm2) were used for the investigation of TCO-less C-DSSC performance with and without reflector geometry. Furthermore, the DSSC performances were analyzed and compared with the commercially available amorphous silicon based solar cell for indoor applications.

Design of Far-Red Sensitizing Squaraine Dyes Aiming Towards the Fine Tuning of Dye Molecular Structure.

Model squaraine dyes having sharp and narrow absorptions mainly in the far-red wavelength region has been logically designed, synthesized and used for their application as sensitizer in the dyesensitized solar cells (DSSC). In order to have fine control on energetics, dyes having same mother core and alkyl chain length varying only in molecular symmetry and position of substituent were designed. It has been found that even keeping all other structural factor constant, only positional variation of substituent leads to not only in the variation of energetics by 0.1 eV but affects the photovoltaic characteristics also. Optimum concentration of dye de-aggregating agent was found to be 100 times with respect to the sensitizing dye concentration. Amongst dyes utilized in this work best performance was obtained for unsymmetrical dye SQ-40 giving a photoconversion efficiency of 4.01% under simulated solar irradiation at global AM 1.5.

Mechanisms of charge accumulation in the dark operation of perovskite solar cells.

In this work, a new current peak at forward bias in the dark current-voltage curves has been identified for standard mesoscopic perovskite solar cells. This characteristic peak appears only under some specific conditions, mainly in the reverse scan (RS) direction and when the solar cells were kept for several seconds under short-circuit conditions before starting the RS measurement. This peak disappears when the above experimental conditions are not applied. It is considered that this uncommon diode shape is obtained because shallow and/or deep trap states located at the interface between either perovskite/p-type or perovskite/n-type transport materials are dynamically filled during the RS voltage scan. To corroborate this hypothesis, the response of hole transport materials (HTMs), small molecule spiro-OMeTAD and polymer P3HT, as well as both HTMs with additives, was compared. Also perovskite absorbers such as CH3NH3PbI3 and all-inorganic perovskite based on cesium (CsPbI3) were also analyzed, achieving in all cases similar trends.

Rapid Synthesis of D-A'-π-A Dyes through a One-Pot Three-Component Suzuki-Miyaura Coupling and an Evaluation of their Photovoltaic Properties for Use in Dye-Sensitized Solar Cells.

Twenty-four D-A'-π-A dyes were rapidly synthesized through a one-pot three-component Suzuki-Miyaura coupling reaction, which was assisted by microwave irradiation. We measured the absorption spectra, electrochemical properties, and solar-cell performance of all the synthesized dyes. The D5 πA4 dye contained our originally designed rigid and nonplanar donor and exerted the highest efficiency at 5.4 %. The short-circuit current (Jsc ) was the most important parameter for the conversion efficiency (η) in the case of the organic D-A'-π-A dyes. Optimal ranges for the D-A'-π-A dyes were observed for high values of Jsc /λmax at λ=560-620 nm, an optical-absorption edge of λ=690-790 nm, and EHOMO and ELUMO values of <1.14 and -0.56 to -0.76 V, respectively.

Rapid Synthesis of Thiophene-Based, Organic Dyes for Dye-Sensitized Solar Cells (DSSCs) by a One-Pot, Four-Component Coupling Approach.

This one-pot, four-component coupling approach (Suzuki-Miyaura coupling/C-H direct arylation/Knoevenagel condensation) was developed for the rapid synthesis of thiophene-based organic dyes for dye-sensitized solar cells (DSSCs). Seven thiophene-based, organic dyes of various donor structures with/without the use of a 3,4-ethylenedioxythiophene (EDOT) moiety were successfully synthesized in good yields based on a readily available thiophene boronic acid pinacol ester scaffold (one-pot, 3-step, 35-61%). Evaluation of the photovoltaic properties of the solar cells that were prepared using the synthesized dyes revealed that the introduction of an EDOT structure beside a cyanoacrylic acid moiety improved the short-circuit current (Jsc) while decreasing the fill factor (FF). The donor structure significantly influenced the open-circuit voltage (Voc), the FF, and the power conversion efficiency (PCE). The use of a n-hexyloxyphenyl amine donor, and our originally developed, rigid, and nonplanar donor, both promoted good cell performance (η=5.2-5.6%).

High reduction of interfacial charge recombination in colloidal quantum dot solar cells by metal oxide surface passivation.

Bulk heterojunction (BHJ) solar cells based on colloidal QDs and metal oxide nanowires (NWs) possess unique and outstanding advantages in enhancing light harvesting and charge collection in comparison to planar architectures. However, the high surface area of the NW structure often brings about a large amount of recombination (especially interfacial recombination) and limits the open-circuit voltage in BHJ solar cells. This problem is solved here by passivating the surface of the metal oxide component in PbS colloidal quantum dot solar cells (CQDSCs). By coating thin TiO2 layers onto ZnO-NW surfaces, the open-circuit voltage and power conversion efficiency have been improved by over 40% in PbS CQDSCs. Characterization by transient photovoltage decay and impedance spectroscopy indicated that the interfacial recombination was significantly reduced by the surface passivation strategy. An efficiency as high as 6.13% was achieved through the passivation approach and optimization for the length of the ZnO-NW arrays (device active area: 16 mm2). All solar cells were tested in air, and exhibited excellent air storage stability (without any performance decline over more than 130 days). This work highlights the significance of metal oxide passivation in achieving high performance BHJ solar cells. The charge recombination mechanism uncovered in this work could shed light on the further improvement of PbS CQDSCs and/or other types of solar cells.

Improved understanding of the electronic and energetic landscapes of perovskite solar cells: high local charge carrier mobility, reduced recombination, and extremely shallow traps.

The intriguing photoactive features of organic-inorganic hybrid perovskites have enabled the preparation of a new class of highly efficient solar cells. However, the fundamental properties, upon which the performance of these devices is based, are currently under-explored, making their elucidation a vital issue. Herein, we have investigated the local mobility, recombination, and energetic landscape of charge carriers in a prototype CH3NH3PbI3 perovskite (PVK) using a laser-flash time-resolved microwave conductivity (TRMC) technique. PVK was prepared on mesoporous TiO2 and Al2O3 by one or two-step sequential deposition. PVK on mesoporous TiO2 exhibited a charge carrier mobility of 20 cm(2) V(-1) s(-1), which was predominantly attributed to holes. PVK on mesoporous Al2O3, on the other hand, exhibited a 50% lower mobility, which was resolved into balanced contributions from both holes and electrons. A general correlation between crystal size and mobility was revealed irrespective of the fabrication process and underlying layer. Modulating the microwave frequency from 9 toward 23 GHz allowed us to determine the intrinsic mobilities of each PVK sample (60-75 cm(2) V(-1) s(-1)), which were mostly independent of the mesoporous scaffold. Kinetic and frequency analysis of the transient complex conductivity strongly support the superiority of the perovskite, based on a significant suppression of charge recombination, an extremely shallow trap depth (10 meV), and a low concentration of these trapped states (less than 10%). The transport mechanism was further investigated by examining the temperature dependence of the TRMC maxima. Our study provides a basis for understanding perovskite solar cell operation, while highlighting the importance of the mesoporous layer and the perovskite fabrication process.

Charge transfer and recombination at the metal oxide/CH3NH3PbClI2/spiro-OMeTAD interfaces: uncovering the detailed mechanism behind high efficiency solar cells.

In recent years, organometal halide perovskite-based solid-state hybrid solar cells have attracted unexpected increasing interest because of their high efficiency (the record power conversion efficiency has been reported to be over 15%) and low fabrication cost. It has been accepted that the high efficiency was mainly attributed to the strong optical absorption (absorption coefficient: 15,000 cm(-1) at 550 nm) over a broader range (up to 800 nm) and the long lifetimes of photoexcited charge carriers (in the order of 10 ns - a few 100 ns) of the perovskite absorbers. However, much of the fundamental photophysical properties of perovskite relating to the high photovoltaic performance are remained to be investigated. The charge separation and recombination processes at the material interfaces are particularly important for solar cell performances. To better understand the high efficiency of perovskite solar cells, we systematically investigated the charge separation (electron and hole injection) and charge recombination dynamics of CH3NH3PbClI2 hybrid solar cells employing TiO2 nanostructures as the electron transfer material (ETM) and spiro-OMeTAD as the hole transfer material (HTM). The measurements were carried out using transient absorption (TA) techniques on a time scale from sub-picoseconds to milliseconds. We clarified the timescales of electron injection, hole injection, and recombination processes in TiO2/CH3NH3PbClI2/spiro-OMeTAD solar cells. Charge separation and collection efficiency of the perovskite-based solar cells were discussed. In addition, the effect of TiO2 size on the charge separation and recombination dynamics was also investigated. It was found that all TiO2-based perovskite solar cells possessed similar charge separation processes, but quite different recombination dynamics. Our results indicate that charge recombination was crucial to the performance of the perovskite solar cells, which could be effectively suppressed through optimising nanostructured TiO2 films and surface passivation, thus pushing these cells to even higher efficiency.

Elucidating the structure-property relationships of donor-π-acceptor dyes for dye-sensitized solar cells (DSSCs) through rapid library synthesis by a one-pot procedure.

The creation of organic dyes with excellent high power conversion efficiency (PCE) is important for the further improvement of dye-sensitized solar cells. We wish to describe the rapid synthesis of a 112-membered donor-π-acceptor dye library by a one-pot procedure, evaluation of PCEs, and elucidation of structure-property relationships. No obvious correlations between ε, and the η were observed, whereas the HOMO and LUMO levels of the dyes were critical for η. The dyes with a more positive E(HOMO), and with an E(LUMO)<-0.80 V, exerted higher PCEs. The proper driving forces were crucial for a high J(sc), and it was the most important parameter for a high η. The above criteria of E(HOMO) and E(LUMO) should be useful for creating high PCE dyes; nevertheless, that was not sufficient for identifying the best combination of donor, π, and acceptor blocks. Combinatorial synthesis and evaluation was important for identifying the best dye.

Control of charge dynamics through a charge-separation interface for all-solid perovskite-sensitized solar cells.

The relationship between the structure of the charge-separation interface and the photovoltaic performance of all-solid dye-sensitized solar cells is reported. This cell is composed of porous a TiO2/perovskite (CH3NH3PbI(x)Cl(3-x))/p-type organic conductor. The porous titania layer was passivated with Al2O3 or Y2O3 to remove surface traps of the porous titania layer. Both passivations were effective in increasing the efficiency of the solar cell. Especially, the effect of Y2O3 passivation was remarkable. After passivation, the efficiency increased from 6.59 to 7.5%. The increase in the efficiency was discussed in terms of the electron lifetime in TiO2, the thermally stimulated current, the measurement of the microwave refractive carrier lifetime, and transition absorption spectroscopy. It was proven that surface passivation resulted in retardation of charge recombination between the electrons in the porous titania layers and the holes in the p-type organic conductors.

CH3NH3SnxPb(1-x)I3 Perovskite Solar Cells Covering up to 1060 nm.

We report photovoltaic performances of all-solid state Sn/Pb halide-based perovskite solar cells. The cell has the following composition: F-doped SnO2 layered glass/compact titania layer/porous titania layer/CH3NH3SnxPb(1-x)I3/regioregular poly(3-hexylthiophene-2,5-diyl). Sn halide perovskite itself did not show photovoltaic properties. Photovoltaic properties were observed when PbI2 was added in SnI2. The best performance was obtained by using CH3NH3Sn0.5Pb0.5I3 perovskite. 4.18% efficiency with open circuit voltage 0.42 V, fill factor 0.50, and short circuit current 20.04 mA/cm(2) are reported. The edge of the incident photon to current efficiency curve reached 1060 nm, which was 260 nm red-shifted compared with that of CH3NH3PbI3 perovskite solar cells.

Huge suppression of charge recombination in P3HT-ZnO organic-inorganic hybrid solar cells by locating dyes at the ZnO/P3HT interfaces.

The charge separation and charge recombination dynamics in P3HT-ZnO and P3HT-dye-ZnO bulk heterojunction organic-inorganic hybrid solar cells (OIHSCs) prepared by a one-pot method were studied using a transient absorption (TA) method, both for optical absorption of P3HT in the visible region and for optical absorption of SQ36 in the NIR region. In the case of P3HT-ZnO, the charge separation was very fast, occurring within 1 ps. On the other hand, high charge recombination between electrons in the surface states and/or the conduction band of ZnO and holes in P3HT was observed. In the case of P3HT-dye-ZnO, we found that the charge recombination could be greatly suppressed by locating the dye at the P3HT/ZnO interfaces while maintaining a fast charge separation rate (a few ps to 10 ps). Our findings provide one methodology for the design of OIHSCs for improving their conversion efficiency, which is to position the dye at the appropriate BHJ interfaces.