•Caprolactam (CPL) is employed as Lewis base for one-step anti-solvent process.•19.2% conversion efficiency of planar perovskite solar cells has been achieved.•Stabilities of perovskite films and devices have been significantly enhanced.•Residual CPL is supposed to well passivate grain boundaries and the interface.Low-cost caprolactam (CPL) has been introduced into one-step anti-solvent process to fabricate efficient and stable planar perovskite solar cells (PSCs) for the first time. It is found that CPL as a Lewis base has the weak coordination ability to Pb(II), and this weak interaction will be easily broken by either anti-solvent dripping or annealing owing to the steric effect, thus leading to a compact film with monolithic grains. Up to 19.2% power conversion efficiency (PCE) has been achieved. Especially, the PSCs exhibit superior stability in ambient condition, which PCE can be retained 95% of its initial PCE after 1200 h. Further investigation suggests that the residual CPL in the perovskite film can well passivate perovskite grain boundaries and the interface by its interaction with Pb2+ cations, thus leading to good anti-moisture and thermal stabilities of perovskite films. This work provides a new way to fabricate highly efficient, stable PSCs by adopting an appropriate Lewis base simultaneously for high quality perovskite films and passivation toward grain boundaries and interfaces.Download high-res image (208KB)Download full-size image

An n-type semiconducting copolymer of perylene diimide and dithienothiophene (PPDIDTT) is used as a dual function interfacial layer to modify the surface of perovskite films in inverted perovskite solar cells. The PPDIDTT layer can remarkably passivate the surface trap states of perovskite through the formation of a Lewis adduct between the under-coordinated Pb in perovskite and S in the dithienothiophene unit of PPDIDTT, and also shows efficient charge extraction and transfer properties. The PPDIDTT modified devices exhibit a maximum power conversion efficiency of 16.5%, superior to that of the control devices without PPDIDTT (15.3%). In addition, the device stability and hysteresis in J–V curves of the modified devices are also improved compared to those of the control devices.

Introduction of Lewis acid–base adducts is an efficient way for achieving high quality perovskite films. In this work, we investigate the influence of Lewis bases on the annealing process of perovskite films, and three Lewis bases with different melting points and boiling points including DMSO, urea and thiourea are chosen. The interaction between the Lewis base and PbI2 as well as the residual time of Lewis bases in perovskite films while annealing has been investigated. In the meantime, directly spin-coating urea or thiourea on perovskite films is also performed to study the effect of Lewis bases on the perovskite crystallinity and morphology during annealing. It is found that the existence of Lewis bases in the annealing process could significantly promote the perovskite grain growth. DMSO can be removed within a few minutes whereas the residual time of thiourea is longer than that of DMSO and urea, consistent with their boiling points. As a result, the grain sizes of the thiourea-based perovskite film increase continuously whereas the DMSO-based sample shows an obvious ripening effect only in the first few minutes. To take advantage of the ripening effect, synergistic systems of DMSO and a small amount of urea or thiourea are further adopted for fabricating perovskite solar cells (PSCs). The device based on DMSO/urea presents the best PCE of 20.06%, higher than 18.8% for the device based on DMSO and 8.35% for the device based on urea.

•A modified prepared method developed for high quality CuInS2 quantum dots.•Careful controlling Cu/In non-stoichiometric ratios to reduce the surface defects.•Up to 8.54% efficiency achieved for CuInS2 quantum dot sensitized solar cells.•Interfacial electron recombination mechanism is suggested.•A simple way to reduce the photo-generated carrier loss for highly efficient QDSCs.Aiming at improving the cell performance of colloidal CuInS2 (CIS) quantum dot sensitized solar cells (QDSCs), a modified synthetic method has been developed to prepare CIS quantum dots (QDs), and Cu/In non-stoichiometric ratios of CIS QDs have been carefully controlled for the first time. It is found that, with the amount of In element increasing, the short-circuit photocurrent density (Jsc), open-circuit voltage (Voc) and fill factor (FF) of CIS QDSCs will gradually increase, leading to the cell performance enhanced. Up to 8.54% PCE has been achieved when the Cu/In precursor molar ratio is 1/4, which is a new record for the CIS-based solar cells. Electrochemical impedance analysis, open-circuit voltage-decay (OCVD) and time-resolved photoluminescence analyses further confirm that In-rich CIS QDs can bring about surface defect states significantly reduced, thus leading to the charge recombination at TiO2/CIS/electrolyte interfaces efficiently inhibited. Interfacial electron recombination mechanism of the solar cells is proposed that photo-generated carrier recombination in the cell is mainly dominated by the electron transfer process from the conduction band of TiO2 to unoccupied defect states of CIS, which has a great influence on the FF of the device. This work provides a new and simple way to reduce the loss of photo-generated carriers, improve the interfacial carrier collection and achieve highly efficient QDSCs.Cu/In non-stoichiometric ratios of CuInS2 (CIS) quantum dot (QDs) are carefully controlled for highly efficient CIS QDs-sensitized solar cells. Up to 8.54% efficiency is achieved as a new record for CIS-based solar cells. Interfacial electron recombination mechanism is proposed, with the In content increasing, surface defects of CIS QDs are significantly reduced, thus leading to interfacial carrier recombination efficiently inhibited.Download high-res image (200KB)Download full-size image

A morphology-controlled CH3NH3PbI3-xClx film is synthesized via two-step solution deposition by spin-coating a mixture solution of CH3NH3Cl and CH3NH3I onto the TiO2/PbI2 film for the first time. It is revealed that the existence of CH3NH3Cl is supposed to result in a preferential growth along the [110] direction of perovskite, which can improve both the crystallinity and surface coverage of perovskite and reduce the pinholes. Furthermore, the formation process of CH3NH3PbI3-xClx perovskite is explored, in which intermediates containing chlorine are suggested to exist. 13.12% of power conversion efficiency has been achieved for the mesoscopic cell, higher than 12.08% of power conversion efficiency of the devices fabricated without CH3NH3Cl via the same process. The improvement mainly lies in the increasing open-circuit photovoltage which is ascribed to the reduction of reverse saturation current density.Keywords: mixed-halide lead perovskite; morphology-control; perovskite formation process; preferential-growth; solar cells; two-step spin coating

We report two simple triphenylamine-based hole-transporting materials (HTMs) containing vinyl derivatives, 3,6-di(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl)-2-thiophene (apv-T) and 3,6-di(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl)-9-ethyl-carbazole (apv-EC) for the perovskite solar cells. According to theoretical calculation and experimental results, their HOMO energy levels are similar to that of conventional spiro-OMeTAD, which is supposed to be favorable for the hole transportation in the device. Up to 12% of light-to-electricity conversion efficiency has been achieved for the mesoporous TiO2/CH3NH3PbI3/apv-EC/Au solar cell without using p-type dopants. Time-resolved photoluminescence (PL) measurement and Electrochemical Impedance Spectra (EIS) further reveal that relatively high hole mobility of the apv-EC and weak recombination occurred at TiO2/CH3NH3PbI3/apv-EC interfaces of the device are crucial to the good performance in comparison with the apv-T. Advantages such as easy synthesis, low cost and relatively good cell performance provide a potential application as the replacement of the expensive spiro-OMeTAD.

CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP (TPA = 4,4′-((1E, 1′E,3E,3′E)-[1,1′-biphenyl]-4,4′-diylbis(buta-1,3-diene-4,1-diyl)); DP = bis(N,N-di-p-tolylaniline); n = 1, 2, 3, 4) as hole-transporting materials (HTMs) have been fabricated. After optimization of the mesoporous TiO2 film thickness, devices based on 2TPA-2-DP with power conversion efficiencies (PCEs) of up to 12.96% have been achieved, comparable to those of devices with (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) (spiro-OMeTAD) as HTM under similar conditions. Further time-resolved photoluminescence (PL) measurements showed a fast charge transfer process at the perovskite/2TPA-2-DP interface. With the aid of electrochemical impedance spectra, a study of the electron blocking ability of 2TPA-2-DP in the device reveals that the presence of 2TPA-2-DP can greatly increase charge transfer resistance at the HTM/Au interface in the device, thus reducing the recombination. Furthermore, the perovskite solar cells based on these four HTMs exhibit good stability after testing for one month.

•Temperature as a key parameter to manipulate the CH3NH3PbI3 film deposition.•Effect of reaction temperatures on morphology and charge transport.•As high as 17.40% of the conversion efficiency by controlling reaction temperatures.Reaction temperature as a key parameter has been introduced to manipulate the film deposition of the CH3NH3PbI3 absorber fabricated by the two-step solution deposition method. It is found that conversion time of dense CH3NH3PbI3 layer can be significantly reduced by raising reaction temperature. CH3NH3PbI3 crystal grain sizes increase with reaction temperature increasing, resulting in rougher surface. CH3NH3PbI3 films deposited at higher temperatures exhibit better charge transport ability, larger built-in heterojunction field and weaker charge recombination, leading to enhanced solar cell performance. By optimizing reaction temperatures, as high as 17.40% and 14.02% of power conversion efficiencies of the mesoscopic and planar perovskite solar cells have been achieved, respectively.

A thin wide band gap organic semiconductor N,N,N′,N′-tetraphenyl-benzidine layer has been introduced by spin-coating to engineer the metal–semiconductor interface in the hole-conductor-free perovskite solar cells. The average cell power conversion efficiency (PCE) has been enhanced from 5.26% to 6.26% after the modification and a highest PCE of 6.71% has been achieved. By the aid of electrochemical impedance spectroscopy and dark current analysis, it is revealed that this modification can increase interfacial resistance of CH3NH3PbI3/Au interface and retard electron recombination process in the metal–semiconductor interface.Keywords: metal−semiconductor (M−S) interface; modification; N,N,N′,N′-tetraphenyl-benzidine (TPB); perovskite solar cell;

Two series of ZnInxS1+1.5x and Ag(y)–ZnInxS1+1.5x+0.5y solid solutions are prepared by hydrothermal methods. The synthetic conditions such as the molar ratio of In/Zn, the pH value, the hydrothermal temperature and the reaction time are found to intensely influence the crystal structure and the morphology of the photocatalyst as well as its photocatalytic activity for H2 generation from water. It is revealed that the ZnIn1.5S3.25 solid solution (In/Zn = 1.5) prepared at 160 °C for 6 h by adding 1 mL hydrochloric acid to the precursor solution shows the highest photocatalytic H2 evolution rate of 1.85 mmol h−1 g−1 in the presence of Ru as the co-catalyst and Na2S/Na2SO3 as sacrificial reagents. Furthermore, after Ag+ doping, the photocatalytic H2 evolution rate remarkably increased to 3.20 mmol h−1 g−1 for the Ag(1.5%)–ZnIn1.5S3.2575 sample. This work provides a new opportunity to develop efficient photocatalysts for photosplitting water into hydrogen.

A new non-traditional organic hole-transporting material (HTM), 4-(4-phenyl-4-α-naphthylbutadienyl)-N,N-bis(4-benzyl)-aniline (PNBA), has been employed in CH3NH3PbI3 perovskite solar cells for the first time. The pore filling of PNBA into mesoporous TiO2/CH3NH3PbI3 scaffold is investigated in detail. As high as 11.4% of light-to-electricity conversion efficiency has been achieved, comparable to corresponding spiro-OMeTAD-based devices under the same conditions. It is revealed that the uniform and thin PNBA film is sufficient as a HTM for perovskite solar cells, and can facilitate hole transport to the metal cathode and also block electron transfer from the perovskite to the metal cathode.

Highly ordered SnO2 inverse opal films with different thicknesses are prepared by our newly developed doctor-blading technique combined with liquid-phase deposition, which have been applied in CdS/CdSe co-sensitized solar cells for the first time. Up to 4.37% of light-to-electricity conversion efficiency with a high open-circuit photovoltage of 700 mV has been achieved under AM 1.5 (100 mW cm–2) illumination. A careful comparison of two SnO2 photoanodes (inverse opal structure and conventional disordered film composed of submicrometer particles) is performed by analysis of optical and photoelectrochemical properties and electrochemical impedance spectra. The comparison reveals that the highly ordered SnO2 inverse opal structure can effectively reduce the charge recombination and increase the open-circuit photovoltage, fill factor, and cell performance.

High quality inverse opal SnO2 films with a large area of more than 10 cm2 were prepared by the doctor-blading technique combined with a new liquid phase deposition. This method is controllable, time saving and suitable for different substrates, and exhibits potential for applications in device fabrication.

The search for semiconductor-sensitized solar cell (SSC) counter electrode alternatives has been a continuous effort and long ongoing work, while the studies in counter electrode kinetic performance and stability are important to improve the overall efficiency. Here, a ternary chalcopyrite compound CuInS2 is first employed as counter electrode (CE) material for CdS/CdSe cosensitized solar cells. Besides, in order to increase the electron transfer activity at the counter electrode/electrolyte interface and stability, an appropriate amount of active carbon/carbon black mixture is introduced to afford CuInS2/carbon composite electrodes. Electron transfer processes in CuInS2-based electrodes are investigated in detail with the aid of electrochemical impedance spectroscopy and I–E measurement. Up to 4.32% of the light-to-electricity conversion efficiency has been achieved for the CdS/CdSe SSCs with the CuInS2/carbon composite electrode. Besides, a preliminary long-term stability test reveals that the new CuInS2/carbon composite counter electrode exhibits good stability after being kept in the dark at room temperature and without current flow for 1000 h.Keywords: carbon; chalcopyrite; composite electrode; counter electrode; semiconductor-sensitized solar cell;

A facile aqueous co-precipitation method is successfully introduced to synthesize ZnS–In2S3 solid solution (labeled In2xZn3(1−x)S3, x = 0–1) and Cu2+ doped In1.4Zn0.9S3 (x = 0.7) solid solution (labeled Cu(y)/In1.4Zn0.9S3, y = 0–4%). Their band gap energies are dependent on the compositional x and y values. The photocatalytic hydrogen generation rate can be remarkably improved to 438 μmol h−1 for Cu(1%)/In1.4Zn0.9S3 solid solution, in comparison with 79 μmol h−1 for un-doped In1.4Zn0.9S3 solid solution. It is suggested that the coupling of the two traditional band engineering strategies of making solid solution and metal ion doping is beneficial to obtain an appropriate conduction band position and effective charge separation of Cu2+ doped In2xZn3(1−x)S3 solid solution, thus improve the photocatalytic activity. This work provides a new opportunity to design novel efficient photocatalysts for water splitting into hydrogen.

A new kind of solvent-free eutectic melt electrolytes for dye-sensitized solar cells (DSCs) has been designed, which are composed of imidazolium salts (i.e. 1-methyl-3-acetylimidazolium iodide (MAII), 1-ethyl-3-methylimidazolium iodide (EMII)) and plastic crystal succinonitrile (SN). Differential scanning calorimetric measurement reveals that the eutectic melt mixture EMII/SN can exhibit stable liquid state in the whole temperature domain from −80 to 80 °C. Besides, the conductivities of the eutectic melt electrolytes are higher than conventional imidazolium-based ionic liquid, 1-propyl-3-methylimidazolium iodide (PMII). Up to 7.46% of light-to-electricity conversion efficiency has been achieved by using the SN-based eutectic melt electrolyte, higher than PMII-based DSCs. Besides, the devices based on eutectic melt electrolytes exhibit good thermal stability. These results show that succinonitrile-based eutectic melts as environmentally friendly electrolytes, possess great potential for large-scale outdoor application of DSCs.

A series of ordered photoanodic architectures (including ordered TiO2 nanotube arrays (TNT), ZnO nanorods, ZnO/TiO2 core/shell nanostructures) for CdS/CdSe sensitized solar cells (QDSCs), were fabricated directly on transparent conductive oxide glasses by a facile sol–gel assisted template process. The morphologies, optical and electrical properties of TNTs and CdS/CdSe co-sensitized TNTs have been demonstrated. The effect of CdSe deposition time on the cell performance was clarified, and the growth mechanism of the CdSe quantum dots on the surface of the TNTs has been proposed as well. Furthermore, the evolution of open-circuit photovoltage (Voc) towards CdSe deposition time has been investigated by electrochemical impedance spectroscopy (EIS). A promising light-to-electricity conversion efficiency of up to 4.61% has been achieved with 3 μm long TNT arrays, which is the best record for sandwich-type ordered TNT-based QDSCs.

A novel freestanding poly (β-hydroxyethyl methacrylate), PHEMA-based organogel electrolyte is developed simply by optimization of the solution polymerization in the same solvent as the organic electrolyte for dye-sensitized solar cells (DSCs). The room temperature ionic conductivity of the gel electrolyte is 4.54 × 10−3 S cm−1, and the conduction behavior can be well described by the free volume model. The quasi-solid-state dye-sensitized solar cell fabricated with this PHEMA-based polymer gel electrolyte can present high energy conversion efficiency up to 7.5%. Preliminary long-term stability test further reveals that this quasi-solid-state electrolyte exhibits good stability after 1000 h thermal test in comparison with the DSCs based on corresponding liquid electrolyte.

A new kind of quasi-solid state electrolytes for dye-sensitized solar cells (DSCs) has been prepared by in situ photopolymerization from the precursor 1,6-hexanediol diacrylate (HDDA) in 1-hexyl-3-methyl imidazolium iodide (HMII). The optimal ratio of polymer/ionic liquid is determined by the conductivities of the electrolytes. In order to further increase the miscibility between ionic liquid and the polymer, oligomer polyethylene glycol dimethyl ether (PEGDME) is introduced. By optimization of the amount of PEGDME in the electrolyte, the DSCs using this kind of solid-state electrolytes can present 6.5% of light-to-electricity conversion efficiency under 41 mW cm−2. In the meantime, the influence of PEGDME additive is detailedly investigated by electrochemical impedance spectrum (EIS) and intensity modulated photovoltage spectroscopy (IMVS) techniques. Preliminary long-term stability test revealed that this in situ photopolymerized electrolyte exhibits good stability after 1000 h thermal test.Highlights► In situ photopolymerized HMII/polymer composite electrolytes for DSCs are obtained. ► Conductivity and cell performance are further increased by adding an oligomer PEGDME. ► 6.4% of efficiency is achieved, and the cells exhibits good thermal stability.

At present, the photovoltaic performance of quantum dot-sensitized solar cells (QDSCs) is still much lower than conventional DSCs. Appropriate porous TiO2 photoanodes for QDSCs need to be further investigated, and optimization of the nanoparticle-based photoanodes is highly desirable as well. In this article, the influence of the structural properties of various TiO2 photoanodes on CdS/CdSe-sensitized solar cells have been systematically studied. Quantitative analyses of light-harvesting efficiency (LHE) and electron-transfer yield (ΦET) for the QDSCs are investigated for the first time. It is revealed that the LHE increases in the long wavelength region with the addition of large size TiO2 particles to the transparent film. In the meantime, the balance between the light scattering and surface area also needs to be controlled, which can significantly restrain the dark current of the device. A double-layer photoanodic structure can give 4.92% of light-to-electricity conversion efficiency with a photoactive area of 0.15 cm2.

Chemically crosslinked polyacrylamide-based hydrogel has been first used as the polymer matrix to prepare quasi-solid-state polysulfide electrolyte for CdS/CdSe co-sensitized solar cells (QDSCs). The room temperature ionic conductivity of the gel electrolyte reaches 0.093 S·cm−1. QDSCs based on this quasi-solid-state electrolyte can present up to 4.0% of light-to-electricity conversion efficiency. Meanwhile, the interfacial recombination at TiO2/electrolyte interface of the cell is also investigated by Electrochemical Impedance Spectroscopy (EIS).

At present, the photovoltaic performance of quantum dot-sensitized solar cells (QDSCs) is still much lower than conventional DSCs. Appropriate porous TiO2 photoanodes for QDSCs need to be further investigated, and optimization of the nanoparticle-based photoanodes is highly desirable as well. In this article, the influence of the structural properties of various TiO2 photoanodes on CdS/CdSe-sensitized solar cells have been systematically studied. Quantitative analyses of light-harvesting efficiency (LHE) and electron-transfer yield (ΦET) for the QDSCs are investigated for the first time. It is revealed that the LHE increases in the long wavelength region with the addition of large size TiO2 particles to the transparent film. In the meantime, the balance between the light scattering and surface area also needs to be controlled, which can significantly restrain the dark current of the device. A double-layer photoanodic structure can give 4.92% of light-to-electricity conversion efficiency with a photoactive area of 0.15 cm2.

A series of ordered photoanodic architectures (including ordered TiO2 nanotube arrays (TNT), ZnO nanorods, ZnO/TiO2 core/shell nanostructures) for CdS/CdSe sensitized solar cells (QDSCs), were fabricated directly on transparent conductive oxide glasses by a facile sol–gel assisted template process. The morphologies, optical and electrical properties of TNTs and CdS/CdSe co-sensitized TNTs have been demonstrated. The effect of CdSe deposition time on the cell performance was clarified, and the growth mechanism of the CdSe quantum dots on the surface of the TNTs has been proposed as well. Furthermore, the evolution of open-circuit photovoltage (Voc) towards CdSe deposition time has been investigated by electrochemical impedance spectroscopy (EIS). A promising light-to-electricity conversion efficiency of up to 4.61% has been achieved with 3 μm long TNT arrays, which is the best record for sandwich-type ordered TNT-based QDSCs.

High quality inverse opal SnO2 films with a large area of more than 10 cm2 were prepared by the doctor-blading technique combined with a new liquid phase deposition. This method is controllable, time saving and suitable for different substrates, and exhibits potential for applications in device fabrication.

A facile aqueous co-precipitation method is successfully introduced to synthesize ZnS–In2S3 solid solution (labeled In2xZn3(1−x)S3, x = 0–1) and Cu2+ doped In1.4Zn0.9S3 (x = 0.7) solid solution (labeled Cu(y)/In1.4Zn0.9S3, y = 0–4%). Their band gap energies are dependent on the compositional x and y values. The photocatalytic hydrogen generation rate can be remarkably improved to 438 μmol h−1 for Cu(1%)/In1.4Zn0.9S3 solid solution, in comparison with 79 μmol h−1 for un-doped In1.4Zn0.9S3 solid solution. It is suggested that the coupling of the two traditional band engineering strategies of making solid solution and metal ion doping is beneficial to obtain an appropriate conduction band position and effective charge separation of Cu2+ doped In2xZn3(1−x)S3 solid solution, thus improve the photocatalytic activity. This work provides a new opportunity to design novel efficient photocatalysts for water splitting into hydrogen.

Two series of ZnInxS1+1.5x and Ag(y)–ZnInxS1+1.5x+0.5y solid solutions are prepared by hydrothermal methods. The synthetic conditions such as the molar ratio of In/Zn, the pH value, the hydrothermal temperature and the reaction time are found to intensely influence the crystal structure and the morphology of the photocatalyst as well as its photocatalytic activity for H2 generation from water. It is revealed that the ZnIn1.5S3.25 solid solution (In/Zn = 1.5) prepared at 160 °C for 6 h by adding 1 mL hydrochloric acid to the precursor solution shows the highest photocatalytic H2 evolution rate of 1.85 mmol h−1 g−1 in the presence of Ru as the co-catalyst and Na2S/Na2SO3 as sacrificial reagents. Furthermore, after Ag+ doping, the photocatalytic H2 evolution rate remarkably increased to 3.20 mmol h−1 g−1 for the Ag(1.5%)–ZnIn1.5S3.2575 sample. This work provides a new opportunity to develop efficient photocatalysts for photosplitting water into hydrogen.

An n-type semiconducting copolymer of perylene diimide and dithienothiophene (PPDIDTT) is used as a dual function interfacial layer to modify the surface of perovskite films in inverted perovskite solar cells. The PPDIDTT layer can remarkably passivate the surface trap states of perovskite through the formation of a Lewis adduct between the under-coordinated Pb in perovskite and S in the dithienothiophene unit of PPDIDTT, and also shows efficient charge extraction and transfer properties. The PPDIDTT modified devices exhibit a maximum power conversion efficiency of 16.5%, superior to that of the control devices without PPDIDTT (15.3%). In addition, the device stability and hysteresis in J–V curves of the modified devices are also improved compared to those of the control devices.