Synthesis and properties of two novel copolymers based on squaraine and fluorene units for solar cell materials

Two novel copolymers based on squaraine and fluorine units have been synthesized through palladium catalyzed Suzuki coupling reaction and Sonogashira coupling reaction,respectively.The structures and properties of the two copolymers were characterized by FT-IR.NMR,UV-vis absorbance（Abs）,gel permeation chromatography（GPC）,thermal gravimetric analysis （TGA）,differential scanning calorimetry（DSC） and cyclic voltammetry（CV）.The solution absorption spectrums of P_1 and P_2 show two distinct absorption bands,one locates at 300-500 nm and the other at 600-800 nm.The absorption spectrums of P_1 and P_2 in films are broadened obviously and the spectral responses are extended up to 900 nm.Thermal gravimetric analysis demonstrates that the polymers are stable.Cyclic voltammetry experiment shows that the band gaps of the copolymers are 1.65 eV and 1.67 eV. respectively,suggesting their potential for applications as solar cells materials.

Progress in hole-transporting materials for perovskite solar cells

In recent years the photovoltaic community has witnessed the unprecedented development of perovskite solar cells（PSCs） as they have taken the lead in emergent photovoltaic technologies. The power conversion efficiency of this new class of solar cells has been increased to a point where they are beginning to compete with more established technologies. Although PSCs have evolved a variety of structures, the use of hole-transporting materials（HTMs） remains indispensable. Here, an overview of the various types of available HTMs is presented. This includes organic and inorganic HTMs and is presented alongside recent progress in associated aspects of PSCs, including device architectures and fabrication techniques to produce high-quality perovskite films. The structure, electrochemistry, and physical properties of a variety of HTMs are discussed, highlighting considerations for those designing new HTMs. Finally, an outlook is presented to provide more concrete direction for the development and optimization of HTMs for highefficiency PSCs.

Truxene-based Hole-transporting Materials for Perovskite Solar Cells

Three star-shaped truxene-based small molecules（namely TXH,TXM,TXO） were synthesized,characterized and used as hole-transporting materials（HTMs） for perovskite solar cells（Pv SCs）. The device based on TXO delivered a respectable power conversion efficiency（PCE） of 7.89% and a high open-circuit voltage（Voc） of 0.97 V,which far exceeded the values of the devices based on other two small molecules. The highest PCE for the device based on TXO is mainly contributed from its lowest series resistance（Rs） value and largest short-circuit current（Jsc） value under the same circumstances. All these results indicate that TXO is a promising HTM candidate for Pv SCs.

A novel phenoxazine-based hole transport material for efficient perovskite solar cell

Based on the previous research work in our laboratory, we have designed and synthesized a small-molecule, hole transport material （HTM） POZ6-2 using phenoxazine （POZ） as central unit and dicyanovinyl units as electron-withdrawing terminal groups. Through the introduction ofa 2-ethyl-hexyl bulky chain into the POZ core unit, POZ6-2 exhibits good solubility in organic solvents. In addition, POZ6-2 possesses appropriate energy levels in combination with a high hole mobility and conductivity in its pristine form. Therefore, it can readily be used as a dopant-flee HTM in perovskite solar cells （PSCs） and a conversion efficiency of 10.3% was obtained. The conductivity of the POZ6-2 layer can be markedly enhanced via doping in combination with typical additives, such as 4-tert-butylpyridine （TBP） and lithium bis（trifluoromethanesulfonyl） imide （LiTFS1）. Correspondingly, the efficiency of the PSCs was further improved to 12.3% using doping strategies. Under the same conditions, reference devices based on the well-known HTM Spiro-OMeTAD show an efficiency of 12.8%.

Titanylphthalocyanine as hole transporting material for perovskite solar cells

Titanylphthalocyanine （TiOPc） as hole transporting material （HTM） was successfully synthesized by a simple process with low cost. Perovskite solar cells using the TiOPc as HTM were fabricated and characterized. TiOPc as HTM plays an important role in increasing the power conversion efficiency （PCE） by minimizing recombi- nation losses at the perovskite/Au interface because TiOPc as HTM can extract photogenerated holes from the perovskite and then transport quickly these charges to the back metal electrode. In the research, the β-TiOPc gives a higher PCE than α-TiOPc for the devices due to sufficient transfer dynamics, The β-TiOPc was applied in perovskite solar cells without clopping to afford an impressive PCE of 5.05% under AM 1.5G illumination at the thickness of 40 nm which is competitive with spiro-OMeTAD at the same condition. The present work suggests a guideline for optimizing the photovoltaic properties ofperovskite solar cells using the TiOPc as the HTM.

Solution‑Processed Thin Film Transparent Photovoltaics:Present Challenges and Future Development

Electrical energy is essential for modern society to sustain economic growths.The soaring demand for the electrical energy,together with an awareness of the environmental impact of fossil fuels,has been driving a shift towards the utilization of solar energy.However,traditional solar energy solutions often require extensive spaces for a panel installation,limiting their practicality in a dense urban environment.To overcome the spatial constraint,researchers have developed transparent photovoltaics(TPV),enabling windows and facades in vehicles and buildings to generate electric energy.Current TPV advancements are focused on improving both transparency and power output to rival commercially available silicon solar panels.In this review,we first briefly introduce wavelength-and non-wavelengthselective strategies to achieve transparency.Figures of merit and theoretical limits of TPVs are discussed to comprehensively understand the status of current TPV technology.Then we highlight recent progress in different types of TPVs,with a particular focus on solution-processed thin-film photovoltaics(PVs),including colloidal quantum dot PVs,metal halide perovskite PVs and organic PVs.The applications of TPVs are also reviewed,with emphasis on agrivoltaics,smart windows and facades.Finally,current challenges and future opportunities in TPV research are pointed out.

Recent theoretical progress in the development of perovskite photovoltaic materials

Since the seminal work by Kojima et al. in 2009, solar cells based on hybrid organic-inorganic perovskites have attracted considerable attention and experienced an exponential growth, with photovoltaic efficiencies as of today reaching above 22%. Despite such an impressive development, some key scientific issues of these materials, including the presence of toxic lead, the poor long-term device stability under heat and humidity conditions, and the anomalous hysteresis of the current-voltage curves shown by various solar cell devices, still remain unsolved and constitute an important focus of experimental and theoretical researchers throughout the world. Density functional theory calculations have been successfully applied to exploring structural and electronic properties of semiconductors, complementing the experimental results in search and discovery of novel functional materials. In this review, we summarize the current progress in perovskite photovoltaic materials from a theoretical perspective. We discuss design of lead-free perovskite materials, humidity-induced degradation mechanisms and possible origins for the observed solar cell hysteresis, and assess future research directions for advanced perovskite solar cells based on computational materials design and theoretical understanding of intrinsic properties.

High efficiency ETM-free perovskite cell composed of CuSCN and increasing gradient CH_(3)NH_(3)PbI_(3)

To enhance device performance and reduce fabrication cost,a series of electron transporting material(ETM)-free perovskite solar cells(PSCs)is developed by TCAD Atlas.The accuracy of the physical mode of PSCs is verified,due to the simulations of PEDOT:PSS-CH_(3)NH_(3)PbI_(3)-PCBM and CuSCN-CH_(3)NH_(3)PbI_(3)-PCBM p-i-n PSCs showing a good agreement with experimental results.Different hole transporting materials(HTMs)are selected and directly combined with n-CH_(3)NH_(3)PbI_(3),and the CuSCN-CH_(3)NH_(3)PbI_(3) is the best in these ETM-free PSCs.To further study the CuSCN-CH_(3)NH_(3)PbI_(3) PSC,the influences of back electrode material,gradient band gap,thickness,doping concentration,and bulk defect density on the performance are investigated.Energy band and distribution of electric field are utilized to optimize the design.As a result,the efficiency of CuSCN-CH_(3)NH_(3)PbI_(3) PSC is achieved to be 26.64%.This study provides the guideline for designing and improving the performances of ETM-free PSCs.

Photoelectrochemical properties of MWCNT-TiO_2 hybrid materials as a counter electrode for dye-sensitized solar cells

The MWCNT-Ti02 hybrid materials were prepared by a simply mixing method and used as a counter electrode （CE） for dye-sensitized solar cells. Compared with the platinum CE, MWCNT-TiO2 CE has the similar redox voltage and current response in the cyclic voltammetry. The electrochemical catalytic activity was characterized by the electrochemical impedance spectroscopy and Tafel curve, including the equivalent circuit, the exchange current density, the limiting diffusion current density, and the diffusion coefficient of triiodide/iodide redox species. The results indicate that the reduction process from triiodide to iodide is determined by the kinetic-controlled and diffusion-limited processes. The device performance is optimal based on the MWCNT-TiO2 （mass ratio of 2：1） CE, such as the open-circuit voltage of 0.72 V, the short-circuit photocurrent density of 15.71 mA/cm2, the fill factor of 0.68, and the photon-to-electron conversion efficiency of 7.69%.

Organic thin-film solar cells:Devices and materials

In recent years, the performance of organic thinfilm solar cells has gained rapid progress, of which the power conversion efficiencies （r/p） of 3%-5% are commonly achieved, which were difficult to obtain years ago and are improving steadily now. The r/p of 7.4% was achieved in the year 2010, and r/p of 9.2% was disclosed and confirmed at website of Mitsubishi Chemical in April, 2011. The promising future is that the r/p of 10% is achievable according to simulation results. Apparently, these are attributed to material innovations, new device structures, and also the better understanding of device physics. This article summarizes recent progress in organic thinfilm solar cells related to materials, device structures and working principles. In the device functioning part, after each brief summary of the working principle, the methods for improvements, such as absorption increment, organic/electrode interface engineering, morphological issues, are addressed and summarized accordingly. In addition, for the purpose of increasing exciton diffusion efficiency, the benefit from triplet exciton, which has been proposed in recent years, is highlighted. In the active material parts, the chemical nature of materials and its impact on device performance are discussed. Particularly, emphasis is given toward the insight for better understanding device physics as well as improvements in device performance either by development of new materials or by new device architecture.

Rapid development in two-dimensional layered perovskite materials and their application in solar cells

Two-dimensional （2D） layered organic-inorganic hybrid perovskite （2D PVK） materials have beenrecently developed as a novel candidate for photovoltaic application with high stability and a maximumpower conversion efficiency of 12.5%. This article summarized these newly emerging 2D PVK materialsand their uses in solar cells. The structural, physical, and chemical properties as well as the classificationof 2D PVK materials are discussed. The photovoltaic performance parameters of various 2D perovsldtesolar cells （2D PSCs） are summarized and their device stability is compared with conventional 3Dperovskite solar cells （3D PSCs）. It has been concluded that 2D PVKs show greater stability upon humidity,heat stress, and light intensity as compared to 3D analogues and act as a class of promising materials forapplication in solar cells.

Recent advances in the design of dopant-free hole transporting materials for highly efficient perovskite solar cells

Continuous success has been achieved for solution-processed inorganic-organic hybrid perovskite solar cells（PVSCs） in the past several years, in which organic charge transporting materials play an important role. At present, most of the commonly used hole-transporting materials（HTMs） such as spiro-OMeTAD derivatives for PVSCs require additional chemical doping process to ensure sufficient conductivity and shift the Fermi level towards the HOMO level for efficient hole transport and collection. However, this doping process not only increases the complexity and cost of device fabrication, but also decreases the device stability. Thus development of efficient dopant-free HTMs for PVSCs is highly desirable and remains as a major challenge in this field. In this review, we will summarize the recent advances in the molecular design of dopant-free HTMs for PVSCs.

Development of electron and hole selective contact materials for perovskite solar cells

Nowadays,both n-i-p and p-i-n perovskite solar cells（PSCs） device structures are reported to give high performance with photo conversion efficiencies（PCEs） above 20%.The efficiency of the PSCs is fundementally determined by the charge selective contact materials.Hence,by introducing proper contact materials with good charge selectivity,one could potentially reduce interfacial charge recombination as well as increase device performance.In the past few years,copious charge selective contact materials have been proposed.Significant improvements in the corresponding devices were observed and the reported PCEs were close to that of classic Spiro-OMeTAD.This mini-review summarizes the state-of-the-art progress of typical electron/hole selective contact materials for efficient perovskite solar cells and an outlook to their development is made.

D-A structural protean small molecule donor materials for solution-processed organic solar cells

Under the synergistic effect of molecular design and devices engineering, small molecular organic solar cells have presented an unstoppable tendency for rapid development with putting forward donor- acceptor （D-A） structures. Up to now, the highest power conversion efficiency of small molecules has exceeded 11%, comparable to that of polymers. In this review, we summarize the high performance small molecule donors in various classes of typical donor-acceptor （D-A） structures and discuss their relationships briefly.

Organic functional materials based buffer layers for efficient perovskite solar cells

In this review, we highlight the recent development of organic π-functional materials as buffer layers in constructing efficient perovskite solar cells（PVSCs）. By following a brief introduction on the PVSC development, device architecture and material design features, we exemplified the exciting progresses made in field by exploiting organic π-functional materials based hole and electron transport layers（HTLs and ETLs） to enable high-performance PVSCs.

Recent progress on advanced transmission electron microscopy characterization for halide perovskite semiconductors

Halide perovskites are strategically important in the field of energy materials. Along with the rapid development of the materials and related devices, there is an urgent need to understand the structure–property relationship from nanoscale to atomic scale. Much effort has been made in the past few years to overcome the difficulty of imaging limited by electron dose,and to further extend the investigation towards operando conditions. This review is dedicated to recent studies of advanced transmission electron microscopy(TEM) characterizations for halide perovskites. The irradiation damage caused by the interaction of electron beams and perovskites under conventional imaging conditions are first summarized and discussed. Low-dose TEM is then discussed, including electron diffraction and emerging techniques for high-resolution TEM(HRTEM) imaging. Atomic-resolution imaging, defects identification and chemical mapping on halide perovskites are reviewed. Cryo-TEM for halide perovskites is discussed, since it can readily suppress irradiation damage and has been rapidly developed in the past few years. Finally, the applications of in-situ TEM in the degradation study of perovskites under environmental conditions such as heating,biasing, light illumination and humidity are reviewed. More applications of emerging TEM characterizations are foreseen in the coming future, unveiling the structural origin of halide perovskite’s unique properties and degradation mechanism under operando conditions, so to assist the design of a more efficient and robust energy material.

Synthesis and properties of novel low band-gap polymers bearing squaraine units

Novel main-chain-conjugated poly(carbazol-alt-squaraine) and poly(dipyridyl-alt-squaraine) were successfully synthesized through direct polycondensation of 9-(2-ethylhexyl)carbazole-bridged or dipyridyl-bridged bispyrrole and squaric acid.The structures and properties of the polymers were characterized using ~1H NMR,FT-IR,UV-vis and cyclic voltammetry.Both polymers exhibit excellent solubility in common organic solvents and good thermal stability.Their UV-vis absorption spectra indicated the polymers have b...

Improved performance and air stability of perovskite solar cells based on low-cost organic hole-transporting material X60 by incorporating its dicationic salt

The development of an efficient, stable, and low-cost hole-transporting material (HTM) is of great significance for perovskite solar cells (PSCs) from future commercialization point of view. Herein, we specifically synthesize a dicationic salt of X60 termed X60(TFSI)2, and adopt it as an effective and stable "doping" agent to replace the previously used lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) for the low-cost organic HTM X60 in PSCs. The incorporation of this dicationic salt significantly increases the hole conductivity of X60 by two orders of magnitude from 10-6 to 10-4 S cm-1. The dramatic enhancement of the conductivity leads to an impressive power conversion efficiency (PCE) of 19.0% measured at 1 sun illumination (100 mW cm-2, AM 1.5 G), which is comparable to that of the device doped with LiTFSI (19.3%) under an identical condition. More strikingly, by replacing LiTFSI, the PSC devices incorporating X60(TFSI)2 also show an excellent long-term durability under ambient atmosphere for 30 days, mainly due to the hydrophobic nature of the X60(TFSI)2 doped HTM layer,which can effectively prevent the moisture destroying the perovskite layer. The present work paves the way for the development of highly efficient, stable, and low-cost HTM for potential commercialization of PSCs.

SYNTHESIS AND CHARACTERIZATION OF COPOLYMERS BASED ON BENZOTHIADIZOLE-THIOPHENE-PHENYLENEVINYLENE

Two novel copolymers based on benzothiadizole-thiophene-phenylenevinylene have been synthesized through palladium catalyzed triple-bond polycondensation method.The copolymers exhibit good solubility in common organic solvents such as CHCl;,CH;Cl;and THF.The structures and properties of the two copolymers are characterized by FT-IR,;H-NMR,UV-Vis absorbance（Abs）,gel permeation chromatography（GPC）,thermal gravimetric analysis and cyclic voltammetry（CV）.The copolymers of P;and P;show absorption spectra with maximum peak at 532 nm and 573 nm in solution,respectively.Compare to their monomers M;and M;,the absorption peaks of P;and P;were red-shifted by 34 nm and 54 nm respectively.Thermal gravimetric analysis demonstrated that the polymers were stable and little weight loss was observed below 300℃.Cyclic voltammetry experiments showed that the band gaps of the copolymers were 1.81 eV and 1.62 eV,respectively,suggesting their potential for applications as organic solar cell materials.