Jianxi Yao

Numerical simulation: Toward the design of high-efficiency planar perovskite solar cells

Organo-metal halide perovskite solar cells based on planar architecture have been reported to achieve remarkably high power conversion efficiency (PCE, >16%), rendering them highly competitive to the conventional silicon based solar cells. A thorough understanding of the role of each component in solar cells and their effects as a whole is still required for further improvement in PCE. In this work, the planar heterojunction-based perovskite solar cells were simulated with the program AMPS (analysis of microelectronic and photonic structures)-1D. Simulation results revealed a great dependence of PCE on the thickness and defect density of the perovskite layer. Meanwhile, parameters including the work function of the back contact as well as the hole mobility and acceptor density in hole transport materials were identified to significantly influence the performance of the device. Strikingly, an efficiency over 20% was obtained under the moderate simulation conditions.

Mesoporous SnO2 nanoparticle films as electron-transporting material in perovskite solar cells

Perovskite solar cells with mesoporous metal oxide films as scaffold layers have demonstrated very impressive advances in performance recently. Here, we present an investigation into mesoporous perovskite solar cells incorporating mesoporous SnO2 nanoparticle films as electron-transporting materials and scaffold layers, to replace traditional mesoporous TiO2 films. We have optimized the SnO2 film thickness and treated the surface of the SnO2 film with an aqueous solution of TiCl4. Due to the TiCl4 treatment the recombination process was significantly retarded. The short-circuit current density (Jsc) and open-circuit voltage (Voc) reached nearly 18 mA cm−2 and 1 V, respectively. Consequently, the power conversion efficiency of the device with the SnO2 film exceeded 10%.

Low-temperature, solution-deposited metal chalcogenide films as highly efficient counter electrodes for sensitized solar cells

Transition metal chalcogenide crystalline films FeSe2, Cu1.8S, and CuSe have been deposited from solution by drop casting their dissolved inks onto a conductive substrate, followed by a mild thermal treatment. We demonstrate that the resulting chalcogenide films exhibit an excellent catalytic activity and function as highly efficient counter electrodes (CEs) for dye- and quantum dot-sensitized solar cells (DSCs and QDSCs). In particular, the FeSe2 and CuSe films produced herein with novel morphologies show better catalytic activity than that of the conventional Pt coated CE used in DSCs and Cu2S in QDSCs, respectively. Ensuing devices present an improved photovoltaic performance with maximum values of 9.10% for DSCs and 4.94% for QDSCs, comparable to those based on Pt and Cu2S CEs. The efficient CE materials developed here from such a facile and scalable route offer strong potential for a broader solar cell application that requires low-cost and large-scale production.

High-efficiency and stable quasi-solid-state dye-sensitized solar cell based on low molecular mass organogelator electrolyte

The highest photoelectric conversion efficiency (9.61%) for a quasi-solid-state DSSC (QS-DSSC) based on a low molecular mass organogelator (LMOG) is achieved using N,N′-1,5-pentanediylbis-dodecanamide as a LMOG in conjunction with a TiO2 photoanode of sub-microspheres sensitized with a high-absorptivity Ru complex (C101). The competition of interfacial kinetic processes between the recombination of the dye cations with photoinjected electrons and the regeneration of the dye cations by I− is investigated by transient adsorption measurements. The data revealed that there is an efficient interfacial charge separation at the TiO2 photoelectrode/electrolyte interface and the dye is rapidly regenerated, which contributes to the high photocurrent. Furthermore, using electrochemical impedance spectroscopy (EIS) and controlled intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), it is determined that the negative shift of TiO2 conduction band edge of the QS-DSSC contributes to the high Voc. Moreover, the QS-DSSC exhibits significantly improved stability during the accelerated thermal and light-soaking test. During the accelerated aging test, there is almost no change in the short-circuit current density (Jsc) in the QS-DSSC, while the Jsc of the liquid electrolyte based DSSC decreases sharply. These results are very important for the application and commercialization of DSSCs.

Tetraphenylmethane-Arylamine Hole-Transporting Materials for Perovskite Solar Cells

A new class of hole-transporting materials (HTM) containing tetraphenylmethane (TPM) core have been developed. After thermal, charge carrier mobility, and contact angle tests, it was found that TPA-TPM (TPA: arylamine derivates side group) showed higher glass-transition temperature and larger water-contact angle than spiro-OMeTAD with comparable hole mobility. Photoluminescence and impedance spectroscopy studies indicate that TPA-TPMs hole-extraction ability is comparable to that of spiro-OMeTAD. SEM and AFM results suggest that TPA-TPM has a smooth surface. When TPA-TPM is used in mesoscopic perovskite solar cells, power conversion efficiency comparable to that of spiro-OMeTAD is achieved. Notably, the perovskite solar cells employing TPA-TPM show better long-term stability than that of spiro-OMeTAD. Moreover, TPA-TPM can be prepared from relatively inexpensive raw materials with a facile synthetic route. The results demonstrate that TPM-arylamines are a new class of HTMs for efficient and stable perovskite solar cells.

Diketopyrrolopyrrole or benzodithiophene-arylamine small-molecule hole transporting materials for stable perovskite solar cells

Two simple small-molecular arylamine derivatives 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (OMeTPA-DPP) and 4,4′-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (OMeTPA-BDT) linked with diketopyrrolopyrrole or benzodithiophene moieties have been synthesized. The new compounds show better thermal stability than spiro-OMeTAD. The steady-state and time-resolved photoluminescence demonstrate that the new compounds have good hole extraction ability. The perovskite solar cells employing OMeTPA-BDT show a comparable power conversion efficiency with that of spiro-OMeTAD. After more than 200 hours of aging under one sun illumination, the residual efficiencies of the PSCs based on OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 8.69%, 11.15% and 9.08%, respectively. The results demonstrate that the newly-developed compounds can act as efficient hole transporting materials for stable perovskite solar cells.

TiO2 SUB-MICROSPHERES AS A BI-FUNCTIONAL SCATTERING LAYER FOR HIGH-PERFORMANCE DYE-SENSITIZED SOLAR CELLS

The sub-microspheres play multiple roles in enhancing dye adsorption and light-scattering to improve the performance of dye-sensitized solar cells (DSSCs). In this work, the well-defined TiO2 sub-microspheres with anatase granular-like nanocrystals are prepared in high yield by combining hydrolytic process with solvothermal treatment. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results indicated that plenty of rhombic nanoparticles with ~ 18 nm diameter having mutual contacts to neighboring nanoparticles were densely self-assembled into sub-microspheres, and abundant mesopores existed in the whole sub-microspheres with superior light scattering ability. The appropriate pore diameter and relatively high specific surface area of the as-obtained sub-microsphere result in a higher dye adsorption. As expected, by using the sub-microspheres as a scattering layer, a higher photovoltaic conversion efficiency of 10.15% is obtained for DSSCs.

Continuous electron transport pathways constructed in TiO2 sub-microsphere films for high-performance dye-sensitized solar cells

TiO2 sub-microspheres applied as photoanodes in dye-sensitized solar cells (DSSCs) have been reported to have dual functions with high specific surface areas and light scattering capabilities. Large voids are left in the sub-microsphere film and lead to the poor connectivity between the neighboring sub-microspheres, which will limit the enhancement of the short-circuit current density (Jsc) and the power conversion efficiency (η). Herein, we develop a closely linked network by introducing TiO2 nanocrystallines into the voids to construct a continuous electron transport pathway. It is found that pore sizes, porosity, and specific surface area could be effectively tuned by simply adjusting the content of TiO2 nanocrystallines. As confirmed by the intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) and electrochemical impedance spectra (EIS), when the content of TiO2 nanocrystallines in the sub-microspheres is 10 wt% (NP10), the electron transport time, electron collection efficiency, and electron diffusion length are optimized compared with the other contents. As a result, the η of the optimized NP10 photoanode based DSSC is up to 11.22%, which is higher than the 10.58% efficiency demonstrated by the pure sub-microsphere photoanode based cell.

BiVO4 semiconductor sensitized solar cells

Semiconductor sensitized solar cells (SSSCs) are promising candidates for the third generation of cost-effective photovoltaic solar cells and it is important to develop a group of robust, environment friendly and visible-light-responsive semiconductor sensitizers. In this paper, we first synthesized bismuth vanadate (BiVO4) quantum dots by employing facile successive ionic layer adsorption and reaction (SILAR) deposition technique, which we then used as a sensitizer for solar energy conversion. The preliminary optimised oxide SSSC showed an efficiency of 0.36%, nearly 2 orders of magnitude enhancement compared with bare TiO2, due to the narrow bandgap absorption of BiVO4 quantum dots and intimate contact with the oxide substrate. This result not only demonstrates a simple method to prepare BiVO4 quantum dots based solar cell, but also provides important insights into the low bandgap oxide SSSCs.

Preparation and characterisation of TiO2 nanorod and nanotube films as photoanodes for dye-sensitised solar cells

TiO2 nanorod arrays are successfully synthesised on the transparent conductive fluorine-doped tin oxide glass through a facile one-step solvothermal route without any surfactant and template, TiO2 nanotube arrays were obtained through acid etching afterwards. Dye-sensitised solar cells based on these two different one-dimensional TiO2 photoelectrodes show the power conversion efficiency of 1.68 and 3.89% for nanorod and nanotube cells, respectively. The nanotube cell has better photoelectric performance than nanorod cell because of lower recombination, faster electron transformation, higher specific surface area for adsorbing more dye molecules and superior light scattering capacity for boosting the light-harvesting efficiency.