Hongkun Cai

Air-processed, efficient CH3NH3PbI3−xClx perovskite solar cells with organic polymer PTB7 as a hole-transport layer

Low-temperature, solution-processed perovskite solar cells (PSCs), which utilized organic poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) as a hole-transport layer (HTL), achieved a power conversion efficiency (PCE) as high as 13.29% when fabricated in ambient air. Through a comparative study, we demonstrate this PCE value to be superior compared to its counterparts with spiro-OMeTAD or P3HT as the HTL; the superiority consists of a higher fill factor (FF) and open-circuit voltage (Voc). By probing the absorption spectrum of CH3NH3PbI3−xClx before and after spin-coating the PTB7, it is discovered that the spin coating of PTB7 has little influence on the quality of the perovskite films. Furthermore, it is shown that PTB7 possesses higher conductivity compared with conventional HTLs, including spiro-OMeTAD, P3HT and PCDTBT. Moreover, in order to further improve device performance, the prevalent additives lithium bis (trifluoromethylsulphony) imide (LiTFSI) and 4-tert-butylpyridine (t-Bp) are investigated, along with a post-annealing process that is applied to the whole device. The results presented here and the overall fabrication method represent a helpful new approach for fabricating highly efficient perovskite-based photovoltaic devices.

UV-Sintered Low-Temperature Solution-Processed SnO2 as Robust Electron Transport Layer for Efficient Planar Heterojunction Perovskite Solar Cells

Recently, low temperature solution-processed tin oxide (SnO2) as a versatile electron transport layer (ETL) for efficient and robust planar heterojunction (PH) perovskite solar cells (PSCs) has attracted particular attention due to its outstanding properties such as high optical transparency, high electron mobility, and suitable band alignment. However, for most of the reported works, an annealing temperature of 180 °C is generally required. This temperature is reluctantly considered to be a low temperature, especially with respect to the flexible application where 180 °C is still too high for the polyethylene terephthalate flexible substrate to bear. In this contribution, low temperature (about 70 °C) UV/ozone treatment was applied to in situ synthesis of SnO2 films deposited on the fluorine-doped tin oxide substrate as ETL. This method is a facile photochemical treatment which is simple to operate and can easily eliminate the organic components. Accordingly, PH PSCs with UV-sintered SnO2 films as ETL were successfully fabricated for the first time. The device exhibited excellent photovoltaic performance as high as 16.21%, which is even higher than the value (11.49%) reported for a counterpart device with solution-processed and high temperature annealed SnO2 films as ETL. These low temperature solution-processed and UV-sintered SnO2 films are suitable for the low-cost, large yield solution process on a flexible substrate for optoelectronic devices.

Toward Revealing the Critical Role of Perovskite Coverage in Highly Efficient Electron-Transport Layer-Free Perovskite Solar Cells: An Energy Band and Equivalent Circuit Model Perspective

Currently, most efficient perovskite solar cells (PVKSCs) with a p-i-n structure require simultaneously electron transport layers (ETLs) and hole transport layers (HTLs) to help collecting photogenerated electrons and holes for obtaining high performance. ETL free planar PVKSC is a relatively new and simple structured solar cell that gets rid of the complex and high temperature required ETL (such as compact and mesoporous TiO2). Here, we demonstrate the critical role of high coverage of perovskite in efficient ETL free PVKSCs from an energy band and equivalent circuit model perspective. From an electrical point of view, we confirmed that the low coverage of perovskite does cause localized short circuit of the device. With coverage optimization, a planar p-i-n(++) device with a power conversion efficiency of over 11% was achieved, implying that the ETL layer may not be necessary for an efficient device as long as the perovskite coverage is approaching 100%.

Organic–inorganic hybrid CH3NH3PbI3 perovskite materials as channels in thin-film field-effect transistors

Organic–inorganic hybrid perovskite materials promise both the superior carrier mobility of inorganic semiconductors and the processability of organic materials, which make organic–inorganic hybrid perovskite materials good substitutes in all the applications put forth for organic materials and extend their application to higher speed devices than is presently possible with either a-Si or organic semiconductors. Recent reports have shown high carrier mobility and long electron–hole diffusion lengths of organic–inorganic hybrid perovskite materials. We have demonstrated a thin-film field-effect transistor with an organic–inorganic hybrid CH3NH3PbI3 material as the semiconducting channel based on these advantages via a low-temperature vapor-assisted solution process. The obvious electrical field effect is obtained in organic–inorganic hybrid CH3NH3PbI3 perovskite TFTs with a field-effect mobility of 396.2 cm2 V−1 s−1, current modulation greater than 104, sub-threshold current of 0.4035 V per decade and threshold voltage of −3.501 V.

Novel Combination of Efficient Perovskite Solar Cells with Low Temperature Processed Compact TiO2 Layer via Anodic Oxidation

In this work, a facile and low temperature processed anodic oxidation approach is proposed for fabricating compact and homogeneous titanium dioxide film (AO-TiO2). In order to realize morphology and thickness control of AO-TiO2, the theory concerning anodic oxidation (AO) is unveiled and the influence of relevant parameters during the process of AO such as electrolyte ingredient and oxidation voltage on AO-TiO2 formation is observed as well. Meanwhile, we demonstrate that the planar perovskite solar cells (p-PSCs) fabricated in ambient air and utilizing optimized AO-TiO2 as electron transport layer (ETL) can deliver repeatable power conversion efficiency (PCE) over 13%, which possess superior open-circuit voltage (Voc) and higher fill factor (FF) compared to its counterpart utilizing conventional high temperature processed compact TiO2 (c-TiO2) as ETL. Through a further comparative study, it is indicated that the improvement of device performance should be attributed to more effective electron collection from perovskite layer to AO-TiO2 and the decrease of device series resistance. Furthermore, hysteresis effect about current density-voltage (J-V) curves in TiO2-based p-PSCs is also unveiled.

Improving Performance of Organic-Silicon Heterojunction Solar Cells Based on Textured Surface via Acid Processing.

UNLABELLED Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT PSS) is widely applied in organic-photoelectronic devices due to its excellent transparency and conductivity. However, when it is used in the organic-silicon heterojunction solar cells with traditional pyramid texturing surface, the device performance is limited by the contact between the PEDOT PSS and silicon wafer at the bottom of the pyramids. We optimized the structure of the bottom of the pyramids via acid isotropic etching (AIE) method with mixed acid solution to ensure that the silicon wafer is fully covered by the PEDOT PSS. In addition, hydrogenated amorphous silicon thin films were deposited with PEVCD method as the passivation and back surface field (BSF) layer to decrease the rear surface recombination rate, thus increasing the long wavelength response. Finally, a power conversion efficiency of 13.78% was achieved after depositing MoO3 on the front of the device as the antireflection layer.

Undesirable role of remnant PbI2 layer on low temperature processed planar perovskite solar cells

Two-step deposition methods for preparing CH3NH3PbI3 films are becoming increasingly competitive when considering fabricating perovskite solar cells (PSCs) under ambient conditions, along with possessing the ability to better control their morphology. The surplus PbI2 phase can be detected within the formed CH3NH3PbI3 films due to partial conversion of PbI2 to CH3NH3PbI3, which causes variation in the PbI2 stoichiometry of the resultant films. In this work, we carefully study the influence of the remnant PbI2 on the performance of planar PSCs including efficiency and thermal stability by varying the PbI2 stoichiometry of the resultant CH3NH3PbI3. Through a further comparative study, the undesirable role of the remnant PbI2 layer on planar PSC performance is exposed, indicating that the remnant PbI2 layer not only greatly impedes carrier extraction and transport, but also accelerates the degradation of the CH3NH3PbI3 film. To further eliminate the interference of this phenomenon, low temperature processed planar PSCs approaching 13% efficiency are attained in ambient atmosphere by the modified two-step method with enhanced thermal stability, which has significant potential for future mass production of PSCs and provides insight into the correlation between device performance and the PbI2 stoichiometry of CH3NH3PbI3 films.

Revealing the unfavorable role of superfluous CH3NH3PbI3 grain boundary traps in perovskite solar cells on carrier collection

In this work, a facile sequential thermal evaporation strategy in low vacuum (STELV) is proposed to achieve good crystallization of CH3NH3PbI3 films with controllable grain size. Through a comparative study, it is observed that the photovoltaic parameters of perovskite solar cells (PSCs) are closely related to the CH3NH3PbI3 grain size, and large grains should contribute to a superior performance and a small hysteresis index. Also, we demonstrate that the declining performance can be ascribed to a high carrier combination rate caused by superfluous perovskite grain boundary (PGB) traps. By combination with Silvaco simulation software based on physical models, the recombination rate distribution within PSCs is clearly observed. In addition, the lateral conduction band energy distribution within CH3NH3PbI3 films and the transient carrier collection efficiency are also displayed, suggesting that PGB traps are prominently responsible for inferior device performance by causing serious lateral transport of photo-generated carriers and reducing their collection efficiency, which is consistent with experimental results.

Influence of the gas flow of Argon and the distance between substrate and plasma on properties of Al-doped zinc oxide films

Al-doped ZnO(ZAO) films were deposited by DC magnetron sputtering using facing zinc oxide targets at room temperature and in argon atmosphere. The effects of the gas flow of Argon and the distance between substrate and plasma on the properties of the ZAO thin films were characterized by several techniques. By optimizing the craft of preparation, the electrical resistivity as low as 3.3×10-4 Ω·cm and the optical transmittance over 80% in the visible range were obtained for these thin ZAO films. Therefore, the ZAO thin films were suitable for the window layers of n-i-p thin film solar cells or transparent conductive films.

Boron doped hydrogenated nanocrystalline silicon thin films prepared by layer-by-layer technique and its application in n-i-p flexible amorphous silicon thin film solar cells

Boron doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films are deposited using layer-by-layer technique in radio frequency (RF) plasma enhance chemical vapor deposition system. Studies about the influence of gas pressure, RF power density and buffer layer in i/p interface on the microstructure and electrical properties of Boron doped nc-Si:H thin films have been carried out. The experimental results have shown that a), as the gas pressure decreases, the grain size of nc-Si:H thin film increases, while its conductivity turns small, and the thin films transform into amorphous silicon when the gas pressure decreases to 100Pa; b), the microstructure of nc-Si:H thin films evolves to more ordered when the power density was enhanced from 280mW/cm2 to 560mW/cm2, and the grain size changes to be larger; c), the thickness of i/p buffer has large effect on the microstructure of Si:H thin films, when the deposition time of buffer layer increases from 0min to 10min, the Si:H thin films changes from amorphous to nanocrystalline, however the optimal time is about 2min. When the boron doped nc-Si:H thin films are applied as the window layer of n-i-p flexible amorphous silicon solar cells, comparing with p type a-SiC:H window layer, the performance of solar cells have been largely improved.