Compared to linear polymers with the same molecular weight, star-shaped polymers have the superiority of drug loading and delivery. The glycyrrhetinic acid (GA) from licorice is remarkably characteristic of liver distribution and liver cells targetability. In this paper, four-armed star-shaped polycaprolactone was synthesized and amino polyethylene glycol was modified by glycyrrhetinic acid (NH2-PEG-GA). Then the condensation reaction between the two above polymers finally produced four-armed star-shaped poly(ethylene glycol)-b-poly(ε-caprolactone) block copolymer (sPCL-b-PEG-GA). The structures of the intermediates and product were characterized by1H NMR. The results indicated that the structure and molecular weight of sPCL-b-PEG-GA can be controlled by the varied ratios of pentaerythritol (PTOL) toε-caprolactone (ε-CL) in the presence of stannous octoate (Sn(Oct)2), and the amphiphilic copolymer sPCL-b-PEG-GA consists of PTOL as core, PCL as inner hydrophobic segments, PEG as external hydrophilic segments, and terminal glycyrrhetic acid as targeting ligand. The work explored a new synthesis route of star poly(ethylene glycol)-b-poly(ε-caprolactone) copolymer with liver targetability. The star-shaped polymer is expected to be an efficient drug carrier.
This paper reports that high-rate-deposition of microcrystalline silicon solar cells was performed by very-highfrequency plasma-enhanced chemical vapor deposition. These solar cells, whose intrinsic µc-Si:H layers were prepared by using a different total gas flow rate (F total ), behave much differently in performance, although their intrinsic layers have similar crystalline volume fraction, opto-electronic properties and a deposition rate of ∼ 1.0 nm/s. The influence of F total on the micro-structural properties was analyzed by Raman and Fourier transformed infrared measurements. The results showed that the vertical uniformity and the compact degree of µc-Si:H thin films were improved with increasing F total . The variation of the microstructure was regarded as the main reason for the difference of the J-V parameters. Combined with optical emission spectroscopy, we found that the gas temperature plays an important role in determining the microstructure of thin films. With F total of 300 sccm, a conversion efficiency of 8.11% has been obtained for the intrinsic layer deposited at 8.5 Å/s (1 Å=0.1 nm).
A series of high rate growth μc-Si:H thin films with different thicknesses were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) process with high power and high pressure. The microstructure of the μc-Si:H thin films was studied by the Raman and XRD spectra. It was found that the crystal fraction and grain size increased with the thickness of the thin film when the thickness was less than 1000 nm, and then came to saturation when the thickness was higher than 1000 nm. However, the performance of solar cells decreased obviously when the thickness increased from 1000 nm to 2000 nm. Considering the microstructure properties and the ion bombardment during the high rate process, we investigated the controlled microstructure evolution and the improved material quality by discharge power profiling, which improved the performance of solar cells. By optimizing the profiling parameters, such as the amount and the rate of change in discharge power, a high efficiency of 9.36% was obtained with an i-layer deposition rate of 1.2 nm/s. Furthermore, we used the improved μc-Si:H cell in an a-Si:H/μc-Si:H double-junction structure and achieved an initial active-area cell efficiency of 1114%.
In the process of the high growth rate μc-Si:H film deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD), the high energy ion impinging on the growing surface could deteriorate the device performance. Incorporation of a low growth rate intrinsic μc-Si:H p/i buffer layer was advanced in this paper. The results show that the introduced low growth rate buffer layer could improve the characteristics of p/i interface and the vertical uniformity of the intrinsic layer. It was found that the defects in intrinsic layer first decreased and then increased with increasing thickness of the buffer layer. These results led to an optimal thickness for the buffer layers. The efficiency of solar cells was increased about 1% when the thickness was optimized. As a result, the efficiency of 8.11% has been achieved at an i-layer deposition rate of 8.5nm/s.
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