| Hydrogenated nano-amorphous silicon (na-Si:H) films, due to their high absorption coefficient (α≈105) like amorphous silicon, wide optical bandgap (Eopt≈1.8-2.0eV) like crystalline silicon, and high stability under illumination, have been widely studied in the recent years for their potential applications in solar cells.Under the preparing conditions of low silane concentration f=SiH4/(SiH4+H2)≤3% (a mixture of silane highly diluted in hydrogen), RF power density p=591mW/cm2, substrate temperature Ts=200℃, pressure P=1.1Torr, intrinsic na-Si:H films were grown by plasma enhanced chemical vapor deposition (PECVD). The effects of the silane concentration on the structural, optical and electrical properties of intrinsic na-Si:H films have been systematically investigated. The mechanism of the formation of nano-sized silicon crystallite was discussed. In addition to the intrinsic na-Si:H films, b-doped p-type na-Si:H films were prepared using borane (B2H6) also by PECVD. The influence of borane concentration r=B2H6/(B2H6+SiH4), Ts, Ï, and P on structural, optical, electrical properties and deposition rate 5 were also studied in this paper.The structure of the deposited na-Si:H film was studied by means of X-ray-diffraction pattern (XRD), Raman scattering spectra (Raman), and high-resolution electron microscopy (HRTEM). The results show that na-Si:H films consist of mixed phase of nano-sized silicon crystallites (4-10nm) embedded in amorphous matrix, with a preferential growth in the orientation of (220), crystalline volume fraction Xc gives the value of 10-40%. For f≥4% samples do not have a detectable crystallization and appear essentially amorphous.The chemical bonding analysis had been performed by FTIR transmission measurements. It exhibits that as the f decreases, 630cm-1 2000cm-1 peaks shift to 610cm-1 2100cm-1, respectively, indicating of the transformation of stretching vibrations mode of SiH and SiH2 into SiH2 and SiH3, and the hydrogen content CH also decreases from 8% to 2%.As regards of optical properties, the absorption coefficient a and optical band gap Eopt were derived from ultraviolet-visible transmission spectra. The films show an enhancement of optical absorption from 100 to 102cm-1 in the near infrared region, whereas in the visible region the absorption is lower than that of a-Si:H. The Eopt of na-Si:H is much wider (1.85-1.94) than that of crystalline silicon (1.12eV) and amorphous silicon (1.5-1.8eV) due to the quantum confinement effect.The photo/dark electrical conductivity, σph/σd, was measured by means of an home-made instrument (100mW/cm2, R.T.). It shows that the conductivity activation energy Ea of na-Si:H approaches 0.40eV, much smaller than that of a-Si:H (0.72eV). The dark conductivity reaches 10-5Ω-1cm-1, larger than that of a-Si:H by five orders of magnitude. No obvious Staebler-Wronski effect was detected during extended light soaking (σph/σd decreased less than 10%). A two-phase model has been used to discuss the relation between electrical properties and microstructures.The film thickness was derived from the Surface Profiler and cross-section SEM. It is estimated that the deposition rate 8 of na-Si:H film is less than 1 A/s, much slower than that of a-Si:H (5-10 A/s).The results of investigation of boron-doped na-Si:H films show that boron light-doping enhances the crystalline degree comparing with intrinsic na-Si:H films, (220) preferred orientation growth, crystalline degree improves as f increases. The hydrogen content CH in B-doped na-Si:H films are much smaller than those in intrinsic na-Si:H films for B atoms partly replaced H atoms which bonding to Si atoms initially. Boron doping also makes σph/σd rapidly decrease from 103 to 10 as the borane concentration increases.Optical band gap Eopt of na-Si:H will be enhanced by boron doping to reach approximately 2.0eV, σd of Boron-doped na-Si:H (10-8-10-3Ω-1cm-1) is larger than that of a-Si:H by 23 orders of magnitude, and can be controlled by adjusting boron concentration.In this work, we find that SiH4/(SiH4+H2)=3%, B2H6/(SiH4+B2H6)=5 × 10-4, Ts=150℃, p=200mW/cm2 and P=0.8Torr are the optimum preparation parameters to obtain B-doped na-Si:H films with high electrical conductivity and wide band gap.As we can see, na-Si:H is likely going to be an ideal new-type intrinsic absorption layer material for thin film solar cells for it's high light absorption, good electrical conductivity and high stability under illumination. Since the boron light-doping na-Si:H has a wide optical bandgap comparable to that of a-SiC:H (2.04eV), it is possible to replace the a-SiC:H to be used as the window layer material for silicon film solar cells.
|
PDF Full Text Request