Influence of the deposition parameters on the microstructure and opto-electrical properties of hydrogenated nanocrystalline silicon films by HW-CVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared at high deposition rates (> 13 Å/s) from pure silane without hydrogen dilution by hot wire deposition method by varying filament-to-substrate distance (d_s-f). In this study we have systematically and carefully investigated the effect of filament-to-substrate distance on structural, optical and electrical properties of the Si:H films. A variety of characterization techniques, including Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FE-SEM), UV-Visible-NIR spectroscopy and electrical dark and photoconductivity measurement were used to characterize these films. Films deposited at d_s-f > 5 cm are amorphous while those deposited at d_s-f < 5 cm are biphasic; a crystalline phase and an amorphous phase with nano-sized crystallites embedded in it. Low angle X-ray diffraction analysis showed that the crystallites in the films have preferential orientation along (111) directions. Decrease in d_s-f, the crystallinity and crystalline size increases whereas hydrogen bonding shifts from mono-hydride (SiH) to di-hydride (SiH₂) and poly-hydride (SiH₂)_n complexes. The band gaps of nc-Si:H films (~ 1.9-2.0 eV) are high compared to the a-Si:H films, while hydrogen content remains < 10 at.%. We attribute the high band gap to the quantum size effect. A correlation between electrical and structural properties has been established. Finally, from the present study it has been concluded that the filament-to-substrate distance is a key process parameter to induce the crystallinity in the films by hot wire method. The ease of depositing films with variable crystallite size and its volume fraction, and tunable band gap is useful for fabrication of tandem/micro-morph solar cells.     

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175 °C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N₂O), hydrogen (H₂), silane (SiH₄), and diborane (B₂H₆) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71 eV to 2.0 eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7 × 10^− 5 S/cm to 5.1 × 10^− 7 S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3 eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (V_oc) = 800 mV, short circuit current density (J_sc) = 16.3 mA/cm², fill-factor (FF) = 72%, and photovoltaic conversion efficiency (η) = 9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. J_sc, FF and V_oc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.     

Characterization of HF-PECVD a-Si:H thin film solar cells by using OES studies

Hydrogenated amorphous silicon (a-Si:H) films show considerable potential for the fabrication of thin film solar cells. In this study, the a-Si:H thin films have been deposited in a parallel-plate radio frequency (RF) plasma reactor fed with pure SiH₄. The plasma diagnostics were performed simultaneously during the a-Si:H solar cell deposition process using an optical emission spectrometer (OES) in order to study their correlations with growth rate and microstructure of the films. During the deposition, the emitting species (SiH*, Si*, H*) was analyzed. The effect of RF power on the emission intensities of excited SiH, Si and H on the film growth rate has been investigated. The OES analysis revealed a chemisorption-based deposition model of the growth mechanism. Finally, the a-Si:H thin film solar cell with an efficiency of 7.6% has been obtained.     

p/i界面掺碳缓冲层沉积时间对非晶硅太阳电池性能的影响

本文研究了pin型非晶硅(a-Si)太阳电池p/i界面掺碳缓冲层(C-buffer layer)沉积时间对电池效率和稳定性的影响.研究发现,随着掺碳缓冲层沉积时间的增加,太阳电池的初始效率有所增加,当沉积时间增加到约60s时,电池的初始效率达最大值,而后随着沉积时间的继续增加,电池效率下降.而在太阳电池的稳定性方面,当缓冲层沉积时间小于50s时,随着沉积时间的增加,电池衰退率增大;大于50s后,电池的衰退率又随沉积时间的增大而减小.     

Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films

Flash lamp annealing (FLA) can form polycrystalline silicon (poly-Si) films with various microstructures depending on the thickness of precursor amorphous Si (a-Si) films due to the variation of crystallization mechanisms. Intermittent explosive crystallization (EC) takes place in precursor a-Si films thicker than approximately 2 μm, and the periodicity of microstructure formed resulting from the intermittent EC is independent of the thickness of a-Si films if their thickness is 2 μm or greater. In addition to the intermittent EC, continuous EC and homogeneous solid-phase crystallization (SPC) also occur in thinner films. These crystallization mechanisms are governed by the ignition of EC at Si film edges and the homogeneous heating of interior a-Si. The results obtained in this study could be applied to control the microstructures of flash-lamp-crystallized poly-Si films.     

数值模拟p/i界面对微晶硅薄膜太阳电池性能的影响

采用美国宾州大学开发的AMPS(Analysis of Microelectronic and Photonic Structures)软件模拟了p/i界面缺陷态密度(Npt/i)和非晶孵化层厚度(d)对pin型氢化微晶硅(μc-Si:H)薄膜太阳电池性能的影响.结果表明:随着Npt/i的增大,电池的开路电压Voc和填充因子FF单调减小,短路电流Jsc基本不变;随着d的增大,Jsc和FF单调减小,Voc反而增大;Npt/i和d值的增大均会导致电池光电转换效率η下降.通过对电池内部的电场及能带的分析,对上述模拟结果进行了解释.     

Enhancement of organic thin-film solar cells by incorporating hybrid Au nanospheres and Au nanorods on a metallic grating surface

Abstract

In this study, we demonstrate the fabrication of hybrid plasmonic solar cells using gold nanoparticles (AuNPs). Two types of AuNPs, gold nanospheres (AuNSs) and gold nanorods (AuNRs), were incorporated in a hole transport layer (HTL) (PEDOT:PSS) on a metallic grating electrode. The organic solar cells (OSCs) structure comprised an indium-tin-oxide (ITO)-coated glass substrate/PEDOT:PSS:AuNSs:AuNRs/P3HT:PCBM/Al grating electrode. Adding AuNPs induced localized surface plasmon resonance (LSPR), while grating structured Al at the interface with a photoactive layer excited the propagating surface plasmons. Compared with a flat reference device, the proposed OSCs exhibited improved photovoltaic properties by increasing both the short-circuit current density (J_SC) and the power conversion efficiency (PCE) with large enhancements of 16.23% and 14.06%, respectively. The efficiency improvement was attributed to increased broadband absorption and improved electrical properties inside the thin-film devices.     

Optimisation of low-temperature silicon epitaxy on seeded glass substrates by ion-assisted deposition

Using single crystalline Si wafer substrates, ion-assisted deposition (IAD) has recently been shown [J. Crystal Growth 268 (2004) 41] to be capable of high-quality high-rate epitaxial Si growth in a non-ultra-high vacuum (non-UHV) environment at low temperatures of about 600 °C. In the present work the non-UHV IAD method is applied to planar borosilicate glass substrates featuring a polycrystalline silicon seed layer and carefully optimised. Using thin-film solar cells as test vehicle, the best trade-off between various contamination-related processes (seed layer surface as well as bulk contamination) is determined. In the optimised IAD process, the temperature of the glass substrate remains below 600 °C. The as-grown Si material is found to respond well to post-growth treatments (rapid thermal annealing, hydrogenation), enabling respectable open-circuit voltages of up to 420 mV under 1-Sun illumination. This proves that the non-UHV IAD method is capable of achieving device-grade polycrystalline silicon material on seeded borosilicate glass substrates.