柔性衬底微晶硅薄膜太阳电池研究

采用等离子增强化学气相沉积(PECVD)技术制备了系列本征微晶硅薄膜材料和nip单结微晶硅太阳电池,研究了硅烷浓度、衬底温度和辉光功率等沉积参数与薄膜材料性能、薄膜电池性能三者之间的关系.拉曼光谱和器件测试结果表明:随硅烷浓度的增加,本征层晶化率逐渐减小,直至转变为非晶硅;沉积温度高于200℃时,电池性能严重恶化;随等离子辉光功率增加,材料晶化率保持不变,而电池开路电压逐渐增大,短波光谱响应逐渐增强.在此基础上,优化了单结微晶硅电池沉积参数,得到效率为6.48％ (AM0,25℃)的单结微晶硅薄膜太阳电池;并将其应用到非晶硅／微晶硅叠层电池中,在不锈钢柔性衬底上得到效率为9.28％( AM0,25℃)的叠层电池.     

非晶孵化层对高速生长微晶硅电池性能的影响

采用高压高功率的甚高频等离子体增强化学气相沉积(VHF-PECVD)技术,以不同的反应气体总流量制备出沉积速率大于1nm/s、次带吸收系数(α0.8eV)小于2.5cm-1且具有相同晶化率的本征微晶硅薄膜,然而将其应用在微晶硅电池中时,电池性能却有明显差异.通过对微晶硅电池的光、暗态J-V,量子效率(QE)和微区拉曼(Raman)测试发现,微晶硅薄膜中非晶孵化层厚度的不同是引起电池性能差异的主要原因.反应气体总流量较低时沉积的微晶硅薄膜具有较厚的非晶孵化层,阻碍了载流子的输运,使电池的长波光谱响应下降,从而降低了电池的短路电流密度与填充因子;而增加总气体流量,有效减小了微晶硅薄膜中的非晶孵化层的厚度,从而使电池性能得到改善.最后在总气体流量为500sccm时,制备得到沉积速率为1nm/s,效率为7.3%的单结微晶硅太阳电池.     

微晶硅电池的制备及提高其效率的优化

采用甚高频等离子体增强化学气相沉积(VHF-PECVD)技术制备了不同硅烷浓度系列的微晶硅电池。结果表明:电池的开路电压随着硅烷浓度的增大而逐渐增加,而电池的短路电流则先增加后减小,在转折点电池的效率达到最大,填充因子则变化不明显;(220)择优取向出现,I(220)/I(111)比值大,电池的短路电流密度也大,电池的效率也最高;在实验的范围内,电池的短路电流密度和厚度成正比例关系;首次在国内制备出了效率达7.3%,短路电流密度(Jsc)为21.7mA/cm2,开路电压(Voc)为0.52V,填充因子(FF)为65%的微晶硅电池。     

薄膜非晶硅/微晶硅叠层太阳电池的研究

采用射频等离子体增强化学气相沉积(RF—PECVD)技术制备非晶硅顶电池，采用甚高频等离子体增强化学气相沉积(VHF—PECVD)技术制备微晶硅底电池，初步优化研究了薄膜非晶硅／微晶硅叠层太阳电池顶电池与底电池的本征吸收层厚度匹配与电池电流匹配，以及氧化锌／金属复合背反射电极对电池的作用。研制出了面积为1．0cm^2效率达9．83％的薄膜非晶硅／微晶硅叠层太阳电池。     

基于溅射后腐蚀ZnO的单结微晶硅太阳电池中的陷光研究

采用甚高频等离子体增强化学气相沉积(VHF-PECVD)技术,研究衬底表面形貌和电学特性的变化对微晶硅单结太阳电池性能的影响。通过对不同腐蚀时间溅射ZnO∶Al衬底及基于此的微晶硅单结电池进行研究发现:衬底表面横纵特征尺寸可通过腐蚀时间进行有效调控,光电性能的权衡使其存在最优化衬底腐蚀时间,从而使微晶硅单结电池达到最大光吸收和高电学性能。对衬底陷光结构和电池工艺进一步调整,获得初始效率达10.01%的单结微晶硅薄膜太阳电池,将其应用到非晶硅/非晶硅锗/微晶硅三结叠层电池中,电池效率可达14.51%。     

微晶硅薄膜的光稳定性与微结构的关系

利用甚高频等离子体增强化学气相沉积技术,通过改变功率密度和沉积压强制备了三系列微晶硅薄膜。采用拉曼光谱、XRD与电导率分析技术,研究在光照条件下微晶硅薄膜的光学特性,光电导衰退与晶化率、沉积速率、晶粒尺寸间的关系。研究发现:随着晶化率的增加,微晶硅薄膜的光电导衰退率逐渐减小;随着沉积气压的增加,相同晶化率的薄膜的光稳定性降低。在光照50h后,薄膜的光电导衰退基本达到饱和。     

微晶硅材料和电池特性的相关研究

主要采用甚高频等离子体增强化学气相沉积技术制备了系列微晶硅材料和电池。通过对材料电学特性、结构特性和电池间性能关系的研究,获得了高效率微晶硅薄膜太阳电池所对应材料的基本特性:暗电导在10~(-8)s/cm量级上,光敏性大于1000,晶化率约50%。进行了制备电池的开路电压和表观带隙之间关系的研究。     

H2稀释在PECVD法制备微晶硅薄膜中的影响

利用等离子增强化学气相沉积（PECVD）技术，研究了H稀释度D=H2／（H2＋Sill。）对在玻璃和不锈钢衬底上低温制备微晶硅薄膜的晶化率、晶粒尺寸、薄膜质量等的影响。结果表明，随着硅烷浓度的降低，样品的晶化率、晶粒尺寸有所改变。当D=99％时，晶粒突然变大，晶化率显著提高。因此，我们认为此时的硅薄膜由非晶硅转化为微晶硅。     

VHF-PECVD法高速率沉积氢化微晶硅薄膜

采用光发射谱(OES)技术对氢化微晶硅(μc-Si：H)薄膜的甚高频等离子体增强化学气相沉积(VHF-PECVD)生长过程进行了原位监测,并对不同沉积条件下VHF等离子体中SiH和H的发光峰强度与薄膜沉积速率之间的关系进行了分析与讨论。通过Raman光谱、X射线衍射与扫描电子显微镜(SEM)测量,研究了μc-Si：H薄膜的结构特征与表面形貌。基于当前的沉积系统,对μc—Si：H薄膜沉积条件进行了初步优化,使μc—Si：H薄膜的沉积速率提高到2．0nm／s。     

氢化微晶硅薄膜制备过程中的氧污染问题

采用光发射谱(OES)测量技术,实时监测了不同本底真空制备条件下氢化微晶硅(μc-SiH)薄膜沉积的氧污染程度.样品的X光电子能谱(XPS)与傅立叶变换红外吸收光谱(FTIR)测量结果表明不同氧污染条件下制备所制备μc-SiH薄膜中,氧以不同的键合模式存在.氧污染程度较低时,氧主要表现为O-O与O-H键合;氧污染严重时,则以Si-O键合占主导.通过Raman光谱、电导率与激活能的测量进一步发现μc-SiH薄膜的结构特性与电学特性随沉积过程中氧污染程度不同发生显著的变化,而且这种变化不同于氧污染对a-SiH薄膜的影响.     

High-rate microcrystalline silicon deposition for p–i–n junction solar cells

We have developed a high-rate plasma process based on high-pressure and silane-depletion glow discharge for highly efficient microcrystalline silicon (μc-Si:H) p–i–n junction solar cells. Under high-rate conditions (2–3 nm/s), we find that the deposition pressure becomes the dominant parameter in determining solar-cell performance. With increasing deposition pressure from 4 to 7–9 Torr, short-circuit current increases by 50% due to a remarkable improvement in quantum efficiencies at the visible and near infrared. As a result, the maximum efficiency of 9.13% has been achieved at an i-layer deposition rate of 2.3 nm/s. We attribute the improved performance of high-pressure-grown μc-Si:H solar cells to the structural evolution toward denser grain arrangement that prevents post-oxidation of grain boundaries.     

Microcrystalline silicon and the impact on micromorph tandem solar cells

Intrinsic microcrystalline silicon opens up new ways for silicon thin-film multi-junction solar cells, the most promising being the “micromorph” tandem concept. The microstructure of entirely microcrystalline p–i–n solar cells is investigated by transmission electron microscopy. By applying low pressure chemical vapor deposition ZnO as front TCO in p–i–n configurated micromorph tandems, a remarkable reduction of the microcrystalline bottom cell thickness is achieved. Micromorph tandem cells with high open circuit voltages of 1.413 V could be accomplished. A stabilized efficiency of around 11% is estimated for micromorph tandems consisting of 2 μm thick bottom cells. Applying the monolithic series connection, a micromorph module (23.3 cm²) of 9.1% stabilized efficiency could be obtained.     

Development of high efficiency large area silicon thin film modules using VHF-PECVD

This paper reviews recent work on the development of thin film silicon solar modules and cost-effective production technology. Noting the potential of VHF-PECVD for high rate and high quality deposition, we initiated development of a-Si solar modules. In the first stage, we succeeded in up-scaling a-Si high quality uniform deposition at a high rate of over 1.0 nm/s to a substrate area of 1.1 × 1.4 m² to achieve high productivity. Next, the large area a-Si solar modules with stable aperture efficiency of 8% were developed, and the commercial production of a-Si solar modules commenced in October 2002. In the second stage, aiming at stable efficiency of 12%, which could make the PV power generating cost below residential electricity prices in combination with cost-effective production technology, we have been developing a-Si/μc-Si tandem solar modules. Recently, tandem modules of 40 × 50 cm² in size with a μc-Si i-layer prepared at a deposition rate of 2.1 nm/s yielded initial conversion efficiencies of 11.1%. As for small sized μc-Si single cells, technologies with a high deposition rate of 2.5 nm/s and efficiency of 8.8% have already been developed. In addition, by improving the up-scaling and light-trapping techniques, we will achieve our current goal of 12% stable efficiency for a-Si/μc-Si tandem modules at a deposition rate of over 2.0 nm/s, leading to cost-effective mass production.     

Development of thin film amorphous silicon oxide/microcrystalline silicon double-junction solar cells and their temperature dependence

We have developed thin film silicon double-junction solar cells by using micromorph structure. Wide bandgap hydrogenated amorphous silicon oxide (a-SiO:H) film was used as an absorber layer of top cell in order to obtain solar cells with high open circuit voltage (V_oc), which are attractive for the use in high temperature environment. All p, i and n layers were deposited on transparent conductive oxide (TCO) coated glass substrate by a 60 MHz-very-high-frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. The p-i-n-p-i-n double-junction solar cells were fabricated by varying the CO₂ and H₂ flow rate of i top layer in order to obtain the wide bandgap with good quality material, which deposited near the phase boundary between a-SiO:H and hydrogenated microcrystalline silicon oxide (μc-SiO:H), where the high V_oc can be expected. The typical a-SiO:H/μc-Si:H solar cell showed the highest initial cell efficiency of 10.5%. The temperature coefficient (TC) of solar cells indicated that the values of TC for conversion efficiency (η) of the double-junction solar cells were inversely proportional to the initial V_oc, which corresponds to the bandgap of the top cells. The TC for η of typical a-SiO:H/μc-Si:H was −0.32%/ °C, lower than the value of conventional a-Si:H/μc-Si:H solar cell. Both the a-SiO:H/μc-Si:H solar cell and the conventional solar cell showed the same light induced degradation ratio of about 20%. We concluded that the solar cells using wide bandgap a-SiO:H film in the top cells are promising for the use in high temperature regions.     

Microcrystalline and micromorph device improvements through combined plasma and material characterization techniques

Hydrogenated microcrystalline silicon (μc-Si:H) growth by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) is studied in an industrial-type parallel plate KAI reactor. Combined plasma and material characterization techniques allow to assess critical deposition parameters for the fabrication of high quality material. A relation between low intrinsic stress of the deposited i-layer and better performing solar cell devices is identified. Significant solar cell device improvements were achieved based on these findings: high open circuit voltages above 520 mV and fill factors above 74% were obtained for 1 μm thick μc-Si:H single junction cells and a 1.2 cm² micromorph device with 12.3% initial (V_oc=1.33 V, FF=72.4%, J_sc=12.8 mA cm⁻²) and above 10.0% stabilized efficiencies.     

Material properties of microcrystalline silicon for solar cell application

The paper reviews the material requirements of microcrystalline silicon (μc-Si) in terms of the device operation and configuration for thin film solar cells and thin film transistors (TFTs). We investigated the material properties of μc-Si films deposited by using 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) from a conventional H₂ dilution in SiH₄. Two types of intrinsic μc-Si films deposited at the high pressure narrow electrode gap and the low pressure wide electrode gap were studied for the solar cell absorption layers. The material properties were characterized using dark conductivity, Raman spectroscopy, and transmission electron microscope (TEM) measurements. The μc-Si quality and solar cell performance were mainly determined by microstructure characteristics. Solar cells adopting the optimized μc-Si film demonstrated high stability with no significant changes in solar cell performance after air exposure for six months and subsequent illumination for over 300 h. The results can be explained that low ion bombardment and high atomic hydrogen density under the PECVD condition of the high pressure narrow electrode gap produce high-quality μc-Si films for solar cell application.     

Microcrystalline silicon solar cell using p-a-Si:H window layer deposited by photo-CVD method

A p-a-Si:H layer, deposited by a photo-assisted chemical vapor deposition (photo-CVD) method, was adopted as the window layer of a hydrogenated microcrystalline silicon (μc-Si:H) solar cell instead of the conventional p-μc-Si:H layer. We verified the usefulness of p-a-Si:H for the p-layer of the μc-Si:H solar cell by applying it to SnO₂-coated glass substrate. It was found that the quantum efficiency (QE) characteristics and solar cell performance strongly depend on the p-a-Si:H layer thicknesses. We applied boron-doped nanocrystalline silion (nc-Si:H) p/i buffer layers to μc-Si:H solar cells and investigated the correlation of the p/i buffer layer B₂H₆ flow rate and solar cell performance. When the B₂H₆ flow rate was 0.2 sccm, there was a little improvement in fill factor (FF), but the other parameters became poor as the B₂H₆ flow rate increased. This is because the conductivity of the buffer layer decreases as the B₂H₆ flow rate increases above appropriate values. A μc-Si:H single-junction solar cell with ZnO/Ag back reflector with an efficiency of 7.76% has been prepared.     

Total SiH4/H2 pressure effect on microcrystalline silicon thin films growth and structure

The effect of the total SiH₄/H₂ gas pressure (1–10 Torr) on the growth rate, the film crystallinity and the nature of hydrogen bonding of microcrystalline silicon thin films deposited by 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD) was investigated under well-controlled discharge conditions. The deposition rate presents an optimum for 2.5 Torr, which does not follow the trend of silane consumption that increases with pressure and is attributed to an increase in plasma density. The film crystallinity increases with pressure from 1–2.5 Torr and then remains almost the same, whereas the films deposited at 1 Torr are highly stressed. On the other hand, hydrogen bonding is also drastically affected.