Degradation analysis of thin film photovoltaic modules

Five thin film photovoltaic modules were deployed outdoors under open circuit conditions after a thorough indoor evaluation. Two technology types were investigated: amorphous silicon (a-Si:H) and copper indium gallium diselenide (CIGS). Two 14 W a-Si:H modules, labelled Si-1 and Si-2, were investigated. Both exhibited degradation, initially due to the well-known light-induced degradation described by Staebler and Wronski [Applied Physics Letters 31 (4) (1977) 292], and thereafter due to other degradation modes such as cell degradation. The various degradation modes contributing to the degradation of the a-Si:H modules will be discussed. The initial maximum power output (P_MAX) of Si-1 was 9.92 W, with the initial light-induced degradation for Si-1 ∼30% and a total degradation of ∼42%. For Si-2 the initial P_MAX was 7.93 W, with initial light-induced degradation of ∼10% and a total degradation of ∼17%. Three CIGS modules were investigated: two 20 W modules labelled CIGS-1 and CIGS-2, and a 40 W module labelled CIGS-3. CIGS-2 exhibited stable performance while CIGS-1 and CIGS-3 exhibited degradation. CIGS is known to be stable over long periods of time, and thus the possible reasons for the degradation of the two modules are discussed.     

Transport and stability studies on high band gap a-Si:H films prepared by argon dilution

Device quality hydrogenated amorphous silicon films (a-Si:H) are deposited at a high deposition rate (4–5 Å/s) using a mixture of argon and hydrogen-diluted silane. The films exhibit good opto-electronic properties and show less degradation upon light soaking. Light-induced changes in conductivity could be annealed at much lower temperature. The presence of Ar^* and atomic hydrogen in plasma replaces the weak Si-Si bonds, which are responsible for light-induced degradation by strong Si-Si bonds. This results in the improved stability of the films.     

Dynamics of hydrogen in hydrogenated amorphous silicon

The problem of hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) is studied semiclassically. It is found that the local hydrogen concentration fluctuations-induced extra potential wells, if intense enough, lead to the localized electronic states in a-Si:H. These localized states are metastable. The trapping of electrons and holes in these states leads to the electrical degradation of the material. These states also act as recombination centers for photo-generated carriers (electrons and holes) which in turn may excite a hydrogen atom from a nearby Si-H bond and breaks the weak (strained) Si-Si bond thereby apparently enhancing the hydrogen diffusion and increasing the light-induced dangling bonds.     

Influence of heterogeneities on the electronic properties of hydrogenated amorphous silicon

An attempt is made to highlight the importance of inhomogeneities in hydrogenated amorphous silicon (a-Si:H), in controlling its electronic properties. We note that hydrogen increases the gap in a-Si:H and that hydrogen is distributed inhomogeneously in it. This gives rise to long-range potential fluctuations, which are mostly uncorrelated and usually ignored. These and other such considerations have not only enabled us to gain new insights into the behaviour of a-Si:H in general, but have also allowed us to resolve several unsolved puzzles. Among these are questions like why undoped a-Si:H is n-type, why the creation of dangling bonds upon light soaking (LS) so inefficient, why a-Si:H degrades more upon LS when it is doped, why the reciprocity fails for light-induced degradation, why presence of nanocrystalline silicon improves stability and so on. We provide evidence to support some of our ideas and make suggestions for verifying the others.     

Hydrogen diffusion and electronic metastability in amorphous silicon

Hydrogen diffusion and its role in the many electronic metastability phenomena in hydrogenated amorphous silicon (a-Si:H) is reviewed. A-Si:H contains about 10 at% hydrogen, most of which is bonded to silicon. The hydrogen diffuses at relatively low temperatures by releasing hydrogen from the Si-H bonds into interstitial sites. The reactions of hydrogen with the silicon dangling bonds and the weak bonds provide a hydrogen-mediated mechanism for electron-structural interactions, which are manifested as electronic metastability. The annealing of light-induced defects, the equilibration of defects and dopants, the stretched exponential relaxation kinetics, and the atomic structure formed during growth, are all attributed to hydrogen diffusion.     

Dispersive processes of light-induced defect creation in hydrogenated amorphous silicon

The growing curve of light-induced dangling bonds under illumination has been observed for various intensities of illumination in a-Si:H. It is fitted to a stretched exponential function and then two parameters β and τ involved in the function are estimated as a function of saturated dangling bond density . The experimental values of β, τ, and are compared with those calculated based on our model of light-induced defect creation in a-Si:H.     

Nuclear magnetic resonance investigation of H, H2 and dopants in hydrogenated amorphous silicon and related materials

Nuclear magnetic resonance has been successfully applied to the study of the microstructure of hydrogenated amorphous silicon and related materials. It has been used to determine the local bonding and structural environment of the host atoms, the hydrogen, and the dopants. First, we review some of these NMR experimental results on the hydrogen microstructure in hydrogentaed amorphous semiconductors and compare the results on plasma deposited hydrogenated amorphous silicon (a-Si:H), remote hydrogen plasma deposited a-Si:H, thermally annealed a-Si:H, doped a-Si:H, microcrystalline Si and amorphous (Si, Ge):H alloys. A common feature is that these materials exhibit a heterogeneous distribution of hydrogen bonded to the semiconductor lattice in dilute and clustered phases. In addition, the lattice contains voids of varying number and size that contain non-bonded molecular hydrogen whose quantity is altered by deposition conditions and thermal treatment. Second, we review some aspects of the local bonding structure of dopants in a-Si:H. A significant fraction of the dopants are found to be in dopant-hydrogen clusters similar to those proposed to explain hydrogen passivation in crystalline silicon. Implications of the determined local structure on the doping efficiency are discussed.     

Optical and electronic properties of hydrogenated amorphous silicon films deposited by square-wave modulated RF discharges of silane-he mixtures

The optoelectronic properties of a-Si: H films deposited under very different plasma conditions have been studied as functions of the square-wave modulation frequency of a 13.56 MHz rf discharge. Results for two extreme deposition conditions are presented. At both high and low deposition rates, the modulation of the rf discharge results in a negligible effect or a deterioration of the film properties, particularly the photoconductivity which we have found to be the most sensitive parameter. At high deposition rates, modulation of the discharge at 10 Hz results in the incorporation of particles into the film which produce enhanced absorption in the film. Therefore, for our reactor configuration, the modulation of the discharge does not produce an improvement but rather a degradation of the film properties.     

Photoluminescence of very thin oxide/a-Si:H structures passivated in HCN solutions

In this contribution we present new experimental facts concerning evolution of the a-Si:H photoluminescence (PL) spectra, recorded at 6 K, induced by both formation of very thin oxide layer in the surface region of the semiconductor by nitric acid solutions and passivation of a-Si:H defect states using HCN aqueous solutions. a-Si:H layers were deposited on both n-type of crystalline Si and the Corning glass. The analysis of the set of PL spectra - their interpretation - indicates that two explanations of blue shifts of the PL band maxima are possible. The first is connected with formation of several structurally different a-Si:H-based phases inside the amorphous matrix and/or with the Street model of recombination of localized electron-hole pairs coupled with the optical phonon in Si.Additionally, as it was stated, the wet chemical oxidation of 1 μm thick a-Si:H layers deposited on the glass can induce formation of hydrogenated micro-sized a-Si grains. Passivation procedure performed in the HCN solution can transform an equivalent part of the a-Si:H semiconductor to a-Si:CN. If the multiphase model is accepted for interpretation of the PL spectra then the following main PL transitions related with different phases were observed: 1.20, 1.25, 1.38, 1.41, 1.44, and 1.48 eV.     

Large supercell molecular dynamics study of defect formation in hydrogenated amorphous silicon

We have performed molecular dynamics simulations of the defect formation associated with the Staebler-Wronski (SW) effect in a-Si:H using 224 and 231 atom supercells and employing semiempirical Si-Si and Si-H total energy functionals. The role of hydrogen in the defect formation within the bond breaking model of the SW effect has been investigated for both large supercells. The results suggest that, within this model, H can be important in weakening the normal Si-Si bonds which break to produce defects in the SW effect.     

Slow structural transitions of hydrogen in hydrogenated amorphous silicon during low temperature annealing

A slow decrease in the number of Si-H bonds in B-doped a-Si:H during annealing at 220°C has been observed. It is shown that during annealing hydrogen is transferred irreversibly from a bonded state into molecular H₂ trapped in the samples. This has important implications for the determination of H-diffusion coefficient by SIMS or ERDA.     

Relationship between optical and structural properties of hydrogenated amorphous silicon

A series of experimental studies has been made on the relationship between optical and structural properties of hydrogenated amorphous silicon (a-Si:H) prepared under various conditions. It has been clarified by analysing the results that the shape of the energy spectrum near the band edge and the distribution of the valence-band tail states depend primarily on the structural disorder of the Si network in a-Si:H. On the other hand, the total content and the bonding mode of bonded hydrogen have little effects on these electronic properties of a-Si:H. It has also been found that the distribution of the valenceband tail states might be related to other unidentified factor(s) besides the structural disorder. The present results have been compared with those of the previous experimental and theoretical studies.     

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si?:?H) and amorphous As₂Se₃ (a-As₂Se₃) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As₂Se₃ than for a-Si?:?H has been observed and this is interpreted in terms of the higher structural flexibility of a-As₂Se₃. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.     

Light-induced metastable state in doping-modulated amorphous silicon superlattices

Photo-induced decrease in conductance after long light exposure has been observed for hydrogenated amorphous silicon (a-Si:H) npnp . doping-modulated superlattices showing large persistent photoconductivity (PPC). The initial increase in PPC is taken over by the decrease even to the negative PPC after long illumination. This light-induced, metastable conductance can be recovered completely by 160°C annealing, which is independent of exposure time. In particular, the metastable defects created in the p-layers are found to be annealed out at 100°C. The subband gap light exposure on the a-Si:H npnp. multilayers showing n-type conduction gives rise to the increase of conductance. On the contrary, the conductance of the multilayers having p-channel conduction decreases by IR exposure. These results strongly suggest the creation and annealing of dangling bonds by light-soaking and annealing in doping-modulated superlattices.     

AlCl3-induced crystallization of amorphous silicon thin films

It is reported that the direct contact between Al and amorphous silicon (a-Si) enhances the crystallization of a-Si films. But the polycrystalline silicon (poly-Si) films crystallized by the direct contact of Al metal film suffer the problems of rough surface. In our study, we utilized the AlCl₃ vapor during the a-Si films deposition instead of Al metal film to enhance crystallization. X-ray diffraction (XRD) shows that the AlCl₃ vapor so successfully enhanced the crystallization of a-Si films that the crystallization was completed in 5 h at 540 °C. And the orientation of the poly-Si film deposited with AlCl₃ vapor is much more random than that of annealed with Al metal under layer. But the average grain size is much larger than that. Moreover, the surface of the AlCl₃-induced crystallized poly-Si film was much smoother than that of the Al-induced poly-Si film. The Al and Cl incorporation into the poly-Si film was confirmed using X-ray photoelectron spectroscopy (XPS) and found that the quantity of Al and Cl incorporated into the Si film was below the detection limit of XPS.     

Chemical recycling of cellulose nitrate waste

A chemical degradation technology for processing cellulose nitrate (CN) waste with any initial moisture content, which provides a high level of process safety, has been developed. Under exposure to chemical reagents, CN waste is converted to fireproof water-soluble nontoxic biodegradable products. Regression equations to relate the reaction conditions and the degree of degradation of CN have been derived.     

用H-W-ECR CVD系统实现氢化非晶硅薄膜在氢气环境下同时进行化学热退火和光诱导退火的研究

为了研究氢化非晶硅薄膜的稳定性，我们设计了一个在原子氢气氛中热退火的同时进行光诱导退火的实验(TLAH)。实验装置是由传统的微波电子回旋共振化学气相沉积系统改造而成为热丝辅助微波电子回旋共振化学气相沉积系统。为了对这一退火方法进行比较，对样品还进行了热退火、热退火同时进行光诱导退火。同时，为了定量地分析光电导衰退，我们假设光电导衰退遵循扩展指数规律：1/σph=1/σs-(1/σs-1/σ0)exp[-(t/τ)β]，这里扩展指数参数β 和时间常数 τ 可从与 lnt 的线性关系中截距和斜率得到, 式中光电导饱和值σs可以通过在对数坐标系中表示的光电导和光照时间关系进行高斯拟合得到。实验结果显示：TLAH 方法可以提高氢化非晶硅薄膜的稳定性、改善其微结构和光电特性，同时还发现，光学带隙明显减小、荧光光谱显著地朝着低能方向移动。     

非晶/微晶相变域硅薄膜及其太阳能电池

采用甚高频等离子体增强化学气相沉积（VHF-PECVD）法，成功制备出从非晶到微晶过渡区 域的硅薄膜. 样品的微结构、光电特性及光致变化的测量结果表明这些处于相变域的硅薄膜 兼具非晶硅优良的光电性质和微晶硅的稳定性. 用这种两相结构的材料作为本征层制备了p- i-n太阳能电池，并测量了其稳定性. 结果在AM15(100mW/cm²) 的光强下曝光 800—5000min后，开路电压略有升高，转换效率仅衰退了29%. 关键词： 相变域硅薄膜 光电特性 太阳能电池     

Role of the hydrogen in the light-induced defects in undoped hydrogenated amorphous silicon

We have investigated the effect of the deposition temperature (i.e. the hydrogen content) on the light-induced effects in undoped hydrogenated amorphous silicon (a-Si:H). Combined junction capacitance-temperature (C-T), ESR and IR absorption measurements are carried out in both the dark annealed state (A) and the saturated light-soaked state (B), as well as after partial annealing of the samples, starting from state B. The experimental results indicate that the films deposited at the highest substrate temperature (i.e. the lowest H content) exhibit a completely different behaviour from those deposited at lower substrate temperature (i.e. with higher H concentration), when the samples are left for long times at room temperature in the dark after partial annealing. These results are discussed in detail in relation to the different models proposed to explain the light-induced effects in a-Si:H.     

后退火增强氢化非晶硅钝化效果的研究

在本征氢化非晶硅(a-Si:H(i))/晶体硅(c-Si)/a-Si:H(i)异质结构上溅射ITO时, 发现后退火可大幅增加ITO/a-Si:H(i)/c-Si/a-Si:H(i)的少子寿命(从1.7 ms到4 ms). 这一增强效应可能的三个原因是: ITO/a-Si:H(i)界面场效应作用、退火形成的表面反应层影响以及退火对a-Si:H(i)材料本身的优化, 但本文研究结果表明少子寿命增强效应与ITO和表面反应层无关; 对不同沉积温度制备的a-Si:H(i)/c-Si/a-Si:H(i)异质结后退火的研究表明: 较低的沉积温度(<175 ℃)后退火增强效应显著, 而较高的沉积温度(>200 ℃)后退火增强效应不明显, 可以确定“低温长高温后退火”是获得高质量钝化效果的一种有效方式; 采用傅里叶红外吸收谱(FTIR)研究不同沉积温度退火前后a-Si:H(i)材料本身的化学键构造, 发现退火后异质结少子寿命大幅提升是由于a-Si:H(i)材料本身的结构优化造成的, 其深层次的本质是通过材料的生长温度和退火温度的优化匹配来控制包括H含量、H键合情况以及Si原子无序性程度等微观因素主导作用的一种竞争性平衡, 对这一平衡点的最佳控制是少子寿命大幅提升的本质原因.