Analysis of the laser ablation processes for thin-film silicon solar cells

A detailed analysis of the monolithical series connection of thin-film silicon modules with ZnO/Ag back contact is presented. In this study, pulsed lasers with wavelengths of 1064 nm and 532 nm were used. The influence of various laser parameters like laser power, pulse overlap, etc., on the different patterning steps is discussed. The focus of this study was on the back contact patterning process. Here (i) the flake formation process during the ablation and (ii) the influence of a NIR-laser source as an alternative approach to the green laser were investigated in detail. The latter would reduce system costs if only one NIR-laser source could be used for all patterning steps.     

Precise selective scribing of thin-film solar cells by a picosecond laser

In this paper, precise scribing of thin-film solar cells (CIGS/Mo/Glass) via a picosecond laser is investigated. A parametric study is carried out for P1 and P2 scribing to study the effects of laser fluence and overlap ratio on scribing quality and ablation depth. Three ablation regimes are observed for P1 scribing in different laser fluence ranges, due to the involvement of different ablation mechanisms. The optimum scribing conditions are determined for both P1 and P2 scribing, and the potential processing speed is significantly increased. The heat accumulation effect at different repetition rates is studied to extrapolate the results from low to high repetition rates. A two-temperature model-based model is developed to simulate the scribing process for multiple thin films, providing decent prediction of the slot depth for both P1 and P2 scribing.     

Influence of shunt conduction on determining the dominant recombination processes in CIGS thin-film solar cells

We investigated the transport and photovoltaic properties of Cu(In_1-xGa_x)Se₂ (CIGS) thin-film solar cells. The shunt-current-eliminated diode current could be obtained from the current–voltage characteristics by subtracting the parasitic shunt leakage current from the total current. The temperature dependence of the open-circuit voltage, extracted from the shunt-eliminated (total) current, suggested that the recombination activation energy is comparable to (much less than) the CIGS bandgap. The low-temperature characteristics of the diode ideality factor supported bulk-dominated recombination in the same cell. This suggests that shunt-current subtraction can provide the proper diode parameters of CIGS solar cells.     

Characterization of probe lasers for thin-film optical measurements

The peak-power-density stability and beam-wander precision of a probe laser are important factors affecting the inspection results in precise thin-film optical measurements. These factors are also key to evaluating a probe laser for in-line long-time operation of precise thin-film optical measurements. The peak-power density and beam wander of liner helium–neon (He–Ne) lasers, random He–Ne lasers, and diode lasers as functions of time are investigated experimentally using a beam profiler. It is found that the linear polarized He–Ne laser is considered to be a promising candidate for a probe laser employed in precise thin-film optical measurements due to better peak-power-density stability and beam-wander precision. Both the peak-power-density stability and beam-wander precision of He–Ne lasers are usually better than that of diode lasers, but an adequate warm-up of He–Ne laser for 30 min is required before thin-film optical measurements are made. After 12 h operation, the linear polarized He–Ne laser is suitable for precise thin-film optical measurements because both the peak-power-density stability and the beam-wander precision reach the minimum level. A cost-effective system composed of two linear polarized He–Ne lasers for long-term operation is proposed. This system can operate for around 0.5–1.2 years in precise thin-film optical measurements under the normal operating life of a He–Ne laser by switching the probe laser every 18 h.     

Surface modification of CIGS film by annealing and its effect on the band structure and photovoltaic properties of CIGS solar cells

The composition of Cu(In,Ga)Se₂ (CIGS) films employed in CIGS solar cells is Cu deficient. There can be point defects, including Cu vacancies, Se vacancies, and metal anti-site defects. The surface composition and defects are not well controlled right after CIGS film fabrication with a three-stage co-evaporation process. This fabrication technique can result in a large variation in cell efficiency. In order to control the CIGS film in a reproducible way, we annealed the CIGS film in air, S, or Se. With this annealing procedure, the Cu content of the CIGS surface was significantly reduced and Ga content was strongly increased. An intrinsic CIGS layer with a lower valence-band maximum and a wider ban gap was formed at the surface. By annealing the CIGS film, the open-circuit voltage and fill factor were significantly improved, which indicates that the surface intrinsic layer acts as a hole-blocking layer so that the surface recombination rate is suppressed. In addition to CIGS film annealing, with subsequent annealing of the completed devices using rapid thermal annealing, the efficiency and reproducibility of CIGS solar cells were markedly improved.     

Nd:YAG laser ablation characteristics of thin CIGS solar cell films

This work reports that the ablation characteristics of thin CuIn_1?x Ga _x Se₂ (CIGS) solar cell film differ significantly with elemental composition and laser pulse energy. From in situ shadowgraphs measured during Nd:YAG laser (1,064 nm) irradiation of CIGS films and crater morphologies, it was found that strong surface evaporation is dominant for low Ga concentration films of which band gap is well below the photon energy. As the band gap of CIGS film becomes close to or over the laser photon energy due to increased Ga content, surface absorption diminishes and at low laser energy, laser heating of the film plays an important role. It is demonstrated that for the CIGS films with Ga/(Ga + In) ratio being approximately over 0.2, the laser irradiation leads to solid phase removal of the film due to thermomechanical fracture at low laser energy but to ablative evaporation at elevated energy.     

对硅薄膜型太阳电池的一些思考

在当前迅速发展的绿色环保能源中,硅太阳电池一直占据着首要地位.然而晶体硅太阳电池(单晶硅和多晶硅)由于价格昂贵和材料短缺已不能满足绿色能源快速发展的需要.因此,薄膜型太阳电池已经被视为今后发展的主要方向.非晶硅薄膜太阳电池虽然在性能上还具有不少缺点,但随着薄膜沉积技术的改进以及膜本身质量的不断提高,它在太阳电池领域中仍占有一席之地.多晶硅薄膜太阳电池集晶体硅与非晶硅电池的优点为一体,也受到人们的关注.然而,后起之秀纳米硅薄膜太阳电池,依靠其本身的优越性以及当前纳米技术的进展,将会成为一个新的亮点.     

Electronic behaviour of thin-film CdTe solar cells

New support is given for one of the controversial models about the electronic consequences of the CdCl₂ treatment of a thin-film CdTe solar cell: the assumption that deep acceptor states are introduced in the bulk of the CdTe layer as a result of the CdCl₂ treatment. A detailed study of the doping profile using capacitance–voltage (C-V) measurements is performed as a first step. The above assumption is numerically simulated with our simulation programme SCAPS. In this way, anomalous features of the C-V measurements are fully explained, and further correspondence between calculated and measurable quantities is found. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

High-temperature processing of crystalline silicon thin-film solar cells

The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the “high-temperature approach” thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 °C. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Effects of Ga contents on properties of CIGS thin films and solar cells fabricated by co-evaporation technique

This study examined the effects of Ga content in the CIGS absorber layer on the properties of the corresponding thin films and solar cells fabricated using a co-evaporation technique. The grain size of CIGS films decreased with increasing Ga content presumably because Ga diffusion during the 2nd stage of the co-evaporation process is more difficult than In diffusion. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. Band gap energy and the net carrier concentration of CIGS films increased with Ga/(In + Ga) ratios. Regarding the solar cell parameters, the short circuit current density (J_SC) decreased linearly with Ga/(In + Ga) ratios due to the lack of absorption in the long-wavelength portion of the spectrum, while the open circuit voltage (V_OC) increase with those. However, V_OC values at high Ga/(In + Ga) regions (>0.35) was far below than those extrapolated from the low Ga contents regions, finally resulting in an optimum Ga/(In + Ga) ratio of 0.28 where the solar cell showed the highest efficiency of 15.56% with V_OC, J_SC and FF of 0.625 V, 35.03 mA cm⁻² and 0.71, respectively.     

Time-resolved electrical and optical measurements in a plasmadisplay panel

We have used a commercially available monochrome plasma display panel (PDP) to study the electrical and optical properties of PDPs. The monochrome PDP was filled with helium at a pressure of 0.5 atm with the visible light emission observed directly from the gas discharge. The PDP is driven by an AC voltage source, operating at ~130 V at 50 kHz. With nanosecond resolution we have measured the current in the panel and the spectrally resolved light emission from the panel as a function of applied voltage. This study of a helium-filled conventional PDP is designed to provide data needed for improved modeling calculations of the plasma discharge. The ultimate goal is to achieve improved designs and efficacies for large-screen color PDP's     

Optimization of the CIGS based thin film solar cells: Numerical simulation and analysis

Chalcopyrite Cu(In,Ga)Se (CIGS) is a very promising material for thin film photovoltaics and offers a number of interesting advantages compared to the bulk silicon devices. CIGS absorbers today have a typical thickness of about 1–2 μm. However, on the way toward mass production, it will be necessary to reduce the thickness even further. This paper indicates a numerical study to optimization of CIGS based thin film solar cells. An optimum value of the thickness of this structure has been calculated and it is shown that by optimizing the thickness of the cell efficiency has been increases and cost of production can be reduces. Numerical optimizations have been done by adjusting parameters such as the combination of band gap and mismatch as well as the specific structure of the cell. It is shown that by optimization of the considered structure, open circuit voltage increases and an improvement of conversion efficiency has been observed in comparison to the conventional CIGS system. Capacitance–voltage characteristics and depletion region width versus applied voltage for optimized cell and typical cell has been calculated which simulation results predict that by reducing cell layers in the optimized cell structure, there is no drastically changes in depletion layer profile versus applied voltage. From the simulation results it was found that by optimization of the considered structure, optimized value of CIGS and transparent conductive oxide thickness are 0.3 μm and 20 nm and also an improvement of conversion efficiency has been observed in comparison to the conventional CIGS which cell efficiency increases from 17.65 % to 20.34%, respectively.     

碲化镉薄膜太阳能电池电学特性参数分析

用inline方式全部近空间升华方法制备n-CdS/p-CdTe取得了～11%的转换效率(AM1.5). 把其中n-CdS层采用磁控溅射方法取得了～10%的转换效率(AM1.5). 基于其电流密度-电压(J-V)曲线和外量子效率曲线, 分析了其拟合关键参数对于电池性能的影响程度, 并从理论分析上把目前器件性能参数与当今前沿性能参数以及其理论值进行比较, 指出了如何提高电池转换效率(η)的方法: 提高开路电压(V_oc)、短路电流(J_sc)和填充因子(FF). 关键词： 碲化镉电池 电流密度-电压曲线 外量子效率曲线 电学特性     

Enhancement of light trapping in thin-film solar cells through Ag

Forward-scattering efficiency (FSE) is first proposed when an Ag nanoparticle serves as the light-trapping structure for thin-film (TF) solar cells because the Ag nanoparticle’s light-trapping efficiency lies on the light-scattering direction of metal nanoparticles.Based on FSE analysis of Ag nanoparticles with radii of 53 and 88 nm,the forward-scattering spectra and light-trapping efficiencies are calculated.The contributions of dipole and quadrupole modes to light-trapping effect are also analyzed quantitatively.When the surface coverage of Ag nanoparticles is 5%,light-trapping efficiencies are 15.5% and 32.3%,respectively,for 53and 88-nm Ag nanoparticles.Results indicate that the plasmon quadrupole mode resonance of Ag nanoparticles could further enhance the light-trapping effect for TF solar cells.     

利用光学薄膜模型研究垂直腔面发射激光器的光学特性

可以把垂直腔面发射激光器看作是多层光学薄膜,应用光学薄膜原理对其光学薄膜的特性进行了研究。计算分析了布拉格反射镜和谐振腔模的反射谱受器件结构变化的影响。通过利用菲涅耳系数矩阵法计算,得到了光在垂直腔面发射激光器器件结构中形成的驻波场分布。结果表明,利用菲涅耳系数矩阵法设计垂直腔面发射激光器的光学薄膜是一种快捷准确的方法。     

Study on optical and electrical properties of gold-doped silicon fabricated by femtosecond laser

Microstructured silicon (Si) materials have been fabricated by femtosecond (fs) laser ablation and have been hyperdoped with gold (Au) impurities. The ablated Si materials showed large and thermostable infrared absorption at 1.1–2.5 μm wavelengths, which was contributed by sub-band absorption and laser-induced defects absorption. The Au–Si alloy was formed after laser irradiation onto the Au-coated Si surface, which was determined by XRD characterization. Using N-type Si substrate, the fabricated Au-doped Si performed lower sheet carrier density due to the self-compensation effect between deep donor and acceptor energy lever of Au in bulk Si material. From Hall measurement, both the p- and n- types of Au-doped Si samples can be obtained by controlling the type of Si substrate.     

A diode laser spectrometer at 634nm and absolute frequency measurements using optical frequency comb

This paper reports that two identical external-cavity-diode-laser (ECDL) based spectrometers are constructed at 634 nm referencing on the hyperfine B-X transition R(80)8-4 of ¹²⁷I₂. The lasers are stabilized on the Doppler-free absorption signals using the third-harmonic detection technique. The instability of the stabilized laser is measured to be 2.8×10^-12 (after 1000 s) by counting the beat note between the two lasers. The absolute optical frequency of the transition is, for the first time, determined to be 472851936189.5 kHz by using an optical frequency comb referenced on the microwave caesium atomic clock. The uncertainty of the measurement is less than 4.9 kHz.     

Degradation of flexible thin-film solar cells due to a mechanical strain

Many variants of thin film technology are nowadays part of the photovoltaic market. The most popular are amorphous silicon, CIS (Copper Indium Selenide)/CIGS (Copper Indium Gallium Selenide)/CIGSS (Copper Indium Gallium Sulphur Selenide), and CdS/CdTe (Cadmium Sulphide/Cadmium-Telluride) cells. All mentioned types allow potentially for a flexible cell structure. Most emitter contacts are currently based on TCOs (Transparent Conductive Oxides), however, wider approach with alternative carbon nanoforms, silver nanolayers and polymer materials, called TCLs (Transparent Conductive Layers) are also in use. Authors decided to investigate influence of mechanical stresses on physical and electrical behaviour of these layers. Consequently, the aim of work is to determine the level and possible mechanisms of flexible a-Si cell parameters degradation due to a deterioration of transparent contact properties.