Analysis of laser scribes at CIGS thin-film solar cells by localized electrical and optical measurements

Laser patterning of thin-film solar cells is essential to perform external serial and integrated monolithic interconnections for module application and has recently received increasing attention. Current investigations show, however, that the efficiency of thin-film Cu(In,Ga)Se₂ (CIGS) modules is reduced due to laser scribing also with ultrashort laser pulses. Hence, to investigate the reasons of the laser-induced material modifications, thin-film CIGS solar cells were laser-scribed with femto- and picosecond laser pulses using different scribing procedures and laser processing parameters. Besides standard electrical current voltage (I–V) measurements, additional electrical and optical analysis were performed such as laser beam-induced current (LBIC), dark lock-in thermography (DLIT), and electroluminescence (EL) measurements to characterize and localize electrical losses due to material removal/modifications at the scribes that effecting the electrical solar cell properties. Both localized as well as distributed shunts were found at laser scribe edges whereas the laser spot intensity distribution affecting the shunt formation. Already laser irradiation below the ablation threshold of the TCO film causes material modification inside the thin film solar cell stack resulting in shunt formation as a result of materials melting near the TCO/CIGS interface that probably induces the damage of the pn-junction.     

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.     

Recent progress in thin-film solar cells

The depletion of energy resources should produce an increase in the range of applications for terrestrial solar cells. The main factor determining the extent of this increase will be cost. In this respect, thin-film solar cells are particularly promising. Recent work on cells based on doped amorphous silicon is reviewed in detail. The properties of this relatively new material are still poorly understood.     

Hydrogen concentration in chalcopyrite thin-film solar cells

A survey of the hydrogen concentration distribution in the various layers of chalcopyrite solar cells is presented. Depth profiles were measured by the nuclear reaction analysis method for the glass substrate, Mo back contact, Cu(In,Ga)S₂ or Cu(In,Ga)Se₂ absorber, CdS buffer, and ZnO window layer. We find that hydrogen is present in all layers in concentrations exceeding the solubility of hydrogen in the corresponding crystalline bulk materials. This indicates that the deposition process and the polycrystallinity of the layers favor the uptake of hydrogen. The measured concentrations range from some 10¹⁸ H/cm³ in the absorber up to some 10²¹ H/cm³ in the CdS buffer layer. Effects of annealing at elevated temperatures are reported.     

Polycrystalline silicon on glass for thin-film solar cells

Although most solar cell modules to date have been based on crystalline or polycrystalline wafers, these may be too material intensive and hence always too expensive to reach the very low costs required for large-scale impact of photovoltaics on the energy scene. Polycrystalline silicon on glass (CSG) solar cell technology was developed to address this difficulty as well as perceived fundamental difficulties with other thin-film technologies. The aim was to combine the advantages of standard silicon wafer-based technology, namely ruggedness, durability, good electronic properties and environmental soundness with the advantages of thin-films, specifically low material use, large monolithic construction and a desirable glass superstrate configuration. The challenge has been to match the different preferred processing temperatures of silicon and glass and to obtain strong solar absorption in notoriously weakly-absorbing silicon of only 1–2 micron thickness. A rugged, durable silicon thin-film technology has been developed with amongst the lowest manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 10–11% energy conversion efficiency range, on the path to 12–13%.     

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     

Research status in thin-film crystalline Si solar cells

Recent research status and future subjects for the development of thin-film crystalline Si solar cells were reviewed. Optimum design of cell configuration and polycrystalline silicon growth by atmospheric pressure chemical vapor deposition (APCVD) were demonstrated. In order to configure high efficiency thin-film poly-Si solar cells, a novel method of quasi-three-dimensional simulation using a cylindrical coordinate system was carried out. Interface recombination velocity at grain boundaries should be less than 10³ cm/s based on the simulation results. Even at a relatively short diffusion length of L_n=50 μm, high efficiency larger than 16% will be expected at a thickness of 5–20 μm. Poly-Si films with columnar structures whose diameter was around 5 μm were successfully deposited on foreign substrates with APCVD at a high growth rate of 0.8 μm/min. Up-to-date status of reported cell performances were discussed in addition to future prospects.     

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.     

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     

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.     

锥形二维光子晶体太阳电池数值模拟

采用时域有限差分方法,模拟研究在本征吸收层引入锥形二维光子晶体(2D PC)后,其结构参数变化对单结微晶硅电池各膜层吸收的影响规律.研究表明,2D PC的纵横比(高度与周期之比)对电池本征吸收具有决定性影响.周期小于1μm时,本征吸收随着纵横比的增大先上升后下降,纵横比为1时达到最大值;周期大于1μm时,本征吸收达到最大值的纵横比小于1,且周期越大,实现本征吸收最大化的纵横比越小.当周期为0.5μm,纵横比为1时,锥形2D PC电池的本征吸收达到峰值,短路电流密度为27.8 mA/cm2;与平面结构相比,短路电流密度提升5.8 mA/cm~2,相对增加27%.该研究突破了以往认为绒面陷光效果主要取决于绒面形貌横向特征尺寸的观点,对实验获取最佳的周期或随机绒面陷光结构具有指导意义.     

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.     

Investigation on antireflection coatings for Al:ZnO in silicon thin-film solar cells

We numerically investigate the role of antireflection (AR) coatings, composed of SiO₂ and/or ZnO, in suppression of interfacial reflections in the presence of the transparent conducting oxide, Al-doped ZnO (AZO). Three structures are simulated: (a) AR coatings in organic light-emitting diodes (OLEDs) and flat panel displays, (b) AR coating located between the glass and the AZO, and (c) same as the case b except the involvement of another AR coating between the AZO and the amorphous silicon layers. The weighted average transmittance according to the AM1.5 solar spectrum, the photovoltaic transmittance (T_pv), suggests that there is no evident difference between the structures a and b, especially when the layer number of AR coatings is less than three. An effective way to improve T_pv is presented in the structure c, where T_pv is increased from 73.54% to 76.32% with a three-layered AR coating located between AZO and a-Si. It implies that the suppression of interfacial reflections, resulting from the considerable mismatch of refractive indices at the interface of AZO and a-Si, would benefit the efficiency improvement of silicon thin-film solar cells.     

Two different effects of Na on Cu2ZnSnS4 thin-film solar cells

Among the many factors that affect the characteristics of Cu₂ZnSnS₄ thin-film solar cells, the effects of Na were examined. Because Na passivates the grain boundaries (GBs) in absorber layers, the GBs increase the minority carrier collection by providing a current pathway through which the minority carriers can reach the buffer and window layer and be collected. However, excess Na remains in the absorber layer and diffuses into the buffer and window layers, where it acts as an impurity and traps the minority carriers, thus decreasing the solar cell characteristics. By applying an annealing process after the deposition of the buffer and window layers, the Na content could be decreased. By improving the solar cell characteristics, a power conversion efficiency of 6.43% was achieved.     

Effect of intrinsic ZnO thickness on the performance of SnS/CdS-based thin-film solar cells

Tin monosulfide (SnS) has promising properties as an absorber material for thin-film solar cells (TFSCs). SnS/CdS-based TFSCs have the following device structure: SLG/Mo/SnS/CdS/i-ZnO/AZO/Al. The optimization of thickness of intrinsic zinc oxide (i-ZnO) for SnS-absorber layers and its impact on SnS/CdS heterojunction TFSCs has been investigated at different thicknesses ranging from 39 nm to 73 nm. With the increase in thickness of i-ZnO from 39 nm to 45 nm, the overall performance improved. The highest PCE of 3.50% (with V_OC of 0.334 V, J_SC of 18.9 mA cm⁻², and FF of 55.5%) was observed for 45 nm-thick i-ZnO layers. Upon a further increase in the i-ZnO thickness to 73 nm, the device performance deteriorated, indicating that the optimum thickness of the i-ZnO is 45 nm. The device performances were analyzed comprehensively for different i-ZnO thicknesses.     

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.