Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

Polycrystalline silicon thin-film solar cells on glass

Poly-Si thin-film solar cells on glass feature the potential to reach single-junction efficiencies of 15% or even higher at low costs. In this paper innovative approaches are discussed, which could lead to substantial efficiency improvements and significant cost reductions: (i) preparation of large-grained poly-Si films using the ‘seed layer concept’ targeting at high material quality, (ii) utilization of ZnO:Al-coated glass enabling simple contacting and light-trapping schemes, (iii) utilization of high-rate electron-beam evaporation for the absorber deposition offering a high potential for cost reduction.     

Crystalline silicon on glass (CSG) thin-film solar cell modules

Crystalline silicon on glass (CSG) solar cell technology was developed to address the difficulty that silicon wafer-based technology has in reaching the very low costs required for large-scale photovoltaic applications as well as the 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 1.4 μm thickness, the thinnest active layer of the key thin-film contenders. A rugged, durable silicon thin-film technology has been developed arguably with the lowest likely manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 8–9% energy conversion efficiency range, on the path to 12–13%.     

Crystalline silicon thin-film solar cells—recent results at Fraunhofer ISE

Crystalline silicon thin-film solar cells combine the advantages of the stability and high-efficiency potential of crystalline silicon solar cell technology with the low material utilization of the thin-film solar cell technology. At Fraunhofer ISE the wafer equivalent concept is currently pursued. Within this concept, the active silicon layers are deposited on high-temperature stable substrates. The resulting substrate/layer sandwich can be processed into a solar cell using the same techniques that are used in conventional crystalline silicon wafer solar cell processing, hence the name wafer equivalent. In the present paper we report on how we realized wafer equivalents and explain in detail our development work on processors for both large-area silicon deposition and for zone melting recrystallization. An overview is given on the solar cell results achieved in this area.     

Crystalline Si solar cells based on solar grade silicon materials

A new method named Chemical Physics (CP) method was developed to produce solar grade silicon feedstock at a company in China. In this paper the characteristic of the solar grade silicon made by CP method was analysed. The results show that the purity of solar grade silicon is above 5 N and most of impurities are below 0.0001 wt.%. Crystalline silicon solar cells were prepared using solar grade silicon wafers based on CP method. Average efficiency of the solar cells is about 15.05%, and the highest efficiency is 15.60% under AM1.5 illumination conditions. The light-induced degradation of the solar cells was examined. Degradation by up to 15% of the initial efficiency of the solar cells is detected. The solar cell results and light-induced characteristic show that the solar cells based on CP methods have desired performance and thus have the potential for large scale production.     

Zone melting recrystallization of silicon films for crystalline silicon thin-film solar cells

Coarse-grained silicon films for crystalline silicon thin-film solar cells have been prepared by zone melting recrystallization. A zone melting heater was modified to obtain better temperature homogeneity of the sample and higher reproducibility of the melt process. Various substrate materials of different purity and surface roughness have been tested concerning their suitability for, silicon deposition, zone melting and solar cell process. Solar cell efficiencies up to 10.5% could be achieved on silicon sheets from powder, capped by an intermediate layer. Silicon films on SiAlON ceramics were successfully processed to solar cells by a completely dry solar cell process.     

The application of grating couplers in thin-film silicon solar cells

As an alternative to randomly textured transparent conductive oxides as front contact for thin-film silicon solar cells, the application of periodic light grating couplers was studied. The periods and groove depths of transparent gratings made of zinc oxide were tuned independently from each other and varied between 1 and 4 μm and 100 and 600 nm, respectively. The one-dimensional grating couplers were realized using photolithography. We have analysed the optical properties of the gratings and the properties of amorphous and microcrystalline silicon solar cells incorporating these grating couplers. The achieved results are discussed with respect to the performance of cells deposited on flat and randomly textured substrates.     

Aluminium-induced crystallisation of silicon on glass for thin-film solar cells

Aluminium-induced crystallisation of amorphous silicon is studied for the formation of continuous polycrystalline silicon thin-films on low-temperature glass substrates. It is shown to be a promising alternative to laser crystallisation and solid-phase crystallisation. Silicon grain sizes of larger than 10 μm are achieved at temperatures of around 475°C within annealing times as short as 1 h. The Al doping concentration of the poly-Si films depends on the annealing temperature, as revealed by Hall effect measurements. A poly-Si/Al/glass structure presented here can serve as a seeding layer for the epitaxial growth of polycrystalline silicon thin-film solar cells, or possibly as the base material with the back contact incorporated.     

A highly efficient light-trapping structure for thin-film silicon solar cells

A highly efficient light-trapping structure, consisting of a diffractive grating, a distributed Bragg reflector (DBR) and a metal reflector was proposed. As an example, the proposed light-trapping structure with an indium tin oxide (ITO) diffraction grating, an a-Si:H/ITO DBR and an Ag reflector was optimized by the simulation via rigorous coupled-wave analysis (RCWA) for a 2.0-μm-thick c-Si solar cell with an optimized ITO front antireflection (AR) layer under the air mass 1.5 (AM1.5) solar illumination. The weighted absorptance under the AM1.5 solar spectrum (A_AM1.5) of the solar cell can reach to 69%, if the DBR is composed of 4 pairs of a-Si:H/ITOs. If the number of a-Si:H/ITO pairs is up to 8, a larger A_AM1.5 of 72% can be obtained. In contrast, if the Ag reflector is not adopted, the combination of the optimized ITO diffraction grating and the 8-pair a-Si:H/ITO DBR can only result in an A_AM1.5 of 68%. As the reference, A_AM1.5 = 31% for the solar cell only with the optimized ITO front AR layer. So, the proposed structure can make the sunlight highly trapped in the solar cell. The adoption of the metal reflector is helpful to obtain highly efficient light-trapping effect with less number of DBR pairs, which makes that such light-trapping structure can be fabricated easily.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Multifunctional integration of thin-film silicon solar cells on carbon-fiber-reinforced epoxy composites

Multifunction integration of solar cells in load-bearing structures can enhance overall system performance by reducing parasitic components and material redundancy. The article describes a manufacturing strategy, named the co-curing scheme, to integrate thin-film silicon solar cells on carbon-fiber-reinforced epoxy composites and eliminate parasitic packaging layers. In this scheme, an assembly of a solar cell and a prepreg is cured to form a multifunctional composite in one processing step. The photovoltaic performance of the manufactured structures is then characterized under controlled cyclic mechanical loading. The study finds that the solar cell performance does not degrade under 0.3%-strain cyclic tension loading up to 100 cycles. Significant degradation, however, is observed when the magnitude of cyclic loading is increased to 1% strain. The present study provides an initial set of data to guide and motivate further studies of multifunctional energy harvesting structures.     

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p⁺⁺-type CZ silicon substrates. Growth was performed from In/Ga solutions, and in a purified Ar/4%H₂ forming gas ambient, rather than pure H₂. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm². Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.     

In-situ transmission measurements as process control for thin-film silicon solar cells

In this work, in-situ transmission measurements using plasma as light source are presented for the determination of growth rate and crystallinity during silicon thin-film growth. The intensity of distinct plasma emission lines was measured at the backside of the transparent substrates on which silicon films, ranging from amorphous to microcrystalline, were deposited. Using this configuration, the growth rate of thin-films was determined with high accuracy. In addition, we show that the crystallinity of the films can be monitored in the most critical range (between 40% and 80%) for microcrystalline silicon solar cells by evaluating the intensity ratio of two transmitted wavelengths in-situ. The gradual change in the absorption behaviour of the films during the phase transition is reflected by this ratio of two wavelengths as demonstrated by the good correlation with the crystallinity fraction determined by ex-situ Raman spectroscopy. This approach of in-situ transmission spectroscopy provides an easy-to-implement monitoring and control system for the industrial deposition of thin-film silicon solar cells, as critical material properties can be determined real-time during the deposition process even on rough substrates that are optimized for light trapping in solar cells.     

Microcrystalline silicon thin-film solar cells prepared at low temperature using PECVD

The low-temperature deposition of μc-Si:H has been found to be effective to suppress the formation of oxygen-related donors that cause a reduction in open-circuit voltage (V_oc) due to shunt leakage. We demonstrate the improvement of V_oc by lowering the deposition temperature down to 140°C. A high efficiency of 8.9% was obtained using an Aasahi-U substrate. Furthermore, by optimizing textured structures on ZnO transparent conductive oxide substrates, an efficiency of 9.4% was obtained. In addition, relatively high efficiency of 8.1% was achieved using VHF (60 MHz) plasma at a deposition rate of 12 Å s⁻¹. Thus, this low-temperature deposition technique for μc-Si:H is promising for obtaining both high efficiency and high-rate deposition technique for μc-Si:H solar cells.     

Microcrystalline/micromorph silicon thin-film solar cells prepared by VHF-GD technique

Hydrogenated microcrystalline silicon prepared at low temperatures by the glow discharge technique is examined here with respect to its role as a new thin-film photovoltaic absorber material. XRD and TEM characterisations reveal that microcrystalline silicon is a semiconductor with a very complex morphology. Microcrystalline p–i–n cells with open-circuit voltages of up to 560–580 mV could be prepared. “Micromorph” tandem solar cells show under outdoor conditions higher short-circuit currents due to the enhanced blue spectra of real sun light and therefore higher efficiencies than under AM1.5 solar simulator conditions. Furthermore, a weak air mass dependence of the short-circuit current density could be observed for such micromorph tandem solar cells. By applying the monolithic series connection based on laser patterning a first micromorph mini-module (total area of 23.6 cm²) with 9% cell conversion efficiency could be fabricated.     

Characterization of crystalline silicon grown by plasma-enhanced CVD for thin-film solar cells

High growth-rate Si epitaxy by plasma-enhanced chemical vapor deposition (PECVD) has been investigated for a thin-film solar cell application. A high growth rate of 50 μm/h was obtained at 1050°C with plasma which is 50% larger than that by the conventional CVD without plasma. The electrical properties are almost the same for epitaxial layers with and without plasma. For undoped n-type layers, the Hall mobility and carrier density were about 600 cm²/V s and low 10¹⁵ cm⁻³, respectively. The electron diffusion length in doped p-type layers was about 20 μm. These electrical properties for the layer with plasma, in spite of higher growth rate, are comparable or better than those without plasma.     

Laser structuring of crystalline silicon thin-film solar cells on opaque foreign substrates

Crystalline silicon thin-film solar cells were fabricated on graphite substrates. A laser ablation process was developed for edge isolation of the thin-film cells. The shunt resistance was comparable to otherwise identical cells isolated by plasma etching, while the reproducibility of the laser isolation process was higher. The solar cells were characterized by current-voltage and light beam induced current measurements (LBiC). No interference was detected along the ablated edges. Spatial variations of the minority carrier lifetime are attributed to the grain structure of the seeding layer obtained by the zone melting recrystallization (ZMR).     

Absorbing photonic crystals for silicon thin-film solar cells: Design,fabrication and experimental investigation

In this paper, we present the integration of an absorbing photonic crystal within a thin-film photovoltaic solar cell. Optical simulations performed on a complete solar cell revealed that patterning the hydrogenated amorphous silicon active layer as a 2D photonic crystal membrane enabled to increase its integrated absorption by 28 % between 300 and 720 nm, comparing to a similar but unpatterned stack. In order to fabricate such promising cells, we developed a high throughput process based on holographic lithography and reactive ion etching. The influences of the parameters taking part in those processes on the obtained patterns are discussed. Optical measurements performed on the resulting “photonized” solar cell structures underline the regularity of the 2D pattern and a significant absorption increase above 550 nm, similarly to what is observed on the simulated absorption spectra. Moreover, our patterned cells are found to be robust with regards to the angle of incidence of the light.     

Influence of the substrate geometrical parameters on microcrystalline silicon growth for thin-film solar cells

The effect of substrate morphology on the growth and electrical properties of single-junction microcrystalline silicon cells is investigated. A large variety of V-shaped and U-shaped substrates are characterized by scanning electron microscopy (SEM) and the growth of thin-film microcrystalline silicon (μc-Si:H) devices is observed by cross-sectional transmission electron microscopy (TEM). It is shown that enhanced electrical properties of solar cells are obtained when U-shaped substrates are used and the effect is universal, i.e. independent of the substrate or feature size. U-shaped substrates prevent the formation of two dimensional “cracks”, which are identified as zones of porous material, from propagating throughout the active part of the solar cell. A numerical growth simulation program reproduces satisfactorily these experimental observations. According to these simulations, shadowing effect due to surface morphology and low adatom surface diffusion length are responsible for the formation of cracks in μc-Si:H material.