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The hydrogen plasma passivation of thin film polycrystalline silicon (pc-Si) was investigated in conjunction with plasma texturing process to make efficient heterojunction solar cells. The pc-Si layers were first treated using direct and remote hydrogen plasma technologies. The heterojunction solar cells were then fabricated by subsequent deposition of i/n+ a-Si:H. Hydrogenation at high temperature (610 °C) results in enhanced dissolution and diffusion of hydrogen in pc-Si by a factor of about 3 and 4, respectively, in comparison with those at low temperature (420 °C). The hydrogen atoms in the pc-Si layer mainly bond to the silicon dangling bonds and form complexes with dopant atoms. In addition, platelets defects are generated by the hydrogen plasma in the sub-surface region of pc-Si hydrogenated at 420 °C and cause higher saturation current in the space charge region whilst they form in the region deeper than 1 μm at 610 °C. Removal of the platelets using SF6/N2O plasma post-texturing after low-temperature hydrogenation not only enhances the short circuit current but also improves the open circuit voltage and the fill factor simultaneously. Combining plasma pre-texturing with high-temperature hydrogenation, the best 2 µm-thick pc-Si heterojunction solar cell reaches an efficiency of 8.54%.

Thin-film polycrystalline silicon (pc-Si) solar cell technology tries to combine the cost benefit of thin-film technology and the quality potential of crystalline Si technology. For this type of solar cells the challenge is to fabricate high quality coarse-grained (grain size of 1–100 µm) layers on non-silicon substrates in combination with superb light management. One possible material fabrication approach is metal induced crystallization (MIC). Many metals lower the crystallization temperature of a-Si but for photovoltaic (PV) solar cell applications aluminum seems to be the most interesting. Approximately a decade after the suggestion of Nast et al. to use aluminum induced layer exchange for thin-film solar cells, this paper will discuss the aluminum induced crystallization process itself as well as the implementation into workable solar cells. The record aluminum induced crystallization (AIC) solar cell today features an energy conversion efficiency of 8.5%, too low to be competitive with other PV technologies. The main problems and limitations as encountered today will be described. We believe that an energy conversion efficiency of 8.5% is not the limit of AIC solar cells. Different possible solutions to improve the AIC process as well as the resulting solar cells will be discussed together with our vision on the direction and future of AIC solar cell research.

Crystalline Si (c-Si) technology is dominating the photovoltaics market. These modules are nonetheless still relatively expensive, in particular because of the costly silicon wafers, which require large thickness mostly to ease handling. Thin-film technologies, on the other hand, use much less active material, exhibit a much lower production cost per unit area, but achieve an efficiency still limited on module level, which increases the total system costs. A meet-in-the-middle is possible and is the object of this paper. The development of c-Si thin-foil modules is presented: first, the fabrication of the active material on a glass module and then the processing of the Si foils into solar cells, directly on module level. The activity of IMEC in this area is put into perspective with regard to worldwide research results. It appears that great opportunities are offered to this cell concept, although some challenges still need to be tackled before cost-effective and reliable industrial production can be launched. Copyright © 2012 John Wiley & Sons, Ltd.

We investigate the effects of grain size inhomogeneity on the performance of thin film polycrystalline silicon solar cells with columnar grains. We focus on cells with an exponential distribution of the grain diameter, which approximates the grain size inhomogeneity in polycrystalline silicon films made via aluminum induced crystallization (AIC) with low temperature annealing. Through equivalent circuit simulation, we demonstrate that the size inhomogeneity will only play a role when intra grain defects, which is common in AIC cells, can be avoided. We further demonstrate that the inhomogeneity itself is not a dominant factor which limits the efficiency of these cells.

Within this work, we have explored the use of scanning spreading resistance microscopy (SSRM) on advanced solar cell structures. Three main topics, corresponding to three important needs, were targeted. First, we have analyzed the highly doped regions at the frontside of solar cells. The influence of the surface roughness, hindering the use of other techniques (e.g., secondary ion mass spectrometry, SIMS), and the phosphorus diffusion along grains for multicrystalline silicon (mc-Si) have been studied quantitatively as they may affect substantially the electrical properties of solar cells. Secondly, we have explored local backside contacts manufactured using new techniques like laser ablation followed by dopant diffusion. Having a better knowledge of the two-dimensional (2D)-dopant distribution is a subject of growing interest. Finally, we have studied electrical properties of grain-boundary and intragrain defects in polycrystalline silicon (pc-Si) layers as they may play a major role in the electrical performances of the solar cells.

The relatively new “thin-film polycrystalline-silicon (pc-Si) (grain size of 0.1-100 µm) solar cell on foreign substrate” technology aims at low-cost devices with energy conversion efficiencies above 12 %. A very promising technique to obtain thin pc-Si layers is aluminum-induced crystallization (AIC). Solar cell absorber layers can be made by epitaxial thickening of these AIC seed layers. So far, we have reached energy conversion efficiencies of up to 8% with this approach. In contrast to what is expected a performance independent of the grain size is found which is explained by the presence of intragrain defects. In this paper the electrical activity of both the intragrain defects as well as the grain boundaries is investigated with electron beam induced current (EBIC) measurements before and after hydrogen plasma passivation. Metal-insulator-semiconductor contacts were used as collecting junction to eliminate the interference of the junction shape with the EBIC measurement as found when diffused emitters where used. It is shown that both grain boundaries and intragrain defects are electrically active before and after hydrogen plasma passivation. Finally we argue that Leff,mono, the diffusion length inside the grains, is probably much closer to 1µm in our layers than equal to 100µm as often expected in the literature.

Polycrystalline silicon (grain size ~ 0.1–100 μm) solar cells on foreign substrates are a promising approach for the next generation silicon solar cells. Aluminum-induced crystallization AIC in combination with epitaxy is a possible way to obtain such absorber layers. It is believed that Si islands present on the surface of AIC seed layers have a negative effect on the epitaxy. The removal of these islands could therefore lead to an increased absorber layer quality and solar cell performance. In this paper, we present a selective island removal procedure based on the Al layer already present after AIC annealing. By selecting an etchant which removes Si at least as fast as Al (in this paper plasma etching using SF 6 ), the Al layer acts as a perfectly aligned etching mask for the fully developed islands.

The integration of two-dimensional (2D) periodic nanopattern defined by nanoimprint lithography and dry etching into aluminum induced crystallization (AIC) based polycrystalline silicon (Poly-Si) thin film solar cells is investigated experimentally. Compared to the unpatterned cell an increase of 6% in the light absorption has been achieved thanks to the nanopattern which, in turn, increased the short circuit current from 20.6 mA/cm2 to 23.8 mA/cm2. The efficiency, on the other hand, has limitedly increased from 6.4% to 6.7%. We show using the transfer length method (TLM) that the surface topography modification caused by the nanopattern has increased the sheet resistance of the antireflection coating (ARC) layer as well as the contact resistance between the ARC layer and the emitter front contacts. This, in turn, resulted in increased series resistance of the nanopatterned cell which has translated into a decreased fill factor, explaining the limited increase in efficiency., Authors' post-print version

The image border is a pattern free dark area around the die on the photomask serving as transition area between the parts of the mask that is shielded from the exposure light by the Reticle Masking (ReMa) blades and the die. When printing a die at dense spacing on an EUV scanner, the reflection from its image border overlaps with the edges of neighboring dies affecting CD and contrast in this area. This is related to the fact that EUV absorber stack has 1-3% reflectance for actinic light. For a 55nm thick absorber the induced CD drop at the edges is found to be 4-5 nm for 27 nm dense lines. In this work we will show an overview of the absorber reflection impact on CD at the edge of the field across EUV scanner generations, for several imaging nodes and multiple absorber heights. Increasing spacing between dies on the wafer would prevent the unwanted exposure but results in an unacceptable loss of valuable wafer real estate thereby reducing the yield per wafer and is thus not a viable manufacturing solution. In order to mitigate the reflection from the image border one needs to create a so called black border. The most promising approach is removal of the absorber and the underlying multilayer down to the low reflective LTEM substrate by multilayer etching. It was shown in the previous study that the impact on CD was reduced essentially for 27 nm dense lines exposed on ASML NXE:3100. In this work we will continue the study of a multilayer etched black border impact on imaging. In particular, 22 nm lines/spaces imaging on ASML NXE:3300 EUV scanner will be investigated in the areas close to the black border as well as die to die effects. We will look closer into the CD uniformity impact by DUV Out-of-Band light reflected from black border and its mitigation. A possible OPC approach will also be evaluated.

Two dimensional (2D) periodic photonic nanostructures, fabricated by nanoimprint lithography (NIL) and dry etching on the front surface of crystalline silicon (c-Si) layers, are investigated experimentally and theoretically in order to characterize their optical properties and demonstrate their relevance to photovoltaic (PV) applications. Nanoimprint lithography is performed on c-Si wafers and ultra-thin c-Si films with various thicknesses. A comparison with state-ofthe- art front side texturing with an antireflection coating is made. The 2D periodic photonic nanostructures result in an enhanced light absorption in the photoactive material. The results are validated through simulations based on Rigorous Coupled Wave Analysis (RCWA). The nanoimprinted substrates result in a similar absorption compared to the state-ofthe- art random pyramid texturing while consuming less than a micron of photoactive material. In contrast to the random pyramid texturing, the nanopatterning exhibits a robust performance for a wide range of incident angles up to 70°. The light trapping mechanism we propose is based on the combination of a graded index effect and the diffraction of light inside the photoactive layer at high angles.

Three methods for fabricating crystalline-silicon solar cells thinner than 40 µm down to 1 µm are presented, together with the integration of three nanostructuring schemes - texturing, photonic crystals, plasmons - to boost their light absorption.

The case for thin-film polycrystalline silicon (pc-Si) solar cells is strong as it combines the cost benefit of thin-films and the quality potential of crystalline Si technology. The challenge is in making high-quality pc-Si layers on non-Si substrates. By studying layers based on aluminum-induced crystallization (AIC) we previously showed that electrically active intragrain defects are a major limiting factor for thin-film polycrystalline silicon solar cells. This paper investigates the use of a recently proposed novel scanning-laser based mixed-phase solidification (MPS) process which results in large grains with a low intragrain defect density, as well as a narrow grain size distribution and strong surface crystallographic texture [6]. Through subsequent epitaxial growth, absorber layers with the desired doping and thickness can be obtained. Defect etching and TEM measurements demonstrate the drastically decreased intragrain defect density compared to the AIC-based samples. The most efficient solar cell so far has an energy conversion efficiency of 5.4% and open circuit voltage (V oc ) of around 500mV. From the preliminary results obtained, we conclude that mixed phase solidification is an attractive technique to crystallize seed layers for thin-film silicon solar cells.

A polycrystalline silicon (pc-Si) thin film with large grains on a low-cost non-Si substrate is a promising material for thin-film solar cells. One possibility to grow such a pc-Si layer is by aluminum-induced crystallization (AIC) followed by epitaxial thickening. The best cell efficiency we have achieved so far with such an AIC approach is 8%. The main factor that limits the efficiency of our pc-Si solar cells at present is the presence of many intra-grain defects. These intra-grain defects originate within the AIC seed layer. The defect density of the layers can be determined by chemical defect etching. This technique is well suited for our epitaxial layers but relatively hard to execute directly on the seed layers. This paper presents a way to reveal the defects present in thin and highly-aluminum-doped AIC seed layers by using defect etching. We used diluted Schimmel and diluted Wright etching solutions. SEM pictures show the presence of intra-grain defects and grain boundaries in seed layers after defect etching, as verified by EBSD analyses. The SEM images of diluted Wright etched pc-Si seed layer shows that grain boundaries become much better visible than with diluted Schimmel etch.

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could substantially lower the price of photovoltaic electricity. We recently showed that good solar cells can be made from pc-Si obtained by epitaxial thickening using thermal CVD of a seed layer made by aluminium-induced crystallization (AIC) of amorphous silicon. We already reported cells in substrate configuration with energy conversion efficiencies up to 8.0% for layers on ceramic alumina substrates. However, much higher efficiencies (η > 10%) are needed for this type of pc-Si solar cells to become cost-effective. To achieve these higher efficiencies, cells will probably have to be made in a superstrate configuration on transparent substrates and advanced light trapping will need to be applied. In this paper we report on our recent progress with pc-Si solar cells made on transparent glass-ceramic substrates.So far, our best pc-Si solar cells in substrate configuration on glass-ceramics showed an efficiency of 6.4%. By using plasma texturing to lower the front side reflection, we increased the current density of our cells by roughly 1 mA cm-2. The Jsc is much lower for cells on glass-ceramic than for cells on alumina. This is the result of the better diffuse back reflectance of alumina compared to glass. The Voc and fill factor are comparable for cells on both substrates.To make pc-Si solar cells on glass in superstrate configuration, we will use a-Si/c-Si rear junction emitters. As a first test of the feasibility of this approach, we measured the illuminated IV parameters of pc-Si cells made for the substrate configuration in superstrate configuration. In superstrate configuration, the current density of the cells is much lower than in substrate configuration due to the non-optimized cell design for superstrate illumination. The Voc is slightly smaller in superstrate configuration due to the lower current density.These results indicate that the glass-ceramic substrates are fully compatible to our poly-Si solar cell process. Furthermore, rear-junction poly-Si cells in superstrate configuration should lead to good cell results once the absorber layer thickness is optimized to the diffusion length of the material and light trapping features adapted to the superstrate configuration are applied.

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could lower the price of photovoltaic electricity substantially. At the MRS conference in 2006, we presented a pc-Si solar cell with an efficiency of 5.9% that had an absorber layer made by aluminum-induced crystallization (AIC) of amorphous silicon followed by high-temperature epitaxial thickening. The efficiency of this cell was mainly limited by the current density. To obtain higher efficiencies, we therefore need to implement an effective light trapping scheme in our pc-Si solar cell process. In this work, we describe how we recently enhanced the current density and efficiency of our cells. We achieved a cell efficiency of 8.0% for pc-Si cells in substrate configuration. Our cell process is based on pc-Si layers made by AIC and thermal CVD on smoothened alumina substrates. The cells are in substrate configuration with deposited a-Si heterojunction emitters and interdigitated top contacts. The front surface of the cells is plasma textured which leads to an increase in current density. The current density is further enhanced by minimizing the back surface field thickness of the cells to reduce the light loss in this layer. Our present pc-Si solar cell efficiency together with the fast progression that we have made over the last few years indicate the large potential of pc-Si solar cells based on the AIC seed layer approach.

To reduce the harmful influence of grain boundaries in polycrystalline Si layers we make absorber layers on foreign substrates with columnar grains with a grain width larger than the grain thickness. Such layers with a grain size in the range of ~1-100 µm can be obtained by aluminum-induced crystallization and epitaxy. Until now however, the open-circuit voltage of solar cells made from such layers was quasi-independent of the grain size. To understand this fact, defect etching and Electron Backscattered diffraction (EBSD) measurements were performed to investigate the crystallographic defects. A very large density (~ 109 cm-2) of intra-grain defects (IGD) was found. Room temperature Electron Beam Induced Current (EBIC) measurements were done to localize and investigate the electrically active defects. The intra-grain defects found with defect etching showed a strong recombination activity. These results indicate that the unexpected quasi-independence on the grain size of the open-circuit voltage of our pc-Si solar cells is due to the presence of numerous electrically active intra-grain defects.

Thin-film polysilicon solar cells are a promising low-cost alternative for bulk silicon solar cells. Due to their reduced material thickness, these solar cells are less dependent on the silicon feedstock price. Until now these devices showed a worse performance compared to bulk Si solar cells due to the small grain size and the high recombination velocity at the grain boundaries. A better understanding of hydrogen passivation is therefore of crucial importance to improve the efficiency of polysilicon solar cells. In this work we characterized fine-grained polysilicon layers with a grain size of only 0.2 μm before and after passivation. Plasma hydrogenation led to a higher hydrogen concentration in the first micron of the layer than nitride passivation. The highest efficiency of 5.0 % was reached when nitride passivation was followed by plasma passivation.

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on foreign substrates could lower the price of photovoltaic electricity. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening at high temperatures seems a good way to obtain efficient pc-Si solar cells. Due to its transparency and low cost, glass is well suited as substrate for pc-Si cells. However, most glass substrates do not withstand temperatures around 1000°C that are needed for high-temperature epitaxial growth. In this paper we investigate the use of experimental transparent glass-ceramics with high strain-point temperatures as substrates for pc-Si solar cells. AIC seed layers made on these substrates showed in-plane grain sizes up to 16 μm. Columnar growth was observed on these seed layers during high-temperature epitaxy. First pc-Si solar cells made on glass-ceramic substrates in substrate configuration showed efficiencies up to 4.5%, fill factors up to 75% and open-circuit voltage (Voc) values up to 536 mV. This is the highest Voc reported for pc-Si solar cells on glass and the best cell efficiency reported for cells made by AIC on glass.

A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) layers. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening is an effective way to obtain large-grained pc-Si layers with excellent properties for solar cells. To obtain efficient solar cells, the electronic quality of the pc-Si material obtained by AIC has to be optimized and the cell design has to be adapted to the material. In this paper, we report on pc-Si solar cells made by AIC in combination with thermal CVD on ceramic alumina substrates. We made pc-Si solar cells on alumina substrates that showed Voc values up to 533 mV and efficiencies up to 5.9%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of silicon was used. We demonstrate that the quality of the pc-Si material can be improved drastically by reducing the substrate roughness using spin-on oxides. We further show that a-Si/c-Si heterojunctions lead to much higher Voc values than diffused homojunctions. A cell concept that incorporates spin-on oxides and heterojunction emitters is therefore best suited to obtain efficient pc-Si solar cells on alumina substrates.

Polycrystalline silicon (pc-Si) thin-films with a grain size in the range of 0.1–100 μm grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization (AIC) of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current (EBIC) measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason fo...

The price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. A promising way to make pc-Si layers for solar cells is to use aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal CVD. To obtain efficient pc-Si solar cells, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this paper, we present our best pc-Si solar cells so far and we discuss some key features of our solar cell process. We also discuss the structural and electronic quality of our pc-Si layers. We obtained efficiencies of up to 8% on polycrystalline-silicon solar cells made by AIC in combination with thermal CVD on alumina substrates. By replacing conventional diffused emitters by a-Si/c-Si heterojunction emitters, much higher V oc values were obtained. The use of plasma texturing led to higher current densities due to an enhanced coupling of light into the pc-Si layers. The efficiency of our pc-Si solar cells is limited by the presence of a high intragrain defect density in our layers. A detailed TEM study revealed that most of these defects are already present in the AIC seed layers prior to epitaxial thickening. To improve the material quality of our pc-Si layers, we will therefore need to improve the seed layer quality.