Jochen Löffler

Depth selective laser scribing for thin-film silicon solar cells on flexible substrates

Roll-to-roll production facilitates flexible PV modules and a significant decrease of production cost for thin-film silicon solar cells. However, no standard processes for monolithic series interconnection on opaque foil substrates are readily available. In this contribution, we present different approaches to achieve depth selective laser scribing of thin-film silicon solar cells on electrically insulated steel foil. Besides paving the way to series interconnection on opaque flexible substrates, this concept also allows to significantly reduce the number of process steps during module manufacturing. The required depth selective scribes can be obtained with all three employed Diode Pumped Solid State-lasers (wavelengths of 355 nm, 532 nm, and 1064 nm). Actually, several laser parameter combinations (wavelength, pulse energy, spot overlap, single/multi-pass) have been found to make scribes that meet the requirements of the device architecture. Currently, the electrical validation of the observed scribes is in progress. In a next step, fully series interconnected modules will be manufactured following the presented device and processing concepts.

PECVD deposition of a-Si:H and μc-Si:H using a linear RF source

ECN is aiming at the development of fabrication technology for roll-to-roll production lines for high efficiency thin film amorphous and microcrystalline silicon solar cells. The intrinsic layer will be deposited with high deposition rate microwave plasma enhanced chemical vapour deposition. This plasma source, however, is not suitable for the deposition of doped layers. Therefore, we use a novel, linear RF source for the deposition of doped layers. In this RF source, the substrate is electrically disconnected from the RF network. As a result, the ion bombardment onto the substrate is very mild, with ion energies typically < 10 eV. The low ion energies make this source very attractive for surface treatments like passivation of crystalline silicon wafers by thin SiNx or a-Si layers. In this contribution, we will introduce the novel RF source and discuss the deposition of device quality amorphous and microcrystalline intrinsic Si layers with the novel linear RF source.

Embossing of light trapping patterns in sol-gel coatings for thin film silicon solar cells

For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest as much as possible of the available sunlight. Typically, one uses randomly textured front TCOs to scatter the light diffusively in pin-cells on glass. Here, we investigate methods to texture the back contact with both random and periodic textures for use in nip-cells on opaque foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by embossing. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of stamps with various random and periodic patterns was investigated, and, using scanning electron microscopy, replicas were found to compare well with the originals. Masters with U-grooves of various submicrometer widths have been used to investigate the optimum dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. Diffraction gratings show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell.

Nanoimprint lithography of light trapping patterns in sol-gel coatings for thin film silicon solar cells

For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest all available sunlight. Typically, randomly textured TCO front layers are used to scatter the light diffusively in p-i-n cells on glass. Here, we investigate methods to texture the back contact with both random and periodic textures, for use in n-i-p cells on opaque foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by nanoimprint. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of masters with various random and periodic patterns was tested, and, using scanning electron microscopy, replicas were found to compare well with the originals. Masters with U-grooves of various sub micrometer widths have been used to investigate the optimal dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. Diffraction gratings show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell.

Laser scribing and printing processes for thin-film PV devices

Generic processes such as depth-selective laser scribing and the printing of insulating and conductive tracks that can be cured at low temperature, are a key requirement to enable cost efficient production of advanced thin-film PV concepts. In contrast to TCO glass based superstrate device concepts, no standard laser scribing processes and manufacturing tools to achieve fully monolithic series interconnection are presently available for so-called substrate type device concepts that are built on — often opaque and/orflexible — substrates. Three key features of such advanced interconnection processes are presented here: laser scribing, printing and electrical modeling. As vehicle to develop and demonstrate these interconnection processes, a novel roll-to-roll concept for fabrication of thin-film silicon solar cells on steel foil is chosen. We show that laser scribing of amorphous silicon solar cells with a 1064 nm ps laser leads to a minor loss in efficiency. The screen printing process of low-temperature curing Ag paste is also proven not to degrade the device performance, and by modeling the module interconnection, we calculated losses below 10% when going from individual cells to modules. Efforts are underway to realize large area solar cells with printed current collecting grids, and finally a fully series interconnected module by combining laser scribing and printing as cost-effective processing operation.

Cost reduction by using micro-fingers in thin film silicon modules

A finite element electrical model is described that can be used to calculate the performance of monolithic thin film photovoltaic modules. The model is suitable for all type of thin film modules, like e.g. p-i-n a-Si:H, CIGS and polymer based modules and it includes losses due to interconnection. Using this model a parameter study is performed for a-Si:H cells with the aim to reduce metal consumption in the cell and interconnection. It is shown that a reduction in metal consumption by a factor 1.3 can be achieved with only marginal loss in performance if short cell are used with very short fingers.

Dynamically deposited thin-film silicon solar cells on imprinted foil using linear PECVD sources

ECN is developing nip silicon solar cells based on amorphous and microcrystalline thin films on foil. To optimise light trapping we create nanoscale texturisation of the back reflector of the cells by imprinting a UV curable coating layer on the foil. This contribution focuses on i) the suitability of imprinted UV curable coating layers on foil as substrate for thin film Si solar cells; ii) inline PECVD of silicon layers, using linear plasma sources. We show that amorphous silicon solar cells deposited on foil with random texture can achieve the same good light trapping as cells on Asahi U-type glass (J sc ∼ 15–16 mA/cm2). Furthermore, we show that a-Si nip cells on foil, processed in dynamic mode in an industrial pilot roll-to-roll system for 30 cm wide foils, can achieve efficiencies (of over 7%) which are only slightly less than for cells made in a UHV lab-scale cluster tool. Future work will focus on developing and implementing optimised periodic nanotextures for μc-Si and micromorph tandem cells and the further development of cells in order to achieve efficiencies of more than 10% at high deposition rates.

Back end monolithic series interconnection of a thin film pv module by laser scribing and screen printing

The combination of back end laser scribing and screen printing introduces a potentially cost-saving decoupling of (vacuum) deposition steps and interconnection steps in the production flow of (roll to roll) thin film PV solar cells. This break makes it possible to optimize the back end series interconnection independent of the front end coating and deposition processes. To facilitate this optimization of depth-selective laser scribing with respect to the subsequent print processes, a generic laser platform has been built. This platform enables the integrated use of multiple laser sources at three different wavelengths (1030u2005nm, 515u2005nm and 343u2005nm). The use of screen printing for the conductive and isolating lines makes an integral approach of laser scribing and printing necessary. The flexibility of the laser is used to compensate for the slower response of the screen printing facility. These coupling concepts are essential for a successful implementation of depth-selective scribing and printed interconnection as a generic industrial concept for interconnection in thin film PV module devices. The integrated laser platform is a next step in our development towards the integrated module production concept, using the laser and print production processes we have developed in the past.The combination of back end laser scribing and screen printing introduces a potentially cost-saving decoupling of (vacuum) deposition steps and interconnection steps in the production flow of (roll to roll) thin film PV solar cells. This break makes it possible to optimize the back end series interconnection independent of the front end coating and deposition processes. To facilitate this optimization of depth-selective laser scribing with respect to the subsequent print processes, a generic laser platform has been built. This platform enables the integrated use of multiple laser sources at three different wavelengths (1030u2005nm, 515u2005nm and 343u2005nm). The use of screen printing for the conductive and isolating lines makes an integral approach of laser scribing and printing necessary. The flexibility of the laser is used to compensate for the slower response of the screen printing facility. These coupling concepts are essential for a successful implementation of depth-selective scribing and printed interconnec...

Depth selective laser scribing of thin-film silicon solar cells on foil

Roll-to-roll production of thin-film photovoltaic (PV) solar cells and modules is expected to decrease substantially the manufacturing costs, and thus enable a breakthrough in the price of solar electricity per kWh. The roll-to-roll concept implies that the fabrication of these PV devices on flexible substrates is significantly different from the production of the glass based devices. This is especially valid for the monolithic series interconnection of thin-film silicon solar cells into modules, where the laser scribing step of thin-films on opaque foils requires depth selectivity. As adjusting the laser wavelength to the absorption profiles of the involved layers is not sufficient, we are investigating the ablation mechanisms leading to removal of the different layers of thin-film silicon solar cells.In this paper, first results of laser scribes into working solar cells are reported using 1064u2005nm nanosecond pulsed lasers. Despite the apparent depth selectivity reported earlier for this type of laser, a reduced diode quality and/or shunting of the solar cells is observed. This is probably due to recast at the wall of the laser scribe either by molten material from the back contact or by re-crystallized silicon. Consequently, a broader wavelength/pulse duration matrix has been experimented to understand better the ablation processes of the individual layers, aiming at a reduction of damage of the PV devices due to the laser process.Finally, laser scribes with reduced damage on working solar cells have been achieved with a near IR picosecond laser, leading to lower losses of the solar cell efficiency than with the 1064u2005nm nanosecond laser.Roll-to-roll production of thin-film photovoltaic (PV) solar cells and modules is expected to decrease substantially the manufacturing costs, and thus enable a breakthrough in the price of solar electricity per kWh. The roll-to-roll concept implies that the fabrication of these PV devices on flexible substrates is significantly different from the production of the glass based devices. This is especially valid for the monolithic series interconnection of thin-film silicon solar cells into modules, where the laser scribing step of thin-films on opaque foils requires depth selectivity. As adjusting the laser wavelength to the absorption profiles of the involved layers is not sufficient, we are investigating the ablation mechanisms leading to removal of the different layers of thin-film silicon solar cells.In this paper, first results of laser scribes into working solar cells are reported using 1064u2005nm nanosecond pulsed lasers. Despite the apparent depth selectivity reported earlier for this type of laser, a ...

Depth selective laser scribing of thin films for roll-to-roll production of silicon solar cells

Significant cost reductions for thin-film silicon solar cells are expected from a transition to roll-to-roll production. However, in contrast to state-of-the-art batch-type fabrication of glass based products, for thin-film photovoltaic modules on foil substrates no standard processes for one essential production step – the monolithic series interconnection-are presently available. Laser scribing is the preferred technology here as it allows fast, non-contact, local and precise removal of the thin films. ECN is currently developing the technology and setting up a pilot line for the production of tandem solar cells based on microcrystalline and amorphous silicon on steel foil substrates [1]. To allow monolithic series interconnection on these electrically conducting substrates, an insulating layer is required. In the presented module concept, first all layers of the solar cell are deposited, and after that series interconnection can be realized in one process step by three depth selective laser scribes which are then filled by insulating and electrically conductive inks. In this contribution, we present the latest status of our process development on nanosecond pulsed lasers with three different wavelengths to achieve depth selective scribing of these flexible thin-film silicon solar cells. To gain more insight into the selectivity of the process, the ablation thresholds of the different layers have been determined. Then, continuous lines were scribed by systematically varying the pulse energy and spot overlap. The required depth selective scribes could be obtained with all employed lasers (wavelengths of 355 nm, 532 nm, and 1064 nm).Significant cost reductions for thin-film silicon solar cells are expected from a transition to roll-to-roll production. However, in contrast to state-of-the-art batch-type fabrication of glass based products, for thin-film photovoltaic modules on foil substrates no standard processes for one essential production step – the monolithic series interconnection-are presently available. Laser scribing is the preferred technology here as it allows fast, non-contact, local and precise removal of the thin films. ECN is currently developing the technology and setting up a pilot line for the production of tandem solar cells based on microcrystalline and amorphous silicon on steel foil substrates [1]. To allow monolithic series interconnection on these electrically conducting substrates, an insulating layer is required. In the presented module concept, first all layers of the solar cell are deposited, and after that series interconnection can be realized in one process step by three depth selective laser scribes whi...