Rutger Schlatmann

Zinc oxide films grown by galvanic deposition from 99% metals basis zinc nitrate electrolyte

The use of relatively low purity zinc nitrate for electrochemical deposition of compact ZnO films is attractive for large scale production because of the cost saving potential. ZnO films were grown on SnO2:F and magnetron sputtered ZnO:Al templates using a three electrode potentiostatic system in galvanic mode. The electrolyte consisted of a 0.1 M zinc nitrate solution (either 99.998% or 99% purity) and 1 mM aluminium nitrate for extrinsic doping, when required. Moderate deposition rates of up to 0.9 nm s−1 were achieved on ZnO:Al templates with lower rates of up to 0.5 nm s−1 on SnO2:F templates. Observation of SEM images of the films revealed a wall-like morphology whose lateral thickness (parallel to the substrate) reduced as aluminium was added to the system either in the electrolyte or from the substrate. However, pre-deposition activation of the template by applying a negative voltage (approximately −2 V) allowed the growth of compact films even for the low purity electrolyte. The optical band gap energy of intrinsically doped films was lower than that of the Al doped films. The composite electrical conductivity of all the films studied, as inferred from sheet resistance and Hall effect measurements of the ZnO/template stacks was much less than that of the uncoated templates. A strong E2 (high) mode at around 437 cm−1 was visible in the Raman spectra for most films confirming the formation of ZnO. However, both the Raman modes and XRD reflections associated with wurtzite ZnO diminished for the Al doped films indicating a high level of mainly oxygen related defects. Based on these data, further studies are underway to improve the doping efficiency of aluminium, the crystalline structure and thus the conductivity of such films.

Flexible a-Si/μc-Si Tandem Modules in the Helianthos Project

In order to reduce the costs of thin-film silicon solar cell production, a manufacturing concept in which amorphous silicon (a-Si) solar cells are produced in a roll-to-roll manner on a temporary metal substrate (temporary-superstrate process) has been introduced in the Helianthos Project. Later in the process the device is transferred to a permanent plastic substrate on which cells are monolithically series connected, resulting in PV modules. In the Helianthos pilot line, modules with efficiencies above 7% are obtained on 1-foot-wide foil. On 30times30 cm2 modules, initial efficiencies of 5.7% have been reached, while 60 cm2 modules cut from the 35 cm wide foil have reached stabilized efficiencies of 5.8%. Lab modules with the deposition of a-Si in a batch process on roll-to-roll deposited SnO2, have reached stabilized efficiencies of 6.7%. In order to increase efficiency, amorphous/microcrystalline (a-Si/muc-Si) tandem solar cells have been implemented in the temporary-superstrate process to fabricate flexible, lightweight tandem modules with monolithic series integration. A first module with an aperture area efficiency of 8.6% has been presented earlier. At present, an initial aperture area efficiency of 9.4% has been reached on a 60 cm2 module

Controlling the thermal impact of ns laser pulses for the preparation of the P2 interconnect by local phase transformation in CIGSe

The thermal impact of nanosecond laser pulses was beneficially employed and well-controlled for the preparation of the P2 interconnect by local phase transformation (i.e., by drawing conductive lines rather than removing the material) in CIGSe mini-modules, which were demonstrated to outperform their conventionally needle-patterned counterparts. Conductivity and elemental composition of the scribed lines as well as the extent of the heat-affected area were analyzed, quantified and taken into account for achieving optimal CIGSe solar module performances. This approach opens new prospects for significant simplification of the serial interconnection, since the P2 und the P3 can be scribed simultaneously after deposition of both the CIGSe and the TCO layer.

Electrical and structural functionality of CIGSe solar cells patterned with picosecond laser pulses of different wavelengths

Electrical and structural functionality of monolithic interconnection of CIGSe solar cells by P1-P3 picosecond laser ablation has been successfully established and evaluated as being competitive to conventional needle scribing. In all three patterning steps the material is selectively and completely removed yielding structurally well-defined trenches at high scribing speeds up to 1 m/s. P1 and P2 ps laser scribing clearly improves the solar cell efficiencies whereas P3 scribing is still challenging due to shunts resulting from laser-induced alteration of the absorber material.

Investigation of Thin-Film CIGS Degradation under P2 Scribe Laser Illumination

We present a study of the degradation of thin-film CIGS material in the vicinity of P2 phase-transformation type of laser scribes. They comprised optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, photoluminescence and Raman spectroscopy. While the optical and electron microscopy measurements have shown a clear change of the morphological properties of CIGS including removal, melting and dislocation of material from the area scribed by the laser, the X-ray analysis revealed an accumulation of O, Zn, Ga, and Cu elements in the melted phase and an evaporation of more volatile In atoms from the region of visually changed CIGS. At the same time Raman and photoluminescence measurements have shown substantial alteration of semiconductor material properties, i.e. of the crystallinity and the bandgap energy even far beyond of this region. More precisely, the amplitudes of the A1 and B2,E Raman peaks were found to increase continuously with increasing distance from the P2 scribes, reaching the distances of approximately three times the 3uf073 diameter of the beam. A similar tendency was also observed for photoluminescence signals, that additionally revealed a systematic shift of the bandgap energy in CIGS as estimated from the maximum of the emission spectrum. Our results indicate that the phase-transformation scribing generates changes in thin-film CIGS material far beyond the heat affected zone. As such they can help to decide on optimal spacing between P1-P3 scribes and thus reduce a dead area of thin-film CIGS solar cells.

ZnO:Al/a-SiOx front contact for polycrystalline-silicon-on-oxide (POLO) solar cells

Polycrystalline-silicon-on-oxide (POLO) junctions and related contacting schemes have shown their capability to facilitate high efficiencies for solar cells with passivating selective contacts [1-3]. In this work the front contacting of two-side contacted POLO cells with sputtered aluminum-doped zinc oxide (ZnO:Al) has been investigated. Different approaches were followed to obtain good lifetimes in cell precursors and keep high Voc values in finished cells. Degradation in minority carrier lifetime and implied Voc (iVoc) was observed after the ZnO:Al sputtering deposition. In order to recover the passivation, various thermal treatments were applied. The necessity to implement a protecting layer to cap the ZnO:Al/poly-Si structures during the annealing treatment to prevent a fill factor degradation in finished cells was observed. Initially an intrinsic a-Si:H layer was used as a temporary protecting layer. However, during the decapping process, to remove the amorphous layer, lifetime and iVoc are significantly degraded. Therefore a permanent a-SiOx protecting layer was implemented for maintaining good passivation (Voc = 710u2005mV). This layer has the additional benefit of improving the optical (AR) behaviour on finished cells (increasing Jsc by 1.5%). The best cell reached a conversion efficiency of 21.7 %.Polycrystalline-silicon-on-oxide (POLO) junctions and related contacting schemes have shown their capability to facilitate high efficiencies for solar cells with passivating selective contacts [1-3]. In this work the front contacting of two-side contacted POLO cells with sputtered aluminum-doped zinc oxide (ZnO:Al) has been investigated. Different approaches were followed to obtain good lifetimes in cell precursors and keep high Voc values in finished cells. Degradation in minority carrier lifetime and implied Voc (iVoc) was observed after the ZnO:Al sputtering deposition. In order to recover the passivation, various thermal treatments were applied. The necessity to implement a protecting layer to cap the ZnO:Al/poly-Si structures during the annealing treatment to prevent a fill factor degradation in finished cells was observed. Initially an intrinsic a-Si:H layer was used as a temporary protecting layer. However, during the decapping process, to remove the amorphous layer, lifetime and iVoc are significa...

Interface engineering of Cu(In,Ga)Se2 and atomic layer deposited Zn(O,S) heterojunctions

Atomic layer deposition of Zn(O,S) is an attractive dry and Cd-free process for the preparation of buffer layers for chalcopyrite solar modules. As we previously reported, excellent cell and module efficiencies were achieved using absorbers from industrial pilot production. These absorbers were grown using a selenization/sulfurization process. In this contribution we report on the interface engineering required to adapt the process to sulfur-free multi source evaporated absorbers. Different approaches to a local sulfur enrichment at the heterojunction have been studied by using surface analysis (XPS) and scanning transmission electron microscopy. We correlate the microstructure and element distribution at the interface with device properties obtained by electronic characterization. The optimized completely dry process yields cell efficiencies >16% and 30 × 30 cm2 minimodule efficiencies of up to 13.9% on industrial substrates. Any degradation observed in the dry heat stress test is fully reversible after light soaking.

Direct pulsed laser interference texturing for light trapping in a-Si:H/μc-Si:H tandem solar cells

We present results on direct pulsed laser interference texturing for the fabrication of diffraction gratings in ZnO:Al layers. Micro gratings of 20 micron diameter with a groove period of 860 nm have been written using single pulses of a 355 nm picosecond laser using a home-built two-beam interference setup. The groove depth depends on the local laser intensity, and reaches up to 120 nm. At too high pulse energies, the grooves vanish due to surface melting of the ZnO. The fast scanning stage and the high repetition rate laser of a laser scribe system have been used to write grating textures of several cm2 in ZnO:Al films with a surface coverage of about 80%. A typical laser written grating texture in a ZnO:Al film showed a haze value of about 9% at 700nm. The total transmission of the film was not lowered compared to the film before texturing, while the sheet resistance increased moderately by 15%. A-Si:H/μc-Si:H solar cells with laser textured ZnO:Al front contact layers so far reach an efficiency of 10% and current densities of 11.0 mA/cm2, and 11.2 mA/cm2 for top and bottom cell, respectively. This is an increase of 16% for the bottom cell current as compared to reference cells on planar ZnO:Al. The voltage of the laser textured cells is not reduced compared to the reference cell when slightly overlapping laser pulses of reduced pulse energy are applied. This method allows to write textures in ZnO:Al films that e.g. have been deposited with strongly varying deposition conditions, or cannot be texture etched in HCl. The method can be improved further by using 2D periodic patterns and optimizing the groove pitch, and may be applicable also to other solar cell technologies.