This work is part of the FLAMINGO PV group which is a joint partnership between TU Delft and HyET Solar, which aims to demonstrate efficiencies above 12% on tandem thin film silicon devices. This thesis focused on the optimization of the aluminium surface morphology as well as the TCO layer at HyET Solar.In order to achieve the desired objectives of the FLAMINGO PV group, it is essential to optimize these layers in order to ensure better device performance for the HyET Solar modules.

Overall, in this thesis, a reliable and efficient method for the characterization of the TCO at HyET Solar has been established which makes it easier to determine whether the TCO exhibits desired electrical and structural properties. An experiment was performed in which different TCO thicknesses were varied in order to determine the optimal thickness for the new aluminium foil at HyET. Overall, thicknesses of 750-790 nm were found to yield desired TCO characteristics.

Furthermore, the thesis also focused in addressing the issues regarding the precipitants which occurred as a result of the treatment process which aims to remove oil/grease and other surface impurities from the aluminium substrate. The focus of this project was to address the precipitants issue which could hinder the performance of the current HyET modules. Overall, ultrasonication appeared to be the method for the removal of the precipitants. Different acids were tried among which ;3CB4 and ;AB3 were found to be the most optimal for removing the precipitants when the samples were dipped in them for atleast 3 minutes. Confirmation tests with different HyET textures need to be implemented however to confirm their presence.

Overall, through this thesis, the electrical and structural properties of the TCO layer at HyET was studied and the optimal thickness of 750-790 nm was selected based on their relatively good structural and electrical performance. Also, the precipitants issue has been tackled with ultrasonication with acids such as ;3CB4 and ;AB3 proving to be the best bet so far for eliminating them.
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Thin-film solar cells are second-generation solar cells and they are gaining more traction than the first-generation c-Si solar cells. This is due to the advantages they have over conventional solar cells. The advantages are that they are lightweight, flexible, cheaper and also have better aesthetics than conventional solar cells. Thin-film cells are being processed on a flexible aluminium substrate at HyET Solar which is a Netherlands based company. Their processing technique involves depositing thin-film silicon solar cells on a temporary flexible aluminium substrate. The cells are laminated on a plastic carrier foil and the Al substrate is etched away. HyET solar produces tandem and amorphous single-junction solar cells. For their tandem solar cells, the bottom layer is crystalline. Hence, single-junction crystalline silicon cells are developed at TU Delft to incorporate them into the tandem modules at HyET solar. The crystalline silicon cells deposited at TU Delft are characterized using SEM and raman. The cells deposited at Delft are processed at HyET solar and characterized. Upon characterization, the crystalline fraction and deposition rates increase with an increase in deposition power in the intrinsic layer. The crystalline fraction and deposition rate of a cell does not change by changing the cell thickness of the i-layer. The cell characteristics also change with a change in the silane flow rate in the i-layer. The optimal deposition power with an optimal crystalline fraction was 40W and the corresponding silane flow rate was 3.3 sccm. The deposition rate for a 40W deposition power was 0.41 nm/s. Cells deposited using these parameters were then processed at HyET solar. The cells developed at HyET Solar was shunted. The cause of the shunts was critically analyzed and depositing an amorphous n-layer with a thickness of 80nm seemed to get rid of the shunts. The JV characteristics of the cell were not significant under illumination but the cells displayed diode behaviour when measured in the dark (dark JV). All the deposition parameters except for the deposition time (thickness of deposition) have been optimized for an nc-Si single-junction cell. Further dedicated research on thin-film silicon cells can be performed to improve the electrical characteristics of the cell. ... Read more

The quest for a cleaner and greener fuel has necessitated the development of renewable energy sources. Sun has been a source of energy for human civilization since agrarian times. With the recent developments in science and technology, Solar energy has evolved as the front runner to power the future energy needs. The plentiful, eco-friendly, and abundant solar energy resource has been evolved from the first generation solar comprising of crystalline silicon based solar panels to second generation photovoltaic cells.

This work is based on the scope of the FlamingoPV project which is the collaboration between TU Delft and HyET Solar, Netherlands, to realize large scale commercialization of second generation thin film flexible solar cells. The second generation solar cells offer flexibility, economic viability, and ease of manufacturing. To reach the desired goals of roll-to-roll production of a-Si:H/nc-Si:H and a-Si:H/nc-Si:H/nc-Si:H with stabilized efficiencies of 13\% and 14\% respectively, various production processes need to be optimized.

The current production line at HyET Solar faces losses in performance due to high open circuit resistance and low parallel (shunt) resistance. Moreover, the lab processing route needs to be optimized to improve the yield of the samples produced. The sputtering process of depositing the back contact is considered as one of the potential causes of low shunt resistances in the lab samples. Hence, the sputtering process is optimized for electrical and optical properties. The effect of sputtering pressure and RF power is understood to develop dense and compact films for the Aluminium doped Zinc Oxide (AZO) films used in the back contact of the solar cell stack. It is found that a lower sputtering pressure provides better optical and electrical properties. Furthermore, optical simulation from GenPro4 suggest an increase in intrinsic layer absorption under specific conditions of AZO deposition.

Furthermore, the monolithic series interconnection process at HyET Solar is a bottleneck in the lab processing route. Additionally, the laser power used for scribing is not optimized for tandem and triple junction solar cells. To ensure faster learning cycles for understanding the quality of the silicon layers deposited a novel way of processing solar cells is developed. This approach helps to distinguish various losses generated in the solar cell and ensures quality control in the production line. Finally, a series of triple junction solar cells deposited on the aluminum substrate is processed without laser scribing to demonstrate the possibilities of the process.

The PECVD tool at HyET Solar needs to be optimized to deposit good quality bottom cell for the micromorph solar cells. The continuous variable thickness series have been used to understand Lambert Beer relation for short circuit current density and deposition thickness. The ASA software is used to simulate case studies for troubleshooting deposition of the bottom cell. The results from the electrical simulation are used to understand the problems with the silane dilution ratios in the previous PECVD runs. Furthermore, the results of the simulation suggest the sensitivity of the process to ensure good quality nanocrystalline silicon deposition. It is observed that the first layer of growth is very critical for the nanocrystalline silicon deposition while after a certain thickness of nanocrystalline silicon inclusion of amorphous material does not affect the properties of the absorber layer.

Overall, the production process has been optimized from the lab scale to the industrial scale. One of the causes of low performance in solar cells has been addressed. The bottleneck in the production of lab samples has been addressed ensuring good quality material for faster learning cycles. The deposition conditions for the growth of nanocrystalline silicon have been analyzed. This can help in troubleshooting the production process and maintaining a high-quality deposition process. ... Read more

The future of our energy supply cannot continue to depend on the use of exhaustible fossil fuels. The successful transition to a society powered by renewable energy sources is one of the major challenges our current generation faces. Renewable systems will only fully establish themselves if high conversion efficiencies can be obtained at a reasonable cost, in order to compete with conventional, carbon-based sources. With this in mind, photovoltaic solar energy can be a promising solution.
This study focuses on the investigation of the potential efficiency of a thin-film tandem solar cell, combining amorphous silicon and microcrystalline silicon for a high performance device. With the use of flexible, light substrates, inexpensive modules can be manufactured in a roll-to-roll configuration to produce high efficiency products for a large scale implementation in society. The semiconductor modelling software ASA was used to optimise the structure of the tandem, identifying the absorber layer thicknesses that maximise the efficiency. Moreover, the impact of possible optical variations in the design of the device on the performance were analysed.
To start, model parameters of single-junction amorphous silicon and microcrystalline silicon cells were calibrated to recreate the performance of such devices based on experimental data. By conducting a sensitivity analysis of the model parameters, this was effectively achieved. The single-junction models were combined in a tandem structure to forecast the operation of the multi-junction device.
It was found that the performance of the tandem was highly sensitive to the choice of refractive indices of the layers at the junction between the two subcells. A high mismatch
between the refractive indices of these layers and the absorber layers of the subcells results in an increased reflection of light to the top cell. This is beneficial for the current production of this subcell, but is too detrimental for the operation of the bottom cell. By bringing the refractive indices closer together, an efficiency of 13.0% was predicted. The inclusion of an encapsulation at the front of the cell boosts this efficiency to 13.7%, at a top/bottom absorber layer thickness combination of 160 nm/0.6 µm.
The possibilities of enhancing the efficiency with an intermediate reflector were examined. The optical conditions for the ideal light distribution with such a reflector were
computed to provide estimations of the potential increase in performance. For the best performing device, an increase of the initial efficiency of 13.0% to 13.2% was foreseen, at slightly thinner top cells and similar bottom cell thicknesses.
All in all, the predicted efficiencies of the amorphous silicon/microcrystalline silicon tandem solar cell evidences the potential of these devices for the production of cheap,
highly efficient modules. The developed analysis of the ideal intermediate reflector can be generalised to other multi-junction solar cells to establish the potential efficiency increase such a layer brings along. This can help in the decision of whether the increase in performance justifies the added manufacturing complexity and costs. ... Read more

Thin film solar cell technologies are gaining more favour over first generation PV technologies such as monocrystalline silicon PV modules. This is due to their special characteristics such as lightweight, flexibility and reduced resource consumption. HyET Solar is a Netherlands based company that makes thin film amorphous silicon modules using a novel processing technique. This construction philosophy involves using a temporary Al substrate to deposit the solar cell layers. In the final step a permanent plastic carrier foil is encapsulated, and the Al foil is removed. HyET Solar has partnered with TU Delft under the FlamingoPV project to produce a-Si:H/nc-Si:H tandem solar cells with efficiencies up to 14%. A starting point for the micromorph cell is a single junction nanocrystalline silicon solar cell on Al foil which has never been attempted at TU Delft. A key concept of cell construction is the use of modulated surface textures (MST) to improve light trapping and growth of high quality nc-Si:H material. MST are optimised on Al foil and used to deposit nc-Si:H layers. The quality of the layers is characterised using various techniques such as Raman spectroscopy, XRD and SEM images. Hydride stretching mode signatures are a non-intrusive way of characterising device grade nc-Si:H layers without undertaking the arduous fabrication process at HyET. Investigations show the substrate texture features have an influence on the material properties. Results on Al foil show both different and similar trends compared to glass substrates. The silane concentration is an important parameter which influences the nature of deposited material. By tuning this parameter, different signatures of the hydride stretching modes are observed. Initial results suggest that just any seed layer is non-conducive to growth of the nc-Si:H material. Seed layers are deposited using silane concentration grading technique. These graded seeds show excellent microstructure homogeneity and reproducibility. The seed layers also enhance the (220) preferential growth orientation of nanocrystalline grains. The graded seed layers are incorporated into p-i-n cells which will serve as good p-i interface, this resulted in high quality material with good thickness homogeneity and high crystalline fractions over 75%. The nc-Si:H solar cells show high shunt resistance values that are orders of magnitude higher than HyET’s modules. This indicates high quality material with limited shunts and cracks which were developed using graded seed layers and MST. When completing the cells at HyET, several challenges were faced due to lack of experience with nc-Si:H solar cells. These include but are not limited to, laser scribing settings and final etching of the Al foil. These issues need to be ironed out at HyET before processing new samples which is out of the control of PVMD. To overcome this bottleneck, an alternate finishing process is researched at TU Delft, despite first results of this alternative approach being unsuccessful, we are confident it can be perfected with more dedicated research. ... Read more

High efficiency silicon heterojunction (SHJ) solar cells have already reached more than 26% (Yoshikawa et al., 2017) efficiency when tunnel oxide passivated contacts solar cells just broke the 26% barrier at the beginning of February 2018 (ISFH, 2018). For such devices, major losses occur at both the front and rear contacts where parasitic recombination can be very high. Carrier-selective contacts use a special design in order to build a barrier that would block one specific charge carrier and let the other one go through. A Hybrid solar cell is a combination between a heterojunction solar cell and a TOPCon device, featuring then carrier-selective contacts at both sides. In this thesis, a p-type TOPCon structure is implemented at the rear when the front contact is made of n-type amorphous silicon (a-Si:H).
Intrinsic amorphous silicon (a-Si:H) used as front passivation layer, is deposited on top of crystalline silicon and requires an interface with as few defects as possible to minimize the parasitic recombination velocity. A new pretreatment method studied in this thesis involves the growth of a silicon oxide (SiO2) on a crystalline silicon substrate, that will allow to get rid of most of the superficial defects after etching and before a-Si:H deposition. Lifetimes of up to 6 ms and saturation current density (J0) as low as 14 fA/cm2 can be reached with a 200 nm thick oxide.
As a-Si:H presents a very low lateral conductivity, a transparent conductive oxide (TCO) is needed to transport the charge carriers towards the front metal contacts. The resistivity of such a material should be as low as possible. While increasing the deposition temperature of Indium Tin Oxide (ITO), it has been possible to decrease the resistivity up to 2.5 ·10-4 Ω.cm at a temperature of 130°C, without reducing the optical properties of such a layer.
Finally, manufacturing defects are often introduced during the fabrication process, leading to some shunt losses (low shunt resistance). As we are fabricating several solar cells per wafer, it is most of the time necessary to cut them to get rid of the shunt before doing the measurements. This sensitive step could be avoided with a better isolation of each solar cell. Patterned ITO and metal have been developed in this thesis, allowing to reduce the shunt power losses in the range of 1-2% without the need to cut the cells.
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To improve the performance of the high efficiency c-Si solar cells within the PVMD group several front metallization methods were investigated. The objective was to decrease the series resistance, which would lead to an increase in FF and thus efficiency. Three different cell structure are examined; i) a poly-poly cell with SiN¬x as an ARC, ii) a hybrid solar cell and iii) a silicon heterojunction (SHJ) device. Last two structures have a conductive TCO layer as a top layer.
For the poly-poly device four sorts of front metallization were examined; Al e-beam evaporation, DC Cu-plating, pulse Cu-plating and Al sputtering. Because Al evaporation was the most commonly used metallization method within the group, this will be the reference cell. The best reference cell had as external parameters a VOC of 689 mV, JSC of 38.3 mA/cm2 and a FF of 73.0%, resulting in a efficiency of 19.3%. With DC Cu-plating fingers with significantly higher aspect ratios (up to 0.28 compared to 0.02 of reference), but unwanted growth of Cu was also detected. Nevertheless, it gave an average increase of 2.3% absolute in FF, which resulted in a higher efficiency of 19.6%. In order to remove the unwanted Cu-growth, pulse Cu-plating was done. When pulse plating was done, no unwanted growth of Cu was detected. The efficiency of the pulse plating device was slightly lower than the other two devices. An efficiency of 18.7%. Sputtering of Al proves to have the same result as Al e-beam evaporation, when the same thickness is used.
Regarding the hybrid solar cells the best results was achieved with Al e-beam evaporation, with a VOC of 704 mV, JSC¬ of 40.7 mA/cm2 and FF of 73.8%. This results in a device with a efficiency of 21.0%. With Cu-plating technique and Ag-paste screen printing, these results were not reached. The Cu-plating technique did show an average increase in FF of 1.8% absolute, due to a lower series resistance. Cu contamination however lowered the VOC and JSC.
For SHJ solar cells the reference solar cell had a maximum efficiency of 17.7%, with an average FF of only 61.0%. Cu-plating on SHJ solar cell had de same contamination problems (low VOC and JSC), but because of the 10.9% absolute increase in FF, it showed higher efficiencies (with a maximum of 18.4%. Ag paste screen printing showed a smaller increase in FF (3.4% absolute).
Optical measurements for the rear reflector were done as well. This resulted in an advised rear contact of 100 nm thermally evaporated Ag and 6 μm of screen printed Ag paste. ... Read more