Periodic Reporting for period 1 - SolArray (Analysis of the performance of innovative nanowire arrays with offset nanoholes Si solar cells for enhanced performance)
Berichtszeitraum: 2021-06-01 bis 2023-05-31
Nowadays, technological devices has become an essential part of all our lives. Therefore, a large amount of energy has to be generated to enable this level of energy consumption. Although, in recent years there has been an increasing interest in renewable energy generation driven by government policies and the social awareness, but, the energy we consume today still is mostly generated from fossil fuels. The key question is how to maintain our live style without affecting environment further. The answer is, move towards a greener energy generation and meet the net-zero target that has been established. The renewable energy generation technologies available can all contribute towards a comprehensive strategy. Photovoltaic solar cells (PVSC) are already playing a key role towards cleaner energy generation. The Earth receives enough sunlight in a single day (free to access for everyone) to satisfy the current level of energy demand for 27 years and the current level of the technology involved is offering very promising performance.
The solar cell market is dominated by crystalline Silicon (c-Si) PVSCs which have experienced a steady improvement to achieve 26.5% Power Conversion Efficiency (PCE) for a single junction, almost reaching it theoretical efficiency limit set by Shockley-Queisser. However, as 180-300 m thick costly c-Si layer is needed for near complete light absorption, which accounts to 40% of the cost, so the payback time although it has been significantly reduced over the last decade, but it is rather long. To reduce the cost, 2nd generation PVSC explored 2-3 m thick thin-film but efficiency has been poor. Current third generation solar cell research, dominated by thin organic perovskite based PVSC, is showing a good efficiency, however, its stability issue and use of toxic Lead (Pb) need to be addressed before commercialization.
As the Si PVSC technology is very mature, only reduction in its price will be the key to encourage its wider and faster adoption. One of the techniques used to improve the efficiency, is texturing the surface of a thin Si PVSC with an array of geometrical patterns such as pyramids, inverted pyramids, NanoWires (NW) and nanoholes. The purpose of introducing a texturing pattern is to effectively increase the number of times sunlight is incident on the solar cell due to multiple reflections between the elements of the pattern which leads to an increase in the absorption. It has been reported that the Lambertian limit of light trapping can be achieved by an ideal rough surface where incident light is randomly scattered increasing the optical path length. The optical path can be increased by 50 times (for Si NW), which can allow to reduce silicon wafer thickness from 200-300 µm to just 4-6 µm, nearly 2 orders of magnitude reduction, resulting a significant potential cost reduction.
Nevertheless, with so many different patterns available, it is important to determine which pattern offers better performance. This is one of the questions that this project has been able to answer. We have also identified how the often-forgotten short NWs can be used to mimic the performance of Anti Reflection Coating layers. Then, following this logic, we have successfully mimic 2 and 3 layers ARC using NWs.
The solar cell market is dominated by crystalline Silicon (c-Si) PVSCs which have experienced a steady improvement to achieve 26.5% Power Conversion Efficiency (PCE) for a single junction, almost reaching it theoretical efficiency limit set by Shockley-Queisser. However, as 180-300 m thick costly c-Si layer is needed for near complete light absorption, which accounts to 40% of the cost, so the payback time although it has been significantly reduced over the last decade, but it is rather long. To reduce the cost, 2nd generation PVSC explored 2-3 m thick thin-film but efficiency has been poor. Current third generation solar cell research, dominated by thin organic perovskite based PVSC, is showing a good efficiency, however, its stability issue and use of toxic Lead (Pb) need to be addressed before commercialization.
As the Si PVSC technology is very mature, only reduction in its price will be the key to encourage its wider and faster adoption. One of the techniques used to improve the efficiency, is texturing the surface of a thin Si PVSC with an array of geometrical patterns such as pyramids, inverted pyramids, NanoWires (NW) and nanoholes. The purpose of introducing a texturing pattern is to effectively increase the number of times sunlight is incident on the solar cell due to multiple reflections between the elements of the pattern which leads to an increase in the absorption. It has been reported that the Lambertian limit of light trapping can be achieved by an ideal rough surface where incident light is randomly scattered increasing the optical path length. The optical path can be increased by 50 times (for Si NW), which can allow to reduce silicon wafer thickness from 200-300 µm to just 4-6 µm, nearly 2 orders of magnitude reduction, resulting a significant potential cost reduction.
Nevertheless, with so many different patterns available, it is important to determine which pattern offers better performance. This is one of the questions that this project has been able to answer. We have also identified how the often-forgotten short NWs can be used to mimic the performance of Anti Reflection Coating layers. Then, following this logic, we have successfully mimic 2 and 3 layers ARC using NWs.
We have established that rather than the shape of the NW, we should focus on the equivalent surface area of the pattern. We found that the absorption is nearly shape independent for equivalent surface area of the pattern. The surface area of the pattern can then be related to the equivalent refractive index of a layer using Rytov’s equation. We have found out that the equivalent refractive index of the optimized texturing pattern layer very closely matches the refractive index for the ideal Anti Reflection Coating (ARC) layer for a air-silicon interface. Furthermore, we have confirmed that there is a range of often-ignored short NWs that mimic the performance of a single layer ARC and we have explored it further. We have analysed the performance of a textured design concept where 2 or 3 NWs (of different dimeter and height) are stacked on top of each other, mimicking 2 and 3 layers ARC. By taking this innovative approach, it is possible to achieve very high total absorption (TABS) (equivalent to that of NWs higher than 4 µm) with an ultra-thin pattern (220 nm thick for the 2NWs ARC design, and 268 nm for the 3NWs ARC design). We also investigated whether the shifting the location of the NWs from the centre would make an impact on the optical performance of the design.
The excellent optical performances of the 2NWs and 3NWs ARC design concepts have been confirmed to also offer excellent electrical performance. We considered the generation rate of the optimal cases from the optical simulations and various doping profiles such as axial and radial pn junctions as well as back reflector and Back-Surface Field layer to enhance the performance achieving 16.8% PCE for the 2NWS ARC design and 17.6% PCE for the 3NWs ARC design whilst considering a silicon wafer of only 4 µm thickness. These are remarkable figures that become even more relevant when studying the impact that surface recombination has on these designs. In traditional NWs surface recombination tends to be high due to the large surface-to-volume ratio. We have carried out a comprehensive analysis where we compare the PCE for traditional values of surface recombination velocity with the PCE values when the surface is perfectly passivated. We have demonstrated that our designs are much less impacted by surface recombination compared with much higher NWs.
The excellent optical performances of the 2NWs and 3NWs ARC design concepts have been confirmed to also offer excellent electrical performance. We considered the generation rate of the optimal cases from the optical simulations and various doping profiles such as axial and radial pn junctions as well as back reflector and Back-Surface Field layer to enhance the performance achieving 16.8% PCE for the 2NWS ARC design and 17.6% PCE for the 3NWs ARC design whilst considering a silicon wafer of only 4 µm thickness. These are remarkable figures that become even more relevant when studying the impact that surface recombination has on these designs. In traditional NWs surface recombination tends to be high due to the large surface-to-volume ratio. We have carried out a comprehensive analysis where we compare the PCE for traditional values of surface recombination velocity with the PCE values when the surface is perfectly passivated. We have demonstrated that our designs are much less impacted by surface recombination compared with much higher NWs.
The Fellow has focused on the 3I’s (i.e. interdisciplinary, intersectoral and international level).
The interdisciplinary and intersectoral nature of the research has been very present during the daily implementation of the project by combining optical and electrical simulations together with the machine learning and data analysis when applicable. Further to this, the Fellow has expanded his knowledge further in photonics, optical and electrical numerical modelling, nanotechnology, device engineering and fabrication processes. Moreover, the researcher has participated in many different workshops at City, IOP and LIMS and in conferences where he has been able to discuss with world leaders in many different scientific topics and even discuss about collaborations.
In terms of internationalisation, the Fellow has been very active by participating in European level out reach activities, but also by presenting his work in internationally recognised competitions such as STEM for Britain and by running for the IOP elections to become a committee member of the Energy group which he won and this year the Fellow has been selected as the secretary of the group. The Fellow this year has also run for the election to become a trustee of the IOP but unfortunately, did not receive sufficient votes.
The societal impact of the project is very significant, the Fellow has engaged in numerous events with primary, secondary and high school students where he has been able to explain about the work carried out during the implementation of the project. The feedback from the audience has also been beneficial for the researcher because the students were able to ask some very interesting questions. Further to this, the Fellow has gathered a lot of experience giving talks and has significantly improved the presentation skills.
The interdisciplinary and intersectoral nature of the research has been very present during the daily implementation of the project by combining optical and electrical simulations together with the machine learning and data analysis when applicable. Further to this, the Fellow has expanded his knowledge further in photonics, optical and electrical numerical modelling, nanotechnology, device engineering and fabrication processes. Moreover, the researcher has participated in many different workshops at City, IOP and LIMS and in conferences where he has been able to discuss with world leaders in many different scientific topics and even discuss about collaborations.
In terms of internationalisation, the Fellow has been very active by participating in European level out reach activities, but also by presenting his work in internationally recognised competitions such as STEM for Britain and by running for the IOP elections to become a committee member of the Energy group which he won and this year the Fellow has been selected as the secretary of the group. The Fellow this year has also run for the election to become a trustee of the IOP but unfortunately, did not receive sufficient votes.
The societal impact of the project is very significant, the Fellow has engaged in numerous events with primary, secondary and high school students where he has been able to explain about the work carried out during the implementation of the project. The feedback from the audience has also been beneficial for the researcher because the students were able to ask some very interesting questions. Further to this, the Fellow has gathered a lot of experience giving talks and has significantly improved the presentation skills.