Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics
<p>Over the past decade, metal halide perovskites have significantly stirred up the field of photovoltaic research. Despite the remarkable increase in efficiency, there is still a lack of comprehensive understanding regarding the device physics and stability of perovskite solar cells. This is...
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| Other Authors: | |
| Format: | Thesis |
| Language: | English |
| Published: |
2023
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| _version_ | 1811140274739478528 |
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| author | Thiesbrummel, J |
| author2 | Snaith, H |
| author_facet | Snaith, H Thiesbrummel, J |
| author_sort | Thiesbrummel, J |
| collection | OXFORD |
| description | <p>Over the past decade, metal halide perovskites have significantly stirred up the field of photovoltaic research. Despite the remarkable increase in efficiency, there is still a lack of comprehensive understanding regarding the device physics and stability of perovskite solar cells. This is problematic because to unlock the full potential of perovskite solar cells and push them towards commercialisation, it is imperative that researchers understand the physical underlying mechanisms that determine whether a solar cell can reliably operate at high efficiency levels over long timescales. This thesis, therefore, concentrates on gaining a better understanding of the device physics and stability of perovskite solar cells, with specific emphasis on two aspects: investigating the effect of mobile ions on the performance and stability of these devices, and understanding performance losses in perovskite-based tandems.</p>
<p>Many perovskite solar cells still suffer from current losses that cannot be attributed to suboptimal device optics and light harvesting. The origin of these losses is investigated, using a combination of voltage-dependent photoluminescence time series and various charge extraction measurements. It is demonstrated that the perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted photoluminescence from the device rises on the exact same timescales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping-induced electronic charges but by the movement of mobile ions towards the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. These findings are generalised to a variety of metal-halide perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism.</p>
<p>Hereafter, the impact of such ion-induced loss mechanisms on the device performance upon ageing of perovskite solar cells is investigated. An increased mobile ion density with a corresponding increased field screening leads to a decrease in the steady-state power conversion efficiency mainly due to a large reduction in current density, while the efficiency at high scan speeds (>1000 V/s) where the ions are immobilised is much less affected. Interfacial recombination does not increase upon ageing, yet the open-circuit voltage decreases as a result of an increase in the mobile ion density upon ageing. Furthermore, similar ionic losses appear under different external stressors, in particular when there are free charges present in the absorber layer. We thus reveal a key degradation mechanism, providing new insights into initial device degradation before chemical or extrinsic mechanical device degradation effects manifest, and highlight the critical role mobile ions play therein.</p>
<p>The second part of this thesis, then, shifts focus towards perovskite-based tandem photovoltaics. Understanding performance losses in all-perovskite tandem photovoltaics is crucial to accelerate advancements toward commercialisation, especially since these tandem devices generally underperform in comparison to what is expected from isolated layers and single junction devices. The individual sub-cells in all-perovskite tandem stacks are selectively characterised to disentangle the various losses. It is found that non-radiative losses in the high-gap subcell dominate the overall recombination in the baseline system, as well as in the majority of literature reports. Through a multi-faceted approach, the open-circuit voltage of the high-gap perovskite subcell is enhanced, and employing a novel (quasi) lossless indium oxide interconnect, this enables all-perovskite tandem solar cells with 2.00 V open circuit voltage and 23.7% stabilised efficiency. Reducing transport losses as well as imperfect energy alignments could boost efficiencies to 25.2% and 27.0% as identified via subcell selective electro- and photo-luminescence. Finally, it is shown how, having improved the open circuit voltage, improving the current density of the low-gap absorber pushes efficiencies even further, reaching 25.9% efficiency stabilised, with an ultimate potential of 30.0% considering the bulk quality of both absorbers measured using photo-luminescence. These insights not only show an optimisation example but also a generalisable evidence-based optimisation strategy utilising optoelectronic sub-cell characterisation.</p>
<p>In 2-terminal perovskite-based tandem photovoltaics, mobile ions play an important role as well, as these devices rely on a carefully engineered current balance. Ageing of such devices, and the corresponding mobile-ion-induced current losses, can be detrimental to device performance. Therefore, it is important to not only understand voltage losses but also ion-induced current losses subcell selectively. To this end, we demonstrate the use of current-voltage scans and current decay measurements as well as photoluminescence measurements to investigate the effect of mobile ions subcell-selectively. Finally, the relationship between radiative and non-radiative losses at short circuit conditions is investigated in an ultimate attempt to directly and subcell-selectively quantify current losses from photoluminescence at short circuit.
Overall, the results of this thesis provide a crucial understanding of mobile ions and their role in performance loss and device degradation. We show that it is important to gain further understanding of how to reduce the mobile ion density, or how to engineer the devices so that the charge collection is insensitive to the redistribution of the ions. These findings also pave the way towards accelerated ageing tests of perovskite solar cells, which can be used to identify losses and test potential mitigation mechanisms. At the same time, we highlight the importance of subcell-selective measurements in perovskite-based tandem devices to understand which factors are limiting the efficiency, and design evidence-based optimisation approaches. These findings combined will ultimately enable us to move closer to the radiative efficiency limit, while at the same time improving stability, which is a key for the commercialisation of perovskite-based solar cells.</p>
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| first_indexed | 2024-09-25T04:19:23Z |
| format | Thesis |
| id | oxford-uuid:bb8ef67a-7257-473d-8024-56f067fa0ab9 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-09-25T04:19:23Z |
| publishDate | 2023 |
| record_format | dspace |
| spelling | oxford-uuid:bb8ef67a-7257-473d-8024-56f067fa0ab92024-07-29T10:05:39ZUnderstanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaicsThesishttp://purl.org/coar/resource_type/c_db06uuid:bb8ef67a-7257-473d-8024-56f067fa0ab9PhysicsMaterials sciencePerovskite solar cellsCondensed matterEnglishHyrax Deposit2023Thiesbrummel, JSnaith, H<p>Over the past decade, metal halide perovskites have significantly stirred up the field of photovoltaic research. Despite the remarkable increase in efficiency, there is still a lack of comprehensive understanding regarding the device physics and stability of perovskite solar cells. This is problematic because to unlock the full potential of perovskite solar cells and push them towards commercialisation, it is imperative that researchers understand the physical underlying mechanisms that determine whether a solar cell can reliably operate at high efficiency levels over long timescales. This thesis, therefore, concentrates on gaining a better understanding of the device physics and stability of perovskite solar cells, with specific emphasis on two aspects: investigating the effect of mobile ions on the performance and stability of these devices, and understanding performance losses in perovskite-based tandems.</p> <p>Many perovskite solar cells still suffer from current losses that cannot be attributed to suboptimal device optics and light harvesting. The origin of these losses is investigated, using a combination of voltage-dependent photoluminescence time series and various charge extraction measurements. It is demonstrated that the perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted photoluminescence from the device rises on the exact same timescales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping-induced electronic charges but by the movement of mobile ions towards the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. These findings are generalised to a variety of metal-halide perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism.</p> <p>Hereafter, the impact of such ion-induced loss mechanisms on the device performance upon ageing of perovskite solar cells is investigated. An increased mobile ion density with a corresponding increased field screening leads to a decrease in the steady-state power conversion efficiency mainly due to a large reduction in current density, while the efficiency at high scan speeds (>1000 V/s) where the ions are immobilised is much less affected. Interfacial recombination does not increase upon ageing, yet the open-circuit voltage decreases as a result of an increase in the mobile ion density upon ageing. Furthermore, similar ionic losses appear under different external stressors, in particular when there are free charges present in the absorber layer. We thus reveal a key degradation mechanism, providing new insights into initial device degradation before chemical or extrinsic mechanical device degradation effects manifest, and highlight the critical role mobile ions play therein.</p> <p>The second part of this thesis, then, shifts focus towards perovskite-based tandem photovoltaics. Understanding performance losses in all-perovskite tandem photovoltaics is crucial to accelerate advancements toward commercialisation, especially since these tandem devices generally underperform in comparison to what is expected from isolated layers and single junction devices. The individual sub-cells in all-perovskite tandem stacks are selectively characterised to disentangle the various losses. It is found that non-radiative losses in the high-gap subcell dominate the overall recombination in the baseline system, as well as in the majority of literature reports. Through a multi-faceted approach, the open-circuit voltage of the high-gap perovskite subcell is enhanced, and employing a novel (quasi) lossless indium oxide interconnect, this enables all-perovskite tandem solar cells with 2.00 V open circuit voltage and 23.7% stabilised efficiency. Reducing transport losses as well as imperfect energy alignments could boost efficiencies to 25.2% and 27.0% as identified via subcell selective electro- and photo-luminescence. Finally, it is shown how, having improved the open circuit voltage, improving the current density of the low-gap absorber pushes efficiencies even further, reaching 25.9% efficiency stabilised, with an ultimate potential of 30.0% considering the bulk quality of both absorbers measured using photo-luminescence. These insights not only show an optimisation example but also a generalisable evidence-based optimisation strategy utilising optoelectronic sub-cell characterisation.</p> <p>In 2-terminal perovskite-based tandem photovoltaics, mobile ions play an important role as well, as these devices rely on a carefully engineered current balance. Ageing of such devices, and the corresponding mobile-ion-induced current losses, can be detrimental to device performance. Therefore, it is important to not only understand voltage losses but also ion-induced current losses subcell selectively. To this end, we demonstrate the use of current-voltage scans and current decay measurements as well as photoluminescence measurements to investigate the effect of mobile ions subcell-selectively. Finally, the relationship between radiative and non-radiative losses at short circuit conditions is investigated in an ultimate attempt to directly and subcell-selectively quantify current losses from photoluminescence at short circuit. Overall, the results of this thesis provide a crucial understanding of mobile ions and their role in performance loss and device degradation. We show that it is important to gain further understanding of how to reduce the mobile ion density, or how to engineer the devices so that the charge collection is insensitive to the redistribution of the ions. These findings also pave the way towards accelerated ageing tests of perovskite solar cells, which can be used to identify losses and test potential mitigation mechanisms. At the same time, we highlight the importance of subcell-selective measurements in perovskite-based tandem devices to understand which factors are limiting the efficiency, and design evidence-based optimisation approaches. These findings combined will ultimately enable us to move closer to the radiative efficiency limit, while at the same time improving stability, which is a key for the commercialisation of perovskite-based solar cells.</p> |
| spellingShingle | Physics Materials science Perovskite solar cells Condensed matter Thiesbrummel, J Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| title | Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| title_full | Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| title_fullStr | Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| title_full_unstemmed | Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| title_short | Understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| title_sort | understanding the physics of perovskite solar cells for stable and efficient multijunction photovoltaics |
| topic | Physics Materials science Perovskite solar cells Condensed matter |
| work_keys_str_mv | AT thiesbrummelj understandingthephysicsofperovskitesolarcellsforstableandefficientmultijunctionphotovoltaics |