Periodic Reporting for period 1 - PeTSoC (Lightweight and Flexible All-Perovskite Triple-junction Solar Cells)
Période du rapport: 2020-10-15 au 2022-10-14
Climate change is a global concern with solar energy as a vital renewable resource. The solar industry relies on crystalline Si cells, limited to 29.4% efficiency due to the Shockley-Queisser limit and Auger recombination (PERC cells reach ~22%). Next-gen multijunction solar cells, though promising, demand expensive III-V materials and complex fabrication. Consequently, there's growing interest in low-cost, highly efficient, tunable bandgap organic-inorganic metal halide perovskite solar cells (PSCs) for multijunction applications. Perovskites have an ABX3 structure: A-sites contain organic cations (e.g. MA or FA) or inorganic elements (e.g. Cs), B-sites typically contain elements like Pb or Sn, and X-sites house halides (I or Br). Altering perovskite composition allows for bandgap tunability (1.2-3.0 eV), crucial for multijunction cells. While perovskite-perovskite tandem solar cells have been intensively researched, perovskite-perovskite-perovskite (PPP) triple-junction solar cells, with higher theoretical efficiency, remain limited in studies. Thermal co-evaporation, a scalable technique, enables versatile, uniform perovskite thin-film production over large surfaces without common toxic solvents (DMF or chlorobenzene), making it attractive to industry. Although full thermal co-evaporation is widely used for single-junctions, it has not been applied to triple-junction all-PSCs.
Overall, the project explored thermal co-evaporation (up to 4 sources) to fabricate PSCs with varied bandgaps (1.5-2.3 eV). It advanced materials science, enabling 4D charge carrier tracking via confocal microscopy and new real-time in-situ transmission electron microscopy sample prep techniques. Successful demonstration of monolithic PPP triple-junction solar cells, produced through thermal co-evaporation, paves the way for future lightweight, flexible multijunction solar cells. These innovations have commercial potential, including (1) charging electronic smartphones/devices, (2) powering vehicles and drones/aircraft, (3) wearable textiles/backpacks, (4) smart windows, and more.
Overall, the project explored thermal co-evaporation (up to 4 sources) to fabricate PSCs with varied bandgaps (1.5-2.3 eV). It advanced materials science, enabling 4D charge carrier tracking via confocal microscopy and new real-time in-situ transmission electron microscopy sample prep techniques. Successful demonstration of monolithic PPP triple-junction solar cells, produced through thermal co-evaporation, paves the way for future lightweight, flexible multijunction solar cells. These innovations have commercial potential, including (1) charging electronic smartphones/devices, (2) powering vehicles and drones/aircraft, (3) wearable textiles/backpacks, (4) smart windows, and more.
The 3 project objectives:
1. Develop thermally co-evaporated single-junction PSCs with specific bandgap ranges for top (1.8-1.9 eV), middle (1.5-1.6 eV), and bottom (1.2-1.3 eV) subcells.
2. Gain insights into ultra-thin nanoscale perovskite layers via thermal co-evaporation, demonstrating a quantum well (superlattice) structure.
3. Demonstrate a PPP triple-junction PSCs using thermal co-evaporation.
For Obj. 1, many single-junction PSCs were fabricated and studied. The top subcell utilized the CsFA Pb mixed-I/Br perovskite composition, requiring 4 sources (PbI2, FAI, CsBr, and PbBr2), allowing effective bandgap adjustment from 1.6-1.85 eV. Increasing the Br:I ratio posed a challenge due to photoinduced phase segregation as discussed by researchers in previous works. Ultimately, a bandgap in the 1.77-1.83 eV range was chosen for the top cell in the final triple-junction devices. The middle subcell used FAPbI3 (~1.5 eV), which posed challenges due to phase transitions in air from the black α-phase to the non-perovskite δ-phase. Extensive project efforts focused on addressing this issue. It was observed that the PbI2:FAI ratio significantly affected film quality, especially without Cs+ and Br-. Though high-quality FAPbI3 films were achievable with a higher PbI2:FAI ratio. Characterization tools included S/TEM, PDS, GIWAX, and TRPL. Collaborative sample exchange between StranksLab (Cambridge) and EMPA (Zurich) allowed for specialized measurements using ToF-SIMs and hyperspectral microscopy. The bottom subcells were solution-processed Pb-Sn based perovskites (1.25 eV) made inside a glovebox to prevent oxidation of the Sn2+ to Sn4+, which can be a serious stability issue over time. Evaporating Sn-based sources (SnI2) in the thermal evaporator caused cross-contamination with the wider-bandgap devices lowering their efficiency, thus an alternative compatible solution-processed route was used instead.
For Obj. 2, attempts were made to create films less than 20 nm thick using the thermal co-evaporation system. A key observation was the discontinuous, non-uniform, island-like layer formation, influenced by surface tension and stress/strain factors. This posed challenges for creating quantum well (superlattice) structures with features in the 2-10 nm range. However, this opened up new research areas, including a 4D method for tracking charge carriers and real-time in-situ transmission electron microscopy techniques, both advancements will benefit future researchers.
For Obj. 3, using the top, middle, and bottom perovskite absorbers developed in objective 1, the first batch of PPP triple-junction PSCs were created by Month 14. This was a challenging task due to the numerous layers involved. The MeO-2PACz hole transport layer (HTL) for the middle subcell was not suitable without a reliable indium tin oxide (ITO) process. Further testing showed that the PEDOT:PSS/PTAA bilayer HTL was appropriate in terms of interface and energy band alignment but had more parasitic absorption. Subsequent batches of monolithic triple-junction PPP solar cells were produced with ongoing improvements, mainly focusing on thickness adjustments and slight bandgap alterations using thermal co-evaporation. The final batch demonstrated excellent EQE short-circuit current matching across all three subcells with less than ±5% variation. Results and findings are being prepared for publication.
1. Develop thermally co-evaporated single-junction PSCs with specific bandgap ranges for top (1.8-1.9 eV), middle (1.5-1.6 eV), and bottom (1.2-1.3 eV) subcells.
2. Gain insights into ultra-thin nanoscale perovskite layers via thermal co-evaporation, demonstrating a quantum well (superlattice) structure.
3. Demonstrate a PPP triple-junction PSCs using thermal co-evaporation.
For Obj. 1, many single-junction PSCs were fabricated and studied. The top subcell utilized the CsFA Pb mixed-I/Br perovskite composition, requiring 4 sources (PbI2, FAI, CsBr, and PbBr2), allowing effective bandgap adjustment from 1.6-1.85 eV. Increasing the Br:I ratio posed a challenge due to photoinduced phase segregation as discussed by researchers in previous works. Ultimately, a bandgap in the 1.77-1.83 eV range was chosen for the top cell in the final triple-junction devices. The middle subcell used FAPbI3 (~1.5 eV), which posed challenges due to phase transitions in air from the black α-phase to the non-perovskite δ-phase. Extensive project efforts focused on addressing this issue. It was observed that the PbI2:FAI ratio significantly affected film quality, especially without Cs+ and Br-. Though high-quality FAPbI3 films were achievable with a higher PbI2:FAI ratio. Characterization tools included S/TEM, PDS, GIWAX, and TRPL. Collaborative sample exchange between StranksLab (Cambridge) and EMPA (Zurich) allowed for specialized measurements using ToF-SIMs and hyperspectral microscopy. The bottom subcells were solution-processed Pb-Sn based perovskites (1.25 eV) made inside a glovebox to prevent oxidation of the Sn2+ to Sn4+, which can be a serious stability issue over time. Evaporating Sn-based sources (SnI2) in the thermal evaporator caused cross-contamination with the wider-bandgap devices lowering their efficiency, thus an alternative compatible solution-processed route was used instead.
For Obj. 2, attempts were made to create films less than 20 nm thick using the thermal co-evaporation system. A key observation was the discontinuous, non-uniform, island-like layer formation, influenced by surface tension and stress/strain factors. This posed challenges for creating quantum well (superlattice) structures with features in the 2-10 nm range. However, this opened up new research areas, including a 4D method for tracking charge carriers and real-time in-situ transmission electron microscopy techniques, both advancements will benefit future researchers.
For Obj. 3, using the top, middle, and bottom perovskite absorbers developed in objective 1, the first batch of PPP triple-junction PSCs were created by Month 14. This was a challenging task due to the numerous layers involved. The MeO-2PACz hole transport layer (HTL) for the middle subcell was not suitable without a reliable indium tin oxide (ITO) process. Further testing showed that the PEDOT:PSS/PTAA bilayer HTL was appropriate in terms of interface and energy band alignment but had more parasitic absorption. Subsequent batches of monolithic triple-junction PPP solar cells were produced with ongoing improvements, mainly focusing on thickness adjustments and slight bandgap alterations using thermal co-evaporation. The final batch demonstrated excellent EQE short-circuit current matching across all three subcells with less than ±5% variation. Results and findings are being prepared for publication.
There has been significant progress beyond the state of the art across the project. Firstly, further insights have been brought to the field of thermally co-evaporated PSCs, which is of great interest to the perovskite community both scientifically and commercially. The 4-sources allowed for investigation of the bandgap tuneability (1.5-2.3 eV) - highly useful for multijunction perovskite photovoltaics. It was found that that surface properties are highly important for thermally co-evaporated perovskite films. The versatility of the technique allowed for the development of the 4D tracking of charge carriers in perovskites or other thin-film semiconductor materials using confocal microscopy. Special real-time in-situ methods for high-res TEM and likely extended to other complementary techniques were developed. There were also some insights into how to tackle and reduce hysteresis of p-i-n PSCs in terms of the optimal J-V measurement conditions. Finally, the demonstration of PPP triple-junction solar cells using thermal co-evaporation with improved current matching due to the excellent control of bandgap and thickness compared to solution-processing, will open up many opportunities for researchers in the field. In future, the idea that perovskite-based multijunction solar cells can be made to be lightweight and flexible will be a special selling point as it builds on the long-term goal of commercialisation of perovskite technology, with wider socio-economic and societal impact for the photovoltaic industry across Europe and worldwide. The ultimate goal is to reduce the reliance of fossil fuel energy sources and instead focus on the generation of renewable energy to lessen the effects of climate change.