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.
