- Performance testing of an optimised 1.25 eV mixed Pb/Sn perovskite with an active area of 1 cm2
- 17.6% steady-state power conversion efficiency of best small area devices (~0.1 cm2)
- 14.5% scanned efficiency achieved for the 1 cm2 device – with a steady-state power conversion efficiency (PCE) of 13.2% measured over 300 seconds in air with no encapsulation.
The initial objective for CHEOPS was to achieve a 15% stable lab scale cell without lead (Pb), or at the least with the lowest possible amount of lead. However, in the course of the project, it appeared clearly that no viable lead free exist for the moment and that in the same, the lead present in the films, if properly managed, e.g. with recycling, would not impede technology development. (read more here[link to LCA]). For this reason, researchers chose not to pursue the mission to develop a 100% lead-free single-junction cell, but instead focused on double perovskites based on M+1M’3+ mixed valence cations where M and M’ are for example Ag+ and Bi3+. The result was an optimised 1.25 eV mixed Pb/Sn perovskite with an active area of 1 cm2. During the course of work on the low-lead single-junction cell, most of the optimisation was performed on smaller device area (~0.1 cm2) and reported in a peer-reviewed scientific publication [link]. The best small area devices of this kind achieved a steady-state PCE of 17.6%. The best performance achieved for the 1 cm2 device was a scanned efficiency of 14.5% – with a steady-state PCE of 13.2% measured over 300 seconds in air with no encapsulation.
The presence of toxic Pb in the best performing perovskite solar cell has raised environmental concerns and was, at the beginning of the project, identified as a potential disadvantage for societal acceptance of this new technology. To mitigate risk, the CHEOPS project therefore assessed the environmental impact of the material and reached the conclusion that it is not a considerable threat if applied in the small doses used in perovskite solar cells. Mixed Pb/Sn halide perovskites offer specific advantages, for instance they have the lowest bandgap achievable for this family of materials (down to 1.25 eV) and have both excellent absorption and charge transport properties. Control of the Pb/Sn content allows for tuning of the band gap which paves the way to engineer an optimal bandgap for single junction devices (i.e. 1.34 eV in the Shockley-Queisser limit) or for bottom cells in perovskite-only multijunction devices. These results can therefore serve as an important foundation for a sustainable development of the next generation perovskite PV technology
The low-lead single-junction perovskite PV cells were developed by the University of Oxford.
The low-lead perovskite single-junction cell result with an efficiency of 14.5% is described further in Deliverable 1.7 available for download under reports.
The low-lead single-junction perovskite PV cells were influenced by life cycle analysis (LCA) also developed as part of the CHEOPS project. Read more about the LCA here.