Inorganic CuFeO₂ Delafossite Nanoparticles as Effective Hole Transport Materials for Highly Efficient and Long-Term Stable Perovskite Solar Cells

The regular architecture (n-i-p) of perovskite solar cells (PSCs) has attracted increasing interest in the renewable energy field, owing to high certified efficiencies in the recent years. However, there are still serious obstacles of PSCs associated with spiro-OMeTAD hole transport material (HTM), such as (i) prohibitively expensive material cost (∼150–500 $/g) and (ii) operational instability at elevated temperatures and high humidity levels. Herein, we have reported the highly photo, thermal, and moisture-stable and cost-effective PSCs employing inorganic CuFeO₂ delafossite nanoparticles as a HTM layer, for the first time. By exhibiting superior hole mobility and additive-free nature, the best-performing cell achieved a power conversion efficiency (PCE) of 15.6% with a negligible hysteresis. Despite exhibiting a lower PCE as compared to the spiro-OMeTAD-based control cell (19.1%), nonencapsulated CuFeO₂-based cells maintained above 85% of their initial efficiency, while the PCE of control cells dropped to ∼10% under continuous illumination at maximum power point tracking after 1000 h. More importantly, the performance of control cells was quickly degraded at above 70 °C, whereas CuFeO₂-based cells, retaining ∼80% of their initial efficiency after 200 h, were highly stable even at 85 °C in ambient air under dark conditions. Besides showing significant improvement in stability against light soaking and thermal stress, CuFeO₂-based cells exhibited superior shelf stability even at 80 ± 5% relative humidity and retained over 90% of their initial PCE. Overall, we strongly believe that this study highlights the potential of inorganic HTMs for the commercial deployment of long-term stable and low-cost PSCs.