Researchers from Korea's Inha University, France's CY Cergy Paris Université and Hanoi University of Science and Technology in Vietnam have developed a new family of molecular‑glass hole‑transport materials (HTMs) - T3A, T3B and T3C - to address the long‑term thermal instability of n‑i‑p perovskite solar cells that rely on spiro‑MeOTAD. Using a bulky 9,9‑diphenylfluorene core linked to di‑p‑methoxyphenylamine, they tuned the glass transition temperature (Tg) from 112 °C for T3A to 120 °C for T3B and 131 °C for T3C, aiming to suppress thermally driven morphology changes in the HTM layer.
Devices were fabricated in a standard n‑i‑p stack, simply swapping spiro‑MeOTAD for T3A, T3B or T3C while keeping LiTFSI/tBP doping and the overall HTM:LiTFSI:tBP molar ratio constant; for T3C, its limited solubility in chlorobenzene forced a lower HTM concentration, already compromising film uniformity. Under these conditions, T3A‑ and T3B‑based cells delivered initial power conversion efficiencies (PCEs) above 18 %, whereas T3C devices reached only 8.6 % because JSC, VOC and fill factor (FF) are all reduced by pinholes and inhomogeneous coverage.
Shelf‑stability tests in ambient air (no encapsulation, 1500 h) showed that T3A devices actually gained about 28 % in PCE over time, driven by increases in VOC and FF attributed to oxidation‑induced p‑doping of the HTM, which enhances conductivity and improves energy alignment with the perovskite valence band. T3B devices instead lost roughly 19 % in FF within 500 h, consistent with aggregation and solid‑state redistribution of their more planar, thiophene‑bridged backbone that initially supports good transport but undermines long‑term morphology. T3C cells exhibited only small net changes under shelf aging, reflecting a high‑Tg glass that resists large‑scale relaxation but starts from a less ideal, defect‑rich morphology.
Under maximum‑power‑point operation (one sun, N2), T3B performed best, retaining about 40 % of its initial PCE after around 1 h, while T3A devices lost more than 90 % of their initial efficiency within 30 min, indicating that different HTMs respond very differently to combined light and bias stress compared with simple storage in air. Most strikingly, during thermal aging at 85 °C in the dark, T3C’s highest Tg translated into the best thermal endurance: despite a 25 % drop in JSC, its devices preserved 70 % of their initial PCE after 1000 h, whereas T3A and T3B suffered stronger performance losses linked to FF and JSC/VOC degradation.
Overall, T3A and T3B demonstrate that similar fluorene‑based cores can yield >18 % PCE but diverge in stability depending on how functional groups control oxidation, packing and aggregation, while T3C shows that very high Tg can secure excellent thermal stability at the cost of initial efficiency when solubility and film formation are not optimized. The work underscores that fine‑tuning HTM functional groups and glass‑forming behavior is a potential route to stress‑dependent stability control - across humidity, light, bias and temperature - and could be crucial for turning laboratory‑scale perovskite solar cells into thermally robust commercial devices.
