Thermally Induced Highly Ordered <i>π</i> – <i>π</i> Stacking in GaAs/CNT Solar Cells Enables a Record Efficiency of 20.03%

ABSTRACT GaAs heterojunction solar cells (HJSC) have attracted attention due to their potential application prospects. However, non‐radiative recombination and non‐ideal band alignment hinder further performance improvement. This study innovatively introduced Copper phthalocyanine (CuPc) into the GaAs/carbon nanotube (CNT) HJSC, successfully fabricating highly efficient and stable organic‐inorganic hybrid photovoltaic devices. Strong π – π interactions and excellent energy level matching between CuPc and CNTs induce electron transfer from CNTs to CuPc, significantly enhancing the p‐type doping of CNTs, as revealed by UV–vis, Raman, XPS, and UPS. Thermal annealing induces the formation of highly ordered face‐to‐face stacking structures in CuPc molecules, effectively improving the vertical charge transport capability, which is elucidated through TEM, GIWAXs, fs‐TA, C‐AFM. Ultimately, the champion device achieved a photoelectric conversion efficiency (PCE) of 20.03% and a fill factor (FF) of over 80%, thereby establishing the highest reported values for a GaAs HJSC. The device maintains over 97% of its initial efficiency after 1000 h in an air environment at 65°C. This study demonstrates the application potential of organic‐inorganic hybrid strategies in high‐performance, high‐stability optoelectronic devices.