Highly efficient and air-stable inverted polymer solar cells are fabricated with a poly(2,7-carbazole) derivative (PCDTBT):fullerene derivative (PC70BM) active layer. Low-temperature-annealed sol-gel-derived zinc oxide (ZnO) and molybdenum oxide films are selected as the electron and hole transporting layers, respectively. Power conversion efficiency (PCE) of 6.3% is demonstrated, which is the highest reported to date for inverted polymer solar cells.