Solution-processable lead selenide (PbSe) colloidal quantum dots (QDs) are promising candidates for photovoltaics due to their efficient multiple exciton generation and carrier transport. However, despite these advantages, currently the best PbSe QD solar cells (QDSCs) still have short-circuit current densities (JSC) of about 25 mA cm-2. Here, we report the introduction a solution-processed trivalent antimony-doped tin oxide buffer layer at the interfaces in the device, which led to significant improvement in the JSC. Consistent with the optical simulations, the external quantum efficiency of the devices was improved in a region corresponding to the PbSe QD/buffer layer interfaces (400-600 nm), implying enhanced electron extraction. The improved performance is attributed to optimized gradient energy level alignment and shunt blocking at the interfaces. With this simple interfacial treatment, the JSC of the champion device was increased significantly to 26.7 mA cm-2, a more than 8% improvement compared to the control device. A further increase in the fill factor was also observed, leading to an over 11% improvement in the champion power conversion efficiency, from 7.1% to 7.9%. This work offers a simple method for interfacial engineering that led to PbSe QDSCs with efficiencies that are among the highest reported in the literature.