Due to their low-cost and adaptable technology, colloidal quantum dots (CQD) have attracted extensive interest, especially for photovoltaic applications. In the last few years, the power conversion efficiency (PCE) of CQD based solar cells has increased from 0.01% to over 10%. Compared to other types of CQD solar cells, CuInSxSe 2-x (CISSe) quantum dot based solar cells draw a lot of attention because of the heavy-metal free and non-toxic character of the material. The highest PCE reported so far in this material system is 11.7%, though this used a liquid electrolyte which is inconvenient for the mass deployment of of devices. Solid state CuInS2 based CQD solar cells, despite significant effort, have a champion PCE of only around 1%. The main ways to improve on these nanoparticle cells are to enhance carrier mobilities, tune band offsets at heterojunctions, increase control of the surface passivation and build better device architectures. In this report, we have focused on improving the device structure and charge carrier extraction by employing tin dioxide as a window layer to improve the electrical mobility and passivate the surface of mesoporous-TiO2 in order to decrease the surface defects. By this approach, a dramatic boost in the current was observed, which lead to the significant increase in the overall device performance.