So far physics of quantum electronic transport has not tackled the problems raised by quantum dot intermediate-band solar cells. Our study shows that this physics imposes design rules for the inter-subband transition. We developed an analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport occurring in this transition. Our results clearly indicate that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor. For that, we propose to assume the dot in a tunnel-shell whose main characteristics can be obtained by a simple analytical formula. Moreover, we show that in a realistic case, the energy transition only needs to be larger than 0.28 eV to obtain a quasi Fermi-level splitting. This quite small value designates the quantum dot solar cell as a serious candidate to be an efficient intermediate-band solar cell. This work gives a framework to design efficient inter-subband transitions and then opens new opportunities for quantum dot intermediate-band solar cells.