Silicon nanocrystals (Si NCs) have attracted much attention nowadays because of their novel properties and potential applications in many kinds of nano-devices. In order to further improve the device performance, realizing effective doping in Si NCs is a crucial subject. However, the doping in nano-scaled semiconductors is quite complicated and difficult as revealed both in theoretical and experimental studies. Here, we report the systematically study on Phosphorus (P) or/and Boron (B) doping in Si NCs-based multilayers. The comparative study on the favourite occupy sites of P、B singly and co-doping in Si NCs has been carried out theoretically. We demonstrate the substitutional doping can be achieved even in the small-sized Si NCs experimentally and high conductivity can be obtained. It is also found that the light emission can be enhanced by suitable P doping More interesting, the sub-band light emission has been observed for P-doped Si NCs and the corresponding model is proposed and the improved sub-band emission is realized by P/B co-doping. The work is supported by NSFC (No. 61921005) and National Key R&D Program of China (2018YFB2200101).