The process of pn junction formation is one of the fundamental steps in the manufacturing process of solar cells. It is the most important factor influencing the efficiency of solar cells. The aim of this research is to investigate the effect of different p-type (different boron concentrations) and n-type (different phosphorus concentrations) resistivity on the efficiency of monocrystalline silicon solar cells. The solar cells were fabricated using p-type silicon doped in boron at concentrations ranging from 6.61×10¹⁵  to 3.03×10¹⁶ cm^-3 and n-type silicon doped in phosphorous at concentrations ranging from 5×10¹⁹  to 10²¹  cm^-3. Then, the effect of boron and phosphorus concentration on solar cell efficiency was investigated. It is found that an increase in the concentration of boron or phosphorus results in an increase in the recombination rate and thus a decrease in the efficiency of the solar cell by reducing the short-circuit current (I_sc). The best efficiency of 18.59 % was obtained by using boron doped silicon with a resistivity of 2.16 Ω.cm, corresponding to 6.61×10¹⁵  cm^-3 boron concentration and n- type sheet resistance 43.5 Ω/□ , corresponding to 10^20  cm^-3    phosphorus concentration. As the study was carried out on a range of  resistivities values (0.54 Ω.cm  to   2.16 Ω.cm) , a range of  sheet resistance (10 Ω/□ to  77 Ω/□).