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We present a detailed study on alloying from screen-printed aluminum pastes containing boron additives (Al-B pastes) to further enhance the efficiency of p- and n-type silicon solar cells with an Al-alloyed back-surface field and rear emitter, respectively. Due to the high B solubility in Si, the additional incorporation of B atoms as acceptors into the Al-alloyed p+ region-referred to as Al-B codoping of Si-provides improved shielding of electrons from the recombination-active surface. Thus, alloying from Al-B pastes allows for significantly thinner p + regions and leads to a considerable reduction of the p + saturation current densities. By comparing surface-passivated p+ regions alloyed from Al-B pastes or conventional Al pastes with each other, we show that a highly recombination-active defect limits the minority carrier lifetime in these p+ regions. We demonstrate that the acceptor concentration profiles of the p+ regions can easily be modified by adding different amounts of aluminum diboride or boron trioxide as B sources to the Al pastes.