Alkali Mono-Pnictides: A New Class of Photovoltaic Materials by Element Mutation

Abstract

Selenium (Se) has been studied for over 140 years as the first solid-state solar cell, yet it has only achieved a maximum power conversion efficiency of 6.5%. To improve the efficiency, we propose derivative structures via element mutation. Specifically, we replace Se with Group 15 pnictogens (Pn = P,As,Sb) and fill the interchain space with alkali metals (M = Li,Na,K,Rb,Cs). Our calculations reveal that the band gaps of MPn span the optimal range for solar absorption. We find that NaP, composed of earth-abundant elements, has excellent properties as a solar cell absorber, including a slightly indirect band gap, high optical absorption coefficient just above the absorption onset, light electron and hole effective masses, and ambipolar dopability. However, carrier capture calculations show that P vacancies may limit its photovoltaic performance. Therefore, we propose solutions to reduce P vacancies through chemical potential control. Finally, we present preliminary results of NaP powder sample growth; this reveals a direct band gap of 1.66 eV, close to the predicted value of 1.62 eV. MPn represents a new class of absorber to rival other emerging photovoltaic technologies.