2, University of Barcelona, Barcelona, Barcelon, Spain
Kesterite based absorbers (Cu2ZnSn(S,Se)4 – CZTSSe) has become one of the most relevant thin film photovoltaic emerging technologies based on earth abundant elements. Nevertheless, and in spite of the impressive progresses achieved in the last years, this technology still needs to demonstrate the possibility to achieve efficiencies higher than 15% in the near future, in order to be attractive for its industrialization. The main technological problem of kesterites lies on the high voltage deficit typically exhibited by the solar cell devices, which is markedly higher than those obtained in commercial thin film technologies like CdTe and Cu(In,Ga)Se2. This can be linked to different origins, all of them related to the complex structure of these materials, including the presence of secondary phases presence, Cu/Zn disorder, Sn volatility and multivalence, macro and micro inhomogeneities, etc., that negatively impact on the electrical and transport charge properties of the material. To solve this very relevant issue, doping and alloying strategies have been revealed as the most promising ways to reduce the current voltage deficit of kesterites.
In this work, the main doping (alkali doping, Ge doping, etc.) and alloying (with Ag, Cd and Ge, etc.) strategies reported in the literature will be reviewed and presented. In the first part, the effect of different doping elements (Li, Na, K, Rb, Cs, Ge, Sb, Bi, etc.) will be analyzed and discussed. In particular, the observed impact onto the optoelectronic properties of solar cell devices, and the possible consequences for the reduction of the voltage deficit will be deeply analyzed, showing that Li and Ge are positioned as the most interesting doping candidates.
In the second part of the talk, different attempts for kesterite alloying by cation substitution will be reviewed, in particular the substitution of Cu by Ag, Zn by Ba, Cd or Mg, and Sn by Ge or Si. The potential of kesterite alloying for the solution of the different problems identified in this technology such as Cu/Zn disorder, Sn multivalence, Sn volatility etc., will be discussed, in regards of their effect onto the solar cell devices properties. Finally, the possible application of these alloying elements for the formation of graded band-gap concepts in kesterites, as well as the most promising strategies to be followed in the future for the improvement of the conversion efficiency of kesterite based solar cells through the reduction of the voltage deficit will be analyzed.