Overview of Ge-incorporated kesterite solar cells

Kesterite (CZTS(e)) solar cell has attracted tremendous interest for its feature: Earth-abundant composition, convenient fabrication process, environmental friendliness and low cost. However, the severe large voltage deficient in CZTS(e) solar cell remains a challenge for further enhancement of the power conversion efficiency (PCE). In this prospect, three mechanisms are proposed: Cu/Zn disorder, potential interfacial losses and Sn-related deep-level defects. Sn-related deep-level defects are the most detrimental and hardest factor to manipulate due to the low energy of the formation of atomic antisite and multivalency nature of Sn. Adding extrinsic elements into the absorber layer is of an important method to passivate Sn-related defects. Among them, incorporation of Ge (CZTS:Ge) shows great potential. Comparing to Sn, Ge is more likely to be stabilized at +4 oxidation state. DFT calculation suggests that the deep-level defect density of CZTS(e):Ge is much smaller than that of CZTS(e). The maximum trap-limited conversion efficiency can be promoted to 24.1% with Ge incorporation compared with 20.9% in intrinsic CZTS(e). Early works show that CZTS(e):Ge solar cells present relative higher V_oc and has reached 12.3% power conversion efficiency (PCE), promising to champion the efficiency of CZTS(e) solar cell. Up to date, the benefits from Ge incorporation can be summarized in three aspects. First, Ge incorporation can induce the formation of Ge-Se low-temperature liquid phase as crystallization flux during annealing, which finally enlarges grain size and promote morphology. In the meantime, Sn-Se phases formation can be minimized through a modified reaction pathway brought by Ge doping. Second, Sn-related deep-level defects experimentally observed will be passivated when Ge is incorporated. Third, the energy difference between kesterite, stannite and primitive mixed CuAu is enlarged in Cu₂ZnGeS₄, indicating that Ge may stabilize kesterite in absorber and suppress band tailing caused by atomic disorder (or phase instability). To break through current efficiency bottleneck of CZTS(e) solar cells, we prospect the futural directions in constructing CZTS(e):Ge. First, Zn composition in absorber should be handled with great care to suppress the formation of Ge_Zn-Cu_Zn further. Next, modulation of the gradient band gap, achieved by vertical Ge-Sn compositional gradient in absorber, is also an important aspect. How to avoid the fast diffusion of Ge to form the vertical Ge-Sn gradient will be of the most essential question. Vertical control of independent Sn composition or Ge incorporation in the post-selenization process may be two potential solutions. Furthermore, the nature of the wide band gap of pure sulfide CZTS(e):Ge makes itself suitable to fabricate the top cell in tandem solar cell. Finally, interface modifications and band engineering in CZTS(e) solar cells will also benefit in constructing CZTS(e):Ge solar cells.