锗掺杂/替位锌黄锡矿太阳能电池研究进展

Overview of Ge-incorporated kesterite solar cells

锌黄锡矿(CZTS(e))太阳能电池作为新兴薄膜太阳能电池的代表之一,以其原材料储量丰富、制备工艺简单、环境友好、成本低廉等优势受到广泛关注.为了进一步提升CZTS(e)太阳能电池的器件效率,在光吸收层内引入掺杂或替位元素成为近年来CZTS(e)太阳能电池的重要研究方向.在众多掺杂或替位元素中,锗元素(Ge)对CZTS(e)太阳能电池器件性能(特别是器件的开路电压(Voc))的提升作用极为明显.本文综述了近年来Ge掺杂或替位CZTS(e)太阳能电池(CZTS(e):Ge)的研究进展,重点介绍了CZTS(e)太阳能电池高开路电压损耗(Vocdeficit,Voc-def)的根源、CZTS(e):Ge太阳能电池的制备方法以及Ge对CZTS(e)太阳能电池器件性能的影响,并对CZTS(e):Ge太阳能电池的未来研究方向进行了展望.

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 deeplevel 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 Voc 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 lowtemperature 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 Cu Au 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 GeZn-CuZn 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.