| The difficulty to combine the defect-tolerance and structural stability is seriously blocking the wide application of high-efficiency solar cells.Generally,covalent and polar semiconductors with a prototype of Si/Cd Te crystals exhibit high structural stability due to compact tetrahedral structure,but have the poor defect-tolerance because of the similar electronegativity of the element compositions.On the contrary,for ionic semiconductors,such as perovskite series materials,the great discrepancy in electronegativity between the anions and cations results in the good electronic properties of intrinsic defects.However,due to the large radius of the cations in perovskites,the octahedral structure composed by these cations is relatively sparse and prone to torsion,leading to the low structural stability of the perovskite.Combining the stable structure of covalent semiconductors and the benign defects of ionic compounds,we found that HgX2S4（X=In,Sc and Y）spinel-structured semiconductors exhibit both the two advantages.The spinel structure is composing of tightly bounded coordinate tetrahedral and octahedral has good structural stability.At the same time,its electronic structure showing ionic characteristics ensures a high defect tolerance.Here,based on first-principles calculations,taking HgX2S4（X=In,Sc and Y）spinel semiconductor as the research object,the electronic structure and performance regulation are studied in depth,including the following five parts:（1）The electronic structure of the HgX2S4 system was calculated.It shows that the band edges of HgX2S4 have typical ionic features,with the conduction band minimum（CBM）mainly contributed by Hg-6s/X-ns（d）orbital and the valence band maximum（VBM）predominantly composed of S-3p orbital,making the structure more defect tolerant.Moreover,the prominent downward bending of the CBM caused by spatially spreading Hg-6s/X-ns（d）orbitals with a large principal quantum number（n）,which not only gives the system a suitable band gap,but also facilitates the formation and transport of n-type carriers.The normalized orbital overlap（NOO）calculations verified the electronic structure of the HgX2S4 with delocalized properties.（2）The optical properties of the HgX2S4 spinel system were investigated.The absorption as a function of incident light energy was calculated,showing that the HgX2S4 system compounds have strong light absorption in the visible light region.（3）The thermodynamic stabilities of the spinel structure are demonstrated by phase diagram calculations.Furthermore,the electronic properties of the intrinsic defects of the system are calculated.The results show that the formation energy of n-type defects in the system is lower than that of p-type defects in the band gap range,indicating that HgX2S4 has n-type characteristics.The transition energy level calculation shows that the transition energy levels of the n-type defects that dominate the n-type conductivity are almost all inside the conduction band.Even if the GW0 method is applied to correct the band gap,the position of the defect energy level is still far from the middle of the band gap,indicating that the intrinsic defects of the HgX2S4system do not cause severe nonradiative recombination.（4）In order to obtain the Fermi level and carrier concentration,the degenerate factor of electron occupying orbital at defect site is calculated,by the Pauli exclusion principle,Hund’s rule,and the splitting of the degenerate energy level in the crystal field into account.Then the Fermi level and carrier concentration of the system are obtained by self-consistently solving the electric neutrality equation of the semiconductor.The Fermi level results confirm that the systems all exhibit n-type characteristics.From the carrier concentration results,it can be seen that among HgX2S4（X=In,Sc and Y）,HgIn2S4 has the highest Fermi level and electron concentration.Hg Y2S4 has high electron concentration at the conditions of Y-poor and Hg-poor,while Hg Sc2S4 has the low electron concentration in the whole chemical potential range which is much lower than the optimal carrier concentration of solar cell semiconductor absorber materials.（5）In the second work,we mainly focus on improving the electron concentration of the system through defect engineering.The external Cl doping is attempted to solve this problem,and the results show that the electron concentration of Hg Y2S4 and Hg Sc2S4 can be effectively adjusted to the appropriate carrier concentration range by Cl doping.What’s more,in order to achieve the practical applications of HgX2S4spinel semiconductor,we designed the photovoltaic device structure according to the energy band alignment of the absorber layer,hole transport layer,and electron transport layer materials,and found that Ti O2/HgIn2S4/Ni O,Sn O2/Hg Y2S4/F8 and Ti O2/Hg Sc2S4/Ni O are more reasonable high-efficiency solar cell device structures.In addition,to solve the problem that spinel contains toxic metal Hg,we proposed the scheme of replacing Hg with cations with similar delocalized outer orbitals under the framework based on spinel structure.Accordingly,Ba and Sr elements are attempted to replace Hg in HgIn2S4.By analyzing the electronic structures of BaIn2S4 and SrIn2S4,it was found that the band-edge composition of the replaced structure is similar to the original system.It shows that the above idea is feasible.Therefore,the toxicity problem of spinel can still be solved further by choosing the above thinking. |
