Doping Regulation Of Inorganic Hole Transport Materials In Perovskite Solar Cells

As one of the most promising renewable energy sources,solar energy has the advantages of wide distribution,abundant reserves,clean and pollution-free.The full utilization of solar energy is expected to satisfy the increasing energy demand of human beings.Solar cell is a kind of common solar energy utilization device,which can realize instant conversion of light energy to electric energy under sunlight irradiation.Although silicon-based solar cells have been commercialized,the higher cost of generating electricity than traditional fossil energy sources is still limiting their widespread use.Over the past decade,perovskite solar cells（PSCs）have achieved certified power conversion efficiency（PCE）of 25.7%,approaching to the performance of silicon-based solar cells.In addition,PSCs can be prepared by solution process,showing the advantage of low production cost,which are expected to be a solution for the fabrication of low-cost photovoltaic device.The hole transport layer is an indispensable functional layer for PSCs,which plays significant roles in collecting holes,blocking electrons and inhibiting interfacial recombination.Organic materials are the main hole transport materials of PSCs due to their advantages of controllable molecular structure,satisfactory solubility and film preparation by simple solution process.However,organic hole transport materials generally obtained by complex synthetic processes and high production cost.In addition,organic hole transport materials are easily degraded under illumination conditions,which is detrimental to the long-term stability of PSCs.Fortunately,inorganic hole transport materials are proved superior in production cost and stability.But there is still a performance gap between them and organic hole transport materials.Therefore,the intensive study of inorganic hole transport materials is of great significance for the development of PSCs.In this paper,from the perspective of designing low-cost and high-performance inorganic hole transport materials,three kinds of novel inorganic hole transport materials were prepared by hydrothermal synthesis method,the properties of them were regulated by doping means,and applied them as the hole transport materials of inverted PSCs.The main contents of this paper are as follows:（1）Gallium oxide（Ga₂O₃）is a kind of ultra-wide band gap semiconductor material,but it is rarely used as the hole transport material of PSCs due to the obvious energy level mismatch at the interfaces between Ga₂O₃ and perovskite layers.We synthesized Cu-doped Ga₂O₃（Ga₂O₃:Cu）nanocrystals by hydrothermal synthesis method,and applied them as hole transport materials for inverted PSCs for the first time.It is found that Cu doping can significantly improve the performance of Ga₂O₃ hole transport layer,and the PCE of device increases from 7.6%to 19.5%.The enhanced photovoltaic performance of PSCs can be attributed to the impurity levels introduced by Cu dopants can match well with the valence-band maximum（VBM）of the perovskite layers,providing additional transport channels for the holes,which can effectively improve the holes extraction ability and inhibit recombination process.In this work,a new mechanism of the impurity levels of Ga₂O₃:Cu hole transport layer can be used as additional hole transport channel of PSCs is proposed,showing the feasibility of impurity levels as hole transport channel and providing a new design idea for hole transport materials.（2）Regulating the hole transport channels provided by impurity levels is the key to further improve the performance of Ga₂O₃:Cu hole transport layers.Cu-doped In_xGa_2-xO₃(In_xGa_2-xO₃:Cu)nanocrystals were synthesized as the hole transport material of PSCs by the hydrothermal method.Comparing with Ga₂O₃:Cu hole transport layers,the energy position of impurity levels and the hole transport ability of In_xGa_2-xO₃:Cu hole transport layers can be regulated by incorporating indium ions.Under the optimized conditions,the hole extraction process and carrier recombination process of In_xGa_2-xO₃:Cu based devices are significantly improved,and the PCE of device is increased from 19.6%to 21.4%.This work provides an efficient inorganic hole transport material for inverted PSCs,and demonstrates the regulation method of hole transport channels by impurity levels.（3）Cu-doped lanthanum hydroxide(La(OH3):Cu)nanorods were prepared by hydrothermal method,and applied as hole transport material for PSCs.La（OH）₃ is rarely used as hole transport material for PSCs due to the limited hole transport performance.However,Cu²⁺ions doping can significantly improve the hole transport ability of La（OH）₃,and the hole extraction ability at the interface between La（OH）₃ and perovskite layer is also significantly improved.Moreover,the scanning electron microscopy（SEM）images and X-ray diffraction（XRD）patterns show that Cu²⁺ions doping can promote the growth of perovskite grains,which is conducive to inhibit carrier recombination in devices.Under the optimized doping concentration of Cu²⁺ions,the PCE of device can be increased from 11.3%to 20.4%.This work demonstrates the feasibility of hydroxide as the hole transport material for PSCs,and broadens the selection of hole transport materials.