The Research Of Inorganic Hole Transport Materials For Inverted Perovskite Solar Cells

The organic-inorganic hybrid perovskite solar cells（PSCs）have attracted considerable attention in recent years due to their rapid improvements in conversion efficiency.The power conversion efficiency（PCE）of single junction traditional n-i-p PSCs has reached 25.8%,which is close to that of the most advanced crystalline silicon solar cells.Despite such high conversion efficiency,the weak stability has become an obstacle to the commercialization of PSCs.Among various PSC device structures,the inverted planar structure has attracted more and more attention due to its advantages of simple preparation,low-temperature processing,low hysteresis,and so on.As the main component of inverted PSCs,hole transport layer（HTL）has an extremely important influence on the light capture,carrier extraction and transport,perovskite crystallization,and stability of PSCs.At present,various organic hole transport materials（HTMs）have been widely used in inverted PSCs,especially poly（3,4-ethylenedioxythiophene）:poly（styrenesulfonic acid）（PEDOT:PSS）and poly（triarylamine）（PTAA）.However,the poor stability and high cost of organic materials hinder their large-scale commercial application.Inorganic p-type semiconductors have attracted much attention due to their high transmittance,high hole mobility and excellent chemical stability.By comparison,several inorganic HTMs such as Cu SCN,NiO_x,CoO_x,CuO_x and WO₃ have been used as substitutes for organic HTMs.In this paper,from the perspective of improving the stability of inverted planar PSCs,the stability and efficiency of PSCs are synergistically improved by optimizing the HTL and passivating the interface.The main research contents are as follows:In order to control the size and energy level of NiO_x nanomaterials cooperatively to maximize the device performance.Herein,a novel synthesis of highly dispersed NiO_x NCs is demonstrated by employing tetraalkylammonium hydroxides（TAAOHs,alkyl,methyl,ethyl,propyl,butyl）as precipitating bases,where the varied alkyl chain lengths of TAAOHs enable the size control of the NiO_x NCs and the subsequent altering of their Ni³⁺contents,leading to tunable energy levels of the NiO_x thin films.With the longest butyl chain,the smallest crystal size and the optimal energy level alignment at the NiO_x/perovskite interface are achieved.After further passivating the detrimental Ni³⁺species on the surface of NiO_xHTL,a remarkable PCEs approaching 23%is obtained,which is one of the highest PCEs reported for NiO_x-based inverted PSCs.Furthermore,the unencapsulated device exhibits excellent ultraviolet（UV）stability,which maintains≈87%of its PCEs after 200 h exposure.Nickel oxide is the most popular inorganic HTL for fabricating stable inverted PSCs thus far.However,the Ni³⁺state on the surface of NiO_x thin film(actually the Ni²⁺vacancy in the NiO_xlattice),are considered as a defects that may cause the degradation of the performance of PSCs due to the redox reaction at the NiO_x/perovskite interface.La NiO₃（LNO）perovskite would be a promising replacement given its suitable electronic structure and stable crystal structure,while the LNO films are generally prepared under a very high temperature nearly 700℃,which is incompatible with the fabrication of PSCs.Here,we report a novel solution-processed method to prepare a LNO HTL under a very low temperature（below 150℃）for the first time.The PSCs prepared with LNO could reach a PCEs of 14.39%.More interestingly,the PSCs based on LNO exhibit much higher UV stability（retained 70%of initial PCE after 250 h,versus retained 52%of initial PCE after 100 h for NiO_x-based device）.After further passivation with[2-（9|H|-Carbazol-9-yl）ethyl]phosphonic Acid（2PACz）,a remarkable PCE of 17.15%is obtained.Furthermore,an unencapsulated LNO/2PACz-based PSCs exhibit an excellent UV stability which could maintain about85%of its initial PCE after 400 hours of a continuous UV irradiation(8 m W cm^-2).The Ni³⁺species on the surface of the NiO_x film may cause redox reactions at the NiO_x/perovskite interface,leading to the degradation of perovskite and hindering the effective transport of holes at the NiO_x/perovskite interface.Therefore,it is difficult to improve the stability of bare NiO_x-based PSCs.CoO_x would be a promising alternative for NiO_x due to its excellent electrical conductivity and suitable electronic structure.Here,a novel inorganic CoO_x HTL prepared by a new green solvent water-based solution processing method was developed,replacing the expensive and toxic organic solvent.The electronic properties were adjusted by doping Mg into the CoO_x lattice and passivating the interface with 2PACz,while the optimal energy level alignment matching at the CoO_x/perovskite interface was achieved.More over,the 2PACz effectively passivated interface defects and eliminated residual stress at the Mg:CoO_x/perovskite interface.A maximum PCE of 23%was obtained,which is the highest PCE reported in CoO_x-based inverted PSCs.Compared with NiO_x-based inverted PSCs,Mg:CoO_x-based inverted PSCs have higher thermal stability.In addition,the unencapsulated Mg:CoO_x/2PACz-based device has excellent air,UV,light,and thermal stability.