Preparation Of Transition Metal Oxides And It's Application In DASH Solar Cells

Although boron or phosphous doped amorphous silicon thin films as carrier selective contacts?CSC?have been widely applied inr traditional heterojunction silicon solar cells with high conversion efficiencies.However,the high optical losses due to a high absprotion for these doped amrphous silicon thin films are a limitation to the high efficiency heterjunction silicon solar cells.Alternative CSC materials with high optical band bands?low light absorption?might be a possible option for achieving much higher efficiency.In this thesis,the hot wire oxidation sublimation deposition?HWOSD?technology was employed to prepare some alternative transition metal oxides?TMOs?including tungsten oxide?WO_x?,molybdenum oxide?Mo O_x?,tungsten-molybdenum alloy oxide(W_xMo_1-xO₃)and WO_x/Mo O_x double layer,which were used as hole selective transport layer?HTL?for application in doped-free asymmetric heterogunction?DASH?solar cell.The material perproperteis of these TMOs and their effects on the performances of solar cells as well as the corresponding mechanisms were systematically investigated.The conversation efficiency for the DASH solar cells was inporved by optimzting the material properties and also the related interface qualitites.HWOSD technology has some advantages such as no particle bombardment,adjustable chemical composition for material and controllable deposition rate etc..It is possible to modify or change the material propreties by employing different prpeparation conditions including reative gas flux,the hot-wire temperature and substrate temperature.In addition to the single-type hot wire used,the multiple hit wires could be installed in the deposition chamber,which could be used to prepared different TMOs including multiple alloy oxide thin films and multi-layer complex metal oxide thin films.Such a deposition technology is significiant in both scientific research and industrial application.The material properties of TMO thin films?WO_x and Mo O_x thin films?preparation by HWOSD technology including deposition rate,optical properties,crystal structure,morphology,internal chemical compositions?including multiple chemical valence states inside the metal elements?,etc.were adjusted by varying the deposition conditions such as the oxygen flux?oxygen partial pressure or working pressure?,hot wire temperature and substrate temperature.It was found that:the TMO thin films prepared at low substrate temperatures were all amorphous;as the oxygen flux?oxygen partial pressure?and the hot wire temperature increase,the deposition rate of the TMO thin films also increases,and at the same oxygen flux and hot wire temperature,the deposition rate of Mo O_x thin films was about three times larger than that of WO_x thin films;the TMO thin films prepared under low oxygen fluxes had low light transmission,which may be due to the light absorption of the excess elemental metal;moreover,the metal elements in transition metal oxides with low oxygen contents also exhibit high concentration ratio of low positive chemical valences to high positive chemical valences.Mo O_x and WO_x were deposited on the surface of single crystal silicon wafers,which can passive the surface defects,and the corresponding effective lifetimes of the minority carrier₀1)1))were 356.1and 168.2,respectively.They play an important role in crystalline silicon solar cells.In addition,the W_xMo_1-xO₃ thin films and WO_x/Mo O_x thin films prepared by HWOSD with multiple hot wires.The characteristics of the various materials and their effects on the performance of crystalline silicon solar cells were systematically compared and studied.The device performance of DASH solar cells?Ag grid/ITO/WO_x/c-Si?n?/Mg/Al back contact structure?based on a variety of TMO HTL CSC thin films?about 10 nm?including conversion efficiency?current-voltage,I-V?,spectral response?EQE?,etc.was systematically studied.At the same time,the dark I-V and capacitance-voltage?C-V?tests were carried out to characterize the other parameters of the device including the diode factor,reverse saturation current and the build-in voltage(V_bi)etc..The highest conversion efficiencies for DASH solar cells based on different TMOs i.e.W_xMo_1-xO₃?Mo O_x?WO_x/Mo O_xand WO_x were 10.5%,9.9%,9.6%and 6.0%,respectively.By the post-annealing treatment on DASH solar cells in term of different annealing temperatures,and at the same time optimizing the material properties of the back-contact electrodes and transparent conductive electrodes,the performance of DASH solar cells had been improved further.The highest conversion efficiency of12.15%has been achieved,in which the V_OC,J_SCand FF were 630 m V,32.7 m A/cm²,and=63%,respectively.However,the conversion efficiency of the optimized DASH solar cells in this study is still low,which could be related to a high a high recombination in the TMO/c-Si heterojunction interface as well as to an insufficient light absorption within the solar cell?inadequate light trapping effect?.Thus,more works should be carried out further.