Applications Of Metal Oxide In Silicon-Based Dopant-Free Heterojunction Solar Cells

Photovoltaics（PV）have become one of the most important renewable energy sources in the competitive market through decades of development.The major challenges for the further progress of PV technologies rely on the cost reduction and cell efficiency improvement.Among the many structures of solar cells,passivation contacts（PSCs）,which provide both the excellent passivation and electrical contact properties,have been considered to be an ideal cell design.Two types of high-efficiency crystalline silicon solar cells,i.e.,amorphous silicon（a-Si）/crystalline Silicon（c-Si）-Hetero Junctions（SHJ）solar cells,and Tunnel Oxide Passivating Contact（TOPCon）solar cells,both feature this design concept.However,traditional PSC materials,overly rely on the complex doping and annealing procedures,which not only cause inevitable optical parasitic absorption or Auger recombination losses,but increase heat budget and cost.Thus,dopant-free heterojunctions,which manipulate surface engineering of silicon wafers by their extreme work functions and ingenious band structures,could address these issues and therefore have received a considerable attention from researchers.Meanwhile,due to their advantages of adjustable work functions and suitable energy band structures,metal oxides have been widely exploited to build dopant-free heterojunctions.However,a series of problems remain to be solved for this type of dopant-free heterojunction PSCs.In terms of passivation,the existed preparation process of electron transport material（taking Ti O_x as an example）,i.e.,atomic layer deposition（ALD）,magnetron sputtering,and plasma chemical meteorological deposition（PECVD）,keeping a balance between the film properties and the control cost,is a troublesome problem.In terms of contact properties,hole-selective contacts（HSCs）deposited on n-type silicon（n-Si）substrates are Schottky heterojunctions other than Ohmic contacts,impeding the accurate extraction ofρ_c values by the conventional Cox&Strack Method（CSM）and Transfer Length Method（TLM）.In addition,the charge carrier transport may be affected by the defect states inherent to the transition metal oxides.Meanwhile,the work functions of transition metal oxides are sensitive to the external enviro nment,which may lead to the S-shaped IV curves and cause severe degeneration of device performance.Targeted to the existing problems in the field of metal oxide dopant-free heterojunction PSCs,this thesis presents a systematic research from three perspectives:passivation,contact and transport properties.In terms of passivation,to balance the passivation performance and fabrication cost,a novel low-temperature oxidation（LTO）process is developed to fabricate an oxides film with high passivation performance.Taking Ti O_x as an example,an electron-selective Ti O_x film was fabricated through this method,reaching a low effective surface recombination velocity of 13.7 cm/s.The high-quality passivation performance is mainly attributed to the excellent chemical passivation of Ti-O-Si bonds associated with the surface carrier manipulation ability of LTO-Ti O_x film.Next,by adding a low work function（WF）layer of Li F_x film,the surface band bending effect is formed and the contact resistivity of Ti O_x-based PSC was reduced.Furthermore,the dual functional LTO-Ti Ox/Li F_x electron-selective PSCs are integrated into Si solar cells,and an improvement of device parameters was achieved,i.e.,V_oc from 592 m V to 639 m V,J_sc from 35.12 m A/cm² to 36.91 m A/cm²,FF from77.20%to 79.74%,yielding a photovoltaic conversion efficiency from 16.1%to18.8%.Finally,a comprehensive simulation analysis followed by a detailed technical roadmap of efficiency improvements predicts that a high efficiency beyond 22%based on LTO-Ti O_x PSCs can be achieved,demonstrating their huge application potential in dopant-free heterojunction solar cells.In terms of contact,for the sake of solving that the specific contact resistances of dopant-free Schottky heterojunctions are difficult to accurately measure,we developed an expanded CSM.The series resistances of Schottky heterojunctions are firstly separated from the rectification effect by the Chueng method,and then utilized the traditional CSM to calculate the specific contact resistances.For clearly demonstrating the procedure of expanded CSM,the expanded CSM and TLM are both utilized to measure the_c values of Mo O_x/n-Si Schottky heterojunctions.The results show that the_c values of simples with 10,20,and 30 nm Mo O_x are 55,160,and 317 mΩ·cm² with generally lower coefficient of variation 27.6%,31.3%,and17.4%by the expanded CSM,respectively.As a comparison,the TLM measurements suggest that theρ_c values of simples with 10,20,and 30 nm Mo O_x are 159,245,and395 mΩ·cm² with coefficient of variation 23.5%,40.9%,and 20.6%,respectively.The similar values are also achieved by the Silvaco simulation,demonstrating the effectiveness of the expanded CSM model for extracting_c of Schottky heterojunctions.In addition to the better test accuracy than TLM,an additional advantage of avoiding the problem of uneven current distribution under the electrodes in the TLM model,is endowed to the expanded CSM.Meanwhile,the expanded CSM method is compatible with the lithography process with higher sample geometry accuracy and suitable to measure the specific contact resistance of Schottky heterojunctions composed of various hole transporting materials with different conductivity and n-type silicon.In terms of carrier transport,targeted to the problem that Mo O_x contains a large nmber of defect states and has a large variation of work functions,which affects the device characteristics,we investigate the ability of Mo O_x/a-Si/c-Si and Li F_x/a-Si/c-Si heterojunctions to regulate the energy bands and carrier concentrations of silicon surface at different voltages based on the interdigitated back contact solar cells.Then we research the influence of the defect density of states（DOS）and work functions of Mo O_x on the carrier transport properties as well as the light-state IV characteristics by means of parameter traversal.It is found that too low DOS will lead to the S-shaped IV characteristic curves,which is due to the direct dependence of trap-assisted tunneling on the defect states.If the work function of Mo O_x is less than 5.3 e V,the device V_oc increases with the increases of the work function of Mo O_x.As the work function of Mo O_x greater than 5.3 e V,the quality of the light-state IV characteristics is improved with the increasing work function of Mo O_x.As a result,the S-shaped light-state curves are partly remitted.Finally,the S-shaped IV behavior disappears when the Mo O_x work function is greater than or equal to 5.7 e V,which is because the crossover between the E_v（top of the valence band）of a-Si and E_c（the bottom of the conduction band）of Mo O_x triggers the direct band-tunneling.Around the three existing problems of dopant-free heterojunctions,i.e.,passivation,contact,and carrier transport,a series of work is done via the well-designed experiments and simulations.A low-temperature oxidation method is developed to prepare high-performance oxide passivation films.The extended Cox&Strack method is explored to test the specific contact resistance of Schottky heterojunctions.The influence of DOS and work function of transition metal oxides on carrier transport properties and device light-state IV characteristics are also investigated by TCAD simulations.Although we have carried out targeted researches on the mentioned problems of dopant-free heterojunction passivation contacts,there are still some pressing issues,such as the abstraction of physical models for specific contact resistance measurement in complex Schottky heterojunctions and experimental testing of defect state distributions of transition metal oxides.We discuss these issues and will address them in the future work.