Development Of Transparent P-N Heterojunctions For Electrical And Optoelectrical Device Applications

To date,amorphous silicon（a-Si）has long been the sovereign of large-area flat panel displays,p-n junction diodes,organic/inorganic photovoltaic cells,and so on.However,the conventional Si-based semiconductors and devices could not meet the current global technological demands regarding electrical and optoelectronic applications,in particular where high transparency is required.This critical situation is adeptly tackled by the transparent semiconducting metal oxides（TSMO）by realizing the next-generation flexible and fully transparent devices and circuits even on plastic substrates at relatively low temperature.As our preliminary studies,the promising binary-oxides,such as zinc oxide（ZnO）and tin monoxide（SnO）have been investigated as n-and p-type transparent semiconductor materials,respectively.ZnO is a well-known wide band gap semiconductor possessing a high mobility and transmittance.Likewise,SnO is a proficient metal oxide（MO）which can exhibit bipolar conduction with a direct band gap of².7 eV.As an initial step,ZnO and SnO thin films have been synthesized by rf-magnetron sputtering at optimized deposition conditions to fabricate all-oxide p-n heterojunction diodes.In order to improve the carrier injection/extraction,a very thin layer of indium-zinc oxide（IZO）as an interfacial layer was sputtered onto ZnO film.In turn,all-oxide p-n heterojunction diode with an architecture design of（top electrode）IZO/ZnO/SnO/ITO（bottom electrode）were successfully fabricated,exhibiting a strong I-V rectifying behavior in the dark with forward to reverse current ratio of⁵00 at 2 V along with turn-on voltage of¹.6 V.Other diode parameters such as ideality factor of 3.7,series resistance of 3.83?,and built-in voltage of 0.5 V were also extracted from I-V and C-V measurements.Based on experimentally measured and already known parameters,the energy band diagram was established to estimate the valence and conduction band offsets（i.e.,ΔE_V¹.08 eV andΔE_C⁰.41 eV）.It is highlighted that the inclusion of a thin interfacial layer can boost the carrier injection/extraction and consequently,the potential barrier dependent carrier transportation across the space charge region.Next,a relatively new ternary alloy zinc-tin nitride（ZnSnN₂）belongs to II-IV-V₂materials was exploited as n-type semiconductor.Owing to its ideal band gap in the ranges of1.42-2.2 eV（i.e.,UV-to-near IR）,this material is regarded as a potential absorber for photovoltaic applications.So,in this phase,a novel p-n heterojunction was fabricated by pairing SnO and ZnSnN₂ materials for solar cell applications.P-type SnO and n-type ZnSnN₂thin films were synthesized via electron beam evaporation（EBE）and dc-magnetron sputtering at room temperature,respectively.Consequently,thin film solar cells based on p-n heterojunction with a structure of Al（top-electrode）/ZTN/SnO/ITO（bottom-electrode）have been successfully fabricated.The device performance was examined through selective deposition of top electrode with different materials（e.g.,Ni/Au or Al）.It is revealed that the process of electron accumulation at the cathode was highly facilitated by selecting the metal electrode with relatively low work function（i.e.,Al-electrode）.The heterojunction demonstrated a remarkable J-V rectifying response in the dark with a forward to reverse current ratio of 3×10³,an ideality factor of 4.2 by incorporating Al-top electrode.Under illumination,the inorganic solar cell displayed the maximum power conversion efficiency（PCE）of 0.37%with J_sc of 4.16 mA/cm²,V_oc of 0.25 V,and FF of 0.36.The proposed study highlights the basic perspectives of ZnSnN₂-based solar cells regarding generation,separation,transportation,and accumulation of photo-generated carriers.The further experimental investigations were performed to enhance the efficiency of solar cells.So,in this phase,different kinds of heterojunction devices have been synthesized based on device structure design and annealing treatment.The materials and their growth conditions were similar to those of former experimental studies,except the thickness of thin films and device structure.A very thin layer of dielectric material Al₂O₃ was introduced as a buffer layer in between n-type ZTN and p-type SnO thin films to mitigate the heterojunction non-idealities.Three-heterojunction devices were fabricated,named as device-1（as-deposited P-N heterojunction）,device-2（as-deposited P-i-N heterojunction）,and device-3（Overall post-annealed P-i-N heterojunction）.The P-i-N heterojunction based devices exhibited rather improved performance as compared to P-N junction one.In fact,the inclusion of Al₂O₃buffer layer provided surface passivation to SnO by serving as a hole-blocking layer to hamper the flow of photo-induced holes from p-SnO to n-ZnSnN₂ to perk up the directional carrier-transportation.Buffer layer also tuned the conduction band discontinuity by rising the built-in-voltage（²-times）to enhance the V_oc.Under 1-sun spectra,it exhibited the V_oc of⁰.36 V,J_sc of⁷.5 mA/cm²,FF of⁰.57,and PCE of¹.54%.Moreover,the post-fabrication annealing was proved rather beneficial regarding tuning of the interface effective-energy-gap to realize a cliff-type band offset,resulting in decreased recombination losses.Our present experimental study realizes a promising approach towards low-temperature,low-cost fabrication of thin film-based heterostructure devices for various electrical and optoelectrical applications.