CuInSe₂Nanocrystals, Thin Films For Photovoltaic Application Based On Polylol-Solution Chemical Colloidal Ink Process

CuInSe₂（CIS） chalcopyrite compound is an important photo-absorbingsemiconductor in the development field of thin film solar cells due to its uniquephotoelectric property. In recent years, there have been increasing efforts indeveloping low-cost, non-vacuum solution process to fabricate CIS-based thin filmsolar cells, in which nanocrystals colloidal ink technique has been proposed. In thisdissertation, single-phase, well-dispersed and stoichiometric CuInSe₂nanoparticles,nanoplates and the mixed nanocrystals with near-granular polyhedral shape and flakeshape were synthesized by novel and facile hot-injection synthesis using polylol assolvents. The synthesized CIS nanocrystals were used to prepare stable colloidal inkswith ethanol solvent, and high-quality CIS thin films were deposited on glasssubstrates by dip-coating method using its colloidal inks. Well crystallized, dense andhighly （112） oriented chalcopyrite CIS thin films were obtained after annealing in Aratmosphere. The CIS thin films were used as light-absorbing layers to assemblephotovoltaic test devices with configuration of Mo/CIS/CdS/i–ZnO, n–ZnO/Ag, Alelectrode and photoelectric performances were measured.The research results showed that the dispersity of CIS nanoparticles synthesized byN2H4H2O-assisted hot-solvent process could be improved by using relatively largemolecule polyol such as triethylene glycol （TEG） when compared with ethylene （EG）and diethylene glycol （DEG）. Well-dispersed chalcopyrite CIS nanoparticles with112phase stoichiometry could be synthesized through TEG solvent process. The averagesize of CIS nanoparticles with polygonal morphology were about10.5nm. Highsynthetic temperature led to large average size and narrow size distribution. Thereaction analysis showed that nucleation of ternary CIS compound was rapid and thenuclei directly grew into CIS nanoparticles with a very fast growth duration within1min. Extending reaction time obviously unchanged the average size of the CISnanocrystals but the Ostwald ripening could lead to slight size increase and narrowsize distribution.Well-dispersed, polycrystalline sphalerite CIS nanoplates with rough plate surfaceand diameter ranged from100to600nm and well-despersed, hexagonal orhexagonal-like, monocrystalline,（111） oritened sphalerite CIS nanoplates withdiameter ranged from200to400nm and diameter-thickness-ratio of about10couldbe synthesized by using TEG as solvent and ethylenediamine （En） as reducing agent under different injection style. The products at different reaction stages wereinvestigated by XRD, TEM, HRTEM, SAED, XPS and EDX analysis. The resultsshowed binary CuSe nanoplates were firstly formed and then transformed to ternaryCIS nanoplates by gradual reduction of cupric Cu²⁺ to cuprous Cu⁺, In³⁺ and activeSe^2-participation at growing lattice sites. The PVP molecule can obviously improvethe dispersity of CIS nanocrystals by coordinating acly group on the surface ofnanocrystals. Well-dispersed sphalerite CIS nanocrystals with granular-andflaky-shaped mixed morphology were synthesized by both adding N₂H₄·H₂O and Enin TEG solvent. The relative ratio of granular-shaped CIS nanocrystals andflaky-shaped CIS nanocrystals could be tuned by the relative adding proportion ofN₂H₄·H₂O and En.The flat, dense and crack-free CIS thin films were prepared by dip-coating processusing well-dispersed CIS nanocrystals inks. Annealed in Ar atmosphere, the densityand crystallinity of the CIS thin films were enhanced. The （112） oriented factor of CISthin films rose from1.2to4.5by using the nanocrystals inks with nanoparticles andnanoplates mixed morphology. The sphalerite CIS phase would transformed to thechalcopyrite CIS phase which was more stable in thermodynamics after annealing at580°C in Ar atmosphere. The density and crystalline size of CIS thin films wereobviously enhanced and the （112） oriented factor reached270.9by using thenanocrystals inks with single nanoplates morphology after annealing at580°C.The CIS colloidal inks were used to deposit light-absorbing layers forphotovoltaic test devices with cell structure of Mo/CIS/CdS/i–ZnO, n–ZnO/Ag, Alelectrode. The V_OC, J_SC, FF and conversion efficience of the PV devices derived fromgranular-and flaky-shaped mixed nanocrystals inks were255mV,2.1mA·cm^-2,0.29and0.16%, respectively, under AM1.5irradiation. The V_OC, J_SC, FF and conversionefficience of PV cells from single CIS nanoplates inks were249mV,1.97mA·cm^-2，0.36and0.177%, respectively.