Study On Flexible Polyimide Substrate For Active Matrix Organic Light Emitting Diodes Display

Large progress about flexible display technology has been obtained in last few years.Flexible AMOLED attracts a lot of attention due to its fast response speed,wide viewing angle,excellent color gamut and ultra-portable properties.PI is one of the polymers that have highest glass transition temperature.Meanwhile,PI possesses good chemical resistance and mechanical properties.It was regarded as the most promising flexible substrate.This thesis summarized the basic requirements of properties of PI needed by the complicated process of flexible AMOLED.Consider to the condensation dehydration process of polyamide acid,I studied the influence of soft-baking on the performance of PI.High performance PI substrate was obtained by modifying the curing profile.Infrared curing technology was also used to facilitate the imidization of the polyamide acid.It can significantly improve the efficiency of thermal imidization of transparent PI.In order to prevent the adhesive force in the PI/glass interface from increasing during the TFT process and thus failure in de-bonding the flexible display,I studied a variety of de-bonding layer and a low damage mechanical de-bonding method was proposed to withstand high annealing temperature in TFT fabricating proess.The releasing of residual stress in device easily causes un-controllable curving phenomenon of ultra-thin flexible AMOLED.Based on Stony equation,curvature method was used to characterize the residual stress of each film in TFTs,as a result mapping the residual stress distribution in the TFT and clarifying the relationship between residual stress and radius of curvature of flexible devices.According to the simple beam theory,13 curving situations induced by residual stress releasing were summarized,and the micro-mechanical behaviors of typical situations were discussed in detail.Finally,4 methods of stress management were proposed to control stress releasing and curving of flexible electronic devices.Consider to the relatively low light out-coupling efficiency in flexible OLED,a new method that fabricating ultra-thin PI substrate with optical structure was reported in this thesis.Transparent PI with a higher refractive index of¹.8?PC,¹.58,PET,¹.5?was used as the flexible substrate,in view of the PI film could be a good candidate to extract the light trapped in the organic/ITO modes when considering the issue of refractive index matching.Moreover,it is a potential candidate not only for lighting but also for display,because the severe image blurring phenomenon in traditional display with light extraction structure can be well controlled by shortening the light horizontal propagation distance in thinner PI substrate.Comparing to the OLEDs fabricated on PI substrates without light extraction structure,the proposed flexible OLEDs showed a maximum gain factor of¹.7 for current efficiency at large viewing angle and 1.37 for external quatum effieciency.The proposed OLEDs exhibit excellent angular optical properties including stable CIE coordinates with?x=-0.006 and?y=0.002 as the viewing angle varies from 0°to 60°.Surface scattering effect of the 3-D photonic structure eliminates the periodic distortion phenomenon in electroluminescence spectrum of flexible OLEDs.The variation of current and normalized luminance of OLED were less than±5%after the 10000 times repeted bending test with bending radius of 20 mm and bending frequency of 1 Hz,showing excellent bending stability.This thesis studied the key technology and related scientific issue in TFT array fabrication process.Short-channel and high mobility TFTs are contributed to the display with high resolution,scale down of TFTs and peripheral circuit integration in display panel.Dry etching method of ICP was introduced to fabricate back-channel etched structure TFT.The key parameters of ICP etching process were studied and the etching rate,etching profile and uniformity of source/drain metal Mo were modified.The influence of SF₆ plasma on the NAIZO active layer was explored in depth by XPS.During the deposition of SiO₂ passivation layer,OH^-was introduced in the back-channel of NAIZO,forming the charge compensation effect with interstitial F atoms which acts as electron traps in the channel,thereby NAIZO TFT was successfully fabricated by ICP.In order to achieve high mobility,dual active layer structure was used.By modifying the structure of back channel,bombardment of SF₆ plasma was suppressed and the front channel of IZO was protected.High performance back channel dry etched type amorphous oxide TFT with mobility up to 57 cm²V^-1s^-1,V_th of 0.46 V,SS of0.137 V/dec and excellent bias stability was fabricated for the first time.