Inkjet printing technology has attracted extensive attention in the application of optoelectronic devices because of its low ink consumption, high-resolution pattern, and flexible production scale. However, one of the primary goals of inkjet printing is to achieve uniform deposition of functional films, which is dependent on stable ink droplet jetting. It is essential to achieve stable jetting by the adjustment to the physical properties of inks as well as the printing parameters. Much effort has been put into the development of inkjet inks, including the rheological properties of the inks, such as viscosity, viscoelasticity, and surface tension, which allows them to produce excellent spatially homogeneous films. Although the quality of the devices has been assured by better consistency in the films, they all suffer from low productivity and long cycles when printing with a single nozzle. There is an urgent need to develop new manufacturing technology with high economic efficiency for large-area uniform and efficient printing production. As a viable alternative, multi-nozzle inkjet printing not only overcomes the limitations of mass production but also provides better performance, higher prospective advantages, and lower costs. Nevertheless, many issues impede its future applications and must be addressed. One of the most challenging issues is the thickness variation of deposited films injected through multiple nozzles simultaneously. The inhomogeneity is caused by the variation in the volume of the droplets injected by different nozzles. Some researchers suggest that the issue should be addressed by a deep-learning-based strategy, which can identify and moderate the droplet jetting status of a single-jet printing process. However, this approach is unwieldy used due to unavoidable external constraints on practical applications.

In the present study, printing PEDOT∶PSS is a key tactic for balancing the interaction among nozzles and overcoming the unevenness of injecting. Our work facilitates the application and development of large-scale inkjet printing technology. For the manufacturing of large-scale luminescent display panels, it is crucial to achieve stable and uniform ink droplet injection for multi-nozzle inkjet printing.

In this study, a PiXDRO LP50 multi-nozzle inkjet printing system, consisting of an optical observation system for ink droplet alignment and a printing device, is employed. The optical observation system is required for temporary regulation to align the substrate and ink droplets and correct the ink droplet condition prior to printing. Certain software is used to randomly activate and number 10 of the 128 nozzles, which all have the same basic properties, a pitch of 508 μm, and a diameter of 27 μm. To achieve stable injection and uniform droplets for multi-nozzle inkjet printing, the PEDOT∶PSS ink is used for the analysis of the multi-nozzle inkjet printing parameters, including driving high voltage V_H, low voltage V_L, rising edge to driving high voltage time T_R, driving high voltage time T_P, and falling edge to driving low voltage time T_F. In addition, a method for calculating the variance is developed to investigate the behavior of droplet volume variances to reveal the influence of printing parameters on droplet uniformity in the multi-nozzle inkjet printing process. Specifically, the states of droplets injected with various printing parameters are obtained and evaluated. The optimal printing parameters are then determined by the droplet volume as a function of the printing parameters, and the relative relevance of various printing parameters is calculated by the range operation.

The multi-ink droplets with excellent volume uniformity are obtained by the adjustment to the driving-voltage waveform parameters for inkjet printing, including driving high voltage V_H, low voltage V_L, rising edge to driving high voltage time T_R, driving high voltage time T_P, and falling edge to driving low voltage time T_F. The ink droplet volume is precisely distributed in the range of 19.3-19.5 pL (Fig. 8). The variations of the volume variance of droplets injected simultaneously by multiple nozzles are evaluated, and the minimum variance of the impact of each change in an inkjet printing parameter on the volume of droplets is about 0.006. The range deviation of inkjet printing factors is determined by the minimum variance, which shows the volume uniformity of droplets injected simultaneously by multiple nozzles with different inkjet printing parameters. A small range deviation indicates that the factor has less ability to impact the collective volume variance of the injected droplets. A large range deviation means parameter changes have a greater impact on the collective volume variance of droplets. According to the range deviation of the volume variance of each group of droplets, the ability of inkjet printing parameters to affect the volume change of droplets is ranked as follows: T_R, T_P, V_H, T_F, and V_L (Table 1).

The utilization of multi-nozzle inkjet printing to inject uniform droplets is the key to obtaining large-area luminescent layers for various types of optoelectronic devices. In this study, PEDOT∶PSS ink is used for multi-nozzle inkjet printing, and the driving voltage waveform parameters for inkjet printing, including high voltage V_H, low voltage V_L, rising edge time T_R, peak time T_P, falling edge time T_F, are analyzed in detail. Moreover, the analysis of the volume variance of inkjet droplets is proposed to obtain stable and uniform inkjet droplets sprayed from multiple nozzles through the adjustment to these parameters. The minimal volume variance of many nozzles operating simultaneously is found to be approximately 0.006. The effect of driving voltage factors on droplet behavior is ranked according to the variations of the drop volume variance for multi-nozzle inkjet printing. The ability of driving-voltage parameters to affect inkjet droplet behavior for multi-nozzle inkjet printing is ranked as follows: T_R, T_P, V_H, T_F, and V_L. The research provides significant guidance for realizing large-area electronic devices by multi-nozzle inkjet printing.