A Study on Device Engineering-Material Properties Relationships Toward Scalable Organic Solar Cells

Abstract

Organic solar cells (OSCs) have received immense attention as they possess properties such as lightweightness, flexibility, and low fabrication cost. To date, the rapid material development and progress of relevant device structures enables dramatic enhancement in power conversion efficiency (PCE) within the range of 18%. The PCE of OSCs has reached the stage of commercialization, therefore, scalable production is regarded as an emerging subject. To produce OSCs on an extensive scale, it is essential to investigate the properties of various materials and appropriate device engineering. Thus, this thesis concentrates on device engineering-material properties relationship in order to fabricate large-area, enhance stability, and reduce material cost, with the goal of scalable OSCs. In the first study, to realize thick-film OSCs for large-area production of OSCs, C60-containing polystyrene (PS-C60) was synthesized and used as a non-volatile polymer additive. The addition of PS-C60 led to an efficient charge extraction and superior charge transport, enabling over 7% PCE at a 450 nm active layer thickness. In addition, the morphology locking properties of PS-C60 additive enabled thermal stability of active layer film, maintaining over 80% of initial PCE even though the film was heated at 150 °C for 8 hours. Toxic solvent fabrication is a factor that significantly hinders mass production of OSCs. Accordingly, in the second study, well-soluble p-type and n-type random copolymers were developed to fabricate nontoxic-solvent-processed high-performance all-polymer solar cells (all-PSC). By controlling the ratio of the well-known accepter unit (FTAZ and BDD units) of the p-type polymer, miscibility, and energy level aligning with the n-type polymer were finely controlled. Consequently, nontoxic-solvent-processed all-PSC with an impressive PCE (~8%) was obtained. Fabrication of defect-free cathode interlayer in conventional device structure is a central issue in large-area fabrication. In the conventional structure, cathode interlayers are fabricated using methanol solvent, which results in a defect when fabricated on top of a hydrophobic photoactive layer. Therefore, in the third work, a defect-free cathode interlayer is realized by fabricating organic passivation layer with a simple vacuum deposition processable T2-CNORH on a photoactive layer. This method significantly reduced the methanol contact angle, enabling the implementation of the defect-free cathode interlayer. Finally, a simple batch-mixing protocol using a formulated equation was developed to achieve polymeric photovoltaic material with predictable molecular weight, which is deemed as an indispensable prerequisite for reducing material cost. Through this protocol, we implemented various molecular weight batches of PM6 with the range of 30 ~ 130 kDa. Among the batches, the batch of molecular weight of 120 kDa shows the optimized PCE of 16.5%, which equals or exceeds the results of the previously reported PM6 based device.