Degradation factors in solution processed organic solar cells

Abstract

Organic solar cell is one of the strongest candidate for next-generation photovoltaics owing to its numerous merits such as solution processing, low cost fabrication, high absorption coefficient, etc. Contrary to the advantages, their commercialization has been delayed because of their relatively low efficiency and operational stability compared to conventional inorganic solar cells, such as Si, GaAs, etc. In order to resolve the low efficiency of organic solar cells, a number of studies regarding synthesis of efficient polymers, interfacial engineering and structural modification have been reported, validated through the certified efficiency of 11.5% by national renewable energy laboratory (NREL). However, intrinsically vulnerable stability has yet to be solved. Although a number of studies regarding degradation of organic solar cells have been reported, only a few of them focused on the detailed degradation mechanism. <br/> In this thesis, we investigate the specific degradation mechanism of organic solar cells focusing on the correlation between internal composites and external degradation factors. Based on the inverted structure, organic solar cells showed different stability against photo-induced degradation with respect to the electron extraction layer (EEL). In particular, devices with ZnO nanoparticles (ZnO NPs) and sol-gel processed ZnO (ZnO sol-gel) as EEL were exposed to 1 sun irradiation, which showed distinct difference in open-circuit voltage (Voc) variation. Taking into account the former researches regarding Voc deficit, the difference was analyzed on the basis of non-radiative recombination and energetic disorder. In order to extract the amount of respective factors, electroluminescence and impedance spectroscopy of organic solar cells were measured. It is revealed that the different Voc degradation with respect to variant EEL is originated from both non-radiative recombination and energetic disorder, where the energetic disorder was found to be the dominant factor for both cases.<br/> Besides, the effect of EEL on the thermal stability has been further studied. Through exposing OSCs in thermal stress of 85°C with different EEL, it is revealed that EEL not only affect the photo stability but also thermal stability. In particular, devices with sol-gel processed Al-doped ZnO (AZO sol-gel) were fabricated and compared to that with pristine ZnO sol-gel in terms of thermal stability. Through introducing AZO sol-gel, the efficiency reached 8.65% and retained 70% of its initial performance even after severe condition of 85°C for 6 hours, which is a 46% improvement in stability compared to pristine ZnO. Based on the trap-related analyses and Mott-Gurney law, the improvement is revealed to be originated from reduction of oxygen defect sates and space-charge limited current (SCLC) region.<br/> Whereas the previous studies focused on enhancing the stability through interfacial engineering, in this chapter we focused on modifying the active layer from conventional bulk heterojunction (BHJ) structure to sequentially deposited structure. In accordance with former studies, it is evident that BHJ structure is unfavorable in terms of thermal stability because of the vulnerable nanophase morphology. However, sequentially deposited active layer, which possess the advantages of both efficient BHJ and stable bilayer structures, makes it possible to fabricate highly efficient and thermally stable OSCs. In this chapter, we figure out the correlation between solvents used in sequential deposition and performance, stability on the basis of crystallinity characterization tool, grazing incidence wide angle X-ray scattering (GIWAXS). It is found that not only bulk crystallinity but also top layer crystallinity in sequentially deposited active layer significantly affects the performance and thermal stability of OSCs with sequentially deposited active layer.<br/> In conclusion, exhaustive investigation on degradation mechanism of OSCs in terms of interfacial layer and active layer has been conducted. In particular, EEL has been revealed to be crucial to both photo- and thermal-induced degradation, which is originated from the different contribution of non-radiative recombination and energetic disorder. Furthermore, adoption of sequentially processed active layer instead of BHJ layer significantly increased the thermal stability, which is confirmed through the crystallinity inside the active layer. These results will provide useful insights into developing highly stable OSCs.<br/>