PSCs have attracted much research attentions these years. It is a promising energy technology due to its potential to produce flexible solar panels using low-cost solution processes. Detailed introduction and analysis are presented in chapter 1.

Recently, a family of high-efficiency donor polymers with strong temperature-dependent aggregation properties have been demonstrated. These polymers can be well dissolved in solution at high temperatures (~100℃), yet they can strongly aggregate when the solution cools down to room temperature. This leads to a highly favorable morphology containing highly crystalline yet reasonably small domains. With this TDA property, polymers can enable thick-film (~ 300 nm) PSCs with high FFs and PCEs.

What’s more, blend morphology is important in OSCs. Cha...[ Read more ]

PSCs have attracted much research attentions these years. It is a promising energy technology due to its potential to produce flexible solar panels using low-cost solution processes. Detailed introduction and analysis are presented in chapter 1.

Recently, a family of high-efficiency donor polymers with strong temperature-dependent aggregation properties have been demonstrated. These polymers can be well dissolved in solution at high temperatures (~100℃), yet they can strongly aggregate when the solution cools down to room temperature. This leads to a highly favorable morphology containing highly crystalline yet reasonably small domains. With this TDA property, polymers can enable thick-film (~ 300 nm) PSCs with high FFs and PCEs.

What’s more, blend morphology is important in OSCs. Characterization and analysis of the morphology is useful in understanding the device performances, thus helpful in further improvement of the PCE. Especially for the polymers with TDA property, the morphology study is not deep enough and needs to be carried out more.

In chapter 2, the Influence of processing parameters and molecular weight on the morphology and properties of high-performance PffBT4T-2OD:PC₇₁BM based OSCs was studied. High spin rate/high temperature conditions are found to significantly reduce polymer crystallinity and change polymer backbone orientation from face-on to edge-on. Most surprisingly, it is found that the median domain sizes of PffBT4T-2OD:PC₇₁BM blends processed at different temperatures/spin rates are nearly identical (around 30nm-40nm), while the average domain purity and the molecular orientation relative to polymer:fullerene interfaces can be significantly changed by the processing conditions. A systematic study is carried out to identify the roles of individual processing parameters including processing temperature, spin rate, concentration, and solvent mixtures. Furthermore, the effect of molecular weight on morphology control is also examined. These detailed studies provide important guidance to optimize various morphological parameters and thus electrical properties of PffBT4T-2OD-type materials for the application in PSC.

In chapter 3, the influence of fluorination was studied. As we know, temperature-dependent aggregation is a key property for some donor polymers to realize favorable BHJ morphologies and high-efficiency (>10%) PSCs. Previous studies find that an important structural feature that enables such temperature-dependent aggregation property is the 2nd position branched alkyl chains sitting between two thiophene units. In this work, we demonstrate that an optimal extent of fluorination on the polymer backbone is a second essential structural feature that enables the strong temperature-dependent aggregation property. We compare the properties of three structurally similar polymers with 0, 2 or 4 fluorine substitutions in each repeating unit through an in-depth morphological study. We show that the non-fluorinated polymer does not aggregate in solution (0.02mg ml^-1 in CB) at room temperature, which results in poor polymer crystallinity and extremely large polymer domains (~ 200nm). On the other hand, the polymer with four fluorine atoms in each repeating unit exhibits an excessively strong tendency to aggregate, which makes it difficult to process and causes a large domain (~55nm). Only the polymer with two fluorine atoms in each repeating unit exhibits a suitable extent of temperature-dependent aggregation property. As a result, its blend film achieves a favorable morphology and high PCE (>10%).

In chapter 4, the influence of carboxylate substitution on the morphological and electronic properties of donor polymers is studied. Two pairs of structurally similar terthiophene or quarter thiophene donor polymers with partial or complete carboxylate substitution on the alkyl side chains were investigated. It is found that the carboxylate substitution can enhance the crystallinity of the donor polymers and introduce larger and purer domains. Moreover, the polymers with the carboxylate substitution exhibit reduced bimolecular recombination due to the improved morphology. For device efficiencies, the terthiophene-based polymer, P3TEA (with 50% carboxylate substitution), exhibits the best performance (~9%). The alkyl side chains on P3TEA provide a typical temperature-dependent aggregation property, allowing for effective morphology control, while the carboxylate substitution deepens the HOMO level and enhances the crystallinity of the polymer. These benefits yield a near optimal morphology and high Voc value, and thus the best device efficiency among the polymers studied.

In chapter 5, domain purity in polymer/small molecule system is studied. Due to the structure similarity of the polymer and small molecule, phase separation in these blend films are not easy to identify. By selecting energy of the soft X ray, material contrast is tuned to be large. Therefore, domain purity is tested, which is useful in understanding the relationship between the material structure and device performance.