Organic solar cells (OSCs) have recently attracted broad attentions due to its advantages of simple structure, low cost, light weight, flexibility, transparency, and portability. The field of non-fullerene OSCs has seen an impressive progress during the last few years mainly due to the development of non-fullerene acceptors (NFAs). Whereas, the structure-property relationships regarding photophysical and morphological aspects remain unexplored. In this thesis, I will introduce my research results through a very small aspect of alkoxy substitution on non-fullerene acceptors, which, however, could have a big effect on the understanding of the side chain effect and structure-property relationship and inspire community to further develop materials and push efficiencies of OSCs to a higher l...[ Read more ]

Organic solar cells (OSCs) have recently attracted broad attentions due to its advantages of simple structure, low cost, light weight, flexibility, transparency, and portability. The field of non-fullerene OSCs has seen an impressive progress during the last few years mainly due to the development of non-fullerene acceptors (NFAs). Whereas, the structure-property relationships regarding photophysical and morphological aspects remain unexplored. In this thesis, I will introduce my research results through a very small aspect of alkoxy substitution on non-fullerene acceptors, which, however, could have a big effect on the understanding of the side chain effect and structure-property relationship and inspire community to further develop materials and push efficiencies of OSCs to a higher level. This thesis is constituted of five chapters:

Chapter I is the general background of organic solar cells, including the working mechanisms, characterization methods, development history of photo-active organic materials, and the motivation of this thesis.

Chapter II presents a novel NFA named IDTN-O by substituting methoxy side chains to a reported molecule named IDTN. Typically, alkoxy substitution reduces the bandgap and the open-circuit (V_oc) of corresponding devices. In our case, methoxy substitution on IDTN yields promising effects on energy levels and aggregation. The LUMO and HOMO levels of IDTN-O are slightly upshifted, which leads to a slightly higher V_oc of corresponding devices. Besides, the methoxy groups can introduce the intramolecular conformation locking effects to enhance the π-π stacking of the molecules to achieve higher mobility and FF of the devices. As a result of methoxy substitution, OSCs based on J71: IDTN-O can achieve efficiencies of 12.05%, which is higher than that of J71: IDTN based devices (10.93%). In addition to higher efficiencies, this work is also one of few examples of structural modifications on the β-position of the outer thiophene unit in IDT core.

Chapter III reports a simple yet highly effective molecular design strategy (for non-fullerene acceptors) that has led to OSCs with a new world-record efficiency of 17.6% and that also has wide-ranging applicability. Using symmetric alkoxy side chain substation on on the β-position of the outer thiophene unit in Y6, the resulting molecule Y6-2O suffered from extremely poor solubility (only 1mg/mL) and performance. However, applying asymmetric alkyl and alkoxy substitution, much imporoved solubility (20mg/mL) and outstanding solar cell performance can be achieved based on the asymmetric molecule named Y6-1O. Our in-depth morphology study also reveals the underlying reasons for the great performance enabled by asymmetric substitution. In short, the oxygen atom on the alkoxy chain can introduce the well-known “conformation locking” effect, which has both positive and negative effects (enhanced photo-voltage and molecular packing but poor solubility). Using asymmetric alkoxy and alkyl substitution strategy, the resulting molecule can largely maintain the beneficial effects, while minimizing the solubility problem, because asymmetric molecules typically have much higher solubility than symmetric molecules. Considering that nearly all non-fullerene acceptors in the OSC field have symmetric side chain substitution. Our design strategy of asymmetric side chain substitution can be readily applied to thousands of known molecules and offer tremendous opportunities to further improve the performance of OSCs.

Chapter IV is the continuous work of Chapter III ,which presents a novel NFA named Y6-O by replacing alkyl chains with alkoxy side chains and changing inner branched alkyl chains from 2-ethylhexyl (2-EH) to 2-butyloctyl (2-BO). In this case, alkoxy substitution on Y6 yields promising effects. The LUMO level of Y6-O is upshifted significantly which leads to an improved V_oc of 0.95 V, 0.13 V higher than that of Y6. As a result of alkoxy substitution, we can achieve high-performance devices with efficiencies up to 16.6%, which is one of the highest value reported for binary OSCs to date. In addition to high efficiencies, this work is also one of few recent examples of structural modifications on Y6.

Chapter V is the summary of the thesis, and future perspectives regarding the further development of high-performance non-fullerene OSCs.