Organic photovoltaics has been caught researcher’s attention as an alternative light harvesting technology to silicon photovoltaics due to their light weight, mechanical flexibility and printing production captivity. Although huge progress has been achieved in the past few years, the limited choice of high performing electron accepting materials hinder the advance of OPV development. Therefore, extensive research on new high performing electron accepting material is needed.

In Chapter 2, a new synthetic route of a regio-regular bay-position-linked perylene diimide (PDI) trimer (PDI3) was proposed. The material properties and solar cell performance of PDI3 is investigated and it was found that PDI3 can achieve a power conversion efficiency (PCE) up to 5.9% with an fill factor (F...[ Read more ]

Organic photovoltaics has been caught researcher’s attention as an alternative light harvesting technology to silicon photovoltaics due to their light weight, mechanical flexibility and printing production captivity. Although huge progress has been achieved in the past few years, the limited choice of high performing electron accepting materials hinder the advance of OPV development. Therefore, extensive research on new high performing electron accepting material is needed.

In Chapter 2, a new synthetic route of a regio-regular bay-position-linked perylene diimide (PDI) trimer (PDI3) was proposed. The material properties and solar cell performance of PDI3 is investigated and it was found that PDI3 can achieve a power conversion efficiency (PCE) up to 5.9% with an fill factor (FF)of 61% in non-fullerene organic solar cells. This study introduces a new approach to design new high-performance small molecule acceptors for non-fullerene organic photovoltaics.

In Chapter 3, a pair of perylene diimide non-fullerene small molecular acceptors (SMAs) based on a phenazine (Phen) core with either benzene or thiophene linkers was reported. The main design rationale is to introduce a relatively parallel orientation for PDI units by connecting two PDI units on the same side of the phenazine core. While we find that the phenazine core is possible to introduce a parallel intramolecular orientation, it is also found that the choice of linker units has a major influence on the molecular geometry. As a result, the two SMAs, TPPhen-4PDI and TTPhen-4PDI, exhibit much different performances. The SMA with benzene linkers, TPPhen-4PDI, achieves a PCE up to 8.3%. The performance difference is explained by the significant difference in charge mobility and morphology from GIWAXS analysis. Our work provides important understanding on linker influence on structure property and provides guidelines to the design of high performance PDI-based SMAs.

In Chapter 4, we design and synthesize a family of SMAs with similar aromatic backbone as a reported high efficiency of 15.7% SMA named Y6 but different alkyl chains. In this paper to investigate the influence of alkyl chains on the properties and performance of Y6 type molecules. First, we synthesize a new molecule by swapping the positions of the 2-ethylhexyl (2EH) branch alkyl chain (on the nitrogen atoms) and C11 straight alkyl chain of Y6. The resulting molecule show much reduce of solubility, excessively large domains and decreased solar cell performance, which indicate that it is important to use branch alkyl chains on the nitrogen atoms in the inner bay area of the Y6. Next, we further optimized the branching position of the branched alkyl chain on the two nitrogen atoms. Comparing the performance of the molecules, the one with 3^rd-position branched alkyl chains exhibits optimal solubility, electronic and morphological properties, thus yielding the best efficiency of 16.0% in binary device. Further device optimisation including ternary strategy allows us to achieve a high efficiency of 16.74%. One of our best devices was certified by Newport under the new stress protocol (asymptotic scans, which are much demanding on stability than regular fast scans), yielding a certified efficiency of 16.42 %, the best certified efficiency reported to date for single-junction organic solar cells.