New porphyrinoid donors and absorption complementary non-fullerene acceptors for high efficiency organic solar cells

Porphyrins are the synthetic compounds that mimic Nature’s chlorophyll for solar energy conversion. Their effectiveness as an organic dye in dye-sensitized solar cells has been successfully demonstrated. As for bulk heterojunction organic solar cell (BHJ OSC) application, neither porphyrin polymers nor small molecules (SM) have less contribution and the main performance impediments are the unfavorable aggregation, short exciton diffusion length and the low charge mobility. In addition, the use of conventional device architecture with fullerene as acceptor could not complement the weak absorption in the range of 500–600 nm, specifically for porphyrin small molecules, which nevertheless have defined molecular structure and molecular weight, high purity, and less batch-to-batch variations in comparison with their polymer counterpart. In order to fully unravel the potential of porphyrin-based small molecule solar energy conversion devices, such intrinsic material properties must be optimized by improving the optical, chemical and electronic properties via novel design and synthesis at a molecular level. This proposal will address the technical issues and solutions.

First, the issues stated above are expected to be resolved by new porphyrin structures. For example, the introduction of heteroatoms (P, S, O, Se, Te) into the core of porphyrin can lead to changes in the cavity size and electronic structure, thereby altering the optical, electrochemical, and photochemical properties. This will lead to a significant enhancement in charge mobility due to the unique donor/acceptor nature formed within the macrocycle. Moreover, less coplanar and metal-free structure due to steric crowding of central larger heteroatom can be expected to suppress effectively the aggregation.

Secondly, alternative non-fullerene acceptors with complimentary absorption to porphyrins have to be developed for overcoming the small photovoltage, high cost and other limitations for fullerene based OSCs. And azadipyrromethene dyes and their complexes were selected as non-fullerene acceptor candidates because of their complementary absorption to the porphyrins and low reduction potentials. The aim is to seek a non-fullerene acceptor that performs efficiently with porphyrin donors.

Moreover, the conventional 3,5-alkyl substituted phenyl ring protruded out of the porphyrin plane has been adopted to improve the solubility and suppress aggregation. However, the suppressed intermolecular π–π stacking results in low charge mobility and consequently a relatively poor photovoltaic performance. Herein, replacing the phenyl ring with direct alkyl chains in different patterns are expected to decrease the tendency of crystallization and enhance the solubility as well as intermolecular π–π stacking, leading to a new class of pophyrins with high charge mobility for SM BHJ OSC.

It is expected that with judicious refinement of porphyrinoid donors and non-fullerene acceptors and selection of processing conditions, and further investigation of mechanism of charge generation, improved efficiencies as high as the latest one can be obtained. There is no substitute in nature for chlorophyll. Why not the same with the artificial solar energy conversion?