Understanding the morphology of solution processed fullerene-free small molecule bulk heterojunction blends

Abstract

Bulk-heterojunction (BHJ) molecular blends prepared from small molecules based on diketopyrrolopyrrole (DPP) and perylene-diimide (PDI) chromophores have been studied using optical absorption, cyclic voltammetry, photoluminescence quenching, X-ray diffraction, atomic force microscopy, and current-voltage measurements. The results provided useful insights into the use of DPP and PDI based molecules as donor-acceptor composites for organic photovoltaic (OPV) applications. Beside optoelectronic compatibility, the choice of active layer processing conditions is of key importance to improve the performance of BHJ solar cells. In this context, post-production treatments, viz. thermal and solvent vapour annealing, and the use of 1,8-diiodooctane as a solvent additive were employed to optimize the morphology of blend films. X-ray diffraction and atomic force microscopy indicated that the aforementioned processing strategies led to non-optimal composite morphologies with significantly large crystallites in comparison to exciton diffusion lengths. Although the open circuit voltage of the OPV devices was satisfactory (0.78 V), it was anticipated that the bulky domains hamper charge dissociation and transport, which resulted in low photovoltaic performance.

Morphology‐Independent Efficient Singlet Exciton Fission in Perylene Diimide Thin Films

“…The red film has a narrower absorption feature, although around 620 nm there is a shoulder in the red part of the spectrum, in the same place where a maximum in the spectrum for the black film is observed. Identical changes in the absorption spectrum for this molecule were also observed by others where solvent vapor annealing or post‐deposition thermal annealing dramatically increased the crystallinity of the PDI thin film . The presence of an absorption shoulder around 620 nm in both the red and black film indicates that, although the organization in the films is sufficiently different to lead to a different color, there are likely to be domains in the red film with the same molecular organization as in the black film.…”

Additive‐Morphology Interplay and Loss Channels in “All‐Small‐Molecule” Bulk‐heterojunction (BHJ) Solar Cells with the Nonfullerene Acceptor IDTTBM

“…We find that both carrier transport and bimolecular recombination patterns improve notably in optimized BHJ active layers compared to "as-cast" films, and show that DR3-and SMPV1-based All-SM devices benefit from a greater balance in hole/electron mobilities and higher carrier collection efficiencies compared to their BTR counterparts. Although challenges exist in the determination and direct visualisation of favorable BHJ morphologies, [9,[23][24][25][26][37][38] our characterization approach using spatially-resolved electron energy loss spectrum (EELS) techniques allows for effectively probing the phase distribution of the nonfullerene "All-SM" solar cells studied in this work, with a look on how adding DIO as processing additive to the blend affects moprhologies and device efficiency. In turn, our approach to the morphological analysis of nonfullerene "All-SM" solar cells should be applicable to other material systems and BHJ solar cell optimization studies.…”

N-Heteroacenes as a New Class of Non-Fullerene Electron Acceptors for Organic Bulk-Heterojunction Photovoltaic Devices

“…Additionally, improvements can be achieved by optimizing the processing conditions of the active layer as has been previously demonstrated for other nonfullerene acceptors. [60][61][62] In summation, this new material system shows a respectable photovoltaic performance up to 2.5% power conversion efficiency without extensive optimization, which highlights a clear potential for its further development.…”