Key factors behind the superior performance of polymer-based NFA blends

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• All-small molecule (ASM) solar cells present advantages like high solubility and simpler synthesis for organic photovoltaics (OPVs).
• ASM solar cells currently show lower efficiencies compared to polymer:small molecule blends.
• Understanding performance differences is crucial for advancing OPV commercialization.

Purpose of the Study:

• To compare an ASM blend (ZR1:Y6) with a polymer:small molecule blend (PM7:Y6) using the same non-fullerene acceptor (NFA).
• To investigate the factors limiting the efficiency of ASM solar cells.
• To elucidate the impact of morphology and charge generation on device performance.

Main Methods:

• Comparative analysis of ASM (ZR1:Y6) and polymer:small molecule (PM7:Y6) blends.
• Measurement of energetic offset between exciton singlet and charge transfer states (ΔEs).
• Evaluation of field-dependency of charge generation and charge carrier mobility using space charge limited current (SCLC) measurements.
• Morphological analysis using techniques like X-ray diffraction and microscopy (implied).

Main Results:

• Both ZR1:Y6 and PM7:Y6 exhibit similar energetic offsets (ΔEs).
• ZR1:Y6 displays a stronger field-dependency of charge generation compared to PM7:Y6.
• Lower charge carrier mobilities were observed in ZR1:Y6, attributed to suppressed charge dissociation.
• ZR1:Y6 morphology shows less crystallinity and more intermixing, hindering continuous electron transport pathways for the NFA.

Conclusions:

• The reduced efficiency in the ZR1:Y6 ASM blend is primarily due to lower charge carrier mobility.
• Morphological characteristics, specifically less ordered and more intermixed domains, impede efficient electron transport.
• Addressing morphological order and improving charge transport are key strategies for enhancing ASM solar cell performance.