Elucidating the impact of molecular weight on morphology, charge transport, photophysics and performance of all-polymer solar cells

Abstract

Understanding the influence of polymer molecular weight on the morphology, photophysics, and photovoltaic properties of polymer solar cells is central to further advances in the design, processing, performance and optimization...

Regioregularity-control of conjugated polymers: from synthesis and properties, to photovoltaic device applications

“…By using these D-A copolymers as both P D s and polymer acceptors (P A s), efficient all-polymer solar cells (all-PSCs) have been achieved. 2,5,[137][138][139][140][141][142][143][144][145][146][147][148][149][150] Engineering of the light absorption and frontier energy levels of both P D and P A affords simultaneous enhancements of the open-circuit voltage (V oc ) and short-circuit current density (J sc ), resulting in high-performance all-PSCs. Importantly, the P D and P A chains can be tie molecules and form entangled networks in the BHJ blends, thus resulting in superior mechanical/ thermal stabilities and making all-PSCs as promising candidates for wearable and stretchable electronics.…”

Polymer Acceptors with Flexible Spacers Afford Efficient and Mechanically Robust All‐Polymer Solar Cells

“…The major hurdles that hamper the performance of all‐PSCs based on PSMAs are the strongly phase‐separated blend morphologies, driven by de‐mixing of high molecular weight P D s and PSMAs, resulting in un‐optimized charge generation and transport. [ 15,16 ] These un‐optimal morphologies typically include numerous defect sites (i.e., sharp domain–domain interfaces and large polymer aggregates) in the blend film, limiting the mechanical robustness and stretchability with low COS. [ 17–19 ] In addition, the phase separation of polymer blends is affected by the aggregation and crystalline behaviors of the P D s and P A s. In particular, the PSMAs containing highly crystalline, rigid SMA unit typically possess very strong crystalline and aggregation properties, causing strongly phase‐separated structure with excessive domain purity, thereby lowering both short‐circuit current density ( J sc ) and fill factor (FF) of all‐PSCs. [ 17,20 ] Consequently, a variety of strategies including modification of the P A structure and optimization of processing conditions have attempted to improve the blend morphology.…”

Highly Efficient and Stable All-Polymer Solar Cells Enabled by Near-Infrared Isomerized Polymer Acceptors

“…The access of optimal BHJ morphologies, which support the photogenerated excitons to dissociate into charge carriers at the donor/acceptor (D/A) interfaces, is usually obtained via trial-and-error processing. ,, It is because excessive interchain entanglement generally exists between the polymer donor ( P D ) and P A , resulting in poor morphological characteristics of all-PSCs. ,, This is the main reason why the photovoltaic performance of all-PSCs lags behind those of relevant small-molecule NFA-based systems. Although various strategies for optimizing the BHJ blend morphology have been explored successfully in multiple degrees, such as material design, ,,, molecular mass, ,, solvent and solid additives, ,,, solvents, , third component, and pre- or post-treatments, , etc., the incident photon-to-electron conversion efficiency is still low so far. Finding effective strategies to improve the blend morphology and photovoltaic performance is still under way.…”

“…Additionally, as compared to the polymer–small-molecule photovoltaic systems, all-polymer photovoltaic systems generally have the merits of superior morphology stability and excellent mechanical properties. ,,− However, many all-PSCs based on various P A s still showed poor storage and aging stabilities. − Moreover, morphological transformation is also easily accelerated by heating the corresponding all-polymer systems or by accumulated heat from sunlight irradiation in a real device. ,, In particular, molecular structures of P A s, as well as their molecular packing, can cause a huge difference in mechanical stability in the corresponding all-PSCs. ,− No doubt all of these factors, including the molecular structure, degree of polymerization, D/A miscibility, BHJ morphology, etc ., play their part in stability issues. However, a much bigger obstacle to improving these degradation mechanisms and facilitating the progress of all-PSCs for possible applications is that the precise correlation between the molecular structure and device stability remains phenomenological. ,,, …”