Comparative Study of the Mechanical Properties of All-Polymer and Fullerene–Polymer Solar Cells: The Importance of Polymer Acceptors for High Fracture Resistance

Abstract

High fracture resistance of polymer solar cells (PSCs) is of great importance to ensure long-term mechanical reliability, especially considering their potential in roll-to-roll printing processes and flexible devices. In this paper, we compare mechanical properties, such as the cohesive fracture energy, elastic modulus, and crack-onset strain, of allpolymer solar cells (all-PSCs) and fullerene-based solar cells (PCBM− PSCs) based on the same, representative low-bandgap polymer donor (PTB7-Th) as a function of acceptor content. The all-PSCs exhibit higher fracture energy (2.45 J m −2 ) than PCBM−PSCs (0.29 J m −2 ) at optimized device conditions. Additionally, a 15-fold higher crack-onset strain is observed in all-PSCs than in PCBM−PSCs. Dramatically different mechanical compliances observed for all-PSCs and PCBM− PSCs are investigated in detail by analysis of the blend morphologies as a function of acceptor content (either P(NDI2HD-T) or PCBM acceptors). The superior fracture resistance of all-PSCs is attributed to the more ductile characteristics of the polymer acceptor and the large degree of plastic deformation during crack growth, in contrast to the brittle nature of PCBM and the weak interaction between the polymer-rich phase and highly aggregated PCBM-rich domains. Therefore, this work demonstrates that replacing a small-molecule acceptor (i.e., PCBM) with polymeric materials can be an effective strategy toward mechanically robust PSCs.

All‐Polymer Solar Cells: Recent Progress, Challenges, and Prospects

“…Degradation of PSC performance,a ttributed to donor/ acceptor semiconductor chemical sensitivity,B HJ morphological reorganization/degradation, and external mechanical stresses,i sacritical limitation for the widespread commercialization of this technology.Recent studies have demonstrated great promise in terms of all-polymer cell stability compared to those based on other heterojunction materials (Figure 8). [43][44][45] Am ajor hurdle for the stability of devices using fullerenes or small molecule semiconductors in the active layer is BHJ photo/thermal microstructural instability due to molecular diffusion, brittleness,and poor adhesion, as well as photo/thermal-induced oxidation and dimerization processes. [5c, 43] Furthermore,m ost fullerene or small molecular based acceptors suffer from an initial PCE loss of up to 30-40 %("burn-in loss") in times as short as several hundred hours with (Figures 8a,b).…”

“…Recently, Kim et al. demonstrated that PBDTT‐FTTE:P(NDI2OD‐T2) APSCs exhibit ductile crack growth, whereas the corresponding cells based on fullerenes are brittle with the emergence of sharp cracks through the entire active‐layer film under tensile strain . Under optimized device conditions, as shown in Figure d, the crack‐onset strain for the APSC cell (15.5±1.2 %) is 15‐fold higher compared with that of the fullerene‐based one (1.1±0.2 %), with the elastic moduli amounting to 0.68±0.04 GPa and 2.64±0.12 GPa, respectively.…”

Sequential Blade‐Coated Acceptor and Donor Enables Simultaneous Enhancement of Efficiency, Stability, and Mechanical Properties for Organic Solar Cells

“…For example, OSCs used as wearable electronics should be able to withstand tensile strain of at least 20–30% . Several reviews have been published on the development of the mechanical property of OSCs, indicating that acceptor type, length of the alkyl side‐chain of polymer, polymer molecular weight, and additive will have an effect on the mechanical property of the device, which provide guidelines for the co‐optimization of both mechanical and photovoltaic properties . The molecular weight information of PTB7‐Th and N2200 is shown in Figure S9, Supporting Information.…”

Polymer Pre‐Aggregation Enables Optimal Morphology and High Performance in All‐Polymer Solar Cells

“…All‐polymer solar cells (all‐PSCs), which are composed of polymer donor and acceptor, offer several distinctive advantages (especially mechanical stability) and show great potential for practical application . However, the morphology control of an all‐polymer blend is highly complicated due to the polymer chain entanglement.…”