New Insights into Morphology of High Performance BHJ Photovoltaics Revealed by High Resolution AFM

Abstract

Direct imaging of the bulk heterojunction (BHJ) thin film morphology in polymer-based solar cells is essential to understand device function and optimize efficiency. The morphology of the BHJ active layer consists of bicontinuous domains of the donor and acceptor materials, having characteristic length scales of several tens of nanometers, that reduces charge recombination, enhances charge separation, and enables electron and hole transport to their respective electrodes. Direct imaging of the morphology from the molecular to macroscopic level, though, is lacking. Though transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy (AFM) in the tapping and nanomechanical modes to investigate the BHJ active layer morphology that, when combined with Ar(+) etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a "face-on" orientation and the morphology at the anode interface was markedly different.

The Novel Additive 1‐Naphthalenethiol Opens a New Processing Route to Efficiency‐Enhanced Polymer Solar Cells

“…[ 14,37 ] These features are consistent with those of the large PC 71 BM-rich aggregates revealed with the GISAXS result (Table 2 ). Especially, the large lateral domain size with the AFM surface profi le ( Figure S9, Supporting Information) corresponds well to the oblate features of the ellipsoid model used in the GISAXS data fi tting with 2 A = 14 nm and 2 B = 260 nm.…”

“…[ 37 ] In contrast, with the most impressively improved µ e but less improved µ h for a much less unbalanced R µ value of 5.04, the active layer processed with DIO additive resulted in moderately improved PCE of 6.73%. Characterized by the bimodal PC 71 BM size dispersion with coexisted large and small PC 71 BM aggregates (Table 2 ), the active layers processed with SH-na have better enhanced µ h and better balanced R u , despite that µ e is not best-improved compared to that of the DIO case.…”

“…Recently, similar worm-like PC 71 BM networks were also visualized with fi lm-depth-dependent AFM images. [ 37 ] Further, X-ray photoelectron spectroscopy (XPS) measurements [ 38 ] were conducted for these PTB7:PC 71 BM active layers to determine the surface compositions after dipping treatment. Note that the SH-na in the fi lms is removed by prolong vacuum pump in the vacuum chamber of XPS.…”

Polymer Solar Cells: Solubility Controls Fiber Network Formation

“…[9][10][11][12] The ideal length scale for a polymer solar cell morphology is in the order of the exciton diffusion length, which is about 5 -10 nm. 13 The width of the polymer fibers in the fiber network thus needs to be on the order of 10 nm or less.…”

Synthesis and photovoltaic properties of two new alkoxylphenyl substituted thieno[2,3-f]benzofuran based polymers

“…29 In addition to material engineering, PSCs' performance is also critically dependent on the morphology and the interfaces. [30][31][32] Solvent treatment, additive and thermal annealing have been recognized as effective methods and have played important roles in obtaining desirable morphology to gain superior performance and promote development of BHJ PSCs. See DOI: 10.1039/c5cp02127h techniques have been applied to probe the formation and evolution of morphology of blend films.…”