Interplay of Interfacial Layers and Blend Composition To Reduce Thermal Degradation of Polymer Solar Cells at High Temperature

Abstract

The thermal stability of printed polymer solar cells at elevated temperatures needs to be improved to achieve high-throughput fabrication including annealing steps as well as long-term stability. During device processing, thermal annealing impacts both the organic photoactive layer, and the two interfacial layers make detailed studies of degradation mechanism delicate. A recently identified thermally stable poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C-butyric acid methyl ester (PTB7:PCBM) blend as photoactive layer in combination with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as hole extraction layer is used here to focus on the impact of electron extraction layer (EEL) on the thermal stability of solar cells. Solar cells processed with densely packed ZnO nanoparticle layers still show 92% of the initial efficiency after constant annealing during 1 day at 140 °C, whereas partially covering ZnO layers as well as an evaporated calcium layer leads to performance losses of up to 30%. This demonstrates that the nature and morphology of EELs highly influence the thermal stability of the device. We extend our study to thermally unstable PTB7:[6,6]-phenyl-C-butyric acid methyl ester (PCBM) blends to highlight the impact of ZnO on the device degradation during annealing. Importantly, only 12% loss in photocurrent density is observed after annealing at 140 °C during 1 day when using closely packed ZnO. This is in stark contrast to literature and addressed here to the use of a stable double-sided confinement during thermal annealing. The underlying mechanism of the inhibition of photocurrent losses is revealed by electron microscopy imaging and spatially resolved spectroscopy. We found that the double-sided confinement suppresses extensive fullerene diffusion during the annealing step, but with still an increase in size and distance of the enriched donor and acceptor domains inside the photoactive layer by an average factor of 5. The later result in combination with comparably small photocurrent density losses indicates the existence of an efficient transport of minority charge carriers inside the donor and acceptor enriched phases in PTB7:PCBM blends.

Visualizing morphological principles for efficient photocurrent generation in organic non-fullerene acceptor blends

“…So far it has not been obvious whether this imaging approach yields contrast at the required resolution between semiconducting polymers and small molecules. For instance, NFA acceptors are-unlike fullerenes 18,19 -close in electronic structure to their donor partner molecule. However, as shown in the normalized electron energy-loss spectra in Fig.…”

“…This contrasts with most polymer:fullerene BHJs. 18,19 Results and discussion Molecular organization in pure ITIC thin films Efficient charge transport is typically attributed to the existence of crystals within a photoactive film. For ITIC, it was shown through molecular dynamics simulations that the molecular packing within a thin film might be different from an ITIC crystal in bulk.…”

Direct Correlation of Nanoscale Morphology and Device Performance to Study Photocurrent Generation in Donor-Enriched Phases of Polymer Solar Cells

“…Solar cells using regular device structures were processed as detailed in earlier works. , Indium tin oxide (ITO) substrates (purchased from Lumtec, 15 Ω/sq) were thoroughly cleaned by sonication in acetone and ethanol, followed by rinsing with water and sonication in isopropyl alcohol and applying ultraviolet–ozone for 10 min. A thin layer of poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (CLEVIOS AI 4083) was spin-coated onto the cleaned ITO precoated glass substrate at the speed of 4000 rpm for 60 s followed by an annealing step on a hot plate at 140 °C for 15 min, leading to a thickness of 40 nm.…”

“…For localized analysis of compositional variations, the near-edge structure in the X-ray absorption signal (NEXAFS) has been used in scanning transmission X-ray microscopy. , Unfortunately, the spatial resolution does not allow visualization of domains or phases with features below 10 nm. On the other hand, spectroscopic imaging of low-energy-loss electrons using a transmission electron microscope (TEM) enables one to directly correlate a specific nanoscale morphology with device performance. , Furthermore, direct visualization of non-fullerene acceptor (NFA) organization inside polymer blends has been recently demonstrated using analytic TEM …”

“…On the other hand, spectroscopic imaging of low-energy-loss electrons using a transmission electron microscope (TEM) enables one to directly correlate a specific nanoscale morphology with device performance. 28,29 Furthermore, direct visualization of non-fullerene acceptor (NFA) organization inside polymer blends has been recently demonstrated using analytic TEM. 30 In this work, we focus on nonoptimized polymer blends as a function of the D/A weight ratio to study photocurrent generation and nanoscale organization of acceptors in large polymer-enriched domains with domain sizes surpassing typical exciton diffusion lengths by applying spectroscopic imaging with scanning transmission electron microscopy (STEM-SI) of low energy-loss for visualization.…”