54 55 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Structures and Surfaces Group Beamline I07 X-rays unravel molecular packing and help create high-efficiency solar cells Related publication: LiW., Xiao Z., Smith J. A., Cai J., Li D., Kilbride R. C., Spooner E. L. K., Game O. S., Meng X., Liu D., Jones R. A. L., Lidzey D. G., Ding L. &WangT. Enhancing the efficiency of PTB7-Th:CO i 8DFIC-based ternary solar cells with versatile third components. Appl. Phys. Rev. 6 , 41405 (2019). DOI: 10.1063/1.5125438 Publication keywords: Organic solar cells; Ternary active layer; Non-fullerene acceptor; Morphology O rganic solar cells can generate low-cost renewable energy. These cells usually use a blend of carbon-based materials that can absorb light and create an electrical current. However, not all combinations work efficiently. The way that the molecules order themselves is fundamentally important in determining device efficiency. Researchers at Wuhan University of Technology, the National Center for Nanoscience and Technology in Beijing and the University of Sheffield explored what occurs in blends at the scale of a single molecule to determine device efficiency.Theywere interested inunderstanding the structure of anewtype of solar cell, basedona blendof three different organic semiconductors. Althoughaddinga third component can improve a solar cell’s ability to absorb light, it does not always enhancedevice efficiency. The I07 beamline at Diamond Light Source offers high-resolution X-ray investigations of the structure of surfaces and interfaces. The research team carried out X-ray scattering experiments on I07, combined with other techniques, to determine why adding a third component can positively or negatively affect organic solar cell efficiency. Their measurements provided critical information on how adding a third component changes the molecular ordering of the blend. They were then able to correlate the thin filmmorphology with the electronic properties of the solar cell. Their results also allow predictions of whether a third component is likely to enhance device efficiency. Their work will help develop high-efficiency solar cells and is another step towards commercialisation of this technology. Organic solar cells (OSCs) usually generate electricity using a photoactive layer consisting of a mixture of two different types of organic semiconductor – so-called electron-donors and electron-acceptors. Here, both donor and acceptor components can absorb photonswhich, following charge transfer fromthe donor to the acceptor, produce the holes and electrons that constitute an electrical current.The synthesis of neworganic semiconductors, e.g. non-fullerene electron acceptors, has recently resulted in a significant improvement of OSC efficiency. However, recent work has shown that additional efficiency enhancements can be realised by mixing three organic semiconductors together creating a 'ternary blend'.Here,byselectingathirdcomponentthathasacomplementaryabsorption tothebinarysystem, it ispossibletoenhancetheabsorptionofopticalabsorption by an OSC device and potentially enhance overall device efficiency. However, the selection of third component materials can be challenging, as such materials can easily change the molecular packing of the binary host system which in turn reduces the efficiency by which photo-generated charges can be generated or extracted from the device 1,2 . This issue has prompted us to make a detailed investigation into themolecular-scale interactions between the third component and the binary-host materials, with our aim being to explore molecular packing and ordering in ternary OSCs and its effects on device efficiency. Our work builds on our recent study published in Applied Physics Reviews in which we presented a practical guide to select materials to fabricate high efficiency non-fullerene based ternary organic solar cells. Here, we started from the binary PTB7-Th:CO i 8DFIC polymer:non-fullerene blend which can be used as the photoactive layer in an OSC, with devices having a maximum power conversion efficiency of around 13%. This efficiency is however highly dependent on the molecular packing of the electron acceptor material CO i 8DFIC. Here we found that when this molecule is cast onto a surface from solution, it can either undergo lamellar-crystallisation or form H- or J-type aggregates. Here, J-type aggregationwas shown to result in light-harvesting at near-infrared wavelengths and is beneficial in enhancing device efficiency 3 . In this work, an electron-donating polymer called PBDB-T-SF and a small molecular acceptor IT-4F were separately mixed with the host PTB7-Th:CO i 8DFIC system, with such 3 rd component materials having a different absorption spectrum compared to the host system (Fig. 1). Our objective was to explore the extent to which the 3 rd component material allowed OSC efficiency to be enhanced. In our experiments we found that by incorporating either 15% of PBDB- T-SF or 10% of IT-4F in the host system, it was possible to enhance the overall optical absorption of the blend over the wavelength range 350 to 1,000 nm due to the complementary light absorbance of the 3 rd component. Such films were fabricated into the active layer of an OSC and it was found that devices had an improved short-circuit current (indicative of enhanced optical absorption) and enhanced power conversion efficiency. However, when the concentration of PBDB-T-SF and IT-4F was increased to 30% and 20% respectively, a significant decrease in absorption was observed over the wavelength range of 900 to 1,100 nm. Such absorption is associated with the J-type π-π stacking characteristic of COi8DFIC; a result that indicates that molecular aggregation of the host acceptor componentCO i 8DFIC ismodifiedbyexcessivequantitiesofeitherPBDB-T-SFor IT- 4F. It was found that such reduced absorption lead to a reduced OSC performance in the ternary OSCs composed of 30%PBDB-T-SF and 20% IT-4F (Fig. 2). To gain a deeper insight into the role of PBDB-T-SF and IT-4F third components in directing the molecular ordering of the PTB7-Th:CO i 8DFIC host blend, we performed grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements at the Wide-Angle Scattering & Diffraction beamline (I07) at Diamond (Fig. 3). Here, PTB7-Th:CO i 8DFIC binary films were found to exhibit a distinct diffraction signal at q xy = 0.28 Å -1 in the in-plane direction and a dispersion ring located at q z = 1.7 Å -1 in the out-of-plane direction, with the former attributed to the alkyl chain stacking of PTB7-Th whilst the latter ascribed to the overlap of π-π stacked PTB7-Th and CO i 8DFIC 4 .When 15% of PBDB-T-SF or 10% of IT-4F were introduced into the host blend, the molecular packing of both PTB7-Th and CO i 8DFIC were found to be unchanged with both enhanced alkyl chain stacking and π-π stacking observed; a characteristic that may well facilitate charge transport in the device 5 . In contrast, when the concentration of PBDB- T-SF and IT-4F were increased to 30% and 20% respectively, the PTB7-Th alkyl chain stacking was found to reduce significantly whilst a new intense diffraction peak at q z = 0.5 Å -1 appeared, with this feature being associated with the edge- on lamellar crystal of CO i 8DFIC. This significantly increased lamellar order of CO i 8DFIC results from enlarged phase separation caused by the presence of an excessive amount of third component.The suppression of J-aggregation at higher loadings of PBDB-T-SF and IT-4F is thought to be responsible for the reduced light absorption at longer wavelengths (Fig. 2a,c). Our results suggest therefore that the relative extent of J-aggregation and lamellar crystallisation of CO i 8DFIC can be stronglymodified by the incorporation of a third component; a process that strongly affects the efficiency of solar cell devices incorporating such materials. By controlling such processes, we were able to increase device efficiency by around 10% compared to devices based on a simple binary blend, with further improvements expected using new generations of materials. References: 1. LiW. et al. Contrasting Effects of EnergyTransfer in Determining Efficiency Improvements inTernary Polymer Solar Cells. Adv. Funct. Mater. 28 , 1704212 (2018). DOI: 10.1002/adfm.201704212 2. LiW. et al. Achieving over 11%power conversion efficiency in PffBT4T-2OD- based ternary polymer solar cells with enhanced open-circuit-voltage and suppressed charge recombination. Nano Energy 44 , 155–163 (2018). DOI: 10.1016/j.nanoen.2017.12.005 3. LiW. et al. Molecular Order Control of Non-fullerene Acceptors for High- Efficiency Polymer Solar Cells. Joule 3 , 819–833 (2019). DOI: 10.1016/j. joule.2018.11.023 4. LiW. et al. Correlating the electron-donating core structure withmorphology and performance of carbon–oxygen-bridged ladder-type non-fullerene acceptor based organic solar cells. Nano Energy 61 , 318–326 (2019). DOI: 10.1016/j.nanoen.2019.04.053 5. Li D. et al. Aggregation of non-fullerene acceptors in organic solar cells. J. Mater. Chem. A 8 , 15607–15619 (2020). DOI: 10.1039/d0ta03703f Funding acknowledgement: This work was supported by the National Natural Science Foundation of China (Grants No. 21774097) and the Natural Science Foundation of Hubei Province (Grant No. 2018CFA055).We also thank the UK STFC for part-funding this work via grant ST/R002754/1“SynchrotronTechniques for African Research and Technology”, and EPSRC for funding studentships via the New and Sustainable Photovoltaics CDT (EP/L01551X/1). GIWAXS measurements were performed on I07 at Diamond Light Source (UK) on beamtime project reference S120419. Corresponding authors: Prof.TaoWang,Wuhan University ofTechnology,
[email protected]; Prof. David G. Lidzey, University of Sheffield,
[email protected] Figure 1: (a) Chemical structure of PBT7-Th, COi8DFIC, PBDB-T-SF and IT-4F; (b) A schematic structure of the organic solar cells explored; (c) Absorbance of PBT7-Th, COi8DFIC, PBDB-T-SF and IT-4F. Figure 2: Absorption spectra and J-V curves of PTB7-Th:COi8DFIC based ternary solar cells containing different concentrations of PBDB-T-SF and IT-4F. Figure 3: 2D GIWAXS patterns and 1D profiles along q z axis for PTB7-Th:COi8DFIC films containing different amounts of PBDB-T-SF or IT-4F.