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Upconversion-Triggered Charge Separation in Polymer Semiconductors Yu Jin Jang, Eunah Kim, Sunghyun Ahn, Kyungwha Chung, Jihyeon Kim, Heejun Kim, Huan Wang, Jiseok Lee, Dong-Wook Kim, and Dong Ha Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02511 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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The Journal of Physical Chemistry Letters

Upconversion-Triggered Charge Separation in Polymer Semiconductors Yu Jin Jang,1,† Eunah Kim,2,† Sunghyun Ahn,3 Kyungwha Chung,1 Jihyeon Kim,1 Heejun Kim,1 Huan Wang,1 Jiseok Lee,3,* Dong-Wook Kim,2,* and Dong Ha Kim1,* 1

Department of Chemistry and Nano Science, Ewha Womans University, 52, Ewhayeodae-gil,

Seodaemun-gu, Seoul 03760, Republic of Korea 2

Department of Physics, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul

03760, Republic of Korea 3

School of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea †

Y. J. Jang and E. Kim contributed equally to this work.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (J. Lee); [email protected] (D.-W. Kim); [email protected] (D. H. Kim)

ACS Paragon Plus Environment

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ABSTRACT

Upconversion is a unique optical property which is driven by a sequential photon pumping and generation of higher energy photons in a consecutive manner. The efficiency improvement in photovoltaic devices can be achieved when upconverters are integrated since upconverters contribute to the generation of extra photons. Despite numerous experimental studies confirming the relationship, fundamental explanations for a real contribution of upconversion to photovoltaic efficiency are still in demand. In this respect, we suggest a new approach to visualize the upconversion event in terms of surface photovoltage (SPV) by virtue of Kelvin probe force microscopy (KPFM). One of the most conventional polymer semiconductors, poly(3-hexyl thiophene) (P3HT), is employed as a sensitizer to generate charge carriers by upconverted light. KPFM measurements reveal that the light upconversion enabled the formation of charge carriers in P3HT, resulting in large SPV of -54.9 mV. It confirms that the energy transfer from upconverters to P3HT can positively impact on the device performance in organic solar cells (OSCs).


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TOC GRAPHICS

A new approach to visualize the upconversion event is demonstrated in terms of the surface photovoltage (SPV) by virtue of Kelvin probe force microscopy (KPFM). Under illumination at 980 nm, light upconversion induces the charge distribution in the poly(3-hexyl thiophene) (P3HT) layer, increasing the SPV. It represents a supporting evidence for a viable contribution of upconversion to photovoltaics.

KEYWORDS Upconversion, Kelvin probe force microscopy (KPFM), Contact potential difference (CPD), Surface photovoltage (SPV), Organic solar cells (OSCs) Upconversion is an interesting phenomenon which is derived from a sequential absorption of more than two photons with low-energy, leading to a conversion into higher energy photons (Scheme S1).1-3 On the basis of the unique optical properties, upconverters have been exploited into diverse areas such as biomedical systems, imaging and energy conversion applications.1,3-7 In the field of photovoltaics, broadband absorption is critical to obtain better efficiency since the photovoltaic performance highly depends on the number of photons absorbed in the device. However, light losses occur due to a narrow absorption band of conventional sensitizers which is generally localized in the UV-visible region. One possible approach to overcome this issue is to combine a component which can effectively capture the incident solar light in the near infrared


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(NIR) region. In this respect, upconversion nanocrystals that emit UV-visible light by absorbing NIR source (Scheme 1a) would be a good candidate material to make a better use of solar light. There have been numerous attempts to enhance the photovoltaic efficiency by utilizing upconversion luminescence technology in Si solar cells, dye-sensitized solar cells (DSSCs), organic solar cells (OSCs) and perovskite solar cells.4,8-11 Disappointingly, the practical improvement in power conversion efficiency (PCE) resulting from an upconversion event has been limited to a level less than 0.5%. This represents a slight improvement compared with the cases employing plasmonic nanoparticles (NPs)12-14 or carbon moieties13,15-16, due to the necessity of high-intensity illumination for excitation as well as a low quantum yield of upconverters.1,3 Thus, further investigation is necessary to figure out the origin of PCE enhancement by the inclusion of upconverters. In this regard, we employ Kelvin probe force microscopy (KPFM) as a tool to investigate the practical effect of upconversion in the solar cell efficiency. KPFM has been widely used to measure contact potential difference (CPD), i.e., the work function (ϕ) difference between the tip and the sample surface.17 ϕ is defined as the gap between the vacuum level (EVL) and the Fermi level (EF) of an object. When the tip and the sample are electrically contacted, the electrons at one material with a lower ϕ flow into the other with a higher ϕ, leading to the alignment of EF and a gradient in EVL. The difference in CPD (∆CPD) in the dark and under illumination corresponds to surface photovoltage (SPV), resulting from generation and redistribution of photogenerated charge carriers.17-19 The CPD measurement exhibited a high sensitivity, reaching a sub-millivolt resolution,18 so the direct mapping of SPV has allowed a quantitative estimation of photo-induced charge carrier distribution at the samples.20-23 In that sense, the photocatalytic and photovoltaic efficiency of a system can be complementarily


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assessed in nanoscale using KPFM along with conventional analytical techniques such as spectroscopy, potentiometry, and chromatography. In this work, we rationally designed a direct contact binary system between Gd3+-doped β-NaYF4:Yb3+/Er3+ nanorods (UCNs) and one of the most conventional organic semiconductors, poly(3-hexylthiophene) (P3HT). Firstly, we spectrally tuned the UCNs emission with the absorption band of P3HT (Scheme 1b). Next, SPV maps were obtained to visualize upconversion effect on the P3HT layer using KPFM under 980 nm illumination (Scheme 1c). Summarizing the critical phenomena: 1) The visible light emission of UCNs is absorbed by P3HT layer; 2) The absorbed light can generate charge carriers such as electrons and holes; and 3) The charge carrier distribution on the P3HT surface determines the surface potential. The SEM image in Figure 1a confirms that UCNs with a size of ~300 nm were synthesized. The XRD peaks of UCNs in Figure 1b are indexed to the hexagonal phase of Gd3+doped NaYF4:Yb3+/Er3+.24 The surface of as-synthesized UCNs was hydrophobic due to oleate capping, but UCNs could be dispersed into ethanol after a successive washing process. By spincoating of the UCNs solution in ethanol, UCNs aggregates were randomly distributed on ITO substrate as shown in Figure 1c. P3HT layer (thickness: 65 nm) covered the UCNs arrays to generate a binary composite. The optical properties of P3HT film and UCNs were studied using UV-visible and photoluminescence (PL) spectroscopies. A broadband absorption of P3HT was observed in the wavelength range of 350-650 nm (Figure 1d, black). It coincides with the PL peaks of UCNs at 522 and 541 nm (Figure 1d, green). It confirms that the emission from UCNs can directly contribute to the excitation of P3HT. A P3HT film on UCN arrays (UCNs/P3HT) was subjected to KPFM measurement in order to examine the conventional upconversion effect on the charge carrier generation of solar


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cells. For comparison, a neat P3HT film was also prepared. Consistent with SEM result in Figure 1c, UCNs/P3HT (Figure 2d) exhibited clear undulating surfaces, while the topographical image of the neat P3HT film showed a flat feature (Figure 2a). CPD scans obtained from two samples in dark and under illumination at 980 nm are displayed in Figure 2b-c and e-f. The color change in CPD maps directly visualizes a spatial distribution of photo-generated carriers in the P3HT film. As expected, CPD maps in dark and under illumination were almost identical in the case of neat P3HT (Figure 2b-c). It indicates that P3HT excitation was not effective by the exposure of 980 nm light source. However, a considerable color change was observed on UCNs/P3HT with illumination at 980 nm (Figure 2f). It was more distinct on P3HT film deposited right onto UCNs, converting the red region into blue. It confirms that the photoexcitation of P3HT followed by the charge generation in P3HT could be realized by the emission from UCNs. SPV values were calculated by surface potential differences measured under illumination and in dark, i.e., SPV = CPDlight – CPDdark. Thus, higher SPV corresponds to the formation of a large number of photo-induced charge carriers. As summarized in Figure 3, the magnitude of SPV value was increased from -16.4 to -54.9 mV under 980 nm light illumination, by the inclusion of UCNs arrays beneath the P3HT thin layer (open black circles). When considering the ϕ values of ITO (ϕ ~4.8) and P3HT (ϕ ~4.2-4.5),25 it is expected that electrons are transferred from the P3HT to ITO at the junction till EF of P3HT and ITO is aligned.17 As a result, both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels at the P3HT surface are lower than those at the P3HT/ITO interface as illustrated in Scheme 2. After the photoexcitation of P3HT, electrons tend to flow to P3HT surface and holes migrate to ITO due to the band bending feature of P3HT coated on ITO.


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According to the previous work performed by Osterloh and coworkers,25 the charge generation under the excitation energy of 1.2-1.6 eV could be detected in terms of SPV.25 Their results well agree with the measured SPV of our ITO/P3HT sample under illumination of the NIR laser and it accounts for the SPV value of 16.4 mV. On UCNs/P3HT sample, 980 nm light illumination resulted in the accumulation of electrons at the P3HT layer surface via the effective photoexcitation of P3HT by upconverted emission. It recovers the band bending of P3HT, leading to the decrease in CPD.26 It produced a large SPV value of -54.9 mV. The discussion above can be described by the schematic band diagram in Scheme 2. Neat P3HT thin layer was introduced in this study instead of a typical binary blend of P3HT and electron acceptors such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) which facilitate the charge dissociation and collection in real solar cell devices. It simplified the sample configuration and allowed us to focus on the observation of charge carrier generation in organic semiconductors by the upconversion process under NIR light illumination.

In a generally

accepted working mechanism revealed by femtosecond transient absorption spectroscopy (TAS) and KPFM study, the charge generation in P3HT occurs as follows25,27-31: 1) Photon absorption of P3HT via direct band-gap (Eg) or charge transfer (CT) band excitation; 2) It immediately induces the generation of singlet excitons and polaron pairs (or geminate pairs) which represent the processes of electron population in LUMO level of P3HT and consequentially hole formation in HOMO level of P3HT, followed by the dissociation of the excitons into the Coulombically bound electron-hole pairs; and 3) The diffusion of polarons which refer to the formation of freely migrating charge carriers (free electrons and holes) along the P3HT molecular chains to the ITO takes place. The singlet excitons can be directly converted into polarons or the transition from polaron pairs to the polarons can also occur. The number of charge carriers generated in a neat


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P3HT film can be estimated as internal quantum efficiency (IQE) of ~80% in a neat P3HT film.31 As confirmed in Figure 1d, the emission from UCNs can contribute to both Eg and CT band excitation of P3HT. Accordingly, the number of free electrons and holes reaching to the ITO should be larger in the presence of UCNs arrays at 980 nm illumination compared with that on a neat P3HT film. However, a practical charge dissociation might be different from the aforementioned mechanism. For instance, the possibility of radiative and non-radiative recombination lowering the charge collection efficiency should be considered.27 The emission intensity of upconverted light is another issue to make it questionable whether it is enough to overcome CT states and generate free carriers. In this regard, SPV measurement of a neat P3HT film was carried out under the irradiation of 532 nm laser source with a power output of 10 mW. The direct Eg excitation at 532 nm generated -50 mV of SPV (yellow open circle in Figure 3), which is almost identical to SPV value (-54.9 mV) obtained at 980 nm light irradiation. It confirms that light upconversion can realize the charge generation and collection in P3HT and the process can be visualized by KPFM. In this context, an increase in SPV from -16.4 to -54.9 mV after the incorporation of UCNs can be interpreted as an evidence of the increase in the number of charge carriers collected at ITO driven by the upconverted light. In summary, P3HT sensitization associated with upconversion event was examined by KPFM measurement for the purpose of demonstrating the existence and the quantity of NIRinduced charge generation. KPFM results strongly support that light upconversion enabled the


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formation of charge carriers in P3HT, and the number of photo-generated charge carriers in P3HT was simply estimated by SPV value of 54.9 mV.


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Scheme 1. Schematic illustrations of a) a conventional upconversion process and b) the excitation (gray) and emission (green) ranges of UCNs; The spectral overlap between emission band of UCNs and absorption band of P3HT (purple) can be induced in the composite. c) Schematic diagram to show a binary system prepared by a sequential deposition of UCNs and P3HT thin layer on ITO substrate for KPFM study.


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Figure 1. a) SEM image of as-synthesized UCNs. b) XRD profile obtained from UCNs; A broad peak at around 2θ of 15-40 degree was originated from an XRD holder. c) SEM image of UCN arrays on ITO substrate. d) The absorption spectrum of the neat P3HT layer deposited on ITO substrate (black); PL spectrum of UCNs arrays on ITO substrates under the irradiation of 980 nm laser source with a power output of 1.5 W (green).


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Figure 2. Atomic force microscopy (AFM) topographies (left column) and corresponding contact potential difference (CPD) maps under dark (middle column) and 980 nm laser irradiation (right column). The results were obtained from a-c) neat P3HT film and d-f) the P3HT film deposited on UCNs array. The scale bar represents 200 nm.


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Figure 3. The plot of SPV values depending on the sample configuration. SPV was calculated by the equation of CPDlight – CPDdark. To obtain the values of CPDlight, the illumination of 980 nm (NIR, black open circle) or 532 nm (Visible, yellow open circle) laser sources were carried out on the samples.


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Scheme 2. Schematic energy band diagram of UCNs/P3HT sample in the dark and under illumination using a 980 nm laser source.


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ASSOCIATED CONTENT Detailed experimental procedures and a general explanation about upconversion process are provided in the Supporting Information. ACKNOWLEDGMENT This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (2014R1A2A1A09005656, 2015001948) and also was supported by the Research Fund (1.140073.01, 1.160094.01) of UNIST (Ulsan National Institute of Science & Technology). We acknowledge the support of Hobeom Kim, Dr. Kyung-Geun Lim, and Prof. Tae-Woo Lee at Seoul National University for the instruction of P3HT film processing. REFERENCES (1) Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling Upconversion Nanocrystals for Emerging Applications. Nat. Nanotechnol. 2015, 10, 924-936. (2)

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