Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Realizing Efficient Charge/Energy Transfer and Charge Extraction in Fullerene-Free Organic Photovoltaics via a Versatile Third Component Hao-Wen Cheng,†,‡ Huotian Zhang,§ Yu-Che Lin,†,‡ Nian-Zu She,∥ Rui Wang,† Chung-Hao Chen,‡ Jun Yuan,†,⊥ Cheng-Si Tsao,#,∇ Atsushi Yabushita,∥ Yingping Zou,⊥ Feng Gao,§ Pei Cheng,† Kung-Hwa Wei,*,‡ and Yang Yang*,† Downloaded via UNIV OF SOUTHERN INDIANA on July 17, 2019 at 04:55:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Department of Materials Science and Engineering, California NanoSystems Institute, University of California, Los Angeles, California 90095, United States ‡ Department of Materials Science and Engineering, Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 3001, Taiwan § Biomolecular and Organic Electronics, IFM Linköping University, Linköping 58183, Sweden ∥ Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan ⊥ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China # Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan ∇ Institute of Nuclear Energy Research Taoyuan 32546, Taiwan S Supporting Information *
ABSTRACT: Solution-processed organic photovoltaics (OPVs) based on bulk-heterojunctions have gained significant attention to alleviate the increasing demend of fossil fuel in the past two decades. OPVs combined of a wide bandgap polymer donor and a narrow bandgap nonfullerene acceptor show potential to achieve high performance. However, there are still two reasons to limit the OPVs performance. One, although this combination can expand from the ultraviolet to the near-infrared region, the overall external quantum efficiency of the device suffers low values. The other one is the low open-circuit voltage (VOC) of devices resulting from the relatively downshifted lowest unoccupied molecular orbital (LUMO) of the narrow bandgap. Herein, the approach to select and incorporate a versatile third component into the active layer is reported. A third component with a bandgap larger than that of the acceptor, and absorption spectra and LUMO levels lying within that of the donor and acceptor, is demonstrated to be effective to conquer these issues. As a result, the power conversion efficiencies (PCEs) are enhanced by the elevated shortcircuit current and VOC; the champion PCEs are 11.1% and 13.1% for PTB7-Th:IEICO-4F based and PBDB-T:Y1 based solar cells, respectively. KEYWORDS: Organic photovoltaics, nonfullerene, ternary blend, charge transfer, energy transfer, charge extraction
R
Fullerenes and their derivatives have been widely used as electron acceptors in OPVs, because of their excellent electrontransporting properties and compatibility with polymer donors to form favorable morphologies. Nevertheless, fullerene-based acceptors have several shortcomings, including poor light absorption in the visible and near-infrared (NIR) regions,9,10 bandgaps with limited tunability (i.e., their chemical structures are difficult to modify),11 and easiy aggregation which leads to devices having low stability;12,13 furthermore, most fullerenebased OPVs have exhibited relatively high open-circuit voltage
enewable energy technologies designed to decrease the demand for fossil fuels, and thus minimize their environmental impact, have been progressing rapidly. Solution-processed organic photovoltaics (OPVs) based on bulk-heterojunctions have gained substantial attractions in the past two decades due to several advantages such as easily being processed, lightweight, semitransparent, flexible, as well as large-area roll-to-roll production.1−7 The steady growth in the power conversion efficiencies (PCEs) of OPVs has been certified by the National Renewable Energy Laboratory (NREL) with the highest PCE of 15.6%.8 Further improvements in PCE, however, is still needed for making the OPV technologies to become commercially viable. © XXXX American Chemical Society
Received: April 1, 2019 Revised: June 14, 2019 Published: July 12, 2019 A
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Chemical structures of PTB7-Th, IEICO-4F, PBDB-T, Y1, and ITCC. The red color represents the PTB7-Th based (PTB7-Th:IEICO4F) system; blue represents the PBDB-T based (PBDB-T:Y1) system; green represents the third component, ITCC.
Figure 2. (a,b) Schematic representation of the energy levels in the (a) PTB7-Th:IEICO-4F:ITCC based and (b) PBDB-T:Y1:ITCC based devices. (c,d) Absorption spectra of films of pristine PTB7-Th, IEICO-4F, PBDB-T, Y1, and ITCC films in the two systems.
(VOC) losses of 0.8−1.0 V.14−17 In the past few years, nonfullerene acceptors (NFAs) have been employed widely to remedy these shortcomings.18−22 In contrast to fullerene-based acceptors, NFAs can have a variety of chemical structures and thereby tunable absorption spectra and energy levels.23−29 Furthermore, unlike fullerene acceptors efficient charge generation can be realized between NFAs and donors even when there is only a negligible energy level offset.29 Some of the nonfullerene-based devices can reach relatively high VOC through a new combination of donors/acceptors or through
incorporating third components.30−32 Moreover, the stability of NFAs-based devices can be better than that of fullerenebased devices.12,33−35 With the development of wide-range bandgap, OPVs combining of a wide-bandgap polymer donor and a narrowbandgap NFA have great potential to achieve relatively high performance.24,36,37 Nonetheless, such combinations have two main drawbacks that limit the performance of OPVs. First, although the resulting devices possess wide absorption ranges, extending from the ultraviolet (UV) to the near-infrared, the B
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a,b) Measured J−V curves and (c,d) EQE spectra of PTB7-Th:IEICO-4F based and PBDB-T:Y1 based devices, prepared with and without ITCC, under illumination of an AM 1.5G solar simulator at 100 mW cm−2.
Table 1. Average and Best PCE Data for Devicesa donor/acceptor/third component (ratio, w/w/w) PTB7-Th:IEICO-4F (10:14) PTB7-Th:IEICO-4F:ITCC (10:14:3) PBDB-T:Y1 (9.5:9.5) PBDB-T:Y1:ITCC (9.5:9.5:4)
VOC (V) 0.71 0.75 0.88 0.91
± ± ± ±
0.04 0.06 0.06 0.03
JSC (mA cm−2) calculated JSC(mA cm−2) 20.7 21.2 20.4 21.1
± ± ± ±
0.4 0.5 0.3 0.4
20.2 20.9 20.0 20.8
FF (%) 68.2 66.0 69.6 67.3
± ± ± ±
0.4 0.7 0.5 0.5
PCE (%)
champion PCE (%)
± ± ± ±
10.1 11.1 12.6 13.1
9.8 10.6 12.5 12.9
0.2 0.4 0.3 0.1
a
Devices based on PTB7-Th:IEICO-4F and PBDB-T:IEICO-4F with or without ITCC as the third component.
Th:IEICO-4F system, because the absorption of ITCC lies within those of PTB7-Th and IEICO-4F, the bandgap of ITCC is greater than that of IEICO-4F, and the LUMO energy level of ITCC is higher than that of IEICO-4F. To demonstrate the universality of this concept, another OPV system based on PBDB-T:Y1 has been adopted with ITCC as the third component. We have found that the values of JSC and VOC of these OPVs were enhanced simultaneously after incorporation of this versatile third component, with the best PCE of these OPVs exceeding 13%. Figure 1 presents the chemical structures of PTB7-Th41 and PBDB-T42 (the polymer donors), IECO-4F40 and Y143 (the NFAs), and ITCC37 (the versatile third component). Energy levels of PTB7-Th:IEICO-4F:ITCC and PBDB-T:Y1:ITCC are shown in Figure 2a,b. The energy levels of the LUMO and the highest occupied molecule orbital (HOMO) of each material in the active layer were obtained using cyclic voltammetry (CV), and the curves are shown in Figure S1. The bandgap of ITCC (1.77 eV) is larger than those of IEICO-4F (1.38 eV) and Y1 (1.52 eV). The LUMO energy level of ITCC is −3.91 V; thus, it lies between those of PTB7Th (−3.65 V) and IEICO-4F (−4.14 V) and also between those of PBDB-T (−3.49 V) and Y1 (−3.95 V). Figure 2c,d provides normalized ultraviolet−visible (UV−vis) absorption spectra of films of pristine PTB7-Th, IEICO-4F, PBDB-T, Y1, and ITCC. It can be seen from Figure 2c that the visible
overall external quantum efficiencies (EQE) remain poor, resulting in low short-circuit currents (JSC).24,38 Second, lowering the bandgap of the acceptor usually leads to a downshift in the energy level of the lowest unoccupied molecular orbital (LUMO), resulting in a low value of VOC.25,38−40 Both of these two shortcomings limit the further performance enhancements of OPVs. Here, we propose the concept of incorporating a versatile third component (as a second acceptor) into the active layer to enhance the value of JSC and VOC of OPV devices. The requirements for the third component are as follows: (i) its bandgap is wider than that of the host acceptor, (ii) the absorption of the third component should lie within the range of those of the donor and the host acceptor, allowing energy transfer and/or charge transfer to enhance the photocurrent (JSC), and (iii) the LUMO energy level of the third component should lie between those of the donor and the host acceptor. When the LUMO energy level of the third component is higher than that of the host acceptor, it provides an additional electron extraction pathway with a higher quasi-Fermi level energy, leading to a higher VOC than that obtained in the absence of the third component. In this work, the device based on PTB7-Th:IEICO-4F was chosen to study because it possesses wide photon absorption. However, the JSC and VOC still suffer low values. ITCC was employed as a versatile third component in the PTB7C
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters absorption of PTB7-Th ranged from 550 to 770 nm, whereas IEICO-4F absorbed broadly in the infrared region with its onset near 1000 nm. ITCC had a much narrower absorption band (from 550 to 650 nm) that was positioned within the absorption signals of PTB7-Th and IEICO-4F. In Figure 2d, the UV−vis absorption of PBDB-T is slightly blue-shifted relative to that of PTB7-Th; likewise, the absorption of Y1 is slightly blue-shifted relative to that of IEICO-4F. Again, the absorption of ITCC lies within the absorption bands of PBDBT and Y1. Figure 3a,b displays the J−V curves of PTB7-Th:IEICO-4F (10:14, w/w) and PBDB-T:Y1 (9.5:9.5, w/w) based devices with and without ITCC, recorded under solar simulation with intensity AM 1.5G at 100 mW cm−2. The fabrication process can be found in the Supporting Information. The performance of devices with different ratio of the third component can be found in Table S2 and Table S3. Table 1 summarizes the average and champion performances of devices. The average data were obtained from 10 individual devices. The PCE of the device based on PTB7-Th:IEICO-4F:ITCC shows higher PCE (10.6%) than that of the device based on PTB7-Th:IEICO-4F (9.8%). We attribute this enhancement to an increase in the values of VOC (average value increased from 0.71 to 0.75 V) and JSC (average value increased from 20.7 to 21.2 mA/cm2) but with a slightly lower fill factor (FF; average value decreased from 68.2 to 66.0%). The champion device based on PTB7Th:IEICO-4F:ITCC showed a PCE of 11.1%. For the PBDBT:Y1 system, the trends were similar. The average PCE of devices based on PBDB-T:Y1:ITCC was higher (12.9%) than that of the PBDB-T:Y1 binary device (12.5%) with the champion PBDB-T:Y1:ITCC based device achieving a PCE of 13.1%. Devices based on PTB7-Th:ITCC exhibited a PCE of 8.9% with a value of VOC of 0.95 V, a value of JSC of 16.1 mA/ cm2, and an FF of 58.2% (Figure S2, Table S1). External quantum efficiency (EQE) spectra of the devices based on PTB7-Th:IEICO-4F and PBDB-T:Y1 with and without ITCC are shown in Figure 3c,d. The maximum EQEs increased after incorporating the third component, mainly because of the strong absorbance of ITCC in the region 550−750 nm (Figure 2c,d). The UV−Vis spectra of PTB7-Th:IEICO-4F blend and PTB7-Th:IEICO-4F:ITCC blend are shown in Figure S3. The changes in the absorption curve around 650 nm after incorporation of ITCC is owing to the characteristic absorption peak of the pristine ITCC. To elucidate the mechanism behind the enhanced EQEs values of the ternary blend devices, energy transfer and charge transfer among the components in the active layer is investigated. As shown in Figure 4a, the photoluminescence (PL) intensity of the PTB7-Th:IEICO-4F:ITCC blend film (10:14:3, w/w/w) was increased by 350% relative to that of the PTB7-Th:IEICO-4F (10:14, w/w) blend film, suggesting that Forster resonance energy transfer (FRET)44−47 could occur from the acceptor ITCC to IEICO-4F, because the PL emission of ITCC and the UV−vis absorption of IEICO-4F overlapped (see Figure S4). In the ternary-blend OPVs, three possible charge transfer pathways may take place between PTB7-Th and IEICO-4F, ITCC and IEICO-4F, and PTB7-Th and ITCC. Figure 4b,c presents the J−V curves of devices based on ITCC/IEICO-4F and PTB7-Th/ITCC. All devices were measured under AM 1.5 G illumination at 100 mW cm−2. To examine the charge transfer between the two acceptors ITCC and IEICO-4F, we fabricated devices based on a blend of IEICO-4F:ITCC (14:3, w/w) and obtained a PCE of 0.05%
Figure 4. (a) Measured PL of films of pristine IEICO-4F, PTB7Th:IEICO-4F, and PTB7-Th:IEICO-4F:ITCC. (b) Measured J−V characteristics of devices based on pristine IEICO-4F, pristine ITCC, and an IEICO-4F:ITCC blend (14:3, w/w). (c) Measured J−V characteristics of devices based on pristine PTB7-Th, pristine ITCC, and a PTB7-Th:ITCC blend (10:3, w/w).
(with a VOC of 0.24 V, a JSC of 0.6 mA cm−2, and FF of 31.6%). The value of JSC obtained is similar to those of devices based on pristine IEICO-4F (with a PCE of 0.03%; a VOC of 0.14 V; a JSC of 0.7 mA cm−2; and a FF of 32.3%) and that of the device based on the pristine ITCC (with a PCE of 0.03%; a VOC of 0.18 V; a JSC of 0.6 mA cm−2; and a FF of 29.0%), suggesting that charge transfer between IEICO-4F and ITCC is negligible. In contrast, devices based on the PTB7Th:IEICO-4F blend exhibited larger values of JSC (Table 1), indicative of efficient charge transfer between these two components. To investigate charge transfer between PTB7-Th and ITCC, we also fabricated devices based on PTB7Th:ITCC blend (10:3, w/w). We obtained a JSC of 1.6 mA cm−2 (with a PCE of 0.21%, VOC of 0.34 V, and FF of 38.6%). This value of JSC is larger than those of devices based on pristine PTB7-Th (JSC of 1.1 mA cm−2; PCE of 0.05%; VOC of 0.18 V; and FF of 31.5%) and pristine ITCC (JSC of 0.6 mA cm−2; PCE of 0.03%; VOC of 0.18 V; and FF of 29.0%), suggesting effective charge transfer between PTB7-Th and D
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. (a,b) Two-dimensional plots of the transient absorption (ΔA) of (a) PTB7-Th:IEICO-4F and (b) PTB7-Th:IEICO-4F:ITCC blend films. (c) Normalized ΔA traces probed at 732 nm for PTB7-Th:IEICO-4F (binary) and PTB7-Th:IEICO-4F:ITCC (ternary) blend films with curves fitted using a biexponential function. (d) Lifetimes of binary (black and red symbols) and ternary (green and blue symbols) samples, estimated from the biexponential fittings.
Figure 6. (a) FTPS-EQE spectra of PTB7-Th:IEICO-4F and PTB7-Th:IEICO-4F:ITCC based devices. (b) EQEEL measurements in the dark for the PTB7-Th:IEICO-4F and PTB7-Th:IEICO-4F:ITCC based devices.
based on PTB7-Th:IEICO-4F blend (Figure 5c). The difference in lifetimes was estimated by fitting the TA traces with the biexponential equation: ΔA(t) = A0 + A1e−t/t1 + A2e−t/t2 (t1 < t2), in the probe wavelength region from 720 to 738 nm where A0 represents the amplitude for a component whose lifetime was much longer than the measured delay region time (800 ps), A1 and A2 are the amplitudes for t1 and t2, respectively. The two lifetimes, t1 and t2, can be assigned to the rates of charge transfer and recombination, respectively, at the D−A interface.48 Photoexcitation at 400 nm generated charges in each layer of the PTB7-Th:IEICO-4F binary blend and the PTB7-Th:IEICO-4F:ITCC ternary blend films (Figure 2c).48 The smaller t1 for the PTB7-Th:IEICO-4F:ITCC sample suggests that charge transfer between the donor and acceptor was more efficient in the system containing ITCC.49,50 The smaller t2 for the PTB7-Th:IEICO-4F sample indicates a greater degree of charge recombination, resulting in
ITCC. From these results, we believe that devices based on PTB7-Th:IEICO-4F:ITCC experienced efficient charge transfer between PTB7-Th and IEICO-4F and between PTB7-Th and ITCC, as well as efficient energy transfer from ITCC to IEICO-4F. Transient absorption (TA) spectroscopy was measured to probe charge transfer processes in PTB7-Th:IEICO-4F blend systems prepared with and without ITCC. Figure 5a,b presents the two-dimensional (2D) plots of the changes in the intensity of transmission at different wavelengths (ΔA) for PTB7Th:IEICO-4F and PTB7-Th:IEICO-4F:ITCC blends with respect to time and photon energy, respectively. The TA spectra were performed by laser pumping of the samples at 400 nm and then probing from 450 to 740 nm. We attribute the negative value of ΔA observed from 600 to 740 nm to groundstate bleaching. The decay of the devices based on PTB7Th:IEICO-4F:ITCC blends was slower than that of those E
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Table 2. Various Energy Loss Parametersa Egap − qVSQ OC (eV)
gap qΔVrad,below OC (eV)
qΔVnon‑rad OC (eV)
donor/acceptor/third component (ratio, w/w/w)
Eg (eV)
qVSQ OC (eV)
qVrad OC (eV)
measured qVOC (eV)
calculated qVOC (eV)
ΔE (eV)
ΔE1
ΔE2
ΔE3
PTB7-Th:IEICO-4F (10:14) PTB7-Th:IEICO-4F:ITCC (10:14:3)
1.32 1.36
1.07 1.10
1.02 1.04
0.72 0.75
0.73 0.75
0.59 0.61
0.26 0.26
0.04 0.05
0.29 0.30
Parameters (ΔE1, ΔE2, and ΔE3) for the PTB7-Th:IEICO-4F and PTB7-Th:IEICO-4F:ITCC based devices.
a
Figure 7. Two-dimensional GIWAXS patterns of the (a) PTB7-Th:IEICO-4F and (b) PTB7-Th:IEICO-4F:ITCC blend films and 1D GIWAXS profiles of the (c) PTB7-Th:IEICO-4F and (d) PTB7-Th:IEICO-4F:ITCC blends films.
(0.04 eV). The ΔE3 values of the PTB7-Th:IEICO-4F:ITCC based and PTB7-Th:IEICO-4F based devices were similar (0.29 and 0.30 eV, respectively). Overall, the ΔE1, ΔE2, and ΔE3 values were approximately the same in these two systems. By definition, the total energy loss (ΔE) is the difference between the value of qVOC and the optical bandgap. A detailed information about of how to determine the optical bandgaps of OPVs has been reported.51 The average bandgap (1.36 eV) of PTB7-Th:IEICO-4F:ITCC based devices is slightly larger than that (1.32 eV) of PTB7-Th:IEICO-4F based devices (Figure S5), possibly because of a change in aggregation in the active layer after introducing ITCC. Therefore, the simultaneous increase of the optical bandgap VOC of the device may have led to similar total energy losses for both the PTB7-Th:IEICO-4F and PTB7-Th:IEICO-4F:ITCC based devices (0.59 and 0.61 eV, respectively). The incorporation of a third component is expected to alter the morphology of the active layer and, typically, decrease the PCE of the device. Figure 7a,b presents 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns of the various films; Figure 7c,d displays their 1D GIWAXS profiles. The molecular packing in PTB7-Th:IEICO-4F:ITCC was the same
a lower JSC value. Thus, the incorporation of ITCC into PTB7Th:IEICO-4F, indeed, enhances charge transfer between the donor and acceptor. The incorporation of ITCC into the PTB7-Th:IEICO-4F blend resulted in an increase in the value of VOC of the device, because the LUMO energy level of ITCC is higher than that of IEICO-4F. As shown in Figure 6, to characterize the change in energy loss of devices based on PTB7-Th:IEICO-4F without and with ITCC, the measurements of Fourier-transform photocurrent EQE (FTPS-EQE) and electroluminescence of quantum efficiency (EQEEL) were carried out to detect low photocurrent signals. Table 2 summarizes the results ΔE1, ΔE2, ΔE3 (details in Supporting Information). ΔE1(Egap − qVSQ OC) represents the radiative recombination originating from the below gap = qVSQ absorption above the bandgap; ΔE2 (qΔVrad, OC OC − rad qVOC) represents the radiative recombination originating from ) the absorption below the optical bandgap; ΔE3 (qΔVnon−rad OC represents the nonradiative recombination. The ΔE1 values are the same in the PTB7-Th:IEICO-4F based and PTB7Th:IEICO-4F:ITCC based devices (0.26 eV). The ΔE2 (0.05 eV) of the PTB7-Th:IEICO-4F:ITCC based device was as low as that of the PTB7-Th:IEICO-4F based device F
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters as that in PTB7-Th:IEICO-4F, however, the intensities of both the out-of-plane (qz) and in-plane (qxy) signals of the PTB7Th:IEICO-4F:ITCC ternary blend were lower than those of the PTB7-Th:IEICO-4F binary blend. This is suggestive of low crystallinity of the PTB7-Th: IEICO-4F:ITCC blend. Thus, the inclusion of ITCC into PTB7-Th:IEICO-4F may have weakened the molecular packing, a situation that is not beneficial to the FFs of the devices. The electron and hole mobilities are calculated as shown in Table 3 and Figure S6.
Pei Cheng: 0000-0002-1012-749X Kung-Hwa Wei: 0000-0002-0248-4091 Yang Yang: 0000-0001-8833-7641 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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Table 3. Hole and Electron Mobilities of PTB7-Th:IEICO4F Based Devices Prepared with and without ITCC donor/acceptor/third component (ratio, w/w/w)
μh (cm2 V−1 s−1)
μe (cm2 V−1 s−1)
PTB7-Th:IEICO-4F (10:14) PTB7-Th:IEICO-4F:ITCC (10:14:3)
2.2 × 10−4 1.7 × 10−4
5.0 × 10−4 3.3 × 10−4
ACKNOWLEDGMENTS Prof. Y.Y. thanks the Air Force Office of Scientific Research (AFOSR) (FA2386-18-1-4094), Office of Naval Research (ONR) (N00014-17-1-2484), and UC-Solar Program (MRPI 328368) for their financial support. H.-W.C., Y.-C.L., and K.H.W. thank the Ministry of Education Subsidies for Universities and Tertiary Colleges to Develop International Bilateral Program to Jointly Train World Class Professionals, Taiwan; the Center for Emergent Functional Matter Science of National Chiao Tung University from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE), Taiwan; and the Ministry of Science and Technology, Taiwan (MOST 106-2221-E-009-132-MY3), for financial support.
The lower crystallinity of the PTB7-Th:IEICO-4F:ITCC blends is likely responsible for lower electron and hole mobilities (3.3 × 10−4 and 5.0 × 10−4 cm2 V−1 s−1, respectively, for electrons, and 1.7 × 10−4 and 2.2 × 10−4 cm2 V−1 s−1, for holes respectively) and lower FFs relative to those of the PTB7-Th:IEICO-4F blends. In summary, we have reported an approach to increase the values of JSC and VOC of OPV devices by incorporating a third component that possesses a larger bandgap wider than that of the acceptor, an absorption within those of the donor and acceptor, and a LUMO energy level between those of the donor and acceptor. The increase in JSC of ITCC-incorporated devices is attributed to greater charge transfer between the donors and acceptors and/or FRET between the third component and the acceptor. The increase in VOC is attributed to the higher LUMO energy level of the third component than that of the acceptor. The total energy loss of the devices remained unchanged after introduction of ITCC. Nevertheless, the FF decreased after introducing the third component because it disrupted the original molecular arrangement (decreased the crystallinity), resulting in lower electron and hole mobilities. The increase in VOC and JSC led to improved PCEs of the OPVs devices from 10.1% to 11.1% for the PTB7Th:IEICO-4F based system and from 12.6% to 13.1% for the PBDB-T:Y1 based system. These findings will stimulate investigations into more efficient versatile third components to enhance the performance of OPVs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01344. Cyclic voltammograms, J−V curves, absorption spectra, average optical bandgaps, mobility measurement, photovoltaics performance, experimental details, supplementary notes (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Yingping Zou: 0000-0003-1901-7243 G
DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.9b01344 Nano Lett. XXXX, XXX, XXX−XXX
