Enhancing the Power Conversion Efficiency for Polymer Solar Cells by

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Enhancing the Power Conversion Efficiency for Polymer Solar Cells by Incorporating Luminescent Nano-solid Micelles as Light Converter Die Wang, Wenfei Shen, Jianguo Tang, Yao Wang, Jixian Liu, Xinzhi Wang, Renqiang Yang, Christopher D. Snow, Linjun Huang, Jiqing Jiao, Yanxin Wang, Wei Wang, and Laurence A. Belfiore ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00217 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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Enhancing the Power Conversion Efficiency for Polymer Solar Cells by Incorporating Luminescent Nano-solid Micelles as Light Converter Die Wang1, ‡, Wenfei Shen1, ‡, Jianguo Tang1,*, Yao Wang1, Jixian Liu1, Xinzhi Wang1, Renqiang Yang2,*, Christopher D. Snow3,*, Linjun Huang1, Jiqing Jiao1, Yanxin Wang1, Wei Wang1, Laurence A. Belfiore1,3,* 1. Institute of Hybrid Materials, National Center of International Research for Hybrid Materials Technology, National Base of International Science & Technology Cooperation, College of Materials Science and Engineering, Qingdao University, 308 Ningxia Road, Qingdao 266071, P. R. China, [email protected]. 2. Qingdao Institute of Bioenergy and Bioprocess Technology Chinese Academy of Sciences, Qingdao 266101, China, [email protected]. 3. Department of Chemical and Biological Engineering, Colorado State University, Fort Collins 80523, USA, [email protected], [email protected]. KEYWORDS: hybrid polymer solar cells, luminescent nano-solid micelles, increase photovoltaic performance, increase light absorption, increase stability.


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ABSTRACT: In this research contribution, the idea is created that Ln3+-doped nano-solid micelles has firstly been applied to enhance the photovoltaic properties of novel hybrid polymer solar cells (HPSCs). Of the publications, we have never found the same or similar work to be published previously. And the new method of Ln3+ doped DBC to develop nano-solid micelles luminescent materials is established. Very importantly the applicable technique to incorporate nano-solid micelles into PSCs by spin-coating on the surface of indium tin oxide (ITO) layer is accomplished after comparing the different cooperation ways. This research contribution elucidates two novel HPSCs containing Ln3+ nano-solid micelles formed by Ln3+-doped diblock copolymer (Ln3+-DBC) and organic conjugated ligands. The Ln3+-DBC luminescent nano-solid micelles（LNSM） are formed via coordination between Ln3+ ions and DBC, and are readily dispersed in a hydrophobic solvent. Nano-solid micelles can be incorporated into PSCs by spincoating on the surface of indium tin oxide (ITO) layer, to increase the absorption of sunlight. Critically, the presence of the LNSMs increase PSC absorption of light and increases the power conversion efficiency (PCE) of PTB7-Th/PC71BM based devices from 8.68% to 9.61%, increased by 10.71%, which mainly caused by the enhanced short-circuit current density (Jsc), increased by 12.69% . Besides, the incorporation of LNSMs improved the stability of devices by protecting the active materials’ degradation from UV light.

Introduction Bulk heterojunction polymer solar cells (PSCs) have attracted great attentions in recent decades due to their low-cost, light-weight, and their suitability for incorporation into flexible devices1-9. Extensive research efforts have been made to increase the power conversion efficiency (PCE) of PSCs by developing low-band-gap donor materials, or utilizing advanced fabrication technologies.7, 10-13 However, the reported PCE for single junction PSCs has been


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limited to approximately 13.0%14-18, which represents lower efficiency than competing solar cell varieties19-20. Better photovoltaic performance is needed to enable future industrialization. Because of the low charge carrier mobility in PSCs, the thickness of the active layer is limited to ~100 nm14, 17, 21-22 to guarantee relatively high extraction efficiency for electrons and holes21, 23-26. A thin active layer leads to insufficient light absorption, which limits PCE. To solve this problem, different donor materials with complementary light absorptions were applied to tandem solar cells27-31. Though light absorption can indeed be enhanced, tandem solar cells are rather complicated to fabricate32, which can greatly increase processing cost. Ternary blend PSCs33-35 with two complementary absorption donor materials and one acceptor material are emerging as an effective method to enhance light absorption for single-junction PSCs while retaining a simple fabricating process. However, only several types of donor materials are feasible for ternary blend PSCs, due to the complexity of the energy transfer between the donor materials and the strict material compatibility requirements33, 36. It is well known that rare earth complex has many advantages such as high purity of emitting colours, high light absorption ability, high conversion efficiency and tuneable emission wavelength37. Many research groups have applied rare earth luminescent materials to inorganic solar cell devices for enhancing the photovoltaic performances38-43. Therefore, utilizing rare earth complex enhancing the light absorption of polymer solar cells are also theoretically feasible. Besides, it is becoming an effective way to enhancing the stability of PSCs by incorporating a UV absorbent. Zhan’s group reported the enhanced device stability by incorporating benzophenone as UV absorbent42. J. Kettle44 achieved good results using a luminescent lanthanide-based metal complex to create a down-shifting layer as an alternative UV filter for organic photovoltaic (OPV) devices based on poly(3-hexylthiophene, P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM).


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However, the PCE of the their devices were rather low (2.82%), and the fabricating processes were relatively complex for preparing luminescent down-shifting layers before fabricating PSCs45. However, it is easy to believe that the luminescent layer outside devices will cause light reflection, scattering and energy losses for the subjacent solar cells. For this issue, incorporating luminescent materials in devices will be a more effective strategy to utilizing the luminescence radiated by the luminescent materials. Typically, rare earth luminescent complex materials adopt irregular structures, which is not ideal when incorporating to precisely structured PSCs. Hence, it’s better to regulating the morphology of luminescent materials before incorporated to polymer solar cells. In this work, we provide a very easy yet effective strategy by directly incorporating morphology regulated Ln3+ based luminescent materials in PSCs to improve their photovoltaic performances. To prepare morphology regulated Ln3+ based luminescent materials, we embedded lanthanide ions in diblock copolymer formed a kind of micelles structure, and we abbreviated nano-solid micelles lanthanide ions in diblock copolymer as LNSMs. The lanthanide ions selected in this work were Eu3+ and Tb3+, and the corresponding LNSMs were named as Eu3+ nano-solid micelles (ENSMs) and Tb3+ nano-solid micelles (TNSMs), respectively. Besides, nano-solid micelles with both Eu3+ and Tb3+ ions in diblock copolymer (ETNSMs) can be formed when a ENSM solution and a TNSM solution were mixed with a certain ratio. During the forming processes of LNSM, lanthanide ions can coordinate with carboxyl group in the second DBC segment which is polyacrylic acid (PAA) and induce self-assembly of the first DBC segment which is polymethyl methacrylate (PMMA)

32-33

. The resulting particles are regular,

spherical, solid micelles with 10-20 nm diameters. These structure regulated nano-solid micelles


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are attractive as possible HPSC additives due to their uniformity and high light absorption performance46-47. Experimental Section Material and Methods Materials. Europium oxide (Eu2O3, 99.99%) and Terbium oxide (Tb4O7, 99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd., (China). α-thenoyltrifluoroacetone (TTA, 99%) and 1,10-phenanthroline (phen, 99%) were obtained from Shanghai Darui Chemical Reagent Co., Ltd., (China). The DBC was prepared by us (see below). The N,Ndimethylformamide (DMF) and o-dichlorobenzene solvents were all commercial analytical reagents. Patterned ITO coated glass with a sheet resistance of 15 Ω/sq was purchased from Shenzhen

Display

(China).

Poly{[4,8-bis-(2-ethl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-

b0]dithiophene-2,6-diyl]-alt-[2-(20-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTTC-T) was purchased from Lumtec and Solarmer Inc. [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM) was obtained from American Dye Sources (ADS). All reagents were used as received without further purification. Preparation of DBC solution. The DBC synthesis methods of PMMA and PAA were adopted from the literature32. The reaction products were characterized by 1H NMR spectrum (Figure 1). The molecular weights of PMMA and DBC were calculated according to 1H NMR spectrum by the end group analysis and the comparison between the monomer number and the mass of PMMA and PAA (after drying) 48. Based on the NMR analysis, the mean number of PMMA units was 15 and the mean number of PAA units was 10. Finally, a 0.04 M DBC solution was obtained by dissolving a certain amount of DBC into DMF solvent.


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Figure 1 1H NMR spectra of (a) PMMA and (b) DBC. Preparation

of

ENSMs

and

ETNSMs.

The

conjugated

organic

molecules

α-

thenoyltrifluoroacetone (TTA) and 1,10-phenanthroline (phen), as the first ligand species for ENSMs and TNSMs, respectively. Carboxylates from the PAA segment of the diblock polymer (DBC: PMMA-b-PAA) can also serve as a ligand. Critically, PMMA-b-PAA is amphiphilic with hydrophilic PAA and hydrophobic PMMA. The LNSM solution was obtained as follows. First, phen, TTA and DBC solutions (all in o-dichlorobenzene) were added into a three-necked roundbottom flask with a reflux device, with a molar ratio (Eu3+:TTA:AA) of 1:3:1 [Figure 2 (a)] and molar ratio (Tb3+:phen:AA) of 1:2:1 [Figure 2 (b)], respectively. Second, europium chloride solution (0.02 M in DMF) and terbium chloride solution (0.02 M in DMF) were added into the three-necked round-bottom flasks drop-wise at 60-70 C in the oil bath with condenser, to yield the ENSM solution and TNSM solution, respectively. Finally, the ETNSM solution was obtained by combining and stirring the ENSM and TNSM solution with the molar ratio (Eu3+:Tb3+) of 1:2 [Figure 2 (c)]. Notably, to avoid aggregation, the ENSM solution and ETNSM solution were subjected to several seconds of ultrasonic disruption (Numerical control ultrasonic cleaner: KQ3200DA, 100 W) prior to testing.


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Figure 2 Morphological structures of (a) ENSMs, (b) TNSMs, and (c) ETNSMs. HPSCs preparation. The active layer of our HPSCs were composed of PBDTTT-C-T:PCBM is a low band-gap (~1.58 eV) polymer with high charge carrier mobility (µh=0.27 cm2·V-1·s-1).49 PC71BM is a fullerene derivative C71-butyric acid methyl ester. A HPSC schematic (Figure 3) illustrates the ITO conductive glass, LNSM layer, hole transport layer (PEDOT:PSS), active layer (PBDTTT-C-T:PCBM), and Ca/Al electrode from bottom to top (in the order of deposition). Six specific HPSC variants with different LNSM concentrations and one reference PSC were tested to investigate the performance effects of LNSM (Table 1). (A) ITO/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al; (B) ITO/ENSMs (2.5 mM)/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al; (C) ITO/ENSMs (1.5 mM)/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al; (D) ITO/ENSMs (0.75 mM)/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al; (E) ITO/ETNSMs (2.5 mM)/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al; (F) ITO/ETNSMs (1.5 mM)/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al; (G) ITO/ETNSMs (0.75 mM)/PEDOT: PSS/PBDTTT-C-T: PC71BM/Ca/Al. The molar ratio (Eu3+:Tb3+) of variants (E), (F) and (G) HPSCs was 2:1.


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Table 1 Technical parameters for HPSC preparation Layer LNSM layer Hole-transport layer Active layer Cathode (Ca) Cathode (Al)

Speed (rpm) 2000 4000 1000 / /

Vapor deposition ratio Time (s) (nm/s) / 40 / 30 / 40 0.01 1000 0.5 200

Thickness (nm) / 30 150 10 100

Figure 3 Schematics of (a) HPSC structure, (b) PBDTTT-C-T, PTB7-Th and PC71BM. ITO conductive glass (15 Ω·sq-1) was ultrasonic cleaned for 20 min in a series of solvents: detergent, de-ionized water, acetone, de-ionized water, and isopropyl alcohol. The last two steps are drying in the oven and 6 min of treatment in a plasma cleaning machine. To prepare the LNSM layer, the particles were dispersed (ultrasonic disruption for ~1 min), spin-cast and freeze dried. The technical parameters of HPSCs preparation are shown in Table 1.


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Measurements. NMR measurements mainly consisted of 1H nuclear magnetic resonance (1H NMR) spectroscopy, which was performed on a JNM-ECP600 (600 MHz) spectrometer (JEOL Ltd., Japan) with CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. LNSM morphology was characterized by transmission electron microscopy (TEM) with JEM1200EX instrument (JEOL Ltd., Japan). Ultraviolet-visible (UV-vis) absorption measurements were performed on a Lambda 25B (Perkin, Japan) spectrophotometer at room temperature. It was equipped with double beam proportion monitoring optical system with wavelength detection range from 200 to 500 nm. The dispersion of the LNSMs on the ITO layer was characterized with a field emission scanning akctron microscope (SEM, JSM-7500F). The fluorescent optical microscopic photographs of LNSMs on ITO were obtained at room temperature with XSP-63XD microscope, equipped with a UV lamp emitting in the wavelength range of 10-800 nm. The current density-voltage (J-V) characteristics of the HPSCs were measured using a source measurement unit (Keithley 2420) and an XES 301 (AM 1.5G) full spectrum solar simulator at an irradiation intensity of 100 mW/cm2. Light intensity was calibrated with a standard silicon solar cell. External quantum efficiency (EQE) was measured using a certified Newport incident photon conversion efficiency measurement system. The integrated EQE values show good agreements with the measured short-circuit current density (Jsc). The quantum efficiency of solid ETNSM was characterized by an FLS 980 spectrometer (Edinburgh Instruments, U.K.). Results and Discussions Morphology structures of ENSMs and ETNSMs. To prepare nano-solid micelles, DBCs of PMMA and PAA were synthesized by the Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT) method. Ln3+ can be coordinated by carboxyl groups in PAA segments of DBC and further induce the self-assembly


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of DBCs by forming a Ln3+:PAA rich core33, 50. From Figure 2, it can be seen that the prepared ENSM, TNSM and ETNSM consist of regular spherical particles with uniform small size. Compared to previous research49 including from our group32, the particle corona is much smaller than the core diameter due to the smaller ratio of PMMA to PAA (15:10). The average micelles diameter is about 50-60 nm, 13-20 nm, and 20-30 nm in Figure 2 (a), (b) and (c), respectively. The small size and narrow size distribution suggest that the particles may be suitable components for PSCs or other optoelectronic devices that require very thin films. The Figure 2 inserts illustrate likely chemical structures for each Ln3+-DBC combination regarding ENSMs, SETPs and ETNSMs, in which Ln3+ serves as a linker by coordinating carboxyl groups from multiple PAA segments. A key difference between the ENSMs and TNSMs is the number and chemical identity of the conjugated molecules used as “light antennas” for Eu3+ and Tb3+ respectively. Based on the energy level matching principle, Eu3+ ion is coordinated with TTA molecules at one carboxyl group in a PAA segment of DBC32, 34. In contrast, Tb3+ ion is coordinated with one phen molecule at double PAA segments of two DBCs.51 Herein the ratios of Eu3+ or Tb3+ to conjugate molecules and to carboxyl groups at PAA segments of DBCs were controlled by the stoichiometry. Meanwhile, the PMMA segments form the corona of the nano-solid micelles, keeping LNSMs uniform distribution via stretching their chains into o-dichlorobenzene solvent. Furthermore, double-core nano-solid micelle with small size (about 20-30 nm) will be formed (Figure 2c) when ENSMs and TNSMs are mixed together according the predetermined 1:2 ratio. This ratio leads to the best luminescence intensity (the ratio was optimized in a study that will be published separately). Photovoltaic performances.


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To investigate the effects of nano-solid micelles on polymer solar cells, LNSMs in different concentration were spin-coated on ITO substrate before fabricating hole transport layers. For all developed HPSCs the characteristics including open-circuit voltage (Voc), Jsc, fill factor (FF) and thus PCE, are summarized in Table 2. The corresponding current density-voltage (J-V) curves are shown in Figure 4. Table 2 Photovoltaic parameters of HPSCs with LNSMs as additives and PBDTTT-CT/PC71BM as active layers under AM 1.5G illumination at 100 mW·cm-2

LNSM

Sample

CLn3+ (mM)

Voc (V)

Jsc FF 2 (mA/cm ) (%)

Reference A 0 0.76 15.93 60.64 PSC B 2.5 0.76 15.89 61.00 ENSM C 1.5 0.77 16.48 61.72 D 0.75 0.76 16.41 60.63 E 2.5 0.76 16.93 61.62 ETNSM F 1.5 0.76 17.27 62.93 G 0.75 0.75 16.94 61.56 a The average PCE values obtained from 10 devices.

PCE Average Maximum enhancement PCE (%)a PCE (%) (%) 7.13±0.12 7.25 / 7.38 7.84 7.56 8.26 8.41 7.95

+1.8 +8.1 +4.3 +13.9 +16 +9.7

7.27±0.11 7.56±0.28 7.34±0.22 8.03±0.23 8.27±0.14 7.76±0.19

Figure 4 Compared to a reference PSC, current density-voltage (J-V) characteristics of HPSCs with LNSMs vary depending on Eu3+ ions concentration: (A)0 mA, (B) 2.5 mM, (C) 1.5 mM and


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(D) 0.75 mM or upon the combined concentration of Eu3+ and Tb3+: (E) 2.5 mM, (F) 1.5 mM and (G) 0.75 mM. Current density-voltage (J-V) characteristics curves (Figure 4) clearly show that LNSMs in different concentration favourably impact cell performance (PCE) relative to the reference PSC. The performance benefit was dependent on LNSM concentration, with superior performance for the 1.5 mM concentration compared to the 2.5 mM and the 0.75 mM. For the 1.5 mM ENSM concentration, the HPSC PCE is increased by 8.1% with respect to the reference PSC (from 7.25% to 7.84%). Among all the HPSC photovoltaic parameters presented in Table 2, the largest efficiency enhancement is for HPSC variant C, in which the short circuit current density (Jsc) increases to16.48 mA/cm2 (from 15.93 mA/cm2 for the reference PSC). Therefore, in our investigated concentration range, the optimal concentration of LNSM for maximal PCE increase of HPSCs is 1.5 mM. The presumed origin for the higher PCE of HPSC is that the LNSMs successfully absorb ultraviolet light (in the 250 nm to 400 nm range) and emit visible light that can be absorbed by active polymer PBDTTT-C-T of PSC. The down-converted ultraviolet light can compensate for the weak absorption of active polymer PBDTTT-C-T in PSC49, 52. From Table 2 it can be also noted that the open-circuit voltages are not changing significantly. This result suggests that there is no change of the HOMO energy level of the donor material or the LUMO energy level of the acceptor material when LNSM was incorporated into PSC52. However, the Jsc and FF have been enhanced to varying through LNSM incorporation, such as ~3.2% and ~8.4% relative enhancements in Jsc for HPSC C and HPSC F, respectively, or ~1.8% and ~3.8% relative enhancements in FF corresponding to HPSC C and HPSC F, respectively.


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Inspired by the enhancement effect of ENSM-incorporated HPSCs, we synthesized LNSMs with double Ln3+ hybridization. Like Eu3+, Tb3+ ion absorbs UV-light and emits visible light. From Table 2, we can find that photovoltaic performances including Jsc, FF and PCEs of HPSCs are clearly increased when compared with the reference PSC. When ETNSMs were doped with a low concentration (i.e. 0.75 mM) in HPSC, the photovoltaic performances of HPSCs can be increased slightly. With a higher concentration (i.e. 2.5 mM in HPSC E) including incorporation of Tb3+ ions, photovoltaic performances increased significantly. The PCE of HPSC E reached 8.26% (from 7.25% for the reference PSC), which is higher than HPSC B at the same concentration. However, when the concentration of ETNSMs is 1.5 mM (HPSC F), a still higher PCE (8.41%) was obtained. We hypothesize that the 1.5 mM ETNSM variant outperformed the 2.5 mM variant due to a blocking phenomenon at higher concentrations (see below). In all, we have significantly enhanced the PSC PCE compared with other reports. For example, our PCE exceeds that of benzo[1,2-b:4,5-b’] dithiophene-based conjugated polymer (78%)53 and optimized bulk hetero-junction PTB7:PC71BM (7.74%) in which the active layer has been treated with a series of alcohols19. Furthermore, our method for incorporating LNSMs is easier than depositing a thermally evaporated tris-(8-hydroxyquinoline) aluminum (Alq3) layer for PSC (PCE of 6.61%)54. Our results further support the superiority of spin casting compared to ink-jet printing or using a blade. These latter methods increase the thickness of the active layer resulting in a lower PCE (3.9%)55. To verify the superior photovoltaic performance of the HPSCs we measured the external quantum efficiency (EQE). The EQE value reaches 100% if all incident photons generate electron-hole extractions. Many factors reduce EQE including the reflection of incident light, weak absorption of photo-active materials, and the recombination of electrons and holes50, 56.


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EQE curves for various HPSCs are shown in Figure 5. The visible-region EQE curves of all HPSCs exhibit two predominant bands that peak at 450 nm and 670 nm. These features originate from the B-band and Q-band of the lanthanide complexes. Figure 5 also clearly illustrates the enhanced EQE due to incorporating ENSMs and ETNSMs. In the case of ENSMs, enhancement was mainly located in 550-700 nm, in accord with the ENSM emission spectra (which will be detailed discussed below). In the case of ETNSMs, we attribute the enhanced EQE to the dual effects of ENSMs and TNSMs. Again, the wavelength range of the EQE enhancement (450-700 nm), is in accord with the emission spectra of ETNSMs. In summary, addition of ENSMs or ETNSMs can significantly enhance current density and PCE, and the EQE clearly ties the wavelength-dependent improvements to the lanthanide luminescence.

Figure 5 External quantum efficiency of HPSCs measured using a certified Newport incident photon conversion efficiency (IPCE) measurement system. In order to verify the applicability of the device structure with luminescent nano-solid micelles to other low band gap donor materials, the devices with PTB7-Th/PC71BM as active


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materials were fabricated, and the corresponding photovoltaic parameters were listed in Table 3(corresponding J-V curves in Figure 6). With the optimized LNSM concentration of 1.5 mM, the devices showed an enhanced PCE of 9.61% compared to 8.68% of reference devices that without LNSM. Same results with PBDTTT-C-T/PC71BM system, enhanced photovoltaic performances were mainly ascribed to the increased Jsc, which was from 16.23 mA/cm2 to 18.29 mA/cm2. Therefore, the device structures with optimized concentration of LNSM were also applicable to PTB7-Th/PC71BM system polymer solar cells. Table 3 Photovoltaic parameters of HPSCs with LNSMs as additives and PTB7-Th/PC71BM as active layers under AM 1.5G illumination at 100 mW·cm-2 CLn3+ (mM)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Reference device

0

0.80

16.23

66.83

8.68

Device with ENSM

1.5

0.80

16.98

66.61

9.05

Device with ETNSM

1.5

0.80

18.29

65.66

9.61

Figure 6 J-V curves of luminescent devices with PTB7-Th:PC71BM as active layers.


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Fluorescence. Spin-cast ENSMs or ETNSMs samples on ITO (Figure 7) show the expected red and red-green colors respectively when illuminated with a UV lamp emitting in the wavelength range of 10-800 nm. As the solution concentration of LNSMs decreases, there were fewer very large aggregates (“islands”) [from Figure 7 (a) to (c) and from Figure 7 (d) to (f)]. These fluorescence measurements may help explain why the PCE was maximized (Table 2) when the Ln3+ ions concentration was only 1.5 mM. With 0.75 mM Ln3+ ion concentration (Figure 7 c f), the LNSMs were spin-cast uniformly but with insufficient LNSM density for optimal sunlight absorption. The relatively uniform dispersion achieved for the 1.5 mM concentration enhances PCE to the maximal extent.

Figure 7 Fluorescence emission photos of Eu3+ containing LNSMs at (a) 2.5 mM, (b) 1.5 mM, or (c) 0.75 mM spin-cast on ITO. The combined Eu3+ and Tb3+ particles ETNSMs at (d) 2.5 mM, (e) 1.5 mM, and (f) 0.75 mM emit both red and green light.


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Spin-cast LNSM morphology. The LNSM dispersion state on ITO can strongly influence the PCE. Therefore, we used SEM and AFM to investigate the surface morphology of LNSMs spincast on ITO (Figure 8). SEM images show various nanoparticles dispersion (Figure 8 a-f). As can be seen, with 1.5 mM concentration, LNSMs disperse well. We check the AFM morphologies with 1.5 mM concentration. As results, No larger LNSM aggregates of LNSMs were observed in the AFM images(Figure 8 i and j). Therefore, we believe that 1.5 mM LNSMs were well dispersed via spin-casting. Uniform dispersion is critical if we attempt to understand performance (Table 2) in terms of LNSM concentration. Higher concentrations of ENSMs and ETNSMs had an unfortunate tendency to aggregate (Figure 8 a d). Particle agglomeration (Figure 8 a) and reunion (Figure 8 d) could cut off charge transport in the hole-transport layer, thereby decreasing PCE57-58. This detrimental effect is in accord with the performance date (Table 2) and fluorescence imaging (Figure 8). In contrast, ETNSMs dispersed without particle reunion (e.g. Figure 8 b e) successfully absorb more sunlight, transfer light to the active polymer and increase PCE. Finally, the lower concentrations of ENSMs and ETNSMs also show sparse dispersion (Figure 8 b e). We therefore attribute the lower performance at this concentration to the sub-optimal LNSM density. While further experimentation will be useful, we believe that we have successfully identified key competing criteria for maximizing PCE via LNSMs incorporation. Energy dispersive X-ray spectroscopy (EDS) was used to further validate the spin-cast LNSM surfaces. For example, the EDS spectrum for ENSM confirms the existence of Eu3+ as well as the fluorine present in the ENSM TTA ligands (Figure 8 g). In the case of ETNSMs, nitrogen from the phen ligands, fluorine from TTA, Eu3+ and Tb3+ were all identified (Figure 8


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h). These results are in accord with the expected elemental composition (Figure 2 inserts) and with the observed fluorescence (Figure 7).

Figure 8 Morphological structure and dispersion of LNSMs spin cast on ITO depend on concentration. ENSM with Eu3+ at (a) 2.5 mM, (b) 1.5 mM, and (c) 0.75 mM; ETNSM with Ln3+ (Eu3+ and Tb3+) at (d) 2.5 mM, (e) 1.5 mM, and (f) 0.75 mM. Correspondingly, (g) and (h) are


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EDS maps of ENSM and ETNSM at 1.5 mM; (i) and (j) are AFM images of ENSM and ETNSM at 1.5 mM. Polymer solar cell stability with respect to time. Stability with respect to time is a key challenge for polymer solar cells. Therefore, we quantified the stability of our HPSC compared to the reference PSC (Figure 9). As expected, the PCE of reference PSC decreases with time. Notably, the hybrid cell with ETNSMs degraded much more slowly. While the PCE of the reference PSC decreased to 0 after 4 days, the HPSC operated at 73%. Although the stability of device with luminescent materials was still bad, which was caused by the unstable normal structure, the incorporation of luminescent materials indeed increases the stability for protecting UV light degradation of the active layer. The PSC degradation may be ascribed to the acidic PEDOT: PSS layer corroding ITO glass51-52. We ascribe the improved stability of the HPSC to the UV-screening effect of ETNSMs. In summary, LNSM incorporation not only enhances efficiency but also can prolong the functional PSC life-time.


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Figure 9 PCEs of un-encapsulated HPSC with ETNSMs and the reference PSC stored in 10 days in N2 filled glove box. Mechanism of UV-light energy transfer to active polymer. While the HPSC absorbed light from the entire visible (400-800 nm) peak absorption was at λpeak,abs=700 nm, which corresponds to the absorption of active polymer PBDTTT-C-T (Figure 10). However, there are side peaks in the 250-400 nm UV range (Supporting Information), which correspond to the absorption peaks of carbon-carbon double bonds in the donor polymer chain of PBDTTT-C-T. Absorption of UV light can cause the degradation of conjugated active polymers. Fortuitously, ENSMs, TNSMs, and ETNSMs respectively provide strong absorption bands of 250-400 nm, 250-350 nm, and 250-400 nm.

Figure 10 Ultraviolet-visible absorption spectra of LNSMs and PBDTTT-C-T. The photo-luminescent (PL) excitation and emission spectra of LNSMs are shown in Figure 11 (a) and (b). The excitation spectra were obtained by monitoring the 615 nm emission


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wavelength ENSMs, 545 nm for TNSMs, and both 545 nm and 615 nm for ETNSMs. The peak excitation wavelength was 385 nm for ENSMs, and 336 nm for TNSMs. When the ENSMs and TNSMs were mixed, there were two characteristic excitation peaks (318 nm and 385 nm), and two characteristic emission peaks (545 nm and 615 nm). This result is in accord with the combined red-green emission observed via fluorescence microscopy (Figure 7 d e f). Besides, the quantum efficiency of solid ETNSM was characterized and the quantum efficiency was 37.74%, which was high enough to be a effective light converter.

Figure 11 (a) Excitation and (b) emission spectra of LNSMs. The active polymer PBDTTT-C-T has a major absorption range from 400 to 800 nm. Therefore, PBDTTT-C-T can absorb the light emitted by LNSMs at 545 nm (attributed to hypersensitive transitions of 5D4→7F5) and emitted at 615 nm (attributed to hypersensitive transitions of 5D0→7F2). The emission peak at 700 nm is particularly well-aligned with the maximal absorption peak of PBDTTT-C-T. Ideally, lanthanide luminescence may serve to compensate for the low UV absorption of PBDTTT-C-T, by shifting the energy to visible wavelengths that more readily absorbed (Figure 10). Enhancing absorption will, in turn, enhance the PCE as observed (Table 2).


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It is worth noting that ETNSMs offer two additional emission peaks (Figure 11 (b)) (482 nm and 545 nm) relative to ENSMs due to Tb3+ ions incorporation. We suggest that the latter peak (λ=545 nm) is absorbed more efficiently by the active layer and therefore plays a dominant role in the superior performance obtained for ETNSM. Conclusions Numerous aspects of PSC will require refinement and invention prior to commercial viability. We suggest that LNSMs can be used to improve both lifetime (via protection from UV light) and performance (via re-emission of incident UV light as visible light). This work characterizes seven HPSCs variants, which incorporate very small, spherical LNSMs with varying concentration. The results demonstrated clear PCE enhancement compared to a reference PSC lacking LNSMs. We attribute the performance benefit to the conversion of incident UV light (250-400 nm) to more useful visible light (450-750 nm) via LNSMs luminescence. The emitted light is suitable for absorption by the active polymer PBDTTT-C-T. The best photovoltaic performance of the hybrid polymer solar cells was achieved when incorporating both europium and terbium (ETNSMs). The improved performance can be rationalized in terms of the balancing of the excitation and emission spectra (Figure 11). Given preceding work on LNSMs, we were able to focus on a relatively narrow Ln3+ concentration range (0.75-2.5 mM). Within this range, a doped Ln3+ ion concentration of 1.5 mM led to maximum current density and PCE. We believe that this research provides a platform suitable for further optimization with other lanthanide ion solid micelles. Improvements are likely to be modular since such particles can be spin-coated onto an ITO layer independently of the subsequent layers. Other potential applications for the reported materials include new organic luminescent materials and functional


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devices. In the future, the HPSCs incorporated LNSMs can be applied in the Distributed Generation station. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The absorption spectrum of active material of PBDTTT-C-T. AUTHOR INFORMATION Corresponding Author Email: [email protected], [email protected], [email protected], [email protected] Author Contributions ‡These authors contributed equally. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style). ACKNOWLEDGMENT This work was supported by (1) the Natural Scientific Foundation of China (Grant No. 51473082, 51373081, 51273096); (2) the Program for Introducing Talents of Discipline to Universities (“111” plan); (3) The National One-Thousand Foreign Expert Program (Grant No. WQ20123700111);

(4)

State

Key

Project

of

International

Cooperation

Research

(2016YFE0110800);（5）The 1st class discipline program of Shandong Province of China.


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293x230mm (72 x 72 DPI)


Enhancing the Power Conversion Efficiency for Polymer Solar Cells by

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