Poly(3,4-ethylenedioxythiophene) Quantum Dot-Sensitized Solar

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Poly(3,4-ethylenedioxythiophene) Quantum Dot-Sensitized Solar Cells in the Solid-State Utilizing Polymer Electrolyte Tea-Yon Kim, Sungjin Lee, Dongmin Jeong, Tae Kyung Lee, Byung Su Kim, Il Seok Chae, and Yong Soo Kang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00218 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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ACS Applied Energy Materials

Poly(3,4-ethylenedioxythiophene) Quantum DotSensitized Solar Cells in the Solid-State Utilizing Polymer Electrolyte Tea-Yon Kim‡, Sungjin Lee‡, Dongmin Jeong, Tae Kyung Lee, Byung Su Kim, Il Seok Chae and Yong Soo Kang* Department of Energy Engineering and Center for Next Generation Dye-Sensitized Solar Cells, Hanyang University, Seoul 04763, Korea *Corresponding Author E-mail addresses: [email protected] (Y.S.K.) Keywords: polymer dots, PEDOT, solar cells, sensitizers, anchoring groups, soild-polymer electrolytes

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ABSTRACT As

a

new

class

of

sensitizers,

poly(3,4-ethylenedioxythiophene)

(PEDOT)

and

poly(hydroxymethyl-3,4-ethylenedioxythiophene) (PEDOT-OH) quantum dots (QDs) are successfully introduced for solid-state QD-sensitized solar cells utilizing polymer electrolyte with I-/I3- redox couple for the first time. These two polymer QDs were prepared by facile topdown nano-precipitation method from dodecyl sulfate-doped PEDOT and PEDOT-OH nanofibers. In particular, PEDOT-OH QDs containing–OH anchoring groups have higher molecular weight, wider range of light absorption, and tighter adsorption onto the TiO2 surface, compared to PEDOT QDs, yielding higher photocurrent density and consequently higher energy conversion efficiency beyond 1% under 1 sun conditions, which is higher than that of most carbon nano-dot sensitizers ever reported. Interestingly the range of the light absorption is further broadened by spontaneous re-doping the polymer QDs by I2 and/or I3- dissolved in the polymer electrolyte. Moreover, QDSCs utilizing both polymer QDs show excellent long-term stability over 360 minutes. These results suggest a new avenue to utilize common polymer QDs as a new sensitizer for solar cells, and therefore expect the much higher efficiency achieved by optimizing the structure of polymer QDs and the configuration of corresponding devices.


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Conducting polymer quantum dots (CPQDs) have been widely used as zero-dimensional nanomaterials for various applications such as sensor, biomedical, and optoelectronics applications owing to their unique characteristics, including high extinction coefficient, short fluorescence lifetime, non-blinking property, and tunable electrical and optical properties in comparison with other conventional sensitizers such as dyes and inorganic QDs.1-6 CPQDs commonly consist of spherical nanosized entities below 100 nm, made of π-conjugated polymers, also exhibiting excitation-wavelength-dependent photoluminescence (PL) behaviours in common with other kinds of QDs.7 Two representative synthetic methods have been used widely to prepare CPQDs: top-down and bottom-up methods. The top-down method using a prefabricated polymer has been known to be more facile and versatile, compared to the bottom-up method.7,8 Various CPQDs have been thus synthesized using the top-down method. For instance, poly(3,4-ethylenedioxythiophene) (PEDOT) QDs originating from PEDOT were recently synthesized and have attracted much attention.8 A PEDOT QD inherits the merits of PEDOT: high molar extinction coefficient, good chemical and photo-stabilities, and facile size tunability up to 2–3 nm in diameter.8 This can be a significant advantage for application in sensitized-mesoscopic solar cells (SSCs, e.g. dye or quantum dot-sensitized solar cells), using a mesoporous photoanode and an electrolyte including redox mediators (I-/I3- or S2-/Sn2-).9,10 As the diameter of a PEDOT QD can be readily prepared below 5 nm, it is easily applicable to existing mesoporous semiconductors (e.g. TiO2 with average pore diameter of approximately 20 nm) in photoanode.8,11 Furthermore, because PEDOT-QD demonstrated good chemical and photo-stabilities with various redox mediators,12 it can be a good alternative sensitizer to the conventional expensive dyes or toxic inorganic QDs in SSCs. Furthermore, carbon nano-dots (CNDs) such as graphene dots and


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carbon dots, a kind of zero-dimensional nanomaterial similar to CPQD, have been actively employed in SSCs, but their efficiencies are still below 1 % under 1 sun conditions mostly owing to their poor photocurrent primarily associated with the mismatching of adsorption ranges with solar spectrum and the inferior electron injection from CNDs to TiO2 conduction band (CB).13-16 In this regard, we introduce two new sensitizers—neat PEDOT QD and hydroxymethylPEDOT (PEDOT-OH QD)—for solid-state quantum dot-sensitized solar cells (sQDSCs) employing a solid-polymer electrolyte (SPE) with I-/I3- redox couple for the first time. The sQDSCs with PEDOT-OH QDs, in particular, demonstrated an energy conversion efficiency (ECE) higher than 1 % under 1 sun conditions, which is even higher than that of most CNDs sensitizers. Notably, both photocurrent density (Jsc) and ECE increased with time, reaching the maximum efficiency of 1 %, mostly owing to the re-doping of PEDOT-OH QD with I2 and/or I3originating from SPE.

Figure 1. (A) Schematic illustration of the preparation procedure of PEDOT and PEDOT-OH QDs dispersed in DMF. (B) HR-TEM images and particle size distribution histograms of asprepared PEDOT and PEDOT-OH QDs. The scale bar is 5 nm.


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PEDOT and PEDOT-OH QDs were obtained using a top-down nano-precipitation method as illustrated in Figure 1A. The nanofibrous PEDOTs and PEDOT-OHs doped with dodecyl sulfate (DS) during polymerization in water were collected and subsequently dispersed in dimethylformamide (DMF), followed by sonication and filtering to produce yellow (PEDOT QD) and red (PEDOT-OH QD) solutions. The aggregation of these PEDOT and PEDOT-OH QDs in DMF may be prevented by DS. The formation of PEDOT and PEDOT-OH QDs was investigated using Fourier transform infrared (FT-IR) spectroscopy. The FT-IR spectra of PEDOT QD and PEDOT-OH QD in DMF are shown in Figure S1. The peaks at 1652, 1400, and 1100 /cm correspond to the C=C stretching, C=C stretching of the thiophene ring, and C-O stretching in the ethylenedioxyl groups, respectively.8,17 The broad -OH peak can be observed at 3500 /cm with PEDOT-OH QD. The particle morphology and size of QDs were measured using high-resolution transmission electron microscopy (HR-TEM). As shown in Figure 1B, the average size estimated using TEM was 2.01 and 3.51 nm for PEDOT and PEDOT-OH QDs, respectively. PEDOT-OH QD could maintain larger particle size, compared to PEDOT QD, owing to possible hydrogen bonds of PEDOT-OH QD. The TEM images also exhibit a well-defined crystal lattice of QDs with a lattice spacing of approximately 0.2 nm. The UV-vis absorption spectra of the solutions and TiO2 films adsorbed with PEDOT and PEDOT-OH QDs are shown in Figure 2A. In the case of the solution, the absorption peak of PEDOT-OH QD shows a significant red shift and the absorbance also drastically increases in the visible light range, compared to the PEDOT QD, suggesting a high potential as a sensitizer for solar cells owing to the enlargement of the overlap area with the solar spectrum.18 Moreover, the calculated maximum molar extinction coefficient of PEDOT-OH QD was 246,365 /M·cm at 480


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nm in Figure S3, which is more than 10 times higher than that of the N719 dye sensitizer and even 2.5 times higher than that of the graphene dot.13,19 In the case of QDs adsorbed onto the TiO2 thin films, the absorption peaks broaden but indicate no significant shift in the peak position in comparison with the solution of QDs, indicating no aggregation of the attached QDs.20 Furthermore, the amount of QDs adsorbed onto the TiO2 surface can be calculated from the absorption spectra of the QD-adsorbed films, demonstrating that almost the same amount of QDs is adsorbed in both films (PEDOT QD: 2.3 × 10-5 M/cm2, PEDOT-OH QD: 3.2 × 10-5 M/cm2). Figure S4 shows that the PL emission spectra of both QDs in the solution state indicate excitation-dependent emission, suggesting the heterogeneity in size and properties of the synthesized QDs.21 And also the quantum confinement effect of PEDOT-OH QD is well verified in Figure S5, indicating that QD size is smaller (3.5 nm 2.3 nm), the absorption range is more blue-shifted (470 nm 439 nm) and the band gap size is larger.


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Figure 2. (A) Normalized absorption spectra of solutions (dash-dot) and TiO2 films adsorbed (line) by PEDOT (blue) and PEDOT-OH QDs (red). (B) Potential energy diagram of PEDOT and PEDOT-OH QDs along with conduction band (CB) and valence band (VB) of TiO2, and I3−/I− redox potentials. (C) XPS for Ti 2p binding energy of neat TiO2 film (black), and PEDOT and PEDOT-OH QD-adsorbed TiO2 films. (D) TRPL spectra of the dispersed solution, and the TiO2 films adsorbed by PEDOT and PEDOT-OH QDs excited at 470 nm. The energy levels of PEDOT and PEDOT-OH QDs versus the normal hydrogen electrode (NHE) and vacuum level are shown in Figure 2B. The highest occupied molecular orbital (HOMO) levels for both QDs were obtained using square wave voltammetry and also the optical band gap of the QDs was estimated using the Tauc plot calculated from the UV-vis absorption spectra (refer to Figure S6). The HOMO levels of QDs do not have much difference and both are


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sufficient for regeneration by the I-/I3- redox couple. Notably, the band gaps show a clear distinction between QDs: PEDOT-OH QD (2.21 eV) has a lower band gap than PEDOT QD (2.59 eV), well matched to the size of QDs as already shown in Figure 1. Moreover, the LUMO levels of both QDs estimated from the combination of the HOMO level and the band gap ensure the necessary driving force for the electron injection into the CB of TiO2. However, although the LUMO level of PEDOT QD may demonstrate thermodynamically more favourable electron injection, compared to that of PEDOT-OH QD, because PEDOT QD (−1.18 V vs. NHE) has a higher LUMO level than PEDOT-OH QD (−0.74 V vs. NHE), the effective injection rate from the LUMO levels of QDs to TiO2 CB can be more strongly affected by the proximity between the sensitizer and TiO2 surface.22 In this regard, the hydroxyl group was introduced as an anchoring group into PEDOT-QD to form PEDOT-OH QD. The difference in the Ti 2p binding energy can be a good measure of the relative proximity between them. According to the X-ray photoelectron spectroscopic (XPS) data shown in Figure 2C, the binding energy of Ti 2p is reduced from neat-TiO2 (458.4 eV) to PEDOT QD-adsorbed (TiO2/PEDOT QD, 458.0 eV) and further PEDOT-OH QD-adsorbed TiO2 films (TiO2/PEDOT-OH QD, 457.5 eV). As the amount of QD adsorption is almost the same in both TiO2/PEDOT and TiO2/PEDOT-OH QD films, the above results indicate that PEDOT-OH QD with a hydroxyl group acting as an anchoring group is more strongly adsorbed onto the TiO2 surface by the coordination bonds of Ti with O atoms26,27 than PEDOT QD without an anchoring group, which would be physically adsorbed onto the TiO2 film. Therefore, PEDOT-OH QD can be adsorbed more tightly onto the TiO2 surface than PEDOT QD, which can have a significant impact on the electron injection. Time-resolved photoluminescence (TRPL) spectroscopy is commonly employed to evaluate the electron injection kinetics from QDs to TiO2 CB.23,24 Figure 2D shows the TRPL emission


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decay spectra for each QD both in solution and on the surface of the TiO2 films excited at 470 nm. The PL lifetime can be correlated to the rate of electron injection as kinj = 1/τfilm-1/τsol where kinj is the electron injection rate constant, τfilm is the emission lifetime of TiO2/QDs, and τsol is the emission lifetime of the solution of QDs.24 Further, kinj for the TiO2/PEDOT-OH QD film is estimated to be 6.08 × 109 /s, which is more than 3 times faster in the electron injection rate than the TiO2/PEDOT QD film (1.75 × 109 /s). Moreover, the conformal images of the fluorescence lifetime for both QD films also indicate a shorter lifetime of the TiO2/PEDOT-OH QD film as shown in Figure S7. Furthermore, in consideration of the non-injection rate constant knon = 1/τsol, the electron injection efficiency (ηinj) can be estimated as ηinj = kinj/(kinj + knon) to be 80 % and 89 % for the TiO2/PEDOT QD and TiO2/PEDOT-OH QD films, respectively.23 These results demonstrate that more efficient electron injection from QD to TiO2 CB occurs in the TiO2/PEDOT-OH QD film, which shows promise for achieving a better photovoltaic performance in terms of Jsc and ECE.


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Figure 3. Photovoltaic characteristics of sQDSCs based on SPE containing I-/I3- with PEDOT QD (blue circle) and PEDOT-OH QD (red square) sensitizers under 1 sun conditions (AM 1.5, 100 mW/cm2). (A) J–V curves, (B) IPCEs, and changes in both (C) ECE and (D) Jsc with time under ambient conditions with the reference sQDSC employing a neat TiO2 photoanode.


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Table 1. Photovoltaic characteristics of sQDSCs with PEDOT QD and PEDOT-OH QD. The numbers in parentheses indicate the average photovoltaic characteristics and standard deviations for the five difference cells.

Sensitizer

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff. (%)

PEDOT QD

0.52

1.07

61

0.35

(0.521 ± 0.000)

(1.064 ± 0.003)

(62.24 ± 0.44)

(0.346 ± 0.004)

0.47

4.48

51

1.07

(0.469 ± 0.001)

(4.482 ± 0.004)

(50.62 ± 0.16)

(1.064 ± 0.005)

PEDOT-OH QD

The photovoltaic characteristics of sQDSCs with PEDOT and PEDOT-OH QD sensitizers and an SPE containing I-/I3- redox couple under 1 sun conditions (AM 1.5, 100 mW/cm2) are presented in Figure 3A and summarized in Table 1. Significantly, sQDSC with PEDOT-OH QD shows higher than four times enhancement in Jsc (1.07 4.48 mA/cm2), compared to that of PEDOT QD. The incident photon-to-electron conversion efficiency (IPCE) spectra in Figure 3B are also well in accordance with the tendency of Jsc. These Jsc results can be strong evidence that the electron injection is efficient in QDSCs with PEDOT-OH QD. Consequently, sQDSC with PEDOT-OH QD yielded over 1 % of ECE (1.07 %), which is the highest record for similar types of QDs such as carbon dots and graphene dots. Notably, sQDSCs with PEDOT-OH QD and SPE have a higher ECE than QDSCs with a conventional liquid electrolyte (refer to Figure S8 and Table S1), suggesting that PEDOT-OH QD is more favourable with SPEs. This could be because


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sQDSCs with PEDOT-OH QD and SPE may have a high electron recombination resistance as already demonstrated in our previous report.25 Figures 3C and 3D show ECE and Jsc as a function of time for sQDSCs with PEDOT and PEDOT-OH QDs with the reference cell of the neat TiO2 photoanode in sQDSCs under a continuous 1 sun illumination condition. Notably, both PEDOT and PEDOT-OH QDs indicate a large increase in both ECE and Jsc with time, compared to that with neat TiO2, which shows no change in performance, and ECE becomes stabilized after approximately 150 min. Notably, PEDOT-OH QD exhibits a higher performance improvement than PEDOT QD up to 200 min: ECE and Jsc manifest in significant performance improvements of 4.6 (0.23 1.07 %) and 6.7 (0.66 4.48 mA/cm2) times or more, respectively. Consequently, the performance in both cases has been stable for more than 360 min. Therefore, these results suggest an improvement in both the cell performance and photostability of PEDOT QDs. The performance improvement could be presumably because the re-doping of I2 and/or I3- present in the electrolyte to PEDOT QDs changes the absorption ability of QDs, thereby increasing Jsc and thus also increasing ECE.


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Figure 4. Optical absorption spectra of (A) PEDOT and (B) PEDOT-OH QDs adsorbed onto the TiO2 films with different doping concentrations of I2. All data are the absorbance of only QD adsorbed on the TiO2 films, excluding the light absorbance of the TiO2.

The light absorbances of PEDOT and PEDOT-OH QDs adsorbed onto the TiO2 films were measured by varying the doping concentration of I2 in Figure 4. The absorbances of both PEDOT and PEDOT-OH QDs markedly increased with the I2 and/or I3- re-doping. However, PEDOT-OH QD showed a significant increase in the absorption in the whole visible light region, whereas PEDOT QD mainly showed an increase in the region of 450 nm or less. The above data can well elucidate the tendencies of ECE and Jsc in Figures 3C and D. Consequently, the continuous I2 and/or I3- re-doping by the electrolyte increases the absorption ability of PEDOT QDs, thereby causing a rapid increase in both Jsc and ECE. As the doping concentration increases with time, the change of absorbance becomes smaller and the increase of Jsc and ECE becomes eventually smaller. These drastic changes in the optical properties of PEDOT and PEDOT-OH QDs can be applied not only for achieving higher performance of solar cells, but also in other optoelectronic devices.


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Conclusions In conclusion, as a new class of conductive polymer sensitizers, PEDOT and PEDOT-OH QDs were successfully introduced for solid-state QDSCs employing a solid-polymer electrolyte. Especially, PEDOT-OH QD yielded ECE higher than 1 % under 1 sun conditions owing to the improvement in Jsc associated with the –OH anchoring group in the QD. The hydroxyl groups introduced could help contact onto the TiO2 surface more tightly, resulting in the enhancement of both the electron injection rate and consequently the injection efficiency from QD to TiO2 CB, thereby improving Jsc. The photovoltaic performance of sQDSCs with PEDOT and PEDOT-OH QDs increased with time up to 150 min mostly owing to the re-doping of I2 and/or I3- present in SPE and was stable up to more than 360 min. The I2 and/or I3- re-doping from SPE to PEDOT QDs resulted in the enhancement of the light absorption ability, which significantly improved the performance of sQDSCs. These unprecedented effects and phenomena observed in PEDOT QDs can be expected to pave the way for new opportunities to develop effective and stable light energy conversion materials for next generation devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at. Experimental details and supporting results AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (Y.S.K.). Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University funded by the Minister of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (Number 2009-0093883). And the authors thank Dr. Weon-Sik Chae, KBSI Daegu Center, for the fluorescence lifetime measurements. S. D. G. REFERENCES (1) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; Güntner, R.; Scherf, U. Novel Approaches to Polymer Blends Based on Polymer Nanoparticles. Nat. Mater. 2003, 2, 408-412. (2) Wu, C.; Szymanski, C.; Cain, Z.; McNeill, J. Conjugated Polymer Dots for Multiphoton Fluorescence Imaging. J. Am. Chem. Soc. 2007, 129, 12904-12905. (3) Wu, C.; Jin, Y.; Schneider, T.; Burnham, D. R.; Smith, P. B.; Chiu, D. T. Ultrabright and Bioorthogonal Labeling of Cellular Targets Using Semiconducting Polymer Dots and Click Chemistry. Angew. Chem., Int. Ed. 2010, 49, 9436-9440. (4) Tian, Z.; Yu, J.; Wu, C.; Szymanski, C.; McNeill, J. Amplified Energy Transfer in Conjugated Polymer Nanoparticle Tags and Sensors. Nanoscale 2010, 2, 1999-2011.


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Poly(3,4-ethylenedioxythiophene) Quantum Dot-Sensitized Solar

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