Energy saving electrochromic windows from bistable low-HOMO level conjugated polymers

Abstract

Energy saving electrochromic windows were established by controlling the interfacial charge transport using low-HOMO level (<−5 eV) π-conjugated polymers as bistable electrochromic films and an ionic liquid as the electrolyte. It provided a long bistability (>90 min) at the voltage-off state with a high color contrast (879 cm² C⁻¹).

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Broader context

Energy loss through windows is about 50% of the total energy consumption in buildings and this loss has increased significantly over the last decades. Smart windows based on electrochromic (EC) materials have emerged as energy saving technology because they change their transmittance according to the applied potential. For such a smart window, the optical memory in the voltage-off state (V-Off) is important for energy saving. Furthermore, the bistability should be imperative because no additional energy consumption is required to maintain the colored or transparent state. In this work, we demonstrate the first bistable smart window by controlling the interfacial charge transport using low-HOMO level π-conjugated polymers as EC films and an ionic liquid as the electrolyte layer. It provided a long bistability (>90 min) at V-Off with a high coloration efficiency (879 cm² C⁻¹).

The ability to control the light transmission through a window has captured intense interest in the field of energy saving windows to block out or manipulate dynamic daylight. Recently, the smart window has emerged as a transparent display to propel color and light transmission technology into the realm of next generation displays, architectures, and transportation. There are several methods to control light transmission including electrochemical mirrors¹ and electrochromic windows by using inorganic² and organic materials,³ and π-conjugated polymers (CPs).⁴ Regarding inorganic materials, electrochromic windows (ECWs) based on WO₃ are the most widely studied.^2,5 Particularly, CPs undergo reversible electrochemical optical changes with a fast response, and this process has garnered increasing interest because of their tunable electronic properties, low-voltage operation, and solution processability.⁶ Of the various CPs, the electrochromic (EC) color and transmittance changes of poly(3,4-propylenedioxythiophene) derivatives (PRs) can reach high color contrasts,^7,8 which should be particularly useful for smart windows.

Smart windows change their transmittance according to the applied potential, reducing the overall energy spending of a building by approximately 25%. It also improves quality of life by increasing comfort levels for people inside the building.⁹

For such an electrochemical switching window, the optical memory (OM) during the voltage-off state (V-Off) is important for energy saving windows or displays. Furthermore, the OM in both the colored and transparent states under V-Off, called bistability, should be compulsory because no additional energy consumption is required to maintain the colored (dark) or transparent state.

Unlike molecular EC dyes (e.g., viologens), which are typically dissolved in electrolyte media to generate an ECW, solid state CP films potentially limit the loss of OM caused by the diffusion of EC materials and dopant ions¹⁰ to produce rather long OMs.^6,11–15 However, the color contrast of ECWs in previous examples has been low (<60% at absorption maxima (λ_max)), and they require complicated processes. Although there have been many efforts, including ECWs with gel electrolytes^11,14,15 and EC molecules anchored with metal oxides or rotaxanes,^3,16 the bistability of ECWs has rarely been reported, and it is a major hurdle in smart windows and organic electronics, including displays. Moreover, the mechanisms to realize bistability in CPs are not well understood.

To achieve bistability in an ECW, it is important to control the spontaneous charge transfer that results in OM loss at V-Off. Fundamentally, the loss of OM is related to the charge transport between the interfaces in the ECW that consist of ITO/CP film/electrolyte/ITO. This spontaneous charge transfer (self-reaction) at V-Off can be described by eqn (1):

(Pol*) + (A⁻)_s ⇄ (Pol⁺·A⁻)_pol + e_ITO⁻ (1)

where (Pol*) represents the active centers in the neutral CP films, which absorb visible light, (A⁻)_s are the counter ions of the electrolyte that are injected/ejected to maintain the electro-neutrality inside the CP films, (Pol⁺·A⁻)_pol represents the transparent CP films with the counter ions, and e_ITO⁻ represents the electrons that are transported to the ITO.

The interfacial electron transfer (IET) between ITO and CP films is highly influenced by the energy levels of the CPs.¹⁷ Therefore, it can be hypothesized that the redox potential of the CP can be an exclusive controller of the IET. Furthermore, at the interface between the CP film and the electrolyte, the spontaneous interfacial dopant ion transport (IDT) should be blocked by the electrochemical double layer (EDL).¹

Therefore, we explored a bistable ECW by controlling the interfacial charge transport in a CP-based device. First, the effect of the redox potential on the OM was examined to control the IET, which can be unfavorable when the HOMO energy level (E_HOMO) of the CPs is lower than the Fermi level (E_F) of the electrode.¹⁸ Thus, a series of CPs with different E_HOMO were synthesized to characterize the IET. Our structural motif was based on PRs because these can be easily chemically modified to alter their redox properties via tailor-made molecular designs and to exhibit a high color contrast. Next, the IDT was controlled by using ionic liquids,¹⁹ which form ion blocking layers through the EDLs.¹ Herein, we report a bistable electrochromic window (BECW) based on a new high color contrast PR derivative that has a low E_HOMO.

The ∼250 nm-thick CP films were easily coated on ITO glass via solution casting polymerization (SCP) of monomers (Fig. 1a).²⁰ As shown in the cyclic voltammogram (CV) (Fig. 1b), the redox potential of the PR films was highly dependent on the monomer structure and exhibited higher oxidation peak potentials (E^p_ox) than the poly(3,4-ethylenedioxythiophene)s (PEDOTs) in a propylene carbonate solution containing 0.1 M tetrabutylammonium perchlorate (TBAP) when using Ag/AgCl as a reference. This high E^p_ox for the PR films can be attributed to the large alkylenedioxy group, which decreases the planarity of the polymer main chain.^21,22 The CPs with electron withdrawing (Cl, Br) and bulky side groups (EHO) exhibit a higher E^p_ox because of the electronic^23,24 and steric hindrance effects,^25,26 respectively. The effects of the substituents on the redox potential of the CPs in an ionic liquid medium that contained 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BIL) were nearly identical to those in the TBAP, except for PR-EHO (Fig. S1 and Table S1, ESI†). The E_HOMO of the CP film was estimated from the CV and was confirmed by UPS (Table 1 and Fig. S5, ESI†).

Fig. 1 (a) Synthetic routes for the monomers and polymers. (b) Cyclic voltammograms of the CPs coated on an ITO in TBAP as the electrolyte at a scan rate of 50 mV s⁻¹ (vs. Ag/AgCl) for PEDOT (red), PE-Cl (green), PR-Me (blue), PR-MeO (cyan), PR-Br (magenta), and PR-EHO (orange).

Table 1 Electrochemical properties of CPs

CPs	E ^pred [V]	E ^pox [V]	E 1/2 [V]	E HOMO [eV]	E LUMO [eV]	E g
a E ^Pred and E^Pox are based on the bigaussian multi-peak deconvolution of the cyclic voltammogram in TBAP, which was used as the electrolyte, vs. Ag/AgCl. b Half-wave potential. c The EHOMO was calculated by using E^Pox and estimated with the empirical relation EHOMO = −4.8e − e[E^Pox − E1/2Ferrocene]. d E LUMO of the CPs is determined from the EHOMO and Eg of the CPs. e E g of the CPs, as determined from the onset of the π–π* transition of the neutral-state CPs.
PEDOT	−0.52 (−0.6^31)	0.28 (0.01^31)	−0.12	−4.65	−2.98	1.67 (1.6–1.7^10)
PE-Cl	−0.42	0.3	−0.06	−4.67	−3.02	1.65
PR-Me	−0.18 (−0.3^31)	0.4 (0.03^31)	0.11	−4.77	−2.9	1.87
PR-MeO	0.16	0.59	0.38	−4.96	−3.14	1.82
PR-Br	0.14	0.65	0.40	−5.02	−3.19	1.83
PR-EHO	0.17 (0.08^26)	0.78 (0.25^26)	0.48	−5.15	−3.21	1.94 (1.97^10)

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The structure of the ECW consisted of an ITO/CP film/electrolyte/ITO. To test the optical stability, the transmittance at λ_max (%T_λmax) of the ECW was monitored for 90 min at V-Off in both TBAP (Fig. 2a) and BIL (Fig. 3a). Interestingly, the OM of the colored states for the PR-Br and PR-EHO were superior to those of the other polymers, and the transmittance change at λ_max under V-Off (Δ%T_off) in the colored state (self-bleaching, S_bl) was almost negligible (<2%) in both TBAP and BIL (Fig. S3, ESI†). However, with PEDOT, the Δ%T_off in the colored state was 15% and 20% after 30 min in TBAP and BIL, respectively, and showed the highest change among the CPs (Fig. 2b and 3b). The OM in the colored state did not depend much on the electrolyte; however, it was highly dependent on the E^p_ox of the CPs (Fig. 2c). When the electricity was shut off in the colored state, the spontaneous optical loss could be ascribed to the IET (Fig. 2d). This IET is possible when the E_HOMO of the CPs is higher than the E_F of the ITO electrode (−4.7 eV).²⁷

Fig. 2 (a) The transmittance change for S_bl and S_col at V-off of an ECW for the PEDOT (dotted red line) and PR-Br (solid black line) films in TBAP and monitored at λ_max. (b) Δ%T_off in the colored (S_bl, closed symbols) and bleached states (S_col, open symbols) for PEDOT (red star), PE-Cl (green diamond), PR-Me (blue triangle), PR-MeO (cyan square), PR-Br (magenta circle), and PR-EHO (orange pentagon). (c) A plot for Δ%T_off in the colored state (dotted red circle in (b)) as a function of the E^p_ox and E_HOMO of the CPs. (d) A schematic diagram of the interfacial electron transport in TBAP for S_bl.

Fig. 3 (a) The transmittance change for S_bl and S_col at V-off of the ECW for the PEDOT (dotted red line) and PR-Br (solid black line) films in BIL and monitored at λ_max. (b) Δ%T_off in the colored (S_bl, closed symbols) and bleached states (S_col, open symbols) for PEDOT (red star), PE-Cl (green diamond), PR-Me (blue triangle), PR-MeO (cyan square), PR-Br (magenta circle), and PR-EHO (orange pentagon). (c) A schematic diagram of the interfacial dopant ion transport (IDT) in BIL for S_col.

The Δ%T_off of the transparent state (self-coloring, S_col) was larger than that of the colored state and was dependent on the CPs in TBAP. The Δ%T_off of PEDOT and PE-Cl were ∼20% within 30 min, which was smaller than those of the other CPs (>40%) (Fig. 2b). However, the OM in BIL was dramatically enhanced (Fig. 3a and b), possibly because of the formation of EDLs. EDLs persist for a long time even after the applied voltage is off, preventing interfacial ion transfer.^1,28 Therefore, the IDT in BIL could be blocked (Fig. 3c).

The stability of the EDLs at V-Off is highly dependent on the size of the ions (Fig. S7 and S8, ESI†). The OM was shorter in BBF₄ that consisted of very small anions (BF₄⁻), which may cause an imbalance between the surface counter ions and the ion–ion steric repulsion.²⁹ Conversely, large anions could be paired compactly with cations. Specifically, BILs have been reported to form more densely packed multi-double layers on a charged surface.³⁰ Therefore, all of the CPs in BIL exhibited low Δ%T_off (<10%) even after 90 min (Fig. 3b, S_col), in contrast to that in TBAP.

The reproducible bistability was successful in an ECW with PR-Br, exhibiting a long OM at V-Off in the colored and bleached states (Fig. 4a and b). The cyclability of the ECW was longer than 500 cycles in air (Fig. S4, ESI†). A larger-area ECW (7 inch) was also fabricated and achieved a high color contrast and a long OM (Fig. 4c).

Fig. 4 (a) The bistability of the ECW with PR-Br in BIL under the IET and IDT controlled conditions upon alternative switching between step potentials (±2.8 V) at V-off. (b) Photographic images of the ECW with PR-Br in the colored and bleached states (±2.8 V) at V-off. (c) Photographic images of the large-area ECW (7 inch) with PR-Br at colored (top) and the transparent (bottom) states at V-off. (d) A plot of as a function of E_1/2 of the CPs for S_bl and (e) for S_col of the ECW using TBAP (red symbol) and BIL (black symbol). (f) A plot of k_app as a function of of the ECW using TBAP (red symbol) and BIL (black symbols) for PEDOT (half-filled square), PE-Cl (diamond), PR-Me (triangle), PR-MeO (square), PR-Br (circle), and PR-EHO (pentagon).

In the colored state, the electron transfer from the CP films to the electrode is spontaneous if the E_F of the electrode is below the E_HOMO of the CPs (high-E_HOMO) upon interface formation according to the integer charge-transfer (ICT) model.³² However, when the E_F of the electrode is higher than the E_HOMO of the CP film (low-E_HOMO), such an electron transfer is unfavourable in the colored state (Fig. S10, ESI†).³³ Therefore, the CPs with a higher E_HOMO compared with the E_F of the electrode (−4.7 eV) should undergo IET, whereas those with a lower E_HOMO stay intact in the colored state. Indeed, this analysis agrees well with the experimental result, confirming the long OM for the CPs that have a low E_HOMO or high E^p_ox (PR-Br and PR-EHO). Thus, the IET is an exclusive controller of the OM in the colored states (Fig. 2b and c, and Table S1, ESI†). The morphology of CP films is an important factor for the charge/discharge processes that are associated with the EC properties.^31,34 However, the average roughness (R_a) of the EC films did not correlate with the optical bistability (Fig. S9, ESI†). This result supports the idea that the OM in the colored state can be controlled by the IET.

The two different mechanisms, IET and IDT, can be supported by the kinetic overpotential (η_kin)³⁵ leading to self-reactions (S_bl and S_col) at V-Off as follows:

where E_F,p represents the E_F pinning position at the interface of ITO/CP films, and E_pol is the energy level of the polymer, as determined by the Nernst equation,³⁶ which describes the shift in the energy level as a function of the fraction of the oxidized polymer (f). Here, n represents the number of electrons transferred from the EC film to the ITO electrode. R is the universal gas constant, T is the absolute temperature, and F is the Faraday constant. Because the fraction of the oxidized polymer is related to the changes in optical density (ΔOD), the following equation was elucidated to analyze the loss of OM. The ΔOD is proportional to the total amount of injected/ejected charge during the self-reaction. Thus, the fraction of the oxidized polymer (f) generated by the given oxidation charge (Q_oxi) was derived from the transmittance (eqn (4)).where T_min and T_max are the minimum and maximum transmittance, respectively, when the CP films are fully switched. A is the area of the CP films. T_t is the transmittance at V-off at a certain time and Q_total is the total injected/ejected charge required to generate a fully reduced or oxidized state. CE is the coloration efficiency (cm² C⁻¹). Q_total and Q_oxi can be represented as eqn (5). Eqn (2) can be rearranged to eqn (6) by using eqn (4) and (5). When the transmittance change reaches an equilibrium state, eqn (6) can be represented by eqn (7). The interfacial electron transfer continues until it reaches an equilibrium between E_F,p and E_pol. When Δ%T_off reaches an equilibrium state, is directly related to E_1/2 during the self-reaction (eqn (7)). This showed roughly a linear relationship between and E_1/2 for S_bl (Fig. 4d), indicating that Δ%T_off becomes smaller when E^p_ox becomes larger (Fig. S12, ESI†). Thus, the OM in the colored state should be long with CPs having a low E_HOMO. However, there is no linear relationship for S_col (Fig. 4e), indicating that the S_col process was not fully controlled by the IET. S_col should be energetically favorable for all polymers because of the small localized band gap energy.³⁷ Under these conditions, any small driving force, such as the blocking of ion diffusion or transfer, will distract S_col.

Because the doped-state CPs can be considered to be positively charged CP films that are in contact with the dopant anions, the dopant ion concentration difference between the CP films and the electrolyte interface can generate a diffusion potential (η_diff). The diffusion potential, which is due to the anion concentration difference, can be obtained from the following eqn (8):

where E_S represents the energy level of the electrolyte and (a_A⁻)_pol and (a_A⁻)_S represent the activities of the anions in the CP films and electrolytes, respectively. For a diffusion controlled electrochemical reaction, the diffusion current (i_d) is described by the Cottrell equation (eqn (9)):where F is the Faraday constant, A is the reacted area of the CPs, D₀ is the diffusion coefficient, t is the time, and [A⁻]_s is the counter ion of the electrolyte that is injected/ejected to maintain the electro-neutrality inside the CP films. [Pol⁺·A⁻] represents the bleached state that has a counter ion for balancing. The consumed charge (Q_d) could be derived from i_d(t) (eqn (10)): The relationship between the transmittance change and the diffusion coefficient can be derived as follows. From the CE (eqn (5)), the fraction of the electrochemically changed polymer (f) can be represented by eqn (11): The apparent rate constant (k_app) was determined from the slope of fitting f and the square root of time with eqn (12) (Table S4, ESI†). The f and square root of time fitting for S_col were determined from Fig. S3 (ESI†) data within 500 s (Fig. S13, ESI†).

Fig. 4f clearly displays that the self-coloring process is diffusion controlled in TBAP. k_app for S_col is proportional to the diffusion term in TBAP. However, all of the k_app in BIL were very low and almost invariant with the values for different polymers, regardless of their large variations of E_HOMO. This low k_app for S_col could be attributed to the EDL. Therefore, the OM in the bleached state was significantly enhanced in BIL.

Conclusions

In summary, low E_HOMO level CP films afforded an energy saving bistable ECW by controlling the interfacial charge transport. The mechanism for self-bleaching was ascribed to the electron transfer between ITO and the CPs (IET), which was supported by correlating Δ%T_off with E^p_ox of the polymers at V-off. The self-coloring process in the ECWs was explained by the ion transfer from the CPs to the electrolytes (IDT), which was blocked by the EDL, providing a long bistability for the CPs with low E_HOMO. These two different mechanisms for the OM in self-bleaching (IET) and self-coloring (IDT) were supported by the energy level and diffusion term, respectively. An ECW from the new CPs (PR-Br) maintained the OM for 90 min at V-off with a large Δ%T_λmax (>70%). This allowed the first preparation of a high contrast, fast-switching, and energy-saving ECW.

Acknowledgements

We acknowledge the financial support of the National Research Foundation (NRF) grant funded by the Korean government (Ministry of Science, ICT & Future Planning, MSIP) through the Active Polymer Center for Pattern Integration (APCPI) (2007-0056091). This work was also supported by the Next-generation Converged Energy Material Research Center (CEMRC) of the Agency for Defense Development (ADD).

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Footnote

† Electronic supplementary information (ESI) available: Description of materials and methods, Tables S1–S4 and Fig. S1–S13. See DOI: 10.1039/c5ee03160e

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