Optical Properties

Figure 4.6 displays the absorption spectra of PBT, PBT/F, and F, measured from dichlorobenzene solutions (PBT and F) and solid films (PBT, PBT/F, and F), and their photophysical data are summarized in Table 4.1. The absorption maximum of PBT solution (in dichlorobenzene) was found to be 505 nm which can be attributed to intramolecular charge transfer between the electron donor (DTP) and acceptor (bithiazole) units.^52-54 Relative to the solution absorption, the absorption maximum is located at 520 nm in the solid film which is slightly red-shifted, indicating intermolecular interactions and aggregations occurred in solid films.^64,109 The absorption maximum of π-conjugated cross-linker F was found to be 369 nm (in the dichlorobenzene solution) and found to be 409 nm (in the solid film), respectively, where the red-shift of 40 nm in the solid film is due to the inter-chain association and π stacking of π-conjugated cross-linker F.¹¹⁰ Compared with PBT (in solid films), a new peak appeared at 385 nm in the UV region of H-bonded polymer network PBT/F by the introduction of F, and a

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shifted (ca. 28 nm, from 520 to 548 nm) absorption peak occurred due to the multiple H-bonded effect.^108b The optical band-gaps (Egopt

) of PBT and PBT/F were found from the cut-offs of the absorption wavelengths to be 1.80 and 1.77 eV, respectively. This result implies that it is an efficient way to tune optical properties of the side-chain polymer through supramolecular intractions to absorb maximum photons from visible regions for enhanced photovoltaic applications. Moreover, the homogeneous incorporation of the organic dye (a small molecule) to the polymer with a complimentary absorption band through multi-H-bonds so as to broaden the total light absorption (as shown in Figure 4.6) and thus to induce enhanced photovoltaic properties, which can be further proven by PCE values and EQE measurements in later discussion.

4.3.3 Electrochemical Properties

Cyclic voltammetry (CV) measurements were employed to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of polymers PBT and PBT/F and their CV curves are provided in Figure 4.7. The CV measurements were carried out in a 0.1M tetrabutylammonium hexafluorophosphate (TBAPF6) solution (in acetonitrile) at a scan rate of 100 mV/s under nitrogen. A carbon electrode, which was coated with the polymer film by dip coating, was used as a working electrode and Ag/AgCl was served as a reference electrode, and it was calibrated by ferrocene (E^1/2ferrocene = 0.45 mV versus Ag/AgCl). The HOMO and LUMO energy levels were estimated by the oxidation and reduction potentials from the reference energy level of ferrocene (4.8 eV below the vacuum level) according to the following equation¹¹¹ : EHOMO/ELUMO = [-(Eonset –Eonset(FC/FC+

vs. Ag/Ag+)) - 4.8] eV and band gap = E_onset^ox – E_onset^red (where 4.8 eV is the energy level of ferrocene below the

105 vacuum level and Eonset(FC/FC+

vs. Ag/Ag+) = 0.45 eV). The HOMO and LUMO levels as well as the electrochemical band-gap (Egec

) were determined from oxidation and reduction potentials (Eonsetox

and Eonsetred

) of both polymers and their values are summarized in Table 4.1. It can be seen that polymer PBT, cross-linker F, and H-bonded polymer network PBT/F possess both quasi-irreversible p-doping/dedoping (oxidation/rereduction) processes at positive potentials and n-doping/dedoping (reduction/ reoxidation) processes at negative potentials.

Figure 4.7 Cyclic voltammograms of polymer PBT and supramolecular polymer network PBT/F and cross-linker F.

The onset potentials of (oxidation; reduction) of polymer PBT, cross-linker F, and those of H-bonded polymer network PBT/F, were found to be (0.77; -0.84) V, and (1.49; -1.11), and (0.89; -0.91) V, respectively. The estimated values of (HOMO; LUMO) levels for polymer PBT,

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cross-linker F, and those of H-bonded polymer network PBT/F were found to be (-5.12; -3.51) eV, (-5.84) and (-3.84), and (-5.24; -3.44) eV, respectively. Compared with polymer PBT, the lower HOMO level of H-bonded polymer network PBT/F (induced by the H-bonded structure) indicates that it could be more stable against oxidation due to the incorporation of highly oxidative stable fluorene in cross-linker F.¹¹² Compared with polymer PBT, the lower HOMO energy level of H-bonded polymer network PBT/F (as a donor material) is desirable for the high open circuit voltage of PSC.¹¹³ The LUMO energy levels of the electron donors (PBT and PBT/F) have to be positioned above the LUMO energy levels of the acceptors (e.g., PCBM) at least 0.3 eV, so the exciton binding energy of polymer could be overcome and result in efficient electron transfer from donors to acceptors.

Table 4.2 Photovoltaic Propertiesa and Film Roughnessesb (Rrms Measured by AFM) of Bulk-Heterojunction PSC Devices Containing PBT and PBT/F with PC61BM and PC71BM in a Blending Wt. Ratio of 1:1.

107 4.3.4 Photovoltaic Properties

PBT and PBT/F were utilized as electron donors to investigate the photovoltaic properties of BHJ photovoltaic devices with a device structure of ITO/PEDOT-PSS/polymers:acceptors/Ca/Al, where PC61BM (or PC71BM) was an electron acceptors. Table 4.2 and Figure 4.8 demonstrate current density-voltage (J-V) curves of PBT and PBT/F blended with PC61BM (or PC71BM) with a wt. ratio of 1:1. (The photovoltaic devices with thicknesses ca.

90 nm were annealed at 70°C and tested under 100mWcm^-2 AM 1.5G solar illumination condition.) It can be clearly seen that H-bonded polymer network PBT/F possessed higher PCE values than polymer PBT. However, Li et al. have reported an analogous D-A copolymer containing DTP and bithiazole moieties only possessed V_oc = 0.28 V, J_sc = 0.51 mA/cm², and a maximum PCE value of 0.06%.⁶⁴ In addition, recently we reported a similar polymer with a PCE of 0.27%.¹¹⁴ The most efficient solar cell device exhibited a PCE of 0.86% with Voc, Jsc, and FF (fill factor) values 0.55 V, 4.97 mA/cm², and 31.5%, respectively. Compared with PBT, the higher PCE value of 0.86% in H-bonded polymer network PBT/F was improved by larger Voc

and Jsc values were due to a lower value of HOMO energy level (to induce a larger Voc value), a higher light harvesting amount (by introducing F to induce a larger J_sc value) in visible regions, and an improved crystallinity (by introducing F to have a higher crystallinity). For effective charge carrier transport, hole-mobility is a key parameter and also equally affects FF values of the solar cell devices.¹⁰⁵ We employed hole-only devices to characterize the hole mobilities of PBT and PBT/F films blended with PC61BM and found to be 1.72×10^-7 and 2.32×10^-6 cm²/Vs, respectively, and thus the low hole-molbilities of both polymers induced low FF values (< 40%, see Table 2).¹⁰⁵

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Figure 4.8 (a) J−V and (b) EQE characteristic curves of polymer PBT and H-bonded polymer network PBT/F blended with PC61BM (or PC71BM) in a wt. ratio of 1:1.

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In order to understand the origins of low current densities, we carried out external quantum efficiency (EQE) measurements for solar cell devices by mixing either PC61BM or PC₇₁BM with polymers PBT and PBT/F (1:1 by wt.), respectively (Figure 4.8). The results illustrate the solar cell devices containing H-bonded polymer network PBT/F are able to have higher absorptions of visible lights, which demonstrates enhancements of EQE values with a maximum of ~20% (ca. 350-550 nm) after incorporation of F with PBT through supramolecuar interactions. Hence, the lower current densities of solar cell devices containing PBT (without F) might be induced by their smaller EQE values and lower hole-mobilities.¹⁰⁴

Figure 4.9 AFM images of blended polymers (a) PBT and (b) PBT/F mixed with PC61BM, respectively; (c) PBT and (d) PBT/F mixed with PC71BM, respectively (in a wt. ratio of 1:1 by spin-coating from dichlorobenzene and annealing at 70°C for 30 min.)

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Surface morphologies of the active layer in solar cell devices are also a key parameter for the device performance.¹¹⁵ AFM images of of blended polymers PBT and PBT/F mixed with either PC61BM or PC71BM (1:1 by wt. and annealed at 70°C for 30 min.) are presented in Figure 4.9 and their root-mean-square roughness (Rrms) values are demonstrated in Table 4.2. The active layers of PBT and PBT/F showed good film formation to have smooth surfaces with no obvious aggregations (only nano-scale phase separations) between polymers and PC61BM (or PC71BM).¹¹³ Mixed with PC61BM (or PC71BM) in a wt. ratio of 1:1, PBT (without F) showed larger Rrms values than H-bonded polymer network PBT/F in the blended polymers. This larger R_rms value resulted in the decreased diffusional escape probabilities for mobile charge carriers, and hence increased their recombinations.¹¹⁵ It is evident from AFM images that lesser aggregation between electron-donor polymer and electron-acceptor PCBM molecules and higher π-π stacking allows for better photogenerated charges and inducing higher J_sc values via supramolecular engineering of solar cell devices.

4.4 Conclusion

In conclusion, we could tune molecular energy levels, morphologies, and device proferemances by a new and straight-forward approach to introducing multiple H-bonded supramolecular structures. The broader light absorption (an extra blue absorption from H-bonded crosslinker F and the red-shifted absorption from H-bonded main-chain polymer PBT), lower HOMO level (to have a higher Voc value), higher hole mobility, larger crystallinity, and better morphorlogy in H-bonded polymer network PBT/F induces better photovoltaic properties than that containing polymer PBT. The preliminary photovoltaic performance showed the solar cell device containing 1:1 wt. ratio of PBT/F and [6,6]-phenyl C₇₁ butyric acid methyl ester

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(PC₇₁BM) offers the best power conversion efficiency (PCE) value of 0.86% with a short-circuit current density (Jsc) of 4.97 mA/cm², an open circuit voltage (Voc) of 0.55 V, and a fill factor (FF) of 31.5%. The highly directional multiple H-bonded strategy between melamine and uracil motifs significantly increased self-assembled behavior as well as π-π stackings, which is an encouraging method for the future researches to adjust the photovoltaic properties of polymer solar cell devices.

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Chapter 5

Enhancement of Photovoltaic Properties in Supramolecular Polymer