From crab shell to solar cell: a gel polymer electrolyte based on N-phthaloylchitosan and its application in dye-sensitized solar cells

Abstract

Chitosan, a biopolymer derived from crab shells which is insoluble in common organic solvents has been converted to the organosoluble N-phthaloylchitosan (PhCh) by reaction with phthalic anhydride in dimethylformamide (DMF). The formation and structure of PhCh was confirmed by the characteristic peaks of phthalimido and aromatic groups observed at 719, 1708 and 1772 cm⁻¹ and two sets of peaks centered at 3.0 and 7.5 ppm obtained from FTIR and ¹H NMR analyses respectively. Gel polymer electrolytes consisting of PhCh, ethylene carbonate (EC), and DMF with various amounts of tetrapropylammonium iodide (TPAI) and iodine were prepared. The interaction behavior between polymer–plasticizer–salt was thoroughly investigated using FTIR spectroscopy. The gel polymer electrolyte consisting of PhCh : EC : DMF : TPAI : I₂ in a weight ratio (g) of 0.1 : 0.3 : 0.3 : 0.12 : 0.012 showed the highest conductivity of 5.46 × 10⁻³ S cm⁻¹ at room temperature and exhibited the best performance in DSSCs with efficiency of 5.0%, with J_SC of 12.72 mA cm⁻², V_OC of 0.60 V and fill factor of 0.66.

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1. Introduction

Chitosan, a derivative of N-deacetylated chitin is the second most abundant natural polymer. It is produced from the deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimp) through alkaline hydrolysis treatment.¹ It has been the subject of much research as it offers many advantages such as its non-toxicity, odorlessness, biorenewability, biofunctionality, biodegradability and biocompatibility in animal tissues.² These interesting properties make it possible to be applied in various research areas such as cosmetic,³ lubricant,⁴ biomedical,^5–7 environmental,⁸ agricultural,⁹ and food industries.¹⁰ Chitosan has also found its use in the development of high conductivity polymeric systems because it demonstrates polyelectrolyte behavior due to the protonated amino group in the polymeric backbone.^11–13

However, chitosan has its limitation as it is only soluble in dilute aqueous acidic solutions and insoluble in organic solvents due to the β-1,4′-glycosidic linkages that give chitosan its structural rigidity and crystalline structure besides promoting the formation of intra-molecular hydrogen bonds.^14,15 This can be overcome by modifying the chitosan structure through the two functional groups, –NH₂ and –OH, present in its backbone.^15–18 One of the modifications is by reacting with phthalic anhydride in order to produce phthaloylchitosan (PhCh).^19,20 The product is soluble in dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO) and pyridine due to the presence of hydrophobic phthaloyl group in chitosan that disrupts the formation of specific hydrogen bond from interacting between its amino and hydroxyl groups with the solvents. Thus PhCh can be a suitable candidate for use as a polymer host for preparing polymer electrolytes or gel electrolytes.

Gel polymer electrolytes (GPEs) have been an ongoing research interest due to its ability to overcome the limitations inherent in liquid electrolytes. Commonly, synthetic polymers such as polyacrylonitrile (PAN),^21–23 polyvinyl alcohol (PVA),^24,25 polyvinylidene fluoride (PVdF),²⁶ polymethyl methacrylate (PMMA)²⁷ have been employed as the host polymer matrix to trap the electrolytic solution containing the ionic salt in order to form GPEs for application in dye sensitized solar cells (DSSCs). However, host polymer matrices consisting of biopolymers such as agarose,^28,29 starch,³⁰ cellulose derivatives,^31–34 carrageenan derivatives,^35,36 and chitosan derivatives^37–40 have also gained traction. Bio-sourced polymers such as these are highly important in the present DSSC community because when compared against conventionally high performing silicone solar cells, their lower level of performance is offset by the energy generated and the profitable invested capital returns in the long run due to its bioavailability, minimal environmental impact,⁴¹ and having a low cost of fabrication due to not requiring highly sensitive conditions typical of conventional solar cells.^42,43

The present study is conducted to improve the efficiency of PhCh-GPE based DSSCs by using DMF as one of the plasticizers and a quaternary ammonium iodide salt. DMF, being a polar aprotic solvent with a dielectric constant of 36.7, can dissolve ionic compounds which are insoluble in most non-polar solvents. It can greatly enhance the nucleophilicity of the anions since it has a more exposed surface moiety with a partial negative charge that can solvate cations whereas its less accessible partial positive charge is on the inside.^44,45 Bulky cations such as tetraalkylammonium generally enhance the iodide anion conductivity in the electrolyte through better ionic dissociation.²⁷ Therefore, it would be natural to explore the combined effect of having DMF and a bulky quaternary ammonium iodide salt in the PhCh-GPE system since it is expected to reduce cationic conductivity while enhancing the iodide anionic conductivity in the electrolyte which is part of the criteria for DSSCs with good performance properties.^22,26

2. Materials and methods

2.1. Phthaloylation of chitosan

All chemicals in this work were procured from Sigma Aldrich and used as received without further purification. Chitosan from crab shells (product no. 48165, viscosity >400 mPa s, 1% in acetic acid at 20 °C) and phthalic anhydride were refluxed at 110 ± 10 °C under nitrogen atmosphere for 6 h in dimethylformamide (DMF). The reaction was allowed to proceed for another 20 h at a reduced temperature of 60 °C after which the mixture became a clear yellow viscous solution. The precipitate obtained by pouring the solution into ice-water was collected by filtration, and then further purified by Soxhlet extraction for 8 h using ethanol. The product was dried in vacuum at 60 °C.

2.2. Preparation of the gel polymer electrolytes

0.3 g of EC and 0.3 g of DMF were stirred well with predetermined amounts (Table 1) of tetrapropylammonium iodide (TPAI). Subsequently, 0.1 g of PhCh was added into each salt solution. The mixture was heated to 60 °C and stirred until it becomes a homogeneous gel. After cooling to room temperature, iodine (10 wt% of TPAI) was added to the obtained polymer mixture and stirred until a homogeneous gel polymer electrolyte was obtained (Fig. 1).

Table 1 Ionic conductivity and activation energy of gel polymer electrolyte samples with different masses of TPAI

Sample	TPAI (g)	Conductivity, σ (×10^−3 S cm^−1)	Activation energy, Ea (eV)
A0	0.00	0.75	0.137
A1	0.02	2.16	0.122
A2	0.04	3.12	0.119
A3	0.06	3.93	0.115
A4	0.08	3.94	0.128
A5	0.10	5.41	0.132
A6	0.12	5.46	0.107
A7	0.14	5.03	0.121

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Fig. 1 Photograph of PhCh based gel polymer electrolytes.

2.3. Characterizations of the gel polymer electrolytes

Proton nuclear magnetic resonance (¹H NMR) was taken with a JNM-GSX270 spectrometer (JEOL, Japan). DMSO-d₆ was used as the solvent with a sample concentration of 20% w/v.

Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectra were recorded with a Spotlight 400 spectrometer (PerkinElmer, UK). The acquisition parameters were done with a total of 32 accumulations at 4 cm⁻¹ resolution with a spectral range from 650–4000 cm⁻¹. The deconvolution analysis with multiple peak fitting was done using Origin Pro 9.1 software with second-order derivatives to determine the hidden peaks. A Gaussian model was used in which maximum error associated with the simulated fits was within ±1%.

The XRD analysis was recorded at room temperature using an Empyrean diffractometer (PANalytical, Netherlands) at 2θ angles between 10° and 60° with a step size of 0.026°, using Cu/Kα₁ irradiation.

A 3531 Z Hi tester (HIOKI, Japan) was used to measure the impedance of the polymer gels in the frequency range from 50 Hz to 1 MHz. The impedance data were processed in a complex impedance plot where the imaginary part, Z_i was plotted against its real part Z_r. The ionic conductivity of the samples was calculated by using the following equation:

where d is the thickness of the sample, R_B is bulk impedance taken at the intersection of the plot with the real impedance axis and A is the electrode/electrolyte contact area.

2.4. Solar cell fabrication and characterization

The photo-anode with two layers of TiO₂ was made on a conducting glass substrate using the procedure reported by Bandara et al.²¹ Fluorine Tin Oxide (FTO) glasses were rinsed with distilled water and ethanol before use. The paste required for the first dense layer was prepared by grinding 0.5 g of P90 TiO₂ powder for ∼30 min with 2 mL of pH 1 nitric acid in a mortar. The resulting slurry was spin-coated on the FTO glass with a multi-speed program: first at 1000 rpm for 2 s and then at 2350 rpm for 60 s. After air drying for 30 min, it was sintered at 450 °C for 30 min. For the second layer, the TiO₂ colloidal suspension was prepared by grinding 0.5 g of P25 TiO₂ powder with 2 mL of pH 1 HNO₃, 0.1 g of carbo wax and a few drops of Triton X-100 in the agate mortar. The TiO₂ colloidal suspension was spread on the first dense layer of TiO₂ on the FTO glass using the doctor blade technique. The TiO₂ electrode was sintered in the furnace at 450 °C for 45 min. The electrode was allowed to cool down to about 60 °C and dipped in a warm ethanol solution containing Ruthenium N3 dye for 24 h. The DSSCs were fabricated by sandwiching the gel polymer electrolyte between the dye coated photo-anode and Pt coated FTO glass.

The performance of the DSSCs was investigated using a 2400 electrometer (Keithley, USA) under light irradiation of 1000 W m⁻² at room temperature. From the J–V (current density–cell potential) characteristic curves, the short circuit current (J_SC), open circuit voltage (V_OC), fill factor (FF) and energy conversion efficiency (η) were calculated from the following equations:

where J_max is the current density at maximum power output, V_max is the voltage at maximum power output, and P_in is the power of the incident light.

3. Results and discussion

3.1. Phthaloylation of chitosan

Scheme 1 shows the reaction for the N-phthaloylation of chitosan. Purity of the product was validated by ¹H NMR (Fig. S1, ESI†), δ (ppm, DMSO, 400 MHz): 7.26–8.14 (m, 4H, aromatic-H), 2.00–5.00 (m, 9H, backbone aliphatic-H), in agreement with previous literature.^19,46 Confirmation of the phthaloylated chitosan structure was also made using FTIR (Fig. S2, ESI†). The NH₂ band appearing at around 1601 cm⁻¹ in the pure chitosan spectrum has disappeared in the PhCh spectrum indicating that substitution has occurred onto the N atom of chitosan. PhCh exhibits new characteristic peaks corresponding to the carbonyl amide at 1708 and 1772 cm⁻¹. There is also an absorption peak at 719 cm⁻¹ indicating the presence of the aromatic ring. These peaks are also in agreement with previous literature.^46–49

Scheme 1 N-Phthaloylation reaction.

Comparison of XRD patterns for chitosan and PhCh can be observed in Fig. 2. Chitosan exhibits two characteristic peaks at 2θ = 15.1° and 20.6°. After being phthaloylated, the peaks merged into one broad peak at 2θ = 21.7° indicating that the end product is less crystalline compared to the raw chitosan. This is because the bulkiness of the phthalimido groups results in the reduction of inter- and intramolecular hydrogen bonds of the chitosan, thus disrupting its crystallinity.⁴⁶.

Fig. 2 XRD patterns for (A) chitosan and (B) N-phthaloylchitosan.

3.2. Ionic conductivity

The GPE without TPAI showed an ionic conductivity of 0.75 × 10⁻³ S cm⁻¹ (Table 1). The ionic conductivity increases with the increase in mass of TPAI and reaches the maximum value of value of 5.46 × 10⁻³ S cm⁻¹ when the mass is 0.12 g (Fig. S3, ESI†). This behavior of the conductivity can be attributed to the change in the number of mobile ions in the gel electrolytes.⁵⁰ The number of mobile ions increases due to the increased salt dissociation with the increasing amount of salt in the electrolyte and results in the conductivity increase. Since TPA⁺ ions are very much larger than I⁻ ions, the conductivity can be assumed to be predominantly due to movement of I⁻ ions as the bulky TPA⁺ ions are expected to be entangled in the polymer matrix. However, at higher TPAI content, the conductivity is observed to decrease to the value of 5.07 × 10⁻³ S cm⁻¹. This is due to the formation of ion pair and/or ion aggregates that do not assist the ionic conductivity and also hinder the mobility of the ions.³⁹

The temperature dependence of the ionic conductivity of the GPEs in the temperature range of 30 °C to 100 °C is shown in Fig. 3 (and Table S1, ESI†). The Arrhenius model was employed based on the following equation:

where σ_o is the pre-exponential factor, E_a is the activation energy of ionic conduction, k is the Boltzmann constant and T is temperature in kelvin. The experimental results showed a good fit of R² > 0.95 for each sample in the series. This indicates that ionic conductivity of the GPEs obeys Arrhenius law and suggests that the conductivity is thermally activated.

Fig. 3 Conductivity–temperature dependence of the PhCh–EC–DMF–TPAI gel polymer electrolytes.

Although the activation energy value for the optimum GPE (A6) was the lowest, the difference in its value when compared with the rest of the samples in the series can be considered to be negligible. Similar observations and explanations have been reported on PVA and PAN systems by other researchers.^22,25 In the case of a solid electrolyte, the ion transport would consist of thermally activated hopping from an occupied site to a vacant site, with an energy barrier equal to the E_a which is dependent on salt concentration. However, in the case of organic liquid electrolytes, it was suggested that the E_a is largely independent of the salt concentration for low and moderate concentrations or the type of salt used but instead depends only on the type of solvent heteroatom which influences the dipole re-orientation.^21,26 Thus, the GPE follows a more liquid-like behavior because it consists of an environment where the ionic salt is dissolved in an organic polar co-solvent EC/DMF entrapped in the PhCh matrix. This is the probable reason for the similar E_a seen in Table 1.

3.3. FTIR analysis

In this work, a large portion of the GPE consists of the plasticizers EC and DMF. Therefore, detailed analyses on these two materials are necessary. Fig. 4 shows the FTIR spectra of the pure EC and DMF. In the EC spectrum, the strong doublet peaks at 1770 and 1796 cm⁻¹ are assigned to the C=O stretching. However, the peak present at 1796 cm⁻¹ can also overlap the overtone from the peak at 889 cm⁻¹ which is due to the ring breathing mode.⁵¹ Other peaks at 1067, 1155, 1390 and 1481 cm⁻¹ are attributed to the scissoring and wagging vibrations of CH₂ and stretching vibration of C–O. In the DMF spectrum, the strong peak at 1656 cm⁻¹ is due to the C=O stretching.⁵² Other intense peaks seen at 657, 1089 and 1385 cm⁻¹ are attributed to O–C–N, CH₃ rocking mode and N–C–H bending modes, respectively. The peaks at 865 and 1256 cm⁻¹ are assigned to the C–N symmetric and asymmetric stretching mode, respectively. Other minor peaks also appear at 1501 and 2928 cm⁻¹ due to N–H bending mode and CH₃ symmetric stretching mode, respectively. According to Zhang et al.,⁵³ although no strong H-bonding donor is present in DMF, the H atom coordinated to C=O can theoretically be considered as one possible donor as the C=O is expected to be the main acceptors. However, since EC and DMF are aprotic solvents which are not hydrogen bond donors and have no positively charged hydrogens to form ion–dipole interaction,⁴⁵ only weak interactions are present between both plasticizers where the autoprotolysis is extremely weak.⁵⁴

Fig. 4 FTIR spectra of (A) ethylene carbonate and (B) dimethylformamide.

The FTIR spectrum typical of the PhCh–EC–DMF–TPAI based GPE is shown in Fig. 5. The interaction behavior of the components can be observed through the identification of the changes in the FTIR spectral features for intensity, bandwidth and position.⁵⁵ FTIR has been used to determine the changes due to the cation–polymer binding sites, crystalline–amorphous domain ratios and extent of H-bonding as the polymer chain consists of heteroatoms that undergo ion complexation through Lewis acid–base interactions forming the basis of an electrolyte medium. The method of FTIR deconvolution of main peaks has proven itself to be useful as it can detect the hidden peaks of certain ionic species and polymer segments that contribute to the relationships of all the components in the electrolytes.⁵⁶ In this study, four possible coordination environments were identified: ether (1000–1200 cm⁻¹), amide (1580–1700 cm⁻¹), carbonyl (1700–1840 cm⁻¹), and amine/hydroxyl groups (3130–3700 cm⁻¹) as shown in Fig. 6.

Fig. 5 FTIR spectra typical of PhCh–EC–DMF–TPAI based GPE.

Fig. 6 Deconvolution of individual FTIR regions: (A) ether (1000–1200 cm⁻¹); (B) amide (1580–1700 cm⁻¹); (C) carbonyl (1700–1840 cm⁻¹); and (D) amine/hydroxyl groups (3130–3700 cm⁻¹).

3.3.1. FTIR spectra of ether group in the range from 1000 to 1200 cm⁻¹. Two strong peaks at ∼1069 and ∼1160 cm⁻¹ can be observed in the ether (C–O–C) region (Fig. 6(A)). The histogram in Fig. 7 shows the percentage area for each contributing peak from the deconvolution. The changes in intensity might be attributed to the interaction of the C–O–C group of PhCh and/or EC with the salts. The peak at 1047 cm⁻¹ is attributed to the H-bonded C–O–C stretching arising from PhCh. The CH₃ rocking mode from the combination of EC and DMF is dominant at 1069 cm⁻¹. The coordination of TPA⁺ cations with C–O–C sites is assigned to the peak at 1079 cm⁻¹ since this peak was not present for the GPE without TPAI content. The peak at 1094 cm⁻¹ is due to the presence of crystalline complexes whereas the peak at 1143 cm⁻¹ is assigned to the stretching mode of free C–O–C which most likely occurs in the amorphous region.⁵⁶ At 1156 cm⁻¹ the peak intensities was higher for the GPE with TPAI content whereas the opposite was true at 1171 cm⁻¹. However, there was negligible change in the intensities of these peaks for GPE with TPAI content which suggests that the complexation of polymer/plasticizer with TPAI in these regions is independent of the concentration.

Fig. 7 Relative FTIR band percentage area in the range of 1000 to 1200 cm⁻¹.

3.3.2. FTIR spectra of amide/carbonyl group in the range from 1580 to 1840 cm⁻¹. The C=O absorption is highly sensitive to the interaction behavior such as hydrogen bonding. The first type of C=O absorption is the free uncoordinated C=O and the second type is the bonded C=O where the electron rich group interacts with the other proton donating groups such as O–H, and N–H. The hydrogen bonded C=O normally appears at a relatively lower wavenumber position from that of the free C=O since the interaction will reduce the electron density, hence the vibration energy will decrease, resulting in the peak position to shift. Other than H⁺, similar observations can be found such as the case for absorption formation of Li⁺ bonded to C=O.⁵⁵

The C=O region has been divided into two parts: (1) 1580 to 1700 cm⁻¹ and (2) 1700 to 1840 cm⁻¹. The first part corresponds to the deconvolution specific to the amide, R(C=O)NR₂, region from 1580 to 1700 cm⁻¹ (Fig. 6(B)). The percentage area of each of the peaks is presented in Fig. 8. Intensity of the peak at 1637 cm⁻¹ takes a plunge after salt was added but generally increases with varying salt content. However, the peak at 1649 cm⁻¹ decreases with additional salt. These peaks are probably due to the ordered or disordered interfacial domains in the polymer structure.⁵⁶ A similar trend can be observed for these two peaks with electrolytes containing single or double salts. The most intense peak centered at 1660 cm⁻¹ is attributed to the carboxamide group present in the chitosan. The peak at 1674 cm⁻¹ is assigned to the amide peak belonging to DMF. These peaks generally increase with addition of salt content and even more so for the peak at 1674 cm⁻¹ due to better mobility of DMF which provides better access to coordination sites of the carbonyl region.

Fig. 8 Relative FTIR band percentage area in the range of 1580 to 1700 cm⁻¹.

The region between 1700 and 1840 cm⁻¹ is due to other carbonyl groups of the GPE (Fig. 6(C)). The relative percentage areas of the deconvoluted peaks are shown in Fig. 9. The deconvoluted carbonyl peaks in this region show less sensitivity when compared with the carbonyl peaks in the amide region; the peak intensities generally showed negligible difference regardless of the variation in TPAI content. However, just the presence of TPAI itself does give an overall change where peak intensities for all GPE with TPAI content (A1 until A7) dropped at 1764 and 1773 cm⁻¹, whereas all other peaks increased when compared against the blank GPE (A0). The peak at 1708 cm⁻¹, which was originally a characteristic feature of the pure PhCh, shifted to 1718 cm⁻¹ indicating a strong interaction between PhCh and TPAI. The peaks at 1773 and 1805 cm⁻¹ belong to the carbonyl group of EC.⁵⁷ Another strong peak that can be associated with EC is present at 1798 cm⁻¹ and is observed to increase with increasing TPAI content. It is thus evident that EC may also have interacted with TPAI salt as the mode of the stretching vibration of C=O is strongly affected by interactions with other molecules.⁵¹

Fig. 9 Relative FTIR band percentage area in the range of 1700 to 1840 cm⁻¹.

3.3.3. FTIR spectra of amine/hydroxyl group in the range from 3130 to 3700 cm⁻¹. Fig. 10 displays the deconvolution plot in the 3130–3700 cm⁻¹ region corresponding to the O–H and N–H group of the GPE (Fig. 6(D)). The peak at 3247 cm⁻¹ is due to the hydrogen coordination of N–H from DMF to the ether oxygen from PhCh backbone.⁵⁶ In the presence of TPAI, these sites instead coordinate to the salt cations hence the sudden drop in peak intensities for GPE of A1 to A7. On the other hand, the peak at 3535 cm⁻¹ saw its peak intensities increased when compared against the blank A0 GPE, which indicates the possibility of it being attributed to coordination of N–H band of DMF with TPA⁺ ions. The peak at 3363 cm⁻¹ was assigned to the hydrogen coordination from N–H with carbonyl. Likewise, with presence of TPAI, it saw a definite decreasing trend in peak intensity when TPAI content increased. The C=O sites no longer being hydrogen coordinated to the O–H and N–H group, is now most likely coordinated to TPA⁺, as discussed earlier. The peak at 3451 cm⁻¹ corresponds to the shifted band of 3442 cm⁻¹ of O–H band from pure PhCh. The delocalization of lone-pair electrons because of strong metal-ions-mediated coordination effectively reduces the O–H bond length as the vibrational energy will increase at higher frequency. The band at 3593 cm⁻¹ may also be attributed to the O–H coordinated with the TPA⁺ cations.

Fig. 10 Relative FTIR band percentage area in the range of 3130 to 3700 cm⁻¹.

3.4. X-ray diffraction

The results of the XRD patterns are shown in Fig. 11 (and Table S2, ESI†). It can be observed that the area and height of the broad peak for samples A0 to A6 generally decreased with increasing TPAI content, from an area and height of 121 913 a.u. and 14 458 a.u. to 22 727 a.u. and 2874 a.u., respectively. This might be due to the decreasing of crystallinity of the GPEs. When there is periodic crystalline arrangement of atoms, the X-rays will be scattered only in certain directions when they hit the lattice planes formed by atoms. This will cause high intensity peaks. As the crystallinity in the GPE gets more disrupted, periodicity of the atoms decreases and gets more randomly distributed in 3D space. The X-rays will be scattered in many directions leading to a broad peak distributed in a wide 2θ range instead of high intensity narrower peaks as is shown for every pattern in Fig. 11. It can be inferred from the FTIR analyses that the interactions of TPAI with the GPE system has also succeeded in further disrupting the crystallinity. The amorphousity was most favorable for the GPE sample A6 (TPAI content 0.12 g) and this results corroborates with the trend seen in the conductivity and solar cell studies, since at sample A7, which is beyond the optimum salt concentration point, the area and height increased to 24 896 and 3324 a.u., respectively, possibly due to the re-association of TPAI ions into its salt aggregates.

Fig. 11 XRD patterns of PhCh–EC–DMF–TPAI based GPE with varying amounts of TPAI.

3.5. Dye-sensitized solar cells

Photocurrent density–voltage curves for the DSSCs fabricated with GPEs having different TPAI mass contents are shown in Fig. 12. The J–V characteristic parameters are given in Table 2. Initially, for the GPE having the lowest content of TPAI, 0.02 g, the cell produced an efficiency of only 2.86% with J_SC, V_OC and fill factor of 7.70 mA cm⁻², 0.64 V and 0.58, respectively. With the increase of TPAI content in the GPE, the efficiency of the DSSCs increased. It should be noted that the efficiency variation follows essentially the same variation as J_SC; it is thus the dominant contributor to the solar cell efficiency. The DSSC based on A6 showed the optimum energy conversion efficiency of 5.00% with parameters J_SC of 12.72 mA cm⁻², V_OC of 0.60 V, fill factor of 0.66. This trend is similar as that observed for the conductivity characterizations observed earlier, therefore its behavior could be attributed to the amount of free iodide ions in the GPE. After the optimum point at 0.12 g TPAI, the dissociation of the salt into its free ions would have to compete with the occurrence of formation of ion pair and/or ion aggregates that do not assist the ionic conductivity and also hinder the mobility of the ions, which explains the drop in efficiency at 0.14 g TPAI.

Fig. 12 Current–voltage curves for DSSCs based on PhCh–EC–DMF–TPAI gel polymer electrolytes with varying amounts of TPAI.

Table 2 Performance parameters of DSSCs based on PhCh–EC–DMF–TPAI gel polymer electrolytes having various TPAI contents

Sample	JSC (mA cm^−2)	VOC (V)	FF	η (%)
A1	7.70	0.64	0.58	2.86
A2	8.65	0.66	0.62	3.51
A3	8.95	0.65	0.64	3.75
A4	10.05	0.63	0.65	4.09
A5	11.29	0.58	0.65	4.19
A6	12.72	0.60	0.66	5.00
A7	9.17	0.57	0.69	3.58

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From Table 2, it is seen that while V_OC does not show any increasing or decreasing trend with increase of TPAI content in the electrolyte, the FF, however, shows a monotonic increase in values. Generally, V_OC is determined by the difference between the quasi Fermi level of the TiO₂ and the redox level of I⁻/I₃⁻ couple. The value of V_OC depends on the size of cations in the electrolyte as smaller ones can adsorb well to the TiO₂ surface and alter position of the Fermi level to some extent. Since larger TPA⁺ cations are involved in the present electrolyte, no such changes can be expected and thus the V_OC does not show any noticeable trend of changes when the concentration of the salt in the electrolyte is changed. The value of the FF, however depends on many factors including the charge accumulation within the solar cell occurring due to unbalanced mobilities of charged species and slower transfer of electrons at interfaces and dye regeneration.⁵⁸ As the TPAI content increases, concentration of I⁻ ions in the electrolyte increases giving rise to more currents due to improved conductivity and increase in FF from the lowering of charge accumulation caused by increased charge transfer at the counter electrode and accelerated dye regeneration. For the A7 electrolyte having more TPAI beyond the optimum amount, the current becomes less due to reduced number of I⁻ ions because of the formation of ion pairs and aggregates but it appears that the charge accumulation has not yet increased to the extent to reduce the FF. When charge accumulation occurs, the shape of the J–V curves deviate very much from the ideal rectangular shape, and as can be seen in Fig. 12, as the TPAI content increases, the shape of the J–V curves progressively becomes closer to a rectangular shape indicating that the charge accumulation is becoming lesser. It should be noted that FF is controlled by several more factors such as electron recombination, poor contacts and current leakages and these factors influence each other as well. Therefore, arriving at a better explanation for the observed FF value changes is quite difficult.

Table 3 shows the comparison for the performance parameters of some DSSCs in recent literature (the values shown are the performance parameters of the best electrolyte system discussed in each authors' work). From our own work, it has been shown that significant improvements have been achieved in improving the efficiency of the GPEs based on chitosan and its derivatives. The DSSC properties are also on par or even better than other types of biopolymer based GPEs. It is also comparable to the best values found in GPEs with synthetic polymer host matrices.

Table 3 Comparison of performance parameters of some DSSCs in recent literature

Electrolyte system (polymer–co-solvents–salt–additive/dye)	JSC (mA cm^−2)	VOC (V)	FF	η (%)	References
Synthetic polymer electrolytes
PAN–EC/PC–KI/TPAI/I2–N719	13.79	0.68	0.57	5.36	Dissanayake et al. (2012)^22
PAN–EC/PC–TPAI/I2–N719	10.78	0.75	0.53	4.30	Bandara et al. (2013)^21
PAN–EC/PC–TPAI/I2–chlorophyll	5.78	0.60	0.57	1.97	Hassan et al. (2014)^23
PVA–DMSO/EC/PC–KI/I2–N719	5.08	0.68	0.79	2.74	Aziz et al. (2014)^25
PVA–DMSO/EC/PC–KI/TPAI/I2–TBP/N3	9.66	0.77	0.44	3.27	Arof et al. (2014)^24
PVdF–EC/PC–KI/TPAI/I2–N719	9.16	0.67	0.63	3.92	Arof et al. (2014)^26
PMMA–EC/PC–KI/TPAI/I2–N719	10.70	0.66	0.57	3.99	Dissanayake et al. (2014)^27
Biopolymer electrolytes
Agarose–H2O–KI/I2–N719	3.27	0.67	0.24	0.54	Singh et al. (2013)^29
Agarose–DMSO/GuSCN/PC/NMBI–AEII/I2–N719	11.45	0.76	0.68	5.89	Hsu et al. (2013)^28
Starch–MPII–NaI/I2–N719	4.78	0.57	0.77	2.09	Khanmirzaei et al. (2015)^30
CM-cellulose/PEO–CH3CN/MPII–NaI/I2–TBP/N719	10.03	0.75	0.69	5.18	Bella et al. (2013)^31
Micro-cellulose–CH3CN/EMISCN/MPII–LiI/I2–TBP/N719	8.39	0.59	0.67	3.33	Salvador et al. (2014)^32
Micro-cellulose–BEMA/PEGMA–NaI/I2–TBP/N719	15.20	0.76	0.61	7.03	Bella et al. (2014)^34
CM-κ-carrageenan/CMC–AcOH–NH4I/I2–N719	0.42	0.50	0.64	0.13	Rudhziah et al. (2015)^36
CM-κ-carrageenan–EC/AcOH–NaI/I2–N719	7.60	0.51	0.53	2.06	Bella et al. (2015)^35
Chitosan–EC/BMII–NH4I/I2–anthocyanin	0.07	0.23	0.22	N/A	Buraidah et al. (2010)^37
PhCh/PEO–DMF/BMII–NH4I/I2–tartaric acid/anthocyanin	3.50	0.34	0.39	0.46	Buraidah et al. (2011)^38
PhCh–EC/PC–TPAI/LiI/I2–N719	7.25	0.77	0.67	3.71	Yusuf et al. (2014)^39
PhCh–EC/DMF–TPAI/I2–N3	12.72	0.60	0.66	5.00	Yusuf et al. (2016) [this work]

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4. Conclusions

N-Phthaloylchitosan (PhCh) has been prepared by reacting chitosan with phthalic anhydride; success of the phthaloylation procedure was confirmed by ¹H NMR, FTIR and XRD characterizations. This modified chitosan, PhCh, showed a great potential as a gel polymer electrolyte host. XRD analyses revealed that addition of tetrapropylammonium iodide (TPAI) further reduced the amorphousity of the PhCh. FTIR analyses indicate significant occurrence of both inter- and intra-hydrogen bonded interactions and evidence of ion complexation between polymer–plasticizer–salt. The entrapment of TPA⁺ in these interactions enabled more mobility for I⁻ ions as TPAI content increases, which in turn lead to higher conductivities and solar cell efficiencies. The optimum conditions was obtained for the gel polymer electrolyte consisting of PhCh : EC : DMF : TPAI : I₂ in weight ratio (g) of 0.1 : 0.3 : 0.3 : 0.12 : 0.012, which showed the highest conductivity of 5.46 × 10⁻³ S cm⁻¹ at room temperature and exhibited the best performance in DSSC with efficiency of 5.0%, with J_SC of 12.72 mA cm⁻², V_OC of 0.60 V and fill factor of 0.66.

Abbreviation

AcOH	Acetic acid
AEII	1-Allyl-3-methylimidazolium
BEMA	Bisphenol A ethoxylate dimethacrylate
BMII	1-Butyl-3-methylimidazolium iodide
CH3CN	Acetonitrile
CM	Carboxymethyl
EMISCN	1-Ethyl-3-methylimidazolium thiocyanate
GuSCN	Guanidinium thiocyanate
KI	Potassium iodide
LiI	Lithium iodide
MPII	1-Methyl-3-propylimidazolium iodide
N719 & N3	Ruthenium dye
NaI	Sodium iodide
NH4I	Ammonium iodide
NMBI	N-Methylbenzimidazole
PC	Propylene carbonate
PEGMA	Poly(ethylene glycol) methyl ether methacrylate
PEO	Polyethylene oxide
SPE	Solid polymer electrolyte
TBP	tert-Butyl pyridine

Acknowledgements

The authors gratefully acknowledge the financial support for this project from the University of Malaya for the IPPP grant PG111-2013A. Ahmad Danial Azzahari would also like to thank University of Malaya for the Bright Spark fellowship (BSP/APP/1903/2013).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04188d

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