Functionalized Thermoplastic Polyurethane as Hole Conductor for

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Functionalized Thermoplast Polyurethane as Hole Conductor for Quantum Dot Sensitized Solar Cell Sunil Kumar, Ishwar C Maurya, Om Prakash, Pankaj Srivastava, Santanu Das, and Pralay Maiti ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00783 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Energy Materials

Functionalized Thermoplast Polyurethane as Hole Conductor for Quantum Dot Sensitized Solar Cell

Sunil Kumar1, Ishwar Chandra Maurya2, Om Prakash1, Pankaj Srivastava2, Santanu Das3 and Pralay Maiti1,

1

*

School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu

University), Varanasi-221005, India 2

Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005 India

3

Department of Ceramic Engineering, Indian Institute of Technology (Banaras Hindu University),

Varanasi 221005 India

*Correspondence should be made to [email protected] (P. Maiti)

ACS Paragon Plus Environment

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Abstract A hole conducting layer for quantum dot sensitized solar cell (QDSS) as a function of redox behavior has been reported. Polyurethanes, comprising hard and soft segments, have been functionalized for its use in solar cell application. Functionalization has been confirmed through NMR and FTIR studies. The functionalization of hard segment results in incorporation of ionic moieties which enhances its electrical conductivity, electrochemical and optical properties and displays a crucial role as a hole transport materials for QDSS cells due to proper work function and reduces energy barrier at the interface of active layer and counter electrode leading to reversible charge transport without decomposition. Cadmium sluphide (CdS) quantum dot has been synthesized using capping agent and the size (4 nm) and shape (spherical) has been confirmed through various techniques including TEM, AFM, SEM and DLS. Energy diagram of whole system has been revealed by measuring HOMO-LUMO and VB-CB energy gap through cyclic voltammetry and UV-vis spectrophotometry. The proper energy level alignment with electron transport layer and electron collecting layer provides suitability to transport hole for continuous harvesting of light. Solar cell device has been fabricated using successful layered design of functionalized polyurethane. The incorporation of a thin polyanilene (PANi) layer helps reducing the electron transport toward reverse direction (cathode) by adjusting the LUMO energy gap of polymer gel electrolyte and confirms re-excitation of dropped electron towards quantum dots (photo anode) through quenching under continuous illumination. The device with structure FTO/TiO2/CdS/PANi/PGE/Pt exhibits a photo current density of (Jsc ~ 2.20 mA/cm2), open circuit voltage Voc of 0.60 V, fill factor of 0.78 and photovoltaic conversion efficiency (PCE) of 1.25% using functionalized polyurethane. Keywords: Functionalized polyurethane; CdS quantum dots; polymer gel electrolyte; Buffer layer; Solar cell assembly.


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Introduction Titanium dioxide (TiO2) is one of the most important semiconductors used as the photo anode material which is the key components in the configuration of quantum dot sensitized solar cells (QDSSCs) [1]. The capping agent (EDTA) is often used to prepare CdS quantum dots [2], which is used as photosensitizer for the preparation of photo anode on TiO2 nanostructures due to its effective charge transfer property and excellent light harvesting ability [3]. Optimally, a thick CdS layer would be beneficial for the device performance [4] because of the limited diffusion length of charge carriers in CdS [5,6]. Surface ligands employed in QDs syntheses serve as a platform for surface passivation to generate monodispersed nanocrystals with very few surface defects and interfacial interaction of polymer [7] with QDs depends on nature of functionalization i.e. side chain or end group [8] and the quantum confinement effect improves the optical and conductive properties [9–11]. Liquid electrolytes have a fundamental limitation for long-term operation owing to their volatilization and leakage due to inadequate sealing of cells, possible desorption and photo degradation of the sensitizer [1]. The hydrophilic property of polymer [12] allows the intimate contact with hydrophobic QDs active layer which facilitates charge transport at the interface [13]. Polymer gel electrolytes have superior mechanical properties [14] and promise long-term stability to act as hole conducting materials [12]. The tailoring in composition and functional group allows polymer to absorb liquid electrolyte by forming a well-defined nanostructured gel which lead to high ionic mobility with photo anode results in enhanced photovoltaic performance [1,13]. Recent works focus on functionalized thermoplast polyurethane ionomer gel as a thin film sandwiched between photo anode and Pt [8] as counter electrode to act as polyelectrolyte due to its semiconducting property [15]. The redox mediator as a gel form is essential to extract the electron reaching the counter electrode and transport it through the electrolyte to regenerate the QD that has been oxidized



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at the photo anode due to electron injection [14]. Cyclic voltammetry analysis of polymer electrolyte provides energy levels alignment with photosensitizer i.e. Energy difference between valence band (VB) of QD and highest occupied molecular orbital (HOMO) of polyelectrolyte is very less in comparison to the energy gap between lowest unoccupied molecular orbital (LUMO) of polymer and conduction band (CV) of QDs [16,17]. The energy barrier between LUMO of polymer and CV of photosensitizer is responsible for electron blocking layer i.e., excited photoelectrons from VB of QD cannot reach to LUMO of polymer that results in good performance of QDSSC. Polyurethane is composed of hard and soft segment in alternate fashion in the polymeric chain [18]. The hard segment content of polymer chain [19] provides the idea about polarity and crystalline nature of urethane linkage [20]. The short chain length extender concentrate >N-H hydrogen bonding between hard segment in inter-polymer chains [21] resulting high thermal stability [22]. The functionalization or incorporation of ionic moieties in a hard segment of polymer chain reduces the extent of hydrogen bonding and increases polarity [23] which have stronger capacity to absorb heat [19] and because of the degradation of hard segment, its thermal stability gets reduced [24]. Thermal stability varies depending upon the degree of functionalization or extent of polarity in the chain [25]. An optimum amount of functionalization results in feasible thermal stability [26]. The electronic structure and electrical property of polyurethane ionomer on the interface of photo sensitizer explains its hole transporting ability [27]. The energy gap of HOMO and LUMO can be tuned by varying the functionalization and its content [28]. Since, polyurethane and its ionomer are composed of bonding, nonbinding and anti-bonding molecular orbitals, HOMOLUMO energy gap decreases with incorporation of high degree of ionic entities. The high polarity in polymer chain increases the nonbonding electron density which is filled successively in anti bonding molecular orbitals. The cause of red shifting of absorption spectra is the destabilization of HOMO


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level and stabilization of LUMO level of ionomers. Electrochemical analysis indicates the degree of functionalization requires for lower oxidation potential. The optimum amount of oxidation potential for a HOMO level provides the feasibility to transport hole of photo sensitizer after excitation of electron from valence band of photosensitizer [29]. However, the use of thermoplast is very limited for the solar energy application [30]. Here, we report the functionalized polyurethane as the hole transporting layer for the application of solar energy conversion using CdS based quantum dot sensitized solar cell.

Experimental Materials: Hexamethylene diisocyanate (HMDI), poly(tetramethylene glycol) (PTMG, Mn=2900 g/mol) ethylene diammine (EDA), sodium hydride (NaH, 60 % dispersion in mineral oil), ϒ-propane sultone (PS) were procured from (Sigma-Aldrich, USA). Catalyst dibutyltin dilaurate, N,N-dimethyl acetamide (DMAc) were procured from Himedia and Merck, India, respectively. Cadmium nitrate tetrahydrate (Merck, India), sodium sulfide (Na2S, Himedia), ethylene diamine tetra acetic acid (EDTA, Himedia), ethanol (Merck, India) were used for the synthesis of quantum dot. Preparation of polyurethane: Ethylene diammine (EDA) based thermoplast polyurethane (PU) was synthesized in three naked round bottom flask combining diisocyanate (HMDI), polyol (PTMG) and chain extender (EDA) in a two-step polymerization process, keeping the hard segment content (HSC) 30% and will be termed as PU [31]. The first step is the pre-polymerization process consisting of PTMG and HMDI which is performed under nitrogen atmosphere for 4h at 70 °C followed by the addition of chain extender EDA, DMF as non-aqueous solvent and catalyst (DBTDL: 0.1 mL of 1 wt. % toluene solution) are added in the second step. The catalyst was used to enhance the reaction rate and complete the polymerization process with constant stirring at 70 oC for



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24 h. The polymer flakes are extracted by using deionized water after complete precipitation and dried under reduced pressure at 70 °C in a vacuum oven for 24 h. Functionalization of polyurethane: Polyurethane was modified chemically using propane sultone to enhance the physical property i.e. redox activity and conductivity. In a typical synthesis method, as reported by Banerjee et al [32], 0.3gmpure polyurethane flake was dissolved in DMAc at 130 °C. It was cooled at low temperature (-5 °C) followed by the addition of 0.6 gm of pure sodium hydride to abstract proton from urethane linkage under continuous purging of nitrogen at constant stirring for 1h. Then, 1 mL of 1, 3-propane sultone was added to above reaction mixture at room temperature followed by heating at 65 °C under nitrogen atmosphere with constant stirring for 3 h. The ionomer was precipitated by pouring it in toluene. Precipitated ionomer was separated from toluene and washed with ethanol several times to remove unreacted chemicals. Finally, it was dried under reduced pressure at 60 °C. Preparation of polymer gel electrolyte: Polymer gel electrolyte [33] was prepared by dissolving excess amount of sulfonated polyurethane in the mixture of highly polar solvent in equal ratio i.e., mixture of DMAc and NMP (N-methyl pyrrolidone). Ionomer was mixed and dissolved at 90 °C through constant stirring in a magnetic stirrer. The stirring was continued to get a homogenous and gelatinized mixture followed by freezing at lower temperature (5 °C) for overnight. The gel contains extensively solvated polyions in aggregated form [34]. Synthesis of capped CdS quantum dot: In a typical synthesis process as depicted by Sadjdi et al, [2] 1.234 gm (0.4 M) cadmium nitrate was dissolved in 10 mL ethanol to get Cd2+ solution and was added to 0.855 gm (0.114 M) of EDTA dissolved in 20 mL deionized water with constant stirring at room temperature. The reaction mixture was further stirred at 100 °C keeping the pH ~5.5 for 2 h. EDTA capped Cd2+ was stabilized at room temperature. A solution of 0.12 gm (0.11 M) sodium


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sulfide, dissolved in 10 mL ethanol, was added drop wise under vigorous stirring for 30 min until light yellow precipitate was obtained and the suspension was centrifuged followed by washing with water and ethanol to remove unreacted species. Thus, different size of CdS quantum dots were synthesized by varying the composition of sulfide ion at a constant weight ratio of Cd(NO3)2.2H2O and EDTA under similar reaction condition. EDTA capped CdS were kept in freezer to prevent its agglomeration and further nucleation. Materials characterization: UV-visible measurements have been carried out using JASCO-650 UVvisible spectrophotometer operating in the spectral range of 200-800 nm using a thin solid film. FTIR was performed in transmittance mode at room temperature from 400 to 4000 cm-1 using a Nicolet 670 instrument with a resolution of 4 cm-1. 1H NMR spectra of pure and functionalized PU were recorded on a JEOL AL 300 spectrometer using d6-DMSO solvent. Chemical shift are recorded in parts per million (ppm) with respect to tetra-methylsilane (TMS). The surface morphology of pure uncapped CdS and capped CdS nanoparticles were examined with a Hitachi H-7100 scanning electron microscope operated at an accelerating voltage of 20 kV and magnification of 30,000× for detecting particle size. All the samples were gold coated by means of a sputtering apparatus under vacuum before observation. A NT-MDT multimode atomic force microscope, Russia, controlled by Solver scanning probe was used for surface morphology study. The particle size was analyzed of ultrafine dispersion using dynamic light scattering (DLS) using Horiba Scientific Nanoparticle analyzer SZ-100 in the scattering angle of 90o at room temperature. Cyclic voltammetry was performed on a diluted solution at room temperature using Palm Sens 3 compact electrochemical interfaces with battery powered potentiostat having a potential range (-5 to +5 V) and current range (100 pA to 10 mA).



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X-ray diffraction experiments were performed using a Bruker AXS D8 advance wide angle X-ray diffractometer with Cu-Kα radiation and a graphite monochromator (wavelength ~ 0.154 nm). The generator was operated at 40 kV and 20 mA. Thin polymer film and QDs power were placed on a quartz sample holder at room temperature and were scanned at diffraction angle 2θ from 5 to 80° at the scanning rate of 2°/ min. Thermal behavior of pure polymer and its functionalized materials were analyzed through Mettler Toledo thermogravimetric analyzer performed in the temperature range of 40 – 600 °C at a heating rate of 20°/min under inert atmosphere. Electrochemical measurements: The solution phase samples were prepared by dissolving 2 mg QDs in 6 ml of NMP followed by ultra sonication to ensure stable and fine dispersion of the particle. Cyclic voltammetry of SPUs were carried out in a three electrode system consisting of glassy carbon as a working electrode, a platinum counter electrode and Ag/AgCl reference electrode. Nitrogen purging was used to provide inert atmosphere. Ferrocene is used in this experiment as known reference to calculate energy of HOMO and LUMO levels[35], including the ferrocene value of -4.4 eV. EHOMO = - eV (EOX onset + 4.4);

ELUMO = -eV (ERED onset + 4.4)

Blank run was carried out for pure NMP to obtain the working potential range. The cyclic voltammetry of prepared samples was performed at a scan rate of 20 mV/s at room temperature taking 5 ml solution phase sample (prepared by dissolving 5 mM of SPUs in N-methyl pyrolidone) and scann in the potential range of -2.5 to +2.5 V.

5 mM of SPUs was dissolved in N-methyl pyrolidone. 5 ml solution phase sample was taken in sample container followed by immersing three electrodes under nitrogen purging. The potential


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range -2.5 to +2.5 V was fixed. The CV experiment of prepared samples was performed with 20 mV/s at room temperature. Preparation of photo anode: Fluorine tin doped oxide (2.2 mm thick, 9.0 ohm/square, Latec) conducting glass substrate was taken for preparing the photo anode. 2.0 × 1.5 cm2 sheet under 0.20 cm2 area was coated with thin film of TiO2 paste (Solaronix) using doctor’s blade. It was kept at 80 o

C for 20 minutes to remove low boiling organic solvent. Further, it was kept in a furnace at 450 oC

for degradation of organic binders that result in crystallization of pure TiO2 thin film. EDTA capped CdS QD ultrasonically dispersed in isopropanol was spin coated on TiO2 thin film (1500 rpm for 10 minutes). In this way, plane CdS film was developed on TiO2 film for its further use as DSSC photoanode. Preparation of counter electrode: A thin film of Pt was deposited on FTO under same active area of 0.20 cm2 with the help of doctor blade followed by drying at 80 °C for 30 minutes. Then, it was annealed at 450 °C for 30 minutes for recrystallization of Pt thin film. Small amount of gel electrolyte was dispersed on active area of photo anode followed by sandwiching the counter electrode on it for the fabrication of cell. The cell was sealed from the two sides and electrical contacts were taken for the output measurements. Solar cell apparatus and measurement: A bipotentiostat (AFRDE4E, Pine Instruments, US) along with an e-corder (Model 201, eDAQ, Australia) were used for all current–potential measurements. A 150 W Xenon arc lamp with lamp housing (Model No. 66057) and power supply (Model No. 68752) from Oriel Corporation, US, was used as the light source. Semiconductor electrode was illuminated after passing the light beam through a 6 inch long water column (to filter IR) and condense it with the help of fused silica lenses (Oriel Corporation, US). A long pass filter (Model No. 51280, Oriel



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Corporation, US) has been used to cut-off the UV part and the light obtained this way is referred as visible light.

Results and discussion Optimization of quantum dots: Capping agent has been used to cover up the nascent nucleus to control the size of quantum dots. SEM images of uncapped and EDTA capped CdS are shown in Fig. 1a indicating significantly smaller particle dimension in capped system (40 nm) as compared to uncapped CdS (130 nm). Usually, larger particle in uncapped CdS are formed in some sort of cubical shape while EDTA surface stabilized CdS shows almost uniformly distributed spherical shape grains. AFM image of EDTA capped CdS exhibits spherical particle dimension of 100 nm (Fig. 1b) while TEM image [36] clearly indicate tiny nanoparticles of average size of 4 nm (Fig. 1c). The distribution of nanoparticle is shown in the inset image indicating the particle distribution from 2 to 5 nm. This is to mention that smallest particle is observed through TEM, where the sampling is made from a very dilute dispersion, but the agglomerated dimension is visible in SEM and AFM micrographs. Ultrafine suspensions of various capped CdS nanoparticles are prepared in absolute ethanol through ultra sonication for dynamic light scattering (DLS) measurement based on the random thermal motion or Brownian motion to measure the relative particle dimension. Different extent of sulphur ions generate particle sizes of 14, 11 and 8 nm of CdS nanoparticle from 0.16, 0.14 and 0.11 M concentration of sodium sulfide in the solution of EDTA capped Cd+2 ions (Fig. 1d). This is worth mentioning that DLS measures the hydrodynamic size of the particles which is apparently bigger than the particle size observed through TEM. However, it is evident that nanometer dimension CdS particles are formed using optimized sulfide ion and EDTA as capping agent. Chemical nature of


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CdS and pure EDTA is evaluated through FTIR absorption bands along with the presence of capping agent over CdS particles (Fig. 1e). The peaks at 1635, 1385, 1094, 1424 and 1408 cm-1 in CdS-4 (capped CdS) are assigned for bending vibration of water, bending vibration of CH3 group in ethanol, C–O stretching vibration, asymmetric and symmetric stretching of –COO– entities on the surface of CdS which confirm the coverage of CdS particle with EDTA. The peaks at 3518 and 3376 cm-1 are due to –OH group in a free and hydrogen bonded state [37]. There are shifts in peak positions of EDTA before and after capping on CdS and these shifts in peak positions of capped CdS particle provide supports for efficient role of EDTA to control the size of CdS particle by preventing from agglomeration and coagulation. Surface coverage of quantum dot shows monodisperse nature and stabilizes the size of QDs by reducing surface energy of the particle. The larger particles have low band gap and is responsible for fast electron-hole pair recombination leads to very less response for photovoltaic effect. It is the energy alignment at the interface of the layers which defines photovoltaic conversion efficiency. Very small tiny QDs have large band gap but its band gap and band structure are suitable with respect to band structure of other layer in the assembled solar cell, comparable to facilitate easy charge transport and, thus, a maximum efficiency is obtained.



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\

Figure 1: (a) SEM image of uncapped CdS and capped CdS QDs for a ultrafine dispersed particle on a simple glass slide; (b) AFM image for capped CdS to analyze surface morphology; (c) TEM image of indicated CdS particle operated at 200 kV and inset bar diagram shows the distribution of particle size; (d) Dynamic light scattering measurement of ultrafine dispersed EDTA stabilized indicated CdS QDs in ethanol for three different CdS nanoparticle composed of varying sulphide concentration mentioning the average particle dimension; (e) FTIR spectra of uncapped CdS, pure EDTA and EDTA surface stabilized CdS showing the change of peak position.

Functionalization of polyurethane: In order to use thermoplastic polyurethane as charge carrier for its possible use in solar cell, polyurethane has been functionalized through chemical reaction as mentioned in the reaction scheme of Fig. 2a. Sulfonate groups have been introduced in the polyurethane main chain through the


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reaction with propane sultone. The evidence of sulfonate group and its extent has been confirmed through 1H NMR spectra of functionalized PU as shown in Fig. 2b. Pure PU shows a characteristic peak at chemical shift δ~ 8.2 ppm for NH proton from urethane linkage while functionalized PU shows closely spaced broad and intense peaks at 8.27 and 8.23 ppm, clearly indicate the presence of proton attached with sulfonated group of functionalized PU. Further, three new peaks for different protons at δ=4.5, 1.9 and 1.6 ppm in functionalized PU are due to propane sultone group (CH2-CH2CH2-SO3-) attached in modified PU. Functionalization of aliphatic polyurethane is also confirmed by recording the shift of peak position in FTIR both in hard and soft segment of polyurethane. The peak at 3336 cm-1 in pure PU is due to hydrogen bonded –N-H group present in hard segment [38], responsible for stacking pattern in PU, [39] while a broad signal appears at 3385 cm-1 supports reduced hydrogen bonded structure between hard segments as a result of chemical modification of propane sultone unit at the N-H group in the hard segment (Fig. 2c). Further, a strong peak assigned at 1187 cm-1 establishes successful sulfonation (>S=O linkage) on PU chain. Other FTIR peaks at 2948 and 2866 cm-1 are due to asymmetric and symmetric stretching of –CH2 entity of soft segment in pure PU while 2937 and 2855 cm-1 peaks represent CH2 group present in sulfonated PU. The carbonyl peak of S-PU appears at 1725 cm-1 against 1691 cm-1 peak position in pure PU. The shifting of peak position is presumably due to interaction in modified PU in presence of propane sultone group in main chain of PU. Other peak positions at 1627 and 1645 cm-1 are due to –N-H bending and the signals appear at 1480 and 1069 cm-1 are due to CH2 bending and –C-N stretching for S-PU chain. However, the modification and introduction of polar sulfonate group in polyurethane has been confirmed through NMR and FTIR spectroscopic techniques. The attachment of sulfonate group in the main PU chain should exhibit different thermal stability as compared to main chain. In order to examine the thermal stability, thermogravimetric



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analysis has been performed and the thermogrammes are presented in Fig. 2d. The degradation temperature decreases in functionalized PU which continues to decrease with increasing the extent of sulfonation. The degradation temperature of pure polyurethane is 339 oC while sulphonated PU degrades at 230 °C for S-PU1 which further reduces to 107 °C for S-PU2 with higher amount of sulfonation. This is to mention that temperature corresponding to 5% weight loss is considered as the degradation temperature. Moreover, two stage degradations are evident for all the specimens while it is prominent for sulfonated species. Initial weight loss is due to the degradation of hard segment, because of its lower thermal stability arising from intermolecular hydrogen bonding, while higher temperature degradation corresponds to soft segment degradation. However, sulfonation and its extent exhibit various thermal stabilities depending on the degree of sulfonation.


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(a)

2 OCN (CH2)6 NCO

+

HMDI

(b)

HO [(CH2)4 O]nH

PTMG

70oC O

H

S-PU2

OCN (CH2)6 N C O [(CH2)4 O]n C N (CH2)6 NCO H

O

DBTDL

H2N (CH2)2 NH2

Catalyst

700C

10

H O H O C O[(CH2)4 O]nC N (CH2)6 N C N (CH2)2 N O H H H

H N (CH2)2 N C N H H O

(CH2)6

S O

10

H

(CH2)3 O

O

O N

(CH2)3

(CH2)3

SO3-

SO3-

6

4

2

0

H

(d)

1722 1632 1187

20 3336

10

Weight fraction

1.0

3385

3600

0

(CH2)3

HO

40 30

2

δ (ppm)

PU S-PU2

50

4

C O [(CH2)4 O]n C N (CH2)6 N C N (CH2)2 N

Sulfonated polyurethane

(c)

8

SO3-

SO3-

(CH2)6

δ (ppm)

Functionalization 60oC

O

H

6

PU

NaH, -50C

O

N (CH2)2 N C N

8

N

Polyurethane

T (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PU S-PU1 S-PU2

0.8 0.6 0.4 0.2

1691

3000

2400

1800 1200 -1 Wavenumber /cm

0.0

100

200

300

400

500

600

Temp. / °C

Figure 2: (a) Reaction scheme to synthesize ethylene diamine extended thermoplast polyurethane (PU) and functionalization of hard segment using propane sultone to generate ionomer; (b) NMR spectra for PU and sulfonated PU (S-PU); (c) FTIR spectra of pure PU and S-PU indicating the change of peak position; and (d) weight loss as a function of temperature of pure PU and two different functionalized S-PU as measured through TGA.



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Electronic properties of functionalized PU and CdS for charge transport: It is now pertinent to understand the behavior of capped CdS and functionalized PU for their use in optoelectronic devices including solar cell. Absorption spectra of capped / uncapped CdS quantum dots are shown in Fig. 3a. The absorption edge of uncapped CdS appears at 512 nm and a systematic blue shift is noticed with reduced size of the nanocrystal after wrapping them with appropriate amount of EDTA for surface coverage. The absorption edge of the smallest particle (~4 nm) is recorded at 460 nm. The observed blue shift in absorption edge [40] arises due to size quantization associated with nucleation effect [41]. The optical band gaps are estimated using Tauc’s plot and are shown in Fig. 3b. Optical band gap is inversely proportional to dimension of nanocrystal [5] (Eg = 1240/λ) and the smallest particle shows the highest energy transition of 2.69 eV vis-à-vis 2.41 eV for uncapped CdS (bulk) with systematic increase with lowering the dimension of CdS quantum dot. UV-visible absorption spectra of TiO2 thin film shows a prominent absorption edge at 356 nm due to large band gap in anatase form. The deposition of CdS QDs on TiO2 film exhibits shift in absorption edge towards visible range primarily due to adsorption of surfaces with interacting particles of two phases (supplementary Fig. S1a). A red shift in absorption profile indicates improved photo harvesting ability of CdS coated on to TiO2 arising from better interaction between the phases. It provides efficient charge transfer from CdS to conduction band (CB) of TiO2. Hence, the combination of nanometer dimension CdS and TiO2 offer lesser chance of electron-hole recombination upon photo excitation, which in turn increase charge density in TiO2. UV-visible absorption spectra of ethylene diamine based polyurethane solid film and sulfonated PU solid ionomer are recorded to estimate HOMO-LUMO energy gap (Fig. 3a). Two absorption peaks of PU film appear due to π→π* (at lower wavelength) and n→π* electronic transition (at longer wavelength). The absorption peak for sulfonated polyurethane becomes wider upon increasing the


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degree of functionalization and is mainly associated with n→π* transition of propyl sulphonated group attached in urethane linkage of polymer chain. The bathochromic shift of absorption edge is due to interaction between the sulfonated groups with rest of the hard segment of PU chain (mainly through enhanced dipole-dipole interaction). Thus, highly functionalized PU shifts its absorption edge towards visible region confirming size quantization effect. Energy gap is estimated through Tauc’s plot and are shown in Fig. 3b. Thus, highly sulfonated PU shows lowest energy transition with energy gap of 2.93 eV as opposed to the value of 4.0 eV in pure PU.

Figure 3: (a) UV-visible spectra of indicated CdS nanoparticle and PU/SPU-4 stacked vertically; (b) Tauc’s plot for the calculation of direct energy gap for CdS nanoparticle and PU/S-PU; (c) Cyclic voltammetry of EDTA capped CdS QDs, PANi and S-PU ultrafine dispersed in NMP with scan rate



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20 mV/sec operated at room temperature; (d) Schematic representation of electronic energy alignment and charge transport across the interface of different layer of device (energy diagram).

Electrochemical characterization and energy diagram: Cyclic voltammetry (CV) is a powerful electrochemical technique to study redox behavior of materials [42]. CV measures the current resulting from an applied potential with a fixed scan rate (mV/s). During redox reaction, reduction makes a polymer chain negatively charged while oxidation produces positively charged species. Two peaks appear in voltammograms, one having negative current corresponds to the reduction and other having positive current refers to oxidation. HOMO and LUMO energy levels are estimated from the onset of oxidation and reduction potential, respectively, obtained from CV measurement. Functionalized PUs, with varying degree, are characterized through CV experiment to study the shifting of peaks and its potentials to measure HOMO energy levels. The characteristic two peaks corresponding to cathodic (reduction) and anodic (oxidation) processes are appeared at negative and positive potential, respectively. The difference between HOMO and LUMO energy levels corresponds to energy gap (Eg). HOMO and LUMO energy levels of the polymer influence holes and electron injection efficiency at the interface of donor and accepter materials. The oxidation potential decreases systematically with increasing degree of sulfonation and the values are 0.90, 0.80, 0.78, 0.75 and 0.66 V for SPU-1, SPU-2, SPU-3, SPU-4 and SPU-5, respectively, with the corresponding Eg values of 3.90, 3.23, 3.15, 3.10, 2.93 eV (Fig. 3c and supplementary Fig. S4a). Functionalization on polymer chain thus influences the electronic energy of HOMO levels. Cyclic voltammetry is also an effective tool to probe electroactive property. CV of EDTA stabilized CdS QDs are performed at room temperature with the scan rate of 10 mV /s in the potential window of NMP and measured the oxidation potential of 0.52, 0.57,


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0.60, and 0.68 V for CdS-1, CdS-2, CdS-3 and CdS-4, respectively, with corresponding Eg values of 2.52, 2.62, 2.65 and 2.69 eV (supplementary Fig. S4b). This is to mention that stable and fine dispersion is achieved for smaller particle dimension encapped with EDTA and the onset peak potentials shifts towards more positive side of smaller size particle for oxidation process, respectively. Hence, the smallest size of QDs have more stabilized valence band and more destabilized conduction band [3]. Now, HOMO energy levels from electrochemical measurements and LUMO energy levels calculated from optical energy gap are combined to draw energy diagram which can explain the flow of electron and hole in proper direction (Fig. 3d). It is evident that the close proximity of valence band of CdS-4 and HOMO level of PGE allows the holes to pass on to electrode (Pt) easily. On the other hand, similar energy level of conduction bands of CdS-4 and TiO2 help transportation of electron towards electrode (FTO), as shown by the arrows. This is worth mentioning that conduction band of CdS-4 and LUMO energy level of PGE are also quite similar which may allow the electron to flow in a reverse direction toward Pt electrode and thereby jeopardize the charge carrier transportation for solar cell activity. In order to circumvent the problem, a polymer layer (polyaniline) is added in between CdS and PGE whose LUMO level is quite different than that of PGE or CdS and thereby this buffer layer will not allow the electron to pass on to Pt electrode and instead it will puss the electron only towards other direction (FTO electrode through TiO2). Based on the energy values, the layer structure of assembly for solar cell device has been made as shown in Fig. 4a. Thin PANi layer between CdS and PGE do interact nicely as evident from the shifting of carboxyl (>C=O) stretching frequency at 1720 cm-1 towards lower wavenumber (1689 cm-1) along with broadened –OH absorption and peaks at 3386 and 3238 cm-1, assigned for –NH non hydrogen bonded and hydrogen bonding with EDTA capped CdS moieties in CdS+PANi composite system



Page 20 of 33

(supplementary Fig. S1b). Similarly, S-PU composite with PANi exhibits lowering of peak position from the original peak at 1719 cm-1, assigned to >C=O peak. Further, the >S=O peak position of SPU at 1185 cm-1 has been shifted to 1192 cm-1 in presence of PANi along with broadened peak at 3360 cm-1 assigned due to N-H peak also support good interaction between PANi and S-PU (Supplementary Fig. S1c). Pure polyaniline shows two absorption peaks at 329 and 611 nm due to π→π* and n→π* transition but the mixture of capped CdS and PANi exhibits broadening in absorption spectra along with red shifting observed in mixed system for n→π* transition peak clearly indicate interactive system arising from lone pair of nitrogen atom of conjugated polyaniline (supplementary Fig. S1b). However, PANi layer favorably interact with both CdS and PGE and thereby help transporting the hole from CdS to PGE, as the valence band and HOMO level matches, while electron transport phenomena is suppressed due to mismatch of LUMO energy level of S-PU and conduction band of CdS and ultimately recombination rate of exitons are reduced leading to better harvesting of light [43].


Page 21 of 33

Anode

(a)

FTO TiO2 CdS PANi PGE Pt FTO Cathode

(b)

(c)

4

2

Dark Light

P ( mW/cm )

Light

2

Jmax PGE (S-PU2) Eg = 2.93 eV

-0.2

-0.4

1

Voc

Vmax

0 0.0

Pmax = 1.25

2

Jsc

2

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.6

Voltage (V)

0 0.0

0.2

0.4

Voltage (V)

0.6

Figure 4: (a) Schematic representation of fabricated QDSS cells showing layered structure of different constituent materials; (b) J-V characteristic measurement to estimate photocurrent density and open circuit voltage under the illumination of light; and (c) Power vs. voltage curve to calculate photovoltaic conversion efficiency (PCE)

Fabrication of quantum dot sensitized solar cell based on polymer gel electrolyte: Based on the favorable energy diagram, device has been fabricated using the layered structure as mention in Fig. 4a. Upon illumination from FTO side, quantum dots layer absorb photon and produce electron–hole pair in QDs layer. The photo generated electron transfer from QDs layer to electron acceptor layer (c-TiO2). But some fraction of excited electron may come back due to less life time in excited level. This deactivated electron is to be trapped by polyaniline layer and re-



Page 22 of 33

excited through conjugated π-MOs towards electron acceptor layer under continuous illumination and holes to hole transport layer with the help of hole mobile buffer layer and finally collected by Pt/FTO to undergo reversible charge transport [44]. The function of electronically functionalized polymeric materials originates at the interface [28]. Sulphonated polyurethane ionomer gel has proper electronic band structure with HOMO (-5.06 eV) and LUMO (-5..13 eV) which makes S-PU a very good candidate for the hole transport as well as electron blocking layer with respect to quantum dot layer. The chloride doped conjugated polyaniline at the QDs-PANi interface plays a very important role for scavenging deactivated charge result in reduction of fluorescent emission [45]. Thus, very thin film of polyaniline in between QDs and S-PU improves hole mobility and reduces electron-hole pair recombination in the ground level. Photovoltaic performance parameter such as overall conversion efficiency and fill factor of the solar cell are calculated by using following equation; FF =

η (%) =

×

×100 or η(%) =

× ×

× 100

where, Jsc is short circuit current density (mA/cm2), Voc open circuit voltage (V) and Jmax (mA/cm2) and Vmax (V) are the current density and voltage, respectively, in the J-V curve at the maximum power output. The J-V curve obtained under visible light illumination of 100 mW/cm2 for cell (device structure FTO/TiO2/CdS/PGE/Pt) based on sulfonated polyurethane ionomer gel based hole transport layer (HTL) linked at the interface of CdS QDs is shown in Fig. 4b. The conversion of J-V curve into power vs. voltage curve is shown in Fig. 4c. It exhibits photo current density of 0.92 mA/cm2, open circuit voltage of 0.52 V, fill factor of 0.21 and efficiency of 0.10% (supplementary Fig. S4a & b). Polymer gel electrolyte is layered on CdS QDs since gel has polar functional group which possesses HOMO energy level slightly lower than VB energy level. It is expected that hole is


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extracted towards PGE during exciton (electron–hole pair) generation but low fill factor and photovoltaic conversion efficiency (PCE) are obtained presumably due to low hole mobility at the interface of active layer and can also be demonstrated through high surface energy of polymer gel that leads to poor electrical contact with sensitizer dye [13]. Hence, a need for the improvement in performance of solar cell by changing the usual device structure is realized. The fine dispersion of reduced graphene oxide (RGO) in isopropanol is spin coated at photo active layer followed by layering of PGE. The complete assembly under illumination showed a very little change in Voc but obtained overall efficiency is quite low (0.04%) (Supplementary Fig. S6a). It can be explained on the basis of slight enhancement in hole mobility [46] but energy orientation at interface of RGO/PGE is not so effective leading to possible recombination of some fraction of electron–hole in the active layer. Subsequently, a very thin film of carbon black is spin coated at the top of active layer followed by layering of PGE. The assembly under the similar illumination conditions results in enhancement of photo current density and overall conversion efficiency (0.25%) which is mainly associated with lesser chance of recombination of charge carriers (Supplementary Fig. S6b). Further, a new device architecture is fabricated by incorporating a thin film of highly conductive polymer i.e, chloride doped PANi at the top of CdS QDs film followed by layering a thin film of PGE under analogous condition which displays a marked enhancement in photovoltaic performance and the efficiency is found to be 1.25% (Fig. 4b). The details of photovoltaic parameters are reported in Table 1. Table 1: Photovoltaic parameters and its corresponding values for different devices structure using PGE as hole transport material (HTM).



S.N .

Photocell Structure PGE as HTL

Page 24 of 33

Jsc (mA/cm2)

Voc (V)

FF

Efficiency (%)

1.

FTO/TiO2/CdS/PGE/Pt

0.92

0.52

0.21

0.10

2.

FTO/TiO2/CdS/RGO/PGE/Pt

0.53

0.54

0.14

0.04

3.

FTO/TiO2/CdS/CB/PGE/Pt

2.74

0.34

0.27

0.25

4.

FTO/TiO2/CdS/PANi/PGE/Pt

2.2

0.60

0.78

1.25

This considerable improvement is illustrated with better interfacial contact [47] associated with π-π stacking of PGE with PANi channel, consequently the diffusion length of hole is increased and also interface resistance is reduced [48]. The surface energy of HTL is reduced giving rise to improved kinetics of charge carrier [49]. Thus, a modification in layering of solar cell device using polymer gel electrolyte make significant improvement in performance [50] leading to the use of functionalized thermoplastic for solar cell energy converter. The orientation and alignment of combined energy levels at the interface of electrodes suggest that HOMO level of PANi (-4.90 eV) with respect to HOMO level of various functionalized PU are not properly matched due to high energy difference. Lower the degree of functionalization of PU, lower (more negative) is the energy level of HOMO which is not so effective in terms of conduction pathway of reversible charge transport, prohibiting recombination of electron–hole pair. In other words, it causes better photo voltaic conversion efficiency. Using other buffer layers e.g. RGO or conducting carbon leads to poor photovoltaic performance due to higher HOMO level lead to greater recombination. Highly functionalized PU has -5.06 eV of HOMO levels which depicts close proximity of energy levels with respect to PANi and thereby, efficient hole extraction [51]. Other combinations of SPU with PANi lower the recombination time at the interface indicating poor efficiency of the photovoltaic. A buffer


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layer (PANi) is inserted between quantum dots and polymer gel electrolyte and it plays an important role to reduce electrical resistance at the interface of film by reorganizing and ordering their band structure. It provides better interfacial contact and reduces energy barrier causing enhancement in charge transport, the recombination time is lowered down resulting reduction in electron – hole pair recombination. Conjugated π MOs of PANi interacted with π MOs of sulfonate group provides easy path by reducing diffusion length of charge transport while CdS/PGE layer is not so effective in terms of electrical contact due to high surface energy of PGE at the interface leading to discontinuous flow of charge giving rise to diffused shape curve in J-V measurement and ultimately poor efficiency is obtained. Any conducting layer is not so effective to play its role since band structure at the interface is important to regulate the dynamics of charge transport. PANi provides right kind of band structure for hole transport and thereby provide greater efficiency. However, a thermoplastic can used as hole transporting layer for the photovoltaic application with its ease of processing and longer duration of the solar cell.

Conclusions CdS quantum dot of 4 nm size has been synthesized using capping agent EDTA to act as active layer. The size variation has been confirmed through SEM, TEM, AFM and particle size measurement using DLS. Thermoplast polyurethane has been functionalized with varying degree of sulfonation, by reacting with propane sultone, which is confirmed through NMR and FTIR spectroscopic techniques and is used as polymer gel electrolyte for hole transporting agent in solar cell device. HOMO-LUMO gap is tuned to 2.93 eV after suitably functionalize the polyurethane. The improved conductivity and proper work function demonstrates it to be a suitable hole extraction material at the interface of photoactive layer. Devices have been fabricated using layered structure of



Page 26 of 33

TiO2, CdS and functionalized PU. HOMO-LUMO and VB-CB gap of CdS and functionalized PU have been determined from electrochemical and UV-vis measurement and an energy diagram has been established to choose the right kind of functionalized PU. HOMO-LUMO energy gap decreases as the degree of sulfonation increases. A thin layer of polyaniline has been inserted between CdS and polymer gel electrolyte to create the mismatch in CB of CdS energy level and LUMO of sulfonated PU for restricting the electron transport and facilitate the hole transport towards Pt electrode. Better photovoltaic performance is due to fine surface coverage and good electrical contact leads to current density of 2.2 mA.cm-2 and overall light-to-electrical conversion efficiency of 1.25 %. The electron– hole pair recombination is reduced by using a buffer layer of PANi which minimizes series resistance at the interface by lowering energy barrier for the carrier. Hence, redox behavior of ionomer gel enhances photovoltaic performance because of excellent adhesion and good film forming ability at the interface with its appropriate band level.

SUPPORTING INFORMATION: The supporting information is available with the manuscript and explains in detail the developmental pathway of device fabrication.

AKNOWLEDGEMENTS The author (Sunil Kumar) would like to thank for award of Senior Research Fellowship of UGC, New Delhi, India for providing financial support. Authors acknowledge CIFC, IIT (BHU) for providing SEM, AFM, TEM and NMR for characterization of materials.


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Table of Content

Functionalized Thermoplast Polyurethane as Hole Conductor for Quantum Dot Sensitized Solar Cell

Sunil Kumar1, Ishwar Chandra Maurya2, Om Prakash1, Pankaj Srivastava2, Santanu Das3 and Pralay Maiti1,

*


Functionalized Thermoplastic Polyurethane as Hole Conductor for

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