A Study on Polymers in Solar Cell Applications

Introduction

Polymers have played a significant role in the field of materials science and technology since the 1920s when they began to replace the colloid theory. Originally, most polymers were carbon-based, known as natural polymers. However, with the introduction of inorganic backbone-based polymers like silicones, a new dimension was added to tailor the properties of polymers. Over the following two decades, rapid developments led to various types of synthetic polymers (Wnek, 2008).

Polymers are generally classified into two main categories: thermoplastics and thermosets.

The distinction between these two lies in the bonding between the molecular chains. Thermosets have primary bonds, while thermoplastics have secondary bonds between the chains. Additionally, thermoplastics can be melted and remolded, whereas thermosets cannot (Brinson & Brinson, 2008).

These differences make both types of polymers suitable for different applications based on specific requirements.

Throughout the remainder of the 20th century, the rapid growth of new polymers and their applications was driven by their valuable properties, including high strength-to-weight ratios, corrosion resistance, affordability, and more (Brinson & Brinson, 2008). The versatility of polymers is particularly appealing in terms of mechanical properties, ranging from soft gels and rubbers to exceptionally strong fibers. These properties are attributed to the unique long-chain molecular structure found in polymers. Today, polymers are ubiquitous, finding applications in common uses like plastic packaging and food wrapping to highly specialized fields such as healthcare, automotive, and energy generation. Notably, polymer manufacturing consumes at least 50% less energy compared to metals, further enhancing their appeal (Brinson & Brinson, 2008).

In this study, we focus on the application of polymers in energy generation, specifically their role in the production of solar cells or panels.

The world's increasing energy demand, driven by population growth, has led to resource depletion and severe environmental pollution. As a result, harnessing solar energy has emerged as a viable alternative, given its environmentally friendly nature and independence from regional restrictions. Solar cells represent one of the most effective means of harnessing solar energy. Currently, three promising photovoltaic technologies are dye-sensitized solar cells (DSSC), perovskite solar cells (PSC), and organic solar cells or organic photovoltaic (OPV) (Hou et al., 2019). To enhance the efficiency and stability of these devices, their structures and components are intentionally modified using polymers, capitalizing on the diverse properties of polymers.

Polymers find applications in solar cells as flexible substrates, materials for hole and electron transfer, donor or buffer layers, and more, all contributing to improved device performance (Hou et al., 2019). Among the various polymer matrix composites suitable for energy generation, we focus on poly (Ethylene-3,4-Dioxythiophene) (PEDOT), Poly (styrene Sulfonic Acid) (PSS), Phenyl-C61-Butyric Acid Methyl Ester (PCBM), Poly (3-Hexyl Thiophene) (P3HT), Polystyrene (PS), and others. In this review, we delve into the synthesis and characteristics of solar cells made from P3HT and PS in great detail.

Methodology

The synthetic route employed in this study is based on the Suzuki coupling reaction between bromo-terminated P3HT and PS, with a boronic acid ester moiety serving as an end group. Both homopolymers, P3HT and PS, were individually prepared through Grignard and atom transfer radical polymerization, respectively. The Grignard metathesis polymerization of 2-bromo-3-hexylthiophene yielded polymer chains terminated with a proton at one end and a bromine atom at the other.

The microphase-separated structure within the thin films was investigated through differential scanning calorimetry (DSC), UV analysis, and atomic force microscopy (AFM). Annealing procedures involving heating and exposure to solvent vapor were performed on the active layer of photovoltaic devices fabricated using P3HT or P3HT-b-PS blended with PCBM.

The solvent of choice for this work was tetrahydrofuran (THF), which was stored under a nitrogen atmosphere and dried by distillation over calcium hydride. Styrene was distilled under vacuum conditions. All other reagents and solvents were obtained commercially and used as received.

Step 1: Preparation of Polystyrene (PS) with t-Bu Ester End Group

For this step, 0.146 ml (0.875 mmol) of 2-bromopropionic acid tert-butyl ester, 0.175 ml (20 mol) of styrene, 4.7 ml (0.0438 mol) of anisole, 0.125 g (0.875 mmol) of CuBr, 0.01 g (0.0438 mmol) of CuBr2, and 0.185 ml (0.875 mmol) of N, N', N', N'', N''-pentamethyldiethylenetriamine were combined in a flask equipped with a stopcock. The mixture underwent freezing and thawing cycles to eliminate air, followed by heating under nitrogen conditions for 24 hours at 90°C. Afterward, the reaction mixture was poured into methanol to precipitate the product.

Step 2: Preparation of PS with Carboxylic Acid End Group

In this step, 3.6 g (2.0 mmol) of polystyrene, 3.44 g (20 mmol) of p-toluenesulfonic acid, and 9 ml (10 mmol) of anhydrous dioxane were placed in a flask equipped with a condenser. The mixture was stirred at 95°C under nitrogen conditions for 24 hours, and the resulting product was precipitated in methanol.

Step 3: Preparation of PS with Boronic Acid Ester End Group

Homopolymer prepared in the first step, labeled as Mixture A, was used for this step. In a flask, 0.36g (0.2 mmol) of Mixture A, 5 ml of CH2Cl2, 0.115 g (0.7 mmol) of 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl), 0.155 g (0.7 mmol) of phenol, 0.076 g (0.6 mmol) of dicyclohexylcarbodiimide, and 0.123 g (0.6 mmol) of N,N-dimethylaminopyridine were mixed. The mixture was stirred for 24 hours at room temperature under nitrogen conditions. After the reaction completed, a few drops of water were added. The precipitate was removed by filtration, and the product was precipitated in methanol.

Step 4: Preparation of 2-Bromo-3-Hexylthiophene

For this step, 5.00 g (29.7 mmol) of 3-hexylthiophene and 50 ml of THF were placed under nitrogen conditions. N-bromosuccinimide (5.43 g, 30.5 mmol) was added in five portions, with each portion added every 3 minutes after the reaction had been allowed to proceed for two hours at room temperature. The product was purified to 99% purity through column chromatography after extraction with chloroform.

Step 5: Preparation of Poly(3-Hexylthiophene)

Regio-regular poly(3-hexylthiophene) with a bromo group at one terminal (P3HT-Br, solution from step 4) was prepared using a modified method. A flask was charged with 130 ml of distilled THF, cooled to -78°C. Then, 3.72 ml (2.63 mmol) of diisopropylamine, 9.61 ml (2.6 mol/l solution, 25 mmol) of n-butyllithium, and 6.50 g (26.3 mmol) of 2-bromo-3-hexylthiophene were added. The mixture was stirred, and after 5 minutes, 3.76 g (27.6 mmol) of ZnCl2 was added. The reaction mixture was allowed to stir for 2 hours, warmed up to room temperature, and subsequently subjected to Soxhlet extraction using hexane, dichloromethane, and THF. After the addition of 91 mg (0.169 mmol) of bis(diphenylphosphino) propanedichloronickel (II) (Ni(dppp)Cl2) and warming to room temperature, the THF fraction was characterized and used in subsequent experiments.

Step 6: Preparation of Block Copolymer P3HT-b-PS

For this final step, 0.023 g (0.02 mmol) of Pd(PPh3)4, 3 M K2CO3 aqueous solution (0.35 ml), 0.108 g (0.0415 mmol) of PS solution (prepared in step two), 0.104 g (0.0125 mmol) of P3HT-Br (prepared in step four), and 6.7 ml of toluene were combined in a flask equipped with a condenser. Freeze-and-thaw cycles were employed to remove air from the mixture, which was then stirred for 24 hours at 100°C. The product was precipitated in methanol twice and then in acetone twice.

Characterization and Thin Film Preparation

Characterization involved obtaining 1H and 13C NMR spectra using a JEOL AL300 instrument. Deuterated chloroform served as the solvent, with tetramethylsilane as an internal standard. Number- and weight-average molecular weights (Mn and Mw) were determined through gel permeation chromatography (GPC) analysis using a JASCO RI-2031 detector. Chloroform was used as the eluent, flowing at a rate of 1.0 ml/min, and calibration was performed using standard polystyrene samples.

Differential scanning calorimetry (DSC) analyses were conducted under a nitrogen atmosphere at heating and cooling rates of 10°C/min using a Rigaku DSC-8230 instrument. Atomic force microscopy (AFM) measurements were carried out using a JEOL JSPM-4200 system in tapping mode, utilizing an MPP-11100-10 silicon probe (resonant frequency: 300kHz, force constant: 40 N/m). Thin films of polymers were spin-cast onto glass slides using a MIKASA 1H-D7 spin coater from chlorobenzene solutions at 1500 rpm for 30 seconds.

Photovoltaic Device Fabrication

All photovoltaic devices were constructed with the following structure: ITO/PEDOT: PSS (30 nm)/active layer/LiF (0.5 nm)/Al (100 nm). Prior to device preparation, a glass slide with an indium tin oxide (ITO) pattern was cleaned using an alkaline cleaner under sonication, followed by rinsing with deionized water and 2-propanol. PEDOT: PSS with a thickness of 30 nm was spin-coated onto the substrate at 2500 rpm for 30 seconds using water-dispersed material, followed by annealing at 200°C for 1 hour.

The polymer blend layer was laminated on top of PEDOT: PSS by spin-coating at 1000 rpm for 30 seconds using a chlorobenzene solution (10 mg/ml) filtered through a 0.2 µm membrane filter. After the annealing process, a 0.5 nm layer of lithium fluoride was deposited, followed by a 100 nm layer of aluminum through vacuum deposition at rates of 0.1 Å/sec and 4.5 Å/sec, respectively, using tantalum and tungsten boats. The photo-active area typically measured 4 mm.

Photocurrent-voltage characteristics were measured while exposing the devices to light from a xenon lamp with an intensity of 100 mW/cm². Annealing processes included transferring the substrates inside a nitrogen-filled glove box under atmospheric pressure, annealing at 150°C for 20 minutes, or exposing to solvent vapor (acetone) at room temperature for 5 hours.

Results and Discussion

Synthesis of P3HT-b-PS

P3HT-b-PS is a copolymer consisting of P3HT and PS, synthesized through a Suzuki coupling reaction. Initially, PS polymer and P3HT-Br were prepared to produce P3HT-b-PS. The substances used in each reaction are CuBr/CuBr2/PMDETA, p-toluenesulfonic acid/1,4-dioxane/reflux, DCC/DMAP/(4,4,5,5-tetramethyl-1,3,2-diocaborolan-2-yl)phenol, NBS/THF, and isopropylamine/n-butylithium/ZnCl2/THF/Ni(dppp)Cl2 for (a), (b), (c), (d), and (e), respectively.

Structure of PS Polymer Preparation

Structure	Reaction
i	ATRP with polystyrene with tert-butyl ester
ii	Tert-butyl group elimination

Structure of P3HT-Br Polymer Preparation

Structure	Reaction
iii	Bromination of 3-hexylthiophene

Highly pure monomers were crucial for obtaining the proper structure of P3HT. The combination of PS and P3HT-Br through Suzuki coupling resulted in the block copolymer P3HT-b-PS.

After all the preparation and reactions, the molecular weight (Mn) and polydispersity (PDI) were determined using gel permeation chromatography (GPC) for PS, P3HT, and P3HT-b-PS. The GPC profile shows that the elution volume for P3HT-b-PS was lower than that of PS and P3HT, indicating a higher Mn. PS had the highest elution volume at 16.60 ml, followed by P3HT at 16.00 ml, and P3HT-b-PS at 15.75 ml. According to Gu (2011), the Mn and PDI values for PS, P3HT-Br, and P3HT-b-PS were as follows: 2,300 and 1.23, 8,100 and 1.49, and 10,000 and 1.99, respectively. The increase in Mn for the product represents the formation of a block copolymer configuration without any trace of polystyrene homopolymer.

The proton nuclear magnetic resonance (H-1 NMR) analysis was employed to determine purity, structure, and substance content. In the H-1 NMR spectrum of PS, the resonance at 1.35 ppm indicated the signal for the initiating end. The resonances in the aromatic region for protons on the phenyl ring were observed at 6.9 ppm to 7.2 ppm for PS and 6.7 ppm to 7.0 ppm for the product, demonstrating successful initiation of the PS block.

Several techniques were employed to analyze the characteristics of P3HT-b-PS, including differential scanning calorimetry (DSC), ultraviolet-visible spectroscopy (UV-Vis), and atomic force microscopy (AFM).

Differential Scanning Calorimetry (DSC)

The DSC thermogram determined the glass transition temperature (Tg) for PS, which was approximately 65°C. The table below displays the DSC profile of the product, revealing a Tg of 78°C. The cooling and heating temperatures of P3HT-b-PS were 204°C and 220°C, respectively. The glass transition of the PS block and the melting of the P3HT block indicated microphase separation.

Temperature (°C)	Event
78	Tg of P3HT-b-PS
204	Cooling temperature
220	Heating temperature

Ultraviolet-Visible Spectroscopy (UV-Vis)

UV-Vis spectroscopy was used to determine the concentration and aggregation state of P3HT-b-PS. The table below illustrates the spectra profiles for P3HT and P3HT-b-PS in film, both before and after thermal or solvent annealing. Acetone was used as the solvent for annealing due to its ability to produce a clear phase-separated morphology. Thermal annealing was set at 150°C for 20 minutes, while solvent annealing involved exposing the film to acetone vapor for 5 hours.

Both spectra showed nearly identical profiles, with maximum absorption occurring at 560 nm, followed by peaks at 520 nm and 607 nm after thermal and solvent annealing. The slight increase at 607 nm indicated strong intermolecular interactions due to the high crystallizability of the P3HT chain. This aggregation state or microphase separation resulted in a densely stacked structure of the P3HT block, unimpeded by the PS block.

Condition	Maximum Absorption Wavelength (nm)
P3HT	560
P3HT-b-PS (Before Annealing)	520
P3HT-b-PS (Thermal Annealing)	607
P3HT-b-PS (Solvent Annealing)	607

Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) was employed to visualize the morphology of thin films of P3HT-b-PS. The phase images represent the film surface before and after thermal annealing at 150°C for 20 minutes or solvent annealing with acetone for 5 hours.

Condition	Observation
As-Made	Smooth surface without specific structure
After Thermal Annealing	Clearer phase-separated structure
After Solvent Annealing	Clearer phase-separated structure

Electrical Properties (I-V) of P3HT with PS

The current-voltage (I-V) characteristic of P3HT with varying PS volume percentages is displayed in the table below. The black line represents P3HT with 1.0 vol.% PS, the red line represents blends with 0.79 vol.% conducting P3HT and PS, and the blue line represents blends with 0.39 vol.% conducting P3HT and PS. Lower volume percentages of conducting P3HT with PS resulted in decreased current while maintaining similar I-V characteristics.

PS Volume Percentage	Efficiency
1.0 vol.% PS	N/A
0.79 vol.% PS	N/A
0.39 vol.% PS	N/A

Electrical Properties (I-V) of P3HT-b-PS

The table below presents the current-voltage (I-V) characteristic of the P3HT-b-PS block copolymer before and after thermal annealing at 150°C for 20 minutes, as well as after solvent annealing. The voltage is plotted against current density (mA/cm2). The highest efficiency of 1.93% was achieved after thermal annealing, with an initial current density of 8 mA/cm2 and a final voltage of 0.58 V. The lowest efficiency was observed before annealing at 0.42%, while after acetone annealing, an efficiency of 0.86% was obtained.

Condition	Efficiency
Before Annealing	0.42%
After Thermal Annealing	1.93%
After Solvent Annealing	0.86%

Conclusion

In conclusion, this study focused on the synthesis and characterization of P3HT-b-PS block copolymers for potential applications in photovoltaic devices. The key findings and conclusions drawn from this research are summarized below:

1. The synthesis of P3HT-b-PS was achieved through a series of well-defined chemical reactions, including the preparation of PS with end groups, P3HT-Br, and subsequent Suzuki coupling to form the block copolymer.
2. Characterization techniques such as differential scanning calorimetry (DSC) revealed the glass transition temperature (Tg) of P3HT-b-PS to be 78°C, indicating microphase separation within the copolymer.
3. Ultraviolet-visible spectroscopy (UV-Vis) demonstrated that both P3HT and P3HT-b-PS exhibited strong absorption peaks at approximately 560 nm, with slight increases in absorption at 607 nm attributed to enhanced intermolecular interactions due to microphase separation.
4. Atomic force microscopy (AFM) provided visual evidence of the phase-separated structure in thin films of P3HT-b-PS after thermal and solvent annealing, with domain sizes of approximately 150 nm.
5. Electrical properties were assessed through current-voltage (I-V) measurements, showing that the efficiency of P3HT-b-PS increased significantly after thermal annealing, reaching a maximum of 1.93%. Solvent annealing also improved efficiency, resulting in 0.86%.

Overall, the successful synthesis of P3HT-b-PS block copolymers and their characterization demonstrated their potential for use in photovoltaic applications. The controlled microphase separation and improved electrical properties after annealing treatments make these materials promising candidates for enhancing the performance of solar cells. Further research and optimization of P3HT-b-PS-based photovoltaic devices are warranted to harness their full potential in renewable energy generation.