Synthesis, characterization, and performance of oligothiophene cyanoacrylic acid derivatives for solar cell applications

New dye sensitizers based on an oligothiophene cyanoacrylic acid derivative were synthesized and characterized for solar cell applications. The structures of the new dyes prepared as sensitizers based on oligothiophenes, namely5,5''-di-2-cyanoacrylic acid [2,2':5',2''-terthiophene] (dye1), [2,2':5',2''-terthiophene]-5-cyanoacrylic acid(dye2), and [2,2':5',2'':5'',2'''-quaterthiophene]-5-cyanoacrylic acid(dye3) were confirmed by elemental analysis, mass spectrometry, and 1 H-NMR spectral data. The P3HT/dye2/nc-TiO 2 solar cell produced the highest efficiency of 0.05% with an open circuit voltage of 0.65V compared to dyes 1 and 3 solar cells. That may have been attributed to the dyes’ molecular structure, which had different chain lengths and numbers of groups of cyanoacrylic connected to the dyes’ thiophene moiety The dark current suppressed in the P3HT/dye2/nc-TiO 2 solar cells indicated the formation of the charge blocking layer, which produced an enhanced open-circuit voltage accompanied by a high onset voltage.


Introduction
*Hybrid photovoltaic cells, which are based on junctions between inorganic Nano-crystalline materials, hole transport layers, and organic dyes, so-called dye-sensitized solar cells(DSSCs) have attracted great attention. DSSCs have the potential for low-cost fabrication and ease of production, which makes them good candidates for commercialization (Baxter, 2012). Zinc porphyrin sensitizer SM371 with carboxylate ligand provides PSEs of 12.5-13.0% when employing a Co (II/III) redox system as an electrolyte (Yella et al., 2011) and efficiencies of 15% perovskite solid-state DSSC (Burschka et al., 2013). Ru complexes with carboxylate bipyridine ligands were first used to sensitize TiO2 single crystals (O'Regan and Grätzel, 1991). Nazeeruddin et al. (1993) prepared a series of mononuclear Ru-complexes, a thiocyanate derivative, cis-(SCN2) bis (2, 2'-bipyridyl-4,4'dicarboxyato) ruthenium (II), coded as N3, exhibited 10% efficiency (Nazeeruddin et al., 1993). Metal-free organic photosensitizers have significant advantages over noble Ru-complex sensitizers due to their low production cost, synthetic facial methodology for diverse molecular structures, and higher extinction coefficients (Selopal et al., 2016). Despite these properties, to date, the performance of DSSCs based on metal-free dyes has lagged behind those with metal-organic dyes as a result of their high recombination losses and lower open-circuit voltage (Voc). A few recent metal-free oligothiophene-based dyes in combination with (I -/I3 -) redox systems showed this approach's potential, as these systems have excellent efficiencies of up to 9.8% (Fig. 1).

Fabrication of the polymer solar cells
In this investigation, the solar cells were fabricated with SnO2 to produce F/nc-TiO2/dye/P3HT/AU solar cells. Before fabricating the solar cells, SnO2: F was cleaned using Decon 90 and rinsed in ultrapure water, hot water, and ethanol, and dried for 5 min under a nitrogen stream. Using the doctor blade technique (Al-Dmour, 2014), the electron transporting nc-TiO2 layers were coated onto SnO2: Felectrodes from a paste of nc-TiO2Solaronix. After drying at 100°C for 15 min, the film was heated to 450°C for 30min and then slowly cooled to room temperature. The nc-TiO2 was typically 2μm thick as determined by a scanning electronic microscope. The dye/nc-TiO2/SnO2: F film was prepared by soaking SnO2/nc-TiO2 electrodes in a dye solution at room temperature for 12h to absorb the dye. In the second step, the substrate was removed, rinsed in ethanol, and dried under a nitrogen flow for 3m. Hole-transporting P3HT was prepared by dissolving 0.03g of P3HT in 2ml of chloroform (Sigma-Aldrich), yielding a concentration of 1% w/w. The P3HT solution was spin-coated onto the SnO2: F/nc-TiO2/dye at a spin speed of 1000 rpm for 60 a. AU electrodes were vacuum deposited on top of the P3HT at pressures below 3×10 -6 Torr to obtain an active area of 0.03cm 2 (Fig. 3). Current-voltage (I-V) characteristics were obtained using a Keithley model 307 source measurement unit. A xenon lamp was used to illuminate the devices through the transparent electrodes, with the light (incident power of 100mW/cm -2 ) constrained by an aperture to fall on areas coinciding with one of the top gold electrodes.

Effect of dye structure on solar cells performance
The dye structure plays an important role in improving the efficiency of metal oxide-organic solar cells (Saleh et al., 2015) since the dye enhances charge separation that occurs at the interfaces between the holes and electron transport layers. Three solar cells were constructed using synthesized dye: Dyes 1, 2, and 3. These dyes were individually inserted between the nc-TiO2 and P3HT films to modify the properties of interfacial areas between the films. The structures of the devices became SnO2: F/nc-TiO2/dye/P3HT solar cells. The main parameters of new solar cells were short circuit current density (Jsc), open-circuit voltage (Voc), maximum output power (Pmax), fill factor (FF), and power conversion efficiency (ηe) extracted from the J-V curves. These properties are summarized in Table 1. The devices' J-V characteristics were measured under illumination (100mA/cm 2 =Pl) as light power as shown in Fig. 8. The short circuit current density (Jsc) increased from 0.11 mA/cm 2 in dye 1 to 0.16mA/cm 2 in dye 2 accompanied by an enhanced open-circuit voltage (Voc) from 0.40V to 0.65V. The fill factor (FF) and power conversion (η) values were calculated using Eqs. 1 and 2: (1)

= max
( 2) where stands for light intensity. In this study, the P3HT/dye2/nc-TiO2 had the best solar cell performance. It produced a fill factor (FF) of 47%and a power conversion efficiency (ηe) of 0.05%, which was twice as high as the other devices.
The P3HT/dye3/nc-TiO2 and P3HT/dye1/nc-TiO2 solar cells had a power conversion efficiency of 0.03% and 0.02%, respectively, that was attributed to the lower response to the light falling on these solar cells. Dye 3's absorption curves demonstrated that it absorbed more light than the other dyes. Fig. 9 shows the current density vs voltage (J-V) of typical solar cells fabricated using the three different dyes under dark conditions. The results showed that the rectification ratios ranged from 10 for dye 1 to 10 4 for dye 2 at ±1 V. As the forward bias voltage applied on the Au electrodes increased, the devices became conducting current at different voltage values. This value is called the onset forward conduction voltage, which decreased from 0.8V in dye 2's solar cells to 0.6 V dye 3's solar cells. Dye 1's solar cells did not show good diode behavior, with small rectifications less than 10. Dye 3'sdark current increased more rapidly than the other devices. Table 1 summarizes the photovoltaic output parameters of the solar cells studied in this work.   In this circuit, the organic solar cells' net current density (Jnet) is expressed as (Li et al., 2009) where, V is the bias voltage, Jph is the photocurrent, KB is the Boltzmann constant, T is the temperature, Js is the reverse dark saturation current density, and n is the ideality factor. Using the equation, V=Voc was obtained when a shunt resistor was larger than the series resistance andJ=0mA/cm 2 . The is described by: As indicated in Eq. 4, Js should be small to achieve a high open-circuit voltage. The is proportional to the Ln value (Jph/Js) when Jph<<Js. At these values, the Voc is small. In general, the dark current is suppressed by adding electrons and/or holeblocking layers at the interfacial layers (Wang et al., 2020).
That effectively weakens surface recombination at the donor and acceptor surfaces and improves the efficiency of electronics devices. In our results, the P3HT/dye 2/nc-TiO2 solar cells suppressed the dark current more than the other devices and behaved like a diode in the dark with high rectification. Consequently, dye 2's layer reduced the charge leakage at the interfaces and improved the photo-charge separation and transport, acting as a charge-blocking layer. That may have been due to dye 2's energy levels, which were suitable for the energy levels of P3HT and nc-TiO2. In principle, for efficient P3HT/dye/nc-TiO2 solar cells, during the regeneration of the sensitizers, the holes should move faster than the recombination of the conduction band electrons with oxidized sensitizers. The dyes' highest occupied molecular orbital (HOMO) was below the hole transporters' energy bands so that the oxidized dyes formed after electron injection into nc-TiO2's conduction band, which was effectively regenerated by accepting electrons from the hole transporters.
Dye 2 had high aggregation, demonstrating a low recombination probability. That was ascribed to better adhesion of dye 2's molecules on the surface of the TiO2 with P3HTaccompanied by dye 2's atoms easily penetrating through small pinholes distributed on the top of the nc-TiO2's surface. These observations resulted from the difference in the molecular weights of dyes 2 and 3. Using basic calculation methods, dye 2 had a lower molecular weight of 345g/mole that played a role in producing high power efficiency solar cells. Conversely, dye 3's molecular weight was 425g/mole, which exhibited high aggregation on the top of the nc-TiO2and did not easily penetrate the pinholes like dye 2's solar cells and produced low-efficiency dye 3 solar cells. Dye 1's solar cells hardly behaved like diodes in the dark,

Rs
with a small rectification and onset voltage. Therefore, interfacial layers were created between the P3HT and nc-TiO2that could not overcome the charge leakage and did not have enough energy to separate excitons. Dye 1 structure had two groups of cyanoacrylic acids connected to the terthiophene moiety. That caused an increase in the energy gaps between the lowest unoccupied molecular orbital(LUMO) and HOMO, suppressing the HOMO energy level far below the P3HTlayer'senergy level and increasing the energy level of dye1 excited state above the TiO2's conduction band. Dye 1 has high molecular weight caused aggregation layers between the nc-TiO2 and P3HT because the dye had difficulty spreading through the pinholes on the nc-TiO2film. That also affected the light absorption and produced a low light current density. According to the equivalent circuit shown in Fig. 10, the shunt resistance was low and produced a small opencircuit voltage with a low onset voltage.

Conclusion
To date, considerable research has been devoted to synthesizing new materials for use in organic/inorganic solar cells. The dyes' materials are considered one of the most important components of three-layer solar cells. In this study, three different dyes were synthesized and inserted between the holes and electron transport layers. Dye2's solar cells had the best high power conversion efficiency and short current density compared with the other dyes. It also suppressed the dark current more than other devices, behaving like diodes with high rectification in the dark. That was attributed to the difference in their molecular structures, which affected the interfacial areas' properties at the junctions.