Thin Films of PNDI(2HD)2T and PCPDTBT Polymers Deposited Using the Spin Coater Technique for Use in Solar Cells

Abstract

Conductive polymers play a crucial role in the advancement of modern technologies, particularly in the field of organic photovoltaics (OPVs). Due to advantages such as flexibility, low specific weight, ease of processing, and low production costs, polymeric materials present an attractive alternative to traditional photovoltaic materials. This study investigates the properties of a polymer blend composed of PCPDTBT (donor) and PNDI(2HD)2T (acceptor), used as the active layer in bulk heterojunction (BHJ) solar cells. The motivation behind this research was the search for a novel n-type polymer material with potentially better properties than the commonly used P(NDI2OD-T2). Comprehensive characterization of thin films made from the individual polymers and their blend was conducted using Fourier Transform Infrared Spectroscopy (FTIR), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Ultraviolet-Visible Spectroscopy (UV-Vis), four-point probe conductivity measurements, and photovoltaic testing. The prepared films were continuous, uniform, and exhibited low surface roughness (R_a < 2.5 nm). Spectroscopic analysis showed that the blend absorbs light in a broad range of the spectrum, with slight bathochromic shifts compared to individual polymers. Electrical measurements indicated that the blend’s conductivity (9.1 µS/cm) was lower than that of pure PCPDTBT but higher than that of PNDI(2HD)2T, with an optical band gap of 1.34 eV. Photovoltaic devices fabricated using the blend demonstrated an average power conversion efficiency (PCE) of 6.45%, with a short-circuit current of 14.37 mA/cm² and an open-circuit voltage of 0.89 V. These results confirm the feasibility of using PCPDTBT:PNDI(2HD)2T blends as active layers in BHJ solar cells and provide a promising direction for further optimization in terms of polymer ratio and processing conditions.

1. Introduction

Chemicals called polymers have been used in industry for many years. They play an extremely important role in the economy and in people’s daily lives. Thanks to their great number of advantages, such as low specific weight, high mechanical strength, durability, flexibility, easy processing, and low production cost, they are used as substitutes for substances such as metals, inorganic glass, wood, etc. Their most important feature is their possibility of physical and chemical modification, which translates into the possibility of obtaining the desired properties and producing special polymeric materials. Among the most common polymeric materials that are produced on a large scale (polyethylene, polyvinyl chloride, polystyrene, or polyethylene terephthalate), macromolecular compounds are now being obtained that have unique properties. Developments in the field of polymeric materials have resulted in the introduction of new compounds, which are conductive (electroactive) and photosensitive polymers. These are the materials that are now making a huge contribution to rapidly developing technologies of all sorts, including those under investigation for use as solar cells [1,2,3,4,5,6,7].

Polymer solar cells are used to convert solar energy into electricity. They are used to generate electricity in a greener and more sustainable way than traditional energy sources. Polymer solar cells, unlike other types of solar cells, use polymers as the active layer that converts solar energy into electricity. These polymers are cheaper and easier to manufacture than the materials used in other types of solar cells, such as silicon or tellurium. Polymer solar cells are also more flexible and can be produced in a variety of shapes and sizes, making them more versatile for practical applications. Among the parameters that characterize polymer solar cells are efficiency values, fill factor, open circuit voltage, and short circuit current. The values of these parameters depend on the type and chemical structure of the polymer, as well as the production and operating conditions of the cells. These solar cells are manufactured by depositing thin layers of polymers with p-type and n-type conductivity to a substrate, which is usually made of transparent plastic or glass. The polymer layer contains dopants that enable the conversion of solar energy into electricity [7,8,9,10,11,12]. Small progress had been made on OPVs until a new architecture called bulk heterojunction (BHJ) was developed. In this case, instead of depositing layer by layer, the donor and acceptor materials are mixed, creating an amorphous structure to serve as the active layer [12,13]. Initially, materials already known from OLEDs, such as P3HT (Poly(3-hexylthiophene-2,5-diyl)), were used. However, such solutions did not allow for the attainment of solar cells exceeding 5% efficiency [6,7,8]. Another breakthrough in obtaining fullerene-free polymer solar cells was the use of the PTB7 (Poly [[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl ]]) polymer. The use of this polymer made it possible to obtain photovoltaic structures with efficiency above 7% [13]. The development of push–pull conjugated polymers featuring repeated donor and acceptor units aimed to address the limitations observed in polymers like P3HT. These polymers exhibit absorption spectra spanning from the deep ultraviolet through the visible and into the near-infrared regions [14,15]. One of the interesting push–pull conjugated polymers is PCPDTBT [(Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]], with HOMO = −5.3 eV and LUMO = −3.6 eV. It has repeating donor units of 4H-cyclopenta dithiopene and acceptor units of benzothiadiazole. It can be seen in the literature that when the push–pull polymer PCPDTBT is doped with regioregular polythiophenes P3HT in a blend, an increase in power conversion efficiency is observed [16,17,18]. Furthermore, among the array of n-type polymers, copolymers based on naphthalenediimide (NDI) have emerged as the most successful polymer acceptors. This is attributed to their outstanding thermal and oxidative stabilities, the high electron affinities of the NDI core, and their remarkable electron-transporting properties. Encouraging outcomes, yielding power conversion efficiencies (PCEs) of up to 5.7%, have been attained in all-polymer solar cells (all-PSCs) based on P(NDI2OD-T2) (Poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) [19,20]. This was accomplished through blending with a range of donor polymers, such as P3HT, PTB7, PTQ1 (Poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl]), BFS4, NT, PPDT2FBT, and PTB7-Th [13,14,19,20,21]. Additionally, the use of fullerene derivatives, such as PCBM, as acceptors in OPV systems has historically resulted in high efficiencies due to their excellent electron mobility and favorable morphology formation. However, recent advances in polymer chemistry have highlighted the advantages of non-fullerene acceptors (NFAs), including PNDI-based polymers, which offer improved thermal stability, tunable energy levels, and broader light absorption. While the incorporation of fullerene into PCPDTBT:PNDI(2HD)2T blends could potentially enhance charge separation and transport, it may also introduce issues related to morphological instability and higher material costs. Therefore, further comparative studies would be necessary to evaluate the trade-offs between performance and production feasibility in fullerene-based versus all-polymer systems [22,23,24].

This article presents the test results of the prepared PNDI(2HD)2T:PCPDTBT polymer blend. The PNDI(2HD)2T ([Poly{[N,N′-bis(2-hexyldecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) polymer could be a better n-type material for use in all-PSCs than the well-known polymer P(NDI2OD-T2) as indicated by current research [20,21], with HOMO = −5.8 eV and LUMO = −3.82 eV. In this work, we present a comprehensive study of the PNDI(2HD)2T:PCPDTBT polymer blend as an active layer in all-polymer solar cells. To our knowledge, this combination has not been thoroughly investigated, and our results provide new insights into the morphology, optoelectronic properties, and photovoltaic performance of this blend. The novelty of our research lies in its demonstrating the potential of PNDI(2HD)2T as a superior alternative to P(NDI2OD-T2) in all-PSC architectures, aiming to address the trade-offs between efficiency, cost, and stability.

2. Materials and Methods

The preparation of solar cells from PCPDTBT and PNDI(2HD)2T compounds required prior preparation for further processing. It was decided to use chlorobenzene to dissolve both substances. The polymers used in this study were purchased from Ossila Ltd., Sheffield, UK. According to the manufacturer, the number-average molecular weight (Mn) and weight-average molecular weight (Mw) for PCPDTBT are approximately 35,000 g/mol and 70,000 g/mol, respectively. For PNDI(2HD)2T, Mn is approximately 25,000 g/mol and Mw is about 55,000 g/mol. The compounds were dissolved at a ratio of 10 mg and 5 mg of compound, respectively, per 1 mL of solution. This corresponds to a 2:1 mass ratio of donor to acceptor in the active layer formulation. This blend ratio was selected based on previously reported literature that showed efficient phase separation and balanced charge transport in similar systems [25]. The prepared blend was heated to 80 °C and stirred with a magnetic stirrer for 8 h. Three different samples were prepared. The first with the PCPDTBT compound deposited, the second with PNDI(2HD)2T, and the third with a blend of both substances. The solutions were centrifuged at 3000 rpm.

To produce solar cells, all the thin films were deposited on special substrates (S211 Ossila glass). First, we spin coat the PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) at 5000 rpm for 30 s to produce a film thickness of about 30–40 nm. A short drying process of 5 min at 120 °C was used to remove any residual solvent. Then, the PCPDTBT:PNDI(2HD)2T blend was centrifuged at 3000 rpm for 15 s to produce the active layer. The layers were not additionally heated. The final stage of preparation was the depositing of the silver contacts via magnetron sputtering method, allowing the samples to be tested. A multi-electrode cathode deposition mask was used to create 8 individual pixels.

Each prepared sample was examined using FTIR, AFM, SEM, and UV-Vis methods. The deposited layers were examined using a Thermo Fisher Nicolet 6300 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The test was performed using a spectrometer equipped with an ATR (attenuated total reflectance) using the attenuated phenomenon total reflection of IR radiation. The surface morphology of the samples was evaluated using a scanning electron microscope (SEM) (Zeiss AG, Oberkochen, Germany). The images were taken with a Zeiss Supra 35 camera (Zeiss AG, Oberkochen, Germany)., and the EHT voltage was 3 kV. The 2D and 3D topographic images of 2 × 2 µm were taken using a Park Systems XE-100 atomic force microscope (AFM) (Park Systems Corp., Suwon, Republic of Korea). Root mean square (R_q) roughness and average coefficient values (R_a) were determined using XEI software (Park Systems Corp., Suwon, Republic of Korea). The optical properties of the thin films were determined using a UV-Vis spectrophotometer from Thermo Fisher Scientific Company (Waltham, MA, USA), model Evolution 220. UV-Vis spectroscopy analysis was performed, and wavelength dependence of absorbance was obtained. The main feature of the absorption band in the ultraviolet is its maximum, which is related to the energy required for electron transitions (from the ground state to the excited state). The resistivity and conductivity of deposited polymer layer was measured by the four-point probe equipment (Ossila Ltd., Sheffield, UK). Using a four-point probe, the sheet resistance was measured, with the probes linearly arranged at equal intervals. This measurement involves current flowing between the two external probes, which, in turn, causes a voltage change between the two middle probes. The sheet resistance of the specimen is calculated by measuring the voltage change [19,20]. Sheet resistance of the specimen can be determined using the formula below (1):

$$R_S={\displaystyle \frac{\pi }{ln2}}\times {\displaystyle \frac{\Delta V}{I}}$$

(1)

where I is the given current and ΔV is the depression in voltage between the middle probes.

Then, correction factors are introduced, depending on the shape and size of the probes, by which the result of the above formula is multiplied. This places limits on the possible current paths through the specimen, which improves the accuracy of the measured value. Additionally, the sheet resistance can be used to calculate the resistivity of the specimen, provided its thickness is known (2):

$${\rho =R}_S\times t$$

(2)

where ρ is the resistivity and t is the thickness of the specimen.

The electrical properties of ready-made solar cells containing the tested polymers in their structure were examined using the Ossila Solar Cell Testing Kit (Ossila Ltd., Sheffield, UK), including both a source measure unit and an LED-based solar simulator operating under AM1.5G conditions at a standard light intensity of 100 mW/cm². This setup allows for consistent and reproducible measurements of photovoltaic parameters. The tested devices had the following architecture: ITO/PEDOT:PSS/active layer (PCPDTBT:PNDI(2HD)2T)/Ca/Al. The contacts were deposited using a magnetron sputtering process on a Kurt Lesker system (Kurt J. Lesker Company, Jefferson Hills, PA, USA). Magnetron sputtering was carried out in a high-vacuum chamber with a base pressure of ~5 × 10⁻⁶ mbar. The sputtering target for the calcium (Ca) layer was pure calcium (99.99% purity), while the aluminum (Al) layer was deposited using an aluminum target (99.99% purity). The deposition was performed at a constant power of 100 W for both layers. The deposition rate for calcium was approximately 0.3 Å/s, while for aluminum, it was approximately 0.5 Å/s. The process was conducted under an Ar atmosphere with a flow rate of 20 sccm, ensuring a sputtering pressure of 3 × 10⁻³ mbar. The substrates were heated to 100 °C during deposition to promote adhesion and uniformity.

3. Results and Discussion

FTIR spectra were recorded between 4000 and 400 cm⁻¹. The spectra of pristine single polymer materials and the spectrum of a mixture of these materials were recorded, and the results are summarized in Figure 1. Both pristine polymer materials have -CH₃ and -CH₂ groups in their structure, which was identified and confirmed by the occurrence of peaks within the wavenumbers of 2958 cm⁻¹ (stretching -CH₃), 2856, and 2925 cm⁻¹ (stretching -CH₂). In both cases, at 1505 cm⁻¹, a band from the stretching vibrations of benzene rings (C=C) was also recorded. In the spectrum of the PCPDTBT polymer, a band from the asymmetric stretching vibrations of the cyclopentadiene ring was recorded at 1400 cm⁻¹, consistent with the literature reports on this material [26]. In the IR spectrum of the PNDI(2HD)2T polymer, a band at 3080 cm⁻¹ was recorded, which comes from the stretching vibrations of bonds in the aromatic ring (C-H) in naphthalene units, which is in agreement with earlier findings [19]. The IR spectrum recorded from the blend of polymeric materials contains bands from both PCPDTBT and PNDI(2HD)2T polymers, indicating physical mixing without chemical alteration of the functional groups.

To obtain high-resolution images of the surface morphology, scanning electron microscopy (SEM) was performed at a magnification of 200,000×. Each of the three prepared samples—pristine PCPDTBT, pristine PNDI(2HD)2T, and their blend—was examined individually. As depicted in Figure 2, the surface of the PCPDTBT sample exhibits pronounced grain boundaries and a coarser texture compared to the other samples (Figure 3 and Figure 4). This observation aligns with previous studies reporting fiber-like structures in PCPDTBT films, indicative of its semicrystalline nature [27]. In contrast, the PNDI(2HD)2T sample displays a smoother surface with less distinct boundaries, suggesting a more amorphous morphology. This is consistent with the literature findings wherein PNDI-based polymers exhibit lower degrees of crystallinity, resulting in more uniform film morphologies [28]. The blend sample demonstrates intermediate characteristics, with a relatively uniform hue and moderately defined boundaries, reflecting a combination of the morphological features of the individual polymers. All deposited layers are continuous and smooth, exhibiting low roughness without significant precipitates, inclusions, or impurities.

Topographic images were taken on a 2 × 2 µm surface using AFM (Figure 5). It can be observed that the samples differ significantly from each other. The main difference is the supramolecular organization, which, in the case of PNDI(2HD)2T, is in a much higher density, while forming a planar structure [29]. PCPDTBT, on the other hand, has large, unevenly distributed supramolecular domains [27]. The combination of the two compounds formed a structure whose surface is more similar to PCPDTBT. PCPDTBT has almost four times the divergence in terms of surface shape. The amplitude of the disparity for this polymer ranges from −8 nm to 8 nm. This means that it is less flat than PNDI(2HD)2T, whose histogram oscillates between −2.5 and 2.5 nm (Figure 6). The surface irregularity of this order is insignificant. In contrast, the blend of the two substances shows that the addition of PNDI(2HD)2T improves the surface shape of PCPDTBT and the histogram oscillates from −4 to 4 nm. The roughness parameters were summarized in

Table 1. Roughness properties of the PCPDTBT, PNDI(2HD)2T, and PCPDTBT:PNDI(2HD)2T blend samples.

Sample	Rq [nm]	Ra [nm]	Max. Irregularity [nm]
PCPDTBT	2.88	2.28	9.80
PNDI(2HD)2T	1.04	0.82	9.66
Blend	2.22	1.76	9.91

. The PNDI(2HD)2T sample has a much lower roughness, with R_q and R_a parameters equal to 1.04 and 0.82 nm. In contrast, the PCPDTBT sample has a higher roughness, which is 2.88 and 1.76 nm for R_q and R_a, respectively. The combination of substances has R_q and R_a roughness parameters of 2.22 and 1.76 nm.

UV-Vis analysis (Figure 7) showed that the substances exhibit absorbance in similar bands of the light spectrum. PNDI(2HD)2T has three absorption maxima of 250, 390, and 690 nm, respectively. For PCPDTBT, the first maximum is at a similar location and is also at 250 nm, but the other two are hypsochromically shifted, i.e., toward shorter wavelengths and equal to 430 and 780 nm. The blend of substances has its maxima at 250, 410, and 740 nm. The compound PNDI(2HD)2T has a much higher absorbance with respect to the other samples, which may indicate a greater layer thickness. The thicknesses of the films presented in Figure 7 were not strictly controlled to be identical, which may contribute to the differences in absolute absorbance values observed in the spectra. The higher absorbance of the PNDI(2HD)2T sample compared to the donor material PCPDTBT, and the blend suggests that the PNDI(2HD)2T layer was thicker, likely due to differences in solution viscosity and film formation behavior during spin coating under the same processing conditions. Furthermore, the weaker visibility of PCPDTBT absorption features in the blend spectrum is likely a combined effect of (1) lower concentration of PCPDTBT in the blend (2:1 donor–acceptor mass ratio); and (2) overlapping absorption bands between the two polymers, especially around 250 nm and 400–450 nm, and potentially a lower film thickness of the donor component within the blend, as the acceptor dominates both spectrally and morphologically.

The surface resistance of the deposited polymer thin films was measured (

Table 2. Electrical and optical properties of the PCPDTBT, PNDI(2HD)2T, and PCPDTBT:PNDI(2HD)2T blend samples.

No	Sheet Resistance [Ω/☐]	Resistivity [kΩ·m]	Conductivity [µS/cm]	Egopt [eV]
PCPDTBT	2.93 × 108	0.29	34.3	1.32
PNDI(2HD)2T	1.59 × 1010	15.87	0.63	1.37
Blend	1.1 × 109	1.1	9.1	1.34

). These measurements allowed for the calculation of the resistivity and conductivity of the deposited polymer thin films and their blend. The PCPDTBT polymer exhibited superior conductivity compared to the PNDI(2HD)2T polymer, with a conductivity of 34.3 µS/cm for PCPDTBT and only 0.63 µS/cm for PNDI(2HD)2T. This difference in conductivity can be attributed to the smaller energy gap (Eg_opt) of the PCPDTBT polymer (1.32 eV) compared to the larger gap in PNDI(2HD)2T (1.37 eV), which indicates better charge transport properties for PCPDTBT [27]. Interestingly, the blend of the two polymers showed a conductivity of 9.1 µS/cm, which is lower than that of pure PCPDTBT (34.3 µS/cm), but still significantly higher than the conductivity of PNDI(2HD)2T (0.63 µS/cm). This suggests that the blend maintains some of the conductive properties of PCPDTBT, although the combination of the two polymers results in a decrease in overall conductivity. The observed decrease in conductivity may be related to the disruption of the more ordered, conductive structures in PCPDTBT when blended with PNDI(2HD)2T, which has a more disordered structure. These results are consistent with findings in the literature, where it has been shown that polymer blends often exhibit intermediate properties between those of the individual components, influenced by the balance between their molecular organization and intermolecular interactions [29]. Additionally, the energy gap (Eg_opt) values from the absorption spectra (Figure 8) corroborate this observation. The smaller energy gap of the PCPDTBT polymer allows for easier electron movement, facilitating better conductivity. The Eg_opt value for the blend (1.34 eV) is closer to that of PCPDTBT, indicating that the energy gap is reduced compared to PNDI(2HD)2T, further supporting the idea that the blend has a mixed nature in terms of its electrical properties.

Polymer solar cells were fabricated on Ossila S211 substrates, which allow for the measurement of eight individual photovoltaic structures per substrate. A total of four substrates were used, yielding thirty-two individual measurements. The tested devices followed the architecture: ITO/PEDOT:PSS/active layer (PCPDTBT:PNDI(2HD)2T)/Ca/Al. This configuration is commonly used for polymer solar cells and ensures effective charge transport and good device performance [27]. The short circuit current density (J_SC) for a typical device was 14.37 mA·cm⁻², the open-circuit voltage (V_OC) was 0.89 V, and the fill factor (FF) was 0.51 (Figure 9). These parameters reflect good overall device performance, with the efficiency of the device averaging 6.45% (

Table 3. Photovoltaic performance parameters of PCPDTBT:PNDI(2HD)2T solar cells (average values ± standard deviation; N = 32).

Parameter	Average Value ± SD	Unit
Short-circuit current density (JSC)	14.37 ± 0.52	mA·cm⁻2
Open-circuit voltage (VOC)	0.89 ± 0.03	V
Fill factor (FF)	0.51 ± 0.02	–
Power conversion efficiency (PCE)	6.45 ± 0.21	%

Note: Results based on thirty-two devices (four substrates, eight pixels each). Measurements performed under simulated AM1.5G illumination using the Ossila Solar Cell Testing Kit.

). This performance is in line with the results obtained for similar polymer blends in previous studies, which showed comparable efficiency values for blends of PCPDTBT with various n-type materials [27]. Table 3 shows the average values of the key photovoltaic parameters and their standard deviations, which demonstrate the consistency of the results across the 32 measurements. The results are quite promising, considering that the power conversion efficiency (PCE) reached 6.45% with a relatively low standard deviation of 0.21%. These values indicate that the PCPDTBT:PNDI(2HD)2T blend has good potential for practical photovoltaic applications.

4. Conclusions

This study explored the development and evaluation of a new donor–acceptor polymer blend composed of PCPDTBT and PNDI(2HD)2T for use as the active layer in bulk heterojunction (BHJ) organic solar cells. The main objective was to investigate whether PNDI(2HD)2T could serve as a more effective n-type material than the widely used P(NDI2OD-T2), while maintaining good compatibility and optoelectronic performance with the donor polymer PCPDTBT. A combination of spectroscopic and microscopic techniques, namely, FTIR, AFM, SEM, and UV-Vis spectroscopy, along with four-point probe conductivity measurements and photovoltaic testing, was employed to fully characterize the blend’s structural, optical, and electrical properties. The FTIR analysis confirmed the physical mixing of the two polymers without chemical interaction. AFM and SEM studies revealed smooth and continuous thin films with low surface roughness, particularly for the PNDI(2HD)2T component. The blend film presented an intermediate morphology with improved uniformity compared to pure PCPDTBT. Optical measurements showed strong absorption across the visible spectrum, with the blend exhibiting an optical band gap of 1.34 eV. Electrical conductivity analysis demonstrated that the blend’s conductivity (9.1 µS/cm) was lower than that of pristine PCPDTBT (34.3 µS/cm) but significantly higher than PNDI(2HD)2T alone (0.63 µS/cm), suggesting partial retention of conductive pathways from the donor polymer. Solar cells fabricated using the PCPDTBT:PNDI(2HD)2T blend exhibited good photovoltaic performance, achieving a power conversion efficiency (PCE) of 6.45%, with an average short-circuit current density of 14.37 mA/cm², an open-circuit voltage of 0.89 V, and a fill factor of 0.51. These results are on par with or exceed those of comparable fullerene-free all-polymer systems reported in the literature. In conclusion, the PCPDTBT:PNDI(2HD)2T blend demonstrates strong potential as an active layer material for efficient, stable, and scalable all-polymer solar cells. The blend combines favorable film-forming properties, complementary absorption characteristics, and promising electrical performance. Future work should focus on optimizing processing parameters and blend composition to further enhance device efficiency and stability. The use of PNDI(2HD)2T as an alternative n-type acceptor opens new avenues for improving the performance and cost-effectiveness of organic photovoltaic technologies.