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Article

A Doublet State Palladium(I) N-Heterocyclic Carbene Complex as a Dopant and Stabilizer for Improved Photostability in Organic Solar Cells

by
Aliah El Astal-Quirós
1,2,3,
Valentina Carrarini
1,
Francesca Zarotti
1,
Atiq Ur Rahman
1,
Agustí Lledós
4,
Cristina G. Yebra
3,
Ernesto de Jesús
3 and
Andrea Reale
1,*
1
CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
2
DGTCSI-ISCTI (Direzione Generale per le Tecnologie delle Comunicazioni e la Sicurezza Informatica-Istituto Superiore delle Comunicazioni e delle Tecnologie Dell’informazione), Ministero dell’Impresa e del Made in Italy (MIMIt), Viale America, 201, 00144 Rome, Italy
3
Departamento de Química Orgánica y Química Inorgánica, Instituto de Investigación Química “Andrés M. del Rio”, Universidad de Alcalá, Campus Universitario, 28805 Alcala de Henares, Spain
4
Department de Química, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain
*
Author to whom correspondence should be addressed.
Energies 2024, 17(15), 3787; https://doi.org/10.3390/en17153787
Submission received: 11 July 2024 / Revised: 23 July 2024 / Accepted: 30 July 2024 / Published: 1 August 2024
(This article belongs to the Section A2: Solar Energy and Photovoltaic Systems)
Browse Figures
Versions Notes

Abstract

:
The effect of doublet state metalloradical complex in a solar cell inside the common active layer poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC60BM) is explored. In this work, it is demonstrated that the role of the bis-[1,3-bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(I) hexafluoridophosphate dopant, [Pd(IPrCl)2][PF6], is crucial because the presence of a stable unpaired electron in the molecule significantly improves the optoelectronic performance of the device. We f the optimal concentration of this molecule in the active layer and demonstrate that the presence of this additive in the active layer helps to significantly improve the morphology of the device. The improvements in optoelectronic and morphological parameters are associated with a remarkable increase in photocurrent generation due to more favorable mechanisms of charge separation at the donor/acceptor (D/A) interfaces of the active layer and reduced recombinations. Moreover, the presence of this additive improves the stability of the unencapsulated solar cell against photochemical degradation produced by sunlight.

Graphical Abstract

1. Introduction

1.1. Organic Photovoltaics (OPVs)

Since a few years ago, research in organic photovoltaic (OPV) solar cells is increasing because of the importance of renewable energy sources nowadays. This technology is interesting because, compared with the traditional materials used for solar cells, OPVs can be fabricated on flexible substrates, making them lightweight and potentially suitable for applications where traditional rigid solar panels are not practical [1]. Also, they can be produced using low-cost printing techniques, such as inkjet or roll-to-roll printing [2,3]. This allows for large-scale and potentially cost-effective manufacturing processes compared to the more complex and expensive methods used for silicon and perovskite solar cells. The materials used in OPVs are generally more abundant and less toxic than some of the materials used in silicon and perovskite solar cells [4,5]. This can have positive implications for both the environmental impact and resource availability. Organic solar cells offer greater versatility in terms of design. Organic solar cells offer greater versatility in terms of design [6,7]. The molecular structures of organic materials can be modified to optimize properties such as absorption spectra and charge transport. OPVs can be used in tandem with other types of solar cells, including silicon and perovskite cells, to create tandem solar cells [8]. Tandem structures have the potential to improve overall efficiency by capturing a broader spectrum of sunlight [9]. However, it is crucial to acknowledge some challenges associated with organic solar cells, including lower efficiency compared to silicon and perovskite cells, stability issues, and a shorter lifespan. Research is ongoing to address these challenges and improve the performance of organic solar cells. This paper discusses one method to stabilize and improve the efficiency of a classic inverted organic solar cell. The bulk heterojunction active layer (BHJ) used in this work is poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC60BM). This blend is commonly used in the field of organic solar cells due to its favorable properties, such as good charge transport and efficient light absorption [10]. This model system is often used for studying the effects of various parameters, including additives [11], on morphology optimization [12] and the control of recombination dynamics [13]. Nowadays, the PCEs of advanced OPVs have exceeded 19% for polymer/small molecular systems [14] and 18% for polymer/polymer systems [15].
The devices are fabricated according to the structure of inverted OSCs. In recent years, compared to the direct configuration, the inverted architecture has drawn academic interest in the field of organic solar cells because it can overcome issues such as device stability, the effect of the acidic and hygroscopic properties of PEDOT:PSS when in direct contact with ITO, the use of low-work function metals as top electrodes, and the tendency to oxidate when exposed to air and water vapor [16]. Moreover, the increase in vertical phase separation [17], improved lifetime, and compatibility with R2R fabrication processes [18] make the inverted design extremely promising for commercialization of OSCs [19]. All these features have led us to choose the use of the above-mentioned architecture in the present work.

1.2. Mechanism for Generation of Photocurrent in Organic Solar Cells (OSCs)

The photocurrent generation in OPVs is based on dissociation into free charges of Frenkel-type excitons [20]. Initially, the excited electron on LUMO (the lowest unoccupied molecular orbital) and the corresponding hole on HOMO (the highest occupied molecular orbital) resulting from the absorption of an incident photon are strongly attracted to each other due to the Coulomb force [21]. In order to generate photocurrent, it is mandatory that the exciton dissociate into free charges. For this reason, a critical aspect of the organic solar cells is the design and architecture of the active layer [20], typically a bulk heterojunction constituted by an electron–donor (D) material and an electron–acceptor (A) material [22]. Once the exciton is photogenerated in the donor material, it diffuses until its electron finds an energy level lower than the LUMO it belongs to, i.e., the LUMO of the acceptor material. Due to the energy gap between the two LUMOs, the exciton dissociates, and ideally the electron separates from the hole it has left behind in the HOMO of the donor material [23]. The two charges are free to move and can be transported and collected at the electrodes [24]. Nevertheless, the generation of photocurrent is not assured upon the absorption of a photon within the photoactive layer of an organic solar cell. Excitons are neutral systems, which are not influenced by an electrical field and move randomly. They need to reach the D/A interface and separate fast before the so-called geminate charge recombination (CR) occurs [20]. For this reason, the phase separation of donor/acceptor semiconductors in the BHJ has to be comparable to the exciton diffusion length L = (Dτ)1/2, where D is the coefficient of diffusion and τ is the lifetime of the exciton [25]. A trade-off is required between an efficient collection of photogenerated carriers without recombination (thin layers) and an efficient absorption of the light (thick layer) [26].

1.3. Effect of Singlet and Triplet States in Organic Solar Cells

As mentioned before, the interpenetrating system (BHJ), consisting of electron–donor and electron–acceptor materials mixed together, assures a level of morphology that facilitates the photogenerated excitons to dissociate into free electrons and holes at the donor/acceptor (D/A) interfaces [27].
Initially, the exciton undergoes a transition into a charge transfer (CT) state, with the electron located on the LUMO of the acceptor material and the hole located on the HOMO of the donor material, as shown in Figure 1, where D* refers to donor excited state. Subsequently, the CT state may either recombine back to the ground state if the process does not occur rapidly or dissociate into free charges through a series of charge-separated (CS) states [28,29]. The geminate recombination and the exciton dissociation compete with each other, and the prevalence of one or the other determines the performance of the organic solar cells. For instance, a decrease in photogenerated current and thus a decrease in efficiency of OSCs is expected if the rate of CR exceeds the rate of generation of free charges [30]. It must be clarified that, generally, due to the selection rule in the electronic dipole transition process, excitons directly formed by absorbing photons are singlets (spin character S = 0) [31]. For singlets, the diffusion length is around 10 nm [32]. This value, compared to the reasonable thickness of bulk heterojunction for efficient absorption of light (around 100 nm), can limit the probability of the exciton dissociation process and the PCE of organic solar cells [33]. A consolidated strategy to limit geminate recombination from CT related to a single state is the promotion in the active layer of triplet charge transfer (3CT) states, which are related to triplet states (S =1). The lowest energy triplet exciton, T1, is typically located a few tenths of an eV below S1 [27]. Triplet state excitons exhibit significantly longer lifetimes than singlet excitons [23]. Generally, the decay times for S1 and T1 in organic semiconductors are in the order of nanoseconds and microseconds, respectively. Therefore, intersystem crossing (consisting of a transition from single state to triplet state) allows S1 excitons to transition to lower energy triplet excited states (Tn) and then relax to the T1 exciton state. T1 can dissociate and form 3CT states, providing free charge separation. Alternatively, if the dissociation is not fast, 3CT states can recombine to T1. Moreover, the charge recombination from the triplet charge transfer state (3CT) to the primary triplet excited state (T1) of donor materials can be avoided if the energy level of 3CT is lower than that of T1 and the driving force related to triplet recombinations is higher than −0.1 eV [27,28]. These considerations justify the efforts made in recent years to design organic solar cells based on materials promoting triplet states and, in particular, the introduction of additives in the active layer with a reasonable chemical and optical compatibility with donors and acceptors [34].

1.4. Organometallic Complexes in OSCs

Recently, a series of studies have reported the electronic contribution of organometallic molecules in OPVs [35,36,37]. Several studies have reported the beneficial effect of organometallic complexes in triplet state in solar cells [38]. Triplet states do not participate in the charge separation of low-gap polymers; they can indirectly affect the efficiency of charge separation by providing direct charge recombination to the low triplet state [39]. The emission state is a triplet state, but the precise expression of the character of the triplet state has long been a hot issue under investigation [40,41]. First, the states dominated by singlets and triplets are coupled through fast intersystem crossing (50 fs) through their metal-to-ligand charge transfer components [42,43]. Second, the triplet state decay is collected, improving the conversion efficiency [44].
Recently, layer-by-layer all-polymer solar cells (APSCs) have seen improved power conversion efficiency (PCE) with the addition of a platinum complex (F-Pt) as an energy donor. Adding 0.2 wt% F-Pt to the PY-IT layer increases PCE from 15.86% to 17.14% due to efficient energy transfer [45]. Moreover, efficient polymer solar cells (PSCs) require optimal vertical phase distribution for exciton dissociation and charge transport. Incorporating 0.3 wt% m-Ir(CPmPB)3 triplet material into the PY-DT acceptor layer of layer-by-layer all-polymer solar cells (APSCs) with PM6 as the donor boosts power conversion efficiency (PCE) from 17.32% to 18.24%. This improvement results from enhanced exciton lifetime and diffusion, confirmed by increased photoluminescence (PL) intensity and prolonged PL lifetime. This method shows potential for more efficient LbL APSCs [34].
N-heterocyclic carbenes (NHCs) are very versatile ligands in transition-metal chemistry [46]. Their electronic and steric properties are unique for stabilizing a large variety of transition-metal complexes with notable applications in catalysis and materials science and have been extensively studied in the last three decades. Although some NHC metal complexes have emissive properties [47], palladium(II) emitters have typically low radiative recombination rates. In contrast, Zysman-Colman and co-workers reported highly luminescent d10 palladium(0) complexes of the formula [Pd(NHC)2] [48]. Nonetheless, practical applications of these palladium(0) complexes are limited by their easy oxidation under aerobic conditions [49]. In a previous work [50], some of us have shown that by starting with a Pd(0) complex (1), bis-[1,3-bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(0), but with a different formulation, bis-[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]palladium(I) hexafluoridophosphate, [Pd(IPr)2][PF6], it can be synthesized as complex 2 by a chemical oxidation, as shown in Figure 2, to generate [PdI(NHC)2]+ complexes [50]. Complex 2 is a rare case of a paramagnetic palladium complex in oxidation state I and, unlike the corresponding Pd0 complex, is indefinitely stable in the solid state in air and for several days in solution. The most important characteristic of this complex related to this investigation is the role of one unpaired electron of a singly occupied molecular orbital (SOMO) inside the P3HT:PC60BM bulk heterojunction of an OPV solar cell.

1.5. Target of This Work

To enhance the photovoltaic performance, in terms of Voc and Jsc, it holds great importance to design and synthesize photovoltaic materials featuring an increased number of 3CT states [51]. Our aim was to explore whether the introduction of doublet state molecules as additives in the OPV BHJ can be helpful in controlling the recombination dynamics and the lifetimes of photogenerated carriers during the charge transfer mechanism, as shown in Figure 1 for singlet–triplet dynamics.
The motivation of this work was, therefore, to investigate the effect of the Pd(I) metalloradical doublet state complex on the performance of a classic organic solar cell with an inverted structure (Figure 3).
We decided to use a P3HT:PC60BM system because the energy levels fit well with the HOMO and LUMO levels of the palladium molecule, favoring the cascade effect that occurs in ternary systems [52].
It is known that non-fullerene acceptors improve the photostability of solar cells [53,54]. In this work, we demonstrate that photochemical degradation can also be reduced in fullerene-based OPVs thanks to the role of suitable Pd-based complexes.

2. Materials and Methods

Commercial glass/ITO substrates were used for cell fabrication. The glass substrates, measuring 2.5 × 2.5 cm2, were coated with indium tin oxide (ITO) with a sheet resistance of 10 Ω/sq, which served as the transparent conducting electrode. The ITO substrates were patterned using a ps UV laser. The investigation was focused on the standard inverted structure with the composition ITO/ZnO/PEIE/P3HT:PC60BM/PEDOT:PSS/Ag (Figure 4a). The patterned ITO/glass substrates were cleaned using the following methodology: a manual cleaning process with a diluted soap solution, LABWASH® PREMIUM extra 1% (VWR International Srl, Milan, Italy), in an aqueous solution. In the next step, the soap residue was removed by rinsing with purified water. Subsequently, the substrates were placed inside a beaker filled with deionized (D.I.) water and subjected to an ultrasonic bath for 15 min. The substrates were then dried with a nitrogen (N2) air gun and sonicated in a beaker filled with acetone for 15 min. After drying with a nitrogen air gun again, they were sonicated in a beaker filled with 2-propanol for 15 min. Finally, the substrates were exposed to the UV Ozone 218 Cleaner L2002A3 (Ossila, Leiden, The Netherlands) for 15 min to remove organic residues and improve the wettability and performance of the first layer. After the cleaning steps, every deposition was carried out under inert conditions inside a MBRAUN glovebox with controlled N2 atmosphere. ZnO ink (Helios ETL Spray H-SZ11034, Genesink, Rousset, France) was filtered by a 0.45 PTFE filter, stirred for 20 min in an ultrasonic bath, deposited above the ITO substrates by spin coating at 2000 rpm for 45 s, and then annealed at 110 °C for 10 min. After this step, polyethylenimine-ethoxylated (PEIE) (80% ethoxylated solution, Sigma-Aldrich, St. Louis, MO, USA) was diluted at 0.1% in EtOH (Sigma-Aldrich, ≥99.50%), stirred for 20 min, deposited by spin coating at 5000 rpm for 45 s, and then annealed at 110 °C for 10 min. After these steps, different ratios of the active layer with the dopant [1,3-bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(I) hexafluoridophosphate, [Pd(IPrCl)2][PF6] (2), were deposited by a spin coater at 800 rpm for 45 s and annealed at 110 °C for 10 min. The active layer was prepared by blending poly(3-hexylthiophene), P3HT (TCI, Tokyo Chemical Industry >98.0%) and [6,6]-phenyl-C61-butyric acid methyl ester, PC60BM (Ossila, 99%) with a 1:1 ratio and 20 mg/mL of concentration, solved in 1,2-dichlorobenzene (Sigma-Aldrich, anhydrous, 99%), and stirred for 20 h at room temperature. Before using it, the solution was stirred at 50 °C for 1 h [55,56,57]. The Pd(I) complex was solved in chlorobenzene (Sigma-Aldrich, 99%, anhydrous) at the proportion of 20 mg/mL and stirred at room temperature for 1 h; subsequentially, it was filtered by a 0.45 PTFE filter before mixing it with the BHJ (see the different masses of Pd complex inside the active layer in Table 1). A PEDOT:PSS HTL Solar (Ossila) solution was diluted with 2-propanol (Sigma-Aldrich, 99.5%, anhydrous) with a 1:5 ratio and stirred for 10 min to deposit it. Then, the deposition was spin-casted at 3000 rpm for 45 s with 20 s of previous casting, followed by annealing at 110 °C for 10 min. The last step of this cell fabrication was evaporating 100 nm of silver electrode with a metal evaporator at 10−6 mbar through a shadow mask. Four devices with 0.09 cm2 area each were fabricated on each substrate.
Device performance was characterized outside the glovebox under a sun simulator. For the measurements, a shadow mask (9 mm2) was used to illuminate only one cell at a time. Before each measurement, the irradiation level was verified at the location and position of the solar cell using a calibrated Pyranometer (Skye model SKS1110, Skye Instruments, Llandrindod Wells, UK).
J−V measurements of the OSCs were performed using a Class A sun simulator (ABET 2000, Abet Technologies Inc., Milford, CT, USA) equipped with an AM1.5G filter (ABET). The sun simulator was calibrated using a Si-based reference cell (RR-226-O, RERA Solutions, Nijmegen, The Netherlands) to achieve 1 sun illumination conditions. The Arkeo platform (Cicci Research S.R.L., Grosseto, Italy) was employed for J−V characterization and maximum power point tracking (MPPT), with a voltage step of 20 mV/s and a scan rate of 200 mV/s.
External quantum efficiency (EQE) characterization was conducted using the Arkeo system (Cicci Research S.R.L.) with a 150 W xenon lamp and a double grating (300 to 1400 nm). A Si photodiode was used for incident light calibration before the EQE measurement. An UV−vis spectrophotometer (Shimadzu UV-2550, Shimadzu, Kyoto, Japan) equipped with an integrated sphere was utilized for acquiring absorbance spectra. Spectra from 200 nm to the NIR region were obtained using a UV−vis-NIR Jasco V-630 double beam spectrophotometer with a single monochromator.
Micro Raman and photoluminescence measurements were performed with a Horiba Jobin Yvonne LabRAM ARAMIS spectrophotometer (Horiba, Kyoto, Japan), equipped with a 457 nm laser source and a 2400 lines/cm edge filter. All spectra were acquired with the laser power set to 10%, an acquisition time of 2 s per point, and 50 acquisitions for each pattern. Thickness measurements were taken using a Bruker DEKTAK XT profilometer (Bruker, Billerica, MA, USA). Photovoltage decay and photostability studies were conducted with dedicated Arkeo platforms (Cicci Research S.R.L.). For DFT calculations, the complexes were optimized in 2-hexanone solvent with the TPSSh functional using the SDD+f basis set for Pd (SDD ECP for internal electrons) and the 6-311G(d,p) basis set for all the non-metal atoms (see details in the Supplementary Information). SEM microscopy images, quantities, and spectra were acquired by the HitachiFE SEMS4000 instrument (Hitachi, Tokyo, Japan).
Syntheses and characterization of the precursor of the additive molecule bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(0), [Pd(IPrCl)2] (1), and the additive bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(I) hexafluoridophosphate, [Pd(IPrCl)2][PF6] (2), are described in the Supplementary Information of this paper (S3–S6).

3. Results and Discussion

3.1. Device Optoelectronic Properties

In this study, we investigated the optimal quantity of the Pd(I) metalloradical complex inside the blend as an additive doping molecule to improve the electrical properties of a classic inverted organic solar cell (Figure 4a). The choice of this additive molecule, apart from its doublet state and its metalloradical character, was evaluated by its HOMO and LUMO values (Table 2), which indicate its perfect fit as a semiconductor material in the active layer (Figure 4b).
In addition, it is a stable Pd(I) molecule that is easy to handle and synthesize through Pd(0) NHC biscarbene precursors that are already well known for their use in OLED devices (Figure 2) [48].
The behavior of P3HT:PC60BM BHJ under the effect of small quantities of the Pd(I) complex additive (2) was explored. As shown Figure 5, we observed that small quantities of the Pd(I) complex improved the IV characteristics. The efficiencies are shown separately in Figure 6. In Figure 5a, we show the difference between the IV of the best case (1 wt%) vs. the reference cell without the Pd additive. Adding 1 wt% of Pd additive also improved the performance of the solar cell in terms of fill factor (FF) (Figure 5b), open-circuit voltage (Voc) (Figure 5c), and short-circuit current (Jsc) (Figure 5d). The improvements started with 0.05 wt%. These improvements increased until 1.0 wt% additive was reached. Consequently, the optimal concentration of the Pd(I) complex was found at 1.0 wt%. In Figure 6, we show how this complex significantly increased the efficiency of the solar cell with low amounts of additive (2). We associate this behavior to the doublet state property of this organometallic molecule, which we assume is favorable for the charge transfer mechanism at the donor/acceptor interface. However, a further increase in the quantity of this molecule, from 1.5 wt% onwards, did not further upgrade the performance of the solar cell. Moreover, at the highest amounts of the additive, from 3 wt% onwards, the electrical performance of the solar cell did not improve, and the situation began to worsen, with the performance being even lower than the reference cell.
We believe that this behavior is associated with a worsening in the bulk heterojunction morphology, as discussed in Section 3.2 regarding SEM images of the active layer. As shown in Table 3, by comparing the optimal quantity of additive (2) inside the blend (1.0 wt%) with the reference (without additive), we calculated the improvements as follows: 40% for efficiency (Eff), 20% for the fill factor (FF), 7% for the short-circuit current (Jsc), and 10% for the open-circuit voltage (Voc). Therefore, the effect of the optimal quantity of additive (2) was positive for all the main photovoltaic parameters.
In addition, IPCE and photoluminescence spectra suggested a change in morphology of the bulk heterojunction (Figure 7 and Figure 8). When additive (2) was added, IPCE trended to show a peak shift towards high wavelengths (red shift) with increasing concentrations of Pd as dopant (Figure 7). This can be related to the higher rate of doublet state excitons in the mechanism of photocurrent generation, introduced by the addition of an organometallic complex to the bulk heterojunction. We observed a gradual decrease in the quantum efficiency for a higher quantity of Pd (4 wt%) due to the fact that heavily doping the photoactive layer of the studied devices can negatively affect the morphology of the BHJ and therefore the process of conversion of incident photons as photogenerated carriers. These trends can explain the morphology changes of the bulk heterojunction, as also observed by the SEM analysis in the following section. The deviation of Jsc values from the J–V curve and integrated IPCE spectra are summarized in Table 4 [58,59].
The photoluminescence spectra showed a slight red shift with increasing doping concentration of Pd dopant (Figure 8). This is because the effect of the organometallic complex allows the generation of a reasonable rate of doublet state excitons with longer lifetimes and the potential for “delayed” photon emission as observed in the triplet state charge transfer mechanism [27].

3.2. Morphological Studies of the Active Layer

Morphological analysis was conducted using Raman, absorbance, SEM, and thickness studies. The palladium molecule has an unpaired electron with a radical character [43]; therefore, we decided to study the possible radical reaction with the fullerene PC60BM by the Raman technique. However, we did not observe any chemical modification associated with the radical reaction (Figure 9b). Accordingly, Raman spectra confirmed that additive metalloradical complex (2) was not having any chemical reaction with the different layers deposited in the solar cells (Figure 9b). Because the Raman spectra were not affected by %Pd content in the BHJ, this behavior suggests that the Pd complex can only be associated with a reorganization of the morphology without changes of donor/acceptor spectral fingerprints. Thickness studies using the profilometer demonstrated that the improvement of the electrical characteristics was not associated with an effect of the palladium complex on the thickness of the bulk heterojunction film (Table 5). Confirmation that the thickness of the solar cell did not vary with the addition of additive (2) was provided by the fact that there were no changes in the absolute values in the absorbance spectra (Figure 8).
Morphological studies by the SEM technique showed the difference in the thin film surfaces with different concentrations of the additive (Figure 10). With increasing Pd quantities, the film appearance became more homogeneous, and the crystalline grains were smaller, up to 1 wt% of the Pd case. However, increasing the concentration up to 6 wt% revealed that the grain size increased again. This result confirms that the morphological changes are directly related to the electrical behavior.
The map distribution and the elemental quantities determined by SEM–EDX confirmed that Pd was homogeneously present in the morphology, as reported in Figure S10 of the Supplementary Information.

3.3. Recombination Analysis

Dark J–V plots (Figure 11) revealed that the 1 wt% optimal quantity of Pd complex (2) optimized the recombination dynamics. This trend was associated with the doublet state property, which reduced the recombinations inside the bulk heterojunction and therefore lowered the reverse saturation current. The diode reverse saturation current is directly proportional to the recombinations; therefore, improved diode rectification was observed upon addition of the Pd complex, reaching a maximum of about 1 wt%. From 3.0 wt% onwards, the rectification decreased but was still better than the reference case.
Furthermore, the shunt resistance and series resistance (Figure 12) were consistent with this diode rectification. The series resistance (Figure 12a) decreased with increasing amounts of Pd additive, reaching the minimum at 1 wt% but increasing from 3 wt% onwards. The shunt resistance exhibited the opposite behavior as expected (Figure 12b).
We also performed photovoltage decay studies of the solar cells. These studies revealed that the Pd(I) additive inside the BHJ decreased the recombination rate in the active layer (Figure 13). This was because a doublet state complex with an unpaired electron helped to decrease the recombination inside the active layer, increasing photocurrent generation due to promoted mechanisms of charge separation at the interfaces donor/acceptor and reducing the recombination times as expected.

3.4. Photostability

The photochemical degradation was tested on devices with different concentrations of the donor material inside the BHJ. The stability was measured under 1 sun illumination (Figure 14) by measuring the maximum power point tracking (MPPT) values. Stability refers to the optimal point at which the product of current (I) and voltage (V) is maximized, resulting in the highest possible power output (p = IV). By adding the additive, the stability of the device improved significantly. Because the photostability is associated with the overall Pd(I) complex concentration, the best result in terms of stability was achieved at the maximum concentration tested in this work, i.e., 6 wt%. In the literature, it is known that this family of metalloradical complexes reacts with dioxygen [50]. Therefore, we believe that the Pd(I) metalloradical acts as an antioxidant, improving the photostability of the active layer.
In Figure 14, the two exponential decay curves help to understand the trend of decay due to the degradation of the unencapsulated devices. We observed that the fast initial decay rate, t1, was strongly dependent on the Pd(I) complex. The reference case without any additive presented a very fast decay rate of t1 = 0.89 h. Increasing the additive content, we observed t1 = 19.22 h for 1.0 wt% and t1 = 30.29 h for 6.0 wt%. This trend clearly shows a great improvement in the stabilization of the solar cell due to the presence of the Pd(I) complex.

4. Conclusions

Doping P3HT:PC60BM with the Pd(I) complex at low concentrations improved the photovoltaic performances of the solar cells. The optimal concentration to improve the performance was 1 wt% of the additive inside the BHJ. This behavior is due to the fact that Pd(I) is a doublet state organometallic molecule with an unpaired electron that has a positive role on the recombination mechanisms, most notably the photocurrent generation inside the active layer. The specificity of this Pd molecule on fullerene acceptors due to its radical characteristic could be evaluated. However, in this work, we observed that there was no chemical reaction between the BHJ and Pd(I). In this work, it was also clearly demonstrated that the Pd(I) molecule had a clear effect on the BHJ morphology, making it a finer grain size with the optimal Pd(I) concentration. We also demonstrated that the doublet state Pd(I) could strongly increase the photostability of the unencapsulated solar cells against photochemical degradation.
However, in the future, further studies will also focus on more efficient blends, either based on FA or NFA acceptors, and compare them to the simple and well-studied P3HT:PC60BM model system, which was selected in this study due to the good fit of the HOMO and LUMO values of the Pd molecule in the P3HT:PC60BM system.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en17153787/s1, 1. Synthesis of palladium complexes (S3–S6); 1.1. General procedures (S3); 1.2. Synthesis and characterization of bis-[1,3-bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(0), [Pd(IPrCl)2] (1) (S3); 1.3. Synthesis and characterization of bis-[1,3-bis-(2,6-diisopropylphenyl)-4,5-dichloroimidazol-2-ylidene]palladium(I) hexafluoridophosphate, [Pd(IPrCl)2][PF6] (2) (S4–S6); 2. DFT calculations of the energies of the highest occupied and lowest unoccupied molecular orbitals (MOs) of [Pd(IPrCl)2][PF6] (2) (S7–S10); 3. SEM data (S11–S15); 3.1. Spectra (S11–S12); 3.2. EDX analysis (S13–S15) [50,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78].

Author Contributions

Conceptualization, discussion of the results, writing, review, and editing: A.E.A.-Q., V.C., C.G.Y., E.d.J. and A.R.; device fabrication: A.E.A.-Q., V.C. and A.U.R.; SEM characterization: F.Z.; DFT characterization: A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by the ITA-NTN “Integrated Terrestrial and Non-Terrestrial Networks” project under the RESTART “RESearch and innovation on future Telecommunications systems and networks, to make Italy more smart” initiative of the PNRR. This work was also supported by the Spanish MCIN/AEI/10.13039/501100011033 and the European Union “NextGenerationEU”/PRTR with projects PID2020-114637GB-I00 and TED2021-129634B-I00.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to thank Prof. Claudia Barolo for the discussions that initially helped to develop the idea of this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of photoinduced energy and charge transfer mechanisms.
Figure 1. Schematic illustration of photoinduced energy and charge transfer mechanisms.
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Figure 2. Reaction of the oxidation of the Pd(0) organometallic complex (1) precursor to obtain the Pd(I) additive (2).
Figure 2. Reaction of the oxidation of the Pd(0) organometallic complex (1) precursor to obtain the Pd(I) additive (2).
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Figure 3. Spin multiplicity and spin angular momentum of the different state configurations.
Figure 3. Spin multiplicity and spin angular momentum of the different state configurations.
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Figure 4. (a) Representation of the inverted architecture of OPV devices investigated in the work. (b) Diagram of energy levels for the solar cell stack.
Figure 4. (a) Representation of the inverted architecture of OPV devices investigated in the work. (b) Diagram of energy levels for the solar cell stack.
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Figure 5. (a) J–V curves of reference (0 wt% Pd) and 1 wt% of Pd complex (2). (b) FF box charts of different wt% of Pd complex (2). (c) Open-circuit voltage box charts of different wt% of Pd complex (2). (d) Short-circuit current box charts of different wt% of Pd complex (2).
Figure 5. (a) J–V curves of reference (0 wt% Pd) and 1 wt% of Pd complex (2). (b) FF box charts of different wt% of Pd complex (2). (c) Open-circuit voltage box charts of different wt% of Pd complex (2). (d) Short-circuit current box charts of different wt% of Pd complex (2).
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Figure 6. Efficiency vs. wt% of Pd complex (2) as additive inside the active layer. Box chart (left) and bar error (right).
Figure 6. Efficiency vs. wt% of Pd complex (2) as additive inside the active layer. Box chart (left) and bar error (right).
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Figure 7. Normalized IPCE and integrated current density of different proportions of Pd inside the bulk heterojunction of the solar cell (a); IPCE (%) (b).
Figure 7. Normalized IPCE and integrated current density of different proportions of Pd inside the bulk heterojunction of the solar cell (a); IPCE (%) (b).
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Figure 8. Normalized photoluminescence spectrum at different wavelengths of different proportions of Pd inside the bulk heterojunction of the solar cell.
Figure 8. Normalized photoluminescence spectrum at different wavelengths of different proportions of Pd inside the bulk heterojunction of the solar cell.
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Figure 9. (a) Absorbance spectrum of different concentrations of complex (2) inside the bulk heterojunction of the solar cell. (b) normalized Raman spectrum of different concentrations of complex (2).
Figure 9. (a) Absorbance spectrum of different concentrations of complex (2) inside the bulk heterojunction of the solar cell. (b) normalized Raman spectrum of different concentrations of complex (2).
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Figure 10. SEM images of 10 µm of the active layer above the solar cell stack with different concentrations of Pd(I) additive using a BSE detector (5 keV).
Figure 10. SEM images of 10 µm of the active layer above the solar cell stack with different concentrations of Pd(I) additive using a BSE detector (5 keV).
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Figure 11. Dark J–V measurements of different wt% of additive (2).
Figure 11. Dark J–V measurements of different wt% of additive (2).
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Figure 12. (a) Series resistance and (b) shunt resistance. Box charts of different wt% Pd complex.
Figure 12. (a) Series resistance and (b) shunt resistance. Box charts of different wt% Pd complex.
Energies 17 03787 g012
Energies 17 03787 g013
Figure 13. Transient photovoltage decay of different concentrations of the Pd dopant complex. Photovoltage time decay as a function of pulse light intensity.
Figure 13. Transient photovoltage decay of different concentrations of the Pd dopant complex. Photovoltage time decay as a function of pulse light intensity.
Energies 17 03787 g013
Energies 17 03787 g014
Figure 14. Photostability studies with a light-soaking instrument with an exponential decay model equation of the different trends. Normalized maximum power point tracking (MPPT) trends.
Figure 14. Photostability studies with a light-soaking instrument with an exponential decay model equation of the different trends. Normalized maximum power point tracking (MPPT) trends.
Energies 17 03787 g014
Table 1. Different proportions of the Pd(I) complex additive in wt% and mg/L with the active layer P3HT:PC60BM.
Table 1. Different proportions of the Pd(I) complex additive in wt% and mg/L with the active layer P3HT:PC60BM.
[Pd]/[P3HT:PC60BM] wt%[Pd] in the Active Layer Solution (µg/mL)
0 (Ref)0
0.0510
0.1020
0.50100
1.00200
1.50300
2.00400
3.00600
4.00800
6.001200
Table 2. Energies (in eV) of the highest occupied and lowest unoccupied molecular orbitals (MOs) of the [Pd(IPrCl2)2]+ complex. The highest occupied alpha MO is the SOMO.
Table 2. Energies (in eV) of the highest occupied and lowest unoccupied molecular orbitals (MOs) of the [Pd(IPrCl2)2]+ complex. The highest occupied alpha MO is the SOMO.
Highest OccupiedLowest Unoccupied
alphabetaalphabeta
−5.75−5.94−1.38−3.62
Table 3. Detailed photovoltaic parameters of the OPV cells with different concentrations of Pd complex (2).
Table 3. Detailed photovoltaic parameters of the OPV cells with different concentrations of Pd complex (2).
wt% [Pd]Voc (V)Jsc (mA cm−2)FF (%)Eff (%)
0.00 (Ref)0.509 ± 0.0496.11 ± 0.3146.8 ± 6.41.46 ± 0.32
0.050.532 ± 0.0436.13 ± 0.0847.2 ± 3.71.55 ± 0.25
0.100.550 ± 0.0336.43 ± 0.1953.4 ± 4.41.83 ± 0.25
0.500.558 ± 0.0116.40 ± 0.2754.1 ± 3.12.00 ± 0.12
1.000.559 ± 0.0236.54 ± 0.2856.1 ± 9.52.04 ± 0.29
1.500.550 ± 0.0295.84 ± 0.3355.1 ± 5.41.78 ± 0.34
2.000.495 ± 0.0686.01 ± 0.4449.5 ± 4.31.54 ± 0.38
3.000.479 ± 0.0396.04 ± 0.4851.2 ± 8.91.42 ± 0.37
4.000.451 ± 0.0245.40 ± 0.3644.5 ± 2.21.09 ± 0.15
6.000.393 ± 0.0204.65 ± 0.2342.0 ± 2.70.73 ± 0.07
Table 4. Comparison of Jsc from the sun simulator and J–V integrated from the IPCE curve.
Table 4. Comparison of Jsc from the sun simulator and J–V integrated from the IPCE curve.
wt% [Pd]Jsc (mA/cm−2)—J–V CurveJ Integrated (mA/cm−2)—IPCEDeviation Value (%)
0.0 (Ref)6.11 ± 0.318.1033
0.16.43 ± 0.197.9323
1.06.54 ± 0.288.0824
4.05.40 ± 0.366.1915
Table 5. Different thicknesses considering the Pd(I) dopant.
Table 5. Different thicknesses considering the Pd(I) dopant.
[Pd]/[P3HT:PC60BM] (wt%)ITO/ZnO/PEIE/ PBDB-T:ITIC:Pd (nm)ITO/ZnO/PEIE/ PBDB-T:ITIC: Pd/PEDOT:PSS (nm)
0 (Ref)107 ± 3126 ± 1
0.1103 ± 4119 ± 1
1.0104 ± 4124 ± 3
3.0106 ± 2117 ± 2
6.0106 ± 2116 ± 1
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El Astal-Quirós, A.; Carrarini, V.; Zarotti, F.; Rahman, A.U.; Lledós, A.; Yebra, C.G.; de Jesús, E.; Reale, A. A Doublet State Palladium(I) N-Heterocyclic Carbene Complex as a Dopant and Stabilizer for Improved Photostability in Organic Solar Cells. Energies 2024, 17, 3787. https://doi.org/10.3390/en17153787

AMA Style

El Astal-Quirós A, Carrarini V, Zarotti F, Rahman AU, Lledós A, Yebra CG, de Jesús E, Reale A. A Doublet State Palladium(I) N-Heterocyclic Carbene Complex as a Dopant and Stabilizer for Improved Photostability in Organic Solar Cells. Energies. 2024; 17(15):3787. https://doi.org/10.3390/en17153787

Chicago/Turabian Style

El Astal-Quirós, Aliah, Valentina Carrarini, Francesca Zarotti, Atiq Ur Rahman, Agustí Lledós, Cristina G. Yebra, Ernesto de Jesús, and Andrea Reale. 2024. "A Doublet State Palladium(I) N-Heterocyclic Carbene Complex as a Dopant and Stabilizer for Improved Photostability in Organic Solar Cells" Energies 17, no. 15: 3787. https://doi.org/10.3390/en17153787

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