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Article

Nonfullerene Small Molecular Acceptor Acting as a Solid Additive Enables Highly Efficient Pseudo-Bilayer All-Polymer Solar Cells

by
Jiayin Liu
1,†,
Yuheng Ni
1,†,
Jiaqi Zhang
1,
Yijun Zhao
1,
Wenjing Xu
1,
Xiaoling Ma
1,2,* and
Fujun Zhang
1,2,*
1
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China
2
Tangshan Research Institute of Beijing Jiaotong University, Tangshan 063000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Energies 2024, 17(11), 2623; https://doi.org/10.3390/en17112623
Submission received: 21 April 2024 / Revised: 22 May 2024 / Accepted: 27 May 2024 / Published: 29 May 2024
(This article belongs to the Topic Organic and Perovskite Optoelectronic Materials and Devices)
Browse Figures
Versions Notes

Abstract

:
In this work, pseudo-bilayer planar heterojunction (PPHJ) all-polymer solar cells (APSCs) were constructed on the basis of the commonly used PY-IT and PM6 as the acceptor and donor, respectively. A nonfullerene small molecular acceptor (NF-SMA) BTP-eC9 was incorporated into the PY-IT layer as the solid additive in consideration of its similar building block to PY-IT. BTP-eC9 can serve as a photon capture reinforcer and morphology-regulating agent to realize more adequate photon capture, as well as a more orderly molecular arrangement for effective carrier transport. By incorporating 2 wt% BTP-eC9, the efficiency of PM6/PY-IT-based PPHJ-APSCs was boosted from 15.11% to 16.47%, accompanied by a synergistically enhanced short circuit current density (JSC, 23.36 vs. 24.08 mA cm−2) and fill factor (FF, 68.83% vs. 72.76%). In another all-polymer system, based on PBQx-TCl/PY-DT as the active layers, the efficiency could be boosted from 17.51% to 18.07%, enabled by the addition of 2 wt% L8-BO, which further verified the effectiveness of using an NF-SMA as a solid additive. This work demonstrates that incorporating an NF-SMA as a solid additive holds great potential for driving the development of PPHJ-APSCs.

1. Introduction

Exploration of solar cells dates back to 1839, when French scientist A.E. Becquerel first discovered the photovoltaic effect. The term ‘photovoltaic’ was coined in 1849. In 1883, the first solar cell was prepared by Charles Fritts, who used a selenium semiconductor coated with a very thin layer of gold to form a semiconductor metal junction. Since then, after more than one hundred and forty years’ development, great progress has been achieved in solar cells with various types and gradually increasing efficiencies. Solar cells can be roughly divided into three generations. The first-generation solar cells are inorganic crystalline silicon solar cells, including monocrystalline silicon solar cells and polycrystalline silicon solar cells. In 1954, Bell Labs prepared the first monocrystalline silicon solar cells, with 4.5% efficiency. Over 27% efficiency was obtained in inorganic crystalline silicon solar cells, which dominate the market for solar cell sales today. However, there are shortcomings to crystalline silicon solar cells, such as their complicated preparation process and lack of natural degradation after use, which limit the further development and multi-scenario application of crystalline silicon solar cells. The second-generation thin-film solar cells were then developed. Thin-film solar cells are mainly made of materials such as cadmium telluride, gallium arsenide, etc. Thin-film solar cells are small in mass and bendable, and the preparation process is simplified compared to inorganic crystalline silicon solar cells. However, thin-film solar cells are also limited in mass production due to the low resource reserves of raw materials such as tellurium and indium and the high toxicity of cadmium. Under these circumstances, the third-generation solar cells were developed, including perovskite solar cells, dye-sensitized solar cells and organic solar cells (OSCs). Among the third-generation solar cells, OSCs have the advantages of wide sources of raw materials, low weight, flexibility, semi-transparency and a large area for preparation, which give them unique advantages and wide application prospects, including building photovoltaic integration, agricultural photovoltaic complementary power generation, wearable applications, etc. [1].
The OSCs can be divided into three types: polymer donor/small molecular acceptor, small molecular donor/small molecular acceptor and polymer donor/polymer acceptor. All-polymer solar cells (APSCs), which constitute a polymer donor and acceptor, have received a surge in interest on account of their morphological robustness, mechanical endurance, mechanical stability and excellent film formation [2,3]. Thanks to materials’ and device optimization technology’s development, the efficiency of APSCs is already over 18% [4,5]. However, it should be noted that the performance of APSCs still lags behind that of polymer donor/small-molecule acceptor systems, which is closely associated with the difficult morphological regulation in an intertwined polymer chain for a traditional bulk heterojunction (BHJ) structure [6,7]. Pseudo-bilayer planar heterojunction (PPHJ) configuration-based APSCs have attracted a lot of attention since the stepwise deposition method is employed to form more desirable vertical phase separation with donor or acceptor enrichment near the anode or cathode [8]. The stepwise deposition method can also provide more opportunities to separately adjust donors’ and acceptors’ morphologies by solvent engineering, n-type doping [9], a dilution strategy [10,11], additive engineering [12,13], annealing treatment, etc. [14]. Additive engineering has been widely recognized as an efficient method in manipulating the photoelectric conversion process by optimizing phase separation and purity, as well as molecular orientation, crystallinity, etc. [15]. For instance, diphenyl ether and 1,8-diiodooctane are commonly employed as high-boiling-point additives in solution, which can selectively dissolve the donor or acceptor, and the active layers’ aggregation state can be adjusted in the process of slow volatilization for a high-boiling-point (BP) additive [16,17]. It is worth noting that absolute volatilization of high-BP additives is quite hard in the process of film formation, which may aggravate the performance degradation of APSCs. Alternatively, a solid additive is gradually becoming popular in constructing APSCs, which simultaneously allows for adjusting active layers’ topography, and for APSCs’ stability to be ameliorated with simple post-processing technology. For example, the volatile solid additive 2-methoxynaphthalene was introduced into all-polymer system based on PM6:PY-DT by Sun et al., resulting in synchronously improved PCE and photostability of APSCs [18]. Yan et al. introduced the solid additive N2200, improving the efficiency of APSCs on the basis of PM6:PY-IT to 16.04%, accompanied by synergistically optimized storage stability and photostability [19]. Suitable solid additives are expected to be applied to PPHJ-APSCs to drive a performance improvement. Today, highly efficient APSCs are mostly constructed with a polymerized small molecular acceptor (PSMA) due to its easily adjusted physicochemical characteristics, which arise by utilizing different nonfullerene small molecular acceptor (NF-SMA) building blocks [20,21]. An NF-SMA with similar building blocks to the host PSMA may act as a solid additive, allowing for adjusting the molecular π–π stacking and ordering. Accordingly, employing an NF-SMA as a solid additive should provide more possibilities for constructing highly efficient APSCs.
In this work, an NF-SMA possessing a similar chemical construction to the host materials was successfully adopted as a solid additive to boost the power conversion efficiency (PCE) of PPHJ-APSCs. The PPHJ-APSCs were prepared based on the commonly used polymer acceptor PY-IT and donor PM6 [22]. The NF-SMA BTP-eC9 was selected as the solid additive in consideration of its similar building blocks to a segment of PY-IT, which should afford good compatibility between BTP-eC9 and PY-IT [23]. The main difference in the backbone structures of the two materials is that BTP-eC9 possesses chlorine atoms in the end unit, for capturing photons in the longer wavelength region. Figure 1a–c display the chemical constructions, energy levels and normalized absorption spectra of the materials employed. These materials display complementary absorption spectra and suitable energy levels, which should be one of preconditions for achieving efficient APSCs. Figure 1d shows the absorption spectra of layered films. By adding less BTP-eC9, the layered films’ absorption intensity can be enhanced, which may be closely associated with the ameliorative molecular stacking and aggregation enabled by BTP-eC9. A similar phenomenon can also be observed from the absorption spectra of acceptor films (Figure S1). The efficiency of PPHJ-APSCs was boosted from 15.11% to 16.47% by incorporating 2 wt% BTP-eC9, deriving from a concurrently enhanced short circuit current density (JSC) and fill factor (FF). BTP-eC9 acting as a solid additive can play a positive role in photon capture, as well as topography regulation, for more valid carrier transport and restrained recombination of carriers in PPHJ devices. Another kind of all-green solvent-processed PPHJ-APSC was constructed with the donor PBQx-TCl and acceptor PY-DT, as well as the solid additive L8-BO, to verify the effectiveness of the strategy by employing an NF-SMA as the solid additive. The efficiency of PPHJ-APSCs based on PBQx-TCl/PY-DT was boosted from 17.51% to 18.07% by introducing 2 wt% L8-BO, which is one of the highest PCEs for binary PPHJ-APSCs processed with a green solvent. This work demonstrates that the introduction of an NF-SMA as a solid additive is an effective and facile method for driving forward the performance improvement of PPHJ-APSCs. In the following section, the materials used, device fabrication process, device characterization equipment and method are described in detail, followed by the detailed data and an analysis of the reasons for the performance improvement of APSCs when incorporating an NF-SMA as a solid additive. The last part of the article gives a summary of this work, as well as the potential positive influence of this work in the field.

2. Materials and Methods

2.1. Materials

The glass substrate was purchased from Yiyang South China Xiangcheng Technology Co., Ltd., Yiyang city, Hunan Province, which possessed patterned indium tin oxide (ITO, 15 Ω per square). The poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) employed as the anode interface layer was purchased from H.C. Starck Co., Ltd. The active layers were composed of commonly used materials, including polymer donor PM6: poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo [1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)], polymer donor PBQx-TCl: Poly[9-(5-{4,8-bis[4-chloro-5-(2-ethylhexyl)thiophen-2-yl]-6-methylthieno[2′,3′:4,5]benzo[b]thiophen-2-yl}-4-(2-butyloctyl)thiophen-2-yl)-6-[4-(2-butyloctyl)-5-methylthiophen-2-yl]dithieno[3,2-f:2′,3′-h]quinoxaline], polymer acceptor PY-IT: Poly[2,2′-((2Z,2′Z)-((12,13-bis(2-octyldodecyl)-3,9-diundecyl-12,13dihydro[1,2,5]thiadiazolo[3,4e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]-indole-2,10-diyl)bis(methanylylidene))bis(5-methyl-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile-co-2,5-thiophene], polymer acceptor PY-DT: Poly[2-{2-[(E)-[3,9-bis(2-butyloctyl)-12,13-bis(2-decyltetradecyl)-10-{[(2E)-1-(dicyanomethylidene)-5-(5-methylthiophen-2-yl)-3-oxo-2,3-dihydro-1H-inden-2-ylidene]methyl}-3a,4b,7b,8a,11a,12,13, 13b-octahydro[1,2,5]thiadiazolo[4,3-e]thieno[2′,3′:4,5]thieno[3,2-b]thieno[2″,3″:4′,5′] thieno[2′,3′:4,5]pyrrolo[3,2-g]indol-2-yl]methylidene]-5-methyl-3-oxo-2,3-dihydro-1H-indenylidene}propanedinitrile], SMA BTP-eC9 and SMA L8-BO: 2,2′-((2Z,2’Z)-((12,13-bis(2-ethylhexyl)-3,9-(2-butyloctyl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′] thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile. Poly[(9,9-bis(3′-(N, Ndimethylamino)propyl)-2,7-fluorene)-alt-5,5-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide] (PNDIT-F3N) purchased from eflexPV Co., Ltd. was employed as the cathode interface layer. Silver (Ag) was employed as the cathode.

2.2. Device Fabrication

The glass/ITO substrates were ultrasonically cleaned in detergent, de-ionized and ethanol step by step and then blow-dried through nitrogen. One minute of plasma treatment was employed to further clean the surface of glass/ITO substrates. About 30 nm PEDOT: PSS films were deposited on the ITO substrates and then annealed at 150 degrees centigrade for ten minutes on a heating stage. After that, the substrates with PEDOT films were transferred into a glove box filled with high-purity nitrogen. In the PM6/PY-IT system, the PM6 layer can be prepared by depositing its chlorobenzene solution (7 mg/mL) with 1200 rpm for 40 s; the PY-IT layer can be fabricated by depositing its chloroform solution (7 mg/mL) with 2000 rpm for 30 s on a PM6 layer. After that, the active layers were annealed at 95 degrees centigrade for five minutes on a heating stage. In the PBQx-TCl/PY-DT system, the PBQx-TCl layer can be prepared by hot depositing its methylbenzene solution (5.5 mg/mL); the PY-DT layer can be fabricated by depositing its methylbenzene solution (5.5 mg/mL) with 1400 rpm for 30 s on a PBQx-TCl layer. After that, the active layers were annealed at 110 degrees centigrade for ten minutes on a heating stage. The preparation process of a PM6/PY-IT+BTP-eC9 or PBQx-TCl/PY-DT+L8-BO system is very similar to that of a PM6/PY-IT or PBQx-TCl/PY-DT system; the only difference is the BTP-eC9 or L8-BO incorporating ratio in the acceptor. The PNDIT-F3N layer was prepared from its methanol solution (0.5 mg/mL) with the addition of 0.25 vol% acetic acid. We evaporated 100 nm Ag in the vacuum condition of 5 × 10−4 Pa. The active area was about 3.8 mm2, which we determined by the crossed area of the anode and cathode.

2.3. Characterization

A Keithley 2400 source meter purchased from tektronix company was employed to chart the current density–applied voltage (J-V) curves of PPHJ APSCs in a glove box filled with nitrogen. The Keithley 2400 can provide a wide dynamic range (10 pA to 10 A, 1 µV to 1100 V, 20 W to 1000 W) and high accuracy of 0.012%, which are sufficient for the measurement of J-V curves. The solar simulator (XES-40S2) was purchased from SAN-EI Electric Co., Ltd. to provide the standard AM 1.5G irradiation at 100 mW cm−2. The light source was standardized with a silicon solar cell purchased from Zolix INSTRUMENTS Co., Ltd. The absorption spectra of films were characterized by employing a UV-3101 PC spectrometer. This instrument can measure absorption spectra in the wavelength range from 200 nm to 3200 nm with a 0.5 nm step interval, which can completely cover the spectrum response range of organic materials. The external quantum efficiency spectra of devices were characterized by employing Zolix Solar Cell Scan 100. Nyquist plots were characterized by Zennium pro equipment, which can provide an impedance frequency range of 10 μHz–12 MHZ, frequency accuracy < 0.0025%, frequency resolution of 0.0025% and 10,000 steps/decade. The structure of electron-only devices is ITO/ZnO/active layer/PNDIT-F3N/Al. The electrical parameter dependent on temperature is not given since the key focus of this work was the influence of NF-SMA introduction as a solid additive on performance rather than a temperature dependence investigation. All the devices were measured under the same condition of room temperature, supporting the comparison between the reference and optimal devices.

3. Results and Discussion

The current density–applied voltage (J-V) curves measured with an AM 1.5G calibrated solar simulator, as well as pivotal photovoltaic parameters of related PPHJ-APSCs with distinct ratios of BTP-eC9 in the PY-IT layer, are presented in Figure S2 and Table S1, respectively. The PM6/PY-IT-based PPHJ-APSCs have 15.11% efficiency, accompanied by JSC, VOC and FF values of 23.36 mA cm−2, 0.94 V and 68.83%, respectively. The performance of PPHJ-APSCs can be promoted by adding less BTP-eC9 to the PY-IT layer. Both JSC and FF display a simultaneously increasing tendency with the BTP-eC9 ratios rising to 2 wt% and then a decreasing tendency with the further addition of BTP-eC9. The VOC of PPHJ-APSCs can be kept constant with a low incorporation of BTP-eC9, indicating that the electron transport channel in active layers will not be disturbed by low incorporation of BTP-eC9. The optimum efficiency of 16.47% is achieved with 2 wt% BTP-eC9 incorporation into the PY-IT layer along with JSC, VOC and FF values of 24.08 mA cm−2, 0.94 V and 72.76%, respectively. The J-V curves and key parameters of reference and optimal PPHJ-APSCs are exhibited in Figure 2a and Table 1. To verify the JSC improvement of PPHJ-APSCs with low BTP-eC9 incorporation, the external quantum efficiency (EQE) spectra of the reference and optimal PPHJ-APSCs were characterized (Figure 2b). The computed JSCs are largely consistent with the measured JSCs, within 3% error. It seems that the EQE spectra of PPHJ-APSCs can be improved in the whole photoresponse region with 2 wt% BTP-eC9 incorporation into the PY-IT layer, suggesting a more effective exciton utilization of layered active layers induced by BTP-eC9.
Figure 3a presents the dependence of the photo-generated current density (Jph) on the effective bias (Veff) of reference and optimal PPHJ-APSCs, which is explored based on the J-V curves of related PPHJ-APSCs under standard illumination or dark conditions [24]. The Jph values can reach saturation at the relatively large Veff of 4 V [25]. The saturated photocurrent density (Jsat) values are 23.73 and 24.36 mA cm−2 for the reference and optimal PPHJ-APSCs. The higher Jsat values manifest improved photon capture in the optimally layered active layers, supported by the increased absorption intensity of optimally layered films in the long wavelength region (Figure 1d). The exciton separation efficiency (ηD) or carrier collection efficiency (ηC) can be estimated based on Jph-Veff curves; the detailed calculation process can be found in the supporting information (Table S2) [26]. The ηD and ηC of PPHJ-APSCs can be, respectively, increased from 98.44% to 98.85% and from 82.89% to 83.58% with the addition of 2 wt% BTP-eC9. The concurrently improved exciton separation and carrier collection processes, when incorporating BTP-eC9, should be among the several factors considered for the FF increment of optimal PPHJ-APSCs [27].
Figure 3b displays the JSC of typical PPHJ-APSCs versus the incident intensity, which may be used to study carriers’ dynamic recombination process of PPHJ-APSCs [28]. The formula JSC∝Plightα can be used to describe the relationship between JSC and Plight [29]. The bimolecular recombination is considered to be completely inhibited with the α value of 1. The fitted α values for the reference and the optimal PPHJ-APSCs are 0.967 and 0.971, respectively, which are close to unity, suggesting weak bimolecular recombination in the active layers [30]. The impedance spectroscopy was characterized at V = VOC and under AM 1.5G irradiation in order to investigate the carrier transmission and kinetic recombination process in more depth. The related Nyquist plots for typical PPHJ-APSCs were plotted, as presented in Figure 3c. In the equivalent circuit model, L represents inductance, which is employed to eliminate the influence of the connecting lines during high-frequency scanning. RCT and ROS represent charge carriers’ transfer resistance, closely related to interfacial carriers’ transmission, and series resistance originating from electrodes as well as bulk resistance. The RCT and ROS values can be decreased from 48.4 Ω to 34.2 Ω and from 45.0 Ω to 44.6 Ω, signifying that the incorporated BTP-eC9 can facilitate effective carrier transmission, as well as inhibit carriers’ recombination in layered active layers. To remedy heterogeneity of the interface in the active layer, a constant phase element (CPE) is introduced. The CPE can be determined by the equation Z = (CPET)−1(iw)CPEP, where w, CPET and CPEP represent the angular frequency, capacitance and inhomogeneous constant. The CPEP value of 1 indicates that the CPE can act as the ideal capacitor. The optimal PPHJ-APSCs show a higher CPEP value of 0.975 than that of 0.928 for the reference PPHJ-APSCs, suggesting that interfacial capacitance can be more electrically ideal in PPHJ-APSCs with BTP-eC9 addition. Through the formula τ = RCT × CPET, the active layers’ average charge carrier lifetime (τ) was estimated. The τ values for reference and optimal PPHJ-APSCs were calculated to be 803.4 and 437.8 ns. The shortened τ value of PPHJ-APSCs, resulting from incorporating less BTP-eC9, should be beneficial to suppress layered active layers’ carrier recombination. The fitted parameters for PPHJ-APSCs according to the impedance spectroscopy are summarized in Table S3. The space-charge-limit-current (SCLC) theory was adopted to evaluate the variation in electron mobility (µe) in films influenced by low incorporation of BTP-eC9 [31,32]. The µe in films was boosted from 6.43 × 10−4 to 9.18 × 10−4 cm2 V−1 s−1 by introducing low BTP-eC9. The boosted µe in films may be closely correlative with molecular packing optimization of layered blend films with low BTP-ec9 as a morphology regulator. The enhanced µe should well support the increment of FF for optimal PPHJ-APSCs [33,34].
To figure out the molecular arrangement of reference and optimal films, GIWAXS characterization was performed with the related data displayed in Figure 4 [35]. The pure PM6 films display obvious (100) as well as (010) diffraction peaks in both OOP and IP directions, meaning the formation of a 3D molecular arrangement of PM6 [36]. The distinguishable diffraction peaks of IP (100) as well as OOP (010), which originated from pure PY-IT films’ GIWAXS profiles, manifest the inclined face-on molecular packing of PY-IT. The molecular packing property can be well-maintained in layered films, which can be confirmed from the corresponding GIWAXS profiles with an apparent OOP (010) diffraction peak, as well as (100) diffraction peaks in two directions. The diffraction intensities of the OOP (010) and IP (100) peaks are simultaneously strengthened by incorporating less BTP-eC9 as the solid additive, manifesting the improved face-on molecular stacking in PPHJ films [37]. The coherence length (LC) of molecular crystallization is calculated on the basis of the Scherrer equation: LC = 2πk/Δq. In this equation, k and Δq are the Scherrer constant (~0.9) and full width at half height of a peak [38,39]. The calculated LC values correlative to the π-π stacking are 17.42 Å and 18.41 Å for the reference and optimal films, respectively. The increased LC values signify a higher degree of crystallinity, which should be conductive to charge transport in optimal PPHJ films.
The storage stability of reference and optimal APSCs was characterized, as displayed in Figure S3. The PPHJ APSCs based on PM6/PY-IT or PM6/PY-IT:BTP-eC9 as the active layer could hold 94.2% and 97.8% of the original PCE after storage for about 210 h in a glove box filled with nitrogen. The ameliorative storage stability of PPHJ APSCs can be ascribed to the improved molecular arrangement induced by the low incorporation of BTP-eC9 [40,41].
To verify the general applicability of the strategy of employing an NF-SMA as the solid additive, another kind of PPHJ-APSC processed with all green solvent was constructed based on PBQx-TCl as the donor, PY-DT as the acceptor and L8-BO as the solid additive [42,43]. Figure 5a displays the chemical structures of PBQx-TCl, PY-DT and L8-BO. Figure 5b and Table 2 present the relevant J-V curves and specific parameters of PBQx-TCl/PY-DT-based APSCs with or without L8-BO. The PCE of PBQx-TCl/PY-DT-based APSCs can be increased from 17.51% to 18.07% after incorporating 2% into the PY-DT layer, accompanied with synergistically elevated JSC (from 24.19 to 24.80 mA cm−2) and FF (from 75.38% to 75.88%). It should be noted that the 18.07% efficiency is among the top-ranked values of eco-friendly solvent-processed PPHJ-APSCs [44,45]. Figure 5c presents the EQE spectra of PBQx-TCl/PY-DT-based APSCs. The integrated JSC from the EQE spectra is within 5% deflection when compared with the measured value required from J-V curves. The EQE values of PPHJ-APSCs based on PBQx-TCl/PY-DT can be enhanced with the incorporation of L8-BO, manifesting an ameliorative photon usage efficiency induced by L8-BO. The effectiveness of the strategy of employing an NF-SMA as a solid additive was further confirmed by the PBQx-TCl/PY-DT system.

4. Conclusions

In conclusion, a sequence of PPHJ-APSCs was fabricated on the basis of the donor PM6, acceptor PY-IT and solid additive BTP-eC9. The PCE of PM6/PY-IT-based PPHJ-APSCs was elevated from 15.11% to 16.47% by adding 2 wt% BTP-eC9 in the PY-IT layer, resulting in a concurrently boosted JSC and FF. The JSC improvement should be correlative with more sufficient photon capture of layered films, which can be confirmed by an enhanced absorption intensity in the long wavelength region. BTP-eC9 can also play a role as a morphology-regulating agent in promoting molecular crystallinity, as well as improving the molecular orientation to realize more effective charge transport, resulting in FF improvement. The universality of the strategy of employing an NF-SMA as a solid additive was confirmed with another system, PBQx-TCl/PY-DT. An 18.07% PCE can be achieved in PBQx-TCl/PY-DT-based APSCs by adding 2 wt% L8-BO, which is one of the top values among PPHJ-APSCs processed with all green solvent. In consideration of the large number of NF-SMAs, using an NF-SMA as a solid additive offers huge possibilities for the further performance improvement of PPHJ-APSCs. It should be also noted that the NF-SMAs are nonvolatile, meaning they do not need to be removed from the active layers. This property helps simplify the device preparation process, which is compatible with future commercial production.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en17112623/s1, Figure S1: Absorption spectra of acceptor films. Figure S2: J-V curves of PPHJ-APSCs with different incorporation ratios of BTP-eC9; Table S1: Key parameters of PM6/PY-IT-based PPHJ-APSCs with different BTP-eC9 ratios; Table S2: The Jph&, Jph* and Jph/Jsat of typical PPHJ-APSCs; Table S3: The fitting parameters of related PPHJ-APSCs according to impedance spectroscopy; Figure S3: The normalized PCEs of APSCs’ dependence on storage time.

Author Contributions

Conceptualization, X.M. and F.Z.; methodology, J.L. and Y.N.; software, J.L. and Y.N.; validation, J.L., Y.N., J.Z. and Y.Z.; formal analysis, W.X.; investigation, J.L. and Y.N.; writing—original draft preparation, X.M.; writing—review and editing, F.Z.; funding acquisition, X.M. and F.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Training Program of Innovation and Entrepreneurship for Undergraduates (2024100041259), the National Natural Science Foundation of China (62175011, 62205276, 62105017 and 52311540151), the Natural Science Foundation of Beijing (4232073, 1232029), the Natural Science Foundation of Hebei Province (F2023105002) and the Chunhui Project Foundation of the Education Department of China (HZKY20220002).

Data Availability Statement

Data is contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Chemical construction, (b) HOMO and LUMO levels, (c) normalized absorption spectra of related materials, (d) absorption spectra of related layered films.
Figure 1. (a) Chemical construction, (b) HOMO and LUMO levels, (c) normalized absorption spectra of related materials, (d) absorption spectra of related layered films.
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Figure 2. (a) J-V curves and (b) EQE spectra of reference and optimal PPHJ-APSCs.
Figure 2. (a) J-V curves and (b) EQE spectra of reference and optimal PPHJ-APSCs.
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Figure 3. (a) Jph-Veff curves, (b) JSC-Plight curves, (c) Nyquist plots and equivalent circuit of the reference and optimal PPHJ-APSCs, (d) ln(JL3/V2)-(V/L)0.5 curves of electron-only devices.
Figure 3. (a) Jph-Veff curves, (b) JSC-Plight curves, (c) Nyquist plots and equivalent circuit of the reference and optimal PPHJ-APSCs, (d) ln(JL3/V2)-(V/L)0.5 curves of electron-only devices.
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Figure 4. (a) Grazing-incidence wide-angle X-ray scattering (GIWAXS) images and (b) GIWAXS intensity profiles along different directions for the reference and optimal films, in which black and red lines represent in-plane (IP) and out-of-plane (OOP) directions.
Figure 4. (a) Grazing-incidence wide-angle X-ray scattering (GIWAXS) images and (b) GIWAXS intensity profiles along different directions for the reference and optimal films, in which black and red lines represent in-plane (IP) and out-of-plane (OOP) directions.
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Figure 5. (a) Chemical structures of PBQx-TCl, PY-DT and L8-BO, (b) J-V curves and (c) EQE spectra of typical PPHJ-APSCs.
Figure 5. (a) Chemical structures of PBQx-TCl, PY-DT and L8-BO, (b) J-V curves and (c) EQE spectra of typical PPHJ-APSCs.
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Table 1. Key photovoltaic parameters of reference and optimal PPHJ-APSCs.
Table 1. Key photovoltaic parameters of reference and optimal PPHJ-APSCs.
Donor
Layer		Acceptor
Layer		JSCCal. JSCVOCFFPCE
(avg. ± dev.) a (mA cm−2)(mA cm−2)(avg. ± dev.) a (V)(avg. ± dev.) a (%)(avg. ± dev.) a
(%)
PM6PY-IT23.36
(23.16 ± 0.32)		22.860.94
(0.937 ± 0.007)		68.83
(68.92 ± 0.91)		15.11
(14.95 ± 0.12)
PM6PY-IT+
BTP-eC9		24.08
(23.87 ± 0.15)		23.580.94
(0.938 ± 0.006)		72.76
(73.02 ± 0.45)		16.47
(16.35 ± 0.09)
a Statistical data collected from 10 independent cells with different batches.
Table 2. Key photovoltaic parameters of PPHJ APSCs based on PBQx-TCl/PY-DT or PBQx-TCl/PY-DT+L8-BO as the active layer.
Table 2. Key photovoltaic parameters of PPHJ APSCs based on PBQx-TCl/PY-DT or PBQx-TCl/PY-DT+L8-BO as the active layer.
Donor LayerAcceptor
Layer		JSCCal. JSCVOCFFPCE
(avg. ± dev.) a (mA cm−2)(mA cm−2)(avg. ± dev.) a (V)(avg. ± dev.) a (%)(avg. ± dev.) a (%)
PBQx-TClPY-DT24.19
(23.97 ± 0.24)		23.080.96
(0.968 ± 0.004)		75.38
(74.69 ± 0.89)		17.51
(17.33 ± 0.13)
PBQx-TClPY-DT+
L8-BO		24.80
(24.59 ± 0.16)		23.730.96
(0.965 ± 0.005)		75.88
(75.82 ± 0.25)		18.07
(17.99 ± 0.08)
a Statistical data collected from 6 independent cells with different batches.
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Liu, J.; Ni, Y.; Zhang, J.; Zhao, Y.; Xu, W.; Ma, X.; Zhang, F. Nonfullerene Small Molecular Acceptor Acting as a Solid Additive Enables Highly Efficient Pseudo-Bilayer All-Polymer Solar Cells. Energies 2024, 17, 2623. https://doi.org/10.3390/en17112623

AMA Style

Liu J, Ni Y, Zhang J, Zhao Y, Xu W, Ma X, Zhang F. Nonfullerene Small Molecular Acceptor Acting as a Solid Additive Enables Highly Efficient Pseudo-Bilayer All-Polymer Solar Cells. Energies. 2024; 17(11):2623. https://doi.org/10.3390/en17112623

Chicago/Turabian Style

Liu, Jiayin, Yuheng Ni, Jiaqi Zhang, Yijun Zhao, Wenjing Xu, Xiaoling Ma, and Fujun Zhang. 2024. "Nonfullerene Small Molecular Acceptor Acting as a Solid Additive Enables Highly Efficient Pseudo-Bilayer All-Polymer Solar Cells" Energies 17, no. 11: 2623. https://doi.org/10.3390/en17112623

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