Polymer-fullerene Solar Cells Research Articles

Performance prediction of polymer-fullerene organic solar cells and data mining-assisted designing of new polymers.

Selecting high performance polymer materials for organic solar cells (OSCs) remains a compelling goal to improve device morphology, stability, and efficiency. To achieve these goals, machine learning has been reported as a powerful set of algorithms/techniques to solve complex problems and help/guide exploratory researchers to screen, map, and develop high performance materials. In present work, we have applied machine learning tools to screen data from reported studies and designed new polymer acceptor materials, respectively. Quantitative structure-activity relationship (QSAR) models were generated using machine learning-assisted simulation techniques. For this purpose, 3000 molecular descriptors are generated. Consequently, molecular descriptors having key effect on power conversion efficiency (PCE) were identified. Moreover, numerous regression models (e.g., random forest and bagging regressor models) were developed to predict the PCE. In particular, new materials were designed based on the similarity analysis. The GDB17 chemical database consisting of 166 million organic molecules in an ordered form is used for performing similarity analysis. A similarity behavior between GDB17 materials and the materials reported in literature is studied using RDKit (a cheminformatics software). Noteworthily, 100 monomers proved to be unique and effective, and PCEs of these monomers are predicted. Among these monomers, four monomers exhibited PCE higher than 14%, which is better than various reported studies. Our methodology provides a unique, time- and cost-efficient approach to screening and designing new polymers for OSCs using similarity analysis without revisiting the reported studies. To perform machine learning analysis, data from reported studies and online databases was collected. Different molecular descriptors were generated for polymer materials utilizing Dragon software. 3D structures of studied molecules were applied as input (SDF; structure data file format). Importantly, about 3000 molecular descriptors were generated. Comma-separated value (.csv) file format was used to export these molecular descriptors. To shortlist best descriptors, univariate regression analysis was performed. These descriptors were further utilized for training machine learning models. Moreover, necessary packages of Python for data analysis and visualization were imported such as Matplotlib, Numpy, Pandas, Scikit-learn, Seaborn, and Scipy. Random forest and bagging regressor models were applied for performing machine learning analysis. A cheminformatics software, RDKit, was applied for similarity analysis.

Microscopic Mechanism of High Efficiency and Stability of Inverted Polymer Fullerene Solar Cells

The performance of organic solar cells has been remarkably improved recently, where the cell structures are important for achieving high efficiency and stability. The formation and accumulation of long-lived charges in the cells are critical for the efficiency and stability of the cells; however, their relations with the cell structures have not yet been clarified from a microscopic viewpoint. Here, we report the microscopic mechanism of high efficiency and stability of inverted polymer fullerene solar cells compared to conventional cells even though the same photoactive layers are utilized. We directly observe the formation and accumulation of long-lived charges in these cells with electron spin resonance at a molecular level. We find the reduced effects of the formation and accumulation of long-lived charges in the inverted cells on their efficiency and stability compared to the case of conventional cells. These findings provide a striking advance in fundamental understanding, which is useful for further clarifying the operation mechanism of organic solar cells as well as further improving their efficiency and stability.

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC 71 BM acceptor, the PBDTTS-DFIQ:PC 71 BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility ( μ h / μ e ), and nice morphology in comparison with PBDTTS-IQ:PC 71 BM blend film. Encouragingly, the PBDTTS-DFIQ:PC 71 BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC 71 BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage ( V oc ) of 0.84 V, short current density ( J sc ) of 13.26 mA cm −2 and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs. Two novel NBG D-A copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric indo [2,3-b]quinoxaline (IQ) unit as acceptor unit through tuning the side chains with halogen atom fluorine (F) engineering, aiming to achieve novel copolymer donors with good photovoltaic performance. • Both narrow bandgap copolymers were developed based on asymmetric planar aromatic backbone indo [2,3-b]quinoxaline as an acceptor unit. • The fluorinated PBDTTS-DFIQ exhibits a lower HOMO energy level, better planarity and higher dipole moment in comparison with PBDTTS-IQ. • The PBDTTS-DFIQ:PC 71 BM-based device presents a maximum PCE of 7.40%. • Fluorinated asymmetric indo [2,3-b]quinoxaline framework is a wonderful acceptor moiety to build light-harvesting copolymers for efficient PSCs.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Investigation of polyaniline doped with camphorsulfonic acid in chloroform solution as a hole transporting layer in PTB7: PCBM and perovskite-based solar cells

Polyaniline was chemically synthesized and protonated (doped) with CSA in chloroform with different PANI:CSA molar ratios, ranging from 6:1 to 1:4. Protonation level of the polymer was monitored by means of UV–Vis spectra of the solution and FTIR measurements of thin films. The results were correlated with the changes of specific conductivity in a function of PANI:CSA molar ratio in the films. A broad maximum of conductivity was found in the range from 1:1 to 1:3, i.e. for over-protonated polymer. The solutions with various PANI doping levels were used as hole transport layers (HTLs) in the polymer-fullerene (PTB7:PC70BM) and perovskite (MAPbI3) solar cells. The highest power conversion efficiency (PCE=3.8%) obtained for the organic cell with molar ratio PANI:CSA=1:1 was only slightly lower than that of the reference cell with PEDOT:PSS HTL (4%). The best perovskite cells achieved PCE of about 4% which was markedly lower that the efficiency of cells with Spiro-OMeTAD HTL. However, some of the perovskite cells with PANI:CSA revealed very high short current density, of about 20 mA cm−2 but relatively low open circuit photovoltage (0.42 V). The changes of the PCE in a function of PANI protonation level were discussed in terms of alignment of HOMO and LUMO levels of HTL, donor and acceptor layers in the solar cells as well as quality of interfacial layers.

Degradation of Polymer Solar Cells: Knowledge Learned from the Polymer:Fullerene Solar Cells

Although power conversion efficiency (PCE) of polymer solar cells (PSCs) has exceeded 18%, the lifetime of PSCs is far away from satisfactory and has become the most critical issue before the commercialization of PSCs. Understanding the degradation mechanism and finding the proper way to suppress the degradation process are of high interests for PSC research. Polymer:fullerene solar cells (PFSCs) have a long research history and the working principle is well‐understood, so they are good models for the degradation mechanism study. This mini‐review summarizes the latest research progress of the PFSCs’ degradation studies. Based on the results achieved, one can identify the extrinsic and intrinsic stress factors that cause different degradation processes. Degradation pathways of the PFSCs at different positions in the cells will be discussed in detail, including the electrode, the buffer layer, and the photoactive layer. Special emphasis will be put on various degradation processes of the photoactive layers. In the meantime, the methods to suppress the degradation processes will be presented accordingly. This mini‐review gives a comprehensive insight into the degradation mechanism and stability‐improvement strategies of PFSCs, which has important guiding significance for the stability‐improvement research of PSCs.

Evidence for Field-Dependent Charge Separation Caused by Mixed Phases in Polymer-Fullerene Organic Solar Cells.

As organic photovoltaic performance approaches 20% efficiencies, causal structure-performance relationships must be established for devices to realize theoretical limits and become commercially competitive. Here, we reveal evidence of a causal relationship between mixed donor-acceptor interfaces and charge generation in polymer-fullerene solar cells. To do this, we combine a holistic loss analysis of device performance with quantitative synchrotron X-ray nanocharacterization to identify a >98% anticorrelation between field-dependent geminate recombination and nanodomain purity. Importantly, our analysis eliminates other possible explanations of the performance trends, a requirement to establish causality. The unprecedented granular level of our analysis also separates field-dependent and field-independent recombination at the interface, where we find for the first time that this system is free of field-independent recombination, a loss channel that plagues high-performance systems, including those with non-fullerene acceptors. This result broadens the case that minimizing mixed phases to promote sharp interfaces between pure aggregated domains is the ideal nanostructure for realizing theoretical efficiency limits of organic photovoltaics.

Naphtho[2,3-c]thiophene-4,9-dione based polymers for efficient fullerene solar cells

Designing and synthesizing new organic photovoltaic materials is an inexhaustible driving force for the development of polymer solar cells (PSCs). For an excellent molecular structure with a certain conjugated skeleton, studying its analogues is of considerable significance for fully exploiting its photovoltaic potential. In this work, naphtho[2,3-c]thiophene-4,9-dione (NTD), an analogue of benzo [1,2-c:4,5-c’]dithiophene-4,8-dione (BDD), was designed and synthesized. Based on this building block, alkoxy side chains and alkylthio side chains were grafted on NTD to build NTD-O and NTD-S unit respectively, and then two new D-A conjugated donor polymers, PBNO and PBNS, were synthesized with NTD-O and NTD-S units as acceptor units and asymmetric benzodithiophene unit (asy-BDTBP) as donor units. Later, the photoelectric properties of these two materials in fullerene PSCs were studied. Compared with BDD based benzodithiophene copolymer (asy-BDTBP-BDD), PBNO and PBNS show similar LUMO energy levels, −3.59 eV and −3.63 eV, respectively, PBNO and PBNS show higher HOMO energy levels, which are −5.25 eV and −5.37 eV, respectively. PBNO blended with fullerene acceptor PC71BM exhibited similar power conversion efficiency (PCE) compared to the copolymer asy-BDTBP-BDD (4.64%). The PCE is 4.71% with short-circuit current density (JSC) of 9.85 mA cm−2, open-circuit voltage (VOC) of 0.784 V and fill factor (FF) of 60.98%. Encouragingly, PBNS presented an obviously enhanced PCE in PC71BM system PSCs, and PCE boosted to 7.58% (JSC, 12.40 mA cm−2; VOC, 0.865 V; FF, 70.83%). This work shows that analogues of excellent molecules have huge photovoltaic potential, and that side-chain modification has a significant impact on improving the photovoltaic performance of PSCs.

Low-cost donors based on a dicarboxylic ester side-chain substituted thieno[3,2b]thiophene unit for efficient polymer solar cells

In this article, both novel wide bandgap (WBG) copolymers, namely, PBTTBO-BT, and PBTTBO-TVT, based on a dicarboxylic ester side-chains substituted thieno[3,2-b]thiophene (BTTBO) unit as weak electron-deficient building skeleton and a simple and low-cost 2,2′-bithiophene (BT) or (E)-1,2- di(thiophen-2-yl)ethylene (TVT) as electron-rich donor unit, were successfully synthesized and applicated as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). With the replacement of BT with TVT group, the resulting copolymer PBTTBO-TVT shows higher carrier mobility, broad absorption and better morphologies of blend films, which is mainly ascribed to better coplanarity and π-conjugated system of TVT unit. Under an optimized blend ratio of copolymer:PC71BM (1:1, w/w), the PC71BM-based PSCs with PBTTBO-TVT as donor, obtained a high power conversion efficiency (PCE) of 7.05%, noticeably higher than that of the PCE of 5.78% in device based on PBTTBO-BT:PC71BM, which is owing to an increased short circuit current density (Jsc) of 12.96 mA cm−2 and fill factor (FF) of 66.34% simultaneously. In considering the merits of low-cost and high efficiency, this type of copolymer should be a hopeful donor material for efficient fullerene-PSCs.

Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells.

Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4, 5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells.

State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer-fullerene BHJ solar cells in terms of efficiency, thermal-, and photostability. Considering the strong light absorption in the near-infrared region (600-1000 nm) for most of the efficient acceptors, the exploration of high-performing large band gap (LBG) polymer donors with complementary optical absorption ranging from 400 to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (-F) and unsaturated nitrogen (-N) substituents along the polymer backbones is used to develop the LBG polymer donor PB[N][F]. Results show that the F- and N-substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with an efficiency of only 3.6% and PB[N] with an efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport, and reduce bimolecular recombination and trap-assisted recombination in the PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F- and N-substituted conjugated polymers are promising electron-donor materials for solution-processed non-fullerene BHJ solar cells.

Metal–organic framework nanosheets for enhanced performance of organic photovoltaic cells

Metal–organic framework nanosheets (MONs) are incorporated into the active layer of bulk heterojunction polymer–fullerene solar cells for the first time, resulting in an almost doubling of power conversion efficiency.

Revisiting the Role of Graphene Quantum Dots in Ternary Organic Solar Cells: Insights into the Nanostructure Reconstruction and Effective Förster Resonance Energy Transfer

Recent studies have introduced the graphene quantum dot (GQD) as a third material for the bulk-heterojunction polymer:fullerene solar cell (PSC) to improve light conversion efficiency. Although exciton generation/dissociation and carrier transport in the GQD-incorporated light-absorbing layer are strongly influenced by the ternary component, detailed analysis on the role of GQD in the light-absorbing layer is lacking. In this study, a perspective on origin of improved photovoltaic performance of GQD-incorporated PSC is provided. The Forster resonance energy transfer (FRET) from GQD to polymer:fullerene and reorganization of the ternary-component film are analyzed. The GQD chemical nature difference after controlling nitrogen functionality affects the quantum yield (QY) and surface energy. Because the GQD is distributed in the fullerene-rich domain, actual improvements in the FRET to polymer-rich phase are not great, despite the improved QY and red-shifted photoluminescence. However, changes in the surface...

Recent Advances, Design Guidelines, and Prospects of All-Polymer Solar Cells.

All-polymer solar cells (all-PSCs) consisting of polymer donors (PDs) and polymer acceptors (PAs) have drawn tremendous research interest in recent years. It is due to not only their tunable optical, electrochemical, and structural properties, but also many superior features that are not readily available in conventional polymer-fullerene solar cells (fullerene-PSCs) including long-term stability, synthetic accessibility, and excellent film-forming properties suitable for large-scale manufacturing. Recent breakthroughs in material design and device engineering have driven the power conversion efficiencies (PCEs) of all-PSCs exceeding 11%, which is comparable to the performance of fullerene-PSCs. Furthermore, outstanding mechanical durability and stretchability have been reported for all-PSCs, which make them stand out from the other small molecule-based PSCs as a promising power supplier for wearable electronic devices. This review provides a comprehensive overview of the important work in all-PSCs, in which pertinent examples are deliberately chosen. First, we describe the key components that enabled the recent progresses of all-PSCs including rational design rules for efficient PDs and PAs, blend morphology control, and light harvesting engineering. We also review the recent work on the understanding of the stability of all-PSCs under various external conditions, which highlights the importance of all-PSCs for future implementation and commercialization. Finally, because all-PSCs have not yet achieved their full potential and are still undergoing rapid development, we offer our views on the current challenges and future prospects.

Unraveling the unstable amorphous phase evolution effect on burn-in loss in polymer-fullerene solar cells

Abstract The performance degradation of organic photovoltaics is one of the main limitations to their commercialization, especially an initial, rapid degradation-period, termed as a burn-in loss. Herein, we demonstrate that burn-in loss is caused by the presence of thermodynamically unstable mixed-amorphous phase in the active layer. By using a combination of grazing incidence small angle X-ray scattering and photophysical process experiments, we observe that [6,6]-Phenyl C61 butyric acid methyl ester (PC61BM) molecules rapidly move from the finely-mixed amorphous regimes into crystalline or ordered regimes under accelerated conditions (ISOS-D-2 protocol), resulting in a fast short circuit current (Jsc) burn-in loss. Following an ultraviolet radiation (UV) light processing to control amorphous phase, the kinetics of thermally induced PC61BM crystallization can be slowed down due to the formation of the photo-induced PC61BM dimers in the active layer leading to a smaller burn-in loss. In the case of UV irradiated films, even if the molecules migrated out of amorphous phase, the PC61BM-rich phase becomes loose, rather than the compact and sphere-shaped aggregation, which has been observed in pristine films. These analyses reveal the evolution of thermodynamically unstable amorphous phases eventually leading to a decline power conversion efficiency within a short period of time. The results provide a further design basis of organic photovoltaic materials with good thermal stability and high efficient simultaneously.

A Chlorinated π-Conjugated Polymer Donor for Efficient Organic Solar Cells

Summary We synthesized a chlorinated benzothiadiazole-T4 polymer donor PBT4T-Cl, in which a chlorine atom had been introduced at the 4 position in the middle thiophene unit to fine-tune the energy level of the final polymers. Compared with its non-chlorinated analog, the PBT4T-Cl-based devices exhibited clear increases in open-circuit voltage and fill factor, achieving power conversion efficiencies (PCEs) up to 11.18% with simple synthesis, which is the highest PCE of the chlorinated polymer-based fullerene polymer solar cells (PSCs) reported to date. Grazing incident wide-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy measurements revealed an optimized morphology of the spin-coated PBT4T-Cl blend films, which supported that chlorine substitution could promote the performance of PSCs. More importantly, the PBT4T-Cl-based devices showed superior stability, with a PCE of 8.16% after 50 days' device storage. Through this research, the chlorination of low-band-gap polymers provides new insight into designing π-conjugated polymer semiconductors and realizing further enhancement of polymer solar cell efficiency as well as stability.

The Impact of Device Polarity on the Performance of Polymer–Fullerene Solar Cells

AbstractDiketopyrrolopyrrole (DPP)‐conjugated polymers are a versatile class of semiconductors for application in organic solar cells because of their tunable optoelectronic properties. A record power conversion efficiency (PCE) of 9.4% was recently achieved for DPP polymers, but further improvements are required to reach true efficiency limits. Using five DPP polymers with different chemical structures and molecular weights, the device performance of polymer:fullerene solar cells is systematically optimized by considering device polarity, morphology, and light absorption. The polymer solubility is found to have a significant effect on the optimal device polarity. Soluble polymers show a 10–25% increase in PCE in inverted device configurations, while the device performance is independent of device polarity for less soluble DPP derivatives. The difference seems related to the polymer to fullerene weight ratio at the ZnO interface in inverted devices, which is higher for more soluble DPP polymers. Optimization of the nature of the cosolvent to narrow the fibril width of polymers in the blends toward the exciton diffusion length enhances charge generation. Additionally, the use of a retroreflective foil increases absorption of light. Combined, the effects afford a PCE of 9.6%, among the highest for DPP‐based polymer solar cells.

Charge generation dynamics in polymer nonfullerene solar cells with low energy loss

Polymer nonfullerene solar cells are emerging as an alternative of polymer fullerene solar cells. However, maximizing the short-circuit density and open-circuit voltage is a critical issue in these solar cells. Here, using ultrafast spectroscopy, we measured exciton relaxation and charge separation dynamics in polymer nonfullerene blend with low driving force. Our study indicates high polaron yield despite the energy loss of as low as 0.59 eV. This suggests that the small driving force has minimum detrimental effect in realizing high performance in polymer nonfullerene solar cells.

Comparative Study of the Mechanical Properties of All-Polymer and Fullerene–Polymer Solar Cells: The Importance of Polymer Acceptors for High Fracture Resistance

High fracture resistance of polymer solar cells (PSCs) is of great importance to ensure long-term mechanical reliability, especially considering their potential in roll-to-roll printing processes and flexible devices. In this paper, we compare mechanical properties, such as the cohesive fracture energy, elastic modulus, and crack-onset strain, of all-polymer solar cells (all-PSCs) and fullerene-based solar cells (PCBM–PSCs) based on the same, representative low-bandgap polymer donor (PTB7-Th) as a function of acceptor content. The all-PSCs exhibit higher fracture energy (2.45 J m–2) than PCBM–PSCs (0.29 J m–2) at optimized device conditions. Additionally, a 15-fold higher crack-onset strain is observed in all-PSCs than in PCBM–PSCs. Dramatically different mechanical compliances observed for all-PSCs and PCBM–PSCs are investigated in detail by analysis of the blend morphologies as a function of acceptor content (either P(NDI2HD-T) or PCBM acceptors). The superior fracture resistance of all-PSCs is attribut...

Fluorinated and Alkylthiolated Polymeric Donors Enable both Efficient Fullerene and Nonfullerene Polymer Solar Cells

AbstractIn this work, four donor (D)–acceptor (A) copolymers based on benzodithiophene (BDT) and benzothiadiazole (BT) with different alkylthiolated and/or fluorinated side chains are developed for efficient fullerene and nonfullerene polymer solar cells (PSCs). The synergistic effect of sulfuration and fluorination on the optical absorption, energy level, crystallinity, carrier mobility, blend morphology, and photovoltaic performance is investigated systematically. By incorporating sulfur atoms onto the side chains, a little blueshifted but significantly increased absorption can be obtained for PBDTS‐FBT compared to PBDT‐FBT. On the other side, a little more blueshifted but much stronger absorption and much lower‐lying highest occupied molecular orbital (HOMO) level can be realized for PBDTF‐FBT when introducing fluorine atoms instead of sulfur atoms. With the combination of both fluorination and sulfuration strategies, PBDTS‐FBT exhibits the best absorption ability, lowest HOMO energy level, and highest crystallinity, which make PBDTSF‐FBT devices show the highest power conversion efficiency (PCE) of 10.69% in fullerene PSCs and 11.66% in nonfullerene PSCs. The PCE of 11.66% is the best value for PSCs based on BT‐containing copolymer donors reported so far. The results indicate that fluorination and sulfuration have a synergistically positive effect on the performance of D–A photovoltaic copolymers and their solar cell devices.