Rational design of perylenediimide-based polymer acceptor for efficient all-polymer solar cells

Perylenediimide (PDI)-based small molecules have significantly contributed to the development of non-fullerene acceptors, whereas the development of PDI-based polymer acceptors is relatively lagging behind. In this study, we designed and synthesized two PDI-based n-type polymers named as PF-PDI and PBDT-PDI, in which PDI was used as electron-deficient unit and fluorene (F) or benzodithiophene (BDT) were used as electronrich components. The density functional theory (DFT) calculations and grazing incidence wide-angle X-ray scattering (GIWAXS) results indicate that the PF-PDI shows larger steric hindrance and relatively weaker lamellar packing than that of PBDT-PDI. Comparing with PBDT-PDI, PF-PDI shows red-shift absorption and lower-lying HOMO level, which agrees well with the DFT results. A well-known wide bandgap polymer donor, PDBT-T1 was employed to fabricate polymer solar cells (PSCs) with the two acceptors. The all polymer solar cells (all-PSCs) based on PDBT-T1:PF-PDI showed a high power conversion efficiency (PCE) of 4.47%, which is approximately 2-fold larger than that of devices with PDBT-T1:PBDT-PDI (PCE = 2.70%).     

Perylene diimide-benzodithiophene D-A copolymers as acceptor in all-polymer solar cells

Two n-type conjugated D-A copolymers with perylene diimide (PDI) as acceptor unit and benzodithiophene (BDT) as donor unit, P(PDI-BDT-Ph) and P(PDI-BDT-Th), were synthesized and applied as electron acceptor in all-polymer solar cells (all-PSCs). P(PDI-BDT-Ph) and P(PDI-BDT-Th) films exhibit similar absorption spectra in the visible region with optical bandgap (E_g) of 1.65 eV and 1.55 eV respectively, and the identical LUMO level of −3.89 eV. The all-PSCs based on P(PDI-BDT-Ph) as acceptor and PTB7-Th as donor demonstrated a power conversion efficiency (PCE) of 4.31% with a short-circuit current density (J_sc) of 11.94 mA cm⁻², an open-circuit voltage (V_oc) of 0.81 V, and a fill factor (FF) of 44.49%. By contrast, the corresponding all-PSCs with P(PDI-BDT-Th) as acceptor showed a relative lower PCE of 3.58% with a J_sc of 11.36 mA cm⁻², V_oc of 0.79 V, and FF of 40.00%.     

High performance all-polymer solar cells employing systematically tailored donor polymers

We successfully designed and synthesized a series of BDT-Qx-T based polymers as the donor polymer used in all polymer solar cells (all-PSCs). Their properties were finely tuned by side-chain modification and the introduction of electron-withdrawing fluorine atoms to polymer acceptor unit. Then we systematically investigated the effect of molecular structure on the polymer morphology and photovoltaic properties in all-PSCs. We revealed that the fluorination can optimize polymer energy level and improve the polymer coplanarity, leading to enhanced intermolecular packing and balanced carrier transport. Meanwhile, the substitution of dodecyl for 2-ethylhexyl side chains can result in improved film morphology and hole transport. As a result of the synergistic effect between fluorination and side-chain modification, we achieved a high PCE of 5.35% for the optimized all-PSCs. More importantly, our approach may become a general and effective way to tailor the polymer molecular structure for achieving high performance all-PSCs.     

Optimizing the morphology of all-polymer solar cells for enhanced photovoltaic performance and thermal stability

Due to the complicated film formation kinetics, morphology control remains a major challenge for the development of efficient and stable all-polymer solar cells(all-PSCs). To overcome this obstacle, the sequential deposition method is used to fabricate the photoactive layers of all-PSCs comprising a polymer donor PTzBI-oF and a polymer acceptor PS1. The film morphology can be manipulated by incorporating amounts of a dibenzyl ether additive into the PS1 layer. Detailed morphology investigations ...     

End-chain effects of non-fullerene acceptors on polymer solar cells

Four acceptor1-acceptor2-donor-acceptor2-acceptor1 (A1-A2-D-A2-A1) structural electron acceptors with different end-chains were designed and synthesized which all possessed indacenodithiophene (IDT) core, benzothiadiazole (BT) bridge as acceptor2, and rhodanine (R) end groups as acceptor1. The non-fullerene acceptor attached with ethyl group is called IDT-BT-R2 and used as control compound. And the other three of them are attached with methoxymethyl, trifluoroethyl and 1-piperidino groups generating IDT-BT-RO, IDT-BT-RF3 and IDT-BT-RN, respectively. The influence of end-chains on their optoelectronic properties were compared between four non-fullerene acceptors. Compared with IDT-BT-R2, the molecule IDT-BT-RF3 show red-shifted light absorption and lower LUMO level because of the electron withdrawing property of fluorine atoms. OSCs based on IDT-BT-RF3 display more efficient charge separation and lower degree of monomolecular recombination, allowing OSCs to show higher short-circuit current (J_sc) than the system of IDT-BT-R2. OSCs based on IDT-BT-RO also show more efficient charge separation and less monomolecular recombination. Due to the elevated LUMO level of the acceptor IDT-BT-RN, organic solar cells (OSCs) utilizing this material as acceptor display high open-circuit voltage (V_oc) of 1.10 eV and low energy loss of 0.49 eV when maintaining a relatively high power conversion efficiency (PCE) of 7.09%. We demonstrated that the end-chain engineering could finely tune the light absorption properties and energy levels of novel non-fullerene acceptors and eventually improved OSCs performance can be harvested.     

Construction of simple and low-cost acceptors for efficient non-fullerene organic solar cells

The flexibility in structural design of organic semiconductors endows organic solar cells (OSCs) not only great function-tunabilities, but also high potential toward practical application. In this work, four simple and low-cost non-fullerene acceptors with fluorene or carbazole as central cores, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c] thiophen-4-ylidene)malononitrile (TC) as terminal groups, and thiophene or furan as linkers, named DTC-T-F, DTC-F-F, DTC-T-C and DTC-F-C, are developed through twostep synthesis, and their photophysical properties, electrochemical behavior and photovoltaic performance are systematically and comparatively studied. The results revealed that fluorene-based acceptors exhibited superior photophysical properties and morphology characteristics than carbazole-based counterparts, and thiophene is more suitable as bridging groups. Combining the advantages of both, the BHJ-OSC based on PTB7-Th:DTC-T-F blend film showed a high PCE of 8.8%, with a V_oc of 0.78 V, a J_sc of 17.46 mA cm⁻², and an FF of 0.65, which is the highest value in the PTB7-Th and fluorene-based acceptors coupled devices, implying its potential application.     

Photo-generated charge behaviors in all-polymer solar cells studied by Kelvin probe force microscopy

Photo-generated charge behaviors in the bulk heterojunctions (BHJs) of all-polymer solar cells (PSCs) are studied by Kelvin probe force microscopy (KPFM). Root-mean-square deviations (RMSDs, R_q) of the contact potential difference (CPD) images are applied to quantitatively characterize the phase segregations of the BHJs. When the BHJs are illuminated, CPD values and R_q of CPD images are changed, which attributes to the photo-generated charge transfer and accumulation. Inner structures of the BHJ are thus extrapolated by studying the charge behaviors, demonstrating KPFM an effective technique to study the relationship between inner structures and photovoltaic activities in all-PSCs.     

Small-molecule acceptors with long alkyl chains for high-performance as-cast nonfullerene organic solar cells

Three fused-ring small-molecule electron acceptors, IDTC16-IC, IDTC16-Th, and IDTC16-4F, were designed and synthesized by introducing indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), fluorinated IC, and a thiophene-based unit as the electron-withdrawing end group. Here, instead of the commonly used n-hexyl or n-hexylphenyl side chains, n-hexadecyl peripheral substituents were employed at the IDT core to study the influence of alkyl groups on photovoltaic performance of the nonfullerene acceptors. The introduction of flexible n-hexadecyl group endowed the three acceptors with excellent solubility in common organic solvents. All the three acceptors presented strong absorption ranging from 450 nm to 720 nm in solution with high molar extinction coefficients. As a result, the as-cast organic solar cells (OSCs) based on IDTC16-IC and the wide bandgap polymer donor PM6 exhibited a power conversion efficiency (PCE) of 5.12%. The OSCs based on PM6:IDTC16-Th and PM6:IDTC16-4F showed much better photovoltaic performance with PCEs of 8.76% and 8.55%, respectively. The PCE values were improved to 5.89%, 9.09%, and 9.42% for the PM6:IDTC16-IC, PM6:IDTC16-Th, and PM6:IDTC16-4F OSCs, respectively, with the addition of the solvent additive 1,8-diiodooctane. These findings demonstrate that the combination of alkyl chains at the fused rings and fluorination or aromatic structure change of the terminal groups leads to greatly enhanced photovoltaic performance of nonfullerene acceptors through improving the photophysical, molecular orbital, and film morphological properties.     

Near-infrared absorbing non-fullerene acceptors with unfused D-A-D core for efficient organic solar cells

Two non-fullerene acceptors based on D-A-D-type unfused central units, i.e., BCPDT-1 and BCPDT-2, were synthesized, employing 3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2ethylhexyl)benzo-[1,2:4,5-c′]-dithiophene-4,8-dione (BDD) unit as the A moiety and 4,4-dialkyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) unit as the D moiety. The two molecules possess identical backbones, but carry different side chains (octyl for BCPDT-1 and 2-ethylhexyl for BCPDT-2) on CPDT units. Both BCPDT-1 and BCPDT-2 presented broad absorption extending to near-infrared region with optical band gaps of 1.36 and 1.39 eV, respectively. Organic solar cells (OSCs) were fabricated with PBDB-T as donor and BCPDT-1 or BCPDT-2 as acceptor. The devices based on BCPDT-2 exhibited efficient exciton dissociation and charge collection as well as weak charge recombination, attributed to the proper film morphology with nano-scale phase separation and favorable molecular orientation. Consequently, the BCPDT-2 based device displayed a higher power conversion efficiency (PCE) of 10.65%, while the BCPDT-1 based device showed an inferior PCE of 7.54%.     

3-Dimensional non-fullerene acceptors based on triptycene and perylene diimide for organic solar cells

Two molecules based on triptycene and perylene diimide (PDI) were designed and synthesized as non-fullerene acceptors for organic solar cells (OSCs). The bay-substituted and the imide-substituted molecules, named as TPBA and TPI, respectively, have rigid three-dimensional backbones, which improved the morphological compatibility with the donor polymers. TPBA and TPI exhibit suitable energy levels as acceptors and efficient absorption in the range of 450–600 nm. Their blended films with PTB7-Th displayed power conversion efficiencies of 2.80% and 3.64%, respectively.     

Ternary organic solar cells based on non-fullerene acceptors: A review

Organic solar cells (OSCs) have reached their second golden age in recent two years with a boosted number of publications. Non-fullerene acceptor (NFA) materials have become a rising star in the field which are widely applied in organic solar cells because of their excellent optoelectronic properties, such as strong light-harvesting ability and tunable energy level. Unlike the low synthetic flexibility and high production cost of fullerene materials, NFAs exhibit flexible structures, and relatively low fabrication costs. Recently, the ternary strategy has become another hot research topic in the field, which introduces a third component into the binary host system for OSCs. The application of a ternary strategy can break the limits of light absorption brought by the host system, improve the morphology and energy level alignment for the active layer and thus improved the efficiency of organic solar cell devices. Benefiting from the advancement in both NFA and ternary strategy, the power conversion efficiency (PCE) of organic solar cell has exceeded over 17.5% to date. A comprehensive review of the recent progress in NFA based ternary OSCs (TOSCs) is needed in the field. Herein, this review mainly focuses on recent research on ternary organic solar cells using NFA materials during the last two years. Firstly, device physics and frequently used active materials in NFA based TOSCs are summarized and discussed. Then, the recent reported high-performance NFA based TOSCs are reviewed. Finally, the outlook and future research direction in the field are proposed. This review aims to provide an insight into NFA based TOSCs and help researchers to explore the full potential of OSCs.     

High-performance alloy-like ternary organic solar cells with two compatible non-fullerene acceptors

Ternary blending is one of the effective strategies to modulate the blend film morphology for achieving high efficiency organic solar cells (OSCs). In this work, high-performance ternary OSCs are fabricated by introducing a non-fullerene acceptor, namely IDTP-4F into the PM6:Y6 binary system to enhance the device performance. Detailed investigations indicate that IDTP-4F can form an alloy phase with Y6, resulting in the optimized morphology, which can facilitate the charge transport and reduce recombination, leading to enhanced open-circuit voltage (V_oc) and fill factor (FF) simultaneously. Consequently, the optimized ternary OSCs exhibit an excellent power conversion efficiency (PCE) of 17.1%, which is much higher than that of PM6:Y6 binary OSCs (15.9%). These results indicate that combining two compatible non-fullerene acceptors is an effective strategy to fabricate high efficiency ternary OSCs.     

Polymer solar cells incorporating one-dimensional polyaniline nanotubes

We have fabricated polymer solar cell devices based on poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) and incorporating one-dimensional nanostructured acid-doped polyaniline nanotubes (a-PANINs) as an interfacial layer. The power conversion efficiency of an annealed device incorporating the a-PANIN layer reached 4.26% under AM 1.5 G (100 mW/cm²) illumination, an increase of ca. 26% relative to that of the annealed device lacking an a-PANIN interfacial layer. The incorporation of the a-PANINs in the solution-processed polymer solar cells was reproducible; the high conductivity, controlled tubular nanoscale morphologies, and mobility of the annealed a-PANIN layer led to efficient extraction of photogenerated holes to the buffer layer and suppression of exciton recombination, thereby improving the photovoltaic performance.     

Recent advances and prospects of asymmetric non-fullerene small molecule acceptors for polymer solar cells

Recently,polymer solar cells developed very fast due to the application of non-fullerence acceptors.Substituting asym-metric small molecules for symmetric small molecule acceptors in the photoactive layer is a strategy to improve the perform-ance of polymer solar cells.The asymmetric design of the molecule is very beneficial for exciton dissociation and charge trans-port and will also fine-tune the molecular energy level to adjust the open-circuit voltage (Voc) further.The influence on the ab-sorption range and absorption intensity will cause the short-circuit current density (Jsc) to change,resulting in higher device per-formance.The effect on molecular aggregation and molecular stacking of asymmetric structures can directly change the micro-scopic morphology,phase separation size,and the active layer's crystallinity.Very recently,thanks to the ingenious design of act-ive layer materials and the optimization of devices,asymmetric non-fullerene polymer solar cells (A-NF-PSCs) have achieved re-markable development.In this review,we have summarized the latest developments in asymmetric small molecule acceptors(A-NF-SMAs) with the acceptor-donor-acceptor (A-D-A) and/or acceptor-donor-acceptor-donor-acceptor (A-D-A-D-A) struc-tures,and the advantages of asymmetric small molecules are explored from the aspects of charge transport,molecular energy level and active layer accumulation morphology.     

Polymer additive assisted crystallization of perovskite films for high-performance solar cells

Perovskite solar cells (PSCs) have attracted much attention as a novel photoelectric converter. The quality of perovskite films plays a key role in the efficiency and stability. Among them, defects in the films surface restrict the performance of solar cells. Surface passivation is an effective route to eliminate defect of perovskite films. In this paper, we introduce PVB as a novel polymer additive, it can assist perovskite films with better crystallinity and morphology, as well as less defects. Perovskite solar cells with 1.5 mg/mL optimized concentration PVB exhibit power conversion efficiency (PCE) of 19.04% than 16.34% of control cells. Meanwhile, the cells with PVB demonstrate less hysteresis than that without additive, as well as excellent reproducibility. Additionally, perovskite solar cells based on PVB can retain around 90% of its original efficiency under fully ambient air of 65 ± 5% relative humidity or under 65 °C after aging for 30 days. The finding provides a potential additive candidate for fabrication of higher performance devices.     

Non-fullerene acceptors based on multiple non-covalent interactions for low cost and air stable organic solar cells

Significant progresses have been made on organic solar cells (OSCs) in last few years, but the industrialization of OSCs is still hindered by high cost and poor device stability. Recently, the most successful OSCs are usually based on fused-ring small molecule non-fullerene acceptors (SM-NFAs) which often involve multiple complex synthetic steps and result in low overall yields and high cost. Herein, two easily accessible non-fused NFAs based on BT, CPDT and IC with different halogen atoms named BT-F and BT-Cl were designed and synthesized. Non-covalent interactions or conformation locks between BT and adjacent CPDT units provided planar conformations, which is beneficial for molecular packing and charge transport. When blended with low cost polymer donor PTQ-10, optimal devices based on BT-F provided a PCE of 8.26%. A higher PCE of 8.65% was obtained from BT-Cl-based devices. Low voltage loss of 0.57 and 0.59 V were observed for BT-F- and BT-Cl-based devices. It is noteworthy that devices based on PTQ-10:BT-F and PTQ-10:BT-Cl exhibited excellent stabilities when stored in ambient atmosphere for 10 days without encapsulation. This work demonstrates an example of pursuing low cost OSCs with excellent air stability.     

Polymer passivation of defects in inorganic perovskite solar cells

Inorganic perovskite solar cells (IPSCs) have attained attention due to their excellent thermal and phase stability. In this work, we demonstrate a novel approach for fabricating IPSCs, using the strategies of interface passivation and anti-solvent before spin-coating perovskite. Poly(methyl methacrylate) (PMMA) and chlorobenzene (CB) are used as passivator and anti-solvent, respectively. The CB improves the perovskite crystal morphology. Meanwhile, PMMA passivates the defects between poly(3, 4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and perovskite layer, thus increasing the short-circuit current. Excitingly, we find that PMMA benefits the grain boundaries (GBs) of perovskite, which makes it more humidity-resistant, increasing the stability of perovskite film. Especially, PMMA mitigates interfacial charge losses, and the devices based on CsPbI3-xBrx passivated by PMMA exhibit the power conversion efficiency (PCE) much higher than those based on pure CsPbI3-xBrx.     

Mixed C60/C70 based fullerene acceptors in polymer bulk-heterojunction solar cells

Different mixtures of identically substituted C60 and C70 based fullerens have been used as acceptors in three polymer:fullerene systems that strongly express various performance limiting aspects of bulk heterojunction solar cells. Results are correlated with, and discussed in terms of e.g. morphology, charge separation, and charge transport. In these systems, there appears to be no relevant differences in either mobility or energy level positions between the identically substituted C60 and C70 based fullerenes tested. Examples of how fullerene mixtures influence the nano-morphology of the active layer are given. An upper limit to the open circuit voltage that can be obtained with fullerenes is also suggested.     

Fused-ring acceptors based on quinoxaline unit for highly efficient single-junction organic solar cells with low charge recombination

Two non-fullerene small molecule acceptors (TFQ-F and TFQ-Cl) based on quinoxaline unit were designed and synthesized for efficient organic solar cells (OSCs). These two acceptors showed intense absorption up to 900 nm and high thermal stabilities with decomposition temperatures over 360 °C due to their fused-ring skeletons. TFQ-F and TFQ-Cl are the A-D-A′-D-A type acceptors (A/A′ for acceptor unit and D for donor unit). TFQ-F and TFQ-Cl have the same D-A′-D fragment, which was flanked with different ending groups. The effect of different ending groups on their photophysical properties, electrochemical behaviors, micro-structures and charge recombination properties of active layers, and device performance were investigated systematically. PM6 with the complementary absorption to the two acceptors was used as the donor material. The pristine PM6:TFQ-F blend films displayed the optimal morphologies as revealed by AFM and TEM measurement. Organic solar cells based on PM6:TFQ-Cl blend film showed high J_SC of 25.19 mA/cm² and PCE of 13.2%. The Voc, J_SC and PCE for PM6:TFQ-F film based device were 0.857 V, 23.70 mA/cm² and 13.51%, respectively. The dependence of V_OC/J_SC on various light intensities indicated that PM6:TFQ-F/Cl based device had low charge recombination.     

Enhancing the performance of non-fullerene solar cells with polymer acceptors containing large-sized aromatic units

In this work, we develop four diketopyrrolopyrrole-based polymer acceptors for application in polymer-polymer solar cells. The polymer acceptors contain different-sized aromatic units, from small thiophene to benzodithiophene and large alkylthio-benzodithiophene units. Although the polymer acceptor with large-sized groups shows small LUMO offset and low energy loss when blended with the donor polymer PTB7-Th, the corresponding solar cells can achieve a high power conversion efficiency (PCE) of 3.1% due to high photocurrent. In contrast, the polymer acceptor with small thiophene units only provides a low PCE of 0.14% in solar cells. These results indicate that polymer acceptors with large-sized aromatic units can be potentially used into high performance non-fullerene solar cells.