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.     

A systematical investigation of non-fullerene solar cells based on diketopyrrolopyrrole polymers as electron donor

Diketopyrrolopyrrole (DPP)-based conjugated polymers have been successfully applied in high performance field-effect transistors and fullerene-based solar cells, but show limited application in non-fullerene solar cells. In this work, we use four DPP polymers as electron donor and a perylene bisimide dye as electron acceptor to construct non-fullerene solar cells. The donors and acceptor have complementary absorption spectra in visible and near-infrared region, resulting in broad photo-response from 300 nm to 1000 nm. The solar cells were found to provide relatively low power conversion efficiencies of 1.6–2.6%, which was mainly due to low photocurrent and fill factor. Further investigation reveals that the low performance is originated from the high charge recombination in photo-active layers. Our systematical studies will help better understand the non-fullerene solar cells based on DPP polymers and inspire new researches toward efficient non-fullerene solar cells with broad photo-response.     

Triphenylamine-cored star-shape compounds as non-fullerene acceptor for high-efficiency organic solar cells: Tuning the optoelectronic properties by S/Se-annulated perylene diimide

Three novel star-shaped S/Se-annulated perylene diimide (PDI) small molecule acceptors with triphenylamine as the core, namely TPA-PDI, TPA-PDI-S and TPA-PDI-Se, were designed and synthesized. Using the wideband-gap polymer PDBT-T1 as the donor and Se-annulated perylene diimide (TPA-PDI-Se) as the acceptor, power conversion efficiencies (PCE) of up to 6.10% was achieved, which is 38% higher than the reference of TPA-PDI without heteroatom annulation. Impressively, the S/Se-annulated perylene diimides as acceptors showed high open-circuit voltage (V_OC) of 1.00 V. The high efficiency for TPA-PDI-Se can be attributed to complementary absorption spectra with the donor material, relatively high-lying LUMO level, balanced carrier transport and favorable morphologies. To the best of our knowledge, this PCE of 6.10% is among the highest values based on star-shaped non-fullerene acceptors so far.     

Towards high-efficiency non-fullerene organic solar cells: Matching small molecule/polymer donor/acceptor

Two low band-gap donors, small molecule p-DTS(FBTTh₂)₂ (S_D) and polymer PBDTTT-C-T (P_D), and two perylene diimide acceptors, small molecule PDI-2DTT (S_A) and polymer PPDIDTT (P_A), were used to fabricate non-fullerene organic solar cells. The effects of four donor/acceptor combinations, P_D/P_A, P_D/S_A, S_D/S_A and S_D/P_A, on the morphology, charge transfer, charge transport and photovoltaic performance were investigated. Power conversion efficiencies (PCEs) of P_D/P_A, P_D/S_A, S_D/S_A and S_D/P_A were 3.04%, 0.28%, 2.52% and 0.29%, respectively. The P_D/P_A blend and S_D/S_A blend exhibited relatively uniform and continuous morphology, efficient photoinduced charge transfer, high mobility and balanced charge transport relative to P_D/S_A and S_D/P_A blends, leading to much higher PCEs.     

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.     

Efficient non-fullerene organic solar cells with low-temperature solution-processing ferrous oxides as hole transport layer

Non-fullerene organic solar cells (NF–OSCs) have recently attracted enormous attention due to the rapid advance of high-performance photoabsorbers. On the other hand, interfacial materials also play a crucial role in further increasing the device efficiency, but those materials in particular effective hole transporting ones for NF–OSCs are less developed. In this work, three low-temperature solution-processing ferrous oxide films (including CoO_x, NiO_x, and FeO_x) are used as hole transporting layer (HTL) for NF–OSCs. By adding a surfactant and treating with the ultraviolet ozone (UVO), uniform ferrous oxide films with adjustable energy bands are achieved. The NF–OSCs based on PBDB-T-2Cl:IT-4F active layer and using CoO_x, NiO_x, and FeO_x as the HTL afford power conversion efficiencies of 11.4%, 10.2% and 6.4%, respectively. The higher performance of NF–OSCs with the UVO-treated CoO_x as the HTL is attributed to its more suitable energy level alignment and better hole transportation property relative to those of the other two counterparts.     

Pivotal factors in solution-processed,non-fullerene,all small-molecule organic solar cell device optimization

The importance of device structure and active-layer processing when screening non-fullerene acceptors was demonstrated through the organic solar cell device performance optimization of a solution processable non-fullerene, all small-molecule bulk heterojunction (BHJ) blend. Key tuning parameters were identified; notably, the largest improvement in performance was achieved by switching from the conventional device architecture (ITO/PEDOT:PSS/D-A BHJ/Ca/Al) to an inverted structure (ITO/ZnO/D-A BHJ/MoOx/Ag), approximately doubling the power conversion efficiency from best cells of 0.5%–1.0%, demonstrating the importance of investigating more than a single architecture when screening novel non-fullerene acceptors.     

Acenaphtho[1,2-b]quinoxaline diimides derivative as a potential small molecule non-fullerene acceptor for organic solar cells

We designed and synthesized a small molecule acenaphtho[1,2-b]quinoxaline diimide derivative AQI-T2 as an electron-accepting material for non-fullerene organic solar cells. This molecule exhibits a relatively broad absorption band from 300 to 650 nm, with a moderately low-lying lowest unoccupied molecular orbital energy level of −3.64 eV. Non-fullerene organic solar cells with conventional structure using PTB7-Th as the electron donor and AQI-T2 as the electron acceptor exhibited moderate photovoltaic performances. The best performance was attained from the pristine device, which showed a power conversion efficiency of 0.77% with a relatively high open-circuit voltage of 0.86 V, a short circuit current of 2.04 mA cm⁻² and a fill factor of 43.98%. These results indicated that this n-type molecule can be a promising electron-accepting material for non-fullerene organic solar cells.     

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.     

2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

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%.     

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.     

Modulating the middle and end-capped units of A2-A1-D-A1-A2 type non-fullerene acceptors for high VOC organic solar cells

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been improved rapidly in the last few years, due to the development of excellent non-fullerene acceptor (NFA) materials. However, compared to the high PCE, the open-circuit voltage (V_OC) of OSCs is still relatively low. Recently, we have demonstrated that the “Same-A-Strategy” (SAS) is feasible to achieve high V_OC, where the same electron-accepting (A) unit is utilized to construct polymer donor and NFA. To investigate the structure-properties relationship, here we chose six benzotriazole-based NFAs (BTA1, BTA11, BTA3, BTA13, BTA7 and BTA17) with A₂-A₁-D-A₁-A₂ type molecular backbone, where indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) as middle electron-donating unit, rhodanine (R), 2-(1,1-dicyanomethylene)rhodanine (RCN), and malononitrile (M) as the terminal A_2, respectively. When paired with a p-type polymer J52-Cl containing BTA unit, OSCs based on six material combinations can realize ultra-high V_OC of 1.09–1.33 V with PCEs of 0.13–10.5%. The results indicate that the middle and end-capped units play a vital role to extend the application of SAS, and IDT and RCN are promising choices as the core and end-capped groups.     

Effect of substituents of twisted benzodiperylenediimides on non-fullerene solar cells

Twisted benzodiperylenediimides (TBDPDI) with large rigid conjugated core and strong absorption is regarded as an excellent acceptor in non-fullerene solar cells. Since side chains of semiconductors play a crucial role in the solar cells, TBDPDI acceptors with different side chains (1-ethylpropyl, C5; 2-ethylhexyl, C8; 1-pentylhexyl, C11; 2-octyldodecyl, C20; 1-undecyldodecyl, C23) were synthesized. In solution, TBDPDI compounds (C5, C11, and C23) with alkyl chains branched at 1-position show significantly different absorption profiles and fluorescence intensity with those (C8 and C20) branched at 2-position, due to stronger aggregation of the latter. Nevertheless, alkyl chains have little effect on the molecular orbital energy levels and optical band gaps, as verified by cyclic voltammetry and solid state absorption. Due to their complementary absorption and matchable energy levels with donor of PCE10, these acceptors and PCE10 were used together to fabricate bulk heterojunction (BHJ) solar cells. Because of inferior phase separation with large domain size around 100 nm and bulky insulated side chains, acceptors (C20 and C23) with long alkyl chains have the low electron mobility (μ_e) around 10⁻⁸ cm² V⁻¹ s⁻¹ and the low power conversion efficiency (PCE) of solar cells. TBDPDI (C11) with 1-pentylhexyl gives the highest PCE of 5.0% under the optimized condition, which is attributed to proper phase separation with domain size around 20 nm and highest μ_e of 10⁻⁶ cm² V⁻¹ s⁻¹.     

On the role of aggregation effects in the performance of perylene-diimide based solar cells

A model bulk-heterojunction of a perylene diimide (PDI) monomeric derivative is studied for interrogating the role of PDI aggregates in the photocurrent generation efficiency (η_PC) of PDI-based organic photovoltaic (OPV) devices. Blend films of the PDI derivative and the poly(indenofluorene) (PIF) polymer annealed between room temperature and 220 °C, are used as the photoactive layers for the fabrication of OPVs. The positive effect of thermal annealing is assigned to the evolution of PDI aggregates in the amorphous PIF matrix. Annealing increases the electron mobility by three orders of magnitude. In contrast, owned to the thermally inert PIF matrix used, hole mobility increases only by a factor of six. High resolution cross-sectional scanning electron microscopy suggests that η_PC in PDI-based OPVs is not limited by the PDI aggregates but by their improper alignment. In situ Raman spectra and density functional theory calculations identify a marker for monitoring the strength of π–π stacking interactions between PDI monomers. It s further demonstrated that the electron-collecting electrode of the PIF:PDI devices dictates their performance. The use of Al is found to impede charge extraction and this is attributed to an unidentified product of the reaction between PDI and Al that leads to the formation of an electron-blocking layer. Device performance rectifies when a Ca/Al electrode is used and the power conversion efficiency is increased by a factor of four.     

Perylene diimide based all small-molecule organic solar cells: Impact of branched-alkyl side chains on solubility,photophysics, self-assembly,and photovoltaic parameters

Perylene diimide derivatives have been under intense investigation to replace fullerenes as the electron accepting component in organic photovoltaics, with molecular complexity continuing to grow. Simple alkyl-substituted perylene diimide monomers at the imide nitrogen position, however, have not been extensively investigated. Herein we demonstrate that subtle alkyl-substitutions at the imide-nitrogen position lead to significant changes in solubility, thin-film self-assembly and optical properties. When blended with a small-molecule donor to form all small-molecule, fullerene-free, solution processed organic solar cells, we show that the photovoltaic device performance and consistency can be tuned via alkyl-chain modifications. In addition we have simplified the device fabrication process by utilizing a silver cathode coupled with a small-molecule-ionic interlayer and achieved comparable performance to devices fabricated with a traditional Ca/Al cathode.     

Laser-induced crystallization and conformation control of poly(3-hexylthiophene) for improving the performance of organic solar cells

Femto-second laser irradiation on P3HT:PCBM solutions have been demonstrated to have a significant impact on the conformational structures and photovoltaic performance of the resultant thin films. The crystallinity and edge-on/face-on conformations of P3HT and the aggregation of PCBM can be manipulated by controlling the wavelength (400–800 nm) and illumination duration (1–3 h) of the lasers. Grazing incidence wide- and small-angle X-ray scattering (GIWAXS and GISAXS) have been simultaneously utilized to characterize the nanostructures of the P3HT:PCBM blend films spin-cast from pristine and laser-irradiated solutions. The results show that the crystallinity, π-π* stacking and face-on conformations of P3HT can be enhanced as a result of the laser irradiation at 500 nm for 3 h. Furthermore, the diffusion and aggregation of PCBM molecules are suppressed by the photo-induced dimerization, as evidenced by the Raman spectra of the films cast from laser-irradiated PCBM solutions. The time-resolved fluorescence decay profiles show the charge transfer efficiency is improved, which may correlate to the supramolecular ordering of the polythiophene chains and the optimized phase separation in P3HT:PCBM composite. In the P3HT:PCBM active layer of the organic solar cells, more efficient charge transport and fine interpenetrating networks can be achieved due to the improved conformational microstructures. Consequently, the short-circuit current densities and power conversion efficiencies can be enhanced in organic solar cells based on the laser-irradiation processed P3HT:PCBM solutions.     

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%.     

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.     

Synthesis and characterization of dianchoring organic dyes containing 2,7-diaminofluorene donors as efficient sensitizers for dye-sensitized solar cells

New metal free dianchoring organic dyes featuring A–π–D–π–D–π–A (acceptor – π bridge – donor – π bridge – donor – π bridge – acceptor) configuration have been designed incorporating fluorene and oligothiophene units and successfully synthesized. Elongating the conjugation pathway between the donor and acceptor units altered the distance between the anchoring sites besides the absorption and redox properties. These dyes exhibited broad and intense absorption when compared to the corresponding monoanchoring donor–π–acceptor congeners. Though the dye containing bithiophene unit exhibited comparatively low V_oc due to low electron life time and facile back electron transfer, showed high power conversion efficiency arising from the good light-harvesting capability attributable to the intense absorption peak in the visible region and enhanced interfacial electron transfer rate. This work demonstrates that the smaller distance of separation between the anchoring units increases the insulating capacity of the molecular layer which retards the back electron transfer.