We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2′,3′-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0–2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550–850 nm) and strong absorption with high extinction coefficients of (2.1–2.5) × 105 M–1 cm–1. Fluorine substitution downshifts the LUMO energy level, red-shifts the absorption spectrum, and enhances electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.

•A small molecule nonfullerene acceptor IDT-DPP-R with A1-A2-D-A2-A1 structure is designed and synthesized.•IDT-DPP-R exhibits strong near-infrared absorption and appropriate energy level.•Organic solar cells based on P3HT:IDT-DPP-R:PC61BM exhibit higher efficiency than those of P3HT:PC61BM and P3HT:IDT-DPP-R.A fused-ring electron acceptor based on indacenodithiophene was designed and synthesized. The nonfullerene acceptor shows strong absorption in 500–850 nm with extinction coefficient up to 1.4 × 105 M−1 cm−1 and relatively high lowest unoccupied molecular orbital energy level (−3.55 eV). As an electron-cascade acceptor and strong absorption material, the new acceptor was incorporated into poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric-acidmethyl-ester blend to fabricate ternary blend polymer solar cells. The introduction of the nonfullerene acceptor can not only broaden the light absorption of the active layer to the near-infrared region, but also increase the open circuit voltage due to the higher lowest unoccupied molecular orbital of the nonfullerene acceptor relative to the fullerene acceptor. Ternary blend devices with 20% of the nonfullerene acceptor exhibited an average power conversion efficiency of 3.84%, higher than those of the binary blends using the fullerene acceptor (3.30%) or the nonfullerene acceptor (1.42%).

A fused hexacyclic electron acceptor, IHIC, based on strong electron-donating group dithienocyclopentathieno[3,2-b]thiophene flanked by strong electron-withdrawing group 1,1-dicyanomethylene-3-indanone, is designed, synthesized, and applied in semitransparent organic solar cells (ST-OSCs). IHIC exhibits strong near-infrared absorption with extinction coefficients of up to 1.6 × 105m−1 cm−1, a narrow optical bandgap of 1.38 eV, and a high electron mobility of 2.4 × 10−3 cm2 V−1 s−1. The ST-OSCs based on blends of a narrow-bandgap polymer donor PTB7-Th and narrow-bandgap IHIC acceptor exhibit a champion power conversion efficiency of 9.77% with an average visible transmittance of 36% and excellent device stability; this efficiency is much higher than any single-junction and tandem ST-OSCs reported in the literature.

•A series of fused-ring electron acceptors are designed and synthesized.•The effects of side chains on molecular properties are investigated.•Organic solar cells based on these acceptors exhibit efficiencies ranging from 3.9% to 7.4%.A series of fused-ring electron acceptors with indacenodithiophene as core, 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile as end groups, thiophene with different side chains as π bridge are designed, synthesized and applied in non-fullerene organic solar cells (OSCs). The effects of alkyl side chains on absorption, energy level, molecular packing, domain size, domain purity, charge transport and photovoltaic performance are systematically investigated. Via the steric hindrance effect, increase in carbon number of side chains slightly downshifts HOMO energy levels, upshifts LUMO energy levels and thereby increases open circuit voltage from 0.84 to 0.95 V gradually. Short circuit current, fill factor and efficiency vary non-monotonically with aliphatic carbon number of alkyl chains and reach maximum value simultaneously when carbon number is six.Download high-res image (208KB)Download full-size image

A fused-ring electron acceptor IDT-2BR1 based on indacenodithiophene core with hexyl side-chains flanked by benzothiadiazole rhodanine was designed and synthesized. In comparison with its counterpart with hexylphenyl side-chains (IDT-2BR), IDT-2BR1 exhibits higher highest occupied molecular orbital (HOMO) energy but similar lowest unoccupied molecular orbital (LUMO) energy (IDT-2BR1: HOMO=−5.37 eV, LUMO=−3.67 eV; IDT-2BR: HOMO=−5.52 eV, LUMO=−3.69 eV), red-shifted absorption and narrower bandgap. IDT-2BR1 has higher electron mobility (2.2×10–3 cm2 V–1 s–1) than IDT-2BR (3.4×10–4 cm2 V–1 s–1) due to the reduced steric hindrance and ordered molecular packing. Fullerene-free organic solar cells based on PTB7-Th:IDT-2BR1 yield power conversion efficiencies up to 8.7%, higher than that of PTB7-Th:IDT-2BR (7.7%), with a high open circuit voltage of 0.95 V and good device stability.

ABSTRACTWe report the synthesis of a series of copolymers, having 2,2′-bithiophene as electron-donating moiety, and perylene diimide (PDI) and/or naphthalene diimide (NDI) as electron-accepting moiety, and employed as non-fullerene acceptors in polymer solar cells (PSCs). All the copolymers show wide absorption varying from 300 to 850 nm in the visible and NIR spectrum. When changing the PDI/NDI ratio in the polymer backbone, The LUMO energy levels vary in the range of −3.90 to −3.80 eV and the HOMO energy levels vary in the range of −6.10 to −5.85 eV. Among PSCs based on PTB7-Th donor and these polymer acceptors, the devices based on PTB7-Th/NDI100 yield the best power conversion efficiency (PCE) of 4.67%, while the PTB7-Th/PDI100-based devices yield a PCE of 1.03%. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 682–689

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).

A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 105m−1 cm−1, higher than that of ITIC1 (1.5 × 105m−1 cm−1). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (−5.43 eV) and lowest unoccupied molecular orbital (LUMO) (−3.80 eV) energy levels relative to ITIC1 (HOMO: −5.48 eV; LUMO: −3.84 eV), and higher electron mobility (1.3 × 10−3 cm2 V−1 s−1) than that of ITIC1 (9.6 × 10−4 cm2 V−1 s−1). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.

In this work, a thiophene-fused benzoxadizole (BXT) unit was designed as a new acceptor and synthesized for the first time to build a D–A conjugated polymer (PBXT-IDT) with 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT) for all polymer solar cells (all-PSCs). Due to the strong electron-withdrawing ability of the BXT unit, PBXT-IDT exhibited a very narrow optical bang gap of 1.43 eV and very strong ICT absorption in the range of 500–850 nm that could be complementary with poly(3-hexylthiophene) (P3TH) in the visible absorption region. Moreover, PBXT-IDT showed relatively low HOMO–LUMO energy levels of −5.33 eV and −3.64 eV, respectively, which can be act as an electron-accepting material to match with P3HT as an electron-donating material for all-PSCs. Therefore, the all-PSC device with a blend of PBXT-IDT and P3HT as the active layer was fabricated and the photovoltaic performances were investigated. The device showed a PCE of 1.09% with a high Voc of 0.84 V and a relatively low energy loss (Eloss) of 0.59 V. This indicates that reasonable structural modification of benzoxadizole (BX) can pave a new way to design a polymer as an electron-accepting material in all-PSCs.

An n-type semiconducting copolymer of perylene diimide and dithienothiophene (PPDIDTT) is used as a dual function interfacial layer to modify the surface of perovskite films in inverted perovskite solar cells. The PPDIDTT layer can remarkably passivate the surface trap states of perovskite through the formation of a Lewis adduct between the under-coordinated Pb in perovskite and S in the dithienothiophene unit of PPDIDTT, and also shows efficient charge extraction and transfer properties. The PPDIDTT modified devices exhibit a maximum power conversion efficiency of 16.5%, superior to that of the control devices without PPDIDTT (15.3%). In addition, the device stability and hysteresis in J–V curves of the modified devices are also improved compared to those of the control devices.

A series of D–A copolymers (P1–P3) based on a thiophene fused benzotriazole (BTAZT) unit and a 4,8-bis(4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTFT) unit have been designed and synthesized as electron-donating materials for non-fullerene polymer solar cells (PSCs) with ITIC as an electron-accepting material. The fusion of a thiophene ring with a BTAZ unit and introduction of a fluorine atom at the BDTT unit have been concurrently performed to regulate the absorption properties and energy levels, and two different alkyl side-chains have also been introduced to BTAZT and BDTFT units, respectively, to investigate the influence of side-chains on the properties of the polymers as well as the photovoltaic performances of the PSCs. The photovoltaic performance evaluation in combination with morphology characterization indicates that the introduction of a 2-ethylhexyl group simultaneously at BTAZT and BDTFT units is favourable for intermolecular π–π interaction that improves the morphologies and crystalline domain of P1:ITIC blend film, thus enhancing the charge mobility and the short-circuit current density (Jsc). But the large steric hindrance of the 2-butyloctyl group to replace the 2-ethylhexyl group at BDTFT (P2) or BTAZT (P3) deteriorates intermolecular interaction that leads to the decrease of charge mobility and poor Jsc. Thus, P1 based PSCs exhibit the highest power conversion efficiency of 7.14% with the best Jsc (14.11 ± 0.27 mA cm−2) and fill factor (63.41 ± 1.46%) among P1–P3 polymers.

Developing electron-donating building blocks for organic semiconductors is still one big chemical challenge to achieve high performance active materials for organic photovoltaics (OPVs). In this work, we have successfully designed and synthesized a novel ladder-type nonacyclic indacenodithieno[3,2-b]indole (IDTI) unit via intramolecular annulation with rigid and coplanar features. Two donor–acceptor copolymers of PIDTI-BT and PIDTI-DTBT were synthesized by utilizing the Suzuki and Stille coupling polymerization method with IDTI units. Both copolymers displayed excellent solubility, high thermal stability, broad absorption and a low band gap. The FET hole mobility reaches 2.1 × 10−2 and 1.4 × 10−2 cm2 V−1 s−1 for PIDTI-BT and PIDTI-DTBT, respectively. The conventional bulk-heterojunction (BHJ) polymer solar cell (PSC) devices based on the PIDTI-BT : PC71BM (1 : 2 in wt%) blend exhibit a moderate PCE of 4.02% with a Voc of 0.82 V, a Jsc of 8.99 mA cm−2 and a FF of 54.6% under AM 1.5G, 100 mW cm−2 illumination, which is among the highest values for polymer donor materials based on multifused TI units. The improved performance may be associated with the extended conjugation length, which optimizes the interchain interactions and improves molecular organization for accelerating charge transport. Our results demonstrate that the multifused nonacyclic TIBDP as the donor unit is very promising for application in PSCs and FETs.

We design and synthesize an amino-functionalized conjugated polymer (PPDI-F3N) based on perylene diimide and use it as a multifunctional interfacial layer of TiO2/perovskite in conventional planar perovskite solar cells. The work function of TiO2 is modulated by PPDI-F3N to better align with the conduction band of the perovskite, leading to efficient charge extraction. PPDI-F3N can passivate the TiO2 surface to reduce the severe recombination loss and rapid degradation caused by oxygen vacancies on the UV-sensitive TiO2 surface. Moreover, modulated polarity of PPDI-F3N is beneficial to optimal perovskite crystallization and morphology. All these features contribute to a higher efficiency (18.3%) of the PSCs with the PPDI-F3N interlayer relative to the control devices without the interlayer (16.7%) as well as improved stability and a reduced hysteresis effect.

Three fused-ring n-type semiconductors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2,3-d]thiophene) end-capped with 1,1-dicyanomethylene-3-indanone substituted by different numbers of fluorine atoms (INIC series) are employed as interfacial materials to modify the surface of the perovskite film in inverted planar perovskite solar cells (PSCs). Due to fast interfacial charge extraction and efficient trap passivation, PSCs based on INIC series exhibit a maximum power conversion efficiency of 19.3% without any hysteresis, which is superior to control devices without INIC series (16.6%). Moreover, the strong water-resistance ability of fluorinated INIC significantly enhances the ambient stability of the PSCs. The effects of fluorine atom number on the device performance are discussed.

A fused-ring electron acceptor based on indacenodithiophene (IDIC) was used to replace TiO2 and work as an electron transport layer in planar n–i–p perovskite solar cells. IDIC improves perovskite crystallinity and film quality due to its hydrophobicity and incompatible wetting surface. IDIC facilitates electron extraction and transport due to its high mobility and suitable energy levels matched with the perovskite. IDIC reduces charge recombination in the devices due to trap passivation at the perovskite surface. The IDIC-based devices exhibit a champion power conversion efficiency of 19.1%, which is higher than that of TiO2-based devices (17.4%). Moreover, the device stability is significantly improved by IDIC.

Two polymers containing (E-2,3-bis(thiophen-2-yl)acrylonitrile (CNTVT) as a donor unit, perylene diimide (PDI) or naphthalene diimide (NDI) as an acceptor unit, are synthesized by the Stille coupling copolymerization, and used as the electron acceptors in the solution-processed organic solar cells (OSCs). Both polymers exhibit broad absorption in the region of 300–850 nm. The LUMO energy levels of the resulted polymers are ca.–3.93 eV and the HOMO energy levels are–5.97 and–5.83 eV. In the binary blend OSCs with PTB7-Th as a donor, PDI polymer yields the power conversion efficiency (PCE) of up to 1.74%, while NDI polymer yields PCE of up to 3.80%.

Polyethylenimine ethoxylated (PEIE) is a classical cathode-modifying material. Here, we report the first example of using PEIE as an anode-modifying material. We use MoO3/PEIE/Ag as a hole extraction layer to enhance the performance of inverted organic solar cells (OSCs). The introduction of an ultrathin PEIE interlayer can markedly reduce the anodic resistance of inverted devices, mostly because the ultrathin PEIE interlayer can separate MoO3 from Ag, thereby suppressing the reduction of MoO3 by gaseous Ag to lower-oxidation state Mo species which decrease the work function of MoO3 and are then detrimental to hole extraction from the photoactive layer. In addition, the PEIE interlayer can act as an optical spacer to optimize the optical absorption of inverted devices. As a result, the novel hole extraction structure of MoO3/PEIE/Ag increases device efficiency by a factor of 16% relative to conventional MoO3/Ag in inverted OSCs.

Four spiro[fluorene-9,9′-xanthene] (SFX)-based hole transporting materials (HTMs) functionalized with four-armed arylamine moieties located at different positions are designed and synthesized. These compounds exhibit highest occupied molecular orbital (HOMO) energy levels of −4.9 to −5.1 eV and a hole mobility of 2.2 to 15 × 10−5 cm2 V−1 s−1 after doping. Perovskite solar cells (PSCs) based on a methylammonium lead iodide (MAPbI3) active layer using one of these HTMs (mp-SFX-2PA) exhibit power conversion efficiencies (PCEs) of up to 16.8%, which is higher than that of the control devices based on benchmark spiro-OMeTAD under the same conditions (15.5%). PSCs based on mp-SFX-2PA exhibit better stability (retain 90% of their initial PCEs after 2000 h storage in an ambient atmosphere) than the control devices based on spiro-OMeTAD (retain only 28% of their initial PCEs). mp-SFX-2PA based devices employing a mixed formamidinium lead iodide (FAPbI3)/methylammonium lead bromine (MAPbBr3) perovskite layer exhibit an improved PCE of 17.7%. The effects of arylamines and their location positions on device performance are discussed.

Organic solar cells (OSCs) present some advantages, such as simple preparation, light weight, low cost and large-area flexible fabrication, and have attracted much attention in recent years. Although the power conversion efficiencies have exceeded 10%, the inferior device stability still remains a great challenge. In this review, we summarize the factors limiting the stability of OSCs, such as metastable morphology, diffusion of electrodes and buffer layers, oxygen and water, irradiation, heating and mechanical stress, and survey recent progress in strategies to increase the stability of OSCs, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation. Some research areas of device stability that may deserve further attention are also discussed to help readers understand the challenges and opportunities in achieving high efficiency and high stability of OSCs towards future industrial manufacture.

Solar cells, a renewable, clean energy technology that efficiently converts sunlight into electricity, are a promising long-term solution for energy and environmental problems caused by a mass of production and the use of fossil fuels. Solution-processed organic solar cells (OSCs) have attracted much attention in the past few years because of several advantages, including easy fabrication, low cost, lightweight, and flexibility. Now, OSCs exhibit power conversion efficiencies (PCEs) of over 10%.In the early stage of OSCs, vapor-deposited organic dye materials were first used in bilayer heterojunction devices in the 1980s, and then, solution-processed polymers were introduced in bulk heterojunction (BHJ) devices. Relative to polymers, vapor-deposited small molecules offer potential advantages, such as a defined molecular structure, definite molecular weight, easy purification, mass-scale production, and good batch-to-batch reproducibility. However, the limited solubility and high crystallinity of vapor-deposited small molecules are unfavorable for use in solution-processed BHJ OSCs. Conversely, polymers have good solution-processing and film-forming properties and are easily processed into flexible devices, whereas their polydispersity of molecular weights and difficulty in purification results in batch to batch variation, which may hamper performance reproducibility and commercialization.Oligomer molecules (OMs) are monodisperse big molecules with intermediate molecular weights (generally in the thousands), and their sizes are between those of small molecules (generally with molecular weights <1000) and polymers (generally with molecular weights >10000). OMs not only overcome shortcomings of both vapor-deposited small molecules and solution-processed polymers, but also combine their advantages, such as defined molecular structure, definite molecular weight, easy purification, mass-scale production, good batch-to-batch reproducibility, good solution processability, and film-forming properties. Therefore, OMs are a good choice for solution-processed reproducible OSCs toward scalable commercialized applications. Considerable efforts have been dedicated to developing new OM electron donors and electron acceptors for OSCs. So far, the highest PCEs of solution-processed OSCs based on OM donors and acceptors are 9–10% and 6–7%, respectively. OM materials have become promising alternatives to polymer and/or fullerene materials for efficient and stable OSCs.In this Account, we present a brief survey of the recent developments in solution-processable OM electron donors and acceptors and their application in OSCs. Rational design of OMs with star- and linear-shaped structures based on triphenylamine, benzodithiophene, and indacenodithiophene units and their impacts on device performance are discussed. Structure–property relationships are also proposed. Furthermore, the remaining challenges and the key research directions in the near future are also addressed. In the next years, an interdisciplinary approach involving novel OM materials, especially electron acceptor materials, accurate morphology optimization, and advanced device technologies will probably bring high-efficiency and stable OSCs to final commercialization.

A planar fused-ring electron acceptor (IC-C6IDT-IC) based on indacenodithiophene is designed and synthesized. IC-C6IDT-IC shows strong absorption in 500–800 nm with extinction coefficient of up to 2.4 × 105 M–1 cm–1 and high electron mobility of 1.1 × 10–3 cm2 V–1 s–1. The as-cast polymer solar cells based on IC-C6IDT-IC without additional treatments exhibit power conversion efficiencies of up to 8.71%.

We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = −5.66 eV, LUMO = −3.93 eV; ITIC: HOMO = −5.48 eV, LUMO = −3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-band-gap polymer donors and wide-band-gap polymer donors. ITIC-Th has higher electron mobility (6.1 × 10–4 cm2 V–1 s–1) than ITIC (2.6 × 10–4 cm2 V–1 s–1) due to enhanced intermolecular interaction induced by sulfur–sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-band-gap and wide-band-gap polymer donors. In one case, a power conversion efficiency of 9.6% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.

Flexible organic solar cells (OSCs) based on a blend of low-bandgap polymer donor PTB7-TH and non-fullerene small molecule acceptor IEIC were fabricated via a roll-coating process under ambient atmosphere. Both an indium tin oxide (ITO)-free substrate and a flexible ITO substrate were employed in these inverted OSCs. OSCs with flexible ITO and ITO-free substrates exhibited power conversion efficiencies (PCEs) up to 2.26% and 1.79%, respectively, which were comparable to those of the reference devices based on fullerene acceptors under the same conditions. This is the first example for all roll-coating fabrication procedures for flexible OSCs based on non-fullerene acceptors with the PCE exceeding 2%. The fullerene-free OSCs exhibited better dark storage stability than the fullerene-based control devices.

A series of polymer photodetectors (PPDs) are fabricated based on P3HT as an electron donor and fullerene-free material DC-IDT2T as an electron acceptor. The only difference among these PPDs is the P3HT:DC-IDT2T doping weight ratios from 2:1 to 150:1. The PPDs with P3HT:DC-IDT2T (100:1, w/w) as the active layers exhibit champion external quantum efficiency (EQE) of 28000% and 4000% corresponding to 390 nm and 750 nm light illumination at −20 V bias, respectively. The photomultiplication (PM) phenomenon should be attributed to the enhanced hole tunneling injection due to the interfacial band bending, which is induced by the trapped electrons in DC-IDT2T near the Al cathode. The high EQE value in the long wavelength range is due to the effect of DC-IDT2T photon harvesting and exciton dissociation on the interfacial trap-assisted hole tunneling injection. Meanwhile, the PPDs with DC-IDT2T as the electron acceptor exhibit superior stability compared with the PPDs with PC71BM as the electron acceptor.

Bathophenanthroline (Bphen), an electron transporting material widely used in organic light-emitting diodes, was added as a third component into a mixed solution of poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) to fabricate simplified inverted polymer solar cells (PSCs). Bphen spontaneously migrated onto the ITO cathode during spin coating, and thereby formed a thin modifying interlayer between the cathode and the active layer. PSCs based on the ternary blend PTB7:PC71BM:Bphen showed a power conversion efficiency of 5.35%, higher than that of the binary blend PTB7:PC71BM in an inverted structure of ITO/photoactive layer/MoO3/Ag without an additional cathode-modifying interlayer (3.43%). The other electron transporting materials, 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) and bathocuproine (BCP), were also confirmed to function as Bphen. This strategy simplifies the structure of inverted PSCs without significant loss in efficiency.

We design and synthesize monodisperse fused-ring oligomer molecules benzo[1,2-b:4,5-b′]dithiophene (BDT) flanked with electron-withdrawing diketopyrrolopyrrole (DPP). A tiny change in the side chain induces significant variation in crystallinity, phase separation, charge transport and photovoltaic properties of the semiconductors. BDTS-2DPP with linear alkylthio substituents exhibits a much higher hole mobility of 1.1 × 10−2 cm2 V−1 s−1 than the branched alkyl substituted BDT-2DPP (3.0 × 10−3 cm2 V−1 s−1). The LUMO (−3.49 eV) and HOMO (−5.28 eV) energy levels of BDTS-2DPP are lower than those of BDT-2DPP (−3.46 eV and −5.23 eV) due to the π-acceptor capability of the sulfur atom. Fullerene-free organic solar cells using BDTS-2DPP as a donor and monodisperse fused-ring oligomer molecule IEIC as an acceptor exhibit higher open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency (PCE, 5.29%) than the BDT-2DPP control devices (PCE = 4.00%) with the conventional structure. The inverted devices based on BDTS-2DPP:IEIC show an improved PCE of 6.03% relative to the conventional devices. Atomic force microscopy, grazing incident wide-angle X-ray diffraction and resonant soft X-ray scattering are used to deeply investigate the molecular packing, phase separation and surface aggregation of the blended films and to understand the effect of molecular side chains. We find that the linear alkylthio substitution in BDTS-2DPP improves the crystallinity and unexpectedly reserves small phase separation domains in the blend.

A series of monodisperse macromolecules with A2–A1–D–A1–A2 structure based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) BDT-4DPP, BDT-DPP-Rhod and BDT-DPP-CA were designed, theoretically calculated and synthesized, and compared with their parent molecules BDT-2DPP and BDTS-2DPP with A1–D–A1 structure. These molecules possessed highly planar molecular geometries and high crystallinity. These molecules exhibited good thermal stability with decomposition temperatures of 322–388 °C, strong visible and near-infrared absorption (500–1000 nm), and HOMO energy levels of −5.38 to −5.19 eV and LUMO energy levels of −3.69 to −3.46 eV. Relative to the parent molecules A1–D–A1, A2–A1–D–A1–A2 molecules exhibited red-shifted and stronger absorption. The charge transport properties of these molecules were investigated by organic field-effect transistors, and their hole mobilities were 0.036–1.12 cm−2 V−1 s−1. Replacing alkyl with alkylthio on BDT led to mobility enhancement by one order of magnitude.

The magnitude of intramolecular charge-transfer (ICT) in push–pull chromophores and the fraction of delocalized excitation in multibranched chromophores and conjugated polymers play a crucial role in high photovoltaic efficiency of a solar cell. In this work, we present a joint theoretical and experimental study aimed to understand the influence of thiophene moiety on photophysical properties of push–pull chromophores for solar cell application. It is found that insertion of a thiophene moiety as the conjugated bridge enhances the magnitude of ICT in push–pull chromophores due to the inductive effect of the thiophene moiety. In addition, introduction of a thiophene moiety as the conjugated side chain significantly increases transition dipole moment of the chromophore, and as a consequence, interchromophoric coupling is enhanced, giving rise to a larger fraction of delocalized excitation within multibranched chromophores. The results presented here show that introduction of a thiophene moiety in push–pull chromophores contributes to the improvement of the photophysical properties necessary for highly efficient solar cell performance.

Triarylamine (TAA) and related materials have dramatically promoted the development of organic and hybrid photovoltaics during the past decade. The power conversion efficiencies of TAA-based organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs) have exceeded 11%, 14%, and 20%, which are among the best results for these three kinds of devices, respectively. In this review, we summarize the recent advances of the high-performance TAA-based materials in OSCs, DSSCs, and PSCs. We focus our discussion on the structure–property relationship of the TAA-based materials in order to shed light on the solutions to the challenges in the field of organic and hybrid photovoltaics. Some design strategies for improving the materials and device performance and possible research directions in the near future are also proposed.

•A Semitransparent, non-fullerene, flexible all-plastic organic solar cell is fabricated.•Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) is used as the top and bottom electrodes.•The fullerene-free device shows better efficiency and better bending stability than the fullerene counterpart.Semitransparent, non-fullerene and flexible all-plastic organic solar cell (OSC) based on a blend of poly(3-hexylthiophene) (P3HT) and non-fullerene acceptor IDT-2BR is fabricated using conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as bottom electrode and film-transfer laminated PEDOT:PSS as top electrode. The solar cell shows average visible transmittance of ca. 50%, which can be potentially used for electricity-generating windows. The all-plastic device shows lower power conversion efficiency (PCE) of 2.88% than the traditional inverted device (4.2%), owing to much higher transmittance and lower conductivity of PEDOT:PSS. Our non-fullerene system P3HT:IDT-2BR shows better performance than fullerene system P3HT:PC61BM (PCE = 2.2%) in all-plastic OSCs. Furthermore, the non-fullerene OSC shows better bending stability than the fullerene counterpart.

A novel planar acceptor IDT-2BR was designed and synthesized. Polymer solar cells (PSCs) based on P3HT:IDT-2BR blended films gave power conversion efficiencies of up to 5.12%, which are much higher than that of PC61BM-based control devices (3.71%) and the highest values reported for P3HT-based fullerene-free PSCs.

Diluting concentrated solution (DCS) is a new, simple, general and effective approach to improve power conversion efficiencies (PCEs) of polymer solar cells (PSCs). PCEs of binary blend PSCs, ternary blend PSCs and all-polymer solar cells fabricated using this method are enhanced by a factor as high as 37% relative to those using the general process.

A nonfullerene electron acceptor (IEIC) based on indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile was designed and synthesized. IEIC exhibited good thermal stability, strong absorption in the 500–750 nm region with an extinction coefficient of 1.1 × 105 M−1 cm−1 at 672 nm, deep LUMO energy level (−3.82 eV) close to those of fullerenes, and a relatively high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. Fullerene-free polymer solar cells (PSCs) based on the blends of the IEIC acceptor and a low-bandgap polymer donor PTB7-TH, using a perylene diimide derivative as a cathode interlayer, showed power conversion efficiencies (PCEs) of up to 6.31%, which is among the best PCEs reported for fullerene-free PSCs.

Organic solar cells (OSCs) with a third component, consisting of a donor material, an acceptor material and a third component (organic or inorganic, semiconductor or insulator), received increasing attention in the last five years and the power conversion efficiencies approached 10%. Compared with the traditional binary (two-component) blend, three-component OSCs presented some advantages: broader and stronger absorption, more efficient charge transfer, more efficient charge transport pathways, better charge extraction at the electrodes and improved stability. Various types of third components, such as light-absorbing small molecules or polymers, fullerene or non-fullerene acceptors, metal- or carbon-based nanomaterials, quantum dots, and polymer or small molecule nonvolatile additives were used in OSCs. In this contribution, we review the recent developments in three-component OSCs using various third components. In particular, the absorption complementation, phase separation and the nanostructure in three-component OSCs are addressed.

Layer by layer (LL) solution processed organic solar cells (OSCs) were fabricated using a squaraine small molecule (DIB-SQ) as a donor and a perylene diimide polymer (PPDIDTT) as an acceptor. DIB-SQ and PPDIDTT exhibited matched energy levels and complementary absorption. Compared with the blend structure, the LL devices exhibited better morphology for exciton dissociation and reducing charge recombination. Moreover, efficient vertical phase separation can also be achieved using the LL method, which is beneficial for charge transport and collection at the appropriate electrodes. The traditional blend OSCs exhibited no photovoltaic response, while the LL devices yielded power conversion efficiencies of up to 1.12%.

A novel thiophene-fused diketopyrrolopyrrole unit (7H-pyrrolo[3,4-a]thieno[3,2-g]indolizine-7,10(9H)-dione, PTI) has been designed as an electron acceptor with the 4,4′-bis-(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole (DTS) unit as a donor to construct a new kind of A–D–A molecule (DTS–2TPTI) for solution-processed solar cells. DTS–2TPTI exhibits excellent thermal stability with the decomposition temperature over 400 °C and shows strong absorption from 550 to 750 nm with a high molar extinction coefficient. The optical band gap (Eg) estimated from the absorption edge of the thin film is about 1.44 eV. The highest occupied molecular orbital energy level of DTS–2TPTI determined from CV is about −4.99 eV. Through optimizing the photovoltaic performances of devices, the DTS–2TPTI/PC71BM-based solution processed bulk-heterojunction solar cell with 0.5% DIO as a solvent additive exhibits the best photovoltaic performance with a JSC of 11.28 mA cm−2, VOC of 0.64 V, FF of 59.5% and power conversion efficiency (PCE) of 4.28%, indicating that the PTI unit can act as an efficient acceptor moiety to construct D–A small molecules for OPVs.

Two novel A–D–A type molecules IDT–2BM and IDTT–2BM with extended fused-ring indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) units as cores and strong electron-withdrawing unit 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM) as the end-capping group were synthesized and investigated as electron acceptors in solution-processed polymer solar cells (PSCs). IDT–2BM and IDTT–2BM exhibited strong and broad absorption from 300 to 800 nm, and appropriate LUMO (−3.8 eV) and HOMO (−5.5 to −5.6 eV) levels matching with the classical polymer donor PBDTTT-C-T. IDT–2BM and IDTT–2BM films exhibited intrinsic electron mobilities of about 3.7 × 10−6 and 1.0 × 10−5 cm2 V−1 s−1, respectively. Fullerene-free PSCs employing PBDTTT-C-T as the donor and IDT–2BM or IDTT–2BM as the acceptor afforded power conversion efficiencies of 4.26% and 4.81%, respectively.

A novel small molecule based on indacenodithiophene and 1,1-dicyanomethylene-3-indanone was synthesized and used as an electron acceptor in solution processed organic solar cells, which exhibited a power conversion efficiency as high as 3.93%.

A series of perylene diimide (PDI) dimers (PnTP, n = 0–3) with oligothiophenes as bridges were designed, theoretically calculated, synthesized, and developed as electron acceptors for polymer solar cells (PSCs). The effects of oligothiophene bridge length on the absorption, energy level, charge transport, morphology and photovoltaic properties of the molecules were investigated. These molecules exhibited good thermal stability with decomposition temperatures of 367–413 °C, broad and strong absorption in the visible region (400–700 nm), and appropriate HOMO (−5.74 to −5.61 eV) and LUMO (−3.84 to −3.72 eV) energy levels. When blending these PDI acceptors with a low bandgap polymer donor PBDTTT-C-T, the PSCs exhibited power conversion efficiencies of 0.76–3.61%.

Five copolymers, having 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b′]-dithiophene as a donor unit, and perylene diimide (PDI) and/or naphthalene diimide (NDI) as acceptor moieties, were synthesized by Stille coupling copolymerization, and used as electron acceptors in solution-processed polymer solar cells (PSCs). All five copolymers exhibited broad absorption in the region of 300–800 nm. The LUMO energy level of the resulting copolymers was from −3.90 to −3.77 eV and the HOMO energy level had little variation from −5.65 to −5.57 eV. Among binary blend PSCs using P3HT as a donor and these polymers as acceptors, PPDI25-co-NDI75-based devices (P3HT:PPDI25-co-NDI75 = 3:1, w/w) yielded the best power conversion efficiency (PCE) of up to 1.54%. Among ternary blend PSCs using P3HT as a donor and PDI polymer PPDI100 and NDI polymer PNDI100 as coacceptors, the P3HT:PPDI100:PNDI100 (3:0.25:0.75, w/w) ternary blend afforded the best PCE of 0.83%. All ternary blends based on P3HT:PPDI100:PNDI100 showed decreased VOC, JSC, FF and PCE compared to the corresponding binary blends based on P3HT:PPDI-co-NDI.

•Small molecule 3 and polymer 5 based on perylene diimide and thienylenevinylene were synthesized.•Compounds 3 and 5 were used as acceptors to fabricate fullerene-free polymer solar cells.•Polymer solar cells based on 5 exhibited higher efficiencies (1.00%) than those of 3 (0.69%).A small molecule and a polymer based on perylene diimide and thienylenevinylene were designed and synthesized. Both small molecule and polymer exhibited excellent thermal stability with decomposition temperatures of >400 °C and strong absorption in the visible region (300–800 nm). These two compounds showed highest occupied molecular orbital levels of −5.57 and −5.70 eV and lowest unoccupied molecular orbital levels of −3.72 and −3.67 eV, respectively. Solution processed fullerene-free polymer solar cells based on the small molecule acceptor and the polymer acceptor afforded power conversion efficiencies of up to 0.69% and 1.00%, respectively. Comparative studies of the absorption, energy levels, charge transport, morphology and photovoltaic properties of the small molecule and polymer were carried out.

•Two spirobifluorene-based isomers (SBF1, SBF2) with 1,1-dicyanomethylene-3-indanone as acceptor groups were synthesized.•SBF1 and SBF2 were used as acceptors to fabricate fullerene-free polymer solar cells.•Effects of isomer configuration on optical and electronic properties and device performance were investigated.Two spirobifluorene-based isomeric acceptors (SBF1 and SBF2) with electron-withdrawing 1,1-dicyanomethylene-3-indanone as end groups were theoretically studied, synthesized and applied as electron acceptors for solution-processed polymer solar cells (PSCs). SBF1 possessed three-dimensional molecular structure while SBF2 was more planar. The effect of molecular configuration on absorption, energy level, charge transport, morphology and PSC performance was investigated. These molecules exhibited good thermal stability with decomposition temperatures of 345–348 °C, broad absorption in visible region (400–650 nm), and proper HOMO (−5.96 to −5.94 eV) and LUMO (−3.86 to −3.78 eV) energy levels. The mobility of SBF2 (7.2 × 10−4 cm2 V−1 s−1) was one order of magnitude higher than that of SBF1 (6.1 × 10−5 cm2 V−1 s−1). When blending with low-bandgap polymer donor PBDTTT-C-T, the power conversion efficiencies of SBF2-based PSCs were ca. 50% higher than those of SBF1-based devices.

Organic small molecules (TPA-BT3T, TPA-PT3T, and TPA-DFBT3T) using triphenylamine as a donor unit, terthiophene as a bridge, and benzo-2,1,3-thiadiazole (BT), [1,2,5]thiadiazolo[3,4-c]pyridine (PT) or 5,6-difluorobenzo[c][1,2,5]thiadiazole (DFBT) as an acceptor unit were designed and synthesized through Suzuki coupling reactions. These molecules exhibited good thermal stability with decomposition temperatures over 380 °C and broad absorption from 300 to 700 nm. Photovoltaic devices were fabricated with these small molecules as donors and PC71BM as an acceptor. The TPA-BT3T based devices exhibited a power conversion efficiency of 2.89%, higher than those of the TPA-PT3T- and TPA-DFBT3T-based devices (1.34% and 1.54% respectively). The effects of electron-withdrawing units on absorption, energy level, charge transport, morphology, and photovoltaic properties also were investigated.

Indene-C60 bisadduct (ICBA) is used as an electron-cascade acceptor material in poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7):[6,6]-phenyl-C71-butyric-acid-methyl-ester (PC71BM) blend to fabricate ternary blend polymer solar cells (PSCs). Due to higher lowest unoccupied molecular orbital (LUMO) energy levels of ICBA relative to PC71BM, the open circuit voltage (VOC) increases with the addition of ICBA. ICBA plays a bridging role between PTB7 and PC71BM, thus providing more routes for charge transfer at the donor/acceptor (D/A) interface. When the ICBA content is much smaller than the PC71BM content, the morphology of the ternary blend active layer is similar to that of the PTB7:PC71BM blend, which guarantees suitable phase separation and efficient charge transport. Ternary blend devices with 15% ICBA content exhibit an average power conversion efficiency (PCE) of 8.13%, higher than that (7.23%) of the PTB7:PC71BM binary blend. Without any further device work (such as interlayer, invert structure and tandem cells), the ternary blend PSCs exhibit PCEs as high as 8.24%, which is the highest reported for ternary blend PSCs and ICBA-related PSCs.

Although fullerenes and their derivatives, such as PCBM, have been the dominant electron-acceptor materials in organic photovoltaic cells (OPVs), they suffer from some disadvantages, such as weak absorption in the visible spectral region, limited spectral breadth and difficulty in variably tuning the band gap. It is necessary to explore non-fullerene electron acceptors that will not only retain the favorable electron-accepting and transporting properties of fullerenes but also overcome their insufficiencies. After a decade of mediocrity, non-fullerene acceptors are undergoing rapid development and are emerging as a hot area of focus in the field of organic semiconductors. Solution-processed bulk heterojunction (BHJ) OPVs based on non-fullerene acceptors have shown encouraging power conversion efficiencies of over 4%. This article reviews recent developments in several classes of solution-processable non-fullerene acceptors for BHJ OPVs. The remaining problems and challenges along with the key research directions in the near future are discussed.

A series of conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene and bithiazole were systematically investigated for applications of polymer field-effect transistors and thin film phototransistors. Grazing incidence X-ray diffraction and atomic force microscopy were employed to investigate the solid state molecular organization and film morphology. The solution processability, solid state molecular organization, film morphology, charge transport and phototransistor performance can be tuned by elaborately optimizing the polymer backbone and side chains synchronously. For the optimized polymer (P5), a charge-carrier mobility of 0.194 cm2 V−1 s−1, a threshold voltage of −6 V and an on/off current ratio of 106 were achieved. Phototransistors based on this polymer showed a highest photoresponsivity of 132 A W−1 and a photocurrent/dark current ratio of 2 × 105.

All-polymer and polymer/fullerene inverted solar cells were fabricated by spin-coating and roll-coating processes. The spin-coated small-area (0.04 cm2) devices were fabricated on indium tin oxide (ITO) coated glass substrates in nitrogen. The roll-coated large-area (1.0 cm2) devices were prepared on ITO-free flexible substrates under ambient conditions. The use of a solvent additive, 1,8-diiodooctane (DIO), facilitated phase separation and enhanced power conversion efficiencies (PCEs). The PCE of polymer/fullerene solar cells increased from 4.58% to 8.12% with 2.5% (v/v) DIO when using the spin-coating process, and increased from 1.37% to 2.09% with 5% (v/v) DIO in the roll-coating process. The PCE of all-polymer solar cells increased from 1.44% to 3.51% with 4% (v/v) DIO when employing the spin-coating process. For the roll-coated large area devices the PCE increased from 0.15% to 0.73% with 9% (v/v) DIO. The optimal amounts of DIO, when using the roll-coating process for the two different active layers (5% and 9% respectively) are significantly higher than those for the spin-coating process (2.5% and 4%, respectively), which is ascribed to a fundamentally different drying mechanism.

Four A-D-A type small molecules using 4,4,9,9-tetrakis(4-hexylphenyl)- indaceno[1,2-b:5,6-b′]dithiophene as central building block, bithiophene or terthiophene as π-bridges, alkyl cyanoacetate or rhodanine as end acceptor groups were synthesized and investigated as electron donors in solution-processed organic solar cells (OSCs). These molecules showed excellent thermal stability with decomposition temperatures over 360 °C, relatively low HOMO levels of −5.18 to −5.22 eV, and strong optical absorption from 350 to 670 nm with high molar extinction coefficient of 1.1 × 105 to 1.6 × 105 M–1 cm–1 in chloroform solution. OSCs based on blends of these molecules and PC71BM achieved average power conversion efficiencies of 2.32 to 5.09% (the best 5.32%) after thermal annealing. The effects of thiophene bridge length and end acceptor groups on absorption, energy level, charge transport, morphology, and photovoltaic properties of the molecules were investigated.Keywords: alkyl cyanoacetate; indacenodithiophene; rhodanine; small molecule solar cells;

•The first N-acylated isoindigo monomer was reported.•Polymers based on the new monomer showed broad absorptions and deep LUMO levels.•The new polymers were studied as semiconductors in OTFT devices.Donor/acceptor polymers with N-acylated isoindigo as acceptor units have been synthesized. The thermal, optical, electrochemical, and charge-transport properties of the polymers have been investigated. The new polymers show broad absorption from 450 to 900 nm. Due to the strong electron accepting characteristic of N-acylated isoindigo, the new polymers exhibit narrow optical band gaps and deep LUMO energy levels compared with the polymers based on N-alkylated isoindigo. The new polymers show n-type transport behavior and balanced ambipolar transport behavior in organic thin film transistors (OTFT) device.

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

A n-type small molecule DC-IDT2F, with 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b′]dithiophene as a central building block, furan as π-bridges, and 1,1-dicyanomethylene-3-indanone as end acceptor groups, was synthesized and used as an electron acceptor in solution-processed organic solar cells (OSCs). DC-IDT2F exhibited good thermal stability, broad and strong absorption in 500–850 nm, a narrow bandgap of 1.54 eV, LUMO of –3.88 eV, HOMO of –5.44 eV and an electron mobility of 6.5 × 10–4 cm2/(V·s). DC-IDT2F-based OSCs with conventional and inverted structures exhibited power conversion efficiencies of 2.26 and 3.08%, respectively. The effect of vertical phase separation and morphology of the active layer on the device performance in the two structures was studied.Fullerene-free organic solar cells with conventional and inverted structures were fabricated, and effect of vertical phase separation and morphology of the active layer were studied.Download high-res image (144KB)Download full-size image

Three new organic dyes based on triphenylamine with a structure of A-D-A-D-A (D1), A-D-A (D2) and D-A (D3) were designed, theoretically calculated and synthesized for dye-sensitized solar cells. Dye D1 exhibits a broader absorption than D2 and D3, due to the intramolecular charge transfer between the donor triphenylamine and the acceptor benzothiadiazole. Dye D1 exhibits a lower HOMO and a lower LUMO than D2 and D3 due to the electron-withdrawing benzothiadiazole. The number of anchoring group cyanoacrylic acid has no obvious influence on absorption and energy levels of D2 and D3. The LUMO of D1 locates on benzothiadiazole rather than cyanoacrylic acid anchoring groups, while the LUMOs of D2 and D3 are localized on cyanoacrylic acid. D2 and D3 give higher short-circuit current density than D1. D3 with one anchoring group gives the highest open-circuit voltage. Consequently, the D3-based device gives the highest efficiency.New triphenylamine-based organic dyes with structure of A-D-A-D-A, A-D-A and D-A were designed, theoretically calculated and synthesized for dye-sensitized solar cells.Download high-res image (112KB)Download full-size image

An n-type semiconducting copolymer of perylene diimide and dithienothiophene (PPDIDTT) is used as a dual function interfacial layer to modify the surface of perovskite films in inverted perovskite solar cells. The PPDIDTT layer can remarkably passivate the surface trap states of perovskite through the formation of a Lewis adduct between the under-coordinated Pb in perovskite and S in the dithienothiophene unit of PPDIDTT, and also shows efficient charge extraction and transfer properties. The PPDIDTT modified devices exhibit a maximum power conversion efficiency of 16.5%, superior to that of the control devices without PPDIDTT (15.3%). In addition, the device stability and hysteresis in J–V curves of the modified devices are also improved compared to those of the control devices.

Four spiro[fluorene-9,9′-xanthene] (SFX)-based hole transporting materials (HTMs) functionalized with four-armed arylamine moieties located at different positions are designed and synthesized. These compounds exhibit highest occupied molecular orbital (HOMO) energy levels of −4.9 to −5.1 eV and a hole mobility of 2.2 to 15 × 10−5 cm2 V−1 s−1 after doping. Perovskite solar cells (PSCs) based on a methylammonium lead iodide (MAPbI3) active layer using one of these HTMs (mp-SFX-2PA) exhibit power conversion efficiencies (PCEs) of up to 16.8%, which is higher than that of the control devices based on benchmark spiro-OMeTAD under the same conditions (15.5%). PSCs based on mp-SFX-2PA exhibit better stability (retain 90% of their initial PCEs after 2000 h storage in an ambient atmosphere) than the control devices based on spiro-OMeTAD (retain only 28% of their initial PCEs). mp-SFX-2PA based devices employing a mixed formamidinium lead iodide (FAPbI3)/methylammonium lead bromine (MAPbBr3) perovskite layer exhibit an improved PCE of 17.7%. The effects of arylamines and their location positions on device performance are discussed.

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted considerable attention in the last two decades. Sequentially solution processed BHJ (s-BHJ) have been developed in recent years. s-BHJ not only maintain some advantages of mixed BHJ (m-BHJ), but also exhibit other advantages over m-BHJ. However, to date, s-BHJ OSCs exhibit relatively lower efficiency and have received much less attention compared with m-BHJ OSCs. Moreover, there have been rare systematic comparisons between m-BHJ and s-BHJ OSCs. In this work, we systematically compare the m-BHJ and s-BHJ OSCs based on a classical system PTB7-TH/PC71BM in terms of film morphology, domain size and purity, molecular orientation and aggregation, vertical phase separation, charge transport, efficiency and stability. The s-BHJ OSCs without additives exhibit efficiencies as high as 8.6%, which is similar to that of m-BHJ OSCs with additives (8.5%) and is the highest reported for s-BHJ OSCs. More importantly, the s-BHJ OSCs show much better device stability than the m-BHJ OSCs. This study demonstrates that employing s-BHJ is a promising strategy towards efficient and stable OSCs.

A series of conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene and bithiazole were systematically investigated for applications of polymer field-effect transistors and thin film phototransistors. Grazing incidence X-ray diffraction and atomic force microscopy were employed to investigate the solid state molecular organization and film morphology. The solution processability, solid state molecular organization, film morphology, charge transport and phototransistor performance can be tuned by elaborately optimizing the polymer backbone and side chains synchronously. For the optimized polymer (P5), a charge-carrier mobility of 0.194 cm2 V−1 s−1, a threshold voltage of −6 V and an on/off current ratio of 106 were achieved. Phototransistors based on this polymer showed a highest photoresponsivity of 132 A W−1 and a photocurrent/dark current ratio of 2 × 105.

Bathophenanthroline (Bphen), an electron transporting material widely used in organic light-emitting diodes, was added as a third component into a mixed solution of poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) to fabricate simplified inverted polymer solar cells (PSCs). Bphen spontaneously migrated onto the ITO cathode during spin coating, and thereby formed a thin modifying interlayer between the cathode and the active layer. PSCs based on the ternary blend PTB7:PC71BM:Bphen showed a power conversion efficiency of 5.35%, higher than that of the binary blend PTB7:PC71BM in an inverted structure of ITO/photoactive layer/MoO3/Ag without an additional cathode-modifying interlayer (3.43%). The other electron transporting materials, 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI) and bathocuproine (BCP), were also confirmed to function as Bphen. This strategy simplifies the structure of inverted PSCs without significant loss in efficiency.

Layer by layer (LL) solution processed organic solar cells (OSCs) were fabricated using a squaraine small molecule (DIB-SQ) as a donor and a perylene diimide polymer (PPDIDTT) as an acceptor. DIB-SQ and PPDIDTT exhibited matched energy levels and complementary absorption. Compared with the blend structure, the LL devices exhibited better morphology for exciton dissociation and reducing charge recombination. Moreover, efficient vertical phase separation can also be achieved using the LL method, which is beneficial for charge transport and collection at the appropriate electrodes. The traditional blend OSCs exhibited no photovoltaic response, while the LL devices yielded power conversion efficiencies of up to 1.12%.

A series of monodisperse macromolecules with A2–A1–D–A1–A2 structure based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) BDT-4DPP, BDT-DPP-Rhod and BDT-DPP-CA were designed, theoretically calculated and synthesized, and compared with their parent molecules BDT-2DPP and BDTS-2DPP with A1–D–A1 structure. These molecules possessed highly planar molecular geometries and high crystallinity. These molecules exhibited good thermal stability with decomposition temperatures of 322–388 °C, strong visible and near-infrared absorption (500–1000 nm), and HOMO energy levels of −5.38 to −5.19 eV and LUMO energy levels of −3.69 to −3.46 eV. Relative to the parent molecules A1–D–A1, A2–A1–D–A1–A2 molecules exhibited red-shifted and stronger absorption. The charge transport properties of these molecules were investigated by organic field-effect transistors, and their hole mobilities were 0.036–1.12 cm−2 V−1 s−1. Replacing alkyl with alkylthio on BDT led to mobility enhancement by one order of magnitude.

A series of perylene diimide (PDI) dimers (PnTP, n = 0–3) with oligothiophenes as bridges were designed, theoretically calculated, synthesized, and developed as electron acceptors for polymer solar cells (PSCs). The effects of oligothiophene bridge length on the absorption, energy level, charge transport, morphology and photovoltaic properties of the molecules were investigated. These molecules exhibited good thermal stability with decomposition temperatures of 367–413 °C, broad and strong absorption in the visible region (400–700 nm), and appropriate HOMO (−5.74 to −5.61 eV) and LUMO (−3.84 to −3.72 eV) energy levels. When blending these PDI acceptors with a low bandgap polymer donor PBDTTT-C-T, the PSCs exhibited power conversion efficiencies of 0.76–3.61%.

Two novel A–D–A type molecules IDT–2BM and IDTT–2BM with extended fused-ring indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) units as cores and strong electron-withdrawing unit 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM) as the end-capping group were synthesized and investigated as electron acceptors in solution-processed polymer solar cells (PSCs). IDT–2BM and IDTT–2BM exhibited strong and broad absorption from 300 to 800 nm, and appropriate LUMO (−3.8 eV) and HOMO (−5.5 to −5.6 eV) levels matching with the classical polymer donor PBDTTT-C-T. IDT–2BM and IDTT–2BM films exhibited intrinsic electron mobilities of about 3.7 × 10−6 and 1.0 × 10−5 cm2 V−1 s−1, respectively. Fullerene-free PSCs employing PBDTTT-C-T as the donor and IDT–2BM or IDTT–2BM as the acceptor afforded power conversion efficiencies of 4.26% and 4.81%, respectively.

Flexible organic solar cells (OSCs) based on a blend of low-bandgap polymer donor PTB7-TH and non-fullerene small molecule acceptor IEIC were fabricated via a roll-coating process under ambient atmosphere. Both an indium tin oxide (ITO)-free substrate and a flexible ITO substrate were employed in these inverted OSCs. OSCs with flexible ITO and ITO-free substrates exhibited power conversion efficiencies (PCEs) up to 2.26% and 1.79%, respectively, which were comparable to those of the reference devices based on fullerene acceptors under the same conditions. This is the first example for all roll-coating fabrication procedures for flexible OSCs based on non-fullerene acceptors with the PCE exceeding 2%. The fullerene-free OSCs exhibited better dark storage stability than the fullerene-based control devices.

Organic solar cells (OSCs) present some advantages, such as simple preparation, light weight, low cost and large-area flexible fabrication, and have attracted much attention in recent years. Although the power conversion efficiencies have exceeded 10%, the inferior device stability still remains a great challenge. In this review, we summarize the factors limiting the stability of OSCs, such as metastable morphology, diffusion of electrodes and buffer layers, oxygen and water, irradiation, heating and mechanical stress, and survey recent progress in strategies to increase the stability of OSCs, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation. Some research areas of device stability that may deserve further attention are also discussed to help readers understand the challenges and opportunities in achieving high efficiency and high stability of OSCs towards future industrial manufacture.

A novel small molecule based on indacenodithiophene and 1,1-dicyanomethylene-3-indanone was synthesized and used as an electron acceptor in solution processed organic solar cells, which exhibited a power conversion efficiency as high as 3.93%.

We design and synthesize monodisperse fused-ring oligomer molecules benzo[1,2-b:4,5-b′]dithiophene (BDT) flanked with electron-withdrawing diketopyrrolopyrrole (DPP). A tiny change in the side chain induces significant variation in crystallinity, phase separation, charge transport and photovoltaic properties of the semiconductors. BDTS-2DPP with linear alkylthio substituents exhibits a much higher hole mobility of 1.1 × 10−2 cm2 V−1 s−1 than the branched alkyl substituted BDT-2DPP (3.0 × 10−3 cm2 V−1 s−1). The LUMO (−3.49 eV) and HOMO (−5.28 eV) energy levels of BDTS-2DPP are lower than those of BDT-2DPP (−3.46 eV and −5.23 eV) due to the π-acceptor capability of the sulfur atom. Fullerene-free organic solar cells using BDTS-2DPP as a donor and monodisperse fused-ring oligomer molecule IEIC as an acceptor exhibit higher open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency (PCE, 5.29%) than the BDT-2DPP control devices (PCE = 4.00%) with the conventional structure. The inverted devices based on BDTS-2DPP:IEIC show an improved PCE of 6.03% relative to the conventional devices. Atomic force microscopy, grazing incident wide-angle X-ray diffraction and resonant soft X-ray scattering are used to deeply investigate the molecular packing, phase separation and surface aggregation of the blended films and to understand the effect of molecular side chains. We find that the linear alkylthio substitution in BDTS-2DPP improves the crystallinity and unexpectedly reserves small phase separation domains in the blend.

A novel thiophene-fused diketopyrrolopyrrole unit (7H-pyrrolo[3,4-a]thieno[3,2-g]indolizine-7,10(9H)-dione, PTI) has been designed as an electron acceptor with the 4,4′-bis-(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole (DTS) unit as a donor to construct a new kind of A–D–A molecule (DTS–2TPTI) for solution-processed solar cells. DTS–2TPTI exhibits excellent thermal stability with the decomposition temperature over 400 °C and shows strong absorption from 550 to 750 nm with a high molar extinction coefficient. The optical band gap (Eg) estimated from the absorption edge of the thin film is about 1.44 eV. The highest occupied molecular orbital energy level of DTS–2TPTI determined from CV is about −4.99 eV. Through optimizing the photovoltaic performances of devices, the DTS–2TPTI/PC71BM-based solution processed bulk-heterojunction solar cell with 0.5% DIO as a solvent additive exhibits the best photovoltaic performance with a JSC of 11.28 mA cm−2, VOC of 0.64 V, FF of 59.5% and power conversion efficiency (PCE) of 4.28%, indicating that the PTI unit can act as an efficient acceptor moiety to construct D–A small molecules for OPVs.

All-polymer and polymer/fullerene inverted solar cells were fabricated by spin-coating and roll-coating processes. The spin-coated small-area (0.04 cm2) devices were fabricated on indium tin oxide (ITO) coated glass substrates in nitrogen. The roll-coated large-area (1.0 cm2) devices were prepared on ITO-free flexible substrates under ambient conditions. The use of a solvent additive, 1,8-diiodooctane (DIO), facilitated phase separation and enhanced power conversion efficiencies (PCEs). The PCE of polymer/fullerene solar cells increased from 4.58% to 8.12% with 2.5% (v/v) DIO when using the spin-coating process, and increased from 1.37% to 2.09% with 5% (v/v) DIO in the roll-coating process. The PCE of all-polymer solar cells increased from 1.44% to 3.51% with 4% (v/v) DIO when employing the spin-coating process. For the roll-coated large area devices the PCE increased from 0.15% to 0.73% with 9% (v/v) DIO. The optimal amounts of DIO, when using the roll-coating process for the two different active layers (5% and 9% respectively) are significantly higher than those for the spin-coating process (2.5% and 4%, respectively), which is ascribed to a fundamentally different drying mechanism.