Yingzhi Jin

In article number 1701405, Hae Jung Son and co-workers develop D₁-A-D₂-A-type random terpolymers. Organic photovoltaics (OPVs) introducing the resulting polymer achieve a high efficiency of 10.31%. Furthermore, due to outstanding solution processability of the random terpolymer, 1 cm² OPVs reproducibly shows a high efficiency of up to 9.42% using thick active layers in the range of 250–380 nm.

Solution-processed perovskite solar cells have great potential for low-cost roll-to-roll fabrication. However, the degradation of aged precursor solutions will become a critical obstacle to mass production. In this report, a small molecule (ITIC-Th) is employed to stabilize the perovskite precursor solution containing mixed cations and halides. It is found that ITIC-Th can effectively suppress the formation of yellow δ-phase in the films made from aged precursor solutions. Consequently, the devices fabricated from the aged precursor solution with ITIC-Th experience much less efficiency drop with the increase of the precursor aging time—from 19.20% (fresh) to 16.55% (39 d), compared with the devices made from conventional precursor solutions dropping from 18.07% (fresh) to 1.76% (39 d). The characterizations suggest that ITIC-Th is beneficial for CH₃NH₃⁺ cations to be incorporated into the crystal structure, facilitating the formation of perovskite phase. Furthermore, the presence of ITIC-Th in the perovskite thin film gives rise to additional photocurrent as well as improved fill factor due to the well-matched energy levels, the passivation of defects, and the complementary absorption spectra, suggesting a new route toward future high-efficiency solar cells—incorporating organic non-fullerene acceptors and halide perovskite materials into the same active layer.

Herein, the authors report a novel route to greatly enhance the long-term stability of perovskite precursor solutions by incorporating a non-fullerene small molecule ITIC-Th. The device fabricated from a 39-day aged precursor solution with ITIC-Th remains at a relatively high efficiency, exceeding 16%, while the device from a 39-day precursor solution without ITIC-Th only shows a PCE of 1.76%.

The wide-spread application of organic phosphoresence materials is yet hampered by an insufficient performance under ambient conditions. Ultralong organic phosphorescence (UOP) efficiency can be improved by an intermolecular π-type halogen bonding construction, as reported by Zhongfu An, Qian Peng, Wei Huang, and their co-workers in article number 1705045. The UOP reaches 13.0% efficiency under ambient conditions, enabling data encryption and decryption under sunlight.

Molecular weight is an important factor determining the morphology and performance of all-polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide-based acceptor polymer are prepared with molecular weight varying from M_n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M_n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M_n = 167 kDa however is found to produce a drop in performance to 3% due to liquid–liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.

Excessively high molecular weights are shown to be detrimental to the performance of all-polymer solar cells. Increasing the molecular weight of the acceptor polymer to M_n = 167 kDa is found to result in liquid–liquid phase separation negatively impacting charge generation and collection. Intermediate molecular weights instead provide an optimum morphology with good carrier mobilities and improved molecular order.

Two wide bandgap star-shaped small molecular acceptors, para-TrBRCN and meta-TrBRCN, are synthesized for efficient nonfullerene polymer solar cells (PSCs). The tiny structural variation by just changing the linkage positions affects largely the inherent properties of the resulting molecules. Both molecules have a nonplanar 3D structure, which can prevent the excessively aggregation to realize the optimized morphology and ideal domain size in their active blends. Compared to para-TrBRCN, meta-TrBRCN exhibits the smaller distortions between the truxene skeleton and the benzothiadiazole units, which would also lead to the enhanced π–π stacking and charge transfer. When blending with PTB7-Th, high power conversion efficiencies (PCEs) of 10.15% and 8.28% are obtained for meta-TrBRCN and para-TrBRCN devices, respectively. To make up the weak absorption of above binary active blend in the longer wavelength region and increase the whole device performance further, low bandgap 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylthienyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC-Th) is added as the second acceptor material to fabricate ternary blend PSCs. After adding 20 wt% of ITIC-Th, the resulting devices exhibit the well-balanced optical absorption and fine-tuned morphology, giving rise to the significantly improved PCE of 11.40% with much higher J _sc of 18.25 mA cm⁻² and fill factor of 70.2%.

Tow star-shaped wide bandgap molecular acceptors with truxene core were synthesized for efficient non-fullerene polymer solar cells. Both acceptors show high absorptions in the short wavelength region, which can match well with those of low bandgap polymer donors, giving rise to high power conversion efficiencies of 10.15% from binary blend devices and 11.40% from ternary blend devices, respectively.

In order to utilize the near-infrared (NIR) solar photons like silicon-based solar cells, extensive research efforts have been devoted to the development of organic donor and acceptor materials with strong NIR absorption. However, single-junction organic solar cells (OSCs) with photoresponse extending into >1000 nm and power conversion efficiency (PCE) >11% have rarely been reported. Herein, three fused-ring electron acceptors with varying core size are reported. These three molecules exhibit strong absorption from 600 to 1000 nm and high electron mobility (>1 × 10⁻³ cm² V⁻¹ s⁻¹). It is proposed that core engineering is a promising approach to elevate energy levels, enhance absorption and electron mobility, and finally achieve high device performance. This approach can maximize both short-circuit current density (  J_SC) and open-circuit voltage (V_OC) at the same time, differing from the commonly used end group engineering that is generally unable to realize simultaneous enhancement in both V_OC and J_SC. Finally, the single-junction OSCs based on these acceptors in combination with the widely polymer donor PTB7-Th yield J_SC as high as 26.00 mA cm⁻² and PCE as high as 12.3%.

Single-junction binary-blend polymer solar cells based on PTB7-Th/F8IC afford efficiency of 10.9%, which is higher than those of F6IC (7.1%) and F10IC (10.2%) counterparts. Furthermore, ternary-blend devices based on PTB7-Th/F8IC/PC₇₁BM exhibit J_SC as high as 26.00 mA cm⁻² and power conversion efficiency as high as 12.3%.

Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near-infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high-performance fullerene-free OSCs based on a combination of a medium-bandgap polymer donor (FTAZ) and a narrow-bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single-junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.

High-performance fullerene-free single-junction organic solar cells with power conversion efficiencies up to 12.5% are reported. Transient absorption spectroscopy reveals that exciting either the donor or acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states.

A new electron-rich central building block, 5,5,12,12-tetrakis(4-hexylphenyl)-indacenobis-(dithieno[3,2-b:2′,3′-d]pyrrol) (INP), and two derivative nonfullerene acceptors (INPIC and INPIC-4F) are designed and synthesized. The two molecules reveal broad (600–900 nm) and strong absorption due to the satisfactory electron-donating ability of INP. Compared with its counterpart INPIC, fluorinated nonfullerene acceptor INPIC-4F exhibits a stronger near-infrared absorption with a narrower optical bandgap of 1.39 eV, an improved crystallinity with higher electron mobility, and down-shifted highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Organic solar cells (OSCs) based on INPIC-4F exhibit a high power conversion efficiency (PCE) of 13.13% and a relatively low energy loss of 0.54 eV, which is among the highest efficiencies reported for binary OSCs in the literature. The results demonstrate the great potential of the new INP as an electron-donating building block for constructing high-performance nonfullerene acceptors for OSCs.

Nonfullerene acceptors (NFAs) featuring indacenobis-(dithieno[3,2-b:2′,3′-d]pyrrol) as an electron-rich central building block are designed. The NFAs extend absorption to 900 nm with an optical bandgap of 1.39 eV. Organic solar cells (OSCs), by blending with PBDB-T as polymer donor, contribute a power conversion efficiency of 13.13%, which is among the highest reported for binary OSCs in the literature.

In the field of organic solar cells (OSCs), tandem structure devices exhibit very attractive advantages for improving power conversion efficiency (PCE). In addition to the well researched novel pair of active layers in different subcells, the construction of interconnecting layer (ICL) also plays a critical role in achieving high performance tandem devices. In this work, a new way of achieving environmentally friendly solvent processed polymeric ICL by adopting poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide] (PNDIT-F3N) blended with poly(ethyleneimine) (PEI) as the electron transport layer (ETL) and PEDOT:PSS as the hole transport layer is reported. It is found that the modification ability of PNDIT-F3N on PEDOT can be linearly tuned by the incorporation of PEI, which offers the opportunity to study the charge recombination behavior in ICL. At last, tandem OSC with highest PCE of 12.6% is achieved, which is one of the best tandem OSCs reported till now. These results offer a new selection for constructing efficient ICL in high performance tandem OSCs and guide the way of design new ETL materials for ICL construction, and may even be integrated in future printed flexible large area module device fabrication with the advantages of environmentally friendly solvent processing and thickness insensitivity.

A new polymeric interconnecting layer (ICL) based on poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide]: poly(ethyleneimine)/PEDOT:PSS is developed and applied for the fabrication of high performance tandem organic solar cells (OSCs). Tandem OSCs employing this ICL achieve a high power conversion efficiency of 12.6% with ICL thickness of 60 nm and even reach to 11.3% with ICL thickness of 140 nm.

A novel wide-bandgap electron-donating copolymer containing an electron-deficient, difluorobenzotriazole building block with a siloxane-terminated side chain is developed. The resulting polymer, poly{(4,8-bis(4,5-dihexylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-4,7-di(thiophen-2-yl)-5,6-difluoro-2-(6-(1,1,1,3,5,5,5-heptamethyltri-siloxan-3-yl)hexyl)-2H-benzo[d][1,2,3]triazole} (PBTA-Si), is used to successfully fabricate high-performance, ternary, all-polymer solar cells (all-PSCs) insensitive to the active layer thickness. An impressively high fill factor of ≈76% is achieved with various ternary-blending ratios. The optimized all-PSCs attain a power conversion efficiency (PCE) of 9.17% with an active layer thickness of 350 nm and maintain a PCE over 8% for thicknesses over 400 nm, which is the highest reported efficiency for thick all-PSCs. These results can be attributed to efficient charge transfer, additional energy transfer, high and balanced charge transport, and weak recombination behavior in the photoactive layer. Moreover, the photoactive layers of the ternary all-PSCs are processed in a nonhalogenated solvent, 2-methyltetrahydrofuran, which greatly improves their compatibility with large-scale manufacturing.

A novel electron-donating copolymer, PBTA-Si, containing a benzotriazole building block with a siloxane-functionalized side chain, is developed and used to fabricate thick-film all-polymer solar cells (all-PSC). By means of ternary blending, the all-PSCs attain a power conversion efficiency of 9.17% with a 350 nm thick active layer and 8.34% with a thickness of 420 nm.

Hole transport layer (HTL) plays a critical role for achieving high performance solution-processed optoelectronics including organic electronics. For organic solar cells (OSCs), the inverted structure has been widely adopted to achieve prolonged stability. However, there are limited studies of p-type effective HTL on top of the organic active layer (hereafter named as top HTL) for inverted OSCs. Currently, p-type top HTLs are mainly 2D materials, which have an intrinsic vertical conduction limitation and are too thin to function as practical HTL for large area optoelectronic applications. In the present study, a novel self-assembled quasi-3D nanocomposite is demonstrated as a p-type top HTL. Remarkably, the novel HTL achieves ≈15 times enhanced conductivity and ≈16 times extended thickness compared to the 2D counterpart. By applying this novel HTL in inverted OSCs covering fullerene and non-fullerene systems, device performance is significantly improved. The champion power conversion efficiency reaches 12.13%, which is the highest reported performance of solution processed HTL based inverted OSCs. Furthermore, the stability of OSCs is dramatically enhanced compared with conventional devices. The work contributes to not only evolving the highly stable and large scale OSCs for practical applications but also diversifying the strategies to improve device performance.

A novel self-assembled quasi-3D nanocomposite is demonstrated to be an effective top hole transport layer (HTL) for both fullerene and non-fullerene inverted organic solar cells. Due to the better conductivity of this nanocomposite HTL, the thickness sensitivity issue of graphene oxide is addressed. Surface recombination is suppressed and the highest power conversion efficiency can reach 12.13%.

Two new wide-bandgap D–A–π copolymer donor materials, PBDT-2TC and PBDT-S-2TC, based on benzodithiophene and asymmetric bithiophene with one carboxylate (2TC) substituent are synthesized by a facile approach for fullerene-free organic solar cells (OSCs). The combination of one carboxylate-substituted thiophene with one thiophene bridge in the backbone substantially reduces the steric hindrance, thereby favoring a planar geometry for efficient charge transport and molecular packing. A reasonable highest-occupied-molecular-orbital energy level in relation to that of the acceptor and balanced hole and electron transport are observed for both polymers. This asymmetric structure unit is flexible and versatile, allowing the absorption, energy levels, and morphology of the blend films to be tailored. Fullerene-free OSCs based on PBDT-S-2TC:ITIC achieve a high power conversion efficiency of 10.12%. More impressively, a successful nonhalogen solvent-processed solar cell with 9.55% efficiency is also achieved, which is one of the highest values for a fullerene-free OSC processed using an ecofriendly solvent.

New wide-bandgap D–A–π copolymers based on an asymmetric bithiophene with one carboxylate substituent were synthesized. The asymmetric structure unit is flexible and versatile, which allows the absorption, energy levels and morphology of the blend films to be adjusted easily. D-A-p copolymers produced a high power conversion efficiency of 10.0% for halogen solvent-processed OSCs and 9.55% for non-halogen solvent-processed devices.

The field effect transistor (FET) is arguably one of the most important circuit elements in modern electronics. Recently, a need has developed for flexible electronics in a variety of emerging applications. Examples include form-fitting healthcare-monitoring devices, flexible displays, and flexible radio frequency identification tags. Organic FETs (OFETs) are viable candidates for producing such flexible devices because they incorporate semiconducting π-conjugated materials, including small molecules and conjugated polymers, which are intrinsically soft and mechanically compatible with flexible substrates. For OFETs to be industrially viable, however, they must achieve not only high charge carrier mobility, but also ideal and comprehensible electrical characteristics. Most recently, nonideal double-slope characteristics in the transfer curves of OFETs (i.e., high slope at low gate voltage and low slope at high gate voltage), have stirred debate, which has led to different mechanistic rationales in the literature. This review focuses on the general observations, mechanistic understanding, and possible solutions associated with phenomena that result in FETs with double-slope characteristics. By surveying and systematically summarizing in a single source relevant literature that deals with the issue of double slope, the experimental framework and theoretical basis for interpreting and avoiding this electrical nonideality in OFETs is provided.

A transfer curve of an organic field-effect transistor is shown on the left. What is the charge carrier mobility of the transistor? A) 5.6 cm² V⁻¹ s⁻¹; B) 14.8 cm² V⁻¹ s⁻¹; C) either of them; D) neither of them.

Since the discovery of conjugated polymers in the 1970s, they have attracted considerable interest in light of their advantages of having a tunable bandgap, high electroactivity, high flexibility, and good processability compared to inorganic conducting materials. The above combined advantages make them promising for effective energy harvesting and storage, which have been widely studied in recent decades. Herein, the key advancements in the use of conjugated polymers for flexible energy harvesting and storage are reviewed. The synthesis, structure, and properties of conjugated polymers are first summarized. Then, their applications in flexible polymer solar cells, thermoelectric generators, supercapacitors, and lithium-ion batteries are described. The remaining challenges are then discussed to highlight the future direction in the development of conjugated polymers.

The key advancements in the use of conjugated polymers for flexible energy harvesting and storage are reviewed. Such flexible energy devices may open up a new direction in multidisciplinary fields across chemistry, physics, biology, and engineering.

Rapid progress in the power conversion efficiency (PCE) of polymer solar cells (PSEs) is beneficial from the factors that match the irradiated solar spectrum, maximize incident light absorption, and reduce photogenerated charge recombination. To optimize the device efficiency, a nanopatterned ZnO:Al₂O₃ composite film is presented as an efficient light- and charge-manipulation layer (LCML). The Al₂O₃ shells on the ZnO nanoparticles offer the passivation effect that allows optimal electron collection by suppressing charge-recombination loss. Both the increased refractive index and the patterned deterministic aperiodic nanostructure in the ZnO:Al₂O₃ LCML cause broadband light harvesting. Highly efficient single-junction PSCs for different binary blends are obtained with a peak external quantum efficiency of up to 90%, showing certified PCEs of 9.69% and 13.03% for a fullerene blend of PTB7:PC₇₁BM and a nonfullerene blend, FTAZ:IDIC, respectively. Because of the substantial increase in efficiency, this method unlocks the full potential of the ZnO:Al₂O₃ LCML toward future photovoltaic applications.

Highly efficient polymer solar cells based on nanopatterned ZnO:Al₂O₃ composite film achieve a peak external quantum efficiency up to 90% and a certified power conversion efficiency of 13.03%. Optical and electrical studies demonstrate enhanced light harvesting due to passivation- and dipole-induced suppression of charge recombination loss and broadband absorption enhancement.

Realizing over 10% efficiency in printed organic solar cells via scalable materials and less toxic solvents remains a grand challenge. In article number 1705485, Harald Ade and co-workers report chlorine-free, in-air blade-coating of a new photoactive combination, FTAZ:IT-M, which is able to yield an efficiency of nearly 11%, despite a high humidity of ≈50%.

Three acceptor–donor–acceptor type nonfullerene acceptors (NFAs), namely, F–F, F–Cl, and F–Br, are designed and synthesized through a halogenation strategy on one successful nonfullerene acceptor FDICTF (F–H). The three molecules show red-shifted absorptions, increased crystallinities, and higher charge mobilities compared with the F–H. After blending with donor polymer PBDB-T, the F–F-, F–Cl-, and F–Br-based devices exhibit power conversion efficiencies (PCEs) of 10.85%, 11.47%, and 12.05%, respectively, which are higher than that of F–H with PCE of 9.59%. These results indicate that manipulating the absorption range, crystallinity and mobilities of NFAs by introducing different halogen atoms is an effective way to achieve high photovoltaic performance, which will offer valuable insight for the designing of high-efficiency organic solar cells.

Through a halogenation strategy onto the end-capping group in the FDICTF-based small-molecule acceptor, red-shifted absorptions, increased crystallinities, and higher charge mobilities are achieved. The device based on F–Br with power conversion efficiency of 12.05% and remarkable FF of 76% is one of only a few organic solar cells with efficiencies over 12% reported to date.

Polymer solar cells (PSCs) continue to be a promising low-cost and lead-free photovoltaic technology. Of critical importance to PSCs is understanding and manipulating the composition of the amorphous mixed phase, which is governed by the thermodynamic molecular interactions of the polymer donor and acceptor molecules and the kinetics of the casting process. This progress report clarifies and defines nomenclature relating to miscibility and its relevance and implications to PSC devices in light of new developments. Utilizing a scanning transmission X-ray microscopy method, the temperature dependences of “molecular miscibility” in the presence of fullerene crystals, now referred to liquidus miscibility, are presented for a number of representative blends. An emphasis is placed on relating the amorphous miscibility of high-efficiency PSC blends at a given processing temperature with their actual device performance and stability. It is shown and argued that a system with an amorphous miscibility close to percolation exhibits the most stable morphology. Furthermore, an approach is outlined to convert liquidus miscibility to an effective Flory–Huggins interaction parameter χ. Crucially, determination of temperature-dependent amorphous miscibility paves a way to rationally optimize the stability and mixing behaviors of PSCs at actual processing and operating temperatures.

The significance of miscibility and its temperature dependence in controlling morphology, performance, and stability of polymer:fullerene solar cells is discussed. Highly variable miscibility is observed for a wide range of systems and can be converted to temperature-dependent effective Flory–Huggins interaction parameter (χ). There is an optimum miscibility near the fullerene percolation threshold for the most efficient and stable solar cells.