Fluorescent Conversion Agent Embedded in Zinc Oxide as an Electron-Transporting Layer for High-Performance Non-Fullerene Organic Solar Cells with Improved Photostability.

Zinc oxide (ZnO) is widely used as an electron transporting layer (ETL) for organic solar cells (OSCs). Here, a low-cost commercial water/alcohol-soluble fluorescent conversion agent, sodium 2,2'-([1,1'-biphenyl]-4,4'-diyldivinylene)-bis(benzenesulfonate) (CBS), is incorporated into ZnO to develop a novel organic-inorganic hybrid ETL for high-performance OSCs. The photoinduced charge transfer from CBS to ZnO significantly improves the charge transport properties of ZnO, resulting in faster electron extraction and reduced charge recombination in OSC devices with ZnO:CBS ETLs. ZnO:CBS-based devices exhibit higher power conversion efficiencies (PCEs) than their pure ZnO-based counterparts, especially in devices with a thicker ETL, which is more suitable for roll-to-roll and large-area module processing. Furthermore, the strong ultraviolet-light absorption capability of CBS inhibits the photodegradation of the active layer, improving the photostability of ZnO:CBS based OSC devices. Therefore, this work provides a simple and effective strategy for realizing high-performance OSCs with high PCE and good photostability, which can further facilitate the commercialization of OSCs.

Identifying structure-absorption relationships and predicting absorption strength of non-fullerene acceptors for organic photovoltaics.

Non-fullerene acceptors (NFAs) are excellent light harvesters, yet the origin of their high optical extinction is not well understood. In this work, we investigate the absorption strength of NFAs by building a database of time-dependent density functional theory (TDDFT) calculations of ∼500 π-conjugated molecules. The calculations are first validated by comparison with experimental measurements in solution and solid state using common fullerene and non-fullerene acceptors. We find that the molar extinction coefficient (ε d,max) shows reasonable agreement between calculation in vacuum and experiment for molecules in solution, highlighting the effectiveness of TDDFT for predicting optical properties of organic π-conjugated molecules. We then perform a statistical analysis based on molecular descriptors to identify which features are important in defining the absorption strength. This allows us to identify structural features that are correlated with high absorption strength in NFAs and could be used to guide molecular design: highly absorbing NFAs should possess a planar, linear, and fully conjugated molecular backbone with highly polarisable heteroatoms. We then exploit a random decision forest algorithm to draw predictions for ε d,max using a computational framework based on extended tight-binding Hamiltonians, which shows reasonable predicting accuracy with lower computational cost than TDDFT. This work provides a general understanding of the relationship between molecular structure and absorption strength in π-conjugated organic molecules, including NFAs, while introducing predictive machine-learning models of low computational cost.

Renewed Prospects for Organic Photovoltaics.

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (Eloss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller Eloss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Emissive Charge-Transfer States at Hybrid Inorganic/Organic Heterojunctions Enable Low Non-Radiative Recombination and High-Performance Photodetectors.

Hybrid devices based on a heterojunction between inorganic and organic semiconductors have offered a means to combine the advantages of both classes of materials in optoelectronic devices, but, in practice, the performance of such devices has often been disappointing. Here, it is demonstrated that charge generation in hybrid inorganic-organic heterojunctions consisting of copper thiocyanate (CuSCN) and a variety of molecular acceptors (ITIC, IT-4F, Y6, PC70 BM, C70 , C60 ) proceeds via emissive charge-transfer (CT) states analogous to those found at all-organic heterojunctions. Importantly, contrary to what has been observed at previous organic-inorganic heterojunctions, the dissociation of the CT-exciton and subsequent charge separation is efficient, allowing the fabrication of planar photovoltaic devices with very low non-radiative voltage losses (0.21 ±  0.02 V). It is shown that such low non-radiative recombination enables the fabrication of simple and cost-effective near-IR (NIR) detectors with extremely low dark current (4 pA cm-2 ) and noise spectral density (3 fA Hz-1/2 ) at no external bias, leading to specific detectivities at NIR wavelengths of just under 1013 Jones, close to the performance of commercial silicon photodetectors. It is believed that this work demonstrates the possibility for hybrid heterojunctions to exploit the unique properties of both inorganic and organic semiconductors for high-performance opto-electronic devices.

Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells.

Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor-acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron-hole encounters at later times, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells.

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π-π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

High-Performance Ternary Organic Solar Cells with Controllable Morphology via Sequential Layer-by-Layer Deposition.

Ternary blending of light-harvesting materials has been proven to be a potential strategy to improve the efficiency of solution-processed organic solar cells (OSCs). However, the optimization of a ternary system is usually more complicated than that of a binary one as the morphology of conventional ternary blend films is very difficult to control, thus undermining the potential of ternary OSCs. Herein, we report a general strategy for better control of the morphology of ternary blend films composed of a polymer donor and two nonfullerene small-molecule acceptors for high-performance OSCs using the sequential layer-by-layer (LbL) deposition method. The resulting LbL films form a bicontinuous interpenetrating network structure with high crystallinity of both the donor and acceptor materials, showing efficient charge generation, transport, and collection properties. In addition, the power conversion efficiencies (PCEs) of the ternary LbL OSCs are less sensitive to the blending ratio of the third component acceptor, providing more room to optimize the device performance. As a result, optimal PCEs of over 11, 13, and 16% were achieved for the LbL OSCs composed of PffBT4T-2OD/IEICO-4F:FBR, PBDB-T-SF/IT-4F:FBR, and PM6/Y6:FBR, respectively. Our work provides useful and general guidelines for the development of more efficient ternary OSCs with better controlled morphology.

Interface-enhanced organic solar cells with extrapolated T80 lifetimes of over 20 years.

With recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) based on novel donor and non-fullerene acceptor (NFAs), improving the stability of these systems has become the most important issue for their practical applications. Herein, an efficient and highly stable OSC, containing a novel polymer donor and a non-fullerene acceptor system, is reported. The OSC is based on an inverted device structure that utilizes a self-assembled fullerene monolayer (C60-SAM) as the cathode modification layer, and an efficient and highly stable OSC composes of a polymer donor of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-3-fluorothie-no[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and a non-fullerene acceptor of (2,2'-((2Z,2'Z)-(((4,4,9,9-Tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4-((2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene -2,1-diylidene))dimalononitrile) (IEICO-4F) is presented, showing a PCE of 10%. It further achieves an extrapolated T80 lifetime (the time required to reach 80% of initial performance) of 34,000 h, operating under one sun illumination equivalent. Based on an estimated solar irradiance of 1500 kWh/(m2 year) for China, a potential lifetime of 22 years is inferred for the OSC. Further investigation reveals that the reported C60-SAM modification stabilizes the OSC active layer morphology by lowering the surface energy of the underlying ZnO electron transport layer and suppressing trap-assisted recombination, thereby improving photostability. The results of this work establish important guidelines for the development of non-fullerene based OSCs with enhanced stability and pave the way for the commercialization of OSC technology.

Fused Benzothiadiazole: A Building Block for n-Type Organic Acceptor to Achieve High-Performance Organic Solar Cells.

Narrow bandgap n-type organic semiconductors (n-OS) have attracted great attention in recent years as acceptors in organic solar cells (OSCs), due to their easily tuned absorption and electronic energy levels in comparison with fullerene acceptors. Herein, a new n-OS acceptor, Y5, with an electron-deficient-core-based fused structure is designed and synthesized, which exhibits a strong absorption in the 600-900 nm region with an extinction coefficient of 1.24 × 105 cm-1 , and an electron mobility of 2.11 × 10-4 cm2 V-1 s-1 . By blending Y5 with three types of common medium-bandgap polymers (J61, PBDB-T, and TTFQx-T1) as donors, all devices exhibit high short-circuit current densities over 20 mA cm-2 . As a result, the power conversion efficiency of the Y5-based OSCs with J61, TTFQx-T1, and PBDB-T reaches 11.0%, 13.1%, and 14.1%, respectively. This indicates that Y5 is a universal and highly efficient n-OS acceptor for applications in organic solar cells.

An Open-Circuit Voltage and Power Conversion Efficiency Study of Fullerene Ternary Organic Solar Cells Based on Oligomer/Oligomer and Oligomer/Polymer.

Variations in the open-circuit voltage (Voc ) of ternary organic solar cells are systematically investigated. The initial study of these devices consists of two electron-donating oligomers, S2 (two units) and S7 (seven units), and the electron-accepting [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) and reveals that the Voc is continuously tunable due to the changing energy of the charge transfer state (Ect ) of the active layers. Further investigation suggests that Voc is also continuously tunable upon change in Ect in a ternary blend system that consists of S2 and its corresponding polymer (P11):PC71 BM. It is interesting to note that higher power conversion efficiencies can be obtained for both S2:S7:PC71 BM and S2:P11:PC71 BM ternary systems compared with their binary systems, which can be ascribed to an improved Voc due to the higher Ect and an improved fill factor due to the improved film morphology upon the incorporation of S2. These findings provide a new guideline for the future design of conjugated polymers for achieving higher performance of ternary organic solar cells.

High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor.

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π-π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices.

Donor-acceptor-type copolymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) scaffold for high-efficiency polymer solar cells.

Four donor-acceptor-type low-bandgap conjugated polymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) (NT) acceptor and different donors bridged by a bithiophene spacer have been synthesized through Suzuki or Stille polymerization reactions. Fluorene (F), carbazole (Cz), alkylidene fluorene (AF), and benzodithiophene (BDT) were selected as the donor units to produce a series of new conjugated polymers. Owing to the different electron-donating ability of the donor units, the energy levels, absorption spectra, bandgaps, and carrier mobilities of the resulting polymers were systematically tuned. Bulk-heterojunction-type polymer solar cells based on the new polymers and [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM) or [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM) were investigated and all of the devices exhibited good photovoltaic performance, with power-conversion efficiencies (PCEs) over 3 %. The best device performance was achieved by PF-C12NT, with an open-circuit voltage (Voc ) of 0.87 V, a short-circuit current density (Jsc ) of 12.19 mA cm(-2) , a fill factor (FF) of 61.36 %, and a PCE of 6.51 % under simulated sunlight (100 mW cm(-2) , AM 1.5G).