Bandgap Narrowing in Non‐Fullerene Acceptors: Single Atom Substitution Leads to High Optoelectronic Response Beyond 1000 nm

Two narrow bandgap non‐fullerene acceptors (NBG‐NFAs), namely, COTIC‐4F and SiOTIC‐4F, are designed and synthesized for the fabrication of efficient near‐infrared organic solar cells (OSCs). The chemical structures of the NBG‐NFAs contain a D′‐D‐D′ electron‐rich internal core based on a cyclopentadithiophene (or dithienosilole) (D) and alkoxythienyl (D′) core, end‐capped with the highly electron‐deficient unit 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (A), ultimately providing a A‐D′‐D‐D′‐A molecular configuration that enhances the intramolecular charge transfer characteristics of the excited states. One can thereby reduce the optical bandgap (E_g^opt) to as low as ≈1.10 eV, one of the smallest values for NFAs reported to date. In bulk‐heterojunction (BHJ) OSCs, NBG‐NFA blends with the polymer donor PTB7‐Th yield power conversion efficiencies (PCE) of up to 9.0%, which is particularly high when compared against a range of NBG BHJ blends. Most significantly, it is found that, despite the small energy loss (E_g^opt ? eV_OC) of 0.52 eV, the PTB7‐Th/NBG‐NFA bulk heterojunction blends can yield short‐circuit current densities of up to 22.8 mA cm^?2, suggesting that the design and application of NBG‐NFA materials have substantial potential to further improve the PCE of OSCs.     

Overcoming Space‐Charge Effect for Efficient Thick‐Film Non‐Fullerene Organic Solar Cells

Organic solar cells (OSCs) containing non‐fullerene acceptors have realized high power conversion efficiency (PCE) up to 14%. However, most of these high‐performance non‐fullerene OSCs have been reported with optimal active layer thickness of about 100 nm, mainly due to the low electron mobility (≈10^?4–10^?5 cm² V^?1 s^?1) of non‐fullerene acceptors, which are not suitable for roll‐to‐roll large‐scale processing. In this work, an efficient non‐fullerene OSC based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′′′‐diyl)] (PffBT4T‐2OD):EH‐IDTBR (consists of electron‐rich indaceno[1,2‐b:5,6‐b′]dithiophene as the central unit and an electron‐deficient 5,6‐benzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group) with thickness‐independent PCE (maintaining a PCE of 9.1% with an active layer thickness of 300 nm) is presented by optimizing device architectures to overcome the space‐charge effects. Optical modeling reveals that most of the incident light is absorbed near the transparent electrode side in thick‐film devices. The transport distance of electrons with lower mobility will therefore be shortened when using inverted device architecture, in which most of the excitons are generated close to the cathode side and therefore substantially reduces the accumulation of electrons in the device. As a result, an efficient thick‐film non‐fullerene OSC is realized. These results provide important guidelines for the development of more efficient thick‐film non‐fullerene OSCs.     

High‐Performance Tandem Organic Solar Cells Using HSolar as the Interconnecting Layer

Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.     

Light Trapping with Dielectric Scatterers in Single‐ and Tandem‐Junction Organic Solar Cells

Efficient dielectric scatterers based on a mixture of TiO₂ nanoparticles and polydimethylsiloxane are demonstrated for light trapping in semitransparent organic solar cells. An improvement of 80% in the photocurrent of an optimized semitransparent solar cell is achieved with the dielectric scatterer with ≈100% diffuse reflectance for wavelengths larger than 400 nm. For a parallel tandem solar cell, the dielectric scatterer generates 20% more photocurrent compared with a silver mirror beneath the cell; for a series tandem solar cell, the dielectric scatterer can be used as a photocurrent balancer between the subcells with different photoabsorbing materials. The power conversion efficiency of the tandem cell in series configuration with balanced photocurrent in the subcells exceeds that of an optimized standard solar cell with a reflective electrode. The characteristics of polydimethylsiloxane, such as flexibility and the ability to stick conformably to surfaces, also remain in the dielectric scatterers, which makes the demonstrated light trapping configuration highly suitable for large scale module manufacturing of roll‐to‐roll printed organic single‐ or tandem‐junction solar cells.     

Modulation of End Groups for Low‐Bandgap Nonfullerene Acceptors Enabling High‐Performance Organic Solar Cells

The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high‐performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near‐infrared region, it is important to develop ultralow‐bandgap SMAs that have extended absorption in the spectral range of 800–1000 nm to maximize light absorption and efficiencies. In this work, three low‐bandgap SMAs, namely, IXIC, IXIC‐2Cl, and IXIC‐4Cl, are designed and synthesized with same fused terthieno[3,2‐b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (E_g) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the V_oc of the devices is reduced but the J_sc is significantly increased. In addition, the addition of chlorine atoms can enhance π–π stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine‐substituted IC and OSCs based on PBDB‐T:IXIC‐2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low‐bandgap SMAs.     

Efficient Non‐Fullerene Organic Photovoltaic Modules Incorporating As‐Cast and Thickness‐Insensitive Photoactive Layers

In this work, a new combination of a wide bandgap polymer poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]‐dithiophene‐alt‐N‐(2‐hexyldecyl)‐5′5‐bis[3‐(decylthio)thiophene‐2‐yl]‐2′2‐bithiophene‐3′3‐dicarboximide] (PBTIBDTT) and a non‐fullerene small molecule acceptor based on a bulky seven‐ring fused core (indacenodithieno[3,2‐b]thiophene) end‐capped with 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile groups with one fluorine substituent (ITIC‐F) is proposed, and as‐cast non‐fullerene organic solar cells (NFOSCs) with 11.2% efficiency are achieved. The device efficiencies are also insensitive to the variation of photoactive layer (PAL) thickness and can maintain over 9% efficiency as PAL thickness increases to 350 nm, which is one of the best results for as‐cast organic solar cells. More importantly, non‐fullerene organic photovoltaic (OPV) modules are demonstrated via laser ablation technique for the first time, which delivers a record efficiency of 8.6% (with active area of 3.48 cm²) among large‐area OPV modules. Furthermore, the morphology and performance evolutions of the as‐cast NFOSCs and the ones processed with solvent additive are systematically investigated. The results demonstrate the great advantage of as‐cast solar cells in achieving constant morphology and high performance with thick PALs. The NFOSCs fabricated with simple procedure, insensitive to film thickness and compatible with large‐area OPV modules, show significant potential for application the future.     

High‐Efficiency As‐Cast Organic Solar Cells Based on Acceptors with Steric Hindrance Induced Planar Terminal Group

A series of alkyl, alkoxyl, and alkylthio substituted A–π–D–π–A type nonfullerene acceptors (NFAs) IDTCN‐C , IDTCN‐O, and IDTCN‐S are designed and synthesized. The introduction of a lateral side chain at the outer position of the π bridge unit can endow the terminal moiety with a confined planar conformation due to the steric hindrance. Thus, compared with nonsubstituted NFA ( IDTT2F ), these acceptors tend to form favorable face‐on orientation and exhibit strong crystallinity as verified with grazing‐incidence wide‐angle X‐ray scattering measurement. Moreover, the variation of side chain can significantly change the lowest unoccupied molecular orbital (LUMO) energy level of acceptors. As state‐of‐the‐art NFAs, a power conversion efficiency of 13.28% (V_oc = 0.91 V, J_sc = 19.96 mA cm^?2, and FF = 73.2%) is obtained for the as‐cast devices based on IDTCN‐O , which is among the highest value reported in literature. The excellent photovoltaic performance for IDTCN‐O can be attributed to its slightly up‐shifted LUMO level and more balanced charge transport. This research demonstrates side chain engineering is an effective way to achieve high efficiency organic solar cells.     

Extended Conjugation Length of Nonfullerene Acceptors with Improved Planarity via Noncovalent Interactions for High‐Performance Organic Solar Cells

Three low‐bandgap nonfullerene acceptors (NFAs) IDTO‐T‐4F, IDTO‐Se‐4F, and IDTO‐TT‐4F with extended conjugation length are designed and synthesized. Various π‐spacers, thiophene, selenophene, and thieno[3,2‐b]thiophene are incorporated to extend the conjugated length and enhance the backbone planarity via noncovalent O···S or O···Se interactions. These NFAs exhibit strong light absorption in the range of 600–900 nm with narrow bandgaps between 1.38 and 1.45 eV. By blending with a wide‐bandgap donor material PBDB‐T, organic solar cells (OSCs) based on these NFAs all yield high efficiency over 10% with low energy losses ranging from 0.52 to 0.59 eV. Importantly, as a result of relatively high lowest unoccupied molecular orbital level, large hole and electron mobility in blend film, and low charge carrier recombination loss, optimized devices based on IDTO‐T‐4F exhibit a large open‐circuit voltage of 0.864 V, a high short‐circuit current density of 20.12 mA cm^?2, and a notable fill factor of 72.7%, leading to an impressive efficiency of 12.62%, which represents the best performance for NFA OSCs using noncovalent interactions in acceptor molecule design. The results indicate that optimizing the conjugation length and backbone planarity via intramolecular noncovalent O···S or O···Se interactions is a useful strategy for NFA materials invention toward high‐performance solar cells.     

Alkyl Chain Regiochemistry of Benzotriazole‐Based Donor Polymers Influencing Morphology and Performances of Non‐Fullerene Organic Solar Cells

The effects of alkyl chain regiochemistry on the properties of donor polymers and performances of non‐fullerene organic solar cells are investigated. Two donor polymers (PfBTAZ and PfBTAZS) are compared that have nearly identical chemical structures except for the regiochemistry of alkyl chains. The optical properties and crystallinity of two polymers are nearly identical yet the PfBTAZ:O‐IDTBR blend exhibits nearly double domain size compared to the blend based on PfBTAZS:O‐IDTBR. To reveal the origins of the very different domain size of two blends, the morphology of neat polymer films is characterized, and it is found that PfBTAZ tends to aggregate into much larger polymer fibers without the presence of O‐IDTBR. This indicates that it is not the polymer:O‐IDTBR interactions but the intrinsic aggregation properties of two polymers that determine the morphology features of neat and blend films. The stronger aggregation tendency of PfBTAZ could be explained by its more co‐planar geometry of the polymer backbone arising from the different alkyl chain regiochemistry. Combined with the similar trend observed in another set of donor polymers (PTFB‐P and PTFB‐PS), the results provide an important understanding of the structure–property relationships that could guide the development of donor polymers for non‐fullerene organic solar cells.     

Inter‐Fullerene Electronic Coupling Controls the Efficiency of Photoinduced Charge Generation in Organic Bulk Heterojunctions

Photoinduced charge generation (PCG) dynamics are notoriously difficult to correlate with specific molecular properties in device relevant polymer:fullerene organic photovoltaic blend films due to the highly complex nature of the solid state blend morphology. Here, this study uses six judiciously selected trifluoromethylfullerenes blended with the prototypical polymer poly(3‐hexylthiophene) and measure the PCG dynamics in 50 fs–500 ns time scales with time‐resolved microwave conductivity and femtosecond transient absorption spectroscopy. The isomeric purity and thorough chemical characterization of the fullerenes used in this study allow for a detailed correlation between molecular properties, driving force, local intermolecular electronic coupling and, ultimately, the efficiency of PCG yield. The findings show that the molecular design of the fullerene not only determines inter‐fullerene electronic coupling, but also influences the decay dynamics of free holes in the donor phase even when the polymer microstructure remains unchanged.     

Structure Evolution of Oligomer Fused‐Ring Electron Acceptors toward High Efficiency of As‐Cast Polymer Solar Cells

The structure evolution of oligomer fused‐ring electron acceptors (FREAs) toward high efficiency of as‐cast polymer solar cells (PSCs) is reported. First, a series of FREAs (IC‐(1‐3)IDT‐IC) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, effects of IDT number (1–3) on their basic optical and electronic properties are investigated, and more importantly, the relationship between device performance of as‐cast PSCs and donor(D)/acceptor(A) matching (absorption, energy level, morphology, and charge transport) of IC‐(1‐3)IDT‐IC acceptors and two representative polymer donors, PTB7‐Th and PDBT‐T1 is surveyed. Then, the most promising D/A system (PDBT‐T1/IC‐1IDT‐IC) with the best D/A harmony among the six D/A combinations, which yields a power conversion efficiency (PCE) of 7.39%, is found. Finally, changing the side‐chains in IC‐1IDT‐IC from alkylphenyl to alkyl enhances the PCE from 7.39% to 9.20%.     

Overcoming Interfacial Losses in Solution‐Processed Organic Multi‐Junction Solar Cells

Organic solar cells are promising in terms of full‐solution‐processing which enables low‐cost and large‐scale fabrication. While single‐junction solar cells have seen a boost in power conversion efficiency (PCE), multi‐junction solar cells are promising to further enhance the PCE. In all‐solution‐processed multi‐junction solar cells, interfacial losses are often encountered between hole‐transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high‐performance donor and acceptor materials in multi‐junction solar cells. Here, the authors report on a systematic study of interface losses in both single‐junction and multi‐junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time‐of‐flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO _x nanoparticles successfully overcomes the interfacial losses in both single‐ and multi‐junction solar cells based on various active layers by reducing interface protonation, promoting better energy‐level alignment, and forming a dense and smooth layer. Solution‐processed single‐junction solar cells are demonstrated to reach the same performance as with evaporated MoO _x (over 7%). Multi‐junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO _x nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO _x nanoparticles show FF smaller than 50% and PCE less than 5%.     

Dopamine Semiquinone Radical Doped PEDOT:PSS: Enhanced Conductivity,Work Function and Performance in Organic Solar Cells

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been is applied as hole transport material in organic electronic devices for more than 20 years. However, the redundant sulfonic acid group of PEDOT:PSS has often been overlooked. Herein, PEDOT:PSS‐DA is prepared via a facile doping of PEDOT:PSS with dopamine hydrochloride (DA·HCl) which reacts with the redundant sulfonic acid of PSS. The PEDOT:PSS‐DA film exhibits enhanced work function and conductivity compared to those of PEDOT:PSS. PEDOT:PSS‐DA‐based devices show a power conversion efficiency of 16.55% which is the highest in organic solar cells (OSCs) with (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithio‐phene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PM6):(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) (Y6) as the active layer. Furthermore, PEDOT:PSS‐DA also exhibits enhanced performance in three other donor/acceptor systems, exhibiting high compatibility in OSCs. This work demonstrates that doping PEDOT:PSS with various amino derivatives is a potentially efficient strategy to enhance the performance of PEDOT:PSS in organic electronic devices.