Ligang Yuan

Organic solar cells based on a terpolymer donor and ITIC nonfullerene acceptor exhibit negligible energy level offsets that still lead to efficient hole and electron transfer. A small amount of PC₇₁BM is then added to optimize the blend film morphology, which yields significantly suppressed charge recombination and affords a maximum power conversion efficiency of 11.4% without any additional treatment.

Abstract

Nonfullerene acceptors (NFAs)‐based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors‐based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady‐state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC‐based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC₇₁BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC₇₁BM yield a better PCE of 11.4% without any additive or annealing treatment.

The effectiveness of using perovskite/Zr‐metal–organic framework (MOF) heterojunction in realizing efficient and stable inverted p–i–n perovskite solar cells (PVSCs) is demonstrated in article number 1801715 by Kevin C.‐W. Wu, Chu‐Chen Chueh, and co‐workers. Using perovskite/Zr‐MOF hybrid heterojunction to fabricate PVSCs is demonstrated to possess improved performance and ambient stability, benefiting from the grain‐locking effects introduced by the hybrid MOFs. It even reveals better merits than the perovskite/MOF bilayer heterojunction.

The efficiency limits in non‐fullerene organic solar cells are examined using a numerical simulator. Power conversion efficiency (PCE) of over 18% using recently reported carrier mobility values and voltage losses, are predicted. Increasing the mobility to >10⁻³ cm² V⁻¹ s⁻¹ and decreasing the recombination constant to <10⁻¹² cm³ s⁻¹ is shown to yield a single‐junction and 2T‐tandem cell with PCEs of >20% and >25%, respectively.

Abstract

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10⁻³ cm² V⁻¹ s⁻¹ in combination with low nongeminate recombination rate constants of 10⁻¹² cm³ s⁻¹ could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics.

Humidity‐stable perovskite films are prepared by exploiting the self‐assembled monolayer formation of water‐soluble triazolium ionic liquids (ILs) on a hydrophilic perovksite surface. This strategy means that the devices exhibit good humidity stability, maintaining around 80% initial efficiency for 3500 h under 40 ± 5% humidity. Tuning the molecule structure of the ILs achieves a maximum power conversion efficiency (PCE) of 20.03%.

Abstract

Despite being a promising candidate for next‐generation photovoltaics, perovskite solar cells (PSCs) exhibit limited stability that hinders their practical application. In order to improve the humidity stability of PSCs, herein, a series of ionic liquids (ILs) “1‐alkyl‐4‐amino‐1,2,4‐triazolium” (termed as RATZ; R represents alkyl chain, and ATZ represents 4‐amino‐1,2,4‐triazolium) as cations are designed and used as additives in methylammonium lead iodide (MAPbI₃) perovskite precursor solution, obtaining triazolium ILs‐modified PSCs for the first time (termed as MA/RATZ PSCs). As opposed to from traditional methods that seek to improve the stability of PSCs by functionalizing perovskite film with hydrophobic molecules, humidity‐stable perovskite films are prepared by exploiting the self‐assembled monolayer (SAM) formation of water‐soluble triazolium ILs on a hydrophilic perovskite surface. The mechanism is validated by experimental and theoretical calculation. This strategy means that the MA/RATZ devices exhibit good humidity stability, maintaining around 80% initial efficiency for 3500 h under 40 ± 5% relative humidity. Meanwhile, the MA/RATZ PSCs exhibit enhanced thermal stability and photostability. Tuning the molecule structure of the ILs additives achieves a maximum power conversion efficiency (PCE) of 20.03%. This work demonstrates the potential of using triazolium ILs as additives and SAM and molecular design to achieve high performance PSCs.

Through the rational molecular design of fluorination, the work function of the conjugated polymer (CP) is enhanced from 4.83 to 5.00 eV. Consequently, the CP can be used to modify efficient active layers consisting of polymer donors with a deep HOMO level, such as PBDB‐T‐2F:IT‐4F, and an outstanding power conversion efficiency of 12.7% is achieved in the corresponding device without V _oc loss.

Abstract

Since the highest occupied molecular orbital (HOMO) level of donors in organic solar cells (OSCs) is being constantly downshifted for achieving high open‐circuit voltage (V _oc), a further enhancement of the anode work function (WF) is required. Herein, an effective approach of fluorination is demonstrated to simultaneously improve the WF and transparency for anode interlayer (AIL) material. By fluorination, in combination with the dialysis treatment in LiCl solution, the WF of PCP‐2F‐Li could be significantly enhanced from 4.86 to 5.0 eV, as compared to PCP‐Na. Meanwhile, the transparency of the polymer is also improved. As a result, PCP‐2F‐Li can be used to modify efficient active layers consisting of polymer donors with deep HOMO levels, such as PBDB‐T‐2F:IT‐4F, and an outstanding power conversion efficiency (PCE) of 12.7% is achieved in the corresponding device with a high V _oc of 0.84 V. This result represents the highest efficiency for the OSCs using a solution‐processed pH‐neutral AIL, which is beneficial to the low‐cost fabrication of high‐performance OSCs with improved stability. More importantly, PCP‐2F‐Li could be processed by blade coating for making large‐area device of 1 cm², and a PCE of 10.6% is achieved, bringing a promising prospect for the large‐area device fabrication.

Solvent polarity decides! β‐[70]‐PCFM fullerenes ([6,6]‐Phenyl‐C61‐Formic acid Methyl ester) are formed as the major product in the reaction of C70 with sulfonium salts, affording a mixture of α‐ and β‐site isomers in good yields. Amazingly, the preference for the α‐ or β‐site isomer can be efficiently controlled by means of the solvent polarity! [70]PCFMs are very efficient in passivating the voids/pin‐holes and/or deep slits in bulk heterojunction perovskite solar cells, with efficiencies around 20 %, depending on the isomeric fullerene composition. More information can be found in the Communication by S. M. Zakeeruddin, M. Grätzel, N. Martín, et al. on https://doi.org/10.1002/chem.201806053page 3224.

The latest breakthroughs in interface and defect engineering as applied to metal halide perovskite solar cells and light‐emitting diodes (LEDs) are reviewed in order to shed light on their necessity and importance in tuning the optoelectronic properties of devices in an attempt to realize the best‐performing solar cells and LEDs.

Abstract

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Ring fusion and backbone fluorination yield a novel ladder‐type building block f‐FBTI2, a desirable “stronger acceptor” for enabling n‐type electron‐acceptor polymers. The resulting polymer semiconductor f‐FBTI2‐T shows an excellent power conversion efficiency of 8.1% with a very small energy loss of 0.53 eV in all‐polymer solar cell devices.

Abstract

A novel imide‐functionalized arene, di(fluorothienyl)thienothiophene diimide (f‐FBTI2), featuring a fused backbone functionalized with electron‐withdrawing F atoms, is designed, and the synthetic challenges associated with highly electron‐deficient fluorinated imide are overcome. The incorporation of f‐FBTI2 into polymer affords a high‐performance n‐type semiconductor f‐FBTI2‐T, which shows a reduced bandgap and lower‐lying lowest unoccupied molecular orbital (LUMO) energy level than the polymer analog without F or with F‐functionalization on the donor moiety. These optoelectronic properties reflect the distinctive advantages of fluorination of electron‐deficient acceptors, yielding “stronger acceptors,” which are desirable for n‐type polymers. When used as a polymer acceptor in all‐polymer solar cells, an excellent power conversion efficiency of 8.1% is achieved without any solvent additive or thermal treatment, which is the highest value reported for all‐polymer solar cells except well‐studied naphthalene diimide and perylene diimide‐based n‐type polymers. In addition, the solar cells show an energy loss of 0.53 eV, the smallest value reported to date for all‐polymer solar cells with efficiency > 8%. These results demonstrate that fluorination of imide‐functionalized arenes offers an effective approach for developing new electron‐deficient building blocks with improved optoelectronic properties, and the emergence of f‐FBTI2 will change the scenario in terms of developing n‐type polymers for high‐performance all‐polymer solar cells.

The unstable properties of hybrid perovskites, such as crystal phase transition, fragmentation of organic molecules, and hydrogen bond dissociation, are reviewed, and the importance of additives in structurally stabilizing formamidinium lead trihalide in place of methylammonium is emphasized.

Abstract

Hybrid lead halide perovskite materials are used in solar cells and show efficiencies greater than 23%. Furthermore, they are applied in light‐emitting diodes, X‐ray detectors, thin‐film transistors, thermoelectrics, and memory devices. Lead trihalide hybrid materials contain methylammonium (MA) or formamidinium (FA) (or a mixture), or long alkylammonium halides, as alternative organic cations. However, the intrinsic stability of hybrid lead halide perovskites is not very high, and they are chemically unstable when exposed to moisture, light, or heat because of their organic contents and low formation energies. Therefore, although improvements in the chemical stability are crucial, changing the material composition is challenging because it is directly related to the desired application requirements. Fortunately, hybrid lead halide perovskites have a very high tolerance toward changes in physical properties arising from doping or addition of different cations and anions, in many cases showing improved properties. Here, the intrinsic instability of hybrid lead halide perovskites is reviewed in relation to the crystal phase and chemical stability. It is suggested that FA should be used for lead halide perovskites for chemical stability instead of MA. Furthermore, additives that stabilize the crystal phase with α‐FAPbI₃ should eschew MA.

The rapid development of n‐type polymers has boosted the efficiency of all‐polymer solar cells, which has improved from 2% to 10 % in only seven years. There is a strong need to summarize the design criteria, synthesis, structure–property relationships and recent advances of n‐type polymers, which is addressed in this review. Moreover, the challenges and prospects for further development of all‐PSCs are briefly discussed.

Abstract

All‐polymer solar cells (all‐PSCs) based on n‐ and p‐type polymers have emerged as promising alternatives to fullerene‐based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n‐type polymers consisting of various electron acceptor units for all‐PSCs. So far, more than 200 n‐type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all‐PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state‐of‐the‐art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure–property relationships of n‐type polymers that have been used in all‐PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all‐PSCs. Finally, the challenges and prospects for further development of all‐PSCs are briefly considered.

Integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells have shown great potential to further improve their performance by combining the advantages of perovskite solar cells and NIR BHJ organic solar cells. Combining with the maintained high V _OC, higher efficiencies are expected by fully optimizing the perovskite layers and NIR BHJ layers through device engineering and materials innovations.

Abstract

The recently emerged integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells (IPOSCs) without any recombination layers have generated wide attention. This type of device structure can take the advantages of tandem cells using both perovskite solar and near‐infrared (NIR) BHJ organic solar materials for wide‐range sunlight absorption and the simple fabrication of single junction cells, as the low bandgap BHJ layer can provide additional light harvesting in the NIR region and the high open‐circuit voltage can be maintained at the same time. This progress report highlights the recent developments in such IPOSCs and the possible challenges ahead. In addition, the recent development of perovskite solar cells and NIR organic solar cells is also covered to fully underline the importance and potential of IPOSCs.

Photocurrent–voltage hysteresis in perovskite solar cells (PSCs) induced by ion migration combined with nonradiative recombination near the interface depends on perovskite composition and device structure. Among the methods attempted to reduce the hysteresis, potassium ion doping is found to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition.

Abstract

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.

A new hybrid lead halide material built of deformable post‐perovskite‐type chains exhibits an enhanced intrinsic white photoemission, setting a new record of 45% for photoluminescence quantum yield. The deformable lattice of this compound enables the self‐trapping of excitons responsible for broadband emission.

Abstract

Hybrid metal halides containing perovskite layers have recently shown great potential for applications in solar cells and light‐emitting diodes. Such compounds exhibit quantum confinement effects leading to tunable optical and electronic properties. Thus, broadband white‐light emission has been observed from diverse metal halides and, owing to high color rendering index, high thermal stability, and low‐temperature solution processability, these materials have attracted interest for application in solid‐state lighting. However, the reported quantum yields for white photoluminescence (PLQY) remain low (i.e., in the range 0.5–9%) and no approach has shown to successfully increase the intensity of this emission. Here, it is demonstrated that the quantum efficiencies of hybrid metal halides can be greatly enhanced if they contain a polymorph of the [PbX₄]²⁻ perovskite‐type layers: the [PbX₄]²⁻ post‐perovskite‐type chains showing a PLQY of 45%. Different piperazines lead to a hybrid lead halide with either perovskite layers or post‐perovskite chains influencing strongly the presence of self‐trapped states for excitons. It is anticipated that this family of hybrid lead halide materials could enhance all the properties requiring the stabilization of trapped excitons.

Organic solar cells achieve over 15% efficiency through the use of a copolymer donor, and simultaneously enhanced open‐circuit voltage and short‐circuit current density are obtained. High‐performance solar cells are adaptable for environment‐friendly solvents using a blade‐coating method, while showing better photostability than the corresponding ternary solar cells.

Abstract

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron‐withdrawing unit, ester‐substituted thiophene, is incorporated into a PBDB‐TF polymer to downshift the molecular energy and broaden the absorption. Copolymer‐based solar cells suitable for large‐area devices can be fabricated by a blade‐coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment‐friendly processing conditions and show relatively low photostability compared to copolymer‐based devices. These results not only demonstrate high‐efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.

The hybridization of 2D materials and organic materials represents a promising domain for the realization of improved or unprecedented features in comparison to those of semiconductor devices. This comprehensive review focuses on emerging 2D–organic heterostructures (from their synthesis and fabrication to their state‐of‐the‐art optoelectronic applications) and highlights the future challenges and opportunities associated with these heterostructures.

Abstract

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low‐cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D–organic heterostructures—from their synthesis and fabrication to their state‐of‐the‐art optoelectronic applications—is presented. Future challenges and opportunities associated with these heterostructures are highlighted.