Chen Weijie

For the first time, the successful discrimination and spatial mapping of all three organic components in the active layer of a ternary organic solar cell are achieved with analytical transmission electron microscopy. Knowledge of this nanomorphology is key to understanding photophysical processes and is thus indispensable to further improve the device performance.

Elucidating the complex materials distribution in the active layers of ternary organic solar cells is one of the greatest challenges in the field of organic photovoltaics. Knowledge of the nanomorphology is key to understanding photophysical processes (e.g., charge separation, adjustment of the recombination mechanism, and suppression of the radiationless and energetic losses) and thus improving the device performance. Herein, for the first time, the successful discrimination and spatial mapping of the active layer components of a ternary organic solar cell are demonstrated using analytical transmission electron microscopy. The material distribution of all three organic components is successfully visualized by multimodal imaging using complementary electron energy loss signals. A complete picture of the morphological aspects can be gained by studying the lateral and cross‐sectional morphology as well as the morphology evolution as a function of the mixing ratio of the polymers. Finally, a correlation between the morphology, photophysical processes, and device performance of the ternary and the reference binary system is achieved, explaining the differences of the power conversion efficiency between the two systems.

A fullerene derivative (indene‐C₆₀ bisadduct) is introduced into organic solar cells via blade coating, which can eliminate the efficiency loss caused by different printing methods. This ternary strategy overcomes the morphology issues, and based on this strategy, the blade‐coating device (1.05 cm²) achieves an efficiency of 13.70%.

Abstract

Regarded as a critical step in commercial applications, scalable printing technology has become a research frontier in the field of organic solar cells. However, inevitable efficiency loss always occurs in the lab‐to‐manufacturing translation due to the different fabrication processes. In fact, the decline of photovoltaic performance is mainly related to voltage loss, which is mainly affected by the diversity of phase separation morphology and the chemical structures of photoactive materials. Fullerene derivative indene‐C₆₀ bisadduct (ICBA) is introduced into a PBDB‐T‐2F:IT‐4F system to control the active layer morphology during blade‐coating process. Accordingly, as a symmetrical fullerene derivative, ICBA can regulate the crystallization tendency and molecular packing orientation and suppress charge carrier recombination. This ternary strategy overcomes the morphology issues caused by weaker shear impulse in blade‐coating process. Benefiting from the reduced nonradiative recombination loss, 1.05 cm² devices are fabricated by blade coating with a power conversion efficiency of 13.70%. This approach provides an effective support for recovering the voltage loss during scalable printing approaches.

By finely optimizing the alkyl chains, the nonfullerene acceptor named BTP‐eC9 is synthesized and a maximum power conversion efficiency of 17.8% in organic photovoltaic cells is recorded. This work demonstrates that the optimization of alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving photovoltaic performance.

Abstract

Optimizing the molecular structures of organic photovoltaic (OPV) materials is one of the most effective methods to boost power conversion efficiencies (PCEs). For an excellent molecular system with a certain conjugated skeleton, fine tuning the alky chains is of considerable significance to fully explore its photovoltaic potential. In this work, the optimization of alkyl chains is performed on a chlorinated nonfullerene acceptor (NFA) named BTP‐4Cl‐BO (a Y6 derivative) and very impressive photovoltaic parameters in OPV cells are obtained. To get more ordered intermolecular packing, the n ‐undecyl is shortened at the edge of BTP‐eC11 to n ‐nonyl and n ‐heptyl. As a result, the NFAs of BTP‐eC9 and BTP‐eC7 are synthesized. The BTP‐eC7 shows relatively poor solubility and thus limits its application in device fabrication. Fortunately, the BTP‐eC9 possesses good solubility and, at the same time, enhanced electron transport property than BTP‐eC11. Significantly, due to the simultaneously enhanced short‐circuit current density and fill factor, the BTP‐eC9‐based single‐junction OPV cells record a maximum PCE of 17.8% and get a certified value of 17.3%. These results demonstrate that minimizing the alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.

Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Tao Jia, Jiabin Zhang, Wenkai Zhong, Yuanying Liang, Kai Zhang, Sheng Dong, Lei Ying, Feng Liu, Xiaohui Wang, Fei Huang, Yong Cao

Current achievements in organic solar cells are reviewed and future challenges from fundamental science to industrial applications are discussed, putting organic photovoltaics in perspective. The article touches on power conversion efficiencies, device stability, material design, printability of organic solar cells and their environmental footprint as well as the unique features of organic photovoltaic applications.

Organic solar cells have come a long way from fundamental considerations of charge carrier dynamics in organic semiconductors to devices with laboratory power conversion efficiencies exceeding 17% and first power harvesting installations. Despite this story of success, these days, the scientific community witnesses a shift of research effort to other solar concepts, leaving behind a high‐potential solar technology with better applicability forecasts than ever before. Very compelling reasons still exist why organic solar cells can become the solar technology of the future that offers design versatility and enables unprecedented applications while offering the lowest energy payback times and ecologic sustainability. This perspective article highlights why organic solar cells remain a research field of the highest socioeconomic relevance, which challenges remain to be overcome in the future, and how organic solar cells can make a difference in the future energy landscape.

With the minor new combinatory side chain, the resulting polymer PQSi05 maintains low synthetic complexity and its related blend system achieves optimal average synthetic complexity and average figure‐of‐merit.

The cost‐effectiveness of polymer solar cells is a very important concern for future applications. In this work, a new combinatory side chain integrating siloxane terminal and alkoxy group is developed, and three polymers, PQSi05, PQSi10, and PQSi25, with 5%, 10%, and 25% contents of the siloxane‐terminated alkoxy side chain, respectively, are successfully synthesized. As the content of the combinatory side chain increases, the surface energy of the corresponding polymer film decreases, showing tunable miscibility in blend films with nonfullerene acceptor IT‐4F. A maximum power conversion efficiency (PCE) of 13.56% is achieved in the PQSi05:IT‐4F‐based device. The minor (5%) combinatory side chain approach retains a low synthetic complexity (SC) of 16.58% for PQSi05. Due to the improved device performance, a low figure‐of‐merit (FOM) of 1.22 is obtained for the PQSi05:IT‐4F blend. Furthermore, the contribution of the IT‐4F acceptor is considered for a comprehensive analysis, yielding an average SC (ASC) of 39.31% and an average FOM (AFOM) of 2.90. After statistical analyses and calculations, the PQSi05:IT‐4F is the best cost‐effective active layer to date. It is revealed that the introduction of the minor combinatory side chain is a promising strategy to develop high‐performing and cost‐effective polymer donors.

Rubidium‐incorporated air‐stable Cs_{(1 −  x )}Rb_xPbI₂Br perovskite solar cells are fabricated through a surface passivation strategy. The resulting devices under optimal conditions yield an efficiency of over 15% with excellent long‐term thermal as well as light‐soaking stability in ambient atmosphere.

Inorganic CsPbI₂Br perovskite has gained growing attention due to its potential for improving device performance and stability. However, the notorious phase transition from the photoactive to photoinctive phase in ambient atmosphere hinders its further development. Herein, air‐stable rubidium (Rb)‐incorporated Cs_{(1 −  x )}Rb_xPbI₂Br perovskite with guanidinium bromide (GABr) post‐treatment is demonstrated. The incorporation of smaller monovalent Rb cation contributes to a contraction of the perovskite crystal, leading to an improvement in structure stability. In addition, GABr modification induces a 2D/3D heterostructure perovskite with high crystallinity, appropriate surface morphology, favorable electronic properties, and significantly reduced trap‐state density. Consequently, the fabricated perovskite solar cells deliver a power conversion efficiency (PCE) of 15.6%, which is much higher than the 12.9% reported for reference CsPbI₂Br‐based devices. Meanwhile, the significantly enhanced long‐term (88% of initial PCE after 60 days), thermal (76% of initial PCE after 30 days) as well as light soaking (90% of initial PCE after 300 min) stability in ambient atmosphere is demonstrated.

The introduction of F5PEA⁺ to partially replace PEA⁺ as the 2D perovskite passivation agent, with a strong noncovalent interaction between the two bulky cations and enhanced charge transport, is reported to improve the performance (from 19.58% to 21.10%) and stability of the corresponding wide‐bandgap perovskite solar cells.

The replacement of a small amount of organic cations with bulkier organic spacer cations in the perovskite precursor solution to form a 2D perovskite passivation agent (2D‐PPA) in 3D perovskite thin films has recently become a promising strategy for developing perovskite solar cells (PSCs) with long‐term stability and high efficiency. However, the long, bulky organic cations often form a barrier, hindering charge transport. Herein, for the first time, 2D‐PPA engineering based on wide‐bandgap (≈1.68 eV) perovskites are reported. Pentafluorophenethylammonium (F5PEA⁺) is introduced to partially replace phenylethylammonium (PEA⁺) as the 2D‐PPA, forming a strong noncovalent interaction between the two bulky cations. The charge transport across and within the planes of pure 2D perovskites, based on mixed ammoniums, increases by a factor of five and three compared with that of mono‐cation 2D perovskites, respectively. The perovskite films based on mixed‐ammonium (F5PEA⁺‐PEA⁺) 2D‐PPA exhibit similar surface morphology and crystal structure, but longer carrier lifetime, lower exciton binding energy, less trap density and higher conductivity, in comparison with those using mono‐cation (PEA⁺) 2D‐PPA. The performance of PSCs based on mixed‐cation 2D‐PPA is enhanced from 19.58% to 21.10% along with improved stability, which is the highest performance for reported wide‐bandgap PSCs.

Lead‐free double perovskite Cs₂AgBiBr₆ is introduced to fabricate transparent photovoltaic (TPV) devices. An average visible transmittance up to 73% is produced from an indirect bandgap of 1.98 eV. Due to the optimized device parameters and narrower bandgap than other ultraviolet‐protective TPV absorbers, a record efficiency of 1.58% in the transparent solar cells with transmittance exceeding 70% is obtained.

Perovskite solar cells have attracted great research interest as a promising candidate for silicon solar cells. Plenty of work has been reported to use perovskites to semitransparent windows and transparent photovoltaic (TPV) devices to obtain multifunctional systems. However, the narrow bandgap and sharp absorption edge of the typical perovskites prevent them from achieving the highest transparency to satisfy the requirements of aesthetic and integration, and the poor stability and toxic Pb compositions hinder their practical application. Herein, lead‐free halide double perovskites with a wide bandgap and indirect bandgap characteristics is introduced to fabricate long‐term stable transparent photovoltaic devices exhibiting high visible transmittance (73%) and considerable energy conversion efficiency (1.56%). Through further theoretical calculation and evaluation, a new strategy using indirect bandgap material on TPV devices is proposed to combine the enhancement of these two parameters. This approach will be a significant compliment to near‐infrared‐absorbing solar cells to selectively harvest light in the invisible region to obtain highly performing multi‐junction smart windows on buildings, vehicles and mobile electronics, providing a new reasonable idea to realize TPVs with high efficiency and transparency simultaneously.

Herein, polyethylene glycol (PEG) is used as an additive for the morphology control of lead‐reduced perovskite films. The power conversion efficiency of lead‐reduced perovskite solar cells with PEG additive improves from 13.7% to 16.1% without J –V hysteresis due to pinhole elimination of the perovskite film.

The organic–inorganic halide perovskite solar cells (PSCs) are rapidly developed in just a few years due to its high power conversion efficiency. However, it still faces some critical issues, one of which is the presence of toxic lead (Pb²⁺). Recent researches show that barium (Ba²⁺) can partially replace the Pb²⁺ in perovskite structure and achieve a promising device performance because of its adequate ionic radius. However, the optimal replacement amount of Ba²⁺ in perovskite is still limited. Herein, the methylammonium (MA)/formamidinium (FA) mixed‐cation perovskite is used as the active layer in PSCs and Pb²⁺ is partially substituted with Ba²⁺. Compared with the pure MA system, the best device efficiency can be achieved using higher Ba²⁺ replacement ratio. In addition, while introducing the appropriate polymer additive, the replacement ratio can be further increased without compromise of device efficiency. Using polyethylene glycol as polymer additive, 10.0 mol% Ba‐doped MA/FA mixed‐cation PSC with an efficiency of 16.1% can be realized. It is believed that this report provides an effective strategy to fabricate high‐performance lead‐reduced PSCs.

Polystyrene is added into PBDB‐T:ITIC active layers of organic solar cells, leading to a power conversion efficiency enhancement of up to 16% relative to the control device. Other insulating polymers can also improve the performance of the organic solar cells for different levels dependent on the polymer‐side chain size. This work provides a guideline for the selection of polymer additives in organic solar cells.

A series of insulating polymers are used as additives in nonfullerene organic solar cells (OSCs) for the first time. A significant relative power conversion efficiency (PCE) enhancement of up to 16% is observed with an introduction of polystyrene for only 5.0 wt% into the active layer of OSCs. Other insulating polymers possessing linear nonconjugated backbones with different side chains are also incorporated into OSCs and the resultant PCE enhancement decreases with the decrease in the side chain size. Another important issue that is noted is the glass transition temperature of the polymer additive. When the glass transition temperature is higher than the thermal annealing temperature of the active layer, the polymer additive plays a negative effect on the device performance and the device efficiency decreases monotonically with the increase in addition amount. So the effect of the insulating polymer additives in nonfullerene OSCs can be attributed to the reconstruction of the active layer films, which increases the crystallinity, carrier mobility, and carrier lifetime of the organic semiconductors in the bulk heterojunction of the devices. This work provides a guideline for the selection of polymer additives in OSCs apart from the consideration on the optoelectronic property of the additives.