Chen Weijie

Publication date: February 2020

Source: Nano Energy, Volume 68

Author(s): Kai Wang, Selina Olthof, Waqas Siddique Subhani, Xiao Jiang, Yuexian Cao, Lianjie Duan, Hui Wang, Minyong Du, Shengzhong (Frank) Liu

Graphical abstract

Their superior optoelectronic properties and stability endow organic–inorganic halide perovskite single crystals great potential for high‐efficiency and stable photovoltaics. This progress report summarizes recent exciting developments and future perspectives for perovskite single crystal solar cells, which may attract more attention and provide guidelines for further development in this emerging field.

Abstract

The efficiency of perovskite solar cells has increased to a certified value of 25.2% in the past 10 years, benefiting from the superior properties of metal halide perovskite materials. Compared with the widely investigated polycrystalline thin films, single crystal perovskites without grain boundaries have better optoelectronic properties, showing great potential for photovoltaics with higher efficiency and stability. Additionally, single crystal perovskite solar cells are a fantastic model system for further investigating the working principles related to the surface and grain boundaries of perovskite materials. Unfortunately, only a handful of groups have participated in the development of single crystal perovskite solar cells; thus, the development of this area lags far behind that of its polycrystalline counterpart. Therefore, a review paper that discusses the recent developments and challenges of single crystal perovskite solar cells is urgently required to provide guidelines for this emerging field. In this progress report, the optical and electrical properties of single crystal and polycrystalline perovskite thin films are compared, followed by the recent developments in the growth of single crystal perovskite thin films and the photovoltaic applications of this material. Finally, the challenges and perspectives of single crystal perovskite solar cells are discussed in detail.

The current status of both low‐bandgap mixed Sn‐Pb perovskite solar cells and all‐perovskite tandem solar cells are summarized in this review. Possible strategies for further improving the performance and stability of the devices based on mixed Sn‐Pb perovskites are also discussed.

Abstract

Efficient organic–inorganic metal halide perovskite absorbers have gained tremendous research interest in the past decade due to their super optoelectronic properties and defect tolerance. Lead (Pb) halide perovskites enable highly efficient perovskite solar cells (PSCs) with a record power conversion efficiency (PCE) of over 23%. However, the energy bandgaps of Pb halide perovskites are larger than the optimal bandgap for single junction solar cells, governed by the Shockley–Queisser (SQ) radiative limit. Mixed tin (Sn)‐Pb halide perovskites have drawn significant attention, since their bandgap can be tuned to below 1.2 eV, which opens a door for fabricating all‐perovskite tandem solar cells that can break the SQ radiative limit. This review summarizes the development of low‐bandgap mixed Sn‐Pb PSCs and their applications in all‐perovskite tandem solar cells. Its aim is to facilitate the development of new approaches to achieve high efficiency low‐bandgap single‐junction mixed Sn‐Pb PSCs and all‐perovskite tandem solar cells.

In parallel with the tremendous progress in the efficiency of perovskite solar cells, research on the issue of instability has attracted enormous attention. In this review, the strategies to enhance the stability from the perspectives of the device structure, the photoactive layer, hole‐ and electron‐transporting layers, electrode materials, and device encapsulation are portrayed.

Abstract

In this review, the factors influencing the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is emphasized. The PCE of PSCs has remarkably increased from 3.8% to 23.7%, but on the other hand, poor stability is one of the main facets that creates a huge barrier in the commercialization of PSCs. Herein, a concise overview of the current efforts to enhance the stability of PSCs is provided; moreover, the degradation causes and mechanisms are summarized. The strategies to improve device stability are portrayed in terms of structural effects, a photoactive layer, hole‐ and electron‐transporting layers, electrode materials, and device encapsulation. Last but not least, the economic feasibility of PSCs is also vividly discussed.

Rapid crystallization is demonstrated to be necessary in achieving high‐quality 2DRP perovskite films by comparing dimethylacetamide (DMAC), N,N‐dimethylformamide, and dimethyl sulfoxide solvents. The improved stability and efficiency are observed using DMAC due to the accelerating crystallization rate of 2DRP perovskite crystals.

Abstract

Due to the additional introduction of bulky organic ammonium and the competition between bulky organic ammonium and methyl ammonium in 2D Ruddlesden‐Popper (2DRP) perovskite, the crystallization process becomes complicated. Here, it is demonstrated that the rapid crystallization controlled by processing solvents plays an important role in achieving high‐quality 2DRP perovskite films. It is found that the processing solvents, e.g., dimethylacetamide (DMAC), N,N‐dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), with a different polarity and boiling point, have almost no effect on crystal structure and phase distribution but have a remarkable effect on crystallization kinetics, crystal growth orientation, and crystallinity of 2DRP perovskite. Compared to polar aprotic solvent DMF and DMSO with a high boiling point, DMAC with low polarity and a suitable boiling point has a weak coordination to lead and ammonium salts and is easy to escape during solution processing, which is able to accelerate the crystallization rate of 2DRP perovskite. Benefitting from the rapid crystallization enabled high‐quality 2DRP perovskite films, the best‐performing device with improved stability and a power conversion efficiency of 12.15% is obtained using DMAC solvent. These findings may give guidance for solvent engineering for highly efficient 2DRP perovskite solar cells in the future.

The application of formamidinium (FA)‐based perovskite solar cells has largely been hindered by phase transition from the dark cubic phase to yellow orthorhombic phase. Here, a highly efficient and phase stable FA‐based perovskite solar cell is fabricated by using NbF₅ as a novel additive. NbF₅ can improve the quality of perovskite films and effectively suppress the formation of the yellow δ‐phase.

Abstract

The HC(NH₂)₂ ⁺(FA⁺) is a well‐known substitute to CH₃NH₃ ⁺(MA⁺) for its capability to extend light utilization for improved power conversion efficiency for perovskite solar cells; unfortunately, the dark cubic phase (α‐phase) can easily transition to the yellow orthorhombic phase (δ‐phase) at room temperature, an issue that prevents its commercial application. In this report, an inorganic material (NbF₅) is developed to stabilize the desired α‐phase perovskite material by incorporating NbF₅ additive into the perovskite films. It is found that the NbF₅ additive effectively suppresses the formation of the yellow δ‐phase in the perovskite synthesis and aging process, thus enhancing the humidity and light‐soaking stability of the perovskite film. As a result, the perovskite solar cells with the NbF₅ additive exhibit improved air stability by tenfold, retaining nearly 80% of their initial efficiency after aging in air for 50 d. In addition, under full‐sun AM 1.5 G illumination of a xenon lamp without any UV‐reduction, the perovskite solar cells with the NbF₅ additive also show fivefold improved illumination stability than the control devices without NbF₅.

Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted CH₃NH₃PbI₃ perovskite solar cells.

Abstract

Interface engineering is critical to the development of highly efficient perovskite solar cells. Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted MAPbI₃ perovskite solar cells (PSCs). This treatment has significantly increased MAPbI₃ photovoltaic performance to 18.8% for the urea treated PEDOT:PSS PSCs from 14.4% for pristine PEDOT:PSS devices. The use of urea controls phase separation between PEDOT and PSS segments, leading to the formation of a unique fiber‐shaped PEDOT:PSS film morphology with well‐organized charge transport pathways for improved conductivity from 0.2 S cm⁻¹ for pristine PEDOT:PSS to 12.75 S cm⁻¹ for 5 wt% urea treated PEDOT:PSS. The urea‐treatment also addresses a general challenge associated with the acidic nature of PEDOT:PSS, leading to a much improved ambient stability of PSCs. In addition, the device hysteresis is significantly minimized by optimizing the urea content in the treatment.

New‐type 2D/3D perovskites are designed by first introducing two hydrophobic ammonium salt cations with halogen functional groups into 3D perovskite. The 2D/3D perovskite devices exhibit an optimal power conversion efficiency as high as 20.08% under 1 sun irradiation and superior stability when exposed to humidity, temperature, and continuous UV irradiation.

Abstract

2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA⁺ and 2‐bromoethylamine, BEA⁺) are introduced into (Cs_0.1FA_0.9)Pb(I_0.9Br_0.1)₃ 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA⁺ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups.

Scalable room‐temperature sputtering deposition of the SnO₂ electron transport layer (ETL) with reduced gap states is demonstrated. Perovskite solar cells using a SnO₂ ETL show an efficiency up to 20.2% and a T₈₀ lifetime of 625 h. Mini‐modules with a 22.8 cm₂ aperture area show efficiencies over 12% and a T₈₀ lifetime of 515 h, which indicates the upscalability of our method.

Abstract

Stability and scalability have become the two main challenges for perovskite solar cells (PSCs) with the research focus in the field advancing toward commercialization. One of the prerequisites to solve these challenges is to develop a cost‐effective, uniform, and high quality electron transport layer that is compatible with stable PSCs. Sputtering deposition is widely employed for large area deposition of high quality thin films in the industry. Here the composition, structure, and electronic properties of room temperature sputtered SnO₂ are systematically studied. Ar and O₂ are used as the sputtering and reactive gas, respectively, and it is found that a highly oxidizing environment is essential for the formation of high quality SnO₂ films. With the optimized structure, SnO₂ films with high quality have been prepared. It is demonstrated that PSCs based on the sputtered SnO₂ electron transport layer show an efficiency up to 20.2% (stabilized power output of 19.8%) and a T₈₀ operational lifetime of 625 h. Furthermore, the uniform and thin sputtered SnO₂ film with high conductivity is promising for large area solar modules, which show efficiencies over 12% with an aperture area of 22.8 cm² fabricated on 5 × 5 cm² substrates (geometry fill factor = 91%), and a T₈₀ operational lifetime of 515 h.

Tetrafluoroborate (BF₄ ⁻) anion can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite solar cells. These advantages result in an improved power conversion efficiency of 20.16% from 17.55% due to enhanced open‐circuit voltage and fill factor.

Abstract

Composition engineering is a particularly simple and effective approach especially using mixed cations and halide anions to optimize the morphology, crystallinity, and light absorption of perovskite films. However, there are very few reports on the use of anion substitutions to develop uniform and highly crystalline perovskite films with large grain size and reduced defects. Here, the first report of employing tetrafluoroborate (BF₄ ⁻) anion substitutions to improve the properties of (FA = formamidinium, MA = methylammonium (FAPbI₃)_0.83(MAPbBr₃)_0.17) perovskite films is demonstrated. The BF₄ ⁻ can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite films and derived solar cells. These advantages benefit the performance of perovskite solar cells (PVSCs), resulting in an improved power conversion efficiency (PCE) of 20.16% from 17.55% due to enhanced open‐circuit voltage (V _OC) and fill factor. This is the highest PCE for BF₄ ⁻ anion substituted lead halide PVSCs reported to date. This work provides insight for further exploration of anion substitutions in perovskites to enhance the performance of PVSCs and other optoelectronic devices.

The latest progress and issues toward scalable fabrication of perovskite solar cells are reviewed in an attempt to provide insights into the development of rational fabrication methods for large‐area perovskite films and solar modules.

Abstract

With the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm². However, PCE loss rate is estimated to be 1.4 × 10⁻²% cm⁻², which is 82 and 3.5 times higher than crystalline Si (1.7 × 10⁻⁴% cm⁻²) and thin film technologies (≈4 × 10⁻³% cm⁻²) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.

Here a method to grow wafer‐size thin halide perovskite multiple crystals on aqueous solution surface is reported. The efficiency of lateral‐structure solar cells based on the single‐crystalline perovskite wafer reaches 5.9%.

Abstract

Solar‐grade single or multiple crystalline wafers are needed in large quantities in the solar cell industry, and are generally formed by a top‐down process from crystal ingots, which causes a significant waste of materials and energy during slicing, polishing, and other processing. Here, a bottom‐up technique that allows the growth of wafer‐size hybrid perovskite multiple crystals directly from aqueous solution is reported. Single‐crystalline hybrid perovskite wafers with centimeter size are grown at the top surface of a perovskite precursor solution. As well as saving raw materials, this method provides unprecedented advantages such as easily tunable thickness and rapid growth of the crystals. These crystalline wafers show high crystallinity, broader light absorption, and a long carrier recombination lifetime, comparable with those of bulk single crystals. Lateral‐structure perovskite solar cells made of these crystals demonstrate a record power conversion efficiency of 5.9%.

The morphology control of nonfullerene blends is very important to improve device performance. Here, the high boiling point cosolvent is added to the P3HT:O‐IDTBR blend, which separates the crystallization process of P3HT and O‐IDTBR, and prolongs film forming time. As a result, the crystallinity, lateral and phase separation structure are optimized simultaneously, leading to high performance of 7.18%.

Abstract

The morphology with the interpenetrating network and optimized vertical phase separation plays a key role in determining the charge transport and collection in polymer:nonfullerene small molecular acceptors (SMAs) solar cells. However, the crystallization of polymer and SMAs usually occurs simultaneously during film‐forming, thus interfering with the crystallization process of each other, leading to amorphous film with undesirable lateral and vertical phase separation. The poly(3‐hexylthiophene) (P3HT):O‐IDTBR blend is selected as a model system, and controlling film‐forming kinetics to solve these problems is proposed. Herein, a cosolvent 1,2,4‐triclorobenzene (TCB) with selective solubility and a high boiling point is added to the solution, leading to prior crystallization of P3HT and extended film‐forming duration. As a result, the crystallinity of both components is enhanced significantly. Meanwhile, the prior crystallization of P3HT induces solid–liquid phase separation, hence rationalizing the formation of the nano‐interpenetrating network. Moreover, the surface energy drives O‐IDTBR to enrich near the cathode and P3HT to migrate to the anode. Consequently, a highly crystalline nano‐interpenetrating network with proper vertical phase separation is obtained. The optimal morphology improves charge transport and suppresses bimolecular recombination, boosting the power conversion efficiency from 4.45% to 7.18%, which is the highest performance in P3HT‐based binary nonfullerene solar cells.

A NH₄Cl additive is introduced in the preparation of AVA₂FA _{n −1}Sn _n I_{3 n +1} (<n> = 5) perovskite, leading to highly vertically oriented tin‐based reduced‐dimensional perovskite films with enhanced efficiency and stability. Herein, under the effects of NH₄Cl additive, the optimized power conversion efficiency of tin‐based quasi‐2D perovskite solar cells increases from 4.19% to 8.71% with enhanced stability.

Abstract

Tin‐based perovskites have exhibited high potential for efficient photovoltaics application due to their outstanding optoelectrical properties. However, the extremely undesired instabilities significantly hinders their development and further commercialization process. A novel tin‐based reduced‐dimensional (quasi‐2D) perovskites is reported here by using 5‐ammoniumvaleric acid (5‐AVA⁺) as the organic spacer. It is demonstrated that by introducing appropriate amount of ammonium chloride (NH₄Cl) as additive, highly vertically oriented tin‐based quasi‐2D perovskite films are obtained, which is proved through the grazing incidence wide‐angle X‐ray scattering characterization. In particular, this approach is confirmed to be a universal method to deliver highly vertically oriented tin‐based quasi‐2D perovskites with various spacers. The highly ordered vertically oriented perovskite films significantly improve the charge collection efficiency between two electrodes. With the optimized NH₄Cl concentration, the solar cells employing quasi‐2D perovskite, AVA₂FA _{n −1}Sn _n I_{3 n +1} (<n> = 5), as light absorbers deliver a power conversion efficiency up to 8.71%. The work paves the way for further employing highly vertically oriented tin‐based quasi‐2D perovskite films for highly efficient and stable photovoltaics.

Causes of intrinsic and extrinsic instability of perovskite materials and related mechanisms are discussed in terms of their chemical‐bonding nature. Understanding the critical mechanisms rationalizes the chemical approaches to mitigate the degradation in perovskite solar cells.

Abstract

Chemical bonding dictates not only the optoelectronic properties of materials, but also the intrinsic and extrinsic stability of materials. Here, the causes of intrinsic and extrinsic instability of perovskite materials are reviewed considering their correlation with the unique chemical‐bonding nature of perovskite materials. There are a number of key standardized stability tests established by the International Electrotechnical Commission for commercialized photovoltaic modules. Based on these procedures, the possible causes and related mechanisms of the material degradation that can arise during the test procedures are identified, which are discussed in terms of their chemical bonds. Based on the understanding of the critical causes, promising strategies for mitigating the causes to enhance the stability of perovskite solar cells are summarized. The stability of the state‐of‐the‐art perovskite solar cells implies a need for the development of improved stability‐testing protocols to move onto the next stage toward commercialization.

Despite the record efficiency exceeding 25%, the long‐term operational stability of perovskite solar cells is limited by the degradation mechanisms accelerated by the presence of vacancies and defects. In this review, recent engineering strategies ranging from grains to interfaces that mitigate degradation and improve efficiencies are discussed.

Abstract

Lead halide perovskite solar cells now show excellent efficiencies and encouraging levels of stability. Further improvements in performance require better control of the trap states which are considered to be associated with vacancies and defects at crystallite surfaces. Herein, a reflection on the ways in which these traps can be mitigated is presented by improving the quality of the perovskite layer and interfaces in fully assembled device configurations. In this review, the most recent design strategies reported in the literature, which have been explored to tune grain orientation, to passivate defects, and to improve charge‐carrier lifetimes, are presented. Specifically, the advances made with single‐cation, mixed‐cation and/or mixed‐halide, and 3D/2D bilayer‐based light absorbers are discussed. The interfacial, compositional, and band alignment engineering along with their consequent effects on the open‐circuit voltage, power conversion efficiency, and stability are a particular focus.

A rapid layer‐specific annealing on perovskite active layers enabled by UV LED is developed, and efficiency close to 19% in a simple planar inverted structure of ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering is demonstrated. The results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it.

Abstract

A rapid layer‐specific annealing on perovskite active layer enabled by ultraviolet (UV) light‐emitting diode (LED) is demonstrated and efficiency close to 19% is achieved in a simple planar inverted structure ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering. These results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it. Different in principle from other photonic treatment techniques that can heat up and damage underlying films, the UV‐LED‐annealing method enables layer‐specific annealing because LED light source is able to provide a specific UV wavelength for maximum light absorption of target film. Moreover, the layer‐specific photonic treatment allows accurate estimation of the crystallization energy required to form perovskite film at device quality level.

Ternary organic solar cells based on nonfullerene 3TP3T‐4F and 3TP3T‐IC guest acceptors and PM:Y6 binary host are investigated. The incorporation of 15% 3TP3T‐4F leads to an impressive efficiency of 16.7% (certified as 16.2%) for PM6:Y6:3TP3T‐4F ternary organic solar cells, higher than that (15.6%) of PM6:Y6:3TP3T‐4F devices, which is mainly ascribed to the compatibility between the third component and the host materials.

Abstract

A ternary structure has been demonstrated as being an effective strategy to realize high power conversion efficiency (PCE) in organic solar cells (OSCs); however, general materials selection rules still remain incompletely understood. In this work, two nonfullerene small‐molecule acceptors 3TP3T‐4F and 3TP3T‐IC are synthesized and incorporated as a third component in PM6:Y6 binary blends. The photovoltaic behaviors in the resultant ternary OSCs differ significantly, despite the comparable energy levels. It is found that incorporation of 15% 3TP3T‐4F into the PM6:Y6 blend results in facilitating exciton dissociation, increasing charge transport, and reducing trap‐assisted recombination. All these features are responsible for the enlarged PCE of 16.7% (certified as 16.2%) in the PM6:Y6:3TP3T‐4F ternary OSCs, higher than that (15.6%) in the 3TP3T‐IC containing ternary devices. The performance differences are mainly ascribed to the compatibility between the third component and the host materials. The 3TP3T‐4F guest acceptor exhibits an excellent compatibility with Y6, tending to form well‐mixed phases in the ternary blend without disrupting the favored bicontinuous transport networks, whereas 3TP3T‐IC displays a morphological incompatibility with Y6. This work highlights the importance of considering the compatibility for materials selection toward high‐efficiency ternary organic OSCs.

A novel benzo[1,2‐b:4,5‐b′]difuran (BDF)‐based copolymer, L2, is designed and synthesized. When blended with a non‐fullerene small molecule acceptor TTPT‐T‐4F, the L2‐based device exhibits an efficiency of 14.0%, which is higher than that (12.72%) of its analogue benzo[1,2‐b:4,5‐b′]dithiophene (BDT) copolymer‐based device. Thus, the performance of the BDF‐based copolymers are equal to or greater than that of the BDT‐based counterparts.

Abstract

The development of high‐performance donor polymers is important for obtaining high power conversion efficiencies (PCEs) of non‐fullerene polymer solar cells (PSCs). Currently, most high‐efficiency PSCs are fabricated with benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based conjugated polymers. The photovoltaic performance of benzo[1,2‐b:4,5‐b′]difuran (BDF)‐based copolymers has lagged far behind that of BDT‐based counterparts. In this study, a novel BDF‐based copolymer L2 is designed and synthesized, in which BDF and benzotriazole (BTz) building blocks have been used as the electron‐sufficient and deficient units, respectively. When blending with a non‐fullerene small molecule acceptor (SMA), TTPT‐T‐4F, the L2‐based device exhibits a remarkably high PCE of 14.0%, which is higher than that of the device fabricated by its analogue BDT copolymer (12.72%). Moreover, PSCs based on the L2:TTPT‐T‐4F blend demonstrate excellent ambient stability with 92% of its original PCE remaining after storage in air for 1800 h. Thus, BDF is a promising electron‐donating unit, and the BDF‐based copolymers can be competitive or even surpass the performance of BDT‐based counterparts.

Halide perovskites are promising materials for development in hot carrier (HC) solar cells, where the excess energy of above-bandgap photons is harvested before being wasted as heat to enhance device efficiency. Presently, HC separation and transfer processes at higher-energy states remain poorly understood. Here, we investigate the excited state dynamics in CH₃NH₃PbI₃ using pump-push-probe spectroscopy. It has its intrinsic advantages for studying these dynamics over conventional transient spectroscopy, albeit complementary to one another. By exploiting the broad excited-state absorption characteristics, our findings reveal the transfer of HCs from these higher-energy states into bathophenanthroline (bphen), an energy selective organic acceptor far above perovskite’s band edges. Complete HC extraction is realized only after overcoming the interfacial barrier formed at the heterojunction, estimated to be between 1.01 and 1.08 eV above bphen’s lowest unoccupied molecular orbital level. The insights gained here are essential for the development of a new class of optoelectronics.

A power conversion efficiency of 8.25% for poly(3‐hexylthiophene)‐based polymer solar cells is realized by pairing a novel star‐shaped small‐molecular acceptor 2,7,12‐tris((2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile‐7‐benzothiadiazole‐2‐)truxene with a smart solution‐processing technology in the green solvent 1,2,4‐trimethylbenzene.

Abstract

A novel molecular acceptor of TrBTIC (2,7,12‐tris((2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile‐7‐benzothiadiazole‐2‐)truxene) is designed by attaching the 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile‐benzothiadiazole (BTIC) electron‐deficient unit to an electron‐rich truxene core. TrBTIC has excellent solubility in common solvents and features good energy level matching with poly(3‐hexylthiophene) (P3HT). Interestingly, P3HT can be readily dissolved in warm 1,2,4‐trimethylbenzene (TMB), a green solvent, but crystallizes slowly with long‐term aging in TMB at room temperature. A prephase separation can thus occur before active blend film deposition, and the separation degree can be easily controlled by varying the aging time. After 40 min of aging, the resulting active blend has the most appropriate phase separation with uniform nanowires, which forms favorable interpenetrating networks for exciton dissociation and charge transport. As a result, the device performance is improved from 6.62% to 8.25%. Excitingly, 8.25% is a new record for P3HT‐based solar cells. The study not only provides an efficient nonfullerene acceptor for matching P3HT donors but also develops a promising processing technology to realize high‐performance P3HT‐based polymer solar cells with an efficiency over 8%.