Chen Weijie

Publication date: September 2020

Source: Applied Materials Today, Volume 20

Author(s): Adam D. Printz, Oliver Zhao, Stephen Hamann, Nicholas Rolston, Olav Solgaard, Reinhold H. Dauskardt

Low‐temperature black‐phase CsPbI₃ evolution processes are designed using chemical bond engineering for the fabrication of efficient and ambient‐air‐stable solar cells. After optimization, the low temperature (160 °C)‐annealed 3% polyvinylpyrrolidone device shows the highest efficiency of 10.0% and sustains ≈80% of its initial power conversion efficiency after 5 months of storage in ambient‐air conditions.

Cesium‐based fully inorganic black‐phase (BP) lead halide perovskites (such as α‐, β‐, and γ‐CsPbI₃) with excellent thermal stability and a decently high photovoltaic performance have attracted increasing attention. However, a below 200 °C fabrication process of the desirable BP CsPbI₃ has rarely been reported. Herein, the detailed crystal structure evolution of ambient‐air‐stable BP CsPbI₃ prepared under low temperature conditions is investigated by exploiting the strong coordination bonding between CO in polyvinylpyrrolidone (PVP) and Pb in CsPbI₃ and inflection effect of PVP under annealing. It is found that ambient‐air‐stable BP CsPbI₃ films are formed and the energy barrier for the long‐term stable BP CsPbI₃ formation is significantly reduced (the required annealing temperature is only 80 °C). After optimization, the highest power conversion efficiencies (PCEs) of ≈4.0% and 10.0% are recorded for the 3% PVP‐added devices with light absorbers annealed at 80 and 160 °C, respectively. More importantly, the 3% PVP device annealed at 160 °C maintains ≈80% of its original PCE after 5 months storage under ambient‐air conditions.

A sulfonyl fluoride‐functionalized phenethylammonium salt (SF‐PEA) is demonstrated as an efficient crystallization modulator for fabricating formamidinium lead iodide (FAPbI₃) perovskite films with high optoelectronic quality and phase stability. A champion power conversion efficiency (PCE) of 21.25% (certified PCE of 20.70%) is achieved, which is the highest among “methylammonium‐free” FAPbI₃ perovskite solar cells.

Bulky organic ammonium cations have been widely used to stabilize lead halide perovskites via surface passivation or dimensionality modulation. Herein, a sulfonyl fluoride‐functionalized phenethylammonium salt (SF‐PEA) is reported as a bifunctional additive to stabilize the formamidinium lead iodide (FAPbI₃) perovskite. The sulfonyl group is found to interact with PbI₂ in the precursor and slow down the crystallization of FAPbI₃ during thermal annealing, leading to improved crystalline quality and decreased structural defects. After annealing, the spontaneous assembly of SF‐PEA on the crystal surface of FAPbI₃ not only passivates the surface defects, but also protects the perovskite from phase transition that is caused by strain or moisture invasion. The resulting FAPbI₃ films are extremely stable, which can maintain their black phase for more than 3 months in air with 40–50% relative humidity, much better than pristine and unsubstituted phenethylammonium (PEA)‐based samples. Because of the greatly improved phase stability and crystallization quality, a champion power conversion efficiency (PCE) of 21.25% (certified PCE of 20.70%) is achieved in planar n–i–p structured solar cells, which is the highest one among “methylammonium‐free” FAPbI₃ perovskite photovoltaics.

A conjugated polyelectrolyte is used for simultaneously tailoring the perovskite adjacent interfaces. Herein, for the first time, poly[(9,9‐bis(3′‐((N,N ‐dimethyl)‐N ‐ethyl‐ammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]di‐iodide (PFN‐I)is exploited in inverted planar perovskite solar cells. At the hole transport layer/perovskite interface, the PFN‐I is beneficial for solving the dewetting issue. At the perovskite/electron transport layer interface, the PFN‐I is advantageous for passivating defects.

Interface engineering is an effective means to enhance the performance of thin‐film devices, such as perovskite solar cells (PSCs). Herein, a conjugated polyelectrolyte, poly[(9,9‐bis(3′‐((N,N ‐dimethyl)‐N ‐ethyl‐ammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]di‐iodide (PFN‐I), is used at the interfaces between the hole transport layer (HTL)/perovskite and perovskite/electron transport layer simultaneously, to enhance the device power conversion efficiency (PCE) and stability. The fabricated PSCs with an inverted planar heterojunction structure show improved open‐circuit voltage (V _oc), short‐circuit current density (J _sc), and fill factor, resulting in PCEs up to 20.56%. The devices maintain over 80% of their initial PCEs after 800 h of exposure to a relative humidity 35–55% at room temperature. All of these improvements are attributed to the functional PFN‐I layers as they provide favorable interface contact and defect reduction.

A new indacenothiophene‐based electron transporting material ITCP‐M with near‐infrared (NIR) absorption is developed and applied in inverted perovskite solar cells (PSCs). Interestingly, ITCP‐M can extend absorption to the NIR region in addition to electron extraction and electron transporting, which contributes to the enhanced photovoltaic performance of MAPbI₃‐based inverted PSCs.

Lead‐based organic–inorganic hybrid perovskite solar cells (PSCs) usually show an absorption edge around 800 nm, while the near‐infrared (NIR) wavelength beyond 800 nm cannot be utilized. Herein, a new indacenothiophene‐based electron transporting material (ETM), namely, ITCP‐M, is developed, which works to enhance electron extraction and electron transporting, and simultaneously extends photoresponses to the NIR region in MAPbI₃‐based inverted PSCs. Notably, the ITCP‐M‐based device exhibits a prominent photoresponse beyond 800 nm as observed from the external quantum efficiency (EQE) spectra, contributing to enhanced short‐circuit current density (J _sc) without sacrificing the open‐circuit voltage and fill factor. As a result, inverted PSCs using ITCP‐M ETM delivers a high efficiency of 19.15%, representing one of the highest efficiencies in inverted PSCs using nonfullerene ETMs. This work provides a new and simple strategy to extend photoresponses to the NIR absorption region for MAPbI₃‐based inverted PSCs that can significantly improve device performance.

2,3,6,7,10,11‐Hexaacetoxytriphenylene (HATP) as a discotic liquid crystal with high mobility can aggregate into a column structure on poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) in organic solar cells. HATP columns facilitate the formation of a 3D charge transportation, which increases the intermolecular charge transport and mobility. In addition, triplet excitons, trap states, and bimolecular recombination are suppressed. Thus, the short‐circuit current density is increased significantly.

In this work, a discotic liquid crystal (DLC) 2,3,6,7,10,11‐hexaacetoxytriphenylene (HATP) is used as the interlayer between poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and the active layer to achieve 3D charge transportation for organic solar cells (OSCs). HATP exhibits a columnar structure with a dominantly edge‐on orientation. For a (non‐)fullerene OSC system based on face‐on orientation, HATP columns are favorable for the expansion of edge‐on and face‐on crystallite in the active layer. According to the surface energy, surrounding the HATP columns are mainly acceptors. The significantly improved electron mobility indicates that it is easier for the electron to hop into HATP columns to transport, which represents the formation of the 3D pathway. This transport mode typically can enhance the intermolecular charge transport and effectively suppress the generation of triplet excitons and recombination. Thus, the short‐circuit current density (J _SC) is increased by 14% and 19% for fullerene and non‐fullerene systems, respectively. The power conversion efficiency is improved for non‐fullerene OSCs with different active layer thicknesses (≥150 nm) and fullerene OSCs with an active layer thickness of 140 nm. Overall, this work demonstrates an approach to introduce HATP columns on a PEDOT:PSS layer that has great potential to form a 3D pathway for achieving high‐performance (non‐)fullerene OSCs.

Polycrystalline FAPbI₃ and monocrystalline MAPbBr₃ are synthesized from low‐grade purity commercial products (FAI, PbI₂, MABr, and PbBr₂). The crystal powder‐derived precursor (CP) and commercial products‐derived typical precursor (TP) are used to fabricate planar inverted (FAPbI₃)_0.85(MAPbBr₃)_0.15 perovskite solar cells. CP devices yield a champion power conversion efficiency of 20.5%, which is higher than TP of 16.7%.

Solution‐processed perovskite precursors, especially for MAPbBr₃‐assisted FAPbI₃ crystallization, has been noted to achieve high power conversion efficiency (PCE) for perovskite solar cells (PSCs). However, this low‐temperature processed (FAPbI₃) _x (MAPbBr₃)_1−x typical precursor derived from commercial products (FAI, PbI₂, MABr, and PbBr₂) suffers from environmental sensitivity, poor film crystallinity and less than ideal device reproducibility. Herein, (FAPbI₃) _x (MAPbBr₃)_1–x (0.80 ≤ x  ≤ 0.90)‐based planar inverted PSCs are fabricated, employing grinded monocrystalline MAPbBr₃ and powdered polycrystalline FAPbI₃ as precursors. The champion device with optimal molar ratio x  = 0.85 comprising highly crystalline larger‐grained perovskite film with enhanced carrier transport kinetics and reduced trap‐state density exhibits boosted efficiency reaching 20.50%, which shows a 22.90% improvement over typical precursors with a PCE of 16.68%. In addition, the crystal powder precursor yields obvious film stability under ambient conditions (23 °C, 65–85% humidity) for 150 days and improved device storage stability in the glove box within two months. This protocol using stock crystal powders for perovskite precursor formulation provides a relatively facile and reproducible device fabrication route for the commercialization of PSCs.

Based on the previously reported acceptors ITIC2 and ITIC‐SF, using side chains and an end group strategy, a two‐dimension (2D) conjugated acceptor BDTSF‐IC is designed with fluorinated engineering and thiophene end group. The PM6:BDTSF‐IC‐based polymer solar cells achieve a photovoltaic performance of 13.1%, higher than those achieved by devices based on PM6:BDTCH‐IC (10.51%).

The previously reported nonfullerene small molecule ITIC‐SF achieved via side chain tuning, promotes the power conversion efficiency of polymer solar cells (PSCs) with PBDB‐T‐SF as the donor from 10.1% and ITIC2 acceptors up to 12.2% for ITIC‐SF acceptors. To further this research, benzene end groups of molecules are herein substituted with thiophene rings, obtaining two new molecules BDTCH‐IC with alkylthio substituents, and BDTSF‐IC with alkylthio and fluorine substituents on their thiophene‐conjugated side chains. The absorption edges of BDTCH‐IC and BDTSF‐IC are red‐shifted to 824 and 793 nm, respectively. Strengthened molecular crystallinity, promoted charge extraction, and upgraded morphology endorse the advancement of photovoltaic performance of the small molecular acceptors. Using donor PM6, the two small molecule acceptors show good photovoltaic performance, although the highest occupied molecular orbit energy offsets are small between donor and acceptor materials. As a combination of side‐chain and end‐group engineering, the photovoltaic performance of the PSCs is increased to 13.1%, together with the best short‐circuit current (J _SC) and fill factor reported thus far for this series of molecules. The results indicate that the modification of side chain and end groups is an effective way to improve the photovoltaic performance of small molecule acceptors.

A high power conversion efficiency and fill factor are obtained by doping in the structure of ITO/PEDOT:PSS/perovskite/C₆₀/Ag without a 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline buffer layer. The introduced cesium and water significantly promote the perovskite crystallinity and suppress the trap state. Also, the negligible hysteresis and enhanced stability are achieved. The enhanced performance can be attributed to the improved charge transport and energy‐level alignment.

Air‐processed perovskite solar cells (PSCs) allay the need for costly fabrication in a controlled atmosphere but currently suffer from disadvantages in power conversion efficiency (PCE) and device stability. Herein, the systematic investigation into CH₃NH₃PbI_3–x Cl _x prepared with an antisolvent‐assisted process in a relative humidity of 40 ± 5% is reported, using cesium‐containing aqueous solutions of various strengths. Diffractometry and microscopy reveal how grain sizes vary among the samples, suggesting an optimum concentration for grain size. Those films are fabricated in air into solar cells, and their electrochemical impedance and current–voltage characteristics under light are measured. The films demonstrating optimum strength achieve PCEs of up to 16.6%, compared with the maximum of 14.4% achieved by untreated films. The air‐processed films also exhibit mitigated hysteresis, with indices decreasing to 0.021. Photoluminescence characterization reveals reduced defect densities in Cs‐doped materials and, together with photoelectron spectroscopy, suggests an upward shift in energy bands. Such changes explain the improvement in photovoltaic performance. Stability tests on unencapsulated cells over 14 days show that those made from the perovskite with optimum Cs‐doping degrade slowest, with their conversion efficiencies falling by <10% every 100 h. Our findings may contribute to the low‐cost commercialization of PSCs.

A new strategy using an abundant and colorful organic dye as the additive to passivate defect states and to produce more n‐type perovskite film is proposed, which remarkably enhances both efficiency and humidity/thermal stability of the perovskite solar cells.

Perovskite solar cells are a highly competitive candidate for next‐generation photovoltaic technology. Defects in the perovskite grain boundaries and on the film surfaces however have significant impacts on both the device efficiency and environmental stability. Herein, a strategy using organic dyes as additives to passivate the defect states and produce more n‐type perovskite films, thereby improving charge transport and decreasing charge recombination, is reported. Based on this strategy, the power conversion efficiency of the perovskite solar cell is significantly increased from 18.13% to 20.18% with a negligible hysteresis. Furthermore, the rich hydrogen bonds and carbonyl structures in the organic dye can significantly enhance device stability both in terms of humidity and thermal stress. The results present a promising pathway using abundant and colorful organic dyes as additives to achieve high‐performance perovskite solar cells.

Ligands around inorganic perovskite nanocrystals (PNCs) play a critical role in improving the PNCs‐based solar cell performance. Herein, a facile hexane/ethyl acetate solvent treatment method to manage the ligand amount around PNCs is reported. Finally, a power conversion efficiency of 12.2%, which is the highest performance reported for mixed‐halide CsPbX₃ NCs solar cells, is achieved.

CsPbX₃ (X = Cl, Br, I) inorganic perovskite nanocrystals (PNCs) not only maintain the excellent optical and electronic properties of bulk material but also possess the features of nano‐materials, such as tunable bandgap and easily processable colloidal ink, and enable them to be suitable for incorporation into various electronic devices and compatible with printing techniques. In contrast to the traditional II‐VI and III‐V semiconductor nanocrystals, the unique defect‐tolerance effect makes the CsPbX₃ PNCs promising materials for optoelectronic applications. The ligands around the PNCs play a critical role in the optoelectronic devices performance. Herein, through a facile hexane/ethyl acetate (MeOAc) solvent treatment method to control the ligand amount around CsPbBrI₂ PNCs, the impact of ligand amount on the performance of solar cell is systematically demonstrated and the ligand amount is quantified precisely via the nuclear magnetic resonance internal standard method. Through controlling the ligand amount, the film quality, charge transfer, and transport properties are largely improved. In addition, a simple annealing process is applied to improve the interface properties by partial crystal fusion. As a consequence, the photovoltaic power conversion efficiency of 12.2% is achieved, which is the highest performance reported for mixed‐halide CsPbX₃ PNCs solar cells.

The significant energy loss in organic solar cells mainly results from the charge transfer loss and the nonradiative recombination loss. In view of this, herein, the recent advances in energy loss reduction according to different strategies are systematically summarized. On this basis, some fundamental questions in this topic are proposed to improve future investigations.

Compared with conventional inorganic solar cells (ISCs), energy loss (E _loss) in organic solar cells (OSCs) is usually much higher, limiting their maximum achievable power conversion efficiency (PCE). In view of this, a hot topic in OSC research is how to make E _loss as low as possible. To date, in some typical organic donor/acceptor (D/A) blends, although E _loss has been reduced to the values comparable with those in ISCs, the PCEs of the corresponding devices still fails to meet expectations. One crucial issue is that the physics behind the photovoltaic process in these D/A blends and the corresponding energy loss remain unclear. Herein, combining with an analysis of the photovoltaic process in OSCs, the mechanisms of different energy loss pathways are first discussed. On this basis, the recent advances focusing on E _loss are systematically summarized according to different strategies: 1) optimizing the energy offset of the D/A blend; 2) optimizing the morphology of the D/A blend; 3) ternary modulation; and 4) spin modulation. Finally, the summary and prospects are presented, where some fundamental questions to be cleared up in the photovoltaic process are proposed, such that more targeted photovoltaic design can be carried out in the future investigations of OSCs.

PM6‐Ir1.5, with an iridium complex‐incorporated polymer backbone, yields highly efficient power conversion efficiencies of over 16.5% for halogenated and non‐halogenated non‐fullerene polymer solar cells, with suitable nanoscale morphology, improved physical dynamics, and eco‐compatible processability.

The field of polymer solar cells (PSCs) has seen rapid development after the reports of high‐performance photovoltaic materials. Herein, iridium (Ir) complexes (1.5 mol%) are introduced to the polymer backbone of PM6, and a new π‐conjugated polymer PM6‐Ir1.5 is developed. Further analysis indicates that this approach can rationally modify the molecular packing order of PM6 to achieve suitable nanoscale morphology, efficient charge transport properties, and reduced carrier recombination losses, and significantly improve the photovoltaic performance of the resulting PSCs based on Y6‐C2 as non‐fullerene acceptor with good solubility in common solvents. Optimized devices obtain a power conversion efficiency (PCE) of 17.09%, whereas the corresponding devices fabricated using the green solvent achieve a remarkable PCE of 16.52%, which is the highest value reported so far in the literature for non‐halogenated PSCs.

Methylammonium thiocyanate additive‐assisted hot‐casting free deposition of a high‐quality 2D Dion–Jacobson perovskite film is reported. The optimized film exhibits high crystallinity, preferred orientation, and decreased defects. The corresponding device exhibits a maximum power conversion efficiency of 16.25%. The unsealed device retains 80% of its original efficiency after 35 days of storage in air with a humidity level of 45 ± 5%.

2D Dion–Jacobson (DJ) perovskite solar cells (PVSCs) with a high power conversion efficiency (PCE) are currently predominately fabricated via a hot‐casting process. The reason lies in the difficulty in preparing high‐quality perovskite films under mild conditions when the application of divalent ammonium removes the weak interaction from the spacer cation layer. Herein, the morphology of the 2D DJ perovskite film with a rigid piperidinium ring is tuned through a room‐temperature spin‐coating method, with the aid of a methylammonium thiocyanate (MASCN) additive. With the optimized amount of MASCN addition, the perovskite films deposited on the poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA)/poly[(9,9‐bis(30‐(N ,N‐dimethylamino)propyl)‐2,7‐uorene)‐alt‐2,7‐(9,9‐dioctylfuorene)] (PFN) substrate exhibit fine crystallinity, preferred orientation, decreased defects, and better energy‐level alignment with the hole transport layer. The device with the inverted planar structure presents a J _SC of 17.91 mA cm⁻², V _OC of 1.19 V, fill factor of 0.76, with a maximum PCE of 16.25%, which is the highest PCE for 2D DJ PVSCs free of hot casting. The unsealed device maintains around 80% of its initial efficiency after 35 days of exposure to air (Hr = 45 ± 5%). A potential route toward high‐performance 2D DJ PVSCs is provided.

The amorphous ICBA is incorporated into the host binary PM6:BTP‐4Cl system to fabricate ternary organic solar cells. Both photovoltaic efficiency and ultraviolet durability are enhanced in optimal ternary devices due to the increased photon harvesting, boosted hole transfer from BTP‐4Cl to PM6, highly efficient Förster resonance energy transfer between ICBA and BTP‐4Cl, more balanced charge transport, and more stable film morphology.

A ternary bulk‐heterojunction (BHJ) strategy that synergistically combines the merits of fullerene and nonfullerene acceptors has been regarded as a promising approach to enhance the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Herein, the fullerene derivative ICBA as the morphology regulator is incorporated into a nonfullerene‐based PBDB‐T‐2F:BTP‐4Cl (PM6:BTP‐4Cl) system to fabricate the high‐performance ternary OSCs. The amorphous ICBA prefers to homogeneously distribute in the BTP‐4Cl phase to form the well‐mixed acceptor domains due to their better miscibility, which distinctly reduces the exciton decay loss driven by the unfavorable phase separation and enhances BHJ morphology stability of ternary blends. The appropriate addition of ICBA induces the efficient long‐range Förster resonance energy transfer to BTP‐4Cl and facilitates the ultrafast hole transfer process from BTP‐4Cl to PM6, thereby contributing to charge carrier generation in the actual devices. Ultimately, the optimal ternary OSCs not only yield an average PCE higher than 16.5% but also show the superior ultraviolet photostability relative to binary control devices due to the increased harvesting of ultraviolet photons, boosted charge transfer, more balanced charge transport, and more stable nanostructural morphology. The results provide new insights to enable the simultaneously improved device performance and ultraviolet durability in state‐of‐the‐art ternary OSCs.

Three chlorinated thiophene–based donor polymers are characterized and their devices based on each nonfullerene acceptors are fabricated and optimized. During the shelf life test without encapsulation, an abnormal decrease in the efficiency is observed in both P(Cl)(F  = 0.5) and P(F‐Cl). However, the P(Cl) device exhibits long‐term stability. To understand this, the correlation between crystallinity and miscibility is systematically studied.

Nonfullerene organic solar cells (NFOSCs) have proven to have greater potential in terms of efficiency than fullerene‐based OSCs. However, the heterogeneity of nonfullerene acceptors (NFAs)‐based blend morphology is complex, making it difficult to understand, especially as its optimization requires that compatibility among the molecules be considered. Herein, P(Cl)(F  = 0.5) is newly synthesized with a type of terpolymer to increase compatibility with NFAs relative to that of conventional polymers. As a result, the combination of P(Cl)(F  = 0.5) with IDIC increases its power conversion efficiency (PCE) to 12.1%, compared with that of P(Cl):ITIC‐Th and P(F‐Cl):IT‐4F. However, during the shelf life stability of optimized devices without encapsulation, a rapid decrease in the efficiency of P(Cl)(F  = 0.5):IDIC and P(F‐Cl):IT‐4F is observed; the PCEs of P(Cl) (F  = 0.5):IDIC and P(F‐Cl):IT‐4F decrease to 24.1% and 43.5% of their initial values for up to 350 and 398 h, respectively. On the contrary, P(Cl):ITIC‐Th exhibits superior longterm air stability with a PCE decrease of −2% (for 317‐h) and 9% (for 2002‐h) compared with the initial PCE. To understand this phenomenon, the correlation between crystallinity and miscibility of blend films is systematically investigated. In short, the balanced crystallinity and miscibility of donor and acceptor induces a relatively more stable morphology.

High‐efficiency monolithic silicon‐based tandem solar cells require an adapted bandgap of the top cell. The perovskite composition FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ has a theoretically optimal bandgap of 1.68 eV. Implementation in p–i–n tandem devices gives highest certified efficiency of 25.1%, whereas a substantial efficiency increase is observed over time. By eliminating remaining interfacial and reflection losses, >30% efficiency is feasible.

Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p–i–n tandem configuration. A methylammonium‐free FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in ≈75 nm thicker absorber layers and 0.7 mA cm⁻² higher short‐circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.

A straightforward approach is developed to decouple the degradation effects occurring at the interfaces between the lead halide absorber with a hole‐transport and electron‐transport layers in perovskite solar cells. The impact of the hole‐transport layer is shown to depend on its composition: materials such as nickel oxide aggressively interact with the perovskite, whereas organic polytriarylamine provides a stable interface.

There is growing evidence that the stability of perovskite solar cells (PSCs) is strongly dependent on the interface chemistry between the absorber films and adjacent charge‐transport layers, whereas the exact mechanistic pathways remain poorly understood. Herein, a straightforward approach is presented for decoupling the degradation effects induced by the top fullerene‐based electron transport layer (ETL) and various bottom hole‐transport layer (HTL) materials assembled in p‐i‐n PSCs. It is shown that chemical interaction of MAPbI₃ absorber with ETL comprised of the fullerene derivative most aggressively affects the device operational stability. However, washing away the degraded fullerene derivative and depositing fresh ETL leads to restoration of the initial photovoltaic performance when bottom perovskite/HTL interface is not degraded. Following this approach, it is possible to compare the photostability of stacks with various HTLs. It is shown that poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and NiO_x induce significant degradation of the adjacent perovskite layer under light exposure, whereas poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) provides the most stable perovskite/HTL interface. A time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) analysis allows identification of chemical origins of the interactions between MAPbI₃ and HTLs. The proposed research methodology and the revealed degradation pathways should facilitate the development of efficient and stable PSCs.

Inorganic and organic electron transport layers (ETLs) have become a popular choice as selective contact materials for perovskite solar cells (PSCs). Herein, an overview of various inorganic and organic ETLs synthesis, properties, and their application in PSCs for different architectures, etc., to achieve high power conversion efficiency and functional stability is provided.

The presence of the electron transport layer (ETL) in perovskite solar cells (PSCs) is critical due to the requirement of enhancing the electron collection selectivity. ETLs are essential for achieving a high open‐circuit voltage (V _OC), high fill factor (FF), better transport of directional charges, better absorption of incoming light, and thermodynamically competent operation of photogenerated carrier populations. ETLs are sorted as organic, inorganic, or mixed, with different stability, cost effect, and directional charge transport ability. For instance, by using metal oxides as ETLs, power conversion efficiencies (PCEs) higher than 23% are reached for PSCs. Despite the advantages of metal oxide–ETLs and other organic or mixed ETLs, some questions still have to be addressed to achieve better PCEs, like how to passivate or eliminate the surface traps, how to upgrade the comprehension of the heterointerface, and optimization of morphology. Herein, different considerations of ETLs in different physical and environmental conditions, and different deposition methods used, are presented. Finally, the current studies and future challenges are analyzed in the domain of highly efficient PSCs with various ETLs.

The degradation of printed organic solar cells based on polymer PBDB‐T‐SF and small molecule IT‐4F is studied in operando for two different donor:acceptor ratios. Grazing incidence small angle X‐ray scattering, simultaneous current–voltage measurements and a theoretical model give insight into morphological changes during operation correlated with a decline of short‐circuit current.

Understanding the degradation mechanisms of printed bulk‐heterojunction (BHJ) organic solar cells during operation is essential to achieve long‐term stability and realize real‐world applications of organic photovoltaics. Herein, the degradation of printed organic solar cells based on the conjugated benzodithiophene polymer PBDB‐T‐SF and the nonfullerene small molecule acceptor IT‐4F with 0.25 vol% 1,8‐diiodooctane (DIO) solvent additive is studied in operando for two different donor:acceptor ratios. The inner nano‐morphology is analyzed with grazing incidence small angle X‐ray scattering (GISAXS), and current–voltage (I–V ) characteristics are probed simultaneously. Irrespective of the mixing ratio, degradation occurs by the same degradation mechanism. A decrease in the short‐circuit current density (J _SC) is identified to be the determining factor for the decline of the power conversion efficiency. The decrease in J _SC is induced by a reduction of the relative interface area between the conjugated polymer and the small molecule acceptor in the BHJ structure, resembling the morphological degradation of the active layer.

A simple dual‐source vapor transport deposition (VTD) process is used to fabricate V‐shaped bandgap Sb₂(S,Se)₃ solar cells in one step. The improved efficiency of 7.27% with V _OC (0.46 V) and J _SC (29.6 mA cm⁻²) being synergistically improved is a new efficiency record of Sb₂(S,Se)₃ solar cells based on vacuum method over the previous record of 6.3%.

Antimony chalcogenides (including Sb₂S₃, Sb₂Se₃, and Sb₂(S,Se)₃ alloy) have emerged as promising solar absorber materials. Notably, the Sb₂(S,Se)₃ alloy possesses continuously tunable bandgap from 1.1 to 1.7 eV, which covers the ideal bandgap for single‐junction photovoltaics governed by the Shockley–Queisser theory. Moreover, the bandgap gradient provides effective ways for photogenerated carriers collection and has the potential for high‐efficient Sb₂(S,Se)₃ alloy solar cells. Herein, a V‐shaped distributional bandgap in Sb₂(S,Se)₃ solar cells is reported through a simple dual‐source vapor transport deposition process, enabling the synergetic increase of the open‐circuit voltage (V _OC) and short‐circuit current (J _SC). Through careful optimization, a power conversion efficiency of 7.27% under AM1.5G illumination is obtained, with V _OC and J _SC of 0.46 V and 29.6 mA cm⁻², respectively. This V‐shaped bandgap engineering provides an effective method to enhance the device performance and can be extended to other chalcogenide thin‐film solar cells such as Sn–X, Ge–X, Cu–Sb–X (X = S and Se), and so on.