Printable CsPbI3 Perovskite Solar Cells with PCE of 19% via an Additive Strategy

Enhancing Extraction and Suppressing Cooling of Hot Electrons in Lead Halide Perovskites by Dipolar Surface Passivation

Slowing hot carrier (HC) cooling and improving HC extraction are considered two pivotal factors for enhancing power conversion efficiency in emerging HC photovoltaic applications of perovskites and other materials. Employing ab initio quantum dynamics simulations, we demonstrate the simultaneous slow cooling and efficient extraction of hot electrons at the C60/CsPbI3 interface through dipolar surface passivation with phenethylammonium and 4-fluorophenethylammonium ligands. The passivation effectively suppresses I-Pb lattice vibrations, weakens the hot electron-phonon interaction in CsPbI3, and thus slows down the HC cooling. At the same time, the dipolar surface passivation elevates the LUMO + 1 state in C60 and reduces the energy gap for HC extraction. Concurrently, higher-frequency vibrations of the dipolar layer enhance the coupling between C60 and CsPbI3, promoting efficient HC extraction further. These phenomena are intensified with increased polarity of the dipolar layer. Furthermore, we find that dipolar passivation has the opposite influence on cold electron collection at the band edge, underscoring the fact that the observed improvement in photovoltaic performance stems preferentially from the effective utilization of HCs rather than cold electrons. The work provides a new strategy for achieving high-performance HC perovskite solar cells.

Strategies for Improving Efficiency and Stability of Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted widespread research and commercialization attention because of their high power conversion efficiency (PCE) and low fabrication cost. The long-term stability of PSCs should satisfy industrial requirements for photovoltaic devices. Inverted PSCs with a p-i-n architecture exhibit considerable advantages because of their excellent stability and competitive efficiency. The continuously broken-through PCE of inverted PSCs shows huge application potential. This review summarizes the developments and outlines the characteristics of inverted PSCs including charge transport layers (CTLs), perovskite compositions, and interfacial regulation strategies. The latest effective CTLs, interfacial modification, and stability promotion strategies especially under light, thermal, and bias conditions are emphatically analyzed. Furthermore, the applications of the inverted structure in high-efficiency and stable tandem, flexible photovoltaic devices, and modules and their main obstacles are systematically introduced. Finally, the remaining challenges faced by inverted devices are discussed, and several directions for advancing inverted PSCs are proposed according to their development status and industrialization requirements.

Elucidating Black α-CsPbI3 Perovskite Stabilization via PPD Bication-Conjugated Molecule Surface Passivation: Ab Initio Simulations

The cubic α-CsPbI3 phase stands out as one of the most promising perovskite compounds for solar cell applications due to its suitable electronic band gap of 1.7 eV. However, it exhibits structural instability under operational conditions, often transforming into the hexagonal non-perovskite δ-CsPbI3 phase, which is unsuitable for solar cell applications because of the large band gap (e.g., ∼2.9 eV). Thus, there is growing interest in identifying possible mechanisms for increasing the stability of the cubic α-CsPbI3 phase. Here, we report a theoretical investigation, based on density functional theory calculations, of the surface passivation of the α-, γ-, and δ-CsPbI3(100) surfaces using the C6H4(NH3)2 [p-phenylenediamine (PPD)] and Cs species as passivation agents. Our calculations and analyses corroborate recent experimental findings, showing that PPD passivation effectively stabilizes the cubic α-CsPbI3 perovskite against the cubic-to-hexagonal phase transition. The PPD molecule exhibits covalent-dominating bonds with the substrate, which makes it more resistant to distortion than the ionic bonds dominant in perovskite bulks. By contrasting these results with the natural Cs passivation, we highlight the superior stability of the PPD passivation, as evidenced by the negative surface formation energies, unlike the positive values observed for the Cs passivation. This disparity is due to the covalent characteristics of the molecule/surface interaction of PPD, as opposed to the purely ionic interaction seen with the Cs passivation. Notably, the PPD passivation maintains the optoelectronic properties of the perovskites because the electronic states derived from the PPD molecules are localized far from the band gap region, which is crucial for optoelectronic applications.

Crystallization Kinetics of Hybrid Perovskite Solar Cells

Metal halide perovskites (MHPs) are considered ideal photovoltaic materials due to their variable crystal material composition and excellent photoelectric properties. However, this variability in composition leads to complex crystallization processes in the manufacturing of Metal halide perovskite (MHP) thin films, resulting in reduced crystallinity and subsequent performance loss in the final device. Thus, understanding and controlling the crystallization dynamics of perovskite materials are essential for improving the stability and performance of PSCs (Perovskite Solar Cells). To investigate the impact of crystallization characteristics on the properties of MHP films and identify corresponding modulation strategies, we primarily discuss the relevant aspects of MHP crystallization kinetics, systematically summarize theoretical methods, and outline modulation techniques for MHP crystallization, including solution engineering, additive engineering, and component engineering, which helps highlight the prospects and current challenges in perovskite crystallization kinetics.

Crystallization Control Based on the Regulation of Solvent-Perovskite Coordination for High-Performance Ambient Printable FAPbI3 Perovskite Solar Cells

The critical requirement for ambient-printed formamidinium lead iodide (FAPbI3 ) lies in the control of nucleation-growth kinetics and defect formation behavior, which are extensively influenced by interactions between the solvent and perovskite. Here, a strategy is developed that combines a cosolvent and an additive to efficiently tailor the coordination between the solvent and perovskite. Through in situ characterizations, the direct crystallization from the sol-gel phase to α-FAPbI3 is illustrated. When the solvent exhibits strong interactions with the perovskite, the sol-gel phases cannot effectively transform into α-FAPbI3 , resulting in a lower nucleation rate and confined crystal growth directions. Consequently, it becomes challenging to fabricate high-quality void-free perovskite films. Conversely, weaker solvent-perovskite coordination promotes direct crystallization from sol-gel phases to α-FAPbI3 . This process exhibits more balanced nucleation-growth kinetics and restrains the formation of defects and microstrains in situ. This strategy leads to improved structural and optoelectronic properties within the FAPbI3 films, characterized by more compact grain stacking, smoother surface morphology, released lattice strain, and fewer defects. The ambient-printed FAPbI3 perovskite solar cells fabricated using this strategy exhibit a remarkable power conversion efficiency of 24%, with significantly reduced efficiency deviation and negligible decreases in the stabilized output.

Unveiling the Potential of Ambient Air Annealing for Highly Efficient Inorganic CsPbI3 Perovskite Solar Cells

Here, we report a detailed surface analysis of dry- and ambient air-annealed CsPbI3 films and their subsequent modified interfaces in perovskite solar cells. We revealed that annealing in ambient air does not adversely affect the optoelectronic properties of the semiconducting film; instead, ambient air-annealed samples undergo a surface modification, causing an enhancement of band bending, as determined by hard X-ray photoelectron spectroscopy measurements. We observe interface charge carrier dynamics changes, improving the charge carrier extraction in CsPbI3 perovskite solar cells. Optical spectroscopic measurements show that trap state density is decreased due to ambient air annealing. As a result, air-annealed CsPbI3-based n-i-p structure devices achieved a 19.8% power conversion efficiency with a 1.23 V open circuit voltage.

Crystallization Control for Ambient Printed FA-Based Lead Triiodide Perovskite Solar Cells

Upscalable printing of high-performance and stable perovskite solar cells (PSCs) is highly desired for commercialization. However, the efficiencies of printed PSCs lag behind those of their lab-scale spin-coated counterparts owing to the lack of systematic understanding and control over perovskite crystallization dynamics. Here, the controlled crystallization dynamics achieved using an additive 1-butylpyridine tetrafluoroborate (BPyBF4 ) for high-quality ambient printed α-formamidinium lead triiodide (FAPbI3 ) perovskite films are reported. Using in situ grazing-incidence wide-angle X-ray scattering and optical diagnostics, the spontaneous formation of α-FAPbI3 from precursors during printing without the involvement of  δ-FAPbI3 is demonstrated. The addition of BPyBF4 delays the crystallization onset of α-FAPbI3 , enhances the conversion from sol-gel to perovskite, and reduces stacking defects during printing. Therefore, the altered crystallization results in fewer voids, larger grains, and less trap-induced recombination loss within printed films. The printed PSCs yield high power conversion efficiencies of 23.50% and 21.60% for a 0.09 cm-2 area device and a 5 cm × 5 cm-area module, respectively. Improved device stability is further demonstrated, i.e., approximately 94% of the initial efficiency is retained for over 2400 h under ambient conditions without encapsulation. This study provides an effective crystallization control method for the ambient printing manufacture of large-area high-performance PSCs.

Interface Engineering with Formamidinium Salts for Improving Ambient-Processed Inverted CsPbI3 Photovoltaic Performance: Intermediate- vs Post-Treatment

Inverted all-inorganic perovskite solar cells (PSCs) have attracted increasing attention owing to excellent thermal stability, easy fabrication, and adaptable application as top cell in tandem solar cells. Apart from efficiency, ambient processing is desirable for practical production. To avoid water invasion in ambient air, surface engineering for perovskites is reported as a valid approach. However, most were performed by post-treatment, which hardly regulates the formation process of perovskite crystals. This work demonstrates a simple but effective surface intermediate-treatment strategy to stabilize CsPbI3 perovskites fabricated in ambient air and compares the different effects yielded on the inverted PSCs. By using formamidinium (FA) salts for intermediate-treatment, the strong interaction between FA cation and [PbI6]4- octahedron improves the moisture resistance, and compared with the post-treatment strategy, the accelerated crystallization rate and the shortened exposure time to moisture reduce the devastation by water during film fabrication process further. Moreover, the greatly passivated defects and optimized energy level matching between perovskite and PCBM suppress the nonradiative recombination. Resultantly, the optimized device shows enhanced efficiency from 11.39% to 15.45%, and long-term stability is improved, with 97.6% efficiency remaining after storage for 1600 h. Therefore, we believe that this work can provide a promising guideline for fabricating all-inorganic inverted PSCs in a low-cost manufacturing scheme.

Bottom Distribution of F-Based Additives in Perovskite Films and Their Effects on Photovoltaic Performance

Various additives have been introduced to assist in film preparation and defect passivation. Herein, fluoroiodobenzene (FIB) molecules with different numbers of F atoms were incorporated into perovskite films to optimize the film quality as well as passivate defects. Based on the calculation and experimental results, it was found that the FIB additives were inclined to exist at the bottom of the film because of the strong affinity between F atoms stemming from FIB molecules and O atoms stemming from TiO2, especially for molecules with more F atoms. By optimization of the FIB molecule, the perovskite film crystallinity was significantly improved, the carrier lifetimes were prolonged, and the charge extraction ability was also enhanced. The device with FIB with one F atom achieved a photoelectrical conversion efficiency as high as 22.89% with a Voc of 1.118 V, fill factor (FF) of 80.44%, and Jsc of 25.45 mA cm-2, which was much higher than that of the control device with an efficiency of 20.87%. Furthermore, FIB molecules with three and five F atoms also achieved higher efficiency than that of the control device. The devices with FIB molecules showed better stability than the devices without additives. The unencapsulated devices with FIB additives held 90% of their original efficiencies in an ambient environment with a temperature of 15-25 °C and a relative humidity of 20-30%, while the control device dropped to 76% after more than 1000 h.

Interface Modification for Energy Level Alignment and Charge Extraction in CsPbI3 Perovskite Solar Cells

In perovskite solar cells (PSCs) energy level alignment and charge extraction at the interfaces are the essential factors directly affecting the device performance. In this work, we present a modified interface between all-inorganic CsPbI3 perovskite and its hole-selective contact (spiro-OMeTAD), realized by the dipole molecule trioctylphosphine oxide (TOPO), to align the energy levels. On a passivated perovskite film, with n-octylammonium iodide (OAI), we created an upward surface band-bending at the interface by TOPO treatment. This improved interface by the dipole molecule induces a better energy level alignment and enhances the charge extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and a high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. Further, to demonstrate the effect of the TOPO dipole molecule, we present a layer-by-layer charge extraction study by a transient surface photovoltage (trSPV) technique accomplished by a charge transport simulation.

Moderate temperature deposition of RF magnetron sputtered SnO2-based electron transporting layer for triple cation perovskite solar cells

The perovskite solar cells (PSCs) are still facing the two main challenges of stability and scalability to meet the requirements for their potential commercialization. Therefore, developing a uniform, efficient, high quality and cost-effective electron transport layer (ETL) thin film to achieve a stable PSC is one of the key factors to address these main issues. Magnetron sputtering deposition has been widely used for its high quality thin film deposition as well as its ability to deposit films uniformly on large area at the industrial scale. In this work, we report on the composition, structural, chemical state, and electronic properties of moderate temperature radio frequency (RF) sputtered SnO₂. Ar and O₂ are employed as plasma-sputtering and reactive gases, respectively. We demonstrate the possibility to grow a high quality and stable SnO₂ thin films with high transport properties by reactive RF magnetron sputtering. Our findings show that PSC devices based on the sputtered SnO₂ ETL have reached a power conversion efficiency up to 17.10% and an average operational lifetime over 200 h. These uniform sputtered SnO₂ thin films with improved characteristics are promising for large photovoltaic modules and advanced optoelectronic devices.

Dopant-Free Polymer Hole Transport Materials for Highly Stable and Efficient CsPbI3 Perovskite Solar Cells

All-inorganic perovskite CsPbI₃ contains no volatile organic components and is a thermally stable photoactive material for wide-bandgap perovskite solar cells (PSCs); however, CsPbI₃ readily undergoes undesirable phase transitions due to the hygroscopic nature of the ionic dopants used in commonly used hole transport materials. In the current study, the popular donor material PM6 in organic solar cells is used as a hole transport layer (HTL). The benzodithiophene-based backbone-conjugated polymer requires no dopant and leads to a higher power conversion efficiency (PCE) than 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD). Moreover, PM6 also shows priorities in hole mobility, hydrophobicity, cascade energy level alignment, and even defect passivation of perovskite films. With PM6 as the dopant-free HTL, the PSCs achieve a champion PCE of 18.27% with a competitive fill factor of 82.8%. Notably, the present PCE is based on the dopant-free HTL in CsPbI₃ PSCs reported thus far. The PSCs with PM6 as the HTL retain over 90% of the initial PCE stored in a glovebox filled with N₂ for 3000 h. In contrast, the PSCs with Spiro-OMeTAD as the HTL maintain ≈80% of the initial PCE under the same conditions.

Fabrication of High-Quality CsPbI3 Perovskite Films with Phosphorus Pentachloride Additive for Highly Stable Solar Cells

Fully inorganic perovskite cesium lead triiodide (CsPbI₃ ) has garnered much attention from researcher for photovoltaic application because of its excellent thermal stability compared with the inorganic-organic hybrid counterparts, along with the potential to serve as the top cell in tandem devices with silicon solar cell. However, the active α-phase cubic CsPbI₃ spontaneously tends to transform into the non-perovskite δ-CsPbI₃ when subjected to ambient condition. This work proposes an effective method to fabricate high-quality and stable α-phase cubic CsPbI₃ films by introducing phosphorus pentachloride (PCl₅ ) as an additive. PCl₅ acts as colloidal binder for modulating crystallization dynamics of perovskites, resulting in high-quality film and a significantly suppressed phase transition. Finally, highly stable CsPbI₃ perovskite solar cells can be achieved with a power conversion efficiency up to 17.85 %, and a long-term stability in N₂ filled glove box.

Manipulating the Formation of 2D/3D Heterostructure in Stable High-Performance Printable CsPbI3 Perovskite Solar Cells

Manipulating the formation process of the 2D/3D perovskite heterostructure, including its nucleation/growth dynamics and phase transition pathway, plays a critical role in controlling the charge transport between 2D and 3D crystals, and consequently, the scalable fabrication of efficient and stable perovskite solar cells. Herein, the structural evolution and phase transition pathways of the ligand-dependent 2D perovskite atop the 3D surface are revealed using time-resolved X-ray scattering. The results show that the ligand size and shape have a critical influence on the final 2D structure. In particular, ligands with smaller sizes and more reactive sites tend to form the n = 1 phase. Increasing the ligand size and decreasing the reactive sites promote the transformation from 3D to n = 3 and n < 3 phases. These findings are useful for the rational design of the phase distribution in 2D perovskites to balance the charge transport and stability of the perovskite films. Finally, solar cells based on ambient-printed CsPbI₃ with n-butylammonium iodide treatment achieve an improved efficiency of 20.33%, which is the highest reported value for printed inorganic perovskite solar cells.

Dimethyl sulfoxide: a promising solvent for inorganic CsPbI3 perovskite

Inorganic CsPbI₃ perovskite is an important photovoltaic material due to its suitable band gap and high chemical stability. However, it is a challenge to grow high-quality CsPbI₃ perovskite because the stability of perovskite phase is low and is sensitive to solvent. So far, most of CsPbI₃ perovskites in high-performance perovskite solar cells (PSCs) were prepared from N,N-dimethylformamide, a highly toxic solvent, and no successful case has been reported for dimethyl sulfoxide (DMSO), which is environmentally-friendly with considerably higher complexation capability. Herein, we reveal that forming DMSO-based adduct is the main cause for limiting the quality of CsPbI₃ perovskite from DMSO-based solutions, which would inhibit the formation of DMAPbI₃ (DMA = dimethylammonium, (CH₃)₂NH₂⁺) intermediate. Then, by introducing a vacuum treatment, DMSO molecules could be efficiently extracted from the adduct to induce the formation of DMAPbI₃ intermediate. After annealing, the intermediate is transitioned to the CsPbI₃ perovskite with enhanced crystallinity, high orientation, low defect density, and high uniformity. By using the CsPbI₃ perovskite as a light absorber, the PSCs based on carbon electrode (C-PSCs) achieve an efficiency of 16.7%, a new record for inorganic C-PSCs.

Single Crystallization of Cs4PbBr6 Perovskite from Supersaturated Organic Solutions Optimized Through Solubility Studies

We demonstrate the fabrication of millimeter-sized single crystals of 0D-Cs₄PbBr₆ grown in a supersaturated solution consisting of organic solvents without HBr (aq). One of the precursors, CsBr, was dissolved in ethylene glycol (EG) mixed with dimethyl sulfoxide, which is a good solvent for the other precursor, PbBr₂. At a solvent ratio of 20 vol % EG, the solubility of cesium bromide decreased and the title compound, Cs₄PbBr₆, was selectively formed, whereas, with an EG ratio of 80 vol %, 3D-CsPbBr₃ was formed. A phase diagram (solubility curve) of Cs₄PbBr₆ in the mixed solvent containing 20 vol % EG was obtained by visually observing dissolution and crystal precipitation while changing the temperature. Because the solubility was proportional to the temperature, the solubility curve demonstrated an upper critical solution phenomenon. The solubility near the boiling point of the solution (150 °C) was approximately 0.14 M. A single crystal of Cs₄PbBr₆ was formed by growing a seed crystal in a supersaturated solution on the low-temperature side of the solubility curve. X-ray analysis established the crystal structure; a fluorescence emission at 520 nm with a full width at half maximum of 20 nm confirms the composition of the single crystal to be Cs₄PbBr₆.

Lewis Acid-Base Adducts for Efficient and Stable Cesium-Based Lead Iodide-Rich Perovskite Solar Cells

All-inorganic cesium-lead-iodide (CsPbI₃ Br_{3- x} (2 < x < 3)) perovskite presents preeminent photovoltaic performance and chemical stability. Unfortunately, this kind of material suffers from phase transition to a nonperovskite phase under oxidative chemical stresses. Herein, the introduction of a low concentration of Lewis acid-base adducts (LABAs) is reported to synergistically reduce defect density, optimize interfacial energy alignment, and improve device stability of CsPbI_2.75 Br_0.24 Cl_0.01 (CsPbTh₃ ) solar cells. Both theoretical simulations and experimental measurements reveal that the noncoordinating anions, PF₆ ^- , as a Lewis base can more effectively bind with undercoordinated Pb²⁺ to passivate iodide vacancy defects than the BF₄ ^- and absorbed I^- , and thus the point defects are well suppressed. In addition, N-propyl-methyl piperidinium (NPMP⁺ ) is selected to assemble with PF₆ ^- in CsPbTh₃ film. The NPMP⁺ can regulate the crystal growth and finally homogenize the grain size and decrease the trap density. Consequently, the LABAs strategy can improve the power conversion efficiency of CsPbTh₃ solar cells to 19.02% under 1-sun illumination (100 mW cm^-2 ). Fortunately, the NPMP⁺ and PF₆ ^- -treated CsPbTh₃ film shows great phase stability after storage in ambient air for 250 days, and the power conversion efficiency of corresponding solar cells is almost 76% of the initial value after 60 days aging under ambient conditions.

Polishing the Lead-Poor Surface for Efficient Inverted CsPbI3 Perovskite Solar Cells

Triiodide cesium lead perovskite (CsPbI₃ ) has promising prospects in the development of efficient and stable photovoltaics in both single-junction and tandem structures. However, achieving inverted devices that provide good stability and are compatible to tandem devices remains a challenge, and the deep insights are still not understood. This study finds that the surface components of CsPbI₃ are intrinsically lead-poor and the relevant traps are of p-type with localized states. These deep-energy-level p traps induce inferior transfer or electrons and serious nonradiative recombination at the CsPbI₃ /PCBM interface, leading to the considerable open-circuit voltage (V_oc ) loss and reduction of fill factor (FF). Compared to molecular passivation, polishing treatment with 1,4-butanediamine can eliminate the nonstoichiometric components and root these intrinsically lead-poor traps for superior electron transfer. The polishing treatment significantly improves the FF and V_oc of the inverted CsPbI₃ photovoltaics, creating an efficiency promotion from 12.64% to 19.84%. Moreover, 95% of the initial efficiency of the optimized devices is maintained after the output operation for 1000 h.

Controlling Phase Transition toward Future Low-Cost and Eco-friendly Printing of Perovskite Solar Cells

Perovskite solar cells (PSCs) have grown increasingly popular over the past few years and are considered to be game-changers in the energy conversion market. It has became vital to transfer the deep understanding of the perovskite film formation process during lab-scale fabrication to large-scale production. Complex phase transition during film formation has been revealed by in situ strategies. However, there is still debate which phase transition is the right route for a future scalable approach. Herein, we briefly summarize perovskite crystallization during scalable printing processes. The critical information about the intermediates involved in phase transition from precursors to perovskite crystals are discussed because it deeply impacts the morphology of printed films. Finally, important strategies to control phase transition and challenges toward future low-temperature and eco-friendly printing of perovskite solar cells are proposed. The information provided by this Perspective will assist the screening and development of the perovskite phase transition for future cost-efficient printed perovskite panels.

Tailoring Defects Regulation in Air-Fabricated CsPbI3 for Efficient Inverted All-Inorganic Perovskite Solar Cells with Voc of 1.225 V

Air fabrication of CsPbI₃ perovskite photovoltaics has been attractive and fast-moving owing to its compatibility to low-cost and up-scalable fabrication. However, due to the inevitable erosions, undesirable traps are formed in air-fabricated CsPbI₃ crystals and seriously hinder photovoltaic performance with poor reproduction. Here, 3, 5-difluorobenzoic acid hydrazide (FBJ) is incorporated as trap regulation against external erosions in air-fabricated CsPbI₃. Theoretical simulations reveal that FBJ molecules feature stronger absorbance on CsPbI₃ than water, which can regulate trap formations for water erosions. In addition, FBJ with solid bonding interaction to CsPbI₃ can enlarge formation energy of various defects during crystallization and further suppress traps. Moreover, profiling to reductive hydrazine groups, FBJ inhibits traps for oxidation erosions. Consequently, a champion efficiency of 19.27% with an impressive V_oc of 1.225 V is realized with the inverted CsPbI₃ devices. Moreover, the optimized devices present superior stability and contain 97.4% after operating at 60 °C for 600 h.

Cesium acetate-assisted crystallization for high-performance inverted CsPbI3perovskite solar cells

Efficient inverted (p-i-n) type CsPbI₃perovskite solar cells (PSCs) have revealed promising applications due to their excellent thermal and photostability. Regulating the nucleation and crystallization of perovskite film is an important route to improving the performance of CsPbI₃PSCs. Herein, we explored cesium acetate (CsAc) as additive to manipulate the crystallization process of CsPbI₃perovskite films. By involving in the intermediate phase DMA_1-xCs_xPbI_3-yAc_yof perovskite, the pseudo-halide acetate (Ac^-) can retard the ion exchange reaction between DMA⁺and Cs⁺, leading to a perovskite with dense morphology, low defect density, and a long carrier lifetime. As a result, the optimal CsPbI₃PSCs yielded a high power conversion efficiency of 18.3%. Moreover, the encapsulated devices showed excellent operational stability and the devices retained their initial performance following 500 h of operation at the maximum power point under one-sun illumination in ambient conditions.

Recent Developments in Upscalable Printing Techniques for Perovskite Solar Cells

Just over a decade, perovskite solar cells (PSCs) have been emerged as a next-generation photovoltaic technology due to their skyrocketing power conversion efficiency (PCE), low cost, and easy manufacturing techniques compared to Si solar cells. Several methods and procedures have been developed to fabricate high-quality perovskite films to improve the scalability and commercialize PSCs. Recently, several printing technologies such as blade-coating, slot-die coating, spray coating, flexographic printing, gravure printing, screen printing, and inkjet printing have been found to be very effective in controlling film formation and improving the PCE of over 21%. This review summarizes the intensive research efforts given for these printing techniques to scale up the perovskite films as well as the hole transport layer (HTL), the electron transport layer (ETL), and electrodes for PSCs. In the end, this review presents a description of the future research scope to overcome the challenges being faced in the printing techniques for the commercialization of PSCs.

Quantifying Efficiency Limitations in All-Inorganic Halide Perovskite Solar Cells

While halide perovskites have excellent optoelectronic properties, their poor stability is a major obstacle toward commercialization. There is a strong interest to move away from organic A-site cations such as methylammonium and formamidinium toward Cs with the aim of improving thermal stability of the perovskite layers. While the optoelectronic properties and the device performance of Cs-based all-inorganic lead-halide perovskites are very good, they are still trailing behind those of perovskites that use organic cations. Here, the state-of-the-art of all-inorganic perovskites for photovoltaic applications is reviewed by performing detailed meta-analyses of key performance parameters on the cell and material level. Key material properties such as carrier mobilities, external photoluminescence quantum efficiency, and photoluminescence lifetime are discussed and what is known about defect tolerance in all-inorganic is compared relative to hybrid (organic-inorganic) perovskites. Subsequently, a unified approach is adopted for analyzing performance losses in perovskite solar cells based on breaking down the losses into several figures of merit representing recombination losses, resistive losses, and optical losses. Based on this detailed loss analysis, guidelines are eventually developed for future performance improvement of all-inorganic perovskite solar cells.

Photocatalytic Fuel Cells for Simultaneous Wastewater Treatment and Power Generation: Mechanisms, Challenges, and Future Prospects

Technological advancement is accompanied by excessive consumption of fossil fuels and affluent uses of chemical substances in many sectors, including transportation and manufacturing companies, and so on. Being an exhaustible resource, the excessive use of fossil fuels and of chemical substances may lead to a serious energy crisis in the long run, and it may additionally impose environmental pollution. Attempts have been made in the solution of such serious issues from every nook and corner. Nonetheless, no method has been found to be a panacea in waste water treatment and subsequent beneficiaries. One of the attempts in the solution to such issues is the application of photocatalytic technology, which could serve as a dual function in environmental remediation and clean energy production. A photocatalytic fuel cell is a tool developed for the recovery of energy from organic wastes. A rational cell construction needs the fabrication of photoelectrodes, the design of a photoanode and a photocathode chamber, in addition to an ion-transport membrane for pollution treatment and electricity generation. In this review, comprehensive fundamental assessments and recent developments in the design of photocatalytic fuel cells, their applications, future prospects, and challenges are covered.

Industrially Compatible Fabrication Process of Perovskite-Based Mini-Modules Coupling Sequential Slot-Die Coating and Chemical Bath Deposition

To upscale the emerging perovskite photovoltaic technology to larger-size modules, industrially relevant deposition techniques need to be developed. In this work, the deposition of tin oxide used as an electron extraction layer is established using chemical bath deposition (CBD), a low-cost and solution-based fabrication process. Applying this simple low-temperature deposition method, highly homogeneous SnO₂ films are obtained in a reproducible manner. Moreover, the perovskite layer is prepared by sequentially slot-die coating on top of the n-type contact. The symbiosis of these two industrially relevant deposition techniques allows for the growth of high-quality dense perovskite layers with large grains. The uniformity of the perovskite film is further confirmed by scanning electron microscopy (SEM)/scanning transmission electron microscopy (STEM) analysis coupled with energy dispersive X-ray spectroscopy (EDX) and cathodoluminescence measurements allowing us to probe the elemental composition at the nanoscale. Perovskite solar cells fabricated from CBD SnO₂ and slot-die-coated perovskite show power conversion efficiencies up to 19.2%. Furthermore, mini-modules with an aperture area of 40 cm² demonstrate efficiencies of 17% (18.1% on active area).

Ionic Liquid Treatment for Highest-Efficiency Ambient Printed Stable All-Inorganic CsPbI3 Perovskite Solar Cells

All-inorganic cesium lead triiodide (CsPbI₃ ) perovskite is well known for its unparalleled stability at high temperatures up to 500 °C and under oxidative chemical stresses. However, upscaling solar cells via ambient printing suffers from imperfect crystal quality and defects caused by uncontrollable crystallization. Here, the incorporation of a low concentration of novel ionic liquid is reported as being promising for managing defects in CsPbI₃ films, interfacial energy alignment, and device stability of solar cells fabricated via ambient blade-coating. Both theoretical simulations and experimental measurements reveal that the ionic liquid successfully regulates the perovskite thin-film growth to decrease perovskite grain boundaries, strongly coordinates with the undercoordinated Pb²⁺ to passivate iodide vacancy defects, aligns the interface to decrease the energy barrier at the electron-transporting layer, and relaxes the lattice strain to promote phase stability. Consequently, ambient printed CsPbI₃ solar cells with power conversion efficiency as high as 20.01% under 1 sun illumination (100 mW cm^-2 ) and 37.24% under indoor light illumination (1000 lux, 365 µW cm^-2 ) are achieved; both are the highest for printed all-inorganic cells for corresponding applications. Furthermore, the bare cells show an impressive long-term ambient stability with only ≈5% PCE degradation after 1000 h aging under ambient conditions.

Rethinking the A cation in halide perovskites

The A cation in ABX₃ organic-inorganic lead halide perovskites (OLHPs) was conventionally believed to hardly affect their optoelectronic properties. However, more recent developments have unraveled the critical role of the A cation in the regulation of the physicochemical and optoelectronic properties of OLHPs. We review the important breakthroughs enabled by the versatility of the A cation and highlight potential opportunities and unanswered questions related to the A cation in OLHPs.

Atomic-Scale Imaging and Nano-Scale Mapping of Cubic α-CsPbI3 Perovskite Nanocrystals for Inverted Perovskite Solar Cells

Colloidal synthesized cubic α-CsPbI₃ perovskite nanocrystals having a smaller lattice constant (a = 6.2315 Å) compared to the standard structure, and nanoscale mapping of their surfaces are reported to achieve superior photovoltaic performance under 45-55% humidity conditions. Atomic scale transmission electron microscopic images have been utilized to probe the precise arrangement of Cs, Pb, and I atoms in a unit cell of α-CsPbI₃ NCs, which is well supported by the VESTA structure. Theoretical calculation using density functional theory of our experimental structure reveals the realization of direct band to band transition with a lower band gap, a higher absorption coefficient, and stronger covalent bonding between the Pb and I atoms in the [PbI₆]^4- octahedral, as compared to reported standard structure. Nanoscale surface mapping using Kelvin probe force microscopy yielding contact potential difference (CPD) and conductive atomic force microscopy for current mapping have been employed on α-CsPbI₃ NCs films deposited on different DMSO doped PEDOT:PSS layers. The difference of CPD value under dark and light illumination suggests that the hole injection strongly depends on the interfaces with PEDOT:PSS layer. The carrier transport through grain interiors and grain boundaries in α-CsPbI₃ probed by the single-point c-AFM measurements reveal the excellent photosensitivity under the light conditions. Finally, inverted perovskite solar cells, employing α-CsPbI₃ NCs film as an absorber layer and PEDOT:PSS layer as a hole transport layer, have been optimized to achieve the highest power conversion efficiency of 10.6%, showing their potential for future earth abundant, low cost, and air stable inverted perovskite photovoltaic devices.

2D Perovsktie Substrate-Assisted CsPbI3 Film Growth for High-Efficiency Solar Cells

High-quality perovskite films are beneficial for fabricating perovskite solar cells (PSCs) with excellent photoelectric performance. The substrate on which the perovskite film grows plays a profound role in improving the crystallization quality of the perovskite film. Here, we proposed a novel method for optimizing CsPbI₃ perovskite films, that is, two-dimensional (2D) perovskite substrate-assisted growth (2D-PSAG) method. The prepared PEA₂PbI₄ 2D perovskite with proper wettability and roughness is used as a substrate to fabricate the high-quality CsPbI₃ film. Moreover, it is found that PEA cations show a vertical gradient distribution within the whole CsPbI₃ film because of their bottom-up self-diffusion. Also, PEA cations induce the moderate distortion of [PbI₆]^4- octahedron and slight lattice contraction of CsPbI₃ by chemically bonding between Pb and N atoms. Surprisingly, the trace amounts of PEA cations lead to a bottom-up gradient phase transition from γ-CsPbI₃ to β-CsPbI₃. Therefore, the energy-level alignment becomes more matched at the interface of the perovskite layer/hole transport layer (poly3-hexylthiophene, P3HT), which denotes a large improvement of hole transport and extraction in PSCs made with the 2D-PSAG method. As a result, the CsPbI₃-based PSCs with P3HT as a hole transport layer exhibit a champion efficiency of 17.13%, while the control device exhibits a PCE of only 14.16%. The PSCs made by the 2D-PSAG method retain above 70% of the initial PCE value after storage of 9 days in air (RH 10-20%), while the control device decomposes completely after 9 days. The improved stability could originate from the steric effects of PEA cations and the high crystallization quality of the mixed-phase CsPbI₃ film. Therefore, 2D-PSAG is a novel and promising strategy to develop all-inorganic PSCs with high performance and stability.

All-Inorganic Perovskite Solar Cells with Tetrabutylammonium Acetate as the Buffer Layer between the SnO2 Electron Transport Film and CsPbI3

All-inorganic CsPbI₃ perovskites have great potential in tandem cells in combination with other photovoltaic devices. However, CsPbI₃ perovskite solar cells (PSCs) still face a huge challenge, resulting in a low power conversion efficiency (PCE) relative to organic-inorganic PSCs. In this work, we introduced tetrabutylammonium acetate (TBAAc) as a buffer layer between the SnO₂ electron-transport layer (ETL) and CsPbI₃ all-inorganic perovskite film interface for the first time. TBAAc not only improved the conductivity of SnO₂ ETL but also formed a 1D TBAPbI₃ layer between the SnO₂ ETL and the 3D CsPbI₃ all-inorganic perovskite film, thereby enhancing the stability and passivating the surface defects of the CsPbI₃ perovskite to fabricate high-efficiency carbon-counter electrode (CE)-based CsPbI₃ solar cells. We fabricated carbon-CE-based hole-transporting layer ( HTL)-free PSCs with an FTO/SnO₂/TBAAc/CsPbI₃/C structure. The open-circuit voltage (V_oc), short circuit current density (J_sc), PCE, and fill factor of the champion CsPbI₃ PSCs simultaneously enhanced to 1.08 V, 17.48 mA/cm², 12.79, and 67.8%, respectively. This PCE is currently one of the high efficiencies reported for the above planar-structured carbon-CE-based CsPbI₃ PSCs to date. Moreover, the optimized device exhibits excellent stability, which retained over 83% of its initial PCE after 350 h. This work provides a facile way of simultaneous optimization of the SnO₂ ETL and the CsPbI₃ perovskite layer to fabricate stable and high-efficiency carbon-CE-based CsPbI₃ PSCs.

Perovskite Solar Cells from the viewpoint of innovation and sustainability

Innovation is seriously investing around the themes of climate change and sustainability. Commercial Photovoltaic (PV) has egregiously contributed to getting to 22.1% share of the gross final energy consumption in...

Homojunction Perovskite Quantum Dot Solar Cells with over 1 µm-Thick Photoactive Layer

The solution-processed solar cells based on colloidal quantum dots (QDs) reported so far generally suffer from poor thickness tolerance and it is difficult for them to be compatible with large-scale solution printing technology. However, the recently emerged perovskite QDs, with unique high defect tolerance, are particularly well-suited for efficient photovoltaics. Herein, efficient CsPbI₃ perovskite QD solar cells are demonstrated first with over 1 µm-thick active layer by developing an internal P/N homojunction. Specifically, an organic dopant 2,2'-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6TCNNQ) is introduced into CsPbI₃ QD arrays to prepare different carrier-type QD arrays. The detailed characterizations reveal successful charge-transfer doping of QDs and carrier-type transformation from n-type to p-type. Subsequently, the P/N homojunction perovskite QD solar cell is assembled using different carrier-type QDs, delivering an enhanced power conversion efficiency of 15.29%. Most importantly, this P/N homojunction strategy realizes remarkable thickness tolerance of QD solar cells, showing a record high efficiency of 12.28% for a 1.2 µm-thick QD active-layer and demonstrating great potential for the future printing manufacturing of QDs solar cells.

Interface modification of an electron transport layer using europium acetate for enhancing the performance of P3HT-based inorganic perovskite solar cells

In recent years, although the power conversion efficiency (PCE) of thermally stable all-inorganic CsPbI₃ perovskite solar cells (PSCs) had shown a great progress, the most reported CsPbI₃ PSCs suffered from the large open-circuit voltage (V_oc) loss, which is related to severe nonradiative recombination and a mismatch in energy level at the transport layer/perovskite interface. In this work, europium acetate (EuAc₃) as a multifunction interface material is chosen to modify the TiO₂/perovskite interface, the crystal quality of CsPbI₃ perovskite films is improved, and both bulk and interfacial defects are reduced effectively. Meanwhile, the energy levels arrangement between TiO₂ and CsPbI₃ perovskites is also optimized, corresponding the raised built-in electric field afford a strength force to accelerate the transport and extraction of charge carriers from CsPbI₃ perovskites to TiO₂. As a result, the performance of CsPbI₃ PSCs is largely enhanced with the PCE of 16.76%. When an Ag electrode was replaced by Au, the PCE further improves to 17.92%, which is the highest for CsPbI₃ PSCs with P3HT as the HTL ever reported. Besides, the CsPbI₃ PSC with the EuAc₃ modification layer maintains 84% of the initial PCE under continuous UV irradiation for 250 h in a nitrogen filled glovebox, being obviously higher than the control devices with only 40% of the initial PCE after UV irradiation for 100 h in the same environment.

Organic Matrix Assisted Low-temperature Crystallization of Black Phase Inorganic Perovskites

All-inorganic perovskites have attracted increasing attention for applications in perovskite solar cells (PSCs) and optoelectronics, including light-emitting devices (LEDs). Cesium lead halide perovskites with tunable I/Br ratios and a band gap aligning with the sunlight region are promising candidates for PSCs. Although impressive progress has been made to improve device efficiency from the initial 2.9 % with low phase stability to over 20 % with high stability, there are still questions regarding the perovskite crystal growth mechanism, especially at low temperatures. In this Minireview, we summarize recent developments in using an organic matrix, including the addition and use of organic ions, polymers, and solvent molecules, for the crystallization of black phase inorganic perovskites at temperatures lower than the phase transition point. We also discuss possible mechanisms for this low-temperature crystallization and their effect on the stability of black phase perovskites. We conclude with an outlook and perspective for further fabrication of large-scale inorganic perovskites for optoelectronic applications.

Dopant-Free Hole Transport Materials Afford Efficient and Stable Inorganic Perovskite Solar Cells and Modules

The emerging CsPbI3 perovskites are highly efficient and thermally stable materials for wide-band gap perovskite solar cells (PSCs), but the doped hole transport materials (HTMs) accelerate the undesirable phase transition of CsPbI3 in ambient. Herein, a dopant-free D-π-A type HTM named CI-TTIN-2F has been developed which overcomes this problem. The suitable optoelectronic properties and energy-level alignment endow CI-TTIN-2F with excellent charge collection properties. Moreover, CI-TTIN-2F provides multisite defect-healing effects on the defective sites of CsPbI3 surface. Inorganic CsPbI3 PSCs with CI-TTIN-2F HTM feature high efficiencies up to 15.9 %, along with 86 % efficiency retention after 1000 h under ambient conditions. Inorganic perovskite solar modules were also fabricated that exhibiting an efficiency of 11.0 % with a record area of 27 cm2 . This work confirms that using efficient dopant-free HTMs is an attractive strategy to stabilize inorganic PSCs for their future scale-up.

Stiffening the Pb-X Framework through a π-Conjugated Small-Molecule Cross-Linker for High-Performance Inorganic CsPbI2Br Perovskite Solar Cells

Inorganic CsPbI₂Br perovskites have witnessed incredible advances as a promising representative for translucent and tandem solar cells, but unfortunately, they are still plagued by serious energy losses and undesired phase instability. Herein, a new type of π-conjugated small molecule of 4-guanidinobenzoic-acid-hydrochloride (4-GBACl) is demonstrated to effectively cross-link the Pb-X framework of perovskites. The strong coordination between 4-GBACl and the [PbX₆]^4- octahedron of perovskites effectively stiffens the Pb-X framework to suppress the ion migration, thus stabilizing the perovskite phase structure against light and thermal conditions. Apart from the physical barrier for phase instability resulting from the hydrophobic benzene ring at grain boundaries (GBs), guanidinium cations and -COOH and Cl^- groups can simultaneously afford the passivation of positively and negatively charged defects at the GBs and surface, including undercoordinated halide species and undercoordinated Pb²⁺ ions, thereby effectively inhibiting the charge trapping/recombination centers. Two-dimensional confocal-fluorescence mapping images provide a visualized sight into the significantly suppressed nonradiative recombination and the prolonged carrier lifetime. It is suggested that the 4-GBACl additive plays multiple roles in grain cross-linking to regulate crystallization, distinctly reducing the trap-state density, ion migration inhibition, and moisture barrier in CsPbI₂Br films. Consequently, the 4-GBACl-treated device exhibits a champion power conversion efficiency (PCE) of 15.59% accompanied with a considerably improved V_oc of 1.28 V and maintains 88% of the initial PCE value after 1200 h aging under 20% relative humidity.

All-Evaporated, All-Inorganic CsPbI3 Perovskite-Based Devices for Broad-Band Photodetector and Solar Cell Applications

Following the rapid increase of organic metal halide perovskites toward commercial application in thin-film solar cells, inorganic alternatives attracted great interest with their potential of longer device lifetime due to the stability improvement under increased temperatures and moisture ingress. Among them, cesium lead iodide (CsPbI₃) has gained significant attention due to similar electronic and optical properties to methylammonium lead iodide (MAPbI₃), with a band gap of 1.7 eV, high absorption coefficient, and large diffusion length, while also offering the advantage of being completely inorganic, providing a higher thermal stability and preventing material degradation. On a device level, however, it seems also essential to replace organic transport layers by inorganic counterparts to further prevent degradation. In addition, devices are mostly fabricated by spin coating, limiting their reproducibility and scalability; in this case, exploring all-evaporated devices allows us to improve the quality of the layers and to increase their reproducibility. In this work, we focus on the deposition of CsPbI₃ by CsI and PbI₂ co-evaporation. We fabricate devices with an all-inorganic, all-evaporated structure, employing NiO and TiO₂ as transport layers, and evaluate these devices for both photodetector and solar cell applications. As a photodetector, low leakage current, high external quantum efficiency (EQE) and detectivity, and fast rise and decay times were obtained, while as a solar cell, acceptable efficiencies were achieved. These all-inorganic, all-evaporated devices represent one step forward toward higher stability and reproducibility while enabling large area compatibility and easier integration with other circuitry and, in future, the possible commercialization of perovskite-based technology.

Efficient (>20 %) and Stable All-Inorganic Cesium Lead Triiodide Solar Cell Enabled by Thiocyanate Molten Salts

Besides widely used surface passivation, engineering the film crystallization is an important and more fundamental route to improve the performance of all-inorganic perovskite solar cells. Herein, we have developed a urea-ammonium thiocyanate (UAT) molten salt modification strategy to fully release and exploit coordination activities of SCN^- to deposit high-quality CsPbI₃ film for efficient and stable all-inorganic solar cells. The UAT is derived by the hydrogen bond interactions between urea and NH₄ ⁺ from NH₄ SCN. With the UAT, the crystal quality of the CsPbI₃ film has been significantly improved and a long single-exponential charge recombination lifetime of over 30 ns has been achieved. With these benefits, the cell efficiency has been promoted to over 20 % (steady-state efficiency of 19.2 %) with excellent operational stability over 1000 h. These results demonstrate a promising development route of the CsPbI₃ related photoelectric devices.

p-Type Charge Transfer Doping of Graphene Oxide with (NiCo)1-y Fey Ox for Air-Stable, All-Inorganic CsPbIBr2 Perovskite Solar Cells

The precise regulation of interfacial charge distribution highly determines the power conversion efficiency of perovskite solar cells (PSCs). Herein, inorganic (NiCo)_1-y Fe_y O_x nanoparticle decorated graphene oxide (GO) is successfully demonstrated as a hole booster for all-inorganic CsPbIBr₂ PSC free of precious metal electrode. Arising from the spontaneous electron transfer induced p-type doping of GO from edged oxygen-containing functional groups to (NiCo)_1-y Fe_y O_x , the best all-inorganic CsPbIBr₂ PSC achieves an efficiency of 10.95 % under one standard sun owing to the eliminated paradox between charge extraction and charge localization in GO surface. Furthermore, the champion device exhibits an excellent long-term stability at 10 % relative humidity without encapsulation over 70 days because of the suppressed ions migration.

Tuning an Electrode Work Function Using Organometallic Complexes in Inverted Perovskite Solar Cells

Low-work-function (WF) metals (including silver (Ag), aluminum (Al), and copper (Cu)) used as external cathodes in inverted perovskite solar cells (PSCs) encounter oxidation caused by air exposure and halogen-diffusion-induced corrosion, which threaten the long-term stability of the device. The cathode interlayer (CIL) has shown promise in reducing the metal WF and thus boosting the device power conversion efficiency (PCE). However, it remains a challenge for current CIL materials to enable high-WF metals (e.g., Au) to be used as cathodes to achieve PSCs with a superior PCE and long-term stability. Here, we use a series of synthesized (carbolong-derived) organometallic complexes as CILs to tune the electrode WF in inverted PSCs. Density functional theory calculations and surface characterizations show that the organometallic complexes that contain anions and cations are prone to form anion-cation dipoles on the metal surface, hence drastically reducing the metal's WF. Photovoltaic devices based on a Ag cathode, which was modified with these organometallic complexes, received a boosted PCE up to 21.29% and a remarkable fill factor that reached 83.52%, which are attributed to the dipole-enhanced carrier transport. The environmental stability of PSCs was further improved after employing Au as a cathode with these organometallic complexes, and the modified devices exhibited no efficiency loss after 4080 h storage measurements.

Interfacial engineering of CuSCN-based perovskite solar cells via PMMA interlayer toward enhanced efficiency and stability

We report a new interfacial engineering strategy to improve the photovoltaic performance of CuSCN-based perovskite solar cells.