Improved Power Conversion Efficiency Research Articles

Improving the Photovoltaic Properties and Stability of Inverted Perovskite Solar Cells with Hydroxylamine‐Based Additives

AbstractOrganic–inorganic halide perovskite solar cells (PVSCs) are considered a promising emerging photovoltaic technology that offer exceptional optoelectronic properties and the potential for economic solar energy conversion. Additive engineering‐based fabrication processes can achieve highly efficient and stable PVSCs that feature well‐controlled perovskite layers with a dense, uniform, “black” α‐phase crystal structure, as well as large grains and few defects. In this study, several hydroxylamine derivatives are introduced as additives to FAPbI3 precursor solutions to investigate their effects on the performance of PVSCs. The addition of hydroxylamine derivatives suppresses the formation of the unwanted δ‐phase and lead iodide, while the α‐phase cubic structure is preferentially formed without changing the bandgap of FAPbI3. Additionally, the additive‐treated perovskite films show improved stability compared with those without additives. Moreover, using X‐ray diffraction and X‐ray photoelectron spectroscopy analyses, it is discovered that the hydroxylamine‐based additives are not incorporated in the crystal lattices but rather resided on the surface or grain boundaries. Notably, the inverted PVSCs added with N‐methylhydroxylamine exhibit an improved power conversion efficiency, higher stability, and minimal hysteresis.

Maximizing Performance and Stability of Organic Solar Cells at Low Driving Force for Charge Separation.

Thanks to the development of novel electron acceptor materials, the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This review discusses recent approaches to finding efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). The authors discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. A comprehensive view is then presented on motifs providing a driving force for charge separation, namely hybridization at the donor:acceptor interface and polarization effects in the bulk, of which quadrupole moments (electrostatics) play a leading role. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, which are essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the review presents recent HT studies finding consolidated structure-property relationships in OPV films and devices from various deposition methods, from research to industrial upscaling.

Data-driven design for enhanced efficiency of Sn-based perovskite solar cells using machine learning

In this study, a novel three-step learning-based machine learning (ML) methodology is developed utilizing 26 000 experimental records from The Perovskite Database Project. A comprehensive set of 29 features encompassing both categorical and numerical data was utilized to train various ML models for various solar cell performance metrics, including open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and power conversion efficiency (PCE). The model accuracy was assessed using four key metrics: mean absolute error, mean square error, root mean square error, and R2 score. Among the constructed models, random forest (RF) emerged as the standout performer, boasting an R2 score of 0.70 for PCE. This RF model was then used for prediction on the large, optimized design pool of Sn-based perovskite data with intent to probe a viable non-toxic substitute to the standard Pb-based absorber. A three-step algorithm was tailored, which led to the discovery of a new set of feature combinations, showcasing a PCE improvement over the existing peak performance of Sn-based devices. The key aspects identified were device architecture, dimensionality, and deposition procedures for essential layers, including the electron transport layer, the hole transport layer, the perovskite absorber layer, and the back-contact. Through consideration of these features, an impressive increase in PCE was achieved. There was a 28.35% increase in PCE from 12.24% to 15.71% for architecture optimization and a 24.6% increase in PCE from 12.24% to 15.25% for deposition method optimization. This study additionally addresses the effective implementation of target encoding applied to a diverse set of categorical feature labels. The data-driven methodology proposed in this study allows scientists to efficiently identify an optimal architecture and deposition parameters for non-toxic Sn-based perovskite materials with a much higher anticipated device PCE compared to traditional trial-and-error analyses. Further exploration and exploitation of the current investigation is expected to lead to successful and sustainable development of highly efficient Sn-based perovskite solar cells.

Dye-sensitized solar cells containing photoelectrodes with TiO2 grids

A 90-nm-thick TiO2 grid is created through photolithography, sputtering and lift-off process in this study. A nanoporous film is deposited on the TiO2 grid. The grid induces a protrusive structure in the nanoporous film. Experimental results demonstrate that introducing the TiO2 grid into a dye-sensitized solar cell increases its power conversion efficiency by 25% compared to a dye-sensitized solar cell lacking a TiO2 grid. Consequently, the TiO2 grid proves beneficial in augmenting the power conversion efficiency of the dye-sensitized solar cell. The increased power conversion efficiency of the dye-sensitized solar cell containing the TiO2 grid is attributed to the large surface area and directional electron transport channels of the protrusive structure. The TiO2 grid offers advantages such as simple fabrication, cost-effective production and improved power conversion efficiency.

Low-cost kesterite (CZTS) bilayers as an effective hole-transport layer for perovskite solar cells

This study investigates the utility of Copper Zinc Tin Sulfide (CZTS) thin films as inorganic hole-transporting layers (HTLs) in low-temperature perovskite solar cells (PSCs). The primary goal is to improve power conversion efficiencies (PCE) in PSCs using CZTS thin films fabricated through solution-based methods, with CH3NH3PbI3 perovskite and ZnO serving as the light-absorbing and electron-transporting components, respectively. Precise control over CZTS thin film growth significantly influences the formation of uniform perovskite layers. The study underscores the importance of achieving dense ZnO films with suitable grain sizes to confirm consistent coverage of perovskite films, enabling efficient charge transport while reducing recombination at the CZTS/perovskite interface. Results reveal that CZTS/CZTS bilayers exhibit superior photoelectrochemical performance compared to conventional CZTS(A)/FTO and CZTS(B)/CZTS(A) configurations. Evaluation of water splitting performance demonstrates that the CZTS/CZTS bilayer photoelectrode achieves the maximum photocurrent density, reaching 12 mA/cm2 vs. Ag/AgCl, with an Incident Photon-to-Current Efficiency of 9 % at 400 nm and an applied potential of 0.4V vs. Ag/AgCl. The study comprehensively examines CZTS impact on PSC photovoltaic properties and stability, culminating in a high-performance device with a PCE potential of up to 7 %. This underscores the pivotal role of CZTS− perovskite contacts in determining PSC photovoltaic performance and highlights CZTS potential as a versatile HTM for various PSCs designs.

Green-based modifiable CaZnBr4 for solar cells application

Future revolution in photovoltaics will be hinged mainly on cost, health implication, and material stability and performance. Based on these criteria, lead-based inorganic photovoltaics, organic–inorganic hybrid, and silicon photovoltaics are screened-out. According to the literature, the lead-free inorganic perovskite solar cell is favorably disposed to cost and safe-health. However, the simultaneous solution to material stability, high defect density, and low power conversion efficiency (PCE) still remains a mystery that has not been solved. This research proposed the green-based modifiable CaZnBr4 as a potential candidate for lead-free solar cell application based on the principle of A-site cation with green-based additive incorporation. The green-based additive was obtained from Kola Nitida, Carica Papaya, Ficus Exasperata, and Musa paradisiaca. The elemental characterization of the green-based additives was performed using X-ray fluorescence spectroscopy (XRF). The optical, crystalline, and electronic properties were characterized using ultraviolet–visible (UV–Vis) spectroscopy, X-ray diffractometry, Quantum Espresso, scanning electron microscopy and SCAPS-1D. The green-base-modified CaZnBr4 showed significant PCE improvement by 3% with significant film and crystallinity formation. The stressed state of the parent compound CaZnBr4 shows that it may be better suited for thermovoltaics application. It is recommended that better results could be obtained when different synthetic routes and green-based additives are used to initiate the defect passivation protocols.

Effects of p-type dopant on photovoltaic performance of pervoskite solar cells using phthalocyanine-tetrabenzoporphyrin hole transport materials: A combined theoretical and experimental study

We demonstrate effects of p-type dopant on photovoltaic performance of pervoskite solar cells (PSCs) using phthalocyanine (C6PcH2)-tetrabenzoporphyrin (C6TBPH2) hole transport materials (HTMs) by combining theoretical and experimental studies. Although C6PcH2 and C6TBPH2 molecules share a similar chemical structure, the p-type dopant induced oxidation was only observed in C6TBPH2 molecules through emerging absorption peaks at 2.9 eV and shifting energy band diagram. As a result, power conversion efficiencies (PCEs) of PSCs utilizing C6TBPH2 HTM were improved from around 4 to 14 % by adding p-type dopants, but those of PSCs utilizing C6PcH2 HTM were remained at around 4 %. Numerical simulations indicate that the improvement of PCEs (from 5.7 to 21.1 %) of the simulated cells utilizing the C6TBPH2 HTMs with p-type dopants were attributed to increases in the acceptor densities and deepening the highest occupied molecular orbital energy levels rather than increases in the HTM hole mobility of the HTMs. We suggest that those results could be an indicator for fabrication of efficient PSCs utilizing organic HTMs with p-type dopants.

A synergistic approach in designing InP/ZnS quantum dot based CIGS solar cell

This research work investigates the implementation of non-toxic InP/ZnS quantum dot (QDs) buffer layer in solar cell. Due to its nontoxic behaviour, it is a better replacement of commonly used CdS buffer layer. InP/ZnS quantum dots buffer layer enhances light absorption, leading to a broader spectral response and improved power conversion efficiency. Graded CIGS (CuIn1-xGaxSe2) is used as absorber layer in this study for better utilization of solar spectrum. The combined utilization of an InP/ZnS QD buffer layer and a graded absorber layer results in a synergistic enhancement of the cell's performance. Further optimization of CIGS layers is achieved by varying the gallium content (x) in the Cu(In1−xGax)Se2. For the gallium content (x) in the range of 0.20 to 0.15, maximum efficiency is achieved which is 31.20 %. Numerical modeling is employed in this study to analyze how variations in thickness, doping concentration, and defect density impact the solar cell performances.

End-capped engineering of carbazole based dopant free hole transporting material with improved power conversion efficiency

Perovskite solar cells (PSCs) belonging to third-generation of solar cells are reported to have better power conversion efficiency, cost-effectiveness, and many other benefits. This computational investigation focuses on designing five indolo(3,2-b) carbazole (ICZ) central core containing small hole transport materials (HTMs) to discover photovoltaic properties as donors for perovskite solar cells accompanied by enhanced charge mobility. New HTMs with A–π–D–π–A skeleton has been designed by thiophene-bridged end-capped acceptor engineering on reference molecule (ICZ-R). Absorption spectra, frontier MOs, density of state analysis, molecular electrostatic potential surfaces, transition density matrix, non-covalent interactions, iso-surface maps, and planarity parameters were analyzed through different softwares for better-predicting performance of perovskite solar cells. All tailored HTMs showed better absorption profiles. Among them, ICZ-D3 exhibited the maximum absorbance λmax at 695 nm in dichloromethane solvent and the greatest full-width half maximum (313.93 nm). ICZ-D2 showed the lowest Eg of 1.62 eV, and the highest dipole moment (8.35 D) in the solvent phase, showing admirable solubility. Designed HTMs exhibited decelerated HOMO values (−4.62 to −5.25), lower values of binding energies (0.0323–0.388 <0.542 eV), better intrinsic charge transfer percentage (10.74–72.71%>7.88), lower hardness (0.81–1.11 <1.78 eV) and greater softness values (0.90–1.23 >0.56 eV) as compared to ICZ-R. All designed HTMs demonstrated higher estimated VOC and power conversion efficiency (PCE=11.81–25.90%) than ICZ-R, leading to fruitful applications. ICZ-D4 resulted in the highest PCE (25.90%) with better VOC (1.25 eV). These outstanding findings paved a new way to develop better-performing HTMs for PSCs.

Exploiting the donor‐acceptor‐additive interaction's morphological effect on the performance of organic solar cells

AbstractOrganic solar cells (OSCs) have demonstrated over 19% power conversion efficiency (PCE) with the help of material innovation and device optimization. Co‐working with newly designed materials, traditional solvent additives, 1‐chloronaphthalene (CN), and 1,8‐diodooctane (DIO) are still powerful in morphology modulation towards satisfying efficiencies. Here, we chose recently reported high‐performance polymer donors (PM6 &amp; D18‐Fu) and small molecular acceptors (Y6 &amp; L8‐BO) as active layer materials and processed them by different conditions (CN or DIO or none). Based on corresponding 12 groups of device results, and their film morphology characterizations (both ex‐situ and in‐situ ones), the property‐performance relationships are revealed case by case. It is thereby supposed to be taken as a successful attempt to demonstrate the importance and complexity of donor‐acceptor‐additive interaction, since the device performance and physics analyses are also tightly combined with morphology variation. Furthermore, ternary blend construction for PCE improvement provides an approaching 19% level and showcases the potential of understanding‐guided‐optimization (UGO) in the future of OSCs.

Improving the minority carrier lifetime of PERC solar cells via bi-layer rear interface passivation strategy

Passivated emitter and rear contact (PERC) based solar cells are dominating the current photovoltaic (PV) market due to their high power conversion efficiency (PCE) and low cost. However, issues like the lower minority carrier lifetime (MCLT) and high density of surface dangling bonds are limiting their further improvement in PCE. To overcome this issue, a bi-layer rear interface passivation strategy is developed with the ultra-thin Al2O3 and SiO2 layers in PERC solar cells. Applying single (Al2O3) and double (SiO2/Al2O3) passivation layers revealed that each process parameter such as plasma conditions, deposition temperature, post-annealing temperature, and the film thickness in the ALD process affect the MCLT values as well as overall device performance. Especially, post-annealing treatment which improves crystallinity, and reduces interfacial defects was found to be most influential for the MCLT value. The single Al2O3 layer applied in PERC solar cells revealed an increase and decrease in MCLT values at a relatively lower (<10 nm) and higher thickness (>15 nm), respectively. Further, integration of SiO2 with the Al2O3 layer showed its more prominent effect than just single Al2O3 and SiO2 passivation layers. An optimal SiO2 thickness combination (4 nm) with an Al2O3 layer (12 nm), demonstrated reduced density of surface dangling bonds and diffusion of Al atoms from Al2O3 layers. As a result, it improved the carrier extraction yield by increasing the negative fixed charge. Finally, SiO2/Al2O3 (4/12 nm) double passivation layer post-annealed at 450 °C for 15 min delivers improved PCE from 19.4% to 19.9% in PERC solar cells.

Molecular engineering of tetraphenyl-based hole transporting materials to enhance photovoltaic properties of perovskite solar cells

Perovskite photovoltaic cells have lately gained appeal due to their excellent operational efficiency, photophysical properties, and high dielectric constant. Five novel hole transport materials namely TADM1–TADM5 were devised to improve power conversion efficiency (PCE) of Perovskite solar cells (PSCs). These molecules have tetraphenyl (TP) as a typical core, donor, and different acceptor units connected via a thiophene spacer. Under the framework of DFT theory, the B3LYP functional along with the 6–31 G basis set was utilized to study their geometries, molecular electrostatic potential, reorganization energies, the density of states, quantum chemical parameters, transition density matrix, binding energies, and charge transfer properties. All the designed HTMs (TADM1–TADM5) displayed excellent photovoltaic properties due to lower (Eg) varying (1.94–2.54 eV), small binding energies (0.04–0.36 eV), and high dipole moments (12.89 D to 26.93 D) that enhanced their electron charge transfer behavior. Small reorganization energy (RE) values for holes and electrons resulted in significant charge mobility. All the designed HTMs (TADM1-TADM5) have higher VOC (1.30–1.43 V) and power conversion efficiency (23.70–26.27%) than the reference molecule SMeTAD (20.94%). Consequently, all of the modelled molecules (TADM1-TADM5) demonstrate that using acceptor moieties is an effective method for achieving acceptable optoelectronic characteristics and outstanding efficiency.

Molecular packing tuning via chlorinated end group enables efficient binary organic solar cells over 18.5%

AbstractDesigning novel nonfullerene acceptors (NFAs) is of vital importance for the development of organic solar cells (OSC). Modification on the side chain and end group are two powerful tools to construct efficient NFAs. Here, based on the high‐performance L8BO, we selected 3‐ethylheptyl to substitute the inner chain of 2‐ethylhexyl, obtaining the backbone of BON3. Then we introduced different halogen atoms of fluorine and chlorine on 2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene) malononitrile end group (EG) to construct efficient NFAs named BON3‐F and BON3‐Cl, respectively. Polymer donor D18 was chosen to combine with two novel NFAs to construct OSC devices. Impressively, D18:BON3‐Cl‐based device shows a remarkable power conversion efficiency (PCE) of 18.57%, with a high open‐circuit voltage (VOC) of 0.907 V and an excellent fill factor (FF) of 80.44%, which is one of the highest binary PCE of devices based on D18 as the donor. However, BON3‐F‐based device shows a relatively lower PCE of 17.79% with a decreased FF of 79.05%. The better photovoltaic performance is mainly attributed to the red‐shifted absorption, higher electron and hole mobilities, reduced charge recombination, and enhanced molecular packing in the D18:BON3‐Cl films. Also, we performed stability tests on two binary systems; the D18:BON3‐Cl and D18:BON3‐F devices maintain 88.1% and 85.5% of their initial efficiencies after 169 h of storage at 85°C in an N2‐filled glove box, respectively. Our work demonstrates the importance of selecting halogen atoms on EG and provides an efficient binary system of D18:BON3‐Cl for further improvement of PCE.

Enhanced marine predators algorithm optimized support vector machine for IGBT switching power loss estimation

As the use of renewable energy generation continues to grow, improving power conversion efficiency has become an urgent task. This study aims to propose a high-precision power module loss estimation method, with a focus on predicting the switching losses of insulated gate bipolar transistor (IGBT) modules, which is crucial for the reliability assessment of IGBT modules. The main objective of this study is to establish a high-precision predictive model for IGBT module switching losses to enhance the reliability and efficiency of these devices. Firstly, a dynamic characteristic test platform was established to acquire relevant data for in-depth analysis of IGBT behavior. Secondly, given the excellent performance of support vector machine (SVM) in handling both strong and small-sized datasets, SVM was chosen as the foundational model for high-precision switching loss estimation. Subsequently, an enhanced marine predatory algorithm (EMPA), known for its superior convergence precision and speed, was introduced to optimize the random parameters of SVM. Finally, a method based on the optimized EMPA-SVM was constructed for predicting IGBT switching power losses. The proposed approach was validated using dynamic characteristic test data. And it indicated that the predictive model achieved the value of R 2 exceeding 99.8% for switching losses. Additionally, the mean absolute error and root mean square error metrics of EMPA-SVM model outperformed other models. Therefore, the research results unambiguously demonstrate the significant benefits of accurately predicting IGBT switching losses in enhancing device performance.

Stable PbS colloidal quantum dot inks enable blade-coating infrared solar cells

Infrared solar cells are more effective than normal bandgap solar cells at reducing the spectral loss in the near-infrared region, thus also at broadening the absorption spectra and improving power conversion efficiency. PbS colloidal quantum dots (QDs) with tunable bandgap are ideal infrared photovoltaic materials. However, QD solar cell production suffers from small-area-based spin-coating fabrication methods and unstable QD ink. Herein, the QD ink stability mechanism was fully investigated according to Lewis acid–base theory and colloid stability theory. We further studied a mixed solvent system using dimethylformamide and butylamine, compatible with the scalable manufacture of method-blade coating. Based on the ink system, 100 cm2 of uniform and dense near-infrared PbS QDs (~ 0.96 eV) film was successfully prepared by blade coating. The average efficiencies of above absorber-based devices reached 11.14% under AM1.5G illumination, and the 800 nm-filtered efficiency achieved 4.28%. Both were the top values among blade coating method based devices. The newly developed ink showed excellent stability, and the device performance based on the ink stored for 7 h was similar to that of fresh ink. The matched solvent system for stable PbS QD ink represents a crucial step toward large area blade coating photoelectric devices.Graphical

Nanopatterned Back-Reflector with Engineered Near-Field/Far-Field Light Scattering for Enhanced Light Trapping in Silicon-Based Multijunction Solar Cells.

Multijunction solar cells provide a path to overcome the efficiency limits of standard silicon solar cells by harvesting a broader range of the solar spectrum more efficiently. However, Si-based multijunction architectures are hindered by incomplete harvesting in the near-infrared (near-IR) spectral range as Si subcells have weak absorption close to the band gap. Here, we introduce an integrated near-field/far-field light trapping scheme to enhance the efficiency of silicon-based multijunction solar cells in the near-IR range. To achieve this, we design a nanopatterned diffractive silver back-reflector featuring a scattering matrix that optimizes trapping of multiply scattered light into a range of diffraction angles. We minimize reflection to the zeroth order and parasitic plasmonic absorption in silver by engineering destructive interference in the patterned back-contact. Numerical and experimental assessment of the optimal design on the performance of single-junction Si TOPCon solar cells highlights an improved external quantum efficiency over a planar back-reflector (+1.52 mA/cm2). Nanopatterned metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via substrate conformal imprint lithography and characterized optically and electronically, demonstrating a power conversion efficiency improvement of +0.9%abs over the planar reference. Overall, our work demonstrates the potential of nanophotonic light trapping for enhancing the efficiency of silicon-based multijunction solar cells, paving the way for more efficient and sustainable solar energy technologies.

Machine learning study on organic solar cells and virtual screening of designed non-fullerene acceptors

Machine learning (ML) is effective to establish the complicated trilateral relationship among structures, properties, and photovoltaic performance, which is fundamental issue in developing novel materials for improving power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, we constructed the database of 397 donor–acceptor pairs of OSCs with photovoltaic parameters and descriptor sets, which include donor–acceptor weight ratio within the active layer of the OSCs, root mean square of roughness, and 1024-bit Morgan molecular fingerprint for donor (Fp-D) and acceptor (Fp-A). The ML models random forest (RF), adaptive boosting (AdaBoost), extra trees regression, and gradient boosting regression trees were trained based on the descriptor set. The metrics determination coefficient (R2), Pearson correlation coefficient (r), root mean square error, and mean absolute error were selected to evaluate ML model performances. The results showed that the RF model exhibits the highest accuracy and stability for PCE prediction among these four ML models. Moreover, based on the decomposition of non-fullerene acceptors L8-BO, BTP-ec9, AQx-2, and IEICO, 20 acceptor molecules with symmetric A–D–A and A–π–D–π–A architectures were designed. The photovoltaic parameters of the designed acceptors were predicted using the trained RF model, and the virtual screening of designed acceptors was conducted based on the predicted PCE. The results indicate that six designed acceptors can reach the predicted PCE higher than 12% when P3HT was adopted as a donor. While PM6 was applied as a donor, five designed acceptors can achieve the predicted PCE higher than 16%.

Heterojunction reconstruction via In doping towards high-efficiency CZTSSe solar cells

The Cu2ZnSn(S,Se)4 solar cells (CZTSSe) are emerging as a hot area of low-cost thin film photovoltaic technology owing to its non-toxicity and excellent photoelectric properties. However, the large open-circuit voltage deficit (Voc,deficit) caused by the inherently defective CdS/ CZTSSe heterojunction interface is still a huge challenge for improving power conversion efficiency. Herein, we reconstruct the heterojunction interface by inserting a CZTSSe:In into the CdS/CZTSSe heterojunction through an In2Se3 post-deposition treatment. The local occupation of In3+ at Zn2+ and Sn4+ sites decreases the disordered Zn- and Sn-related deep defects and generates benign InZn donors within CZITSSe transition layer simultaneously, which effectively enhances the surface p-to-n conversion. The combination of chemical and field-effect passivation regulates the charge extraction and passivates the charge recombination. As a result, the In element-buried absorber enables the Voc boost from 488 to 527 mV with an impressive active-area efficiency of 13.59 %. These results demonstrate the great potential of the In element-buried absorber in alleviating the Voc,deficit and enhancing the performance of CZTSSe solar cells.

Dearomative Ring‐Fused Azafulleroids and Carbazole‐Derived Metallofullerenes: Reactivity Dictated by Encapsulation in a Fullerene Cage

AbstractHerein, we report divergent additions of 2,2′‐diazidobiphenyls to C60 and Sc3N@Ih‐C80. In stark contrast to that of the previously reported bis‐azide additions, the unexpected cascade reaction leads to the dearomative formation of azafulleroids 2 fused with a 7‐6‐5‐membered ring system in the case of C60. In contrast, the corresponding reaction with Sc3N@Ih‐C80 switches to the C−H insertion pathway, thereby resulting in multiple isomers, including a carbazole‐derived [6,6]‐azametallofulleroid 3 and a [5,6]‐azametallofulleroid 4 and an unusual 1,2,3,6‐tetrahydropyrrolo[3,2‐c]carbazole‐derived metallofullerene 5, whose molecular structures have been unambiguously determined by single‐crystal X‐ray diffraction analyses. Among them, the addition type of 5 is observed for the first time in all reported additions of azides to fullerenes. Furthermore, unexpected isomerizations from 3 to 5 and from 4 to 5 have been discovered, providing the first examples of the isomerization of an azafulleroid to a carbazole‐derived fullerene rather than an aziridinofullerene. In particular, the isomerism of the [5,6]‐isomer 4 to the [5,6]‐isomer 5 is unprecedented in fullerene chemistry, contradicting the present understanding that isomerization generally occurs between [5,6]‐ and [6,6]‐isomers. Control experiments have been carried out to rationalize the reaction mechanism. Furthermore, representative azafulleroids have been applied in organic solar cells, thereby resulting in improved power conversion efficiencies.

Dearomative Ring-Fused Azafulleroids and Carbazole-Derived Metallofullerenes: Reactivity Dictated by Encapsulation in a Fullerene Cage.

Herein, we report divergent additions of 2,2'-diazidobiphenyls toward C60 and Sc3N@Ih-C80. In stark contrast to that of the previously reported bis-azide additions, the unexpected reactivity leads to the cascade dearomative formation of unprecedented azafulleroids 2 fused with a 7-6-5-membered ring system in the case of C60. In contrast, the corresponding reaction with Sc3N@Ih-C80 switches to the cascade C-H insertion pathway, resulting in multiple isomers, including a carbazole-derived [6,6]-azametallofulleroid 3 and a [5,6]-azametallofulleroid 4 and an unusual 1,2,3,6-tetrahydropyrrolo[3,2-c]carbazole-derived metallofullerene 5, whose molecular structures have been unambiguously determined by single crystal X-ray diffraction analyses. Among them, the addition type of 5 is observed for the first time in all reported additions of azides to fullerenes. Furthermore, unexpected isomerizations from 3 to 5 and from 4 to 5 have been discovered, representing the first example of isomerization from an azafulleroid to a carbazole-derived fullerene rather than an aziridinofullerene. In particular, the isomerism from [5,6]-isomer 4 to [5,6]-isomer 5 is unprecedented in fullerene chemistry, contradicting the present understanding that isomerization generally occurs between [5,6]- and [6,6]-isomers. Control experiments have been carried out to rationalize the reaction mechanism. Furthermore, representative azafulleroids have been applied in organic solar cells, resulting in improved power conversion efficiency.