Chen Weijie

2D/3D/2D-structured perovskite films are constructed by depositing phenethylammonium bromide (PEABr) on top and 2D perovskite PEA₂FA₂Pb₃Br₁₀ at the bottom of 3D perovskite. Finally, a power conversion efficiency of 24.2% is achieved, and the encapsulated device maintains 93.5% of its initial efficiency under 1 sun equivalent illumination of 1000 h in air with a humidity of 50 ± 5%.

Abstract

2D/3D-structured perovskites are promising for high-performance and stable solar cells due to the passivation ability and hydrophobic character of the large organic cations. However, most of the 2D/3D-structured perovskites prepared by adding ammonium halides into the precursors or depositing ammonium halides on top of a 3D perovskite film ignore the buried interface which is crucial for the device efficiency and stability. Here, 2D/3D/2D-structured perovskite films with optimized buried interface are constructed by depositing phenethylammonium bromide (PEABr) on top and 2D perovskite PEA₂FA₂Pb₃Br₁₀ at the bottom of 3D Cs_0.07FA_0.85MA_0.08Pb(I_0.95Br_0.05)₃ perovskite. Thus, a power conversion efficiency (PCE) of 24.2% is achieved due to the minimal nonradiative recombination. The encapsulated device also shows an improved stability, maintaining 93.5% of the initial value after 1000 h under 1 sun equivalent illumination at the maximum power point. This strategy may provide important insights into designing perovskite film structures to achieve high-performance solar cells with high stability.

For the first time, multiple non-covalent interactions between polyvinyl chloride and the conjugated polymer PM6 were discovered, resulting in excellent mechanical properties in PM6/polyvinyl chloride thin films and high thermal/bending stability in organic solar cells.

Abstract

The incorporation of insulating polymers into conjugated polymers has been widely explored as a strategy to improve mechanical properties of flexible organic electronics. However, phase separation due to the immiscibility of these polymers has limited their effectiveness. In this study, we report the discovery of multiple non-covalent interactions that enhances the miscibility between insulating and conjugated polymers, resulting in improved mechanical properties. Specifically, we have added polyvinyl chloride (PVC) into the conjugated polymer PM6 and observed a significant increase in solution viscosity, indicative of favorable miscibility between these two polymers. This phenomenon has been rarely observed in other insulating/conjugated polymer composites. Thin films of PM6/PVC exhibit a much-improved crack-onset strain of 19.35 %, compared to 10.12 % for pristine PM6 films. Analysis reveal that a “cyclohexyl-like” structure formed through dipole-dipole interactions and hydrogen bonding between PVC and PM6 acted as a cross-linking site in the thin films, leading to improved mechanical properties. Moreover, PM6/PVC blend films have demonstrated excellent thermal and bending stability when applied as an electron donor in organic solar cells. These findings provide new insights into non-covalent interactions that can be utilized to enhance the properties of conjugated polymers and may have potential applications in flexible organic electronics.

Complete peripheral fluorination was performed on a two-dimensional conjugation extended molecular platform of CH-series small molecular acceptors (SMAs), rendering the acceptor CH8F with eight fluorine atoms on the molecular backbone. More importantly, the highest efficiency of 19.28 % for CH-series SMAs-based organic solar cells has been achieved so far.

Abstract

Due to the intrinsically flexible molecular skeletons and loose aggregations, organic semiconductors, like small molecular acceptors (SMAs) in organic solar cells (OSCs), greatly suffer from larger structural/packing disorders and weaker intermolecular interactions comparing to their inorganic counterparts, further leading to hindered exciton diffusion/dissociation and charge carrier migration in resulting OSCs. To overcome this challenge, complete peripheral fluorination was performed on basis of a two-dimensional (2D) conjugation extended molecular platform of CH-series SMAs, rendering an acceptor of CH8F with eight fluorine atoms surrounding the molecular backbone. Benefitting from the broad 2D backbone, more importantly, strengthened fluorine-induced secondary interactions, CH8F and its D18 blends afford much enhanced and more ordered molecular packings accompanying with enlarged dielectric constants, reduced exciton binding energies and more obvious fibrillary networks comparing to CH6F controls. Consequently, D18:CH8F-based OSCs reached an excellent efficiency of 18.80 %, much better than that of 17.91 % for CH6F-based ones. More excitingly, by employing D18-Cl that possesses a highly similar structure to D18 as a third component, the highest efficiency of 19.28 % for CH-series SMAs-based OSCs has been achieved so far. Our work demonstrates the dramatical structural multiformity of CH-series SMAs, meanwhile, their high potential for constructing record-breaking OSCs through peripheral fine-tuning.

Stereoisomerism plays an important role in regulating molecular interaction, aggregation structures, and photovoltaic performance. Racemic mixtures made of equal molar ratio of enantiomerically pure (S,S)- and (R,R)-BTP-4F exhibit a higher power conversion efficiency as compared to individual chiral molecules, benefiting from long polaron lifetime, fast hole transfer, and efficient charge transport.

Abstract

Chiral alkyl chains are ubiquitously observed in organic semiconductor materials and can regulate solution processability and active layer morphology, but the effect of stereoisomers on photovoltaic performance has rarely been investigated. For the racemic Y-type acceptors widely used in organic solar cells, it remains unknown if the individual chiral molecules separate into the conglomerate phase or if racemic phase prevails. Here, the photovoltaic performance of enantiomerically pure Y6 derivatives, (S,S)/(R,R)-BTP-4F, and their chiral mixtures are compared. It is found that (S,S) and (R,R)-BTP-4F molecule in the racemic mixtures tends to interact with its enantiomer. The racemic mixtures enable efficient light harvesting, fast hole transfer, and long polaron lifetime, which is conducive to charge generation and suppresses the recombination losses. Moreover, abundant charge diffusion pathways provided by the racemate contribute to efficient charge transport. As a result, the racemate system maximizes the power output and minimizes losses, leading to a higher efficiency of 18.16% and a reduced energy loss of 0.549 eV, as compared to the enantiomerically pure molecules. This study demonstrates that the chirality of non-fullerene acceptors should receive more attention and be designed rationally to enhance the efficiency of organic solar cells.

Double-side passivation with phenyltrimethylammonium iodide (PTMAI) improves the performance and stability of PSCs. PTMAI influences the perovskite nanostructures, effectively reducing various defective states. As a result, solar cells with double-side passivation achieve a high power conversion efficiency of 21.87%, withstanding temperatures as high as 60 °C and continuous exposure to 1 sun for over 1800 and 1000 h, respectively.

Abstract

Perovskite solar cells (PSCs) are in the spotlight as promising renewable energy devices by their appealing properties. However, they face challenges both in power conversion efficiency (PCE) and long-term stability. The presence of surface defects in the PSCs is a significant obstacle to achieving both high efficiency and stability, as these defects cause nonradiative recombination and degradation. Herein, a novel double-side surface passivation method using phenyltrimethylammonium iodide (PTMAI) salt is applied to remove electronic defects effectively. Furthermore, double-side passivation with PTMAI contributes to the enhancement of the perovskite crystallinity with the relaxed non-uniform distribution of local strains. Finally, the efficiency of PSC is significantly improved by double-side passivation with PTMAI, achieving a PCE of 21.87%. Furthermore, the passivated cell exhibited enhanced long-term stability, maintaining over 80% of initial PCE after 1860 and 1030 h under 60 °C and 1 sun illumination, respectively.

Two structurally similar analogues of ADT and NDT are developed as solid additive to exploit their effect in regulating the molecular aggregation and π-stacking of photoactive layer for organic solar cells. It is found that the molecular packing behaviors and volatilization of solid additive show significant impact in regulating the morphological properties of photoactive layer, and thus photovoltaic performance.

Abstract

Regulating molecular packing and aggregation of photoactive layer is a critical but challenging issue in developing high-performance organic solar cells. Herein, two structurally similar analogues of anthra[2,3-b : 6,7-b′]dithiophene (ADT) and naphtho[1,2-b : 5,6-b′]dithiophene (NDT) are developed as solid additive to exploit their effect in regulating the molecular aggregation and π-stacking of photoactive layer. We clarify that the perpendicular arrangements of NDT can enlarge the molecular packing space and improve the face-on stacking of Y6 during the film formation, favoring a more compact and ordered long-range π-π stacking in the out-of-plane direction after the removal of NDT under thermal annealing. The edge-to-face stacked herringbone-arrangement of ADT along with its non-volatilization under thermal annealing can induce the coexistence of face-on and edge-on stacking of blend film. As a result, the NDT treatment shows encouraging effect in improving the photovoltaic performance of devices based on various systems. Particularly, a remarkable PCE of 18.85 % is achieved in the PM6 : L8-BO-based device treated by NDT additive, which is a significant improvement with regard to the PCE of 16.41 % for the control device. This work offers a promising strategy to regulate the molecular packing and aggregation of photoactive layer towards significantly improved performance and stability of organic solar cells.

Publication date: 1 December 2023

Source: Nano Energy, Volume 117

Author(s): Hui Chen, Jiabao Yang, Qi Cao, Tong Wang, Xingyu Pu, Xilai He, Xingyuan Chen, Xuanhua Li

Nature, Published online: 11 September 2023; doi:10.1038/s41586-023-06610-7

The enhanced interaction between 4′-cyanogroup-[1,1′-biphenyl]−4-acyl acrylate and PbI₂ during annealing leads to the formation of a new intermediate complex, which not only induces diffusion-controlled growth but also increases diffusion activation energy, leading to delayed crystallization. Therefore, the optimized device has high efficiency (24.53%) and excellent stability (maintaining 89% of the initial efficiency after 6600 h aging in ambient).

Abstract

Solution crystallization in film devices has attracted broad interest from various fields such as perovskite solar cells. However, the detailed perovskite crystallization kinetics remain unclear due to the difficulty of in situ observation of grain cluster growth during annealing. This article presents the development of an in situ laser scanning confocal polarized microscopy with a temperature-controlled stage to observe nucleation and growth of perovskite crystal clusters. It is found that enhanced interactions by a liquid crystal with perovskite form a new intermediate complex that induces diffusion-controlled growth according to Avrami equation. The retarded cluster growth (63 nm s⁻¹) originates from enlarged diffusion activation energy 40 kJ mol⁻¹ compared with 152 nm s⁻¹ and 37 kJ mol⁻¹ for the Control film during annealing. Finally, the optimized perovskite films with enhanced crystallographic and optical characteristics are applied in solar cells to achieve a champion efficiency of 24.53% with open circuit voltage of 1.172 V and fill factor of 82.78%. The bare device without any protection maintains 89% of its initial efficiency after 6600 h of aging in ambient environment. This work implies that the in situ observation using fluorescence microscopy is a critical for understanding of crystallization kinetics in film devices.

Lewis-base thioacetamide (TAA) is added to the simple FASnI₃ perovskite solution to produce high-quality Pb-free perovskites for high-performance solar cells with effectively controlled perovskite film crystallinity and grain size.

Perovskite solar cells (PSCs) have made great strides in recent years, operating inside the theoretical Scholkely–Quisser efficiency limit with a certified power conversion efficiency (PCE) of 26.1%. However, lead toxicity from Pb-based PSCs can harm the environment. As a result, the search for nontoxic and environmentally friendly substances to replace Pb in perovskites is the need of the hour. Tin has emerged as the most viable choice to replace Pb, due to its favorable electronic properties and smaller bandgaps of Sn-based perovskites between 1.1 and 1.4 eV, strong charge carrier mobility, and high theoretical efficiency of 32%. Sn vacancies and point defects, on the other hand, are easily produced in Sn perovskites, leading to nonradiative recombination. Furthermore, interfacial flaws and traps impede further performance improvement. In this research, to produce high-quality Pb-free perovskites for high-performance PSCs, a Lewis-base thioacetamide (TAA) is added to the simple FASnI₃ perovskite solution. FASnI₃ and TAA additive-based films effectively control perovskite film crystallinity and grain size via Lewis acid–base reaction. The champion FASnI₃ + TAA-based PSC achieves a maximum PCE of 10.67% while paving a facile way for other compositional perovskite analogues to be integrated into highly efficient and operationally stable PSCs.

For the first time, the successful mitigation of potential-induced degradation in glass-encapsulated perovskite solar cells is reported. The modified devices are able to withstand a high electric field for 96 h due to the suppression of Na⁺ ions by a NiO _x blocking layer. The underlying mitigation mechanism will possibly lead a pathway toward successful commercialization of perovskite solar cells.

While the efficiency of perovskite solar cells has been increasing rapidly in recent years, studies on potential-induced degradation (PID) in these devices have so far been very limited. Herein, the first successful mitigation of PID in glass-encapsulated perovskite solar cells is reported. A thin NiO _x blocking layer between the indium tin oxide and the self-assembled monolayer is proposed to suppress the PID in these solar cells. Two groups of devices are fabricated, with and without the presence of the NiO _x layer. When −1000 V is applied between the short-circuited solar cell and the front glass pane, the original devices without NiO _x only retain about 27% of the initial efficiency, and the modified devices with NiO _x retained ≈65% after an extended stress duration of 96 h. The Na⁺ ions from the glass pane occupy NiO _x vacancies under high voltage stress and, as a result, the Na _x ion migration toward the perovskite layer is heavily suppressed due to the introduction of NiO _x . The modified devices also demonstrate good long-term stability, retaining more than 80% of initial efficiency after the PID stress test. The results of this work are helpful on the journey toward successful commercialization of perovskite solar cells.

Perovskite–organic tandem solar cells with minimized open-circuit voltage loss and suppressed phase segregation for wide-bandgap perovskite front cells are achieved. A buried interfacial dielectric layer structure and a simplified metal-free interconnecting layer of ambipolar SnO _x contributed to the reduced interfacial defect states and an efficiency of 22.31% for the tandem solar cells.

Abstract

The wide-bandgap (WBG) perovskite solar cells (PSCs) and narrow-bandgap organic solar cells (OSCs) integrated tandem solar cells (TSCs) show great potential for overwhelming single junction structure, especially the advantage of applying orthogonal solvents for allowing solution processed of each subcell. However, the WBG perovskite with high Br content suffers from serious phase segregation and voltage loss. The commonly used interconnection layer (ICL) in TSCs requires a vacuum-deposited thin metal recombination layer leading to remarkable optical loss. Herein, WBG perovskite with a bandgap of 1.77 eV yields an impressive open-circuit voltage (V _OC) of 1.33 V and a minimum voltage loss of 0.44 V by an elaborate dielectric interface structure reducing the interfacial recombination. Furthermore, the WBG perovskite with a simple SnO _X buffer layer exhibits significantly suppressed phase segregation and improved performance. Consequently, a simplified buffer layer based on the SnO _X that serves as the ICL in perovskite–organic TSCs contributing enhanced light harvesting in the near-infrared region is developed, yielding an efficiency of 22.31%. The simplified ICL that does not involve a metal layer is a potential strategy for scalable and flexible perovskite-based TSCs.

In this work, 2-(2-aminoethyl)–2-thiopseudourea dihydrobromide (AET) is introduced into the tin (Sn)-based perovskite to suppress its oxidation process from Sn²⁺ to Sn⁴⁺ and alleviate its crystallization dynamics. As a result, an impressive fill factor of 80.26% is achieved for the Sn-based perovskite solar cells (TPSCs) with AET. Moreover, encapsulated TPSCs maintain 80% of their initial performance over 1 month.

Abstract

The low toxicity and a near-ideal bandgap endow tin (Sn)-based perovskite solar cells (TPSCs) a promising future in photovoltaic technologies. However, in general the Sn-based perovskites suffer from poor resistance to oxidation and fast crystallization dynamics, resulting in low fill factors (FFs) and power conversion efficiencies (PCEs). Herein, a strategy to improve the performance of TPSCs by incorporating 2-(2-aminoethyl)–2-thiopseudourea dihydrobromide (AET) into the Sn-based perovskites is proposed. AET interacts strongly with the Sn-based perovskites, and thus effectively inhibits the oxidation from Sn²⁺ to Sn⁴⁺ and slows down the crystallization process. As a result, the TPSCs with AET achieve a high FF of 80.26% and a high PCE of 13.55%. Notably, this FF value represents the highest reported in the field of TPSCs with narrow bandgaps. Furthermore, encapsulated TPSCs demonstrate good stability, maintaining 80% of their initial performance over the course of 700 h.

An aqueous solution of KBF₄ is doped into poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), which effectively enhances the conductivity of PEDOT:PSS and reduces the ratio of PSS to PEDOT. The grain size of perovskite increases significantly, and the interfacial defects are obviously reduced. Meanwhile, the power coversion efficiency increases from 16.75% to 19.76%, and the long-term stability is more than 1,000 h.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is widely used as a hole transport layer in inverted perovskite solar cells (PSCs) due to its simple fabrication process and high stability. However, there are severe interface defects between PEDOT:PSS and the perovskite layer, leading to poor power conversion efficiency (PCE) of the inverted devices. Herein, a hole transport layer (PEDOT:PSS) doped with potassium fluoroborate is introduced. The aqueous solution of KBF₄ diluted with PEDOT:PSS is cleverly utilized to compare with PEDOT:PSS diluted with deionized aqueous solution. X-ray photoelectron spectroscopy of PEDOT:PSS confirms the reduction of PSS chains and an increase in conductivity after KBF₄ doping. Moreover, KBF₄ doping promotes crystal growth, resulting in larger grain size of the perovskite film. Additionally, the defects at the PEDOT:PSS/perovskite interface are effectively passivated, suppressing nonradiative recombination. The results show improved short-circuit current density, open-circuit voltage, and fill factor, resulting in a PCE of 19.76%, which is a 17.9% enhancement compared to the original device's 16.75%. The optimized device also exhibits long-term stability exceeding 1000 h, providing a simple and effective strategy for improving the PCE and stability of inverted PSCs.

A comprehensive approach for optimizing the crystallization kinetics of mixed halides in wide-bandgap perovskite and tandem photovoltaics is introduced, allowing wide-bandgap perovskite solar cells to achieve some of the highest efficiencies recorded. This research lays the groundwork for tackling phase separation and inhomogeneous crystallization in bromine-rich perovskite components, ultimately facilitating the advancement of high-performance perovskite-based wide-bandgap and tandem photovoltaics.

Abstract

Wide-bandgap (WBG) perovskites have attracted considerable attention due to their adjustable bandgap properties, making them ideal candidates for top subcells in tandem solar cells (TSCs). However, WBG perovskites often face challenges such as inhomogeneous crystallization and severe nonradiative recombination loss, leading to high open-circuit voltage (V _OC) deficits and poor stability. To address these issues, a multifunctional phenylethylammonium acetate (PEAAc) additive that enhances uniform halide phase distribution and reduces defect density in perovskite films by regulating the mixed-halide crystallization rate, is introduced. This approach successfully develops efficient WBG perovskite solar cells (PSCs) with reduced V _OC loss and enhanced stability. By applying this universal strategy to the FAMACsPb(I_{1− x} Br _x )₃ system with a range of bandgaps of 1.73, 1.79, 1.85, and 1.92 eV, power conversion efficiencies (PCE) of 21.3%, 19.5%, 18.1%, and 16.2%, respectively, are attained. These results represent some of the highest PCEs reported for the corresponding bandgaps. Furthermore, integrating WBG perovskite with organic photovoltaics, an impressive PCE of over 24% for two-terminal perovskite/organic TSCs, with a record V _OC of ≈ 2.2 V is achieved. This work establishes a foundation for addressing phase separation and inhomogeneous crystallization in Br-rich perovskite components, paving the way for the development of high-performance WBG PSCs and TSCs.

A delicate selection of spacer cations and polarity-matched molecular additives is presented to manipulate the 2D perovskite lattice, improving related texture and stability. The optimized FA-based 2D perovskite photovoltaic devices achieve a remarkable power conversion efficiency of 20.03% (certified efficiency of 19.30%). Furthermore, the devices exhibit less than 1% efficiency degradation after operation at maximum power point for 1000 h.

Abstract

Formamidinium (FA)-based 2D perovskites have emerged as highly promising candidates in solar cells. However, the insertion of 2D spacer cations into the perovskite lattice concomitantly introduces microstrain and unfavorable orientations that hinder efficiency and stability. In this study, by finely tuning the FA-based 2D perovskite lattice through spacer cation engineering, a stable lattice structure with balanced distortion, microstrain relaxation, and reduced carrier–lattice interactions is achieved. These advancements effectively stabilize the inherently soft lattice against light and thermal-aging stress. To reduce the photocurrent loss induced by undesired crystal texture, a polarity-matched molecular-type selenourea (SENA) additive is further employed to modulate the crystallization kinetics. The introduction of the SENA significantly inhibits the disordered crystallization induced by spacer cations and drives the templated growth of the quantum well structure with a vertical orientation. This controlled crystallization process effectively reduces crystal defects and enhances charge separation. Ultimately, the optimized FA-based perovskite photovoltaic devices achieve a remarkable power conversion efficiency (PCE) of 20.03% (certified steady-state efficiency of 19.30%), setting a new record for low-n 2D perovskite solar cells. Furthermore, the devices exhibit less than 1% efficiency degradation after operating at maximum power point for 1000 h and maintain excellent stability after thermal aging and cycles of cold–warm shock, respectively.

Publication date: 20 September 2023

Source: Joule, Volume 7, Issue 9

Author(s): Zhaojian Xu, Helen Bristow, Maxime Babics, Badri Vishal, Erkan Aydin, Randi Azmi, Esma Ugur, Bumin K. Yildirim, Jiang Liu, Ross A. Kerner, Stefaan De Wolf, Barry P. Rand

The interface issue between the carbon electrode and perovskite absorber is considered as the main barrier preventing the development of hole transport material-free carbon-based perovskite solar cells. An interlayer comprised of nano-sized perovskite quantum dots with surface capping ligands can be employed as hole extractor, defect passivator, and morphology changer, concurrently, for achieving high-performance photovoltaic devices.

The realization of improved charge transport with suppressed recombination at the interface of perovskite and carbon electrode is the main key for remarkably increasing the power conversion efficiency of carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs). Herein, a strategy that builds a perovskite quantum dot (PQD) interlayer is demonstrated, for the first time, to bridge the perovskite absorber and carbon electrode for solving the interface issue in HTM-free PSCs. It is found that the introduced PQD interlayer concurrently functions as a morphology changer, a defect passivator, and a photogenerated hole extractor. Compared with the pristine perovskite film, the PQD-modified perovskite absorber shows increased contact area and high compatibility with carbon electrode, prolonged carrier lifetime, deduced defect density as well as suppressed recombination. These positive effects, combined with a heterostructure created by perovskite bulks and PQDs facilitating hole transport at the interface, enable an improvement in device efficiency from 16.71% to 17.93%.

Herein, improved printable mesoscopic perovskite solar cells (p-MPSCs) use cross-interface bidentate passivation molecules with alternating sulfur atom distributions to bridge crystal boundaries and improving intercrystalline charge transport. Dual-sided four-terminal tandem assembly with an artificial reflector device and the passivating agent significantly improve power conversion efficiency and open-circuit voltage in series-connected devices, addressing interfacial passivation and layered assembly challenges in p-MPSCs.

For printable mesoscopic perovskite solar cells (p-MPSCs), the formation of numerous grain boundaries due to the microstructural confinement of mesoporous materials is a key factor that leads to concentrated defects and limits intercrystalline charge transport. Herein, the electron-donating and electron-withdrawing effects of atoms are harnessed, respectively, and a unique molecular configuration with alternating distributions of sulfur atoms in thieno[3,2-b]thiophene to construct a cross-interface bidentate passivation molecule, 2,5-dibromothiophene [3,2-b] thiophene (TT-2Br), is utilized. By exploiting its lattice's selective matching, efficient chelation with undercoordinated lead ions, bridging crystal boundaries, is achieved. Furthermore, to overcome the limitation of nonlayered assembly caused by the carbon electrode's full light absorption in p-MPSCs, an artificial reflector device to achieve dual-sided four-terminal tandem assembly with TT-2Br serving as the small-molecule interfacial passivating agent is used. This approach significantly improves the bifacial power output (BPO) to 20.44 mW cm⁻² and the open-circuit voltage to 2.03 V in series-connected devices. In parallel-connected devices, the BPO increases to 25.84 mW cm⁻², and the short-circuit current reaches 39.96 mA cm⁻². This innovative strategy effectively addresses the challenges of interfacial passivation and layered assembly in p-MPSCs.

Herein, p-toluene sulfonate effectively releases the residual strain in wide-bandgap perovskite films, as well as improving the crystallinity and passivating the defect states, which are observed to be highly effective in inhibiting phase segregation in perovskite films. The improved power conversion efficiency of solar cell is accomplished with the lowest open-circuit voltage deficit (410 mV).

Mixed-halide perovskites with wide-bandgap (WBG) have been widely applied for tandem solar cells. Nevertheless, WBG perovskite solar cells (PSCs) easily suffer from photo-induced phase segregation, deteriorating their device efficiency and stability. It is a desirable way that the suppression of phase segregation from the strain relaxation in mixed-halide WBG perovskite films. Herein, p-toluene sulfonate (p-TS) can effectively release the residual strain in WBG perovskite films by occupying the empty orbitals of the uncoordinated Pb²⁺ and halide ion vacancies, as well as improving the crystallinity and passivating the defect states. The outcomes of this strain relaxation are observed to be highly effective in inhibiting phase segregation in WBG perovskite films. Based on the WBG perovskite with p-TS, the significantly improved power conversion efficiency (PCE) of 21.12% is accomplished with the lowest open-circuit voltage deficits (410 mV) for the n-i-p PSCs with an optical bandgap of 1.66 eV. And satisfactory stability of PSC has been gained; the encapsulated device maintains efficiency over 82% of the original efficiency after 1000 h continuous illumination (1-Sun, ≈40 °C). Besides, the p-TS PSC maintains 98.9% of the initial PCE after 4000 h storage in the dry condition (≈10%–20% relative humidity, ≈30 °C).

The researchers introduce protic amine carboxylic acid ion liquids (PACA-ILs) into the perovskite precursor to regulate the phase transition and crystal growth processes. Preferentially oriented perovskite grains can be obtained by inhibiting the formation of MA₂Pb₃I₈·2DMSO phases. The optimized devices show high efficiency over 24% on small-area for all ILs and 21.26% on large-area module for methylammonium butyrate ILs.

Abstract

The α-phase formamidinium lead tri-iodide (α-FAPbI₃) has become the most promising photovoltaic absorber for perovskite solar cells (PSCs) due to its outstanding semiconductor properties and astonishing high efficiency. However, the incomplete crystallization and phase transition of α-FAPbI₃ substantially undermine the performance and stability of PSCs. In this work, a series of the protic amine carboxylic acid ion liquids are introduced as the precursor additives to efficiently regulate the crystal growth and phase transition processes of α-FAPbI₃. The MA₂Pb₃I₈·2DMSO phase is inhibited in annealing process, which remarkably optimizes the phase transition process of α-FAPbI₃. It is noted that the functional groups of carboxyl and ammonium passivate the undercoordinated lead ions, halide vacancies, and organic vacancies, eliminating the deleterious nonradiative recombination. Consequently, the small-area devices incorporated with 2% methylammonium butyrate (MAB) and 1.5% n-butylammonium formate (BAFa) in perovskite show champion efficiencies of 25.10% and 24.52%, respectively. Furthermore, the large-area modules (5 cm × 5 cm) achieve PCEs of 21.26% and 19.27% for MAB and BAFa additives, indicating the great potential for commercializing large-area PSCs.

The functionality and molecular size of the guanidium cations play an important role in forming the porous PbI₂ layers and passivation of perovskite films in a two-step process, where Lewis base sites in the multi-amine functional group and a steric hindrance to the highly polarized imine group should be simultaneously considered for efficient and stable perovskite solar cells.

Abstract

In a two-step procedure for fabricating perovskite films, the PbI₂ layer formed on the substrate is converted to perovskite by reacting PbI₂ with organic iodide. Excess PbI₂ left after forming perovskite composition, however, might have an ill effect on device stability and current–voltage hysteresis, although it positively affects efficiency improvement.  Additive engineering is reported here on to control the residual PbI₂ in a two-step procedure. A series of organic multi-ammonium chloride derivatives are introduced into the PbI₂ precursor solution for the first-step coating, which results in an increase in the perovskite grain size. In addition, carrier lifetime is elongated due to the reduced trap density and the energetics are adjusted to facilitate the extraction of photogenerated carriers. The aminoguanidinium-containing precursor leads to an improved power conversion efficiency (PCE) as compared to the bare PbI₂ precursor mainly due to the significantly enhanced open-circuit voltage and fill factor. Consequently, a PCE of 23.46% is achieved from the hysteresis-less photovoltaic parameters and 93% of the initial PCE is maintained after aging for 1000 h in ambient conditions.

The volatile halogenated solid additives can interact with nonfullerene acceptors to form halogen bonds, enhancing the molecular stacking of acceptors and leading to proper donor–acceptor phase segregations. A champion power conversion efficiency of 17.9% in organic solar cells comprising PM6:Y6 with improved devices’ stability and reproducibility is demonstrated. Additionally, the additives exhibit remarkable capability with acceptors as Y6 derivatives.

Abstract

The halogenated volatile solid additives can delicately optimize the active layer morphology of organic solar cells, improving the devices' performance, stability, and reproducibility. However, what type of intermolecular interaction occurs between the solid additives and the active layer and whether the interaction truly impacts the donor or acceptor remains debatable. Herein, the focus is on halogenated volatile solid additives with conjugated benzene rings and their influence on the morphology of the active layer composed of PM6:Y6 as they evaporated. The absorbance spectra exhibit apparent red-shift features in Y6 absorption regions, while the donor part is unaffected. The theoretical calculation results reveal that the additives can stay between two Y6 molecules and form halogen bonds, affecting the π–π aggregation properties of Y6. As a result, the crystalline features of the active layer are altered, leading to increased charge carrier mobilities, extended charge carrier diffusion lengths, reduced bimolecular charge recombination, and thus the device performance. Especially when 1,3,5-tri bromobenzene is used, a champion power conversion efficiency of 17.9% is attained, among the best-performed organic solar cells comprising PM6:Y6. The findings shed light on theoretical and experimental guidelines for designing and developing volatile solid additives for highly efficient nonfullerene organic solar cells.

Nature Communications, Published online: 06 September 2023; doi:10.1038/s41467-023-41263-0

Nature Energy, Published online: 06 September 2023; doi:10.1038/s41560-023-01313-9