Chen Weijie

The low-cost dopant-free hole-transporting materials (HTMS) based on imidazolium are synthesized and characterized for inverted perovskite solar cell (PSC) applications. The new HTMs can passivate defects and improve efficiency. As a result, DIM with double imidazolium units has an outstanding power conversion efficiency of 19.15%.

The hole-transporting material (HTM) in perovskite solar cells (PSCs) provides an ideal interface with the perovskite layer for hole collection while influencing the perovskite crystalline structure and morphology. This article presents SIM and DIM, two new dopant-free imidazolium-based HTMs with charged units accompanied by counter ions that show the potential to function as ionic HTMs and ionic liquid additives and help improve the efficiency and stability of inverted PSCs. DIM-based PSCs demonstrates exceptional performance with a high-power conversion efficiency of 19.15%, attributed to improved passivation of perovskite interface defects. Furthermore, these new imidazolium salt-based HTMs can be manufactured economically, making them a viable option for mass production and PSC commercialization.

The block copolymer acceptor with broad absorption, b-PYT, is synthesized by combining narrow- and medium-bandgap segments, BTTP-T and BTP-T. This design of b-PYT enables the corresponding photovoltaic devices to operate effectively under AM1.5G and white light-emitting diode illumination with enhanced thermal stability. This approach shows promise in developing high-performance materials for dual-function photovoltaic devices for efficient indoor and outdoor applications.

All-polymer photovoltaics (all-PPVs) that operate under both indoor and outdoor lighting conditions require active layers with appropriately adjusted optical-absorption ranges. However, the optical absorption of a conventional donor–acceptor binary blend is restricted to the combined absorption bands of its components. Herein, a new conjugated block copolymer (CBC) acceptor, b-PYT, is designed by integrating polymer acceptor blocks of wide and narrow bandgaps in a single structure. Such combination results in the wide absorption range (550–850 nm) of b-PYT that matches the emission of both artificial and solar light. The b-PYT CBC acceptor is more crystalline than the corresponding random terpolymer, r-PYT, owing to improved interactions between its macromolecular acceptor units. Despite exhibiting slightly inferior outdoor performance compared to that of devices using the homopolymer BTTP-T, the PM6:b-PYT-based devices deliver superior power conversion efficiency (PCE) under indoor light-emitting diode (LED) light owing to better matched absorption and emission spectra of b-PYT and a cold white LED, respectively. Additionally, it is worth highlighting that PM6:b-PYT-based all-PPVs can maintain approximately 87% of the initial PCE even after 600 min of thermal aging at 150 °C, which demonstrates the superior thermal stability compared with those of all-PPVs that use traditional binary active layers.

Multiscale simulations examined CsPbBr _x I_3−x perovskite materials’ microscale properties and their impact on device performance. By combining density functional theory (DFT) and finite element method, the study analyzed energy bands, density of states, optical properties, and defect concentrations’ influence on photovoltaic performance. These groundbreaking findings are crucial for designing stable, efficient all-inorganic perovskite solar cells (PSCs).

Organic-inorganic hybrid perovskite solar cells (PSCs) have swiftly emerged as a prominent contender in the photovoltaic industry, owing to their unparalleled optoelectronic capabilities. Nevertheless, the commercial viability of organic-inorganic hybrid PSCs is significantly hindered by their limited hygrothermal stability. Therefore, based on multiscale simulation technology, we systematically investigated the microscopic properties of CsPbBr _x I_3−x (0 ≤ x ≤ 3) perovskite materials and the corresponding photovoltaic device performance. Multiscale simulation technology is numerical simulation method that combines the density functional theory (DFT) with finite element method (FEM). DFT is used first to study the energy band structure, density of states, and optoelectronic parameters of CsPbBr _x I_3−x (0 ≤ x ≤ 3). Then, FEM is used to study the optical properties of all-inorganic PSCs based on the results of DFT simulation. Finally, the bulk defect concentration (N _t) of the perovskite material, and the defect concentration between the perovskite and the charge transport layer (CTL) on the photovoltaic performance of the device are calculated and analyzed using CsPbI₃ PSCs as an example. We finally designed the CsPbI₃ all-inorganic PSCs with a theoretical efficiency of 22.65%. Our theoretical simulation methods and corresponding results present a groundbreaking approach to crafting highly efficient and exceptionally stable all-inorganic PSCs.

Addition of phenyltrimethylammonium (PTMA) cations and smaller Br anions causes strain relaxation and changes in crystal orientation, causing the perovskite to have efficient charge transfer and less defective interfaces. The resultant solar cells demonstrate improved efficiency and thermal/light stabilities, with PTMA-bromide-alloyed devices surviving at both 60 °C and 1 sun for 1400 h or more.

As for the commonly used formamidinium lead iodide (FAPbI₃) perovskite, adding small amount of larger organic cations is a viable strategy to overcome its instability caused by formamidinium (FA) or iodide (I). Herein, FAPbI₃ perovskite is modified by addition of quaternary cation phenyltrimethylammonium (PTMA), which forms 2D perovskite phase. With the presence of PTMA cations and small-sized Br anions, the local strain of perovskite layer is mitigated. Also, X-ray analyses reveal that PTMA cation directs FAPbI₃ to have more vertically oriented structure, which aids the enhanced photocarrier generation and extraction. Defect analyses suggest that PTMA bromide alloying also greatly reduces the trap sites. Due to these combined effects, the PTMABr-modified device results in the improved open-circuit voltage plus stability under 60 °C or 1 sun illumination, maintaining 80% of its initial efficiency after 1800 and 1400 h, respectively.

Na₂SSeO₃ solutions using hydrothermal method may be the mixing solutions containing Na₂SSeO₃, Na₂S₂SeO₆, Na₂S₂Se₂O₆, and Na₂Se and can be applied as a cheap and high-activity Se source to prepare Sb₂S _y Se_3−y thin films by chemical bath deposition. When Sb₂S _y Se_3−y thin films with annealing are etched by N-butyldithiocarbamic acid solution in DMF, the corresponding solar cell achieves power conversion efficiency of 8.43%.

Herein, Na₂SSeO₃ solutions are prepared by hydrothermal method at 140 °C for 8 h using Se powder and Na₂SO₃ solution (the molar ratios of Se:Na₂SO₃ = 1:4, 1:5, 1:6) and the chemical composition of Na₂SSeO₃ solutions is analyzed by X-ray photoelectron spectroscopy. Sb₂S _y Se_3−y thin films are prepared by chemical bath deposition (CBD) at 90 °C for 5 h and subsequently annealed at 375 °C for 10 min and etched by N-butyldithiocarbamic acid solution in DMF. The effect of the Na₂SSeO₃ solution, growth solution volume, etching on the thickness and chemical composition of Sb₂S _y Se_3−y thin films, and the power conversion efficiency (PCE) of the corresponding solar cells is systematically investigated. The result reveals that the Na₂SSeO₃ solutions may be the mixing solutions containing Na₂SSeO₃, Na₂S₂SeO₆, Na₂S₂Se₂O₆, Na₂Se. When the molar ratio of Se:Na₂SO₃ and growth solution volume is 1:5 and 56 mL and the Sb₂S _y Se_3−y thin films with annealing are etched, the PCE of Sb₂S _y Se_3−y solar cells is 8.00% with spiro-OMeTAD and 8.43% with spiro-OMeTAD:TMT-TTF. The PCE of 8.43% is the highest value for Sb₂S _y Se_3−y solar cells using CBD and demonstrates that the composition engineering is an important strategy for improving the PCE of Sb₂S _y Se_3−y solar cells using CBD.

The mechanism of MACl-assisted crystallization on high-performance formamidine perovskite photovoltaics is not fully understood, especially the introduction of methylamine cations. The addition of MACl would cause the non-stoichiometric ratio in perovskite film, then the non-synchronous volatilization of methylamine cations and chloride ions would lead to the formation of halogen vacancy defects. The pseudo-halogen passivation method shows better performance, which emphasizes the improvement effect of this method on perovskite devices.

Abstract

Methylammonium chloride (MACl) additive is almost irreplaceable in high-performance formamidine perovskite photovoltaics. Nevertheless, Some of the problems that can arise from adding MACl are rarely mentioned. Herein, it is proposed for the first time that the addition of MACl would cause the non-stoichiometric ratio in the perovskite film, resulting in the halogen vacancy. It is demonstrated that the non-synchronous volatilization of methylamine cations and chloride ions leads to the formation of halogen vacancy defects. To solve this problem, the NH₄HCOO is introduced into the perovskite precursor solution to passivate the halogen vacancy. The HCOO⁻ ions have a strong force with lead ions and can fill the halogen vacancy defects. Consequently, the champion devices' power conversion efficiency (PCE) can be improved from 21.23% to 23.72% with negligible hysteresis. And the unencapsulated device can still retain >90% of the initial PCE even operating in N₂ atmosphere for over 1200 h. This work illustrates another halogen defect source in the MACl-assisted formamidine perovskite photovoltaics and provides a new route to obtain high-performance perovskite solar cells.

High-quality FA-based perovskite films with few lattice defects, free of strain and suppressed ion migration are successfully obtained by adding Cs⁺ and TOPO molecule simultaneously, resulting in a high-power conversion efficiency of 22.71% for solar cells with a significantly improved operational stability.

Abstract

Formamidine lead triiodide (FAPbI₃) perovskites have attracted increasing interest for photovoltaics attributed to the optimal bandgap, high thermal stability, and the record power conversion efficiency (PCE). However, the materials still face several key challenges, such as phase transition, lattice defects, and ion migration. Therefore, external ions (e.g., cesium ions (Cs⁺)) are usually introduced to promote the crystallization and enhance the phase stability. Nevertheless, the doping of Cs⁺ into the A-site easily leads to lattice compressive strain and the formation of pinholes. Herein, trioctylphosphine oxide (TOPO) is introduced into the precursor to provide tensile strain outside the perovskite lattice through intermolecular forces. The special strain compensation strategy further improves the crystallization of perovskite and inhibits the ion migration. Moreover, the TOPO molecule significantly passivates grain boundaries and undercoordinated Pb²⁺ defects via the forming of P═O─Pb bond. As a result, the target solar cell devices with the synergistic effect of Cs⁺ and TOPO additives have achieved a significantly improved PCE of 22.71% and a high open-circuit voltage of 1.16 V (voltage deficit of 0.36 V), with superior stability under light exposure, heat, or humidity conditions.

1,3-diaminopropane dihydroiodide (PDADI) is introduced to modify the perovskite/electron transporting layer (ETL) interface. Based on this strategy, the PDADI-modified p-i-n perovskite solar cells deliver an impressive efficiency of 25.09% (certified 24.58%) at the laboratory scale (0.071 cm²) with an open-circuit voltage of 1.184 V.

Abstract

At present, one of the major factors limiting the further improvement of inverted (p-i-n) perovskite solar cells (PSCs) is trap-assisted non-radiative recombination at the perovskite/electron transporting layer (ETL) interface. Surface passivation with organic ammonium salt is a powerful strategy to improve the performance of PSCs. Herein, an effective method by using propylamine hydroiodide (PAI) and 1,3-diaminopropane dihydroiodide (PDADI) is reported to modify the perovskite/ETL interface. These two ammonium salts do not form new perovskite but directly passivate the defects on the perovskite surface after annealing. The results show that the PDADI-modified perovskite films possess a lower surface defect density and less non-radiative recombination as well as improved charge carrier transport. Based on this strategy, the PDADI-modified p-i-n PSCs deliver an impressive efficiency of 25.09% (certified 24.58%) with an open-circuit voltage of 1.184 V. Furthermore, the unencapsulated PDADI-modified PSCs also exhibit good storage stability, retaining 91% of initial PCE at 65 °C in a N₂ glove box for 1300 h. This strategy provides an efficient route to fabricate highly efficient and stable inverted p-i-n structured PSCs.

The 4,5-Cl-2PACz self-assembled monolayer has an improved intrinsic photostability compared to that of 3,6-Cl-2PACz, supporting the stabilized vertical distribution of donor and acceptor components, reducing the energetic disorder, alleviating non-radiative recombination losses, and thus prolonging the T₈₀ lifetime of devices from ~470 to over 760 h for binary and over 1140 h for ternary devices under continuous illumination.

Abstract

Organic photovoltaics (OPVs) have recently achieved efficiencies of over 19% and are well underway toward practical applications. However, issues concerning operational stability remain a major challenge ahead of OPV commercialization. Here, when replacing the conventional hole-transporting layer PEDOT:PSS with a self-assembled monolayer of [2-(3,6-dichloro-9H-carbazol-9-yl)ethyl]phosphonic acid (3,6-Cl-2PACz) or [2-(4,5-dichloro-9H-carbazol-9-yl)ethyl]phosphonic acid (4,5-Cl-2PACz) it is found that the T₈₀ lifetime of PM6:BTP-eC9-based devices can be improved from ~100 to ~470 and over 800 h, respectively. The power conversion efficiency is also improved from 17.29% to 18.17% and 18.67%, respectively. The improved performance and prolonged photostability in 4,5-Cl-2PACz-based devices stem from the stabilized vertical distribution of donor and acceptor components, reducing the energetic disorder and thus alleviating non-radiative recombination losses. It is further found that the surface energy of 4,5-Cl-2PACz-modified substrates stays constant under prolonged illumination due to the improved intrinsic photostability of 4,5-Cl-2PACz, supporting the robust active layer morphology. Applying 4,5-Cl-2PACz in a ternary device of PM6:BTP-eC9:L8-BO-F delivered an efficiency of 19.05% and a T₈₀ lifetime over 1140 h.

Wide bandgap perovskite films with over 1 µm thickness are necessary for tandem application with small-size textured Si cells. Thicker perovskite film usually leads to serious fill factor (FF) losses in cells compared with thinner ones. Here, the hole transport bilayers could improve the charge-carrier extraction speed and enable the over 1 µm thick perovskite solar cells to achieve FFs around 80%.

Abstract

Achieving high efficiencies in halide perovskite solar cells with thicknesses >1 µm is necessary for developing perovskite-Si tandem cells based on small pyramidal structures. To achieve this goal, not only is the perovskite layer quality to be optimized but also the properties of the charge-transport layers must be tuned to reduce charge-collection losses. The transport layers provide a non-ohmic resistance that modulates the Fermi-level splitting inside the perovskite absorber. The finite conductivity of the transport layers can lead to losses in the fill factor (FF) and short-circuit current, even at infinite charge-carrier mobility in the absorber layer. These losses notably scale with the absorber layer thickness, which implies that higher-conductivity transport layers are required for thicker perovskite absorbers. One strategy to improve charge collection and thereby FFs in thick inverted perovskite solar cells is to use bilayers of hole-transport layers. In this study, the combination of poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] with self-assembled monolayers provides the best photovoltaic performance in single-junction devices.

Single-component iodonium initiators with strong oxidability and different electron delocalization properties were designed to replace lithium bis(trifluoromethanesulfonyl)imide and 4-tert-butylpyridine in doped 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD), which can oxidize Spiro-OMeTAD without air assistance. The devices based on this oxidizing Spiro-OMeTAD showed excellent efficiencies of 25.16 % (certified: 24.85 % for 0.062-cm²) and 20.71 % for a 15.03-cm² module as well as remarkable overall stability.

Abstract

To date, perovskite solar cells (pero-SCs) with doped 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) hole transporting layers (HTLs) have shown the highest recorded power conversion efficiencies (PCEs). However, their commercialization is still impeded by poor device stability owing to the hygroscopic lithium bis(trifluoromethanesulfonyl)imide and volatile 4-tert-butylpyridine dopants as well as time-consuming oxidation in air. In this study, we explored a series of single-component iodonium initiators with strong oxidability and different electron delocalization properties to precisely manipulate the oxidation states of Spiro-OMeTAD without air assistance, and the oxidation mechanism was clearly understood. Iodine (III) in the diphenyliodonium cation (IP⁺) can accept a single electron from Spiro-OMeTAD and forms Spiro-OMeTAD⋅⁺ owing to its strong oxidability. Moreover, because of the coordination of the strongly delocalized TFSI⁻ with Spiro-OMeTAD⋅⁺ in a stable radical complex, the resulting hole mobility was 30 times higher than that of pristine Spiro-OMeTAD. In addition, the IP-TFSI initiator facilitated the growth of a homogeneous and pinhole-free Spiro-OMeTAD film. The pero-SCs based on this oxidizing HTL showed excellent efficiencies of 25.16 % (certified: 24.85 % for 0.062-cm²) and 20.71 % for a 15.03-cm² module as well as remarkable overall stability.

Dual regulation of SAM and perovskite is achieved via a facile posttreatment by formamidinium chloride (FACl) solution in N,N-Dimethylformamide (DMF). DMF removes SAM clusters by solvation reaction, while FACl acts as a crystallization scaffold that assists the preferential orientation of perovskite, which facilitates the charge transportation and suppresses nonradiative recombination at the buried interface of perovskite solar cells.

The application of self-assembled molecules (SAM) allows inverted-structural perovskite solar cells to accomplish high efficiencies, by virtue of their high hole conductivity and negligible parasitic absorption. However, amphiphilic SAM tend to form spherical micelles in commonly used alcoholic solvents, leading to aggregation in the resulting film. In addition, hydrophobic groups of SAM are exposed to the surroundings after being deposited on transparent substrates, responsible for the poor crystallinity of perovskite and interface quality. Therefore, it is important to study the treatment of SAM layers or SAM/perovskite interfaces for better device performance. Herein, dual regulation of SAM and perovskite is achieved via a facile posttreatment by formamidinium chloride (FACl) solution in N,N-Dimethylformamide. First, by taking advantage of solvent rinsing, aggregations at the surface of SAM layer are alleviated. Second, FACl on the top of the SAM layer assists the crystallization of perovskite through interaction with PbI₂ and reduces the defect concentration via passivating the halide vacancies at the buried interface of the device. With a more uniform SAM layer, better-crystallized perovskite, and suppressed nonradiative recombination between them, the power conversion efficiency of the target device is improved by 20%.

A simple and efficient method for accelerating the oxidation process of spiro-OMeTAD is presented. By simply replacing the conventional processing solvent chlorobenzene with ethyl acetate, the oxidation process time is substantially reduced. Without postoxidation treatment and any other additional oxidizing agents, a high PCE of 23.3% is achieved, which marks the highest efficiency obtained from nonhalogenated solvent-processed spiro-OMeTAD.

As one of the most widely used hole transport materials (HTMs) in optoelectronics devices, 2,2’,7,7’-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) has endowed perovskite solar cells (PSCs) with record power conversion efficiencies (PCEs). However, the long oxidation time in the ambient atmosphere is time-consuming. Furthermore, commonly used solvents for spiro-OMeTAD film processing such as chlorobenzene (CB) are toxic, which severely hinders the commercialization of PSCs. Herein, ethyl acetate (EA) serves as a superior green solvent alternative to the typical processing solvent, which substantially reduces the oxidation time of spiro-OMeTAD. Without postoxidation treatment and any other additional oxidizing agents, the resultant device delivers a high PCE of 23.3%, significantly outperforming the CB-based device. It is also the highest efficiency obtained from nonhalogenated solvent-processed spiro-OMeTAD. The electrostatic potential mapping demonstrates that the electron density at nitrogen atoms of EA-processed spiro-OMeTAD is more delocalized, leading to a faster oxidation. Furthermore, the processing solvent EA is a green solvent with low toxicity, and the amount of HTMs used in the fabrication of solar cells can be dramatically reduced. This study provides a new solution for processing spiro-OMeTAD, which is ideal for the mass production of PSCs.

Constructing a front surface gradient, which increases minority carrier lifetime without defect passivation, is challenging for two-step processed perovskite solar cells. A front surface layer assembled of ZnCdSeS-based quantum dots enables a 73 mV gain in open-circuit voltage and a power conversion efficiency of 24.37% by forming a valance band gradient and minimizing the lattice mismatch with the absorber.

Abstract

A front surface gradient of the absorber valence band can effectively reduce the open-circuit voltage (V _OC) loss of perovskite solar cells by suppressing the minority carrier concentration near the front surface. However, the existing method is limited to the one-step fabrication process, resulting in underachieved photon harvesting and power conversion efficiency (PCE). To solve the problem, ZnCd-based alloy quantum dots (QDs) are utilized to create a valence-band-maximum gradient at the front surface of a two-step processed FAPbI₃ absorber. This design significantly enhances V _OC without requiring surface passivation. Furthermore, it is demonstrated that reducing the QD-perovskite lattice mismatch while maintaining QD's energy levels mitigates nonradiative recombination without compromising the front surface gradient effect. As a result, normal-structured perovskite solar cells achieve a V _OC equivalent to 93% of the Schockley–Queisser limit and a PCE of 24.37%.

Exceeding thermal disorder-induced strain (TDIS) over light-induced strain can eliminate the photoexcitation-induced strain gradient responsible for segregation. Simultaneously, dynamic disorder-inducing polaron localization significantly reduces excess carrier density, thereby mitigating the impact of light-induced strain. As a result, entropy dominates the system's free energy. These insights highlight the importance of strain homogenization for stable-phase, mixed-halide perovskites.

Abstract

The reversal of halide ions is studied under various conditions. However, the underlying mechanism of heat-induced reversal remains unclear. This work finds that dynamic disorder-induced localization of self-trapped polarons and thermal disorder-induced strain (TDIS) can be co-acting drivers of reverse segregation. Localization of polarons results in an order of magnitude decrease in excess carrier density (polaron population), causing a reduced impact of the light-induced strain (LIS – responsible for segregation) on the perovskite framework. Meanwhile, exposing the lattice to TDIS exceeding the LIS can eliminate the photoexcitation-induced strain gradient, as thermal fluctuations of the lattice can mask the LIS strain. Under continuous 0.1 W cm⁻² illumination (upon segregation), the strain disorder is estimated to be 0.14%, while at 80 °C under dark conditions, the strain is 0.23%. However, in situ heating of the segregated film to 80 °C under continuous illumination (upon reversal) increases the total strain disorder to 0.25%, where TDIS is likely to have a dominant contribution. Therefore, the contribution of entropy to the system's free energy is likely to dominate, respectively. Various temperature-dependent in situ measurements and simulations further support the results. These findings highlight the importance of strain homogenization for designing stable perovskites under real-world operating conditions.

A semiconducting homopolymer, hailing from an electron-rich peri-xanthenoxanthene-based polycyclic aromatic monomer, produces uniform films with heightened hole mobility, amplified modulus, and constrained species diffusion. These attributes enable the construction of perovskite solar cells with an initial average efficiency of 24.6 %, which also exhibit enhanced thermostability at 85 °C and operational durability.

Abstract

Highly efficient perovskite solar cells typically rely on spiro-OMeTAD as a hole transporter, achieving a 25.7 % efficiency record. However, these cells are susceptible to harsh 85 °C conditions. Here, we present a peri-xanthenoxanthene-based semiconducting homopolymer (p-TNI2) with matched energy levels and a high molecular weight, synthesized nearly quantitatively through facile oxidative polymerization. Compared to established materials, p-TNI2 excels in hole mobility, morphology, modulus, and waterproofing. Implementing p-TNI2 as the hole transport layer results in n-i-p perovskite solar cells with an initial average efficiency of 24.6 %, rivaling 24.4 % for the spiro-OMeTAD control cells under identical conditions. Furthermore, the p-TNI2-based cells exhibit enhanced thermostability at 85 °C and operational robustness.

In this study, a strategy is developed to initiate a complex by adding multifunctional imidazolidinyl urea additive into PbI2 precursor solution. The complex can be used as nucleation sites and templates for perovskite crystallization , thereby regulating the growth of perovskite films, passivating defects, improving hydrophobicity, and inhibiting lead leakage. Finally, the efficiency and stability of perovskite solar cells were improved.

Abstract

High-quality perovskite absorption layer is the fundamental basis for efficient and stable perovskite solar cells (PSCs). Due to the ionic nature of perovskite components, plentiful charged defects and suspension bonds remain on the surface of perovskite grains after continuous high-temperature annealing. Here, the complex initiated by the introduction of a multifunctional imidazolidinyl urea (IU) additive into the PbI₂ precursor solution could serve as nucleation sites and crystallization templates for perovskite crystals to optimize the growth of high-quality perovskite films. By anchoring at the grain boundaries of perovskite films, IU molecules could passivate various types of defects, improve the hydrophobic properties, and inhibit lead leakage. Attributed to reduced defect density, improved charge transport, and inhibited α-FAPbI₃ transition, the PSCs prepared based on IU additives achieved a champion power conversion efficiency of 23.18% (21.51% for the control PSCs) with negligible hysteresis and satisfactory stability.

Addressing the challenge of selecting appropriate ILs for interfacial modification, this work introduces a versatile strategy starting from IL chemical structures. Various ILs with diverse anions are systematically tested for modification at perovskite/SnO₂ interfaces. The study analyzed the relationship between IL structures and their impact on energy level arrangement, work function, perovskite crystallization, interface stress, charge transfer, and device performance.

Abstract

Ionic liquids (ILs) have emerged as versatile tools for interfacial engineering in perovskite photovoltaics. Their multifaceted application targets defect mitigation at SnO₂-perovskite interfaces, finely tuning energy level alignment, and enhancing charge transport, meanwhile suppressing non-radiative recombination. However, the diverse chemical structures of ILs present challenges in selecting suitable candidates for effective interfacial modification. This study adopted a systematic approach, manipulating IL chemical structures. Three ILs with distinct anions are introduced to modify perovskite/SnO₂ interfaces to elevate the photovoltaic capabilities of perovskite devices. Specifically, ILs with different anions exhibited varied chemical interactions, leading to notable passivation effects, as confirmed by Density Functional Theory (DFT) calculation. A detailed analysis is also conducted on the relationship between the ILs' structure and regulation of energy level arrangement, work function, perovskite crystallization, interface stress, charge transfer, and device performance. By optimizing IL chemical structures and exploiting their multifunctional interface modification properties, the champion device achieved a PCE of 24.52% with attentional long-term stability. The study establishes a holistic link between IL structures and device performance, thereby promoting wider application of ILs in perovskite-based technologies.

This work presents a thorough and effective strategy to improve the performance of perovskite solar cells (PSCs) by incorporating phytic acid (PA) into SnO₂ as an electron transport layer (ETL). The multiple functions of PA make the efficiency of the flexible device up to 21.08%, and the efficiency of the rigid device up to 21.82%.

Abstract

The power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs) are significantly reduced by defect-induced charge non-radiative recombination. Also, unexpected residual strain in perovskite films leads to an unfavorable impact on the stability and efficiency of PSCs, notably flexible PSCs (f-PSCs). Considering these problems, a thorough and effective strategy is proposed by incorporating phytic acid (PA) into SnO₂ as an electron transport layer (ETL). With the addition of PA, the Sn inherent dangling bonds are passivated effectively and thus enhance the conductivity and electron mobility of SnO₂ ETL. Meanwhile, the crystallization quality of perovskite is increased largely. Therefore, the interface/bulk defects are reduced. Besides, the residual strain of perovskite film is significantly reduced and the energy level alignment at the SnO₂/perovskite interface becomes more matched. As a result, the champion f-PSC obtains a PCE of 21.08% and rigid PSC obtains a PCE of 21.82%, obviously surpassing the PCE of 18.82% and 19.66% of the corresponding control devices. Notably, the optimized f-PSCs exhibit outstanding mechanical durability, after 5000 cycles of bending with a 5 mm bending radius, the SnO₂-PA-based device preserves 80% of the initial PCE, while the SnO₂-based device only remains 49% of the initial value.

Annealing-free and light-soaking-free tin dioxide (SnO₂) electron transport layers are developed and applied to non-fullerene organic solar cells (OSCs) based on PTB7-Th:IEICO-4F. The OSCs fabricated with pristine SnO₂ exhibited 8.34% low efficiency without annealing. However, upon annealing the pristine SnO₂ at 150 °C, the efficiency recovered to the normal value of 11.21%. The solar cell using γ-ray treated SnO₂ demonstrated a comparable efficiency of 11.18% without any annealing process. Furthermore, the light-soaking effect disappeared in the γ-ray-treated SnO₂.

Abstract

The electrode buffer layer is crucial for high-performance and stable OSCs, optimizing charge transport and energy level alignment at the interface between the polymer active layer and electrode. Recently, SnO₂ has emerged as a promising material for the cathode buffer layer due to its desirable properties, such as high electron mobility, transparency, and stability. Typically, SnO₂ nanoparticle layers require a postannealing treatment above 150°C in an air environment to remove the surfactant ligands and obtain high-quality thin films. However, this poses challenges for flexible electronics as flexible substrates can't tolerate temperatures exceeding 100°C. This study presents solution-processable and annealing-free SnO₂ nanoparticles by employing y-ray irradiation to disrupt the bonding between surfactant ligands and SnO₂ nanoparticles. The SnO₂ layer treated with y-ray irradiation is used as an electron transport layer in OSCs based on PTB7-Th:IEICO-4F. Compared to the conventional SnO₂ nanoparticles that required high-temperature annealing, the y-SnO₂ nanoparticle-based devices exhibit an 11% comparable efficiency without postannealing at a high temperature. Additionally, y-ray treatment has been observed to eliminate the light-soaking effect of SnO₂. By eliminating the high-temperature postannealing and light-soaking effect, y-SnO₂ nanoparticles offer a promising, cost-effective solution for future flexible solar cells fabricated using roll-to-roll mass processing.

We incorporated robust quadrupole-quadrupole interactions to regulate the crystal growth of 2D Ruddlesden-Popper (RP) perovskites. This was achieved through the development of two unique semiconductor spacers, namely PTMA and 5FPTMA, with different dipole moments. Devices utilizing films incorporating these interactions exhibited a significant efficiency enhancement, increasing from 15.66 % to 18.56 %.

Abstract

The crystal growth and orientation of two-dimensional (2D) perovskite films significantly impact solar cell performance. Here, we incorporated robust quadrupole-quadrupole interactions to govern the crystal growth of 2D Ruddlesden–Popper (RP) perovskites. This was achieved through the development of two unique semiconductor spacers, namely PTMA and 5FPTMA, with different dipole moments. The ((5FPTMA)_0.1(PTMA)_0.9)₂MA_n−1Pb_nI_3n+1 (nominal n=5, 5F/PTMA−Pb) film shows a preferred vertical orientation, reduced grain boundaries, and released residual strain compared to (PTMA)₂MA_n−1Pb_nI_3n+1 (nominal n=5, PTMA−Pb), resulting in a decreased exciton binding energy and reduced electron-phonon coupling coefficients. In contrast to PTMA−Pb device with an efficiency of 15.66 %, the 5F/PTMA−Pb device achieved a champion efficiency of 18.56 %, making it among the best efficiency for 2D RP perovskite solar cells employing an MA-based semiconductor spacer. This work offers significant insights into comprehending the crystal growth process of 2D RP perovskite films through the utilization of quadrupole-quadrupole interactions between semiconductor spacers.

An ingenious synthesis strategy is proposed for novel tetramers with higher glass transition temperatures (T _g), simplifying intricate reaction steps and reducing synthetic complexity. The resulting Tet-1-based organic solar cell (OSC) demonstrates superior photovoltaic performance. Importantly, the higher T _g and lower diffusion coefficient contribute to excellent stability in the binary device.

Abstract

Oligomer acceptors in organic solar cells (OSCs) have garnered substantial attention owing to their impressive power conversion efficiency (PCE) and long-term stability. However, the simple and efficient synthesis of oligomer acceptors with higher glass transition temperatures (T _g) remains a formidable challenge. In this study, we propose an innovative strategy for the synthesis of tetramers, denoted as Tet-n, with elevated T _gs, achieved through only two consecutive Stille coupling reactions. Importantly, our strategy significantly reduces the redundancy in reaction steps compared to conventional methods for linear tetramer synthesis, thereby improving both reaction efficiency and yield. Furthermore, the OSC based on PM6:Tet-1 attains a high PCE of 17.32 %, and the PM6:L8-BO:Tet-1 ternary device achieves an even more higher PCE of 19.31 %. Remarkably, the binary device based on the Tet-1 tetramer demonstrates outstanding operational stability, retaining 80 % of the initial efficiency (T ₈₀) even after 1706 h of continuous illumination, which is primarily attributed to the enhanced T _g (247 °C) and lower diffusion coefficient (1.56×10⁻²⁷ cm² s⁻¹). This work demonstrates the effectiveness of our proposed approach in the straightforward and efficient synthesis of tetramers materials with higher T _gs, thus offering a viable pathway for developing high-efficiency and stable OSCs.

The self-regulation effect is well demonstrated by finely tuning the aggregation behavior of nonfullerene acceptor (NFA) Z9, where the tethered phenyl groups worked as endogenous additives and participated in intermolecular packing enabling the formation of 3D charge transport channels with super-exchange electron coupling. NFA Z9 forms optimal nanomorphology with polymer donors without external additives, and a high PCE of 19.0 % is achieved.

Abstract

The blend nanomorphology of electron-donor (D) and -acceptor (A) materials is of vital importance to achieving highly efficient organic solar cells. Exogenous additives especially aromatic additives are always needed to further optimize the nanomorphology of blend films, which is hardly compatible with industrial manufacture. Herein, we proposed a unique approach to meticulously modulate the aggregation behavior of NFAs in both crystal and thin film nanomorphology via self-regulation effect. Nonfullerene acceptor Z9 was designed and synthesized by tethering phenyl groups on the inner side chains of the Y6 backbone. Compared with Y6, the tethered phenyl groups participated in the molecular aggregation via the π–π stacking of phenyl-phenyl and phenyl-2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) groups, which induced 3D charge transport with phenyl-mediated super-exchange electron coupling. Moreover, ordered molecular packing with suitable phase separation was observed in Z9-based blend films. High power conversion efficiencies (PCEs) of 19.0 % (certified PCE of 18.6 %) for Z9-based devices were achieved without additives, indicating the great potential of the self-regulation strategy in NFA design.