Chen Weijie

The favorable double cascading charge transfer paths pave an interesting possibility to spatially separate electrons upon visible light excitation and holes upon NIR photon absorption simultaneously at interfaces, significantly suppressing non-radiative bimolecular recombination and reaching the photocurrent density as high as 27.48 mA cm⁻² and power conversion efficiency of 20.31%.

Abstract

Nowadays, nearly 48.7% near-infrared (NIR) irradiation (>800 nm) of the full solar spectrum has actually not been fully utilized since the state-of-the-art perovskite film usually can only absorb the most UV–vis sunlight radiation. Herein, high efficiency integrated Cs_0.15FA_0.85PbI₃ perovskite/organic bulk (PC₆₁BM:D18:Y6) heterojunction solar cells with enhanced low energy photon harvest until 931 nm and a high maintained open circuit voltage of 1.04 V is successfully obtained. In particular, the favorable double cascading charge transfer paths pave an interesting possibility to spatially separate electrons upon visible light excitation and holes upon NIR photon absorption simultaneously at interfaces, significantly suppressing non-radiative bimolecular recombination and reaching the photocurrent density as high as 27.48 mA cm⁻² and power conversion efficiency of 20.31%. Besides, the strong hydrophobicity of the ternary organic film has effectively prevented ambient humidity penetration and improves the stability of the perovskite in the continuous aging test (humidity > 60%) compared with the control device. This work has opened a significantly new window to improve the NIR light harvest for next generation highly efficient solar cells with full spectrum response.

Mainstream silicon solar cell concepts are strongly limited in the short-circuit current they can generate by parasitic absorption. Silicon carbide–based transparent passivating contacts can be used to reduce the parasitic absorption. On these cells, the generated short-circuit current can be increased by selecting an appropriate antireflective coating like magnesium fluoride, enabling very high short-circuit current densities.

Herein, an optical loss analysis of the recently introduced silicon carbide–based transparent passivating contact (TPC) for silicon heterojunction solar cells is presented, the most dominant losses are identified, and the potential for reducing these losses is discussed. Magnesium fluoride is applied as an antireflective coating to reduce the reflective losses by up to 0.8 mA cm^{− 2}. When applying the magnesium fluoride, the passivation quality of the layer stack degrades, but is restored after annealing on a hot plate in ambient air. Afterwards, a road map for TPC solar cells toward an efficiency of 25% is presented and discussed. The largest part in efficiency gain is achieved by reducing the finger width and by increasing the passivation quality. Furthermore, it is shown that TPC solar cells have the potential to achieve short-circuit current densities above 42 mA cm^{− 2} if the finger width is reduced and the front-side indium tin oxide (ITO) layer can be replaced by an ITO silicon nitride double layer.

Self-powered smart windows are of great importance for light and heat management in the future. Highly visible-transparent and color-neutral MAPbCl₃-based transparent perovskite solar cell is integrated with electrochromic devices to realize the reversible transparency change between dark and bright states.

Electrochromic smart windows can reduce the energy consumption of buildings by managing the light and heat transmission. However, they need external power to work, raising the installation cost and compromising the aesthetics of the buildings. Self-powered smart windows without external power sources have great potential in practical applications. Transparent solar cells can be integrated with smart windows and serve as their power sources. High optical transmittance, good color neutrality, and high power conversion efficiency (PCE) are required for the transparent solar cells to meet the optical and power requirements of self-powered smart windows. Herein, an efficient MAPbCl₃-based transparent perovskite solar cell (TPSC) using a solvent-assisted two-step approach is developed. The transparency and color-neutrality of the TPSCs are optimized through delicately selecting and pairing the charge transport layers and transparent electrodes. The TPSCs achieve a PCE up to 1.06% and average visible transmittance up to 72%. Self-powered smart windows powered by the TPSCs show fast and reversible modulation of visible light from 55% to 5% without external power input. This work demonstrates the prospect of deploying TPSCs in a self-powered smart window for energy saving and sustainable buildings.

The energy level alignment at buried interfaces between perovskites and an organic charge transport layer can undergo significant changes under illumination compared to dark. This is caused by selective charge carrier accumulation within the perovskite layer.

Tremendous progress in employing metal halide perovskites (MHPs) in a variety of applications, especially in photovoltaics, has been made in the past decade. To unlock the full potential of MHP materials in optoelectronic devices, an improved understanding of the electronic energy level alignment at perovskite-based interfaces is required. This particularly pertains to such interfaces under device operation conditions, e.g. under illumination with visible light such as in a solar cell. Herein, it is revealed that the energy level alignment at the buried interface between a double cation lead halide perovskite film and charge-selective organic transport layers changes upon white light illumination. This is found from photoemission experiments performed with the samples in dark and under illumination, and the interfacial energy level shift is reversible. The underlying mechanism is attributed to the accumulation of one charge carrier type within the perovskite film at the interface under illumination, as a result of the charge-selective nature of the organic layer. The fact that the interfacial energy level alignment at MHP-based junctions under illumination can differ from that in dark is to be taken into account to fully rationalize device characteristics.

Urea (Ur) and methyl-substituted urea (Me-Ur) are used as additives to modulate the lattice structure and crystallinity of CsPbI₂Br perovskite. The Me-Ur can attenuate the strong hydrogen bonding networks in the urea, leading to better defect passivation and suppression of the lattice distortion. A device efficiency of 16.5% with an open-circuit voltage of 1.33 V is obtained from CsPbI₂Br+Me-Ur solar cells.

Inorganic perovskite solar cells (PSCs) have witnessed extraordinary advances owing to their prominent stability against thermal aging. However, they suffer from a phase transition from black phase to yellow phase under ambient conditions and serious energy losses relative to the optical bandgap. Herein, urea (Ur) and methyl-substituted urea (Me-Ur) additives are used to modulate the lattice structure and crystallinity of the CsPbI₂Br, facilitating phase stability and high device performance. The Me-Ur can attenuate the strong hydrogen bonding networks in the Ur, which leads to stronger coordination of the carbonyl group with undercoordinated Pb²⁺, more efficiently passivating the defect states and suppressing the lattice distortion of the [PbI₆]⁴⁻ octahedra in the CsPbI₂Br perovskite. Consequently, a champion power conversion efficiency of 16.5% with an open-circuit voltage up to 1.33 V is obtained for the CsPbI₂Br+Me-Ur-based PSCs, accompanied by enhanced stability under continuous illumination at a temperature of 45 ± 5 °C. These results emphasize the importance of regulating the lattice distortion by the urea derivative to implement efficient and stable inorganic CsPbI₂Br PSCs.

It is demonstrated that high-quality CsPbI₂Br perovskite films could be prepared by using 2D non-layered materials as additives, such as In₂S₃ nanoflakes (Nano-In₂S₃) with well-matched lattices and unsaturated dangling bonds on the surface. As a result, the organic-free perovskite solar cells with inverted configuration exhibit improved device performance along with excellent stability.

Organic-free perovskite solar cells (PSCs) have been of rising interest due to their remarkable resistance toward long-term thermal stress. Nevertheless, the inorganic perovskite films usually suffer from poor crystallization and high-density defects in bulk and near/at the interfaces, which leads to significant charge recombination loss and hence inferior device performance. Herein, it is demonstrated that high-quality CsPbI₂Br perovskite films could be prepared by using 2D non-layered materials as additives, such as In₂S₃ nanoflakes (Nano-In₂S₃) with well-matched lattices and unsaturated dangling bonds on the surface. In addition, it is found that the introduction of Nano-In₂S₃ results in not only defect passivation but also remarkable quasi-Fermi level splitting across the perovskite film due to its gradient doping behavior, thereby enhancing the built-in electric field in the inverted PSCs. As a result, the optimal devices based on Nano-In₂S₃:CsPbI₂Br absorber and all-inorganic interfacial layers deliver a champion power conversion efficiency of 15.17% along with excellent ambient and thermal stabilities, superior to those of the pristine devices and comparable to the best organic-free PSCs. A novel strategy for highly efficient and stable organic-free photovoltaics by using 2D non-layered materials as multifunctional additives is demonstrated.

CsPbBrCl₂: Ln³⁺ PQDs are employed in a perovskite solar cell to achieve a “lattice to lattice” doping effect and passivate the intrinsic defects in MAPbI₃-based PSCs. CsPbBrCl₂: Ln³⁺ PQDs can adjust work function, optimize bandgap alignment, and form stronger Ln-X bonds, and displays a power conversion efficiency of 22.52% and a high V _oc of 1.20 V.

Abstract

The passivation effect of inorganic perovskite quantum dots (PQDs) is a promising method to attain outstanding performance in perovskite solar cells (PSCs), which has ignited widespread interest recently. Lanthanides (Ln) doped PQDs demonstrate unique properties, but nevertheless, are not explored in PSCs. In this work, four kinds of Ln³⁺ doped CsPbBrCl₂ PQDs (Ln³⁺ = Yb³⁺, Ce³⁺, Eu³⁺, Sm³⁺) are firstly introduced into PSCs, which displays the synergistic effect of composition engineering and defect engineering. The results indicate that the introduction of CsPbBrCl₂: Ln³⁺ can not only improve the crystallinity and passivate the intrinsic and surface defects of the MAPbI₃ layer through ion and ligand passivation, but also form a stronger LnI bond than PbI, adjust work function (W _F), and optimize band alignments. CsPbBrCl₂:Sm³⁺ PQDs possess the best performance and exhibit remarkable promotions of open-circuit voltage (V _oc) from 1.13 to 1.20 V and power conversion efficiency from 18.54% to 22.52%. The humid-resist, thermal-resist abilities, and the long-term stability of PSCs are energetically improved due to enhanced structure stability by Sm³⁺ doping and the hydrophobic characteristic. The strategy of Ln³⁺ doped PQDs applied to PSCs provide an approach to achieve high-performance PSCs.

The effect of formamidinium (FA⁺) on modulating methylammonium (MA⁺) based (mixed-halide wide-bandgap preovskites) MWPs (MA_1.06PbI₂Br(SCN)_0.12) crystallization properties for achieving high-quality perovskite films is evaluated. Based on the optimized MA_0.96FA_0.1PbI₂Br(SCN)_0.12 film, a monolithic perovskite/organic tandem solar cells with a new record high-efficiency of 21.2% is achieved.

Abstract

Monolithic perovskite/organic tandem solar cells have attracted increasing attention due to their potential of being highly efficient while compatible to facile solution fabrication processes. One of the limiting factors for improving the performance of perovskite/organic tandem cells is the lack of wide-bandgap perovskites with suitable bandgap, film quality, and optoelectronic properties for front cells. In addition, the development of low-bandgap organic bulk-heterojunction (BHJ) rare cells with extended absorption in the infrared range is also critical for improving tandem cells. This work has carefully optimized mixed halide wide-bandgap perovskite (MWP) films by introducing a small amount of formamidinium (FA⁺) cations into the basic composition of MA_1.06PbI₂Br(SCN)_0.12, which provides an effective means to modulate the crystallization properties and phase stability of the films. At optimized conditions, the MA_0.96FA_0.1PbI₂Br(SCN)_0.12 forms high-quality films with grain boundaries homogeneously passivated by PbI₂, leading to a reduction in defect states and an enhancement in phase stability, enabling the fabrication of perovskite solar cells with a power conversion efficiency(PCE) of 17.4%. By further integrating the MWP front cell with an organic BHJ (PM6:CH1007) rare cell composed of a nonfullerene acceptor with absorption extended to 950 nm, a tandem cell with PCE over 21% is achieved.

It is demonstrated in the present study that efficient grain boundaries engineering via laser generated nanocrystals with tailored surface states for improved carriers dynamics can lead to the construction of perovskite solar cells with pronounced environmental stability and increased champion power conversion efficiency up to 23.74%.

Abstract

Grain boundaries (GBs) engineering of hybrid perovskite films is of significance for accessing high performance perovskite solar cells (PSCs), owing to the abundant defect states existed therein originating from the low temperature film processing. Nanocrystals embedding at GBs has shown profound advantages in carrier dynamics modulation, while the surface defects on nanocrystals in turn lead usually to the trapping of carriers at GBs. The authors herein demonstrate the efficient GBs engineering via laser generated nanocrystals with tailored surface states for improved carriers dynamics and environmental stability of PSCs. The embedding of La doped BaSnO₃ (LBSO) nanocrystals with bare surfaces in perovskite provides an additional channel to facilitate the effective carrier extraction and reduce the carrier recombination, leading to a maximum power conversion efficiency (PCE) of 21.11% with negligible hysteresis for the mixed-cation PSCs. To clarify the influence of surface defect states of the laser generated nanocrystals on the performance of PSCs, 1H,1H-perfluorooctylamine is grafted on LBSO nanocrystals during the laser irradiation, resulting in improved champion PCE up to 21.65% and pronounced environmental stability. The universal embedding of the LBSO nanocrystals with tailored surface states in different perovskite by fabricating FAPbI₃ PSCs with a champion PCE of 23.74% is further demonstrated.

Ln³⁺-based halide Cs₃TbCl₆ QDs are synthesized and introduced to the interface of perovskite films to promote better bandgap alignment and reduce interface defects density. The Cs₃TbCl₆ QDs modified device has achieved a super high open-voltage of 1.235 V. Cs₃TbCl₆ QDs and black phosphorus dual modified device has yielded a champion photoelectric conversion efficiency of 23.49% and a filling factor of 80.32%.

Abstract

Interfacial engineering is one of the most effective means to improve the photoelectric conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). In this work, Ln³⁺-based halide Cs₃TbCl₆ quantum dots (QDs) are synthesized through a modified hot-injection method, which displays an excitonic emission centered at 431 nm and the characteristic emission peaks of Tb³⁺ ions. Then, the Ln³⁺-based halide Cs₃TbCl₆ QDs are introduced to the interface of Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite films in the PSCs, which can regulate the energy levels, fill the grain boundaries and remove the ionic defects. Surprisingly, the Cs₃TbCl₆ QDs modified devices achieve a champion PCE of 22.89% with a super high open-voltage of 1.235 V. The high open-voltage can be mainly attributed to the better bandgap alignment, enhanced interface, and reduced defects density. Afterward, the hole transport layer (HTL) is modified by the black phosphorus QDs (BPQDs), yielding a champion PCE of 23.49% and a filling factor of 80.32%. The Cs₃TbCl₆ QDs modified unencapsulated device possesses well environmental stability and humidity stability. This work demonstrates a new kind of Ln³⁺-based metal QDs and explores a new approach to fabricate the PSCs with high open-voltage, high efficiency, and good stability through the QD-based passivation techniques.

All-small-molecule organic solar cells (ASM-OSCs) have received a lot of attention owing to several advantages such as well-defined molecular weights, easy purification, and satisfactory batch-to-batch repeatability. Herein, two isomeric small-molecule donors (SMDs) containing S···O noncovalently conformational locks (NoCLs) are reported. Upon regio-isomerization of NoCLs, high power conversion efficiencies of 13.99% and 15.34% are achieved for binary and ternary ASM-OSCs.

Abstract

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have surpassed 19% thanks to the innovation of polymer donors and molecular acceptors. However, the batch-to-batch variations in polymer materials are detrimental to the reproducibility of the device performance. In comparison, small-molecule donors (SMDs) possess some unique advantages, such as well-defined molecular weights, easy purification, and excellent batch-to-batch repeatability. Herein, a pair of regioisomeric SMDs (BT-O1 and BT-O2) has been synthesized with alkoxy groups as S···O noncovalently conformational locks (NoCLs) at the inner and outer position, respectively. Theoretical and experimental results reveal that the regioisomeric effect has a significant influence on the light-harvest ability, energy levels, molecular geometries, internal reorganization energy, and packing behaviors for the two SMDs. As a result, BT-O2-based binary device shows an impressive PCE of 13.99%, much higher than that of BT-O1 based one (4.07%), due to the better-aligned energy level, more balanced charge transport, less charge recombination, lower energy loss, and more favorable phase separation. Furthermore, the fullerene derivative PC₇₁BM is introduced into BT-O2:H3 as the third component to achieve a notable PCE of 15.34% (certified 14.6%). Overall, this work reveals that NoCLs is a promising strategy to achieve high-performance SMDs for all-small-molecule OSCs.

With the emergence of high-performance non-fullerene acceptors, significant enhancement in power conversion efficiencies (PCEs) of organic solar cells (OSCs) has been achieved. However, the modest open-circuit voltage imposed by relatively large non-radiative recombination energy loss (ΔE ₃) limits further improvement of PCEs. This review summarizes the recent advance in ΔE ₃ of OSCs from material design, morphology manipulation, ternary strategy, and mechanism.

Abstract

Impressive short-circuit current density and fill factor have been achieved simultaneously in single-junction organic solar cells (OSCs) with the emergence of high-performance non-fullerene acceptors. However, the power conversion efficiencies (PCEs) of OSCs still lag behind those of inorganic and perovskite solar cells, mainly due to the modest open-circuit voltage (V _OC) imposed by relatively large energy loss (E _loss). Generally, E _loss in solar cells can be divided into three parts. Among them, ΔE ₁ is inevitable for all photovoltaic cells and depends on the optical bandgap of solar cells, while radiative recombination energy loss, ΔE ₂, in OSCs can approach the negligible value via finely matching donor with acceptor material in the blend. The relatively large non-radiative recombination energy loss, ΔE ₃, becomes the main barrier to further reduce E _loss and thus enhance PCE in non-fullerene acceptor-based OSCs. In this review, the recent studies and achievements about ΔE ₃ in non-fullerene acceptor-based OSCs have been summarized from the aspects of material design, morphology manipulation, ternary strategy, mechanism, and theoretical study. It is hoped that this review helps to get a deep understanding and boost the advance of ΔE ₃ study in OSCs.

Here, indaceno[1,2-b:5,6-b']dithiophene-oxindole complex is chosen as a multifunctional additive, which can simultaneously passivate Pb²⁺ and I⁻ on the surface and in grain boundaries in the perovskite absorber.

Abstract

Despite the swift development in perovskite solar cells (PSCs), suppressing the ion defects in the perovskite bulk and further extending the long-lasting stability of the cells remain the concerned issues that are yet to be solved. Here, a symmetrical organic acceptor−donor−acceptor (A−D−A) molecule with the core architecture of indaceno[1,2-b:5,6-b']dithiophene (IDT) and bilateral arms of oxindole, named IDT-OD, as a versatile defect passivation agent, is adopted to inactivate the nonradiative recombination sites in the perovskite absorber. The S element in the IDT unit and carbonyl group CO in the bilateral acceptor unit as the Lewis-base contributes to the passivation sites that are the under-coordinated Pb²⁺ cation defects and the N−H group in oxindole unit interacts with halide dangling bonds. The molecular structure with its symmetrical double arms assists the formation of a superior perovskite layer with enlarged grain size, smooth surface topography, hydrophobic property, and low density of defect state. Consequently, the corresponding PSCs with the proper IDT-OD additive yield a remarkable increase in efficiency from 22.77% to 24.04%, along with excellent long-term environmental and thermal stabilities. This study offers a propitious approach for ionic defect passivation engineering toward high-performance PSCs.

Scalable deposition of high-quality Dion–Jacobson perovskite films via tailoring crystallization kinetics is reported. Notably retarded crystallization is realized by using a ternary solvent, which yields a film with improved crystallinity, highly vertical orientation, and graded phase distribution. The prepared solar devices exhibit an impressive open-circuit voltage of 1.21 V and remarkable stability under stimuli of light, heat, and humidity.

Abstract

Two-dimensional perovskites have attracted substantial attention for solar cell applications because of their higher stability as compared to their 3D analogs. To achieve efficient charge transport in thin-film devices, obtaining high crystalline perovskite crystals perpendicularly aligned to the substrate is of great importance. This article reports the scalable printing of high-quality Dion–Jacobson (DJ) perovskite thin films via tailoring crystallization kinetics. Introducing a small amount of 1-methyl-2-pyrrolidinone to the conventional N,N-dimethylformamide:dimethyl sulfoxide-based precursor, the strong coordination with ammonium spacers enables a notably retarded crystallization, which results in perovskite films with distinctly enhanced crystallinity, highly vertical orientation, and graded phase distribution. Accordingly, efficient charge generation and ultrafast interphase charge transfer are realized. The champion DJ perovskite device delivers a high current density of 17.10 mA cm^–2, an impressive open-circuit voltage of 1.21 V, leading to a stabilized efficiency of 16.19%. In addition, the devices processed from the ternary solvent exhibit remarkably improved stability under stimuli with light, heat, and humidity, benefiting from their superb phase stability. This work demonstrates an important advancement in scalable deposition of DJ perovskite thin films for efficient and stable photovoltaic devices.

Fluorinated bis(perfluorophenyl)pimelate (BF7), a novel non-volatile additive, improves the efficiency of the Y6:PM6-based solar cell to 17.01% with exceptional morphological stability against thermal annealing. X-ray scattering analysis and theoretical calculations reveal that BF7 selectively interacts with Y6 via preferred F–π noncovalent supramolecular interactions to obtain the optimized film morphology for elevated device performance and thermal stability.

Abstract

A novel non-volatile additive, fluorinated bis(perfluorophenyl)pimelate (BF7), is demonstrated to effectively improve both the efficiency and thermal stability of a highly efficient organic solar cell (OSC), comprising fluorinated Y6 as the small-molecule acceptor and PM6 as the polymer donor. Processed with optimized 0.5 wt% BF7 in solution, the PM6:Y6:BF7 device achieves an elevated power conversion efficiency (PCE) of 17.01%, compared to 15.16% of that processed without BF7. Moreover, the BF7-elevated PCE can sustain 95% of the best PCE over 100 °C annealing for 72 h. Grazing incidence X-ray scattering and differential scanning calorimetry results consistently indicate that BF7 in the PM6:Y6:BF7 device interacts preferentially with Y6, resulting in improved fractal-like network structures of the active layer with optimized size and orientation of Y6 nano-crystallites and elevated thermal stability. Molecular simulation also supports that the observed structure and thermal stability is associated with the F–π noncovalent supramolecular interactions between the perfluorophenyl moieties of BF7 and difluorophenyl-based FIC-end-groups of Y6. Similar bifunctional BF7 effects are also observed in the well-known PM6:IT-4F system, suggesting that adding BF7 for concomitantly improved PCE and thermal stability might extend generally to OSCs that feature small molecule acceptors of difluorophenyl end-groups.

The role of structural and energetic disorder is studied in the non-fullerene PM6:Y6 blend in comparison to PM6:N4. For both materials, disorder influences the V _OC at room temperature, but also its progression with temperature. The present work highlights the need to understand the reasons behind energetic disorder in these blends, with the goal to further reduce open-circuit losses.

Abstract

Non-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing temperature-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage V _OC on temperature and illumination intensity. Disorder is found to influence the value of the V _OC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend.

Charge separation is key to efficient photocatalytic solar energy conversion. The distribution of surface charge on photocatalysts can be identified and the driving forces of charge separation quantitatively determined at the nanoscale by the spatially resolved surface photovoltage technique. The identification of photocatalytic charge separation mechanisms will enable the rational design of efficient photocatalytic systems.

Abstract

The photocatalytic conversion of solar energy offers a potential route to renewable energy, and its efficiency relies on effective charge separation in nanostructured photocatalysts. Understanding the charge-separation mechanism is key to improving the photocatalytic performance and this has now been enabled by advances in the spatially resolved surface photovoltage (SRSPV) method. In this Review we highlight progress made by SRSPV in mapping charge distributions at the nanoscale and determining the driving forces of charge separation in heterogeneous photocatalyst particles. We discuss how charge separation arising from a built-in electric field, diffusion, and trapping can be exploited and optimized through photocatalyst design. We also highlight the importance of asymmetric engineering of photocatalysts for effective charge separation. Finally, we provide an outlook on further opportunities that arise from leveraging these insights to guide the rational design of photocatalysts and advance the imaging technique to expand the knowledge of charge separation.

An organic ferroelectric material poly(vinylidene fluoride):dabcoHReO₄ as a perovskite dopant can be partially polarized by the built-in electric field of perovskite solar cell (pero-SC) itself, which produces an additional electric field, thus promoting the charge-carrier transportation. A promising 24.23% power conversion efficiency (PCE) (certified PCE of 23.45%) and robust operational stability are obtained.

Abstract

The built-in electric field (BEF) intensity of silicon heterojunction solar cells can be easily enhanced by selective doping to obtain high power conversion efficiencies (PCEs), while it is challenging for perovskite solar cells (pero-SCs) because of the difficulty in doping perovskites in a controllable way. Herein, an effective method is reported to enhance the BEF of FA_0.92MA_0.08PbI₃ perovskite by doping an organic ferroelectric material, poly(vinylidene fluoride):dabcoHReO₄ (PVDF:DH) with high polarizability, that can be driven even by the BEF of the device itself. The polarization of PVDF:DH produces an additional electric field, which is maintained permanently, in a direction consistent with that of the BEF of the pero-SC. The BEF superposition can more sufficiently drive the charge-carrier transport and extraction, thus suppressing the nonradiative recombination occurring in the pero-SCs. Moreover, the PVDF:DH dopant benefits the formation of a mesoporous PbI₂ film, via a typical two-step processing method, thereby promoting perovskite growth with high crystallinity and a few defects. The resulting pero-SC shows a promising PCE of 24.23% for a 0.062 cm² device (certified PCE of 23.45%), and a remarkable PCE of 22.69% for a 1 cm² device, along with significantly improved moisture resistances and operational stabilities.

A series of polymer acceptors named PY-X have been synthesized. By optimizing the length of branched alkyl chains, PY-HD (2-hexyldecyl substitution)-based all polymer solar cell (all-PSC) delivers a high efficiency of 16.76%, with an open-circuit voltage of 0.949 V, and an extremely low non-radiative voltage loss of 0.18 V, representing the highest efficiency for binary all-PSCs.

Abstract

The power conversion efficiencies (PCEs) of small molecule acceptor (SMA)-based organic solar cells have already exceeded 18%. However, the development of polymer acceptors still lags far behind their SMA counterparts mainly due to the lack of efficient polymer acceptors. Herein, a series of polymer acceptors named PY-X (with X being the branched alkyl chain) are designed and synthesized by employing the same central core with the SMA L8-BO but with different branched alkyl chains on the pyrrole motif. It is found that the molecular packing of SMA-HD featuring 2-hexyldecyl side chain used in the synthesis of PY-HD is similar to L8-BO, in which the branched alkyl chains lead to condensed and high-order molecular assembly in SMA-HD molecules. When combined with PM6, PY-HD-based all polymer solar cell (all-PSC) exhibits a high PCE of 16.41%, representing the highest efficiency for the binary all-PSCs. Moreover, the side-chain modification on the pyrrole site position further improves the performance of the all-PSCs, and the PY-DT-based device delivers a new record high efficiency of 16.76% (certified as 16.3%). The work provides new insights for understanding the structure–property relationship of polymer acceptors and paves a feasible avenue to develop efficient conjugated polymer acceptors.

A heterojunction post-annealing treatment is utilized to suppress the nonradiative recombination for a highly competitive power conversion efficiency of 8.64% and a record open-circuit voltage (V _OC) of 520 mV in Sb₂Se₃ thin-film solar cells. The V _OC deficit of the device is lower than that of any other reported efficient antimony chalcogenide solar cells.

Abstract

Despite the fact that antimony triselenide (Sb₂Se₃) thin-film solar cells have undergone rapid development in recent years, the large open-circuit voltage (V _OC) deficit still remains as the biggest bottleneck, as even the world-record device suffers from a large V _OC deficit of 0.59 V. Here, an effective interface engineering approach is reported where the Sb₂Se₃/CdS heterojunction (HTJ) is subjected to a post-annealing treatment using a rapid thermal process. It is found that nonradiative recombination near the Sb₂Se₃/CdS HTJ, including interface recombination and space charge region recombination, is greatly suppressed after the HTJ annealing treatment. Ultimately, a substrate Sb₂Se₃/CdS thin-film solar cell with a competitive power conversion efficiency of 8.64% and a record V _OC of 0.52 V is successfully fabricated. The device exhibits a much mitigated V _OC deficit of 0.49 V, which is lower than that of any other reported efficient antimony chalcogenide solar cell.

Tin oxide electron transport layer (ETL) for perovskite solar cells (PSCs) has become one of the most suitable candidates to replace the TiO₂ for low-temperature process on a flexible substrate. A comprehensive picture of the recent progress and challenges of different SnO₂ ETL deposition techniques and the performance of PSC devices is provided.

Organic inorganic halide perovskites have drawn great attention in the past decade, due to their superior photovoltaic performance with an efficiency over 25%. For planar heterojunction structure perovskite solar cells (PSCs) tin oxide based electron transport layers (ETLs) have become one of the most suitable candidates to replace titanium oxide to make flexible devices because of their low-temperature processing. The deposition techniques of SnO₂ can be categorized into chemical deposition, such as sol-gel, chemical bathing or atomic layer deposition, and physical deposition, such as thermal evaporation or sputtering. Depending on the deposition technique, defects, and morphology in the SnO₂ layer may vary drastically, leading to poor performance of PSCs. In this review, we have provided a comprehensive picture of the recent progress and challenges of different SnO₂ ETL deposition techniques and the performance of PSC devices. The additional modifications on SnO₂ mentioned in this article are also effective ways to eliminate the intrinsic defects in the film. The drawbacks and benefits of SnO₂ ETLs and the corresponding actions for this are also discussed. We hope this review will help with the comprehensive understanding of the relationship between the property of SnO₂ and its structure of ETLs to enhance PSCs' performance.