Chen Weijie

Thermal degradation in perovskite solar cells is still an unsettled issue that limits its further development. In this study, 2-(1H-pyrazol-1-yl)pyridine is introduced into lead halide 3D perovskites, which allows 1D–3D hybrid perovskite materials to be obtained. The heterostructural 1D–3D perovskites are proved to be capable of remarkably prolonging the photoluminescence decay lifetime and suppressing charge carrier recombination in comparison to conventional 3D perovskites. The intrinsic properties of thermodynamically stable yet kinetically labile 1D materials allow the system to alleviate the lattice mismatch and passivate the interface traps of heterojunction region of 1D–3D hybrid perovskites that may occur during the crystal growth process. Importantly, the as-fabricated 1D–3D perovskite solar cells display a thermodynamic self-healing ability, which is induced through blocking the ion-migration channels of A-site ions by the flexible 1D perovskite with less densely close-packed structure. Particularly, the power conversion efficiency of as-fabricated unencapsulated 1D–3D perovskite solar cells is demonstrated to be reversible under temperature cycling (25–85 °C) at 55% relative humidity, which largely outperforms the pure 3D perovskite solar cell. The present study provides a facile approach to fabricate 1D–3D perovskite solar cells with high efficiency and long-term stability.

1D–3D hybrid perovskite is applied as absorber in a solar cell for the first time with remarkable thermodynamic self-healing capability. The power conversion efficiency of as-fabricated unencapsulated 1D–3D perovskite solar cells is demonstrated to be reversible under temperature cycling (25–85 °C) at 55% relative humidity, which largely outperforms the pure 3D perovskite solar cell.

Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second-order recombination losses dominate the shape of the current density–voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric-field dependence of first-order losses, which includes electric-field-dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first- and second-order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.

By varying the fullerene fraction above and below the electron percolation threshold in the model high efficiency OPV system PTB7:PC71BM, the importance of electric-field-dependent first-order losses is shown. The results also shed light on the limitations of widely used and accepted transport models below the percolation threshold of the bulk heterojunction system.

Besides broadening of the absorption spectrum, modulating molecular energy levels, and other well-studied properties, a stronger intramolecular electron push–pull effect also affords other advantages in nonfullerene acceptors. A strong push–pull effect improves the dipole moment of the wings in IT-4F over IT-M and results in a lower miscibility than IT-M when blended with PBDB-TF. This feature leads to higher domain purity in the PBDB-TF:IT-4F blend and makes a contribution to the better photovoltaic performance. Moreover, the strong push–pull effect also decreases the vibrational relaxation, which makes IT-4F more promising than IT-M in reducing the energetic loss of organic solar cells. Above all, a power conversion efficiency of 13.7% is recorded in PBDB-TF:IT-4F-based devices.

Two critical factors (miscibility and vibrational relaxation) of nonfullerene molecular acceptors with the intramolecular electron push–pull effect are analyzed and related to their photovoltaic properties in organic solar cells (OSCs). A power conversion efficiency of 13.7% is recorded in OSCs by using a nonfullerene acceptor IT-4F, which shows a stronger intramolecular electron push–pull effect than its nonfluorinated counterpart.

Limited by the various inherent energy losses from multiple channels, organic solar cells show inferior device performance compared to traditional inorganic photovoltaic techniques, such as silicon and CuInGaSe. To alleviate these fundamental limitations, an integrated multiple strategy is implemented including molecular design, interfacial engineering, optical manipulation, and tandem device construction into one cell. Considering the close correlation among these loss channels, a sophisticated quantification of energy-loss reduction is tracked along with each strategy in a perspective to reach rational overall optimum. A novel nonfullerene acceptor, 6TBA, is synthesized to resolve the thermalization and V_OC loss, and another small bandgap nonfullerene acceptor, 4TIC, is used in the back sub-cell to alleviate transmission loss. Tandem architecture design significantly reduces the light absorption loss, and compensates carrier dynamics and thermalization loss. Interfacial engineering further reduces energy loss from carrier dynamics in the tandem architecture. As a result of this concerted effort, a very high power conversion efficiency (13.20%) is obtained. A detailed quantitative analysis on the energy losses confirms that the improved device performance stems from these multiple strategies. The results provide a rational way to explore the ultimate device performance through molecular design and device engineering.

Comprehensive optimization on organic solar cells is conducted, including molecular design, interfacial engineering, optical manipulation, and tandem architecture construction. Synergistical application of multiple strategies improves the balance of the energy losses from transmission, insufficient light trapping, thermalization, and carrier dynamic loss. An impressively high device performance up to 13.2% is achieved.

A durable flexible perovskite solar cell that employs graphene as transparent anode and carbon nanotubes as cathode is successfully developed by Ning Wang, Kaili Jiang, Hong Lin, Zhanhu Guo, and co-workers in article number 1706777. All-carbon-electrode-based devices exhibit promising efficiency, excellent flexibility, and stability, providing a new avenue for construction of cheap and large-scale flexible perovskite solar cells.

Processing solvent additives in polymer:fullerene bulk heterojunction systems are known as a promising method to enhance photovoltaic performance. It is generally agreed that solvent additives enable polymers to have a high degree of molecular order which increases the device performance. However, the understanding of the efficiency enhancement is not complete. There is a lack of insight regarding the quantitative determination of the molecular miscibility between polymer and fullerene as well as the inner morphology changes induced by the additives. In this work, understanding of the influence of the solvent additive 1,8-octanedithiol (ODT) is provided on the classic system poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) films. The impact on polymer crystallinity, surface structure, inner morphology, and quantitative molecular miscibility of P3HT and PCBM is studied as a function of ODT volume concentration. The crystallinity is probed with absorption spectroscopy and grazing incidence wide-angle X-ray scattering. The morphology and miscibility are characterized via atomic force microscopy and time-of-flight grazing incidence small angle neutron scattering. Besides an increased crystallinity and prominent phase separation, ODT increases the solubility of PCBM in P3HT and reduces the size of amorphous P3HT domains. Moreover, solvent processing with a high ODT concentration alters the vertical material composition of the active layer.

The function of 1,8-octanedithiol (ODT) on polymer:fullerene bulk heterojunction systems is comprehensively studied for the well-established model system poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester. Besides the positive influence of ODT on crystallinity and surface morphology, the influence on the molecular miscibility between polymer and fullerene is probed, providing a complete correlation between morphology and solar cell efficiency.

Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high-performance organic nonfullerene BHJ blends PBDB-T:ITIC and PBDB-T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer-polymer (PBDB-T:N2200) blend are more tolerant to composition changes, relative to polymer-molecule (PBDB-T:ITIC) devices. In both systems, short-circuit current density (J_sc) is tracked closely with PCE, indicating that exciton dissociation and transport strongly influence PCEs. With dilute acceptor concentrations, polymer-polymer blends maintain high electron mobility relative to the polymer-molecule blends, which explains the dramatic difference in PCEs between them as a function of D:A blend ratio. In addition, polymer-polymer solar cells, especially at high D:A blend ratio, are stable (less than 5% relative loss) over 70 d under continuous heating at 80 °C in a glovebox without encapsulation. This work demonstrates that all-polymer solar cells show advantage in operational lifetime under thermal stress and blend-ratio resilience, which indicates their high potential for designing of stable and scalable solar cells.

Based on two representative and high performance organic nonfullerene bulk heterojunction blends, PBDB-T:ITIC and PBDB-T:N2200, the effect of blend ratio on device performance and the relevant device stability is investigated in-depth. Solar cell devices incorporating polymer–polymer blends exhibit stable device performance in a wide range of blend ratios and excellent stability under dark and thermal stress.

A new electron-rich central building block, 5,5,12,12-tetrakis(4-hexylphenyl)-indacenobis-(dithieno[3,2-b:2′,3′-d]pyrrol) (INP), and two derivative nonfullerene acceptors (INPIC and INPIC-4F) are designed and synthesized. The two molecules reveal broad (600–900 nm) and strong absorption due to the satisfactory electron-donating ability of INP. Compared with its counterpart INPIC, fluorinated nonfullerene acceptor INPIC-4F exhibits a stronger near-infrared absorption with a narrower optical bandgap of 1.39 eV, an improved crystallinity with higher electron mobility, and down-shifted highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Organic solar cells (OSCs) based on INPIC-4F exhibit a high power conversion efficiency (PCE) of 13.13% and a relatively low energy loss of 0.54 eV, which is among the highest efficiencies reported for binary OSCs in the literature. The results demonstrate the great potential of the new INP as an electron-donating building block for constructing high-performance nonfullerene acceptors for OSCs.

Nonfullerene acceptors (NFAs) featuring indacenobis-(dithieno[3,2-b:2′,3′-d]pyrrol) as an electron-rich central building block are designed. The NFAs extend absorption to 900 nm with an optical bandgap of 1.39 eV. Organic solar cells (OSCs), by blending with PBDB-T as polymer donor, contribute a power conversion efficiency of 13.13%, which is among the highest reported for binary OSCs in the literature.

The trap states at grain boundaries (GBs) within polycrystalline perovskite films deteriorate their optoelectronic properties, making GB engineering particularly important for stable high-performance optoelectronic devices. It is demonstrated that trap states within bulk films can be effectively passivated by semiconducting molecules with Lewis acid or base functional groups. The perovskite crystallization kinetics are studied using in situ synchrotron-based grazing-incidence X-ray scattering to explore the film formation mechanism. A model of the passivation mechanism is proposed to understand how the molecules simultaneously passivate the Pb–I antisite defects and vacancies created by under-coordinated Pb atoms. In addition, it also explains how the energy offset between the semiconducting molecules and the perovskite influences trap states and intergrain carrier transport. The superior optoelectronic properties are attained by optimizing the molecular passivation treatments. These benefits are translated into significant enhancements of the power conversion efficiencies to 19.3%, as well as improved environmental and thermal stability of solar cells. The passivated devices without encapsulation degrade only by ≈13% after 40 d of exposure in 50% relative humidity at room temperature, and only ≈10% after 24 h at 80 °C in controlled environment.

Introducing semiconducting molecules with Lewis acid or base functional groups into a sol–gel MAPbI₃ film promotes uniform decoration of grain boundaries within the bulk film. These molecules holistically passivate under-coordinated Pb²⁺ vacancies or Pb–I antisite defects, leading to significant enhancements of the power conversion efficiency as well as improved environmental and thermal stability of solar cells.

Highly efficient planar heterojunction perovskite solar cells (PVSCs) with dopamine (DA) semiquinone radical modified poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (DA-PEDOT:PSS) as a hole transporting layer (HTL) were fabricated. A combination of characterization techniques were employed to investigate the effects of DA doping on the electron donating capability of DA-PEDOT:PSS, perovskite film quality and charge recombination kinetics in the solar cells. Our study shows that DA doping endows the DA-PEDOT:PSS-modified PVSCs with a higher radical content and greater perovskite to HTL charge extraction capability. In addition, the DA doping also improves work function of the HTL, increases perovskite film crystallinity, and the amino and hydroxyl groups in DA can interact with the undercoordinated Pb atoms on the perovskite crystal, reducing charge-recombination rate and increasing charge-extraction efficiency. Therefore, the DA-PEDOT:PSS-modified solar cells outperform those based on PEDOT:PSS, increasing open-circuit voltage (V _oc) and power conversion efficiency (PCE) to 1.08 V and 18.5%, respectively. Even more importantly, the efficiency of the unencapsulated DA-PEDOT:PSS-based PVSCs are well retained with only 20% PCE loss after exposure to air for 250 hours. These in-depth insights into structure and performance provide clear and novel guidelines for the design of effective HTLs to facilitate the practical application of inverted planar heterojunction PVSCs.

The power output and operation lifetime of inverted perovskite solar cells (PVSCs) can be simultaneously improved by incorporating dopamine-copolymerized poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (DA-PEDOT:PSS). The higher radical content and stronger surface trap passivation capability of DA-PEDOT:PSS relative to PEDOT:PSS enhance photovoltaic performance and suppress charge recombination loss, leading to highly efficient and stable PVSCs with a power conversion efficiency of 18.5% (V _oc: 1.08 V).

Metal halide perovskites are exceptional candidates for inexpensive yet high-performing optoelectronic devices. Nevertheless, polycrystalline perovskite films are still limited by nonradiative losses due to charge carrier trap states that can be affected by illumination. Here, in situ microphotoluminescence measurements are used to elucidate the impact of light-soaking individual methylammonium lead iodide grains in high-quality polycrystalline films while immersing them with different atmospheric environments. It is shown that emission from each grain depends sensitively on both the environment and the nature of the specific grain, i.e., whether it shows good (bright grain) or poor (dark grain) luminescence properties. It is found that the dark grains show substantial rises in emission, while the bright grain emission is steady when illuminated in the presence of oxygen and/or water molecules. The results are explained using density functional theory calculations, which reveal strong adsorption energies of the molecules to the perovskite surfaces. It is also found that oxygen molecules bind particularly strongly to surface iodide vacancies which, in the presence of photoexcited electrons, lead to efficient passivation of the carrier trap states that arise from these vacancies. The work reveals a unique insight into the nature of nonradiative decay and the impact of atmospheric passivation on the microscale properties of perovskite films.

Metal halide perovskites are an exciting class of materials for low-cost optoelectronics but their performance remains limited by nonradiative losses. The surface adsorption of different atmospheric molecules to different types of grains in perovskite films can have a profound impact on the local luminescence of that grain under continual illumination depending on whether the grain has few (bright grain) or many (dark grain) defects. Oxygen molecules are shown to bind particularly strongly to iodide vacancies which, in the presence of photoexcited electrons, leads to passivation of the carrier trap states that arise from these vacancies.

In order to utilize the near-infrared (NIR) solar photons like silicon-based solar cells, extensive research efforts have been devoted to the development of organic donor and acceptor materials with strong NIR absorption. However, single-junction organic solar cells (OSCs) with photoresponse extending into >1000 nm and power conversion efficiency (PCE) >11% have rarely been reported. Herein, three fused-ring electron acceptors with varying core size are reported. These three molecules exhibit strong absorption from 600 to 1000 nm and high electron mobility (>1 × 10⁻³ cm² V⁻¹ s⁻¹). It is proposed that core engineering is a promising approach to elevate energy levels, enhance absorption and electron mobility, and finally achieve high device performance. This approach can maximize both short-circuit current density (  J_SC) and open-circuit voltage (V_OC) at the same time, differing from the commonly used end group engineering that is generally unable to realize simultaneous enhancement in both V_OC and J_SC. Finally, the single-junction OSCs based on these acceptors in combination with the widely polymer donor PTB7-Th yield J_SC as high as 26.00 mA cm⁻² and PCE as high as 12.3%.

Single-junction binary-blend polymer solar cells based on PTB7-Th/F8IC afford efficiency of 10.9%, which is higher than those of F6IC (7.1%) and F10IC (10.2%) counterparts. Furthermore, ternary-blend devices based on PTB7-Th/F8IC/PC₇₁BM exhibit J_SC as high as 26.00 mA cm⁻² and power conversion efficiency as high as 12.3%.

Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near-infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high-performance fullerene-free OSCs based on a combination of a medium-bandgap polymer donor (FTAZ) and a narrow-bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single-junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.

High-performance fullerene-free single-junction organic solar cells with power conversion efficiencies up to 12.5% are reported. Transient absorption spectroscopy reveals that exciting either the donor or acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states.

Moving away from the high-performance achievements in organometal halide perovskite (OHP)-based optoelectronic and photovoltaic devices, intriguing features have been reported in that photocarriers and mobile ionic species within OHPs interact with light, electric fields, or a combination of both, which induces both spatial and temporal changes of optoelectronic properties in OHPs. Since it is revealed that the transport of photocarriers and the migration of ionic species are affected not only by each other but also by the inhomogeneous character, which is a consequence of the route selected to deposit OHPs, understanding the nanostructural evolution during OHP deposition, in terms of the resultant structural defects, electronic traps, and nanoscopic charge behaviors, will be valuable. Investigation of the film-growth mechanisms and strategies adopted to realize OHP films with less-defective large grains is of central importance, considering that single-crystalline OHPs have exhibited the most beneficial properties, including carrier lifetimes. Critical factors governing the behavior of photocarriers, mobile ionic species, and nanoscale optoelectronic properties resulting from either or all of them are further summarized, which may potentially limit or broaden the optoelectronic and photovoltaic applications of OHPs. Through inspection of the recent advances, a comprehensive picture and future perspective of OHPs are provided.

With giant steps regarding organometal halide perovskite (OHP)-based optoelectronic and photovoltaic devices having been made, OHPs are being driven toward applications beyond photovoltaics. Recent progress regarding the various characteristics of OHPs and their impact on photovoltaic devices are reviewed, from microstructural evolution coupled with nanostructural/electronic disorder to photoinduced charge-carrier dynamics; the implications for potential applications are also outlined.

Lead-free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb2+ cation with a divalent cation or substituting three Pb2+cations with two trivalent cations. Herein we report the substitution of two Pb2+cations with one monovalent Ag+ and one trivalent Bi3+ cations to synthesize Cs2AgBiX6 (X= Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, we have elucidated the charge carrier relaxation mechanism in the double perovskite NCs. The Cs2AgBiBr6 NCs exhibit ultrafast hot-carrier cooling (<1 ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs2AgBiCl6 NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects.