Ligang Yuan

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.

Delicate engineering of chromaticity is required to faithfully reproduce colors in a backlit display, this is extremely difficult for green downconverters because the human eye is highly sensitive to green colors. The central challenge is to achieve finely tunable green emissions in the narrow range of 525–535 nm while keeping the full width at half maximum (FWHM) <25 nm at the same time. Here, a room-temperature ion-exchange-mediated self-assembly strategy for preparing FAPbBr₃ (FA = CH(NH₂)₂⁺) nanoplates (NPs) to fulfill this goal is introduced. 2D layered OA₂PbBr₄ (OA is octadecylamine) NPs are first synthesized by spontaneous reprecipitation, and are then transformed into FAPbBr₃ NPs through a OA⁺-to-FA⁺ exchange induced self-assembly of HP monolayers. A c-axis contraction in this process makes a relative large thickness variation in OA₂PbBr₄ NPs, which can be realized by simply varying the precursor concentration, only result in a small thickness change in subsequent FAPbBr₃ NPs, thereby enabling finely tunable emissions in the range of 525–535 nm along with FWHM <25 nm and a quantum yield up to 85%. As a downconverter, the FAPbBr₃ NPs realize an ultrapure green backlight that covers ≈95% Rec. 2020 standard in the CIE 1931 color space.

A room-temperature ion-exchange-mediated self-assembly strategy is developed to prepare FAPbBr₃ nanoplates. This strategy enables finely tunable, ultrapure green emissions in the highly desirable narrow range of 525–535 nm along with narrow linewidths and high photoluminescence quantum yields, therefore enabling the “greenest” backlight for displays.

Two new small-molecule donors, DTSi(FBTTh₂Cy)₂ and DTGe(FBTTh₂Cy)₂, were synthesized by replacing hexyl-end-side with cyclohexyl-end-side groups on dithieno[3,2-b:2′,3′-d]silole (DTSi) and dithieno[3,2-b:2′,3′-d]germole (DTGe)-based cores. The current results indicate that the cyclized-end groups in small conjugated materials possess a high potential for improving organic solar cells.

The conformation adopted by single reduced graphene oxide flakes dispersed in a P3HT matrix is determined by the type of organic substituents grafted covalently to their surface. When these composite materials are employed as hole-transporting materials in perovskite solar cells, their morphology determines the performance: with the smooth ones providing reproducible devices with good power conversion efficiency (PCE), while the crumpled ones generating undesired local short circuits that ultimately reduce PCE.

In article number 1705955, Wen-Yong Lai, Wenming Su, and co-workers develop screen-printed poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) grids as ITO-free anodes for flexible organic light-emitting diodes (OLEDs). The great potential of the method is demonstrated by manufacturing printed large-area flexible grid electrodes. The image illustrates the screen printing process as well as the potential application of the flexible grid electrodes to construct flexible OLEDs.

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.

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.

An effective and universal sequential solution deposition process is reported, which realizes spontaneous improvements in the film quality and interfacial engineering (dual-effect device) in planar perovskite solar cells (PSCs). Compared with the conventional devices, the PSCs fabricated via this process exhibit high power conversion efficiency (PCE) of up to 19.40% with 26.9% increment, compared to the control device (PCE = 15.29%).

A new type of composite hole-transport materials based on a new metal-organic copper complex (CuH) and Spiro-OMeTAD have been developed for perovskite solar cells. As a result, the conversion efficiencies obtained for perovskite solar cells based on the composite HTM system reach as high as 18.83%, which is superior to solar cells based on the individual hole-transport materials CuH (15.75%) or Spiro-OMeTAD (14.47%) under the same working conditions.

Molecular weight is an important factor determining the morphology and performance of all-polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide-based acceptor polymer are prepared with molecular weight varying from M_n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M_n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M_n = 167 kDa however is found to produce a drop in performance to 3% due to liquid–liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.

Excessively high molecular weights are shown to be detrimental to the performance of all-polymer solar cells. Increasing the molecular weight of the acceptor polymer to M_n = 167 kDa is found to result in liquid–liquid phase separation negatively impacting charge generation and collection. Intermediate molecular weights instead provide an optimum morphology with good carrier mobilities and improved molecular order.

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.

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.

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%.

The carrier concentration of the electron-selective layer (ESL) and hole-selective layer can significantly affect the performance of organic–inorganic lead halide perovskite solar cells (PSCs). Herein, a facile yet effective two-step method, i.e., room-temperature colloidal synthesis and low-temperature removal of additive (thiourea), to control the carrier concentration of SnO₂ quantum dot (QD) ESLs to achieve high-performance PSCs is developed. By optimizing the electron density of SnO₂ QD ESLs, a champion stabilized power output of 20.32% for the planar PSCs using triple cation perovskite absorber and 19.73% for those using CH₃NH₃PbI₃ absorber is achieved. The superior uniformity of low-temperature processed SnO₂ QD ESLs also enables the fabrication of ≈19% efficiency PSCs with an aperture area of 1.0 cm² and 16.97% efficiency flexible device. The results demonstrate the promise of carrier-concentration-controlled SnO₂ QD ESLs for fabricating stable, efficient, reproducible, large-scale, and flexible planar PSCs.

SnO₂ quantum dots (QDs) are synthesized by a simple and reproducible two-step low-temperature method, in which the carrier concentration of colloidal SnO² QDs is controlled. Planar perovskite solar cells with the efficiencies of 20.8% in small size (0.09 cm²), ≈19% in large size (1 cm²), and 16.97% for flexible devices with low-temperature processed SnO₂ QD electron-selective layers are obtained.

Despite the rapid progress in solar power conversion efficiency of archetype organic–inorganic hybrid perovskite CH₃NH₃PbI₃-based solar cells, the long-term stability and toxicity of Pb remain the main challenges for the industrial deployment, leading to more uncertainties for global commercialization. The poor stabilities of CH₃NH₃PbI₃-based solar cells may not only be attributed to the organic molecules but also the halides themself, most of which exhibit intrinsic instability under moisture and light. As an alternative, the possibility of oxide perovskites for photovoltaic applications is explored here. The class of lead-free stable oxide double perovskites A₂M(III)M(V)O₆ (A = Ca, Sr, Ba; M(III) = Sb³⁺ or Bi³⁺; M(V) = V⁵⁺, Nb⁵⁺, or Ta⁵⁺) is comprehensively explored with regard to their stability and their electronic and optical properties. Apart from the strong stability, this class of double perovskites exhibits direct bandgaps ranging from 0.3 to 3.8 eV. With proper B site alloying, the bandgap can be tuned within the range of 1.0–1.6 eV with optical absorptions as strong as CH₃NH₃PbI₃, making them suitable for efficient single-junction thin-film solar cell application.

The class of lead-free stable oxide double perovskites A₂M(III)M(V)O₆ (A = Ca, Sr, Ba; M(III) = Sb³⁺ or Bi³⁺; M(V) = V⁵⁺, Nb⁵⁺, or Ta⁵⁺) is comprehensively explored with regard to their stability and their electronic and optical properties. Apart from the strong stability, this class of double perovskites exhibits direct bandgaps ranging from 0.3 to 3.8 eV. With proper B site alloying, the bandgap can be tuned within the range of 1.0–1.6 eV with strong optical absorptions, making them suitable for efficient single-junction thin-film solar cell application.

In article number 1701405, Hae Jung Son and co-workers develop D₁-A-D₂-A-type random terpolymers. Organic photovoltaics (OPVs) introducing the resulting polymer achieve a high efficiency of 10.31%. Furthermore, due to outstanding solution processability of the random terpolymer, 1 cm² OPVs reproducibly shows a high efficiency of up to 9.42% using thick active layers in the range of 250–380 nm.

A novel and low-temperature processed in-situ grown ZnO nanorod arrays on AZO electrode are applied as electron transport materials (ETMs) in perovskite solar cells. The assembled flexible device shows excellent mechanical robustness, retaining 90% of efficiency after 1000 bending cycles. In addition, the fabrication process is time-saving and can be recyclable, paving the way for industrial production.