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Controlled functionalization of 2D black phosphorus (BP) nanosheets provides unique opportunities to tune their chemical, physical, and electronic properties. Herein, the preparation of single‐atom nickel‐doped BP (Ni–BP) sheets using a simple solution‐based strategy is reported. Using the Ni–BP sheets as a passivation layer on top of a perovskite film leads to standard perovskite solar cells (PSCs) with improved performance. The standard n–i–p PSCs with the Ni–BP interlayer achieve maximum power conversion efficiencies of over 22%, with negligible hysteresis and a modest improvement in stability when subjected to different testing conditions. The perovskite films prepared with Ni–BP sheets‐based passivation are found to have reduced defect densities as well as improved charge‐transfer properties and carrier lifetimes. Density‐functional theory calculations support the experimental results through showing that the atomic Ni‐doping increases the work function of the BP interlayer, enabling better hole extraction, as well as increasing the surface hydrophobicity of the BP layer, hence reducing water sorption into the perovskite film. [ABSTRACT FROM AUTHOR]

Materials engineering is key to improving the stability and photovoltaic parameters of inverted perovskite solar cells (PSCs). This work presents the effect of two different fluorinated additives on the performance of PSCs containing the archetypal three‐dimensional perovskite, methylammonium lead triiodide (MAPbI3). 3‐(2,3,4,5,6‐Pentafluorophenyl)propylammonium iodide (FPAI) is added to the anode modifying layer and (2,3,4,5,6‐pentafluorophenyl)methylammonium bromide (FMABr) is blended into the perovskite layer. The inverted devices containing FPAI in the anode modifying layer and 0.32 mol% of FMABr from the perovskite precursor solution had hysteresis‐free current density‐voltage characteristics and a maximum power conversion efficiency of 22.3%, which is an absolute increase of 1.7% compared to the MAPbI3 device (20.6%) of the same architecture but without the additives. [ABSTRACT FROM AUTHOR]

Bulk heterojunction organic solar cells continue to show steady photoconversion efficiency improvements. However, single component organic solar cells are a particularly attractive alternative due to the relative simplicity of device manufacture. It has been proposed that organic semiconductors with a high dielectric constant (≈10) could give rise to spontaneous free charge carrier generation upon photoexcitation. In this manuscript, factors are explored that affect the dielectric constant of organic semiconductors, particularly the optical‐frequency dielectric constant. The properties of monomers, dimers and trimers of two isoelectronic families of materials that have acceptor units composed of one or two dicyanovinylbenzothiadiazole moieties and one to three donor units are compared. The donor components are composed of either fluorenyl or cyclopentadithiophene moieties with the same glycol‐based solubilizing groups. It is found that chromophore planarity and orientation with respect to the substrate, and film density affect the optical and electronic properties of the materials, especially the high‐frequency dielectric constant. The results also indicate that delocalization of the highest occupied and lowest unoccupied molecular orbitals is a critical factor. The dimer with two dicyanovinylbenzothiadiazole moieties and two dithienocyclopentadiene units is found to have the highest optical frequency dielectric constant and overall performance. [ABSTRACT FROM AUTHOR]

Efficient and stable perovskite solar cells with a simple active layer are desirable for manufacturing. Three-dimensional perovskite solar cells are most efficient but need to have improved environmental stability. Inclusion of larger ammonium salts has led to a trade-off between improved stability and efficiency, which is attributed to the perovskite films containing a two-dimensional component. Here, we show that addition of 0.3 mole percent of a fluorinated lead salt into the three-dimensional methylammonium lead iodide perovskite enables low temperature fabrication of simple inverted solar cells with a maximum power conversion efficiency of 21.1%. The perovskite layer has no detectable two-dimensional component at salt concentrations of up to 5 mole percent. The high concentration of fluorinated material found at the film-air interface provides greater hydrophobicity, increased size and orientation of the surface perovskite crystals, and unencapsulated devices with increased stability to high humidity. Efficient and stable perovskite solar cells with simple active layers are desirable for manufacturing, yet formation of a two-dimensional component in the perovskite film compromises the performance. Here, the authors report low temperature fabrication of highly efficient and stable inverted solar cells by adding a fluorinated lead salt. [ABSTRACT FROM AUTHOR]

Effects of different cathode metals, such as aluminum, calcium, and silver, and difference in their reflectivity on the photocurrent generation in pentacene-C60 heterojunction solar cell are presented. Using optical transfer matrix calculations, we find that metal reflectivity has a profound impact on the electrical field confinement within the multilayer device structures. Silver as cathode offers better optical-field confinement close to the pentacene-C60 interface over generally preferred aluminum cathode. External quantum efficiency measurements confirm higher exciton dissociation efficiency and high photocurrent generation ability of silver over aluminum cathode making the choice of cathode metal an important parameter in device optimization. [ABSTRACT FROM AUTHOR]

Current–voltage hysteresis of perovskite solar cells (PSCs) has raised the concern of accurate performance measurement in practice. Although various theories have been proposed to elucidate this phenomenon, the origin of hysteresis is still an open question. Herein, the use of guanidinium cation (Gu+)‐dopant is demonstrated to tailor the crystal structure of mixed‐cation formamidinium‐cesium lead triiodide (FA0.83Cs0.17PbI3) perovskite, resulting in an improved energy conversion efficiency and tunable current–voltage hysteresis characteristic in planar solar cells. Particularly, when the concentration of Gu‐dopant for the perovskite film increases, the normal hysteresis initially observed in the pristine PSC is first suppressed with 2%‐Gu‐dopant, then changed to inverted hysteresis with a higher Gu‐dopant. The hysteresis tunability behavior is attributed to the interplay of charge/ion accumulation and recombination at interfaces in the PSC. Furthermore, compared to the cell without Gu+‐dopant, the optimal content of 2% Gu+‐dopant also increases the device efficiency by 14%, reaching over 17% under one sun illumination. A new method to control current–voltage hysteresis of planar structured FA0.83Cs0.17PbI3 perovskite solar cells (PSCs) is presented while enhancing device efficiency through tailoring the crystal structure of the perovskite compound with a guanidinium cation (Gu+)‐dopant. New insights into the correlation of the dynamics of device hysteresis with the interfacial charge recombination and accumulation in the PSCs are revealed. [ABSTRACT FROM AUTHOR]

The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes. [ABSTRACT FROM AUTHOR]

Non-fullerene acceptors are now capable of being used in high efficiency bulk heterojunction (BHJ) donor-acceptor organic solar cells. Acceptors comprising single or multiple linked chromophores have been used. We have developed a new non-fullerene molecular acceptor as well as two non-polymeric macromolecular materials that contain four equivalents of a similar chromophore, but can adopt different spatial arrangements of the chromophores. We compare the effect of having single and multiple chromophores within a macromolecule on the charge generation processes in P3HT:non-fullerene acceptor BHJ films using Transient Absorption Spectroscopy (TAS) and Time Resolved Microwave Conductivity (TRMC) measurements. It was found from the TAS measurements that at low weight percent (5 wt%) the single chromophore formed more polarons than the acceptors in which chromophores were linked, due to it having a more even distribution within the film. At higher concentrations (50 wt%) the trend was reversed due to the single chromophore forming crystalline domains, which reduced the interface area with the P3HT donor. The TRMC measurements showed that more mobile carriers were formed in the macromolecular acceptors when used at low concentrations in the blend and, independent of concentration, mobile carriers had a longer lifetime when compared to films containing the molecular material, which we ascribe to the charges being able to sample more than one chromophore and thus reduce recombination events. [ABSTRACT FROM AUTHOR]

Solution-processable non-polymeric organic semiconductors are attractive for opto-electronic applications due to their relatively simple synthetic reproducibility and characterization, and enhanced capability for fine-tuning of their properties. We report the synthesis, charge transport, photophysics, and photovoltaic properties of three non-polymeric materials based upon bisarylthiophenyl diketopyrrolopyrrole [DPP(ThAr) 2 ]. The DPP(ThAr) 2 molecules are comprised of solubilizing alkyl groups, 2-ethylhexyl (for 1a ) and 2-octyldodecyl (for 1b and 2 ), which are capped with "electron accepting" moieties - fluorenone ( 1 ) or benzothiadiazole ( 2 ). While the materials could be prepared under standard Suzuki-Miyaura cross-coupling conditions, a simple direct arylation afforded 2 and 1b in good yields. We found 1a with a short alkyl chain lacked sufficient solubility for solution processing but 1b and 2 are solution processable and form good quality films. All three materials exhibited ambipolar charge transport in field effect transistors, with 1b showing balanced charge mobilities of about 10 −3 cm 2 V −1 s −1 . Bulk heterojunction solar cells of 1b or 2 with PC71BM were found to have high open-circuit voltages (up to 0.9 V) with power conversion efficiencies of up to 4.5% and 2.6%, respectively. [ABSTRACT FROM AUTHOR]

Photocurrent generation in organic bulk heterojunction (BHJ) solar cells is most commonly understood as a process which predominantly involves photoexcitation of the lower ionization potential species (donor) followed by electron transfer to the higher electron affinity material (acceptor) [i.e., photoinduced electron transfer (PET), which we term Channel I]. A mirror process also occurs in which photocurrent is generated through photoexcitation of the acceptor followed by hole transfer to the nonexcited donor or photoinduced hole transfer (PHT), which we term Channel II. The role of Channel II photocurrent generation has often been neglected due to overlap of the individual absorption spectra of the donor and acceptor materials that are commonly used. More recently Channel II charge generation has been explored for several reasons. First, many of the new high-efficiency polymeric donors are used as the minority component in bulk heterojunction blends, and therefore, the acceptor absorption is a significant fraction of the total; second, nonfullerene acceptors have been prepared, which through careful design, allow for spectral separation from the donor material, facilitating fundamental studies on charge generation. In this article, we review the methodologies for investigating the two charge generation channels. We also discuss the factors that affect charge generation via Channel I and II pathways, including energy levels of the materials involved, exciton diffusion, and other considerations. Finally, we take a comprehensive look at the nonfullerene acceptor literature and discuss what information about Channel I and Channel II can be obtained from the experiments conducted and what other experiments could be undertaken to provide further information about the operational efficiencies of Channels I and II. [ABSTRACT FROM AUTHOR]

A deposition strategy is demonstrated for fabricating perovskite solar cells (PSCs) under ambient laboratory conditions. A nitrogen-flow treatment followed by a short (3 min), low-temperature (135 °C) anneal enables control of the nucleation and crystal growth of slot-die coated perovskite layers without the need for an anti-solvent for devices. PSCs using either glass or poly(ethylene terephthalate) (PET) substrates were prepared with an n-i-p device architecture and comprised a perovskite layer containing formamidinium and cesium cations, with or without the additive of 2-(2,3,4,5,6-pentafluorophenyl)-ethylammonium iodide (FEAI). The perovskite layer in glass-based devices was slot-die coated onto a stationary substrate with a moving coating head, whereas the deposition of the perovskite layer in PET-based devices was achieved by slot-die coating onto a moving (roll-to-roll, R2R) web using a stationary coating head. The processing conditions enabled the fabrication of PSCs displaying excellent temperature and humidity tolerance (up to 50% relative humidity). Devices fabricated on the glass substrates had power-conversion efficiencies (PCEs) of more than 18%, with PCEs of more than 15% measured for cells on the PET film substrates. The use of the FEAI additive improved the environmental stability of encapsulated glass-based PSC devices, which retained up to 70% of their initial PCE after 1000 h at maximum power point under 1-sun illumination in air. The scalability of the fabrication techniques used in this work, along with the low annealing temperature and ability to process in an ambient laboratory environment, makes this a promising approach for translation to industrial-scale R2R production processes. • Nitrogen-flow strategy for slot-die coated perovskite films at the short annealing time and reduced annealing temperature. • Fluorine-containing additive improves the stability and performance of perovskite solar cells (PSCs). • Characteristics of the perovskite films are unimpacted by well controlling the amounts of additives. • Power conversion efficiencies of 18.35% with slot-die coated and 15.00% with roll-to-roll printed PSCs are achieved in ambient conditions. [ABSTRACT FROM AUTHOR]

The performance of bulk heterojunction (BHJ) organic solar cells can be affected by a range of factors including the materials combination, processing solvent, post deposition annealing, and/or whether they are used in a conventional or inverted architecture. In this study we compared conventional and inverted BHJ solar cells composed of a non-polymeric donor (5 Z ,5′ Z)‐5,5'‐[(5‴,5‴''''‐{4,8‐bis[5‐(2‐ethylhexyl)‐4‐ n -hexylthiophen‐2‐yl]benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl}bis{3′,3'',3‴‐trihexyl‐[2,2':5′,2'':5'',2‴‐quaterthiophene]‐5‴,5‐diyl})bis(methanylylidene)]bis[3‐ n -hexyl‐2‐thioxothiazolidin‐4‐one] (BQR) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor. It was found that the conventional device structure had power conversion efficiencies nearly two and a half times that of the inverted device, 9.0% versus 4.0%. Through a combination of Shockley equivalent circuit fitting, optical modelling, and light intensity dependent photocurrent measurements we identified that the origin of the power losses for the inverted architecture relative to the conventional device structure arose from a larger component of bimolecular recombination. Neutron reflectometry measurements showed that the origin of the larger bimolecular recombination losses for the inverted device was due to the PC 71 BM phase separating, with a PC 71 BM rich layer located near the anode reducing the hole extraction efficiency. [Display omitted] • BQR and PC 71 BM conventional and inverted BHJ solar cells are compared. • The conventional device structure was two and a half times more efficient. • Increased bimolecular recombination identified as the origin of the power losses for the inverted architecture. • A PC 71 BM rich layer near the anode reduced the hole extraction efficiency in inverted device. [ABSTRACT FROM AUTHOR]

• Three-dimensional naphthalene diimide-based acceptor for BHJ solar cells. • Acceptor contains four moieties attached to a central tetraphenylethylene unit. • Acceptor showed balanced hole and electron transport. • A 1:1 donor:acceptor weight ratio gave devices with the best PCE of 3.0 %. Non-fullerene acceptors are proving to be effective for high efficiency polymer bulk heterojunction solar cells. We report the synthesis and properties of a three-dimensional naphthalene diimide-based acceptor where four dithienylnaphthalene moieties are attached to a central tetraphenylethylene unit. In spite of the material being designed as an electron acceptor it was found to have similar and good hole and electron mobilities of 1.5 × 10−4 and 1.8 × 10−4 cm2/Vs, respectively. When blended with PTB7-Th, which had previously been used with other three-dimensional acceptors, at a weight ratio of 1:1 the hole mobility was found to increase slightly but the electron mobility decreased such that there was an imbalance of two orders of magnitude. The blend with a 1:1 donor:acceptor weight ratio was found to give devices with the best power conversion efficiency of 3.0 % in an inverted device architecture, which is amongst the best for non-polymeric naphthalene diimide acceptors. A combination of transient absorption and intensity dependent measurements, along with the charge mobility results indicated that while exciton separation was rapid, the performance of the devices was limited by recombination processes that decreased the extraction of the photogenerated charges. [ABSTRACT FROM AUTHOR]