Efficient perovskite solar cells via improved carrier management.

Metal halide perovskite solar cells (PSCs) are an emerging photovoltaic technology with the potential to disrupt the mature silicon solar cell market. Great improvements in device performance over the past few years, thanks to the development of fabrication protocols1-3, chemical compositions4,5 and phase stabilization methods6-10, have made PSCs one of the most efficient and low-cost solution-processable photovoltaic technologies. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Despite much effort, the performance of the best-performing PSCs is capped by relatively low fill factors and high open-circuit voltage deficits (the radiative open-circuit voltage limit minus the high open-circuit voltage)11. Improvements in charge carrier management, which is closely tied to the fill factor and the open-circuit voltage, thus provide a path towards increasing the device performance of PSCs, and reaching their theoretical efficiency limit12. Here we report a holistic approach to improving the performance of PSCs through enhanced charge carrier management. First, we develop an electron transport layer with an ideal film coverage, thickness and composition by tuning the chemical bath deposition of tin dioxide (SnO2). Second, we decouple the passivation strategy between the bulk and the interface, leading to improved properties, while minimizing the bandgap penalty. In forward bias, our devices exhibit an electroluminescence external quantum efficiency of up to 17.2 per cent and an electroluminescence energy conversion efficiency of up to 21.6 per cent. As solar cells, they achieve a certified power conversion efficiency of 25.2 per cent, corresponding to 80.5 per cent of the thermodynamic limit of its bandgap.

Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene).

Perovskite solar cells typically comprise electron- and hole-transport materials deposited on each side of a perovskite active layer. So far, only two organic hole-transport materials have led to state-of-the-art performance in these solar cells1: poly(triarylamine) (PTAA)2-5 and 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD)6,7. However, these materials have several drawbacks in terms of commercialization, including high cost8, the need for hygroscopic dopants that trigger degradation of the perovskite layer9 and limitations in their deposition processes10. Poly(3-hexylthiophene) (P3HT) is an alternative hole-transport material with excellent optoelectronic properties11-13, low cost8,14 and ease of fabrication15-18, but so far the efficiencies of perovskite solar cells using P3HT have reached only around 16 per cent19. Here we propose a device architecture for highly efficient perovskite solar cells that use P3HT as a hole-transport material without any dopants. A thin layer of wide-bandgap halide perovskite is formed on top of the narrow-bandgap light-absorbing layer by an in situ reaction of n-hexyl trimethyl ammonium bromide on the perovskite surface. Our device has a certified power conversion efficiency of 22.7 per cent with hysteresis of ±0.51 per cent; exhibits good stability at 85 per cent relative humidity without encapsulation; and upon encapsulation demonstrates long-term operational stability for 1,370 hours under 1-Sun illumination at room temperature, maintaining 95 per cent of the initial efficiency. We extend our platform to large-area modules (24.97 square centimetres)-which are fabricated using a scalable bar-coating method for the deposition of P3HT-and achieve a power conversion efficiency of 16.0 per cent. Realizing the potential of P3HT as a hole-transport material by using a wide-bandgap halide could be a valuable direction for perovskite solar-cell research.

Carrier-resolved photo-Hall effect.

The fundamental parameters of majority and minority charge carriers-including their type, density and mobility-govern the performance of semiconductor devices yet can be difficult to measure. Although the Hall measurement technique is currently the standard for extracting the properties of majority carriers, those of minority carriers have typically only been accessible through the application of separate techniques. Here we demonstrate an extension to the classic Hall measurement-a carrier-resolved photo-Hall technique-that enables us to simultaneously obtain the mobility and concentration of both majority and minority carriers, as well as the recombination lifetime, diffusion length and recombination coefficient. This is enabled by advances in a.c.-field Hall measurement using a rotating parallel dipole line system and an equation, ΔµH = d(σ2H)/dσ, which relates the hole-electron Hall mobility difference (ΔµH), the conductivity (σ) and the Hall coefficient (H). We apply this technique to various solar absorbers-including high-performance lead-iodide-based perovskites-and demonstrate simultaneous access to majority and minority carrier parameters and map the results against varying light intensities. This information, which is buried within the photo-Hall measurement1,2, had remained inaccessible since the original discovery of the Hall effect in 18793. The simultaneous measurement of majority and minority carriers should have broad applications, including in photovoltaics and other optoelectronic devices.

Morphology and Performance Enhancement through the Strong Passivation Effect of Amphoteric Ions in Tin-based Perovskite Solar Cells.

Despite the optoelectronic similarities between tin and lead halide perovskites, the performance of tin-based perovskite solar cells remains far behind, with the highest reported efficiency to date being ≈14%. This is highly correlated to the instability of tin halide perovskite, as well as the rapid crystallization behavior in perovskite film formation. In this work, l-Asparagine as a zwitterion plays a dual role in controlling the nucleation/crystallization process and improving the morphology of perovskite film. Furthermore, tin perovskites with l-Asparagine show more favorable energy-level matching, enhancing the charge extraction and minimizing the charge recombination, leading to an enhanced power conversion efficiency of 13.31% (from 10.54% without l-Asparagine) with remarkable stability. These results are also in good agreement with the density functional theory calculations. This work not only provides a facile and efficient approach to controlling the crystallization and morphology of perovskite film but also offers guidelines for further improved performance of tin-based perovskite electronic devices.

Compositional engineering of perovskite materials for high-performance solar cells.

Of the many materials and methodologies aimed at producing low-cost, efficient photovoltaic cells, inorganic-organic lead halide perovskite materials appear particularly promising for next-generation solar devices owing to their high power conversion efficiency. The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium lead halide materials. Here we combine the promising-owing to its comparatively narrow bandgap-but relatively unstable formamidinium lead iodide (FAPbI3) with methylammonium lead bromide (MAPbBr3) as the light-harvesting unit in a bilayer solar-cell architecture. We investigated phase stability, morphology of the perovskite layer, hysteresis in current-voltage characteristics, and overall performance as a function of chemical composition. Our results show that incorporation of MAPbBr3 into FAPbI3 stabilizes the perovskite phase of FAPbI3 and improves the power conversion efficiency of the solar cell to more than 18 per cent under a standard illumination of 100 milliwatts per square centimetre. These findings further emphasize the versatility and performance potential of inorganic-organic lead halide perovskite materials for photovoltaic applications.

Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells.

Organolead trihalide perovskite materials have been successfully used as light absorbers in efficient photovoltaic cells. Two different cell structures, based on mesoscopic metal oxides and planar heterojunctions have already demonstrated very impressive advances in performance. Here, we report a bilayer architecture comprising the key features of mesoscopic and planar structures obtained by a fully solution-based process. We used CH3NH3 Pb(I(1-x)Br(x))3 (x = 0.1-0.15) as the absorbing layer and poly(triarylamine) as a hole-transporting material. The use of a mixed solvent of γ-butyrolactone and dimethylsulphoxide (DMSO) followed by toluene drop-casting leads to extremely uniform and dense perovskite layers via a CH3NH3I-PbI2-DMSO intermediate phase, and enables the fabrication of remarkably improved solar cells with a certified power-conversion efficiency of 16.2% and no hysteresis. These results provide important progress towards the understanding of the role of solution-processing in the realization of low-cost and highly efficient perovskite solar cells.

o-Methoxy substituents in spiro-OMeTAD for efficient inorganic-organic hybrid perovskite solar cells.

Three spiro-OMeTAD derivatives have been synthesized and characterized by (1)H/(13)C NMR spectroscopy and mass spectrometry. The optical and electronic properties of the derivatives were modified by changing the positions of the two methoxy substituents in each of the quadrants, as monitored by UV-vis spectroscopy and cyclic voltammetry measurements. The derivatives were employed as hole-transporting materials (HTMs), and their performances were compared for the fabrication of mesoporous TiO2/CH3NH3PbI3/HTM/Au solar cells. Surprisingly, the cell performance was dependent on the positions of the OMe substituents. The derivative with o-OMe substituents showed highly improved performance by exhibiting a short-circuit current density of 21.2 mA/cm(2), an open-circuit voltage of 1.02 V, and a fill factor of 77.6% under 1 sun illumination (100 mW/cm(2)), which resulted in an overall power conversion efficiency (PCE) of 16.7%, compared to ~15% for conventional p-OMe substituents. The PCE of 16.7% is the highest value reported to date for perovskite-based solar cells with spiro-OMeTAD.

Ethanol purification enables high-quality α-phase FAPbI3 perovskite microcrystals for commercial photovoltaic applications.

Reliable quality and sustainable processes must be developed for commodities to enter the commercial stage. For next-generation photovoltaic applications such as perovskite solar cells, it is essential to manufacture high-quality photoactive perovskites via eco-friendly processes. We demonstrate that ethanol, an ideal green solvent, can be applied to yield efficient alpha-phase FAPbI3 perovskite microcrystals.

Shallow-level defect passivation by 6H perovskite polytype for highly efficient and stable perovskite solar cells.

The power conversion efficiency of perovskite solar cells continues to increase. However, defects in perovskite materials are detrimental to their carrier dynamics and structural stability, ultimately limiting the photovoltaic characteristics and stability of perovskite solar cells. Herein, we report that 6H polytype perovskite effectively engineers defects at the interface with cubic polytype FAPbI3, which facilitates radiative recombination and improves the stability of the polycrystalline film. We particularly show the detrimental effects of shallow-level defect that originates from the formation of the most dominant iodide vacancy (VI+) in FAPbI3. Furthermore, additional surface passivation on top of the hetero-polytypic perovskite film results in an ultra-long carrier lifetime exceeding 18 µs, affords power conversion efficiencies of 24.13% for perovskite solar cells, 21.92% (certified power conversion efficiency: 21.44%) for a module, and long-term stability. The hetero-polytypic perovskite configuration may be considered as close to the ideal polycrystalline structure in terms of charge carrier dynamics and stability.

Quantum barriers engineering toward radiative and stable perovskite photovoltaic devices.

Efficient photovoltaic devices must be efficient light emitters to reach the thermodynamic efficiency limit. Here, we present a promising prospect of perovskite photovoltaics as bright emitters by harnessing the significant benefits of photon recycling, which can be practically achieved by suppressing interfacial quenching. We have achieved radiative and stable perovskite photovoltaic devices by the design of a multiple quantum well structure with long (∼3 nm) organic spacers with oleylammonium molecules at perovskite top interfaces. Our L-site exchange process (L: barrier molecule cation) enables the formation of stable interfacial structures with moderate conductivity despite the thick barriers. Compared to popular short (∼1 nm) Ls, our approach results in enhanced radiation efficiency through the recursive process of photon recycling. This leads to the realization of radiative perovskite photovoltaics with both high photovoltaic efficiency (in-lab 26.0%, certified to 25.2%) and electroluminescence quantum efficiency (19.7 % at peak, 17.8% at 1-sun equivalent condition). Furthermore, the stable crystallinity of oleylammonium-based quantum wells enables our devices to maintain high efficiencies for over 1000 h of operation and >2 years of storage.

Efficient inorganic-organic hybrid perovskite solar cells based on pyrene arylamine derivatives as hole-transporting materials.

A set of three N,N-di-p-methoxyphenylamine-substituted pyrene derivatives have successfully been synthesized and characterized by (1)H/(13)C NMR spectroscopy, mass spectrometry, and elemental analysis. The optical and electronic structures of the pyrene derivatives were adjusted by controlling the ratio of N,N-di-p-methoxyphenylamine to pyrene, and investigated by UV/vis spectroscopy and cyclic voltammetry. The pyrene derivatives were employed as hole-transporting materials (HTMs) in fabricating mesoporous TiO2/CH3NH3PbI3/HTMs/Au solar cells. The pyrene-based derivative Py-C exhibited a short-circuit current density of 20.2 mA/cm(2), an open-circuit voltage (Voc) of 0.886 V, and a fill factor of 69.4% under an illumination of 1 sun (100 mW/cm(2)), resulting in an overall power conversion efficiency of 12.4%. The performance is comparable to that of the well-studied spiro-OMeTAD, even though the Voc is slightly lower. Thus, this newly synthesized pyrene derivative holds promise as a HTM for highly efficient perovskite-based solar cells.

Rationally Designed Eco-Friendly Solvent System for High-Performance, Large-Area Perovskite Solar Cells and Modules.

The important but remained issue to be addressed to achieve the mass production of perovskite solar modules include a large-area fabrication of high-quality perovskite film with eco-friendly, viable production methods. Although several efforts are made to achieve large-area fabrication of perovskite, the development of eco-friendly solvent system, which is precisely designed to be fit to scale-up methods are still challenging. Herein, this work develops the eco-friendly solvent/co-solvent system to produce a high-quality perovskite layer with a bathing in eco-friendly antisolvent. The new co-solvent/additive, methylsulfonylmethane (MSM), efficiently improves the overall solubility and has a suitable binding strength to the perovskite precursor, resulting in a high-quality perovskite film with antisolvent bathing method in large area. The resultant perovskite solar cells showed high power conversion efficiency of over 24% (in reverse scan), with a good long-term stability under continuous light illumination or damp-heat condition. MSM is also beneficial to produce a perovskite layer at low-temperature or high-humidity. MSM-based solvent system is finally applied to large-area, resulting in highly efficiency perovskite solar modules with PCE of 19.9% (by aperture) or 21.2% (by active area) in reverse scan. These findings contribute to step forward to a mass production of perovskite solar modules with eco-friendly way.

Transparent Electrodes with Enhanced Infrared Transmittance for Semitransparent and Four-Terminal Tandem Perovskite Solar Cells.

This report addresses indium oxide doped with titanium and tantulum with high near-infrared transparency to potentially replace the conventional indium tin oxide transparent electrode used in semitransparent perovskite devices and top cells of tandem devices. The high near-infrared transparency of this electrode is possibly explained by the lower carrier concentration, suggesting less defect sites that may sacrifice its optical transparency. Incorporating this transparent electrode into semitransparent perovskite solar cells for both the top and bottom electrodes improved the device performance through possible reduction of interfacial defect sites and modification in energy alignment. With this indium oxide-based semitransparent perovskite top cell, we also demonstrated four-terminal perovskite-silicon tandem configurations with improved photocurrent response in the bottom silicon cell.

Roll-to-roll gravure-printed flexible perovskite solar cells using eco-friendly antisolvent bathing with wide processing window.

Driven by recent improvements in efficiency and stability of perovskite solar cells (PSCs), upscaling of PSCs has come to be regarded as the next step. Specifically, a high-throughput, low-cost roll-to-roll (R2R) processes would be a breakthrough to realize the commercialization of PSCs, with uniform formation of precursor wet film and complete conversion to perovskite phase via R2R-compatible processes necessary to accomplish this goal. Herein, we demonstrate the pilot-scale, fully R2R manufacturing of all the layers except for electrodes in PSCs. Tert-butyl alcohol (tBuOH) is introduced as an eco-friendly antisolvent with a wide processing window. Highly crystalline, uniform formamidinium (FA)-based perovskite formation via tBuOH:EA bathing was confirmed by achieving high power conversion efficiencies (PCEs) of 23.5% for glass-based spin-coated PSCs, and 19.1% for gravure-printed flexible PSCs. As an extended work, R2R gravure-printing and tBuOH:EA bathing resulted in the highest PCE reported for R2R-processed PSCs, 16.7% for PSCs with R2R-processed SnO2/FA-perovskite, and 13.8% for fully R2R-produced PSCs.

Gravure-Printed Flexible Perovskite Solar Cells: Toward Roll-to-Roll Manufacturing.

Recent advances in perovskite solar cells (PSCs) have resulted in greater than 23% efficiency with superior advantages such as flexibility and solution-processability, allowing PSCs to be fabricated by a high-throughput and low-cost roll-to-roll (R2R) process. The development of scalable deposition processes is crucial to realize R2R production of flexible PSCs. Gravure printing is a promising candidate with the benefit of direct printing of the desired layer with arbitrary shape and size by using the R2R process. Here, flexible PSCs are fabricated by gravure printing. Printing inks and processing parameters are optimized to obtain smooth and uniform films. SnO2 nanoparticles are uniformly printed by reducing surface tension. Perovskite layers are successfully formed by optimizing the printing parameters and subsequent antisolvent bathing. 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene is also successfully printed. The all-gravure-printed device exhibits 17.2% champion efficiency, with 15.5% maximum power point tracking efficiency for 1000 s. Gravure-printed flexible PSCs based on a two-step deposition of perovskite layer are also demonstrated. Furthermore, a R2R process based on the gravure printing is demonstrated. The champion efficiency of 9.7% is achieved for partly R2R-processed PSCs based on a two-step fabrication of the perovskite layer.

Achieving Long-Term Operational Stability of Perovskite Solar Cells with a Stabilized Efficiency Exceeding 20% after 1000 h.

Perovskite solar cells (PSCs) with mesoporous TiO2 (mp-TiO2) as the electron transport material attain power conversion efficiencies (PCEs) above 22%; however, their poor long-term stability is a critical issue that must be resolved for commercialization. Herein, it is demonstrated that the long-term operational stability of mp-TiO2 based PSCs with PCE over 20% is achieved by isolating devices from oxygen and humidity. This achievement attributes to systematic understanding of the critical role of oxygen in the degradation of PSCs. PSCs exhibit fast degradation under controlled oxygen atmosphere and illumination, which is accompanied by iodine migration into the hole transport material (HTM). A diffusion barrier at the HTM/perovskite interface or encapsulation on top of the devices improves the stability against oxygen under light soaking. Notably, a mp-TiO2 based PSC with a solid encapsulation retains 20% efficiency after 1000 h of 1 sun (AM1.5G including UV) illumination in ambient air.

Understanding how excess lead iodide precursor improves halide perovskite solar cell performance.

The presence of excess lead iodide in halide perovskites has been key for surpassing 20% photon-to-power conversion efficiency. To achieve even higher power conversion efficiencies, it is important to understand the role of remnant lead iodide in these perovskites. To that end, we explored the mechanism facilitating this effect by identifying the impact of excess lead iodide within the perovskite film on charge diffusion length, using electron-beam-induced current measurements, and on film formation properties, from grazing-incidence wide-angle X-ray scattering and high-resolution transmission electron microscopy. Based on our results, we propose that excess lead iodide in the perovskite precursors can reduce the halide vacancy concentration and lead to formation of azimuthal angle-oriented cubic α-perovskite crystals in-between 0° and 90°. We further identify a higher perovskite carrier concentration inside the nanostructured titanium dioxide layer than in the capping layer. These effects are consistent with enhanced lead iodide-rich perovskite solar cell performance and illustrate the role of lead iodide.

Spatial Distribution of Lead Iodide and Local Passivation on Organo-Lead Halide Perovskite.

We identify nanoscale spatial distribution of PbI2 on the (FAPbI3)0.85(MAPbBr3)0.15 perovskite thin film and investigate the local passivation effect using confocal based optical microscopy of steady state and time-resolved photoluminescence (PL). Different from a typical scanning electron microscope (SEM) morphology study, confocal based PL spectroscopy and microscopy allow researchers to map the morphologies of both perovskite and PbI2 grains simultaneously, by selectively detecting their characteristic fluorescent bands using band-pass filters. In this work, we compare the perovskite samples without and with excess PbI2 incorporation and unambiguously reveal PbI2 distribution for the PbI2-rich sample. In addition, using the nanoscale time-resolved PL technique we show that the PbI2-rich regions exhibit longer lifetime due to suppressed defect trapping, compared to the PbI2-poor regions. The measurement on the PbI2-rich sample indicates that the passivation effect of PbI2 in perovskite film is effective, especially in localized regions. Hence, this finding is important for further improvement of the solar cells by considering the strategy of excess PbI2 incorporation.

Indolo[3,2-b]indole-based crystalline hole-transporting material for highly efficient perovskite solar cells.

We have designed and synthesized fluorinated indolo[3,2-b]indole (IDID) derivatives as crystalline hole-transporting materials (HTM) for perovskite solar cells. The fluorinated IDID backbone enables a tight molecular arrangement stacked by strong π-π interactions, leading to a higher hole mobility than that of the current HTM standard, p,p-spiro-OMeTAD, with a spherical shape and amorphous morphology. Moreover, the photoluminescence quenching in a perovskite/HTM film is more effective at the interface of the perovskite with IDIDF as compared to that of p,p-spiro-OMeTAD. As a consequence, the device fabricated using IDIDF shows superior photovoltaic properties compared to that using p,p-spiro-OMeTAD, exhibiting an optimal performance of 19%. Thus, this remarkable result demonstrates IDID core-based materials as a new class of HTMs for highly efficient perovskite solar cells.

Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells.

The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions. The concentration of defect states, which reduce a cell's performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible. We show that the introduction of additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreases the concentration of deep-level defects. The defect-engineered thin perovskite layers enable the fabrication of PSCs with a certified power conversion efficiency of 22.1% in small cells and 19.7% in 1-square-centimeter cells.