Symposium ES1 : Perovskite Solar Cells—Towards Commercialization

Lead sulfide (PbS) colloidal quantum dots (CQDs) have emerged as attractive candidates for thin film photovoltaic applications for their tunable near-infrared absorption, multiple exciton generation effect and solution processability. In PbS CQD solar cells, however, insulating ligands such as 1,2-ethanedithiol (EDT) and 3-mercaptopropionic acid (MPA) are often used and do not contribute to optical absorbance, thereby limiting short-circuit current density (J_sc). To solve this issue, we chose CH₃NH₃PbI₃ perovskite as a promising capping ligand of PbS CQDs by taking advantage of its long carrier diffusion length and complementary optical spectrum with PbS CQDs. In our work, a solid-state ligand exchange method instead of complicated solution-phase ligand exchange method is employed to obtain CH₃NH₃PbI₃ capped PbS CQDs, which exhibit significantly improved optical and electrical properties than neat PbS CQDs. Saturated solution of CH₃NH₃PbI₃ in acetonitrile enables to attain thick PbS films with complete ligand replacement in the layer-by-layer deposition process. In inverted solar cells of CH₃NH₃PbI₃ capped PbS CQDs, CH₃NH₃PbI₃ ligands can not only create a p/n heterojunction with PbS to facilitate charge separation, but also act as an energy relay between PbS and the TiO₂ layer to form the cascade energy alignment and reduce the energy loss. Thus the optimal solar cell reaches an impressive J_sc of ~25 mA/cm² and a power conversion efficiency (PCE) of 4.25%. Furthermore, upon the addition of EDT-capped PbS CQDs, the bilayer solar cells yields a remarkably enhanced PCE up to 5.28%, due to more balanced and efficient charge transport in the device.

Reference:
J. Peng, Y. Chen, X. Zhang, A. Dong, Z. Liang, Solid-State Ligand-Exchange Fabrication of CH₃NH₃PbI₃ Capped PbS Quantum Dot Solar Cells. Adv. Sci. 3, 1500432 (2016).

Hybrid organic-inorganic perovskite materials are now attracting more and more attention as the very promising candidate for the next generation low cost and high efficiency solar cell technology. The certified power conversion efficiency (PCE) of the perovskite solar cells (PSCs) rapidly increased from 3.8% to 22.1% in the past several years. The traditional lead source used in organic-inorganic perovskite precursor solution is halide lead such as PbI₂ or PbCl₂. However, some complex fabrication processes like anti-solvent method, two-step solution method or long time annealing is needed to convert lead iodide (PbI₂) or lead chloride (PbCl₂)-based precursor solution into good perovskite film. Lead acetate (PbAc₂) is the most potential material to replace the halide lead because of the simple fabrication process and short annealing time to form high quality film. Furthermore, there is still plenty of room to improve the efficiency of PbAc₂-based PSCs. Here, we present two simple approaches: crystallization optimization and controlled interfacial engineering. The crystallization of the perovskite film was controlled through the pre-treatment of the substrate and the precursor solvent. The optimized perovskite films were obtained by suitable temperature treatment, and as a result, the PbAc₂-based PSCs achieved the PCE of 18.2% for 0.1 cm² active area and 8.8% efficiency for large active area (2 cm²) devices. Moreover, a thin ionic liquid layer was introduced to modify the PEDOT:PSS hole transporting layer. The introduction of ionic liquid enhanced the charge transportation between the perovskite film and PEDOT:PSS layer. The device with optimized crystallization and controlled interfacial engineering delivered a champion PCE of 19.2% for 0.1 cm² active area, which also exhibited no hysteresis under the scanning conditions.

The development of PV has seen quite a few alternatives (to c-Si), but only few are still with us, and even fewer have really impacted PV’s role as a more sustainable source of electrical power than fossil fuel-based generation. By looking for ways to put Halide Perovskite(HaP) cells in perspective with respect to other PV cells, we can suggest issues, both fundamental scientific and more practical development ones, that can be important for allowing HaP-based PV to influence the world electrical energy scene. These include real, perceived or imagined characteristics of the materials. At the meeting I will discuss these issues, with some emphasis on the low recombination rate (à high photovoltage), one of the more attractive features of HaP-based PV.

Work done with Gary Hodes

Support: Israel National Nano-initiative, Weizmann Institute of Science's Alternative sustainable Energy Research Initiative; Israel Ministry of Science.

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 22% and further improvements are expected up to 25%. These novel organometal halide perovskite absorbers which possess exceptionally strong and broad light absorption enable to approach the performances of the best thin film technologies. To commercialize these great solar cells, there are many bottlenecks such as long term stability, large scale fabrication process, and environmental issues.
In this presentation, we introduce our recent efforts to facilitate commercialization of perovskite solar cells. For examples, we introduce a recycling technology of perovskite solar cells, which will facilitate the commercialization as well as solve the environmental issues of perovskite solar cells. Moreover, Br-concentration gradient perovskite materials were realized by using HBr treatment of perovskite materials. The enhancement in hole extraction was verified from measurement of photoluminescence spectroscopy. Also, we are going to discuss about stability issue of perovskite materials regarding charge generation and extraction.

We introduce a one-step solution processing technique to facilitate photovoltaics commercialization – in situ perovskite polycrystal formation via substrate-free growth to produce micron to nano scale (< 500 nm), redispersable methylammonium lead halide crystals (CH₃NH₃Pbl_3, CH₃NH₃PbBr₃, CH₃NH₃PbCl₃ and their mixed halides). Herein, an in situ solution-assisted preparation technique will be described that differs from conventional perovskite growth methods (i.e. one-step or two-step approach in DMF, followed by a hot-plate annealing). As a substrate-free approach, crystal pre-growth eliminates annealing and provides large-area deposition of crystals at room temperature with good film uniformity. This process is a temperature-assisted in situ crystallization method based on varying experimental conditions such as temperature, time, solution concentration and choice of solvent. Controlled homogeneous crystal growth is achieved by an immediate crystallization in solution upon mixing precursors (methylammonium iodide/bromide/chloride and lead (II) iodide/bromide/ chloride) in alcohols (methyl, ethyl, isopropyl, 1-butyl and 2-butyl) or in toluene through a nucleophilic substitution reaction at the solvent boiling points, followed by direct precipitation of the perovskite material.
Our solution growth technique results in dispersive perovskite crystals derived from polar protic alcohols or non-polar toluene as solvents not only eliminates deposition of a film onto a substrate and avoids heating to induce crystallization on the substrate, but also improves surface coverage of the films of the perovskite crystals and shows improvement in air/moisture stability (<1 month for the mixed halides and <2 months for single halide perovskites, tested in a fume hood at ~20°C under varying laboratory humidity level) as well as thermal stability in argon (~150°C for CH₃NH₃PbCl₃, ~ 200°C for CH₃NH₃Pbl₃ and ~300°C for CH₃NH₃PbBr₃) in their dry powder form. The use of a variety of solvents is advantageous over conventional high boiling point solvents such as DMF. The resulting perovskite material has been found redispersable in polar aprotic solvents such as acetonitrile, hexane, or in acetone and toluene, which was extensively characterized by in situ techniques such as high energy synchrotron XRD, wide angle x-ray scattering (WAXS), Fourier transform infrared (FTIR) in transmission/reflection geometry (ATR, %Reflection), UV-Vis-NIR (%Reflection), and micro Raman spectroscopy. In situ FTIR and Raman spectra were combined with thermal annealing experiments, run at 20-400°C in argon/air, explained by first principles calculations. We also performed solid state ¹H, ¹³C, ²⁰⁷Pb –MAS NMR, steady-state/time-resolved photoluminescence measurements, XPS, TGA, AFM, and SEM. (1) M. Acik et al. (2016) in preparation. (2) M. Acik et al. J. Mater. Chem. (2016) A 4 (17), 6185. (3) Gong et al. Energy Environ. Sci. 8 (2015) 1953. (4) Yue et al. Solar Energy 105 (2014) 669.

ACKNOWLEDGEMENT
Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.

Perovskite solar cell (PSC) unprecedentedly developed in recent years due to its excellent photovoltaic performance and simple solution processing method. This time we introduce our main achievements obtained from inverted PSC devices with working area over one centimetre square. Firstly, reproducibility of device performance was improved through controlling the morphology and uniformity of perovskite layer and charge extraction layers in the solar cells. The thick charge extraction layers with high conductivity, such as NiO and TiO₂ heavily doped with Mg²⁺, Li⁺ and Nb⁵⁺, were used to reduce pinhole of the film and increase the long-term stability of device.¹ Secondly, we developed efficient inverted-structure with a perovskite-fullerene graded heterojunction (GHJ), which is featured with a gradient distribution of electron accepting material in the light absorption perovskite layer. This structure is found capable of enhancing the PCE of inverted structured PSCs as it improves the photoelectron collection and reduces recombination loss, especially for the formamidinium (FA) cation based perovskites that have superior spectra response and thermal stability. The conformal fullerene coating on perovskite during the GHJ deposition facilitates a full coverage of fullerene with reduced layer thickness, thus minimize the resistive loss in larger size devices. Though these efforts, we have successfully achieved certified efficiency of 18.2% based on a cell with an aperture area larger than one centimetre square.² Furthermore, design of the modules and the cost simulation of PSCs module were also introduced. The costs of modules are found to be only one third of that of commercial silicon solar cells.³

(1) W. Chen, et al., Science, 350, 944 (2015).
(2) Y. Wu, et al., Nature Energy, 1, 16148 (2016).
(3) M. Cai, et al., Advanced Science, 2016, 1600269.

Perovskite based solar cells, mostly employ solution processed perovskite layers. Evaporated methylammonium lead iodide perovskite layers have also been reported and been employed in solar cells. Our group has developed several perovskite based solar cells, using vacuum based perovskite preparation methods. These metal oxide free p-i-n type perovskite cells exhibit high power-conversion efficiencies. We have extended this work to fully evaporated perovskite devices reaching power conversion efficiencies as high as 20 % in a planar single junction device. We further will report on details related to the n-i-p and p-i-n devices as well as identifiy key properties from the stack. Avenues to further increase the device performance by using multiple cation perovskite prepared via sublimation will also be presented.

Recently, organic-inorganic lead halide perovskites have been intensively studied for forming optoelectronic devices (e.g., solar cells, light-emitting diodes, and photodetectors) because of their low cost and high performance. While high-temperature sintering and annealing processes are typically used to prepare good-quality perovskites, thermal annealing for extended periods is known to cause perovskite decomposition, which degrades device performance, stability, and reproducibility. Therefore, it is highly desirable to develop room-temperature processed methods for controllably forming perovskite films.
In this work, we propose a room-temperature scheme of controllable ligand-assisted formation of perovskite films featuring large grain-sizes, and high crystallinity while being free of impurities. Different representative ligands with different features (molecular size, volatility, solubility, and reac-tivity) are experimentally investigated to unravel the ligand effects on the formation kinetics, morphology, and optoelec-tronic properties of perovskite. The mechanism of ligand-assisted perovskite formation is theoretically elucidated through the thermodynamics and chemical kinetics studies. With the experimental and theoretical studies, a selection rule for identifying ideal ligands for formation of high-quality perovskite films is established. This work offers a fundamental understanding of ligand effects on the formation of perovskite films for the future design of high-performance and low-cost perovskite-based optoelectronics.

Organometallic lead halide perovskite material has gained high popularity among all other semiconductors because of its high performance in a variety of applications in solar cells, sensors, LED, detectors, etc. The ease of processing these materials through solution routes, such as spin-coating, blade-coating and spray coating makes them particularly attractive. Solvent engineering, where precursors are dissolved in a mixture of solvent and anti-solvent drip is applied during the coating process, has recently emerged as a critical step for controlling the microstructural and morphological outcome of ink drying and solidification and has been shown to produce pin-hole-free films with high power conversion efficiency, justifying its broad adoption across the field. This talk will describe our recent investigations into the crucial role of anti-solvent dripping on the solution-to-solid phase transformation of MAPbI3, FAPbI3 and FAxCs1-xPbI3 perovskite systems. To do so, we utilize a suite of in situ diagnostic probes including high speed optical microscopy, optical reflectance and absorbance, and grazing incidence wide angle x-ray scattering (GIWAXS), all performed during spin coating, to monitor the solution thinning behavior, changes in optical absorbance, and nucleation and growth of crystalline phases of the precursor and perovskite. Our investigations reveal that the choice of processing solvent, namely DMSO and GBL, and their ratios, strongly impact the effectiveness of the anti-solvent drip. Our time-resolved observations reveal the anti-solvent drip must be applied before the crystallization of the precursor solvates, which also depends upon the solvent mixture, placing additional importance on the timing of the drip in achieving optimal morphology and device performance reproducibility. The insight provided by in situ studies should improve perovskite thin film manufacturability in terms of solar cell performance, yield and reproducibility.

In the last few years, high efficiency perovskite solar cells of small area have been developed in research lab. However, in order to enable commercialization of this technology, it is pivotal to increase the device area and develop solar cell interconnection processes for making solar modules. For these reasons, we have developed laser scribing processes and monolithically connected perovskite mini-modules.
In upscaling of device area a drop in efficiency due to resistive losses is expected. Therefore, the dimensions of the serial interconnected single solar cells should be properly defined according to the sheet resistance of the transparent conducting oxide (TCO) layer. Moreover, the uniformity of the constituent layers and quality of interconnections must be taken into account to obtain high efficiency solar modules. In this work, we show a reproducible and scalable method to fabricate large-area, high quality perovskite films using a two-step deposition method which combines vacuum- and solution-based processes. Interconnection patterning is obtained entirely by laser scribing using picosecond laser pulses with a wavelength of 355 nm to minimize dead area losses between interconnected solar cells. Preliminary development of monolithically interconnected mini-modules of 6.73 cm² aperture area has already yielded promising results with stabilized efficiencies up to 11%. Devices in planar heterojunction configuration were developed, avoiding the use of high-temperature processes for mesoporous scaffolds. The electron transport layer stack was grown by vacuum deposition techniques, ensuring a uniform coating for large area substrates. The low-temperature process is suitable for flexible plastic foils and roll-to-roll manufacturing. The paper would present results on upscaling of small area perovskite solar cells to mini-module sizes and discuss the challenges and prospects of laser scribing and large area deposition techniques for perovskite solar modules development on different substrates.

Hybrid perovskite solar cell, reaching power conversion efficiency (PCE) above 22%, exert a maximal merit in cost reduction of manufacture by combining cheap materials with low process temperature, the latter leading to high speed production. Low temperature printing process (<120^oC) can be applied to hybrid perovskite cells by using organic conductive materials (PEDOT, PCBM, etc.) and/or nanocrystalline SnO₂, ZnO, and their composites for carrier transporting layers. With high voltage >1.1V, PCE of these cells can compete with sintered metal oxide (TiO₂, etc.)-based perovskite cells on glass substrates. The printing process also meets fabrication of flexible cells on plastic substrates. SnO₂ works as a good electron collector exhibiting durable and hysteresis-less photovoltaic performance, due to good interfacial connects at the junction with perovskite crystals by suppressing recombination. This indicates the quality of heterojunction interfaces is essential to high performing device. We showed that low temperature-based Al₂O₃/ZnO/SnO₂ composite works as good collector of MAPbI₃ with PCE>15% and high stability [1]. On ZnO collector, FAPbI₃ was found to be chemically more stable than MAPbI₃, exhibiting PCE>16% [2]. Chemically most stable TiO₂ generally needs high temperature sintering process to prepare mesoporous layer. As an exception, brookite nanocrystal is capable of strong interparticle necking at temperature <120^oC through dehydration condensation reaction that enables formation of dense uniform layer. Brookite-based plastic (ITO-PEN) film perovskite cell prepared on amorphous SnO_x compact layer shows non-hysteretic performance with PCE 14% and high mechanical stability against bending [3].

Our best efficiencies of low-temperature based perovskite solar cell and triple-cation based cell, both made on TiO₂ electron collectors, were 21.6% and 20.8%, respectively. Despite high efficiency of lead halide perovskite, continuous effort to explore lead-free materials is highly important. Lead-free halide perovskite solar cells can also be made on low-temperature prepared metal oxide collectors. We have fabricated Bi-based perovskite device on various metal oxide underlayers, which exhibited strong influence on the morphology of (CH₃NH₃)₃Bi₂I₉ [4,5]. By interfacial engineering using organic additives, dendritic absorber layer was converted to flat uniform layer, leading to improvement in photovoltaic performance. All above experiments corroborate importance of the quality of metal oxide collector, which determines the quality of current-rectifying interfaces against recombination loss.

References
[1] J. Song, E. Zheng, X.-F. Wang, W. Tian, T. Miyasaka, Solar Ener. Mat. Solar Cells, 2016, 144, 623-630.
[2] J. Song, W. Hu, X.-F. Wang, G. Chen, W. Tian, and T. Miyasaka, J. Mater. Chem. A, 2016,4, 8435-8443.
[3] A. Kogo, M. Ikegami, and T. Miyasaka, Chem. Commun., 2016, 52, 8119-8122.
[4] T. Singh, A. Kulkarni, M. Ikegami, and T. Miyasaka, ACS Appl. Mater. Interfaces, 2016, 8, 14542-14547.
[5] A. Kulkarni, T. Singh, M. Ikegami, and T. Miyasaka, submitted.

Planar heterojunction perovskite solar cells (PSCs) with both high efficiency and good stability under operating conditions remains a challenge. Here, we consider that the TiO₂ electron transfer layer (ETL) is illuminated during operation of PSCs and show that the photoexcited state properties of the compact ETLs play critical roles in defining device performance in terms of efficiency and stability. Both the photoconductivity and photocatalytic activity of TiO₂ are greatly affected by deep level trap states associated with native point defects. In this work, we have engineered a defective TiO₂ thin film containing such hole trap states and used it as the ETL for planar heterojunction PSCs. Photoconductive measurements show that the defective TiO₂ thin film has an internal gain higher than 10³ at a small bias of 0.5 V, which is attributed to the minority carrier trapping mechanism in the film. By coupling the highly photoconductive TiO₂ ETL with a high quality perovskite light absorber synthesized using a low-pressure vapor annealing process, a highly efficient planar heterojunction PSC with relatively small hysteresis is achieved. The maximum PCEs under reverse scan, forward scan, and steady-state are 19.0%, 17.1%, and 17.6%, respectively. Moreover, the stability of the planar heterojunction PSC is greatly improved by using defective TiO₂ as the ETL. TiO₂ is known to be an effective UV photocatalyst for oxidizing organic materials and the hybrid perovskite can be photocatalytically degraded under UV illumination, which typically leads to poor stability for the PSCs. The incorporation of hole-trap states in the TiO₂ ETL reduces its activity for photocatalytic oxidation. As a result, the stability of fabricated planar heterojunction PSCs is greatly improved and we show, for the first time, steady-state PCE greater than 15.4% during continuous operation for 100 h. These results demonstrate that engineering defects in the TiO₂ ETL is a promising way to improve the efficiency and stability of planar heterojunction PSCs.

As an emerging photovoltaic device with commercial potential, hybrid organic-inorganic perovskite solar cells (PSCs) have developed rapidly in the recent years. With a certified 22.1% power conversion efficiency, compatibility with flexible substrates and low fabrication energy consumption, PSCs are attracting enormous interest in both the academic and industrial field. Although the charge transfer and recombination mechanisms are different for solid-state dye-sensitized solar cells (ssDSSCs) and perovskite solar cells, similar hole transport materials and additives are applied in both of them. For instance, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and 4-tert-Butylpyridine (tBP) are both used in PSCs and ss-DSSCs as the hole transport layer additives. However, the only difference is indicated in their functions. In ssDSSCs, tBP is the charge recombination inhibitor while LiTFSI is the dopant.

This research is characterized the roles of tBP and LiTFSI in PSCs. A spectrum-dependent HTL oxidation mechanism in perovskite was proposed which in association with LiTFSI. On the other hand, based on focused ion beam (FIB) and bright filed transimission electron microscopy (BF-TEM), it has been discovered that tBP controls the morphology of HTL in PSCs. The study also indicates that, the existence of tBP can dramatically reduce the hygroscopicity of HTL and enhance the stability of PSCs. Atom probe tomography was applied for the first time in this field to illustrate this phenomenon. Moreover, this research has concluded that LiTFSI and tBP can form a series of complexes. These complexes improve the device performance and also slow down the further degradation from perovskite to its precursors.



Reference

1 Wang, S., Yuan, W., & Meng, Y. S. (2015). Spectrum-Dependent Spiro-OMeTAD Oxidization Mechanism in Perovskite Solar Cells. ACS applied materials & interfaces, 7(44), 24791-24798.

2 Wang, S., Sina, M., Parikh, P., Uekert, T., Shahbazian, B., Devaraj, A., & Meng, Y. S. (2016). Role of 4-tert-Butylpyridine as a Hole Transport Layer Morphological Controller in Perovskite Solar Cells. Nano Letters, 16(9), 5594-5600.

The greatest challenge facing the development of organic and perovskite photovoltaics is combining high efficiency and long-term stability. We recently demonstrated the first realization of stability testing methodology using concentrated sunlight that allows independent control of light intensity, the sample temperature and environment during the exposure.[1] Accelerated photo-stability testing of hybrid perovskite MAPbX₃ films (X = I or Br) by exposure to 100 suns showed that the evolution of light absorption and the corresponding structural modifications were dependent on the type of halide ion and the sample temperature. The degradation in absorption of MAPbI₃ films after exposure at elevated sample temperature (~45-55^oC), due to decomposition of the hybrid perovskite material, was documented, while no photobleaching or decomposition of MAPbBr₃ films were observed after exposure to similar stress conditions. This improved stability was related to differences in Br-related bond strengths and in the perovskites' crystalline forms.[2] The stability of pure MAPbI₃ and MAPbBr₃ films was found to be superior to that of mixed halide compositions MAPb(I_1-xBr_x)₃, possibly due to stressing its crystal structure, inducing more structural defects and/or grain boundaries compared to pure halide perovskites. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials.[3] Furthermore, the synthesis sequence of the Perovskite deposition process was found to affect its stability, due to the effect of PbI₂ residue in the film.[4]


[1] I. Visoly-Fisher, A. Mescheloff, M. Gabay, C. Bounioux, L. Zeiri, M. Sansotera, A. E. Goryachev, A. Braun, Y. Galagan, E.A. Katz, Sol. Ener. Mater. & Sol. Cells 134 (2015), 99–107.

[2] R. K. Misra, S. Aharon, B. Li, D. Mogilyanski, I. Visoly-Fisher, L. Etgar, E. A. Katz, J. Phys. Chem. Lett. 6 (2015), 326−330.

[3] R. K. Misra, L. Ciammaruchi, S. Aharon, D. Mogilyanski, L. Etgar, I. Visoly-Fisher, E. A. Katz, ., ChemSusChem 9 (2016), 2572 – 2577.

[4] In prep.