Planar Solar Cells Research Articles

All‐Inorganic CsPb1−xGexI2Br Perovskite with Enhanced Phase Stability and Photovoltaic Performance

Compared with organic-inorganic perovskites, all-inorganic cesium-based perovskites without volatile organic compounds have gained extensive interests because of the high thermal stability. However, they have a problem on phase transition from cubic phase (active for photo-electric conversion) to orthorhombic phase (inactive for photo-electric conversion) at room temperature, which has hindered further progress. Herein, novel inorganic CsPb1-x Gex I2 Br perovskites were prepared in humid ambient atmosphere without a glovebox. The phase stability of the all-inorganic perovskite was effectively enhanced after germanium addition. In addition, the highest power conversion efficiency of 10.8 % with high open-circuit voltage (VOC ) of 1.27 V in a planar solar cell based on CsPb0.8 Ge0.2 I2 Br perovskite was achieved. Furthermore, the highest VOC up to 1.34 V was obtained by CsPb0.7 Ge0.3 I2 Br perovskite, which is a remarkable record in the field of all-inorganic perovskite solar cells. More importantly, all the photovoltaic parameters of CsPb0.8 Ge0.2 I2 Br perovskite solar cells showed nearly no decay after 7 h measurement in 50-60 % relative humidity without encapsulation.

Solvent-Antisolvent Ambient Processed Large Grain Size Perovskite Thin Films for High-Performance Solar Cells

In recent years, hybrid organic-inorganic halide perovskites have been widely studied for the low-cost fabrication of a wide range of optoelectronic devices, including impressive perovskite-based solar cells. Amongst the key factors influencing the performance of these devices, recent efforts have focused on tailoring the granularity and microstructure of the perovskite films. Albeit, a cost-effective technique allowing to carefully control their microstructure in ambient environmental conditions has not been realized. We report on a solvent-antisolvent ambient processed CH3NH3PbI3−xClx based thin films using a simple and robust solvent engineering technique to achieve large grains (>5 µm) having excellent crystalline quality and surface coverage with very low pinhole density. Using optimized treatment (75% chlorobenzene and 25% ethanol), we achieve highly-compact perovskite films with 99.97% surface coverage to produce solar cells with power conversion efficiencies (PCEs) up-to 14.0%. In these planar solar cells, we find that the density and size of the pinholes are the dominant factors that affect their overall performances. This work provides a promising solvent treatment technique in ambient conditions and paves the way for further optimization of large area thin films and high performance perovskite solar cells.

Planar Metasurfaces Enable High-Efficiency Colored Perovskite Solar Cells.

The achievement of perfect light absorption in ultrathin semiconductor materials is not only a long‐standing goal, but also a critical challenge for solar energy applications, and thus requires a redesigned strategy. Here, a general strategy is demonstrated both theoretically and experimentally to create a planar metasurface absorber comprising a 1D ultrathin planar semiconductor film (replacing the 2D array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector. Guided by derived formulisms, a new type of macroscopic planar metasurface absorber is experimentally demonstrated with light near‐perfectly and exclusively absorbed by the ultrathin semiconductor film. To demonstrate the power and simplicity of this strategy, a prototype of a planar metasurface solar cell is experimentally demonstrated. Furthermore, the device model predicts that a colored planar metasurface perovskite solar cell can maintain 75% of the efficiency of its black counterpart despite the use of a perovskite film that is one order of magnitude thinner. The displayed cell colors have high purities comparable to those of state‐of‐the‐art color filters, and are insensitive to viewing angles up to 60°. The general theoretical framework in conjunction with experimental demonstrations lays the foundation for designing miniaturized, planar, and multifunctional solar cells and optoelectronic devices.

Rigorous wave-optical treatment of photon recycling in thermodynamics of photovoltaics: Perovskite thin-film solar cells

The establishment of a rigorous theory on thermodynamics of light management in photovoltaics that accommodates various loss mechanisms as well as wave-optical effects in the absorption and reemission of light is at stake in this contribution. To this end, we propose a theoretical framework to calculate the open-circuit voltage enhancement resulting from photon recycling ($\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$) with rigorous wave-optical treatment. It can be applied to both planar thin-film and nanostructured single-junction solar cells. We derive an explicit expression for $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$, which reveals its dependence on internal quantum luminescence efficiency, parasitic reabsorption, and on photon escape probabilities of reemmited photons. While the internal quantum luminescence efficiency is an intrinsic material property, both latter quantities can be determined rigorously for an arbitrary solar cell architecture by three-dimensional electrodynamic dipole emission calculations. We demonstrate the strengths and validity of our framework by determining the impact of photon recycling on the $V_{\mathrm{oc}}$ of a conventional planar organo-metal halide perovskite thin-film solar cell and compare it to established reference cases with perfect antireflection and Lambertian light scattering. Our calculations reveal $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ values of up to 80 mV for the considered device stack in the absence of angular restriction and up to 240 mV when the escape cone above the cell is restricted to $\theta_{\mathrm{out}}=2.5^\circ$ around the cell normal. These improvements impose severe constraints on the parasitic absorption as a parasitic reabsorption probability of only 2\% reduces the $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ to 100 mV for the same angular restriction. Our work here can be used to provide design guidelines.

Influence of the alkyl chain lengths in perylenetetracarboxylic diimide (PTCDI) derivatives on the photovoltaic properties of planar organic solar cells

Abstract Control of the lowest unoccupied molecular orbital (LUMO) energy offset between an electron-donating polymer (D) and electron-accepting molecule (A) is important for the efficient dissociation of photo-induced excitons into free carriers and for improving the power conversion efficiencies (PCEs) of organic photovoltaics (OPVs). The distance between D and A also affects the dissociation of charge transfer (CT) excitons into free carriers, because the exciton binding energy is inversely proportional to the spatial separation in electron-hole coupling. In addition, the distance between D and A influences the probability that photo-induced excitons in the D (or A) domain become CT excitons through the D/A interface. In this study, the influence of D-A separation on OPV characteristics was investigated by fabricating planar OPVs composed of perylenetetracarboxylic diimide derivatives with different alkyl chain lengths (PTCDI-Cn) as A and a low band-gap polymer (PTB7) as D. The D-A separation distance was controlled by the alkyl chain length of the PTCDI-Cn, which functions as a spacer separating D and A. The open-circuit voltage (Voc) improved at the expense of short-circuit current densitie (Jsc) with increasing alkyl chain length in PTCDI-Cn. To examine the electron-hole coupling between A and D, the electroluminescence (EL) from CT states was measured. A flat CT EL was observed for the OPVs with PTCDI-C3 (perylenetetracarboxylic diimide with propyl groups), while the maxima of CT EL bands were clearly seen for other PTCDI-Cn-based OPVs. This result indicates the existence of CT excitons with different electron-hole coupling distances at the PTB7/PTCDI-C3 interface, suggesting the existence of a variety of molecular orientations of PTCDI-C3 on the surface of PTB7. This may be the origin of the increased PCE for the PTCDI-C3-based OPVs compared to other PTCDI-Cn-based OPVs.

Planar perovskite FAxMA1-xPbI3 solar cell by two-step deposition method in air ambient

Perovskite material presents prospective application potential in the third generation solar cell for their broad spectral absorption, small exciton binding energy and long exciton diffusion length. In this paper, low temperature and stable mixed-cation perovskite FAxMA1-xPbI3 solar cells fabrication method in air ambient was reported. The inverted planar ITO/PEDOT:PSS/FAxMA1-xPbI3/PCBM/Ag structure was fabricated in air ambient, which shows better performance than either FAPbI3 or MAPbI3 devices, and FA0.4MA0.6PbI3 demonstrates the highest PCE of 9.7% among the FAxMA1-xPbI3 solar cell devices. In order to decrease the potential barrier between the PCBM and Ag electrode, a layer of Bphen was inserted between PCBM and Ag electrode, the PCE of ITO/PEDOT:PSS/FAxMA1-xPbI3/PCBM/Bphen/Ag perovskite solar cell was increased to 10.86%. We simulated and calculated the absorption and band energy of perovskite materials to verify the accuracy of experimental results. Our result provides a progress towards fabricating perovskite solar cells in the air ambient.

N-Type Doping of Sb2S3 Light-Harvesting Films Enabling High-Efficiency Planar Heterojunction Solar Cells.

Sb2S3 is a kind of new light-absorbing material possessing high stability in ambient environment, high absorption coefficient in the visible range, and abundant elemental storage. To improve the power conversion efficiency of Sb2S3-based solar cells, here we control the defect in Sb2S3 absorber films. It is found that the increase of sulfur vacancy is able to upgrade photovoltaic properties. With the increase in sulfur vacancy, the carrier concentrations are increased. This n-type doping gives rise to an upshift of the Fermi level of Sb2S3 so that the charge transport from Sb2S3 to the electron selection material becomes dynamically favorable. The introduction of ZnCl2 in film fabrication is also found to regulate the film growth for enhanced crystallinity. Finally, the photovoltaic parameters, short-circuit current density, open-circuit voltage, and the fill factor of the device based on the Sb2S3 film are all considerably enhanced, boosting the final power conversion efficiency from 5.15 to 6.35%. This efficiency is the highest value in planar heterojunction Sb2S3 solar cells and among the top values in all kinds of Sb2S3 solar cells. This research provides a fundamental understanding regarding the properties of Sb2S3 and a convenient approach for enhancing the performance of Sb2S3 solar cells.

Optical Study and Experimental Realization of Nanostructured Back Reflectors with Reduced Parasitic Losses for Silicon Thin Film Solar Cells.

We study light trapping and parasitic losses in hydrogenated amorphous silicon thin film solar cells fabricated by plasma-enhanced chemical vapor deposition on nanostructured back reflectors. The back reflectors are patterned using polystyrene assisted lithography. By using O2 plasma etching of the polystyrene spheres, we managed to fabricate hexagonal nanostructured back reflectors. With the help of rigorous modeling, we study the parasitic losses in different back reflectors, non-active layers, and last but not least the light enhancement effect in the silicon absorber layer. Moreover, simulation results have been checked against experimental data. We have demonstrated hexagonal nanostructured amorphous silicon thin film solar cells with a power conversion efficiency of 7.7% and around 34.7% enhancement of the short-circuit current density, compared with planar amorphous silicon thin film solar cells.

Aqueous-Solution-Based Approach Towards Carbon-Free Sb2 S3 Films for High Efficiency Solar Cells.

Sb2 S3 is a new kind of photovoltaic material that is promising for practical application in solar cells owing to its suitable bandgap, earth-abundant elements, and excellent stability. Here, we report on an aqueous-solution-based approach for the synthesis of Sb2 S3 films from easily accessible Sb2 O3 as antimony source. In this reaction, 3-mercaptopropionic acid was applied as both solvent and sulfur precursor, aqueous ammonia was employed as a solvent. After simple annealing at a temperature as low as 270 °C, the spin-coated precursor solution can generate compact, flat, uniform, and well-crystallized Sb2 S3 film. Mechanistic study showed that the formation of Sb-complex with ammonium carboxylates leads to the successful dissolution of Sb2 O3 powder. A suitable annealing process was able to generate carbon-free Sb2 S3 films. Planar heterojunction solar cell based on the as-prepared Sb2 S3 film delivered a power conversion efficiency of 5.57 %, which is the highest efficiency of solution-processed planar heterojunction Sb2 S3 solar cells and a high value in all kinds of Sb2 S3 solar cells. This research provides a convenient approach for the fabrication of device-quality Sb2 S3 films, and highlights solution processing of carbon-free metal chalcogenide thin films as a suitable process for application in optoelectronic devices.

A novel in situ hydrothermal preparation route for Sb2S3 and its solar cell application

In this study, we reported a novel in situ hydrothermal approach for fabricating Sb2S3 films. Metal Sb films were first deposited on substrates using room-temperature radio frequency magnetron sputtering and then undergo an in situ transformation to Sb2S3 films through a novel low-temperature hydrothermal reaction. Characteristics of the obtained Sb2S3 films prepared by our method were investigated. Planar Sb2S3 solar cells utilizing in situ fabricated Sb2S3 film as absorber has also been fabricated and yielded a power conversion efficiency of 1.32%.

Surface polarization and recombination in organic-inorganic hybrid perovskite solar cells based on photo- and electrically induced negative capacitance studies

Abstract Recent studies have shown that organic-inorganic hybrid perovskites (OIHPs) exhibit unprecedented optoelectronic properties for photovoltaic applications. Complete realizations of photo- and electrically induced surface polarization and recombination are critically important for extensively upgrading solar cell performance. We have explored such effects in a planar solar cell comprising ITO(glass)/PEDOT:PSS/CH3NH3PbI3-xClx/PC60BM/PEI/Ag with a maximum power conversion efficiency (PCE) of 15.4%. A remarkable frequency-dependence of surface polarization was observed for the operating device under illumination (i.e. short-circuit current J s c condition) and at steady state (i.e. open-circuit voltage V o c condition). A sign reversal of the capacitance (i.e. negative capacitance) associated with the surface polarization and recombination was detected at some specific steady states. Based on capacitance-voltage (C-V) spectroscopic measurements and theoretical fittings, they can be understood that, (i) a naturally formed surface depletion region due to energy mismatch can be fully suppressed at a very low fluence and moderate forward bias voltage; (ii) a surface capacitance ( C s ) appears at lower ac-modulation frequencies only; (iii) remarkable trap-assist and band to band surface recombination may play the dominant roles for generating the negative capacitive effect in the CH3NH3PbI3-xClx based photovoltaic system.

Spin-coated planar Sb2S3 hybrid solar cells approaching 5% efficiency.

Antimony sulfide solar cells have demonstrated an efficiency exceeding 7% when assembled in an extremely thin absorber configuration deposited via chemical bath deposition. More recently, less complex, planar geometries were obtained from simple spin-coating approaches, but the device efficiency still lags behind. We compare two processing routes based on different precursors reported in the literature. By studying the film morphology, sub-bandgap absorption and solar cell performance, improved annealing procedures are found and the crystallization temperature is shown to be critical. In order to determine the optimized processing conditions, the role of the polymeric hole transport material is discussed. The efficiency of our best solar cells exceeds previous reports for each processing route, and our champion device displays one of the highest efficiencies reported for planar antimony sulfide solar cells.

Laser-Induced Flash-Evaporation Printing CH3NH3PbI3 Thin Films for High-Performance Planar Solar Cells.

Organic-inorganic hybrid perovskites have been emerging as promising light-harvesting materials for high-efficiency solar cells recently. Compared to solution-based methods, vapor-based deposition technologies are more suitable in preparing compact, uniform, and large-scale perovskite thin films. Here, we utilized flash-evaporation printing (FEP), a laser-induced ultrafast single source evaporation method employing a carbon nanotube evaporator, to fabricate high-quality methylammonium lead iodide perovskite thin films. Stoichiometric films with pure tetragonal perovskite phase can be achieved using a controlled methylammonium iodide to lead iodide ratio in evaporation precursors. The film crystallinity and crystal grain growth could further be promoted after postannealing. Planar solar cells (0.06 cm2) employing these perovskite films exhibit a champion power conversion efficiency (PCE) of 16.8% with insignificant hysteresis, which is among the highest reported PCEs using vapor-based deposition methods. Large-area (1 cm2) devices based on such perovskite films also achieved a stabilized PCE of 11.2%, indicating the feasibility and scalability of our FEP method in fabricating large-area perovskite films for other optoelectronic applications.

(Invited) Quantum Dot/Wide-Bandgap Semiconductor Hetero-Junction Interface Engineering for Efficient Infrared Solar Cells

Fulfilling the potential of solar power is essential toward the realization of a low-carbon society. In doing so, ultra-high efficiency solar cells play a crucial role, and several different concepts for ultra-high efficiency have been proposed: e. g. multiple exciton generation, hot-carrier extraction, intermediate band, multi-junction (M-J). Among these concepts, M-J solar cells based on III-V semiconductors are the only solar cells that have exceeded the single-junction limit (approximately 30% under one-sun illumination). However, the solar cells rely heavily on high cost solar cell technology, which makes a wide implementation of the solar cells difficult. We have been exploiting the potential of solution-processed M-J solar cells. In constructing M-J solar cells, OPVs, and perovskite solar cells can be used for the top and/or middle cells of M-J solar cells. However, there are few materials to choose from for the bottom cells that can operate in the short-wave infrared. Therefore, the development of low cost and efficient short-wave infrared solar cells is required. One of the promising candidates for use as the middle and/or bottom cells is lead chalcogenide colloidal quantum dots (CQDs) because the bandgap of bulk PbS is located in the infrared region (3.1 μm) and can be readily tuned by controlling quantum dot synthesis conditions. CQDs are compatible with low-temperature solution-based technologies. Although we see recent progress in solar cell performance of PbS CQD-based solar cells, there are many problems to be solved. The active layer thickness of the solar cells is needed to be thick enough to absorb solar energy in a wide range of the solar spectrum, particularly in the near-infrared and short-wave infrared region. The typical exciton diffusion length of the solar cells is approximately 200 nm in PbS QD/ZnO planar solar cells (Fig. 1 insert (a)). We then focused on PbS CQDs and ZnO nanowire (NW) to construct heterojunction structures to achieve simultaneous enhancement in carrier transport and light harvesting efficiency ((Fig. 1 insert (b)). Unlike PbS QD/ZnO planar structures, PbS QD/ZnO NW hybrid structures forming bulk-heterojuntion allow almost all the photo-generated carriers to reach the PbS QD/ZnO NW interface even when the active layer gets thicker than the carrier diffusion lengths [J. Phys. Chem. Lett., 4, 2455 (2013)]. Our recent study revealed that NW-type solar cells give an effective carrier diffusion length of over 1 μm [J. Phys. Chem. C, 119, 27265 (2015)]. The charge recombination at the interface is a major issue that degrades solar cell performance. Therefore, passivation of the surface of ZnO NWs is an effective way to suppress the recombination process at the interface. In fact, polyethylenimine treatment was successfully performed to decrease the intensity of the PL from defect states of ZnO NWs, which results in increasing the Voc of the NW-type solar cells by approximately 10%, compared to the untreated solar cells. Narrowing the bandgap is necessary to efficiently utilize the sun light. We synthesized different-sized PbS CQDs showing absorption bands in the wavelength region ranging from the visible to short-wave infrared (from 300 nm to 2000 nm). The external quantum efficiency (EQE) spectra of the ZnO NW type solar cells give an EQE peak originating from the first exciton absorption. From the EQE spectra, we confirmed that the solar cells were able to convert photon energy to electricity from 300 nm to 2000 nm (Fig. 1) [ACS Energy Lett., 2, 2110 (2017)]. The solar cells using PbS CQDs having the first exciton absorption peak locating at approximately 1510 nm produced an EQE of 30% at the peak, which is the highest value ever reported on the solution-processed solar cells. These features of PbS QD/ZnO NW solar cells show the high potential for the subcells of solution-processed M-J solar cells. The solar cells that can operate in the infrared region have other interesting features such as low Voc loss as well as relatively high Voc, which is as high as that of Ge solar cells. PbS QD/ZnO NW solar cells have been confirmed to have long-term stability in the air and under illumination [Phys. Stat. Sol. rrl, 8 (12), 961 (2014)]. 5-year air stability was also verified [Sci. Technol. Adv. Mater., 19, 336 (2018)]. Figure 1

Enhanced Crystallization by Methanol Additive in Antisolvent for Achieving High‐Quality MAPbI3 Perovskite Films in Humid Atmosphere

Perovskite solar cells have attracted considerable attention owing to their easy and low-cost solution manufacturing process with high power conversion efficiency. However, the fabrication process is usually performed inside a glovebox to avoid moisture, as organometallic halide perovskites are easily dissolved in water. In this study, we propose a one-step fabrication of high-quality MAPbI3 perovskite films in around 50 % relative humidity (RH) humid ambient air by using diethyl ether as an antisolvent and methanol as an additive into this antisolvent. Because of the presence of methanol, the water molecules can be efficiently removed from the gaps of the perovskite precursors and the perovskite film formation can be slightly controlled, leading to pinhole-free and low roughness films. Concurrently, methanol can be used to tune the DMSO ratio in the intermediate perovskite phase to regulate perovskite formation. Planar solar cells fabricated by using this method exhibited the best efficiency of 16.4 % with a reduced current density-voltage hysteresis. This efficiency value is approximately 160 % higher than the devices fabrication by using only diethyl ether treatment. From the impedance measurement, it is also found that the recombination reaction is suppressed when the device is prepared with methanol additive in the antisolvent. This method presents a new path for controlling the growth and morphology of perovskite films in humid climates and laboratories with uncontrolled environments.

Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics

Summary Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity.

High near-infrared wavelength response planar silicon-heterojunction solar cells

Perovskite/silicon-heterojunction solar cell is a very promising cell structure for using the solar spectrum efficiently and hence improving the energy conversion efficiency. However, perovskite solar cell based on solution technique requires the planar silicon-heterojunction bottom solar cell, which needs a different fabrication process from the textured substrate. In this paper, we developed the high performance planar silicon-heterojunction solar cell by introducing a novel design for the n-type back field and p-type emitter. The results indicate that a wide bandgap and low refractive index nc-SiOx:H for back field improves the open-circuit voltage, fill factor and short-circuit current density by the increased near-infrared long wavelength response. In addition, a nc-Si:H buffer layer between the a-Si:H passivation layer and p-nc-SiOx:H emitter increases the crystallinity in the emitter and then improves device performance further. Combining the developed nc-SiOx:H back field and p-nc-SiOx:H emitter, we achieved an efficiency of 18.77% (Certified 19.1%) on a smooth c-Si wafer. Realization of perovskite/silicon monolithic tandem solar cells with an over 1.7 V output voltage shows the potential application for solar to fuel generation and it also proves that the above strategies are very effective for improving the performance of bottom cell in the tandem solar cells.

Phase Transition Control for High-Performance Blade-Coated Perovskite Solar Cells

Summary Here, we have identified that the key issue for rational transitioning from spin-coating to blade-coating processes of perovskite films arises from whether intermediate phases participate in the phase transition. In situ characterizations were carried out to provide a comprehensive picture of structural evolution and crystal growth mechanisms. These findings present opportunities for designing an effective process for blade-coating perovskite film: a large-grained dense perovskite film with high crystal quality and photophysical properties can be obtained only via direct crystallization for both spin-coating and blade-coating processes. As a result, the blade-coated MAPbI3 films deliver excellent charge-collection efficiency at both short circuit and open circuit, and photovoltaic properties with efficiencies of 18.74% (0.09 cm2) and 17.06% (1 cm2) in planar solar cells. The significant advances in understanding how the phase transition links spin-coating and blade-coating processes should provide a path toward high-performance printed perovskite devices.

Ultra-compact titanium oxide prepared by ultrasonic spray pyrolysis method for planar heterojunction perovskite hybrid solar cells

The electron transport layer of planar-heterojunction perovskite solar cells was produced by the ultrasonic spray pyrolysis (USP) of organotitanium precursors and the spin-coating of TiO2 sol-gel. Although the TiO2 products prepared from both methods have the same anatase crystal structure, the former route yields an ultra-compact, bulk-like film with reduced defects. Results from the electrochemical impedance spectroscopy measurements reveal the electron transfer resistance through the TiO2/CH3NH3PbIxCl3−x interface for the titanium oxide prepared by ultrasonic spray pyrolysis (TiO2-USP) method is lower than that for the titanium oxide prepared by sol-gel (TiO2-SG) method. Interesting, the TiO2-USP device exhibits relative low interfacial resistance of charge carrier through the CH3NH3PbIxCl3−x/spiro-OMeTAD interface as well. As a result, replacing TiO2-SG with TiO2-USP substantially improves the power conversion efficiency from 12.98% to 16.13%, demonstrating the USP is a facile approach to fabricate high-quality TiO2 for developing high-performance perovskite solar cells.

Perovskite seeding growth of formamidinium-lead-iodide-based perovskites for efficient and stable solar cells

Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI3-based solar cells have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI2 film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI3-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.