N-type Perovskite Research Articles

Reducing nonradiative recombination for highly efficient inverted perovskite solar cells via a synergistic bimolecular interface

Reducing interface nonradiative recombination is important for realizing highly efficient perovskite solar cells. In this work, we develop a synergistic bimolecular interlayer (SBI) strategy via 4-methoxyphenylphosphonic acid (MPA) and 2-phenylethylammonium iodide (PEAI) to functionalize the perovskite interface. MPA induces an in-situ chemical reaction at the perovskite surface via forming strong P-O-Pb covalent bonds that diminish the surface defect density and upshift the surface Fermi level. PEAI further creates an additional negative surface dipole so that a more n-type perovskite surface is constructed, which enhances electron extraction at the top interface. With this cooperative surface treatment, we greatly minimize interface nonradiative recombination through both enhanced defect passivation and improved energetics. The resulting p-i-n device achieves a stabilized power conversion efficiency of 25.53% and one of the smallest nonradiative recombination induced Voc loss of only 59 mV reported to date. We also obtain a certified efficiency of 25.05%. This work sheds light on the synergistic interface engineering for further improvement of perovskite solar cells.

Synthesis of [60]Fullerene-Fused Lactones via Carboxylic Acid Group-Directed C-H Bond Activation and Further Retro Baeyer-Villiger Reaction.

An efficient palladium-catalyzed reaction of [60]fullerene with benzoic acids via carboxylic acid group-directed C-H bond activation is achieved. The obtained [60]fullerene-fused lactones can undergo a retro Baeyer-Villiger reaction to provide [60]fullerene-fused ketones via apparent reduction in the presence of triflic acid. A representative ketone product obtained by the reduction reaction can be employed as an overcoating layer for the electron-transporting layer in an n-type perovskite solar cell.

Strain relaxation and multidentate anchoring in n-type perovskite transistors and logic circuits

Strain relaxation and multidentate anchoring in n-type perovskite transistors and logic circuits

Reductive Sn2+ Compensator for Efficient and Stable Sn-Pb Mixed Perovskite Solar Cells.

Tin-lead (Sn-Pb) mixed perovskite with a narrow bandgap is an ideal candidate for single-junction solar cells approaching the Shockley-Queisser limit. However, due to the easy oxidation of Sn2+, the efficiency and stability of Sn-Pb mixed perovskite solar cells (PSCs) still lag far behind that of Pb-based solar cells. Herein, highly efficient and stable FA0.5MA0.5Pb0.5Sn0.5I0.47Br0.03 compositional PSCs are achieved by introducing an appropriate amount of multifunctional Tin (II) oxalate (SnC2O4). SnC2O4with compensative Sn2+ and reductive oxalate group C2O4 2- effectively passivates the cation and anion defects simultaneously, thereby leading to more n-type perovskite films. Benefitting from the energy level alignment and the suppression of bulk nonradiative recombination, the Sn-Pb mixed perovskite solar cell treated with SnC2O4 achieves a power conversion efficiency of 21.43%. More importantly, chemically reductive C2O4 2- effectively suppresses the notorious oxidation of Sn2+, leading to significant enhancement in stability. Particularly, it dramatically improves light stability.

Inhibition of ion diffusion/migration in perovskite p-n homojunction by polyetheramine insert layer to enhance stability of perovskite solar cells with p-n homojunction structure.

Perovskite p-n homojunctions (PHJ) have been confirmed to play a crucial role in facilitating carrier separation/extraction in the perovskite absorption layer and provide an additional built-in potential, which benefits the inhibition of carrier recombination in perovskite solar cells (PSCs) and ultimately improves device performance. However, the diffusion and migration of ions between n-type and p-type perovskite films, particularly under operational and heating conditions, lead to the degradation of PHJ structures and limit the long-term stability of PSCs with PHJ structure (denoted as PHJ-PSCs). In this study, we propose an insert layer strategy by directly introducing an ultra-thin polyetheramine (PEA) layer between the n-type and p-type perovskite films to address those challenges arising from ion movements. Femtosecond transient absorption (fs-TAS) and photoluminescence (PL) measurements demonstrate that the PHJ (without and with the insert layer) enhances carrier separation/extraction compared to the single n-type perovskite film. Monitoring the evolution of bromine element distribution reveals that the insert layer can efficiently suppress ion diffusion between perovskite films, even under long-term illumination and heating conditions. Consequently, an efficiency of 23.53% with excellent long-term operational stability is achieved in the optimized PHJ-PSC with the insert layer.

Interfacial engineering eliminates energy loss at perovskite/HTL junction.

Realizing efficient FAPbI3-based devices with high open-circuit voltage (VOC) is still challenging, due to severe energy loss between the n-type perovskite and p-type hole-transporting layer (HTL). Here, we developed a strategy involving controlling the formation of iodine vacancies in order to induce formation of p-type perovskite and hence mitigate such energy loss. Post-deposition of n-butylamine iodide was discovered to induce an n-to-p-type transition in the FAPbI3 perovskite and hence form the p-type perovskite/p-type HTL junction. The resultant device realized a VOC of as high as 1.12 V, a value ∼14.3% higher than that of the corresponding n-type FAPbI3 device (0.98 V).

P-n Homojunction perovskite solar cells: effects of ionic density and thickness of the doped layers

The p-n homojunction perovskite solar cells are promising in comparison to planar heterojunction perovskite solar cells. It is observed that the p-n homojunction provides an efficient built-in electric field for the photo-generated electron-hole pairs to dissociate into free charge-carriers and orient them to transport to the respective electrodes, thereby reducing the overall losses due to recombination processes. However, the existence of p-n homojunction, while in operation, is currently undergoing experimental tests. In this study, we simulated the current-voltage characteristics of both planar heterojunction and p-n homojunction perovskite solar cells. For simulation, we utilised a one-dimensional drift-diffusion equation. The current-voltage characteristics show a high fill factor for the p-n homojunction devices indicating better power conversion efficiency as compared to that of planar heterojunction devices. In addition, we report the spatial distribution of electron- and hole-density with variations in (i) the mobile ionic density, inherently present in the perovskite material and (ii) the thickness of p- and n-type perovskite layers. It is observed that the role of p-n homojunction ceases when the ionic density is higher than 1017 cm−3 or when one of the doped layers is substantially thicker as compared to the other. We correlate the cease of p-n homojunction to (i) the ionic effect which screens the built-in field, and (ii) the predominance of the effect of one doped layer over the other. The study provides impetus to the development of the p-n homojunction perovskite solar cells.

High performance direct current-generating triboelectric nanogenerators based on tribovoltaic p-n junction with ChCl-passivated CsFAMA perovskite

Herein, we develop direct current-generating triboelectric nanogenerators (DC-TENGs) based on the tribovoltaic p-n junction using n-type perovskite (CsFAMA) and p-type conductive polymer (PEDOT:PSS); CsFAMA based DC-TENG is shown to generate a high DC power output of about 2.1 µA cm−2 (current) and 0.33 V (voltage). We also introduce choline chloride (ChCl) to passivate the defects inside CsFAMA to improve the power-generating performance of DC-TENG by increasing triboelectric charge density, carrier mobility, and built-in potential, which are the key factors that determine device performance. Due to the synergetic effect of reduced defect sites (5.0×1015 cm−3 to 1.0×1015 cm−3), enhanced electron mobilities (1.0×10−2 cm2 V−1 s−1 to 2.3×10−2 cm2 V−1 S−1), and modulated work function, the passivated CsFAMA-based DC-TENG generates an output current density of 11 µA cm−2 and an output voltage of 0.80 V. Our results are expected to contribute to the development of high-performance DC-TENGs by presenting a promising strategy that involves controlling the triboelectric semiconducting interface.

Stronger binding force improving surface passivation of perovskites for High-Performance inverted solar cells

The surface properties of perovskites to a large extent affect the charge carrier dynamics of perovskites, which is critical to realize highly efficient and stable perovskite solar cells (PSCs). Herein, the surface properties of the perovskite are manipulated using dimethylamine hydroiodide (DMAI) and 1-[4-(Trifluoromethyl) phenyl] hydrazine hydrochloride (TFMPHC) synergistic system for inverted planar PSCs with a p-i-n device architecture. The results of systemically experimental studies and theoretical calculations reveal that the DMAI and TFMPHC could be chemically coupled through N-H···F hydrogen bond, which could significantly enhance the binding force for the DMAI-TFMPHC system and the perovskites. The DMAI-TFMPHC system shows a stronger binding force with the perovskite than that of the single passivating agent, thereby improving the passivation effect of perovskite films. Meanwhile, the DMAI-TFMPHC treatment could make a more n-type perovskite surface, facilitating the charge extraction and reducing interfacial nonradiative recombination at the interface of the perovskite/electron transport layer. Due to a combination of the stronger binding force and hydrophobicity of trifluoromethyl, the DMAI-TFMPHC-based inverted PSC shows superior photovoltaic performance and stability.

P-Type Carbon Dots for Effective Surface Optimization for Near-Record-Efficiency CsPbI2 Br Solar Cells.

Interface modification to minimize charge recombination and trapping for efficient charge transport is crucial for the performance of perovskite solar cells (PSCs). Herein, functionalized p-type blue carbon dots (B-CDs) are ventured as an interface passivation layer to enhance the efficiency and long-term stability of all-inorganic CsPbI2 Br PSCs. It is found that first the blue carbon dots with abundant NH, CN, CO, and CO functional groups effectively passivate defects by reacting with I- and Pb2+ ions in the perovskite through hydrogen and coordinative bonds. Second, the p-type B-CDs modifiers form a P-N junction with the n-type perovskite to provide efficient pathways for hole transfer and electron blocking. Third, the B-CDs increase the hydrophobicity of the perovskite film to improve the stability of CsPbI2 Br PSCs. With the above advantages, the CsPbI2 Br PSC with B-CDs modification shows an efficiency as high as 16.76%, one of the highest for its type. In addition, the modification renders significant improvement of air and light stability, with 95.33% of the initial PCE retained after storage in the ambient environment for 1000 h. This work demonstrates the great potential of B-CDs application in perovskite-based optoelectronic devices.

Cu(I)-Catalyzed Synthesis of [60]Fullerene-Fused Lactams and Further Electrochemical Functionalization.

A novel and efficient Cu(I)-catalyzed radical heteroannulation reaction of [60]fullerene (C60) with α-bromo acetamides has been disclosed for the direct synthesis of diverse C60-fused lactams. Furthermore, the formed C60-fused lactams can be served as a versatile platform for further electrochemical functionalization to prepare 1,2-, 1,4-, 1,2,3,16-, and 1,4,9,25-adducts of C60. In addition, a representative fullerene product has been applied as an overcoating layer of the electron-transporting layer in n-type perovskite solar cell.

Performance optimization of homojunction perovskite solar cells by numerical simulation

Perovskite materials possess n-type and p-type electrical conductivity through a control over perovskite precursor composition and stoichiometry during the film formation. A p-n homojunction can be constituted between the p-type perovskite and the n-type perovskite and a built-in electric field is formed, which can promote the oriented transportation of the photon-generated carriers and reduce the carrier recombination losses. In this paper, the numerical simulation of p-n homojunction methylammonium lead iodide (MAPbI 3 ) solar cells has been performed and the effects of different electron transport layer (ETL) materials and hole transport layer (HTL) materials, defect density and thickness of the absorber layer and interface defect density on the device photovoltaic performance have been studied. Simulation results show that TiO 2 and Spiro-OMeTAD are the most promising ETL material and HTL material, respectively. To improve device performance, the defect density of the p-type MAPbI 3 absorber layer needs to be as low as 10 15 cm −3 and an optimal absorber thickness of 0.4 μm needs to be guaranteed. Moreover, the device performance is further improved by maintaining the interface defect density less than or equal to 10 9 cm −2 . With these optimizations, a conversion efficiency of 27.10% is achieved for the homojunction perovskite solar cells with a structure consisting of FTO/TiO 2 /n-type MAPbI 3 /p-type MAPbI 3 /Spiro-OMeTAD/Au. Hence, the device structure with perovskite homojunction provide an effective approach towards development of highly efficient solar cells. • Homojunction MAPbI 3 solar cells have been simulated with efficiency of 27.10%. • The effect of different electron transport layer and hole transport layer materials has been investigated. • The impact of bulk defect density and interface defect density has also been studied.

P–n heterojunction perovskite solar cell with bilateral Ohmic contacts

The tunability of the carrier/conduction polarity of halide perovskite via intrinsic defect self-doping makes it possible to implement a simple p–n heterojunction perovskite solar cell. Here, approaching 16% efficient prototype device of the p-type spiro-MeOTAD/n-type MAPbI3 heterojunction perovskite solar cell is demonstrated via synergetic energy band and interface engineering. The n-type perovskite is enabled by self-doping associated with excess PbI2, avoiding a redundant electron transport layer, while the oxygen-induced p-type extrinsic doping (oxidation) of spiro-MeOTAD ensures its high hole conductivity and Ohmic contact with Au back electrode. Furthermore, ITO's effective work function is reduced by the polar molecular modification layer to ensure its Ohmic contact with perovskite. Also, the functional groups of this modification layer effectively passivate the front interface defects, contributing to the enhanced device performance observed. This work highlights the great potential of facile energy band and interface engineering for efficient and simple-structured perovskite solar cells.

Microstructure and thermoelectric properties of Sr0.9La0.1TiO3/TiO2 biphase composite ceramics

As a well-known n-type perovskite thermoelectric oxides, SrTiO3 has always captured attention to find strategies for achieving high thermoelectric performance. In this paper, a series of reduced Sr0.9La0.1TiO3/TiO2 biphase composite ceramics were prepared via the solid-state reaction method. The effects of TiO2 on the microstructure and thermoelectric properties of the ceramics were investigated. The addition of TiO2 enhanced the carrier mobility via the formation of A-site cation vacancies and increased the carrier concentration probably through introducing oxygen vacancies in rutile TiO2 during the reduction process. Moreover, the additional phonon scattering provided by the interface between the second phase and matrix and oxygen vacancies decreased the thermal conductivity. When the TiO2 addition ratio was optimized to 25 wt%, a large power factor of 1.66 mW/K2/m and the ZT value of 0.24 were achieved at 447 K. The present strategy in this work may be feasible to broaden the range of operating temperature of SrTiO3-based oxide thermoelectric materials.

Highly Crystalline and (110)-Oriented n-Type Perovskite Films with Excellent Structural Stability via Cu Doping

The crystallinity improvement of a perovskite film is one of the most important factors that determines its final photovoltaic performance. In this work, we unusually initiated the perovskite film ...

(Invited) Surface Properties of Perovskite Oxynitrides Influence Solar Water Splitting Activity

Photoelectrochemical (PEC) water splitting using semiconductors has been a promising means to directly produce hydrogen gas from water. Semiconductors capable of absorbing a wide wavelength range of visible light are in the limelight at the moment, because the favourable optical property causes thermodynamically high solar-to-hydrogen (STH) conversion efficiency. The n-type perovskite oxynitrides AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) have intensive absorption bands in visible light above 600 nm so that they are one of the leading candidates for the solar water splitting.1 For instance, LaNbON2 has an Eg of 1.7 eV (wavelength λ = 750 nm) and thus potential for the STH conversion efficiency of 29.5% in reference to an incident photon-to-current efficiency of unity. However, the water splitting activity using the oxynitrides is relatively low and unstable under long-term illumination.1-2 The synthesis of oxynitrides, namely, the nitridation of starting oxides to the corresponding oxynitrides, is performed mostly in harsh conditions including a high temperature, long nitridation time, and a high flow rate of NH3. Such the severe nitridation obviously increases a bulk crystallinity of oxynitrides, which it permits efficient light absorption and charge separation under irradiation. In particular, it is necessary to prepare high-crystalline BaBO2N because the oxynitrides have the absence of stoichiometric, crystalline oxide precursors (with Ba/B ratio of unity) for nitridation.3-4 Unfortunately, the severe nitridation enhances reductions of B-site cations to a lower oxidation state (e.g., Nb5+ to Nb4+ or Nb3+) leading to anion vacancy and/or surface impurity phases such as NbO x N y , owing to the high electronegativity of the B-site cations. The surface defect sites, where the photoreaction takes place, promotes recombination of photoexcited holes and electrons during the PEC water oxidation, thus resulting in low photocurrent density. There were several reports on how to decrease defect density of oxynitrides during the nitridation, including flux-assisted nitridation and etching treatment by an aqua regia.4-5 However, these methods should be positively considered the suitable flux media and unintended doping depending on each oxynitride, and the partial loss of oxynitride particles. There was also a limitation in improving the photocurrent density.Herein we present the active and stable solar water oxidation over oxynitride photoanodes, which was remarkably improved by surface annealing in an inert Ar flow to increase the degree of crystallinity and as well to suppress the generation of surface defects.6 Also, we report the quantitative relationship between the surface defect density of the oxynitride and its photocurrent density, demonstrating that surface properties of oxynitrides greatly influence the solar water splitting performance.7 Oxides (or carbonates) contains A-site cations and B2O5 (B=Ta, Nb) were blended and calcined in air at different temperatures from 1273 to 1573 K for 30 h to obtain crystalline oxides such as Ba5B4O15. The starting oxides were nitrided at different temperatures and durations under a NH3 flow. The products were washed by distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed under an Ar flow at an appropriate temperature depending on oxynitride. For PEC measurements, particulate oxynitride photoanodes were prepared by a particle transfer method or an electrophoretic deposition.The nitridation of Ba5B4O15 caused highly-defective BaBO2N surface due to Lewis base and Ba-rich conditions of the starting oxide. The annealing in Ar both enhances the crystallinity of amorphous surface and effectively suppresses the formation of reduced defects. As a result, the annealed BaNbO2N photoanode exhibited greatly enhanced photocurrent of 5.2 mA cm-2 at 1.23 VRHE during sunlight-driven water oxidation, which is approximately twenty times higher than that of as-prepared BaNbO2N. BaTaO2N is thermally more stable in an Ar flow as compared with BaNbO2N, so that the high-temperature annealing at 1073 K was found to improve both the bulk and surface properties of inactive as-prepared BaTaO2N. Consequently, the particulate BaTaO2N photoanode followed by the Ar annealing produced a half-cell STH energy conversion efficiency of 1.4% at 0.88 VRHE. In addition, the solar water oxidation retained 79% of the initial photocurrent over 24 h. Surface and bulk characterizations of perovskite oxynitrides annealed in Ar and the corresponding photoresponse will be discussed in detail in the presentation.References J. Seo et al., Angew. Chem. Int. Ed., 2018, 57, 2.T. Hisatomi et al., Energy Environ. Sci., 2013, 6, 3595.J. Seo et al., Adv. Energy Mater., 2018, 1800094.S. Jadhav et al., J. Mater. Chem. A, 2020, 8, 1127.M. Matsukawa et al., Nano Lett., 2014, 14, 1038.J. Seo et al., ACS Appl. Energy Mater., 2019, 2, 5777.J. Seo et al., J. Mater. Chem. A, 2019, 7, 493.

An Energy Harvesting Perspective of a Perovskite Based Thermoelectric Module: Fabrication and Evaluation

Oxide thermoelectric materials based on n-type perovskite Ca0.99Dy0.005Lu0.005MnO3 and p-type layered Ca3Co4O9 were developed and their application towards energy harvesting from waste heat was demonstrated. The n- and p-type oxides were synthesized by a sol–gel route followed by hot-press sintering to achieve densities that were 95–98% of the theoretical values. The doping concentrations in the thermoelectric materials were optimized to achieve lower electrical resistivity with moderate Seebeck coefficients that yielded power factors close to 550 μW m−1 K−2 at 950 K. A 14-element thermoelectric (TE) module was fabricated by connecting the developed n- and p-type oxide materials electrically in series and thermally in parallel. The electrical connectivity was achieved using Ag conductive paste followed by sandwiching the n and p-type materials between two insulating alumina plates. The module was tested in a simulated environment of hot gas flow with hot-side temperatures up to 623 K. The module generated an output voltage of 0.34 V with a power density of 0.9 mW cm−2 at 623 K.

Visible-Light-Driven Water Splitting Using Perovskite-Type Oxynitride Photoanodes

The n-type perovskite oxynitrides having a general formula AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) are capable of absorbing a wide range of visible light above 600 nm, theoretically leading to high solar-to-hydrogen (STH) conversion efficiency. Interestingly, band-gap energies (Eg ) of the oxynitrides can be tuned by different combinations of elements at the A and B sites. For example, BaNbO2N has an Eg of 1.8 eV (wavelength λ = 740 nm) and thus potential for the STH conversion efficiency of 28.6% in conjunction with an incident photon-to-current efficiency of unity. Despite the favourable optical properties, the photoelectrochemical (PEC) water splitting using the oxynitrides, except LaTiO2N (λ = 600 nm), has rarely been studied up to the present and the photoactivity under sunlight irradiation remains relatively low.1) The presence of B species in AB(O,N)3, which are readily reduced during nitridation because of the high electronegativity of B, promotes the generation of defects and/or impurities at oxynitride surface.1) In fact, Nb5+ in ANbO2N is reduced to Nb4+ and/or Nb3+ during high temperature nitridation.2) Also, the absence of facile, crystalline oxide precursors with Ba/B stoichiometry to BaBO2N (B=Ta, Nb) produces amorphous surface of BaBO2N after the nitridation.3) These mainly lead to low crystallinity of the oxynitrides, which enhances recombination of photoexcited holes and electrons and thus results in low photocurrent for the water splitting. Therefore, the control of defects and impurities during the synthesis of oxynitrides is crucial to enhancing the water splitting activity. Herein we report a novel synthesis route of BaNbO2N as a challenging material, including mild nitridation of a crystalline oxide and subsequent annealing in an inert Ar flow, which led to highly increased anodic photocurrent density for solar-driven water oxidation.4) Moreover, we discuss surface states of BaNbO2N subjected to the Ar annealing at various temperatures and the effects of such surface states on the photocurrent density under sunlight irradiation.5) Perovskite-type BaNbO2N was synthesized by the thermal nitridation of Ba-rich crystalline Ba5Nb4O15 at 1173 K for 20 and 40 h. The nitrided products were washed with copious amounts of distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed in Ar flow at different temperatures such as 773, 873, 973 and 1073 K for 1 h. For PEC water oxidation trials, the particulate BaNbO2N photoanodes were prepared by a particle transfer method.6) The mild nitridation of Ba5Nb4O15 for 20 h produced amorphous surface of the as-prepared BaNbO2N owing to Lewis base and Ba-rich conditions of the starting oxide, as presented in Figure 1(a). The prolonged nitridation for 40 h, to increase surface and bulk crystallinity of the oxynitride, rather led to NbN impurity nanoparticles covered on the oxynitride surface shown in (b). The photoanode using this BaNbO2N caused very limited improvement in water oxidation activity. Alternatively, the annealing in Ar as a post-nitridation treatment was carried out to modify the surface property of as-prepared oxynitride and compared with that in a reducing NH3. Both the annealing treatments enhanced surface crystallinity, demonstrating clear lattice fringes of single BaNbO2N planes presented in (c) and (d). Furthermore, the annealing in Ar was very effective at suppressing the generation of surface defects and impurities. As a result, the particulate BaNbO2N photoanode exhibited a remarkably enhanced photocurrent density of 5.2 mA cm-2 at 1.23 VRHE toward visible-light-driven water splitting (AM 1.5G), which has been never reported for an oxynitride responsive at wavelengths above 600 nm. Surface and bulk characterizations of BaNbO2N treated in different annealing conditions and the corresponding photoelectroactivity will be discussed in more detail in the presentation. References J. Seo, H. Nishiyama, T. Yamada, K. Domen, Angew. Chem. Int. Ed., 2018, 57, 2.T. Hisatomi, C. Katayama, Y. Moriya, T. Minegishi, M. Katayama, H. Nishiyama, T. Yamada, K. Domen, Energy Environ. Sci., 2013, 6, 3595.J. Seo, Y. Moriya, M. Kodera, T. Hisatomi, T. Minegishi, M. Katayama, K. Domen, Chem. Mater., 2016, 28, 6869.J. Seo, T. Hisatomi, M. Nakabayashi, N. Shibata, T. Minegishi, M. Katayama, K. Domen, Adv. Energy Mater., 2018, 1800094.J. Seo, M. Nakabayashi, T. Hisatomi, N. Shibata, T. Minegishi, M. Katayama, K. Domen, J. Mater. Chem. A, 2018, DOI: 10.1039/C8TA09950B.T. Minegishi, N. Nishimura, J. Kubota, K. Domen, Chem. Sci., 2013, 4, 1120. Figure 1 . HRTEM images of BaNbO2N particles as-prepared by the nitridation at 1173 K for 20 (a) and 40 h (b) and subsequently annealed at 873 K for 1 h at NH3 (c) and Ar atmospheres (d). (Reprinted from ref. 4) Figure 1

Planar p–n homojunction perovskite solar cells with efficiency exceeding 21.3%

Perovskite solar cells (PSCs) have emerged as an attractive photovoltaic technology thanks to their outstanding power conversion efficiency (PCE). Further improvement in the device efficiency is limited by the recombination of the charge carriers in the perovskite layer even when employing heterojunction-based architectures. Here, we propose and demonstrate a p-type perovskite/n-type perovskite homojunction whose built-in electric field promotes oriented transport of the photo-induced carriers, thus reducing carrier recombination losses. By controlling the stoichiometry of the perovskite precursors, we are able to induce n-type or p-type doping. We integrate the homojunction structure in a planar PSC combining a thermally evaporated p-type perovskite layer on a solution-processed n-type perovskite layer. The PSC with a MAPbI3 homojunction achieves a PCE of 20.80% (20.5% certified PCE), whereas the PSC based on a FA0.15MA0.85PbI3 homojunction delivers a PCE of 21.38%. We demonstrate that the homojunction structure is an effective approach, beyond existing planar heterojunction PSCs, to achieve highly efficient PSCs with reduced carrier recombination losses. Carrier recombination limits the power conversion efficiency of perovskite solar cells. Here the authors construct a planar p–n homojunction perovskite solar cell to promote the oriented transport of carriers and reduce recombination, thus enabling power conversion efficiency of 21.3%.

Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations.

We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.