Ligang Yuan

I/Br/Cl triple‐anion perovskite material with bandgap of 1.8 eV is tailored for indoor light harvesting, which realizes a record high indoor efficiency of 36.2% with increased open circuit voltage (V _oc) and minimal short‐circuit current ( J _sc) loss. The I/Br halide segregation is restrained by Cl‐involvement, realizing a long‐term stability of over 95% after 2000 h.

Abstract

Indoor photovoltaics are promising to enable self‐powered electronic devices for the Internet of Things. Here, reported is a triple‐anion CH₃NH₃PbI_{2− x} BrCl _x perovskite film, of which the bandgap is specially designed for indoor light harvesting to achieve a record high efficiency of 36.2% with distinctive high open circuit voltage (V _oc) of 1.028 V under standard 1000 lux fluorescent light. The involvement of both bromide and chloride suppresses the trap‐states and nonradiative recombination loss, exhibiting a remarkable ideality factor of 1.097. The introduction of chloride successfully restrains the halide segregation of iodide and bromide, stabilizing the triple‐anion perovskite film. The devices show an excellent long‐term performance, sustaining over 95% of original efficiency under continuous light soaking over 2000 h. These findings show the importance and potential of I/Br/Cl triple‐anion perovskite with tailored bandgap and suppressed trap‐states in stable and efficient indoor light recycling.

Near‐infrared (NIR) emitting host–guest copolymers with different host units are synthesized, characterized and employed in light‐emitting electrochemical cells (LECs). The selection of host unit influences the LEC performance strongly, and the best NIR‐LEC delivers a second‐fast turn to a strong radiance >100 µW cm^‐2 at a low voltage of 3.6 V and a good operational stability of >350 h.

Abstract

A set of host–guest copolymers with alternating benzodithiophene and benzotriazole (BTz) derivatives as host units and 4,7‐bis(5‐bromothiophen‐2‐yl)‐benzo[c][1,2,5]thiadiazole as the minority guest are synthesized, characterized, and evaluated for applications. A light‐emitting electrochemical cell (LEC) comprising such a host–guest copolymer delivers fast‐response near‐infrared (NIR) emission peaked at 723 nm with a high radiance of 169 µW cm⁻² at a low drive voltage of 3.6 V. The NIR‐LEC also features good stability, as the peak NIR output only drops by 8% after 350 h of continuous operation. It is, however, found that the LEC performance is highly sensitive to the detailed chemical structure of the host backbone, and that the addition of electron‐donating thiophene bridging units onto the BTz unit is highly positive while the inclusion of fluorine atoms results in a drastically lowered performance, presumably because of the emergence of hydrogen bonding within the active material.

Representative research studies on free carrier, exciton, and phonon dynamics in lead‐halide perovskites, performed with terahertz (THz) time‐domain spectroscopy (THz‐TDS) and time‐resolved THz spectroscopy (TRTS), are reviewed. The power of THz techniques to elucidate the photophysics, and hence the outstanding photovoltaic performance of the halide perovskite materials, is illustrated.

Abstract

Terahertz time‐domain spectroscopy is a noncontact, coherent technique that can probe dynamics of carriers, phonons and excitons, and the interplay among these degrees of freedom, which determines the functionalities of the system. In the past few years, lead‐halide perovskites have shown to be a promising class of materials in the areas of solar cells, light emitting diodes, lasers, photodetectors, and field‐effect transistors. In these electronic and photonic devices, the knowledge of the dynamics of charge carriers and other low‐energy excitations is crucial to understand the underlying physics that ultimately determines the device performances. Here, some of the most representative works in the area of halide perovskite research using terahertz time‐domain spectroscopy and time‐resolved terahertz spectroscopy are reviewed, highlighting the power of ultrafast terahertz techniques on this class of material system.

Spatial heterogeneity in the functional optical properties of CH₃NH₃PbI₃ thin films is mapped on the microscopic scale using femtosecond pump‐probe transient absorption and time‐resolved photoluminescence microscopy. The coregistration of short‐ (sub‐picosecond) and long‐time (nanosecond) contrast provides information about the origin of the heterogeneity. Furthermore, the variation of functional properties is found to be only partially correlated with physical structure.

Abstract

The spatial heterogeneity of carrier dynamics in polycrystalline metal halide perovskite (MHP) thin films has a strong influence on photovoltaic device performance; however, the underlying cause is not yet clearly understood. Here, the sub‐micrometer scale mapping of charge carrier dynamics in CH₃NH₃PbI₃ thin films using time‐resolved nonlinear optical microscopy, specifically transient absorption microscopy (TAM) with sub‐picosecond (ps) and time‐resolved photoluminescence (PL) microscopy with nanosecond temporal resolution is reported. To study the influence of physical morphology on charge carrier dynamics, MHP thin films having granular‐ and fibrous structures are investigated. On both types of films, spatial regions with short‐lived transient gain signals (fast nonradiative relaxation within ≈1 ps) typically show slower charge recombination via radiative relaxation, which is attributed to the presence of additional energy states near the band edge. In addition, fibrous films show longer PL lifetimes. Interestingly, the functional contrast shown in TAM images exhibits fundamental differences from the structural contrast shown in scanning electron microscopy images, implying that the variation of trap density in the bulk contributes to the observed spatial heterogeneity in carrier dynamics.

Using the single‐component Eu³⁺–Tb³⁺‐containing metallopolymer Poly(NVK‐co‐2‐co‐7) with the straightforward high‐quality white‐lights as the emitting layer, its reliable WPLED with stepwise alignments of both HOMO and LUMO levels affords the record‐renewed electroluminescent performance among previous organo‐Ln³⁺‐based WOLEDs/WPLEDs.

Abstract

Despite the excellent physical properties of single‐component Eu³⁺–Tb³⁺‐containing metallopolymers, the development of their flexible white polymer light‐emitting diodes (WPLEDs) for portable full‐color flat displays remains a formidable challenge. Herein, the WPLEDs from a metallopolymer Poly(NVK‐co‐2‐co‐7) are reported, in which [Eu(DBM)₃(4‐vp‐PBI)] (2) and [Tb(tba‐PMP)₃(4‐vp‐PBI)] (7) with different localized circumstances are grafted into poly(N‐vinyl‐carbarzole) (PVK). In this design, both Dexter and Förster energy transfers occur, which endow a photoluminescent quantum yield up to 22.3% of the straightforward high‐quality white‐lights. Contributing from the stepwise alignment of frontier molecular orbitals of Poly(NVK‐co‐2‐co‐7) as the emitting layer in combination with CBP‐ and BCP‐assisted carrier‐transports, a reliable WPLED with the record‐renewed electroluminescent performance (L ^Max = 388.0 cd m⁻², η_c ^Max = 31.1 cd A⁻¹, η_p ^Max = 15.0 lm W⁻¹, η_EQE ^Max = 18.1%, and weak efficiency‐roll‐off) among previous organo‐Ln³⁺‐based white organic light‐emitting diodes/WPLEDs is achieved. This finding renders a single‐component Eu³⁺–Tb³⁺‐containing metallopolymers a potential new platform to cost‐effective flexible WPLEDs for practical applications.

CH₃NH₃PbI₃ exhibits a superior piezophotocatalytic hydrogen generation rate upon concurrent light and mechanical stimulations, much higher than that of piezocatalytic and photocatalytic hydrogen evolution rate as well as their sum. Combining piezocatalysis and photocatalysis of semiconductor photocatalysts to attain a collective piezophotocatalysis may represent an appealing strategy for efficient solar energy conversion, including water splitting, organic fuel production, etc.

Abstract

To alleviate photoinduced charge recombination in semiconducting nanomaterials represents an important endeavor toward high‐efficiency photocatalysis. Here a judicious integration of piezoelectric and photocatalytic properties of organolead halide perovskite CH₃NH₃PbI₃ (MAPbI₃) to enable a piezophotocatalytic activity under simultaneous ultrasonication and visible light illumination for markedly enhanced photocatalytic hydrogen generation of MAPbI₃ is reported. The conduction band minimum of MAPbI₃ is higher than hydrogen generation potential (0.046 V vs normal hydrogen electrode), thereby rendering efficient hydrogen evolution. In addition, the noncentrosymmetric crystal structure of MAPbI₃ enables its piezoelectric properties. Thus, MAPbI₃ readily responds to external mechanical force, creating a built‐in electric field for collective piezophotocatalysis as a result of effective separation of photogenerated charge carriers. The experimental results show that MAPbI₃ powders exhibit superior piezophotocatalytic hydrogen generation rate (23.30 µmol h⁻¹) in hydroiodic acid (HI) solution upon concurrent light and mechanical stimulations, much higher than that of piezocatalytic (i.e., 2.21 µmol h⁻¹) and photocatalytic (i.e., 3.42 µmol h⁻¹) hydrogen evolution rate as well as their sum (i.e., 5.63 µmol h⁻¹). The piezophotocatalytic strategy provides a new way to control the recombination of photoinduced charge carriers by cooperatively capitalizing on piezocatalysis and photocatalysis of organolead halide perovskites to yield highly efficient piezophotocatalysis.

The interface and bulk defects of perovskite solar cells are distinguished and quantified, and are for the first time traced in situ using an expanded admittance model. A fullerene derivative [6, 6]‐phenyl‐C61‐butyric acid (PCBA) is introduced into the TiO₂/perovskite interface to release the interface stress.

Abstract

The stability issue that is obstructing commercialization of the perovskite solar cell is widely recognized, and tremendous effort has been dedicated to solving this issue. However, beyond the apparent thermal and moisture stability, more intrinsic semiconductor mechanisms regarding defect behavior have yet to be explored and understood. Herein, defects are quantified; especially interface defects, within the cell to reveal their impact on device performance and especially stability. Both the bulk and interface defects are distinguished and traced in situ using an expanded admittance model when the cell degrades in its efficiency under illumination or voltage. The electric field‐induced interface, rather than bulk defects, is found to have a direct correlation to stability. Releasing the interface strain using a fullerene derivative is an effective way to suppress interface defect formation and improve stability. Overall, this work provides a quantitative approach to probing the semiconductor mechanism behind the stability issue, and the inherent correlation discovered here among the electric field, interface strain, interface defects, and cell stability has important implications for ongoing device stability engineering.

A rational core–shell design of open air low temperature in situ processable CsPbI₃ quasi‐nanocrystals is proposed. A bifunctional ligand 4‐fluorophenethylammonium iodide and new compound H₂PbI₄ increase crystal stability, charge extraction, and assist divalent ion doping, respectively. The best p‐i‐n solar cell with 13.4% efficiency can retain 72% beyond 500 h in ambient air without encapsulation.

Abstract

As a promising alternative, inorganic perovskite nanocrystals allow reinforced stability of photovoltaic device. Unfortunately, directly assembling these nanocrystals into film is uncontrollable. Instead, in situ assembling technology under low temperature in open air is attractive but limited due to the tendency of nonperovskite transition. The adverse shell ligands and unstable core lattices are known as the fundamental problems. In order to address this issue, here proposed is a rational core–shell design: 1) with respect to ligands, a new one, 4‐fluorophenethylammonium iodide, is used to enhance bonding force and charge coupling between ligands and nanocrystals; 2) with respect to lattices, a novel compound H₂PbI₄ is employed to assist divalent ion (Mn²⁺) doping into perovskite lattices. By low temperature in situ processing CsPbI₃ quasi‐nanocrystal film, the highest power conversion efficiency of 13.4% for p‐i‐n solar cells is achieved, which retains 92% after 500 h in ambient air. The current study underlines the significance of rational hierarchical design of inorganic perovskite nanocrystals, especially for low temperature in situ processable electronic devices.

Hysteresis in the dark, attributable to bias induced degradation of the p‐type interface, is investigated and eliminated in NiO‐based inverted perovskite solar cells. Enhanced stability to forward bias is obtained with the introduction of a low‐temperature hybrid magnesium‐based interlayer.

Abstract

In perovskite solar cells (PSCs), the interfaces are a weak link with respect to degradation. Electrochemical reactivity of the perovskite's halides has been reported for both molecular and polymeric hole selective layers (HSLs), and here it is shown that also NiO brings about this decomposition mechanism. Employing NiO as an HSL in p–i–n PSCs with power conversion efficiency (PCE) of 16.8%, noncapacitive hysteresis is found in the dark, which is attributable to the bias‐induced degradation of perovskite/NiO interface. The possibility of electrochemically decoupling NiO from the perovskite via the introduction of a buffer layer is explored. Employing a hybrid magnesium‐organic interlayer, the noncapacitive hysteresis is entirely suppressed and the device's electrical stability is improved. At the same time, the PCE is improved up to 18% thanks to reduced interfacial charge recombination, which enables more efficient hole collection resulting in higher V _oc and FF.

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation.

Abstract

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In perovskite solar cells, the research community is just starting to understand their effects on performance and long‐term durability. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation. In this essay, the latest reports are summarized and the authors' perspective on this very important subject is given.

Spiro‐OMeTAD has been widely used as a promising hole conductor for metal halide perovskite solar cells due to its ability to deliver highly efficient devices. However, additives such as lithium salt and O₂ exposure are still required to modify the electrical properties due to the poor conductivity of pristine spiro‐OMeTAD. In article number https://doi.org/10.1002/aenm.2019015191901519, Jianfeng Lu, Udo Bach and co‐workers employ the oxidized form of spiro‐OMeTAD as a dopant to improve the efficiency of the spiro‐OMeTAD‐based lithium‐free perovskite solar cells from 10% to 19.3% while simultaneously enhancing the device stability.

Herein, a facile method to improve the long‐term stability of perovskite solar cells using N719 dye molecules as additives is presented. Perovskite‐dye hybrid films show better crystallinity, enhanced light absorption, and boosted moisture stability. Due to the greatly retarded hydration process, the degradation process of perovskite‐dye solar cells is three times longer than that for pristine devices.

Metal‐halide perovskite solar cells (PSC) have shown great success in achieving high efficiencies but less satisfaction in achieving long‐term stability. Perovskites are prone to forming perovskite hydrates in humid environments, which leads to the decomposition of the perovskite materials. Herein, a common and cheap dye molecule, called cis‐di(thiocyanato)bis(2,2‐bipyridyl4,4‐dicarboxylate)ruthenium(II), denoted as N719, is introduced into mixed‐cation mixed‐halide perovskites for better moisture stability. It is discovered that the N719 molecules form perovskite‐dye complexes in the precursor solution, leading to larger grains and better film crystallinity by slowing down the crystallization process. Fourier‐transform infrared spectroscopy and X‐ray diffraction characterizations suggest that the N719 molecules exist in the crystallized perovskite films but are not incorporated into the perovskite crystal lattice. The presence of N719 molecules in perovskite films greatly retards the formation of perovskite hydrates due to a three‐times‐increased water migration barrier. Owing to these improvements, nonencapsulated N719‐PSC retain over 80% of their original efficiencies after aging under a high relative humidity of 60% for 250 h, which is three times longer than that for pristine cells. A cheap and effective route for controlling the perovskite crystallization process and improving the stability of PSC without sacrificing device efficiency is represented.

A fullerene derivative, [6,6]‐phenyl‐C₆₁‐butyric acid‐N,N‐dimethyl‐3‐(2‐thienyl)propanam ester (PCBB‐S‐N), is designed and synthesized to correct defects in electron‐transporting layers (ETLs) and perovskite films. Its use leads to a promising power conversion efficiency (PCE) of 21.08% for perovskite solar cells. Importantly, devices containing PCBB‐S‐N simultaneously realize excellent thermal stability and water resistance.

Abstract

The poor long‐term stability of organic–inorganic hybrid halide perovskite solar cells (pero‐SCs) remains a big challenge for their commercialization. Although strategies such as encapsulation, doping, and passivation have been reported, there remains a lack of understanding of the water resistance and thermal stability of pero‐SCs. A fullerene derivative, [6,6]‐phenyl‐C₆₁‐butyric acid‐N,N‐dimethyl‐3‐(2‐thienyl)propanam ester (PCBB‐S‐N) containing a functional sulfur atom and C_60, is synthesized and employed as electron transporting layer (ETL)/intermediary layer to targetedly heal the multitype defects in pero‐SCs or assist the growth of ETL, such as [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), in planar p‐i‐n pero‐SCs. The repaired pero‐SCs can not only dramatically improve their power conversion efficiencies, but also address stability issues under moisture and high temperature. The corresponding mechanism of PCBB‐S‐N with targeted therapy effect in a device is systematically investigated by both experiments and theoretical calculation. This work demonstrates that the proposed fullerene derivative with finely tuned chemical structure can be a promising ETL candidate or intermediary to approach stable and efficient planar p‐i‐n pero‐SCs.

A new method is developed to synthesize SnO _x ‐Cl colloids and to realize an in situ and spontaneous ion‐exchange reaction during the perovskite film crystallization process. It is found that such ion exchange can perfectly passivate the interface defects and reduce energy loss at the interface.

Abstract

Interface engineering is of great concern in photovoltaic devices. For the solution‐processed perovskite solar cells, the modification of the bottom surface of the perovskite layer is a challenge due to solvent incompatibility. Herein, a Cl‐containing tin‐based electron transport layer; SnO _x ‐Cl, is designed to realize an in situ, spontaneous ion‐exchange reaction at the interface of SnO _x ‐Cl/MAPbI₃. The interfacial ion rearrangement not only effectively passivates the physical contact defects, but, at the same time, the diffusion of Cl ions in the perovskite film also causes longitudinal grain growth and further reduces the grain boundary density. As a result, an efficiency of 20.32% is achieved with an extremely high open‐circuit voltage of 1.19 V. This versatile design of the underlying carrier transport layer provides a new way to improve the performance of perovskite solar cells and other optoelectronic devices.

The studies from the steady‐state and time‐dependent measurements indicate that the extended absorption range, short charge carrier extraction time, and high charge carrier mobility by the non‐fullerene electron acceptors in the photoactive layer are responsible for enhanced photocurrent in ternary organic solar cells.

Ternary organic solar cells, a single active layer comprising three different components, are demonstrated to be one of the most efficient ways to approach high‐performance organic solar cells. But nevertheless, most of the ternary organic solar cells are characterized by steady‐state measurements, which are helpful but inadequate to fully understand the underlying charge carrier behavior at a short time scale. Herein, a comparison of the steady‐state and time‐dependent measurements is used to investigate the functionality of non‐fullerene electron acceptors in ternary organic solar cells. The steady‐state measurements indicate that non‐fullerene electron acceptors enlarge the absorption range of the photoactive layer, suppress charge carrier recombination, reduce charge carrier transfer resistance, and thereby increase photocurrent in ternary organic solar cells. The time‐dependent measurements demonstrate that a short charge carrier extraction time and a high charge carrier mobility are responsible for enhanced photocurrent in ternary organic solar cells. A comprehensive method understanding the underlying of enhanced efficiency of ternary organic solar cells is provided herein.

Record‐efficiency near‐infrared fluorescent organic light‐emitting diodes (OLEDs) are demonstrated using energy transfer from a thermally activated delayed fluorescence sensitizer to a cyanine pyrrolopyrrole derivative. The OLEDs show a maximum external quantum efficiency of 5.4% and spectrally narrow IR emission centered at λ = 790 nm. The cohost energetics are found to play an important role in determining the device efficiency.

Abstract

Through various triplet‐harvesting approaches, fluorescent organic light‐emitting diodes (OLEDs) that emit in the visible spectrum can now be fabricated with efficiencies rivaling those of their phosphorescent counterparts. However, achieving high efficiencies in the near‐infrared (NIR) is considerably more challenging. This is in part due to the low quantum yield of most fluorescent NIR emitters and inefficient triplet exciton harvesting in such devices. Here, fluorescent NIR OLEDs with an external quantum efficiency of 5.4% and a peak emission wavelength of 790 nm are demonstrated. The OLEDs are fabricated by combining a deep‐red host that undergoes thermally assisted delayed fluorescence with a near‐infrared cyanine dye that emits with high efficiency. The devices show nearly pure NIR emission with a NIR cut‐on wavelength of 749 nm and >90% emitted power at wavelengths above 750 nm. It is also shown that the host polarity strongly affects the device performance.

Using the single‐component Eu³⁺–Tb³⁺‐containing metallopolymer Poly(NVK‐co‐2‐co‐7) with the straightforward high‐quality white‐lights as the emitting layer, its reliable WPLED with stepwise alignments of both HOMO and LUMO levels affords the record‐renewed electroluminescent performance among previous organo‐Ln³⁺‐based WOLEDs/WPLEDs.

Abstract

Despite the excellent physical properties of single‐component Eu³⁺–Tb³⁺‐containing metallopolymers, the development of their flexible white polymer light‐emitting diodes (WPLEDs) for portable full‐color flat displays remains a formidable challenge. Herein, the WPLEDs from a metallopolymer Poly(NVK‐co‐2‐co‐7) are reported, in which [Eu(DBM)₃(4‐vp‐PBI)] (2) and [Tb(tba‐PMP)₃(4‐vp‐PBI)] (7) with different localized circumstances are grafted into poly(N‐vinyl‐carbarzole) (PVK). In this design, both Dexter and Förster energy transfers occur, which endow a photoluminescent quantum yield up to 22.3% of the straightforward high‐quality white‐lights. Contributing from the stepwise alignment of frontier molecular orbitals of Poly(NVK‐co‐2‐co‐7) as the emitting layer in combination with CBP‐ and BCP‐assisted carrier‐transports, a reliable WPLED with the record‐renewed electroluminescent performance (L ^Max = 388.0 cd m⁻², η_c ^Max = 31.1 cd A⁻¹, η_p ^Max = 15.0 lm W⁻¹, η_EQE ^Max = 18.1%, and weak efficiency‐roll‐off) among previous organo‐Ln³⁺‐based white organic light‐emitting diodes/WPLEDs is achieved. This finding renders a single‐component Eu³⁺–Tb³⁺‐containing metallopolymers a potential new platform to cost‐effective flexible WPLEDs for practical applications.

Publication date: 18 September 2019

Source: Joule, Volume 3, Issue 9

Author(s): Henk J. Bolink

Herein, a facile strategy that can carry out double passivation to improve the performance of perovskite solar cells (PSCs) is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film; double passivation improves the performance of PSCs significantly.

Material passivation is essential to enhance the quality of perovskite materials and boost the performance of perovskite solar cells (PSCs). However, most of the previous reports only paid attention to improving the quality of perovskite films by adopting single passivation. Here, a facile strategy that can carry out double passivation to improve the performance of PSCs is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film, and by combining PEABr and PbI₂, the double passivation improves the performance of PSCs significantly. Very high short‐circuit current density of 24.30 mA cm⁻², open‐circuit voltage of 1.10 V, and fill factor of 79.75% are achieved which lead to a surprising efficiency of 21.32% for the passivated device. The improved efficiency is mainly according to the available surface passivation of the perovskite material, leading to repressed nonradiative recombination and unhindered charge collection. In addition, the passivated device exhibits better power conversion efficiency stability relative to the control device.

Photocurrent–voltage hysteresis in perovskite solar cells (PSCs) induced by ion migration combined with nonradiative recombination near the interface depends on perovskite composition and device structure. Among the methods used in the attempt to reduce the hysteresis, potassium‐ion doping is found to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition.

Abstract

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.

Positive photoconductivity in CH₃NH₃PbBr₃ turns into negative photoconductivity after Bi doping. This transition is due to photogenerated DX‐like centers in Bi‐doped CH₃NH₃PbBr₃.

Abstract

Halide perovskites are known to be photoconductive for more than half a century, and their efficient photocarrier generation gives rise to positive photoconductivity (PPC). In this work, the discovery of negative photoconductivity (NPC) in hybrid perovskite CH₃NH₃PbBr₃ after Bi doping is reported. Transient photoconductivity measurements reveal a surprising bipolar behavior with a fast positive response followed by exponential negative photocurrent decay, resulting in an equilibrium photocurrent even below the dark level. The NPC effect in Bi‐doped CH₃NH₃PbBr₃ is among the largest ones reported so far for semiconductors. It is proposed that the transition to negative photoconductance is related to the presence of DX‐like centers in Bi‐doped halide perovskites, similar to doped III–V and chalcopyrite semiconductors. Such photogenerated DX‐like centers in the Bi‐doped CH₃NH₃PbBr₃ can trap mobile band electrons and enhance charge recombination, thus reducing the conductivity. This mechanism is consistent with the observations of crossover from PPC to NPC as functions of temperature, composition, and illumination. The results underscore the importance of defect engineering for tuning the optoelectronic properties of halide perovskites.