Mahui

Solution‐processed 2D Nb₂O₅(001) nanosheets (c‐Nb₂O₅ NS) are prepared and combined with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as an electron transport layer (ETL) for inverted inorganic CsPbI₂Br perovskite solar cells (PeSCs) with a high performance. The c‐Nb₂O₅ NS not only facilitate the electron transport, blocking the hole transport, but also contribute to the defect passivation and retard the iodine ions diffusion toward the Ag electrode.

Herein, solution‐processed 2D Nb₂O₅(001) nanosheets (c‐Nb₂O₅ NS) are prepared and combined with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as an electron transport layer (ETL) for inverted inorganic CsPbI₂Br perovskite solar cells (PeSCs). The PeSCs with a c‐Nb₂O₅/PC₆₁BM bilayer ETL yield a high power conversion efficiency (PCE) up to 11.74%, remarkably outperforming the devices with only PC₆₁BM (9.10%) and those with the state‐of‐the‐art ZnO/C₆₀ ETL (10.65%) prepared under the same conditions. More importantly, the nonencapsulated PeSCs with c‐Nb₂O₅ exhibit a high thermal stability with only 20% PCE loss after 400 h thermal aging at 85 °C. Such an impressive performance and a high stability can be attributed to the introduction of c‐Nb₂O₅(001) NS with favorable band levels, strong acid nature, and the small lattice fringe spacing along the large lateral (001) surface, which not only facilitate the electron transport, blocking the hole transport, but also contribute to defect passivation and retard the iodine ions diffusion toward the Ag electrode. This study thus provides a deeper insight for the interfacial design in inverted inorganic PeSCs and contributes to PCE improvement in the future.

Device simulations show that the low electron carrier concentration in the near‐interface region of the buffer layer is the fundamental factor that limits carrier transport and thus J _SC of Cu(In,Ga)Se₂ solar cells with a large spike‐like conduction band offset. New strategies are proposed for increasing J _SC through increasing the near‐interface electron concentration.

A spike‐like conduction‐band offset (CBO) in heterojunction solar cells is shown to limit the short‐circuit current density (J _SC) dramatically when the spike height is large. It is widely believed that the spike‐like CBO produces a potential barrier, which resists the photogenerated carriers flowing through the junction interface and thus decreases J _SC. However, our device simulation studies on Cu(In,Ga)Se₂ (CIGS) solar cells reveal that a large spike‐like CBO causes an extremely low electron carrier concentration in the near‐interface region of the buffer layer, which is the major factor that limits the carrier transport and thus J _SC. If the near‐interface electron concentration is increased, J _SC can be increased despite the fact that the large spike‐like CBO and potential barrier are still present. These results indicate that the near‐interface electron concentration is the fundamental factor limiting the J _SC, more fundamental than the potential barrier. Therefore, not only the commonly adopted band‐alignment engineering, but also various other methods, for example, choosing buffer materials with suitable effective density of states, introducing favorable interface defects, or increasing the doping level, can be adopted for improving the current collection in heterojunction solar cells. Therefore, J _SC can always be increased even when the large spike‐like CBO is inevitable.

Environmentally benign solvent N ‐methyl‐pyrrolidone (NMP) is for the first time used to make a precursor solution that achieves a 10.23% efficient CuIn(S,Se)₂ (CISSe) solar cell. Annealing the NMP‐based precursor film in air favors decomposition of organic species and results in higher quality CISSe absorber material and better device performance than annealing in the glove box.

A safe and environmentally benign solvent is a requisite for mass production of thin film solar cells via solution methods. Here, a highly industry suitable solvent N‐methyl‐pyrrolidone (NMP) is used for the first time to make a precursor solution with simple compounds of CuCl, InCl₃·4H₂O and thiourea and fabricate CuIn(S,Se)₂ (CISSe) solar cells. A power conversion efficiency of 10.23% has been achieved from the NMP‐based solution when the precursor film was processed in air, which is only 8.55% for film processed in glove box. Characterizations using XRD, Raman, SEM, EDX, and FTIR show air annealing favors decomposition of organic species in the precursor film, which results in high quality absorber materials. Further improvement in device efficiency can be expected by gallium alloying and optimization of device fabrication conditions. The results demonstrate highly efficient thin film solar cells can be fabricated from an industry suitable NMP precursor solution in air, which is promising for simplifying film processing and reducing manufacture cost.

Incorporation of a small amount of benzo[1,2‐c:4,5‐c]dithiophene‐4,8‐dione (BDD) block into the alkylsilyl functionalized copolymer by random copolymerization provides a beneficial trade‐off that the slightly reduced periodic sequence promotes the compatibility with an acceptor, whereas the introduction of planar units allows a preferred face‐on orientation with enhanced π–π stacking of the random copolymer to facilitate charge transfer.

Herein, an alkylsilyl functionalized alternative (D‐A1) copolymer with high crystallization property as the polymer matrix and planar [1,2‐c:4,5‐c]dithiophene‐4,8‐dione (BDD) block as the second acceptor unit (A2) are selected to construct two D‐A1‐D‐A2 type random copolymers PBDT‐TZ‐BDD‐1/19 and PBDT‐TZ‐BDD‐1/9. It is found that incorporation of a small amount of BDD block into the alkylsilyl functionalized copolymer by random copolymerization can effectively manipulate the energy levels, light absorption, molecular packing and the photovoltaic properties when blended with ITIC (indacenodithieno[3,2‐b]thiophene (IT) as the central donor unit and 2‐ (3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (IC) as end groups). More importantly, random copolymerization provides a beneficial trade‐off that the slightly reduced periodic sequence promotes the compatibility with the acceptor, whereas introduction of planar BDD units allows a preferred face‐on orientation with enhanced π–π stacking of the random copolymer to facilitate the charge transfer. As a result, the random copolymer PBDT‐TZ‐BDD‐1/19 delivers a significantly higher power conversion efficiency (11.02%) than the alternative binary copolymer counterpart together with the remarkably improved short circuit current and fill factor. These results demonstrate that random polymerization of a small amount of planar units into the highly crystalline polymer matrix is a promising strategy to develop high‐performance polymer solar cells.

Formation of lead‐free methylammonium bismuth iodide (MA₃Bi₂I₉) films is investigated in situ by X‐ray scattering and optical spectroscopy and contrasted to methylammonium lead iodide (MAPbI₃). The Bi‐based compound crystallizes directly from solution, whereas the Pb‐based perovskite initially forms a solvated crystalline phase requiring annealing for conversion. Both materials benefit from anti‐solvent dripping while the as‐cast film is in the disordered sol–gel state.

Organic–inorganic lead‐based halide perovskite compounds currently yield thin film solar cells with a power conversion efficiency (PCE) of >23%. However, replacing the lead with less‐toxic elements while maintaining a high PCE remains a challenge. For this reason, there has been significant effort to develop Pb‐free compounds, including methylammonium bismuth iodide (MA₃Bi₂I₉), but such systems severely underperform when compared with the prototypical Pb‐based methylammonium lead iodide (MAPbI₃). For the latter, it is known that lead complexes with polar solvents, such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF), to form iodoplumbates which can co‐crystallize into solvated phases. Herein, the solidification and growth behaviors of Bi‐ and Pb‐based films is investigated using multi‐probe in situ characterization methods. It is shown that the Bi‐based compound crystallizes directly and rapidly into a textured polycrystalline microstructure from a precursor solution without evolving through intermediate crystalline solvated phases, in contrast to MAPbI₃. This solidification process produces isolated crystals and challenges the growth of continuous and crystalline films required for solar cells. It is revealed that solvent engineering with antisolvent dripping is crucial to enable the formation of continuous polycrystalline films of MA₃Bi₂I₉ and functional solar cells thereof.

A low‐cost, high‐yield synthesis of a chorine‐substituted thiophene donor polymer material P(Cl) is presented. The material is characterized, and its performance within a polymer solar cell (PSC) with a nonfullerene acceptor is evaluated. Excellent power‐conversion efficiency combined with a low synthetic complexity is demonstrated for P(Cl), and hence, it is a promising candidate for the development in commercial PSC applications.

The industrialization of polymer solar cells (PSCs) requires high‐performance devices with high efficiencies and stabilities. Although high‐performance PSCs are achieved via outstanding research into their component materials and device structures, several challenges still need to be overcome, including the synthetic complexity (SC) of producing the active material. In this study, donor polymers based on two heterocyclic rings and simple donor–acceptor structures are designed to obtain a low‐cost material for PSCs. An inexpensive and high‐performance donor polymer P(Cl) is realized by the introduction of a chlorine‐atom substitution. P(Cl), which has lower SC than commercial donor polymers, has many advantages, such as high overall yield, low number of synthetic steps, and inexpensive raw materials. Moreover, fabricated P(Cl)‐based PSCs exhibit a high power‐conversion efficiency (PCE) of 12.14%. Through the shelf protocol of the international summit on organic photovoltaics stability in dark testing‐1 (ISOS‐D‐1) measurements, superior long‐term stability is demonstrated for P(Cl)‐based devices both without and with encapsulation; their PCEs are maintained at 91% and 100% of the initial values for up to 2002 and 2858 h, respectively, under ambient conditions. Therefore, P(Cl) is a promising donor polymer for commercial PSC applications.

Leadless hybrid perovskites are obtained by the epitaxial synthesis of CsPbX₃ (X = Cl, Br, I) perovskite quantum dots through surface chemical conversion of Cs₂GeF₆ double perovskites with PbX₂ (X = Cl, Br, I). The obtained CsPbBr₃/Cs₂GeF₆ products show high color purity and enhanced stability, indicating their potential application in lighting devices.

Abstract

Lead halide perovskites (LHPs) have received increased attention owing to their intriguing optoelectronic and photonic properties. However, the toxicity of lead and the lack of long‐term stability are potential obstacles for the application of LHPs. Herein, the epitaxial synthesis of CsPbX₃ (X = Cl, Br, I) perovskite quantum dots (QDs) by surface chemical conversion of Cs₂GeF₆ double perovskites with PbX₂ (X = Cl, Br, I) is reported. The experimental results show that the surface of the Cs₂GeF₆ double perovskites is partially converted into CsPbX₃ perovskite QDs and forms a CsPbX₃/Cs₂GeF₆ hybrid structure. The theoretical calculations reveal that the CsPbBr₃ conversion proceeds at the Cs₂GeF₆ edge through sequential growth of multiple PbBr₆ ⁴⁻ layers. Through the conversion strategy, luminescent and color‐tunable CsPbX₃ QDs can be obtained, and these products present high stability against decomposition due to anchoring effects. Moreover, by partially converting red emissive Cs₂GeF₆:Mn⁴⁺ to green emissive CsPbBr₃, the CsPbBr₃/Cs₂GeF₆:Mn⁴⁺ hybrid can be employed as a low‐lead hybrid perovskite phosphor on blue LED chips to produce white light. The leadless CsPbX₃/Cs₂GeF₆ hybrid structure with stable photoluminescence opens new paths for the rational design of efficient emission phosphors and may stimulate the design of other functional CsPbX₃/Cs‐containing hybrid structures.

Optical anisotropy in hybrid metal halide perovskites is demonstrated and the first account of the linear electro‐optic effect in these materials reported. These findings, along with the flexibility and solution‐processability of these materials, may suggest new applications for hybrid metal halide perovskites in polarizing optics and electro‐optic modulators.

Abstract

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro‐optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on‐chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in‐plane birefringence and linear electro‐optic response. Phenylmethylammonium lead chloride (PMA₂PbCl₄) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA₂PbCl₄ is reported and its EO coefficient is determined to be 1.40 pm V⁻¹. This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in‐plane birefringence and electro‐optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.

It is found that 2D planar defects in multilayered 2D crystals can be healed by grain boundary (GB) sliding, which works like a “wiper blade” to correct all metastable phases into thermodynamically stable phases along its trace. The driving force for GB sliding is the gain in interlayer binding energy. The study highlights the role of the often‐neglected interlayer interactions for defect repair, which have significant potential for obtaining large‐scale defect‐free 2D films.

Abstract

Understanding the mechanisms and kinetics of defect annihilations, particularly at the atomic scale, is important for the preparation of high‐quality crystals for realizing the full potential of 2D transition metal dichalcogenides (TMDCs) in electronics and quantum photonics. Herein, by performing in situ annealing experiments in an atomic resolution scanning transmission electron microscope, it is found that stacking faults and rotational disorders in multilayered 2D crystals can be healed by grain boundary (GB) sliding, which works like a “wiper blade” to correct all metastable phases into thermodynamically stable phases along its trace. The driving force for GB sliding is the gain in interlayer binding energy as the more stable phase grows at the expanse of the metastable ones. Density functional theory calculations show that the correction of 2D stacking faults is triggered by the ejection of Mo atoms in mirror twin boundaries, followed by the collective migrations of 1D GB. The study highlights the role of the often‐neglected interlayer interactions for defect repair in 2D materials and shows that exploiting these interactions has significant potential for obtaining large‐scale defect‐free 2D films.

The efficiency and photostability of all‐inorganic mixed‐halide perovskite solar cells (PVSCs) can be simultaneously enhanced by introducing an amino‐functionalized polymer PN4N as a novel cathode interlayer and dopant‐free PDCBT hole‐transporting layer. The favorable interaction between perovskite crystal and PN4N/PDCBT can effectively improve CsPbI₂Br film quality, with power conversion efficiency over 16%.

Abstract

A synergic interface design is demonstrated for photostable inorganic mixed‐halide perovskite solar cells (PVSCs) by applying an amino‐functionalized polymer (PN4N) as cathode interlayer and a dopant‐free hole‐transporting polymer poly[5,5′‐bis(2‐butyloctyl)‐(2,2′‐bithiophene)‐4,4′‐dicarboxylate‐alt‐5,5′‐2,2′‐bithiophene] (PDCBT) as anode interlayer. First, the interfacial dipole formed at the cathode interface reduces the workfunction of SnO₂, while PDCBT with deeper‐lying highest occupied molecular orbital (HOMO) level provides a better energy‐level matching at the anode, leading to a significant enhancement in open‐circuit voltage (V _oc) of the PVSCs. Second, the PN4N layer can also tune the surface wetting property to promote the growth of high‐quality all‐inorganic perovskite films with larger grain size and higher crystallinity. Most importantly, both theoretical and experimental results reveal that PN4N and PDCBT can interact strongly with the perovskite crystal, which effectively passivates the electronic surface trap states and suppresses the photoinduced halide segregation of CsPbI₂Br films. Therefore, the optimized CsPbI₂Br PVSCs exhibit reduced interfacial recombination with efficiency over 16%, which is one of the highest efficiencies reported for all‐inorganic PVSCs. A high photostability with a less than 10% efficiency drop is demonstrated for the CsPbI₂Br PVSCs with dual interfacial modifications under continuous 1 sun equivalent illumination for 400 h.

The efficiency and photostability of all‐inorganic mixed‐halide perovskite solar cells (PVSCs) can be simultaneously enhanced by introducing an amino‐functionalized polymer PN4N as a novel cathode interlayer and dopant‐free PDCBT hole‐transporting layer. The favorable interaction between perovskite crystal and PN4N/PDCBT can effectively improve CsPbI₂Br film quality, with power conversion efficiency over 16%.

Abstract

A synergic interface design is demonstrated for photostable inorganic mixed‐halide perovskite solar cells (PVSCs) by applying an amino‐functionalized polymer (PN4N) as cathode interlayer and a dopant‐free hole‐transporting polymer poly[5,5′‐bis(2‐butyloctyl)‐(2,2′‐bithiophene)‐4,4′‐dicarboxylate‐alt‐5,5′‐2,2′‐bithiophene] (PDCBT) as anode interlayer. First, the interfacial dipole formed at the cathode interface reduces the workfunction of SnO₂, while PDCBT with deeper‐lying highest occupied molecular orbital (HOMO) level provides a better energy‐level matching at the anode, leading to a significant enhancement in open‐circuit voltage (V _oc) of the PVSCs. Second, the PN4N layer can also tune the surface wetting property to promote the growth of high‐quality all‐inorganic perovskite films with larger grain size and higher crystallinity. Most importantly, both theoretical and experimental results reveal that PN4N and PDCBT can interact strongly with the perovskite crystal, which effectively passivates the electronic surface trap states and suppresses the photoinduced halide segregation of CsPbI₂Br films. Therefore, the optimized CsPbI₂Br PVSCs exhibit reduced interfacial recombination with efficiency over 16%, which is one of the highest efficiencies reported for all‐inorganic PVSCs. A high photostability with a less than 10% efficiency drop is demonstrated for the CsPbI₂Br PVSCs with dual interfacial modifications under continuous 1 sun equivalent illumination for 400 h.

Halide perovskite single crystal films that inherit the distinct properties of both bulk single crystal and polycrystalline film such as low trap density, high carrier mobility, and well‐defined thickness have aroused widespread attention. In article no. 1800294, Dai‐Bin Kuang and co‐workers review recent progresses in the fabrication methodologies, physical and chemical properties, as well as the optoelectronic applications of perovskite single crystal thin films.

In article no. 1800313, Yan‐Zhen Zheng, Xia Tao, and co‐workers report bromide‐induced room temperature crystallization of highly photoactive black phase formamidinium‐based perovskite films with high performance and reproducibility.

A block copolymer F127 passivation strategy in conjunction with the solvent annealing process significantly enhances the performance and stability of planar perovskite solar cells. Hydrophilic tails of F127 passivate defects at grain boundaries through hydrogen bonding, whereas the dangling hydrophobic groups suppress perovskite decomposition against moisture and heat.

Organic–inorganic metal halide perovskite solar cells (PSCs) exhibit excellent photovoltaic performance but have the drawbacks of instabilities against moisture and heat due to the inherent hydroscopic nature and volatility of their organic components. Herein, it is reported that using the block copolymer F127 as the passivation reagent in conjunction with the solvent annealing process can efficiently improve the performance and stability of corresponding organic–inorganic PSCs. It is anticipated that the hydrophilic poly(ethylene oxide) tails of F127 polymers connect with contiguous perovskite crystals and passivate defects at perovskite grain boundaries, whereas the dangling hydrophobic poly(phenyl oxide) centers suppress perovskite decomposition caused by moisture and heat. After the optimization of the F127 additive, planar PSCs with champion power conversion efficiencies of 21.01% and 18.71% are achieved on rigid and flexible substrates, respectively. The F127 passivation strategy provides an effective approach for fabricating high‐efficiency and stable PSCs.

A novel surface passivation of a perovskite surface is reported using the polyfluoro organic compound tris(pentafluorophenyl)boron (TPFPB), which can yield large grains, reduced defect densities, and improved charge transport and phase stability for the perovskite film. Using this strategy, a champion perovskite solar cell achieves a high power conversion efficiency of 21.6% as well as significantly improved air and light stabilities.

In planar perovskite solar cells (PSCs), defect‐induced recombination at the interface between the perovskite and hole transport layer (HTL) leads to a large potential loss and performance deterioration. Therefore, an effective method for improving interfacial properties is critical to boost the performance and stability of PSCs. Herein, a novel surface engineering technology is reported for passivating the perovskite surface with the polyfluoro organic compound tris(pentafluorophenyl)boron (TPFPB), which can yield large perovskite grains, reduced defect densities, and improved charge transport and phase stability for the perovskite film, and enhanced power conversion efficiency (PCE) and stability for PSCs. Using this strategy, a champion FA_0.85MA_0.15PbI₃ perovskite cell achieves a high PCE of 21.6% as well as significantly improved air and light stabilities. This work demonstrates that TPFPB is a promising material for crystallization control and defect passivation and paves a new path for mitigating defects and further increasing the performance of planar PSCs.

An n‐doping of SnO₂ is successfully realized through the use of the triphenylphosphine‐oxide molecule, where electrons are revealed to be transferred from the R₃P⁺O⁻ σ‐bond to the peripheral tin atoms and delocalized. That novel effect enlarges the built‐in‐field from 0.01 to 0.07 eV and reduces the energy‐barrier from 0.55 to 0.39 eV at the SnO₂–perovskite interface enabling a device conversion‐efficiency from 19.01% to 20.69%.

Abstract

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n‐type doping of oxide semiconductors. Herein, successful n‐type doping of SnO₂ is demonstrated by a simple, air‐robust, and cost‐effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P⁺O⁻σ‐bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi‐photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO₂, which enlarges the built‐in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO₂/perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

A simple but powerful sequential vapor deposition technique is introduced to fabricate high‐quality mixed‐cation mixed‐halide perovskite films for perovskite solar cells. Using copper phthalocyanine as a hole transport layer and sputtered SnO₂ as an unprecedentedly employed electron transport layer in all‐vacuum‐deposited mixed‐perovskite solar cells, high‐performance devices are achieved, envisioning future cost‐effective commercialization.

The incorporation of various cations and halides to form mixed perovskites has enabled perovskite solar cells (PSCs) to exceed 20% power conversion efficiencies (PCEs). However, they are primarily prepared by solution methods, which limit film uniformity and scalability. Although co‐evaporation is used to prepare all‐vacuum‐deposited PSCs with a decent performance, it involves multiple sources and quartz crystal monitors (QCMs) to simultaneously control deposition rates and film thicknesses, which increase production cost and fabrication complexity and interfere QCMs’ reading precision. Herein, a simple and cost‐effective sequential vapor deposition involving only one QCM and two sources is demonstrated as an advantageous and reliable method to fabricate high‐quality and uniform mixed‐cation mixed‐halide perovskite films with microscale grain sizes and extraordinary morphology for the PSC application. In addition, for the first time, radio frequency (RF)‐sputtered SnO₂ is implemented into all‐vacuum‐deposited PSCs as an electron transport layer (ETL). Together with evaporated copper phthalocyanine (CuPc) as a thermally and chemically stable low‐cost hole transport layer (HTL), alternative to the commonly used 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamino)‐9,9′‐spirobifluorene (Spiro‐OMeTAD), which is costly, highly hygroscopic, and deliquescent, a respectable PCE of 15.14% is achieved with a promising device stability and negligible hysteresis.

An in situ back‐contact passivation strategy is adopted to optimize the photovoltaic performance of n–i–p planar perovskite solar cells. Devices with a flat‐band alignment between the perovskite and polymer passivation layer achieve a high photovoltage of 1.15 V and fill factor of 83% with 1.53 eV bandgap perovskite, leading to a stabilized power conversion efficiency of 21.6%.

Abstract

Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.