JIANG ZHI XUAN

A homochiral molecular ferroelectric was incorporated into a perovskite film to enlarge the built‐in electric field of the perovskite solar cell (PSC), thereby facilitating charge separation and transportation. The molecular ferroelectric component of the PSC passivates the defects in the perovskite active layers to induce an approximately eightfold enhancement in photoluminescence intensity and a reduction in electron trap‐state density.

Abstract

The nonradiative recombination of electrons and holes has been identified as the main cause of energy loss in hybrid organic–inorganic perovskite solar cells (PSCs). Sufficient built‐in field and defect passivation can facilitate effective separation of electron–hole pairs to address the crucial issues. For the first time, we introduce a homochiral molecular ferroelectric into a PSC to enlarge the built‐in electric field of the perovskite film, thereby facilitating effective charge separation and transportation. As a consequence of similarities in ionic structure, the molecular ferroelectric component of the PSC passivates the defects in the active perovskite layers, thereby inducing an approximately eightfold enhancement in photoluminescence intensity and reducing electron trap‐state density. The photovoltaic molecular ferroelectric PSCs achieve a power conversion efficiency as high as 21.78 %.

A simple and effective sulfur‐doping method–based chemical bath deposition is introduced to improve interface contact between NiO and perovskite for efficient inverted perovskite solar cells. Sulfur doping leads to promoted perovskite film quality with reduced amount of grain boundaries and trap‐assisted charge recombination. The champion efficiency based on MAPbI₃ reaches 20.43% in the NiO‐based inverted photovoltaic (PV) device.

As one of the most promising hole‐transporting materials for perovskite solar cells (PSC), NiO is widely used in the inverted p–i–n cell structure due to its high stability, decent hole conductivity, and easy processability for hysteresis‐free cells. However, the efficiency of NiO‐based PSCs is still low, due largely to the poor perovskite/NiO interface. Herein, a sulfur‐doping strategy to modify NiO surface via ion exchange reaction by a simple and scalable chemical bath deposition technique is introduced, which greatly improves the photovoltaic (PV) performance of the derived devices. A systematic investigation is shown where sulfur doping leads to favorable interfacial energetics with a reduced V _oc loss. Sulfur doping at the interface also improves the contact between NiO and perovksite and facilitates the formation of high‐quality perovskite films. Carrier dynamics studies demonstrate reduced defect states and trap‐assisted recombination with sulfur doping, which promote the PV performance of the devices. These merits contribute concurrently to low‐loss charge transfer across the perovskite/NiO interface and facilitate charge transport through the perovskite films, leading to a high champion efficiency of 20.43% of the p–i–n structure solar cell devices.

The salt of the cation (Mes₂B⁺; Mes: mesitylene) and the anion [B(C₆F₅)₄]⁻ is introduced as superior p‐type dopant for organic semiconductors. The doping mechanism involves electron transfer from the semiconductor to Mes₂B⁺, and the positive charge is stabilized by [B(C₆F₅)₄]⁻. For poly(3‐hexylthiophene), the anion even stabilizes bipolarons. The effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]⁻ is estimated to be 5.9 eV.

Abstract

Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto‐) electronics. Beyond single‐component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes₂B⁺; Mes: mesitylene) and the tetrakis(penta‐fluorophenyl)borate anion [B(C₆F₅)₄]⁻ is demonstrated as p‐type dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes₂B⁺, and the positive charge on the polymer is stabilized by [B(C₆F₅)₄]⁻. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C₆F₅)₄]⁻ even enables the stabilization of polarons and bipolarons in poly(3‐hexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]⁻ is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors.

In article number https://doi.org/10.1002/adma.2020014762001476, Xugang Guo and co‐workers develop two ultranarrow‐bandgap n‐type polymer semiconductors, which enable efficient electron transport in organic thin‐film transistors with a highest electron mobility of 1.72 cm² V⁻¹ s⁻¹ and which deliver remarkable photovoltaic performance with a highest power conversion efficiency of 10.22% and short‐circuit current up to 22.52 mA cm⁻². The emergence of such polymers will guide materials innovation for realizing highperformance fully flexible all‐polymer solar cell modules.

A high‐performance all‐polymer solar cell (PCE of 12.06%) is achieved based on a novel polymer acceptor with a voltage loss of 0.52 eV, which is one of the smallest values reported for all‐polymer solar cells to date.

Abstract

Although the field of all‐polymer solar cells (all‐PSCs) has seen rapid progress in device efficiencies during the past few years, there are limited choices of polymer acceptors that exhibit strong absorption in the near‐IR region and achieve high open‐circuit voltage (V _OC) at the same time. In this paper, an all‐PSC device is demonstrated with a 12.06% efficiency based on a new polymer acceptor (named PT‐IDTTIC) that exhibits strong absorption (maximum absorption coefficient: 2.41 × 10⁵ cm⁻¹) and a narrow optical bandgap (1.49 eV). Compared to previously reported polymer acceptors such as those based on the indacenodithiophene (IDT) core, the indacenodithienothiophene (IDTT) core has further extended fused ring, providing the polymer with extended absorption into the near‐IR region and also increases the electron mobility of the polymer. By blending PT‐IDTTIC with the donor polymer, PM6, a high‐efficiency all‐PSC is achieved with a small voltage loss of 0.52 V, without sacrificing J _SC and FF, which demonstrates the great potential of high‐performance all‐PSCs.

Publication date: 15 September 2020

Source: Solar Energy Materials and Solar Cells, Volume 215

Author(s): Damir Aidarkhanov, Zhiwei Ren, Chang-Keun Lim, Zhuldyz Yelzhanova, Gaukhar Nigmetova, Gaukhar Taltanova, Bakhytzhan Baptayev, Fangzhou Liu, Sin Hang Cheung, Mannix Balanay, Aidos Baumuratov, Aleksandra B. Djurišić, Shu Kong So, Charles Surya, Paras N. Prasad, Annie Ng

2D organic-inorganic lead iodide perovskites hold great promise for functional optoelectronic devices. However, their performances have been seriously limited by poor long-term stability in ambient environment. Here, we perform a systematic study for the stability improvement of a typical 2D organic-inorganic lead iodide perovskite (PEA) 2 PbI 4 . The degradation of the (PEA) 2 PbI 4 films can be attributed to the interaction with the humidity in environment, which leads to decomposition of the perovskite components. Then, we demonstrate that polymer passivation provides an effective approach for improving the crystal quality and stability of the (PEA) 2 PbI 4 films. Correspondingly, the photoemission of the polymer-passivated (PEA) 2 PbI 4 films has been enhanced due to the decreased trap states. More importantly, a hydrophobic polymer (Poly(4-Vinylpyridine), PVP) will protect the (PEA) 2 PbI

A zwitterionic conjugated polyelectrolyte presents high hole mobility, compatible covalence level, and the ability for passivating surface defects of the perovskite film. The formation of a weak double‐layer capacitance, which is not strong enough to induce the migration of MA⁺ ions, contributes to low carrier transport resistance and interfacial charge accumulation, leading to high efficiency and stability.

Achieving rapid extraction and equivalent transport of charge carriers is an effective way to improve the performance of perovskite solar cells (PSCs). Herein, a thiophene‐based zwitterionic conjugated polyelectrolyte (poly(5‐amino‐5‐carboxy‐3‐oxapentyl)‐2,5‐thiophene [POWT]) is introduced into PSCs as a hole‐transporting and interfacial material. The polyelectrolyte has a high hole mobility of 5.74 × 10⁻³ cm² V⁻¹ s⁻¹ (similar to that of poly(triarylamine) [PTAA]) and compatible covalence level relative to the perovskite. Terminated with a zwitterionic pair of a‐amino acid, POWT layer builds up a weak double‐layer capacitance at the interface, which is not strong enough to induce the migration of MA⁺ ions in the perovskite layer. Deep electrical study on the PSC with the structure of indium tin oxide (ITO)/POWT/FA_0.2MA_0.8PbI_2.9Br_0.1/C₆₀/bathocuproine (BCP)/Ag discloses that the device has low carrier transfer resistance, low leakage current density, and minor interfacial charge accumulation. The open‐circuit voltage and the short‐circuit current density are much improved, and the power conversion efficiency (PCE) is up to 17.5%. With a‐amino acid zwitterions, POWT passivates the surface charge defects and grain boundaries of the perovskite film. The PSC presents negligible hysteresis and high stability. After 56 days, the unencapsulated PSC still remains at 85% of the original efficiency.

A new fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is utilized to passivate the surface of CsPbI₂Br perovskite for solar cells with enhanced efficiency as well as improved stability.

Surface modification is demonstrated as an efficient strategy to enhance the efficiency and stability of perovskite solar cells (PVSCs). Fluorinated organic ammonium salts featuring a strong hydrophobic nature are seldom used as passivation agents for the surface modification of CsPbI₂Br perovskites. Herein, a fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is incorporated into the surface of CsPbI₂Br perovskite for the first time. After the CFPEAI modification, the defects of CsPbI₂Br perovskite are significantly passivated with reduced trap densities. The best‐performance PVSC with CFPEAI modification shows an excellent power conversion efficiency (PCE) of 16.07% with a high fill factor (FF) of 84.65%, a short‐circuit current density (J _SC) of 15.45 mA cm⁻², and an open‐circuit voltage (V _OC) of 1.23 V. In contrast, the control PVSCs without the surface modification exhibit a lower PCE of 14.50% with a FF of 80.56%, a J _SC of 15.05 mA cm⁻², and a V _OC of 1.20 V. With CFPEAI passivation, the CsPbI₂Br perovskite film exhibits enhanced hydrophobicity, thereby leading to improved stability for the corresponding PVSC in comparison with the control PVSC without any surface modification.

Passivation is like a pair of magic hands, which can heal defective perovskite cubes to a perfect light absorption layer. Herein, the origin of various defects as well as their detrimental effects on perovskite solar cells (PSCs) performance and the targeted passivation strategies for specific defects are summarized. Finally, the future development trend on passivation is provided.

With a certificated record efficiency of 25.2%, organometal halide perovskite (OHP) solar cells have experienced unprecedentedly rapid development in the past decade due to their extraordinary photoelectronic properties. However, because of the rapid processing conditions and complex precursor compositions, there are a large number of defects in polycrystalline OHP films, including point defects and 2D defects along grain boundary and on the surface. Unfortunately, these defects serve as the nonradiative recombination centers and exert negative effects on the degradation and performance of OHP layers, heavily limiting their further application for efficient photovoltaic devices. Herein, the formation origin of various defects as well as their detrimental effects on the efficiency and stability of perovskite solar cells (PSCs) are discussed, and recent passivation strategies for specific defects to minimize defect state density in the perovskite films are summarized. Finally, a brief outlook on the development trend of future passivation engineering is provided for deeper understanding of efficient and stable PSCs.

A novel strategy of tuning perovskite crystal orientation toward ≈45° inclination with respect to the substrate is proposed with incorporating 2,3‐diaminopropionic acid monohydrochloride (2,3‐DAPAC) into FASnI₃, which facilitates charge transport in the perovskite film from bottom to top. The solar cells with 2,3‐DAPAC acquire a champion power conversion efficiency of 7.23% and improved stability.

Despite a higher power conversion efficiency (PCE) than other lead‐free perovskite solar cells (PSCs) due to intrinsically excellent optoelectronic properties and suitable bandgaps of tin (Sn) perovskites, Sn‐based PSCs still suffer from issues of stability and efficiency for practical applications. Herein, a novel strategy of tuning perovskite crystal orientation toward ≈45° with respect to the substrate by doping 2,3‐diaminopropionic acid monohydrochloride (2,3‐DAPAC) into formamidinium tin iodide (FASnI₃) is proposed, which facilitates charge transport in the perovskite film and consequent device performances. In addition, the incorporation of 2,3‐DAPAC into FASnI₃ enables dense and smooth high‐quality perovskite films with less Sn vacancies. Applications of the 2,3‐DAPAC‐treated FASnI₃ films into PSCs acquire a champion PCE of 7.23%, showing 37.2% enhancement compared with 5.27% of the control device. Moreover, the storage stabilities of both perovskite films and PSCs are significantly prolonged with improved film quality.

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Yehao Deng, Zhenyi Ni, Axel F. Palmstrom, Jingjing Zhao, Shuang Xu, Charles H. Van Brackle, Xun Xiao, Kai Zhu, Jinsong Huang

Barriers with compact morphology/structure and shielding capability can be designed/ integrated in perovskite solar cells to prevent issues like product volatilization, ion diffusion, electrode corrosion, and ingress of the harmful components brought about by the intrinsic interface failure or the attack of heat, sunlight, electric bias, and H₂O/O₂, leading to robust stability of the whole device.

Abstract

Perovskite solar cells (PSCs) have attracted much attention in the past decade and their power conversion efficiency has been rapidly increasing to 25.2%, which is comparable with commercialized solar cells. Currently, the long‐term stability of PSCs remains as a major bottleneck impeding their future commercial applications. Beyond strengthening the perovskite layer itself and developing robust external device encapsulation/packaging technology, integration of effective barriers into PSCs has been recognized to be of equal importance to improve the whole device’s long‐term stability. These barriers can not only shield the critical perovskite layer and other functional layers from external detrimental factors such as heat, light, and H₂O/O₂, but also prevent the undesired ion/molecular diffusion/volatilization from perovskite. In addition, some delicate barrier designs can simultaneously improve the efficiency and stability. In this review article, the research progress on barrier designs in PSCs for improving their long‐term stability is reviewed in terms of the barrier functions, locations in PSCs, and material characteristics. Regarding specific barriers, their preparation methods, chemical/photoelectronic/mechanical properties, and their role in device stability, are further discussed. On the basis of these accumulative efforts, predictions for the further development of effective barriers in PSCs are provided at the end of this review.

A series of copolymers via a random copolymerization approach are designed and synthesized. The well‐defined fibril interpenetrating morphology with appropriate phase separation in PT2‐based blends can efficiently suppress the unfavorable aggregation, resulting in excellent morphological stability and high efficiency. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable polymer solar cells.

Abstract

Morphological stability is crucially important for the long‐term stability of polymer solar cells (PSCs). Many high‐efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene‐4,8‐dione (T1‐Cl) via a random copolymerization approach. It is found that all the copolymers can self‐assemble into a fibril nanostructure in films. By altering the T1‐Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT‐4F, O‐INIC3, EH‐INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

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A general decomposition pathway from tetragonal CH₃NH₃PbI₃ and cubic CH₃NH₃PbBr₃ to lead halides is revealed, through the formation of an intermediate superstructure CH₃NH₃PbX_2.5 with ordered vacancies. A carbon coating is demonstrated to be effective in stabilizing the perovskite framework, and thus slowing down the decomposition.

Abstract

Organic–inorganic hybrid perovskites (OIHPs) have generated considerable excitement due to their promising photovoltaic performance. However, the commercialization of perovskite solar cells (PSCs) is still plagued by the structural degradation of the OIHPs. Here, the decomposition mechanism of OIHPs under electron beam irradiation is investigated via transmission electron microscopy, and a general decomposition pathway for both tetragonal CH₃NH₃PbI₃ and cubic CH₃NH₃PbBr₃ is uncovered through an intermediate superstructure state of CH₃NH₃PbX_2.5, X = I, Br, with ordered vacancies into final lead halides. Such decomposition can be suppressed via carbon coating by stabilization of the perovskite structure framework. These findings reveal the general degradation pathway of OIHPs and suggest an effective strategy to suppress it, and the atomistic insight learnt may be useful for improving the stability of PSCs.

Nature Chemistry, Published online: 06 July 2020; doi:10.1038/s41557-020-0488-2

The strength of electrostatic interactions in semiconductors strongly affects their performance in optoelectronic devices. Now, doping two-dimensional naphthalene-based lead halide perovskites with tetrachloro-1,2-benzoquinone has been shown to introduce donor–acceptor interactions within the organic network, without disrupting the inorganic sublattice. This in turn altered the energy of the materials’ electron–hole electrostatic Coulomb interactions.