JIALINGBO

A dopant‐free polymeric hole transport material (HTM) is synthesized to fabricate perovskite solar cells. The carbonyl groups can passivate defects of under‐coordinated Pb atoms that exist in the surface of perovskite films. A PBT1‐C based device shows a power conversion efficiency of 19.06% with a fill factor of 81.22%, which is the highest value among the dopant‐free polymeric HTMs.

Abstract

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high‐performance device results are based on the doped small molecule hole‐transporting material (HTM), spiro‐OMeTAD, which affects their long‐term stability. In addition, some defects from under‐coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant‐free HTM based on a donor‐acceptor polymer, PBT1‐C, synthesized from the copolymerization between the benzodithiophene and 1,3‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)‐5,7‐bis(2‐alkyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione units is introduced. PBT1‐C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant‐free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1‐C can effectively passivate both surface and grain boundary traps of the perovskite.

Recent progress in bandgap engineering strategies including the two main, widely used impurity and pressure as well as intermediate band, external electric field, and steric methods are reviewed comprehensively. Their underlying mechanism, achievements, and challenges are outlined. Additionally, future research directions are provided to realize direct and gap size continually tunable perovskites for further enhancing solar cell performance.

Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single‐junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

Herein, amorphous‐Zn₂SnO₄ (am‐ZTO) is used to provide a large free energy difference (ΔG) to improve electron injection from perovskite to electron transport layers. In addition, the introduction of the am‐ZTO also leads to a dense physical contact between the am‐ZTO and the FTO substrate, leading to decreased leakage current. The optimized device exhibits a power conversion efficiency of 20.04%.

Charge extraction by electron transport layers (ETLs) plays a vital role in improving the performance of perovskite solar cells (PSCs). Here, PSCs with four different types of ETLs, such as SnO₂, amorphous‐Zn₂SnO₄ (am‐ZTO), am‐ZTO/SnO₂, and SnO₂/am‐ZTO, are successfully synthesized. The interface recombination behavior and the charge transport properties of the devices affected by four types of ETLs are systematically investigated. For dual am‐ZTO/SnO₂ ETLs, compact am‐ZTO ETL prepared by the pulsed laser deposition method provides a dense physical contact with FTO than the spin coating films, decreasing leakage current and improving charge collection at the interface of ETL/FTO. Moreover, dual am‐ZTO/SnO₂ ETLs lead to large free energy difference (ΔG), improving electron injection from perovskite to ETLs. One additional electron pathway from perovskite to am‐ZTO is formed, which can also improve electron injection efficiency. A power conversion efficiency of 20.04% and a stabilized efficiency of 19.17% are achieved for the device based on dual am‐ZTO/SnO₂ ETLs. Most importantly, the devices are fabricated at a low temperature of 150 °C, which offers a potential method for large‐scale production of PSCs, and paves the way for the development of flexible PSCs. It is believed that this work provides a strategy to design ETLs via controlling ΔG and interface contact to improve the performance of PSCs.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

The passivation of the SnO₂ electron transport layer by fullerenes in metal halide perovskite solar cells is studied with X‐ray photoelectron spectroscopy depth profiling. Interfacial binding of fullerenes to the SnO₂ surface is essential for reproducible and effective passivation and improved solar cell performance.

Abstract

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO₂ as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO₂ ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO₂ interface. Through a comparative study using [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C₆₁‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO₂ interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO₂ ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C₆₀ and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO₂ surface.

Herein, acetate anion (Ac⁻) is used to partially replace I⁻ in the CsPbI₂Br framework. Ac⁻ doping changes the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite solar cells exhibit a power conversion efficiency of 15.56%, an open circuit voltage of 1.30 V, and great air stability.

Abstract

The Cs‐based inorganic perovskite solar cells (PSCs), such as CsPbI₂Br, have made a striking breakthrough with power conversion efficiency (PCE) over 16% and potential to be used as top cells for tandem devices. Herein, I⁻ is partially replaced with the acetate anion (Ac⁻) in the CsPbI₂Br framework, producing multiple benefits. The Ac⁻ doping can change the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite films present lower trap densities, longer carrier lifetimes, and fast charge transportation compared to the host CsPbI₂Br films. Interestingly, the CsPbI_{2− x} Br(Ac) _x PSCs exhibit a maximum PCE of 15.56% and an ultrahigh open circuit voltage (V _oc) of 1.30 V without sacrificing photocurrent. Notably, such a remarkable V _oc is among the highest values of the previously reported CsPbI₂Br PSCs, while the PCE far exceeds all of them. In addition, the obtained CsPbI_{2− x} Br(Ac) _x PSCs exhibit high reproducibility and good stability. The stable CsPbI_{2− x} Br(Ac) _x PSCs with high V _oc and PCE are desirable for tandem solar cell applications.

In article number https://doi.org/10.1002/aenm.2019007201900720, Yumin Liu, Li Yu, Huai Yang and co‐workers report the design of smart photovoltaic windows with a series of working modes that are enabled by coupling of multi‐responsive liquid crystal/polymer composite films and semi‐transparent perovskite solar cells, providing stable electrical power generation, energy savings, and privacy protection.

In article number https://doi.org/10.1002/aenm.2019012571901257, Yana Vaynzof and co‐workers introduce π‐extended phosphoniumfluorene electrolytes as hole‐blocking layers in planar perovskite solar cells. The electrolytes drastically alter the energetic landscape of the device, introducing a strong dipole between the fullerene electron extraction layer and the silver electrode. This results in a substantial enhancement in the built‐in potential of the device, increasing its open‐circuit voltage by up to 120 meV.

Cheap plastic in solar cells! TiO₂ nanoparticles‐based perovskite solar cells (PVSCs) with a polystyrene (PS) interface modification layer and a poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) hole‐transporting layer exhibit encouraging photovoltaic performances and stabilities. The use of a cheap plastic material in PVSCs would allow the fabrication of low‐cost PVSCs for commercial use

Abstract

Interface engineering of TiO₂ nanoparticles (NPs)‐based perovskite solar cells (PVSCs) is often necessary to facilitate the extraction and transport of charge carriers. In this work, poly[{9,9‐bis[3′‐(N,N‐dimethyl)propyl]‐2,7‐fluorene}‐alt‐2,7‐(9,9‐dioctylfluorene)] (PFN) and polystyrene (PS) are demonstrated to be effective surface modifiers of the TiO₂ NPs electron‐transporting layer in n‐i‐p PVSCs. The low‐cost insulating polymer PS performs better than the PFN conjugated polymer owing to its high film quality, low surface energy and insulating characteristics. A peak power conversion efficiency (PCE) of 15.09 % with an open‐circuit voltage (V _OC) of 1.05 V and a PCE of 17.13 % with an ultrahigh V _OC of 1.18 V is achieved with TiO₂ NPs/PS‐based PVSCs using poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) and spiro‐OMeTAD, respectively, as the hole‐transporting material.

The OH…I⁻ hydrogen bonding interactions between poly(vinyl alcohol) (PVA) and FASnI₃ have the effects of introducing nucleation sites, slowing down crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the ions. By adding PVA, the FASnI₃–PVA perovskite solar cells attain improved power conversion efficiency and stability.

Abstract

Tin‐based perovskites with narrow bandgaps and high charge‐carrier mobilities are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, the crystalline rate of tin‐based perovskites is much faster, leading to abundant trap states and much lower open‐circuit voltage (V _oc). Here, hydrogen bonding is introduced to retard the crystalline rate of the FASnI₃ perovskite. By adding poly(vinyl alcohol) (PVA), the OH…I⁻ hydrogen bonding interactions between PVA and FASnI₃ have the effects of introducing nucleation sites, slowing down the crystal growth, directing the crystal orientation, reducing the trap states, and suppressing the migration of the iodide ions. In the presence of the PVA additive, the FASnI₃–PVA PSCs attain higher power conversion efficiency of 8.9% under a reverse scan with significantly improved V _oc from 0.55 to 0.63 V, which is one of the highest V _oc values for FASnI₃‐based PSCs. More importantly, the FASnI₃–PVA PSCs exhibit striking long‐term stability, with no decay in efficiency after 400 h of operation at the maximum power point. This approach, which makes use of the OH…I⁻ hydrogen bonding interactions between PVA and FASnI₃, is generally applicable for improving the efficiency and stability of the FASnI₃‐based PSCs.

In article no. 1900199, Ana F. Nogueira and co‐workers modify perovskite surfaces with alkylammonium chloride, which increases the stability of the solar cells, making it last longer when exposed to environmental conditions. After the modification, 2D/3D structures are formed and their chemical structures are identified. This mixture makes the films more humidity tolerant.

In article no. 1900078, Changlei Wang, Xiaofeng Li, Yanfa Yan, and co‐workers report that the introduction of block copolymer F127 could passivate grain boundaries and enhance the hydrophobicity of perovskite films simultaneously, resulting in highly efficient planar and flexible perovskite solar cells with good stability.

Low‐cost and efficient organic small molecules are desired as hole transporting materials for high‐performance perovskite solar cells. Two new molecules containing a benzodithiophene core and triphenylamine side chains are synthesized from cheap starting materials by a simple and low‐cost method. Lead‐free, tin‐based perovskite solar cells employing these new benzodithiophene‐based hole transporting materials achieve good efficiencies.

Abstract

Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

This review provides a systematic overview of self‐powered integrated systems based on perovskite solar cells, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices are discussed to further the understanding of fundamental device physics. The current challenges and future perspective are provided.

Integrated smart portable devices (e.g., self‐powered devices) that utilize the environment‐friendly energy (e.g., solar energy) by means of photovoltaic technology (e.g., solar cell) are a popular concept in the current technological development trend. As a key component of integrated devices, photovoltaic devices acting as a bridge between solar energy and working devices play an important role in the whole system performance. The emergence of perovskite solar cells (PSCs) with high power conversion efficiencies (over 25%) allows for the possibility and appearance of many multifunctional self‐powered integrated devices. In this review, a systematic overview of self‐powered integrated devices based on PSCs that are reported so far is provided, including integrated energy storage devices, integrated artificial photosynthesis devices, and other self‐powered integrated devices. The key strategies for fabricating these devices and performance are also discussed to further the understanding of fundamental device physics. Finally, the current challenging issues and future perspective are provided to promote the development of self‐powered integrated devices based on PSCs in the near future.

Laminated perovskite layers with different crystal sizes and optical and electrical characteristics are achieved by using aniline as the solvent in the perovskite precursor solution. Inverted planar perovskite solar cells with the laminated films as active layers achieve an average power conversion efficiency of 20.65%, originating from the high V _OC 1.112 V and fill factor of 80.8%.

Abstract

Laminated multilayers of perovskite films with different optical and electronic characteristics will easily realize high‐performance optoelectronic devices because it is widely demonstrated that differential distribution of film properties in the vertical direction of devices plays particularly important roles in device performance. However, the existing laminated perovskite films are hardly prepared by a solution process because there is no solvent with sufficient selectivity of solubility for different perovskite materials. Here, it is demonstrated that aniline (AN) has a largely different solubility toward the perovskite MAPbI₃ and the MAPbI₃ blend with an additive of hydrochloride diethylammonium chloride. By using AN as the solvent in the perovskite precursor solution, two laminated perovskite layers with different crystal size and optical and electrical characteristics are achieved. Inverted perovskite solar cells with the laminated films as active layers achieve an averaged power conversion efficiency of 20.65% originating from the high V _OC 1.112 V and fill factor of 80.8%. The devices maintain 98% efficiency after 400 h under 65% RH. This work provides a very simple and feasible method for production of laminated perovskite films to achieve high‐performance perovskite solar cells.