JIALINGBO

The surface treatment of the electron transport layer, PCBM, is done using stearic acid. The treated surface consists of perpendicularly aligned monolayers of stearic acid which repel water, creating a hydrophobic film on top of PCBM. Amide linkages which crosslink stearic acid and the methyl ester group of PCBM, act as a barrier by preventing iodine ions which migrate from the active layer, reacting with the aluminum electrode.

Having achieved power conversion efficiencies higher than 22%, perovskite solar cells (PSCs) look set to be game changers in the field of photovoltaics. Their instability in humid environments, however, reduces their potential for commercialization. In this study, the role chemical degradation plays in moisture‐affected devices is investigated, and, based on this concept, a technique that enhances the device stability of p‐i‐n PSCs is developed. By surface treatment of the [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer with hydrophobic stearic acid and ethylenediamine, increased moisture resistivity of PCBM is achieved. The treated surface of the PCBM layer improves hydrophobicity, with a contact angle of 108°, and also prevents water ingress in the perovskite layer longer than non‐treated surfaces. In addition, interfacial stability is enhanced by the suppressed interaction between the ions and the electrodes, resulting in treated devices exhibiting improved stability in their photovoltaic parameters compared to non‐treated devices when exposed to a dark environment with a relative humidity of 45%.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Here, recent progress in interface modification for OSCs and PSCs aimed to improve interfacial charge extraction and mitigate surface recombination, and to enhance trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

Perovskite solar cells have experienced a rapid development since the first report in 2012 with the power conversion efficiency approaching the theoretical limit. Device stability is still one of the remaining challenges for commercialisation. In this Review, the authors address the important role the charge selective contacts play in the long‐term stability of perovskite solar cells.

Abstract

Lead halide perovskite solar cells have rapidly achieved high efficiencies comparable to established commercial photovoltaic technologies. The main focus of the field is now shifting toward improving the device lifetime. Many efforts have been made to increase the stability of the perovskite compound and charge‐selective contacts. The electron and hole selective contacts are responsible for the transport of photogenerated charges out of the solar cell and are in intimate contact with the perovskite absorber. Besides the intrinsic stability of the selective contacts themselves, the interfaces at perovskite/selective contact and metal/selective contact play an important role in determining the overall operational lifetime of perovskite solar cells. This review discusses the impact of external factors, i.e., heat, UV‐light, oxygen, and moisture, and measured conditions, i.e., applied bias on the overall stability of perovskite solar cells (PSCs). The authors summarize and analyze the reported strategies, i.e., material engineering of selective contacts and interface engineering via the introduction of interlayers in the aim of enhancing the device stability of PSCs at elevated temperatures, high humidity, and UV irradiation. Finally, an outlook is provided with an emphasis on inorganic contacts that is believed to be the key to achieving highly stable PSCs.

The surface treatment of the electron transport layer, PCBM, is done using stearic acid. The treated surface consists of perpendicularly aligned monolayers of stearic acid which repel water, creating a hydrophobic film on top of PCBM. Amide linkages which crosslink stearic acid and the methyl ester group of PCBM, act as a barrier by preventing iodine ions which migrate from the active layer, reacting with the aluminum electrode.

Having achieved power conversion efficiencies higher than 22%, perovskite solar cells (PSCs) look set to be game changers in the field of photovoltaics. Their instability in humid environments, however, reduces their potential for commercialization. In this study, the role chemical degradation plays in moisture‐affected devices is investigated, and, based on this concept, a technique that enhances the device stability of p‐i‐n PSCs is developed. By surface treatment of the [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer with hydrophobic stearic acid and ethylenediamine, increased moisture resistivity of PCBM is achieved. The treated surface of the PCBM layer improves hydrophobicity, with a contact angle of 108°, and also prevents water ingress in the perovskite layer longer than non‐treated surfaces. In addition, interfacial stability is enhanced by the suppressed interaction between the ions and the electrodes, resulting in treated devices exhibiting improved stability in their photovoltaic parameters compared to non‐treated devices when exposed to a dark environment with a relative humidity of 45%.

A low‐temperature solution‐processed TFB is demonstrated as an ideal hole‐transporting layer to push the PCE of the inverted perovskite solar cells (PVSCs) up to 20.2%. Moreover, this TFB‐based inverted PVSC exhibits good stability, retaining 90% of its original efficiency after storage for 30 days in ambient air.

Abstract

Low‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH₃NH₃PbI₃ (MAPbI₃)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI₃‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.

Understanding how excess lead iodide precursor improves halide perovskite solar cell performance

Understanding how excess lead iodide precursor improves halide perovskite solar cell performance, Published online: 17 August 2018; doi:10.1038/s41467-018-05583-w

The molecular order of nonfullerene electron acceptor INPIC‐4F is manipulated by varying the self‐organization time during solution casting. With the presence of solvent vapor, INPIC‐4F grows into spherulites with poor efficiency. On the contrary, casting on hot substrates promotes face‐on π−π stacking, which improves absorption as well as efficient exciton dissociation and balanced charge mobility for a maximum efficiency of 13.1%.

Abstract

Developing a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB‐T:INPIC‐4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC‐4F in achieving high performance is demonstrated. When PBDB‐T:INPIC‐4F is cast with the presence of solvent vapor to prolong the organization time, INPIC‐4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face‐on π−π stacking of INPIC‐4F. This molecular transformation of INPIC‐4F significantly enhances the absorption ability of INPIC‐4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.

Ternary organic solar cells with improved power conversion efficiency (PCE) and ambient stability are developed by combining a nonfullerene acceptor and a fullerene acceptor. Such a ternary system is insensitive to the content of the third component, and PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of the binary reference device.

Abstract

The ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC₇₁BM) are reported. It is found that the addition of PC₇₁BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC₇₁BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.

In article number 1802323, Chih‐Ping Chen and co‐workers demonstrate hydrophilic carbon nanodots efficient additives in perovskite solar cells (PSC). The p‐i‐n PSC device incorporating these additives demonstrated a power conversion efficiency of 20.2% and exhibited excellent air‐stability, maintaining high PCEs (25 °C and a humidity of 40%) for over 500 h.