Chen Weijie

Nature Communications, Published online: 25 November 2019; doi:10.1038/s41467-019-13136-y

Publication date: February 2020

Source: Nano Energy, Volume 68

Author(s): Cong Chen, Xinmeng Zhuang, Wenbo Bi, Yanjie Wu, Yanbo Gao, Gencai Pan, Dali Liu, Qilin Dai, Hongwei Song

Graphical abstract

The chemical decomposition inhibition strategy was introduced in perovskite films through iodine bromide to modify the crystal defects, leading to PSCs with suppressed hysteresis effects, attractive PCE of 21.5% and superior durability of 5000 h.

Publication date: Available online 23 November 2019

Source: Nano Energy

Author(s): Haonan Si, Chenzhe Xu, Yang Ou, Guangjie Zhang, Wenqiang Fan, Zhaozhao Xiong, Ammarah Kausar, Qingliang Liao, Zheng Zhang, Abdul Sattar, Zhuo Kang, Yue Zhang

Graphical abstract

Using diluted poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the hole transport layer (HTL), Sn–Pb‐based low‐E _g perovskite solar cells (PSCs) with a maximum power conversion efficiency (PCE) of up to 19.58% and J _sc of 29.81 mA cm⁻² are achieved. Then, an all‐perovskite four‐terminal tandem cell with a PCE of 23.26% is demonstrated with this low‐E _g PSC as the bottom cell. This easy and effective approach also reduces the cost of devices.

Recently, Sn–Pb low‐bandgap (E _g) perovskite solar cells (PSCs) have attracted enormous interest as an ideal bottom cell for all‐perovskite tandem solar cells. However, due to the lack of high‐performance Sn–Pb low‐E _g PSCs, the development of all‐perovskite tandem solar cells is severely constrained. Herein, the performance of Sn–Pb low‐E _g (1.2 eV) PSC is improved significantly using diluted poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a hole transport layer with a maximum power conversion efficiency (PCE) up to 19.58% and short‐circuit current density of 29.81 mA cm⁻². The four‐terminal (4‐T) all‐perovskite tandem solar cell is constructed using an optical splitting system with this high‐efficient low‐E _g PSC as the bottom cell and a wide‐E _g (1.6 eV) PSC as the top cell. The best all‐perovskite 4‐T tandem solar cell shows a PCE of 23.26%.

A compact TiO₂/WO₃ bilayer film is fabricated as electron transport layer (ETL) in perovskite solar cells. Compared to the single WO₃ layer, the bilayer efficiently covers the fluorine‐doped tin oxide (FTO), avoids the direct contact between perovskite and FTO, decreases the risk of recombination. Finally, the bilayer ETL based device achieves a superior power conversion efficiency of 20.14%.

Abstract

It is crucial to retard the carrier recombination and minimize the energy loss at the transparent electrode/electron transport layer (ETL)/perovskite absorber interfaces to improve the performance of the perovskite solar cells (PSCs). Here, a bilayered TiO₂/WO₃ film is designed as ETL by combining atomic layer deposition (ALD) technology and spin‐coating process. The ALD‐TiO₂ underlayer fills the fluorine‐doped tin oxide (FTO) valleys and makes the surface smoother, which effectively avoids the shunt pathways between perovskite layer and FTO substrate and thereby suppresses electron–hole recombination at the interface. Moreover, the presence of hydrophilic TiO₂ underlayer is helpful in forming a uniform and compact WO₃ layer which is beneficial for extracting electron from perovskite to ETL. Meanwhile, the lower valance band minimum level of TiO₂ relative to WO₃ can efficiently enhance the hole‐blocking ability. By employing the optimized TiO₂ (7 nm)/WO₃ bilayer as ETL, the resulting cell exhibits an obviously enhanced power conversion efficiency of up to 20.14%, which is much better than the single WO₃ or TiO₂ ETL based device. This work is expected to provide a viable path to design ultrathin and compact ETL for efficient PSCs.

Large‐area perovskite films are deposited by a scalable blade coating method on flexible glass substrates at room temperature and in an ambient environment. Additive engineering by ammonium chloride effectively controls the perovskite crystallization and improves film quality. The flexible perovskite module achieves a record efficiency of 15.86% on a large aperture area of 42.9 cm².

Abstract

Perovskite materials are good candidates for flexible photovoltaic applications due to their strong absorption and low‐temperature processing, but efficient flexible perovskite modules have not yet been realized. Here, a record efficiency flexible perovskite solar module is demonstrated by blade coating high‐quality perovskite films on flexible Corning Willow Glass using additive engineering. Ammonium chloride (NH₄Cl) is added into the perovskite precursor solution to retard the nucleation which prevents voids formation at the interface of perovskite and glass. The addition of NH₄Cl also suppresses the formation of PbI₂ and reduces the trap density in the perovskite films. The implementation of NH₄Cl enables the fabrication of single junction flexible perovskite solar devices with an efficiency of 19.72% on small‐area cells and a record aperture efficiency of 15.86% on modules with an area of 42.9 cm². This work provides a simple way to scale up high‐efficiency flexible perovskite modules for various applications.

An overview is provided on the recent advances in the fundamental understanding of how the interfaces between the perovskite film and the charge transport layers influence the performance of halide perovskite solar cells. Furthermore, the various design strategies for the improvement of interfacial materials and interfacial phenomena are discussed.

Abstract

Organic–inorganic hybrid perovskite solar cells (HPSCs) have achieved an impressive power conversion efficiency (PCE) of 25.2% in 2019. At this stage, it is of paramount importance to understand in detail the working mechanism of these devices and which physical and chemical processes govern not only their power conversion efficiency but also their long‐term stability. The interfaces between the perovskite film and the charge transport layers are among the most important factors in determining both the PCE and stability of HPSCs. Herein, an overview is provided on the recent advances in the fundamental understanding of how these interfaces influence the performance of HPSCs. Firstly, it is discussed how the surface energy of the charge transport layer, the energy level alignment at the interfaces, the charge transport in interfacial layers, defects and mobile ions in the perovskite film, and interfacial layers or at the interfaces affect the charge recombination as well as hysteresis and light soaking phenomenon. Then it is discussed how the interfaces and interfacial materials influence the stability of HPSCs. At the same time, an overview is also provided on the various design strategies for the interfaces and the interfacial materials. At the end, the outlook for the development of highly efficient and stable HPSCs is provided.

A postdeposition treatment is designed to modify the photoactive layer of perovskite solar cell (PSC) by spin‐coating p‐aminobenzoic acid (PABA). The PABA treatment can enhance V _OC, fill factor, and power conversion efficiency. The performance improvement is attributed to the suppression of carrier trap states. PABA post‐treatment provides a promising strategy and potential option for high performance solar cells.

Abstract

Various approaches of interface engineering are shown to be effective in improving the device performance of organic–inorganic hybrid perovskite solar cells (PSCs). The modification of the photoactive layer of PSC, CH₃NH₃PbI₃ (MAPbI₃), by spin‐coating a layer of p‐aminobenzoic acid (PABA), which can significantly enhance the open‐circuit voltage (V _OC), the fill factor (FF), and the power conversion efficiency (PCE) of PSCs, is herein reported. The champion device shows a short‐circuit current (J _SC) of 22.83 mA cm⁻², V _OC of 1.167 V, FF of 0.768, and PCE of 20.47%. The improvement in photovoltaic performance is attributed to the suppression of carrier trap states and the improvement in the morphologies of perovskite films. This work demonstrates a simple and effective protocol to enhance the device performance, and provides an insight into the influence of PABA post‐treatment on the charge carrier dynamics.

An electron‐selective contact that simultaneously enables efficient electron selectivity and suppression of parasitic infrared absorption is achieved by a thick ZnO film capped with LiF _x /Al. Combining such an electron‐selective contact with a MoO _x ‐based stack as hole‐selective contact, a fully dopant‐free heterocontact Si solar cell is demonstrated with a record efficiency of 21.4%.

Abstract

Although charge‐carrier selectivity in conventional crystalline silicon (c‐Si) solar cells is usually realized by doping Si, the presence of dopants imposes inherent performance limitations due to parasitic absorption and carrier recombination. The development of alternative carrier‐selective contacts, using non‐Si electron and hole transport layers, has the potential to overcome such drawbacks and simultaneously reduce the cost and/or simplify the fabrication process of c‐Si solar cells. Nevertheless, devices relying on such non‐Si contacts with power conversion efficiencies (PCEs) that rival their classical counterparts are yet to be demonstrated. In this study, one key element is brought forward toward this demonstration by incorporating low‐pressure chemical vapor deposited ZnO as the electron transport layer in c‐Si solar cells. Placed at the rear of the device, it is found that rather thick (75 nm) ZnO film capped with LiF _x /Al simultaneously enables efficient electron selectivity and suppression of parasitic infrared absorption. Next, these electron‐selective contacts are integrated in c‐Si solar cells with MoO _x ‐based hole‐collecting contacts at the device front to realize full‐area dopant‐free‐contact solar cells. In the proof‐of‐concept device, a PCE as high as 21.4% is demonstrated, which is a record for this novel device class and is at the level of conventional industrial solar cells.

Halide perovskites exhibit extraordinary properties of slow hot‐carrier cooling, long‐range hot‐carrier transport, and efficient hot‐carrier extraction, and are capable of unlocking disruptive high‐efficiency hot‐carrier photovoltaics which will overcome the Shockley–Queisser limit. The intricate photophysical mechanisms behind the novel phenomena are distilled, an engineering and developmental toolkit is assembled, and the challenges and opportunities in this fledging area are examined.

Abstract

Rapid hot‐carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot‐carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot‐carrier properties: slow hot‐carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long‐range hot‐carrier transport (up to ≈600 nm), and highly efficient hot‐carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot‐carrier cooling properties in halide perovskites and prospects for perovskite hot‐carrier solar cells are also discussed.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Low‐toxicity tin‐based perovskites have excellent optical and electrical properties, and are a good alternative for lead‐based perovskites. However, the performance and stability of tin‐based perovskites are not comparable. The properties of tin‐based perovskite films and the performance of tin‐based perovskite solar cells are reviewed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Abstract

The tremendous interest focused on organic–inorganic halide perovskites since 2012 derives from their unique optical and electrical properties, which make them excellent photovoltaic materials. Pb‐based halide perovskite solar cells, in particular, currently stand at a record efficiency of ≈23%, fulfilling their potential toward commercialization. However, because of the toxicity concerns of Pb‐based perovskite solar cells, their market prospects are hindered. In principle, Pb can be replaced with other less‐toxic, environmentally benign metals. Sn‐based perovskites are thus the far most promising alternative due to their very similar and perhaps even superior semiconductor characteristics. After years of effort invested in Sn‐based halide perovskites, sufficient breakthroughs have finally been achieved that make them the next runners up to the Pb halide perovskites. To help the reader better understand the nature of Sn‐based halide perovskites, their optical and electrical properties are systematically discussed. Recent progress in Sn‐based perovskite solar cells, focusing mainly on film fabrication methods and different device architectures, and highlighting roadblocks to progress and opportunities for future work are reviewed. Finally, a brief overview of mixed Sn/Pb‐based systems with their anomalous yet beneficial optical trends are discussed. The current challenges and a future outlook for Sn‐based perovskites are discussed.

Rapid advancement of perovskite solar cells confronts the challenges of reliable measurement, which is important for data analysis and results reproduction. Major measurement methods and the key factors affecting evaluation are summarized. A measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.

Abstract

Perovskite solar cells (PSCs) have undergone an incredibly fast development and attracted intense attention worldwide owing to their high efficiency and low‐cost fabrication. However, it is challenging to make a reliable measurement of PSCs, which creates great difficulty for researchers to compare and reproduce published results. Herein, the major measurement methods and key factors affecting evaluation of PSCs are summarized, such as hysteresis in current–voltage measurement, calibration of solar simulators for less mismatch in spectra and light intensity, and the area for the calculation of current density and power conversion efficiency. PSCs are also compared with n–i–p or p–i–n structures that exhibit different feedback under the same measurement methods. Finally, a measurement proposal is provided to help researchers obtain reliable measurement results close to those certified by public test centers.