Wangyikai

Publication date: 15 February 2020

Source: Journal of Photochemistry and Photobiology A: Chemistry, Volume 389

Author(s): Seyed Abolghasem Kahani, Elham Maleki, Maryam Ranjbar

Abstract

Graphical abstract

Four low‐cost copolymer donors of poly(thiophene‐quinoxaline) (PTQ) derivatives are developed to investigate the effect of their fluorination forms on charge‐separation and voltage loss (V _loss) of their polymer solar cells. The device based on the PTQ derivative with a bifluorine substituent on its quinoxaline A‐unit demonstrates a high power conversion efficiency of 16.21%, benefitting from the efficient charge separation and low V _loss.

Abstract

Four low‐cost copolymer donors of poly(thiophene‐quinoxaline) (PTQ) derivatives are demonstrated with different fluorine substitution forms to investigate the effect of fluorination forms on charge separation and voltage loss (V _loss) of the polymer solar cells (PSCs) with the PTQ derivatives as donor and a A–DA'D–A‐structured molecule Y6 as acceptor. The four PTQ derivatives are PTQ7 without fluorination, PTQ8 with bifluorine substituents on its thiophene D‐unit, PTQ9, and PTQ10 with monofluorine and bifluorine substituents on their quinoxaline A‐unit respectively. The PTQ8‐ based PSC demonstrates a low power conversion efficiency (PCE) of 0.90% due to the mismatch in the highest occupied molecular orbital (HOMO) energy levels alignment between the donor and acceptor. In contrast, the devices based on PTQ9 and PTQ10 show enhanced charge‐separation behavior and gradually reduced V _loss, due to the gradually reduced nonradiative recombination loss in comparison with the PTQ7‐based device. As a result, the PTQ10‐based PSC demonstrates an impressive PCE of 16.21% with high open‐circuit voltage and large short‐circuit current density simultaneously, and its V _loss is reduced to 0.549 V. The results indicate that rational fluorination of the polymer donors is a feasible method to achieve fast charge separation and low V _loss simultaneously in the PSCs.

In article number https://doi.org/10.1002/adma.2019035591903559, Edward H. Sargent, Zhijun Ning, and co‐workers intentionally include a secondary amine, dimethylamine, in MAPbI₃ perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. NiO _x ‐based inverted perovskite solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% with excellent operational stability under continuous light soaking.

Here, a 3.19%‐external quantum efficiency all‐solution‐processed green perovskite light‐emitting diode is reported by employing 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene (TPBi) doped conjugated amino‐alkyl substituted polyfluorene poly[(9,9‐bis(3′‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)] (PFN) as electron injection layer. The doping of TPBi into PFN not only enhances the capability of electron injection, but also significantly suppresses the emission quenching of perovskite caused by the charge transfer between perovskite and PFN.

Abstract

Metal halide perovskites have attracted considerable attention in the field of light‐emitting diodes due to their high color purity and solution processability. However, most perovskite light‐emitting diodes (PeLEDs) employ thermally deposited charge transport layers (CTLs) on top of perovskite layers. In order to realize low‐cost and scalable fabrication of PeLEDs, all‐solution process is highly desired, but still remaining great challenges. Here, an efficient all‐solution‐processed green PeLEDs is reported by incorporating 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene (TPBi) doped conjugated amino‐alkyl substituted polyfluorene poly[(9,9‐bis(3′‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)] (PFN) electron injection layer, achieving a maximum luminance of 9875 cd m⁻², a high current efficiency of 10.41 cd A⁻¹, and an external quantum efficiency of 3.19%. Since the solvents used for perovskite precursors and PFN are orthogonal, the protected and complete interface of perovskite film and CTL is effectively obtained by solution processes. The doping of TPBi into PFN not only enhances the capability of electron injection, but also significantly suppresses the emission quenching of perovskite films caused by the charge transfer between perovskite and PFN due to the reduced difference in their work functions. This work provides an efficient approach for the development of all‐solution‐processed PeLEDs.

Herein, it is demonstrated that solution‐processed dopant‐free spiro molecules can serve as superior hole‐transport materials (HTMs) to fabricate efficient inverted (p‐i‐n) perovskite solar cells. An entirely solution process is achieved by rational choice of orthogonal solvent, which allows to deposit uniform and pinhole‐free perovskite films without compromising the hole‐extraction capability of the spiro interlayers.

Spiro‐linked compounds have been used as benchmark hole‐transport materials (HTMs) for the construction of efficient normal architecture (n‐i‐p) perovskite solar cells (PSCs). However, the heavy reliance on the use of dopants not only complicates the device fabrication but imposes long‐term stability concern of the devices. Herein, it is reported that solution‐processed dopant‐free spiro molecules can serve as superior HTMs to fabricate efficient inverted (p‐i‐n) PSCs. Rational choice of orthogonal solvent allows us to solution deposit uniform and pinhole‐free perovskite films without compromising the hole‐extraction capability of the spiro‐based interface layers. To illustrate the generality of the strategy, three spiro‐linked molecules are investigated side by side as HTMs in one‐step solution‐processed CH₃NH₃PbI₃ PSCs. Due to the favored energy‐level alignment and high hole mobility, solar cells based on the HTM of spiro‐TTB yield a high efficiency of 18.38% with open‐circuit voltages (V _OC) up to 1.09 V. These results suggest that small molecular HTMs commonly developed for normal structure devices can be of great potential to fabricate cost‐effective and highly efficient inverted PSCs.

Solution‐processed MoO₃ (SM), synthesized by a simple low‐temperature process, is utilized as an efficient and stable anode interfacial layer for organic solar cells (OSCs). The ultrasmooth SM film, without pinholes, exhibits excellent photovoltaic performance and device stability in OSCs, maintaining ≈92% of its initial solar cell efficiency over 2500 h storage in inert conditions.

The interfacial layer (IL) in organic solar cells (OSCs) can be an important boosting factor for improving device efficiency and stability. Herein, a facile and cost‐effective approach to form a uniform molybdenum oxide (MoO₃) film with desirable stability is provided, based on solution processing at low temperatures by simplified precursor solution synthesis. The solution‐processed MoO₃ (SM) film, with oxygen vacancies induced by the hydroxyl group, functions as an efficient anode IL in conventional OSCs. The hole‐transporting performance of SM is well demonstrated in nonfullerene‐based OSCs exhibiting over 10% of power conversion efficiency. The enhanced device performance of SM‐based OSCs over that of poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is investigated by analyzing the morphology, electronic state, and electrical conductivity of such a hole‐transporting layer, as well as the charge dynamics in the completed devices. Furthermore, the high stability of the SM films in OSCs is examined under various environmental conditions, including long‐term and thermal stability. In particular, fullerene‐based OSCs with SM maintain over 90% of their initial cell performance over 2500 h under inert conditions. It is shown that solution‐processed metal oxides can be viable ILs with high functionality and versatility, overcoming the drawbacks of conventionally adopted conducting polymer interlayers.

In parallel with the tremendous progress in the efficiency of perovskite solar cells, research on the issue of instability has attracted enormous attention. In this review, the strategies to enhance the stability from the perspectives of the device structure, the photoactive layer, hole‐ and electron‐transporting layers, electrode materials, and device encapsulation are portrayed.

Abstract

In this review, the factors influencing the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is emphasized. The PCE of PSCs has remarkably increased from 3.8% to 23.7%, but on the other hand, poor stability is one of the main facets that creates a huge barrier in the commercialization of PSCs. Herein, a concise overview of the current efforts to enhance the stability of PSCs is provided; moreover, the degradation causes and mechanisms are summarized. The strategies to improve device stability are portrayed in terms of structural effects, a photoactive layer, hole‐ and electron‐transporting layers, electrode materials, and device encapsulation. Last but not least, the economic feasibility of PSCs is also vividly discussed.

Rapid crystallization is demonstrated to be necessary in achieving high‐quality 2DRP perovskite films by comparing dimethylacetamide (DMAC), N,N‐dimethylformamide, and dimethyl sulfoxide solvents. The improved stability and efficiency are observed using DMAC due to the accelerating crystallization rate of 2DRP perovskite crystals.

Abstract

Due to the additional introduction of bulky organic ammonium and the competition between bulky organic ammonium and methyl ammonium in 2D Ruddlesden‐Popper (2DRP) perovskite, the crystallization process becomes complicated. Here, it is demonstrated that the rapid crystallization controlled by processing solvents plays an important role in achieving high‐quality 2DRP perovskite films. It is found that the processing solvents, e.g., dimethylacetamide (DMAC), N,N‐dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), with a different polarity and boiling point, have almost no effect on crystal structure and phase distribution but have a remarkable effect on crystallization kinetics, crystal growth orientation, and crystallinity of 2DRP perovskite. Compared to polar aprotic solvent DMF and DMSO with a high boiling point, DMAC with low polarity and a suitable boiling point has a weak coordination to lead and ammonium salts and is easy to escape during solution processing, which is able to accelerate the crystallization rate of 2DRP perovskite. Benefitting from the rapid crystallization enabled high‐quality 2DRP perovskite films, the best‐performing device with improved stability and a power conversion efficiency of 12.15% is obtained using DMAC solvent. These findings may give guidance for solvent engineering for highly efficient 2DRP perovskite solar cells in the future.

Publication date: February 2020

Source: Solar Energy Materials and Solar Cells, Volume 205

Author(s): Çağla Odabaşı, Ramazan Yıldırım

Their superior optoelectronic properties and stability endow organic–inorganic halide perovskite single crystals great potential for high‐efficiency and stable photovoltaics. This progress report summarizes recent exciting developments and future perspectives for perovskite single crystal solar cells, which may attract more attention and provide guidelines for further development in this emerging field.

Abstract

The efficiency of perovskite solar cells has increased to a certified value of 25.2% in the past 10 years, benefiting from the superior properties of metal halide perovskite materials. Compared with the widely investigated polycrystalline thin films, single crystal perovskites without grain boundaries have better optoelectronic properties, showing great potential for photovoltaics with higher efficiency and stability. Additionally, single crystal perovskite solar cells are a fantastic model system for further investigating the working principles related to the surface and grain boundaries of perovskite materials. Unfortunately, only a handful of groups have participated in the development of single crystal perovskite solar cells; thus, the development of this area lags far behind that of its polycrystalline counterpart. Therefore, a review paper that discusses the recent developments and challenges of single crystal perovskite solar cells is urgently required to provide guidelines for this emerging field. In this progress report, the optical and electrical properties of single crystal and polycrystalline perovskite thin films are compared, followed by the recent developments in the growth of single crystal perovskite thin films and the photovoltaic applications of this material. Finally, the challenges and perspectives of single crystal perovskite solar cells are discussed in detail.

In general, mixed cations and anions containing formamidinium (FA), methylammonium (MA), caesium, iodine, and bromine ions are used to stabilize the black α-phase of the FA-based lead triiodide (FAPbI₃) in perovskite solar cells. However, additives such as MA, caesium, and bromine widen its bandgap and reduce the thermal stability. We stabilized the α-FAPbI₃ phase by doping with methylenediammonium dichloride (MDACl₂) and achieved a certified short-circuit current density of between 26.1 and 26.7 milliamperes per square centimeter. With certified power conversion efficiencies (PCEs) of 23.7%, more than 90% of the initial efficiency was maintained after 600 hours of operation with maximum power point tracking under full sunlight illumination in ambient conditions including ultraviolet light. Unencapsulated devices retained more than 90% of their initial PCE even after annealing for 20 hours at 150°C in air and exhibited superior thermal and humidity stability over a control device in which FAPbI₃ was stabilized by MAPbBr₃.

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.

Publication date: Available online 15 November 2019

Source: Solar Energy Materials and Solar Cells

Author(s): Shina Li, Ruixin Ma, Xing zhao, Jiahui Guo, Yuchun Zhang, Chenchen Wang, He Ren, Yong Yan

A carrier‐resolved photo‐Hall characterization technique is employed to simultaneously access majority/minority carrier properties as a function of light intensity for CH₃NH₃PbI₃ perovskite films processed without and with photovoltaic performance‐enhancing additives. Measurements on films with variable grain size reveal the passivation of bulk defects and n‐doping effect with PbI₂ excess and relative insensitivity to grain boundary density and thiocyanate additive concentration.

Abstract

With power conversion efficiencies now exceeding 25%, hybrid perovskite solar cells require deeper understanding of defects and processing to further approach the Shockley‐Queisser limit. One approach for processing enhancement and defect reduction involves additive engineering—, e.g., addition of MASCN (MA = methylammonium) and excess PbI₂ have been shown to modify film grain structure and improve performance. However, the underlying impact of these additives on transport and recombination properties remains to be fully elucidated. In this study, a newly developed carrier‐resolved photo‐Hall (CRPH) characterization technique is used that gives access to both majority and minority carrier properties within the same sample and over a wide range of illumination conditions. CRPH measurements on n‐type MAPbI₃ films reveal an order of magnitude increase in carrier recombination lifetime and electron density for 5% excess PbI₂ added to the precursor solution, with little change noted in electron and hole mobility values. Grain size variation (120–2100 nm) and MASCN addition induce no significant change in carrier‐related parameters considered, highlighting the benign nature of the grain boundaries and that excess PbI₂ must predominantly passivate bulk defects rather than defects situated at grain boundaries. This study offers a unique picture of additive impact on MAPbI₃ optoelectronic properties as elucidated by the new CRPH approach.

The reverse bias dark current density in organic bulk heterojunction photodiodes and its activation energy are measured and successfully reproduced in terms of thermal charge injection from the contacts into a broadened density of states at the interface. The agreement is excellent for five polymer semiconductors with widely varying energy levels and charge transport characteristics.

Abstract

Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias (  J _d) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of J _d in state‐of‐the‐art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of J _d are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating J _d to material parameters, insights into the origin of J _d are obtained that enable the future selection of successful OPD materials.

Publication date: February 2020

Source: Solar Energy Materials and Solar Cells, Volume 205

Author(s): Huibin Yin, Shiyuan Gao, Jian Liu

Publication date: February 2020

Source: Solar Energy Materials and Solar Cells, Volume 205

Author(s): Yu Kawano, Jakapan Chantana, Takahito Nishimura, Takashi Minemoto

The efficiencies of semitransparent perovskite device and four‐terminal perovskite/silicon multijunction/tandem solar cells rise to 18.3% and 27.0%, respectively. This is the highest recorded efficiency for semitransparent perovskite solar cells thus far. The high efficiencies originate from good transparency and high conductivity of the nanomesh‐structured gold top electrode.

Abstract

Multijunction/tandem solar cells have naturally attracted great attention because they are not subject to the Shockley–Queisser limit. Perovskite solar cells are ideal candidates for the top cell in multijunction/tandem devices due to the high power conversion efficiency (PCE) and relatively low voltage loss. Herein, sandwiched gold nanomesh between MoO₃ layers is designed as a transparent electrode. The large surface tension of MoO₃ effectively improves wettability for gold, resulting in Frank–van der Merwe growth to produce an ultrathin gold nanomesh layer, which guarantees not only excellent conductivity but also great optical transparency, which is particularly important for a multijunction/tandem solar cell. The top MoO₃ layer reduces the reflection at the gold layer to further increase light transmission. As a result, the semitransparent perovskite cell shows an 18.3% efficiency, the highest reported for this type of device. When the semitransparent perovskite device is mechanically stacked with a heterojunction silicon solar cell of 23.3% PCE, it yields a combined efficiency of 27.0%, higher than those of both the sub‐cells. This breakthrough in elevating the efficiency of semitransparent and multijunction/tandem devices can help to break the Shockley–Queisser limit.

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.

An ultrathin functional polymer layer is used to enhance the nucleation of atomic layer deposited (ALD) SnO₂ contacts in metal‐halide perovskite solar cells. These nucleation‐enhanced ALD layers act as “built‐in” barriers to both internal and external degradation pathways, significantly improving the long‐term operational stability of high efficiency unencapsulated devices (>18%) in air.

Abstract

Metal‐halide perovskites show promise as highly efficient solar cells, light‐emitting diodes, and other optoelectronic devices. Ensuring long‐term stability is now a major priority. In this study, an ultrathin (2 nm) layer of polyethylenimine ethoxylated (PEIE) is used to functionalize the surface of C₆₀ for the subsequent deposition of atomic layer deposition (ALD) SnO₂, a commonly used electron contact bilayer for p–i–n devices. The enhanced nucleation results in a more continuous initial ALD SnO₂ layer that exhibits superior barrier properties, protecting Cs_0.25FA_0.75Pb(Br_0.20I_0.80)₃ films upon direct exposure to high temperatures (200 °C) and water. This surface modification with PEIE translates to more stable solar cells under aggressive testing conditions in air at 60 °C under illumination. This type of “built‐in” barrier layer mitigates degradation pathways not addressed by external encapsulation, such as internal halide or metal diffusion, while maintaining high device efficiency up to 18.5%. This nucleation strategy is also extended to ALD VO _x films, demonstrating its potential to be broadly applied to other metal oxide contacts and device architectures.