Chen Weijie

Remarkable advancement has been made in the efficiency of organic solar cells (OSCs) in recent times, mostly due to novel fused ring electron acceptors (FREAs). Here, structural evolution of FREAs to enhance efficiency is comprehensively discussed. Moreover, recent progress in polymer design, semi‐transparent OSCs, ternary, and tandem OSCs is provided. The challenges and future development of FREAs are briefly addressed.

Abstract

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution‐processed bulk‐heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA‐based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all‐small‐molecule OSCs, semi‐transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA‐OSCs for future material design and challenges ahead is provided.

Poly[(5,6‐bis(2‐hexyldecyloxy)benzo[c][1,2,5]thiadiazole‐4,7‐diyl)‐alt‐(5,50‐(2,5‐difluoro‐1,4‐phenylene)bis(thiophen‐2‐yl))] (PDTBTBz‐2Fanti)‐based organic photovoltaics (OPVs) show an exceptionally high efficiency of 23.1% under a 1000‐lx light‐emitting diode lamp.

Abstract

The unique electro‐optical features of organic photovoltaics (OPVs) have led to their use in applications that focus on indoor energy harvesters. Various adoptable photoactive materials with distinct spectral absorption windows offer enormous potential for their use under various indoor light sources. An in‐depth study on the performance optimization of indoor OPVs is conducted using various photoactive materials with different spectral absorption ranges. Among the materials, the fluorinated phenylene‐alkoxybenzothiadiazole‐based wide bandgap polymer—poly[(5,6‐bis(2‐hexyldecyloxy)benzo[c][1,2,5]thiadiazole‐4,7‐diyl)‐alt‐(5,50‐(2,5‐difluoro‐1,4‐phenylene)bis(thiophen‐2‐yl))] (PDTBTBz‐2F_anti)‐contained photoactive layer—exhibits a superior spectrum matching with indoor lights, particularly a light‐emitting diode (LED), which results in an excellent power absorption ratio. These optical properties contribute to the state‐of‐the‐art performance of the PDTBTBz‐2F_anti:[6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM)‐based OPV with an unprecedented high power‐conversion efficiency (PCE) of 23.1% under a 1000 lx LED. Finally, its indoor photovoltaic performance is observed to be better than that of an interdigitated‐back‐contact‐based silicon photovoltaic (PCE of 16.3%).

Here, intermediate‐controlled crystal growth is reported via solvent tuning to prepare highly oriented 2D perovskite films with faster transport, longer carrier lifetime, and lower defect density.

Abstract

Reduced‐dimensional hybrid perovskite semiconductors have recently attracted significant attention due to their promising stability and optoelectronic properties. However, the issue of poor charge transport in 2D perovskites limits its application. Here, studies on intermediate‐controlled crystal growth are reported to improve charge carrier transport in 2D perovskite thin films. It is shown that the coordination strength of solvents with perovskite precursor affects the initial state of intermediate phase formation as well as the subsequent perovskite layer growth. Tuning the solvent composition with a mixture (5:5) of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) leads to the growth of highly orientated 2D perovskite films with much‐improved optoelectronic properties (faster transport by ≈50x, longer carrier lifetime by ≈4x, and lower defect density by ≈30x) than the film prepared with pure DMF. Consequently, perovskite solar cells based on DMF/DMSO (5:5) show >80% efficiency improvement than the devices based on pure DMF.

Publication date: July 2019

Source: Nano Energy, Volume 61

Author(s): Yu Chen, Zhou Yang, Xuguang Jia, Yihui Wu, Ningyi Yuan, Jianning Ding, Wen-Hua Zhang, Shengzhong (Frank) Liu

Graphical abstract

Zn²⁺ doped CuGaO₂ nanocrystals have been synthesized via a facial hydrothermal approach and employed as efficient mesoporous hole transporting layer for inverted perovskite solar cells. High device efficiency with superior thermal stability has been achieved due to the excellent hole transporting property and chemical stability of inorganic Zn²⁺ doped CuGaO₂.

Publication date: June 2019

Source: Nano Energy, Volume 60

Author(s): Shuyan Shao, Jingjin Dong, Herman Duim, Gert H. ten Brink, Graeme R. Blake, Giuseppe Portale, Maria Antonietta Loi

Graphical abstract

Device simulations show that the low electron carrier concentration in the near‐interface region of the buffer layer is the fundamental factor that limits carrier transport and thus J _SC of Cu(In,Ga)Se₂ solar cells with a large spike‐like conduction band offset. New strategies are proposed for increasing J _SC through increasing the near‐interface electron concentration.

A spike‐like conduction‐band offset (CBO) in heterojunction solar cells is shown to limit the short‐circuit current density (J _SC) dramatically when the spike height is large. It is widely believed that the spike‐like CBO produces a potential barrier, which resists the photogenerated carriers flowing through the junction interface and thus decreases J _SC. However, our device simulation studies on Cu(In,Ga)Se₂ (CIGS) solar cells reveal that a large spike‐like CBO causes an extremely low electron carrier concentration in the near‐interface region of the buffer layer, which is the major factor that limits the carrier transport and thus J _SC. If the near‐interface electron concentration is increased, J _SC can be increased despite the fact that the large spike‐like CBO and potential barrier are still present. These results indicate that the near‐interface electron concentration is the fundamental factor limiting the J _SC, more fundamental than the potential barrier. Therefore, not only the commonly adopted band‐alignment engineering, but also various other methods, for example, choosing buffer materials with suitable effective density of states, introducing favorable interface defects, or increasing the doping level, can be adopted for improving the current collection in heterojunction solar cells. Therefore, J _SC can always be increased even when the large spike‐like CBO is inevitable.

Environmentally benign solvent N ‐methyl‐pyrrolidone (NMP) is for the first time used to make a precursor solution that achieves a 10.23% efficient CuIn(S,Se)₂ (CISSe) solar cell. Annealing the NMP‐based precursor film in air favors decomposition of organic species and results in higher quality CISSe absorber material and better device performance than annealing in the glove box.

A safe and environmentally benign solvent is a requisite for mass production of thin film solar cells via solution methods. Here, a highly industry suitable solvent N‐methyl‐pyrrolidone (NMP) is used for the first time to make a precursor solution with simple compounds of CuCl, InCl₃·4H₂O and thiourea and fabricate CuIn(S,Se)₂ (CISSe) solar cells. A power conversion efficiency of 10.23% has been achieved from the NMP‐based solution when the precursor film was processed in air, which is only 8.55% for film processed in glove box. Characterizations using XRD, Raman, SEM, EDX, and FTIR show air annealing favors decomposition of organic species in the precursor film, which results in high quality absorber materials. Further improvement in device efficiency can be expected by gallium alloying and optimization of device fabrication conditions. The results demonstrate highly efficient thin film solar cells can be fabricated from an industry suitable NMP precursor solution in air, which is promising for simplifying film processing and reducing manufacture cost.

Publication date: June 2019

Source: Nano Energy, Volume 60

Author(s): Qiaoshi An, Jian Wang, Fujun Zhang

Graphical abstract

The optimized ternary PSCs achieve a PCE of 14.13% by incorporating alloyed donor (PBDB-T-2F and J71) with a non-fullerene acceptor Br-ITIC. More than 11% PCE improvement is achieved by employing ternary strategy on the basis of binary PSCs with PCE over 12%, which is mainly attributed to the enhanced photon harvesting and optimized phase separation of the ternary active layer.

Publication date: June 2019

Source: Nano Energy, Volume 60

Author(s): Jiahua Tao, Xiaobo Hu, Yixin Guo, Jin Hong, Kanghua Li, Jinchun Jiang, Shaoqiang Chen, Chengbin Jing, Fangyu Yue, Pingxiong Yang, Chuanjun Zhang, Zhuangchun Wu, Jiang Tang, Junhao Chu

Graphical abstract

Solution‐processed 2D Nb₂O₅(001) nanosheets (c‐Nb₂O₅ NS) are prepared and combined with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as an electron transport layer (ETL) for inverted inorganic CsPbI₂Br perovskite solar cells (PeSCs) with a high performance. The c‐Nb₂O₅ NS not only facilitate the electron transport, blocking the hole transport, but also contribute to the defect passivation and retard the iodine ions diffusion toward the Ag electrode.

Herein, solution‐processed 2D Nb₂O₅(001) nanosheets (c‐Nb₂O₅ NS) are prepared and combined with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) as an electron transport layer (ETL) for inverted inorganic CsPbI₂Br perovskite solar cells (PeSCs). The PeSCs with a c‐Nb₂O₅/PC₆₁BM bilayer ETL yield a high power conversion efficiency (PCE) up to 11.74%, remarkably outperforming the devices with only PC₆₁BM (9.10%) and those with the state‐of‐the‐art ZnO/C₆₀ ETL (10.65%) prepared under the same conditions. More importantly, the nonencapsulated PeSCs with c‐Nb₂O₅ exhibit a high thermal stability with only 20% PCE loss after 400 h thermal aging at 85 °C. Such an impressive performance and a high stability can be attributed to the introduction of c‐Nb₂O₅(001) NS with favorable band levels, strong acid nature, and the small lattice fringe spacing along the large lateral (001) surface, which not only facilitate the electron transport, blocking the hole transport, but also contribute to defect passivation and retard the iodine ions diffusion toward the Ag electrode. This study thus provides a deeper insight for the interfacial design in inverted inorganic PeSCs and contributes to PCE improvement in the future.

A specific bidentate molecule, 2‐mercaptopyridine, is demonstrated to substantially enhance anchoring strength at surface of metal halide perovskites, which improves the passivation efficacy and stability synchronously relative to monodentate counterparts. The highly stable bidentate anchoring based passivation on CH₃NH₃PbI₃ not only advances power conversion efficiency from 18.35% to 20.28%, but also leads to a champion lifetime in humid air.

Abstract

Chemical passivation is an effective approach to suppress the grain surface dominated charge recombination in perovskite solar cells (PSCs). However, the passivation effect is usually labile on perovskite crystal surface since most passivating agents are weakly anchored. Here, the use of a bidentate molecule, 2‐mercaptopyridine (2‐MP), to increase anchoring strength for improving the passivation efficacy and stability synchronously is demonstrated. Compared to monodentate counterparts of pyridine and p‐toluenethiol, 2‐MP passivation on CH₃NH₃PbI₃ film results in twofold improvement of photoluminescence lifetime and remarkably enhanced tolerance to chlorobenzene washing and vacuum heating, which improve the power conversion efficiency of n–i–p planar structured PSCs from 18.35% to 20.28%, with open‐circuit voltage approaching 1.18 V. Moreover, the CH₃NH₃PbI₃ films passivated with 2‐MP exhibit unprecedented humid‐stability that they can be exposed to saturated humidity for at least 5 h, mainly due to the passivation induced surface deactivation, which renders the unencapsulated devices retaining 93% of the initial efficiency after 60 days aging in air with relative humidity of 60–70%.

To optimize inorganic perovskite based solar cells, a lanthanide halide is employed to modify the electron transport layer/perovskite interface and form a gradient energy band, which can restrain the charge recombination at the interface and inside the perovskite. Eventually, an efficiency as high as 10.88% is obtained, representing the highest efficiency of CsPbIBr₂ perovskite solar cells.

Abstract

Inorganic cesium lead halide perovskite solar cells (PSCs) have received enormous attention due to their excellent stability compared with that of their organic–inorganic counterparts. However, the lack of optimization strategies leads the inorganic PSCs to suffer from low efficiency arising from significant recombination. To overcome this dilemma, a surface modification of the electron transport layer (ETL)/perovskite interface is undertaken by using SmBr₃ to improve the crystallization and morphology of the perovskite layer for enhanced ETL/perovskite interface interaction. Encouragingly, a gradient energy band is created at the interface with an outstanding hole blocking effect. As a result, both the charge recombination occurring at the interface and the nonradiative recombination inside the perovskite are suppressed, and, simultaneously, the charge extraction is improved successfully. Therefore, the power conversion efficiency of the CsPbIBr₂ PSCs is increased to as high as 10.88% under one sun illumination, which is 30% higher than its counterparts without the modification. It is logically inferred that this valuable optimization strategy can be extended to other analogous structures and materials.

An n‐doping of SnO₂ is successfully realized through the use of the triphenylphosphine‐oxide molecule, where electrons are revealed to be transferred from the R₃P⁺O⁻ σ‐bond to the peripheral tin atoms and delocalized. That novel effect enlarges the built‐in‐field from 0.01 to 0.07 eV and reduces the energy‐barrier from 0.55 to 0.39 eV at the SnO₂–perovskite interface enabling a device conversion‐efficiency from 19.01% to 20.69%.

Abstract

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n‐type doping of oxide semiconductors. Herein, successful n‐type doping of SnO₂ is demonstrated by a simple, air‐robust, and cost‐effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P⁺O⁻σ‐bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi‐photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO₂, which enlarges the built‐in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO₂/perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

The efficiency and photostability of all‐inorganic mixed‐halide perovskite solar cells (PVSCs) can be simultaneously enhanced by introducing an amino‐functionalized polymer PN4N as a novel cathode interlayer and dopant‐free PDCBT hole‐transporting layer. The favorable interaction between perovskite crystal and PN4N/PDCBT can effectively improve CsPbI₂Br film quality, with power conversion efficiency over 16%.

Abstract

A synergic interface design is demonstrated for photostable inorganic mixed‐halide perovskite solar cells (PVSCs) by applying an amino‐functionalized polymer (PN4N) as cathode interlayer and a dopant‐free hole‐transporting polymer poly[5,5′‐bis(2‐butyloctyl)‐(2,2′‐bithiophene)‐4,4′‐dicarboxylate‐alt‐5,5′‐2,2′‐bithiophene] (PDCBT) as anode interlayer. First, the interfacial dipole formed at the cathode interface reduces the workfunction of SnO₂, while PDCBT with deeper‐lying highest occupied molecular orbital (HOMO) level provides a better energy‐level matching at the anode, leading to a significant enhancement in open‐circuit voltage (V _oc) of the PVSCs. Second, the PN4N layer can also tune the surface wetting property to promote the growth of high‐quality all‐inorganic perovskite films with larger grain size and higher crystallinity. Most importantly, both theoretical and experimental results reveal that PN4N and PDCBT can interact strongly with the perovskite crystal, which effectively passivates the electronic surface trap states and suppresses the photoinduced halide segregation of CsPbI₂Br films. Therefore, the optimized CsPbI₂Br PVSCs exhibit reduced interfacial recombination with efficiency over 16%, which is one of the highest efficiencies reported for all‐inorganic PVSCs. A high photostability with a less than 10% efficiency drop is demonstrated for the CsPbI₂Br PVSCs with dual interfacial modifications under continuous 1 sun equivalent illumination for 400 h.

A block copolymer F127 passivation strategy in conjunction with the solvent annealing process significantly enhances the performance and stability of planar perovskite solar cells. Hydrophilic tails of F127 passivate defects at grain boundaries through hydrogen bonding, whereas the dangling hydrophobic groups suppress perovskite decomposition against moisture and heat.

Organic–inorganic metal halide perovskite solar cells (PSCs) exhibit excellent photovoltaic performance but have the drawbacks of instabilities against moisture and heat due to the inherent hydroscopic nature and volatility of their organic components. Herein, it is reported that using the block copolymer F127 as the passivation reagent in conjunction with the solvent annealing process can efficiently improve the performance and stability of corresponding organic–inorganic PSCs. It is anticipated that the hydrophilic poly(ethylene oxide) tails of F127 polymers connect with contiguous perovskite crystals and passivate defects at perovskite grain boundaries, whereas the dangling hydrophobic poly(phenyl oxide) centers suppress perovskite decomposition caused by moisture and heat. After the optimization of the F127 additive, planar PSCs with champion power conversion efficiencies of 21.01% and 18.71% are achieved on rigid and flexible substrates, respectively. The F127 passivation strategy provides an effective approach for fabricating high‐efficiency and stable PSCs.

A novel surface passivation of a perovskite surface is reported using the polyfluoro organic compound tris(pentafluorophenyl)boron (TPFPB), which can yield large grains, reduced defect densities, and improved charge transport and phase stability for the perovskite film. Using this strategy, a champion perovskite solar cell achieves a high power conversion efficiency of 21.6% as well as significantly improved air and light stabilities.

In planar perovskite solar cells (PSCs), defect‐induced recombination at the interface between the perovskite and hole transport layer (HTL) leads to a large potential loss and performance deterioration. Therefore, an effective method for improving interfacial properties is critical to boost the performance and stability of PSCs. Herein, a novel surface engineering technology is reported for passivating the perovskite surface with the polyfluoro organic compound tris(pentafluorophenyl)boron (TPFPB), which can yield large perovskite grains, reduced defect densities, and improved charge transport and phase stability for the perovskite film, and enhanced power conversion efficiency (PCE) and stability for PSCs. Using this strategy, a champion FA_0.85MA_0.15PbI₃ perovskite cell achieves a high PCE of 21.6% as well as significantly improved air and light stabilities. This work demonstrates that TPFPB is a promising material for crystallization control and defect passivation and paves a new path for mitigating defects and further increasing the performance of planar PSCs.