Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Kan, Bin; Zhang, Jie; Liu, Feng; Wan, Xiangjian; Li, Chenxi; Ke, Xuezhi; Wang, Yunchuang; Feng, Huanran; Zhang, Yamin; Long, Guankui; Friend, Richard H.; Bakulin, Artem A.; Chen, Yongsheng

doi:10.1002/adma.201704904

Cited by 222 publications

(147 citation statements)

References 51 publications

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“…[4][5][6] Benefiting from the great efforts devoted to the design of new materials, [7][8][9][10][11][12][13] optimization of the blend morphology, [14][15][16][17][18] understanding the charge generation mechanism, [19][20][21][22][23][24][25][26] significant progress has been achieved in the last few years. [4][5][6] Benefiting from the great efforts devoted to the design of new materials, [7][8][9][10][11][12][13] optimization of the blend morphology, [14][15][16][17][18] understanding the charge generation mechanism, [19][20][21][22][23][24][25][26] significant progress has been achieved in the last few years.…”

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Eco‐Compatible Solvent‐Processed Organic Photovoltaic Cells with Over 16% Efficiency

et al. 2019

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inorganic photovoltaic cells based on crystal-silicon, the solution processability of OPV cells makes it suitable for the largescale solution production at room temperature via the roll-to-roll method, which promises low-cost and large area device fabrication onto flexible substrates. [4][5][6] Benefiting from the great efforts devoted to the design of new materials, [7][8][9][10][11][12][13] optimization of the blend morphology, [14][15][16][17][18] understanding the charge generation mechanism, [19][20][21][22][23][24][25][26] significant progress has been achieved in the last few years. Recently, the power conversion efficiencies (PCEs) of OPV cells have surpassed ≈15%. [27][28][29] However, the devices with cutting-edge performance are fabricated using highly toxic solvents like chlorinated and/or aromatic solvents, which is not adaptable for large-scale production and becoming a severe problem that hinders the mass production of the OPV cells. Therefore, the material design of OPV materials should take both the efficiency and processability into consideration.At present, the commonly used processing solvents for high-performance OPV materials are chlorobenzene (CB) and chloroform (CF), as they possess excellent dissolving capability of the highly conjugated structures. [30][31][32] Over the last few years, the design and application of nonfullerene acceptors (NFAs) have achieved Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large-area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco-compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco-compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP-4F-8 (also known as Y6) are modified and BTP-4F-12 is synthesized. Combining with the polymer donor PBDB-TF, BTP-4F-12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB-TF is replaced by T1 with better solubility, various eco-compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm 2 OPV cells by the blade-coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents. Organic PhotovoltaicsOrganic photovoltaic (OPV) cells consisting of organic layers as photoactive materials are one of the most promising next-generation photovoltaic technologies to harvest clean and renewable solar energy. [1][2][3] When compared with the conventional

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Eco‐Compatible Solvent‐Processed Organic Photovoltaic Cells with Over 16% Efficiency

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“…[6][7][8] However, this leads to large energy losses (>0.60 eV), as defined by E loss = E g opt − qV oc , (E g opt is the optical bandgap, V oc is the open-circuit voltage, and q is elementary charge), and limits the power conversion efficiency (PCE) to less than 12% [9][10][11][12][13][14] after decades of effort. [22][23][24][25][26][27][28][29][30][31][32][33] For such a material combination, a large ΔE LUMO always exists, which reduces the highest occupied molecular orbital (HOMO) offset [ΔE HOMO = E HOMO(D) − E HOMO(A) ] to minimize energy loss [34][35][36][37] while enhancing the light collection in near-infrared (NIR) [15][16][17][18][19][20][21] To form a complementary absorption, current popular NFA OSCs are based on the combination of a widebandgap donor and a narrow-bandgap acceptor.…”

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Accurate Determination of the Minimum HOMO Offset for Efficient Charge Generation using Organic Semiconducting Alloys

Zhang

Liu

Zhou

et al. 2019

Advanced Energy Materials

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reveal the bright future of this greenenergy technology. [1][2][3] Exciton dissociation into free charges is the most important optoelectronic process, which is driven by the energy-level difference between donor and acceptor materials. [4,5] For OSCs utilizing fullerene derivatives as electron acceptors, a lowest unoccupied molecular orbital (LUMO) offset [ΔE LUMO = E LUMO(D) − E LUMO(A) ] of no less than 0.30 eV was generally proposed to guarantee efficient electron transfer. [6][7][8] However, this leads to large energy losses (>0.60 eV), as defined by E loss = E g opt − qV oc , (E g opt is the optical bandgap, V oc is the open-circuit voltage, and q is elementary charge), and limits the power conversion efficiency (PCE) to less than 12% [9][10][11][12][13][14] after decades of effort. Thus far, the development of donor (D)-acceptor (A)-type nonfullerene molecular acceptors (NFAs) with well-tunable electronic structures has opened up a great opportunity in this active topic because these NFAs have realized high PCEs over 15%. [15][16][17][18][19][20][21] To form a complementary absorption, current popular NFA OSCs are based on the combination of a widebandgap donor and a narrow-bandgap acceptor. [22][23][24][25][26][27][28][29][30][31][32][33] For such a material combination, a large ΔE LUMO always exists, which reduces the highest occupied molecular orbital (HOMO) offset [ΔE HOMO = E HOMO(D) − E HOMO(A) ] to minimize energy loss [34][35][36][37] while enhancing the light collection in near-infrared (NIR) Current research indicates that exciton dissociation into free charge carriers can be achieved in material combinations with the highest occupied molecular orbital (HOMO) offset lowered to 0 eV in non-fullerene organic solar cells. However, the quantitative relationship between the HOMO offset and exciton dissociation has not been established because of the difficulty inachieving continuously tunable HOMO offsets. Here, the binary blends of PTQ10:ZITI-S and PTQ10:ZITI-N are combined to form the positive and negative HOMO offsets of 0.20 and −0.07 eV, respectively. While the PTQ10:ZITI-S binary blend delivers a decent power conversion efficiency (PCE) of 10.69% with a shortcircuit current (J sc ) of 16.94 mA cm −2 , the PTQ10:ZITI-N with the negative offset shows a much lower PCE of 7.06% mainly because of the low J sc of 12.03 mA cm −2 . Because the tunable HOMO levels can be realized in organic semiconducting alloys based on ZITI-N and ZITI-S acceptors, the transformation of the HOMO energy offset from negative to positive values is achieved in the PTQ10:ZITIN:ZITI-S ternary blends, delivering much-improved PCEs up to 13.26% with a significant, 74% enhancement of J sc to 20.93 mA cm −2 .With detailed investigations, the study reveals that the minimum HOMO offset of ≈40 meV is required to achieve the most-efficient exciton dissociation and photovoltaic performance.

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“…[22] However, the vast majority of those device performances were obtained with layer-thicknesses at around 100 nm [23][24][25][26][27][28][29] and decreased drastically with the increase of the active layer thickness, which limits their application in the roll to roll large-scale solution printing technology. And for tandem solar cells, the PCE has reached 17%.…”

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High Performance Thick‐Film Nonfullerene Organic Solar Cells with Efficiency over 10% and Active Layer Thickness of 600 nm

Zhang

Feng

Meng

et al. 2019

Advanced Energy Materials

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been reported. And for tandem solar cells, the PCE has reached 17%. [22] However, the vast majority of those device performances were obtained with layer-thicknesses at around 100 nm [23][24][25][26][27][28][29] and decreased drastically with the increase of the active layer thickness, which limits their application in the roll to roll large-scale solution printing technology. [30,31] Furthermore, 20%-40% of the incident photon flux were wasted in such a active layer thickness, [32] which directly limits the short circuit current density (J sc ) of the corresponding OSCs. Thus, it is necessary to develop high efficiency OSCs with tolerance of the active layer thickness. However, a lot of research work have proved that the charge collection efficiency of a device is inversely proportional to the square of the film thickness of the active layer. [33,34] It was reflected in the decline of fill factor (FF) with the increase of film thickness. Also, the J sc will decrease due to the severe bimolecular recombination. [35,36] Thus, it is still a challenge to obtain high efficiency devices with active layer thickness tolerance. To date, in the reported cases with active layer thickness tolerance, most are based on fullerene derivative acceptors and it has been found that the donor materials with high crystallinity and balanced mobility with fullerene derivative acceptors manifested better performance with high film thickness. [35][36][37][38] Compared with fullerene derivatives based OSCs, it is much more challenging to realize thick-film NFAs OSCs with high performance since the electron mobilities of NFAs are usually lower than that of fullerene derivative acceptors. [39,40] Thus, the charge transport and collection process in those NFA based thick films are not efficient. So far, great attentions have been drawn on the NFA based thick film OSCs and much progress have been made. For examples, Yip and co-workers reported devices based on PffBT4T-2OD:EH-IDTBR and realized a PCE of 9.1% with an active layer thickness of 300 nm by optimizing device architectures to overcome the space-charge effects. [41] Zhang and co-workers reported devices based on PM6:IDIC with PCEs of 11.9% under the film thickness of 150 nm and 11.3% under the condition of 255 nm condition. Although the device performance is good enough, the cases with thicker active layers Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick-film organic solar cells employing a nonfullerene acceptor F-2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achi...

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Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Cited by 222 publications

References 51 publications

Eco‐Compatible Solvent‐Processed Organic Photovoltaic Cells with Over 16% Efficiency

Eco‐Compatible Solvent‐Processed Organic Photovoltaic Cells with Over 16% Efficiency

Accurate Determination of the Minimum HOMO Offset for Efficient Charge Generation using Organic Semiconducting Alloys

High Performance Thick‐Film Nonfullerene Organic Solar Cells with Efficiency over 10% and Active Layer Thickness of 600 nm

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