Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

https://science.sciencemag.org/content/sci/early/2018/08/08/science.aat2612.full.pdf

Organic and solution-processed tandem solar cells with 17.3% efficiency

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within t...

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

A new nonfullerene small molecule with acceptor–donor–acceptor (A–D–A) structure, namely, NFBDT, based on a heptacyclic benzodi(cyclopentadithiophene) (FBDT) unit using benzo[1,2-b:4,5-b′]dithiophene as the core unit, was designed and synthesized. Its absorption ability, energy levels, thermal stability, as well as photovoltaic performances were fully investigated. NFBDT exhibits a low optical bandgap of 1.56 eV resulting in wide and efficient absorption that covered the range from 600 to 800 nm, and suitable energy levels as an electron acceptor. With the widely used and successful wide bandgap polymer PBDB-T selected as the electron donor material, an optimized PCE of 10.42% was obtained for the PBDB-T:NFBDT-based device with an outstanding short-circuit current density of 17.85 mA cm–2 under AM 1.5G irradiation (100 mW cm–2), which is so far among the highest performance of NF-OSC devices. These results demonstrate that the BDT unit could also be applied for designing NF-acceptors, and the fused-ring b...

Small-Molecule Acceptor Based on the Heptacyclic Benzodi(cyclopentadithiophene) Unit for Highly Efficient Nonfullerene Organic Solar Cells

Tandem Organic Solar Cell with 20.2% Efficiency

A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells

A key feature of organic electronic devices is their mechanical flexibility. However, the performance of flexible organic optoelectronic devices still lags behind the performance of devices on rigid substrates. This is due, in particular, to the lack of flexible transparent electrodes that simultaneously offer low resistance, high transparency and a smooth surface. Here, we report flexible transparent electrodes created using water-processed silver nanowires and a polyelectrolyte. Due to ionic electrostatic charge repulsion, the nanowires form grid-like structures in a single step, leading to smooth, flexible electrodes that have a sheet resistance of around 10 Ω □−1 and a transmittance of around 92% (excluding the substrate). To illustrate the potential of the approach in organic electronics, we use the flexible electrodes to create organic photovoltaic devices. The devices are tested with different types of donors and acceptors, and exhibit performance comparable to devices based on commercial rigid electrodes. Furthermore, flexible single-junction and tandem devices achieve power conversion efficiencies of 13.1% and 16.5%, respectively. Flexible transparent electrodes made from silver nanowires that form grid-like structures due to ionic electrostatic charge repulsion can be used to create flexible single-junction and tandem organic photovoltaic devices with power conversion efficiencies of 13.1% and 16.5%, respectively.

Flexible organic photovoltaics based on water-processed silver nanowire electrodes

Fabricating solar cells with tandem structure is an efficient way to broaden the photon response range without further increasing the thermalization loss in the system. In this work, a tandem organic solar cell (TOSC) based on highly efficient nonfullerene acceptors (NFAs) with series connection type is demonstrated. To meet the different demands of front and rear sub-cells, two NFAs named F-M and NOBDT with a whole absorption range from 300 to 900 nm are designed, when blended with wide bandgap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) and narrow bandgap polymer PTB7-Th, respectively, the PBDB-T: F-M system exhibits a high Voc of 0.98 V and the PTB7-Th: NOBDT system shows a remarkable Jsc of 19.16 mA cm-2 , which demonstrate their potential in the TOSCs. With the guidance of optical simulation, by systematically optimizing the thickness of each layer in the TOSC, an outstanding power conversion efficiency of 14.11%, with a Voc of 1.71 V, a Jsc of 11.72 mA cm-2 , and a satisfactory fill factor of 0.70 is achieved; this result is one of the top efficiencies reported to date in the field of organic solar cells.

Nonfullerene Tandem Organic Solar Cells with High Performance of 14.11.

A new acceptor–donor–acceptor (A–D–A) type nonfullerene acceptor, 3TT‐FIC, which has three fused thieno[3,2‐b]thiophene as the central core and difluoro substituted indanone as the end groups, is designed and synthesized. 3TT‐FIC exhibits broad and strong absorption with extended onset absorption to 995 nm and a low optical bandgap of 1.25 eV. The binary device based on 3TT‐FIC and the polymer PTB7‐Th exhibits a power conversion efficiency (PCE) of 12.21% with a high short circuit current density (   J sc) of 25.89 mA cm−2. To fine‐tune the morphology and make full use of the visible region sunlight, phenyl‐C71‐butyricacid‐methyl ester (PC71BM) is used as the third component to fabricate ternary devices. In contrast to the binary devices, the ternary blend organic solar cells show significantly enhanced EQE ranging from 300 to 700 nm and thus an improved  J sc with a high value of 27.73 mA cm−2. A high PCE with a value of 13.54% is achieved for the ternary devices, which is one of the highest efficiencies in single junction organic solar cells reported to date. The results provide valuable insight for the ternary devices in which the external quantum efficiency (EQE) induced by the third component is evidently observed and directly contributed to the enhancement of the device efficiency.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201800307

A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary‐Blend Organic Solar Cells with Efficiency over 13%

Achieving high efficiency and well-kept ductility in ternary all-polymer organic photovoltaic blends thanks to two well miscible donors

DOI: 10.1002/aenm.201901024 partially because of their characteristics such as easily tuned absorptions and energy levels, facile synthesis and purification, etc.[1–3] Thus, over 15% for single junction devices[4–7] and 17% for tandem devices[8] have been achieved. Based on theoretical prediction, it is highly possible to achieve 18% for single junction cell and 20% for tandem cells.[8] To achieve such goals, both the Voc and Jsc need to be improved in addition to achieving high fill factor (FF). Based on the state of the art works, almost all high-performance OSC utilized a medium bandgap donor with a narrow bandgap acceptor (often a NFA), and absorb the sunlight only to ≈900 nm.[9–14] This means a large portion of sunlight (≈>40%) has been wasted completely. Thus, it is clear that optimized materials with absorption above 900 nm need to be designed. Intrinsically, these materials would have a lower lowest unoccupied molecular orbital (LUMO), which thus would generate a lower Voc. Indeed, some great effect has been dedicated to improve the performance in individually improving Jsc or Voc but with sacrificing the other parameter. For example, extending the length of conjugation systems (6TIC,[18] 2,2′-((2Z,2′Z)-(5,10-dihexylnaphtho[1,2′b:5,6-b′]di(4,4bis(4-hexylphenyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene2,7-diyl)bis(5,6-difluoro-3-(dicyanomethylene)-2-methyleneindan-1-one),[19] IDCIC,[14] FNIC2,[20] NCBDT,[21] etc.) or introducing electron withdrawing end group ((4,4,10,10tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydrodithienyl[1,2-b:4,5b′]benzodi-thiophene-2,8-diyl)bis(2-(3-oxo2,3-dihydroinden-5,6-dichloro-1-ylidene)malononitrile),[22] 6TIC-4F,[23] etc.) has been applied to enhance the Jsc, mainly by increasing the sunlight absorption. Similarly, side chain conjunction optimization (ITIC2)[24] or end group modulation (IT-M, IT-DM,[11] (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene),[25] F-M,[26] etc.) has been explored to enhance the Voc individually. It is a great challenge to simultaneously improve the two tangled parameters, open circuit voltage (Voc) and short circuit current density (Jsc) for organic solar cells (OSCs). Herein, such a challenge is addressed by a synergistic approach using fine-tuning molecular backbone and morphology control simultaneously by a simple yet effective side chain modulation on the backbone of an acceptor–donor–acceptor (A–D–A)-type acceptor. With this, two terthieno[3,2-b]thiophene (3TT) based A–D–A-type acceptors, 3TT-OCIC with backbone modulation and 3TT-CIC without such modification, are designed and synthesized. Compared with the controlled molecule 3TT-CIC, 3TT-OCIC shows power conversion efficiency (PCE) of 13.13% with improved Voc of 0.69 V and Jsc of 27.58 mA cm−2, corresponding to PCE of 12.15% with Voc of 0.65 V and Jsc of 27.04 mA cm−2 for 3TT-CIC–based device. Furthermore, with effective near infrared absorption, 3TT-OCIC is used as the rear subcell acceptor in a tandem device and gave an excellent PCE of 15.72%.

Yanna Sun

Papers

Flexible organic photovoltaics based on water-processed silver nanowire electrodes

Nonfullerene Tandem Organic Solar Cells with High Performance of 14.11.

A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary‐Blend Organic Solar Cells with Efficiency over 13%

Achieving high efficiency and well-kept ductility in ternary all-polymer organic photovoltaic blends thanks to two well miscible donors

Achieving Both Enhanced Voltage and Current through Fine‐Tuning Molecular Backbone and Morphology Control in Organic Solar Cells