Prof. Yongsheng Chen earns the State Natural Science Award, stimulus hopes a shot in the arm for OPV.

The State Science and Technology Prizes is the highest honor in China in science and technology, in order to recognize citizens and organizations who have made remarkable contributions to scientific and technological progress and promote the development of science and technology. There are five State prizes in science and technology.

January 8, 2019, Prof. Yongsheng Chen’s team earns the State Natural Science Award, the Second Class Award, with their project, "Research on Organic and Carbon Nanomaterials for Energy Conversion and Storage", which is implemented by Prof. Yongsheng Chen, Xiangjian Wan, Huang Yi and Chengyang Tian from Nankai University and Prof. Chengyang Wang from Tianjin University. 
Prof. Yongsheng Chen, distinguished professor of Chemistry at Nankai University, the State Key Laboratory and Institute of Elemento-Organic Chemistry, Director for the Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, is devoted to organic and polymeric functional materials, carbon-based nanomaterials, green energy devices using graphene and nanotubes, including OPV and supercapacitor, etc.

Prof. Yongsheng Chen has made breakthroughs in organic solar cells filed since 2016 and has become one of the leading professionals in OPV field. In August 2018, Prof. Yongsheng Chen team set record for organic tandem solar cells with 17.3% efficiency. (The previous record, set in 2006 by the same group, was 12.7%.) Power conversion efficiency of organic solar cells takes a giant leap from 12.7% to 17.3%, which makes stimulus hopes for OPV field.

Let’s review, how the team of Prof. Yongsheng Chen achieved the high efficiency in organic tandem solar cells!
Prof. Yongsheng Chen cooperated with Liming Ding from Center for Excellence in Nanoscience (CAS), and Prof. Hin-Lap Yip from Institute of Polymer Optoelectronic Materials and Devices at South China University of Technology and have developed a highly efficient 2-terminal monolithic solution processed tandem OPV, thanks to tandem cell strategy and the high diversity and tunable band structure of organic materials. The research, “Organic and solution-processed tandem solar cells with 17.3% efficiency.” which was published in Science.

The low charge mobility of organic materials is the fundamental reason to limit the efficiency.
Though organic photovoltaic cells (OPVs) have many advantages, their performance still lags far behind that of other photovoltaic platforms. One of the most fundamental reasons for this is the low charge mobility of organic materials, leading to a limit on the active layer thickness and efficient light absorption. The team uses a semi-empirical model analysis and the tandem cell strategy to overcome such issues, also takes advantage of the high diversity and easily tunable band structure of organic materials to develop a higher effective OPV device.

A semi-empirical model analysis and the great diversity of organic materials
Tandem cell strategy is an effective way to simultaneously address the above issues for OPV, and furthermore, it probably works best for OPV. First, it would overcome the above thickness issue in the single junction cell due to the low mobility of organic materials since wide and efficient absorptions could be achieved with the active layers stacked with complementary absorption in tandem cell. On the other side and more importantly, it can take one of the most important and fundamental advantages of organic materials, i.e., great diversity. This is because that in tandem cells, in principle, the active organic materials in each subcell required with different but matching band structures could be designed and obtained due to the high diversity, easily tunable band structures and advanced synthesis of organic materials these days. In addition, benefiting from this, the thermalization and transmission loss can be reduced in the tandem cells.

The team has found that the non-fullerene acceptor molecule COi8DFIC (also named O6T-4F) could roughly meet the above requirements, as it has infrared absorption onset of ~ 1050 nm (optical bandgap Eg 1.20 eV) and when blending with PTB7-Th as donor and PC71BM as the secondary acceptor, the single junction device based on it exhibited a high Jsc with EQE over 70% in the infrared absorption range, a FF of 69.7%, and a low Eloss of 0.51 eV.

The performance of the each subcell was first studied and optimized using the inverted structures. Note the single junction device for front cell based on the selected active material of PBDB-T:F-M has a high EQE response in the range of 300-750 nm, giving a current of 15.96 mA/cm2, together with a high Voc of ~ 0.94 V and FF of 69.8%. Note the performance of this inverted device is slightly different from that of the regular device and the Eloss for this single junction cell is 0.71 eV. The optimal Jsc is close to the value predicted for the front subcell required in the best tandem cell. But the optimized single junction devices using PTB7-Th:O6T-2F:PC71BM for the rear unit gave a high current of 27.98 mA/cm2 in the range from 300 to 1050 nm with a Voc of 0.69 V and decent FF of 69.7% and rather low Eloss of 0.51 eV, similar as that in literature. Though the device showed a broad and decent EQE response from 300 to 1050 nm, it only has an integrated current of 11.2 mA/cm2 in the range of 720-1050 nm desired for the rear cell, lower than the best predicted current of 15.5 mA/cm2 designed above if assuming a clear cut off absorption for the rear sub unit exists at 720 nm.
Fig. (A) Molecular structures of PBDB-T, F-M, PTB7-Th, O6T-4F and PC71BM.(Cited from the research)

▲Fig. (B) Normalized absorption spectra of PBDB-T:F-M and PTB7-Th:O6F-4F:PC71BM films. (C) J-V curves.(Cited from the research)

The performance of the optimized tandem cells show good stability behavior, with only a minor performance degradation of 4% after continuing 166 days-testing. The significantly higher PCE of 17.36% than the state of the art levels, together with the semi-empirical analysis provided, indicate that OPV if using tandem cell has much more potential than previously thought, and PCE >25% should be achievable using the already achieved best EQE of 80%, Eloss of 0.45 eV and FF of 0.75. Considering its other advantages, OPV should be as competitive as other solar cell technologies for industry application in future if the stability issue of OPV could be addressed.

Predicted PCEs of 2T tandem solar cells based on semi-empirical analysis under AM 1.5G. 
(A) PCEs versus EQE and λonset, rear cell with assuming the Eloss of each subcell in the range of 0.4-0.8 eV and a fixed FF of 0.75. (B) PCEs versus EQE and λonset, rear cell with assumed Eloss of 0.6 eV and FF of 0.75. (C) PCEs versus λonset, rear cell with Eloss of 0.4, 0.5, 0.6, 0.66, 0.7, 0.76 and 0.8 eV, FF of 0.75 and EQE of 75%. (D) PCEs versus Eloss and λonset of rear cell with assumed EQE of 75% and FF of 0.75. (Cited from the research)

The team uses Enli Tech QE-R Solar cell Quantum Efficiency Measurement System, measuring the EQE curves of the single-junction devices based on PBDB-T:F-M and PTB7-Th:O6F-4F:PC71BM. The absorption spectra range of PBDB-T:F-M and PTB7-Th:O6F-4F:PC71BM films is 300-1050 nm, and it can be covered by QE-R, whose wavelength range is 300-1100 nm.

(Cited from the research)

Optical simulation and photovoltaic performance of the tandem cells.
(A) Device architecture of the tandem cell. (B) Energy level diagram of the tandem solar cell. (C) Simulated current density generated in a tandem cell as a function of the thicknesses of the active layers. (D) J-V curve. (E) EQE and 1-reflectance of the optimized tandem solar cell and (F) J-V curve of the tandem cells under different light intensity from 4.97 to 112.68 mW/cm2. The dot vertical line at 720 nm in (E) is the cross point of the EQE plots of the two subcells, and the dot vertical line at 985 nm indicates the effective absorption up position (assuming 50% EQE) of the rear subcell.

Organic and solution-processed tandem solar cells with 17.3% efficiency.
Lingxian Meng, Yamin Zhang, Xiangjian Wan, Chenxi Li, Xin Zhang, Yanbo Wang, Xin Ke, Zuo Xiao, Liming Ding, Ruoxi Xia, Hin-Lap Yip, Yong Cao, Yongsheng Chen.
Science  14 Sep 2018: Vol. 361, Issue 6407, pp. 1094-1098, DOI: 10.1126/science.aat2612