The development of organic and  perovskite solar cells has seen significant advances in materials  innovation, power conversion efficiencies (PCEs), and device stability.  With that in mind, the researchers in the Department of Materials Science and Engineering (MSE) at Southern University of Science and Technology (SUSTech) have made continuous efforts for significant progress in this field.

MSE Professor Xugang Guo has led his research team to publish several important papers in high-level journals over the last few months, including Advanced Materials (IF = 27.398), SCIENCE CHINA Chemistry (IF = 6.356), Energy & Environmental Science (IF = 30.289), Advanced Functional Materials (IF = 16.836), and Journal of the American Chemical Society (IF = 14.612).

In a recent paper titled “A Narrow-Bandgap n-Type Polymer with an  Acceptor-Acceptor Backbone Enabling Efficient All-Polymer Solar  Cells,”published in Advanced Materials , the research group led by  Professor Xugang Guo developed a new narrow-bandgap polymer acceptor L14  (Figure 1), featuring an acceptor–acceptor (A–A) type backbone, which  is synthesized by copolymerizing a dibrominated fused-ring electron  acceptor (FREA) with distannylated bithiophene imide. L14 shows not only  a narrow bandgap and high absorption coefficient but also low-lying  frontier molecular orbital (FMO) levels. Such FMO levels yield improved  electron transfer characteristics, and unexpectedly, without sacrificing  open-circuit voltage (Voc), which is attributed to a small  non-radiative recombination loss of 0.22 eV. Benefiting from the  improved photocurrent along with the high fill factor and Voc, an  excellent efficiency of 14.3% is achieved, which is among the highest  values for all-polymer solar cells (all-PSCs). The results demonstrate  the superiority of narrow-bandgap A–A type polymers for improving  all-PSC performance and pave a way toward developing high-performance  polymer acceptors for all-PSCs.

Figure 1. (a) The J–V  characteristics of the all-PSCs based on L14 with an acceptor-acceptor  (A-A) backbone and its donor-acceptor (D-A) analogous polymer L11. (b)  Plots of PCE against V_oc for all-PSCs reported previously with PCEs over 8% and this work. (Adv. Mater. 2020, 32, 2004183)

Recent advances in the development of polymerized FREA have promoted  the PCE of all-PSCs over 13%. However, the monomer of an FREA typically  consists of a mixture of three isomers due to the regioisomeric  brominated end groups. In this work, the two isomeric end groups are  successfully separated to resolve the  regioisomeric issue. Three  polymer acceptors, namely PY-IT, PY-OT, and PY-IOT (Figure 2), are  developed, where PY-IOT is a random terpolymer with the same ratio of  the two acceptors. Interestingly, from PY-OT, PY-IOT to PY-IT, the  absorption edge gradually redshifts which should be beneficial for  photocurrent improvement in solar cells. Theoretical calculation  indicates that the lowest unoccupied molecular orbitals (LUMOs) are  distributed on the entire molecular backbone of PY-IT, contributing to  the enhanced electron transport.

Consequently, the PM6: PY-IT system achieves an excellent PCE of  15.05%, which is significantly higher than those for PY-OT (10.04%) and  PY-IOT (12.12%) and also the highest value in All-PSCs. This work  demonstrates that the site of polymerization on FREAs strongly affects  device performance, offering insights into the development of efficient  polymer acceptors for all-PSCs. This work is recently published in  Advanced Materials, titled “Precisely Controlling the Position of  Bromine on the End Group Enables Well-Regular Polymer Acceptors for  All-Polymer Solar Cells with Efficiencies over 15%”.

Figure 2. Three polymer acceptors (PY-IT,  PY-OT, and PY-IOT) without regioisomeric issues are developed. The  all-PSCs based on PM6: PY-IT achieve an excellent PCE of 15.05%,  significantly higher than those based on PY-OT (10.04%) and PY-IOT  (12.12%). (Adv. Mater. 2020, 32, 2005942)

The Voc of all-PSCs is typically lower than 0.9 V even for the most  efficient ones. Large energy loss is the main reason for limiting Voc  and efficiency of all-PSCs. In their third paper published in SCIENCE  CHINA Chemistry and entitled “Reducing energy loss via tuning energy  levels of polymer acceptors for efficient all-polymer solar cells.”, the  team used electron-deficient building blocks based on bithiophene  imides along with materials design to effectively tune the lowest LUMO  energy levels of polymer acceptors (Figure 3), which resulted in a  reduced energy loss induced by charge generation and recombination loss  due to the suppressed charge-transfer (CT) state absorption. Despite a  negligible driving force, all-PSC based on the polymer donor and  acceptor combination with well-aligned energy levels exhibited efficient  charge transfer and achieved an external quantum efficiency over 70%  while maintaining a large Voc of 1.02 V, leading to a 9.21% efficiency.  Through various spectroscopy approaches, this work sheds light on the  mechanism of energy loss in all-PSCs, which provides a new avenue to  achieve efficient all-PSCs with large Voc and drives further development  of all-PSCs.

Figure 3. Molecule structures and basic  properties of new polymer acceptors based on BTIn and the classical  polymer acceptor N2200. (Sci. China Chem. 2020, DOI: 10.1007/s11426-020-9826-4)

The ternary strategy has many advantages in improving the performance  of organic solar cells, which has aroused great interest in  researchers. It is worth noting that adding the third component to the  main binary system will result in a more complex mixed morphology of the  active layer. Poor material compatibility can lead to serious molecular  disorder and large-scale phase separation, thereby reducing the  performance of solar cells. Compared with two small molecules, the  compatibility of the two polymers is usually poor. This is the main  reason for the lower success rate of the ternary battery based on two  polymer donors compared to the ternary battery based on two small  molecule receptors. In their fourth paper entitled “Two  compatible polymer donors contribute synergistically for ternary  organic solar cells with 17.53% efficiency”and published in Energy & Environmental Science,  the team introduced a highly efficient polymer donor material S3 and  added it to the classic PM6: Y6 binary system to prepare a ternary  organic solar cell. S3 and PM6 have complementary absorption spectra and  good compatibility, which is highly favorable for the optimization of  the photon capture and morphology of the ternary blend film, thereby  simultaneously increasing the short-circuit current density (J_SC)  and fill factor (FF). At the same time, the highest occupied molecular  orbital (HOMO) energy level of S3 is slightly lower than that of PM6,  which makes the non-radiative energy loss of ternary organic solar cells  lower than that of binary solar cells based on PM6, thereby achieving  higher open-circuit voltage. The energy conversion efficiency of the  optimized ternary cell is as high as 17.53% (Figure 4), which is one of  the highest values reported in ternary organic solar cells. This study  further proves that the ternary strategy is a universal method to  improve the performance of organic solar cells and provides an effective  way to realize high-efficiency and low-cost organic solar cells.

Figure 4. The J–V  characteristics of organic solar cells based on PM6: Y6, S3: Y6, PM6:S3:  Y6 blend film. (Energy Environ. Sci.2020, DOI:10.1039/D0EE02516J)

Developing organic solar cells based on ternary active layer is one  of effective strategies to improve their photovoltaic performance.  However, this strategy is very limited success in all-polymer solar  cells due to the scarcity of high-performance narrow bandgap polymer  acceptor and the challenge of morphology optimization in ternary  bulk-heterojunction all-polymer blend films. In Advanced Materials,  Professor Xugang Guo group has previously reported polymer acceptor  DCNBT-TPC with the ultra-narrow bandgap (ultra-NBG) up to 1.38 eV. This  polymer has strong absorption in long wavelength region, which breaks  the absorption bottleneck limiting the performance of all-PSCs. In their  fifth paper in Advanced Functional Materials and titled  “Highly Efficient Ternary All-Polymer Solar Cells with Enhanced  Stability.”, the team reported here a ternary system based on an  ultra-NBG polymer acceptor DCNBT-TPC, a medium bandgap polymer donor  PTB7-Th, and a wide bandgap polymer donor PBDB-T. The optimized ternary  all-PSCs yield an excellent PCE of 12.1% with a remarkable short-circuit  current density of 21.9 mA cm⁻², which is the champion  efficiency in ternary all-polymer solar cells (Figure 5). This work  demonstrates that the utilization of a ternary all-polymer system based  on ultra-NBG polymer acceptor blended with compatible polymer donors is  an effective strategy to advance the all-polymer solar cells.

Figure 5. The active layer component, thin film absorption, and J–V characteristics of all-polymer solar cells. (Adv. Funct. Mater. 2020, 2008494)

The charge transporting layers in perovskite solar cells (PSCs) have  attracted more and more attention in recent years, which plays a  critical role in determining their PCEs and device stability. Specially,  the hole-transporting materials (HTMs) have received a great deal of  interest. As a way to expedite the commercialization process, three  paramount concerns must be addressed when developing new HTMs: i) high  performance (superior PCE and device stability); ii) low cost; iii)  eco-friendly processability. However, there has been few reports on HTMs  simultaneously with low-cost preparation, environmentally friendly  processability, and high performance. Recently, Professor Xugang Guo  group has designed and synthesized a novel HTM (MPA-BT-CA) involving  2-cyanoacrylic acid (CA) group and donor-acceptor molecular backbone,  which shows low-cost production and good alcohol solubility.  Encouragingly, a remarkably high PCE of 21.24%, along with good device  stability, was achieved in inverted PSCs based on MPA-BT-CA. More  importantly, a PCE as high as 20.5% can still be maintained when using  ethanol as a processing solvent (Figure 6). The corresponding research  results were published in the Journal of the American Chemical Society,  titled “Teaching an Old Anchoring Group New Tricks: Enabling Low-Cost,  Eco-Friendly Hole-Transporting Materials for Efficient and Stable  Perovskite Solar Cells,” and selected as “Spotlights on Recent JACS  Publications.”

Figure 6.  Chemical structures of the recently developed low-cost HTMs,  green-solvent processable HTMs, and HTMs enabling state-of-the-art  performance (first row) and chemical structures reported in this work  along with a schematic diagram of the multifunctionality of MPA-BT-CA  (second row). (J. Am. Chem. Soc. 2020, 142, 16632)

Figure 7. The above work was featured on the cover of Journal of the American Chemical Society.

The above work was supported by the team of Professor Chuluo Yang of  Shenzhen University, the team of Professor He Yan of the Hong Kong  University of Science and Technology, the team of Professor Feng Gao of  Linköping University in Sweden, the team of Professor Hongzheng Chen of  Zhejiang University, the team of Professor Fujun Zhang of Beijing  Jiaotong University, the team of Professor Long Ye of Tianjin  University, and Dr. Zhipeng Kan, researcher of the Chinese Academy of  Sciences, Associate Professor Anqiao Shi from Beijing Institute of  Technology, Professor Antonio Facchetti and Tobin J. Marks from  Northwestern University in the United States, and SUSTech Core Research  Facilities （SCRF）.

Paper link：

https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202004183

https://onlinelibrary.wiley.com/doi/10.1002/adma.202005942

https://link.springer.com/article/10.1007/s11426-020-9826-4

https://pubs.rsc.org/en/content/articlelanding/2020/ee/d0ee02516j#!divAbstract

https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202008494

https://pubs.acs.org/doi/10.1021/jacs.0c06373