Lian Zhong

Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency

Non-fullerene polymer solar cells (PSCs) with solution-processable n-type organic semiconductor (n-OS) as acceptor have seen rapid progress recently owing to the synthesis of new low bandgap n-OS, such as ITIC. To further increase power conversion efficiency (PCE) of the devices, it is of a great challenge to develop suitable polymer donor material that matches well with the low bandgap n-OS acceptors thus providing complementary absorption and nanoscaled blend morphology, as well as suppressed recombination and minimized energy loss. To address this challenge, we synthesized three medium bandgap 2D-conjugated bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymers J52, J60, and J61 for the application as donor in the PSCs with low bandgap n-OS ITIC as acceptor. The three polymers were designed with branched alkyl (J52), branched alkylthio (J60), and linear alkylthio (J61) substituent on the thiophene conjugated side chain of the benzodithiophene (BDT) units for studying effect of the substituents on the photovoltaic performance of the polymers. The alkylthio side chain, red-shifted absorption down-shifted the highest occupied molecular orbital (HOMO) level and improved crystallinity of the 2D conjugated polymers. With linear alkylthio side chain, the tailored polymer J61 exhibits an enhanced JSC of 17.43 mA/cm(2), a high VOC of 0.89 V, and a PCE of 9.53% in the best non-fullerene PSCs with the polymer as donor and ITIC as acceptor. To the best of our knowledge, the PCE of 9.53% is one of the highest values reported in literature to date for the non-fullerene PSCs. The results indicate that J61 is a promising medium bandgap polymer donor in non-fullerene PSCs.

A near-infrared non-fullerene electron acceptor for high performance polymer solar cells

Low-bandgap polymers/molecules are an interesting family of semiconductor materials, and have enabled many recent exciting breakthroughs in the field of organic electronics, especially for organic photovoltaics (OPVs). Here, such a low-bandgap (1.43 eV) non-fullerene electron acceptor (BT-IC) bearing a fused 7-heterocyclic ring with absorption edge extending to the near-infrared (NIR) region was specially designed and synthesized. Benefitted from its NIR light harvesting, high performance OPVs were fabricated with medium bandgap polymers (J61 and J71) as donors, showing power conversion efficiencies of 9.6% with J61 and 10.5% with J71 along with extremely low energy loss (0.56 eV for J61 and 0.53 eV for J71). Interestingly, femtosecond transient absorption spectroscopy studies on both systems show that efficient charge generation was observed despite the fact that the highest occupied molecular orbital (HOMO)–HOMO offset (ΔEH) in the blends was as low as 0.10 eV, suggesting that such a small ΔEH is not a crucial limitation in realizing high performance of NIR non-fullerene based OPVs. Our results indicated that BT-IC is an interesting NIR non-fullerene acceptor with great potential application in tandem/multi-junction, semitransparent, and ternary blend solar cells.

Non-fullerene polymer solar cells based on a selenophene-containing fused-ring acceptor with photovoltaic performance of 8.6%

In this work, we present a non-fullerene electron acceptor bearing a fused five-heterocyclic ring containing selenium atoms, denoted as IDSe-T-IC, for fullerene-free polymer solar cells (PSCs). This molecule exhibits a low band gap (Eg = 1.52 eV), strong absorption in the 600–850 nm region and a high LUMO level (−3.79 eV). When a large band gap polymer J51 (Eg = 1.91 eV) was used as the donor, complementary absorption of the polymer donor and acceptor was obtained in the wavelength range of 350–850 nm. The solar cell based on J51:IDSe-T-IC gives a maximum PCE of 8.6%, with a high Voc of 0.91 V, a Jsc of 15.20 mA cm−2 and a fill factor (FF) of 62.0%. Moreover, this performance is much higher than that of J51:PC71BM based PSCs under similar device fabrication conditions (PCE = 6.0%). The trade-off features of the Jsc and Voc existing in PSCs with fullerene acceptors have been minimized in the fullerene-free PSCs based on IDSe-T-IC and J51. The results demonstrate that fine-tuning the absorption and electronic energy levels of non-fullerene acceptors, and properly selecting a polymer donor to achieve complementary absorption, is a promising way to further improve the performance of the PSCs.

Alkoxy substituted benzodithiophene-alt-fluorobenzotriazole copolymer as donor in non-fullerene polymer solar cells

A new benzodithiophene (BDT)-alt-fluorobenzotriazole (FBTA) D-A copolymer J40 was designed and synthesized by introducing 2-octyldodecyloxy side chains on its BDT units, for expanding the family of the BDT-alt-FBTA-based copolymers and investigating the side chain effect on the photovoltaic performance of the polymer in non-fullerene polymer solar cells (PSCs). J40 exhibits complementary absorption spectra and matched electronic energy levels with the n-type organic semiconductor (n-OS) (3, 9-bis(2-methylene-(3-(1, 1-dicyanomethylene)-indanone))-5, 5, 11, 11-tetrakis(4-hexylphenyl)-dithieno[2, 3-d:2′, 3′-d′]-s-indaceno[1, 2-b:5, 6-b′]dithiophene) (ITIC) acceptor, and was used as polymer donor in the non-fullerene PSCs with ITIC as acceptor. The power conversion efficiency (PCE) of the PSCs based on J40:ITIC (1:1, w/w) with thermal annealing at 120 °C for 10 min reached 6.48% with a higher open-circuit voltage (Voc) of 0.89 V. The high Voc of the PSCs is benefitted from the lower-lying highest occupied molecular orbital (HOMO) energy level of J40. Although the photovoltaic performance of the polymer J40 with alkoxy side chain is lower than that of J60 and J61 with alkylthio-thienyl conjugated side chains, the PCE of 6.48% for the J40-based device is still a relatively higher photovoltaic efficiency in the non-fullerene PSCs reported so far. The results indicate that the family of the BDT-alt-FBTA-based D-A copolymers are high performance polymer donor materials for non-fullerene PSCs and the side chain engineering plays an important role in the design of high performance polymer donors in the non-fullerene PSCs.

High performance as-cast semitransparent polymer solar cells

Semi-transparent polymer solar cells (ST-PSCs) have attracted great attention recently because of their potential for application in smart windows, etc. Here, we fabricated ST-PSCs based on a low band-gap conjugated polymer, PTB7-Th, as the donor and a narrow band-gap n-type organic semiconductor (n-OS), ITVfIC, as the acceptor. The active layer (with a thickness of 100 nm) of the ST-PSC exhibits a high average transmittance (AT) of 73.46% in the wavelength range of 400–600 nm. The as-cast ST-PSC with 15 nm Ag as the cathode without any additive or thermal-annealing treatment shows a higher power conversion efficiency (PCE) of 8.21% with an AT of 33.7%, which is one of the highest values for ST-PSCs without extra treatment. In addition, the ST-PSCs show good thermal stability with 91% of their original PCE value retained after high temperature treatment at 200 °C for 2 hours. The higher PCE and good stability indicate that the ST-PSC has potential for practical application and the narrow bandgap ITVfIC could be a promising n-OS acceptor for the fabrication of ST-PSCs.

Short-axis substitution approach on ladder-type benzodithiophene-based electron acceptor toward highly efficient organic solar cells

Short-axis substitution, as an effective way to change the optical and electronic properties of the organic semiconductors for organic photovoltaics (OPVs), is a readily approach to modify non-fullerene acceptors, especially for the linear fused rings system. Here, two new fused-ring electron acceptors (CBT-IC and SBT-IC) were designed and developed by short-axis modification based on the dithienyl[1,2-b:4,5-b′]benzodithiophene (BDCPDT) system. Combined with a medium bandgap polymer donor J71, both of the OPV devices exhibit high power conversion efficiency (PCE) over 11%, and ~70% external quantum efficiencies. To better understand how this kind of substitution affects the BDCPDT based acceptors, a comparative analysis is also made with the the plain acceptor BDT-IC without this modification. We believe this work could disclose the great potential and the versatility of BDCPDT block and also enlighten other ladder-type series for further optimization.