Haijun Bin

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.

11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor

Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si–C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm−2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.

Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells.

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.

High-Efficiency Nonfullerene Polymer Solar Cells with Medium Bandgap Polymer Donor and Narrow Bandgap Organic Semiconductor Acceptor

A nonfullerene polymer solar cell with a high efficiency of 9.26% is realized by using benzodithiophene-alt-fluorobenzotriazole copolymer J51 as a medium-bandgap polymer donor and the low-bandgap organic semiconductor ITIC with high extinction coefficients as the acceptor.

9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor

In the last two years, polymer solar cells (PSCs) developed quickly with n-type organic semiconductor (n-OSs) as acceptor. In contrast, the research progress of nonfullerene organic solar cells (OSCs) with organic small molecule as donor and the n-OS as acceptor lags behind. Here, we synthesized a D-A structured medium bandgap organic small molecule H11 with bithienyl-benzodithiophene (BDTT) as central donor unit and fluorobenzotriazole as acceptor unit, and achieved a power conversion efficiency (PCE) of 9.73% for the all organic small molecules OSCs with H11 as donor and a low bandgap n-OS IDIC as acceptor. A control molecule H12 without thiophene conjugated side chains on the BDT unit was also synthesized for investigating the effect of the thiophene conjugated side chains on the photovoltaic performance of the p-type organic semiconductors (p-OSs). Compared with H12, the 2D-conjugated H11 with thiophene conjugated side chains shows intense absorption, low-lying HOMO energy level, higher hole mobility and ordered bimodal crystallite packing in the blend films. Moreover, a larger interaction parameter (χ) was observed in the H11 blends calculated from Hansen solubility parameters and differential scanning calorimetry measurements. These special features combined with the complementary absorption of H11 donor and IDIC acceptor resulted in the best PCE of 9.73% for nonfullerene all small molecule OSCs up to date. Our results indicate that fluorobenzotriazole based 2D conjugated p-OSs are promising medium bandgap donors in the nonfullerene OSCs.

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.

Fine‐Tuning of Molecular Packing and Energy Level through Methyl Substitution Enabling Excellent Small Molecule Acceptors for Nonfullerene Polymer Solar Cells with Efficiency up to 12.54%

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520-740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (JSC ) of 18.41 mA cm-2 , significantly higher than that of the device based on J71:ITCPTC (11.63% with a JSC of 17.52 mA cm-2 ). The higher JSC of the PSC based on J71:MeIC can be attributed to more balanced μh /μe , higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

Indacenodithienothiophene–naphthalene diimide copolymer as an acceptor for all-polymer solar cells

An alternating copolymer (P(IDT-NDI)) containing indacenodithienothiophene (IDT) and naphthalene diimide (NDI) units was synthesized for application as an acceptor material in all-polymer solar cells (all-PSCs). The polymer possesses a low bandgap of 1.51 eV, a suitable LUMO level of −3.84 eV and a HOMO level of −5.75 eV for use as an acceptor material instead of PCBM. Three conjugated polymers including J50 and J51 with a medium bandgap (ca. 1.9 eV) and PTB7-Th with a low bandgap (1.59 eV) were selected as donor materials for the investigation of the photovoltaic performance of the nonfullerene acceptor P(IDT-NDI). The champion all-PSCs with P(IDT-NDI) as an acceptor demonstrated power conversion efficiencies of 3.63%, 4.12% and 5.33% for the polymer donors PTB7-Th, J50 and J51, respectively. The results indicate that the complementary absorption of the polymer donor with polymer acceptor is very important for high performance all-PSCs and P(IDT-NDI) is a promising polymer acceptor for all-PSCs.

Synthesis and optoelectronic properties of new D–A copolymers based on fluorinated benzothiadiazole and benzoselenadiazole

Two donor–acceptor (D–A) copolymers (PBDTBT and PBDTBSe) based on bithienyl-benzodithiophene (BDT) as the donor (D) unit, fluorinated benzothiadiazole (BT) or benzoselenadiazole (BSe) as the acceptor (A) unit, were designed and synthesized. The optoelectronic properties of the two polymers, including the absorption spectra, electronic energy levels, hole mobility and photovoltaic properties, were fully studied and compared. The effects of the replacement of S by Se in the BT unit and F substitution on their optoelectronic properties were investigated in detail. The replacement of S by Se in the benzothiadiazole unit red-shifted the absorption spectra and increased the hole mobility of the D–A copolymers. Polymer solar cells (PSCs) based on the polymers as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as the acceptor were fabricated, and the photovoltaic performance of the PSCs was optimized by optimizing the device fabrication conditions, including the solvent, donor–acceptor blend ratio, thermal annealing and solvent additive. The highest power conversion efficiency (PCE) of the optimized PSCs based on the polymers reached 5.06% and 2.20% for PBDTBT and PBDTBSe, respectively. The lower photovoltaic performance of the PSCs based on PBDTBSe is due to the poorer morphology of its active layer.