Lingwei Xue

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.

All‐Polymer Solar Cells Based on Absorption‐Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%

High-efficiency all-polymer solar cells with less thickness-dependent behavior are demonstrated by using a low bandgap n-type conjugated polymer N2200 as acceptor and an absorption-complementary difluorobenzotriazole-based medium-bandgap polymer J51 as donor.

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.

Constructing a Strongly Absorbing Low-Bandgap Polymer Acceptor for High-Performance All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) offer unique morphology stability for the application as flexible devices, but the lack of high-performance polymer acceptors limits their power conversion efficiency (PCE) to a value lower than those of the PSCs based on fullerene derivative or organic small molecule acceptors. We herein demonstrate a strategy to synthesize a high-performance polymer acceptor PZ1 by embedding an acceptor-donor-acceptor building block into the polymer main chain. PZ1 possesses broad absorption with a low band gap of 1.55 eV and high absorption coefficient (1.3×105  cm-1 ). The all-PSCs with the wide-band-gap polymer PBDB-T as donor and PZ1 as acceptor showed a record-high PCE of 9.19 % for the all-PSCs. The success of our polymerization strategy can provide a new way to develop efficient polymer acceptors for all-PSCs.

New generation perovskite solar cells with solution-processed amino-substituted perylene diimide derivative as electron-transport layer

In this study, for the first time, we introduced amino-substituted perylene diimide derivative (N-PDI) as an alternative electron transport layer (ETL) to replace the commonly used TiO2 in planar heterojunction perovskite solar cells. Two types of device structures, i.e., glass/FTO/N-PDI/CH3NH3PbI3−xClx/spiro-MeOTAD/Au and polyethylene terephthalate PET/ITO/N-PDI/CH3NH3PbI3−xClx/spiro-MeOTAD/Au, were fabricated on both rigid and flexible substrates using room-temperature solution processing technique. Based on the proposed device structures, power conversion efficiency (PCE) of 17.66% was obtained based on glass/FTO rigid substrates, and a PCE of 14.32% was achieved based on PET/ITO flexible substrates. The results showed that the terminal amino group in N-PDI enhanced the wetting capability of the surfaces to perovskite, reduced the surface work function of the FTO substrate and passivated the surface trap states of the perovskite films. These results confirm that small molecule semiconductor N-PDI can serve as an effective electron-transport material for achieving high-performance perovskite solar cells and draw molecular design guidelines for electron-selective contacts with perovskite.

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.

Random terpolymer with a cost-effective monomer and comparable efficiency to PTB7-Th for bulk-heterojunction polymer solar cells

A new random terpolymer PTB7-Th-T2 has been designed and synthesized by incorporating a significantly lower cost monomer, 2,2′-bithiophen, for application as a donor material in polymer solar cells (PSCs). By replacing 25 mol% of extremely expensive 3-fluorothieno[3,4-b]thiophene-2-carboxylate monomer in the famous PTB7-Th by a >30-times lower cost bithiophene, the new terpolymer PTB7-Th-T2 shows a comparable HOMO energy level and increased absorption intensity in the wide wavelength range of 400–700 nm. In addition, in a polymer/PC71BM blended film, the hole mobility has improved from 1.67 × 10−4 for PTB7-Th to 2.49 × 10−4 cm2 V−1 s−1 for PTB7-Th-T2. A power conversion efficiency (PCE) of 7.05% has been achieved in a polymer bulk heterojunction photovoltaic device with the structure of ITO/PEDOT:PSS/PTB7-Th-T2:PC71BM (1:1.5, w/w)/Ca/Al by using 3% of 1,8-diiodooctane (DIO) as a solvent additive. Furthermore, through introducing an amino-substituted perylene diimide (PDIN) as the cathode interlayer, a high fill factor (FF) of 67.38% and PCE of 8.19% have been obtained; these values are higher than those of the control polymer PTB7-Th of 63.26% and 7.93%, respectively at the same device conditions.

Insights into the working mechanism of cathode interlayers in polymer solar cells via [(C8H17)4N]4[SiW12O40]

A low-cost ( 90%), alcohol soluble surfactant-encapsulated polyoxometalate complex [(C8H17)4N]4[SiW12O40] has been synthesized and utilized as a cathode interlayer (CIL) in polymer solar cells (PSCs). A power conversion efficiency of 10.1% can be obtained for PSCs based on PTB7-Th (poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]-dithiophene][3-fluoro-2[(2-ethylhexyl) carbonyl] thieno [3,4-b]-thiophenediyl]]):PC71BM ([6,6]-phenyl C71-butyric acidmethyl ester) due to the incorporation of [(C8H17)4N]4[SiW12O40]. Combined measurements of current density–voltage characteristics, transient photocurrent, charge carrier mobility and capacitance–voltage characteristics demonstrate that [(C8H17)4N]4[SiW12O40] can effectively increase the built-in potential, charge carrier density and mobility and accelerate the charge carrier extraction in PSCs. Most importantly, the mechanism of using [(C8H17)4N]4[SiW12O40] as the CIL is further brought to light by X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) of the metal/[(C8H17)4N]4[SiW12O40] interface. The findings suggest that [(C8H17)4N]4[SiW12O40] not only decreased the work function of the metal cathodes but also was n-doped upon contact with the metals, which provide insights into the working mechanism of the CILs simultaneously improving the open circuit voltage, short circuit current and fill factor in the PSCs.