Yankang Yang

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.

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.

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.

Naphthalenediimide‐alt‐Fused Thiophene D–A Copolymers for the Application as Acceptor in All‐Polymer Solar Cells

Three n-type alternating D-A copolymers based on a naphthalenediimide (NDI) acceptor (A) unit and three different donor (D) units with varied electron-donating strength including thiophene (P(NDI-T)), thieno[3,2-b]thiophene (P(NDI-TT)), and thieno[3,2-b;4,5-b]dithiophene (P(NDI-TDT)), were synthesized, for the application as acceptor materials in all-polymer solar cells (all-PSCs). The effect of the donor units of thiophene, thienothiophene (TT) and thienodithiophene (TDT) on the physicochemical and photovoltaic properties of the n-type D-A copolymers was systematically investigated. It was found that the absorption spectrum is red-shifted and the energy band gap (Eg ) is reduced for the NDI-based D-A copolymers with increasing number of thiophene rings in the thiophene or fused thiophene donor units. All-PSCs were fabricated with the medium band gap conjugated polymer J51 (Eg of ca 1.9 eV) as polymer donor and the n-type D-A copolymers as acceptor. The power conversion efficiency reached 2.59 %, 3.70 % and 5.10 % for the all-PSCs with P(NDI-T), P(NDI-TT), and P(NDI-TDT) as acceptor, respectively. The results indicate that a larger conjugated fused molecular plane with more thiophene rings as donor units in the NDI-based D-A copolymers is beneficial to reduce the band gap, broaden the absorption and enhance the photovoltaic performance of n-type D-A copolymer acceptors.

Effect of furan π-bridge on the photovoltaic performance of D-A copolymers based on bi(alkylthio-thienyl)benzodithiophene and fluorobenzotriazole

The medium band gap donor-acceptor (D-A) copolymer J61 based on bi(alkylthio-thienyl)benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit and thiophene as π-bridge has demonstrated excellent photovoltaic performance as donor material in nonfullerene polymer solar cells (PSCs) with narrow bandgap n-type organic semiconductor ITIC as acceptor. For studying the effect of π-bridges on the photovoltaic performance of the D-A copolymers, here we synthesized a new D-A copolymer J61-F based on the same donor and acceptor units as J61 but with furan π-bridges instead of thiophene. J61-F possesses a deeper the highest occupied molecular orbital (HOMO) level at −5.45 eV in comparison with that (−5.32 eV) of J61. The non-fullerene PSCs based on J61-F:ITIC exhibited a maximum power conversion efficiency (PCE) of 8.24% with a higher open-circuit voltage (Voc) of 0.95 V, which is benefitted from the lower-lying HOMO energy level of J61-F donor material. The results indicate that main chain engineering by changing π-bridges is another effective way to tune the electronic energy levels of the conjugated D-A copolymers for the application as donor materials in non-fullerene 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.