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 (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 (1.3×10 cm ). 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.

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 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.

Suppression of carrier recombination is critically important in realizing high-efficiency polymer solar cells. Herein, it is demonstrated difluoro-substitution of thiophene conjugated side chain on donor polymer can suppress triplet formation for reducing carrier recombination. A new medium bandgap 2D-conjugated D-A copolymer J91 is designed and synthesized with bi(alkyl-difluorothienyl)-benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit, for taking the advantages of the synergistic fluorination on the backbone and thiophene side chain. J91 demonstrates enhanced absorption, low-lying highest occupied molecular orbital energy level, and higher hole mobility, in comparison with its control polymer J52 without fluorination on the thiophene side chains. The transient absorption spectra indicate that J91 can suppress the triplet formation in its blend film with n-type organic semiconductor acceptor m-ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2,3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene). With these favorable properties, a higher power conversion efficiency of 11.63% with high V of 0.984 V and high J of 18.03 mA cm is obtained for the polymer solar cells based on J91/m-ITIC with thermal annealing. The improved photovoltaic performance by thermal annealing is explained from the morphology change upon thermal annealing as revealed by photoinduced force microscopy. The results indicate that side chain engineering can provide a new solution to suppress carrier recombination toward high efficiency, thus deserves further attention.

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 (J ) of 18.41 mA cm , significantly higher than that of the device based on J71:ITCPTC (11.63% with a J of 17.52 mA cm ). The higher J of the PSC based on J71:MeIC can be attributed to more balanced μ /μ , 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.

We synthesized two wide bandgap A−D−A structured p-type organic semiconductor (p-OS) small molecules with weak electron-withdrawing ester end groups: SM1 with cyano group (CN) on the ester group and SM2 without the CN group. SM1 showed stronger absorption, lower-lying HOMO energy level, and higher hole mobility in comparison with that of SM2 without the CN groups. The all-smallmolecule organic solar cell (SM-OSC) with SM1 as donor and a narrow bandgap n-OS IDIC as acceptor demonstrated a high power conversion efficiency (PCE) of 10.11% and a high fill factor (FF) of 73.55%, while the PCE of the device based on SM2:IDIC is only 5.32% under the same device fabrication condition. The PCE of 10.11% and FF of 73.55% for the SM1-based device are the highest values for the nonfullerene SM-OSCs reported in the literature so far. The results indicate that the cyano substitution in SM1 plays an important role in improving the photovoltaic performance of the p-OS donors in the nonfullerene SM-OSC. In addition, the photoinduced force microscopy (PiFM) was first used in OSCs to characterize the morphology of its donor/acceptor blend active layer.

Two medium-bandgap p-type organic small molecules H21 and H22 with an alkylsily-thienyl conjugated side chain on benzo[1,2-b:4,5-b']dithiophene central units are synthesized and used as donors in all-small-molecule organic solar cells (SM-OSCs) with a narrow-bandgap n-type small molecule 2,2'-((2Z,2'Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) as the acceptor. In comparison to H21 with 3-ethyl rhodanine as the terminal group, H22 with cyanoacetic acid esters as the terminal group shows blueshifted absorption, higher charge-carrier mobility and better 3D charge pathway in blend films. The power conversion efficiency (PCE) of the SM-OSCs based on H22:IDIC reaches 10.29% with a higher open-circuit voltage of 0.942 V and a higher fill factor of 71.15%. The PCE of 10.29% is among the top efficiencies of nonfullerene SM-OSCs reported in the literature to date.