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In recent years, conjugated polymers have attracted great attention in the application as photovoltaic donor materials in polymer solar cells （PSCs）. Broad absorption, lower-energy bandgap, higher hole mobility, relatively lower HOMO energy levels, and higher solubility are important for the conjugated polymer donor materials to achieve high photovoltaic performance. Side-chain engineering plays a very important role in optimizing the physicochemical properties of the conjugated polymers. In this article, we review recent progress on the side-chain engineering of conjugated polymer donor materials, including the optimization of flexible side-chains for balancing solubility and intermolecular packing （aggregation）, electron-withdrawing substituents for lowering HOMO energy levels, and two-dimension （2D）-conjugated polymers with conjugated side-chains for broadening absorption and enhancing hole mobility. After the molecular structural optimization by side-chain engineering, the 2D-conjugated polymers based on benzodithiophene units demonstrated the best photovoltaic performance, with powerconversion efficiency higher than 9%.

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 n-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 n-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 n-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 n-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.

The open-circuit voltage（V_（oc））is one of the important parameters that influence the power conversion efficiency（PCE）of polymer solar cells.Its value is mainly determined by the energy level offset between the highest occupied molecular orbital（HOMO）of the donor and the lowest unoccupied molecular orbital（LUMO）of the acceptor.Therefore,decreasing the HOMO value of the polymer could lead to a high Voc and thus increasing the cell efficiency.Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene（BDT）unit.The polymer with the methoxyl functionl group（POBDT（S）-T1）exhibited a HOMO value of-5.65 eV,which is deeper than that（-5.52 eV）of polymer without methoxyl unit（PBDT（S）-T1）.As a result,POBDT（S）-T1-based solar cells show a high V（oc of 0.98 V and PCE of 9.2%.In contrast,PBDT（S）-T1-based devices show a relatively lower Voc of 0.89 V and a moderate PCE of 7.4%.The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.

Four metallated conjugated oligothiophenes, S-1, S-2, S-3 and S-4, with platinum（II） aryleneethynylenes as the electron-rich building block were synthesized to investigate their physicochemical and photovoltaic properties. These small molecules possess fairly low-lying HOMO energy levels which match with the LUMO energy level of the electron acceptor PC70BM （[6,6]-phenyl-C71-butyric acid methyl ester）. Using the simple process of spin-coating solution fabrication technique, S-I：PC70BM （1：4, w/w） based organic solar cells exhibiting a high Voc of 0.913 V, with a PCE value of 0.88% were developed. In contrast, the OSC device based on S-2：PC70BM （3：7, w/w） displayed a higher PCE of 1.59% with a higher Jsc value of 5.89 mA cm^-2. The device based on S-4：PCToBM （1：4, w/w） exhibited a PCE value of 1.56%, with a Voc of 0.917 V.

This paper reports the synthesis and characteristics of a series of alkyl-substituted planar polymers. The physical properties are carefully tuned to optimize their photovoltaic performance. Depending on the length of soluble alkyl side chains which modify the structural order and orientation substantially in polymer backbones, the device performance can be improved significantly. The tuning of HOMO energy levels optimized polymers’ spectral coverage of absorption and their hole mobility, as well as miscibility with fullerene; all these efforts enhanced polymer solar cell performances. The short-circuit current, Jsc for polymer solar cells was increased by adjusting polymer chain packing ability. It was found that films with well distributed polymer/fullerene interpenetrating network exhibit improved solar cell conversion efficiency. Enhanced efficiency up to 5.8% has been demonstrated. The results provide important insights about the roles of flexile chains in structure-property relationship for the design of new polymers to be used in high efficient solar cells.

Donor-acceptor （D-A）-conjugated polymers P（BT-C1） and P（BT-C2）, with dithieno[2,3-b;7,6-b]carbazole （C1） or dithi- eno[3,2-b;6,7-b]carbazole （C2） as D-unit and benzothiadiazole （BT） as A-unit, were synthesized. The optical bandgaps of the polymers are similar （1.84 and 1.88 eV, respectively）. The structures of donor units noticeably influence the energy levels and backbone curvature of the polymers. P（BT-C1） shows a large backbone curvature; its highest occupied molecular orbital （HOMO） energy level is -5.18 eV, whereas P（BT-C2） displays a pseudo-straight backbone and has a HOMO energy level of -5.37 eV. The hole mobilities of the polymers without thermal annealing are 1.9×10^-3 and 2.7×10^-3 cm^2 V-1 s^-1 for P（BT-C1） and P（BT-C2）, respectively, as measured by organic thin-film transistors （OTFTs）. Polymer solar cells using P（BT-C1） and P（BT-C2） as the donor and phenyl-Cyl-butyric acid methyl ester （PCyLBM） as the acceptor were fabricated. Power conversion efficiencies （PCEs） of 4.9% and 5.0% were achieved for P（BT-C1） and P（BT-C2）, respectively. The devices based on P（BT-C2） exhibited a higher Voc due to the deeper HOMO level of the polymer, which led to a slightly higher PCE.

4,4＇-dibromo-2-nitro-biphenyl and 4,4＇-dibromo-2,3＇-dinitro-biphenyl have been synthesized via nitration reaction with 4,4＇-dibromobiphenyl as the raw material. Three novel thiophene derivatives, 4, 4＇-di（4-hexyl-thiophen-2-yl）biphenyl, 4,4＇-di（4-hexyl-thiophen-2-yl）-2- nitro-biphenyl and 4,4＇-di（4- hexyl-thiophen-2-yl）-2,3＇-dinitrobiphenyl were synthesized through Stille coupling reaction, followed by polymerization in the presence of FeC13, respectively. UV-vis absorption spectra, fluorescence spectra, photoluminescence spectra and electrochemical properties of the polymers were investigated. And the band-gap （Eg）, HOMO orbital energy （EHCMO）, and LUMO orbital energy （EUJMO） of the polymers were calculated. Among the polymers, polymer PBTN and PBTD show lower band-gap （2.67 and 2.63 eV）, lower HOMO energy level （-5.38 and -5.4 eV） and broader wavelength （432 and 438 nm） than that of polymer PBTB （2.69 eV, -5.36 eV and 424 nm） with incorporation of one nitro group or two nitro groups in the main chain, respectively.

A new silole monomer with two 4-（N,N-dimethylamino）phenyl substitutions on silicon atom as designed and synthesized. Three copolymers PF-N-HPS1, PF-N-HPS10 and PF-N-HPS20 were then obtained by copolymerizations of 2,7-fluorene deriva- tives with the silole monomer at feed ratios of 1%, 10%, and 20%. Their UV-vis absorption, electrochemical, photolumines- cent, and electroluminescent （EL） properties were investigated. PF-N-HPS possessed HOMO levels of -5.25-5.58 eV, and showed green emissions. Using PF-N-HPS as the emissive layer, three different polymer light-emitting diodes were fabricated as device A with ITO/PEDOT/PF-N-HPS/A1, device B with ITO/PEDOT/PF-N-HPS/Ba/A1, and device C with ITO/PEDOT/ PF-N-HPS/TPBI/Ba/A1. For the device A, PF-N-HPS only showed very low EL efficiency of 0.06-0.33 cd/A, indicating that the A1 cathode could not inject electron efficiently to the emissive polymers containing the 4-（N,N-dimethylamino）phenyl groups. For the device B, low work function Ba supplied better electron injections, and the EL efficiency could be improved to 0.85-1.44 cd/A. TPBI with a deep HOMO level of -6.2 eV could enhance electron transport and hole blocking. Thus modi- fied recombinations and largely elevated EL efficiency of 4.56-7.96 cd/A were achieved for the device C. The separation of the emissive layer and metal cathode with the TPBI layer may also suppress exciton quenching at the cathode interface.