Fenghong Li

A water-soluble metallophthalocyanine derivative as a cathode interlayer for highly efficient polymer solar cells

A novel organic small molecule water-soluble poly-N-alkylpyridine substitued metallophthalocyanine derivative VOPc(OPyCH3I)8, namely 2,3,9,10,16,17,23,24-octakis-[N-methyl-(3-pyridyloxy)] vanadylphthalocyanine iodide (1 : 8), was synthesized and applied in polymer solar cells (PSCs). Notably, a power conversion efficiency (PCE) of 8.12% for the working area of 2 × 2 mm2 and a PCE of 7.23% for the working area of 4 × 4 mm2 have been achieved in the PSCs with this molecule as a cathode interlayer. They are comparable with the higher values of PCE of the PSCs reported currently, indicating that VOPc(OPyCH3I)8 is a new promising candidate as a good cathode interlayer for highly efficient PSCs.

Highly efficient polymer solar cells based on a universal cathode interlayer composed of metallophthalocyanine derivative with good film-forming property

A new cathode interlayer (CIL) material metallophthalocyanine (MPc) derivative 1,4,8,11,15,18,22,25-octaoctyloxy-2,3,9,10,16,17,23,24-octa-[N-methyl-(3-pyridyloxy)] zinc-ylphthalocyanine iodide (1 : 8) (ZnPc(OC8H17OPyCH3I)8) was synthesized and applied in polymer solar cells (PSCs) based on PTB7:PC71BM (PTB7 = thieno[3,4-b]thiophene/benzodithiophene, PC71BM = [6,6]-phenyl C71-butyric acidmethyl ester), P3HT:PC61BM (P3HT = poly(3-hexylthiophene), PC61BM = [6,6]-phenyl C61-butyric acidmethyl ester) or PCDTBT:PC71BM (PCDTBT = poly[N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]) as an active layer. As a result, power conversion efficiency (PCE) values of the PSCs were 8.52%, 4.02% and 6.88%, respectively, which are much higher than those of corresponding PSCs with the Al-only cathode. It indicates that ZnPc(OC8H17OPyCH3I)8 is a new promising candidate as a universal CIL for highly efficient PSCs. Compared to VOPc(OPyCH3I)8 (2,3,9,10,16,17,23,24-octakis-[N-methyl-(3-pyridyloxy)] vanadylphthalocyanine iodide (1 : 8)), the PSC with ZnPc(OC8H17OPyCH3I)8 as a CIL has higher short-circuit current and fill factor because ZnPc(OC8H17OPyCH3I)8 can form a better, denser, and more uniform film on the active layer than VOPc(OPyCH3I)8 as demonstrated by atomic force microscopy (AFM), energy-dispersive spectrum mapping on scan electron microscopy (SEM-EDS mapping) and contact angle measurements.

N-type cathode interlayer based on dicyanomethylenated quinacridone derivative for high-performance polymer solar cells

A new π-conjugated electrolyte bis(dicyanomethylene)-quinacridone with two octyl-pyridium (DCNQA-PyBr) has been synthesized and employed as a solution-processed cathode interlayer (CIL) for polymer solar cells (PSCs). The devices exhibited simultaneously increased open-circuit voltage (Voc), short-circuit current (Jsc) and fill factor (FF). Overall, the PSCs with PCDTBT (poly[N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]) as a donor and PC71BM ([6,6]-phenyl C71-butyric acid methyl ester) as an acceptor incorporating a 13 nm DCNQA-PyBr interlayer exhibit a power conversion efficiency (PCE) of 6.96%, which is 1.3 times of that of the Al-only device. Most importantly, compared to the reference π-conjugated electrolyte QA-PyBr, DCNQA-PyBr shows much improved electron transport ability and conductivity. As a result, the DCNQA-PyBr based devices only show a slight decrease in electron transport upon increasing the thickness of the CIL, thus allowing a high PCE with a wide CIL thickness range from 5 nm to 40 nm. Furthermore, introducing DCNQA-PyBr as a CIL into the devices based on P3HT:PC61BM (P3HT = poly(3-hexylthiophene), PC61BM = [6,6]-phenyl C61-butyric acid methyl ester) and PTB7:PC71BM (PTB7 = polythieno[3,4-b]-thiophene-co-benzodithiophene) also leads to significantly enhanced device performance, showing high PCEs of 3.91% and 8.23%, respectively. These results confirm DCNQA-PyBr to be a promising CIL material for solution-processed large-area PSCs.

Improving the efficiency of polymer solar cells via a treatment of methanol : water on the active layers

Significant improvement in the power conversion efficiency (PCE) of polymer solar cells (PSCs) has been observed when the active layer was treated with a mixture of methanol and water (M : W). For the PSCs based on ITO/PEDOT:PSS/PCDTBT:PC71BM/Al, the PCE was 6.58% when the surface of the PCDTBT:PC71BM film was treated with pure methanol. However M : W (6 : 1) treatment on the PCDTBT:PC71BM film could further improve the PCE to 7.44% which is even higher than the PCE values of the PSCs with LiF (6.00%) or PFN (6.88%) as a cathode interlayer. Similarly in the PSCs based on ITO/PEDOT:PSS/PTB7:PC71BM/LiF/Al, M : W (6 : 1) treatment on the surface of the PTB7:PC71BM film can increase the PCE to 8.47% while the PCE values of the PSCs with methanol treatment and without any treatment on the PTB7:PC71BM film are 8.14% and 7.41%, respectively. Combined contact angle measurements and X-ray photoemission spectroscopy depth profiling demonstrated that the M : W treatments resulted in a more favorable redistribution of the polymer and PC71BM in the active layer because evaporation of the solvents drove PC71BM to migrate from the interior of the blend film to the top (air) surface.

Alcohol/water-soluble porphyrins as cathode interlayers in high-performance polymer solar cells

Three alcohol/water-soluble porphyrins (Zn-TPyPMeI:zinc(II) meso-tetra(N-methyl-4-pyridyl) porphyrin tetra-iodide, Zn-TPyPAdBr:zinc(II) meso-tetra[1-(1-adamantylmethyl ketone)-4-pyridyl] porphyrin tetra-bromide and MnCl-TPyPAdBr:man-ganese(III) meso-tetra[1-(1-adamantylmethyl ketone)-4-pyridyl] porphyrin tetra-bromide were employed as cathode interlayers to fabricate polymer solar cells (PSCs). The PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) and PCDTBT (poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)])-blend films were used as active layers in polymer solar cells (PSCs). The PSCs with alcohol/water-soluble porphyrins interlayer showed obviously higher power conversion efficiency (PCE) than those without interlayers. The highest PCE, 6.86%, was achieved for the device with MnCl-TPyPAdBr as an interlayer. Ultraviolet photoemission spectroscopic (UPS), carrier mobility, atomic force microscopy (AFM) and contact angle (θ) characterizations demonstrated that the porphyrin molecules can result in the formation of interfacial dipole layer between active layer and cathode. The interfacial dipole layer can obviously improve the open-circuit voltage (Voc) and charge extraction, and sequentially lead to the increase of PCE.

Metallophthalocyanine derivatives utilized as cathode interlayers for polymer solar cells: a practical approach to prepare a uniform film

Three metallophthalocyanine derivatives: 2,3,9,10,16,17,23,24-octakis-[N-ethyl-(3-pyridyloxy)]vanadylphthalocyanine bromine (1 : 8) (VOPc(OPyC2H5Br)8), 2,3,9,10,16,17,23,24-octakis-[N-butyl-(3-pyridyloxy)]vanadylphthalocyanine bromine (1 : 8) (VOPc(OPyC4H9Br)8) and 2,3,9,10,16,17,23,24-octakis-[N-hexyl-(3-pyridyloxy)]vanadylphthalocyanine bromine (1 : 8) (VOPc(OPyC6H13Br)8) were synthesized and applied in polymer solar cells (PSCs) based on PTB7:PC71BM (PTB7 = thieno[3,4-b]thiophene/benzodithiophene, PC71BM = [6,6]-phenyl C71-butyric acidmethyl ester) as an active layer. As a result, the highest power conversion efficiency (PCE) value of the PSCs is 7.99% for the device with VOPc(OPyC2H5Br)8 as a cathode interlayer. Although increasing the alkyl side chains attached to the pyridine functional groups of phthalocyanine leads to a slight decrease of PCE, VOPc(OPyC6H13Br)8 can form a better, denser and more uniform film on the active layer as demonstrated by atomic force microscopy, energy dispersive spectrum mapping and contact angle measurements.

Benzothiadiazole-oligothiophene flanked dicyanomethylenated quinacridone for non-fullerene acceptors in polymer solar cells

A new class of benzothiadiazole-oligo(3-hexylthiophene) flanked dicyanomethylenated quinacridone derivatives DCNQA-BT-Tn (n = 1–3) has been designed and synthesized in good yield by iterative bromination and Suzuki coupling reactions, followed by Knoevenagel condensation. The photophysical and electrochemical properties of the compounds have been fully investigated. They showed intense and broad absorption at the visible and near-infrared regions, a suitable LUMO energy level at about −3.7 eV, and the appropriate film-formation properties. The bulk heterojunction (BHJ) photovoltaic devices based on the blend film of P3HT/DCNQA-BT-T1 (P3HT = poly(3-hexylthiophene)) showed a power conversion efficiency (PCE) of 0.73% under AM 1.5, 100 mW cm−2 irradiation. Compared with the traditional P3HT/PC61BM (PC61BM = [6,6]-phenyl-C61-butyric acid methyl ester) device, the P3HT/DCNQA-BT-T1 device displayed intense absorption in the near-infrared region (650–750 nm) and effectively contributed to the photocurrent. These results indicate that DCNQA derivatives are promising non-fullerenes acceptors for the BHJ polymer solar cells.

Alcohol-Soluble Isoindigo Derivative IIDTh-NSB as a Novel Modifier of ZnO in Inverted Polymer Solar Cells

Alcohol-soluble isoindigo derivative with thiophene groups and sulfobetaine zwitterions, IIDTh-NSB was applied as a novel modifier of ZnO in inverted polymer solar cells (i-PSCs). When IIDTh-NSB (0.2 mg/mL) was spin-coated on ZnO as an electron transport layer (ETL), power conversion efficiency (PCE) of the PTB7:PC71BM based i-PSCs reached 8.88%, which is a 20% improvement of that of 7.40% for the device with the ZnO-only ETL. If ZnO was doped by IIDTh-NSB of 1.0 wt %, the PCE of 8.50% could be achieved in the i-PSCs. Combined measurements of capacitance-voltage characteristics, carrier mobility, and photocurrent density-effective voltage characteristics revealed that incorporating IIDTh-NSB as the modifier of ZnO by coating or doping enhanced the built-in potential, charge carrier density and mobility, exciton dissociation, and charge carrier extraction in the i-PSCs because of the improved interfacial contact between the photoactive layer and ZnO as shown in water contact angle measurements and atomic force microscopy images. Finally, impedance spectroscopy investigation provided strong lines of evidence that incorporating IIDTh-NSB as the modifier of ZnO led to the great enhancement in short-circuit current density and fill factor. Furthermore, all the devices with IIDTh-NSB as a modifier of ZnO presented better stability than the device with ZnO-only. These findings suggest that IIDTh-NSB is an effective and competitive material for modification of ZnO in the i-PSCs.

Anode Engineering of Highly Efficient Polymer Solar Cells Using Treated ITO

ITO substrates were treated with organic solvent cleaning(OSC), SC1 treatment[V(NH4OH):V(H2O2): V(H2O)=1:1:5], O2 plasma and UV ozone, respectively. Combined investigations of atom force microscopy(AFM), water contact angle measurements, ultraviolet photoemission spectroscopy(UPS) and X-ray photoemission spectroscopy(XPS) demonstrated that UV ozone treatment could give rise to the smoothest surface, the most hydrophilic property and the highest work function(WF) of ITO due to the removal of hydrophobic C―O impurity from the ITO surface and the enrichments of more oxygen on the ITO surface. When PEDOT:PSS film[(poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)] was deposited on the ITO substrates treated with UV ozone, it showed a lower root-mean- square roughness in AFM images, a higher transmission in UV-Vis transmission spectra and a higher WF in UPS spectra than the PEDOT:PSS films deposited on the ITO substrates treated by other three methods. As a result, the power conversion efficiency of polymer solar cells(PSCs) based on PTB7:PC71BM as an active layer and ITO treated by UV ozone as an anode can reach 8.48% because of the simultaneously improved short circuit current, open circuit voltage and fill factor compared to the PSCs with ITO treated with other three methods.

A naphthodithieno[3,2-b]thiophene-based copolymer as a novel third component in ternary polymer solar cells with a simultaneously enhanced open circuit voltage, short circuit current and fill factor

A dialkoxyl-substituted naphthodithieno[3,2-b]thiophene-based copolymer (PV12) was applied as a third component for ternary polymer solar cells (PSCs) based on PCDTBT1−x:PV12x:PC71BM. In Al-only ternary PSCs with x = 0.15, a power conversion efficiency (PCE) of 6.73% was achieved due to simultaneous enhancement in the open circuit voltage, short circuit current and fill factor, which is much higher than the PCE of 5.28% for Al-only binary PSCs based on PCDTBT:PC71BM and 2.93% for Al-only binary PSCs based on PV12:PC71BM. The PCE reached 7.69% when IIDTh-NSB as a cathode interlayer was introduced between the ternary active layer and Al. Ultraviolet photoemission spectroscopy measurements demonstrated a cascade-type energy level alignment at the ternary PCDTBT(D1)/PV12(D2)/PC71BM(A) junction. As a result, the voltage limitation was overcome at the D1/D2/A junction. Ultraviolet-visible absorption and external quantum efficiency spectra showed that PV12 had complementary absorption to PCDTBT in the solar spectrum. Atomic force microscopy and transmission electron microscopy images showed that the morphology and phase separation of the active layer were optimized by adding PV12. Photoluminescence spectral investigations suggested that the Forster resonance energy transfer from PCDTBT to PV12 occurred in the ternary PSCs under illumination. Finally, the PCEs of the ternary PSCs are not as sensitive to the thickness of the active layer as those of the binary PSCs, which is important for the roll-to-roll coating processing of organic photovoltaic modules.