Jegadesan Subbiah

Dithienogermole as a fused electron donor in bulk heterojunction solar cells.

We report the synthesis and bulk heterojunction photovoltaic performance of the first dithienogermole (DTG)-containing conjugated polymer. Stille polycondensation of a distannyl-DTG derivative with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD) results in an alternating copolymer which displays light absorption extending to 735 nm, and a higher HOMO level than the analogous copolymer containing the commonly utilized dithienosilole (DTS) heterocycle. When polyDTG-TPD:PC(70)BM blends are utilized in inverted bulk heterojunction solar cells, the cells display average power conversion efficiencies of 7.3%, compared to 6.6% for the DTS-containing cells prepared in parallel under identical conditions. The performance enhancement is a result of a higher short-circuit current and fill factor in the DTG-containing cells, which comes at the cost of a slightly lower open circuit voltage than for the DTS-based cells.

Toward Large Scale Roll‐to‐Roll Production of Fully Printed Perovskite Solar Cells

Fully printed perovskite solar cells are demonstrated with slot-die coating, a scalable printing method. A sequential slot-die coating process is developed to produce efficient perovskite solar cells and to be used in a large-scale roll-to-roll printing process. All layers excluding the electrodes are printed and devices demonstrate up to 11.96% power conversion efficiency. It is also demonstrated that the new process can be used in roll-to-roll production.

A molecular nematic liquid crystalline material for high-performance organic photovoltaics

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

The effect of molybdenum oxide interlayer on organic photovoltaic cells

Both small molecule and polymer photovoltaic cells were fabricated with molybdenum oxide interlayer at the indium tin oxide electrode. Enhancement in power efficiencies was observed in both small molecule and polymer cells. Specifically, the power conversion efficiencies of small molecule cells with the molybdenum oxide interlayer were enhanced by a maximum of 38% due to a significant enhancement in the fill factor. The improved fill factor is attributed to the reduction in series resistance. Our ultraviolet photoemission spectroscopy data indicate that the formation of band bending and the built-in field at the interface due to the interlayer leads to enhancement in hole extraction from the photoactive layer toward the anode.

Organic Solar Cells Using a High‐Molecular‐Weight Benzodithiophene–Benzothiadiazole Copolymer with an Efficiency of 9.4%

A high molecular weight donor-acceptor conjugated polymer is synthesized using the Suzuki polycondensation method. Using this polymer, a single-junction bulk-heterojunction solar cell is fabricated giving a power conversion efficiency of 9.4% using a fullerene-modified ZnO interlayer at the cathode contact.

Energy level evolution of air and oxygen exposed molybdenum trioxide films

The evolution of electronic energy levels of controlled air and oxygen exposed molybdenum trioxide (MoO3) films has been investigated with ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and x-ray photoemission spectroscopy. We found that while most of the electronic levels of as deposited MoO3 films remained largely intact, the reduction in the work function (WF) was substantial. The gradual surface WF change from 6.8 to 5.3 eV was observed for air exposed film, while oxygen exposed film the surface WF saturated at ∼5.7 eV. Two distinct stages of exposure are observed, the first dominated by oxygen adsorption for 1013 L.

Energy level evolution of molybdenum trioxide interlayer between indium tin oxide and organic semiconductor

The thickness dependance of molybdenum trioxide (MoO3) interlayer between conducting indium tin oxide (ITO) and chloro-aluminum pthalocyanine (AlPc-Cl) has been investigated with ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy. It was found that the MoO3 interlayer substantially increased the surface workfunction (WF). The increase was observed to saturate at 20 A of MoO3 coverage. The increased WF results in hole accumulation and a band-bendinglike situation in the subsequently deposited AlPc-Cl. From these observations, a possible explanation is deduced for the observed reduction in series resistance by the insertion of the MoO3 insulating layer.

An isoindigo and dithieno[3,2-b:2′,3′-d]silole copolymer for polymer solar cells

The copolymer of isoindigo and dithieno[3,2-b:2′,3′-d]silole, P(iI-DTS), is reported as prepared by Stille coupling to yield a soluble high molecular weight material absorbing light throughout the visible spectrum up to 800 nm. With deep HOMO and LUMO energy levels (high ionization potential and electron affinity) electrochemically measured at −5.55 and −3.95 eV respectively, this new p-type polymer enabled the fabrication of high open circuit voltage polymer solar cells when blended with fullerene derivatives. By employing solvent additives, the morphology of the devices was optimized to yield power conversion efficiencies of 4%.

MoO3/poly(9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine) double-interlayer effect on polymer solar cells

A double interlayer composed of MoO3 and poly(9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine) (TFB) was used as an anode contact for bulk heterojunction polymer solar cells. Using this strategy, photovoltaic cells with poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]: [6,6]-phenyl-C61 butyric acid methyl ester (MDMO-PPV:PCBM) blend as a photoactive layer were fabricated. An enhancement in power conversion efficiency of 53% was observed in cells with a double interlayer compared with cells having a PEDOT: PSS interlayer. The enhancement is attributed to the combined effects of electron blocking and enhanced charge extraction from the photoactive layer to the anode.

High-performance polymer solar cells with a conjugated zwitterion by solution processing or thermal deposition as the electron-collection interlayer

We report highly efficient polymer solar cells (PSCs) with rhodamine 101, a conjugated zwitterion with positive and negative charges on the same molecule, as the electron-collection interlayer. Rhodamine 101 can be processed by solution processing techniques or thermal deposition. The rhodamine 101 interlayer simultaneously improves the short-circuit current, open-circuit voltage, fill factor so as to improve the photovoltaic efficiency of the PSCs with poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active materials in comparison with control PSCs with Al or Ca/Al as the cathode. The photovoltaic efficiency reaches 6.15% for the PSCs with rhodamine 101/Al as the cathode under AM1.5G illumination, whereas the efficiencies are only 3.81% and 4.57% for the control PSCs with Al and Ca/Al as the cathodes, respectively. On the other hand, the improvement on the photovoltaic performance by the rhodamine 101 interlayer is less remarkable for the PSCs with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or PC71BM as the active materials. The photovoltaic efficiency is 4.25% for the PSCs with rhodamine 101/Al as the cathode, almost the same as that (4.20%) of the control PSCs with Ca/Al as the cathode. The high efficiency of the PSCs with the rhodamine 101 interlayer is ascribed to the lowering of the work function of metals by rhodamine 101 that has a strong dipole moment. The different effects of the rhodamine 101 interlayer on the two PSCs with PCDTBT and P3HT as the donor materials are attributed to different reactivities of the two polymers with active metals like Ca and Al. PCDTBT consisting of both electron-donating and electron-withdrawing units is more reactive with the active metals. The reaction between PCDTBT and Ca leads to low photovoltaic efficiency of the PSCs with Ca/Al as the cathode.