No abstract

No abstract

To understand the origin of the open-circuit voltage of heterojunction photovoltaic (HJ-PV) cells with small-molecular-weight organic thin films, HJ-PV cells with ITO/donor (20 nm)/fullerene (40 nm)/bathocuproine (10 nm)/Al were studied using three kinds of donors and four types ITOs with different work functions.Since the first report of an organic photovoltaic (PV) cell with the donor (D)/acceptor (A) heterojunction (HJ) by Tang, 1 organic PV cells have attracted attention owing to their ease of fabrication, potential for low-cost productions, and compatibility with flexible substrates.2 A fundamental understanding of the excitonic nature of organic materials is essential for device engineering.2d By the introduction of device concepts such as the mixed 3a and blended 3b,3c DA HJs, the exciton-blocking layer (EBL), 3d and the tandem cell, 3e the performance of small-molecular-weight 2b and polymer 2c,2e organic PV cells has been improved dramatically.To achieve high power conversion efficiencies of such PV cells, the increase in the open-circuit voltage (V oc ) as well as the short-circuit current density (J sc ) and the filling factor (FF) is required. However, the origin of V oc is not well understood. 2,4aFor metal/insulator/metal (MIM) type PV cells, the built-in potential (V bi ) results from the work function difference Á between the electron-collecting (EC) and hole-collecting (HC) electrodes. For so-called ''bulk HJ'' (Type I) PV cells, Al or Ag with low work functions () (4.2 or 4.3 eV) in contact with an A molecule [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) blended with one of various conjugated D polymers is used as the EC electrode, while the polymer film of poly(3,4-ethylenedioxythiophene (PEDOT) doped with poly-(styrenesulfonate) (PSS) acts as the HC electrode. of PEDOT:PSS (5.2-5.3 eV) is higher than that of indium-tinoxide (ITO) (4.5-4.8 eV). Therefore, Á is kept almost constant to be %1:0 eV, since the Fermi level of the metal electrode is considered to be pinned to the lowest unoccupied molecular orbital (LUMO) level of PCBM (4.2-4.3 eV).2c,4a Under such high Á in Type I cells, V oc has been found to change linearly with the highest occupied molecular orbital (HOMO) level or the oxidation potential (E ox ) of Ds. 2c,4b In contrast, Á is 0.3-0.6 eV for HJ small-molecular-weight organic thin film PV (Type II) cells (i.e., ITO/D/A(C 60 )/bathocuproine(BCP)/Al(or Ag)), 2a where the bare ITO electrode is mainly used as the HC electrode and the BCP layer functions as EBL and improves electron transport from the C 60 film to the EC electrode. 3d,5 Recently, V oc of Type II cells with a boron subphthalocyanine chloride (SubPc) D layer was found to be so low as 0.57-0.58 eV, 6 although HOMO of SubPc was estimated electrochemically to be 5.6 eV and thus V oc was expected to be >1:0 V from the V oc vs. HOMO level (or E ox ) relationship described above. It was also found that closer values (0.97-0.98 V) to the expected V oc were measured only when the thickness of the D layer became very t...

No abstract

The authors report the use of chemically modified indium tin oxide (ITO) with different binding groups (–COCl and –PO2Cl2) of p-chlorobenzene derivatives forming effective monolayers to control the work function of ITO and hence to enhance the hole injection. The enhanced hole injection is studied by measuring current density–voltage (J-V) characteristics. The behavior of J-V characteristics caused by varying the ITO work function in hole-only single-carrier devices with a hole transport layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine is examined. Upon grafting with p-chlorophenylphosphoryl dichloride, the J-V characteristics show a space-charge-limited conduction behavior. Such modified ITO anodes lead to improvements in the device properties.