Influence of the Work Function of Chemically Modified Indium–Tin–Oxide Electrodes on the Open-circuit Voltage of Heterojunction Photovoltaic Cells

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...

Role of Polymer Fractionation in Energetic Losses and Charge Carrier Lifetimes of Polymer: Fullerene Solar Cells

“…Thus, it varies with the energy difference between the highest occupied molecular orbital of the donor (HOMO D ) and the lowest unoccupied molecular orbital of the acceptor (LUMO A ). [1][2][3] In practice the HOMO D -LUMO A difference is insufficient to predict V oc due to differences in non-radiative recombination. 4 The highest possible V oc , denoted V oc,rad , can be achieved when only radiative recombination occurs and the absorbed photon flux is balanced exactly by the emitted flux under open-circuit 3 conditions; this is called the radiative limit.…”

Improvement of Open-Circuit Voltage in Organic Photovoltaic Cells with Chemically Modified Indium-Tin Oxide

“…The molecular approach allows for fine-tuning the work function using organic molecules on ITO depending upon magnitude and direction of the dipole moment [34]. The effective work functions formed by chemical modification of ITO shown in Figure 1 were estimated from the contact potential difference (CPD) values [34,35]. Figure 2 shows the effect of ITO work function on the current density-voltage (J-V bias ) characteristics under 100 mW  cm −2 illumination and in dark of four kinds of the PV cells with various surface treatments of ITO.…”

The effect of structural order on solar cell parameters, as illustrated in a SiC-organic junction model

“…It is generally agreed in organic PV (including dye-sensitized solar cells) that the energetics of the D/A interface define a theoretical upper limit to the open-circuit voltage (V OC ) via the ionization energy (IE) of the donor and the electron affinity (EA) of the acceptor, and because of the dependence of the short-circuit current density (J SC ) of the solar cell on the difference between the EA values of the D and A materials [1][2][3][4][5][6][7][8] . It is generally agreed in organic PV (including dye-sensitized solar cells) that the energetics of the D/A interface define a theoretical upper limit to the open-circuit voltage (V OC ) via the ionization energy (IE) of the donor and the electron affinity (EA) of the acceptor, and because of the dependence of the short-circuit current density (J SC ) of the solar cell on the difference between the EA values of the D and A materials [1][2][3][4][5][6][7][8] .…”