The possibility of the increase in open-circuit voltage of organic photovoltaic cells based primarily indium-tin oxide (ITO)/rubrene/fullerene/Al structure by changing the work function of ITO anodes and Al cathodes was described in this work. To change built-in potential preferably in order to increase the open-circuit voltage, the work function of ITO should be increased and work function of Al should be decreased. The correlation between the change in work functions of electrodes and performance of the organic photovoltaic cells before and after surface modifications was examined in detail. The enhancement of open-circuit voltage depends on a function of work function change of both ITO and Al electrode. We could show that the built-in potential in the cells played an important role in open-circuit voltage.
Organic photovoltaic (PV) cells have been attracted much attention in recent decades due to their potentials as fabrication, low-cost production, and technological advantages of semiconductor materials [1-5]. Since the first report of donor-acceptor heterojunction with a power conversion efficiency (hp) of about 1% by Tang [
To obtain large open-circuit voltage (Voc), Taima et al. introduced a p-type semiconductor 5, 6, 11, 12-tetraphenylnaphthacene (rubrene), which has the HOMO level of 5.4 eV. They obtained the Voc of 0.91 V [
Indium-tin-oxide (ITO) is the most widely used as a transparent anode in organic PV cells due to its high conductivity, work function, and transparency in the visible spectral range [
To investigate the possibility of increase in Voc by controlling the work functions of the electrodes, we report here the use of chemically modified ITO with different terminal groups (Hand Cl-) of p-benzenesulfonyl chlorides and p-chlorophenyldichlorophospate (-P) forming effective monolayers. We examine the correlation between the change in the work function of ITO and the performance of the PV cells by the chemical modification and find that the large increase in Voc. In this work, we selected tris(8-hydroxyquinoline)aluminum (Alq3) as an electron transport layer (ETL) to substitute for bathocuproine (BCP) in cells based on rubrene (Rub)/buckminsterfullerene (C60) heterojunction. Moreover, to examine the further improvement of Voc, we used a lithium carboxylate (C6H5COOLi) [
ITO coated glass substrates with a sheet resistance of ca. 15 W/square (Sanyo Vacuum Industries) were cleaned by sonication successively in two detergents (Extran MA 03, pH 6.8, MERCK and Kontaminon O, pH 10, WAKO), rinsed with deionized water, and stored in isopropanol until being required. After cleaning with acetone and isopropanol (this cleaned ITO will be called hereafter “as-cleaned ITO” with notation of “ac”) the ITO substrates were immersed for 5 min in dichloromethane solutions containing 1 mM of (Hand Cl-) of p-benzenesulfonyl chlorides (Tokyo Chemical Industry) and p-chlorophenyldichlorophospate (Tokyo chemical industry). The modified ITO anodes were rinsed in pure dichloromethane and then vacuum dried for ~1 h.
C60 (purity > 99%) (Tokyo Chemical Industry), the sublimed grade rubrene (Aldrich Co.) and Alq3 (Dojindo Labs), the reagent grade BCP (Kanto Chemical), and lithium benzoate (purity~99%) (Aldrich Co.) were used without further purification. All the materials were deposited using vacuum evaporation under a pressure of 5 - 7 ´ 10−6 Torr at deposition rates of 1 - 1.5 Ǻ/s for organic layers and 3 - 4 Ǻ/s for Al cathode. The active area for all the cells was defined to be 5 ´ 5 mm2 by using a shadow mask. The current density-voltage (J-V) curves were measured under illumination of a simulated solar light with 100 mW ´ cm−2 (AM1.5G) by a solar simulator (Yamashita Denso, YSS-50). Electric data were taken using an Advantest R6145 DC voltage current source unit at room temperature in ambient atmosphere.
The absorption spectral data for all the thin film were taken using an UV-visible spectrophotometer (UV-265 FW, Shimadzu) at room temperature in ambient atmosphere.
For a cell based on exciton dissociation by charge transfer at a donor-acceptor (D/A) interface, hp is the product of the efficiencies [
In order to decrease the work function of the Al electrode, we have to put another layer of less electron affinity than C60. As such materials, we examined Alq3 and BCP [10,16,20,29-31] LUMO levels of which are higher (i.e., less electron affinity) than that of C60. In fact, the organic side for these interfaces is charged positively, making this side more comfortable (low energy) for an electron, and making the sign of D negative. Taking into account the D at Alq3/Al interface of ~−1.0 eV [
Next, we discuss the work function control of the anode side. The molecular approach allows for fine-tuning the work function using organic molecules on ITO depending upon magnitude and direction of the dipole moment [
An interface dipole with its negative end pointing toward the organic layer and its positive end toward the electrode surface increases the ITO work function (i.e., the Fermi energy is down) and HOMO energy level in the organic layer is relatively up by adding an electrostatic energy [