e., ITO/nc-TiO2/P3HT:PCBM/Ag cell. After five cycles of CdS deposition, the cell of ITO/nc-TiO2/CdS(n)/P3HT:PCBM/Ag gives rise to a significant increase in V oc, which increases from 0.15 to 0.60, 0.40, and 0.33 V for n = 5, 10, and 15, respectively.
This result can be explained as follows. On one hand, it is known that V oc is mainly dominated by the energy level difference between the donor highest occupied molecular orbital (HOMO) and the acceptor lowest unoccupied molecular orbital (LUMO) levels in the polymer bulk heterojunction solar cells. In our case, before the deposition of CdS, the electron acceptor materials are TiO2 and PCBM. However, after the introduction of CdS, CdS also works as an electron acceptor. Apparently, www.selleckchem.com/products/Rapamycin.html the effective LUMO level of the acceptor should be determined by three acceptor materials, i.e., TiO2,
PCBM, and CdS. Importantly, the CB level (−3.7 eV) of CdS is higher than that (−4.2 eV) of TiO2, which probably enhances the effective LUMO level of the acceptor and the energy level difference between the HOMO of donor and the LUMO of acceptor levels, ultimately increasing Belnacasan the V oc of the cells with CdS compared to the ITO/nc-TiO2/P3HT:PCBM/Ag cell without CdS. On the other hand, V oc may also be affected by charge recombination in the cells under open-circuit condition. CdS as an electron-selective layer can prevent the electron from escaping the TiO2 to the active layer, which can be characterized by the shunt resistance (R sh), calculated from the inverse slope of I-V characteristics under illumination at V = 0 V. A higher R sh is more beneficial to the increase of V oc. This explanation is supported by the shunt resistance of the ITO/nc-TiO2/CdS(n)/P3HT:PCBM/Ag cells: 620,
350, and 290 Ω/cm2, for n = 5, 10, and 15, respectively, indicating an increased shunt resistance compared to the ITO/nc-TiO2/P3HT:PCBM/Ag without CdS. Besides, the improvement in both I sc and FF of the ITO/nc-TiO2/CdS(n)/P3HT:PCBM/Ag cells PDK4 is also found. There are several reasons for I sc enhancement. The first one may be the reduced charge recombination from TiO2 to the P3HT:PCBM film when introducing CdS nanoparticles. It can be seen from the energy diagram shown in Figure 1b that the photogenerated electrons are injected from CdS and P3HT to TiO2 and PCBM, part of which may combine with the holes in P3HT. However, compared to the cells without CdS, the recombination in the cells with CdS is reduced because of the formation of the CdS energy barrier layer, which is similar to the case of CdS-sensitized TiO2 nanotube arrays . The increased interfacial area between the donor and acceptor as shown in Figure 2 after the deposition of CdS on TiO2 may be the second reason, which makes more excitons dissociate into free electrons and holes.