Temperature dependence of HOMO-LUMO levels and open circuit voltage for P3HT:PCBM organic solar cells

Abstract

In this study, the open circuit voltage (V OC ) of poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells was measured at temperatures ranging from 300 K to 400 K. The temperature dependence of the vacuum shift and of the highest occupied molecular orbital (HOMO) energy level of P3HT and PCBM were measured by ultraviolet photoelectron spectroscopy (UPS) in the same temperature range. The temperature dependence of the absorption edge was also studied in the same temperature range to obtain the temperature variation of the optical band gap energy (E g ). The measured V OC of the devices showed a clear decreasing trend with increasing operating temperature and the total decrease was found to be about 0.1 V. Although the origin of V OC is still not fully understood it is generally believed that the energy level offset between the HOMO of the donor and the LUMO of the acceptor minus the exciton binding energy (0.3 eV) directly determines the value of V OC . However, by utilizing the measured values of the HOMO for the P3HT (donor) and of the LUMO for the PCBM (acceptor), we have found that the calculated values of V OC and its temperature dependence do not agree with the measured V OC values. This indicates that factors other than the offset between the HOMO of the donor and the LUMO of the acceptor materials are impacting V OC .

Synergistic effect of CdSe quantum dots (QDs) and PC61BM on ambient-air processed ZnO QDs/PCDTBT: PC61BM:CdSe QDs/MoO3 based ternary organic solar cells

“…This significant enhancement, in the photovoltaic parameter J SC is attributed to the better charge transportation from PCDTBT to PC 61 BM via CdSe QDs in the ternary OSC [49]. We know that the V OC of organic solar cell mainly depends on the energy difference (offset) between the lowest unoccupied molecular orbital (LUMO) level of the acceptor and the highest occupied molecular orbital (HOMO) level of the donor [50,51]. Therefore, this improvement in the V OC from 699 to 854 mV by the synergistic effect of CdSe QDs and PC 61 BM is due to the energy offset enhancement which could be seen clearly in the energy band diagram in figure 4.…”