Abstract
Open-circuit voltage (VOC) in organic solar cells (OSCs) is currently still not well-understood. A generally acceptable view is that VOC is mainly determined by the energy level offset between donor and acceptor materials. Recently in ternary blend OSCs, VOC is found to be dependent on the blend composition. But contrary to expectation, this dependence is not a simple linear relationship, which adds complications to understanding on VOC. Here, in order to figure out the origin of VOC, we performed a series of experiments on both binary and ternary blend OSCs in a wide temperature range from 15 K to 300 K. It is observed that the devices behave like Schottky barrier (SB) diode. By fitting the experimental results with SB diode model, the detailed device parameters of ternary blend OSCs are extracted and it is found that VOC is determined by the energetics of organic molecules and metal at the cathode interface, and the inhomogeneity of the SB also play a great role in the origin of VOC at low temperatures. This work not only paves the way to deep understanding on the origin of VOC, but also opens a door to further exploring the general working principle of OSCs.
Highlights
Open-circuit voltage (VOC) in organic solar cells (OSCs) is currently still not well-understood
In conventional binary blend devices, VOC is determined by the energy level offset between donor and acceptor material, which is expressed by the following equation[11]: VOC = ( E highest occupied molecular orbital (HOMO),D − E lowest unoccupied molecular orbital (LUMO),A − 0.3 eV)/q, (1)
The light and dark J-V curves of phenyl-C61-butyric acid methyl ester (PCBM) device are shown in Fig. 1. (J-V curves of other four devices are shown in Supplementary Figure S1.) All the parameters for the five devices extracted in J-V measurements are listed in Supplementary Table S1, it should be noted here that these measurements are performed under AM 1.5G illumination of 0.8 suns intensity, so the JSCs are lower than the values in standard tests
Summary
Open-circuit voltage (VOC) in organic solar cells (OSCs) is currently still not well-understood. Street et al have pushed this work a step further, they confirmed the continuous variation in energy of the HOMO (LUMO) level of two donor (acceptor) materials in the blend by using photocurrent spectral response (PSR) to measure the electronic states of ternary blend They still observed that the optically excited exciton states do not reflect the average composition, but retain individual molecular characteristics, which indicates the formation of alloy[5]. Analyses are performed on the current density-voltage (J-V) characteristics with the thermionic emission (TE) theory from inorganic SB diodes, and SB heights are calculated for the devices with different blend composition These results show that VOC is mainly determined by the SB formed at the cathode interface and influenced by the surface states and inhomogeneity of SB at the interface
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