lt-in electric field formed from the orbital bending near the junction.

(a) (b)

Figure 3. Comparative J-V response, (a) with and without LaF3 and (b) between the two different layers.

Figure 4. J-V response of the fabricated OSC with LaF3 buffer layer with and without heat treatment.

(a) (b)

Figure 5. The vacuum level shift and energy levels in OSCs (a) without and (b) with LaF3 CBL.

Due to increased electrical field the electrons can tunnel through 3 nm LaF3 layer efficiently [35] [36] . Again, LaF3 film is an electrical insulator and due to its exceptionally large surface polarization [37] [38] thin LaF3 layer is expected to decrease the surface potential of the Al cathode and thus the effective work function, resulting in favorable electron extraction. These are all in good consistence with the obtained enhancement in JSC and VOC.

As shown in Figure 5(b) the LaF3 layer can act as hole-blocking layer as the ionization potential of LaF3 (12.4 eV) is much larger than that of PCBM (6.3 eV). LaF3 also has a high electron affinity that enhances electron extraction from PCBM and blocks holes as a result probability of radiative recombination is greatly improved. Moreover, recombination at the metal/polymer interface can be avoided as the LaF3 layer separates the recombination zone from the metal electrode.

4. Conclusion

Thin layer of LaF3 as a novel cathode buffer layer between the Al cathode and photoactive polymer in an organic solar cell (OSC) in organic solar cell has been investigated in this article. Thinner LaF3 could induce LUMO/HOMO levels to have a more feasible level alignment for electron transport, and block holes. Substantial increase in the short-circuit current density and open-circuit voltage of the conventional OSC was observed.

Table 1. Summary of the experimental results.

Extremely smooth surface of amorphous LaF3 as obtained by AFM image favored its diffusion barrier characteristics for ambient oxygen and Al ions that degraded the organic layer and thereby reduced solar cell stability and lifetime. Therefore along with performance enhancement LaF3 is expected to enhance the stability and lifetime of organic solar cell. All together, very thin LaF3 can be considered as a very promising material that can be used in organic solar cells as a cathode buffer layer.

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