| UP to now, a variety of electron tansport materials have been reported for polymer solar cells, including metal oxides, metal fluorides, and organic compounds such as conjugated polyelectrolyte, n-type small molecules, fullerene derivatives, etc. However, many of inorganic electron transport materials show poor compatibility with organic active layer or with the low-cost solution-processing technique. As for organic electron transport materials, their feature not only low stabilities but also high-cost due to multi-step synthnsis, which make them far from being applicable in the field of low-cost polymer solar cells. Thus, there is still a big challenge for us to design effective, solution-processable and low-cost electron transport materials.In this study, we designed and prepared a new type graphene composite of rGO-K4 PTC. It has been characterized by means of scan electron microscopy(SEM), X-ray photoelectron(XPS), Fourier transfer infrared(FTIR) and Ultraviolet-visible(UV-vis) absorption spectroscopies. Moreover, the work functions of rGO-K4 PTC and K4 PTC were calculated to be 4.03 eV and 3.99 eV, respectively, according to the ultraviolet photoelectron spectra(UPS). These values are significantly lower than the work functions of rGO(4.38 eV) and Al(4.3 eV), but match well with the lowest unoccupied molecular orbital(LUMO)(4.0-4.1 eV) of the widely used acceptor materials(i.e. PC61 BM, PC71BM). Thus, it is expected that r GO-K4 PTC and K4 PTC are promising electron transport materials, which might facilitate the electron extraction from the active layer to the cathode in organic solar cells.In order to investigate their photovoltaic properties, various polymer solar cells were prepared using rGO-K4 PTC or K4 PTC as the electron transport layer and P3HT:PC61BM, PTB7-th:PC61BM or PTB7-th:PC71BM as the active layer, respectively. As compared to the reference devices without electron transport layer, the power conversion efficiencies(PCEs) of the organic solar cells with K4 PTC and rGO-K4 PTC as respective ETL were increased by ~35% and even comparable to those of standard Ca/Al devices. Moreover, monochromatic incident photon to electron conversion efficiency(IPCE) spectra revealed significant improvement in the range of 400-650 nm for rGO-K4 PTC and K4PTC-based devces as compared with that of Al-only device, consistent with the results obtained from the current density-voltage(J-V) tests. Therefore, it is concluded that inserting rGO-K4 PTC or K4PTC-based layer between the active layer and cathode Al can improve the photovoltaic performance of the device.Furthermore, technique of atomic force microscopy(AFM) was applied to study the surface morphologies of the P3HT:PC61BM with or without ETL. It was revealed that the addition of rGO-K4 PTC or K4PTC-based ETL reduced the surface root-mean-square(RMS) roughness of P3HT:PC61BM. In addition, space charge limited current(SCLC) measurements demonstrated higher electron mobilities for rGO-K4 PTC or K4PTC-based devices relative to that of Al-only device. These results indicated that inserting rGO-K4 PTC or K4PTC-based ETL can efficiently improve the contact between the active layers and Al electrode and reduce the series resistance(Rs) of the device. On the other hand, the degradation tests performed in moist air revealed that rGO-K4PTC-based device is much more stable than the reference Ca/Al device as well as the K4PTC-based device. This might be attributed to the presence of rGO in ETL, which protects the active layer against oxidation and degradation.In addition, we prepared a series of N,P-doped graphene quantum dots(GQDs) and investigated their potenialas HTL or ETL. Our results showed that the as-prepared GQDs can not work well as HTL, but can fairly work as ETL. Though the PCE of GQD-based device is still lower than that of the standard Ca/Al device, we believe that some improvement would be achieved by optimizing of the thickness and morphyology of the ETL. |
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