Methylammonium lead halide perovskites have been intensively studied as promising photo absorption and carrier transporter materials in solar cells due to excellent semiconducting properties, a broad range of light absorption, and a high extinction coefficient. Although PCE of perovskite solar cells has been increased remarkably, a few reports have considered solution-processed planar heterojunction (SP-PHJ) structure solar cells without using a mesoporous or compact semiconducting metal oxide (e.g, TiO2) layer processed by high-temperature sintering, and the SP-PJH solar cells to date have shown lower PCE than those with a mesoporous or compact TiO2 layer. Commercialization of perovskite solar cells requires easy, scalable and low-temperature methods to fabricate them efficiently by a solution process without sintering. PEDOT:PSS can be considered as a good HEL because of simple solution processibility, planarization effect on the underlying ITO layer, and a low-temperature annealing process. . However the work function WF of PEDOT:PSS (4.9 to 5.2 eV depending on the ratio of PEDOT to PSS) is lower than the ionization potential IPof perovskite (e.g. 5.4 eV for methylammonium lead iodide (CH3NH3PbI3)), so the potential energy loss at PEDOT:PSS/Perovskite interface decreased built-in potential in perovskite solar cells.
The WF in HELs can be tuned by using molecular surface engineering to control the surface composition in HEL films, which depends on the surface-enriched molecules and their concentration relative to the conducting polymer. Thus we used a self-organized HEL (SOHEL) which is composed of a conducting polymer composition (e.g., PEDOT:PSS) and a perfluorinated ionomer (PFI), i.e., tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer. Here, we present solution-processed methylammonium lead iodide CH3NH3PbI3-based perovskite solar cells with a high-WF SOHEL for good energy level alignment with the IP level of CH3NH3PbI3. The SOHEL at the hole extraction interface can increase the built-in potential, the photocurrent, and thus the PCE of perovskite solar cells. We obtained high PCE of 11.7% in SP-PHJ perovskite solar cells under 100-mW/cm2 illumination. We also demonstrated flexible perovskite solar cells on a poly(ethylene terephthalate) (PET) substrate; they had PCE as high as 8.0%.
Pohang University of Science and Technology (POSTECH)
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