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A high-efficiency inverted-type CH₃NH₃PbI₃ (MAPbI₃) solar cell was fabricated by using a ultrathin poly[3-(4-carboxybutyl)thiophene-2,5-diyl]-Na (P3CT-Na) film as the hole transport layer. The averaged power conversion efficiency (PCE) can be largely increased from 11.72 to 18.92% with a double-filtering process of the P3CT-Na solution mainly due to the increase in short-circuit current density (<i>J_SC</i>) from 19.43 to 23.88 mA/cm², which means that the molecular packing structure of P3CT-Na thin film can influence the formation of the MAPbI₃ thin film and the contact quality at the MAPbI₃/P3CT-Na interface. Zeta potentials, atomic-force microscopic images, absorbance spectra, photoluminescence spectra, X-ray diffraction patterns, and Raman scattering spectra are used to understand the improvement in the <i>J_SC</i>. Besides, the light intensity-dependent and wavelength-dependent photovoltaic performance of the MAPbI₃ solar cells shows that the P3CT-Na thin film is not only used as the hole transport layer but also plays an important role during the formation of a high-quality MAPbI₃ thin film. It is noted that the PCE values of the best P3CT-Na based MAPbI₃ solar cell are higher than 30% in the yellow-to-near infrared wavelength range under low light intensities. On the other hand, it is predicted that the double-filtering method can be readily used to increase the PCE of polymer based solar cells.