Research Article: Alkali Metal Doping for Improved CH3NH3PbI3 Perovskite Solar Cells

Date Published: December 21, 2017

Publisher: John Wiley and Sons Inc.

Author(s): Wangen Zhao, Zhun Yao, Fengyang Yu, Dong Yang, Shengzhong (Frank) Liu.


Organic–inorganic hybrid halide perovskites are proven to be a promising semiconductor material as the absorber layer of solar cells. However, the perovskite films always suffer from nonuniform coverage or high trap state density due to the polycrystalline characteristics, which degrade the photoelectric properties of thin films. Herein, the alkali metal ions which are stable against oxidation and reduction are used in the perovskite precursor solution to induce the process of crystallization and nucleation, then affect the properties of the perovskite film. It is found that the addition of the alkali metal ions clearly improves the quality of perovskite film: enlarges the grain sizes, reduces the defect state density, passivates the grain boundaries, increases the built‐in potential (Vbi), resulting to the enhancement in the power conversion efficiency of perovskite thin film solar cell.

Partial Text

Organic–inorganic hybrid halide perovskites commonly adopting the ABX3 structure: where A is a monovalent organic or inorganic cation, e.g., methylammonium (MA+), formamidinium (FA+), Cs+ etc.; B is a divalent metal ion (Pb2+, Sn2+, Ge2+ etc.) and X is a monovalent anion (Cl−, Br−, I−, SCN− etc.) have attracted great attention owing to their superior photoelectronic properties.1, 2, 3, 4, 5 As a type of star material in the photovoltaic field, the performance of perovskite‐based thin film solar cells have gone through tremendous advance, almost caught up with or even gone beyond the efficiency of crystalline silicon, CIGSe (copper–indium–gallium diselenide) and CdTe solar cells only in a few years.1, 6, 7, 8, 9, 10 And the high efficiency perovskite solar cell depends on the high‐quality absorbing layer governed by its deposition process including nucleation and thin film growth. However, some undesirable properties, such as the phase transition, instability to humidity, UV (ultraviolet), and heat are often accompanied due to perovskite absorber material own properties.11, 12, 13 In addition, the perovskite films with the polycrystalline nature which crack in the periodic crystal structure lead to the formation of a large number of grain boundaries (GBs). These GBs in the perovskite layer are proven to be recombination centers, which has negative impact on device performance.14, 15 So, it is desired to prepare larger crystallites with reduced GBs for higher performance. Therefore, further treatment for defect passivation or reduction of GBs is critically vital.16, 17, 18

First, to check the quality of perovskite absorber layer, morphological characterization based on field‐emission scanning electron microscopy (FE‐SEM) was carried out to determine the shape and coverage of the MAPbI3 grains and thin films prepared in the presence of additives and the control sample (without any additive). It was found that all of the perovskite films exhibited high coverage, which are displayed in Figure1a–c. Comparative SEM analysis brought out the variations in film evenness and grain size. For a clear comparison, Figure S1 (Supporting Information) provides statistical grain size distribution based on the top‐view images in Figure 1. It is exhibited that without additive, average grain size is ≈140 nm, and the average grain sizes increase to ≈220, 230 nm when Na+ and K+ doping were used, respectively. Meanwhile, lower surface roughness of perovskite film with alkali metal cation doped are observed compared with the control sample by AFM (atomic force microscope) in Figure 1d–f, with the root‐mean‐square of Rq = 8.99 (Na doped), 7.94 (K doped), 9.91 (control sample) nm. The perovskite films also appear to be entirely covered based on AFM, being consistent with the FE‐SEM results. Synthesizing above consequences, we found that the additives apparently help to improve the perovskite growth into much large crystallite grains and smoother films.

An effective additive method has been developed based on one‐step, solution crystallization process to prepare the perovskite film. It has been demonstrated that the addition of alkali metal cations in the perovskite precursors significantly improve the grain size, and reduce the trap states, which is vital for achieving high‐efficiency polycrystalline thin film solar cells. In addition, we observed enhanced luminescent intensity, lengthened carrier lifetime by alkali metal cations doping, which help to push the performance of the planar perovskite solar cells to a higher level. The approach proposed in this work, which is simple and facile yet very effective, allows versatility with widespread applicability since the additive is readily applicable to other bulk‐heterojunction systems with great adaptability, thus facilitating the practical application of photovoltaic technologies.

Chemicals: PbI2, lithium bis (trifluoromethanesulfonyl)imide (Li‐TFSI), 4‐tert‐butyl pyridine (tBP), chlorobenzene were obtained from Sigma‐Aldrich. Spiro‐OMeTAD was purchased from Shenzhen Feiming Technology Co. γ‐butyrolactone, dimethylsulfoxide, HI (57 wt% in water), methylamine (CH3NH2, 40 wt% in aqueous solution), diethyl ether, sodium iodide (NaI), potassium iodide (KI) were bought from Aladdin Inc. All the chemicals were used without further purification.

The authors declare no conflict of interest.