Molecularly Engineered Hole Transporting Materials for High Performance Perovskite Solar Cells

Abstract

Perovskite solar cells have rapidly revolutionized the photovoltaic research showing an im-pressively dynamic progress on power conversion efficiency from 3.8 to 22% in only several years, a record for a nascent technology. Furthermore, inexpensive precursors and simple fabrication methods of perovskite materials hold a great potential for future low-cost energy generation enabling the global transition to a low-carbon society. The best performing device configuration of perovskite solar cell is composed of an electron transporting material, typi-cally a mesoporous layer of titanium dioxide, which is infiltrated with perovskite material and coated with a hole transporting material. However, although perovskite solar cells have achieved high power conversion efficiency values, there are several challenges limiting the industrial realization of low-cost, stable, and high-efficiency photovoltaic devices. To date, spiro-OMeTAD and PTAA are hole transporting materials of choice in order to main-tain the highest efficiency, however, the prohibitively high price hinders progress towards cheap perovskite solar cell manufacturing and may contribute to more than 30% of the overall module cost. Additionally, such wide bandgap hole transporting materials typically require doping in order to match necessary electrical conductivity and the use of additives is prob-lematic, since hygroscopic nature of doping makes the hole transporting layer highly hydro-philic leading to rapid degradation, negatively influencing the stability of the entire device. In order to overcome these problems, the rational design, synthesis, and characterization of a variety of small molecule-based hole transporting materials have been on a focus of this the-sis. Through judicious molecular engineering four innovative hole transporting materials KR131, KR216, KR374, and DDOF were developed via alternative synthetic schemes with the minimized number of steps and simple workup procedures allowing cost-effective upscale. Employing various characterization methods, the relationship between the molecular struc-ture of the novel hole transporting materials and performance of perovskite solar cells was investigated, leading to a fundamental understanding of the requirements of the hole trans-porting materials and further improvement of the photovoltaic performance. Furthermore, the synthesis of the dopant-free hole transporting materials based on push-pull architecture is presented. Highly ordered characteristic face-on organization of KR321 hole transporting molecules benefits to increased vertical charge carrier transport within a perov-skite solar cell, leading to a power conversion efficiency over 19% with improved durability. The obtained result using pristine hole transporting material is the highest and outperforms most of the other dopant-free hole transporting materials reported to date. Highly hydropho-bic nature of KR321 may serve as a protection of perovskite layer from the moisture and pre-vent the diffusion of external moieties, showing a promising avenue to stabilize perovskite solar cells.