Molecularly Engineered Hole Transporting Materials for High Performance Perovskite Solar Cells

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

Effect of alkyl chain length on the properties of triphenylamine-based hole transport materials and their performance in perovskite solar cells

A new series of diacetylide-triphenylamine (DATPA) derivatives with five different alkyl chains in the para position, MeO, EtO, nPrO, iPrO and BuO, were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells (PSC) studied. Their thermal, optical and electrochemical properties were investigated along with their molecular packing and charge transport properties to analyse the influence of different alkyl chains in the solar cell parameters. The shorter alkyl chain facilitates more compact packing structures which enhanced the hole mobilities and reduced recombination. This work suggests that the molecule with the methoxy substituent (MeO) exhibits the best semiconductive properties with a power conversion efficiency of up to 5.63%, an open circuit voltage (Voc) of 0.83 V, a photocurrent density (Jsc) of 10.84 mA cm−2 and a fill factor of 62.3% in perovskite solar cells. Upon replacing the methoxy group with longer alkyl chain substituents without changing the energy levels, there is a decrease in the charge mobility as well as PCE (e.g. 3.29% for BuO-DATPA). The alkyl chain length of semiconductive molecules plays an important role in achieving high performance perovskite solar cells

One Step Facile Synthesis of Novel Anthanthrone Dye Based, Dopant-Free Hole Transporting Material for Efficient and Stable Perovskite Solar Cells

Perovskite solar cell (PSCs) technology has made a tremendous impact in the solar cell community due to their exceptional performance, as the power conversion efficiency (PCE) surged to world record 22% within just last few years. Despite this high efficiency value, the commercialization of PSCs for large area applications at affordable prices is still pending due to the low stability of devices in ambient atmospheric conditions and a very high cost of the hole transporting materials (HTM) used as the charge transporting layer in such devices. To cope with these challenges, the use of cheap HTMs can play a dual role in terms of lowering the overall cost of the perovskite technology as well as protecting the perovskite layer to achieve higher stability. In order to achieve these goals, various new organic hole transporting materials (HTMs) have been proposed. In this work we use a unique and novel anthanthrone (ANT) dye as a conjugated core building block and an affordable moiety to synthesize a new HTM. The commercially available dye was functionalized with an extended diphenylamine (DPA) end capping group. The newly developed HTM, named DPA-ANT-DPA, was one-step synthesized and used successfully in mesoporous perovskite solar cell devices, achieving a PCE of 11.5% under 1 sun condition with impressive stability. The obtained device efficiency is amongst the highest, as per D-A-D molecular design and low band gap concern. Such kind of low cost HTM based on inexpensive starting precursor anthanthrone dye paves the way for economical and large-scale production of stable perovskite solar cells

Efficiency vs. stability: dopant-free hole transporting materials towards stabilized perovskite solar cells

In the last decade, perovskite solar cells have been considered a promising and burgeoning technology for solar energy conversion with a power conversion efficiency currently exceeding 24%. However, although perovskite solar cells have achieved high power conversion efficiency, there are still several challenges limiting their industrial realization. The actual bottleneck for real uptake in the market still remains the cost-ineffective components and instability, to which doping-induced degradation of charge selective layers may contribute significantly. This article overviews the highest performance molecular and polymeric doped and dopant-free HTMs, showing how small changes in the molecular structure such as different atoms and different functional groups and changes in substitution positions or the length of the pi-conjugated systems can affect photovoltaic performance and long-term stability of perovskite solar cells.GM

Efficiency vs. stability: dopant-free hole transporting materials towards stabilized perovskite solar cells

In the last decade, perovskite solar cells have been considered a promising and burgeoning technology for solar energy conversion with a power conversion efficiency currently exceeding 24%. However, although perovskite solar cells have achieved high power conversion efficiency, there are still several challenges limiting their industrial realization. The actual bottleneck for real uptake in the market still remains the cost-ineffective components and instability, to which doping-induced degradation of charge selective layers may contribute significantly. This article overviews the highest performance molecular and polymeric doped and dopant-free HTMs, showing how small changes in the molecular structure such as different atoms and different functional groups and changes in substitution positions or the length of the pi-conjugated systems can affect photovoltaic performance and long-term stability of perovskite solar cells