Triphenylamine-based hole transport materials for perovskite solar cells

The rapid development in perovskite solar cells (PSC) has generated a tremendous interest in the photovoltaic community. The power conversion efficiency (PCE) of these devices has increased from 3.8% in 2009 to a recent certified efficiency of over 20% which is mainly the product of the remarkable properties of the perovskite absorber material. One of the most important advances occurred with the replacement of the liquid electrolyte with a solid state hole conductor which enhanced PCE values and improved the device stability. Spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)- 9,9′-spirobifluorene) is the most common hole transport material in perovskite solar cells. Nevertheless, the poor conductivity, low charge transport and expensive synthetic procedure and purification have limited its commercialisation. Triphenylamines (TPA) like Spiro-OMeTAD are commonly employed due to the easy oxidation of the nitrogen centre and good charge transport. Other triarylamines have similar properties to Spiro-OMeTAD but are easier to synthesise. The aim of this doctoral thesis is to investigate different types of hole transport materials in perovskite solar cells. Three different series of triphenylamine-based HTM were designed, synthesised, characterised and studied their function in perovskite solar cells. A series of five diacetylide-triphenylamine (DATPA) derivatives (Chapter 3) with different alkyl chain length in the para position was successfully synthesised through a five step synthesis procedure. A range of characterisation techniques was carried out on the molecules including; optical, electrochemical, thermal and computational methods. The results show that the new HTMs have desirable optical and electrochemical properties, with absorption in the UV, a reversible redox property and a suitable highest occupied molecular orbital (HOMO) energy level for hole transport. Perovskite solar cell device performances were studied and discussed in detail. This project studied the effect of varying the alkyl chain length on structurally similar triarylamine-based hole transport materials on their thermal, optical, electrochemical and charge transport properties as well as their molecular packing and solar cell parameters, thus providing insightful information on the design of hole transport materials in the future. The methoxy derivative showed the best semiconductive properties with the highest charge mobility, better interfacial charge transfer properties and highest PCE value (5.63%). The use of p-type semiconducting polymers are advantageous over small molecules because of their simple deposition, low cost and reproducibility. Styrenic triarylamines (Chapter 4) were prepared by the Hartwig-Buchwald coupling followed by their radical polymerization. All monomers and polymers were fully characterised through electrochemical, spectroscopic and computational techniques showing suitable HOMO energy levels and desirable optoelectrochemical properties. The properties and performance of these monomers and polymers as HTMs in perovskite solar cells were compared in terms of their structure. Despite the lower efficiencies, the polymers showed superior reproducibility on each of the device parameters in comparison with the monomers and spiro-OMeTAD. Finally, star-shaped structures combine the advantages of both small molecules, like well-defined structures and physical properties, and polymers such as good thermal stability. Two star-shaped triarylamine-based molecules (Chapter 5) were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells studied. These materials afford a PCE of 13.63% and high reproducibility and device stability. In total this work provided three series of triarylamine-based hole transport materials for perovskite solar cells application and enabled a comparison of the pros and cons of different design structures: small-molecule, polymeric and star-shaped

Star-shaped triarylamine-based hole-transport materials in perovskite solar cells

Two novel star-shaped triarylamine-based hole transport materials with triphenylamine (STR1), or a partially oxygen-bridged triphenylamine (STR0), as core and para-substituted triphenylamine side arms were synthesized, fully characterized and studied in perovskite solar cells. Their thermal, optical, electrochemical and charge transport properties were examined and compared in the context of their molecular structure. Due to its more planar configuration, STR0 showed a red-shifted absorption in comparison with STR1. STR0 also forms a more stable amorphous glassy state and showed higher glass transition temperature than STR1 and spiro-OMeTAD. These HTMs were tested in perovskite solar cells using a device configuration of FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/HTM/Au showing a power conversion efficiency of 13.3% for STR0 and 11.5% for STR1. The STR0-based devices showed higher fill factor and better reproducibility than spiro-OMeTAD-based cells. Without dopant additives, solar cells based on STR0 exhibited a good photocurrent density of 16.63 mA cm−2 and the efficiency improved from a starting PCE of 3.9% to 6.6% after two weeks of storage

Improved Stability of Inverted and Flexible Perovskite Solar Cells with Carbon Electrode

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.We demonstrate highly efficient, stable, and flexible perovskite solar cells of large areas, utilizing a carbon back-contact electrode in a p–i–n cell configuration. We enabled good electronic contact at the interface with carbon by inserting an ultrathin buffer layer before the carbon coating. Solar cells of such structure reach a power conversion efficiency of 15.18% on PET foil (device area of 1 cm2). We performed impedance spectroscopy and transient decay measurements to understand the electron transport characteristics. Furthermore, we demonstrate excellent operational (maximum power point) and thermal (85 °C) stability of these devices over 1000 h of aging

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

Solution-processable perylene diimide-based electron transport materials as non-fullerene alternatives for inverted perovskite solar cells

Perylene diimide derivatives with different functional groups (OR) in the bay position were synthesised (PDI-1, OR = OC6H4OMe; PDI-2, OR = OC6H4CH2CH2NHBoc; PDI-3, OR = OC6H4CO2Me) and their optoelectronical properties were characterised. The derivatives were applied as alternative electron transport materials (ETMs) to replace the commonly used PCBM in inverted perovskite solar cells (PSCs). Devices with the structure ITO/PTAA/Cs0.04(MA0.17FA0.83)0.96Pb(I0.83Br0.17)3/ETM/Ag (ETM = PCBM or PDI-1, -2 or -3) were fabricated through solution processing techniques. A competitive power conversion efficiency (PCE) of 16.8% was obtained for the PDI-3-based device, which was comparable to the PCBM-based device with PCE of 17.3%. It was found that the electronic nature of the functional groups plays an important role in the charge extraction and band alignment of these small molecular semiconductors

Solution-processable perylene diimide-based electron transport materials as non-fullerene alternatives for inverted perovskite solar cells

Financial support from the National Council of Science and Technology (Conacyt) Mexico. Funding from the Foundation of Polish Science (First TEAM/2017-3/30).Perylene diimide derivatives with different functional groups (OR) in the bay position were synthesised (PDI-1, OR = OC6H4OMe; PDI-2, OR = OC6H4CH2CH2NHBoc; PDI-3, OR = OC6H4CO2Me) and their optoelectronical properties were characterised. The derivatives were applied as alternative electron transport materials (ETMs) to replace the commonly used PCBM in inverted perovskite solar cells (PSCs). Devices with the structure ITO/PTAA/Cs0.04(MA0.17FA0.83)0.96Pb(I0.83Br0.17)3/ETM/Ag (ETM = PCBM or PDI-1, -2 or -3) were fabricated through solution processing techniques. A competitive power conversion efficiency (PCE) of 16.8% was obtained for the PDI-3-based device, which was comparable to the PCBM-based device with PCE of 17.3%. It was found that the electronic nature of the functional groups plays an important role in the charge extraction and band alignment of these small molecular semiconductors.Publisher PDFPeer reviewe

Correcting unintended changes in electroluminescence perturbation for reliable light intensity modulated spectroscopies

Light intensity modulated photocurrent and photovoltage spectroscopies, IMPS and IMVS respectively, are characterization techniques for studying charge carrier transport and recombination properties of photosensitive samples such as photovoltaic solar cells. In these techniques controlling the modulated light flux is key to obtaining accurate results. Typically, the electroluminescence of the light source is considered frequency-independent and therefore, it may be estimated from the modulated current delivered by the power source. However, some anomalies may appear when the experimental requirements demand large variations in the measurement conditions. Herein, an analysis is presented on the unusual low-frequency response of IMPS and IMVS which appears for some light sources at high illumination intensities. We found that a frequency-dependent modulation of the light source electroluminescence should be accounted for, rather than the traditional steady-state calibration of the setup, as it may affect the accuracy and even produce undesired artifacts during the measurements. A protocol for detecting the modulation of the electroluminescence is proposed, combining the simultaneous use of the IMPS of a reference photodiode and the impedance spectroscopy of the light source. Discerning whether these low-frequency signal 'tails' are due to the measurement setup or the sample is of major importance to avoid misinterpretations in any study. This is particularly important for preventing misinterpretations in studies on perovskite solar cells whose instability and ion-conductivity phenomena relate to the low-frequency region of the spectra

Light-stable methylammonium-free inverted flexible perovskite solar modules on PET exceeding 10.5% on a 15.7 cm2 Active Area

[Image: see text] Perovskite solar modules (PSMs) have been attracting the photovoltaic market, owing to low manufacturing costs and process versatility. The employment of flexible substrates gives the chance to explore new applications and further increase the fabrication throughput. However, the present state-of-the-art of flexible perovskite solar modules (FPSMs) does not show any data on light-soaking stability, revealing that the scientific community is still far from the potential marketing of the product. During this work, we demonstrate, for the first time, an outstanding light stability of FPSMs over 1000 h considering the recovering time (T(80) = 730 h), exhibiting a power conversion efficiency (PCE) of 10.51% over a 15.7 cm(2) active area obtained with scalable processes by exploiting blade deposition of a transporting layer and a stable double-cation perovskite (cesium and formamidinium, CsFA) absorber