Dr Imalka Jayawardena

The development of perovskite solar cells (PSCs) with low recombination losses, at low processing temperatures is an area of growing research interest as it enables compatibility with roll-to-roll processing on flexible substrates as well as with tandem solar cells. The inverted or p-i-n device architecture has emerged as the most promising PSC configuration due to the possibility of using low temperature processable organic hole transport layers and more recently, self-assembled monolayers such as, [4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid (Me-4PACz). However, devices incorporating these interlayers suffer from poor wettability of the precursor leading to pin hole formation and poor device yield. Here, we demonstrate the use of alumina nanoparticles (Al2O3 NPs) for pinning the perovskite precursor on Me-4PACz, thereby improving the device yield. While similar wettability enhancements can also be achieved by using poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), a widely employed surface modifier, the incorporation of Al2O3 NPs results in significantly enhanced Shockley-Read-Hall recombination lifetimes exceeding 3 μs, which is higher than those on films coated directly on Me-4PACz and on PFN-Br modified Me-4PACz. This translates to a champion power conversion efficiency of 19.9% for PSCs fabricated on Me-4PACz modified with Al2O3, which is a ∽20% improvement compared to the champion device fabricated on PFN-Br modified Me-4PACz.

Perovskite solar cells (PSCs) have shown rapid development recently, whereas nonideal stability remains the chief obstacle toward commercialization. Thus, it is of utmost importance to probe the degradation pathway for the entire device. Here, the extrinsic stability of inverted PSCs (IPSCs) is investigated by using standard shelf-life testing based on the International Summit on Organic Photovoltaic Stability protocols (ISOS-D-1). During the long-term assessment of 1700 h, the degraded power conversion efficiency is mainly caused by the fill factor (53% retention) and short-circuit current density (71% retention), while the open-circuit voltage still maintains 97% of the initial values. Further absorbance evolution and density functional theory calculations disclose that the perovskite rear-contact side, in particular for the perovskite/fullerene interface, is the predominant degradation pathway. This study contributes to understanding the aging mechanism and enhancing the durability of IPSCs for future applications.

X-ray detectors currently employed in dosimetry suffer from a number of drawbacks including the inability to conform to curved surfaces and being limited to smaller dimensions due to available crystal sizes. In this study, a hybrid X-ray detector (HXD) has been developed which offers real-time response with added advantages of being highly sensitive over a broad energy range, mechanically flexible, relatively inexpensive, and able to be fabricated over large areas on the desired surface. The detector comprises an organic matrix embedded with high-atomic-number inorganic nanoparticles which increase the radiation attenuation and within the device allows for simultaneous transfer of electrons and holes. The HXD delivers a peak response of 14 nA cm(-2), which corresponds to a sensitivity of 30.8 mu C Gy(-1) cm(-2), under the exposure of 6-MV hard X-rays generated by a medical linear accelerator. The angular dependence of the HXD has been studied, which offers a maximum variation of 26% in the posterior versus lateral beam directions. The flexible HXD can be conformed to the human body shape and is expected to eliminate variations due to source-to-skin distance with reduced physical evaluation complexities.

Curved X-ray detectors have the potential to revolutionise diverse sectors due to benefits such as reduced image distortion and vignetting compared to their planar counterparts. While the use of inorganic semiconductors for curved detectors are restricted by their brittle nature, organic-inorganic hybrid semiconductors which incorporated bismuth oxide nanoparticles in an organic bulk heterojunction consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) are considered to be more promising in this regard. However, the influence of the P3HT molecular weight on the mechanical stability of curved, thick X-ray detectors remains less well understood. Herein, high P3HT molecular weights (>40 kDa) are identified to allow increased intermolecular bonding and chain entanglements, resulting in X-ray detectors that can be curved to a radius as low as 1.3 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard dark current of

Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 μm due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1–1 mm thickness regime is demonstrated, having a high sensitivity of ∼160 μC mGy–1 cm–3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.

The remarkable optoelectronic performance of metal halide perovskites has instigated fundamental research into their photophysics and photochemistry, including the fundamentals of light absorption and emission, and electrical transport. Recent studies have revealed unexpected hot carrier phenomena in metal halide perovskites, whereby at energies high above the band gap, and at high excitation intensities, long-lived hot carriers have been observed 1 which can undergo long-range transport. 2 This has opened the possibility of harvesting and exploiting hot carriers in semiconductor devices, for instance to overcome the Schockley-Queisser limit in photovoltaic cells.

Interface engineering is an effective means to enhance the performance of thin‐film devices, such as perovskite solar cells (PSCs). Herein, a conjugated polyelectrolyte, poly[(9,9‐bis(3′‐((N,N‐dimethyl)‐N‐ethyl‐ammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]di‐iodide (PFN‐I), is used at the interfaces between the hole transport layer (HTL)/perovskite and perovskite/electron transport layer simultaneously, to enhance the device power conversion efficiency (PCE) and stability. The fabricated PSCs with an inverted planar heterojunction structure show improved open‐circuit voltage (Voc), short‐circuit current density (Jsc), and fill factor, resulting in PCEs up to 20.56%. The devices maintain over 80% of their initial PCEs after 800 h of exposure to a relative humidity 35–55% at room temperature. All of these improvements are attributed to the functional PFN‐I layers as they provide favorable interface contact and defect reduction.

Mixed halide Perovskite solar cells (PSCs) are commonly produced by depositing PbCl2 and CH3NH3I from a common solvent followed by thermal annealing, which in an up-scaled manufacturing process is likely to take place under ambient conditions. However, it has been reported that, similar to the effects of thermal annealing, ambient humidity also affects the crystallisation behaviour and subsequent growth of the Perovskite films. This implies that both of these factors must be accounted for in solar cell production. In this work, we report for the first time the correlation between the annealing time, relative humidity (RH) and device performance for inverted, mixed halide CH3NH3PbI(3−x)Cl x PSCs with active area ≈1 cm2. We find a trade-off between ambient humidity and the required annealing time to produce efficient solar cells, with low humidities needing longer annealing times and vice-versa. At around 20% RH, device performance weakly depends on annealing time, but at higher (30%–40% RH) or lower (0%–15% RH) humidities it is very sensitive. Processing in humid environments is shown to lead to the growth of both larger Perovskite grains and larger voids; similar to the effect of thermal annealing, which also leads to grain growth. Therefore, samples which are annealed for too long under high humidity show loss of performance due to low open circuit voltage caused by an increased number of shunt paths. Based on these results it is clear that humidity and annealing time are closely interrelated and both are important factors affecting the performance of PSCs. The findings of this work open a route for reduced annealing times to be employed by control of humidity; critical in roll-to-roll manufacture where low manufacturing time is preferred for cost reductions.

Optical pump terahertz probe spectroscopy (OPTP) is a versatile non-contact technique that measures transient photoconductance decays with femtosecond temporal resolution. However, its maximum temporal range is limited to only a few nanoseconds by the mechanical delay lines used. We extended the temporal range of OPTP to milliseconds and longer while retaining sub-nanosecond resolution. A separate pump laser was electrically synchronized to the probe pulses, allowing the pump-probe delay to be controlled with an electronic delay generator. We demonstrated the capabilities of this technique by examining the photoconductance decays of semiconductors with lifetimes ranging over six orders of magnitude: III-Vs, metal halide perovskites, germanium, and silicon. A direct comparison of results on silicon from OPTP and inductively coupled photoconductance decay highlighted the higher spatial and temporal resolution of OPTP, which allowed in-plane and out-of-plane carrier diffusion to be studied.

Printing techniques have been widely adopted in the fabrication of flexible electronic components. However, its application is still limited in complex control and communication circuitry due to the low performance and low fabrication uniformity amongst printed devices, compared to conventional electronics. Thus, the electronic systems in real-world applications are hybrid integrations of printed and conventional electronics. Here we demonstrate a low-cost, low-complexity, fully-printable flexible photodetector that can withstand over 100 1 mm-radius bending cycles using a simple and scalable two-step fabrication process. The prototypes are implemented in an augmented book system to automatically detect the ambient light through optical apertures on paper of a printed book, and then transmit the information to an adjunct device. This technique demonstrates the utility of low-cost materials and processes for robust large area sensing applications and could act as a gateway to pertinent multimedia information.

The deposition of amorphous carbon electrical contacts on a diamond radiation detector by Pulsed Laser Deposition (PLD) is introduced as a novel technique for producing tissue equivalent X-ray dosimeters. Three devices were fabricated with the following electrical contacts: pure amorphous carbon (labelled Poly-C), amorphous carbon mixed with Nickel (PLD) (labelled Poly-C/Ni) and conventional sputtered Pt (labelled Poly-Pt). To examine the performance of PLD carbon as a contact, a set of X-ray detection characteristics was studied and compared to those of Poly-Pt. This investigation includes current–voltage characteristics, linearity and dose rate dependence, sensitivity and specific sensitivity, photoconductive gain, stability, reproducibility and time response (rise and fall-off times). The experimental results suggest that Poly-C/Ni is suitable for an X-ray dosimeter. It shows a high signal to noise ratio (SNR) of ~ 3300, approximately linear relationship between the photocurrent and the dose rate and a sensitivity of 65 nC/Gy. In addition the current signal is stable and reproducible (within 0.26%) and the rise and fall-off times are less than 1.1 and 0.4 s, respectively.

The remarkable optoelectronic performance of metal halide perovskites has instigated fundamental research into their photophysics and photochemistry, including the fundamentals of light absorption and emission, and electrical transport. Recent studies have revealed unexpected hot carrier phenomena in metal halide perovskites, whereby at energies high above the band gap, and at high excitation intensities, long-lived hot carriers have been observed 1 which can undergo long-range transport. 2 This has opened the possibility of harvesting and exploiting hot carriers in semiconductor devices, for instance to overcome the Schockley-Queisser limit in photovoltaic cells.

Electronic skins (e-skins), which can seamlessly adapt and adhere to the body to mimic the functionality of human skin, are a rapidly emerging research area. Such e-skins have the potential to revolutionize artificial prosthetics, robotics, human-machine interfacing, and health monitoring applications. Powering the e-skin is a critical challenge at present due to strict performance criteria, including flexibility, stretchability, mobility, and autonomous operation. One of the most promising approaches to overcome some of these challenges is to scavenge energy from the human body's movements and its surrounding environment. This paper outlines some of the key potential developments that enable energy harvesting through mechanical, thermal affects, and low light sources, as well as energy management and storage technologies, which could lead toward the construction of autonomous e-skin modules and self-powered sensing systems.

A new model which comprehensively explains the working principles of contact-mode Triboelectric Nanogenerators (TENGs) based on Maxwell’s equations is presented. Unlike previous models which are restricted to known simple geometries and derived using the parallel plate capacitor model, this model is generic and can be modified to a wide range of geometries and surface topographies. We introduce the concept of a distance-dependent electric field, a factor not taken in to account in previous models, to calculate the current, voltage, charge, and power output under different experimental conditions. The versatality of the model is demonstrated for non-planar geometry consisting of a covex-conave surface. The theoretical results show excellent agreement with experimental TENGs. Our model provides a complete understanding of the working principles of TENGs, and accurately predicts the output trends, which enables the design of more efficient TENG structures.

Triple cation CsFAMA perovskite films fabricated via a one-step method have recently gained attention as an outstanding light-harvesting layer for photovoltaic devices. However, questions remain over the suitability of one-step processes for the production of large-area films, owing to difficulties in controlling the crystallinity, in particular, scaling of the frequently used anti-solvent washing step. This can be mitigated through the use of the two-step method which has recently been used to produce large-area films via techniques such as slot dye coating, spray coating or printing techniques. Nevertheless, the poor solubility of Cs containing salts in IPA solutions has posed a challenge for forming triple cation perovskite films using the two-step method. In this study, we tackle this challenge through fabricating perovskite films on a caesium carbonate (Cs2CO3) precursor layer, enabling Cs incorporation within the film. Synergistically, we find that Cs2CO3 passivates the SnO2 electron transport layer (ETL) through interactions with Sn 3d orbitals, thereby promoting a reduction in trap states. Devices prepared with Cs2CO3 treatment also exhibited an improvement in the power conversion efficiency (PCE) from 19.73% in a control device to 20.96% (AM 1.5G, 100 mW cm−2) in the champion device. The Cs2CO3 treated devices (CsFAMA) showed improved stability, with un-encapsulated devices retaining nearly 80% efficiency after 20 days in ambient air.

Tin (Sn)-based perovskites are promising for lower band gap and tandem solar cells due to their red-shifted band gap towards the infrared region. However, Sn based hybrid metal halide perovskite is well known to suffer rapid photoconductivity decay due to the oxidation of Sn 2+ to Sn 4+ under atmospheric oxygen. Here, the charge carrier dynamics of tin based metal halide perovskite is investigated using optical pump-terahertz probe (OPTP) spectroscopy. Also, the performance of the perovskite films at different annealing temperatures is compared and the best performing temperature is identified. The Sn based perovskite films exhibited a remarkable charge carrier mobility and diffusion length.