A significant volume of literature exists that describe minor changes in composition and/or microstructure in perovskite solar cells (PSCs) as the driving force for incremental improvements in device performance. Many authors cite crystallinity as a fundamental driver of performance improvements, yet often do so without quantitatively defining crystallinity or, importantly, addressing the important questions behind their observations. Here I will discuss two recent case studies where processing modifications have been investigated as a means of controllably varying active layer crystallinity and describe the impact on device performance. i) We demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3,MAPI). We show that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where a small excess of methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV and, in parallel, that the photoluminescence (PL) yield increases 15x, indicative of a suppression of non-radiative recombination pathways. This is coupled with the development of crystallographic texture (110) in the MAPI. In-situ transient optoelectronic measurements of the charge concentration in PSCs under operation suggest that the concentration of trapped charges (either at interfaces or in the bulk) is some 5x lower at matched Voc. We believe these trap states originate in/near the disordered or amorphous areas between MAPI crystallites, resulting at least in part from orientation mismatch between crystalline domains. ii) Secondly, we identify previously unobserved nanoscale defects residing within individual grains of solution processed MAPI thin films. Using scanning transmission electron microscopy (STEM) we identify the defects to be inherently associated with the established solution-processing methodology and introduce a facile processing modification to eliminate such defect formation. Specifically, defect elimination is achieved by co-annealing the as deposited MAPI layer with the electron transport layer PCBM resulting in devices that significantly outperform devices prepared using the established methodology, achieving PCE increases from 13.6 % to 17.7 %. The use of TEM allows us to correlate the performance enhancements to improved intra-grain crystallinity and show that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intra-grain crystallinity drives an improvement of charge collection, a reduction of surface recombination at the MAPI/PCBM interface and a change in the density of local sub-gap states.
Nanoscale optoelectronic devices based on coplanar nanogap electrodes, when compared with traditional vertical devices, exhibit attractive characteristics, such as high density of integration, high sensitivity, fast response and multifunctionality. Moreover, their low-cost high-throughput fabrication on flexible disposable substrates opens up several new applications in sectors ranging from telecommunications and consumer electronics to healthcare - to name a few. However, their commercial exploitation has been hitherto impeded by technological bottlenecks, owing to the incompatibility of currently available fabrication techniques, eg. e-beam lithography, with industrial upscaling.
Adhesion lithography is a nanopatterning technique that allows the facile high yield fabrication of coplanar metal electrodes separated by a sub-15 nm gap on large area substrates of any type, including plastic. These electrodes, when combined with solution-processed and/or low-dimensional nanostructured materials deposited at low, plastic-compatible, temperatures give rise to nanoscale optoelectronic devices with intriguing properties.
It will be shown that both nanoscale light-emitting and light-sensing devices can be fabricated upon using light-emitting polymers along with self-assembling surface modifiers, and lead halide perovskites and functionalised colloidal PbS quantum dots, respectively. Emphasis will be given in recent advances in flexible nanoscale photodetectors fabricated with nanogap coplanar electrodes, operating in DUV up to NIR part of the spectrum. These devices exhibit high responsivity, sensitivity and fast response speed (hundreds of nanoseconds) owing to the extreme downscaling of key device dimensions. These results demonstrate that adhesion lithography combined with advanced materials concepts constitutes a new fabrication paradigm enabling a plethora of advanced applications within the field of flexible electronics.
The formation of a well-defined, reproducible ZnO nanorod scaffold for hybrid photovoltaic applications has been investigated. A standard hydrothermal growth method was used and the influence of chemical additions in controlling length, width, density, and orientation was studied. The nanostructures prepared have been characterized by scanning electron microscopy, x-ray diffraction, UV-visible spectroscopy in addition to measurement of the wetting behavior. A standard procedure for the production of vertically orientated nanorods with a narrow size distribution, high areal density, and good wettability in aqueous solutions is presented.
We introduce a novel nanostructuring method for bulk heterojunction solar cells which is aimed at overcoming current
limitations associated with short exciton diffusion lengths and poor charge transport. By employing a nanosphere
templating technique porous interconnected films of copper phthalocyanine (CuPc) have been prepared. Subsequent
infiltration of the CuPc structures with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) results in the formation of
three dimensionally structured nanocomposites, consisting of interpenetrating and interconnected networks. The
lengthscale separation in the composite can be engineered to match exciton diffusion lengths and the interconnectivity is
compatible with good charge transport. We propose this templating strategy as a widely applicable solution to the
continued development of low-cost organic photovoltaics.
We describe a simple technique for the selective area modification of the bandgap in planar 3-D photonic crystals (PhC). The PhCs are grown by controlled drying of monosized polystyrene spheres. Uniaxial pressure of 41 MPa can produce a shift in the bandgap of ~90 nm from 230 nm spheres. An unexpected broadening of the bandgap is attributed to the change in topology associated with large necks formed between spheres at pressures greater than 10 MPa.