Timothy W. Jones

An applied light-beam induced current study of dye-sensitised solar cells: Photocurrent uniformity mapping and true photoactive area evaluation

The conditions for light-beam induced current (LBIC) measurement were experimentally optimised for dye-sensitised solar cells. The impacts of too fast a laser diode modulation frequency (f) and too short a dwell time (t0) were investigated for their distortions, artefacts, and noise on the overall photocurrent map image. Optimised mapping conditions for fastest measurement were obtained at a f = 15 Hz and t0 = 900 ms. Whole device maps (nominal area 4 × 4 mm2) were obtained on devices in which fabrication defects were intentionally induced. The defects were readily resolved with the LBIC setup and conditions. The inclusion of defects had the effect of broadening the photocurrent distribution and producing a sub-optimal tail to photocurrent histograms. Photoactive areas were derived from LBIC maps and were larger than those predicted by the projected screen printing pattern by up to 25%, which has obvious implications for efficiency measurements made on nominal projected active area.

Energy yield potential of perovskite-silicon tandem devices

Metal-halide perovskite photovoltaic devices have recently become of great interest to the research community with efficiencies of the thin film devices already reported to exceed a single junction cell efficiency of 20%. Of particular interest with this type of device is its potential to be integrated with the well established silicon photovoltaic technology into a monolithic tandem device with the potential to deliver efficiencies of greater than 30% In such devices the most attractive prospect is to monolithically integrate the perovskite and silicon materials into a planar single device for ease of deposition and device construction. Such a series connected device comes with inherent current matching restrictions on the operating performance of the individual junctions when working in tandem. These restrictions significantly complicate the calculation of potential energy yield for such a tandem device due to the impact of spectrum, angle of incidence and temperature. We model the performance of a tandem device using experimentally determined performance characteristics combined with representative resource and environment datasets to evaluate the energy yield potential of perovskite-silicon tandem under outdoor conditions. We observe that the largest impact is caused by the spectral irradiance distribution (7.6%) followed by angle of incidence (2.7%) and temperature response differential (0.7%). Our modelling indicates that these combine to alter the energy yield for a tandem device by 4.5%.

Tunable Crystallization and Nucleation of Planar CH3NH3PbI3 through Solvent-Modified Interdiffusion

A smooth and compact light absorption perovskite layer is a highly desirable prerequisite for efficient planar perovskite solar cells. However, the rapid reaction between CH3NH3I methylammonium iodide (MAI) and PbI2 often leads to an inconsistent CH3NH3PbI3 crystal nucleation and growth rate along the film depth during the two-step sequential deposition process. Herein, a facile solvent additive strategy is reported to retard the crystallization kinetics of perovskite formation and accelerate the MAI diffusion across the PbI2 layer. It was found that the ultrasmooth perovskite thin film with narrow crystallite size variation can be achieved by introducing favorable solvent additives into the MAI solution. The effects of dimethylformamide, dimethyl sulfoxide, γ-butyrolactone, chlorobenzene, and diethyl ether additives on the morphological properties and cross-sectional crystallite size distribution were investigated using atomic force microscopy, X-ray diffraction, and scanning electron microscopy. Furthermore, the light absorption and band structure of the as-prepared CH3NH3PbI3 films were investigated and correlated with the photovoltaic performance of the equivalent solar cell devices. Details of perovskite nucleation and crystal growth processes are presented, which opens new avenues for the fabrication of more efficient planar solar cell devices with these ultrasmooth perovskite layers.