Dr Jay Patel

Abstract:

Impressive power conversion efficiencies coupled with the relative ease of fabrication have made perovskite solar cells a front runner for next-generation photovoltaics. Although perovskite films and optoelectronic devices have been widely studied, relatively little is known about the chemistry of the precursor solutions. Here, we present a study on the hydrolysis of N,N-dimethylformamide, correlating how pH changes related to its degradation affect the crystallization of MAPbI3xClx perovskite films. By careful manipulation of the pH, and the resulting colloid distribution in precursor solutions, we fabricate perovskite films with greatly improved crystallinity, which when incorporated into photovoltaic devices reproducibly yield efficiencies of over 18%. Extending this method to the mixed cation, mixed halide perovskite FA0.83MA0.17Pb(I0.83Br0.17)3, we obtain power conversion efficiencies of up to 19.9% and open-circuit voltages of 1.21 V for a material with a bandgap of 1.57 eV, achieving the lowest yet reported loss in potential from bandgap to a VOC of only 360 mV.

Abstract:

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon re-absorption which masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50nm and 533nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon re-absorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient, of value 6.8x10(-10)cm(3)s(-1), is common to all samples irrespective of film thickness. Hence we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples, with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, while thinner films or index-matched surrounding layers can reduce the possibility for photon re-absorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

Abstract:

Organohalide perovskites have emerged as promising light-sensing materials because of their superior optoelectronic properties and low-cost processing methods. Recently, perovskite-based photodetectors have successfully been demonstrated as both broadband and narrowband varieties. However, the photodetection bandwidth in perovskite-based photodetectors has so far been limited to the near-infrared regime owing to the relatively wide band gap of hybrid organohalide perovskites. In particular, short-wavelength infrared photodiodes operating beyond 1 μm have not yet been realized with organohalide perovskites. In this study, narrow band gap organic dyes are combined with hybrid perovskites to form composite films as active photoresponsive layers. Tuning the dye loading allows for optimization of the spectral response characteristics and excellent charge-carrier mobilities near 11 cm 2 V -1 s -1 , suggesting that these composites combine the light-absorbing properties or IR dyes with the outstanding charge-extraction characteristics of the perovskite. This study demonstrates the first perovskite photodiodes with deep near-infrared and short-wavelength infrared response that extends as far as 1.6 μm. All devices are solution-processed and exhibit relatively high responsivity, low dark current, and fast response at room temperature, making this approach highly attractive for next-generation light-detection techniques.

Abstract:

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2 )2 ]0.83 Cs0.17 Pb(Br0.2 I0.8 )3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm(2) V(-1) s(-1) . Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.