Enhanced optoelectronic quality of perovskite thin films with hypophosphorous acid for planar heterojunction solar cells

Abstract

Solution-processed metal halide perovskite semiconductors, such as CH3NH3PbI3, have exhibited remarkable performance in solar cells, despite having non-negligible density of defect states. A likely candidate is halide vacancies within the perovskite crystals, or the presence of metallic lead, both generated due to the imbalanced I/Pb stoichiometry which could evolve during crystallization. Herein, we show that the addition of hypophosphorous acid (HPA) in the precursor solution can significantly improve the film quality, both electronically and topologically, and enhance the photoluminescence intensity, which leads to more efficient and reproducible photovoltaic devices. We demonstrate that the HPA can reduce the oxidized I2 back into I−, and our results indicate that this facilitates an improved stoichiometry in the perovskite crystal and a reduced density of metallic lead.

Alkali Metal Doping for Improved CH3NH3PbI3 Perovskite Solar Cells

“…These peaks are associated with Pb 2+ in the perovskite. It was noteworthy to point out that another pair of peaks with lower binding energies (141.6 and 136.8 eV) with the signature of Pb 0 were detected when the perovskite films were etched by argon ion seen in Figure S3 (Supporting Information) 48, 49. The peaks of Na 1s and K 2p were also observed, confirm the existence of Na + and K + .…”

“…In this regard, combination of solvent and dopants have been tested, such as: the introduction of different source of metal or the solvent, the utilization of single or mixed solvent,19, 20 a variety of lead salt precursors,21, 22 as well as so‐called engineering step23 or the exotic additive: ionic liquid,24 Lewis base25, 26, 27 or other organic and inorganic additives (MACl,28 Guanidinium,29 water,30, 31 H 3 PO 2 ,32, 33 1,8‐diiodooctane (DIO),34 1‐chloronaphthalene (CN),35 hydroiodic acid (HI),36, 37 aluminum acetylacetonate (Al‐acac 3 ),38 LiI,39 Na + 40 etc. ), which induce the nucleation and crystallization process of perovskite thin films.…”

High Open‐Circuit Voltages in Tin‐Rich Low‐Bandgap Perovskite‐Based Planar Heterojunction Photovoltaics

“…[30] Especially, for the low-bandgap perovskite (60% and 80% Sn content) based PV devices, the obtained V OC 's are significantly high given the fact that planar heterojunction-based PV device configuration is generally known to demonstrate low V OC 's in comparison with device architectures incorporating mesoporous charge extraction layers. [35,48,49] The EQEs measured for these devices are lower at higher wavelengths due to lower absorption in the IR absorption compared to that in the visible region (see the Supporting Information, Figure S1) and this could potentially limit the J SC 's that we get in our devices. This problem has been attributed to the presence of MAPb x Sn 1-x Cl 3 up to 33% of the total material in the thin film that does not absorb visible light due to its large bandgap (>3 eV), [50] and the longer wavelength absorption and EQE can be increased by adding molecular iodine into the precursor solution that reduces this pure chloride perovskite phase to below 10%, hence, enhancing the longer Adv.…”

“…PDS is capable of measuring 4-5 orders of magnitude dynamic range absorption data that allow for assessing the perovskite semiconductor quality and measuring disorder in terms of Urbach energy "E U " with a higher degree of sensitivity compared to linear absorption measurement techniques. [34][35][36] Urbach energy can be derived from the following expression:…”

Crystallization Kinetics and Morphology Control of Formamidinium–Cesium Mixed‐Cation Lead Mixed‐Halide Perovskite via Tunability of the Colloidal Precursor Solution

“…Furthermore, it has been shown that additives to the precursor solution can impact the nucleation rate, morphology, grain size, and crystallinity of the perovskite. Additives, such as polymers, [13] small molecules, [14,15] metal ions, [16] water, [17] or acids, [18][19][20][21][22][23][24] can play a vital role in material quality, enabling improvements in material stability, optoelectronic properties, and photovoltaic performance.Work by Yan et al characterized the 1:1 CH 3 NH 3 I:PbI 2 stoichiometric precursor solution as a colloidal dispersion in a mother solution, rather than a pure solution. [25] When fabricated on planar substrate, this colloidal solution forms films with poor morphology, due to the presence of rodshaped colloids comprised of a soft coordination complex in the form of a lead polyhalide framework.…”