Efficient perovskite solar cells by metal ion doping

Abstract

BROADER CONTEXTPerovskite solar cells, which promise to deliver the highest efficiency, lowest cost nextgeneration PV technology, have been largely advanced over the last few years by improvements in the polycrystalline thin film quality. So far, improvements in film uniformity and smoothness, have mainly been at the expense of crystalline grain-size, and charge recombination losses at defect sites. High luminescence efficiency, which is an indication of better optoelectronic quality, has generally been found in films with polycrystalline grains of many micrometres in scale. This suggests a current compromise between ideal morphology and ideal optoelectronic quality.For traditional semiconductors and crystalline solids, the influence of impurity ion doping has been studied extensively and can either alter the crystallisation or induced electronic positive or negative type doping. However, in the perovskite community, impurity doping has been largely unexplored. Here, we show that doping the perovskite solution with Al 3+ , which has a much smaller ionic radius than Pb 2+ , has profoundly positive influenced on the crystalline and optoelectronic quality of the perovskite absorber layer: We demonstrate a two-fold increase in the photoluminescence quantum efficiency and a significantly reduced electronic disorder, despite the films still having polycrystalline grains on the order of one micrometer is scale. This largely overcomes the trade-off between film smoothness and optoelectronic quality, and these improvements translate into highly efficient planar heterojunction perovskite solar cells.Our work paves the way for further improvement of the optoelectronic quality of perovskite thin films, and subsequent devices, via highlighting a new avenue for investigation of the role of dopant impurities upon crystallisation and controlling the electronic defect density in the perovskite thin films. ABSTRACTRealizing the theoretical limiting power conversion efficiency (PCE) in perovskite solar cells requires a better understanding and control over the fundamental loss processes occurring in the bulk of the perovskite layer and at the internal semiconductor interfaces in devices. One of the main challenges is to eliminate the presence of charge recombination centres throughout the film which have been observed to be most densely located at regions near the grain boundaries.Here, we introduce aluminium acetylacetonate to the perovskite precursor solution, which improves the crystal quality by reducing the microstrain in the polycrystalline film. At the same time, we achieve a reduction in the non-radiative recombination rate, a remarkable improvement in the photoluminescence quantum efficiency (PLQE) and a reduction in the electronic disorder deduced from an Urbach energy of only 12.6 meV in complete devices. As a result, we demonstrate a power-conversion efficiency (PCE) of 19.1% with negligible hysteresis in planar heterojunction solar cells comprising all organic p and n-type charge collection layers. Our work shows that...

Influence of Interface Morphology on Hysteresis in Vapor‐Deposited Perovskite Solar Cells

“…In contrast, for the type D, which we will refer to as an "inverted" device architecture, the PCBM ETL was deposited after the MAPbI 3 was evaporated on a poly(4butylphenyldiphenylamine) HTL. [27] Figure 1a shows the J-V characteristics of the champion devices for each architecture type A-D. For the type A (cTiO 2 ETL) a clear hysteresis can be seen between the forward and reverse J-V sweep. While the standard method to determine the PCE of a solar cell is based on analysis of the J-V curve, [28] it has been proposed that an alternate method measuring the "stabi lized power output" (SPO) may be more appropriate in assessing the working efficiency of hysteretic cells.…”

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

“…However, recently several studies, using different perovskite systems, have shown a good correlation between reduced microstrain and enhanced optoelectronic properties. [16,43,44] We suggest that either microstrain itself, or the crystal defects responsible for microstrain, or both are closely related to the defects responsible for negating high optoelectronic quality in these materials. Hence, with respect to attempting to realize an ideal perovskite semiconductor, minimizing microstrain in the polycrystalline thin films is likely to be a good metric to assess, and lead to improved optoelectronic quality.…”

“…[39][40][41] Here, we quantify the effects of microstrain in our perovskite films by analyzing the peak broadening in the diffraction patterns according to the modified Williamson-Hall method. [16,40,42,43] Additional information is provided in the Supporting Information. We show in Figure 4A, a clear reduction in microstrain as we dissolve the colloids in the 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.…”

In Situ Studies on Phase Transitions of Tris(acetylacetonato)-Aluminum(III) Al(acac)3

“…The huge number of papers is due to possible applications, i.e., Al(acac) 3 is a common precursor for the preparation of alumina Al 2 O 3 , [36,[42][43][44] for thin films via CVD [45,46] or nanoparticles [42] and several polymers, e.g., polycarbosilanes (PACS) [47,48]. It is also used as charging additive for liquid electrostatic toners [49] and as additive for perowskite precursor solutions improving crystal quality and reducing strain in the films for perowskite solar cells [50].…”