Influence of Film Thickness on the Electronic Band Structure and Optical Properties of P–I–N CH₃NH₃PbI_3−xCl_x Perovskite Solar Cells

Abstract

The outstanding properties of hybrid organic-inorganic perovskite semiconductors, such as high charge carrier mobility, [1] high absorption of the solar spectrum, [2] low exciton binding energies, [3] and long diffusion lengths, [4][5][6] have motivated a worldwide effort to develop their use in solar cells devices. Consequently, their power conversion efficiencies (PCEs) have increased more quickly than any other emerging photovoltaic technology. Currently, the highest certified PCE achieved with perovskite solar cells (PSCs) has reached over 25%. [7] Planar heterojunction P-I-N and N-I-P PSCs are promising structures compared with their counterpart mesoporous PSCs because of the high annealing temperature and additional processing difficulties required for preparing mesoporous TiO 2 . [8] Among different methylammonium lead halide perovskite, such as CH 3 NH 3 PbI 3 , CH 3 NH 3 PbBr 3 , and CH 3 NH 3 PbI 3Àx Cl x , the CH 3 NH 3 PbI 3Àx Cl x structure is more suitable for planar heterojunction PSCs, owing to its remarkable properties such as long diffusion lengths (L D ) for both holes and electrons [2,9,10] and an appropriate bandgap. [11] The L D of CH 3 NH 3 PbI 3 is about 100 nm; [5] however, it can be increased up to 1 μm upon the addition of chlorine (Cl) to form the CH 3 NH 3 PbI 3Àx Cl x structure. [4] This long L D and wider bandgap contribute significantly to improving the open-circuit voltage (V oc ) in photovoltaic devices. [2] Nevertheless, the amount of Cl that is retained in the structure after processing remains a debated issue. [12] Some studies report the presence of Cl, [13][14][15][16][17][18][19][20][21] whereas others indicate that Cl does not exist in the final structure, [22][23][24][25][26][27][28][29][30][31][32] for instance, because the X-ray diffraction (XRD) peaks of CH 3 NH 3 PbI 3Àx Cl x and CH 3 NH 3 PbI 3 are identical. Despite this fact, the Cl addition plays a pivotal role in improving morphology [29] and has much better stability against moisture, [33] which makes it an unprecedently strong candidate for PSCs. There are two common methods known for the fabrication of CH 3 NH 3 PbI 3Àx Cl x : one is the physical vapor evaporation method, [33][34][35] and the second, more frequently used method is spin coating [12,31] followed by thermal annealing. Vapor deposition yields smooth and uniform films with variable thickness and allows precise control of the film composition. [28][29][30][31][32][33][34] However, the expense and low observed PCEs of vapor processing make spin coating [34] a more feasible and widely used approach, which is better suited for the mass production of PSCs.Despite the importance of active layer thickness, few studies have examined the effect of film thickness on the properties of CH 3 NH 3 PbI 3Àx Cl x films. A layer-by-layer approach using both vapor and solution deposition demonstrated CH 3 NH 3 PbI 3Àx Cl x perovskite films with variable thickness, uniform morphologies,

Lithium Polystyrene Sulfonate as a Hole Transport Material in Inverted Perovskite Solar Cells

“…As shown in Figure 4(a)–(c), perovskite films deposited on Li:PSS had smooth morphologies with pin‐holes and small grain sizes. Poor morphology has a detrimental effect on device performance since pin‐holes and smaller grains act as trap states which are potential sites for recombination and may result in poor device parameters [47–49] . This is consistent with the observation that devices using Li:PSS only suffered from poor performance with low FF values.…”

Multifunctional Additives to Enhanced Perovskite Solar Cell Performance

“…E_{\text{cutoff}} - E_{\text{onset}} \left.\right)$$where hv = 21.22 eV is the emission energy from He irradiation. [ 47 ] In brief, the E VB values of 5‐ADI and control are −5.76 and −5.88 eV, respectively. According to the calculated optical bandgap, the corresponding conduction band minimum ( E CB ) values are defined to be −4.24 and −4.36 eV.…”

A Simple Cu(II) Polyelectrolyte as a Method to Increase the Work Function of Electrodes and Form Effective p‐Type Contacts in Perovskite Solar Cells

“…The details of this method are described in a previous report. [59] The optical field distribution in a device using PEDOT:PSS as an HTL is shown in Figure 3d; it can be seen that the optical field is largely attenuated in the active layer for wavelengths up to the absorption onset of the material. In order to investigate the influence of the HTL on the optical field, the optical field distribution was re-calculated assuming a completely transparent HTL (κ = 0).…”