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

Abstract

] Of relevance to this work is the binary metal perovskite CH 3 NH 3 (Pb x Sn 1-x )I 3 [0 ≤ x ≤ 1]. [30,31] Interestingly, the bandgap bows and becomes lower when Sn 2+ is substituted by Pb 2+ for samples with 80% and 60% Sn content compared to 100% Sn-based perovskite, in line with previous observations. [30,31] While such tin-based perovskites offer tunable bandgaps down to 1.1 eV, the fabrication of efficient optoelectronic devices has been impeded by factors including poor semiconductor quality and low surface coverage. [30] As a consequence, solar cells made using these perovskites often exhibit very low efficiencies, with typical PCEs < 1% obtained for planar heterojunction devices. [30] To overcome this challenge, we have developed a novel elevated temperature processing method (depicted in Figure 1A), [32] for preparing CH 3 NH 3 (Pb x Sn 1-x )I 3 perovskites on a Poly(3,4-ethylenedioxythiophene):poly(styrenesulf onate) (PEDOT:PSS)/nickel oxide (NiO) bilayer, which results in the formation of large micron-sized grains ( Figure 1B) with almost complete substrate coverage. Our semiconductors not only exhibit relatively low energetic and structural disorder but also impart high PCEs when fabricated into a PV device. For PVs prepared using the lowest bandgap perovskites, open circuit voltages (V OC 's) approaching the prediction of the Shockley-Queisser (S-Q) model are demonstrated. Such promising performance metrics are obtained against a backdrop of fast radiative recombination and low photoluminescence quantum efficiencies (PLQEs), pointing toward the crucial role of high intrinsic charge carrier mobility in these low-bandgap semiconductors.To study the optical properties of the CH 3 NH 3 (Pb x Sn 1-x )I 3 [0 ≤ x ≤ 1] perovskite thin films, linear absorption and photoluminescence (PL) were measured as shown in Figure S1 (Supporting Information). It can be observed in Figure 1C that the bandgap bows as we substitute Pb 2+ in place of Sn 2+ (until 40% Sn 2+ ions are replaced by Pb 2+ ) and results in a nonmonotonic bandgap lowering similar to what was observed previously by Hao etal. [31] Briefly, the bandgap of the 60% and 80% Sn content films exhibit a lower bandgap than the 100% Sn-substituted films. A similar trend can also be traced in the PL spectra (see Figure S1B of the Supporting Information) where the PL spectra of 80% and 60% Sn content thin-film samples are red-shifted compared to the 100% Sn content thinfilm sample, which is consistent with the absorption spectra. Such anomalous bandgap bowing and lack of conformity with Vegard's law [31,33] have been attributed to the competition The performance of organometallic halide (hybrid) perovskite solar cells has improved dramatically in just a few years, with photovoltaic (PV) power conversion efficiencies (PCEs) now exceeding 22% for state-of-the-art devices. [1][2][3][4][5] This remarkable result, coupled with their low cost, tunability, and versatile lowtemperature preparation methods, makes hybrid perovskites one of the most promising semiconduct...

Vapor transport deposition of antimony selenide thin film solar cells with 7.6% efficiency

“…This means that trap-assisted recombination is present in both solar cells and is the dominating loss mechanism 24, 25 . The light-intensity-dependent V OC can provide critical insights into the recombination mechanism in the solar cells 26 . For V OC measurement, the device is open circuit, so there is no current extraction from the devices, and all photo-generated carriers recombine in Sb 2 Se 3 film.…”

“…For V OC measurement, the device is open circuit, so there is no current extraction from the devices, and all photo-generated carriers recombine in Sb 2 Se 3 film. Thus, the carrier recombination process can be reflected based on the relationship of V OC ∝ n ( k B T/q )ln( I ), where k B is the Boltzmann constant, T is the temperature and q is elementary charge 26–29 . n ( k B T/q ) is the slope of V OC vs. the natural logarithm of light-intensity ln( I ).…”

“…n ( k B T/q ) is the slope of V OC vs. the natural logarithm of light-intensity ln( I ). For trap-free solar cells, the slope of V OC vs. ln( I ) should be k B T/q (i.e., n  = 1) 26, 29 . For our devices, as shown in Fig.…”

The Emergence of the Mixed Perovskites and Their Applications as Solar Cells

“…Similar to previous reports, the partial Sn substitution in lead halide PVSK resulted in broadened absorption from 800 nm to 900 nm. Most recently, Sadhanala et al [116] reported a novel elevated temperature processing method to fabricate MASn x Pb 1−x I 3 (0 ≤ x ≤ 1) thin films, obtaining large micron-sized grains with almost complete surface coverage (Figure 12c,d). In their film fabrication process, the temperature of the hotplate was elevated to 240 °C after the deposition of PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) layer.…”

Photorefractive Effect in Organic–Inorganic Hybrid Perovskites and Its Application to Optical Phase Shifter

“…Since the excitation photon energy is higher than the bandgap, this PL signal is caused by one-photon excitation. [14,33] Therefore, the sharp band-edge absorption profile reflects the low defect density of the single crystal. By using a 1028 nm excitation pulse with a pulse fluence of 36 µJ cm −2 , the PL spectrum under two-photon excitation was clearly observed at 579 nm as shown by the black solid line in Figure 1a.…”