Open-circuit Voltage Loss in CH₃NH₃SnI₃ Perovskite Solar Cells

Abstract

Organicinorganic perovskite solar cells based on tin halides exhibit a small open-circuit voltage (V OC ) because of a large photon energy loss from band-gap energy, as large as 0.81.0 eV. In this study, we discussed the origin of the V OC loss in CH 3 NH 3 SnI 3 -based devices by measuring the temperature dependence of V OC . As a result, we found that the large loss in V OC is mainly due to the surface recombination at the interface rather than the bulk recombination in the perovskites. Organicinorganic perovskite solar cells based on lead halides (CH 3 NH 3 PbI 3¹x Cl x ) have attracted much attention of many researchers owing to their superior optical and electronic properties, such as high absorption coefficient and charge carrier mobility, small exciton binding energy, and long diffusion length of charge carriers. 13 Moreover, they have shown a steep increase in the power conversion efficiency (PCE) from 3.8% in 2009 4 to more than 22% in 2016 5,6 and hence are recently considered as a promising next-generation photovoltaic device. However, the use of toxic material (Pb) still remains a major obstacle for the commercialization of these devices. Thus, it is highly desirable for environmental friendly devices to replace lead(II) with other nontoxic metal elements, such as germanium(II), tin(II), and bismuth(III). 712Very recently, several groups have successfully fabricated organicinorganic perovskites solar cells based on tin halides (CH 3 NH 3 SnI 3 ), which yields the best PCE in the range of 56%. 8,11,12 However, the device performance of tin-based perovskite solar cells still lags far behind that of lead-based counterparts. In particular, the open-circuit voltage (V OC ) is typically much smaller than the band-gap energy (E g ). In other words, the photon energy loss (E loss ) from E g is as large as 0.8 1.0 eV, which is larger by 0.30.5 eV than that for lead-based devices.1315 Hence, it is highly required to study the origin of the V OC loss for further improvements in the device performance of tin-based perovskite solar cells.Herein, we have fabricated the CH 3 NH 3 SnI 3 -based solar cells with a layer structure as shown in Figure 1a, and studied the origin of the photon energy loss E loss = E g ¹ qV OC (q is the elementary charge) by measuring the temperature dependence of V OC . As a result, we found that the dominant recombination mechanism in these devices is the surface recombination at the interface rather than the bulk recombination of the perovskite layer, eventually resulting in a large loss in V OC .Organicinorganic perovskite solar cells based on tin halides (CH 3 NH 3 SnI 3 ) were fabricated as follows. A 20-nmthick layer of dense TiO 2 (d-TiO 2 ) was coated onto an F-doped SnO 2 (FTO; Asahi Glass) substrate by plasma-enhanced atomic layer deposition (a source material was titanium(IV) isopropoxide (TTIP)). Mesoporous TiO 2 (mp-TiO 2 ) films were deposited onto the d-TiO 2 /FTO substrate by spin-coating a TiO 2 paste (Nikki Syokubai Kasei, PST-18NR) diluted by ethanol (1:3 by weight). The...

Formation of Stable Tin Perovskites Co‐crystallized with Three Halides for Carbon‐Based Mesoscopic Lead‐Free Perovskite Solar Cells

“…Organic–inorganic lead halide perovskite (e.g., CH 3 NH 3 PbI 3 ) solar cells (PSCs) have attracted much attention because their device performance has progressed rapidly . Because of a concern about conventional PSCs containing toxic lead, lead‐free PSCs, such as methylammonium tin tri‐iodide perovskite (MASnI 3 ) solar cells have been developed . The problem with the pure tin PSC was its unstable device performance, such that the reported efficiencies of power conversion (PCE), 5–6 %, were difficult to reproduce .…”

Suppression of Charge Carrier Recombination in Lead-Free Tin Halide Perovskite via Lewis Base Post-treatment

“…Another reason is due to the tin and iodide vacancies as a result of the weak bonding energy of these elements . Fast crystallization of tin halide perovskite promotes the formation of pinholes and a rough surface, finally leading to a higher charge carrier recombination rate. , All these result in a large open-circuit voltage loss of more than 0.8 eV …”

Charge traps in lead-halide perovskites with different grain sizes

“…1,2) As shown in Fig. 2 against the photon energy E. [20][21][22][23] The bandgap energy increased from 1.57 to 1.59 eV with decreasing grain size of perovskites. These results are consistent with previous reports.…”