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Origin and Whereabouts of Recombination in Perovskite Solar Cells Lidia Contreras Bernal, Manuel Salado, Anna Todinova, Laura Calio, Shahzada Ahmad, Jesus Idígoras, and Juan Antonio Anta J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017
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The Journal of Physical Chemistry
Locus of Recombination in Perovskite Solar Cells 158x107mm (72 x 72 DPI)
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Origin and Whereabouts of Recombination in Perovskite Solar Cells
Lidia Contreras-Bernala, Manuel Saladoa,b, Anna Todinovaa, Laura Caliob, Shahzada Ahmadb , Jesús Idígorasa,*, Juan A. Antaa,* a
Área de Química Física, Universidad Pablo de Olavide, E-41013, Sevilla, Spain.
b
Abengoa Research, C/Energía Solar n° 1, Campus Palmas Altas, 41014 Sevilla, Spain
ABSTRACT. The success of metal halide perovskite solar cells stems from high absorption combined with a low recombination rate. Despite the fact these properties are inherent to the perovskite material, the choice of selective contacts is critical to achieve high voltages according to experimental evidence. In this work, the impedance and the open-circuit photopotential are measured for two excitation wavelengths (blue and red light), two illumination directions (back and front) and at different temperatures. The open-circuit recombination characteristics of two different perovskite compositions, i.e. pure MAPbI3 and mixed ion-based (FAPbI3)0.85(MABr3)0.15, and with two different hole selective layers (Spiro-OMeTAD and P3HT) have been studied. Our results indicate that, for the studied devices, the recombination process that determines the open-circuit potential is governed by the bulk of the perovskite layer via a trap-limited mechanism, but surface-mediated recombination cannot be ruled out for P3HT contact or degraded devices. Further we propose a model that provides a general interpretation of the nature of recombination in perovskite solar cells.
INTRODUCTION
Recent photovoltaics has been revolutionized by the advent of metal halide perovskite (MHP) solar cells. This new family of materials has made it possible to achieve a certified current efficiency record above 22% in just a time span of a few years.1 One of the most exciting properties of MHPs is the capability of reaching open-circuit voltages very close to the thermodynamic limit, indicative of very low non-radiative recombination losses.2–4 Although this property seems to be related with the nature of the perovskite material itself,5–8 the fact is that the choice of electron and hole selective
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contact impacts significantly the apparent recombination rate and the open-circuit voltage that can be reached in working devices.9–11,1 This latter observation points to an important contribution of recombination events taking place at the perovskite interfaces, with some materials allowing for a faster recombination and hence producing a deleterious effect on the photovoltage. Recently Zarazúa and coworkers12 have proposed a surface recombination model that actually involves surface photogenerated carriers in the recombination kinetics of perovskite solar cells.
In this work we aim to address the concern, where in working devices the main recombination pathway is occurring. In other words, is photovoltage (open-circuit potential) determined by the bulk of the absorbing perovskite layer, by interfacial recombination or by a combination of both? To unravel this, we combine a collection of experiments where the optical generation spatial profile produced by the optical excitation is varied. On the other hand, we work with working devices at open circuit and at illumination intensities close to 1-sun. This is an important point because many previous studies on recombination kinetics has been based on time-resolved emission and absorption experiments, done (1) under excitation intensities not necessarily coincident with those relevant at 1-sun photovoltaic working conditions and (2) with isolated perovskite layers deposited on different substrates.2,13,14 However, it must be noted that the recombination kinetics, and thus the mechanism, varies significantly with charge density.15 Furthermore, the morphology and stability of the perovskite layer can be very different depending on the substrate in which it was deposited and the preparation conditions, which are likely to affect the recombination mechanism as well.16,17
Experiments were made with MHP solar cells based on two different perovskite compositions (MAPbI3 = MAI and (FAPbI3)0.85(MABr3)0.15 = MIX) and two different hole selective layers (HSL; 2,2´,7,7´-Tetrakis(N,N-di-pmethoxyphenylamine)-9,9´-spirobifluorene = Spiro and poly(3-hexylthiophene-2,5-diyl) = P3HT). For simplicity we will use the following notation to refer to the device configurations: MAI/P3HT, MAI/Spiro and MIX/Spiro. In addition, by controlling the concentration of the perovskite precursors, we were able to fabricate devices of varying thickness.
EXPERIMENTAL SECTION Device fabrication details FTO-coated glass (TEC15, Pilkington) patterned by laser etching was employed for the fabrication of perovskite based solar cells. Prior to perovskite deposition, the substrates were cleaned by ultrasonication in three different solutions. First a Hellmanex® solution (2%) were used and rinsed with deionized water and ethanol. Then samples were ultrasonicated in acetone, and 2-propanol and finally they were dried by using nitrogen. After this step, a TiO2 compact layer was deposited by spray pyrolysis at 450oC using titanium diisopropoxide bis(acetyl acetonate) precursor solution
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(75% in 2-propanol, Sigma Aldrich) using dry air as a carrier gas. The TiO2 blocking layer was then annealed for 30 minutes at 450 ºC for the formation of anatase phase. Once samples achieve room temperature, a TiO2 mesoporous layer (Dyesol, 30NRD) was deposited by spin coating (4,000 rpm for 20 s) and annealed at 500 ºC with a progressively heating. Subsequently, pure methyl ammonium (MAPbI3) or mixed cation perovskite ((FAPbI3)0.85(MAPbBr3)0.15) was then deposited. A variety of precursors concentrations (0.8M, 1.2M and 1.4M) were used in order to obtain perovskite films with different thickness. The perovskite solution was spin coated in a two steps setup at 1000 and 6000 rpm for 10 and 20s respectively. During the second step, 110µL of chlorobenzene was dropped on the spinning substrate 15 seconds before the end of the spinning program. The samples were then annealed (100°C) for 1h in an argon filled glove box. In particular, for precursor concentrations of 0.8 M and 1.2M, thicknesses of about 300 and 500 nm were obtained for MAI-based devices respectively, whereas thicknesses of about 650 and 950 nm were obtained for MIXbased devices (Table S1, Figure S1). Later perovskite deposition, 35 µL of a Spiro-OMeTAD solution was then spin coated at 4000 rpm for 20 seconds as hole transporting material. Spiro-OMeTAD material (70mM) were dissolved in 1 mL of chlorobenzene using standard additives as 17.5 µL of a lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) stock solution (520 mg of LiTFSI in 1mL of
acetonitrile),
21.9
µL
of
a
FK209
(Tris(2-(1H-pyrazol-1-yl)-4-tert-
butylpyridine)cobalt(III)Tris(bis(trifluoromethylsulfonyl)imide))) stock solution (400 mg in 1 mL of acetonitrile) and 28.8 µL of 4-tert-butylpyridine (t-BP). In the case of using P3HT material as hole selective layer, 15 mg/mL solution in chlorobenzene was prepared and doped with 6.8ml of a 28.3 mg/mL stock solution of LiTFSI in acetonitrile. P3HT solution was spun coated at 3000 rpm for 30 s atop of the perovskite layer. For all devices, 80 nm of gold was deposited by thermal evaporation under a vacuum level between 1·10-6 and 1·10-5 torr. All solutions were prepared inside an argon glove box under controlled moisture and oxygen conditions (H2O level:
