Effect of Low Temperature on Charge Transport in Operational Planar

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Effect of low temperature on charge transport in operational planar and mesoporous perovskite solar cells Miloš Petrovi#, Tao Ye, Vijila Chellappan, and Seeram Ramakrishna ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14019 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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ACS Applied Materials & Interfaces

Effect of low temperature on charge transport in operational planar and mesoporous perovskite solar cells

Miloš Petrović†,‡, Tao Ye†,‡, Vijila Chellappan‡*, Seeram Ramakrishna†*

†

Department

of

Mechanical

Engineering

and

Centre

of

Nanofibers

and

Nanotechnology (NUSCNN), National University of Singapore 117576, Singapore ‡

Institute of Materials Research and Engineering (IMRE), Agency for Science,

Technology and Research (A*STAR), #08-03, 2 Fusionopolis Way, Innovis, 138634, Singapore. *Corresponding authors: [email protected]; [email protected]

Keywords: perovskites, charge transport, low-temperature, recombination, carrier lifetime, photo-CELIV

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Abstract Low-temperature optoelectrical studies of perovskite solar cells using MAPbI3 and mixed-perovskite absorbers implemented into planar and mesoporous architectures reveal fundamental charge transporting properties in fully assembled devices operating under light bias. Both types of devices exhibit inverse correlation of charge carrier lifetime as a function of temperature, extending carrier lifetimes upon temperature reduction, especially after exposure to high optical biases. Contribution of bimolecular channels to overall recombination process should not be overlooked, since the density of generated charge surpasses trap-filling concentration requirements. Bimolecular charge recombination coefficient in both device types is smaller than Langevin theory prediction and its mean value is independent of applied illumination intensity. In planar devices, charge extraction declines upon MAPbI3 transition from tetragonal to orthorhombic phase, indicating connection between trapping/detrapping mechanism and temperature. Charge extraction by linearly increasing voltage studies further support this assertion, as charge carrier mobility dependence on temperature follows multiple trapping predictions for both device structures. Monotonously increasing trend following the rise in temperature opposes behavior observed in neat perovskite films and indicates importance of transporting layers and effect they have on charge transport in fully assembled solar cells. Low temperature phase transition shows no pattern of influence on thermally activated electron/hole transport.

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Introduction Perovskite based photovoltaics went a long way from Kojima’s breakthrough work in 20091 to reach the present point of maturity when elucidation of their inner workings becomes an important factor necessary to complete the big picture regarding these organo-metallic light harvesters. Despite the impressive volume of research articles concerning perovskite solar cells, most of the published results are focused on device performance,2–4 stability,5,6 as well as engineering of hole/electron transporting layers and their interface with lead halide absorber.7,8 Only recently this focus shifted towards deeper understanding of device photophysics,9 mainly interplay between major charge carrier parameters like lifetime, mobility, amount of extracted charge and transport across the interface with blocking layers.10–13 However, such studies conducted on fully operational solar cells exposed to a wide range of ambient temperatures are too few and far between and mostly dealing with the origin of hysteresis and the effect it has on the solar cell performance.14,15 Although such studies offer answers to critical questions regarding the working principle of perovskite photovoltaics, there is still a lot of ground to be covered towards better understanding of the relationship between aforementioned parameters. Another issue is evident lack of studies concerning the effect of illumination under these conditions, despite the fact that connection between parameters describing device photophycs and light exposure stems from the very nature of photovoltaic devices. Our work presented herein aims to bridge this gap and shed more light on charge transport, 3



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collection and recombination properties within the framework of two most frequently encountered device architectures: planar (ITO/PEDOT:PSS/MAPbI3/PCBM/Al) and mesoporous

employing

PbI2-enriched

mixed-perovskite

absorber

(FTO/TiO2/FA0.81MA0.15Pb(I0.836Br0.15)3/Spiro-OMeTAD/Au) sample, under a wide range of applied temperatures (50-300K). Device having planar structure is a good representative of simple and fast fabrication process, while mixed-perovskite incorporated into mesoporous structure is a common example of high-performance perovskite solar cells. Wide span of applied temperatures accounts for the low temperature phase transition within the crystal structure of MAPbI3 lead halide perovskites and takes into the consideration its effect on the main photophysics parameters. Exhaustive studies related to charge recombination, extraction and transport properties of these two structure types were conducted using a set of optoelectrical techniques operating under variable illumination levels. Common characterization methods used to evaluate the behavior of perovskite devices include field-effect-transistors (FET),16 space charge limited current (SCLC)17 and time-of-flight (TOF),15 each of them having a set of advantages and shortcomings in respect to one another.18–21 In principle, TOF is a well established and reliable technique for estimation of charge carrier mobilities, however in the case of planar samples, film thickness is insufficient to fulfil the criteria stating that transit time needs to significantly exceed RC constant, which effectively renders it unreliable in the case of sub-micron thick solar cells. Hence, we opted for small perturbation transient photovoltage/photocurrent extraction (TPV/TPC) techniques to probe charge 4


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carrier lifetime and charge concentration in both planar and mesoporous samples, whilst ensuring back illumination induces open circuit voltages encountered in complete solar cells operating under realistic conditions. Within the framework of small amplitude transient approach, intensity of background illumination is fluctuated in such way that applied bias causes generation of charge concentrations within devices in a manner corresponding to the open circuit voltage values normally established when illumination intensity is close to, or less than one sun. In this way, non-idealized conditions encountered during outdoors photoconversion can be replicated. Moreover, throughout the experiments, intensity of laser ping was adjusted in order to constantly maintain the peak of transient curve below 10% threshold of the background

light

bias,

thus

ensuring

relaxation

time

constant

follows

single-exponential decay, which in turn allows direct measurement of fastest carrier lifetimes and prevents all injected charge carriers to exceed concentration necessary to disturb the field distribution established within the cell. Furthermore, photoinduced charge extraction by linearly increasing voltage (Photo-CELIV)22,23 technique was employed to describe the effect of temperature and excitation power on the electron/hole transport. Analogous approach to the one in TPV/TPC measurement was followed with photo-CELIV as well, while illumination levels were governed by intensity of laser pulse and corresponding system response enabled us to investigate charge carrier mobilities, bimolecular recombination contribution and electron/hole transport energy, as well as mapping of their behaviour in relation to illumination levels and temperature. Data gathered in this way helped us to identify any disparities 5



between charge transport mechanism within neat perovskite film and fully-assembled solar cells. Furthermore, we examined whether the established trend is governed by structural transition within crystal lattice from tetragonal to orthorhombic phase, occurring at temperatures below 160K in methylammonium lead iodides. On the other hand, mixed-perovskites assume cubic arrangement at the room temperature and show no such phase transitions upon cooling.24

Experimental Section Planar devices were prepared via consecutive spin-coating of PEDOT:PSS (Heraeus, Clevios PVP AI 4083), solution of MAPbI3 and PC71BM (Sigma-Aldrich), on pre-cleaned ITO patterned substrates. First, stock solution of PEDOT:PSS was filtered through 0.2 µm PVDF filter and spin-casted at 5000 rpm, followed by temperature treatment at 135°C over the period of 10 minutes. After cooling down, substrates were transferred to nitrogen filled glovebox, where deposition of perovskite absorber was completed using prepared 400 mg/mL solution made from MAI (Dyesol) and PbI2(TCI) in N,N-dymethylformamide (Sigma-Aldrich). Film was deposited at the speed of 3000 rpm followed by dynamic dispersion of 100 µL of chlorobenzene 4 seconds upon achieving desired rotation velocity, in order to improve overall film quality. Such newly formed MAPbI3 films were annealed for 15 minutes at 100°C and left cooling down to 25°C prior the deposition of electron transporter. PC71BM was deposited at 2000 rpm over 40 s, from 20 mg/mL solution in chlorobenzene (Sigma-Aldrich), previously filtered through 0.45 µL PTFE membrane filter and left 6


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overnight in evacuated antechamber prior evaporation of 110 nm thick Al electrode over the shadow-mask. Fabrication of mesoporous samples followed similar deposition sequence, starting with two-step deposition of electron transporter consisting of 30 nm blocking and 150 nm mesoporous TiO2 layers. Blocking film was made from titanium isopropoxide (Sigma-Aldrich) and 12 M HCl (Sigma-Aldrich) precursor solution in 10 mL of ethanol, spun over the FTO substrate at 6000 rpm and calcinated for 30 mins in the air at 450°C. Mesoporous layer was deposited from titania nanoparticle paste 30NR-D (Dyesol) diluted in ethanol at 1:5 ratio, spin-coated over the blocking layer at 6000 rpm and finally fired up at 500°C for half an hour. During the next step, substrates were moved to a glovebox and deposition of perovskite layer was performed via two-step process from 0.5 mL of N,N-dimethylformamide/dimethylsulfoxide solution of methylammonium bromide (TCI), formamidinium iodide (Dyesol), lead iodide (TCI) and lead bromide (Sigma-Aldrich) with 1:8.5:24:3.4 weight ratio in respect to MABr. Detailed procedure for synthesis of PbI2-enriched mixed-perovskite can be found in the literature.25 First spin-casting step lasted for 10 s at the speed of 2000 rpm, followed by a second one for 35 s at 6000 rpm and analogously to fabrication of planar devices, 100 µL of chlorobenzene was dropped after 10 seconds into the second spin step in order to finalize formation of pinhole-free layer after annealing at 100°C for 1 hour. Hole transporting layer was deposited from precursor solution of commercially available

2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene

(spiro-OMeTAD, Merck) in anhydrous chlorobenzene (74 mg/mL) with the addition 7



of following salts to improve hole extraction during solar cell operation: tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl) imide (29 µL from 100 mg/mL of stock solution), lithium bis (trifluoromethylsulphonyl) imide (17.5 µL from 520 mg/mL of stock solution) and 4-tert-butylpyridine (28.5 µL). Samples were afterwards transferred to evaporation chamber and 80 nm thick Au electrode was laid on top of them, thus finalizing the fabrication step. In both planar and mesoporous samples, shadow mask used during metal evaporation confined final device area to 9 mm2. Scanning electron microscopy and Energy Dispersive X-ray spectroscopy measurements were performed on FE-SEM: JEOL JSM7600F (2kV and 5kV accelerating voltages) with EDX mapping attachment (OXFORD Instruments). Examined lead halide films were spin-coated over ITO covered glass substrates from MaPbI3 and FA0.81MA0.15Pb(I0.836Br0.15)3 solutions and both samples were exposed to ambient air conditions for 16 hours prior to the measurements. XRD patterns of perovskite films deposited on quartz substrates were recorded at 0.4s steps on D8-Advance setup from Bruker, using X-ray generator with Cu radiation operating at 40 kV and 40mA. Temperature dependent measurements were performed using closed-cycle helium cooled cryostat pressurized by Edwards turbo pumping station T75 to a working pressure of 1e-6-2e-7 torr, while temperature was monitored on the Lakeshore 311 temperature controller and samples were stabilized for 10 minutes at each temperature point prior to the measurement. Photo-CELIV transients were induced using Nd:YAG laser (NT341A–10–AW) operating at 532 nm (1 Hz frequency) 8


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Effect of Low Temperature on Charge Transport in Operational Planar

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