Copper Thiocyanate Inorganic HoleTransporting Material for High-Efficiency Perovskite Solar Cells Vinod E. Madhavan,§,† Iwan Zimmermann,§,‡ Cristina Roldán-Carmona,‡ Giulia Grancini,‡ Marie Buffiere,† Abdelhak Belaidi,*,† and Mohammad Khaja Nazeeruddin*,‡ †
Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, P.O. Box 5825, Doha, Qatar Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland
‡
S Supporting Information *
ABSTRACT: In this Letter we show that the mixed perovskite in the form of (FAPbI3)0.85(MAPbBr3)0.15 in combination with CuNCS as p-type hole conductor leads to over 16% power conversion efficiency (PCE) under full sun illumination and yields a remarkable monochromatic incident photon-to-electron conversion efficiency of 85%. The devices displayed a short-circuit current density (Jsc) of 21.8 mA/cm2, open-circuit voltage (Voc) of 1100 mV, fill factor (FF) of 0.69, and a PCE of 16.6%. Under similar conditions, the device without CuSCN shows a PCE of 9.5%, with a significant decrease in the Jsc (from 21.8 mA/cm2 to 15.64 mA/cm2) and Voc (from 1100 mV to 900 mV). The high Jsc with CuSCN is mainly due to the effective charge transfer between perovskite and CuSCN, followed by the fast hole transport through CuSCN to the Au. In comparison, the spiro-OMeTAD reference cells showed efficiencies up to 19.65%. Different from most organic holetransporting materials is the transparency and high hole mobility of CuSCN, which represent a paradigm shift in perovskite solar cells particularly for tandem solar cells. and polymers. CuI was first employed by Christians et al.,10 and conversion efficiencies of 6% have been achieved. CuSCN was introduced as a HTM by Chavhan et al.11 and Qin et al.12 The reported highest efficiency for an n-i-p architecture is 12.4%, and more recently a CuSCN-based inverted PSC (p-i-n architecture) showing 16.6% was reported.13 It is worth noting that Cu2O was also used in an ultrathin film form as HTM in an inverted structure to fabricate PSCs.14,15 Among the Cu-based materials, CuSCN was giving the highest PCEs reported to date. However, a systematic study of the effect of the CuSCN layer thickness on the efficiency of PSC has not been reported. CuSCN is an inorganic p-type semiconducting material with good transparency, high hole mobility (0.01−0.1 cm2 V−1 s−1), and high chemical stability.16 Different methods were adopted to prepare CuSCN thin films such as spray coating,16 doctor blading,12 and electrodeposition,13 resulting in PCEs of 9.2%, 12.4%, and 16.6%, respectively. More recently, Jung et al. claimed to have prepared a perovskite solar cell using CuSCN as HTM deposited by spin coating that led to an efficiency of
O
rganic−inorganic perovskites have attracted considerable attention in the photovoltaic community over the past few years because of their competitive optoelectronic properties and solution-processed device architecture. The perovskite solar cells (PSCs) are important players in the field of thin-film solar technology, and solar cells with different architectures, i.e., n-i-p, p-i-n, n-i-metal, p-i-metal, have been fabricated.1−5 Device efficiencies quickly raised to over 22% today.6−9 Up to now, most of the research carried out in the field of perovskite photovoltaics was focused on achieving the best possible device efficiencies through either engineering the perovskite absorbing material or developing new, effective hole-transporting materials (HTMs) or electrontransporting materials (ETM). Little attention has been given to long-term stability and material costs of PSCs. With PSCs already matching the efficiencies of conventional silicon solar cells today, those parameters are of paramount importance when considering the commercialization stage. Inorganic HTMs can be directly applied in their pristine form and do not need any additives or dopants as usually used in polymers or small molecules, which is an important asset for long-term stability of PSC devices. Recently, inorganic Cu-based HTMs were used as alternatives to the organic-based small molecules © 2016 American Chemical Society
Received: October 4, 2016 Accepted: November 1, 2016 Published: November 1, 2016 1112
DOI: 10.1021/acsenergylett.6b00501 ACS Energy Lett. 2016, 1, 1112−1117
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
Figure 1. (a) Schematic drawing of the device architecture. PSC devices were prepared on FTO substrates from a stack of 20 nm compact TiO2, 250 nm mesoporous TiO2 infiltrated with perovskite, 700 nm perovskite capping layer, different thickness HTMs, and 80 nm Au electrode. The perovskite material used in this study is a mixed anion mixed cation compositional modification based on the nominal formula (CH2(NH3)2PbI3)0.85(CH3NH3PbBr3)0.15 with 5% PbI2 excess. The perovskite layers were fabricated using a one-step spin-coating process based on the antisolvent addition (see Experimental Methods). The finished perovskite films were annealed for 1 h at 100 °C. The CuSCN HTM was employed by either spin-coating or doctor-blading technique for preparing different thickness films. (b) Corresponding energy levels of PSC layers.
Figure 2. SEM cross-sectional images of perovskite-based solar cell. The cell structure is glass/FTO/compact TiO2/mesoporous TiO2/ perovskite/HTM/Au contact. The different HTMs are (a) spiro-OMeTAD, (b) doctor-bladed (DB) CuSCN, (c) spin-coated (SC) CuSCN, and (d) no HTM device with glass/FTO/compact TiO2/mesoporous TiO2/perovskite/Au contact.
18% and stabilized at 15.6%.17 Scaling of CuSCN is easy with spin coating or the doctor blade method. The first is a relatively simple process and suitable for medium size area deposition; however, it suffers because of high material wastage during spinning. Doctor blade coating is a low-cost method having advantages such as simplicity, scalability, low-temperature processing, and high throughput and is a solution-based thinfilm deposition technique that is compatible with role-to-role fabrication of large-area devices. This method adapted for depositing perovskite layers is already reported.18 The factors affecting film quality are precursor solution concentration, blading speed, and the substrate−blade distance during deposition. A robotic deposition is ideal for such deposition methods. In this work the deposition of CuSCN as an inorganic HTM onto the mixed-cation mixed-halide perovskite (FAPbI3)0.85(MAPbBr3)0.15 (FA = formamidinium; MA = methylammonium) is investigated. The lead-based perovskite films are deposited following a one-step process method and using mixed cations and halides. Because of the low solubility of the CuSCN material, spin-coating and doctor-blading deposition techniques are employed to obtain variable layer thicknesses. Spin coating of the CuSCN film resulted in relatively thin films
that are however still able to give promising device efficiencies. Comparable results were obtained employing thicker layers with the doctor-blading technique. The thickness of the CuSCN layers prepared in our study span from 30 to 500 nm; to our knowledge, this is the first time good PCEs have been obtained using ultrathin CuSCN layers prepared from spin coating in an n-i-p architecture. In addition, timedependent photoluminescence (PL) measurements are carried out in order to gain insight into the dynamics of the photogenerated holes in the perovskite material upon injection into the CuSCN HTM. Schematic drawings of the device architecture as well as energy levels of the different solar cell components are displayed in Figure 1. Because of the close alignment with the perovskite valence band, CuSCN is a potential candidate for efficient hole extraction in perovskite solar cells. The challenge for solution processing of an inorganic material onto perovskite is finding a suitable solvent that does not dissolve the perovskite material. Thin films of the inorganic HTM CuSCN were prepared from solutions in propyl sulfide, which was found to not affect the perovskite layer, but provided reasonable solubility for CuSCN of up to around 15 mg/mL. Because of the limited solubility of CuSCN 1113
DOI: 10.1021/acsenergylett.6b00501 ACS Energy Lett. 2016, 1, 1112−1117
Letter
ACS Energy Letters
to 15.43% even though the maximum thickness of the CuSCN layer was only around 30 nm. It is worth noting that the thickness of CuSCN was limited to 30 nm though we tried to build thicker films by increasing the spin coating sequentially. This is not the case with results reported in ref 17, in which the spin coating gave thicker films. Our results show that even ultrathin layers of CuSCN are sufficient for an effective hole extraction. Employing thicker CuSCN layers prepared using the doctor blade technique allowed further improvement of the device efficiency up to 16.6% for layers having a thickness of around 500 nm. This enhancement is mainly attributed to the increase in the open-circuit voltage (Voc) and the short-circuit current (Jsc) . The reference cell having spiro-OMeTAD as a HTM showed a PCE of up to 19.65%. It is important to highlight the relatively high value of the Voc of 1.18 V compared to 1.1 V for the CuSCN layers. Such a high value of Voc could be attributed to the low charge recombination at the interface of perovskite and spiro-OMeTAD. Indeed, Pydzińska et al.20 performed impedance measurements on two similar perovskite devices where only the Spiro-OMeTAD was exchanged by CuSCN. Their measurements revealed that the interface perovskite/CuSCN exhibits a charge recombination that is faster than that of the interface perovskite/spiro-OMeTAD. Continuous wave photoluminescence and time-resolved PL (TRPL) measurements have been carried out on the perovskite and perovskite/CuSCN layers to understand the interfacial interactions. In particular, to selectively monitor the chargetransfer processes at the perovskite/CuSCN interface, we deposited the perovskite on glass substrates to eliminate any possible injection effects due to the presence of any coated substrate. Figure 4a shows the ultraviolet−visible (UV−vis) spectra of the perovskite samples along that of with the perovskite/CuSCN sample. The absorption of the sample is all very similar, reflecting the similar perovskite thickness (around 700 nm) for all the samples fabricated. This is an important point because it allows us to keep the same excitation density for optical investigation for both the perovskite sample with and without the HTM on top. Absorption edges of all samples are observed at ∼770 nm, according to the literature.21 Figure 4b shows the PL emission of the perovskite layer. The perovskite PL emission peak is centered at around 795 nm. When the CuSCN layer is on top, we observe a dramatic quenching of the PL signal. Note that the same excitation density is used; therefore, this quenching is qualitatively and quantitatively significant. With respect to the pristine perovskite, we estimate a quenching of about 95% in the presence of the HTM. This shows that in the presence of the CuSCN, the quenching of the PL signal reflects an efficient hole transfer to the CuSCN. This result is in agreement with published work17,20 in which it was shown that the timeresolved PL emission decay in the case of CuSCN is faster than that in the case of spiro-OMeTAD, indicating efficient chargetransfer dynamics. The authors attributed that to the rapid hole injection into CuSCN. This confirms the suitability of CuSCN as an effective HTM material in PSCs. From the PL decay we observe a fast component in the presence of the CUSCN with a time constant of around 1 ns (see Table 2) that dominates (see relative amplitudes in Table 2) the decay. The fast quenching gives evidence that hole transfer happens at the perovskite/CuSCN interface within 1 ns. It was observed by Pydzińska et al.20 that the hole transfer to CuSCN is three times faster thanthat of spiro-OMeTAD. It was also suggested that a possibility of interfacial charge
in propyl sulfide, different techniques such as spin coating and doctor blading were used to obtain variable layer thicknesses. The CuSCN single layers deposited by spin coating is much more uniform but are limited to a thickness of up to around 30 nm. Doctor blading, however, allowed further increases in the thickness up to 500 nm. SEM cross-sectional images from champion devices prepared using different HTM layers are shown in Figure 2. The perovskite devices used in this study were prepared from a stack of FTO/TiO2/perovskite/HTM/Au metal contact, except in the case where no HTM is deposited. Detailed procedure of the solar cell fabrication can be found in Experimental Methods. In Figure 2a, the reference cell is shown where a homogeneous film of spiro-OMeTAD deposited by spin coating was used as a HTM. The thickness is around 300 nm. In Figure 2b, a thick CuSCN film of around 500 nm resulting from the doctorblading method is shown. Figure 2c displays an ultrathin CuSCN layer of around 30 nm deposited by spin coating. In the case of no-HTM samples, the Au top contact is directly deposited onto the perovskite layer (Figure 2d). The performance of the PSC devices with CuSCN as an inorganic HTM was analyzed and compared to that of standard devices with spiro-OMeTAD and devices having no HTM present. The devices were measured using simulated 1 sun AM 1.5G (100 mW/cm2) radiation. Reverse current−voltage (J−V) plots of the champion devices employing the different HTMs are shown in Figure 3, and corresponding results are summarized in Table 1.
Figure 3. J−V curves of mixed perovskite solar cells based on CuSCN, spiro-OMeTAD, and no hole-transporting materials. The cell structure is glass/FTO/compact TiO2/mesoporous TiO2/ perovskite/HTM/Au contact.
Table 1. Summary of Photovoltaic Parameters of the Champion Devices spiro-OMeTAD CuSCN DB CuSCN SC no HTM
Voc [mV]
Jsc [mA cm−2]
FF [%]
η [%]
1180 1100 1060 900
22.70 21.80 20.86 15.64
73.3 69.2 70.1 67.6
19.65 16.60 15.43 9.50
As expected, devices with no HTM showed low efficiency with a maximum PCE of 9.50%. This value is in good agreement with what has been published previously.19 Spincoated CuSCN layers significantly improved the efficiency up 1114
DOI: 10.1021/acsenergylett.6b00501 ACS Energy Lett. 2016, 1, 1112−1117
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Figure 4. (a) UV−vis absorption spectra of the perovskite and perovskite/CuSCN layer samples on glass substrates. (b) CW-PL spectra of the samples, excitation at 550 nm. Note that the pristine perovskite films (black dotted line) have been divided by 4.4 for a clearer rescaled visualization. (c) Normalized PL dynamics in the first 30 ns time window, upon excitation at 460 nm; excitation density, 1015 photons/cm3. The samples were encapsulated with a thin film of PMMA on top to prevent any degradation or oxygen reaction during the measurements. Solid lines represent the outcome of the exponential fitting, y = A1 exp(−x/t1) + A2 exp(−x/t2). See Table 2 for parameters.
process for 20 min. Then a mesoporous TiO2 layer with a thickness of around 250 nm was deposited by spin coating from a dispersion of TiO2 nanoparticles (DYESOL, 30 NR-D) in ethanol. The substrates were then annealed at 450 °C for 10 min, cooled to room temperature, and treated with a doping agent Li-TFSI (10 mg/mL in acetonitrile) followed by a sintering step at 450 °C for 10 min, as previously reported in the literature.22,23 These substrates were transferred into a glovebox with N2 controlled atmosphere for further processing. As the photoactive material, a perovskite layer was prepared based on the composition (FAPbI3)0.85(MAPbBr3)0.15 (FA = formamidinium, MA = methylammonium) with 5% of PbI2 excess,24 using PbI2 (TCI), MABr (Dyesol), FAI (Dyesol), and PbBr2 (TCI) as precursors. For the precursor solution, all materials were weighed in stoichiometric ratio and dissolved in DMSO:DMF solvents with a volume ratio 1:4, affording a solution of 1.25 M. The perovskite layers were fabricated by a single-step spin-coating procedure based on the antisolvent addition previously reported by Soek and co-workers.25 In short, the precursor solution is spun at 4000 rpm for 30 s, with an acceleration of 2000, and 10 s prior to the end, 100 μL of chlorobenzene was added onto the spinning film to obtain dense, pinhole-free perovskite layers. The finished films were annealed at 100 °C for 1 h prior to the deposition of the HTM layer. For CuSCN deposition, a solution containing 10 and 15 mg/mL of CuSCN in dipropyl sulfide solvent (Aldrich, purity 97%) was stirred overnight and spin coated at 500 rpm for 60 s followed by annealing at 65 °C or deposited by doctor-blade at 80 °C. Because of the limitation in solubility of this material, sequential depositions were tested to increase the film thickness. Several sequential depositions were performed followed by a low-temperature annealing at 65 or 80 °C. For comparison, Spiro-OMeTAD was used as a reference HTM material. A 300 nm layer of doped spiro-OMeTAD was spin coated as the hole-transporting material at 4000 rpm for 20s. The doped spiro-OMeTAD was prepared following a method described elsewhere.26 Spiro-OMeTAD solutions were prepared from a 60 mM solution in chlorobenzene using tertbutylpyridine, Li-TFSI, and FK209 Co-dopant as additives. Finally, the top contacts were applied by thermal evaporation of 80 nm of Au using a shadow mask. Characterization Techniques. Scanning electron microscopy (SEM) pictures were taken with a FEI Teno SEM instrument equipped with an upper in-lens detector (T2) operated at 5 keV and 0.4 nA at a working distance of 4 mm. The thickness
Table 2. Exponential Fitting Parameters from Figure 4c perovskite perov. + CuSCN
A1 [%]
A2 [%]
t1
t2
100 87
− 13
>30 ns 1.3 ns
− >30 ns
recombination for CuSCN exists, thereby limiting the performance of the PSCs. Our results and ref 17 show higher efficiencies (16.6% and 18%, respectively), which indicates that the rate of recombination is not very significant with respect to spiro-OMeTAD. More studies are required on the CuSCN films properties to understand the carrier diffusion length and formation and size of the grain structure of the spin-coated and doctor-bladed film. This can give an insight into the superior PCE we obtained in our PSC. In summary, employing CuSCN as an inorganic HTM in mixed anion mixed cation PSCs resulted in PCE values of up to 15.43% for spin-coated thin films of around 30 nm. Thicker CuSCN layers of around 500 nm fabricated by doctor blading afforded even better PCEs up to 16.6%, indicating the quality of the interface is still retained. In comparison, the control sample with spiro-OMeTAD showed a PCE of 19.65%. These promising results open up a huge potential for the usage of CuSCN as HTM in PSCs as a more stable and cheap alternative to the commonly used small molecule or polymerbased HTMs. Both methods adapted in this work (spin coating and doctor blading) for the deposition of HTM layer are promising for large-area deposition. Time-dependent PL measurements recorded on CuSCN-coated perovskite layers show fast quenching rates (∼1 ns) and thus efficient hole injection into the CuSCN HTM material.
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EXPERIMENTAL METHODS Device Preparation. All chemicals were purchased from commercial sources and used without further purification. The devices were fabricated onto fluorine-doped tin oxide (FTO) coated glass substrates employing a stack of compact TiO2/mesoporous-TiO2/perovskite/HTM/gold. Conductive FTO glass (NSG10) was sequentially cleaned by Helmanex and isopropanol in an ultrasonic bath for 20 min before exposure to UV-ozone treatment for 15 min. The compact TiO2 film was applied on FTO substrates by spray pyrolysis at 450 °C from a precursor solution containing 600 μL of titanium di-isopropoxidebis(acetylacetonate) and 400 μL of acetyleacetone in 9 mL of ethanol, followed by a sintering 1115
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of the films was also measured with a stylus profilometry (Bruker). The photovoltaic device performance was analyzed using a VeraSol LED solar simulator (Newport) producing 1 sun AM 1.5G (100 W/cm2) sunlight. Current−voltage curves were measured in air with a potentiostat (Keithley). The light intensity was calibrated with an NREL certified KG5 filtered Si reference diode. The solar cells were masked with a metal aperture of 0.16 cm2 to define the active area. The current− voltage curves were recorded scanning at 20 mV s−1. Absorption spectra were measured on a PerkinElmer UV−vis spectrophotometer Lambda 950s. For measuring the PL spectra, a perovskite film and a CuSCN/perovskite interface were separately spin coated on a glass plate under the same conditions and followed by PMMA encapsulation coating for isolating the films from atmospheric conditions during measurement. PL spectra were recorded by exciting the perovskite films at 550 nm with a standard 450 W xenon CW lamp. The signal was recorded by a Gilden Photonics spectrofluorometer. Time-resolved PL experiments were performed using a pulsed source at 460 nm (Ps diode lasers BDS-SM, pulse with
