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Ultra-thin high efficiency semitransparent perovskite solar cells
Nano Energy (2015) 13, 249–257
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Ultra-thin high efficiency semitransparent perovskite solar cells Enrico Della Gasperaa,1, Yong Pengb,1, Qicheng Houb, Leone Spicciac, Udo Bacha,b,d, Jacek J. Jasieniaka,n, Yi-Bing Chengb,nn a
CSIRO Manufacturing Flagship, Bayview Ave, Clayton, Victoria 3168, Australia Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia c School of Chemistry, Monash University, Victoria 3800, Australia d Melbourne Centre for Nanofabrication, Clayton, Victoria 3168, Australia b
Received 23 December 2014; received in revised form 17 February 2015; accepted 17 February 2015 Available online 26 February 2015
KEYWORDS
Abstract
Transparent electrode; Molybdenum oxide; Photovoltaics; Methylammonium lead iodide; Thin film
Solar cells based on organometal trihalide perovskite light absorbers have recently emerged as one of the most promising class of photovoltaic devices. Given their high performances, along with their low fabrication costs, such devices are appealing for building integrated photovoltaics, especially if made semitransparent and used as solar windows. In this study, efficient perovskite solar cells with high transparency in the visible spectrum have been developed. These devices comprise a dielectric–metal–dielectric multilayered top electrode with high transparency and conductivity, and an optimized solution-based thin and continuous methylammonium lead iodide film. Through this combination of processing and device architecture developments, semitransparent solar cells with high visible transmittance and record power conversion efficiency have been prepared, including devices with a perovskite thickness as low as 55 nm. & 2015 Elsevier Ltd. All rights reserved.
Introduction n
Corresponding author. Tel.: +61 395452446. Corresponding author. Tel.: +61 399054930. E-mail addresses: [email protected] (J.J. Jasieniak), [email protected] (Y.-B. Cheng). 1 Enrico Della Gaspera and Yong Peng contributed equally to this work. nn
http://dx.doi.org/10.1016/j.nanoen.2015.02.028 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Organometal trihalide perovskites have rapidly emerged as one of the most promising class of absorber materials for high efficiency solar cells [1–3]. Some of the reasons for their success are their optimal band gap ( 1.55 eV for methylammonium lead iodide, CH3NH3PbI3), high absorption coefficients
250 (104–105 cm 1) [4], and long carrier diffusion lengths [5]. Moreover, perovskite solar cells (PSCs) can be fabricated with simple and straightforward solution-based procedures that are amendable to continuous, high-throughput processes [6–8]. Despite being a very young technology, the record power conversion efficiency (PCE) of PSCs has already exceeded 20%, with numerous research groups demonstrating PSCs with PCEs above 15% [9–14]. This rapid growth of PSC research and the concurrent improvement in device performances have enabled the fabrication of efficient semitransparent PSCs that could find applications, for example, in building integrated photovoltaics [15,16]. In order to prepare a highly efficient and semitransparent device, all layers, excluding the light absorber, should be highly transparent. Considering a planar PSC architecture, the absorber material is deposited on a transparent conductive oxide (TCO, for example tin doped indium oxide, ITO, or fluorine doped tin oxide, FTO) covered with a hole blocking layer (e.g. TiO2). The hole transport layer (HTL) is then deposited on the perovskite, and the cell is completed with a thick metal electrode, such as Ag or Au. A semi-transparent device can be fabricated if the opaque metal electrode is replaced by a transparent conductive electrode. Several examples of transparent electrodes have been used in solar cells and more generally in optoelectronic devices, including thin metal films [17], conductive oxides (e.g. ITO) [18], metal nanowires [19], conductive polymers (e.g. PEDOT:PSS) [20], carbon nanotubes [21], graphene [22], etc. These materials are sometimes combined to enhance the transmittance, reduce the resistivity and improve the mechanical properties of the electrode [23–26]. However, many of these options present disadvantages when used in semitransparent PSCs, such as low conductivity, high production cost, and/or require solvents and processing temperatures that are not compatible with the organometal trihalide perovskites. In this work, the opaque metal cathode has been replaced with a dielectric–metal–dielectric (DMD) multilayer. DMD structures rely on interference effects provided by the two dielectric layers, which increase the light transmission through a thin metal film [27]. Such DMD stacks have been widely adopted as visible-transparent and IR-reflective coatings (heat mirrors) [27,28], and as transparent conductors for various optoelectronic devices, including organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs) [29–33]. Critical to their success as semi-transparent windows is the precise selection and thickness optimization of the individual dielectric and metal films [27,32,33]. Here, we present a DMD transparent electrode composed of a MoO3–Au–MoO3 stack. Molybdenum trioxide was chosen because of its well-established hole injection properties [34] and because it provides a good nucleation surface for Au, which allows thin and uniform metal films to be deposited. Common metals employed within PSCs are Au, Ag and Al. Au was selected for this study because of its higher stability towards air oxidation compared to Al and lower reactivity towards halides compared to Ag. In addition to the transparency of the electrode, the inherent transmittance of the absorbing layers must also be considered for semitransparent devices. Naturally, this depends on the thickness of the CH3NH3PbI3 layer for PSC, whose deposition is known to be inherently challenging. Using our recently developed gas-assisted deposition technique [11], CH3NH3PbI3 layers
E. Della Gaspera et al. as thin as 50 nm have been deposited, while still obtaining a continuous and homogeneous layer that results in devices with high efficiency and high transparency values in the visible spectrum. By combining this controllable CH3NH3PbI3 layer deposition with a DMD electrode structure, record PCEs between 5.5% and 13.6% have been achieved for devices with average visible transmittance (AVT) values of 31% and 7%, respectively.
Materials and methods Materials All chemicals have been used as received without any further purification. Methylamine (CH3NH2, 33% in ethanol), hydroiodic acid (HI, 57% in water), N,N-dimethylformamide (DMF, 99.8%), lead iodide (PbI2, 99.999%), 4-tert-butylpyridine (TBP, 97%), lithium bis(trifluoromethylsulphonyl)imide (LiTFSi, 99.95%), chlorobenzene (99.8%), acetonitrile (99.8%) and molybdenum trioxide (MoO3, 99.5%) have been purchased from Sigma Aldrich. Gold (Au) has been purchased from A&E Metals. 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD, 99.5%), has been purchased from Luminescence Technology Corp.
Samples fabrication Details of the perovskite solar cell (PSC) fabrication, excluding the composite top electrode, have been published elsewhere [11]. Briefly, methylammonium iodide (CH3NH3I) was synthesized by reacting CH3NH2 with HI and eventually purified via recrystallization. To prepare the CH3NH3PbI3 precursor solution, a 1:1 M ratio of CH3NH3I and PbI2 was dissolved in DMF at various concentrations, namely 20, 30, 35, 45 wt%. For this purpose, 59 mg, 102 mg, 128 mg and 194 mg of a 1:1 CH3NH3I: PbI2 molar mixture was dissolved in 250 mL DMF, respectively. A dense TiO2 layer was screen printed on top of patterned and cleaned FTO glass (NEG glass, 9 Ω/□) using a screen printing paste (Dyesol, BL-1) and annealed at 450 1C for 10 min. The CH3NH3PbI3 films were subsequently deposited by spin coating the precursor solution at 6500 rpm for 30 s, using a gas-assisted spin coating method [11], and annealed at 100 1C for 10 min. The thickness of the perovskite layer was controlled by varying the concentration of the CH3NH3PbI3 solution. A Spiro-OMeTAD film was then deposited by spincoating at 3000 rpm for 30 s, a solution prepared by dissolving 41.6 mg Spiro-OMeTAD, 7.5 μL of a stock solution of 520 mg mL 1 LiTFSi in acetonitrile and 16.9 μL TBP in 0.5 mL chlorobenzene. The solar cells were completed by thermal evaporation under vacuum (3 10 6 Torr) of the DMD electrodes composed of: (i) bottom MoO3 (b-MoO3) layer with thicknesses of 0, 1, 5, 10, 20 or 35 nm; (ii) a thin Au film (10 nm); and (iii) a top MoO3 (t-MoO3) layer (35 nm). The deposition rate was set to be 0.1 nm/s for MoO3 and 0.05 nm/s for Au. These electrodes were deposited also on plain glass slides and silicon substrates for additional characterization.
Characterization techniques The surface morphology and the cross sections of selected samples were analyzed using a FEI Helios Nanolab 600
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Figure 1 (a) Schematic illustration (not to scale) of the PSC architecture. (b) SEM image in cross section of a complete device. For the uncolored image refer to Figure S1 in the Supporting information. (c) Enlarged view of the multilayer top electrode and schematic of its structure. (d) Simulations (shaded dashed lines) and experimental data (solid lines) showing the transmittance of Au (black), b-MoO3/Au (red) and b-MoO3/Au/t-MoO3 (blue). (e) SEM image of Au film. (f) SEM image of b-MoO3/Au film. The insets show photos of the two samples.
Scanning Electron Microscope (SEM). Optical absorption, transmission and reflection spectra of the DMD electrodes deposited on glass substrates, the CH3NH3PbI3 films deposited on FTO–glass and the completed semitransparent devices were measured using a Perkin-Elmer Lambda1050 spectrophotometer equipped with an integrating sphere. Ellipsometry quantities Ψ and Δ of MoO3 films on either Si or glass substrates were measured using a J.A. Woollam M-2000 spectroscopic ellipsometer and the refractive index (n) and absorption coefficient (k) were evaluated from Ψ and Δ data using the CompleteEASE data analysis software. The obtained n and k values for MoO3 as a function of wavelength were then used together with bulk Au optical constants to model the DMD stack transmission using the Fresnel matrix method, assuming infinite, flat, parallel, isotropic and homogeneous layers with non-interpenetrating interfaces. Sheet resistances of the DMD electrodes deposited on glass were measured by using a Jandel RM3000 4-point probe and averaged over at least 5 measurements per sample. X-Ray Diffraction (XRD) patterns of the CH3NH3PbI3 films deposited on FTO–glass substrates were collected using a Bruker D8 diffractometer equipped with a Cu-Kα radiation source and operated at 40 mV and 40 mA. J–V curves of solar cells were measured using a Keithley 2400 source meter scanning the devices at 100 mV/s under illumination of simulated sunlight (100 mW/cm2) provided by an Oriel solar simulator with an AM 1.5 filter. A black metal mask was employed to define a device area of 0.16 cm2. IPCE spectra were recorded on a Keithley 2400 source meter under the irradiation of a 300 W Xenon lamp with an Oriel Cornerstone™ 260 1/4 m monochromator.
Results and discussion The PSC architecture is presented in Figure 1a–c. A uniform and continuous CH3NH3PbI3 layer ( 300 nm thick) was first deposited on top of an FTO/glass substrate coated with a dense and thin ( 30 nm) n-type TiO2 film. Spiro-OMeTAD ( 200 nm thick) was then introduced as a HTL and the cell was completed by precisely depositing a transparent DMD top electrode using thermal evaporation. The advantage of the DMD multilayer architecture is that it provides improved transparency when compared to neat metal films. This is clearly shown in Figure 1d where the experimental and simulated transmittance for Au, b-MoO3/ Au and b-MoO3/Au/t-MoO3 layers on glass are presented (see Materials and methods for details). A neat 10 nm thick Au film shows low transmission because of the high reflection at the Au/air and Au/glass interfaces. However, when the thin Au film is sandwiched between the two MoO3 dielectric layers, destructive interference in the visible range can be achieved through simple tuning of the exact layer thicknesses to provide an increase in transparency. The precise control of the optical properties of such DMD structures can only be achieved if each layer is continuous. When Au is deposited directly on glass or on Spiro-OMeTAD (the HTL used in the PSC architecture adopted here) it nucleates into separate islands, which reach the percolation threshold at a thickness of 10 nm (Figure 1e) [35]. These metallic islands possess a localized surface plasmon resonance (SPR) that results in a reduction of the overall transmittance compared to an ideal thin Au metal film. In contrast, when a
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Figure 2 (a) Current–Voltage characteristics of PSCs with/without b-MoO3 measured under dark and 1 sun (AM 1.5) illumination. (b) PCE of PSCs and sheet resistance of DMD electrodes deposited on glass as a function of the b-MoO3 film thickness. (c) Absorption spectrum (Absorption=100 Transmittance Reflectance) of 300 nm thick CH3NH3PbI3 film and IPCE of PSC with 300 nm CH3NH3PbI3 film measured under illumination from the FTO side (dark blue) and from the DMD side (light blue), respectively. (d) Photocurrent density and PCE measured as a function of time for a device held at 0.77 V forward bias, which corresponds to the voltage at maximum power point for that particular device.
very thin MoO3 layer (even 1–2 nm in thickness) is used as a seeding layer, the surface provides for the appropriate conditions to grow continuous and smooth Au films (Figure 1f). This is verified by the excellent agreement between experimental transmittance of the b-MoO3/Au layers and thin-film optical modeling that assumes bulk optical properties. When comparing both systems, Au films evaporated on glass are green–blue in color, which indicates the presence of a localized SPR, whereas Au films deposited on MoO3/glass appear yellow. This difference in morphology and optical properties is also accompanied by a large variation in electrical conductivity. The Au films deposited on MoO3 exhibit a sheet resistance of 13 Ω/□ compared to 250 Ω/□ when Au is deposited on glass or on Spiro-OMeTAD. If thicker Au films are deposited, high conductivity can eventually be achieved without the MoO3 interlayer, but with the drawback of a lower transmission in the visible spectrum. Therefore, such films are not desirable for semitransparent devices. Moreover, by using MoO3 as a nucleation layer, very thin (6–7 nm) continuous and smooth Au films can be deposited, with even higher transparency, but with a slightly higher sheet resistance ( 20 Ω/□, see Figure S2). The thickness of both the bottom and top dielectric films plays an important role in defining the transmission window of DMD structures [27]. By optimizing the t-MoO3 thickness to a value of 30–40 nm and by progressively increasing the bMoO3 thickness, a slight redshift in the transparency window is observed (see Figure S3). However, the thickness of b-
MoO3 also influences the charge extraction properties of the PSCs due to the limited conductivity of MoO3. For these reasons, the effects of the b-MoO3 layer thickness on the solar cell performance were investigated. Figure 2 and Table 1 illustrate the performance of PSCs prepared with different transparent top electrodes. If a thin (10 nm) Au layer is deposited directly on top of SpiroOMeTAD, the devices exhibit poor performances (PCEo1%). A high series resistance (4.3 103 Ω cm2) and non-ideal J–V curves (Figure 2a and Figure S4) are observed with a large variability in the experimental data (see Table 1). This is consistent with the results presented in Figure 1e and f, which show that glass and Spiro-OMeTAD surfaces cause the growth of a resistive and inhomogeneous Au film that does not provide efficient charge collection. When a very thin (1 nm) MoO3 film is deposited on top of the Spiro-OMeTAD layer, highly conductive, continuous and smooth Au films are grown with a sheet resistance of 13–15 Ω/□. The inclusion of this MoO3 layer therefore enables solar cells to be fabricated with much lower series resistances (between 10 and 15 Ω cm2), and much higher efficiencies (11% on average, with top devices exceeding 13%, Figure 2b and Table 1). Similar results were observed for b-MoO3 layers with thicknesses up to 20 nm. However, thicker b-MoO3 films also cause a slight decrease in efficiency. This decrease is mainly due to a reduction in fill factor (Figure 2b and Table 1), suggesting that thick b-MoO3 layers hinder the complete extraction of charges from the HTL. A similar effect has
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Table 1 Summary of PSC performances (open circuit voltage, VOC; short circuit current, JSC; fill factor, FF; power conversion efficiency, PCE; series resistance, RS) measured under 1 sun illumination (AM 1.5) as a function of the thickness of the b-MoO3 layer. The thickness of the Au (10 nm) and t-MoO3 (35 nm) layers is kept constant in all devices.a b-MoO3 (nm)
VOC (mV)
JSC (mA cm 2)
FF (%)
Average PCE (%)
Best PCE (%)
RS (Ω cm2)
0 1 5 10 20 35
6307150 984726 988723 969713 998724 969716
8.073.7 18.070.7 20.470.4 18.570.4 17.772.2 19.870.3
972 6176 5879 6272 6177 4974
0.670.3 10.971.6 11.771.7 11.270.6 10.872.3 9.371.0
1.2 13.0 13.6 12.2 13.3 10.5
430073000 11.574.2 13.676.7 10.271.0 13.476.1 15.272.7
a
All values excluding the best PCE are the average of 12 devices prepared under identical conditions. The errors associated with the measurements are 71 standard deviation.
been observed recently for PSCs prepared using a MoO3/Al top contact [36]. If a thicker Au layer is deposited, high efficiency can be achieved even without b-MoO3, because the gold film reaches full percolation and ensures high conductivity and efficient charge extraction. Reference devices prepared without MoO3 and using a thick, opaque Au cathode (70 nm) consistently exhibited a PCE above 14% (see Figure S5). For completeness we note that analogous device performances were observed for solar cells prepared using a DMD multilayered top electrode that utilized a thin layer of Ag instead of Au (see Figure S6). This highlights the possibility to successfully use cheaper materials for the DMD electrode. However, we did notice that these electrodes were not very stable in the presence of the perovskite layers. Figure 2c compares the absorption spectrum of a 300 nm thick CH3NH3PbI3 layer and the incident photon-to-current efficiency (IPCE) spectra of PSCs, comprising a b-MoO3 layer with a thickness of 20 nm, measured by illuminating through either the FTO or the DMD electrodes. IPCE values above 80% were achieved at lower wavelengths when passing through the FTO. In contrast, the IPCE curve shows a sharp drop below 430 nm and also a slight decrease in the 450–550 nm range when illuminating from the DMD side. These effects are ascribed to the strong absorption of Spiro-OMeTAD [37] and to the slightly reduced transparency of the DMD electrode at wavelengths below 600 nm (see Figure 1d and Figure S3), respectively. The device characteristics of this sample illuminated from both sides are reported in Table S1. For both illumination directions the IPCE curve decreases slightly at long wavelengths (550–800 nm) due to incomplete light absorption, as can be inferred from the absorption profile of the CH3NH3PbI3 film, which shows that the incoming light is not fully absorbed by the perovskite layer in that wavelength range. Standard devices prepared using a thick metal cathode usually show fairly flat IPCE spectra, even at wavelengths approaching the perovskite band gap. This occurs because the metal film reflects the incident light back into the absorbing layer, increasing the efficacy of light absorption by CH3NH3PbI3 due to a second light pass. As our top electrode is highly transparent, the amount of light reflected back into the device is negligible, and so the IPCE curve is consistent with the absorption profile of a 300 nmthick CH3NH3PbI3 film.
In light of the recently reported hysteresis observed in the J–V curves of planar PSCs [38,39], our devices were measured in the conventional manner (from forward bias to short circuit, FB–SC) and also in the reverse direction (from short circuit to forward bias, SC–FB) to assess any variation in the J–V characteristics. Typical J–V curves for one of the top performing devices are reported in Figure S7 in the Supporting information. The PCE values associated with the two scan directions were 13.1% and 10.2%, respectively. It has been demonstrated that the most reliable procedure to determine the true power output of a solar cell is to measure the stabilized photocurrent and efficiency by holding the device at the voltage corresponding to the maximum power point for that device under illumination, defined as Vmp. The same solar cell used for the J–V curves presented in Figure S7 was held at a forward bias of 0.77 V (corresponding to Vmp) and the photocurrent density and PCE were recorded as a function of time (Figure 2d). The photocurrent stabilizes within a few seconds and results in a PCE of 12.9% at 150 s, which is only slightly reduced (by a relative 2%) from the value calculated from the J–V curves measured in FB–SC direction, confirming the reliability of the efficiency values obtained from the J–V curves. Having evaluated the role of the transparent top electrode, attention was then focused on the absorber layer. Due to the high absorption coefficient of CH3NH3PbI3 in the visible wavelength range, even moderately thin films (400 nm) absorb almost all visible light. In order to make a semitransparent device, the thickness of the absorber layer has to be substantially reduced. Recently, other research groups have shown the deposition of thin (50–100 nm) perovskite films of high quality using vacuum-based techniques [15,40]. However, achieving thin and continuous CH3NH3PbI3 films using solution processing still remains a challenge, even if perovskite “islands” resulting from dewetting of CH3NH3PbI3 crystals during spin coating have been exploited for the preparation of neutral-colored semitransparent devices [16,41]. We have recently developed a gas-assisted solution method which enables for the deposition of high quality, dense and continuous CH3NH3PbI3 films (300–350 nm thick) [11]. Here, we have adapted this method to fabricate continuous perovskite films with a thickness as low as 50 nm. Figure 3 shows the SEM images in cross section and top view of CH3NH3PbI3 films prepared from solutions with different precursor
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E. Della Gaspera et al.
Figure 3 Characterization of PSCs with CH3NH3PbI3 films of different thicknesses. (a) Cross section and (b) top view SEM images of CH3NH3PbI3 films deposited on FTO. The average thickness (a) and grain size (b) are shown at the top of each image. The scale bar is 500 nm and it applies to all images. c) J–V curves of PSCs under AM 1.5 (1 sun) illumination. All devices were illuminated from the FTO side. The inset shows a photograph of a semitransparent PSC. (d) IPCE and (e) transmittance spectra of complete PSCs with different CH3NH3PbI3 film thicknesses. (f) Plot of PCE (average and best) as a function of AVT (370–740 nm) and comparison with other semitransparent PSC. The arrows show the performance of our best devices if the AVT is calculated between 400 and 800 nm, for a fair comparison with previously published results [15].
concentrations. A progressive increase in CH3NH3PbI3 film thickness, from 50 nm up to 300 nm was achieved by gradually increasing the precursor concentration. For clarity purposes, from now on these samples will be referred as 55, 105, 140 and 290 nm, consistent with their thickness. The CH3NH3PbI3 grain size evaluated from the top view SEM images is smaller in thin films (100 nm) than in the 300 nm film. However, there is a large variation in the grain size (high polydispersity), with grains as big as 500 nm and as small as 150 nm being observed in the thicker samples (see also Figure S8). No obvious defects or pinholes are observed, and the films appear continuous and homogeneous at every thickness assessed. This is particularly noteworthy when the
large surface roughness of the FTO–glass substrate is considered (see Figure S9). XRD measurements confirmed the deposition of CH3NH3PbI3 without any contaminant phases (Figure S10). Importantly, apart from a variation in intensity of the diffraction peaks, which scales proportionally with the amount of probed material (i.e. film thickness), no differences in diffraction patterns are observed between samples of different thicknesses. The optical transmission spectra of these CH3NH3PbI3 films deposited on glass–FTO substrates are presented in Figure S11. As expected, a higher transmittance is observed for thinner perovskite films. For reference, the absorption feature of Spiro-OMeTAD at around 400–450 nm, as well as its effect in increasing the
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Table 2 Summary of device performances under 1 sun (AM 1.5) illumination from the FTO side as a function of the thickness of the CH3NH3PbI3 layer using an optimized DMD top electrode (AVT =average visible transmittance).a CH3NH3PbI3 thickness (nm)
AVT (%)
VOC (mV)
JSC (mA cm 2)
FF (%)
Average PCE (%)
Best PCE (%)
RS (Ω cm2)
54724 107722 141729 289726
31 19 16 7
718744 941741 950741 988723
9.770.1 13.770.5 14.770.4 20.470.4
6674 6373 6575 5879
4.670.4 8.170.5 9.371.1 11.771.7
5.3 8.8 10.1 13.6
10.671.3 12.572.6 11.873.7 13.676.7
a
All values excluding the best PCE are the average of 12 devices prepared under identical conditions. The errors associated with the measurements are 71 standard deviation.
transmittance of the films by reducing scattering and reflection at the CH3NH3PbI3/air interface, is clearly seen. Using these perovskite films, a series of semitransparent devices was prepared in which the transparent top electrode was composed of 5 nm b-MoO3, 10 nm Au and 35 nm t-MoO3. This electrode gave the best results when used in PSC with thick (300 nm) CH3NH3PbI3 films (see Table 1). The J–V characteristics, IPCE spectra and transmission spectra of representative devices are presented in Figure 3b–d, respectively, and the device performances are summarized in Table 2. A clear trend in device characteristics with decreasing CH3NH3PbI3 thickness can be observed: thinner devices generate less photocurrent as a consequence of the poor light absorption (see Figure 3c), while VOC and FF do not change substantially. Only the devices prepared with 55 nm thick perovskite films show a reduced open circuit voltage. Considering the high surface roughness of FTO and the very low thickness of such CH3NH3PbI3 layers, the presence of pinholes cannot be excluded, even if there is no direct evidence of contact between the FTO–TiO2 substrate and the SpiroOMeTAD layer (see Figure S12). An evaluation of the average visible transmittance (AVT, calculated between 370 nm and 740 nm) of the full devices confirmed the higher light transmission through solar cells with the thinnest perovskite layer (AVT=31%), while thicker CH3NH3PbI3 films cause a reduction in transparency (AVT= 19% for 105 nm thick films, AVT=16% for 140 nm thick films and AVT=7% for 290 nm thick films). These devices are orange–brown in color, which is more distinct at higher CH3NH3PbI3 thicknesses. Conversely, PSCs incorporating very thin CH3NH3PbI3 layers have a color closer to neutrality (see Figure S13). Figure 3f shows the evolution of average and best PCE as a function of AVT for the prepared devices, and the comparison with the state of the art of semitransparent PSCs. The devices developed in this study have afforded the highest PCEs reported to date at every measured AVT. This is especially the case if the AVT is calculated between 400 nm and 800 nm, as has been done in previously published work [15].
Conclusions In conclusion, high efficiency semitransparent perovskite solar cells have been prepared by combining a transparent multilayered top electrode with an optimized solutionbased deposition of CH3NH3PbI3. Through the judicious control of the different layers comprising the top electrode and of the deposition parameters affecting the perovskite
thickness, semitransparent devices have been developed which show a PCE of 5.3% with 31% AVT, and a record high 13.6% PCE with 7% AVT. These results represent a step forward towards the fabrication of solar cells with high efficiency and high transparency for building-integrated photovoltaics and other semitransparent applications.
Acknowledgments We acknowledge the Australian Research Council for providing equipment and fellowship support, the Australian Renewable Energy Agency and the Australian Centre for Advanced Photovoltaics for financial support. The work was also funded through CSIRO Manufacturing Flagship as part of an Office of the Chief Executive Postdoctoral Fellowship (E.D.G.). J.J.J. acknowledges the Australian Research Council for funding through the grant DP110105341. U.B. thanks the CSIRO for providing support through an OCE Science Leader position. Part of this work was performed at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).
Appendix A.
Supplementary Information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.02.028.
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E. Della Gaspera et al. Enrico Della Gaspera obtained his Ph.D. in Materials Science and Engineering from the University of Padova, Italy in 2011. After spending 18 months as a postdoctoral fellow at the University of California, Los Angeles (UCLA) in Prof. Bruce Dunn's group, he moved to CSIRO in 2013 where he currently holds an OCE postdoctoral fellowship. His research focuses mainly on solution processed inorganic materials and nanocomposites, with applications in transparent conductors, optoelectronic devices, sensors and energy harvesting and storage. Yong Peng received his Ph.D. degree in 2008 from Wuhan University of Technology. He focused his Ph.D. research interests on synthesizing and designing structural materials including ceramics, super alloy and light metals. In 2008 he started working in the Department of Materials Engineering at Monash University and changed his research interests to Dye-Sensitized Solar Cells (DSC) and emerging solar cells technologies. At present, Dr. Yong Peng is majorly working on continuously printing large scale DSC modules and on perovskite solar cells.
Qicheng Hou is currently a Ph.D. student in the Department of Materials Engineering at Monash University. He received his B.S. degree in Materials Engineering from Monash University (Australia) and Central South University (China) in 2014. His main research interest is perovskite-silicon tandem solar cells.
Leone Spiccia obtained his Ph.D. degree in Physical and Inorganic Chemistry from the University of Western Australia in 1984. After postdoctoral positions at the University of Calgary, Université de Neuchâtel and the Australian National University, he took up an academic appointment at Monash University in 1987, where he is currently a Professor of Chemistry. His research interests include solar, water splitting, dye-sensitized solar cells, thin film solar cells, coordination and bio-inorganic chemistry, biomimetic chemistry, biosensors, therapeutic and diagnostic applications of nanomaterials and radiolabelled bioconjugates. Udo Bach received his Ph.D. from the Swiss Federal Institute of Technology (EPFL) and worked for 3 years in a technology start-up company in Dublin (Ireland). After spending 15 months as a postdoc in the group of Prof. Paul Alivisatos at UC Berkeley (USA), he moved to Monash University in 2005 to establish his own research group. He has been an Australian Research Fellow since 2006, a CSIRO OCE Science Leader and a Melbourne Centre for Nanofabrication Tech Fellow since 2011. Prof. Bach is involved in fundamental and applied research in the area of photovoltaics, nanofabrication, DNA-directed self-assembly, nanoprinting and plasmonics.
Ultra-thin high efficiency semitransparent perovskite solar cells Jacek Jasieniak completed his Ph.D. from the University of Melbourne in 2008 before moving to CSIRO for postdoctoral work on solution-processed optoelectronics. He continued this work as a Fulbright fellow at the University of California, Santa Barbara (UCSB) with Prof. Alan Heeger before returning back to CSIRO in 2012. He is currently the Group Leader of Agile Manufacturing at CSIRO and project area leader of solution-processed inorganic optoelectronics.
257 Yi-Bing Cheng is a professor in the Department of Materials Engineering at Monash University, Australia. He completed his undergraduate (1978) and Master (1983) studies at Wuhan University of Technology, China and received a Ph.D. degree from University of Newcastle-upon-Tyne, U.K. in 1989. He joined Monash University in 1991. He specialises in inorganic materials and composites. He started working on dye sensitised solar cells in 2001 and is mainly interested in solution processed solar cells.
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RAPID COMMUNICATION
Ultra-thin high efficiency semitransparent perovskite solar cells Enrico Della Gasperaa,1, Yong Pengb,1, Qicheng Houb, Leone Spicciac, Udo Bacha,b,d, Jacek J. Jasieniaka,n, Yi-Bing Chengb,nn a
CSIRO Manufacturing Flagship, Bayview Ave, Clayton, Victoria 3168, Australia Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia c School of Chemistry, Monash University, Victoria 3800, Australia d Melbourne Centre for Nanofabrication, Clayton, Victoria 3168, Australia b
Received 23 December 2014; received in revised form 17 February 2015; accepted 17 February 2015 Available online 26 February 2015
KEYWORDS
Abstract
Transparent electrode; Molybdenum oxide; Photovoltaics; Methylammonium lead iodide; Thin film
Solar cells based on organometal trihalide perovskite light absorbers have recently emerged as one of the most promising class of photovoltaic devices. Given their high performances, along with their low fabrication costs, such devices are appealing for building integrated photovoltaics, especially if made semitransparent and used as solar windows. In this study, efficient perovskite solar cells with high transparency in the visible spectrum have been developed. These devices comprise a dielectric–metal–dielectric multilayered top electrode with high transparency and conductivity, and an optimized solution-based thin and continuous methylammonium lead iodide film. Through this combination of processing and device architecture developments, semitransparent solar cells with high visible transmittance and record power conversion efficiency have been prepared, including devices with a perovskite thickness as low as 55 nm. & 2015 Elsevier Ltd. All rights reserved.
Introduction n
Corresponding author. Tel.: +61 395452446. Corresponding author. Tel.: +61 399054930. E-mail addresses: [email protected] (J.J. Jasieniak), [email protected] (Y.-B. Cheng). 1 Enrico Della Gaspera and Yong Peng contributed equally to this work. nn
http://dx.doi.org/10.1016/j.nanoen.2015.02.028 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
Organometal trihalide perovskites have rapidly emerged as one of the most promising class of absorber materials for high efficiency solar cells [1–3]. Some of the reasons for their success are their optimal band gap ( 1.55 eV for methylammonium lead iodide, CH3NH3PbI3), high absorption coefficients
250 (104–105 cm 1) [4], and long carrier diffusion lengths [5]. Moreover, perovskite solar cells (PSCs) can be fabricated with simple and straightforward solution-based procedures that are amendable to continuous, high-throughput processes [6–8]. Despite being a very young technology, the record power conversion efficiency (PCE) of PSCs has already exceeded 20%, with numerous research groups demonstrating PSCs with PCEs above 15% [9–14]. This rapid growth of PSC research and the concurrent improvement in device performances have enabled the fabrication of efficient semitransparent PSCs that could find applications, for example, in building integrated photovoltaics [15,16]. In order to prepare a highly efficient and semitransparent device, all layers, excluding the light absorber, should be highly transparent. Considering a planar PSC architecture, the absorber material is deposited on a transparent conductive oxide (TCO, for example tin doped indium oxide, ITO, or fluorine doped tin oxide, FTO) covered with a hole blocking layer (e.g. TiO2). The hole transport layer (HTL) is then deposited on the perovskite, and the cell is completed with a thick metal electrode, such as Ag or Au. A semi-transparent device can be fabricated if the opaque metal electrode is replaced by a transparent conductive electrode. Several examples of transparent electrodes have been used in solar cells and more generally in optoelectronic devices, including thin metal films [17], conductive oxides (e.g. ITO) [18], metal nanowires [19], conductive polymers (e.g. PEDOT:PSS) [20], carbon nanotubes [21], graphene [22], etc. These materials are sometimes combined to enhance the transmittance, reduce the resistivity and improve the mechanical properties of the electrode [23–26]. However, many of these options present disadvantages when used in semitransparent PSCs, such as low conductivity, high production cost, and/or require solvents and processing temperatures that are not compatible with the organometal trihalide perovskites. In this work, the opaque metal cathode has been replaced with a dielectric–metal–dielectric (DMD) multilayer. DMD structures rely on interference effects provided by the two dielectric layers, which increase the light transmission through a thin metal film [27]. Such DMD stacks have been widely adopted as visible-transparent and IR-reflective coatings (heat mirrors) [27,28], and as transparent conductors for various optoelectronic devices, including organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs) [29–33]. Critical to their success as semi-transparent windows is the precise selection and thickness optimization of the individual dielectric and metal films [27,32,33]. Here, we present a DMD transparent electrode composed of a MoO3–Au–MoO3 stack. Molybdenum trioxide was chosen because of its well-established hole injection properties [34] and because it provides a good nucleation surface for Au, which allows thin and uniform metal films to be deposited. Common metals employed within PSCs are Au, Ag and Al. Au was selected for this study because of its higher stability towards air oxidation compared to Al and lower reactivity towards halides compared to Ag. In addition to the transparency of the electrode, the inherent transmittance of the absorbing layers must also be considered for semitransparent devices. Naturally, this depends on the thickness of the CH3NH3PbI3 layer for PSC, whose deposition is known to be inherently challenging. Using our recently developed gas-assisted deposition technique [11], CH3NH3PbI3 layers
E. Della Gaspera et al. as thin as 50 nm have been deposited, while still obtaining a continuous and homogeneous layer that results in devices with high efficiency and high transparency values in the visible spectrum. By combining this controllable CH3NH3PbI3 layer deposition with a DMD electrode structure, record PCEs between 5.5% and 13.6% have been achieved for devices with average visible transmittance (AVT) values of 31% and 7%, respectively.
Materials and methods Materials All chemicals have been used as received without any further purification. Methylamine (CH3NH2, 33% in ethanol), hydroiodic acid (HI, 57% in water), N,N-dimethylformamide (DMF, 99.8%), lead iodide (PbI2, 99.999%), 4-tert-butylpyridine (TBP, 97%), lithium bis(trifluoromethylsulphonyl)imide (LiTFSi, 99.95%), chlorobenzene (99.8%), acetonitrile (99.8%) and molybdenum trioxide (MoO3, 99.5%) have been purchased from Sigma Aldrich. Gold (Au) has been purchased from A&E Metals. 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD, 99.5%), has been purchased from Luminescence Technology Corp.
Samples fabrication Details of the perovskite solar cell (PSC) fabrication, excluding the composite top electrode, have been published elsewhere [11]. Briefly, methylammonium iodide (CH3NH3I) was synthesized by reacting CH3NH2 with HI and eventually purified via recrystallization. To prepare the CH3NH3PbI3 precursor solution, a 1:1 M ratio of CH3NH3I and PbI2 was dissolved in DMF at various concentrations, namely 20, 30, 35, 45 wt%. For this purpose, 59 mg, 102 mg, 128 mg and 194 mg of a 1:1 CH3NH3I: PbI2 molar mixture was dissolved in 250 mL DMF, respectively. A dense TiO2 layer was screen printed on top of patterned and cleaned FTO glass (NEG glass, 9 Ω/□) using a screen printing paste (Dyesol, BL-1) and annealed at 450 1C for 10 min. The CH3NH3PbI3 films were subsequently deposited by spin coating the precursor solution at 6500 rpm for 30 s, using a gas-assisted spin coating method [11], and annealed at 100 1C for 10 min. The thickness of the perovskite layer was controlled by varying the concentration of the CH3NH3PbI3 solution. A Spiro-OMeTAD film was then deposited by spincoating at 3000 rpm for 30 s, a solution prepared by dissolving 41.6 mg Spiro-OMeTAD, 7.5 μL of a stock solution of 520 mg mL 1 LiTFSi in acetonitrile and 16.9 μL TBP in 0.5 mL chlorobenzene. The solar cells were completed by thermal evaporation under vacuum (3 10 6 Torr) of the DMD electrodes composed of: (i) bottom MoO3 (b-MoO3) layer with thicknesses of 0, 1, 5, 10, 20 or 35 nm; (ii) a thin Au film (10 nm); and (iii) a top MoO3 (t-MoO3) layer (35 nm). The deposition rate was set to be 0.1 nm/s for MoO3 and 0.05 nm/s for Au. These electrodes were deposited also on plain glass slides and silicon substrates for additional characterization.
Characterization techniques The surface morphology and the cross sections of selected samples were analyzed using a FEI Helios Nanolab 600
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Figure 1 (a) Schematic illustration (not to scale) of the PSC architecture. (b) SEM image in cross section of a complete device. For the uncolored image refer to Figure S1 in the Supporting information. (c) Enlarged view of the multilayer top electrode and schematic of its structure. (d) Simulations (shaded dashed lines) and experimental data (solid lines) showing the transmittance of Au (black), b-MoO3/Au (red) and b-MoO3/Au/t-MoO3 (blue). (e) SEM image of Au film. (f) SEM image of b-MoO3/Au film. The insets show photos of the two samples.
Scanning Electron Microscope (SEM). Optical absorption, transmission and reflection spectra of the DMD electrodes deposited on glass substrates, the CH3NH3PbI3 films deposited on FTO–glass and the completed semitransparent devices were measured using a Perkin-Elmer Lambda1050 spectrophotometer equipped with an integrating sphere. Ellipsometry quantities Ψ and Δ of MoO3 films on either Si or glass substrates were measured using a J.A. Woollam M-2000 spectroscopic ellipsometer and the refractive index (n) and absorption coefficient (k) were evaluated from Ψ and Δ data using the CompleteEASE data analysis software. The obtained n and k values for MoO3 as a function of wavelength were then used together with bulk Au optical constants to model the DMD stack transmission using the Fresnel matrix method, assuming infinite, flat, parallel, isotropic and homogeneous layers with non-interpenetrating interfaces. Sheet resistances of the DMD electrodes deposited on glass were measured by using a Jandel RM3000 4-point probe and averaged over at least 5 measurements per sample. X-Ray Diffraction (XRD) patterns of the CH3NH3PbI3 films deposited on FTO–glass substrates were collected using a Bruker D8 diffractometer equipped with a Cu-Kα radiation source and operated at 40 mV and 40 mA. J–V curves of solar cells were measured using a Keithley 2400 source meter scanning the devices at 100 mV/s under illumination of simulated sunlight (100 mW/cm2) provided by an Oriel solar simulator with an AM 1.5 filter. A black metal mask was employed to define a device area of 0.16 cm2. IPCE spectra were recorded on a Keithley 2400 source meter under the irradiation of a 300 W Xenon lamp with an Oriel Cornerstone™ 260 1/4 m monochromator.
Results and discussion The PSC architecture is presented in Figure 1a–c. A uniform and continuous CH3NH3PbI3 layer ( 300 nm thick) was first deposited on top of an FTO/glass substrate coated with a dense and thin ( 30 nm) n-type TiO2 film. Spiro-OMeTAD ( 200 nm thick) was then introduced as a HTL and the cell was completed by precisely depositing a transparent DMD top electrode using thermal evaporation. The advantage of the DMD multilayer architecture is that it provides improved transparency when compared to neat metal films. This is clearly shown in Figure 1d where the experimental and simulated transmittance for Au, b-MoO3/ Au and b-MoO3/Au/t-MoO3 layers on glass are presented (see Materials and methods for details). A neat 10 nm thick Au film shows low transmission because of the high reflection at the Au/air and Au/glass interfaces. However, when the thin Au film is sandwiched between the two MoO3 dielectric layers, destructive interference in the visible range can be achieved through simple tuning of the exact layer thicknesses to provide an increase in transparency. The precise control of the optical properties of such DMD structures can only be achieved if each layer is continuous. When Au is deposited directly on glass or on Spiro-OMeTAD (the HTL used in the PSC architecture adopted here) it nucleates into separate islands, which reach the percolation threshold at a thickness of 10 nm (Figure 1e) [35]. These metallic islands possess a localized surface plasmon resonance (SPR) that results in a reduction of the overall transmittance compared to an ideal thin Au metal film. In contrast, when a
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Figure 2 (a) Current–Voltage characteristics of PSCs with/without b-MoO3 measured under dark and 1 sun (AM 1.5) illumination. (b) PCE of PSCs and sheet resistance of DMD electrodes deposited on glass as a function of the b-MoO3 film thickness. (c) Absorption spectrum (Absorption=100 Transmittance Reflectance) of 300 nm thick CH3NH3PbI3 film and IPCE of PSC with 300 nm CH3NH3PbI3 film measured under illumination from the FTO side (dark blue) and from the DMD side (light blue), respectively. (d) Photocurrent density and PCE measured as a function of time for a device held at 0.77 V forward bias, which corresponds to the voltage at maximum power point for that particular device.
very thin MoO3 layer (even 1–2 nm in thickness) is used as a seeding layer, the surface provides for the appropriate conditions to grow continuous and smooth Au films (Figure 1f). This is verified by the excellent agreement between experimental transmittance of the b-MoO3/Au layers and thin-film optical modeling that assumes bulk optical properties. When comparing both systems, Au films evaporated on glass are green–blue in color, which indicates the presence of a localized SPR, whereas Au films deposited on MoO3/glass appear yellow. This difference in morphology and optical properties is also accompanied by a large variation in electrical conductivity. The Au films deposited on MoO3 exhibit a sheet resistance of 13 Ω/□ compared to 250 Ω/□ when Au is deposited on glass or on Spiro-OMeTAD. If thicker Au films are deposited, high conductivity can eventually be achieved without the MoO3 interlayer, but with the drawback of a lower transmission in the visible spectrum. Therefore, such films are not desirable for semitransparent devices. Moreover, by using MoO3 as a nucleation layer, very thin (6–7 nm) continuous and smooth Au films can be deposited, with even higher transparency, but with a slightly higher sheet resistance ( 20 Ω/□, see Figure S2). The thickness of both the bottom and top dielectric films plays an important role in defining the transmission window of DMD structures [27]. By optimizing the t-MoO3 thickness to a value of 30–40 nm and by progressively increasing the bMoO3 thickness, a slight redshift in the transparency window is observed (see Figure S3). However, the thickness of b-
MoO3 also influences the charge extraction properties of the PSCs due to the limited conductivity of MoO3. For these reasons, the effects of the b-MoO3 layer thickness on the solar cell performance were investigated. Figure 2 and Table 1 illustrate the performance of PSCs prepared with different transparent top electrodes. If a thin (10 nm) Au layer is deposited directly on top of SpiroOMeTAD, the devices exhibit poor performances (PCEo1%). A high series resistance (4.3 103 Ω cm2) and non-ideal J–V curves (Figure 2a and Figure S4) are observed with a large variability in the experimental data (see Table 1). This is consistent with the results presented in Figure 1e and f, which show that glass and Spiro-OMeTAD surfaces cause the growth of a resistive and inhomogeneous Au film that does not provide efficient charge collection. When a very thin (1 nm) MoO3 film is deposited on top of the Spiro-OMeTAD layer, highly conductive, continuous and smooth Au films are grown with a sheet resistance of 13–15 Ω/□. The inclusion of this MoO3 layer therefore enables solar cells to be fabricated with much lower series resistances (between 10 and 15 Ω cm2), and much higher efficiencies (11% on average, with top devices exceeding 13%, Figure 2b and Table 1). Similar results were observed for b-MoO3 layers with thicknesses up to 20 nm. However, thicker b-MoO3 films also cause a slight decrease in efficiency. This decrease is mainly due to a reduction in fill factor (Figure 2b and Table 1), suggesting that thick b-MoO3 layers hinder the complete extraction of charges from the HTL. A similar effect has
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Table 1 Summary of PSC performances (open circuit voltage, VOC; short circuit current, JSC; fill factor, FF; power conversion efficiency, PCE; series resistance, RS) measured under 1 sun illumination (AM 1.5) as a function of the thickness of the b-MoO3 layer. The thickness of the Au (10 nm) and t-MoO3 (35 nm) layers is kept constant in all devices.a b-MoO3 (nm)
VOC (mV)
JSC (mA cm 2)
FF (%)
Average PCE (%)
Best PCE (%)
RS (Ω cm2)
0 1 5 10 20 35
6307150 984726 988723 969713 998724 969716
8.073.7 18.070.7 20.470.4 18.570.4 17.772.2 19.870.3
972 6176 5879 6272 6177 4974
0.670.3 10.971.6 11.771.7 11.270.6 10.872.3 9.371.0
1.2 13.0 13.6 12.2 13.3 10.5
430073000 11.574.2 13.676.7 10.271.0 13.476.1 15.272.7
a
All values excluding the best PCE are the average of 12 devices prepared under identical conditions. The errors associated with the measurements are 71 standard deviation.
been observed recently for PSCs prepared using a MoO3/Al top contact [36]. If a thicker Au layer is deposited, high efficiency can be achieved even without b-MoO3, because the gold film reaches full percolation and ensures high conductivity and efficient charge extraction. Reference devices prepared without MoO3 and using a thick, opaque Au cathode (70 nm) consistently exhibited a PCE above 14% (see Figure S5). For completeness we note that analogous device performances were observed for solar cells prepared using a DMD multilayered top electrode that utilized a thin layer of Ag instead of Au (see Figure S6). This highlights the possibility to successfully use cheaper materials for the DMD electrode. However, we did notice that these electrodes were not very stable in the presence of the perovskite layers. Figure 2c compares the absorption spectrum of a 300 nm thick CH3NH3PbI3 layer and the incident photon-to-current efficiency (IPCE) spectra of PSCs, comprising a b-MoO3 layer with a thickness of 20 nm, measured by illuminating through either the FTO or the DMD electrodes. IPCE values above 80% were achieved at lower wavelengths when passing through the FTO. In contrast, the IPCE curve shows a sharp drop below 430 nm and also a slight decrease in the 450–550 nm range when illuminating from the DMD side. These effects are ascribed to the strong absorption of Spiro-OMeTAD [37] and to the slightly reduced transparency of the DMD electrode at wavelengths below 600 nm (see Figure 1d and Figure S3), respectively. The device characteristics of this sample illuminated from both sides are reported in Table S1. For both illumination directions the IPCE curve decreases slightly at long wavelengths (550–800 nm) due to incomplete light absorption, as can be inferred from the absorption profile of the CH3NH3PbI3 film, which shows that the incoming light is not fully absorbed by the perovskite layer in that wavelength range. Standard devices prepared using a thick metal cathode usually show fairly flat IPCE spectra, even at wavelengths approaching the perovskite band gap. This occurs because the metal film reflects the incident light back into the absorbing layer, increasing the efficacy of light absorption by CH3NH3PbI3 due to a second light pass. As our top electrode is highly transparent, the amount of light reflected back into the device is negligible, and so the IPCE curve is consistent with the absorption profile of a 300 nmthick CH3NH3PbI3 film.
In light of the recently reported hysteresis observed in the J–V curves of planar PSCs [38,39], our devices were measured in the conventional manner (from forward bias to short circuit, FB–SC) and also in the reverse direction (from short circuit to forward bias, SC–FB) to assess any variation in the J–V characteristics. Typical J–V curves for one of the top performing devices are reported in Figure S7 in the Supporting information. The PCE values associated with the two scan directions were 13.1% and 10.2%, respectively. It has been demonstrated that the most reliable procedure to determine the true power output of a solar cell is to measure the stabilized photocurrent and efficiency by holding the device at the voltage corresponding to the maximum power point for that device under illumination, defined as Vmp. The same solar cell used for the J–V curves presented in Figure S7 was held at a forward bias of 0.77 V (corresponding to Vmp) and the photocurrent density and PCE were recorded as a function of time (Figure 2d). The photocurrent stabilizes within a few seconds and results in a PCE of 12.9% at 150 s, which is only slightly reduced (by a relative 2%) from the value calculated from the J–V curves measured in FB–SC direction, confirming the reliability of the efficiency values obtained from the J–V curves. Having evaluated the role of the transparent top electrode, attention was then focused on the absorber layer. Due to the high absorption coefficient of CH3NH3PbI3 in the visible wavelength range, even moderately thin films (400 nm) absorb almost all visible light. In order to make a semitransparent device, the thickness of the absorber layer has to be substantially reduced. Recently, other research groups have shown the deposition of thin (50–100 nm) perovskite films of high quality using vacuum-based techniques [15,40]. However, achieving thin and continuous CH3NH3PbI3 films using solution processing still remains a challenge, even if perovskite “islands” resulting from dewetting of CH3NH3PbI3 crystals during spin coating have been exploited for the preparation of neutral-colored semitransparent devices [16,41]. We have recently developed a gas-assisted solution method which enables for the deposition of high quality, dense and continuous CH3NH3PbI3 films (300–350 nm thick) [11]. Here, we have adapted this method to fabricate continuous perovskite films with a thickness as low as 50 nm. Figure 3 shows the SEM images in cross section and top view of CH3NH3PbI3 films prepared from solutions with different precursor
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Figure 3 Characterization of PSCs with CH3NH3PbI3 films of different thicknesses. (a) Cross section and (b) top view SEM images of CH3NH3PbI3 films deposited on FTO. The average thickness (a) and grain size (b) are shown at the top of each image. The scale bar is 500 nm and it applies to all images. c) J–V curves of PSCs under AM 1.5 (1 sun) illumination. All devices were illuminated from the FTO side. The inset shows a photograph of a semitransparent PSC. (d) IPCE and (e) transmittance spectra of complete PSCs with different CH3NH3PbI3 film thicknesses. (f) Plot of PCE (average and best) as a function of AVT (370–740 nm) and comparison with other semitransparent PSC. The arrows show the performance of our best devices if the AVT is calculated between 400 and 800 nm, for a fair comparison with previously published results [15].
concentrations. A progressive increase in CH3NH3PbI3 film thickness, from 50 nm up to 300 nm was achieved by gradually increasing the precursor concentration. For clarity purposes, from now on these samples will be referred as 55, 105, 140 and 290 nm, consistent with their thickness. The CH3NH3PbI3 grain size evaluated from the top view SEM images is smaller in thin films (100 nm) than in the 300 nm film. However, there is a large variation in the grain size (high polydispersity), with grains as big as 500 nm and as small as 150 nm being observed in the thicker samples (see also Figure S8). No obvious defects or pinholes are observed, and the films appear continuous and homogeneous at every thickness assessed. This is particularly noteworthy when the
large surface roughness of the FTO–glass substrate is considered (see Figure S9). XRD measurements confirmed the deposition of CH3NH3PbI3 without any contaminant phases (Figure S10). Importantly, apart from a variation in intensity of the diffraction peaks, which scales proportionally with the amount of probed material (i.e. film thickness), no differences in diffraction patterns are observed between samples of different thicknesses. The optical transmission spectra of these CH3NH3PbI3 films deposited on glass–FTO substrates are presented in Figure S11. As expected, a higher transmittance is observed for thinner perovskite films. For reference, the absorption feature of Spiro-OMeTAD at around 400–450 nm, as well as its effect in increasing the
Ultra-thin high efficiency semitransparent perovskite solar cells
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Table 2 Summary of device performances under 1 sun (AM 1.5) illumination from the FTO side as a function of the thickness of the CH3NH3PbI3 layer using an optimized DMD top electrode (AVT =average visible transmittance).a CH3NH3PbI3 thickness (nm)
AVT (%)
VOC (mV)
JSC (mA cm 2)
FF (%)
Average PCE (%)
Best PCE (%)
RS (Ω cm2)
54724 107722 141729 289726
31 19 16 7
718744 941741 950741 988723
9.770.1 13.770.5 14.770.4 20.470.4
6674 6373 6575 5879
4.670.4 8.170.5 9.371.1 11.771.7
5.3 8.8 10.1 13.6
10.671.3 12.572.6 11.873.7 13.676.7
a
All values excluding the best PCE are the average of 12 devices prepared under identical conditions. The errors associated with the measurements are 71 standard deviation.
transmittance of the films by reducing scattering and reflection at the CH3NH3PbI3/air interface, is clearly seen. Using these perovskite films, a series of semitransparent devices was prepared in which the transparent top electrode was composed of 5 nm b-MoO3, 10 nm Au and 35 nm t-MoO3. This electrode gave the best results when used in PSC with thick (300 nm) CH3NH3PbI3 films (see Table 1). The J–V characteristics, IPCE spectra and transmission spectra of representative devices are presented in Figure 3b–d, respectively, and the device performances are summarized in Table 2. A clear trend in device characteristics with decreasing CH3NH3PbI3 thickness can be observed: thinner devices generate less photocurrent as a consequence of the poor light absorption (see Figure 3c), while VOC and FF do not change substantially. Only the devices prepared with 55 nm thick perovskite films show a reduced open circuit voltage. Considering the high surface roughness of FTO and the very low thickness of such CH3NH3PbI3 layers, the presence of pinholes cannot be excluded, even if there is no direct evidence of contact between the FTO–TiO2 substrate and the SpiroOMeTAD layer (see Figure S12). An evaluation of the average visible transmittance (AVT, calculated between 370 nm and 740 nm) of the full devices confirmed the higher light transmission through solar cells with the thinnest perovskite layer (AVT=31%), while thicker CH3NH3PbI3 films cause a reduction in transparency (AVT= 19% for 105 nm thick films, AVT=16% for 140 nm thick films and AVT=7% for 290 nm thick films). These devices are orange–brown in color, which is more distinct at higher CH3NH3PbI3 thicknesses. Conversely, PSCs incorporating very thin CH3NH3PbI3 layers have a color closer to neutrality (see Figure S13). Figure 3f shows the evolution of average and best PCE as a function of AVT for the prepared devices, and the comparison with the state of the art of semitransparent PSCs. The devices developed in this study have afforded the highest PCEs reported to date at every measured AVT. This is especially the case if the AVT is calculated between 400 nm and 800 nm, as has been done in previously published work [15].
Conclusions In conclusion, high efficiency semitransparent perovskite solar cells have been prepared by combining a transparent multilayered top electrode with an optimized solutionbased deposition of CH3NH3PbI3. Through the judicious control of the different layers comprising the top electrode and of the deposition parameters affecting the perovskite
thickness, semitransparent devices have been developed which show a PCE of 5.3% with 31% AVT, and a record high 13.6% PCE with 7% AVT. These results represent a step forward towards the fabrication of solar cells with high efficiency and high transparency for building-integrated photovoltaics and other semitransparent applications.
Acknowledgments We acknowledge the Australian Research Council for providing equipment and fellowship support, the Australian Renewable Energy Agency and the Australian Centre for Advanced Photovoltaics for financial support. The work was also funded through CSIRO Manufacturing Flagship as part of an Office of the Chief Executive Postdoctoral Fellowship (E.D.G.). J.J.J. acknowledges the Australian Research Council for funding through the grant DP110105341. U.B. thanks the CSIRO for providing support through an OCE Science Leader position. Part of this work was performed at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).
Appendix A.
Supplementary Information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.02.028.
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E. Della Gaspera et al. Enrico Della Gaspera obtained his Ph.D. in Materials Science and Engineering from the University of Padova, Italy in 2011. After spending 18 months as a postdoctoral fellow at the University of California, Los Angeles (UCLA) in Prof. Bruce Dunn's group, he moved to CSIRO in 2013 where he currently holds an OCE postdoctoral fellowship. His research focuses mainly on solution processed inorganic materials and nanocomposites, with applications in transparent conductors, optoelectronic devices, sensors and energy harvesting and storage. Yong Peng received his Ph.D. degree in 2008 from Wuhan University of Technology. He focused his Ph.D. research interests on synthesizing and designing structural materials including ceramics, super alloy and light metals. In 2008 he started working in the Department of Materials Engineering at Monash University and changed his research interests to Dye-Sensitized Solar Cells (DSC) and emerging solar cells technologies. At present, Dr. Yong Peng is majorly working on continuously printing large scale DSC modules and on perovskite solar cells.
Qicheng Hou is currently a Ph.D. student in the Department of Materials Engineering at Monash University. He received his B.S. degree in Materials Engineering from Monash University (Australia) and Central South University (China) in 2014. His main research interest is perovskite-silicon tandem solar cells.
Leone Spiccia obtained his Ph.D. degree in Physical and Inorganic Chemistry from the University of Western Australia in 1984. After postdoctoral positions at the University of Calgary, Université de Neuchâtel and the Australian National University, he took up an academic appointment at Monash University in 1987, where he is currently a Professor of Chemistry. His research interests include solar, water splitting, dye-sensitized solar cells, thin film solar cells, coordination and bio-inorganic chemistry, biomimetic chemistry, biosensors, therapeutic and diagnostic applications of nanomaterials and radiolabelled bioconjugates. Udo Bach received his Ph.D. from the Swiss Federal Institute of Technology (EPFL) and worked for 3 years in a technology start-up company in Dublin (Ireland). After spending 15 months as a postdoc in the group of Prof. Paul Alivisatos at UC Berkeley (USA), he moved to Monash University in 2005 to establish his own research group. He has been an Australian Research Fellow since 2006, a CSIRO OCE Science Leader and a Melbourne Centre for Nanofabrication Tech Fellow since 2011. Prof. Bach is involved in fundamental and applied research in the area of photovoltaics, nanofabrication, DNA-directed self-assembly, nanoprinting and plasmonics.
Ultra-thin high efficiency semitransparent perovskite solar cells Jacek Jasieniak completed his Ph.D. from the University of Melbourne in 2008 before moving to CSIRO for postdoctoral work on solution-processed optoelectronics. He continued this work as a Fulbright fellow at the University of California, Santa Barbara (UCSB) with Prof. Alan Heeger before returning back to CSIRO in 2012. He is currently the Group Leader of Agile Manufacturing at CSIRO and project area leader of solution-processed inorganic optoelectronics.
257 Yi-Bing Cheng is a professor in the Department of Materials Engineering at Monash University, Australia. He completed his undergraduate (1978) and Master (1983) studies at Wuhan University of Technology, China and received a Ph.D. degree from University of Newcastle-upon-Tyne, U.K. in 1989. He joined Monash University in 1991. He specialises in inorganic materials and composites. He started working on dye sensitised solar cells in 2001 and is mainly interested in solution processed solar cells.