Low-temperature processed efficient and colourful semitransparent perovskite solar cells for building integration and tandem applications

Accepted Manuscript Low-temperature processed efficient and colourful semitransparent perovskite solar cells for building integration and tandem applications Mushfika Baishakhi Upama, Md Arafat Mahmud, Haimang Yi, Naveen Kumar Elumalai, Gavin Conibeer, Dian Wang, Cheng Xu, Ashraf Uddin PII:

S1566-1199(18)30623-2

DOI:

https://doi.org/10.1016/j.orgel.2018.11.037

Reference:

ORGELE 5000

To appear in:

Organic Electronics

Received Date: 7 August 2018 Revised Date:

23 October 2018

Accepted Date: 25 November 2018

Please cite this article as: M.B. Upama, M.A. Mahmud, H. Yi, N.K. Elumalai, G. Conibeer, D. Wang, C. Xu, A. Uddin, Low-temperature processed efficient and colourful semitransparent perovskite solar cells for building integration and tandem applications, Organic Electronics (2018), doi: https://doi.org/10.1016/ j.orgel.2018.11.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Low-temperature processed efficient and colourful semitransparent perovskite solar cells for building integration and tandem applications a,

Mushfika Baishakhi Upama *, Md Arafat Mahmud c,

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a, b

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, Haimang Yi , Naveen Kumar

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Elumalai *, Gavin Conibeer , Dian Wang , Cheng Xu , Ashraf Uddin

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E-mail: [email protected], [email protected]

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School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, 2052, Sydney, Australia b Centre for Sustainable Energy Systems, Research School of Engineering, The Australian National University, Canberra, Australia c Department of Mechanical Engineering, Faculty of Engineering and Science, Curtin University, Malaysia

ABSTRACT

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Due to excellent performance and low fabrication costs, perovskite solar cells have become an attractive choice for building integrated photovoltaics (BIPV), especially semitransparent windows. Semitransparent perovskite solar cells are also useful as the upper sub-cell in tandem applications. In this study, we demonstrate efficient perovskite solar cells with good transparency in the visible spectrum (400-800 nm). The device efficiency and transparency were controlled by varying the perovskite layer thickness, including devices with a perovskite thickness as low as 40 nm. A dielectric/metal/dielectric (D/M/D) electrode was used as the transparent back electrode. The best performing device exhibited >9% efficiency at 12.4% average visible transparency. By reducing the perovskite layer thickness, the transparency further increased to 20.5% at an efficiency of 3.5%. Furthermore, perovskite layer thickness-dependent hysteresis behaviour, colour perception in human eye and month-long systematic degradation patterns are investigated and discussed comprehensively. The resulting solar cells are one of the highest performing semitransparent perovskite solar cells using D/M/D back electrode and a significant step forward to BIPV and tandem applications.

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KEYWORDS: MAPbI3, AVT, PC71BM, MoO3/Ag/MoO3, CIE diagram, D/M/D electrode

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1. Introduction

Organic–inorganic metal halide perovskite materials have emerged as one of the most promising and intriguing absorber materials for high efficiency solar cell within a short period of time [1-6]. The certified power conversion efficiency (PCE) of perovskite solar cells (PSCs) has recently exceeded 22% [7]. The compelling success story of PSCs has been possible due to their close to optimal bandgap (~1.55 eV for methylammonium lead iodide (CH3NH3PbI3 or MAPbI3)) [8], small exciton binding energy [9], balanced electron/hole mobility [10], high absorption coefficients [11], large dielectric constant, long carrier diffusion lengths [12], solution-based high throughput ability [13], and low-temperature processing using inexpensive and abundant materials [14]. Another interesting feature of these high efficiency solar cells is the possibilities of making semitransparent (ST) PSCs. Such ST cells have exciting future applications in building integrated photovoltaics (BIPV) [15-17], 1

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colorfulness for aesthetic purposes [18], wearable electronics and photovoltaic vehicles [19] and as tandem cell elements. Typically state-of-art devices are opaque due to the use of highly reflective metal back electrodes (Pt, Al, Au or Ag) [10, 20, 21]. On the other hand, ST PSCs can be fabricated by reducing the thickness of the photoactive layer and using a transparent back electrode [19]. As a result, a tradeoff exists between the generated photocurrent and the device transmittance [22]. Although ST PSCs can be fabricated simply by using thin Au or Ag metal film as the back electrode, these electrodes can be highly reflective [23] and hence causes energy loss [15]. There is a variety of alternative approaches to fabricate transparent electrodes [24]. Suitable candidates for transparent candidates include thin metal films sandwiched or covered by wide band gap dielectric materials, metallic nanowires, graphenes, carbon nanotubes, transparent conductive oxides, and conducting polymers [24-26]. In order to reduce the energy loss of reflective metal thin film electrode, an additional dielectric layer, such as MoO3, is employed outside the metal layer in fabricating ST devices with better performance, typically known as dielectric/metal/dielectric (D/M/D) electrode [23] [27-29].

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D/M/D structures consist of thin, semitransparent metal films embedded between two dielectric layers. The dielectric layers serve as charge-transport and light-coupling layer or optical spacer, respectively [30, 31]. A variety of material combinations has been used in these multilayer electrodes, typically based on ultrathin (~10-12 nm thick) films of high-work function metals (e.g., Au, Ag, Cu) and several metal oxides, including WO3, MoO3, V2O5, NiO, and ZnSnO/ZnSnO3 [31]. In our work, for the D/M/D electrodes, we employed an evaporated MoO3/Ag/MoO3 multilayer structure, based on ultrathin Ag film with a thickness of 10 nm. The inner MoO3 layer was implemented as hole injection/extraction layer with a thickness of 6 nm. The outer MoO3 capping layer plays role as an optical spacer. It can control the reflectivity of the D/M/D electrode. In the absence of an outer layer, ST solar cells demonstrate low Jsc [30]. On the other hand, when its thickness is relatively higher (≥ 80 nm) and the light is incident from the bottom side (ITO glass side), the reflectivity of the D/M/D anode reportedly increases which results in higher Jsc. However, the FF and Voc values decrease due to increased series resistance [30]. In some cases, higher capping layer thickness can also reduce the light transmission and the device AVT. Typically, in ST solar cells with D/M/D top electrode, 35-50 nm thickness is reported to be optimum for MoO3 capping layer [30-32]. The sheet resistance for such D/M/D electrode is ~40 Ω/□ which is on the same order as that of commercial indium Nn oxide (ITO) thin films (36 Ω/□) [22, 33], providing necessary conductivity for efficient charge transport. Hence, in this work, 6/10/35 nm was used as the thickness for the D/M/D top electrode. Previous reports, using D/M/D back electrode, usually also included compact and/or mesoporous TiO2 as the electron transport layer (ETL) [23, 27, 34] in an n-i-p architecture or PEDOT:PSS as the hole transport layer (HTL) [28, 29] in p-i-n architecture. However, the fabrication of TiO2 requires high temperature processing (>450 °C) [23, 35-39], which is one of the barriers towards mass production of PSCs as it exceeds the requirement of a 100–150 °C temperature range for flexible substrates with roll-to-roll processing [40-43]. On the other hand, PEDOT: PSS has acidic and hygroscopic properties, which have been reported as being detrimental to long-term device stability [32, 44]. Furthermore, Bag et al. [45] 2

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In our previous report, we proposed and developed a simple n-i-p perovskite device architecture that replaces TiO2 by fullerene ETL layers ([6,6]-phenyl C71 butyric acid methyl ester (PC71BM) or [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM)) and PEDOT:PSS by the inclusion of a Spiro-OMeTAD/MoO3 layer [46]. Our new device architecture can be fabricated at a processing temperature of only 100 °C. PC71BM and PC61BM are commonly used as acceptor materials in bulk heterojunction organic soar cells [47-52]. PC61BM layer has been proved to be effective in perovskite solar cells as an alternative of traditional TiO2 ETL [53, 54]. In our device structure, the fullerene layer acts as an efficient electron acceptor and also helps to increase photocurrent by broadening the overall device absorption window, thus yielding excellent photocurrent and photovoltaic performance [46]. Although this device structure enabled low-temperature processing, the possibility of growing high quality thin perovskite films directly on top of the fullerene layer and hence, the fabrication of ST PSCs was not investigated. Such thin PV can aid to the large-scale worldwide energy demand through BIPV applications. BIPV can reduce the electrical transmission losses and installation costs, and require less storage capacity as the generated electricity is close to its utilization site during peak demand [55]. Another application of thin PV is in tandem solar cell. Tandem structure is an effective way to minimize the fundamental optical and thermal losses under the Shockley–Queisser assumptions. It is composed of several semiconductors with different bandgaps for better coverage of the solar spectrum and each semiconductor is responsible for narrow-band absorption to reduce the thermal loss [56]. ST PSCs meet these criteria to be an effective sub-cell for efficient tandem solar cell design. In addition, the understanding of these thin PSCs with fullerene ETL can provide versatility in interfacial layer selection of tandem structure.

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In this work, we demonstrate the first successful fabrication of n-i-p configured ST PSCs using fullerene/perovskite thin films and D/M/D back electrodes. ST devices were fabricated using thin MAPbI3 films, with thicknesses as low as 40 nm. MoO3/Ag/MoO3 was used as the transparent back electrode. For simplicity, only PC71BM was used as the ETL as devices with a PC71BM ETL exhibit significantly higher suppression of hysteresis than those with PC61BM ETL, which is highly recommended for PSCs for greater accuracy and stability [46]. The highest PCE of the ST PSCs was 9.23%, with a 93 nm perovskite layer thickness, which is almost two-third of the PCE of the best opaque PSC with identical structure and a reflective metal (Ag) electrode [46], and is one of the highest reported PCEs for ST PSCs with a D/M/D electrode [19, 23, 28, 34]. A graphical representation, comparing contemporary ST PSC efficiencies with various transparent electrodes (such as D/M/D, dielectric mirror, graphene and Ag nanowire) [19, 23, 28, 34] and the best efficiency reported in this work, is shown in Fig. 1a. By varying the thickness of the perovskite layer, we controlled the transparency of the device, resulting in colourful, tunable and efficient ST PSCs. PCEs between 3.40% and 9.23% were achieved for devices with average visible transmittance (AVT) values of 20.5% 3

ACCEPTED MANUSCRIPT and 14.1%, respectively. The analysis provides compelling insight into future BIPV and tandem applications of low-temperature solution-processed ST PSCs.

2. Experimental section 2.1 Device fabrication

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Patterned ITO glass substrates (12 mm × 12 mm), from Lumtec, were cleaned via ultrasonication in soapy DI water, DI water, acetone and isopropanol, sequentially, each for 10 min. PC71BM (1- Material, Inc.) was dissolved in chlorobenzene to make a 20 mg/mL solution and spin-coated on top of the ITO coated glass substrates at 1000 rpm for 60 s to deposit a 53 ± 3.3 nm film. The deposited film was annealed at 80 °C for 5 min.

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460 mg lead iodide (PbI2) and 159 mg methyl ammonium iodide (CH3NH3I or MAI) (1:1 molar ratio) were dissolved in different volumes of DMF (N, N–dimethylformamide) to make 40, 25 and 18 wt. % solutions. The solutions were stirred vigorously overnight at room temperature in a N2 filled glovebox. The MAPbI3 solutions, with different wt. %, were spincast on the PC71BM layer at different spin speeds (ranging from 2500-5000 rpm) to achieve different perovskite layer thicknesses. Later, the substrates were annealed at 100 °C for 10 min.

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Spiro-OMeTAD HTL was spin-coated on top of the perovskite layer in a process reported in our earlier work [46]. In brief, 72.3 mg Spiro-OMeTAD (2,2',7,7'-Tetrakis (N,N-di-pmethoxyphenylamino)-9,9'-spirobifluorene) was dissolved in 1 mL chlorobenzene and doped with 17.5 µl Li-TFSI (520 mg/mL in acetronitrile) and 28.8 µL 4-TBP in chlorobenzene. The solution was spin coated on the perovskite layer dynamically at 2500 rpm for 40 s in order to prevent the PC71BM layer from damage since both Spiro-OMeTAD and PC71BM can dissolve in chlorobenzene. For better hole transport, an additional layer of thin MoO3 film (6 nm) was deposited on top of the Spiro-OMeTAD layer via thermal evaporation.[46] Finally, 10 nm thin Ag and 35 nm MoO3 layers were deposited on the Spiro-OMeTAD/MoO3 HTL layer through a shadow mask via thermal evaporation in vacuum at a pressure of 1 × 10 mBar. The device area was fixed to be 0.12 cm2 by using a metal shadow mask. For each perovskite layer thickness, at least six ST cells were fabricated.

2.2 Device characterization The PC71BM and perovskite film thicknesses were measured by an Alpha-Step D600 profiler. A MATLAB script was developed based on the code developed by Burkhard and Hoke [57] to simulate the optical electric field inside the device and the resulting exciton generation rate at different perovskite layer thicknesses. The surface roughness of the perovskite layers was extracted from atomic force microscopy (AFM) images using a Bruker Dimension ICON SPM. FEI Nova NanoSEM 230 FE-SEM was used to capture the perovskite surface topology images. X-ray diffraction (XRD) with Cu Kα radiation was performed at an angle ranging from 10° to 60° by step-scanning with a step size of 0.02°. The current density–voltage (J–V) 4

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measurements were performed using an IV5 solar cell I–V testing system from PV Measurements, Inc. (using a Keithley 2400 source meter) under an illumination power of 100 mW/cm2 by an AM 1.5G solar simulator (Oriel model 94023A; 100 mW/cm2). During the J-V measurements of the ST cells, the illumination was always from the ITO side (bottom illumination). For the optical characterization of the ST films, a UV–Vis spectrophotometer (LAMBDA 950 UV/Vis Spectrophotometer) was used. External quantum efficiency (EQE) measurements were performed using a QEX10 spectral response system (PV measurements, Inc.). The resistivity of the MoO3/Ag/MoO3 back electrode was measured by using a four point probe system (Jandel model RM3).

3. Results and discussion

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Figure 1b shows the architecture of the fabricated n-i-p ST PSCs: ITO/PC71BM/MAPbI3 perovskite/Spiro-OMeTAD/MoO3/Ag/MoO3. Here, PC71BM is used as the ETL, SpiroOMeTAD is the HTL and ultrathin MoO3/Ag/ MoO3 (6/10/35 nm) is the transparent back electrode. The inner MoO3 is mainly used to enhance the hole transport at the interface [46]. The device materials, with different bandgaps and energy levels, are shown in Fig. 1c. MAPbI3 was selected as the absorbing material for fabricating tinted ST PSCs as this material is blessed with direct bandgap of ~1.55 eV, high carrier mobility (~10 cm2 V-1s-1) and efficient exciton dissociation even at room temperature [40]. ST cells with different transparency were achieved by varying the perovskite layer thickness. First, 40 wt. % MAPbI3 solution was spin-coated on top of the ETL at different spin speeds (in rpms). Spin speeds of 3500 and 5000 rpm yielded 336 ± 12 and 247 ± 18 nm thick perovskite layers, respectively. ~300 nm is a typical perovskite thickness for opaque PSCs; for better visual transparency, thinner films are required [58]. However, when the spin speed exceeded 5000 rpm, the film surface coverage became uneven at the substrate corners, leading to visibly non-uniform films. For the n-i-p device structure under discussion, complete perovskite surface coverage on top of the PC71BM layer is a prerequisite, otherwise the Spiro-OMeTAD solution can destroy the underlying PC71BM layer [46]. In order to obtain thinner perovskite films, the MAPbI3 solution concentration was reduced to 25 and 18 wt. %. For each concentration, the spin speed was varied from 2500 to 5000 rpm and a broad range of perovskite layer thicknesses (from 160 ± 16 to 40 ± 12 nm) was achieved, with complete coverage of the perovskite layer on PC71BM. The relationship of the MAPbI3 solution concentration (wt. %) and spin speed (rpm) with the MAPbI3 layer thickness (nm) is summarized in Fig. S1. Eight different thicknesses (40-336 nm), in total, were used to fabricate the ST PSCs with different transparencies. In order to investigate the deposited thin perovskite film quality and the surface morphology and topography, AFM, SEM and XRD characterisations were employed. Figure 2a, c, e and 2b, d, f show the two and three dimensional AFM images of perovskite films, with different thicknesses (336, 138, 45 nm) and MAPbI3 precursor solution wt. %, on top of the PC71BM ETL film, respectively. From the images, it can be observed that the thick (336 nm) perovskite film, grown from a relatively high concentration of precursor solution (40 wt. %), 5

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contains larger grains and has a rougher surface than that of 138 nm and 45 nm films, grown from precursor solutions of lower concentrations (25 and 18 wt. %, respectively). The extracted average and RMS (root mean square) surface roughness of the fabricated perovskite films are listed in Table S1. As can be seen from Table S1, the 336 nm perovskite film demonstrates approximately 60% higher RMS surface roughness (138 nm: Rq= 6.5 nm, 45 nm: Rq= 6.6 nm) compared to the thinner perovskite films. Higher perovskite film roughness can imply higher leakage current in the perovskite film [59, 60], when used as a photoactive layer in a complete PSC. Nonetheless, uniformly distributed and densely packed, large crystal grains in perovskite films can lead to weak hysteresis behaviou [61]. In addition, SEM was conducted on these perovskite layers on top of PC71BM. The SEM images are shown in Fig. 3a-c. The relatively large, compact perovskite crystal grains of 336 nm film are confirmed from the SEM images (Fig. 3a). The grain size decreases when the perovskite precursor solution wt. % and the resulting perovskite film thickness decreases. The grain size can be impacted by the concentration of perovskite precursor solution. Liang et al. [62] also showed a similar relationship between the perovskite grain size and the concentration of the precursor solutions. Relatively low precursor concentrations resulted in decreased grain size due to unfavorable wetting properties of precursor solution on substrate. However, Liang et al. used spray-coating for perovskite film deposition and observed undesirable perovskite films having incomplete surface coverage when lower precursor concentrations were used. In spray-coating, the precursor solutions with low concentrations were unable to supply sufficient precursor species for the growth of the perovskite grains, leading to large intervals between the grains and hence incomplete surface coverage [62]. In contrast, we have used a one-step, solution-processed perovskite film deposition technique, and despite different grain size and precursor concentrations, all the perovskite films displayed a pinhole-free, compact surface morphology (Fig. 3) which is required for the successful subsequent deposition of the solution-processed layer (Spiro-OMeTAD). Pinhole-free perovskite surface morphology has also been reported to reduce charge recombination and thus enhance the device efficiency of PSCs [63]. The XRD patterns of the perovskite films, with different thicknesses, were also recorded and are displayed in Fig. 4a. For all the perovskite films, (110), (112), (211), (220), (312), (224), and (330) characteristic diffraction peaks were present at 14.12°, 19.89°, 23.46°, 28.45°, 31.90°, 40.69°,and 43.16°,respectively, which indicates the presence of a tetragonal crystal structure [64, 65]. However, the 45 nm film did not show the (202) peak and both the 138 and 45 nm films did not show the (411) peak. Furthermore, the peak intensities of these films were relatively lower than those of the 336 nm film, indicating lower perovskite crystallinity [59] in the thinner films with lower precursor concentrations (25 and 18 wt. %, respectively). Next, we fabricated ST PSCs in conjunction with the fabricated perovskite films, with different thicknesses (40-336 nm), to investigate their performance as photo-active absorber layers. The J-V curves of the devices with various thicknesses are shown in Fig. 4b. The inset of Fig. 4b shows examples of two ST PSCs with different MAPbI3 thicknesses (138 nm for the top and 45 nm for the bottom photograph, respectively). The J-V curves for PSCs, with all of the 8 different thicknesses, are plotted in Fig. S2. Table 1 lists the average as well as the best performance of the fabricated ST PSCs for each case. The average values are 6

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presented with the corresponding standard deviation values of at least six samples from a random batch. In addition, Table S2 contains the best performance of the fabricated PSCs at both FB-SC (forward bias to short circuit) and SC-FB (short circuit to forward bias) directions at a scan rate of 0.2 V/s. As expected, the generated photocurrent of the PSCs is heavily dependent on the absorbing MAPbI3 layer thickness. The short circuit current density (Jsc) of the PSCs reduces with the decrease in the perovskite layer thickness, owing to lower photocurrent generation with less light absorption. Jsc is the highest (14.97 ± 0.39 mA/cm2) for the thickest perovskite layer (336 ± 12 nm; 40 wt. % MAPbI3). This Jsc (14.97 ± 0.39 mA/cm2) is lower than the typical Jsc of their opaque counterparts (22.1 ± 1.01 mA/cm2) [46] due to the transmitted light lost through the transparent back electrode (which would have been reflected from a reflective electrode and have a second chance to be absorbed). The Jsc reduces slightly for the ST devices with a 247 ± 18 nm MAPbI3 layer (40 wt. % MAPbI3). There is a sharp decline in the Jsc when the MAPbI3 thickness is reduced below 200 nm by using lower concentration of the MAPbI3 precursor. The Jsc of the devices with 40 ± 12 nm MAPbI3 is almost half that of the devices with 160 ± 16 nm MAPbI3. The effect of the variation of perovskite layer thickness was also optically simulated in MATLAB using transfer matrix modelling (TMM). The TMM was used to enumerate the optical electric field distribution, |En|2, and the exciton generation rate inside the devices (Fig. S3 and S4, respectively). In Fig. S3, increasing the normalized modulus squared value (|En|2) increases the number of photons available for absorption. For 336 nm perovskite thickness, E-field intensity is the strongest with two interference peaks inside the perovskite layer (Fig. S3a). Hence, this device shows the highest exciton generation rate in Fig. S4a. Experimentally, the 336 nm devices yielded the highest Jsc (14.97 ± 0.39 mA/cm2), which is coherent with the simulation result. Only one peak is observed in both 160 nm and 138 nm devices. The E-field is the weakest in the 45 nm devices, with a deep notch in the exciton generation profile; these devices exhibited significantly reduced Jsc (6.97±0.47 mA/cm2).

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Despite their higher Jsc, the optimized thickness of the perovskite layer is not in the 247-336 nm range (40 wt. % MAPbI3). The best performing devices come from the 93- 160 nm range (25 wt. % MAPbI3) with 8.25 ± 0.56 % average PCE for 138 ± 9 nm and >9% best PCE for 93 ± 12 nm MAPbI3 thickness. The reason for the lower efficiency of the thicker layer ST PSCs (247-336 nm) is the high device series resistance (Rs). The Rs of these devices is almost an order of magnitude higher than that of the optimized devices with 93- 160 nm MAPbI3 thickness (Table 1). Such high Rs value can be attributed to the relatively rougher surface morphology of the thicker films (as can be seen from the AFM images in Fig. 2) and the resulting poor contact with the thin top electrode. For the same reason, S-shaped curve near Voc is observed in the J-V measurements of the thicker devices (Fig. S2). As the MAPbI3 solution is deposited and grown on top of PC71BM in each case, it is expected that the PC71BM/ MAPbI3 interface does not undergo a change when the perovskite thickness is increased, rather this increase in Rs is related to the increased contact resistance at the MAPbI3/HTL/(D/M/D) interface due to a rough perovskite surface morphology [66, 67]. The combined thickness of the D/M/D back electrode, used in the ST PSCs, is only 51 nm (MoO3/Ag/MoO3: 6/10/35 nm), in contrast to the thickness of the back electrode of their opaque counterparts at ~100 nm. We hypothesize that the thin top D/M/D electrode, on 7

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MAPbI3 film thickness (nm)

Jsc 2 (mA/cm )

Voc (mV)

FF (%)

Avg. PCE (%)

336±12

14.97±0.39

0.904±0.02

48.96±0.80

6.62±0.09

247±18

14.91±0.90

0.873±0.058

46.35±2.37

6.01±0.36

160±16

14.11±1.82

0.978±0.025

56.37±3.27

138±9

13.47±0.88

0.982±0.033

64.39±1.62

93±12

12.28±1.52

0.983±0.009

50±9

8.81±0.48

45±7 40±12

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Table 1 Photovoltaic parameters of the semitransparent perovskite solar cells with different active layer 2 thicknesses under AM1.5G light irradiance at 100 mW/cm . All the values (average ± standard deviation) were evaluated for at least six cells.

Series resistance, Rs 2 (Ω.cm )

Shunt resistance, Rsh 2 (Ω.cm )

AVT (%)

6.76

67.74±8.09

1408±50.28

10.3

6.31

103.08±11.67

1261.2±338.60

10.6

7.80±1.25

9.00

13.72±2.97

1290.6±134.49

10.7

8.25±0.56

9.13

14.07±4.34

2289.6±895.51

12.4

63.09±1.54

7.59±0.95

9.23

12.04±1.19

2252±792.02

14.1

0.933±0.014

50.49±2.66

3.54±0.62

4.13

29.96±6.51

475.5±177.93

17.5

6.97±0.47

0.994±0.006

53.48±1.56

3.65±0.35

3.91

26.58±2.50

584.8±166.28

19.0

7.24±0.06

0.921±0.016

49.04±1.73

3.45±0.25

3.50

28.96±2.30

532.8±221.78

20.5

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Max. PCE (%)

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The FF decreases again from the optimized value when the perovskite layer thickness is 50 nm or below (18 wt. % MAPbI3). As a result, the device efficiency rapidly decreases to 3.43.6%. Unlike the thickest devices (40 wt. % MAPbI3), the thinnest (18 wt. % MAPbI3) devices exhibit a drastic reduction in the device shunt resistance (Rsh) (Table 1). This observation is consistent with earlier literature [68]. Pinhole short formation in the thinnest perovskite films can lead to this low device shunt resistance,[68] which is approximately a quarter that of the optimized ST PSCs (Table 1). A small RMS surface roughness of even 6.6 nm can be detrimental for 40-50 nm thick perovskite films. However, the largest contributor to the reduced efficiency of the thinnest PSCs remains the low Jsc (~7-9 mA/cm2), due to the reduction in photocurrent generation in the thin absorbing layers. Another fundamental aspect of ST solar cells is the device transparency. The transmittance spectra of the ST PSCs with MAPbI3 thicknesses from 45-160 nm are plotted in Fig. 4c. The transmittance spectra for the D/M/D back electrode and devices with all the 08 different perovskite thicknesses are shown in Fig. S5 and S6. The full device transmittance reduces gradually across the visible spectrum with the increase in the perovskite thickness. To evaluate the optical properties of the ST devices, the AVT (%), which is a well-known index for ST device transparency, was calculated from the transmittance spectra, using the following expression [58, 69]:

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Here, λ is the wavelength and T(λ) (%) the transmittance at λ. The AVT values are listed in Table 1. The AVT values indicate that the highest light transmission is through solar cells with the thinnest perovskite layer (maximum AVT=20.5% at thickness of 40 nm), while thicker MAPbI3 films yield a reduction in device transparency (for example, AVT= 14.1% for 93 nm thick films, AVT= 12.4% for 138 nm thick films and AVT=10.6% for 247 nm thick films). The lowest AVT (~10%) belongs to the thickest film (336 nm). The variation in AVT within a broad range from 10% to 21% demonstrate a direct and flexible management of device transparency by solely controlling the perovskite layer thickness. The relationship of the device PCE and AVT with the perovskite layer thickness is summarized in Fig. 4d.

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To confirm the device Jsc, the EQEs of the fabricated solar cells were measured in the wavelength range 300-800 nm as shown in Fig. 5a. The thickest layer (336 nm) shows the highest EQE. The EQE gradually reduces, especially in the long wavelength region (550-800 nm), when the thickness decreases from 336 nm to 138 nm and 45 nm. The integrated Jsc from the EQE are 14.2, 12.8 and 6.6 mA/cm2 for 336 nm, 138 nm and 45 nm, respectively, which are close to the Jsc values measured from J-V measurements (14.9, 13.4 and 7.2 mA/cm2, respectively). The ~800 nm cut-off wavelength correlates with the 1.55 eV bandgap of the MAPbI3 perovskite active layer [8]. Figure 5b displays the absorption spectra of the perovskite layer as a percentage of the total absorption of the ST PSCs. The absorption of the thickest layer is the highest which is consistent with the EQE (Fig. 5a) and transmittance spectra (Fig. 4c), resulting in the highest Jsc. Although the absorption profiles of 138 nm and 45 nm perovskite layers are almost similar in the short wavelength region (< 500 nm), the EQE for 45 nm devices is consistently lower compared to that of 138 nm devices throughout the visible spectrum. Since the absorption profiles of the thinner perovskite layers are almost similar in the short wavelength region, the reduction in EQE of 45 nm devices can be attributed to the inferior charge transport properties of the devices. As can be seen from Table 1, the FF of the 138 nm and 45 nm devices are 64.4% and 53.5%, respectively, due to reduced shunt resistance and possible pinhole short formation in the latter. Apart from device efficiency and transparency, hysteresis in the J–V curve is a familiar and major issue for PSCs, that can produce a different current response depending on the history of applied bias [68]. So, we also probed the hysteresis behaviour of the fabricated ST PSCs, although the devices are based on a hysteresis- free opaque device architecture [46]. The J-V curves of the devices with 336 nm (40 wt. % MAPbI3), 138 nm (25 wt. % MAPbI3) and 45 nm (18 wt. % MAPbI3) perovskite films in both FB-SC and SC-FB directions are presented in Fig. 6a, b, c, respectively. From Fig. 6, the devices with thick 336 nm perovskite showed negligible hysteresis similar to their opaque counterparts, while both of the thinner devices (138 nm and 45 nm) showed some degree of hysteresis at the slower scan rate of 0.2 V/s. To determine the hysteresis phenomenon quantitatively, the hysteresis index (HI) of the devices were calculated using the following equation [70, 71]:

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and 0.307 for devices with 336 nm (40 wt. % MAPbI3), 138 nm (25 wt. % MAPbI3) and 45 nm (18 wt. % MAPbI3) perovskite layer, respectively. The negligible hysteresis behaviour of the thick perovskite devices can be attributed to the higher crystallinity and larger grains [61, 72] of the thicker MAPbI3 layers on PC71BM ETL (as previously observed from AFM, SEM and XRD in Fig. 2, 3 and 4a, respectively), containing few defect sites.

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We have also evaluated the human eye perception of the fabricated ST devices to investigate their feasibility in semitransparent applications, such as aesthetic BIPV and automobile applications. Since the human eye’s recognition is heavily dependent on the spectral response of the objects, the human perception of colour and transparency needs to be considered as well [73]. In order to evaluate the perception of the ST PSCs across the visible spectrum, the tristimulus value (, ), * and the colour coordinates +, , were calculated from the device transmittance spectra, using the following formula:

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The corresponding colour coordinates are listed in Table 2. The coordinates on the Commission International de L’Eclairage (CIE) 1931 chromaticity diagram are illustrated in Fig. 6d. The devices from three different MAPbI3 precursor concentrations are located in three different regions in the CIE colour space. The thickest devices (40 wt. % MAPbI3) are located at the farthest corner from the so-called ‘white point’ at (0.33, 0.33), representing a deep reddish-brown colour. The AVT of these devices is the lowest at ~10%. For the optimized devices (25 wt. % MAPbI3) with the highest PCE (>9%) and moderate AVT (1110

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14%), the coordinates are in the region appearing bright brownish-orange to the human eye. The highest AVTs (17.5-20.5%) were achieved for thinner devices (from 18 wt. % MAPbI3). These devices are in the lighter brownish-orange region in the CIE colour space. For comparison of the transparency and human eye perception of the devices with different thicknesses, actual photographs of a building through these devices are presented in Fig. S7. As can be seen from the chromaticity diagram in Fig. 6d and the photographs, three different shades of colour, with varied transparencies, can be obtained from the fabricated ST PSCs. These tinted ST PSCs can be useful for BIPV and automobile applications. Table 2 Device AVT (%) and colour coordinates (x, y) on the CIE 1931 colour space as a function of the perovskite layer thickness.

Colour coordinates (x, y) (0.57,0.42) (0.57, 0.42) (0.48, 0.44) (0.46, 0.45) (0.47, 0.45) (0.42, 0.42) (0.44, 0.43) (0.43, 0.42)

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Avg. PCE (%)

Max. PCE (%)

AVT (%)

336±12 247±18 160±16 138±9 93±12 50±9 45±7 40±12

6.62±0.09 6.01±0.36 7.80±1.25 8.25±0.56 7.59±0.95 3.54±0.62 3.65±0.35 3.45±0.25

6.76 6.31 9.00 9.13 9.23 4.13 3.91 3.50

10.3 10.6 10.7 12.4 14.1 17.5 19.0 20.5

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MAPbI3 film thickness (nm)

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Finally, a month-long systematic degradation study was carried out for the ST PSCs to assess their dark stability. The un-encapsulated ST devices were stored in a N2 filled glovebox for 30 days and the photovoltaic performance was measured at regular intervals at room temperature in a humidity controlled environment (relative humidity: 35–40%). Figs. 7a-d illustrate the normalized PCE, Voc, Jsc and FF values of the PSCs, respectively, from the day of fabrication up to 30 days. Daily PCE, Voc, Jsc and FF values of the PSCs, with all the 8 different MAPbI3 thicknesses, have been tabulated in Tables S3-S10.

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Remarkably, the thickest devices (336 and 247 nm; 40 wt. % MAPbI3) retain about 100% of their initial PCE even after 30 days, whereas PCE drops down to ~70% and ~63% of the initial PCE, for 93-160 nm (25 wt. % MAPbI3) and 40-50 nm (18 wt. % MAPbI3) perovskite thicknesses, respectively, during the same period of time. Over 1.40 and 1.58 times higher stability for the thickest devices (336 and 247 nm; 40 wt. % MAPbI3) was observed compared to the optimized (93-160 nm; 25 wt. % MAPbI3 and the thinnest devices (40-50 nm; 18 wt. % MAPbI3). The excellent stability of the thickest devices can be attributed to the superior crystallinity and to the more compact and larger grains, compared to the thinner devices. On the other hand, the moderate reduction in dark stability of the optimized devices can be correlated to a simultaneous reduction in all the photovoltaic parameters (~10% reduction in both Jsc and Voc, and ~14% in FF, respectively) and is comparable to the typical dark stability of opaque MAPbI3 perovskite cells reported in previous literature [7476]. The decrease in the optimized device stability compared to the thick devices can be 11

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correlated with the decrease in crystallinity in the perovskite films of 93-160 nm (as can be seen from the XRD patters in Fig. 4a), which can allow leakage current [77] over an extended time period since fabrication. For the thinnest devices (40-50 nm), there is a significant reduction in the device efficiency (~40 % reduction in the final PCE) over the period of 1 month and the main contributing factor of the efficiency reduction is the rapid decline in the device Voc (~25 % reduction in the final Voc). Two possible degradation mechanisms of these ST PSCs are: - (1) relatively lower crystallinity of the perovskite films compared to the thicker devices (from both the 25 and the 18 wt. % MAPbI3 precursors, Fig. 4a) and (2) chemical reaction between the Ag electrode and the perovskite over long storage times caused by direct contact between the ultrathin perovskite layer (40-50 nm) and the metal electrode. The device stability of the thinner ST PSCs can be improved further by increasing the crystallinity of the thinner perovskite layers; however that experiment is beyond the scope of the present study. On the other hand, the thicker ST devices show impressive efficiency (6-9%) and month-long stability and are a notable step toward BIPV and tandem applications.

4. Conclusions

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In summary, we have successfully demonstrated colourful and efficient semitransparent perovskite solar cells, employing a simple low-temperature and solution-processed methodology. The efficiency and transparency of the ST PSCs were controlled by tuning the perovskite layer thickness. A multilayered D/M/D was used as the transparent back electrode that does not require any sophisticated nanostructures or dielectric mirrors. The optimum perovskite layer thickness was found to be ~93 nm, deposited from a 25 wt. % MAPbI3 precursor solution. An average PCE of 8.25% and an AVT of 12.4% in the wavelength range of 300-800 nm were achieved at the optimum thickness, accompanied by moderate device stability. The best performing device produced >9% efficiency, which is one of the highest reported efficiencies for ST PSCs with a D/M/D electrode. The maximum AVT of ~21% was obtained for a 40 nm perovskite layer at an average efficiency of 3.45%. The impact of the perovskite absorbing layer thickness on the perovskite morphology, device efficiency, hysteresis and stability has been discussed. In addition, the human eye perception of the fabricated ST PSCs was evaluated and analyzed by their position in the CIE 1931 colour space. The colour coordinates of the devices varied significantly with the perovskite layer thicknesses and the precursor solution concentrations. Three different shades of brownish colour were achieved from the ST PSCs which can be a great addition to aesthetic BIPV applications. Our results suggest that the n-i-p configuration of the ST MAPbI3 perovskite cells, with a low-temperature processed PC71BM ETL and a D/M/D back electrode, is a promising candidate for constructing efficient and stable semitransparent perovskite solar cells for future BIPV and tandem applications.

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Acknowledgements

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The authors would like to thank the Australian Centre for Advanced Photovoltaics, UNSW staff and technicians for their support. We acknowledge Future Solar Technologies for providing funding. We also thank the staff of the School of Photovoltaic and Renewable Energy Engineering (SPREE), the Electron Microscope Unit (EMU), and the Solid State and Elemental Analysis Unit (SSEAU) of the University of New South Wales (UNSW).

Supplementary Information

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MAPbI3 perovskite layer thickness (nm) as a function of MAPbI3 wt. % and spin speed (rpm); RMS and average surface roughness (in nm) of different perovskite films; J-V curves and photovoltaic parameters of ST PSCs; transmittance spectra of transparent back electrode and ST PSCs; photographs and daily Jsc, Voc, FF and PCE values of ST PSCs.

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Figure 1 (a) Graphical representation of contemporary semitransparent perovskite solar cell efficiencies with various transparent electrodes and the best efficiency reported in this work; (b) schematic representation of fabricated MAPbI3 based semitransparent perovskite solar cells with MoO3/Ag/MoO3 back electrode; (c) the energy levels of the device structure related materials in eV.

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Figure 2 Two and three dimensional atomic force microscopy (AFM) images of the MAPbI3 perovskite film at different thicknesses and MAPbI3 wt. %: (a, b) 336 nm (40 wt. %), (c, d) 138 nm (25 wt. %), and (e, f) 45 nm (18 wt. %). 19

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Figure 3 Top view scanning electron microscopy (SEM) images of MAPbI3 perovskite film at different thicknesses and MAPbI3 wt%: (a) 336 nm (40 wt. %), (b) 138 nm (25 wt. %), and (c) 45 nm (18 wt. %).

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Figure 4 (a) XRD patterns of MAPbI3 perovskite film of different thicknesses on PC71BM layer, showing the major diffraction peaks and the corresponding crystal orientation; (b) J-V curves of semitransparent perovskite solar cells with different thicknesses of MAPbI3 perovskite layers at room temperature under AM1.5G illumination at 100 mW/cm2 (insetrepresentative photographs of the semitransparent devices for the thick and thin groups of perovskite thickness); (c) UV–Vis transmittance of the corresponding films; (d) device efficiency (%) and AVT (%) with error bars as a function of perovskite layer thicknesses (nm).

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Figure 5 (a) External quantum efficiency (EQE) curves for semitransparent perovskite solar cells with different thicknesses of MAPbI3 perovskite layers, and (b) absorption spectra of the corresponding perovskite layers inside the device.

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Figure 6 Photocurrent hysteresis phenomena in fabricated semitransparent perovskite solar cells with different MAPbI3 wt. % at FB-SC (forward bias to short-circuit) and SC-FB (shortcircuit to forward bias) scan directions and 0.2 V/s scan rate: (a) 40 wt. %, (b) 25 wt. %, and (c) 18 wt. %; (d) representation of colour coordinates (x, y) of the semitransparent MAPbI3 solar cells with different thicknesses of perovskite layer under standard D65 illumination light source on CIE 1931 chromaticity diagram.

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Figure 7 Normalized device performances of the ST perovskite devices with 8 different thicknesses as a function of sample storage time, stored in a N2 filled glove box: (a) normalized PCE, (b) normalized Voc, (c) normalized Jsc and (d) normalized FF. All the J-V parameters were obtained with SC-FB (short-circuit to forward bias) scan direction with a scan rate of 0.2 V/s.

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Low-temperature processed efficient and colourful semitransparent perovskite solar cells using fullerene/perovskite thin films and dielectric/metal/dielectric back electrode for BIPV and tandem applications

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Low-temperature semitransparent perovskite solar cells with fullerene ETL Exhibiting >9% efficiency at 12.4% average visible transparency Perovskite thickness-dependent hysteresis, colour perception & degradation studied Suitable for roll-to-roll BIPV and tandem applications

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