Delayed Annealing Treatment for High-Quality CuSCN: Exploring Its

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Delayed Annealing Treatment for High-Quality CuSCN: Exploring its Impact on Bifacial Semitransparent N-I-P Planar Perovskite Solar Cells Lin Fan, Yuelong Li, Xin Yao, Yi Ding, Shan zhen Zhao, Biao Shi, Changchun Wei, Dekun Zhang, Baozhang Li, Guangcai Wang, Ying Zhao, and Xiaodan Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00001 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Energy Materials

Delayed Annealing Treatment for High-Quality CuSCN: Exploring its Impact on Bifacial Semitransparent N-I-P Planar Perovskite Solar Cells Lin Fan,a,b,c,d Yuelong Li,a,c,d Xin Yao,a,c,d Yi Ding,a,c,d Shanzhen Zhao,a,c,d Biao Shi,a,c,d Changchun Wei,a,c,d Dekun Zhang,a,c,d Baozhang Li,a,c,d Guangcai Wang,a Ying Zhao,a,c,d and Xiaodan Zhanga,c,d* a

Institute of Photoelectronic Thin Film Devices and Technology of Nankai University, Tianjin 300071, P. R. China; Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Tianjin 300071, P. R. China b Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, PR China; National Demonstration Center for Experimental Physics Education, Jilin Normal University, Siping 136000, PR China c Key Laboratory of Optical Information Science and Technology of Ministry of Education, Tianjin 300071, P. R. China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China *Corresponding author E-mail address: [email protected] (X. Zhang)

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ABSTRACT: Inorganic p-type copper (I) thiocyanate (CuSCN) hole-transporting material (HTM) belongs to a promising class of compounds integral for the future commercialization of perovskite solar cells (PSCs). However, deposition of high-quality CuSCN films is a challenge for fabricating n-i-p planar PSCs. Here we demonstrate a pinhole-free and ultra-smooth CuSCN films with high crystallinities and uniform coverage via delayed annealing treatment at 100°C, which can effectively optimize the interfacial contact of between the perovskite absorber and the electrode for efficient charge transport. A satisfactory efficiency of 13.31% is achieved from CuSCN-based n-i-p planar PSC. In addition, due to the superior transparency of p-type CuSCN HTM it is also possible to prepare bifacial semitransparent n-i-p planar PSC, which eventually permits a maximum efficiency of 12.47% and 8.74% for the front and rear illumination, respectively. The low-temperature process developed in this work is also beneficial for those applications such as flexible and tandem solar cells on heat-sensitive substrates.

KEYWORDS: CuSCN hole-transporting material, delayed annealing treatment, interfacial contact, n-i-p planar perovskite solar cell, bifacial semitransparent n-i-p planar device, low-temperature process


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INTRODUCTION Significant developments of organic-inorganic hybrid metal halide PSCs have been reported in recent years due to their potential of high-performance capabilities and wide range applications. This can be attributed to the advantageous properties of the organometallic halide perovskite materials ABX3 (where A=CH3NH3+, NH2CH=NH2+, CH3CH2NH3+; B=Pb2+, Sn2+; X=Cl-, Br- or I-), such as an almost ideal direct bandgap, high absorption coefficient, small binding energy, ambipolar charge transport, long charge-carrier diffusion lengths (≈1 µm) and lifetimes (≈100 ns).1-3 Recently, the highest certified power conversion efficiency (PCE) for PSCs is 22.7%, which is rapidly approaching the performance of crystalline silicon, cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) solar cells.4-7 Planar structure PSCs have drawn much attention owing to their low-temperature fabrication process which is simpler comparing to the mesoscopicones.8,9 The process also allows for the development of flexible devices10 and perovskite-based monolithic tandem devices.11-13 An effective approach to further improve the efficiency of planar PSCs is to produce bifacial semitransparent planar devices that can convert solar energy into electricity from both the front and rear sides of the device.14 This bifacial semitransparent structure holds great promise for applications in perovskite-based tandem solar cells and building-integrated photovoltaics.15,16 Furthermore, the other types of bifacial solar cells have also been reported and achieved some results, which further reveals the reliability and feasibility of bifacial structure.17 Common

planar

PSCs

generally use

costly

organic

2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-(spirobifluorene)

HTMs

such

as

(spiro-OMeTAD),18,19

poly(3,4-ethylenedioxythio-phene):polystyrene sulfonate (PEDOT:PSS),20 poly-3-hexylthiophene (P3HT)21 and poly(triarylamine) (PTAA),22 in the hope of achieving high photovoltaic performance. However, their relatively high cost may restrict the practical applications of PSCs, which would become a potential barrier for future commercialization. Among these HTMs, the spiro-OMeTAD has demonstrated the best planar device performance,7,23,24 but it has an obvious absorption between 300 nm and 430 nm in the UV-vis-NIR range.25 This feature results in a loss of efficiency in bifacial semitransparent or tandem devices due to an overlap with the absorption of the perovskite absorber in the rear incidence situation. Therefore, it is necessary to exploring its



alternatives. Solution processed inorganic copper-based p-type semiconductors, such as hexagonal CuSCN,26 CuOx,27 CuI28 and CuS,29 are highly promising substitutes for organic HTMs because of their wide bandgap with high conductivity, and lower cost. Thus, the integration of inorganic selective contacts may have impact on the production costs of the PSCs. The intrinsic p-type semiconductor CuSCN, so-called β-CuSCN phase, is the most potential candidate due to its excellent hole mobility (≈0.1 cm2 V−1 s−1)30 and superior transparency in the entire visible-light range with the favorable valence band (VB) and conduction band (CB) energy levels of EVB=-5.3 eV and ECB =-1.8 eV.31 There are reports of CuSCN-based inorganic n-i-p planar, p-i-n planar and mesostructured PSCs with maximum efficiencies of 9.6%,32 18.1%33 and 20.2%,34 respectively (excluding simulation). In addition, the temperature and thickness of CuSCN HTMs have been carefully optimized. Jen et al. reported that the thermal annealing beyond 100℃ would deteriorate the device performance, which was caused by CuSCN film morphology coarsening and CuSCN decomposition to Cu2O.35 Lee et al. pointed out the thickness of CuSCN HTM plays a key role on the crystallization quality of CuSCN and device efficiency.36 However, it has been observed that reported PCE of the CuSCN-based inorganic n-i-p planar PSC is the lowest than that of three devices, especially the open circuit voltage (VOC) and fill factor (FF), which may be attributed to (1) solution processed CuSCN film could more easy to damage the relatively thin perovskite absorber (compared with mesostructure)32 and (2) the hasty crystallization processes of the conventional annealing treated CuSCN film would deteriorate film quality.32,37-39 Therefore, the search for an appropriate processing technology to prepare high-quality CuSCN films and hence high-performance CuSCN-based inorganic n-i-p planar devices is of great importance. In this work, we design and implement a facile method of depositing CuSCN films via delayed-thermal-annealing treatment. After spin-coating a CuSCN solution onto the perovskite FA0.5MA0.5PbI3-xClx (where FA=formamidinium; MA=methylammonium) absorber, we put the CuSCN ink-coated substrate in a nitrogen environment at room temperature (R.T.) for ~10 minutes until the ink has completely dried. This is followed with an annealing at 100°C for 5 minutes. We successfully obtain smooth, pinhole-free uniform films of highly crystalline CuSCN films. The corresponding n-i-p planar device efficiency is enhanced to 13.31% with a VOC of 1.03 V and a FF of 60.4%. In addition, we have also prepared a bifacial semitransparent n-i-p planar


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PSC using CuSCN, which achieved a maximum PCE of 12.47% for the front incidence and 8.74% for the rear incidence, respectively. All processes were conducted at a temperature of 120°C or less, which is desirable for a wide range of applications, such as flexible and tandem solar cells on thermal sensitive substrates.

EXPERIMENTAL SECTION Materials Fluorine-doped tin oxide (FTO) coated glass was purchased from Wuhan lattice solar energy technology co., LTD. Lead iodide (Pbl2), lead chloride (PbCl2) (99.9%), methylammonium iodide (MAI, CH3NH3I) and formamidinium iodide (FAI, HC(NH2)2I) were purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO) and isopropanol were purchased from Tianjin Guangfu Fine Chemical Research Institute. The inorganic Copper(I) thiocyanate (CuSCN) was purchased from Tianjin sheehan, Mr Buder technology co., LTD and 1,1''-thiobisethane were purchased from Beijing inoke Technology Co. Ltd. All reagents were of analytical grade and used without further purification. The purity of the gold for thermal evaporation was 99.99%. N-I-P Planar Perovskite Solar Cell and Bifacial Semitransparent Device Fabrication The

n-i-p

planar

PSC

structure

is

comprised

of

the

FTO-glass/compact

TiO2/perovskite/CuSCN/Au and the bifacial semitransparent n-i-p planar device using ultra-thin Au as the transparent rear incidence. The FTO-glass was first cleaned in an ultrasonic bath in a solution of mild detergent and deionized water for 1 hour and then left to dry under a stream of nitrogen. A ~12-nm-thick compact TiO2 electron-transporting material (ETM) was deposited onto the cleaned FTO-glass by an Atomic Layer Deposition (ALD) method, its thickness can be controlled by adjusting the reaction cycles. The perovskite films were prepared by using a simple two-step sequential spin-coating method. In the first step, the precursor solution containing 1.6 M PbI2 and PbCl2 (molar ratio is 1:1) is dissolved in a 1 mL of DMSO solvent (held at 70°C) and then spin-cast on the compact TiO2 ETMs at a speed of 3000 r.p.m. for 1 minute. This was then followed by a drying process at room temperature (R.T.) for 1 hour. For the second step, the transformation from the precursor films to the perovskite films was accomplished by dripping a



mixture of FAI and MAI solutions dissolved in isopropanol (12 mg mL-1 each). The solution loading time was fixed before spin coating at 3000 r.p.m. for 1 minute, followed by annealing at 120°C for 8 minutes. The inorganic HTM was prepared by dissolving CuSCN (50 mg) in 1 mL of 1,1''-thiobisethane which was then spin-casted onto the top of the perovskite absorbers at different rotational speeds (CuSCN films thickness can be controlled by adjusting the rotational speed). This was then followed by a drying process at R.T., and the samples were then annealed at 100°C for 5 minutes in order to form the CuSCN crystals. Finally, a Au electrode (99.99%, ~80 nm) or ultra-thin Au (99.99%, ~10 nm) electrode was thermally evaporated on top by use of a shadow mask as the rear contact.40 Apart from the electrode evaporation, all fabrication processes were completed in a nitrogen filled glovebox. The finished devices were not encapsulated but stored under dry conditions and characterized in ambient conditions at a temperature of 25°C. Material Characterizations and Solar Cell Performance Measurements The surface morphology was characterized by using a scanning electron microscope (SEM, Jeol JSM-6700F) and an atomic force microscopy (AFM, NanoNavi-SPA400). The crystal structure of samples was examined by X-ray diffraction (Rigaku ATX-XRD) using Cu Kα radiation as the radiation source (λ=1.5405 Å) across a 2θ range of 3° to 80°. In this paper, grazing incidence X-ray diffraction (GIXRD) data was measured with the same X-ray incident angles (grazing incidence angle 0.3°). The time-resolved photoluminescence (TRPL) spectroscopy was measured with a PL spectrometer (Edinburgh Instruments, FLS 920), and a pulsed laser with a wavelength and frequency of 635 nm and 1 MHz was employed as the excitation source. A long pass filter of 655 nm was used to filter out the excitation light in the transient PL measurements. The transmission spectra at R.T. were recorded with a Varian Cary 5000 UV-visible-NIR spectrophotometer within a wavelength range of 300 to 1100 nm. The electrical properties of samples were characterized by conductivity using a Keithley 617 programmable electrometer. Electrochemical impedance spectra (EIS) of the PSCs were performed on a Princeton potentiostat electrochemical workstation (PARSTAT 4000) in dark in the frequency ranging from 1Hz to 1MHz without a bias. Photocurrent density-voltage (J-V) curves of PSCs were measured at 25°C under the AM 1.5G (100 mW cm-2) illumination in a nitrogen filled glovebox. Unless otherwise specified, bias


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scan from 1.2 V to -0.2 V firstly (reverse scan) and return back (forward scan) with a step of 70 mV and scan speed of 30 mV s-1. Reverse curve is mainly adopted to evaluate the device performance. The spectral response was taken by an external quantum efficiency (EQE) measurement system (QEX10, PV Measurement) under normal ambient atmosphere, which is equipped with a monochromator, a lock-in ampliﬁer, a Xe lamp, and a current-voltage amplifier.

RESULTS AND DISCUSSION It is important to study the morphology and crystal structure of the CuSCN to improve its performance in PSC and therefore the preparation technology and growth parameters of CuSCN films have to be carefully tailored. Here we reveal two different methods to achieve CuSCN HTMs, namely by a direct annealing treatment and delayed annealing treatment. The detailed fabrication processes are schematically depicted in Figure 1. A ~12-nm-thick compact TiO2 ETM was deposited onto a clean FTO-glass by an ALD method. The perovskite (FA0.5MA0.5PbI3-xClx) absorber was prepared by a two-step sequential spin-coating method.41 Moreover, the corresponding EDS elemental mapping images are also shown in Figure S1. The inorganic HTM was prepared by dissolving CuSCN (50 mg) in 1 mL of 1,1''-thiobisethane which was then spin-casted onto the top of the perovskite absorber. For the direct annealing treatment, the CuSCN ink-coated substrate was directly annealed without any drying process, resulting in CuSCN to form a porous structure. This can lead to serious interfacial contact problems after Au electrode (rear contact) deposition, which can deteriorate influence device performance. On the other hand, for the delayed annealing treatment, the CuSCN ink-coated substrate was left in a nitrogen environment at R.T. for ~10 minutes until the ink had completely dried. It was then annealed at 100°C for 5 minutes. By doing so, a pinhole-free smooth, uniform and dense surface can be obtained with the modified process. Therefore the delayed annealing treatment can effectively improve interfacial contact. It is probably because the retarded evaporation process of solvent in the delayed annealing treatment can ensure sufficient assembling time for CuSCN solution to form a dense and smooth precursor film, at the same time allows for a complete removal of the solvent from the wet CuSCN film. After annealing at 100°C for 5 minutes, a high-quality CuSCN film



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exhibiting high crystallinity and uniform coverage can be easily realized. On the contrary, the direct annealing process causes the CuSCN film to form a porous structure full of voids and unavoidable structure defects and deformations due to the hasty solvent evaporation and fast crystallization processes. This is also confirmed by the corresponding atomic force microscopy (AFM) images, where the roughness average (Ra) and the root mean square (RMS) were reduced effectively in the delayed annealing process as shown in Figure 1.

Ra=6.25 nm; RMS=8.00 nm

Ra=4.79 nm; RMS=6.03 nm

Figure 1. Schematic illustrations of the FTO-glass/compact TiO2/perovskite/CuSCN substrates fabrication processes, and the top-view SEM images and AFM images of the CuSCN films prepared by direct annealing treatment and delayed annealing treatment, respectively. X-ray diffraction (XRD) spectra of the CuSCN films fabricated by the two annealing methods are illustrated in Figure 2a. The typical diffraction peaks related to the (003) and (101) planes of the rhombohedral phase (β-phase) CuSCN are present,34 indicating a high level of phase purity. Compared with the direct annealing treatment, using the modified process, the diffraction peak intensity of the delayed annealing treated CuSCN film increases, and the orientated growth through (101) direction can be clearly observed. Combined with the corresponding scanning electron microscope (SEM) results, it is recognized that the delayed annealing treatment method is effective promising way for preparing high-quality and uniform coverage of the CuSCN film. This in turn promotes the interfacial contact and improves device performance.


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In order to reveal the photo-induced charge transfer and recombination characteristics of the CuSCN films, the steady state photoluminescence (PL) and time resolved photoluminescence (TRPL) measurements were conducted.42 The inset in Figure 2b shows the steady state PL spectra of the FTO-glass/compact TiO2/perovskite/CuSCN substrates prepared by the two different processes. The PL intensity of the delayed annealing treated CuSCN HTM is weaker compared to that of the direct annealing treated sample indicating effective charge transfer at the interface. The corresponding TRPL decays are shown in Figure 2b. Curve can be fitted with an exponential equation as shown below: f (t)=A0+A1exp(-t/τ1)+A2exp(-t/τ2),

(1)

where A0 is a constant for the base-line offset, A1 and A2 are the corresponding decay amplitude, τ1 is the fast decay time responsible for the interface recombination and τ2 is the slow decay time responsible for the bulk recombination.43 Obtained parameters of the TRPL data are listed in Table S1. High crystalline quality and uniform coverage of delayed annealing treated CuSCN HTM leads to a faster PL quenching (both at the interface and in the bulk), indicating an improved carrier separation and injection, while this is crucial to suppress carrier recombination for current improvement and better device efficiency. To gain further insight into the charge transport and recombination at the interface, electrochemical impedance spectra (EIS) measurements were carried out on the corresponding n-i-p planar PSCs.44,45 The EIS was measured at a voltage bias of 0.8 V in the absence of light and resulting Nyquist plot is shown in Figure 2c. The equivalent circuit model (inset of Figure 2c) is composed of a series resistance (RS), transfer resistance (Rtr) at the ETM/perovskite and the perovskite/HTM interfaces (corresponding to the high-frequency arc), and a recombination resistance (Rrec) forming a parallel circuit with the capacitors (corresponding to the low-frequency arc).42 Note that the TiO2/perovskite interface is identical in all cases, the Rtr and Rrec are mainly associated with the perovskite/CuSCN interface, hence its value reflects the charge transport/recombination properties at the perovskite/CuSCN interface. It is clear that the device based on the delayed annealing treated CuSCN HTM exhibits a smaller high-frequency arc and a larger low-frequency arc, indicating better charge transport properties and lower carrier



recombination processes at the perovskite/CuSCN interface, which are conducive to increase device performance.

(a) (101)

Delayed annealing

Direct annealing

15

20

25

30

35

40

45

50

55

200000

10000

5000

150000

0

0

10000

20000

30000

100000

50000 Direct annealing Delayed annealing

0

0

20000

40000

60000

40

80

100

25 20

15

10 Direct annealing

5

0 0.0

80000

60

Time (ns)

(d)

15000

Delayed annealing

Wavelength (nm)

20

Current density (mA/cm2)

250000

Direct annealing

650 675 700 725 750 775 800 825 850

60

2θ θ (degree)

(c)

PL intensity (arb.u.)

(003)

Intensity (arb.u.)

Intensity (arb.u.)

(b)

10

-Z'' (Ω Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Delayed annealing 0.2

0.4

Z' (Ω Ω)

0.6

0.8

1.0

1.2

Voltage (V)

Figure 2. (a) XRD patterns of the CuSCN films prepared by the direct annealing and delayed annealing treatments. (b) PL decay profiles of the corresponding FTO-glass/compact TiO2/perovskite/CuSCN substrates (inset: steady state PL spectra of the above two substrates). (c) Nyquist plot (inset: magnification of the high-frequency region and equivalent circuit) and (d) the reverse and forward J-V curves of the corresponding n-i-p planar perovskite devices.

Table 1. Detailed photovoltaic parameters of the n-i-p planar perovskite devices prepared by the direct annealing treated and delayed annealing treated CuSCN HTMs

Scanning

JSC

Samples

VOC -2

FF

PCE

Rs

RSh 2

2

direction

(mA cm )

(V)

(%)

(%)

(Ω cm )

(Ω cm )

Direct

Reverse

19.42

1.01

52.6

10.32

6.79

2162

annealing

Forward

17.69

1.01

51.4

9.19

7.03

190

Delayed

Reverse

21.39

1.03

60.4

13.31

5.78

4625

annealing

Forward

21.40

1.03

61.2

13.49

5.92

3938


σph

σd

(S/cm)

(S/cm)

3.89×10-5

1.88×10-5

7.92×10-5

4.00×10-5

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Figure 2d presents the corresponding photocurrent density-voltage (J-V) curves measured under different bias scan directions (simulated AM1.5G, 100 mW cm−2) and the detailed photovoltaic parameters derived from the J-V curves are summarized in Table 1. All device parameters such as the short circuit current (JSC), VOC, FF, RS, shunt resistance (RSh) and conductivity (light and dark) are improved substantially with the delayed annealing treated CuSCN HTM. In addition, a negligible hysteresis phenomenon is observed in this device. These improvements are mainly attributed to the smooth and uniform coverage resulting from the delayed

annealing

treatment.

The

modified

treatment

can

effectively improve

the

perovskite/CuSCN and CuSCN/Au interface contacts, suppress carrier interfacial recombination and thus ensure effective carrier extraction and transport. On the other hand, the relatively poor electrical performance of the device prepared by the direct annealing treatment is mainly due to the redundant voids and poor coverage, which leads to a severe carrier interfacial recombination and thus a poor device performance. This is also consistent with the above PL, TRPL and EIS results. Ultimately, the reverse PCE of device is enhanced from 10.42% to 13.31% with a high VOC of 1.03 V and a high FF of 60.4%. As far as we know, this is a satisfactory performance for CuSCN-based inorganic n-i-p planar PSCs by now (excluding simulation), and the reported performance of the CuSCN-based n-i-p perovskite devices is shown in Table S2.25,32,34,36-39,46-51 However, one can see that the FF of the n-i-p planar perovskite devices based on delayed annealing treated CuSCN HTMs are still generally low, which may be attributed to the following two reasons: (1) the 1,1''-thiobisethane solvent is a highly polar solvent, it can corrupt the underlying perovskite layers (as shown in Figure S2a-b and S3) and thus damage the perovskite/CuSCN interface more or less;32,37-39 (2) the lower FF may also originate from potential induced degradation of the CuSCN/Au contact.34 Therefore, further improving the interfacial properties of the perovskite/CuSCN and CuSCN/Au interfaces, such as developing a protective layer (for the perovskite/CuSCN interface) and a spacer layer (for the CuSCN/Au interface), will become the subject in our coming research. Furthermore, the parameter deviations of the n-i-p planar perovskite devices based on two CuSCN films are shown in Figure S4. For each annealing condition, 20 devices were fabricated to achieve an average reverse PCE of ~8.92% and ~12.37% with the direct or delayed annealing treatments, respectively. It reveals again that the quality of the CuSCN film is decisive for device



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performance. Afterwards, the stability measurement on the device based on delayed annealing treated CuSCN film was also carried out. Figure S5a-b show the time dependent of the device parameters. It is necessary to mention here that the device was not encapsulated and just stored in the nitrogen-filled glove box, and no UV cutoff filters were used during the illumination. A stable PCE of approximately 12.48% has been achieved, with the average photocurrent of 20.97 mA cm-2, the average VOC is 0.99 V, and the average FF is 59.43%, indicating a relatively good stability. In addition, p-type CuSCN HTM exhibits superior transparency in the entire UV-vis-NIR range, which is almost equivalent to FTO-glass and FTO-glass/TiO2 substrates (Figure 3a). This high transparency guarantees sufficient light harvest for perovskite layer in the rear incidence and potentially higher photocurrent in the solar cells. As a result, the wide band-gap CuSCN (Eg=3.6 eV) can be used as an ideal transparent HTM to achieve bifacial semitransparent device architecture. The energy diagram and device architecture of the bifacial semitransparent n-i-p planar PSC demonstrated in this work are presented in Figure 3b and c, respectively. This bifacial semitransparent

device

structure

is

comprised

of

a

FTO-glass/compact

TiO2/perovskite/CuSCN/ultra-thin Au (99.99%, ~10 nm) which can harvest light from both sides. The performance of the bifacial semitransparent n-i-p planar PSC can be optimized by carefully adjusting the thickness of the delayed annealing treated CuSCN samples. Figure 3d-f show the cross-sectional SEM images of the FTO-glass/compact TiO2/perovskite/CuSCN substrates with thicknesses of ~300 nm, ~200 nm and ~100 nm thick, respectively. The transmittance spectra of the FTO-glass/CuSCN substrates fabricated with the three different CuSCN thickness show that the visual transparency as well as the measured transmittance tends to improve a little with decreasing thickness (Figure 3g). The corresponding reverse J-V characteristics are measured under AM1.5G illumination (Figure 3h) and the detailed photovoltaic parameters are summarized in Table 2.


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(b)

(a)

(c)

100

Transmittance (%)

80

60

40

FTO CuSCN TiO2

20

0 300

400

500

600

700

800

Wavelength (nm)

(e)

(f)

(g)

(h)

90

~300 nm

~200 nm ~100 nm

200 nm

200 nm

200 nm

80 70 60 50 40 30

300 nm

20

200 nm 100 nm

10 0 300

400

500

600

700

800


(d)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


22.5 Rear incidence

20.0

300 nm

17.5

200 nm 100 nm

15.0

Front incidence

12.5

300 nm

10.0

200 nm

7.5

100 nm

5.0 2.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Wavelength (nm)

Figure 3. (a) Transmittance spectra of a bare FTO-glass substrate, FTO-glass/TiO2 substrate and FTO-glass/CuSCN substrate, respectively. (b) The energy level diagram of each layer in the n-i-p planar perovskite solar cell. (c) The device architecture of the bifacial semitransparent n-i-p planar perovskite device fabricated in this study. (d-f) The cross-sectional SEM images of the FTO-glass/compact TiO2/perovskite/CuSCN substrates fabricated by the delayed annealing treated CuSCN films with thicknesses of ~300 nm, ~200 nm and ~100 nm thick, respectively. (g) Transmittance spectra of the FTO-glass/CuSCN substrates fabricated using the above three different CuSCN thickness. (h) The reverse J-V curves of the corresponding bifacial semitransparent n-i-p planar devices for the front and rear incidence, respectively. Devices were measured under AM1.5G illumination.

Table 2. Characteristics of the bifacial semitransparent n-i-p planar devices based on different thickness of the delayed annealing treated CuSCN HTMs for the front and rear incidences, respectively

Incidence Thickness

JSC (mA cm-2)

VOC (V)

FF (%)

PCE (%)

Rs (Ω cm2)

RSh (Ω cm2)

direction ~300 nm

10.95

0.90

46.7

4.64

18.6

250

~200 nm

20.39

0.89

60.1

10.90

8.1

585

~100 nm

19.49

0.87

47.0

7.97

10.9

184

~300 nm

7.29

0.77

38.6

2.16

32.1

272

~200 nm

8.11

0.87

62.2

4.38

16.3

2063

~100 nm

7.68

0.89

64.6

4.41

14.2

2536

Front incidence

Rear incidence



Note that the bifacial semitransparent device performances for the rear incidence are lower than those for the front incidence due to the following two reason: (1) the transmittance of the FTO-glass/CuSCN/ultra-thin Au substrate is inferior to those of the FTO-glass substrate and the FTO-glass/CuSCN substrate in the entire UV-vis-NIR range examined (as shown in Figure S6), which is mainly attributed to the relatively lower transparency of the ultra-thin Au substrate; (2) the transport characteristics of photogenerated carriers for the rear incidence are worse than that for the front incidence, and the corresponding explanations will be discussed in the following. Therefore, how to improve light transparency of the electrode and enhance transport characteristics of photogenerated carriers for the rear incidence will be the subject in the coming works. The poorest electrical performances are observed both for the front and rear incidences in the device employing ~300 nm-thick delayed annealing treated CuSCN HTM. This is mainly due to (1) a longer transfer distance for holes from the perovskite absorber to the rear electrode and (2) a severer charge recombination around the abundant grain boundaries. The device performance is improved dramatically for the ~200 nm-thick samples, in particular JSC and FF. A pinhole-free smooth and uniform coverage of CuSCN film with an appropriate thickness and transparency can permit efficient perovskite light absorption, prohibit shunt paths through direct contact with the perovskite absorber and rear electrode, and ensure more efficient hole extraction and electron blocking. However, a further decrease in film thickness deteriorates the device performance significantly in the case of the front incidence. This may be because that ~100 nm-thick layer may leading to a local inhomogeneous coverage and subsequently insufficient carrier transport. This is despite the fact that it has a relatively higher transparency which is better light absorption of the perovskite layer for the rear incidence. Ultimately, the device fabricated using ~200 nm-thick delayed annealing treated CuSCN films show the best reverse performance under AM1.5G irradiation: JSC of 20.39 mA cm−2, VOC of 0.89 V, FF of 60.1%, and PCE of 10.90% for the front incidence and JSC of 8.11 mA cm−2, VOC of 0.87 V, FF of 62.2%, and PCE of 4.38% for the rear


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incidence.

(b)

20

Front incidence 15

10

Rear incidence

5

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

(c) 100

25

25

Front incidence 20

80

Front incidence

EQE (%)



(a)

15

10 5

20

Rear incidence

60

15

40

10

20

5

Rear incidence 0

0

100

200

300

400

0 300

400

600

700

800

0

Wavelength (nm)

Time (s)

Voltage (V)

500

Integrated photocurrent (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) The reverse and forward J-V curves, (b) steady-state photocurrent measured at a bias voltage near the maximum power point and (c) corresponding EQE spectra of the champion bifacial semitransparent n-i-p planar device for the front and rear incidence, respectively. The device was measured under standard test conditions (simulated AM1.5G, 100 mW cm−2).

Table 3. Photovoltaic parameters of the champion bifacial semitransparent n-i-p planar device for the front and rear incidence, respectively Scanning

JSC (mA cm-2)

VOC (V)

FF (%)

PCE (%)

Reverse

20.66

1.03

58.6

12.47

7.68

740

Forward

20.59

1.03

60.8

12.89

7.33

780

Reverse

13.95

1.03

60.9

8.74

8.14

675

Forward

13.95

0.96

57.5

7.60

12.78

188

Incidence direction

Rs (Ω cm2)

RSh (Ω cm2)

direction Front incidence

Rear incidence

Figure 4a shows the forward and reverse scanning J-V curves of the champion bifacial semitransparent device for the front and rear incidence, respectively, and the photovoltaic parameters are summarized in Table 3. For the front illumination, it attains a PCE of 12.47% in the reverse scan curve is obtained and 12.89% for the forward scan curve, respectively. The discrepancy of the PCEs derived from the two curves is only 0.4%, indicating a negligible hysteresis. By holding a bias near the maximum power output point (0.68 V), a stabilized photocurrent density of 17.74 mA cm-2 was obtained (Figure 4b, red curve). Device performance shows an obvious distinction depending on light illumination direction. The detailed mechanisms are interpreted as follows. Light with different wavelength has different absorption coefficient for certain materials. Generally speaking light with longer wavelength will penetrate deeper in materials due to its relatively lower absorption coefficient as illustrated in Figure 5. As a result, the distributions of photogenerated carriers will change



according to the light illumination direction.

Figure 5. The energy band schematic representations of CuSCN (green), perovskite (red) and TiO2 (violet). For each material, the highest and the lowest energy levels are the conduction band (CB) and valence band (VB). Process 1 and Process 2 are defined as the front incidence and rear incidence, respectively. In the case of front illumination, light will penetrate from TiO2 to perovskite film as shown in Process 1 (Figure 5). Photogenerated carrier distribution shows the maxima point near the TiO2/perovskite interface and decreases in the light illumination direction. The relatively large conduction band offset ΔECB between the perovskite and TiO2 film (ΔECB = CB of the perovskite - CB of TiO2 = 0.4 eV), is theoretically beneficial in accelerating interfacial electrons (negative circles) transportation from perovskite to TiO2 film. As a result, carrier recombination will be substantially suppressed in the perovskite film due to the deficiency of electrons. Hence, both steady-state efficiency and hysteresis-free behaviour for the front incidence were achieved.52 On the other hand, for the rear incidence as shown in Process 2, PCE exhibits a decrease from 8.74% in the reverse scan curve to 7.60% in the forward scan curve and by holding a bias near the maximum power output point (0.74 V), we obtained photocurrent density which has evidently decreased (Figure 4b, black curve). Photogenerated carrier distribution shows the maxima point near the perovskite/CuSCN interface and decreases in the light illumination direction for the rear incidence. The valance band offset ΔEVB between CuSCN and perovskite


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film is relatively small (ΔEVB = VB of CuSCN - VB of the perovskite = 0.1 eV), thus the interfacial holes (positive circles) can not be extracted promptly and adequately. As a result, interfacial hole accumulation occurs near the perovskite/CuSCN interface under continuous illumination, and space charged limitation happens, which finally gives rise to a serious interface recombination. As a result, an obvious hysteresis phenomenon and a subsequent decay in the photocurrent density were observed for the rear incidence. The corresponding external quantum efficiency (EQE) spectra are shown in Figure 4c, they display a plateau across a wide range from ~400 nm to ~650 nm with a high value around 80% for the front incidence. However, for the rear incidence, the bifacial semitransparent device shows a lower spectrum response at wavelengths between 300 nm to 500 nm and its EQE peaks at 550 nm reach to only 55%, which is not only due to the FTO-glass/CuSCN/ultra-thin Au substrate has a poor transparency in the entire UV-vis-NIR range (as shown in Figure S6), but also due to the relatively poor transport characteristics of photogenerated carriers for the rear incidence (as shown in Figure 4b).

Rear incidence

16

12

Eff. (%)

Eff. (%)

15

9 6

FF (%)

FF (%)

66 55

14 12 10

65 52 39

44 1.32

1.26

1.10

Voc (V)

Voc (V)

Front incidence

8 78

3 77

0.88 0.66

1.05 0.84 0.63 30

Jsc (mA/cm2)

Jsc (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17.5 14.0 10.5 7.0 0

2

4

6

8

10

12

14

16

18

20

25 20 15 10

0

2

4

6

8

10

12

14

16

18

20

Samples

Samples

Figure 6. Statistical distributions of the bifacial semitransparent n-i-p planar device parameters (for the front and rear incidence, respectively) obtained in 20 devices fabricated under optimized conditions.

Reproducibility is critical for the practical use of PSCs. By fabricating 20 devices under the same conditions, average reverse PCE of ~11.50% and ~8.29% for the front and rear incidence



were achieved. This implies good reproducibility for the optimized bifacial semitransparent n-i-p planar PSCs. The corresponding statistical distribution of the photovoltaic properties is presented in Figure 6.

CONCLUSIONS In summary, we have designed and implemented a facile method to deposit high-quality p-type CuSCN films via delayed annealing treatment. During the drying process, the slow evaporation of the solvent provides sufficient assembly time for the CuSCN molecules to form a dense, smooth and uniform precursor film, which is then fully crystallized at 100°C. A satisfactory PCE of 13.31% is achieved for the CuSCN-based n-i-p planar PSC. Moreover, by taking advantage of the superior transparency of the CuSCN films, we demonstrated bifacial semitransparent n-i-p planar PSCs to avoid compromising the absorption of the perovskite absorber for the rear incidence. Finally, we have achieved a maximum PCE of 12.47% and 8.74% for the front and rear incidence of bifacial PSCs, respectively. In addition, the low-temperature fabrication process developed here shows potential applications such as flexible and tandem solar cells where thermal-sensitive substrate are involved.

ACKNOWLEDGMENTS The authors gratefully acknowledge the supports from National Natural Science Foundation of China (61474065, 61674084), Tianjin Research Key Program of Application Foundation and Advanced Technology (15JCZDJC31300), Key Project in the Science & Technology Pillar Program of Jiangsu Province (BE2014147-3), and the 111 Project (B16027).


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Supporting Information Available: Figure S1. (a) EDS image of the perovskite absorber. (b-d) The corresponding EDS elemental mapping images. Table S1. Obtained parameters from the TRPL curves Table S2. Summary of performance of the CuSCN-based n-i-p perovskite solar cells. The reported performance of the CuSCN-based n-i-p perovskite devices is shown for reference as well. Figure S2. (a) XRD patterns of bare perovskite film, CuSCN films and perovskite/CuSCN substrates fabricated by two kinds of annealing method. (b) The corresponding diffraction peaks intensity of the CuSCN coated on perovskite films. Figure S3. XRD spectra of the bare perovskite absorber and the perovskite/1,1''-thiobisethane substrates prepared by direct annealing treatment and delayed annealing treatment, respectively. Figure S4. The parameter deviations of the n-i-p planar perovskite devices based on two CuSCN films. Figure S5. (a-b) The kinetic performances of the delayed annealing treated CuSCN-based device measured under continuous AM1.5G light illumination.

Figure

S6.

Transmittance

spectra

of

FTO-glass,

FTO-glass/CuSCN/ultra-thin Au.


FTO-glass/CuSCN

and


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