RF sputtered CdS films as independent or buffered electron transport layer for efficient planar perovskite solar cell

Solar Energy Materials and Solar Cells 178 (2018) 186–192

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RF sputtered CdS films as independent or buffered electron transport layer for efficient planar perovskite solar cell ⁎

T

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Yixin Guoa, Jinchun Jianga,b, , Shaohua Zuoa,b, , Fuwen Shia,b, Jiahua Taoa, Zhigao Hua, Xiaobo Hua, Gujin Hub, Pingxiong Yanga, Junhao Chua,b,c a

Department of Electronic Engineering, East China Normal University, Shanghai 200241, China Shanghai Center for Photovoltaics, Shanghai 201201, China c Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Perovskite solar cell Low temperature CdS/TiO2 composite layer Sputtering Thin films

Metal sulfide has the potential to take the place of high temperature sintered TiO2 as electron transportation layer for perovskite solar cell (PSC) with improved light stability and suppressed hysteresis. In this work, CdS films were used as independent or buffered electron transport layer for planar perovskite solar cell by a lowtemperature RF sputtering method for the first time. The effects of surface roughness and optical absorption of CdS films on the photovoltaic performance of PSCs were discussed. The PSC with sputtered CdS film shows a higher open-circuit voltage (Voc) and efficiency of 13.17% than high temperature sintered TiO2 ETL (12.71%). Moreover, a RF sputtered CdS buffer layer between TiO2 and perovskite could tune the conduction band edge of TiO2 and perovskite and passivating the surface defects. Time resolved photoluminescence results indicate the RF sputtered CdS film buffer layer could accelerate charge transportation and a higher conversion efficiency over 16% has thus been achieved, with enhanced air stability and minimized hysteresis. These findings offer new research directions for low-temperature sputtered metal sulfide film as a promising electron transport material for stable and high efficient planar perovskite solar cell.

1. Introduction With the increase of world's energy needs, great efforts have been made in searching for cheap and efficient photovoltaic materials. Photovoltaic cell using organic–inorganic hybrid perovskite as the absorber material has achieved a highest record efficiency up to 22.1% [1]. Compared with mesoscopic structure perovskite solar cell, planar perovskite solar cell simplifies the structure of solar cell and fabrication process. The electron transportation layer (ETL) is necessary in planar perovskite solar cell and their quality directly plays an important role in device performance. Crystallized TiO2 is a wide band gap semiconductor with considerable electron mobility and has been considered as ideal ETL material for PSC [2]. However, high temperature annealing (above 450 °C) condition was required for crystallized TiO2 film [3] which increases the cost of time and energy and impedes the perovskite solar cell commercialization on a large scale. Recently, SnO2 [4], In2O3 [5] and CeOx [6] have been used as ETL in PSCs by low temperature solution-method with a high efficiency over 10%. However, additional annealing processes were still needed in those ETLs which increase the cost of preparation.

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Owing to the good electron mobility, appropriate optical band gap and low temperature preparation condition, several metal sulfides, such as ZnS [7], CdS [8] and In2S3 [9], have been viable alternative to crystallized TiO2. However, the conduction band offset of ZnS was proved to be unmatched with Pb-based perovskite material and rare material In in In2S3 would improve the cost of solar cell module. On the contrary, CdS, as one of the most common n-type material, has drawn much interest of the researchers due to its low cost and widely application in CdTe [10], Cu2ZnSnS4 (CZTS) [11] and Cu(In,Ga)Se2(CIGS) [12] solar cell. CdS thin films could be easily synthesized by wet chemical method, such as sol-gel spinning method [13,14], chemical bath deposition [15], hydrothermal process [16] et al. The perovskite solar cell based on chemical bath deposited CdS ETL has achieved a high efficiency over 15% [17]. However, CdS deposited by wet chemical method could cause serious environmental problems due to the large amount of cadmium-containing waste during the preparation process. A ultra-high vacuum thermal evaporation method has also been used to prepared CdS film and a champion efficiency of 12.2% has been achieved for a photostable planar perovskite solar cell [8]. However, the efficiency is still lower than that of PSC based on solution method

Corresponding authors at: Department of Electronic Engineering, East China Normal University, 500 DongChuan Road, Shanghai 200241, China. E-mail addresses: [email protected] (J. Jiang), [email protected] (S. Zuo).

https://doi.org/10.1016/j.solmat.2018.01.017 Received 28 July 2017; Received in revised form 12 November 2017; Accepted 9 January 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Cd/S ratio and electrical properties of CdS films with different thickness; (b) The optical transmittance of CdS films with various thicknesses; AFM images for CdS films with different thicknesses for (c) 10 nm; (d) 20 nm; (e) 30 nm.

prepared CdS films. In addition, the compactness and uniformity of CdS film cannot be well-controlled by a vacuum thermal evaporation method. Radio-frequency (RF) sputtering to process a compact CdS layer as electron transport layer or buffer layer has been reported in CdTe [18], Cu2ZnSnS4 [19] solar cell, which is suitable to reduce the environmental impact and thin film solar cells can be manufactured by an in-line process for massive production. Based on above consideration, planar perovskite solar cell with CdS and TiO2/CdS films as the ETL are fabricated. The CdS layer is prepared by radio-frequency sputtering process. The structural, morphological, optical and photoelectric properties of prepared films were investigated in detail. The power conversion efficiency of TiO2/CdS based device can be as high as 16%, as compared to 12.71% for TiO2 based device and 13.17% for CdS based device.

100 W RF power and 0.2 Pa work pressure. The substrate temperature was kept at 423 K and CdS layer thicknesses were obtained by controlling sputtering time. 2.1.3. Perovskite solar cell device preparation Given the demand of convenience and repeatability in further massive production, a highly reproducible one-step method [20] without antisolvent process has been taken to prepare perovskite films. 37 wt% precursor solutions of 3:1 MAI:Pb(Ac)2 were spin on substrate at 2000 rpm for 30 s. The layers were further annealed on the hot plate at 90 °C for 5 min and then coated with a hole-transporting material solution (68 mM spiro-OMeTAD, 26 mM Li-TFSI and 55 mM TBP in chlorobenzene) at 2000 rpm for 30 s. The substrate was transferred to a evaporation equipment where ~ 80 nm Au back electrodes were deposited under high vacuum (6 × 10−4 Pa).

2. Material and methods 2.2. Characterization 2.1. Preparation detail The structure of films were obtained by X-ray diffraction (XRD, Bruker D8 Advance). Surface morphology were collected by Scanning electron microscopy (SEM,JEOL) and atomic force microscopy (AFM, Bruke). The optical parameters of the films were measured by UV–VIS–NIR spectrophotometer (Cary5000). The thickness of the films were measured by stylus profilometry (Veeco, dektak 150). The chemical component of the films were estimated by X-ray fluorescence (XRF) spectrometer (XRF, SHIMADZU EDX-7000). The ultraviolet photoelectron spectra (UPS) was obtained by a Thermo Scientific ESCALab 250Xi. Time resolved photoluminescence (TRPL) measurements were investigated with an excitation wavelength of 420 nm. The incident photo-to-current conversion efficiency (IPCE) was obtained by Qtest Station 1000AD with a calibrated Si-cell as reference. Current density–voltage (J–V) characteristics were measured with a Keithley

2.1.1. Preparation of TiO2 layer Prior to deposition process, chemically FTO glass was cleaned with detergent solution, acetone, ethanol and then dried by nitrogen gas. To prepare TiO2 ETL, 350 µl Titanium isopropoxide was diluted in 5 mL isopropanol with 0.013 M HCl. The FTO substrates were coated by spin coating the precursor solution for 60 s with different revolutions, and annealed at 500 °C in a rapid thermal process device (RTP-500，Beijing East Star Research Office of Applied Physics). 2.1.2. Preparation of CdS layer The CdS films were sputtered from 3-in. CdS ceramic target. The base pressure was kept at 5 × 10−4 Pa and the work gas was pure Ar. RF magnetron sputtering was used for depositing CdS film with the 187

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model 2400 digital source meter and a solar simulator (Oriel Sol3A, Newport) calibrated to AM 1.5, 100 mW/cm2 with a standard silicon photodiode. The cells were masked with a 0.09 cm2 active area and all the measurements were carried out in air with a humidity of ~ 60%.

Table 1 The J–V characteristics of PSCs based on CdS films with varied thickness.

3. Results and discussion Fig. 1(a) shows the Cd/S ratio and electrical properties of CdS films with different thickness. As seen in Fig. 1(a), the Cd/S ratio and carrier concentration decrease with CdS film thickness. As reported before [21], donor defects such as sulfur vacancy would increase the carrier concentration in CdS bulk. However, too thin and discontinuous film would increase the scattering probability and decrease the carrier mobility of CdS film as shown in Fig. 1(a). Higher carrier mobility could accelerate the carrier transportation and benefit device performance. Fig. S1 shows XRD pattern of CdS film on glass substrate, only a weak peak at 26.5° corresponding to (111) planes of cubic CdS could be observed. Since CdS is a light absorb material with a 2.4 eV band gap [22], investigation was required to judge whether the CdS film would prevent the incident light into perovskite absorb layer. The optical transmittance spectrum of CdS films with various thicknesses is presented in the wavelength range of 300–800 nm. As seen in Fig. 1(b), when thickness of CdS film is 10 nm, CdS film still holds a relatively high average optic transmittance above 80%. With the increase of CdS film thickness, the optical transmittance of CdS film in 300–600 nm show obvious decline where CdS would start to absorb the light in the film. Fig. 1(c-e) depicts the AFM photos of CdS films with different thickness. The 5 µm × 5 µm AFM image of the 10 nm CdS layer shows an inhomogeneous film with a large root-mean-square (RMS) roughness of 15.7 nm. With the increase of CdS film thickness, the surface looks more uniform and smooth, as reflected by the decreased RMS roughness from 15.7 to 12.8 nm. The RMS further increases to 13.5 nm when CdS thin film thickness increase to 30 nm. The smoother ETL surface would lead to a denser perovskite film without appreciable pinholes to decrease the leakage current of solar cell. The growth of perovskite film influenced by CdS film thickness was investigated by cross-sectional SEM images (Fig. S2). As seen in Fig. S2, the perovskite film deposited on 10 nm CdS film has the minimal thickness (244 nm) which may be due to largest RMS and pinholes as shown in surface SEM image of CdS films (Fig. S3). Fig. 2(a) shows schematic configuration of planar perovskite solar cell based CdS ETL, the simple planar device structure is considered to have excellent prospect towards industrialization. J–V curves of PSCs with various thickness CdS thin films are shown in Fig. 2(b) and their parameters are listed in Table 1. As seen in Fig. 2(b) and Table 1, device based on 10 nm CdS shows the worst performance with a low short-

Sample

Jsc (mA cm−2)

JIPCE (mA cm−2)

Voc (V)

FF

PCE (%)

CdS (10 nm) CdS (20 nm) CdS (30 nm)

13.58 20.71 17.63

12.68 18.12 15.97

1.06 1.10 1.10

0.46 0.58 0.52

6.68 13.17 10.10

circuit current (JSC) of 13.58 mA cm−2 and fill factor (FF) of 0.46. Although 10 nm CdS has the highest optical transmittance, too thin ETL film may not block hole effectively which could cause serious electronhole recombination in perovskite film. As CdS film became thicker, the parameters increased to Voc = 1.1 eV, JSC = 20.71 mA cm−2 and FF = 0.58. When the thickness of the CdS film increases to 30 nm, the Voc remains but Jsc and FF decrease dramatically, Thicker electron transport layer could increases the distance of electron transportation, resulting in the increase of charge recombination probabilities. And the lower transmittance also could decrease the photoelectric performance of device. Moreover, the change of light absorber layer thickness may also affect device performance [23,24]. As observed in cross-sectional SEM images (Fig. S2), the perovskite film deposited on 20 nm CdS film has the maximal thickness (287 nm) which may collection photon more efficiently. Consequently, the 20 nm CdS film is considered as the optimal thickness in this study. To verify the reproducibility of the devices, PCEs from 20 PSCs based on the 20 nm CdS layer were measured and the results are shown in Fig. S4, while the average PCE and highest PCE are 11.88% and 13.17% for the device with 20 nm CdS. To ensure the accuracy of J-V measurement, J-V curve of PSC with 20 nm CdS film was scanned for forward and reverse direction and a moderate hysteresis is observed in Fig. S5. As the photocurrent of solar cells is directly related with external quantum efficiency (EQE), incident photonto-electron conversion efficiency (IPCE) of these devices is measured in Fig. 2(c). The IPCE data indicates that 20 nm CdS shows the highest external quantum efficiency of solar cells in the whole visible light region which is in accordance with the photo-current density in Fig. 2(b). In order to compare with traditional high temperature annealed TiO2, optimized PSC with ~ 30 nm TiO2 as ETL was prepared, the optimized parameters are listed in Table S1. As seen in Fig. 3(a), PSC with TiO2 shows higher FF but lower Voc in comparison with PSC based on CdS ETL. Although device with 20 nm CdS film has the larger Voc and Jsc than device with optimized TiO2 ETL, the poor FF limit the further improvement of solar cell performance. The Series Resistance (Rs) and Shunt Resistance (Rsh) were calculated by J-V curve and added in Fig. 3(a). Compared with devices with TiO2 ETL, the lower Rsh may

Fig. 2. (a) Schematic configuration of planar perovskite solar cell based CdS ETL; (b) J–V curves of PSCs based on CdS films with varied thickness; (c) IPCE spectral of solar cell devices.

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Fig. 3. (a) J–V curves for TiO2 and CdS based PSCs; (b) Schematic diagram for band alignment of planar perovskite solar cell with TiO2 and CdS as electron transportation layering.

transportation from perovskite to the ETL. Thus, CdS could be an appropriate buffer layer between TiO2 and perovskite solar cell to promote electron transportation besides being an independent ETL layer. As mentioned above, low transmittance would impede the light into perovskite film which may deteriorate the photovoltaic conversion performance. Thus, a thin CdS film (10 nm) was considered as appropriate buffer layer to decrease the absorption loss of photons in ETL. Fig. 4(a) reveals the XRD patterns of TiO2, TiO2/CdS and TiO2/CdS/ perovskite films. Peaks at 14°, and 28.44° can be attributed to (110), (220) reflections of CH3NH3PbI3 respectively. No obvious diffraction peaks related to TiO2 and CdS could be found due to the too thin film

be the main reason for the poorer FF of device which indicates larger leakage current in device with CdS ETL than device with TiO2 ETL. Fig. 3(b) demonstrates the energy level diagram for the perovskite solar cell device with different electron transport materials (CdS and TiO2). The valence band maximum (VBM) values (−6.2 eV) and work function (−4.1 eV) of sputtered CdS film were determined by UPS, and the conduction band minimum (CBM) values (−3.9 eV) were calculated by the optical band gap from the UV–visible absorption spectra (Fig. S6). The CBM and VBM of other layers were from references [6,7] with same preparation method. According to the energy diagram, CdS materials possess relatively higher Fermi energy, which may facilitate the charge

Fig. 4. (a) X-ray diffraction patterns of TiO2, TiO2/CdS and TiO2/CdS/perovskite films; (b) The optic transmittance of TiO2 film and TiO2/CdS films; Scanning microscopic images for (c) FTO/TiO2; (d) FTO/TiO2/CdS; (e) FTO/TiO2/CdS/ perovskite films.

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Fig. 5. (a) J–V curves of PSCs with varied thickness CdS films; (b) Statistic PCEs from 20 devices based on CdS or TiO2/CdS ETL; (c) Schematic illustration explaining the mechanism of enhanced performance of PSC by the CdS modified on the TiO2 surface; (d) IPCE spectral of corresponding solar cell devices.

and shown in Fig. 5(b) to verify the reproducibility. Device with TiO2/ CdS ETL shows an average PCE of 15.19% which is considerably higher than that of device with TiO2 ETL (11.66%). The calculated Rsh for device with TiO2/CdS ETL is 16,004 Ω cm2 which is larger than that of device with TiO2 ETL. The higher Rsh indicates minimized leakage current in device with TiO2/CdS ETL, which leads to an improved FF. In addition to FF, other device parameters (Jsc and Voc) also increase. As mentioned above, the higher conduction band edge of CdS may cause a greater tuning of the energy levels of the perovskite film with TiO2 as demonstrate in Fig. 5(c). Therefore, the electron transfer from the perovskite to the TiO2 more efficiently, leading to reduced recombination rate and enhanced charge collection efficiency with larger Voc. Furthermore, as indicated in Fig. 5(c), CdS buffer layer could passivating the surface defects of the TiO2 [25] which would act as recombination sites during the separation stage of photogenerated carriers. Although the incorporation of CdS could cause light absorption as shown in Fig. 4(b), the decrease of non-radiative recombination in TiO2 surface could improve the photon-to-electron conversion ability leading to an increased short current density, which consequently improves the PCE. The incident photo-to-current conversion efficiency spectra of the PSCs based on TiO2 and CdS/TiO2 are shown in Fig. 5(d). In a comparison to the bare TiO2 and CdS device, the device with TiO2/ CdS shows outstanding increases of EQE between 350 and 800 nm which is in accordance with analysis above. Generally, TiO2 based planar PSCs tend to exhibit a significant hysteresis than mesoscopic PSCs which would affect the accurate

thickness. Fig. 4(b) describes the optical transmittance of TiO2 and TiO2/CdS film. As seen in Fig. 4(b), the pure TiO2 film exhibits the high optical transmittance over 80%. When 10 nm CdS film was deposited on TiO2 surface, TiO2/CdS bilayer film still maintains a high average optic transmittance above 75%. Fig. 4(c-d) depicts the surface SEM photos of TiO2 and TiO2/CdS thin film. Morphology of TiO2 film without inserted CdS exhibits rough surface with small grains. The TiO2/CdS bilayer thin film shows a smoother surface morphology with less voids which may decrease probability of direct contact of perovskite and FTO substrate. Fig. 4(e) shows the morphology of perovskite film on TiO2/CdS bilayer thin films and a high coverage and less cracks surface was obtained. J–V curves of PSCs with TiO2 and TiO2/CdS thin film are shown in Fig. 5(a), and their parameters are listed in Table 2. As seen in Fig. 5(a) and Table 2, the introduction of thin CdS film could hugely improve the performance of PSC with a champion efficiency of 16.01%. Statistic PCEs from 20 devices based on CdS or TiO2/CdS ETL were measured

Table 2 The J–V characteristics of PSCs with CdS,TiO2 and TiO2/CdS films. Sample

Jsc (mA cm−2)

JIPCE (mA cm−2)

Voc (V)

FF

PCE (%)

CdS TiO2 CdS/TiO2

13.58 20.16 20.93

12.68 18.02 19.47

1.06 0.95 1.02

0.46 0.66 0.75

6.68 12.71 16.01

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Fig. 6. (a) J-V curve from different scan directions for TiO2 and TiO2/CdS based PSCs; (b) Time resolved photoluminescence spectral of perovskite film on bare FTO, TiO2/FTO and TiO2/ CdS/FTO.

listed in Fig. S7. All devices were kept in a desiccator with ~ 20% humidity. TiO2 based device only maintained 30% of the initial PCE after storing in ambient air for 120 h, whereas the TiO2/CdS-based device remained 95% of the initial PCE, suggesting that a thin CdS buffer layer could effectively improve the PSC device stability.

measurement of J-V curve [26]. The hysteretic behavior of the perovskite solar cells based on TiO2 and TiO2/CdS as ETL are examined with forward and reverse bias. As seen in Fig. 6(a), PSCs of TiO2 tends to exhibit a lower J–V curve when scanned from a forward direction than with a reverse. By contrast, the PSCs with TiO2/CdS electron transport layer shows less hysteresis phenomenon,which indicates that the carrier traps created at the interface between the perovskite and ETL is reduced. Although ion migration [27] and ferroelectric performance [28] could explain hysteresis to some extent, the most common explanation for hysteresis is defects located at TiO2 surface leading to low carrier collection rate [29]. Fullerenes [30] were reported as being effective capping layer on TiO2 surface to reduce hysteresis performance. Compared with expensive and air-unstable fullerene, low-cost and stable CdS could be a possible material to reduce hysteresis of TiO2 based planar PSCs. In order to further evaluate the recombination of devices, TRPL measurements were applied to characterize the charge transportation ability of perovskite films with different ETL. In order to analyse the dynamics of recombination, The TRPL decay curves were fitted to a bi-exponential rate law:

4. Conclusion In summary, CdS films were synthesized by low temperature RF sputtering and used as electron transport layer and buffer layer for perovskite solar cell. The structure, morphological, optical and photoelectrical properties of the obtained samples were studied. The CdS film thickness was optimized and a PCE over 13% was obtained. A thin CdS layer at the interface between TiO2 and perovskite layers could significantly improve the photoelectric properties of PSCs. The results indicate RF sputtered CdS and TiO2/CdS films can be suitable candidates for replacing high temperature annealed TiO2 films in perovskite solar cell devices. Acknowledgments

t t f (t ) = A1 exp ⎛− ⎞ + A2 exp ⎛− ⎞ + B ⎝ τ1 ⎠ ⎝ τ2 ⎠ ⎜

⎟

⎜

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This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Y2K4401DG0), Major State Basic Research Development Program of China (Grant No. 2013CB922300), National Natural Science Foundation of China (Grant No. 61376129, No. 61604055 and No. 61474045), Shanghai Pujiang Program (16PJ1402600).

where A1 and A2 are the relative amplitudes, τ1 and τ2 are the lifetimes for the fast and slow recombination, respectively. τ1 was usually used to measure electron extraction ability of ETL layer due to it reflects the information of interface recombination between perovskite and ETL layer and τ2 represents the radiative recombination lifetime [31–33]. The average decay time was calculated using the following equation:

τavg =

∑ Ai τi/ ∑

Appendix A. Supporting information

Ai Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2018.01.017.

A shorter average lifetime is preferred since it reflects a faster electron transfer process between perovskite and ETL layer. As indicated in Fig. 6(b), FTO/perovskite film shows a long average lifetime of 53 ns while a dramatic decrease of the average lifetime from 53 ns to 3.18 ns and 2.6 ns are observed from FTO/perovskite film to TiO2/ perovskite film and TiO2/CdS/perovskite film. The shorter lifetime indicates that charge transfer from perovskite to TiO2/CdS is faster than that of the TiO2, which may benefit the charge collection and separation in devices. The efficient charge transfer in the CdS/perovskite interface, compared to the TiO2/perovskite interface, could lead to reduced charge accumulation at the CdS/CH3NH3PbI3 interface, which may also explains the minimized hysteresis in the TiO2/CdS based device in comparison with the bare TiO2-based device. Stability measurement of PSCs based on TiO2 and TiO2/CdS ETL was performed by recording the PCEs at intervals of 24 h and corresponding data are

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