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Efficient and stable hole-conductor-free mesoscopic perovskite solar cells using SiO2 as blocking layer
Organic Electronics 58 (2018) 69–74
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Efficient and stable hole-conductor-free mesoscopic perovskite solar cells using SiO2 as blocking layer
T
Huang Liua, Bingchu Yanga,∗, Hui Chena, Kangming Lia, Gang Liua, Yongbo Yuana, Yongli Gaoa,b, Conghua Zhoua,∗∗ a Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, PR China b Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: SiO2 Perovskite solar cell Blocking layer Hole-conductor-free Carbon
Efficient and stable hole-conductor-free mesoscopic perovskite solar cells were fabricated using SiO2 as blocking layer which was inserted between the top carbon electrode and bottom electron-transport-layer in the device. Power conversion efficiency of 13.09% (AM1.5G, 100 mW/cm2) was obtained, which was comparable to those using ZrO2 as the blocking layer. Besides that, prolonged stability has been obtained. After being stored at ambient air for 104 days (relative humidity between 50% and 70%), 94.19% of the starting efficiency was preserved for devices without any encapsulation. Moreover, effect of thickness of SiO2 blocking layer on device performance was studied by tuning the concentration of SiO2 paste. A moderate concentrate was found to be beneficial to device performance. Without blocking layer, poor performance was observed due to short cutting between top electrode and bottom electron-transport-layer; when concentration of SiO2 paste is relative higher (which corresponded to relatively thicker films), cracks appeared which caused short-cutting. Finally, due to the relative higher preservation of Si element (more than 1000 times of Zr) in earth, applying SiO2 as blocking layer could further lower down the cost of the carbon based hole-conductor-free mesoscopic perovskite solar cells.
1. Introduction Perovskite solar cells (PSCs) have attracted extensive attentions during the past few years because of high power conversion efficiency (PCE) and low cost [1–5]. Due to the continuous efforts that have been paid in several branches like the control over coarsening dynamics of perovskite crystallites [6–11], optimization over the interfacial charge extraction processes [12–17], as well as advanced architecture designation [2,18–20], the efficiency has been upgraded to higher than 20%, and a certified 22.7% in 2017 [21]. However, these efficiencies has usually been obtained with assistant of evaporated top electrode like Au, Ag [22–24], and expensive hole-transport-materials [18,25], like spiro-OMeTAD. The adaption of these materials not only increases the cost, but also brings risk over device instability due to corrosion. To solve the problem, Han et al. proposed a kind of mesoscopic holeconductor-free PSCs in 2013, where carbon film was used as both top electrode and hole-extraction layer, in addition the device could be fully printable [2]. In 2014, they achieved a certified PCEs of 12.8% (AM 1.5G) as well as stability of 1000 h (without encapsulation) [1],
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (C. Zhou).
∗∗
https://doi.org/10.1016/j.orgel.2018.04.008 Received 16 February 2018; Received in revised form 23 March 2018; Accepted 3 April 2018 Available online 04 April 2018 1566-1199/ © 2018 Published by Elsevier B.V.
then in early 2017, they further prompted the efficiency to 15.6% [19]. It is noted that, during the fabrication procedure of Han's mesoscopic PSCs, a blocking layer is inserted between bottom charge-extraction-layer and top carbon electrode, so as to avoid short-cutting. And since then, only a wide-band semiconductor of ZrO2 has been chosen [2]. Anyhow, considering the idea of “stability-efficiency-cost” [26], it would also be meaningful to explore other material which is not only insulating, but also cheaper. Basing on this consideration, SiO2 might be a realistic candidate [27]. Since it is insulating and used widely in semiconducting industry, in addition, it is abundant in earth. Element of Si is far more than Zr, Si element is more than 1000 times of Zr element in earth [28]. Giving possible replacement of ZrO2, cost of the carbon based PSCs could be further decreased. Here in the work, SiO2 is used to fabricate blocking layer in the mesoscopic PSCs. PCE of 13.09% has been obtained under AM 1.5 G illumination (100 mW/cm2), which is comparable to ZrO2 based devices when adopting same fabrication procedures. Moreover, shelfstability of more than 100 days has also been demonstrated for the SiO2 based device without any encapsulation.
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H. Liu et al.
blading and annealed 350 °C in oven for 30 min, whereas colloidal TiO2 nanoparticles were uesd as binders as proposed in our last report [29]. Perovskite precursor was prepared by dissolving 2.514 g PbI2, 0.828 g MAI and 0.05998 g 5-AVAI in 5.6 ml GBL. Then 30 μL of such precursor was dripped onto the architecture, being followed by drying at 50 °C for 12 h in open air, which helped the formation of perovskite (PVSK here after) described by (5-AVA)x(MA) (1-x)PbI3. 2.3. Material characterization and performance evaluation of PSCs Crystallographic and morphological properties of materials were investigated with X-ray diffraction (XRD, D500, Siemens) and scanning electron microscopy (FE-SEM, Nova Nano SEM230 and FE-SEM, JSM6490LV). Film thickness was measured by profiler (Dectak 150, Veeco). Current-voltage (IV) characteristic of PSCs were recorded by a digital source meter (Model 2400, Keithley Inc.) under illumination of solar simulator (Newport 91160S, AM1.5G, 100 mW/cm2). Dwell time of 10 ms and step voltage of 12 mV were used, light soaking of 40s is applied before IV scanning. All of the tests were done in open air except for SEM. For shelf-stability test, devices were un-encapsulated and stored in ambient air (dark), with relative humidity (RH) recorded.
Fig. 1. (a) to (e) Animation for the fabrication of carbon based hole-conductorfree PSCs using SiO2 as blocking layer.(f)Schematic of the device. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2. Experimental section 2.1. Materials and reagents
3. Results and discussions
Graphite powder (99.85%), carbon black nanoparticles (99%), SiO2 nanoparticles (A380, Degussa), ZrO2 nanoparticles (99.7%), TiO2 nanocrystallites (P25, 20 nm, Degussa), lead iodide (PbI2, 99%, Sigma), methylammoniumiodide (CH3NH3I, MAI, 99%, Dyesol), HOOC(CH2) 4NH3I (5-AVAI, 99%), g-Butyrolactone (GBL, 99.9%), anhydrous N,N–dimethylformamide (DMF, 99.9%), Ammonium chloride (NH4Cl, 99.99%), diethanol amine (DEA, 99%), butyl titanate (TTBT, 99%), acetone (99%), ethanol (99%), ethylene glycol (99%) were all used as received without further purification. Deionized water was prepared in laboratory.
3.1. Characterization of devices with SiO2 as blocking layer Fig. 2 (a) shows a typical cross-sectional SEM image of the SiO2 based hole-conductor-free mesoscopic PSC. The layered structure is same to those using mesoporous ZrO2 film as the blocking layer. The SiO2 layer is mesoporous, as will be seen later, thus is beneficial to subsequent filling of PVSK crystallites in the architecture. To explore the crystallographic properties of the formed PVSK, XRD is applied on the device before and after formation of PVSK. As shown in Fig. 2 (b), before formation, few peaks could be observed. For example, one lying at 26.6° and the other at 54.8° both of which belong to graphite plates [29]. Anyhow, due to the relative higher thickness of carbon film on top (facing X-ray beam in the characterization), XRD peaks of other components could hardly be distinguished, like TiO2, SiO2 as well as FTO. After formation of PVSK, several peaks appear in the pattern, peaks at 14.17°, 20.08°, 23.55°, 28.51°, 31.94°, 35.03°, 40.53° as well as 43.13° could be assigned to crystal planes of (110), (112), (111), (004), (310), (221), (202) and (212) of PVSK crystallites [1,29]. Besides that, no peaks are shown for PbI2. As a result, the PVSK crystallites could be well crystalized in the mesoporous substrate.
2.2. Device fabrication and characterization As shown in Fig. 1 (f), architecture of “FTO/c-TiO2/mp-TiO2/ Blocking layer/Carbon” is utilized to prepare the hole-conductor-free PSCs [19], whereas “c/mp” denote “compact/mesoporous” respectively, and mesoporous SiO2 film is imported as the blocking layer. Before device fabrication, FTO (F doped tin oxide) substrates were ultrasonically cleaned in acetone, deionized water and ethanol each for 10 min, and then were dried at oven and treated by UV-ozone for 20 min. c-TiO2 and mp-TiO2 layers were prepared by spin-coating as described before [29]. For SiO2 layer, it was also prepared by spincoating (3000 RPM), followed by annealing at 500 °C in oven for 30 min. SiO2 paste was prepared by ball-milling SiO2 powders in ethanol with ethylene glycol (5% in volume ratio) as binder. To optimize the thickness of SiO2 layer, four concentrations were prepared as 0.42, 0.84, 1.26 and 1.68 mol/L. Carbon films were coated by doctor
3.2. Effect of concentration of SiO2 on device performance of PSCs Photo-to-electric conversion properties of the SiO2 based hole-conductor-free PSCs are shown in Fig. 3. It is noticed that, without blocking Fig. 2. (a) Typical cross-sectional SEM image of the SiO2 based hole-conductor-free mesoscopic PSC. (b) XRD spectra of the device before and after infiltration of perovskite precursor. Crystal planes marked by blue are due to graphite in the carbon film. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. (a) Typical current density-voltage curves of PSCs with respect to concentration of SiO2. (b) Effect of concentration of SiO2 on performance parameters of PSCs. Showing from top to bottom is open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) and power conversion efficiency (PCE). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
concentration of SiO2 is relatively higher, the films become uneven, and cracks appear, as could be reflected by Fig. 4(d) and (e). While at relatively lower concentrations, uniform films are obtained. The difference between surface morphological properties tells the reason for the evolution of device performance observed above. At higher concentrations, because of cracks in the blocking layer, direct contact is possible between top electrode of carbon and bottom electron-transport-layer of TiO2, which produces charge recombination, and deteriorates device performance. Also due to the uneven distribution of such cracks, wide distribution appears in device performance. However, at relative lower concentrations, uniform SiO2 layer is obtained, which leads to narrowed distribution of device performances. Anyhow, the existence of cracks in SiO2 layer might arise from the material chemistry of SiO2 paste, for example the concentration of binders in the paste, and also the surface chemistry of SiO2 nanoparticles. However, according to observation shown in Fig. 4, thin layer (∼1.5 μm) is enough for SiO2 to act as the blocking layer. It is also noted that, hysteresis exists in current devices, as shown in Fig. 5. To cut down such hysteresis, formation of PVSK crystallites was manipulated. Similar like that done in literature [19,31], a NH4Cl (DMF) assisted route was utilized. As shown in Fig. 5 (a), no obvious hysteresis could be seen after the replacement. It is noted that, for NH4Cl based devices, light-soaking of only 1s was used before IV scanning. In addition, a parameter of hysteresis index (HI) is defined using equation (1):
layer, poor device performance is obtained. For example, open circuit voltage (VOC) is around 500 mV, short circuit current (JSC) is around 17 mA/cm [2], while PCE is about 5%. Such performance is well ascribed to charge recombination because of the direct contact between TiO2 layer and carbon electrode. Also the recombination could be reflected by the obvious leakage current in dark J-V curves, as shown in bottom of Fig. 3 (a). Anyhow, the recombination behavior is different from short-cutting or shunt, whereas linearity usually appears. After blocking layer of SiO2 is added, device performance is upgraded. One can see overall improvement in all of the four parameters. For example, VOC increases to near 860 mV, JSC goes to higher than 20 mA/cm [2], while PCE rises to exceed 10%, nearly doubled when comparing to the device without blocking layer. Similar behavior was also observed by Han et al. [30]. The improvement is due to retarded charge recombination contributed by the insulating layer of SiO2. As a result, SiO2 is also efficient when acting as the blocking layer. Besides that, device performance is observed to be sensitive to concentration of SiO2 in the paste. As seen in Fig. 3 (b), higher device performance could be harvested when the concentration is relatively lower, so is the yield of the performance. This could be well reflected when comparing device in concentrations of 0.42–0.84 mol/L and 1.26–1.68 mol/L. When concentration is small (the former), performance gathers around 10%; while at higher concentrations, the performance disperses from 5% to 10%, even drops down to those without blocking layer. Later in Fig. 4 the relationship between concentration and thickness of SiO2 layer will be shown. It could be found that, when using SiO2 as blocking layer, quite thin film (∼1.5 μm) is enough. And again, the device performance will be further discussed with assistant of morphological studies. Fig. 4 (a) shows top view SEM image of mp-TiO2 layer grown on cTiO2/FTO. As expected, porous morphology appears. After SiO2 layer is added on top, similar porous structure is preserved. However, when
HI =
PCE − − PCE + × 100% PCE − + PCE +
(1)
where “−/+” denotes PCEs obtained from reverse/positive currentvoltage scanning respectively. As is shown in Fig. 5 (b), for 5AVA-I (GBL) based devices, HI scales from 2.49% to 10.15% (centers at Fig. 4. (a) Typical top-view SEM image of mp-TiO2 film, and (b) to (e), top view SEM images of SiO2 layers prepared from with different concentration: (b) 0.42M, (c) 0.84M, (d) 1.26M, (e) 1.68M . Insets in (c) and (e) depict corresponding cross-section images. (f) Relationship between film thickness of SiO2 layer and concentration in SiO2 paste. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 5. Comparison between the hole-conductor-free PSCs using PVSK crystallites formed from solution basing on either 5AVA-I (GBL) or NH4Cl (DMF). (a) Typical current-voltage curves, and (b) distribution of hysteresis index (HI) of the two kinds of PSCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Comparison between hole-conductor-free PSCs using either SiO2 or ZrO2 as blocking layer: (a) VOC, (b) JSC, (c) FF, and (d) PCE. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
distribution is narrowed, ranging from 770 mV to 910 mV, while centering at 870 mV. Both of the two kinds of devices show PCEs around 11%, however, the SiO2 based devices might hold the advantage of relative lower cost for the abundant preservation of element Si on earth.
6.05%), while for NH4Cl(DMF) based devices, a narrowed range between −4.42% and 3.77% is observed (centers at −0.65%). The reduced HI index coincides well with literature. 3.3. Comparison with ZrO2 based PSCs
3.4. Stability Besides hysteresis, it is also meaningful to compare the device performance between SiO2 based PSCs and ZrO2 based ones. To help comparison, ZrO2 based devices were prepared like that done in our last work [29], and again, same device architecture are used, as well as the PVSK solution (basing on GBL). The resultant performance parameters are all collected and shown in Fig. 6. It could be found that, they come out with quite similar PCEs. However, there exists slight difference in VOC. For VOC, SiO2 based devices showed relatively higher value than those basing on ZrO2. For ZrO2 based devices, it distributes from 740 mV to 900 mV, centers at 830 mV; while for SiO2 based devices, the
Stability of the SiO2 based hole-conductor-free PSCs (without any encapsulation) has also been tested. The first is the shelf-stability. As shown in Fig. 7 (a), the initial efficiency is 13.09%. 18 days later, it increases slightly to 13.63%; After being stored for 104 days (with relative humidity between 50% and 70%), 94.19% of the starting value or 12.33% was preserved. The increase in the first 18 days might be ascribed to the continuous formation of crystallites as revealed by Yang et al. [32]. The slight decrease after 104 days-storage is mainly due to the corrosion by the penetrated H2O/O2. However, the stability is better 72
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Fig. 7. Shelf-stability test of the SiO2 based hole-conductor-free PSCs. (Active device area was 0.18 cm2, testing area was 0.0514 cm2). Relative humidity (RH) of storage environment is also shown for reference. (a) Current-voltage curves recorded at 0, 48 and 104 days for the same device (Reverse scanning). (b) Evolution of performance parameters with storage time.(c) Evolution of PCE, FF, JSC and VOC of PSCs (normalized to initial value) under continuous illumination (AM1.5G,100mW/cm2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
than those PSCs using evaporated metal films as top electrode (especially silver). Besides that, operational stability is also tested since it is more comparable to practical usage [33]. Fig. 7 (c) shows the evolution of PCE, FF, JSC, VOC of the SiO2 based hole-conductor-free PSCs under continuous illumination (AM 1.5G, 100 mA/cm2, open circuit condition, again, no encapsulation was used and the test was performed in open air, with relative humidity of about 40%). It could be seen that, during the first 10 h illumination, the efficiency changes from 11.09% to 9.64% (86.93% of the starting value) for the initial 1 h, but then remains unchanged for the afterwards 9hs. After being stored in dark for 2 days, the efficiency is recovered to 12.88%, then similar behavior is observed. The observation shows that, the SiO2 based hole-conductor-free PSCs could work stably. The sound stability is due to the thick and condensed carbon films, which provides a heavy barrier for air (H2O/O2) to penetrate, and also the chemical stability of SiO2, thus is appealing to long term applications. Anyhow, the slight decrease in the first 1 h in the operational stability test might be ascribed to ion migration in the devices, but needs further detailed investigation.
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4. Conclusion In conclusion, efficient and stable hole-conductor-free perovskite solar cells were successfully fabricated using SiO2 as blocking layer. An efficiency of 13.09% was obtained, as well as shelf-stability up to 104 days (without encapsulation). Acknowledgements B. Yang thanks support of NSFC (No. 61172047). C. Zhou thanks support of NSFC (No. 61774170), and partially by Scientific and Technological Project of Hunan Provincial Development and Reform Commission, Y. Yuan thanks support from NSFC (No.51673218). Y. Gao thanks support from NSFC (No.11334014), and the support from NSF (CBET-1437656). 73
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Efficient and stable hole-conductor-free mesoscopic perovskite solar cells using SiO2 as blocking layer
T
Huang Liua, Bingchu Yanga,∗, Hui Chena, Kangming Lia, Gang Liua, Yongbo Yuana, Yongli Gaoa,b, Conghua Zhoua,∗∗ a Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, PR China b Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: SiO2 Perovskite solar cell Blocking layer Hole-conductor-free Carbon
Efficient and stable hole-conductor-free mesoscopic perovskite solar cells were fabricated using SiO2 as blocking layer which was inserted between the top carbon electrode and bottom electron-transport-layer in the device. Power conversion efficiency of 13.09% (AM1.5G, 100 mW/cm2) was obtained, which was comparable to those using ZrO2 as the blocking layer. Besides that, prolonged stability has been obtained. After being stored at ambient air for 104 days (relative humidity between 50% and 70%), 94.19% of the starting efficiency was preserved for devices without any encapsulation. Moreover, effect of thickness of SiO2 blocking layer on device performance was studied by tuning the concentration of SiO2 paste. A moderate concentrate was found to be beneficial to device performance. Without blocking layer, poor performance was observed due to short cutting between top electrode and bottom electron-transport-layer; when concentration of SiO2 paste is relative higher (which corresponded to relatively thicker films), cracks appeared which caused short-cutting. Finally, due to the relative higher preservation of Si element (more than 1000 times of Zr) in earth, applying SiO2 as blocking layer could further lower down the cost of the carbon based hole-conductor-free mesoscopic perovskite solar cells.
1. Introduction Perovskite solar cells (PSCs) have attracted extensive attentions during the past few years because of high power conversion efficiency (PCE) and low cost [1–5]. Due to the continuous efforts that have been paid in several branches like the control over coarsening dynamics of perovskite crystallites [6–11], optimization over the interfacial charge extraction processes [12–17], as well as advanced architecture designation [2,18–20], the efficiency has been upgraded to higher than 20%, and a certified 22.7% in 2017 [21]. However, these efficiencies has usually been obtained with assistant of evaporated top electrode like Au, Ag [22–24], and expensive hole-transport-materials [18,25], like spiro-OMeTAD. The adaption of these materials not only increases the cost, but also brings risk over device instability due to corrosion. To solve the problem, Han et al. proposed a kind of mesoscopic holeconductor-free PSCs in 2013, where carbon film was used as both top electrode and hole-extraction layer, in addition the device could be fully printable [2]. In 2014, they achieved a certified PCEs of 12.8% (AM 1.5G) as well as stability of 1000 h (without encapsulation) [1],
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (C. Zhou).
∗∗
https://doi.org/10.1016/j.orgel.2018.04.008 Received 16 February 2018; Received in revised form 23 March 2018; Accepted 3 April 2018 Available online 04 April 2018 1566-1199/ © 2018 Published by Elsevier B.V.
then in early 2017, they further prompted the efficiency to 15.6% [19]. It is noted that, during the fabrication procedure of Han's mesoscopic PSCs, a blocking layer is inserted between bottom charge-extraction-layer and top carbon electrode, so as to avoid short-cutting. And since then, only a wide-band semiconductor of ZrO2 has been chosen [2]. Anyhow, considering the idea of “stability-efficiency-cost” [26], it would also be meaningful to explore other material which is not only insulating, but also cheaper. Basing on this consideration, SiO2 might be a realistic candidate [27]. Since it is insulating and used widely in semiconducting industry, in addition, it is abundant in earth. Element of Si is far more than Zr, Si element is more than 1000 times of Zr element in earth [28]. Giving possible replacement of ZrO2, cost of the carbon based PSCs could be further decreased. Here in the work, SiO2 is used to fabricate blocking layer in the mesoscopic PSCs. PCE of 13.09% has been obtained under AM 1.5 G illumination (100 mW/cm2), which is comparable to ZrO2 based devices when adopting same fabrication procedures. Moreover, shelfstability of more than 100 days has also been demonstrated for the SiO2 based device without any encapsulation.
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blading and annealed 350 °C in oven for 30 min, whereas colloidal TiO2 nanoparticles were uesd as binders as proposed in our last report [29]. Perovskite precursor was prepared by dissolving 2.514 g PbI2, 0.828 g MAI and 0.05998 g 5-AVAI in 5.6 ml GBL. Then 30 μL of such precursor was dripped onto the architecture, being followed by drying at 50 °C for 12 h in open air, which helped the formation of perovskite (PVSK here after) described by (5-AVA)x(MA) (1-x)PbI3. 2.3. Material characterization and performance evaluation of PSCs Crystallographic and morphological properties of materials were investigated with X-ray diffraction (XRD, D500, Siemens) and scanning electron microscopy (FE-SEM, Nova Nano SEM230 and FE-SEM, JSM6490LV). Film thickness was measured by profiler (Dectak 150, Veeco). Current-voltage (IV) characteristic of PSCs were recorded by a digital source meter (Model 2400, Keithley Inc.) under illumination of solar simulator (Newport 91160S, AM1.5G, 100 mW/cm2). Dwell time of 10 ms and step voltage of 12 mV were used, light soaking of 40s is applied before IV scanning. All of the tests were done in open air except for SEM. For shelf-stability test, devices were un-encapsulated and stored in ambient air (dark), with relative humidity (RH) recorded.
Fig. 1. (a) to (e) Animation for the fabrication of carbon based hole-conductorfree PSCs using SiO2 as blocking layer.(f)Schematic of the device. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2. Experimental section 2.1. Materials and reagents
3. Results and discussions
Graphite powder (99.85%), carbon black nanoparticles (99%), SiO2 nanoparticles (A380, Degussa), ZrO2 nanoparticles (99.7%), TiO2 nanocrystallites (P25, 20 nm, Degussa), lead iodide (PbI2, 99%, Sigma), methylammoniumiodide (CH3NH3I, MAI, 99%, Dyesol), HOOC(CH2) 4NH3I (5-AVAI, 99%), g-Butyrolactone (GBL, 99.9%), anhydrous N,N–dimethylformamide (DMF, 99.9%), Ammonium chloride (NH4Cl, 99.99%), diethanol amine (DEA, 99%), butyl titanate (TTBT, 99%), acetone (99%), ethanol (99%), ethylene glycol (99%) were all used as received without further purification. Deionized water was prepared in laboratory.
3.1. Characterization of devices with SiO2 as blocking layer Fig. 2 (a) shows a typical cross-sectional SEM image of the SiO2 based hole-conductor-free mesoscopic PSC. The layered structure is same to those using mesoporous ZrO2 film as the blocking layer. The SiO2 layer is mesoporous, as will be seen later, thus is beneficial to subsequent filling of PVSK crystallites in the architecture. To explore the crystallographic properties of the formed PVSK, XRD is applied on the device before and after formation of PVSK. As shown in Fig. 2 (b), before formation, few peaks could be observed. For example, one lying at 26.6° and the other at 54.8° both of which belong to graphite plates [29]. Anyhow, due to the relative higher thickness of carbon film on top (facing X-ray beam in the characterization), XRD peaks of other components could hardly be distinguished, like TiO2, SiO2 as well as FTO. After formation of PVSK, several peaks appear in the pattern, peaks at 14.17°, 20.08°, 23.55°, 28.51°, 31.94°, 35.03°, 40.53° as well as 43.13° could be assigned to crystal planes of (110), (112), (111), (004), (310), (221), (202) and (212) of PVSK crystallites [1,29]. Besides that, no peaks are shown for PbI2. As a result, the PVSK crystallites could be well crystalized in the mesoporous substrate.
2.2. Device fabrication and characterization As shown in Fig. 1 (f), architecture of “FTO/c-TiO2/mp-TiO2/ Blocking layer/Carbon” is utilized to prepare the hole-conductor-free PSCs [19], whereas “c/mp” denote “compact/mesoporous” respectively, and mesoporous SiO2 film is imported as the blocking layer. Before device fabrication, FTO (F doped tin oxide) substrates were ultrasonically cleaned in acetone, deionized water and ethanol each for 10 min, and then were dried at oven and treated by UV-ozone for 20 min. c-TiO2 and mp-TiO2 layers were prepared by spin-coating as described before [29]. For SiO2 layer, it was also prepared by spincoating (3000 RPM), followed by annealing at 500 °C in oven for 30 min. SiO2 paste was prepared by ball-milling SiO2 powders in ethanol with ethylene glycol (5% in volume ratio) as binder. To optimize the thickness of SiO2 layer, four concentrations were prepared as 0.42, 0.84, 1.26 and 1.68 mol/L. Carbon films were coated by doctor
3.2. Effect of concentration of SiO2 on device performance of PSCs Photo-to-electric conversion properties of the SiO2 based hole-conductor-free PSCs are shown in Fig. 3. It is noticed that, without blocking Fig. 2. (a) Typical cross-sectional SEM image of the SiO2 based hole-conductor-free mesoscopic PSC. (b) XRD spectra of the device before and after infiltration of perovskite precursor. Crystal planes marked by blue are due to graphite in the carbon film. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. (a) Typical current density-voltage curves of PSCs with respect to concentration of SiO2. (b) Effect of concentration of SiO2 on performance parameters of PSCs. Showing from top to bottom is open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) and power conversion efficiency (PCE). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
concentration of SiO2 is relatively higher, the films become uneven, and cracks appear, as could be reflected by Fig. 4(d) and (e). While at relatively lower concentrations, uniform films are obtained. The difference between surface morphological properties tells the reason for the evolution of device performance observed above. At higher concentrations, because of cracks in the blocking layer, direct contact is possible between top electrode of carbon and bottom electron-transport-layer of TiO2, which produces charge recombination, and deteriorates device performance. Also due to the uneven distribution of such cracks, wide distribution appears in device performance. However, at relative lower concentrations, uniform SiO2 layer is obtained, which leads to narrowed distribution of device performances. Anyhow, the existence of cracks in SiO2 layer might arise from the material chemistry of SiO2 paste, for example the concentration of binders in the paste, and also the surface chemistry of SiO2 nanoparticles. However, according to observation shown in Fig. 4, thin layer (∼1.5 μm) is enough for SiO2 to act as the blocking layer. It is also noted that, hysteresis exists in current devices, as shown in Fig. 5. To cut down such hysteresis, formation of PVSK crystallites was manipulated. Similar like that done in literature [19,31], a NH4Cl (DMF) assisted route was utilized. As shown in Fig. 5 (a), no obvious hysteresis could be seen after the replacement. It is noted that, for NH4Cl based devices, light-soaking of only 1s was used before IV scanning. In addition, a parameter of hysteresis index (HI) is defined using equation (1):
layer, poor device performance is obtained. For example, open circuit voltage (VOC) is around 500 mV, short circuit current (JSC) is around 17 mA/cm [2], while PCE is about 5%. Such performance is well ascribed to charge recombination because of the direct contact between TiO2 layer and carbon electrode. Also the recombination could be reflected by the obvious leakage current in dark J-V curves, as shown in bottom of Fig. 3 (a). Anyhow, the recombination behavior is different from short-cutting or shunt, whereas linearity usually appears. After blocking layer of SiO2 is added, device performance is upgraded. One can see overall improvement in all of the four parameters. For example, VOC increases to near 860 mV, JSC goes to higher than 20 mA/cm [2], while PCE rises to exceed 10%, nearly doubled when comparing to the device without blocking layer. Similar behavior was also observed by Han et al. [30]. The improvement is due to retarded charge recombination contributed by the insulating layer of SiO2. As a result, SiO2 is also efficient when acting as the blocking layer. Besides that, device performance is observed to be sensitive to concentration of SiO2 in the paste. As seen in Fig. 3 (b), higher device performance could be harvested when the concentration is relatively lower, so is the yield of the performance. This could be well reflected when comparing device in concentrations of 0.42–0.84 mol/L and 1.26–1.68 mol/L. When concentration is small (the former), performance gathers around 10%; while at higher concentrations, the performance disperses from 5% to 10%, even drops down to those without blocking layer. Later in Fig. 4 the relationship between concentration and thickness of SiO2 layer will be shown. It could be found that, when using SiO2 as blocking layer, quite thin film (∼1.5 μm) is enough. And again, the device performance will be further discussed with assistant of morphological studies. Fig. 4 (a) shows top view SEM image of mp-TiO2 layer grown on cTiO2/FTO. As expected, porous morphology appears. After SiO2 layer is added on top, similar porous structure is preserved. However, when
HI =
PCE − − PCE + × 100% PCE − + PCE +
(1)
where “−/+” denotes PCEs obtained from reverse/positive currentvoltage scanning respectively. As is shown in Fig. 5 (b), for 5AVA-I (GBL) based devices, HI scales from 2.49% to 10.15% (centers at Fig. 4. (a) Typical top-view SEM image of mp-TiO2 film, and (b) to (e), top view SEM images of SiO2 layers prepared from with different concentration: (b) 0.42M, (c) 0.84M, (d) 1.26M, (e) 1.68M . Insets in (c) and (e) depict corresponding cross-section images. (f) Relationship between film thickness of SiO2 layer and concentration in SiO2 paste. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 5. Comparison between the hole-conductor-free PSCs using PVSK crystallites formed from solution basing on either 5AVA-I (GBL) or NH4Cl (DMF). (a) Typical current-voltage curves, and (b) distribution of hysteresis index (HI) of the two kinds of PSCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Comparison between hole-conductor-free PSCs using either SiO2 or ZrO2 as blocking layer: (a) VOC, (b) JSC, (c) FF, and (d) PCE. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
distribution is narrowed, ranging from 770 mV to 910 mV, while centering at 870 mV. Both of the two kinds of devices show PCEs around 11%, however, the SiO2 based devices might hold the advantage of relative lower cost for the abundant preservation of element Si on earth.
6.05%), while for NH4Cl(DMF) based devices, a narrowed range between −4.42% and 3.77% is observed (centers at −0.65%). The reduced HI index coincides well with literature. 3.3. Comparison with ZrO2 based PSCs
3.4. Stability Besides hysteresis, it is also meaningful to compare the device performance between SiO2 based PSCs and ZrO2 based ones. To help comparison, ZrO2 based devices were prepared like that done in our last work [29], and again, same device architecture are used, as well as the PVSK solution (basing on GBL). The resultant performance parameters are all collected and shown in Fig. 6. It could be found that, they come out with quite similar PCEs. However, there exists slight difference in VOC. For VOC, SiO2 based devices showed relatively higher value than those basing on ZrO2. For ZrO2 based devices, it distributes from 740 mV to 900 mV, centers at 830 mV; while for SiO2 based devices, the
Stability of the SiO2 based hole-conductor-free PSCs (without any encapsulation) has also been tested. The first is the shelf-stability. As shown in Fig. 7 (a), the initial efficiency is 13.09%. 18 days later, it increases slightly to 13.63%; After being stored for 104 days (with relative humidity between 50% and 70%), 94.19% of the starting value or 12.33% was preserved. The increase in the first 18 days might be ascribed to the continuous formation of crystallites as revealed by Yang et al. [32]. The slight decrease after 104 days-storage is mainly due to the corrosion by the penetrated H2O/O2. However, the stability is better 72
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Fig. 7. Shelf-stability test of the SiO2 based hole-conductor-free PSCs. (Active device area was 0.18 cm2, testing area was 0.0514 cm2). Relative humidity (RH) of storage environment is also shown for reference. (a) Current-voltage curves recorded at 0, 48 and 104 days for the same device (Reverse scanning). (b) Evolution of performance parameters with storage time.(c) Evolution of PCE, FF, JSC and VOC of PSCs (normalized to initial value) under continuous illumination (AM1.5G,100mW/cm2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
than those PSCs using evaporated metal films as top electrode (especially silver). Besides that, operational stability is also tested since it is more comparable to practical usage [33]. Fig. 7 (c) shows the evolution of PCE, FF, JSC, VOC of the SiO2 based hole-conductor-free PSCs under continuous illumination (AM 1.5G, 100 mA/cm2, open circuit condition, again, no encapsulation was used and the test was performed in open air, with relative humidity of about 40%). It could be seen that, during the first 10 h illumination, the efficiency changes from 11.09% to 9.64% (86.93% of the starting value) for the initial 1 h, but then remains unchanged for the afterwards 9hs. After being stored in dark for 2 days, the efficiency is recovered to 12.88%, then similar behavior is observed. The observation shows that, the SiO2 based hole-conductor-free PSCs could work stably. The sound stability is due to the thick and condensed carbon films, which provides a heavy barrier for air (H2O/O2) to penetrate, and also the chemical stability of SiO2, thus is appealing to long term applications. Anyhow, the slight decrease in the first 1 h in the operational stability test might be ascribed to ion migration in the devices, but needs further detailed investigation.
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4. Conclusion In conclusion, efficient and stable hole-conductor-free perovskite solar cells were successfully fabricated using SiO2 as blocking layer. An efficiency of 13.09% was obtained, as well as shelf-stability up to 104 days (without encapsulation). Acknowledgements B. Yang thanks support of NSFC (No. 61172047). C. Zhou thanks support of NSFC (No. 61774170), and partially by Scientific and Technological Project of Hunan Provincial Development and Reform Commission, Y. Yuan thanks support from NSFC (No.51673218). Y. Gao thanks support from NSFC (No.11334014), and the support from NSF (CBET-1437656). 73
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