20January2017

Nano-Micro Letters

Effective Improvement of the Photovoltaic Performance of Carbon- Based Perovskite Solar Cells by Additional Solvents

Chenxi Zhang, Yudan Luo, Xiaohong Chen, Yiwei Chen, Zhuo Sun, Sumei Huang*

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Nano-Micro Letters, Nano-Micro Letters,Volume 8, Issue 4pp 347–357

First Online: 31 May 2016  (Article)

DOI: 10.1007/s40820-016-0094-4

*Corresponding author.   *Corresponding author. E-mail: smhuang@phy.ecnu.edu.cn

 

 

Abstract

 


Optical and SEM images of CH3NH3PbI3 layers fabricated without drop-casting solvent (a, b) or with toluene (c, d) or chlorobenzene (e, f) as a drop-casting solvent. (g) Cross section FESEM image of the PSCs.

A solvent assisted methodology has been developed to synthesize CH3NH3PbI3 perovskite absorber layers. It involved use of a mixed solvent of CH3NH3I, PbI2, g-butyrolactone and dimethylsulphoxide (DMSO) followed by addition of chlorobenzene. The method produced ultra-flat and dense perovskite capping layers atop mesoporous TiO2 films, enabling a remarkable improvement on the performance of free hole-transport material (HTM) carbon electrode based perovskite solar cells. Toluene was also studied as an additional solvent for comparison. At the annealing temperature of 100 °C, the fabricated HTM-free perovskite solar cells based on drop-casting chlorobenzene demonstrated power conversion efficiency (PCE) of 9.73 %, which is 36 % and 71 % higher than those fabricated from the perovskite films using toluene or without adding an extra solvent, respectively. The interaction between the PbI2-DMSO- CH3NH3I intermediate phase and the additional solvent was discussed. Furthermore, the influence of the annealing temperature on the absorber film formation, morphology, and crystalline structure was investigated and correlated with the photovoltaic performance. Highly efficient, simple and stable HTM-free solar cells with PCE of 11.44 % were achieved by utilizing the optimum perovskite absorbers annealed at 120 °C.


 

Keywords

Halide perovskite; Solar cell; Spin-coating; Carbon counter electrode; Free-hole-transporting material

 

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1 Introduction

 

During the past five years, there has been a surging interest in the study of organic-inorganic hybrid perovskite compounds for applications in photovoltaic devices because of low cost and simple fabrication process and high efficiency solar power conversion [1-6]. Typically, perovskite solar cells (PSCs) employed a mesoporous titania or alumina scaffold, a methylammonium lead iodide perovskite light absorber, an organic hole transport material (HTM), characteristically spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’-bifluorene), and an Au or Ag electrode. PSCs achieved a power conversion efficiency (PCE) of >10 % in 2012 [2, 3]. TiO2/spiro-OMeTAD is regarded as a successful couple owing to their good optical transparency and perfect band alignment with respect to CH3NH3PbI3. Later, the PCEs were improved to 15 % by using a two-step sequential deposition technique, involving spin-coating of a PbI2 followed by exposure to a solution of CH3NH3I to form CH3NH3PbI3, or a dual-source vapor deposition technique to fabricate a planar heterojunction solar cell [4, 5]. Then, a planar structured perovskite solar cell using a polyethyleneimine ethoxylated modified ITO electrode, yttrium-doped TiO2 layer, mixed halide perovskite CH3NH3PbI3–xClx absorber and spiro-OMeTAD reached a PCE of 19.3 % [6]. Very recently, triple Cs/ methylammonium (MA)/formamidinium (FA) cation PSCs reached a power output of 21.1 % [7].

Currently, the most research PSC devices employ gold as a back contact, in conjunction with organic hole conductors acting as electron-blocking or hole transport layers [4-7]. These cells are expensive because of the high cost of pure hole transportation materials. Besides, hole conductors such as the widely used spiro-OMeTAD are not only expensive but can cause moisture-induced degradation in PSCs, especially with hygroscopic dopants such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) [8]. To overcome the lifetime-limiting problems with the organic hole-transporters, HTM-free perovskite photovoltaics were proposed and reported by Etgar et al. in 2012 [9]. HTM-free PSCs achieved efficiencies about 10 % with gold as a back electrode [10-12]. The precious Au electrode also requires a high-vacuum and high-cost evaporation technique, thereby limiting its future application. Nowadays, the efficiency of the best perovskite solar cells is competitive with current commercial technologies, and they are potentially much cheaper. However, commercial solar cells must last 20-30 years with minimal degradation. The biggest challenge facing perovskite solar cells is long-term stability in a wide range of environments [7].

Low-cost nano-carbon can be an ideal material to substitute Au as a back contact in PSCs because its function is similar to that of Au. In the past decade, carbon nanomaterials have been demonstrated to be excellent counter-electrode candidates for use in dye-sensitized solar cells (DSSCs) owing to their various fascinating properties, including high electrical conductivity, thermal stability, good optical transparency, unique nanostructure, excellent electrocatalytic activity, low cost and abundance [13-16]. Substantial gains have been made in the application of carbon nano-materials in DSSCs, however, perovskite organic lead iodide is unstable at high temperatures or in some solvents [17]. Thus, the direct preparation of a carbon layer faces some problems. HTM-free perovskite solar cells with carbon contacts were first fabricated by infiltrating CH3NH3PbI3 or a mixed-cation 5-ammoniumvaleric (5-AVA) and methylammonium (MA) perovskite (5-AVA)x(MA)1-xPbI3 into a high-temperature prefabricated monolithic device, which consists of four layers including TiO2 dense, TiO2 mesoporous, ZrO2 mesoporous and carbon layers [18]. The (5-AVA)x(MA)1-xPbI3 perovskite device achieved a PCE of 12.8 %. The cells have shown promising stability under long term light soaking and long term heat exposure. But, these devices employed complex structures and required processing temperatures of up to 400 °C to remove solvents and organic binders in the printed ZrO2 space and carbon black/graphite electrodes. The complicate fabrication and high-temperature processing increased material or energy consumption and limited their mass-production and fabrication on a plastic substrate. Very recently, carbon counter electrodes were prepared using low-temperature (LT) processed (70-100 °C) carbon pastes and applied in HTM-free perovskite/TiO2 heterojunction solar cells to substitute noble metallic anodes. Under optimized conditions, a PCE in the range of 8.31-9.00 % has been demonstrated with these carbon counter electrodes [19, 20]. The HTM-free solar cell with the LT carbon counter-electrode can have a much simpler structure, benefiting for reducing the cost and improving the overall stability of perovskite solar cells. However, the PSCs based on the LT carbon contact are low in photovoltaic performance. Therefore, it is worth developing and improving the performance of HTM-free and carbon based PSC solar cells.

The perovskite layers with a well-defined grain structure, full surface coverage, and small surface roughness allow realization of an efficient solar cell [5, 21]. Therefore, various morphology control protocols including sequential deposition [4], thermal evaporation deposition [5], compositional engineering [22, 23], additive-assisted deposition [24, 25], solvent engineering [26-28] and intramolecular exchange processing [7] were investigated for high-quality perovskite absorbers. Jeon et al. [26] reported a solvent engineering method for highly uniform CH3NH3Pb(I1-xBrx)3 (x=0.1-0.15) perovskite layers and high-efficiency organic HTM based devices using a mixed solvent of g-butyrolactone (GBL) and dimethylsulphoxide (DMSO) followed by a toluene drop-casting (DC). Although DMSO based solvent engineering [26, 28] is a very potential experimental technique, the related processing involves the anfractuous coupling of fluid rheology, solvent evaporation and molecular self-assembly, and the formation of high quality perovskite film is the result of a complex dynamic process still under investigation. Furthermore,  the morphology control methods have been mainly developed for organic HTM based PSCs, and rarely explored for HTM-free PSCs. However, the ideal morphology of HTM-free PSC also requires a uniform, highly crystalline and high-coverage perovskite capping layer on the top of the mesoporous TiO2 [29].

In this work, we report a chlorobenzene (CB) based solvent-assisted process toward synthesizing simple, stable and efficient HTM-free perovskite solar cells with carbon counter-electrodes. CH3NH3PbI3 absorber layers were synthesized on mesoporous TiO2 films via spin-coating the mixed solution of CH3NH3I, PbI2, GBL and DMSO, followed by drop-casting CB or toluene (TO) while spinning. Toluene was studied as a drop-casting solvent for comparison. The effects of the drop-casting solvent and the annealing temperature on thin film morphology, crystal structure, and the solar cell performance were investigated. Through changing the drop-casting solvent and optimizing the annealing temperature, extremely uniform and dense perovskite capping layers atop mesoporous TiO2 films were obtained, enabling the fabrication of remarkably improved HTM-free perovskite solar cells. The efficiency of simple structured HTM-free solar cells was high up to 11.42 %. The formed carbon based PSC devices have shown much more promising stability than the HTM devices.

 

2 Experimental Section

 

2.1 Materials

 

Unless specified, all materials were purchased from either Alfa Aesar or Sigma-Aldrich, and used as received. Spiro-MeOTAD was purchased from Merck KGaA and Luminescence Technology Corp. Methylammonium iodide (MAI) CH3NH3I was synthesized according to a previous study [30].

 

2.2 Device Fabrication

 

Fluorine-doped tin oxide coated glass (Pilkington TEC 15) was patterned by etching with Zn powder and 2 M HCl. The etched substrate was then cleaned with surfactant and rinsed with acetone and ethanol and de-ionized water. A 50 nm-thick compact TiO2 (c-TiO2) thin layer was synthesized by a procedure reported in our previous work [31]. The porous TiO2 (p-TiO2) layer was deposited by spin coating at 5000 rpm for 30 s using a commercial TiO2 paste (Dyesol 18NR-T) diluted in ethanol (1:2.5 weight ratio), and consequently heating at 500 °C for 30 min. After cooling to room temperature, the as-prepared nanoporous TiO2 films were then dipped into a 40 mM TiCl4 aqueous solution for 30 min at 70 °C, dried at ambient atmosphere and then sintered at 500 °C for 30 min.

CH3NH3PbI3 perovskite absorber layers were synthesized by modifying the solvent-engineering method reported by Jeon et al. [26]. This was done in a glove box maintaining 10 % RH level. The synthesized CH3NH3I (0.1975 g) powders and lead iodide PbI2 (0.5785 g) were stirred in a mixture of g-butyrolactone (GBL) (700 mL) and dimethylsulphoxide (DMSO) (300 mL) at 60 °C for 12 h. The formed precursor solution was deposited onto p-TiO2/c-TiO2/FTO substrate by a successive two-step spin-coating process at 2000 rpm for 50 s and at 3500 rpm for 50 s, respectively. During the second step, anhydrous chlorobenzene or toluene was dripped onto the center of the sample 30 s prior to the end of the program. The perovskite-precursor coated substrate was heated and dried on a hot plate at a temperature 50-140 °C for 10 min.

The carbon electrodes were prepared by doctor-blade coating a low-temperature conductive carbon ink (bought from Shanghai Materwin New Materials Co., Ltd) on the grown perovskite absorber, followed by drying at 100 °C for 30 min. For comparison, spiro-OMeTAD was deposited on the perovskite absorber, and the metal cathode around 100 nm was deposited on the spiro-OMeTAD HTM layer by thermal evaporation under the base pressure of 6 ´10-4 Pa [31, 32].

 

2.3 Characterization

 

The morphologies of the perovskite absorbers using different post-heating temperatures, and various drop-casting solvents were characterized by field emission scanning electron microscope (FESEM, Hitachi S4800). The structures of the formed perovskite absorbers were identified by X-ray diffractometer (XRD, Bruker D8 Davinci instrument, Cu-Kα: λ=0.15406 nm). Photocurrent density–voltage (J-V) measurements were performed using an AM 1.5 solar simulator equipped with a 1000 W Xenon lamp (Model No. 91192, Oriel, USA). The solar simulator was calibrated by using a standard Silicon cell (Newport, USA). The light intensity was 100 mW cm-2 on the surface of the test cell. J-V curves were measured using a computer-controlled digital source meter (Keithley 2440) with the reverse direction. During device photovoltaic performance characterization, a metal aperture mask with an opening of about 0.09 cm2 was used.

 

3 Results and Discussion

 

Initially, we investigated the effect of drop-casting solvent on the morphology of CH3NH3PbI3 perovskite film grown on the porous TiO2 via the solvent-engineering method. The post-heating temperature for the CH3NH3PbI3 absorber is 100 °C. Figure 1 shows optical and SEM images of CH3NH3PbI3 layers fabricated without drop-casting solvent or with chlorobenzene or toluene as a drop-casting solvent. For the sample without using drop-casting solvent, non-homogeneous perovskite film was formed, rather large branch-like grains with a significant portion of the substrate ( the p-TiO2 layer) being exposed without CH3NH3PbI3 coverage are seen in the Fig. 1a, b, which is in accordance with previous observations [5, 33]. A higher magnification SEM image of these large branch-like grains reveals that the grain structure consists of crystals with sizes of 50-400 nm as shown in the inset of Fig. 1b. Crystals of semiconductor perovskite CH3NH3PbI3 with a wide range of sizes were also reported when the perovskite was deposited by a single-step spin-coating from a solution of CH3NH3I and PbI2 in GBL or N,N-dimethylformamide (DMF) [4]. When toluene was used as a drop-casting solvent, the large branch-like grains disappeared in the optical image in Fig. 1c,and some smaller (35-140 nm) and more uniform crystals started to form, leading to reduced pinhole sizes and enhanced surface coverage of perovskites as shown in Fig. 1d. In contrast, when chlorobenzene was used as a drop-casting solvent, the mesoporous TiO2 layer was covered with interconnected crystals with a full surface coverage as clearly shown in the inset of Fig. 1f. The CH3NH3PbI3 films produced by the addition of chlorobenzene are composed of sub-micron (100-550 nm) sized grains, which are obviously larger than those in the absorber layer using toluene as drop-casting solvent. For the former, the top surface exhibits a dense-grained morphology. The differences in the surface coverages of perovskite films on the p-TiO2 layer likely affect the device characteristics [29, 34]. The cross section FESEM image of the PSCs is shown in Fig. 1g. The formed carbon film is very thick compared to the other functional layers in the device. The thickness of the carbon layer is about 11.6 mm. Its sheet resistance is about 14.6 W sq-1. The carbon film achieved good and tight adhesion to the underlayered CH3NH3PbI3 and provided complete coverage over the absorber.

Optical and SEM images of CH3NH3PbI3 layers fabricated without drop-casting solvent (a, b) or with toluene (c, d) or chlorobenzene (e, f) as a drop-casting solvent. (g) Cross section FESEM image of the PSCs.

Optical and SEM images of CH3NH3PbI3 layers fabricated without drop-casting solvent (a, b) or with toluene (c, d) or chlorobenzene (e, f) as a drop-casting solvent. (g) Cross section FESEM image of the PSCs.

The XRD patterns of the above three samples are shown in Fig. 2. The red, blue and purple curves exhibit XRD spectra measured from CH3NH3PbI3 layers without drop-casting solvent and with chlorobenzene or toluene as the drop-casting solvent, respectively. Diffraction peaks observed at 14.02°, 28.32°, 31.76°, 40.46°, and 43.02° correspond to the reflections from (110), (220), (310), (224), and (314) crystal planes of the tetragonal perovskite structure, respectively [35, 36]. It was observed that compared to the case of the pristine absorber without drop-casting solvent modification, intense diffraction peaks on both the (110) and (220) facets became significantly enhanced with the addition of chlorobenzene, indicative of the improvements in the crystalline property of the CH3NH3PbI3 film. Also, the optical and SEM images indicate the enlarged crystalline domains in the lateral direction with the morphology evolution from branch-like to plate-like (Fig. 1). However, compared to the case without drop-casting solvent, both (110) and (220) peaks became obviously reduced with the addition of toluene, which can be attributed to the reduced quality of the perovskite crystals in the latter as shown in inset of Fig. 1b, d.

To investigate the effect of the additional solvent on the HTM-free device performance, carbon electrode based PSCs were fabricated without drop-casting solvent or with chlorobenzene or toluene as a drop-casting solvent. The post-heating temperature for the CH3NH3PbI3 absorber is 100 °C. For each device fabrication condition, 8-12 PSCs were fabricated in an identical manner. The average device characteristics are shown in Fig. 3. The corresponding photovoltaic parameters including short circuit current density (JSC), open circuit voltage (VOC), PCE and fill factor (FF) are listed in Table 1. As shown in Fig. 3 and Table 1, the PSC with the pristine perovskite film showed a JSC of 16.10 mA cm-2, a VOC of 0.77 V and a FF of 0.46, therefore an overall PCE of 5.70 %. By using toluene droplets, the fabricated device showed enhanced VOC of 0.80 V, FF of 0.55 and similar JSC of 16.31 mA cm-2, and thus, improved PCE of 7.17 %, leading to an about 26 % enhancement of PCE. Remarkably, through introduction of chlorobenzene into the perovskite precursor layer, the PCE surged to 9.73 %. The introduction of chlorobenzene droplets resulted in simultaneous improvement of all the device parameters, e.g. JSC increased from 16.10 to 19.21 mA cm-2, VOC from 0.77 to 0.83 V, and FF from 0.46 to 0.61, and thus, an about 71 % enhancement of PCE in the device.

XRD spectra of CH3NH3PbI3 layers fabricated without drop-casting (DC) solvent or with chlorobenzene (CB) or toluene (TO) as a DC solvent.

XRD spectra of CH3NH3PbI3 layers fabricated without drop-casting (DC) solvent or with chlorobenzene (CB) or toluene (TO) as a DC solvent.

The morphology and the crystal structure evolution of the perovskite film with chlorobenzene droplet treatment shown in Figs. 1 and 2 could be attributed to formation of CH3NH3I-PbI2-DMSO intermediate phase during spin-coating from MAI, PbI2 and DMSO [26]. This process can be regarded as the transformation of PbI2-DMSO-MAI into MAPbI3, similar to the case with a toluene drop-casting. Jeon et al. [26] deposited high-quality CH3NH3Pb(I1-xBrx)3 (x=0.1-0.15) perovskite layers through use of a combination of DMSO/GBL followed by a toluene drip. They found that the spin-coated PbI2-DMSO-MAI intermediate phase possessed an extremely uniform and flat morphology, the intermediate phase was partly transformed into perovskite phases at 100 °C, and completely transformed into CH3NH3Pb(I1-xBrx)3 took place at an annealing temperature of 130 °C. But, from Fig. 2, pure perovskite phases were obtained at 100 °C for both chlorobenzene and toluene treatment in our synthesis system. Our results are in accordance with those reported in Ref. 27. High-quality CH3NH3PbI3 films were obtained for a fast deposition-crystallization procedure at 100 °C with the assistance of CB in the latter.

J-V curves for carbon based solar cells with CH3NH3PbI3 layers fabricated without drop-casting (DC) solvent or with chlorobenzene (CB) or toluene (TO) as a DC solvent.

J-V curves for carbon based solar cells with CH3NH3PbI3 layers fabricated without drop-casting (DC) solvent or with chlorobenzene (CB) or toluene (TO) as a DC solvent.

Thermal energy directly dominates the thermodynamics of the crystalline perovskite film formation in solution processes. Controlling the thermal annealing process for the perovskite precursor film is a key to achieving high performance [17, 34]. In this work, we found that annealing temperature played an important role in the quality of the perovskite absorber. Figure 4 shows XRD patterns of the perovskite films grown on the p-TiO2 via the solvent-assisted method with different annealing temperatures. The CH3NH3PbI3 layers were fabricated with chlorobenzene treatment. The post-heating temperature was changed from 50-140 °C. When a low annealing temperature of 50 °C was used, XRD peaks at low angles of 7.21 ° and 9.17° can be attributed to the MAI-PbI2-DMSO intermediate phase in the film [26]. When the annealing temperature was increase to 70 °C, only a very weak peak at 9.17° was observed. When the annealing temperature was equal to or greater than 100 °C, XRD peaks at the low angles disappeared completely. Besides, when the annealing temperature was equal to or greater than 70 °C, peaks at 14.02°, 28.32°, 31.76°, 40.46°, and 43.02°, corresponding to the reflections from (110), (220), (310), (224), and (314) crystal planes of the tetragonal perovskite structure, respectively [35, 36], was detected. The XRD results indicated that the MAI-PbI2-DMSO intermediate phase was fully transformed into perovskite phases at 100 °C for our pure halide material system with chlorobenzene drop-casting. Moreover, a small peak at 12.66°, which is associated with the PbI2 film, is visible in the XRD pattern for the absorber formed at 120-140 °C. The intensity of this PbI2 phase peak increased with the temperature rising from 120 to 140 °C, while the perovskite phase peaks decreased with the temperature increase. It is worth mentioning that annealing at 120 °C brought about majority conversion of the precursor film to the active perovskite phase and retention of a small fraction of PbI2, as exhibited by the small peak at 12.66°. Although this peak expresses incomplete conversion of the PbI2-DMSO-MAI film to active perovskite phase, the presence of a little residual PbI2 has been found to be advantageous for device performance [37, 38], likely due to passivation of surface and grain boundary states [37].

XRD patterns of the perovskite films grown on the p-TiO2 via chlorobenzene drop-casting at different annealing temperatures of 50-140 °C.

XRD patterns of the perovskite films grown on the p-TiO2 via chlorobenzene drop-casting at different annealing temperatures of 50-140 °C.

Figure 5 shows optical and SEM images of the perovskite films grown on the p-TiO2 via the solvent-assisted method with various annealing temperatures of 120-140 °C and chlorobenzene drop-casting. Annealing at 120 °C resulted in the formation of plate-like, uniform and well crystallized perovskite layer. The entire layer is composed of homogeneous, interconnected and perfectly crystallized crystals with 100 % surface coverage atop the p-TiO2 shown in the inset of Fig. 5b. The perfect absorber layer is composed of sub-micron (250-750 nm) sized grains, which are more uniform and larger than those in the CH3NH3PbI3 film formed with chlorobenzene treatment and annealing temperature of 100 °C shown in Fig. 1e, f.  When the annealing temperature increased further from 120 °C, however, the quality of the resulted absorber degraded and the surface coverage of perovskites decreased. At 130 °C, the grown absorber layer is composed of sub-micron sized grains with some portion of the p-TiO2 layer being exposed without CH3NH3PbI3 coverage as clearly shown in the inset of Fig. 5d. Annealing at 140 °C led to larger pinhole sizes and lower surface coverage of perovskites as exhibited in the inset of Fig. 5f.

Optical and SEM images of the perovskite films grown on the p-TiO2 via chlorobenzene drop-casting at different annealing temperatures. (a, b) 120 °C, (c, d) 130 °C and (e, f) 140 °C.

Optical and SEM images of the perovskite films grown on the p-TiO2 via chlorobenzene drop-casting at different annealing temperatures. (a, b) 120 °C, (c, d) 130 °C and (e, f) 140 °C.

Photovoltaic performance of solar cells with CH3NH3PbI3 layers fabricated via assistance of chlorobenzene at different annealing temperatures of 120-140 °C.

Photovoltaic performance of solar cells with CH3NH3PbI3 layers fabricated via assistance of chlorobenzene at different annealing temperatures of 120-140 °C.

To investigate the effect of annealing temperature on the HTM-free device performance, PSCs were fabricated with CH3NH3PbI3 absorbers obtained via drop-casting chlorobenzene and annealing at different temperatures from 100 to 140 °C. For each temperature, 8-12 PSCs were fabricated in an identical manner. The average device characteristics are presented in Fig. 6, the corresponding photovoltaic parameters of which are summarized in Table 2. A clear correlation was observed between the annealing temperature of the perovskite and the photovoltaic performance of the device. Samples fabricated with 120 °C gave the highest PCE, 11.44 %, as a result of the JSC, VOC and FF, 21.43 mA cm−2, 0.89 V, 0.60, respectively. Devices grown with 130 °C also exhibited a quite high PCE of 10.21 %, as a result of the JSC, VOC and FF, 19.29 mA cm−2, 0.84 V, 0.63, respectively. But, as the annealing temperature was increased further above 130 °C, the JSC, VOC and FF values considerably decreased, leading to a dramatic fall in PCE. The best photovoltaic performance of the device with 120 °C can be mainly associated with the highest crystalline, morphological and surface coverage quality of its absorber shown in Figs. 4 and 5. These experimental observations are consistent with the results reported in refs. [17, 34]. The corresponding monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra of these four devices are shown in Fig. 7. Aside from the PSCs with 140 °C, another three exhibited broad and efficient photoelectric conversion covering the range of visible light. Especially, the devices with 120 and 130 °C achieved much higher IPCEs than the cell with 140 °C over the whole spectral range between 300 and 800 nm, matching the difference in photocurrents obtained for these devices shown in Fig. 6 and Table 2.

J-V curves for the solar cells with CH3NH3PbI3 layers fabricated via chlorobenzene drop-casting at different annealing temperatures of 120-140 °C.

J-V curves for the solar cells with CH3NH3PbI3 layers fabricated via chlorobenzene drop-casting at different annealing temperatures of 120-140 °C.

IPCE spectra of the solar cells with CH3NH3PbI3 layers fabricated via chlorobenzene drop-casting at different annealing temperatures of 120-140 °C.

IPCE spectra of the solar cells with CH3NH3PbI3 layers fabricated via chlorobenzene drop-casting at different annealing temperatures of 120-140 °C.

Moreover, the series resistance (RS) and the shunt resistances (RSH) were estimated from the J–V characteristics shown in Figs. 3 and 6. The RS and RSH data are summarized in Tables 1 and 2. For solar cells, maintaining the RS as low as possible is vitally important because large RS will decrease JSC, VOC, FF, and consequently PCEs [31, 39, 40]. For the devices with 100 °C, the RS from the PSC with addition of chlorobenzene is 73 W cm2, which is smaller than 121 or 129 W cm2 from the device with introduction of toluene or without drop-casting solvent, respectively. In the case of the chlorobenzene treatment, when the temperature increased from 100 to 130 °C, the RS was kept at low values about 70 W cm2. But, when the temperature increased up to 140 °C, the RS of the device rose to 293 W cm2. The low RS from the device with addition of chlorobenzene and annealing temperatures of 100-130 °C is due to small contact resistance and low bulk resistance of the high-quality perovskite layer, indicating that high currents can flow through the cell at low applied voltages [39]. Typically, the shunt resistance (RSH) is due to p-n junction non-idealities and impurities near the junction, which bring about partial shorting of the junction, especially near cell edges. The RSH must be higher to avoid current loss at the junction [40], dwindling the photocurrent and consequently the solar cell performance. It has been diffusely reported that the pinholes formed in the solution-processed CH3NH3PbI3–xClx absorbers can cause direct contact of the p-type spiro-OMeTAD and the TiO2 compact layer, leading to a shunting path that is probably partially responsible for the low fill factor and open-circuit voltage in devices [5, 17]. In the case of our HTM-free PSCs with 100 °C shown in Table 1, the pinhole or voids in the capping layer on the p-TiO2 layer caused the least RSH, resulting in the lowest FF and VOC in the device with pristine absorber, and induced the second least RSH, leading to the second lowest FF and VOC in the device via dropwise toluene application. The smoother perovskite layer due to introducing toluene or chlorobenzene into the spin-coated perovskite precursor layer from the mixed solution of CH3NH3I, PbI2, GBL and DMSO decreased the series resistance and increased the shunt resistance of cell, resulting in the higher photovoltaic effects (Table 1). In the case of different annealing temperatures with the chlorobenzene treatment shown in Table 2, as the temperature increased from 100 to 130 °C, the shunt RSH had the higher and higher resistive value, but, when the temperature increased up to 140 °C, the pinhole or voids in the capping layer on the p-TiO2 layer caused the least RSH, resulting in the lowest FF and VOC in the device. Therefore, the improved device performance can be associated with the evolved and promoted morphological and crystalline properties of CH3NH3PbI3 film by use of chlorobenzene or toluene droplets and optimized annealing temperature, as exhibited by optical, SEM and XRD measurements in Figs. 1, 2, 4, and 5. The improved device performance unambiguously verified the significance of chlorobenzene droplets and annealing temperature on PSC optimization. The possible mechanisms for improving the quality of the perovskite absorber layer with drop-casting solvent and enhancing the resulted PSC performance are further explored below.

Stability profile (PCE) of non-sealed FTO/c-TiO2/p-TiO2/CH3NH3PbI3 /spiro-MeOTAD/Ag (squares) and FTO/c-TiO2/p-TiO2/CH3NH3PbI3/carbon (triangles) perovskite solar cells. The devices were kept in a dry cabinet of electronics without nitrogen (10 % RH, room temperature).

Stability profile (PCE) of non-sealed FTO/c-TiO2/p-TiO2/CH3NH3PbI3 /spiro-MeOTAD/Ag (squares) and FTO/c-TiO2/p-TiO2/CH3NH3PbI3/carbon (triangles) perovskite solar cells. The devices were kept in a dry cabinet of electronics without nitrogen (10 % RH, room temperature).

The boiling points of the traditional solvents used for growing perovskite absorbers are relatively high, having values of 153 °C and 204-205 °C for N,N-dimethylmethanamide (DMF) and GBL, respectively. Such relatively high boiling points can result in the formation of large-area thickness variations or shrinkage/de-wetting of the casting precursor solution due to the relatively prolonged drying times of solution-coated films. In our work, the proposed solvent- assisted technology involved spin-coating the mixed solution of CH3NH3I, PbI2, GBL/DMSO, followed by drop-casting chlorobenzene or toluene while spinning. Toluene (boiling point: 111 °C, vapor pressure: 22 mmHg) and chlorobenzene (boiling point: 132 °C, vapor pressure: 11.8 mmHg) have much lower boiling point and tremendously higher vapor pressure than DMSO (boiling point: 189 °C, vapor pressure: 0.6 mmHg) and GBL (boiling point: 204-205 °C, vapor pressure: 1.5 mmHg) at room temperature. Considering the physical properties of the solvents, at the beginning stage of the spin coating process, the film was composed of MAI and PbI2 dissolved in the DMSO/GBL solvent mixture, the precursor film continued to thin because liquid flowed radially owing to the action of centrifugal force, and solvent evaporation was neglected. While in the intermediate stage, the composition of the film was concentrated by the evaporation of GBL due to its higher vapor pressure than DMSO. Then, the introduction of toluene or chlorobenzene droplets with greatly higher vapor pressure during the second spinning stage induced the immediate freezing of the constituents on spinning and the rapid formation of the MAI-PbI2-DMSO phase, producing a full and even precursor layer. The use of DMSO helped to retard the rapid reaction between the inorganic PbI2 and CH3NH3I components of perovskite during the evaporation of solvents in the spin-coating process because DMSO has stronger coordination ability with PbI2 than that of the usually used DMF [41]. At last, the greatly homogeneous and flat precursor film was converted into a pure crystalline CH3NH3PbI3 perovskite layer after annealing at 100 °C. As displayed in Figs. 1-3 and Table 1, CH3NH3PbI3 films via the dropwise chlorobenzene application showed better morphological, crystalline and photovoltaic properties than those of perovskite layers using toluene droplet treatment. For a highly volatile solvent such as toluene, one can expect to have very significant evaporation during the spin-off stage. The strong evaporation of the casting toluene solvent possibly affects fluid rheology and vice versa [42, 43], tending to give relatively rough precursor films. In contrast, chlorobenzene has a lower evaporation rate due to its relatively high boiling point and lower vapour pressure compared to those of toluene, which should be a benefit to form a flatter, smoother and more uniform MAI-PbI2-DMSO intermediate phase layer. The higher-quality intermediate phase layer resulted in more highly crystalline and more homogeneous CH3NH3PbI3 perovskite capping layer atop the mp-TiO2 after heating treatment at 100 °C, which caused the lower series resistance, the higher shunt resistance and better photovoltaic performance in the perovskite solar cell (Fig. 3 and Table 1). The fabricated HTM-free perovskite solar cells based on chlorobenzene treatment demonstrated PCE of 9.73 %, which is 71 and 36 % higher than that of the control device fabricated from the pristine perovskite film or via toluene droplet treatment, respectively. The improved device performance unambiguously verified the significance of chlorobenzene droplets on HTM-free PSC optimization. After further optimizing the annealing temperature, the HTM-free perovskite solar cells based on the incorporation of chlorobenzene achieved a PCE of 11.44 %.

Photovoltaic performance of solar cells with CH3NH3PbI3 layers fabricated without assisted solvent or with chlorobenzene or toluene as an assisted solvent.

Photovoltaic performance of solar cells with CH3NH3PbI3 layers fabricated without assisted solvent or with chlorobenzene or toluene as an assisted solvent.

Moreover, the fabricated HTM-free solar cells are significantly more stable than the spiro-OMeTAD HTM based devices with Ag electrode shown in Fig. 8. The PCE of carbon based device was over 8 % after 120 days, while the efficiency of the HTM based cell dropped to 3 % only after only 5 days. We hope that our findings can provide better understanding of the crystalline perovskite film formation in solvent- assisted processes and make a contribution for the development of low-cost and stable perovskite solar cells.

 

4 Conclusions

 

The type of the drop-casting solvent and annealing temperature employed during the preparation of CH3NH3PbI3 films via a solvent-assisted process have considerable impact on the resulted absorber morphologies, crystalline structures and device photovoltaic performances. The advantages using chlorobenzene as an assisted solvent become apparent upon comparing the J-V curves and PCEs of the carbon based HTM-free CH3NH3PbI3 devices prepared using CB or TO as a DC solvent or without DC solvent. The CH3NH3PbI3 films grown using CB show better morphological, surface coverage and crystalline properties than those of perovskite layers formed by addition of TO during the processing under the same conditions. The pristine perovskite layers without using DC solvent exhibit the even poorer morphological and crystalline properties. The HTM-free devices based on CH3NH3PbI3 fabricated by incorporation of chlorobenzene show superior PCEs. The effects of the annealing temperature on CH3NH3PbI3 film morphology, crystal structure, and the solar cell performance were also investigated for the chlorobenzene assisted process. High-efficiency carbon based HTM-free solar cells with a PCE of 11.44 % were produced by using an optimized annealing temperature of 120 °C. This work provides an effective protocol for fabricating efficient, simple, stable and low-cost inorganic-organic hybrid heterojunction solar cells.

 

Acknowledgements

 

This work was supported by National Natural Science Foundation of China (No. 11274119, 61275038).

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[24] Y.-J. Jeon, S. Lee, R. Kang, J.-E. Kim, J.-S. Yeo, S.-H. Lee, S.-S. Kim, J.-M. Yun, D.-Y. Kim, Planar heterojunction perovskite solar cells with superior reproducibility. Sci. Rep. 4, 6953 (2014). doi:10.1038/srep06953

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Citation Information

Chenxi Zhang, Yudan Luo, Xiaohong Chen, Yiwei Chen, Zhuo Sun, Sumei Huang, Effective Improvement of the Photovoltaic Performance of Carbon- Based Perovskite Solar Cells by Additional Solvents. Nano-Micro Lett. 8(4), 347-357 (2016). http://dx.doi.org/10.1007/s40820-016-0094-4

History

Received: 22 March 2016 / Accepted: 10 May 2016


Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    Effective Improvement of the Photovoltaic Performance ofCarbon- Based Perovskite Solar Cells by Additional Solvents

  • Author: Chenxi Zhang, Yudan Luo, Xiaohong Chen, Yiwei Chen, Zhuo Sun, Sumei Huang
  • Year: 2016
  • Volume: 8
  • Issue: ?
  • Journal Name: Nano-Micro Letters
  • Publisher: OPEN ACCESS HOUSE SCIENCE & TECHNOLOGY
  • ISSN: 2150-5551
  • URL: http://dx.doi.org/10.1007/s40820-016-0094-4
  • Abstract:

    A solvent assisted methodology has been developed to synthesize CH3NH3PbI3 perovskite absorber layers. Itinvolveduse of a mixed solvent of CH3NH3I, PbI2, g-butyrolactone and dimethylsulphoxide (DMSO) followed by addition of chlorobenzene.The methodproduced ultra-flat and denseperovskite capping layers atop mesoporous TiO2 films,enabling a remarkable improvement on the performance of free hole-transport material (HTM) carbon electrode based perovskite solar cells.Toluene was also studied as an additional solvent for comparison. At the annealing temperature of 100 °C, the fabricated HTM-free perovskite solar cells based on drop-casting chlorobenzene demonstrated power conversion efficiency (PCE) of 9.73 %, which is 36 % and 71 % higher than those fabricated from the perovskite films using toluene or without adding an extra solvent, respectively. The interaction between the PbI2-DMSO- CH3NH3I intermediate phase and the additional solvent was discussed. Furthermore, the influence of the annealing temperature on the absorber film formation, morphology, and crystalline structure was investigated and correlated with the photovoltaic performance. Highly efficient, simple and stable HTM-free solar cells with PCE of 11.44 % were achieved by utilizing the optimum perovskite absorbers annealed at 120 °C.

  • DOI: 10.1007/s40820-016-0094-4