J Solid State Electrochem DOI 10.1007/s10008-017-3749-2
ORIGINAL PAPER
Improving the performance of polymer solar cells by efficient optimizing the hole transport layer-graphene oxide Xinxin Huang 1,2 & Huangzhong Yu 1 & Zuping Wu 1,2 & Yanping Li 1,2
Received: 17 May 2017 / Revised: 21 August 2017 / Accepted: 22 August 2017 # Springer-Verlag GmbH Germany 2017
Abstract Graphene oxide (GO) materials have emerged as a promising alternative for hole transport layer (HTL) in polymer solar cells (PSCs) due to their unique structures and properties. However, insulating properties and eco-contaminative production of GO still need to be solved. Here, we report on the preparation of GO through an improved Hummers method without using NaNO3, which is an eco-friendly option because it avoids the emissions of NO2 and N2O4 toxic gases. Subsequently, the GO as HTL in PSCs is reduced by simple heat treatment of different temperatures in air, and the performance of devices is obviously improved. The FT-IR and XPS spectra show oxygenated functional groups in GO thin films are gradually removed with the increase of annealing temperature, which restores sp2 hybridized graphitic structure, and makes the GO thin films more conducive to the charge transfer. The highest power conversion efficiency of PSCs based on the P3HT: PC71BM system with GO as HTL is 3.39%, which approaches that of PSCs with PEDOT: PSS as HTL (3.41%). Moreover, the devices with annealed GO as HTL have better stability compared to devices with PEDOT: PSS. Keywords Graphene oxide . Polymer solar cells . Hole transport layer . Heat treatment Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-017-3749-2) contains supplementary material, which is available to authorized users. * Huangzhong Yu
[email protected]
1
School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510640, China
2
School of Materials Science & Engineering, South China University of Technology, Guangzhou 510640, China
Introduction In recent decades, energy shortage and environmental pollution are more and more serious, which is ringing the alarm bell to humanity. These are due to the dramatic increase of the consumption of fossil energy with the large increase of world population, which leads to a large number of greenhouse gases and produces a lot of waste water. Currently, clean and renewable energy are being sought by people. The large amounts existence of solar energy makes it become a most promising candidate [1–5]. The solar cells are mostly silicon based solar cells. However, the 3rd generation solar cells including organic solar cells, dye-sensitized solar cells, quantum dot solar cells and perovskite solar cells are gradually being widely studied, which is due to the solution processable 3rd generation solar cells presenting the possibility of large industrial production [1–4, 6, 7]. Polymer solar cells (PSCs) as a kind of organic solar cell have attracted vast attention for their potential applications as clean energy sources. The power conversion efficiencies (PCE) of PSCs have a rapidly improvement in recent years and exceed 11% reported at present [8]. This can be attributed to the tremendous development of conjugated active layer materials [9, 10], device structures [11, 12], and modified electrode interface materials [13, 14]. The modified electrode interface layers play an important role in charge separation and charge collection for PSCs [15, 16]. For increasing the efficiency and the stability of PSCs, a hole transport layer (HTL) is inserted between the anode and active layer, and an electron transport layer (ETL) is induced between the cathode and active layer [17, 18]. PEDOT: PSS is a traditional HTL material, which is due to its good electrical properties and excellent solution process ability. However, it suffers from strong acidity (PH = 1) and hygroscopicity features which will seriously affect the longterm stability of PSCs [19, 20]. Therefore, many new HTL
J Solid State Electrochem
materials are researched to replace PEDOT: PSS. Metal oxides such as V2O5, NiO and MoO3 are one of them and have been widely investigated as HTL in PSCs [21–25]. However, metal oxides mostly need to be thermally deposited under high vacuum, which is adversed to large scale production in PSCs [26]. Currently, graphene has become a hot spot of scientific research, which is due to its unique two-dimensional (2D) single atomic-thick sp2-hybridized carbon sheet structure and excellent electrical conductivity [5, 15, 18, 26]. However, the graphene sheets are insoluble in water, which are incompatible with the solution-processable of PSCs [19, 27]. To solve these problems, the solution-processable graphene oxide (GO) as an HTL in PSCs has been researched. GO is an oxidized graphene sheet, and there are a lot of oxygen functional groups on its basal planes (e.g. epoxy groups and hydroxyl) and edges (carboxyl), which makes it easy functionalized. Functionalized oxygen groups can improve solubility of GO, but the C-O bonds disrupt the sp2 conjugation of the hexagonal graphene sheets and make GO an insulator, which limits its application in PSCs [28]. In order to make GO from insulator to semiconductor, Jin-Mun Yun et al. reported that using chemical reduction methods removed the oxygen functional groups of GO to increase the electric conductivity [26]. Although the chemical reduction methods have been supposed to a promising route for producing solutionprocessable reduced grapheme oxide (RGO), making it easy to form a film on flexible substrate. Moreover, these methods also allow to adjust band gap of RGO by chemical treatment [29–31]. However, the manufacturing process of RGO by chemical methods is expensive and toxic, which is contrary to the concept of environmentally friendly development [26]. Therefore, the preparation of RGO by thermal reduction [32], electron beam-induced reduction [33] and γ-ray irradiation [34] reduction has been researched to replace the chemical reduction methods. In this context, we synthesized GO using an improved Hummers method without using NaNO3. Then, the GO with various annealing temperatures as HTL in PSCs was investigated, and the influence of GO thickness and annealing time on the performance of PSCs was simultaneously researched. We observed that the thickness, annealing time and annealing temperature had significantly affected on the electrical conductivity of GO films. Here, we further went deep into investigating the influence of annealing temperature on the performance of GO films. Annealed GO films were used as a novel class of HTLs to improve charge-collection and modify the electrode surfaces. We found that the device with GO film spin-coated in 2000 rpm and then annealed at 280 °C for 30 min in air as HTL had the highest PCE. The performance of device based on P3HT:PC71BM (3.39%) and based on PTB7:PC71BM (7.62%) with the optimal processing conditions of GO film as HTL approached that of PEDOT: PSS as HTL (3.41%, 7.66%). Moreover, when the GO film was
inserted as HTL, the PSCs exhibited long-term stability. Thus, the annealed GO films as HTLs in PSCs presented the advantages of facile solution-processable, low-cost and good interfacial characteristics and could be a generally applicable hole transport layer for ambient stable and high-efficiency PSCs.
Experimental section Regioregular electron-donor poly[[4,8-bis[(2-ethylhexyl) oxy]benzo [1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-(2ethylhexy) carbonyl] thieno[3,4-b] thiophenediyl]] (PTB7), poly(3-hexyltthiophene (P3HT) and acceptor [6, 6]-pheny C71-butyric acid methy ester (PC71M), indene-C60bisadduct (ICBA) were purchased from Luminescent Technology, Inc. And the PEDOT: PSS (Clevios 4083) solution was purchased from H.C. Starck. Preparation of graphene oxide (GO) GO was prepared from natural graphite powder according to a Hummers’ method with an improvement of removing NaNO3 from the reaction process [35]. In more detail, a capacity of 500 ml of round bottom flask containing 68 ml of H2SO4 was placed in ice bath (≤10 °C). Natural graphite power (3.0 g) was added to H2SO4 and strongly stirred at 5 °C for 1 h to make the reactants adequately mixing. KMnO4 (12.0 g) was divided into four times to add in mixed solution over a period of 1 h, maintaining vigorous stirring. The suspended solution was continuously left in ice bath for 1 h to mix reaction before being heated to 35 °C for 3 h. At the end 30 min of above 3 h, 120 ml of deionized water was slowly added into the pasty mixture. Sequentially, the temperature started to fast rise to 95 ° C and continued to react for 15 min, and then 360 ml deionized water and 20 ml 3% H2O2 were poured into the reaction mixture for removing the residual KMnO4, and the color of the solution turned from dark brown to yellow. Finally, the resulting suspension was washed with 5% HCl aqueous solution and deionized water for several times to remove metal ions and make PH of the solution approximate neutral, and the product had been freeze-dried in a freeze drier for 3 days before being used. Fabrication of polymer solar cells devices Normal architecture BHJ PSCs were fabricated on ITO-coated glass substrates (10 Ω sq.−1). Before the preparation of devices, the substrates were cleaned in a sonication bath in detergent, deionized water, ethanol, acetone, and isopropyl alcohol, respectively. Then, the ITO glasses were dried for a few hours in drying oven. After the cleaning and drying processes, the surface of the ITO was treated with O2 plasma treatment for 10 min to remove the organic contamination and increase
J Solid State Electrochem
hydrophilicity. Then the PEDOT: PSS was spin-coated on the ITO glass at 3500 rpm for 40 s and annealed at 150 °C for 15 min. The aqueous solution of GO (1 mg ml−1) was spincoated on cleaned ITO substrates at different spin-coated speeds from 1000 to 4000 rpm for 40 s to achieve appropriate thickness of GO films, and then different annealing temperatures (No annealing, 100, 180, 230, 280 and 320 °C) were used to form GO films under 2000 rpm spin-coated speed and 30 min annealing time. Subsequently, further optimization of reaction conditions, GO were annealed for different times (10, 20, 30 and 60 min) in air, respectively. Then, prepared ITO/HTL substrates were transferred to a N2-filled glovebox (O2 < 0.1 ppm, H2O < 0.1 ppm). Both 1, 2-dichlorobenzene solution containing P3HT:PC71BM (1:1, w/w) and a mixed solvent of chlorobenzene and 1, 8-diiodooctane (97:3, v/v) containing PTB7:PC 71BM (1:1.5, w/w) were stirred in glovebox at 50 °C overnight. After cooling to room temperature, these solutions were filtered through a 0.2 μm polytetrafluoroethylene (PTFE) filter before used. The active layers were spin-coated on HTLs in a glovebox. The thickness of P3HT: PC71BM and PTB7:PC71BM layers are about 200 and 100 nm, respectively. The Ca (10 nm) and Al (100 nm) electrode were deposited continuously in a deposition chamber under a basic vacuum pressure lower than 3 × 10−4 Pa. The shadow mask was used during thermal evaporation to define the active area of 0.16 cm2. All devices were unencapsulated. The structure of the PSC devices and GO used in the PSC devices are shown in Fig. 1. Characterization of GO and GO films The micromorphology of GO was observed on a EV018 scanning electron microscope (SEM, Carl Zeiss Jena, Germany) at an acceleration voltage of 10.0 kV. X-ray diffraction (XRD) was performed on a Bruker D8 Advance Diffractometer using Cu Kα radiation source (λ1 = 1.54060 Å, λ2 = 1.54439 Å) and aLynxEye-XE detector to attain the change of interplanar spacing. The weight change of GO with improving temperature was characterized by using Thermogravimetric analysis (TGA, NETZSCH TG 209F 3 TGA209F3A 0430 L). The Infrared absorption spectrum (FT-IR) was obtained by using a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) from 3800 to 800 cm−1 with a resolution of 4 cm−1. The Raman spectra was performed using a Raman spectrometer (inVia Reflex, inVia-58P056), the wavelength of focused excitation laser was 532 nm (2.33 eV). Chemical analysis of the samples was recorded on X-ray photoelectron spectroscopy (XPS, Kratos Axis Ulra DLD, UK) under conditions of AlKα monochromatic X-ray source and three electron takeoff angles (30°, 60° and 90°). Thickness of all active layer and GO films in I-V curve were measured by using alpha-step surface profiler. The current density–voltage (J–V) characteristics of the devices were performed using a computer
controlled Keithley 2400 source-meter under illumination with calibrated AM 1.5 G (100 mW cm−2) at room temperature. The fluorescence spectra (PL) of the P3HT films with different GO films as HTLs were measured by using a SPEX 1681 automated spectrofluorometer and the 330 nm excitation wavelength was used. The hydrophilic of annealed GO thin films was characterized by contact angle meter (DSA100, Germany) equipped with a video capture using 10 μl of water as probe liquid at room temperature.
Result and discussion Characterization of GO Fig. 2a shows the typical SEM image of GO. The sample was prepared by planting the freeze-dried of GO powder on conductive adhesive. The image indicates that large lateral size of GO is synthesized [36]. Furthermore, the thickness and morphology of a single layer of GO is shown in Fig. S1. It is estimated that the thickness of a single layer of GO is about 1.1 nm. XRD pattern is shown in Fig. 2b. It is clearly observed that a single strong peak in graphite powder sample appearing at 27.05°corresponds to an average interlayer separation of 3.29 Å. The graphite powder is oxidized to achieve GO, and 2θ peak of GO at 10.75°appears, corresponding a interlayer separation of about 8.22 Å. The interlayer separation value of the GO is about 2.5 times greater than that of the natural graphite powder. The different interlayer separation between natural graphite powder and GO is due to their different structure. Natural graphite powder composes of stacked a lot of monolayer planar sp2-hybridized carbon atoms and GO is an individual layer of graphite sheet with oxygenated functional groups such as hydroxyl, carboxyl and epoxy on its basal plane, which increases the interlayer separation of GO. This is consistent with other reports [37]. Fig. 2c is the TGA curve of GO. The curve shows that the weight loss under 100 °C (95.1% wt) is mainly caused by the evaporation of surface H2O molecules among GO sheets [38]. The distinct weight loss between 100 °C (95.1% wt) and 280 °C (53.7% wt) is attributed to the decomposition of less stable oxygenated functional groups on GO sheets (such as hydroxyl, aldehydes and carboxylic groups) and the evaporation of structure water in GO. A weak weight loss between 280 °C (53.7% wt) and 800 °C (44.6% wt) is thought to the removal of stable functional groups (mainly epoxides) [39]. Considering the withstand temperature of ITO and the TGA curves of GO, several appropriate temperatures are selected in the range of 100–320 °C
J Solid State Electrochem Fig. 1 a Scheme of the PSC devices structure used in this study, b chemical structure of GO used in the PSC devices
to research the performance of PSCs with different annealing temperatures GO films as HTLs. Characterization of GO films To confirm chemical functional groups of the GO thin films with various annealing temperatures, the FT-IR analysis was performed to characterize GO films on
quartz substrate. As can be shown in Fig. 3a, the highly broadened and intense peak of GO is observed in the range of 3000–3700 cm−1 when the GO films are not annealed, which is due to the stretching modes of trapped H 2 O, −COOH groups and -C-OH groups. H o w e v e r, t h e p e a k i n t e n s i t y o f G O i n 3 0 0 0 – 3700 cm−1 is gradually decreased with the increase of annealing temperature, and almost disappeared at
Fig. 2 a SEM morphology images of GO. b XRD pattern of GO and graphite. c TGA curve of GO
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Fig. 3 a FT-IR spectra of the GO thin films present the evolution of various functional groups by various annealing temperatures. b Raman spectra of GO films by various annealing temperatures in air
180 °C, which is due to the remove of trapped H2O, −COOH groups and -C-OH groups. The other vibrational modes of GO are also found in the spectra, which are -C = O (1670–1780 cm−1), −COOH (1620–1670 cm−1), the bending modes of H2O (1570–1620 cm−1), asymmetric vibrational stretching of C = C (1500– 1570 cm−1) and C-O-C (1248–1490 cm−1), separately. The range of 850–1500 cm−1 is usually divided into three regions including α region (900–1020 cm−1) for all ether derivatives, β region (1020–1248 cm−1) for ketonic species along with peroxides, pyran and γ region (1248–1490 cm −1) for epoxides. Moreover, the peak intensities of these oxygenated functional groups are also evidently reduced with the increase of thermal temperature. Thus, lots of oxygenated functional groups
in β region and γ region between the carbon basal sheets in GO are removed by thermal reduction, which makes the greatly improvement of GO conductivity [40, 41]. That’s going to be shown through the I-V curve in the following. Raman spectroscopy is a powerful tool to characterize carbonaceous materials and distinguishing the disorder in the crystal structures of carbon [42, 43]. The Raman spectra of GO films by different annealing temperatures are showed in Fig. 3b (All spectra are after the substrates reduced). All samples were recorded after dropping the aqueous dispersions (3 mg ml−1) on glass substrates and then annealed with different temperatures to evaporate the solvent. Measurements were carried out using 20X objective at 532 nm laser excitation. In the Raman spectra of GO in Fig. 3b, two prominent peaks of the D band and G band are clearly observed, which are located in approximately 1347 and 1587 cm−1, respectively. The D band peak arises from the structural defects created by the attachment of oxygen groups on the carbon basal plane, corresponding to the breathing mode of κ-point phonons of A1g symmetry. The G peak represents the first-order scattering of the E2g mode related to vibration of sp2-bonded carbon atoms [44–46]. Furthermore, the G band not only represents the size of the sp2 domain but also is used to the reference of D band. The relative intensity ratio of D band and G band (ID/IG) is used to measure the extent of disorder. According to the Raman spectra in Fig. 3b, the ID/IG ratios of GO thin films annealed by different temperatures are estimated from 0.65 at no annealing, 0.67 at 100 °C, 0.78 at 180 °C, 0.80 at 230 °C, and 0.81 at 280 °C to 0.89 at 320 °C. This result indicates that the size of sp2 domains is decreased by reduction, and the remove of oxygenated function in GO makes the smaller size and more numerous reduced GO presented. From Raman spectra’s analysis, we can speculate the lower performance in 320 °C for PSCs which due to the excessive defects in GO films and make the leakage current undue increase. To further survey the change of oxygenated functional groups in the annealed GO films, the X-ray photoelectron spectroscopy (XPS) is shown in Fig. 4. The GO thin films are prepared by dropping the GO (1 mg/ml) aqueous solution on ITO substrates. The binding energy (BE) obtained in the XPS analysis is corrected for specimen charge by referencing the C1s peak to 284.6 eV. Fig. 4a shows the full scan spectra of the annealed GO films, the characteristic peaks of the O1s (284.6 eV) and C1s (533.0 eV) levels are shown in the full spectra of the GO films, and the peaks of In atom (around 445.0 eV) are found in GO thin films unannealing, 100 °C annealing, and 320 °C annealing. We think that low temperature annealed GO thin films have the poor light absorption, which makes the XPS scanning light easily reach ITO substrate. So the characteristic peak at 445.0 eV of In atom in the ITO substrate can be discovered in Fig. 4a. Moreover, when the annealing is at 320 °C, the indium tin oxide partly
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Fig. 4
a Survey XPS full scan spectra of the GO films of various annealing temperatures. b Detail C 1s XPS spectra of the GO films of various annealing temperatures
decomposes and diffuses into the GO films, which results in the decrease of conductivity and the increase of surface roughness in annealed GO thin films. Fig. 4b depicts the detail C1s regions for the annealed GO thin films. In these spectra, C1s peak can be resolved into four peaks, corresponding to C-C bonding (284.6 eV), C-O bonding (286.1 eV), C = O bonding (287.7 eV) and O-C = O bonding (288.9 eV). Their peak positions are similar to the reported values [41, 47]. Here the change of C-O, C = O and O-C = O bonding peaks is mainly cared in the reduced GO thin films. It is observed that the intensity of the peaks of C-O, O-C = O and C = O is gradually decreased with the increase of annealing temperature from 100 °C to 180 °C. When the GO thin films are annealed at 230 °C, 280 °C, and 320 °C respectively, the intensity of the peaks corresponding to C-O, C = O, and O-C = O is rapidly decreased with the increase of annealing temperature. The result is in good agreement with FT-IR spectra. The detailed relative ratios of C and O in all the samples are calculated using area of the C and O as listed in Table 1. Performance of PSC devices with various annealed GO thin films as HTLs GO and annealed GO (RGO) with substantial sp3 fraction are used as HTL materials in devices, the performance of PSCs is sensitive to the GO film thickness. To investigate the effect of annealed GO films of various thicknesses as HTLs on the photovoltaic performance of PSCs, we selected the P3HT:PC71BM blended composite as the active layer and optimized the P3HT:PC71BM based devices fabricated with GO thin films as HTLs under various spin-coating speeds, the structure of the devices was ITO/HTL/P3HT:PC71BM/Ca/Al. For comparison, the PSCs without a HTL were also fabricated. Fig. 5a shows J-V curves of the P3HT:PC71BM based devices fabricated with GO thin films under various spincoating speeds and then annealed at 280 °C for 30 min. In the case of using the ITO without HTL, the device has a PCE of only 1.68% with low open-circuit voltage (Voc) of 0.51 V, short-circuit current density (Jsc) of 7.88 mA cm−2, and fill factor (FF) of 41.7%. By contrast, the performance of the devices with GO as HTL is sharply improved, the Table 1 Ratio values of C/O in GO thin films with various annealing temperatures Annealing temperature/°C
No Annealing
100 180 230 280 320
C/O
0.56
0.76 0.83 1.24 1.89 1.92
performance parameters are shown in Table 2. The devices with 2000 rpm GO as HTL show an optimal performance with a Jsc of 10.96 mA cm−2, a Voc of 0.59 V, a FF of 52.1% and an overall PCE of 3.37%. More than two times PCE enhancement of the devices with GO as HTL is achieved when compared with the devices with only ITO. With further increase of spin-coating speed, both the FF and the Jsc of the devices slowly decrease. This may be because overquick rotate speed leads to the poor film of GO, even the uncovered completely on ITO, which produces plenty of leakage current and not well ohmic contact. For the device with 1000 rpm GO as HTL, the devices have larger series resistance (Rs) of 13.62 Ω cm2 due to the thicker GO film compared to the 2000 rpm of spincoated, which will result in lower PCE (3.18%). In order to examine the influence of GO thin films annealed by various temperatures as HTLs on the performance of devices, the P3HT:PC71BM based devices with the optimal spincoating speed (2000 rpm) and annealing 30 min were prepared and the J-V characteristics of under illumination and in the dark were shown in Fig. 5b and c. According to the detailed performance data illustrated in Table 3, we can find that the annealing temperature has a great effect on the performance of PSCs, and the performance of the devices using the annealed GO thin films as HTLs improves with the improvement of heat treatment temperatures of GO thin films, the P3HT:PC71BM based devices using the annealed GO thin films as HTL exhibits an optimal PCE of 3.39% with a Voc of 0.59 V, a Jsc of 10.93 mA cm−2 and a FF of 52.6% at 280 °C, which is approach that using the PEDOT:PSS as HTL (PCE = 3.41%). According to the FI-IR and XPS spectra, we know that the more removal of trapped H2O molecular and oxygenated functional groups in the GO thin films with the increase of annealing temperatures, which restores sp2 hybridized graphitic structure, makes the GO thin films more conducive to the electron transfer, and the increase of conductivity of GO thin films (Fig. 6) helps the increase of Jsc and FF of the solar cells. That is also explained by the increase of Rsh and the decrease of Rs from no annealing to 280 °C in Table 3. However, the too high temperature (320 °C) annealing GO thin films leads to the reduction of PCE of PSCs. This result can be explained by the following several aspects. Firstly, the too high temperature of heat treatment increases the resistance of ITO, which will enhance the Rs of the solar cells and reduce the performance of the solar cells. The resistivity of ITO films is related with the creation of oxygen vacancies in the ITO films, annealed at 320 °C in air, the free oxygen in air will react with the ITO films. The reaction will decrease the oxygen vacancies and the carrier concentration, which leads to the increase of the resistivity of the ITO films [48]. Secondly, we know that the ITO will be decomposed by annealed at 320 °C, and then In atom diffuses to reduced GO thin films, above Fig. 4a also illustrates the phenomenon, which lowers conductivity of GO thin films and damage the performance of the PSCs.
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Fig. 5 a J–V curves of P3HT:PC71BM based devices fabricated with GO thin films under various spin-coating speeds. b J-V features of the P3HT:PC70BM based devices with different temperature annealing. c
J-V characteristics of corresponding b in dark state. d J–V characteristics of the P3HT:PC70BM based devices inserted GO thin films under various annealing time with 2000 rpm spin-coating
Thirdly, we learn that the ITO thin films annealed at 320 °C have more defects from above Fig. 3b, the removal of defective oxygen functional groups at high temperature of 320 °C induces the formation of topological defects on the basal plane resulted from the creation of CO and CO2 [49]. High temperature heat treatment also makes GO sheets curled, and increases the surface roughness of GO film. So increased resistance, added defects and improved surface roughness of GO/
ITO film at high temperature of 320 °C make that the hole cannot be well collected, which decreases the performance of the PSCs. To further optimize the device performance, different annealing times (10, 20, 30 and 60 min) was used to evaluate the performance of optimized P3HT:PC71BM based devices fabricated with GO (1 mg ml−1) under 2000 rpm spin-coating. Fig. 5d shows J-V curves of the devices under 100 mW cm−2
Table 2 Performance parameters of P3HT:PC71BM based devices fabricated with GO films under various spin-coating speeds
Spin-coating speeds
Jsc (mA cm−2)
Voc (V)
FF (%)
PCE (%)
Rsh (Ω cm2)
Rs (Ω cm2)
ITO only 1000 rpm 2000 rpm 3000 rpm 4000 rpm
7.88 10.83 10.96 9.05 8.26
0.51 0.58 0.59 0.57 0.56
41.7 50.6 52.1 50.3 46.8
1.68 3.18 3.37 2.59 2.16
186.94 178.67 293.88 386.10 326.09
18.74 13.62 11.47 14.60 17.60
J Solid State Electrochem Table 3 Performance parameters of P3HT:PC71BM based devices implanted GO thin films and PEDOT: PSS under various annealing temperatures for 30 min with the optimized spincoating speed of 2000 rpm
Jsc (mA cm−2)
Voc (V)
FF (%)
PCE (%)
Rsh (Ω cm2)
PEDOT:PSS
10.09
0.62
54.5
3.41
692.97
15.55
No Annealing 100
2.62 7.44
0.56 0.57
20.5 28.0
0.30 1.19
166.47 167.79
376.96 77.23
T (°C)
Rs (Ω cm2)
180
9.15
0.58
52.8
2.80
435.31
15.33
230
10.08
0.59
53.9
3.21
526.63
14.68
280 320
10.93 10.44
0.59 0.58
52.6 51.1
3.39 3.09
429.44 374.34
14.47 15.83
illumination (AM 1.5G) and the performance parameters are summarized in Table 4. The performance characteristics including the Jsc, Voc and PCE in PSCs are gradually increased with the longer annealing time in the beginning. However, when annealing reaches 30 min, the performance tends to be stable with the PCE 3.39%, and then the longer annealing time almost has no effect on the device performance.
Fig. 6 a PL spectra of various temperatures annealed GO films/P3HT and PEDOT: PSS/P3HT on ITO substrates. b I-V characteristics of ITO/ annealed GO thin films/Al devices
To get further insight into the charge extraction properties of the photogenerated carriers from the active layer to HTLs, the samples’ steady state PL spectra were measured and analyzed as shown in Fig. 6. The intensity of PL spectra responses from the quenching ability of the P3HT films fabricated on ITO/GO surface of various annealing temperatures. When the intensity of PL quenching is increased, the holes generated in P3HT absorbers are more efficiently transferred into HTLs. The intensity of the PL response from the P3HT film is gradually reduced with the increase of annealing temperatures of GO, which shows the holes generated in P3HT absorbers can be more efficiently transferred into HTLs with the rise of annealing temperatures of GO [50]. This also explains the change of Jsc in PSCs. When the quenching ability of the active layer increases, the Jsc of the device will rise. The GO annealed at 280 °C exhibits the best PL quenching ability, proving that the GO annealed at 280 °C has more efficiently enhanced rate of carrier extraction at the HTLs/active layer [51, 52]. To further support the PL results and understand the reason of the performance changes in PSCs, the I-V characteristics were recorded in sandwich cells composed by ITO/HTL/Ag, with different temperatures annealed GO used as HTLs. According to Fig. 6b, the direct current conductivity (σ0) can be calculated from the slope of I-V plot, using the equation I = σ0Ad−1V [53, 54], where d is the thickness of GO (30 nm, 3 mg ml−1 GO were spin-coated in 1500 rpm) and the A is the area of sample (0.16 cm2). The conductivity of GO thin films with no annealing was 1.2×10−4 S m−1, and the conductivity of annealed GO thin films were 3.4×10−3 S m−1 at 100 °C, 1.0×10−2 S m−1 at 180 °C, 1.7×10−2 S m−1 at 230 °C, 1.9×10−2 S m−1 at 280 °C and 1.4×10−2 S m−1 at 320 °C, respectively. As a result, it is expected that the photogenerated charge carriers in P3HT:PC71BM based absorber are more efficiently transported to GO films with the increase of thermal annealing. The higher conductivity of GO thin films also explains the observed PL quenching, originating from the improved charge extraction from the active layers to the HTLs. Another considered is the good contact and interface stability between electrode and active layer. Forming a good contact between the electrode and active layer is critical to prevent interfacial dewetting and delamination. From the
J Solid State Electrochem Table 4 Performance parameters of P3HT:PC71BM based devices fabricated with GO thin films as HTLs under different annealing times
Annealing time (min)
Jsc (mA cm−2)
Voc (V)
FF (%)
PCE (%)
Rsh (Ω cm2)
Rs (Ω cm2)
10
9.20
0.57
31.2
1.67
146.75
33.16
20 30
10.14 10.93
0.58 0.59
48.1 52.6
2.83 3.39
310.48 429.59
15.42 14.47
60
10.89
0.58
52.8
3.33
402.62
13.86
measurement of contact angles in Fig. 7, we can see that the surfaces of annealed GO thin films are distinctly more hydrophobic than the surfaces of PEDOT: PSS, and contact angles become lager with improving the annealing temperatures. The contact angles of annealed GO thin films sharply increase from 10.5°at no annealing, 26.6°at 100 °C, 35.2°at 180 °C, 45.2°at 230 °C, 49.6°at 280 °C to 66.7°at 320 °C, and the contact angle of PEDOT:PSS is only 8.5°. We attribute this to the trapped H2O and oxygenated function groups are removed, which results in the formation of a wrinkle and the decrease of surface energy. The hydrophobic surface can promote an ordered growth of P3HT films [55, 56]. In order to further testify the possibility of general usage of the GO thin films as HTLs under the thermal reduction in PSCs, we prepared the PTB7:PC71BM based solar cells inserted GO thin films spin-coated with the optimized speed of 2000 rpm and treated with various annealing temperatures
for 30 min. The structure of the devices is ITO/HTLs/ PTB7:PC71BM/Ca/Al, HTLs are used with different temperatures annealed GO films and PEDOT:PSS. The J–V characteristics of the PTB7:PC71BM based devices under AM 1.5G illumination are presented in Fig. 8, and device parameters are summarized in Table 5. As can be clearly shown in Table 5, the performance of the devices with various temperatures annealed GO are strongly linked with the annealing temperatures. With the increase of The temperature of the heat treatment GO from no annealing to 280 °C, the parameters (Voc, Jsc, FF, PCE) of the devices with GO as HTL significantly increase. The GO based devices annealed at 280 °C show the best performance with a Jsc = 18.44 mA cm−2, FF = 56.6%, PCE = 7.62%, which is close to the PEDOT:PSS based devices (7.66%). It is further indicated that our prepared GO thin films with appropriate annealing temperature have the good interfacial modification potential in the photoelectric devices.
Fig. 7 Hydrophily of PEDOT:PSS and GO thin films of various annealing temperatures by measuring their contact angles: 10.5° for
PEDOT:PSS, 10.5° at no annealing, 26.6° at 100 °C, 35.2° at 180 °C, 45.2° at 230 °C, 49.6° at 280 °C and 66.7° at 320 °C
J Solid State Electrochem
Fig. 8 J-V characteristic curve of PTB7:PC71BM based solar cells fabricated with GO films of various annealing temperatures under illumination
Fig. 9 Stability of P3HT:PC71BM based PSCs with various annealing temperatures GO thin films and PEDOT: PSS as HTLs
Apart from device efficiency, the device stability is another important aspect that affects the realization of commercial PSCs. Normalized PCE and J–V characteristics of the PSCs as a function of storage time are depicted in Fig. 9. All of these devices were placed in the air without any encapsulation. The P3HT:PC71BM based devices with GO thin films (Annealing temperature 280 °C for 30 min, optimal spin-coating speed 2000 rpm) exhibit good long-term stability. The PCE of P3HT:PC71BM based devices with GO thin films can remain about 30% of their initial efficiency after 60 h. However, the efficiency of PEDOT:PSS based devices (Annealing temperature 150 °C, spin-coating speed 4000 rpm) are approach zero after 60 h. By comparing, the PEDOT: PSS based devices have a large decay in performance, which is due to the sharply decreased of Jsc and FF. Thus, the GO as HTL can protect the device against degradation and promise long-term working life of the PSCs. Moreover, we find the lifetime change of PSCs with 230 °C and 280 °C annealed GO thin films as HTLs is nearly similar, which indicates that the annealing temperature has no important effect on the lifetime of devices with GO as HTLs. The enhanced stability and lifetime compared with the PEDOT: PSS based PSCs are due to the
increase of hydrophobic of annealed GO thin films. So the GO as HTL is expected to be a better material for PSCs.
Table 5 Performance parameters of PTB7:PC71BM based devices implanted GO thin films and PEDOT:PSS under various annealing temperatures for 30 min with the optimized spincoating speed of 2000 rpm
Conclusions In summary, we first have investigated the preparation of GO by using the improved Hummer’s method without sodium nitrate, which have avoided the generation of toxic gases (NO2 and N2O4) and decreased the cost of GO synthesis. Subsequently, we use the GO thin films as HTLs with different spin-coating speeds, various annealing times and diverse annealing temperatures to optimize. The results show that the thickness, annealing time and annealing temperature all play an important role in the performance of devices. Too thick GO films increase the Rs, and sharply lower the performance of the PSCs. Too thin GO films make the surface incompletely covered and leakage current increase. We have also explored the effect of various annealing temperatures on the GO films. The FT-IR and XPS spectra show oxygenated functional groups in GO thin films are gradually removed with the increase of annealing temperature, which results in the electrical
T (°C)
Jsc (mA cm−2)
Voc (V)
FF (%)
PCE (%)
Rsh (Ω cm2)
Rs (Ω cm2)
PEDOT:PSS No Annealing 100 180 230 280 320
18.63 5.35 9.47 16.18 17.19 18.44 17.26
0.75 0.59 0.66 0.73 0.73 0.73 0.72
54.8 21.8 26.2 48.3 56.9 56.6 51.6
7.66 0.69 1.64 5.67 7.14 7.62 6.43
461.89 95.53 80.75 382.27 456.56 546.28 471.82
11.101 151.35 77.21 15.68 10.23 9.24 14.00
J Solid State Electrochem
conductivity of GO films to increase. Thus, the performance of devices with GO as HTL is improved with the increase of annealing temperature. The PCE of P3HT:PC 71 BM based (3.39%) and PTB7:PC 71 BM based (7.62%) PSCs with 280 °C annealed GO as HTL exhibits an approximate PEDOT:PSS as HTL (3.41% and 7.66%), and have long-term stability. These results indicate the GO films can be a generally applicable charge transport layer for ambient stable and high-efficiency PSCs. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 61474046 and 61176061).
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