Efficient and Air-Stable Planar Perovskite Solar Cells Formed on Graphene-Oxide-Modified PEDOT:PSS Hole Transport Layer
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As a hole transport layer, PEDOT:PSS usually limits the stability and efficiency of perovskite solar cells (PSCs) due to its hygroscopic nature and inability to block electrons. Here, a graphene-oxide (GO)-modified PEDOT:PSS hole transport layer was fabricated by spin-coating a GO solution onto the PEDOT:PSS surface. PSCs fabricated on a GO-modified PEDOT:PSS layer exhibited a power conversion efficiency (PCE) of 15.34%, which is higher than 11.90% of PSCs with the PEDOT:PSS layer. Furthermore, the stability of the PSCs was significantly improved, with the PCE remaining at 83.5% of the initial PCE values after aging for 39 days in air. The hygroscopic PSS material at the PEDOT:PSS surface was partly removed during spin-coating with the GO solution, which improves the moisture resistance and decreases the contact barrier between the hole transport layer and perovskite layer. The scattered distribution of the GO at the PEDOT:PSS surface exhibits superior wettability, which helps to form a high-quality perovskite layer with better crystallinity and fewer pin holes. Furthermore, the hole extraction selectivity of the GO further inhibits the carrier recombination at the interface between the perovskite and PEDOT:PSS layers. Therefore, the cooperative interactions of these factors greatly improve the light absorption of the perovskite layer, the carrier transport and collection abilities of the PSCs, and especially the stability of the cells.
KeywordsPerovskite solar cells Moisture resistance Wettability Stability Graphene oxide PEDOT:PSS
PSCs (perovskite solar cells) with GO (graphene-oxide)-modified PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate) exhibit a higher efficiency and better stability.
GO-modified PEDOT:PSS surfaces exhibit superior wettability relative to PEDOT:PSS.
Hole extraction selectivity of GO inhibits carrier recombination of PSCs.
Perovskite solar cells (PSCs) have attracted considerable attention because of their low-cost and high-efficiency_ENREF_1. The power conversion efficiency (PCE) of mesoporous PSCs has already reached more than 22.1% . Although a low-temperature processing method has been reported for mesoscopic PSCs using nanoparticles , high-temperature (500 °C) processing is still required to produce the most efficient mesoscopic PSCs [3, 4]. Relative to mesoporous PSCs, planar PSCs are expected to consume less energy and save cost, and promise to be a more feasible means of fabricating cells on a flexible plastic substrate, while showing capability of being integrated into tandem solar cells [5, 6, 7].
Currently emergent inverted (p-i-n) planar PSCs yield a 20.3% PCE , which is almost comparable to that of mesoporous PSCs. Inverted planar PSCs, which typically use a MAPbI3-PCBM (-phenyl-C 61-butyric acid methyl ester) bilayer junction, offer some advantages in terms of their simple fabrication and lower hysteresis [9, 10]. Poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) which offers a high level of conductivity has been widely applied as a hole-selective material in both polymer solar cells [11, 12, 13] and inverted PSCs [5, 14, 15, 16]. However, the use of PEDOT:PSS would greatly limit the lifetime and performance of cells due to its hygroscopic nature and inability to block electrons . Therefore, p-type inorganic materials, such as CuSCN , NiO , and CuI , have been proposed as promising alternative to PEDOT:PSS to enhance the stability of PSCs. However, the low conductivity of inorganic materials limits the performance of PSCs. Recently, heavily p-doped LiNiMgO was developed as a hole transport layer (HTL) by Chen et al. , allowing PSCs to attain a PCE of 16.2% with an area of 1 cm2. However, high-temperature processes are usually used to improve the conductivity of the inorganic hole transport layer, including the LiNiMgO layer. To develop a better hole transport layer with a low-temperature solution process, some hole inorganic material and PEDOT:PSS composites have also been developed to overcome these shortcomings. GeO2-doped PEDOT:PSS , NiOx-PEDOT:PSS , and MoOx-PEDOT:PSS  have been demonstrated to be efficient hole-transporting materials for planar PSCs with efficiencies exceeding 15% due to the better coverage of the perovskite films and the hole transport ability.
Graphene oxide (GO) is a graphene sheet functionalized with oxygen-containing functional groups, in the form of epoxy and hydroxyl groups on the basal plane and various other groups, such as carboxyl, carbonyl, phenol, and quinone, typically at the sheet edges [24, 25]. GO and reduced graphene oxide (RGO) have been used as the HTL in PSCs  and polymer solar cells . The use of oxygen-containing functional groups of GO as an amphiphilic functional layer improves the wettability of the perovskite precursor solution, which improves the surface coverage and grain uniformity of the perovskite film. However, the poor conductivity of GO due to its high oxygen content makes the performance of cells highly sensitive to the thickness of the GO film . The GO and PEDOT:PSS composite compensates for the drawbacks of single GO and conventional PEDOT:PSS in that it exhibits a higher PCE of 9.7% and a better stability in air compared to that of PSCs with a PEDOT:PSS layer . Li et al.  reported on the use of GO as interface modifier, inserted into the interface between the perovskite and a spiro-MeOTAD hole transport layer, thus improving the contact between the perovskite and spiro-MeOTAD layer, by passivating the surface defects in the perovskite film and preventing charge recombination.
In this work, a GO solution was spin-coated onto the PEDOT:PSS surface to form a GO-modified PEDOT:PSS layer. The performance of PSCs based on both unmodified and GO-modified PEDOT:PSS was investigated. It was found that PSCs modified with GO exhibited a higher PCE and better stability in air. The enhanced performance of the GO-modified PSCs is attributed to the superior wettability of the GO-modified PEDOT:PSS surface, retarding carrier recombination, and partial removal of the PSS material at the PEDOT:PSS surface.
3 Experimental Section
GO powder was first dispersed in ethanol using a bath sonicator, at a concentration of 1 mg mL−1, and then diluted to 30 mg L−1. The GO was synthesized from graphite powder using a modified Hummer method . A perovskite precursor solution was prepared by dissolving 1-M methyl ammonium iodide (MAI, 1-Material Inc.) and 1-M lead (II) iodide (PbI2, 99.9%, Aladdin Reagents) in a mixture of dimethyl sulphoxide (DMSO, AR 99% GC, Aladdin Reagents) and γ-butyrolactone (GBL, AR 99% GC, Aladdin Reagents) (7:3 v/v). Cleaned ITO-coated glass with a sheet resistance of 10 Ω/□ was treated with ultraviolet ozone for 20 min. PEDOT:PSS solutions (CLEVIOS PVP Al 4083, Heraeus) were spin-coated at 4000 rpm for 1 min to form a 20-nm film, followed by annealing at 140 °C for 20 min. Then, GO was spin-coated onto the surface of the PEDOT:PSS layer at 4000 rpm for 1 min in the atmosphere. Next, the samples were transferred into an argon-filled glove box and retained for 1 h before further spin-coating of the perovskite layer. The CH3NH3PbI3 film was spin-coated onto PEDOT:PSS or GO-modified PEDOT:PSS layers by a consecutive two-step spin-coating process at 2000 and 4000 rpm for 20 and 60 s, respectively. During the second spin-coating step, toluene drop-casting was employed to replace DMSO solvent. The samples were dried at 100 °C for 10 min. After being cooled, a 70-nm PCBM layer was formed on the CH3NH3PbI3 film by spin-coating a solution of PCBM in chlorobenzene (10 mg mL−1) at 1200 rpm for 60 s. Finally, a 100-nm Ag electrode was thermally evaporated onto the PCBM layer at a base pressure of 5 × 10−4 Pa. The process for producing PSCs with a GO-modified PEDOT:PSS layer is shown in Fig. S1.
The surface morphologies of the samples were investigated by field-emission scanning electron microscopy (SEM, Hitachi S-4800) and atomic force microscopy (AFM, JPK Instruments, Germany). The contact angles were measured with an optical contact angle meter (JC2000D3) at room temperature. X-ray diffraction (XRD) patterns of the perovskite films were measured with a PW3040/60 instrument (Holland Panalytical PRO) using a Cu Kα radiation source (30 kV, 25 mA). X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo Scientific) was performed using Al-Kα monochromatic radiation. The photoluminescence (PL) spectra were measured using a fluorescence spectrophotometer (FluoroMax-4, HORIBA Jobin–Yvon). The transmission and absorption spectra of the samples were recorded using a UV–Vis spectrophotometer (U-3900, Hitachi). The current density–voltage (J–V) characteristics were measured using a Keithley model 2440 source meter and a Newport solar simulator system with AM1.5G and 100-mW cm−2 illumination. The incident photon to current conversion efficiency (IPCE) over a wavelength range of 300–800 nm was measured using an optical power meter (2936-R, Newport). EIS measurements were obtained in the dark using a PGSTAT 302-N electrochemical workstation (Autolab). Ultraviolet photoelectron spectroscopy (UPS) spectra were measured with a monochromatic He II light source (40.8 eV) and an R4000 analyzer (VG Scienta).
4 Results and Discussion
In summary, the PCE of PSCs with the GO modification can be as high as 15.34%, which is obviously higher than that (11.90%) of PSCs fabricated with the unmodified PEDOT:PSS. The PCE of PSCs fabricated with the GO-modified PEDOT:PSS remains at 83.5% of the initial PCE value after aging for 39 days, indicating that the use of the GO-modified PEDOT:PSS instead of the unmodified PEDOT:PSS as a hole transport layer is a good strategy for efficiently improving the performance and stability of PSCs. The ethanol in the GO solution can partially remove the hydrophilic PSS material at the PEDOT:PSS surface during the spin-coating of the GO solution, which can improve the moisture resistance and decrease the contact barrier at the PEDOT:PSS surface. The scattered distribution of the GO pieces can further limit the hydrophobicity of the PEDOT:PSS surface due to the partial isolation of the GO pieces, which can also improve the moisture resistance. The superior wettability of the GO-modified PEDOT:PSS surface helps to form a high-quality perovskite layer with better crystallinity and fewer pin holes. Furthermore, the GO exhibits selectivity in the hole extraction, which can inhibit the recombination of holes and electrons at the interface between the perovskite and PEDOT:PSS. Therefore, the cooperative interactions of these factors can greatly improve the absorption of the perovskite layer, the carrier transport and collection abilities of PSCs, and especially the stability of the cells. Our results could form a basis for a strategy to overcome the weakness of PEDOT:PSS as a typical hole transport material and fully develop its advantages for application to perovskite solar cells.
This work was supported by National Natural Science Foundation of China (Grant Nos. 61275038 and 11274119).
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