Enhanced performance of perovskite solar cells with P3HT hole-transporting materials via molecular p-type doping

Abstract

The conducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) has been widely used as a polymeric hole-transporting material (HTM) in inorganic–organic perovskite solar cells (PSCs). However, pristine P3HT-based PSC devices typically exhibit mediocre overall performance, mainly due to its relatively low conductivity. Herein, we successfully introduced tetrafluoro-tetracyano-quinodimethane (F4TCNQ) as an efficient p-type dopant for P3HT as a HTM in mesoscopic PSCs. The overall performance was significantly enhanced after the introduction of F4TCNQ into P3HT. Under an optimal doping condition (1.0%, w/w), an impressive power conversion efficiency (PCE) of 14.4% was achieved, which was considerably higher than the pristine P3HT based devices (10.3%). The dramatic improvement of the PCE originated from the increase of the photocurrent density and fill factor, strongly correlated to the significant increase of the bulk conductivity of F4TCNQ doped P3HT. After doping with 1.0% F4TCNQ, the conductivity of the P3HT film was significantly increased by more than 50 times. UV-Vis and Fourier transform infrared spectroscopy (FTIR) measurements indicated that p-doping occurs via the electron transfer from the highest occupied molecular orbital (HOMO) level of P3HT to the lowest unoccupied molecular orbital (LUMO) level of the F4TCNQ, which led to a substantial increase of the bulk conductivity. Furthermore, PSCs based on the P3HT:F4TCNQ composite as a HTM also exhibited superior long-term stability under ambient conditions with a humidity of 40%. F4TCNQ was thus demonstrated to be an effective p-dopant for P3HT to improve the electrical properties and thereby the overall performance for highly efficient and stable PSCs.

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1. Introduction

Inorganic–organic halide perovskites with the composition ABX₃ [A = CH₃NH₃⁺ (MA), NH = CHNH₃⁺ (FA) or Cs⁺; B = Pb or Sn; X = Cl, Br, I] have received a great deal of research interest as light absorbers in solid-state thin-film solar cells.^1–4 The record power conversion efficiency (PCE) has been rapidly increased to over 22% in only few years.⁵ Perovskites possess several unique characteristics, such as a broad spectral absorption range, high absorption coefficient, high charge carrier mobility and diffusion length etc., which make them promising light-absorbing materials in solar cell devices. Moreover, perovskite materials can be solution-processed from cheap starting materials. Hence, the high efficiency together with facile fabrication routes render PSCs as the forerunner among the low-cost next generation solar cell technologies.

The state-of-the-art PSC devices routinely employed hole-transporting materials (HTMs), which are essential components for achieving high solar cell efficiencies.⁶ The HTMs play key roles in extracting and transporting the photo-generated holes from the perovskites to the metal electrodes, and thus minimize undesired recombination losses at the interfaces. Thus, searching for an efficient HTM has been one of the hottest research topics in PSCs. A wide range of HTMs have been developed in PSCs, which are mainly composed of organic hole-conductors and inorganic p-type semiconductors. Inorganic p-type semiconductors, such as copper iodide (CuI) and copper thiocyanate (CuSCN), exhibit high conductivity and low production costs. However, PSC devices based on CuI and CuSCN typically displayed unsatisfactory overall efficiencies, with the highest reported PCE of only 12.4%.^7–9 Small molecule based hole-conductors have been intensively studied as HTMs in PSCs.^6,10 A large number of small molecule HTMs have been developed in PSCs, including triphenylamine (TPA)-based, carbazole-based, donor–acceptor (D–A) conjugated small molecules, tetrathiafulvalene and pentacene derivatives etc.^11–18 Among them, spiro-type small molecule HTMs have exhibited the superior overall efficiencies, such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), with the highest PCE of over 20%.¹⁹

Conducting polymers constituted another major class of organic hole-conductors in PSCs, owing to their excellent charge carrier mobility, suitable energy levels and good processability. A great number of conducting polymers have been applied as HTMs in PSCs.⁶ Yet, the overall performance of PSC devices based on these polymeric HTMs were typically unsatisfactory. Poly(triarylamine) (PTAA) has been the only example of conducting polymer-based HTMs used in PSCs that could work as efficiently as spiro-type small molecules. The highest reported PCE of PSCs based on PTAA also reached more than 20%.²⁰ However, the price of PTAA is extremely expensive (50 times higher than gold), which significantly impedes its potential large-scale application in the future.²¹

Poly(3-hexylthiophene-2,5-diyl) (P3HT) (Fig. 1a), which has been widely used as model polymeric hole-conductors in organic electronic devices, were extensively studied in PSCs.^22–28 However, the devices based on pristine P3HT as HTMs typically exhibited mediocre performance, largely due to its relatively low conductivity in its pristine form.²⁷ The enhancement of the conductivity of P3HT has been a key issue in order to obtain high overall performance in PSCs using such a polymeric HTM. With this in mind, several doping methods have been introduced to improve the conductivity of pristine P3HT and thus the overall performance. Nakamura et al. doped P3HT with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) and a pyridine-based additive 2,6-di-tert-butylpyridine.²² The incorporation of these p-type additives/dopants increased the carrier density and resulted in a two orders of magnitude larger hole conductivity, which led to a significant increase of the overall efficiency from 9.2% to 12.4%. However, lithium salts are hydroscopic, which may potentially accelerate the degradation of PSC devices as reported previously.¹⁷ Therefore, seeking alternative p-type dopants for P3HT is of great importance to improve both of the overall photovoltaic performance and long-term stability. So far, highly efficient P3HT-based HTMs in PSCs have been mainly dominated by doping with highly conductive carbon materials. Wang and co-workers introduced bamboo-structured carbon nanotubes (BCNs) into P3HT, which not only improved the hole conductivity of P3HT, but also reduced charge carrier recombination at perovskite/HTM interface correlated with a superior film morphology.²⁷ These positive effects led to a significant increase of the overall efficiency from 3.6% (pristine) to 8.3% (1 wt% BCNs). Snaith and co-workers functionalized P3HT with single-walled carbon nanotubes (SWNTs), which resulted in a maximum PCE of 15.3% with an average efficiency of over 10%.²⁹ In another report, Li et al. doped P3HT with a highly conductive carbon material graphdiyne (GD).²⁸ The incorporation of GD facilitated the hole-transporting property of P3HT, due to the strong π–π stacking interaction between GD particles and P3HT. A maximum PCE of 14.58% was obtained for PSCs based on the HTM composite. Additionally, such PSC devices also exhibited good long-term stability.

Fig. 1 (a) Schematic diagram of electrons transfer from P3HT to F4TCNQ (b) energy level diagram of P3HT and F4TCNQ.

Tetrafluoro-tetracyano-quinodimethane (F4TCNQ) (Fig. 1a) has been successfully demonstrated to be an effective molecular p-dopant for P3HT.^30–32 The lowest unoccupied molecular orbital (LUMO) of F4TCNQ matches well matches well withthe highest occupied molecular orbital (HOMO) of P3HT (Fig. 1b). p-Type doping occurs via electrons transfer from the HOMO level of the P3HT to the LUMO of the F4TCNQ.³⁰ Hence, more P3HT positive charges (P3HT⁺) are expected to be produced in the mixture, thus increasing the bulk conductivity of P3HT film. In this work, we successfully introduced F4TCNQ doped P3HT composite as an efficient HTM in mesoscopic PSCs. In comparison to pristine P3HT, the overall efficiency of F4TCNQ-doped device (1.0%, w/w) was dramatically enhanced from 10.3% to 14.4% under 1 sun irradiation (100 mW cm⁻², AM 1.5G), mainly due to the significant increases of the short-circuit current density (J_sc) and fill factor (FF). The conductivity of P3HT doped with 1.0% F4TCNQ was significantly increased by more than 50 times, which made a great contribution to solar cell performance. UV-Vis and Fourier transform infrared spectroscopy (FTIR) measurements indicated that p-doping occurs via the electron transfer from HOMO level of P3HT to the LUMO level of the F4TCNQ, which led to a substantial increase of the bulk conductivity. Furthermore, this P3HT:F4TCNQ composite HTM based PSCs also exhibited superior long-term stability under ambient condition with a humidity of 40%. F4TCNQ was thus demonstrated to be an effective molecular p-dopant for P3HT to improve the electrical property and the overall performance in highly efficient and stable PSCs.

2. Experimental

2.1 Materials

All the chemicals and reagents were used as received from chemical companies, including PbI₂ (>98%, TCI), PbBr₂ (99%, Sigma-Aldrich), HI (48% in water, Sigma-Aldrich), HBr (48% in water, Sigma-Aldrich), CH₃NH₂ (33 wt% in absolute ethanol, Sigma-Aldrich), formamidine acetate (99%, Sigma-Aldrich), titanium diisopropoxide bis(acetylacetonate) 75% in isopropanol (Tiacac, Sigma-Aldrich), mesoporous-TiO₂ paste (18NR-T, Dyesol), Li-bis(trifluoromethanesulfonyl) imide (LiTFSI, Sigma-Aldrich), 4-tert-butylpyridine (TBP, 96%, Sigma-Aldrich), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ, 97%, Sigma-Aldrich), poly(3-hexylthiophene-2,5-diyl) (P3HT, regioregular, Sigma-Aldrich).

The NH=CHNH₃I was synthesized according to a previously literature.¹⁹ 28.1 ml of HI was slowly dropped into 10 g formamidine acetate in methanol solution cooled at 0 °C with vigorous stirring for 5 hours. The resulting solution was concentrated by rotary evaporation at 80 °C until all the precipitate dissolve out, then the crude solid was dissolved by methanol and re-precipitated in diethyl ether. After recrystallization for 3 times, the white powder was dried at 80 °C under vacuum for 2 days to afford the desired pure NH=CHNH₃I.

The CH₃NH₃Br (MABr) was synthesized by slowly dropping 31.1 ml of HBr into 33.8 ml CH₃NH₂ ethanol solution cooled at 0 °C with vigorous stirring for 5 hours. The resulting solution was concentrated by rotary evaporation at 50 °C until all the precipitate dissolve out, then the crude solid was dissolved by ethanol and re-precipitated in diethyl ether. After recrystallization for 3 times, the white powder was dried at room temperature under vacuum for 2 days to afford the desired pure MABr.

The mixed-cation perovskite precursor solution of (FAPbI₃)_1−x(MAPbBr₃)_x (x = 0.15) was prepared in a glovebox, by dissolving the FAI (1 M), MABr (0.2 M), PbI₂ (1.1 M) and PbBr₂ (0.2 M) in a mixed solvent of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) (4 : 1, v/v), as reported previously.^19,20

2.2 Device fabrication and characterization

Fluorine-doped tin oxide (FTO)-coated glass (Pilkington TEC 15) was firstly patterned by etching with Zn powder and 2 M HCl. The etched substrate was then sequentially cleaned by using detergent, de-ionized water and ethanol. Remaining organic residues were removed under oxygen plasma for 30 min. A compact TiO₂ blocking layer (BL) of roughly 30–40 nm was deposited on the cleaned FTO glasses by spray pyrolysis of titanium diisopropoxide bis(acetylacetonate) diluted in anhydrous ethanol at a volumetric ratio of 1 : 10 and then heated at 500 °C for 30 min. A mesoporous TiO₂ layer was deposited by spin-coating TiO₂ paste (Dyesol 18NR-T) diluted in anhydrous ethanol at ratio of 1 : 5 by weight at 5000 rpm for 30 s. The layers were then sintered in air at 500 °C for 30 min. The mixed-cation perovskite films were deposited onto the mesoporous TiO₂/BL TiO₂/FTO substrates from the precursor solution by a two-step spin-coating procedure, at 1000 rpm for 30 s and then 5000 rpm for 20 s. During the second step, 200 μl of chlorobenzene was dropped onto the substrates 10 s prior to the end of the program. The substrate was directly heated on a hotplate at 100 °C for 60 min. After cooling to room temperature, different doping level of P3HT:F4TCNQ composites were deposited on the perovskite layers at 2000 rpm for 30 min via solution process. 20 mg ml⁻¹ P3HT solution was prepared by dissolving 20 mg P3HT in 1 ml ortho-dichlorobenzene (DCB), stirred at 70 °C for 30 min. p-Type doping material F4TCNQ in DCB solution with a concentration of 2 mg ml⁻¹ was also stirred at 70 °C for 30 min before adding to the P3HT solution. In addition, for comparison, 9.1 μl of a stock solution of 28.5 mg ml⁻¹ LiTFSI in acetonitrile (ACN) and 4.5 μl 4-tert-butylpyridine (TBP) was mixed into 1 ml P3HT (20 mg) solution as HTM solution. After spin coating of the HTM layers, the substrates were heated at 65 °C for 15 min. Finally, a layer of 100 nm Au was deposited on top of the HTM layer under high vacuum (<4 × 10⁻⁴ Pa) by thermal evaporation.

The photocurrent–voltage (J–V) characteristics of the solar cells were measured using a Keithley 2400 Source-measure unit under illumination of a simulated sunlight (AM 1.5G, 100 mW cm⁻²) provided by an Oriel Sol3A solar simulator (Newport USA, Model: 94023A) with an AM 1.5 filter in ambient air. Light intensity was calibrated with a Newport calibrated standard Si reference cell (SER. no. 506/0358). A black mask with a circular aperture (0.09 cm²) smaller than the active area of the square solar cell (0.20 cm²) was applied on top of the cell. The J–V curves were obtained from forward bias to short-circuit at a scan rate of 20 mV s⁻¹. The incident photo-to-current conversion efficiency (IPCE) was measured by Oriel IQE 200 (Newport USA, Model: 94023A/PVIV-212V/IQE-AC-QTH-SI-220). Prior to measurement, a standard silicon solar cell was used as reference.

2.3 Characterizations

The top view and cross-section scanning electron microscopy (SEM) images were obtained by HR-SEM performed with FEI (Field Emission Instruments: Nova Nano SEM 450), the USA. Conductivity measurements were performed as follows.³³ Glass substrates were sequentially cleaned by detergent, de-ionised water and ethanol. Remaining organic residues were removed under oxygen plasma for 30 min. A thin layer of compact TiO₂ (∼30 nm) was coated on the glass substrates. After sintering the TiO₂ film at 500 °C for 30 min, the film was cooled to room temperature. A solution of HTM in DCB was spin-coated onto the TiO₂ substrate, whereas the concentration was the same as in case for the photovoltaic device. Finally, a 200 nm-thick of Ag was deposited on the top of the HTM by thermal evaporation under high vacuum (<4 × 10⁻⁴ Pa). A two-point probe setup was used with a Keithley 2400 source meter for measuring linear current–voltage curves. The electrochemical impedance spectroscopy (EIS) measurements were carried out at different applied bias in the dark condition using an impedance/gain-phase analyser (Zahner Model: Zennium, Serial No. 40037, German) electrochemical workstation with the scanning frequency range from 10⁶ to 0.1 Hz. The magnitude of the alternative signal was 10 mV. The UV-Vis spectra were measured by Agilent 8453 spectrophotometer (Model: HP 8435, China). The infrared spectra were measured by Fourier transform infrared spectrometer (FTIR) mode on 6700 (ThermoFisher, USA).

3. Results and discussion

The photovoltaic performance of P3HT doped with varied doping concentrations of F4TCNQ as HTMs were first examined in mesoscopic PSCs. The schematic illustration and cross-sectional scanning electron microscopy (SEM) image of the device architecture are depicted in Fig. 2a and b. The solar cell devices were fabricated with a structure of FTO glass/compact TiO₂ (∼30–40 nm)/mesoporous TiO₂ (∼200 nm)/perovskite/HTM/Au. The mixed-cation perovskite light absorber (FAPbI₃)_0.85(MAPbBr₃)_0.15 was prepared by using a solvent-engineering technique as reported previously.^19,20 Perovskite crystals grew inside the pores of scaffold and additionally formed a capping layer with a total thickness of about 600 nm. P3HT with varied doping levels of F4TCNQ (0–2%, w/w) were deposited on the as-prepared perovskite substrates as HTMs. From top-view SEM images as shown in Fig. S1 in the ESI,† it can be clearly seen that P3HT:F4TCNQ film uniformly covered onto the compact perovskite layer.

Fig. 2 (a) Schematic device architecture of perovskite solar cells studied. (b) Cross-sectional SEM image of the complete PSC device containing FTO glass/compact TiO₂/mesoporous TiO₂/perovskite/P3HT:F4TCNQ/Au. (c) J–V characteristics of the PSC devices based on P3HT as HTMs with different concentrations of F4TCNQ.

The current density–voltage (J–V) characteristics of PSC devices based on varied doping levels of P3HT as HTMs measured under 100 mW cm⁻² illumination (AM 1.5G) are displayed in Fig. 2c and photovoltaic parameters are summarized in Table 1. PSC device based on pristine P3HT as a HTM shows the lowest PCE of only 10.3%, with an open-circuit voltage (V_oc) of 0.96 V, a J_sc of 19.5 mA cm⁻² and a FF of 0.55, respectively. The molecularly p-doping of P3HT with F4TCNQ resulted in dramatic enhancements of the overall efficiencies for all the three different doping levels tested, due largely to the great improvements of the J_sc and FF. Under an optimal condition (doping with 1.0% F4TCNQ), a maximum PCE of 14.4% is achieved, with a V_oc of 0.97 V, a J_sc of 23.9 mA cm⁻² and a FF of 0.62, respectively. This is one of the highest PCE achieved for P3HT-based HTMs in PSCs. P3HT doped with 1.0% F4TCNQ will be used as the optimal condition for the following experiments without otherwise stated. The incident-photon-to-current conversion efficiency (IPCE) spectra for PSCs based on P3HT with and without F4TCNQ (Fig. S2, ESI†). Both of these two PSC devices display a wide spectra response to a long wavelength over 800 nm. As compared to pristine P3HT, F4TCNQ doped one shows a remarkable improvement of IPCE over the whole region measured, which agrees well with results from the J–V measurements. For comparison purpose, the most commonly used additives LiTFSI and TBP in P3HT with optimal concentrations were also tested under identical conditions, as shown in Fig. 2c. A peak PCE of 12.6% is obtained, with a V_oc of 0.92 V of, a J_sc of 22.5 mA cm⁻² and a FF of 0.61, respectively. The J_sc and FF of PSCs based on P3HT using these two additives are relatively improved as compared to pristine P3HT, whereas the extent of the enhancements are not as significant as the F4TCNQ doped one, in particular for the J_sc.

Table 1 Photovoltaic parameters of PSCs using P3HT as HTMs doped with different concentrations of F4TCNQ measured under 100 mW cm⁻² illumination (AM 1.5G)

Doping ratio (w/w)	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)
0%	0.96	19.5	0.55	10.3
0.5%	0.96	20.7	0.59	11.7
1.0%	0.97	23.9	0.62	14.4
2.0%	0.98	22.6	0.56	12.5

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In order to elucidate the origin of the enhancements of the photovoltaic parameters for the F4TCNQ doped devices, we carried out the conductivity measurements using two-point probe methodology based on glass/compact TiO₂/HTM/Ag device as reported previously (Fig. 3a).³³ Conductivity was calculated following the formula: σ = L/(Rμd), where L is the channel length; R is the film resistance calculated from gradient of the curve; μ is the channel width; d is the film thickness. The conductivity of pristine P3HT is calculated to be 1.06 × 10⁻⁵ S cm⁻¹. By stark contrast, doping with F4TCNQ resulted in more than 50 times increases of the conductivity, amounting to 5.59 × 10⁻⁴ S cm⁻¹. The higher conductivity obtained for F4TCNQ doped devices results in a minimized series resistance of 7.4 Ω cm⁻² (derived from J–V measurement) as compared to pristine P3HT (10.6 Ω cm⁻²), thus leading to a higher FF. The remarkable enhancement of the J_sc for PSC devices based on F4TCNQ doped P3HT should be largely attributed to the higher conductivity, strongly correlated to a more efficient charge collection. The conductivity of P3HT doped with conventional additives LiTFSI and TBP was also tested for comparison, calculated to be 6.65 × 10⁻⁵ S cm⁻¹ that is only 5 times higher than the pristine one.

Fig. 3 (a) Current vs. voltage curves of glass/compact TiO₂/HTM/Ag hole-only device. Linear fits to these curves are used to measure resistance and calculate the conductivity of different doping HTMs. (b) UV-Vis absorption spectra of F4TCNQ solution and P3HT solutions with different concentrations of F4TCNQ. (c) FTIR spectra of pristine F4TCNQ and P3HT doped with 1% of F4TCNQ thin film.

To gain more insight into the charge recombination process at TiO₂/perovskite/HTM interfaces, electrochemical impedance spectroscopy (EIS) was carried out on complete PSC devices employing pristine and 1% F4TCNQ doped P3HT as HTMs under dark conditions with varied bias voltage in the frequency range from 10⁶ to 0.1 Hz. The Nyquist plots are displayed in Fig. S3a and b.† According to the equivalent circuit model as reported previously,³⁴ the main arc is supposed to be a combination of the recombination resistance (R_rec) and the chemical capacitance of the film. By fitting the Nyquist plots, the relationship between the R_rec and the bias voltage is obtained, as presented in Fig. S3c.† It can be noted that the device with 1% F4TCNQ doped P3HT shows a larger R_rec as compared to the pristine one at the same bias voltage. It indicates a more efficient charge collection for the 1% F4TCNQ doped devices, which leads to a higher J_sc obtained.

As mentioned previously, p-type doping occurs via electrons transfer from the HOMO level of P3HT to the LUMO of F4TCNQ, resulting in a significant increase of the bulk conductivity. To prove this hypothesis, we first implemented the UV-Vis spectroscopy measurements for P3HT with and without F4TCNQ, as shown in Fig. 3b. Pristine P3HT shows the main π–π* absorption at 460 nm. When doped with 1.0% F4TCNQ, sub-gap absorption peaks in the infrared region was clearly observed. Similar phenomena were also found for other doping concentrations of F4TCNQ (0.5, 1.5 and 2%, w/w), as shown in Fig. S4 in the ESI.† The sub-gap absorption was not seen for both pristine P3HT and F4TCNQ solutions, separately. These observations indicate the presence of ground-state charge transfer from P3HT to F4TCNQ molecules, initiated by effective electron removal of electrons due to the electron-poor characteristic of F4TCNQ molecule.³⁰ A distinct colour change in P3HT solution with varied concentrations of F4TCNQ (Fig. S5, ESI†) also supports the ground-state charge transfer happening after F4TCNQ doping. No significant difference of the absorption in the infrared region was observed between pristine P3HT and the one doped with LiTFSI and TBP. FTIR spectroscopy measurements further confirmed the ground-state charge transfer from the HOMO level of P3HT to the LUMO level of F4TCNQ, as presented in Fig. 3c. The absorption peak of cyano bond (C≡N) is a sensitive indicator of the presence of charges on F4TCNQ molecules.³⁰ The C≡N absorption peak of neutral F4TCNQ in solid state is centred at 2228 cm⁻¹. In the F4TCNQ doped P3HT film, this C≡N absorption peak is red-shifted and splits into a few peaks. Similar phenomena were also observed for other doping concentrations of F4TCNQ (Fig. S6, ESI†). These observations indicate the presence of F4TCNQ anion radical state by accepting electrons from P3HT. Above all, spectroscopic analysis confirmed effective p-type doping occurs when P3HT is mixed with F4TCNQ, thereby forming holes on P3HT polymer and enhancing the charge transport property of such polymeric HTM.

The statistical data of photovoltaic parameters for 20 PSC devices based on pristine and doped P3HT are presented and summarized in Fig. 4 and Table S1 (ESI),† respectively. The devices containing F4TCNQ exhibited good reproducibility with an average PCE of 13.8 ± 1.0%. Pristine and LiTFSI and TBP doped devices showed average PCEs of 9.9 ± 1.6% and 12.7 ± 0.8%, respectively. The high average efficiency obtained for F4TCNQ doped devices should be mainly attributed to the high photocurrent densities, which are strongly associated with the high conductivity.

Fig. 4 Statistical data of photovoltaic parameters for 20 PSC devices based on pristine and doped P3HT as HTMs measured under 100 mW cm⁻² illumination (AM 1.5G).

The steady-state efficiency of a representative PSC device based on F4TCNQ doped P3HT was also measured at a constant bias of 0.7 V over 100 seconds under 1 sun condition (AM 1.5G), as presented in Fig. 5a. This device exhibits a steady-state efficiency of 11.9% and current density of 17.1 mA cm⁻² during the testing period. The stability of PSCs device based on F4TCNQ doped P3HT, which were stored without encapsulation at ambient condition with humidity at ∼40% in the dark, was investigated for 960 hours (Fig. 5b). For comparison purpose, the stability of device containing hydroscopic additive LiTFSI was also probed under equivalent conditions. The efficiency of the device containing LiTFSI dropped rapidly during the aging time. After 960 hours, only 20% of its initial PCE was remained. From Fig. 5c, it can be obviously seen that, after 480 hours, the colour of the perovskite films for devices containing LiTFSI already started to partially change to yellow, indicative of the formation of PbI₂ originated from the attack of moisture to the perovskite film. By stark contrast, the stability of F4TCNQ doped device improved remarkably, maintaining over 80% of its initial efficiency after 960 hours. No obvious colour change was observed for F4TCNQ doped devices during the testing period. As shown in Fig. 5d, the water contact angle for F4TCNQ doped P3HT film is 105.49°, which is significantly larger than the corresponding value for LiTFSI doped one (80°). The hydrophobic characteristic of F4TCNQ doped P3HT can effectively prevent the water penetration into the perovskite layer, thus resulting in an improved long-term durability. Therefore, we are convinced that F4TCNQ in place of LiTFSI and TBP as a p-type dopant for P3HT could substantially improve the long-term stability of PSCs.

Fig. 5 (a) PCE (red) and current density (black) as a function of time under illumination at a fixed voltage of 0.70 V for a representative device based on P3HT as HTM doped with 1% F4TCNQ. (b) Efficiency variations of PSC devices based on HTM P3HT doped with F4TCNQ (Group 1) and LiTFSI (Group 2). The device without encapsulations were stored at ambient conditions in the dark at room temperatures with a humidity of 40% measured under 100 mW cm⁻² illumination (AM 1.5G). (c) Images of un-encapsulated PSC devices over time: F4TCNQ (top)and LiTFSI (bottom) doped P3HT as HTMs. (d) Water contact angles of pure and doped P3HT thin films (F4TCNQ and LiTFSI) coated on FTO glass substrates.

4. Conclusions

In summary, we have successfully introduced F4TCNQ doped P3HT as an efficient HTM in mesoscopic PSCs. The overall performance was significantly enhanced after the introduction of F4TCNQ into P3HT. Under an optimal doping condition (1.0%, w/w), an impressive PCE of 14.4% was achieved, which was considerably higher than the pristine P3HT based devices (10.3%). The dramatic improvement of the PCE should be largely attributed to the increases of the photocurrent density and fill factor, strongly correlated to the significant increase of the bulk conductivity of F4TCNQ doped P3HT. After doping with 1.0% F4TCNQ, the conductivity of P3HT film was significantly increased by more than 50 times. UV-Vis and FTIR measurements indicated that p-doping occurs via the electron transfer from the HOMO level of P3HT to the LUMO level of F4TCNQ, which led to a substantial increase of the bulk conductivity. Furthermore, this P3HT:F4TCNQ composite HTM based PSCs also exhibited excellent long-term stability under ambient atmosphere with a humidity of 40%. More intriguingly, both the overall efficiency and long-term stability of PSC device containing F4TCNQ outperformed the most commonly used additive LiTFSI for P3HT. Hence, F4TCNQ was demonstrated to be an effective p-dopant for P3HT-based HTMs in PSCs combining high overall performance and good long-term durability. The present finding will shed interesting lights on further optimizing a wide range of other polymeric HTMs for highly efficient and stable PSCs in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21606039, 21120102036, 91233201), the National Basic Research Program of China (973 program, 2014CB239402), the Swedish Energy Agency, as well as the Knut and Alice Wallenberg Foundation.

References

1. M. A. Green, A. Ho-Baillie and H. J. Snaith, Nat. Photonics, 2014, 8, 506–514.
2. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338, 643–647.
3. H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Grätzel and N.-G. Park, Sci. Rep., 2012, 2(591), 1–7.
4. W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Grätzel and L. Han, Science, 2015, 350, 944–948.
5. http://www.nrel.gov/ncpv/images/efficiencychart.jpg.
6. Z. Yu and L. Sun, Adv. Energy Mater., 2015, 5, 1500213/1–17.
7. J. A. Christians, R. C. M. Fung and P. V. Kamat, J. Am. Chem. Soc., 2013, 136, 758–764.
8. P. Qin, S. Tanaka, S. Ito, N. Tetreault, K. Manabe, H. Nishino, M. K. Nazeeruddin and M. Grätzel, Nat. Commun., 2014, 5, 3834.
9. G. A. Sepalage, S. Meyer, A. Pascoe, A. D. Scully, F. Huang, U. Bach, Y.-B. Cheng and L. Spiccia, Adv. Funct. Mater., 2015, 25, 5650–5661.
10. S. Ameen, M. A. Rub, S. A. Kosa, K. A. Alamry, M. S. Akhtar, H.-S. Shin, H.-K. Seo, A. M. Asiri and M. K. Nazeeruddin, ChemSusChem, 2016, 9, 10–27.
11. B. Xu, D. Bi, Y. Hua, P. Liu, M. Cheng, M. Grätzel, L. Kloo, A. Hagfeldt and L. Sun, Energy Environ. Sci., 2016, 9, 873–877.
12. D. Bi, B. Xu, P. Gao, L. Sun, M. Grätzel and A. Hagfeldt, Nano Energy, 2016, 23, 138–144.
13. Y. Hua, J. Zhang, B. Xu, P. Liu, M. Cheng, L. Kloo, E. M. J. Johansson, K. Sveinbjörnsson, K. Aitola, G. Boschloo and L. Sun, Nano Energy, 2016, 26, 108–113.
14. Y. Liu, Z. Hong, Q. Chen, H. Chen, W.-H. Chang, Y. Yang, T.-B. Song and Y. Yang, Adv. Mater., 2016, 28, 440–446.
15. M. Cheng, B. Xu, C. Chen, X. Yang, F. Zhang, Q. Tan, Y. Hua, L. Kloo and L. Sun, Adv. Energy Mater., 2015, 1401720/1–9.
16. B. Xu, E. Sheibani, P. Liu, J. Zhang, H. Tian, N. Vlachopoulos, G. Boschloo, L. Kloo, A. Hagfeldt and L. Sun, Adv. Mater., 2014, 26, 6629–6634.
17. J. Liu, Y. Wu, C. Qin, X. Yang, T. Yasuda, A. Islam, K. Zhang, W. Peng, W. Chen and L. Han, Energy Environ. Sci., 2014, 7, 2963–2967.
18. S. Kazim, F. J. Ramos, P. Gao, M. K. Nazeeruddin, M. Grätzel and S. Ahmad, Energy Environ. Sci., 2015, 8, 1816–1823.
19. D. Bi, W. Tress, M. I. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P. Correa Baena, J.-D. Decoppet, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel and A. Hagfeldt, Sci. Adv., 2016, 2, e1501170/1–7.
20. W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo and S. I. Seok, Science, 2015, 348, 1234–1237.
21. Y. Rong, L. Liu, A. Mei, X. Li and H. Han, Adv. Energy Mater., 2015, 5, 1501066/1501061–1501016.
22. Y. Guo, C. Liu, K. Inoue, K. Harano, H. Tanaka and E. Nakamura, J. Mater. Chem. A, 2014, 2, 13827–13830.
23. B. Conings, L. Baeten, C. De Dobbelaere, J. D'Haen, J. Manca and H.-G. Boyen, Adv. Mater., 2014, 26, 2041–2046.
24. T. Meng, C. Liu, K. Wang, T. He, Y. Zhu, A. Al-Enizi, A. Elzatahry and X. Gong, ACS Appl. Mater. Interfaces, 2016, 8, 1876–1883.
25. D. Bi, L. Yang, G. Boschloo, A. Hagfeldt and E. M. J. Johansson, J. Phys. Chem. Lett., 2013, 4, 1532–1536.
26. A. Abrusci, S. D. Stranks, P. Docampo, H.-L. Yip, A. K. Y. Jen and H. J. Snaith, Nano Lett., 2013, 13, 3124–3128.
27. M. Cai, V. T. Tiong, T. Hreid, J. Bell and H. Wang, J. Mater. Chem. A, 2015, 3, 2784–2793.
28. J. Xiao, J. Shi, H. Liu, Y. Xu, S. Lv, Y. Luo, D. Li, Q. Meng and Y. Li, Adv. Energy Mater., 2015, 5, 1401943/1–7.
29. S. N. Habisreutinger, T. Leijtens, G. E. Eperon, S. D. Stranks, R. J. Nicholas and H. J. Snaith, Nano Lett., 2014, 14, 5561–5568.
30. K.-H. Yim, G. L. Whiting, C. E. Murphy, J. J. M. Halls, J. H. Burroughes, R. H. Friend and J.-S. Kim, Adv. Mater., 2008, 20, 3319–3324.
31. E. F. Aziz, A. Vollmer, S. Eisebitt, W. Eberhardt, P. Pingel, D. Neher and N. Koch, Adv. Mater., 2007, 19, 3257–3260.
32. J. Gao, J. D. Roehling, Y. Li, H. Guo, A. J. Moule and J. K. Grey, J. Mater. Chem. C, 2013, 1, 5638–5646.
33. T. Leijtens, I. K. Ding, T. Giovenzana, J. T. Bloking, M. D. McGehee and A. Sellinger, ACS Nano, 2012, 6, 1455–1462.
34. S. Lv, L. Han, J. Xiao, L. Zhu, J. Shi, H. Wei, Y. Xu, J. Dong, X. Xu, D. Li, S. Wang, Y. Luo, Q. Meng and X. Li, Chem. Commun., 2014, 50, 6931–6934.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21775c

‡ These authors contributed equally to this work.

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