17% Non‐Fullerene Organic Solar Cells with Annealing‐Free Aqueous MoO_x

Abstract

A charge transport layer based on transition metal‐oxides prepared by an anhydrous sol–gel method normally requires high‐temperature annealing to achieve the desired quality. Although annealing is not a difficult process in the laboratory, it is definitely not a simple process in mass production, such as roll‐to‐roll, because of the inevitable long cooling step that follows. Therefore, the development of an annealing‐free solution‐processable metal‐oxide is essential for the large‐scale commercialization. In this work, a room‐temperature processable annealing‐free “aqueous” MoO_x solution is developed and applied in non‐fullerene PBDB‐T‐2F:Y6 solar cells. By adjusting the concentration of water in the sol–gel route, an annealing‐free MoO_x with excellent electrical properties is successfully developed. The PBDB‐T‐2F:Y6 solar cell with the general MoO_x prepared by the anhydrous sol–gel method shows a low efficiency of 7.7% without annealing. If this anhydrous MoO_x is annealed at 200 °C, the efficiency is recovered to 17.1%, which is a normal value typically observed in conventional structure PBDB‐T‐2F:Y6 solar cells. However, without any annealing process, the solar cell with aqueous MoO_x exhibits comparable performance of 17.0%. In addition, the solar cell with annealing‐free aqueous MoO_x exhibits better performance and stability without high‐temperature annealing compared to the solar cells with PEDOT:PSS.

Annealing‐free solution‐processable aqueous MoO_x are developed and applied in bulk‐heterojunction polymer solar cells based on non‐fullerene system PBDB‐T‐2F:Y6. The solar cells with aqueous MoO_x exhibit higher efficiencies and better stabilities without high‐temperature annealing compared to the solar cells with PEDOT:PSS.

1. Introduction

Polymer solar cells (PSCs) based on organic semiconductors are considered the most promising candidate as a future mobile energy source due to their advantages, including light weight, low cost, flexibility, and simple fabrication process with the possibility of printable or roll‐to‐roll mass production.^{[ 1} , ² , ³ , ⁴ , ⁵ , ^{6 ]} The power conversion efficiency (PCE) of single junction polymer solar cells has steadily increased and recently reached over 16% due to the development of novel high quality donor and acceptor materials.^{[ 7} , ⁸ , ⁹ , ^{10 ]} The increment of the efficiency of polymer solar cells was achieved by not only the development of higher performing new donor and acceptor materials, but also by improving the quality of charge transport layers (or called electrode buffer layers) such as the hole transport layer (HTL) and electron transport layer (ETL) inserted between the active layer and electrodes.^{[ 11 ]} It is clearly known that the inserted charge transport layers induce significantly improved efficiency by enhancing the charge extraction property and reducing leak currents which occur due to undesirable opposite charge flow. In addition, charge transport layers also have a significant influence on the stability of PSCs.

In earlier studies on polymer solar cells, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was the most popular hole transporting material because of its high conductivity and well‐matched work‐function (≈5.1 eV) for the highest occupied molecular orbital level of general donor polymers.^{[ 12} , ¹³ , ¹⁴ , ^{15 ]} However, since it is known that the acidity of PEDOT:PSS may cause degradation of both the indium tin oxide (ITO) electrode and active layer,^{[ 16} , ¹⁷ , ^{18 ]} there has been extensive research on HTL materials to replace PEDOT:PSS. Among the several materials that can be utilized as an alternative to PEDOT:PSS, the most prominent candidate materials are transition metal oxides such as MoO₃, V₂O₅, NiO, and WO₃ due to their good charge transport properties and high stability.^{[ 19} , ²⁰ , ²¹ , ^{22 ]} In particular, MoO₃ is the most widely utilized as an HTL in PSCs.

There are many ways to prepare the molybdenum oxide buffer layer, including a wet chemical sol–gel method, thermal evaporation, chemical vapor deposition, and pulsed laser deposition.^{[ 23} , ²⁴ , ²⁵ , ^{26 ]} Normally, solution‐processable MoO_x prepared by the sol–gel method has been mainly utilized for PSCs with a conventional structure. In the case of inverted PSCs, the thermal evaporation method is preferred for the deposition of the MoO_x HTL. This is because of the necessity of a high temperature annealing step to achieve the desired hole transporting property in the case of solution‐processable MoO_x. Unfortunately, normal organic‐based photovoltaic materials are not able to withstand high temperature annealing well. Moreover, the high‐temperature annealing process also acts as a disadvantage in the fabrication of flexible devices because general flexible substrates are not able to withstand high‐temperature annealing above 150 °C. Therefore, the development of annealing‐free solution‐processable metal oxides including MoO_x is essential for the large‐scale commercialization of flexible PSCs.

Depending on the nature of the base solvent, the sol–gel method can be classified into two routes: an aqueous sol–gel method and anhydrous sol–gel method. The difference between these two routes is whether water is used as the base solvent or not. In the aqueous sol–gel route, water and alcoholic solvents are used as the reaction medium, while in the anhydrous sol–gel route, only organic solvents are used as the reaction medium.^{[ 27 ]} Typically, MoO_x prepared by the anhydrous sol–gel method using MoO₂(acac)₂ is needed for high‐temperature curing to result in high quality. For example, Li and coworkers reported that MoO_x HTL deposited by spin coating of a precursor solution prepared through the dilution of MoO₂(acac)₂ in anhydrous isopropanol is needed for high temperature annealing over 150 °C in air.^{[ 28 ]} Riedl et al. also reported that the MoO_x film prepared by solution processing required high temperature annealing over 150 °C in an N₂ atmosphere to obtain a high work function of 5.3 eV.^{[ 29 ]}

In our present work, we developed annealing‐free solution‐processable “aqueous” MoO_x (aq‐MoO_x), and applied it in a bulk‐heterojunction (BHJ) PSC. An important reason for the high‐temperature annealing step after MoO_x film deposition is to remove the ligands. We found that the acetylacetonate ligand of MoO₂(acac)₂ can be easily removed through the simple hydrolysis caused by a small amount of water. By adjusting the concentration of water in the sol–gel route for the MoO_x precursor, we successfully developed an annealing‐free MoO_x HTL with excellent electrical properties. The non‐fullerene solar cell based on a blend of poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo][1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione) (PBDB‐T‐2F, also called PM6) and non‐fullerene acceptor (2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2,“3”:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) (Y6) with the general MoO_x layer prepared by the anhydrous sol–gel method showed a poor efficiency of 7.7% without annealing. If this anhydrous MoO_x (an‐MoO_x) layer was annealed at 200 °C, the efficiency was recovered to 17.1%, which was normal value typically observed in conventional structure PBDB‐T‐2F:Y6 solar cells. However, without any annealing process, the solar cell with aq‐MoO_x exhibited comparable performance of 17.0%. In addition, the solar cell with annealing‐free aq‐MoO_x exhibited better performance and stability without high‐temperature annealing compared to the solar cells with PEDOT:PSS. A similar trend was observed in a fullerene system solar cell based on a blend of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM). The PTB7‐Th:PC₇₁BM solar cell with non‐annealed an‐MoO_x showed very low efficiency of 1.37%, while the solar cell with annealing‐free aq‐MoO_x exhibited a higher efficiency and better stability without high‐temperature annealing compared to the solar cells with PEDOT:PSS.

2. Results and Discussion

Figure  1a shows the schematic diagrams of the PSCs with a conventional structure and the chemical structures of the materials used in this study. Molybdenyl acetylacetonate [MoO₂(acac)₂] was used as the starting material for preparing the an‐MoO_x and aq‐MoO_x solutions. The details regarding the fabrication of the an‐MoO_x and aq‐MoO_x solutions are described in the Experimental Section. Figure 1b shows the energy level diagram of the PSC with the conventional structure. To investigate the effect of aq‐MoO_x on non‐fullerene solar cells, a combination of PBDB‐T‐2F and non‐fullerene acceptor Y6,^{[ 30} , ^{31 ]} and a combination of poly[1‐(5‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐6‐methylbenzo[1,2‐b:4,5‐b′]dithiophen‐2‐yl)‐4‐hexylthiophen‐2‐yl)‐3‐(4‐hexyl‐5‐methylthiophen‐2‐yl)‐5‐octyl‐4H‐thieno[3,4‐c]pyrrole‐4,6(5H)‐dione] (PBDTTPD‐HT) and 2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IDIC) were utilized as the donor and acceptor materials.^{[ 32 ]} For the fullerene‐based active layer, PTB7‐Th and PC₇₁BM were used as the photoactive materials. For the electrons transport layer, solution‐processable ZnO nanoparticles (nanoparticle suspension) that can be processed at room temperature were used.

Figure 1.

a) Schematic diagrams of the PSCs with a conventional structure as well as the chemical structure of the materials and b) the energy level diagram of the PSC with a conventional structure.

Figure  2a,d shows the current density (J) versus voltage (V) characteristics of the PBDB‐T‐2F:Y6 non‐fullerene solar cells (Figure 2a) and PTB7‐Th:PC₇₁BM solar cells (Figure 2d) fabricated using aq‐MoO_x or an‐MoO_x, respectively. In addition, the J–V characteristics and photovoltaic parameters of the PBDTTPD‐HT:IDIC were shown in Figure S1 and Table S1, Supporting Information, respectively. The J–V results obtained from the device with the PEDOT:PSS HTL are presented for comparison. Note that the PEDOT:PSS film was annealed at 150 °C after spin‐coating. The external quantum efficiency (EQE) spectra of the PBDB‐T‐2F:Y6, PBDTTPD‐HT:IDIC non‐fullerene solar cells, and PTB7‐Th:PC₇₁BM solar cells are presented in Figure S2, Supporting Information. The details related to the performance of PTB7‐Th:PC₇₁BM and PBDB‐T‐2F:Y6 solar cells are listed in Table  1 . The statistics of photovoltaic parameters of the conventional polymer solar cell based on PTB7‐Th:PC₇₁BM, PBDTTPD‐HT:IDIC, and PBDB‐T‐2F:Y6 were shown in Figures S3–S5, Supporting Information, respectively. The PBDB‐T‐2F:Y6 solar cell with the non‐annealed an‐MoO_x HTL showed a very low power conversion efficiency of 7.7% with a relatively low short‐circuit current density (J _sc) of 20.50 mA cm⁻², an open‐circuit voltage (V _oc) of 0.826 V, and a fill factor (FF) of 45.2%. These results are inevitable consequences of the general non‐annealed MoO_x HTL prepared by the anhydrous sol–gel method because there was no chance to remove the ligands from the MoO_x HTL without high‐temperature annealing. In fact, if the MoO_x HTL is prepared with high‐temperature annealing at 200 °C, the efficiency of the solar cell fabricated with the an‐MoO_x HTL recovered to a normal level of 17.1% with a J _sc of 27.43 mA cm⁻², a V _oc of 0.845 V, and an FF of 73.8%. However, the solar cell with aq‐MoO_x prepared by the aqueous sol–gel method showed almost comparable performance to the solar cell with the annealed an‐MoO_x HTL, even without high‐temperature annealing. Note that the aq‐MoO_x HTL was kept at 50 °C for 5–10 min to dry the surface because a hydrophobic organic active layer was not deposited well on top of the aq‐MoO_x HTL without removing surface moisture. The solar cell with aq‐MoO_x yields a PCE of 17.0% with a J _sc of 27.53 mA cm⁻², a V _oc of 0.843 V, and an FF of 73.1%. The solar cell with aq‐MoO_x exhibited better performance compared to the solar cell with the PEDOT:PSS HTL. Moreover, the performance of PBDB‐T‐2F:Y6 solar cells with the aq‐MoO_x HTL did not change even after annealing at high temperatures of up to 200 °C, as shown in Figure 2b (Table S2, Supporting Information). The optical properties of these three MoO_x HTLs did not differ significantly from each other, as shown in Figure 2c. However, the MoO_x HTL exhibited a slightly higher transmittance in the visible region compared to PEDOT:PSS.

Figure 2.

a) J–V characteristics of the PBDB‐T‐2F:Y6 solar cells, b) J–V characteristics of the PBDB‐T‐2F:Y6 solar cells with aq‐MoO_x annealed at various temperatures, c) transmittance spectra of PEDOT:PSS, an‐MoO_x, and aq‐MoO_x, and d) J–V characteristics of the fullerene‐based solar cell based on PTB7‐Th:PC₇₁BM.

Table 1.

Summarized photovoltaic parameters of the conventional polymer solar cell based on PTB7‐Th:PC₇₁BM and non‐fullerene solar cell based on PBDB‐T‐2F:Y6

Active layer	Description	JV J sc [mA cm−2]	EQE J sc [mA cm−2]	V oc [V]	FF [%]	Best PCE [%]
PBDB‐T‐2F and Y6	PEDOT:PSS	26.50	26.14 (98.6%)	0.854	0.735	16.6
	Anhydrous MoOx	20.50	20.02 (97.6%)	0.826	0.452	7.7
	Anhydrous MoOx (200 °C)	27.43	26.87 (97.9%)	0.845	0.738	17.1
	Aqueous MoOx	27.53	26.91 (97.7%)	0.843	0.731	17.0
PTB7‐Th and PC71BM	PEDOT:PSS	15.67	14.88 (94.9%)	0.791	0.673	8.3
	Anhydrous MoOx	4.25	4.12 (96.9%)	0.683	0.479	1.4
	Anhydrous MoOx (200 °C)	16.60	16.35 (98.5%)	0.791	0.682	9.0
	Aqueous MoOx	16.69	16.20 (97.1%)	0.790	0.671	8.9

A similar trend was observed in the fullerene‐based solar cell using PTB7‐Th:PC₇₁BM (see Figure 2d). The solar cell with the non‐annealed an‐MoO_x HTL showed a PCE of just 1.4% with a poor FF (47.9%) and a very low J _sc (4.25 mA cm⁻²) together with a larger series resistance. When the an‐MoO_x HTL was annealed at 200 °C, however, the efficiency was recovered to 9.0% (see Figure S6 and Table S3, Supporting Information). With the aq‐MoO_x HTL, the PTB7‐Th:PC₇₁BM solar cell exhibited a comparable efficiency of 8.9% even without any annealing process. The J–V curves and photovoltaic parameters of polymer solar cells based on PTB7‐Th:PC₇₁BM with aq‐MoO_x with various H₂O concentrations were shown in Figure S7 and Table S4, Supporting Information.

In the evaluation of the solar cell performance, the aq‐MoO_x HTL clearly performed better than the PEDOT:PSS HTL. Figure  3a shows the charge extraction (CE) current, which was measured under 1‐sun illumination and V _oc conditions. From this CE current, the CE density was deduced, as shown in Figure 3b. The CE density results were consistent with the J–V characteristics. In the case of the device with the PEDOT:PSS HTL, the calculated CE density was 1.09 × 10¹⁴ cm⁻². The device with the non‐annealed an‐MoO_x HTL showed a one order of magnitude lower CE density of 6.93 × 10¹³ cm⁻² than the device with the PEDOT:PSS HTL. However, similar to the J–V results, the CE density of the device with the an‐MoO_x HTL significantly increased with high temperature annealing at 200 °C. In the case of the device with the aq‐MoO_x HTL, although the calculated CE density value was slightly lower than that of the device with the annealed an‐MoO_x HTL (1.35 × 10¹⁴ cm⁻²), it clearly showed a better CE density value (1.23 × 10¹⁴ cm⁻²) than that of the device with the PEDOT:PSS HTL.

Figure 3.

a) Charge extraction (CE) current transient under 1 sun illumination and V _oc conditions, b) calculated charge extraction density, and c) Nyquist plot of the impedance spectroscopy measurements of the polymer solar cells with various MoO_x HTLs under dark conditions.

The improved electrical properties of the aq‐MoO_x HTL were also observed in the impedance spectroscopy (IS) study. The Nyquist plots of the IS results measured under dark conditions are shown in Figure 3c. All of the devices exhibited one semicircle without a transmission line (TL), which indicates that all devices undergo strong recombination.^{[ 33 ]} In general, this feature can be interpreted using the Gerischer impedance model, where the square root of the product of the recombination resistance (R _rec) and transportation resistance (R _tr) can be described as the Gerischer resistance (R _G).^{[ 34} , ^{35 ]} The device with the annealed an‐MoO_x HTL or aq‐MoO_x HTL exhibited a higher R _G (or R _rec, which is inversely proportional to the recombination rate) than the device with the non‐annealed an‐MoO_x HTL or PEDOT:PSS. Since all devices have the same structure except for the MoO_x layer, the difference of R _G definitely resulted from the charge transportation at the MoO_x layer and charge recombination at the interface. A higher R _G corresponds to a lower recombination probability, which indicates better charge transport ability without recombination. Therefore, the IS results shown in Figure 3c clearly indicate that the charge transport properties of the aq‐MoO_x HTL was better than that of the PEDOT:PSS HTL.

Normally, in metal oxide layers prepared by an anhydrous sol–gel method, high‐temperature curing is conducted to remove ligands, which provide solubilizing power in metal oxide nanoparticles. To explore the chemical structure differences between the an‐MoO_x HTL and aq‐MoO_x HTL, Fourier transform infra‐red (FTIR) spectroscopy was performed, as shown in Figure  4 . The non‐annealed an‐MoO_x layer showed clear evidence of the characteristic peaks of acetylacetonate in the fingerprint region including a C=C stretch at 1573 cm⁻¹, C=O stretch at 1530 cm⁻¹, C=O stretch + C—H bending at 1422 cm⁻¹, CH₃ symmetrical bending at 1354 cm⁻¹, C=C stretch + C—CH₃ stretch at 1280 cm⁻¹, and CH₃ rocking at 1029 cm⁻¹.^{[ 36} , ³⁷ , ^{38 ]} In addition, the non‐annealed an‐MoO_x layer also exhibited a broad peak at 3300 cm⁻¹, which is attributed to the O—H stretch vibration of ethanol, and two C—H stretch peaks near 2980 cm⁻¹, which are attributed to acetylacetonate (Figure 4b).^{[ 39 ]} However, in the an‐MoO_x layer cured at 200 °C, all carbon related peaks disappeared, which indicates that acetylacetonate ligands were successfully removed by the curing process. The chemical structure change of the an‐MoO_x layer as a function of the curing temperature is presented in Figure S8, Supporting Information. The removal of acetylacetonate peaks from the an‐MoO_x layer was only completed at 200 °C, which is in good agreement with previous literatures.^{[ 28} , ^{29 ]} For the aq‐MoO_x layer, however, the acetylacetonate peaks almost vanished in the FTIR spectra even without a curing process. The aq‐MoO_x layer showed a bending and stretching vibrations of H₂O at 1620 and 3300 cm⁻¹, respectively. Therefore, it can be concluded that all of the acetylacetonate ligands were already removed from the solution state by the hydrolysis effect resulting from the added water. The FTIR spectra of the aq‐MoO_x layer as a function of the curing temperature is shown in Figure S9, Supporting Information. With increasing curing temperature, only water peaks were removed. However, the presence of water does not seem to significantly influence the performance of the solar cells (Figure 2c).

Figure 4.

Fourier transform infra‐red (FTIR) spectroscopy of MoO_x HTLs: a) fingerprint region, b) function group region, and c) CH₂ and CH₃ stretch mode vibration peaks of non‐annealed an‐MoO_x films.

The non‐annealed an‐MoO_x exhibited peaks at 960, 880, 800, and 564 cm⁻¹. The peak at 800 cm⁻¹ was characterized as the C—H out‐of‐plane bending mode originating from acetylacetonate. The doublet peak at 960 and 880 cm⁻¹ was characterized as the stretching mode of cis‐MoO₂, O=Mo=O, while the peak at 564 cm⁻¹ was identified as the bending mode of Mo—O—Mo.^{[ 40} , ^{41 ]} The peak in the range of 850–600 cm⁻¹ was identified as the stretching mode of Mo—O—Mo.^{[ 28} , ³⁶ , ^{42 ]} In addition, only non‐annealed an‐MoO_x exhibited the CH₂ stretching mode at 2927 cm⁻¹ (Figure 4c), which indicates the possibility of interaction between ethanol and MoO₂(acac)₂. Based on all of the FTIR results, it can be concluded that the non‐annealed an‐MoO_x film could be described as Mo₂O₅(acac)₂(EtOH)₂.^{[ 40 ]} In the an‐MoO_x film annealed at high temperature, the C—H out‐of‐plane bending mode peak at 800 cm⁻¹ disappeared due to the decomposition of the acetylacetonate ligand. Furthermore, the cis‐MoO₂ peak at 880 cm⁻¹ disappeared and the peak at 960 cm⁻¹ was slightly shifted to 985 cm⁻¹, which is attributed to Mo=O stretching. In addition, the CH₂ stretching mode peak also disappeared. All of these phenomena observed after high‐temperature annealing can be interpreted as the process of conversion from the cis‐MoO₂ form with ligands to a form close to MoO₃.^{[ 43 ]} For the aq‐MoO_x, the overall features of the spectrum in the 1100–500 cm⁻¹ region were similar to annealed an‐MoO_x. No C—H related peaks were observed. The board peak at 880 cm⁻¹ in aq‐MoO_x was interpreted as a MoO₃(H₂O)_x stretching mode. The IR spectrum of aq‐MoO_x showed a similar feature of molybdenum trioxide hydrates.^{[ 40} , ⁴³ , ^{44 ]} Although the intermediate hydrate Mo₂O₅(acac)₂(H₂O)₂ state was not observed in our study, the overall FTIR results clearly indicate that MoO₂(acac)₂ was successfully converted to (MoO₃)_n(H₂O)_{2 n +1} without high temperature curing due to hydrolysis with added water.

X‐ray photoelectron spectroscopy (XPS) analysis was carried out to more clearly elucidate the state of the Mo atoms. Figure  5a,b shows the XPS spectra of the Mo 3d and O 1s peaks, respectively. For the Mo 3d peaks, by using the Gaussian fitting method, the two Mo 3d peaks were divided into two doublets originating from the spin‐orbital splitting. The major doublet (higher intensity) at 232.8 and 236.02 eV belonged to the Mo⁶⁺ oxidation state, while the minor doublet (lower intensity) at 231.7 and 234.5 eV belonged to the Mo⁵⁺ oxidation state.^{[ 29 ]} The atomic concentration ratio of Mo⁶⁺/Mo⁵⁺ was determined from the proportion of the integrated peak area of each component. The Mo⁶⁺/Mo⁵⁺ ratios were 3.86 and 4.59 for an‐MoO_x without annealing and aq‐MoO_x, respectively. For an‐MoO_x with annealing, the Mo⁶⁺/Mo⁵⁺ ratio was 5.13. The O 1s peak was analyzed as two peaks by Gaussian fitting. The lower binding energy peak (oxygen‐LBE) at 530.7 eV is assigned to bonding with the Mo atom, while the higher binding energy peak (oxygen‐HBE) at 531.7 eV is attributed to the oxygen associated with the acetylacetonate ligand.^{[ 28 ]} The oxygen‐HBE/oxygen‐LBE ratios were 1,28, 0.41, and 0.94 for an‐MoO_x without annealing, an‐MoO_x with annealing, and aq‐MoO_x, respectively.

Figure 5.

XPS spectra of a) Mo 3d and b) O 1s core level peaks obtained from the MoO_x layers.

Among the three MoO_x HTLs, the annealed an‐MoO_x showed a higher Mo⁶⁺/Mo⁵⁺ ratio and lower oxygen‐HBE/oxygen‐LBE ratio compared to an‐MoO_x without annealing, which means that the Mo⁶⁺ fraction and oxygen‐LBE fraction were enhanced after high‐temperature annealing. These results clearly indicate that Mo⁵⁺ was further oxidized to Mo⁶⁺ due to decomposition of the acetylacetonate ligand.^{[ 45 ]} A similar trend was observed in the comparison between annealed an‐MoO_x and aq‐MoO_x. The aq‐MoO_x showed a higher Mo⁶⁺/Mo⁵⁺ ratio and lower oxygen‐HBE/oxygen‐LBE ratio compared to an‐MoO_x without annealing. Therefore, we can conclude that water removed the acetylacetonate ligand from MoO₂(acac)₂ and oxidized Mo atoms to the highest oxidation state of Mo⁶⁺. Interestingly, the Mo:O stoichiometry of all MoO_x layers was below than 1:3, which indicates that all MoO_x layers were sub‐stoichiometric. This is the reason why we used MoO_x instead of MoO₃. The Mo:O stoichiometry is listed in Table S5, Supporting Information. Consequently, we clearly confirmed that the acetylacetonate ligand can be removed not only by high‐temperature curing but also by the hydrolysis process of water. Based on the results of the FTIR and XPS studies, the expected chemical structures of the an‐MoO_x and aq‐MoO_x precursor solutions are illustrated in Figure  6 .

Figure 6.

Schematic illustration of the expected chemical structures of the an‐MoO_x and aq‐MoO_x precursor solutions.

To investigate the band structure difference between an‐MoO_x and aq‐MoO_x, ultraviolet photoelectron spectroscopy (UPS) analysis was conducted, as shown in Figure S10, Supporting Information. The summary of the UPS analysis is illustrated in Figure  7 as band diagrams. The Fermi energy (E _F) was calculated from the ITO surface and all other spectra are plotted with respect to this value. The calculated E _F values were 4.74 and 5.14 eV for ITO and PEDOT:PSS, respectively. The vacuum levels (VLs) were determined by linear extrapolation of the secondary electron cutoffs on the high binding energy side. The valence band (VB) maximum was extracted from the onset on the low binding energy side. The relative position of the VB maximum level was determined by comparing the shift of the onset to the E _F of ITO. The conduction band (CB) minimum level was estimated using the extracted VB maximum and the measured optical gaps extracted from the UV–vis absorption spectra (Figure S11 and Table S6, Supporting Information). All MoO_x layers exhibited an optical band gap of 3.22 eV, which are in agreement with the literature data.^{[ 46 ]} The workfunction (WF) of the non‐annealed an‐MoO_x HTL was 4.74 eV, which is comparable to that of the ITO substrate. The WF of the an‐MoO_x HTL with annealing was enlarged due to the VL shift of 0.39 eV. In the case of aq‐MoO_x, the measured WF was 4.92 eV. A similar trend was observed for the calculated hole injection barrier (φ _h). The deduced φ _h values were 0.74, 0.49, and 0.60 eV for the non‐annealed an‐MoO_x HTL, an‐MoO_x HTL with annealing, and aq‐MoO_x, respectively. The an‐MoO_x HTL with annealing showed more favorable energy level alignment for hole transport. The φ _h value of aq‐MoO_x was slightly higher than that of the annealed an‐MoO_x HTL. This energy level difference played an important role in the performance of the polymer solar cell. All of the solar cell results were consistent with this φ _h difference. Of course, the energy level alignment of aq‐MoO_x was not the best, but it is worth noting that a significant improvement was achieved without high temperature curing. To get more insight into the charge transport property at the interface between HTLs and active layer, we have measured hole mobilities using hole‐only devices with structure ITO/HTLs/active layer/evaporated MoO_x/Ag. The active layer thickness was 150 nm and the thickness of HTLs was shown in Table S7, Supporting Information. The active layer permittivity was 3.5, as previously stated in literature.^{[ 47 ]} The hole mobilities of various hole transport layers (HTLs)/active layer were determined by the steady‐state space‐charge‐limited current (SCLC) method. The SCLC region and hole mobility of various HTLs/active layer were shown in Figure S12 and Table S8, Supporting Information, respectively. The hole mobility of PEDOT:PSS/PBDB‐T‐2F:Y6 was similar to other studies reported in literature.^{[ 30 ]} The distinguishable difference in the hole mobility of an‐MoO_x with and without annealing showed a clear effect of high temperature annealing in the removal of the organic ligand. Furthermore, the hole mobility of aq‐MoO_x was higher than that of PEDOT:PSS. Overall, the hole mobility data in Table S8, Supporting Information, is consistent with the J–V curves of polymer solar cells.

Figure 7.

Energy band diagrams of aq‐MoO_x and an‐MoO_x with and without annealing layers obtained from UPS measurements.

In addition to the energy level alignment, morphology changes can also influence the charge transport properties. However, a significant surface morphology change was not detected in the atomic force microscope (AFM) measurements (Figure S13, Supporting Information). Furthermore, the morphology of PBDB‐T‐2F:Y6 casted on various hole transport layers was also examined (Figure S14, Supporting Information). As shown in Figure S14, Supporting Information, all AFM images showed similar fibril network morphology. Therefore, we concluded that the difference of the performance of the solar cell was not due to a morphology change at the interface between the MoO_x HTL and active layer.

Since the aq‐MoO_x HTL does not require an annealing process, the water added during the sol–gel process remains in the film (Figure 4). Of course, it is expected to be in a weakly bonded form, but it is doubtful that the remaining water will affect the stability of the device. To confirm this point, we evaluated the operation stability for the PBDB‐T‐2F:Y6 non‐fullerene solar cells. This stability test was conducted in air without any encapsulation. Figure  8 shows the changes of J _sc, V _oc, FF, and PCE measured continuously over 25 h with various HTLs as PEDOT:PSS, aq‐MoO_x without annealing, and aq‐MoO_x annealed at 150 °C. Because of well‐known weak stability of non‐fullerene solar cells based on Y6, both solar cell with the PEDOT:PSS and solar cell with MoO_x HTL showed continuous decreases of PCE from the beginning.^{[ 48 ]} However, after 25 h of operation, the solar cell with aq‐MoO_x showed better condition compared to the solar cell with PEDOT:PSS.

Figure 8.

Changes of a) J _sc, b) V _oc, c) FF, and d) PCE of non‐fullerene solar cell based on PBDB‐T‐2F:Y6 during continuous operation in air without any encapsulation.

Storage stability performed for PTB7‐Th:PC₇₁BM solar cells also showed similar trend (Figure S15, Supporting Information). The PTB7‐Th:PC₇₁BM solar cell with the PEDOT:PSS HTL showed continuous decreases of J _sc and FF from the beginning. After 1000 h, the PCE significantly decreased from 8.43% to 4.9%. In the case of the solar cell with the aq‐MoO_x HTL, however, all of the solar cell parameters were well maintained compared to the solar cell with PEDOT:PSS. The PCE decreased by only 9% from 8.81% to 8.01%. Therefore, it can be concluded that the aq‐MoO_x HTL does not significantly affect the stability even if it contains water in the film.^{[ 49 ]}

The most notable advantage of annealing‐free aq‐MoO_x is that it allows the fabrication of an inverted solar cell employing only a solution process. Some organic photoactive materials become significantly degraded with high temperature annealing including PTB7‐Th:PC₇₁BM and PBDTTPD‐HT:IDIC combinations. The PTB7‐Th and PC₇₁BM active layer allows only a maximum temperature of 80 °C. Thus, in most cases, PTB7‐Th and PC₇₁BM solar cells were fabricated with room temperature drying by keeping them in a glove box for 2–3 h. The PBDTTPD‐HT:IDIC active layer is similar as it also allows only a maximum of 100 °C annealing after active layer deposition. Therefore, the fabrication of inverted solar cells was not possible using general solution‐processable HTL materials, which require high‐temperature curing over 150 °C. In most cases, a vacuum deposition method was utilized for the HTL construction. The use of PEDOT:PSS was also limited in the fabrication of inverted solar cells because PEDOT:PSS also needed to be dried at a temperature of at least 100 °C. However, with our aq‐MoO_x HTL, active layer degradation by high temperature annealing is no longer a serious problem. Figure  9 shows the J–V characteristics of the PTB7‐Th:PC₇₁BM and PBDTTPD‐HT:IDIC solar cell fabricated with an inverted structure. The HTLs were deposited on active layer in N₂ or in air. In both PTB7‐Th:PC₇₁BM and PBDTTPD‐HT:IDIC active layer combinations, the solar cells with aq‐MoO_x showed slightly better performance compared to the solar cell with the PEDOT:PSS HTL. Somewhat lower performance compared to the solar cell with the conventional structure was obtained because of the adsorption property. Since aq‐MoO_x is based on a hydrophilic solvent, it is somewhat difficult to deposit it on a hydrophobic organic photoactive layer by spin‐coating. Therefore, to enhance the adsorption property, we added 2 mg mL⁻¹ of Triton X‐100 into the PEDOT:PSS and aq‐MoO_x solution.^{[ 50} , ^{51 ]} This performance decrease was most likely caused by adding Triton X‐100.

Figure 9.

a) J–V characteristics of the inverted polymer solar cells based on PTB7‐Th:PC₇₁BM and the b) J–V characteristics of polymer solar cell based on PBDTTPD‐HT:IDIC. The hole transport layers were deposited on active layer in N₂ or air. To obtain better adsorption with an organic active layer, 2 mg mL⁻¹ of Triton X‐100 was added into the PEDOT:PSS and aq‐MoO_x solution.

3. Conclusion

In summary, we developed annealing‐free solution‐processable aqueous MoO_x and applied it in a BHJ polymer solar cell based on a blend of PBDB‐T‐2F:Y6 and PTB7‐Th:PC₇₁BM. In general, the charge transport layer based on moly oxides prepared by an anhydrous sol–gel method normally requires high‐temperature annealing in air to achieve desired quality. As a result, the non‐fullerene solar cell with the general an‐MoO_x HTL prepared by the sol–gel method showed a poor efficiency of 7.7% without annealing. This poor efficiency was recovered to the normal value typically observed in PBDB‐T‐2F:Y6 solar cells when this an‐MoO_x layer was annealed at 200 °C. However, the solar cell with aq‐MoO_x prepared by the aqueous sol–gel method showed almost comparable performance to the solar cell with the annealed an‐MoO_x HTL, even without high‐temperature annealing. Moreover, the solar cell with aq‐MoO_x exhibited a higher efficiency and better stability without high‐temperature annealing compared to the solar cells with PEDOT:PSS. A similar trend was observed in the fullerene‐based solar cell based on PTB7‐Th:PC₇₁BM. In addition, annealing‐free aq‐MoO_x allowed the successful fabrication of inverted PTB7‐Th:PC₇₁BM solar cell, which was impossible to fabricate using a general anhydrous MoO_x HTL which requires a high‐temperature annealing process. Although high‐temperature annealing is not a difficult process in laboratory scale production, it is definitely not a simple process to apply in roll‐to‐roll processes because of the inevitable long cooling step that follows. Thus, our aqueous method based on the hydrolysis effect can be an essential technique for large‐scale roll‐to‐roll mass production of flexible devices.

4. Experimental Section

Preparation of the MoO_x Solution

Molybdenum dioxydiacetylacetonate (MoO₂(acac)₂ 99%) was purchased from Sigma‐Aldrich and used as the precursor material for the aqueous and anhydrous MoO_x solutions. For the anhydrous solution, MoO₂(acac)₂ (12 mg mL⁻¹) was diluted in 10 mL of anhydrous ethanol 99.9% and stirred for 1 h to obtain a clear and colorless solution. In the case of the aqueous solution, MoO₂(acac)₂ (12 mg mL⁻¹) was diluted in a mixture of 1 mL anhydrous of ethanol 99.9% (Sigma‐Aldrich) and 9 mL of deionized water. Ethanol was added to improve the solubility of MoO₂(acac)₂ and hydrophilicity of aq‐MoO_x solution. The aqueous solution was stirred for 1 h to obtain a light‐blue solution. Both aqueous and anhydrous solutions were kept at room temperature for aging over 10 days, which were needed for complete hydrolysis process of MoO_x solution, to obtain ready to use dark‐blue color solutions. To prepare the MoO_x solution used for the inverted polymer solar cell, Triton X‐100 (Sigma‐Aldrich) was mixed with the MoO_x aqueous solution at a ratio of 2:4 (mg:mL), which was denoted as ratio 0.5. The Triton X‐100:MoO_x solution was ultrasonicated for 2 h and kept at room temperature.

Device Fabrication

The conventional BHJ polymer solar cells were fabricated using an ITO/MoO_x/PTB7‐Th:PC₇₁BM/ZnO/Al structure. The patterned indium‐tin oxide (ITO) substrates were subjected to ultrasonication in deionized water, acetone, and isopropyl alcohol, respectively. They were then dried in an oven at 100 °C for several hours. Once dried, the substrates were treated with UV for 60 min to increase the hydrophilicity. The MoO_x film was coated on the cleaned ITO substrates by spin coating the aqueous MoO_x solution or anhydrous MoO_x solution at 5000 rpm followed by annealing at 50 or 200 °C for 10 min in air. Then, the MoO_x film was cooled to room temperature and then placed into an N₂‐filled glovebox to make the solar cell active layer. The PTB7‐Th:PC₇₁BM (1:1.5 by weight) blend solution was prepared by dissolving in chlorobenzene with 3% DIO as an additive to achieve better phase separation. The PBDTTPD‐HT:IDIC (1:1.5 by weight) blend solution was prepared by dissolving in chloroform with 0.6% DIO additive. The PBDB‐T‐2F:Y6 (1:1.2 by weight) blend solution was prepared by dissolving in chloroform with 0.5% CN additive. The solution was stirred for 1 day before using. After passing through a 0.45 µm PTFE syringe filter, the blend solution was spin‐cast onto the MoO_x layer with a spin speed ranging from 900 to 1100 rpm to create an approximately 100 nm thick active layer of PTB7‐Th:PC₇₁BM. In case of PBDTTPD‐HT:IDIC and PBDB‐T‐2F:Y6, a spin speed ranging from 3000 to 4000 rpm was used. A ZnO nanoparticle solution (2.5 wt% in IPA) diluted in IPA at a ratio of 1:5 was spin‐coated on top of the active layer at 5000 rpm. The devices were kept inside the glovebox for 1 h for drying at room temperature. Then, the devices were placed in the evaporation chamber to make 100 nm of Al under a high vacuum pressure of less than 10⁻⁶ Torr. The active surface area of the device, as defined by a metal shadow mask, was 0.13 cm². For the reference device, PEDOT:PSS (CLEVIOS PH1000), which was substituted for the MoO_x layer, was spin‐coated on top of ITO substrate at 5000 rpm and annealed at 150 °C in air.

The inverted BHJ polymer solar cells were fabricated using an ITO/ZnO/PTB7‐Th:PC₇₁BM/MoO_x/Ag structure. A ZnO nanoparticle solution (2.5 wt% in IPA) diluted in IPA at a ratio of 1:1 was spin‐coated on top of the ITO substrate at 5000 rpm and annealed at 85 °C for 10 min in air. Then, the ZnO film was cooled to room temperature and then placed in an N₂‐filled glovebox to make the solar cell active layer. The PTB7‐Th:PC₇₁BM blend solution was spin‐cast onto the ZnO layer at 1000 rpm. A Triton X‐100:MoO_x aqueous solution was spin‐coated on top of the active layer at 3000 rpm and the devices were dried inside the glovebox for 1 h without using any post‐treatment annealing temperature. For the reference device, modified PEDOT:PSS (M‐PEDOT) was spin‐coated on top of the active layer at 5000 rpm and annealed at 80°C for 10 min. Then, the devices were placed in the evaporation chamber to make 100 nm of Ag under a high vacuum pressure of less than 10⁻⁶ Torr and the active surface area of the device was 0.13 cm². The non‐fullerene polymer solar cell was fabricated using the same procedure described above, except that PBDTTPD‐HT:IDIC was used as the polymer active layer.

Device Characteristics

The current density–voltage curves of the solar cell devices were obtained using a Keithley 2401 source measurement unit under AM1.5G simulated illumination (100 mW cm⁻²). A standard Si photodiode detector with a KG‐3 filter (Newport Co., Oriel) was used to calibrate the intensity of the simulated sunlight. All devices were measured under an inert environment using a solar simulator inside an N₂‐filled glove box. The EQE of the optimized solar cell performance was determined by the IQE‐200B (Newport Co., Oriel). The absorption and transmittance spectra of MoO_x were measured by using a UV–vis spectrophotometer (Varian, Carry 5000). The surface morphologies of the MoO_x layers and Triton X‐100:MoO_x layer were measured by using an atomic force microscope (AFM, Nanocute, SII NanoTechnology). UPS and XPS were carried out with an ESCALAB 250‐XI surface analysis system equipped with a He‐discharge lamp providing He‐I photons with an energy of 21.22 eV for UPS analysis and a monochromatic Al‐Kα X‐ray gun with a photon energy of 1486.6 eV for the XPS investigation. The base vacuum pressure of the analysis system was ≈10⁻⁷ Torr. The Fermi edge was calibrated using a clean Au film and all spectra were plotted with respect to the determined Fermi level. All XPS measurements were calibrated with reference to the Au 4f_7/2 core level (83.98 eV) of a freshly deposited Au film. All binding energies were normalized with the C 1s peak as an internal standard. CE and CELIV were conducted using the CE and CELIV analyzer functions of an organic semiconductor parameter test system (McScience T4000) under 1.0 sun at V _OC conditions. FTIR was conducted using a Bruker Invenio R spectrometer. Impedance spectroscopy (IS) was carried out by using an impedance analyzer (IVIUM tech, IviumStat) under dark conditions, which measured the optoelectronic frequency response in the frequency range of 1 MHz to 1 Hz. The thickness of various hole transport layers was determined by using Stylus Profilometry (Bruker Dektak XT). The hole mobilities of various hole transport layers (HTLs)/active layer were measured by the SCLC method. The structures of the hole‐only device were ITO/HTLs/PBDB‐T‐2F:Y6/MoO_x/Ag. The hole mobility of HTLs/active layer was determined by Mott–Gurney law in the SCLC trap free regime, by using the slope of J ^0.5 versus V.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Tran H. N., Park S., Wibowo F. T. A., Krishna N. V., Kang J. H., Seo J. H., Nguyen‐Phu H., Jang S.‐Y., Cho S., 17% Non‐Fullerene Organic Solar Cells with Annealing‐Free Aqueous MoOx . Adv. Sci. 2020, 7, 2002395 10.1002/advs.202002395