Effect of solvent additive on active layer morphologies and photovoltaic performance of polymer solar cells based on PBDTTT-C-T/PC₇₁BM

Abstract

Photovoltaic properties of polymer solar cells (PSCs) are strongly affected by surface and bulk morphologies of their active layers. Herein, three solvent additives with different boiling points (BP) and asymmetric solvencies with respect to the polymer donor and fullerene-based acceptor were used to control the surface and bulk morphologies of the blend active layers based on PBDTTT-C-T and PC₇₁BM. Based on the detailed results obtained from photovoltaic measurements and morphological characterization, the correlations between the nature of the solvent additives, the surface and bulk morphologies of the blend films and the photovoltaic performance of the devices are rationally demonstrated. We found that the V_oc is determined by the contact surface potential, which is affected by the donor/acceptor composition at the top surface, while the J_sc and FF are heavily influenced by the sizes and the relative compositional fluctuations of the aggregations in the blends. It was also found that the relative solvencies of the additives to the solutes and the large difference of saturated vapor pressure (P^o) values between the host solvent (o-dichlorobenzene, o-DCB) and the guest solvent (additives with high BP) play the key roles in affecting surface and bulk morphologies of the PBDTTT-C-T : PC₇₁BM blend films. Overall, this work provides informative and useful guidance to select the best solvent additive for morphology control of polymer solar cells.

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Introduction

Polymer solar cells (PSCs) have attracted considerable attention because of their advantages of low cost, easy fabrication, light weight, and capability to be fabricated into flexible large-area devices.^1–5 Recently, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) polymer solar cells, consisting of conjugated polymers as the donor and fullerene derivatives as the acceptor, have reached up to ∼10%.⁶ The morphology⁷ of active layers of BHJ solar cells plays a critical role affecting the device performance. The ideal morphology formed in the active layer not only provides sufficient interfaces for efficient charge separation but also good percolation pathways for charge carrier transport to the respective electrodes so as to minimize the recombination of free charges.⁸ Several strategies including thermal⁹ or solvent annealing,¹⁰ solvent selection,¹¹ mixed solvent¹² and solvent additives^13–15 have been applied to modify or control the nanoscale morphologies of the D/A blends to a favorable state with suitable phase separation and interpenetrating network for getting better photovoltaic performance.

Since the pioneer work by Bazan et al.,¹³ the addition of solvent additives with high boiling points (BP) have attracted much attention and have been used as a simple and effective way to improve the morphology of the active layer of BHJ PSC devices. That is, solvent mixtures based on a host solvent with lower BP and a guest solvent with relatively higher BP are used as processing solvents to fabricate the BHJ layers. Since the two solvents have different BP and thus different vaporizing speed, during the coating process, the composition of the solvent mixture will be changed gradually and thus the solvency of the solvent mixture to the solutes will also be changed gradually. Consequently, morphologies of the BHJ layers can be modified by changing composition of the solvent mixtures or selecting different additives. The effect of the solvent additives on morphology control can be attributed to two properties: their selective solvency to the ingredients in the BHJ layers and their low volatility.¹⁴ High BP solvents like 1,8-octanedithiol (OT), 1,8-diiodooctane (DIO) and N-methyl pyrrolidone (NMP) have been selectively used to optimize the morphologies of active layers for PSCs based on different polymers.^14,16 Furthermore, many reports have been made about the effects of different solvent additives on photovoltaic performance of PSCs.¹⁷ However, to our knowledge, there is rare systematic study on how the variation of the surface and bulk morphologies caused by the different additives influences the photovoltaic parameters in one photovoltaic material system so far. Therefore, to investigate the influence of different additives on the variation of every photovoltaic parameter would be of the great importance to guide the additive selecting for the morphology control and thus photovoltaic performance optimization.

In this work, the blend of poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo-[1,2-b:4,5-b']dithiophene-2,6-diyl)-alt-(2-(2-ethylhexanoyl)-thieno[3,4-b]thiophen-4,6-diyl) (PBDTTT-C-T) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) (1 : 1.5, w/w) was chosen as the model system and three solvent additives, including DIO, NMP and OT, (see Scheme 1) were used to tune morphologies of the blend films. Interestingly, when different additive was used during spin-coating process, the photovoltaic parameters including J_sc, V_oc and FF of the devices varied distinctly. Therefore, the blend films processed with these three additives were characterized to provide morphological properties at the surface and in the bulk of blend films.

Scheme 1 Molecular structures of PBDTTT-C-T, PC₇₁BM, DIO, NMP, and OT.

Results and discussion

Photovoltaic properties

PSC devices with a structure of ITO/PEDOT:PSS (35 nm)/PBDTTT-C-T : PC₇₁BM (1 : 1.5, w/w) (100 nm)/Ca (20 nm)/Al (100 nm) were fabricated. The active layers of the devices were prepared by spin-coating the blend solutions of PBDTTT-C-T : PC₇₁BM using o-dichlorobenzene (o-DCB) as host solvent and high BP solvent additive as guest solvent. In the previous work,¹⁸ the optimum amount of additives was determined to be 3%, (DIO/o-DCB, v/v). Fig. 1 shows current density-voltage (J–V) curves and external quantum efficiency (EQE) curves of PSC devices under the illumination of AM 1.5G, 100 mW cm⁻². The absorption spectra of the active layers are provided as ESI (see Fig. S1†). The photovoltaic data of the devices are summarized in Table 1. As observed, the photovoltaic parameters of the PSCs changed greatly by using these different solvent additives. The PSC based on the blend film cast from o-DCB shows a PCE of 5.43% with a V_oc of 0.83 V, a J_sc of 13.1 mA cm⁻² and a FF of 0.50. By using 3% DIO as the additive, J_sc and FF of the device increased to 15.6 mA cm⁻² and 0.63, respectively, and as a result, the overall efficiency was improved to 7.37%, while its V_oc dropped to 0.75 V. Compared to the device processed from pure o-DCB, the device processed from the o-DCB/NMP mixture exhibited little influence on V_oc but positive influence on J_sc and FF, and as a result, a PCE of 7.44% was recorded. The device processed by the o-DCB/OT mixture showed positive influence on FF but negative influence on J_sc and V_oc, and hence the overall efficiency was only 4.48%. In order to investigate the effect of extensive addition of the different additives on the photovoltaic performance, the more DIO or NMP with contents of 5%, 7%, 10%, 15% were used as additive to fabricate devices. The J–V curves are shown in Fig. S2 and S3,† and the corresponding parameters are summarized in Tables S1 and S2 in ESI.† It can be found that the device performances gradually become inferior with the increasing of the additive contents and the optimal PCEs are achieved when using 3% additives. In order to verify the repeatability of the findings in different polymer: fullerene systems, we used other two classical polymers, PBDTTT-E-T and PBDTTT-C to fabricated the devices and investigate their photovoltaic performance. Fig. S4† shows current density–voltage (J–V) curves, and the corresponding photovoltaic parameters of the devices are summarized in Table S3.† It is quite clear that the same trend as we observed in the PBDTTT-C-T : PC₇₁BM system was obtained. Furthermore, we also studied the effects of different additives in the ternary blend system, i.e. PTB7 : PC₇₁BM : ICBA. Fig. S5† show J–V curves and EQE of the PSCs. Table S4† shows the photovoltaic performance of the devices without or with different additives. We can find that using DIO and NMP as additives can effectively improve photovoltaic performance of the PSCs. The PCE increased from 5.02% for the devices processed without additive to 7.09% and 6.97% for the devices processed with DIO and NMP as additive, respectively. However, when using OT as additive, the devices show a slight enhanced PCE. These results are coincident with that obtained in the binary blend system.

Fig. 1 (a) J–V curves and (b) EQE of the PSCs based on PBDTTT-C-T : PC₇₁BM (1 : 1.5, w/w) without additive or with different additives under the illumination of AM 1.5G, 100 mW cm⁻².

Table 1 Photovoltaic performance of the devices based on PBDTTT-C-T : PC₇₁BM (1 : 1.5, w/w) without or with different additives under the illumination of AM1.5G, 100 mW cm⁻²

Additive	Voc (V)	Jsc^a (mA cm^−2)	FF	PCE (PCEave^b) (%)	RS (Ω cm^−2)
a Calculated Jsc values from the IPCE spectra.b The average PCE is obtained from over 20 devices.
Without	0.83	13.1 (12.8)	0.50	5.43 (5.35)	4.47
DIO	0.75	15.6 (15.2)	0.63	7.37 (7.20)	2.28
NMP	0.82	14.8 (14.4)	0.61	7.44 (7.32)	2.90
OT	0.68	12.2 (12.3)	0.54	4.48 (4.35)	4.53

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Surface morphology

Atomic Force Microscopy (AFM) was used to investigate surface morphology of the blend films processed from different solvents. As shown in Fig. 2a and b, the film processed by pure o-DCB showed very smooth surface with R_q = 0.37 nm and almost no phase separation can be observed from the phase image. When 3% DIO was used, the roughness of the film increased obviously, i.e. R_q = 2.15 nm is observed (see Fig. 2c), and also distinct phase separation can be observed (see Fig. 2d). When 3% NMP was used, as shown in Fig. 2e and f, R_q of the film changed to 0.59 nm, which is rougher than the film processed by pure o-DCB but smoother than the film processed with the use of DIO, and similar trend can also be distinguished for phase separation of the film. Interestingly, when 3% OT was used as solvent additive, the film shows very rough surface (R_q = 6.72 nm) (see Fig. 2g and h). The large feature in Fig. 2h could be the thickness variations or compositional phase separation. We employ Scanning Transmission X-ray Microscopy (STXM) to distinguish these two possibilities clearly. Fig. S8a in ESI† shows X-ray optical density images obtained at photon energy of 287.1 and 284.1 eV where polymer and fullerene are selectively absorbing, respectively. In 287.1 eV image dark region is PC₇₁BM phase and in 284.1 eV image the phase is nearly reversed. This demonstrates that the large, round dispersions in Fig. 2h are phase-separated PC₇₁BM domains, which have depleted the surrounding matrix of PC₇₁BM. Furthermore, surface potentials of the top surfaces of these films were investigated by using the Peak Force-Kelvin Probe Force Microscopy (PF-KPFM). The PF-KPFM surface potential images and the corresponding analysis curves of contact potential difference of the top surfaces are shown in Fig. 3. The average surface contact potentials were deduced with typical scale of 500 nm. According to the PF-KPFM measurements, the contact surface potentials (φ) of these four types of films can be arranged by the order of φ_o-DCB > φ_o-DCB/NMP > φ_o-DCB/DIO > φ_o-DCB/OT, which is similar as the order of the V_oc of the devices (V_oc,o-DCB > V_oc,o-DCB/NMP > V_oc,o-DCB/DIO > V_oc,o-DCB/OT).

Fig. 2 AFM topography images (5 μm × 5 μm) (a, c, e, g) and phase contrast images (b, d, f, h) of PBDTTT-C-T : PC₇₁BM blend films (1 : 1.5, w/w); (a) and (b) processed by pure o-DCB; (c) and (d) processed with 3% DIO; (e) and (f) processed with 3% NMP; (g) and (h) processed with 3% OT.

Fig. 3 PF-KPFM surface potential images (1 μm × 1 μm) of PBDTTT-C-T/PC₇₁BM blend films (a) processed by pure o-DCB; (b) processed with 3% DIO; (c) processed with 3% NMP; (d) processed with 3% OT; (e) the analysis curves of contact potential differences of the top surfaces for these four blend films.

Moreover, in order to find out the origin of the variations in contact surface potentials of the films, the composition of the top surfaces of these four blend films were characterized by X-ray Photoelectron Spectroscopy (XPS), which is an useful tool to determine the composition of thin film surface (6–8 nm) and the stoichiometric ratio of the components.¹⁹ Considering that the typical thickness used in this work is ca. 100 nm, the XPS results can be seen as useful evidences for the polymer/PC₇₁BM compositions on the top surfaces of blend films. The detailed results of XPS measurements of blend films are collected in Table 2 and the spectra are shown in Fig. S7 in ESI.† The XPS measurement of the pure polymer film showed an atomic ratio of 8.10 for C 1s and S 2p peaks, which is close to the calculated values of C/S stoichiometric ratio (7.94), indicating that our test is reliable. In the PBDTTT-C-T/PC₇₁BM blend, sulfur can be used as the characteristic element of the polymer, because PC₇₁BM does not contain sulfur. Therefore, the higher C/S ratio (R_C/S) is observed, the more PC₇₁BM is depleted on the top surface. The R_C/S values of the blend films can be arranged by the order of R_C/S,o-DCB > R_{C/S,o-DCB/NMP} > R_{C/S,o-DCB/DIO} ≈ R_C/S,o-DCB/OT, which is the same as the reversed order of the contact surface potentials of the films (φ_o-DCB > φ_o-DCB/NMP > φ_o-DCB/DIO > φ_o-DCB/OT) and also similar as order of the V_oc of the devices (V_oc,o-DCB > V_oc,o-DCB/NMP > V_oc,o-DCB/DIO > V_oc,o-DCB/OT).

Table 2 XPS element analysis results of the pure polymer film and the blend films without or with the use of additives

Film	Processing solvent	C1s (%)	S2p (%)	C/S ratio	Real D/A ratio on top surface^d
a Theoretical value calculated for C48H56OS6.b The blend films are prepared by the same processes condition as those used in device fabrication.c Theoretical value calculated for PBDTTT-C-T/PC71BM (w/w = 1 : 1.5).d These values are calculated according to the theoretical elemental compositions of C and S or the real C and S compositions obtained from XPS measurements.
Pure polymer	Theoretical value^a	87.27	10.99	7.94	1 : 0
	Pure o-DCB	87.06	10.75	8.10	1 : 0
D/A Blend^b (w/w = 1 : 1.5)	Theoretical value^c	92.22	4.40	20.96	1 : 1.5
	Pure o-DCB	91.04	5.74	15.86	1 : 0.88
	o-DCB with 3% DIO	89.86	7.33	12.26	1 : 0.48
	o-DCB with 3% NMP	91.06	6.05	15.05	1 : 0.84
	o-DCB with 3% OT	89.77	7.45	12.05	1 : 0.45

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Bulk morphology

Grazing Incidence Wide-angle X-ray Scattering (GIWAXS) was used to analyze crystallinity of the PBDTTT-C-T/PC₇₁BM blend films with or without additives. Thin films were spin-cast on PSS-treated Si substrates and have a thickness of 100 nm.²⁰ Fig. 4a and b show the 2D patterns and the out-of plane (q_z) and in plane (q_xy) profiles of the films processed with or without additives. GIWAXS patterns show only broad (100) polymer lamellar reflections at q ≈ 0.3 Å⁻¹ for all the blend films. The PC₇₁BM aggregation peaks are shown at q near 1.3 Å⁻¹. No clear π–π stacking reflection peaks can be observed for all the blends. Overall, a rather amorphous nature of the PBDTTT-C-T is observed and little differences are observed between the blend films processed with different additives. All the blend films exhibit weak peaks at q ≈ 0.4 Å⁻¹, which is originated from PEDOT:PSS reflection when incident angle of X-ray beam is large enough to penetrate the film. As a reference, PSS only film reflection profiles are plotted at the bottom of Fig. 4b. In conclusion, these four blend films processed by different solvents show very similar and low crystallinities, implying that the use of these three additives has no impact on molecular packing structure in the PBDTTT-C-T : PC₇₁BM blend films and the materials are largely amorphous and disordered.

Fig. 4 2D pattern (a) out-of-plane and in-plane profile (b) GIWAXS data of PBDTTT-C-T : PC₇₁BM blend films.

Transmission Electron Microscopy (TEM) was used to complement the morphological characterization. As shown in Fig. S8 in ESI,† TEM images are coincident with the morphologies observed in AFM measurements. When pure o-DCB was used, the interpenetrating networks are not well developed, and the donor and acceptor domains are difficult to be distinguished (see Fig. S8a in ESI†). After the addition of DIO or NMP, phase separation can be observed (see Fig. S8b and S6c in ESI†). However, when OT was used as additive, severe phase separation occurred, i.e. micron-scale domains can be clearly distinguished in Fig. S8d in ESI.†

As the AFM provides only surface structure and the contrast in TEM is low, resonant Soft X-ray Scattering (R-SoXS) is further employed to provide detail information about the bulk morphology of the blend films, i.e. the median characteristic length scales of the morphology and the average composition fluctuations 〈Δc〉.²¹ A photon energy 284.2 eV was utilized to provide high material contrast between PBDTTT-C-T and PC₇₁BM.^21,22 Fig. 5 shows the R-SoXS profile of blend films processed with or without additives. The distribution of scattering profile can be fitted by log-normal functions (see Fig. S9 in ESI†) and represents the distribution function of spatial frequency, s (s = q/2π). The median of the distribution s_median corresponds to the characteristic median length scale, ξ, of the corresponding log-normal distributions in real space with ξ = 1/s_median, a model independent statistical quantity. When the blend film was processed with pure o-DCB, the ξ in the blend is 26.7 nm; when DIO or NMP was used as additive, the scattering profile shows two log-normal distributions. The corresponding ξ are 69.6 and 29.5 nm for DIO and 56.5 and 34.4 nm for NMP. It is noted that the film processed by o-DCB/DIO shows more intense and smaller domains than NMP in high-q (see Fig. S9 in ESI†). As demonstrated previously, smaller domains are more favorable to generate higher photocurrent.²³ This explains why J_sc of the device processed with DIO is higher than the device processed with NMP. For the blend film processed by o-DCB/OT, the extremely large phase separation observed with AFM is out of the q-range that can be captured in the R-SoXS measurement. Only a peak with a characteristic median length scale of ξ 24.5 nm revealed by R-SoXS is generally consistent with AFM/TEM measurement for the blend films.

Fig. 5 R-SoXS information of PBDTTT-C-T/PC₇₁BM blend films without or with additives.

The average composition fluctuations 〈Δc〉 (referred as relative domain purity within a two phase amorphous: amorphous morphology framework as suggested by the WAXS data) can be extracted by integrating the scattering profile and calculating the Total Scattering Intensity (TSI).^22,23c,24 The relative purity is 0.74, 1, 0.94 and 0.64 for blend processed without and with DIO, NMP and OT additive, respectively. We find that domains become purer when DIO and NMP are used as additives. It is noted that the relative purity does not indicate the domains are 100% pure, but the purest in this study. The large PC₇₁BM rich dispersions observed for OT-based devices reduce the amount of PC₇₁BM available for the nano-phase separation. Thus, despite having nearly identical nano-phase morphology to the devices without additives as indicated by the similar shape and location of the R-SoXS peak, the relative purity of the nano-phase separated domains in OT-devices is reduced relative to the devices without additives.

Mobility

The mobility was measured by the space charge limited current (SCLC)²⁵ method by a hole-only device with a structure of ITO/PEDOT:PSS/active layer/Au or an electron-only device with a structure of ITO/Al/active layer/Al and estimated through the Mott-Gurney equation , where ε is the dielectric constant of the polymer, ε₀ is the permittivity of the vacuum, μ₀ is the zero-field mobility, E₀ is the characteristic field, J is the current density, L is the thickness of the blended films layer, V = V_appl − V_bi, V_appl is the applied potential, and V_bi is the built-in potential which results from the difference in the work function of the anode and the cathode (for the electron-only devices, V_bi = 0.2 V; for the electron-only devices, V_bi = 0 V). The detailed hole and the electron mobilities (μ_h and μ_e) are shown in Table 3. Since a PSC device should keep the electric neutrality during whole photoelectric conversion process, the carrier transport ability should be determined by the ‘bucket effect’, that is, carrier transport capability of the BHJ blend is determined by the lower one of μ_h and μ_e. According to the mobility results shown in Table 3, all the blend films casted from the mixture of o-DCB and different additives showed similar hole mobilities with that of the blend casted from pure o-DCB, resulted from the similar molecular packing of the blend film. For all of these four devices, the μ_h values are one or two orders of magnitude higher than their μ_e values. Therefore, carrier transport capabilities of these devices are mainly determined by their electron mobilities. The μ_e values of these four devices can be arranged by the order of μ_e,o-DCB/DIO > μ_e,o-DCB/NMP > μ_e,o-DCB/OT > μ_e,o-DCB. This order is the same as the relative domain purity, which indicates that purer domains can lead to better electron mobility. Since balanced μ_h and μ_e in the BHJ blend is the key to realize good FF,²⁶ we may use the ratio of μ_h/μ_e (R_(h,e)) to qualitatively signify the carrier transport properties of the blend films. For these four types of devices, their R_(h,e) values can be arranged by the order of R_{(h,e),o-DCB/DIO} < R_{(h,e),o-DCB/NMP} < R_{(h,e),o-DCB/OT} < R_(h,e),o-DCB, which is corresponding to the order of their FF. The higher and more balanced μ_h and μ_e were obtained for the blend films processed with DIO as additive, resulting in the higher J_sc, FF and thus PCE of the PSCs.

Table 3 The mobilities of the blend films of PBDTTT-C-T : PC₇₁BM (1 : 1.5, w/w) without or with different additives

Additive	μh (cm^2 V^−1 s^−1)	μe (cm^2 V^−1 s^−1)	μh/μe	FF (%)
Without	6.68 × 10^−3	6.37 × 10^−5	104.9	49.75
DIO	8.73 × 10^−3	8.37 × 10^−4	10.43	65.31
NMP	7.75 × 10^−3	1.88 × 10^−4	41.22	61.05
OT	6.37 × 10^−3	8.08 × 10^−5	78.8	54.38

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Solubility and saturated vapor pressure

The solubilities of PBDTTT-C-T and PC₇₁BM in different solvents were measured as follows: firstly, the optical densities of varied concentrations of PBDTTT-C-T and PC₇₁BM solutions were measured respectively to make the working curves. Secondly, PBDTTT-C-T or PC₇₁BM powder was then added into different solvents, respectively, to make saturated solutions and was centrifuged at 13 500 rpm for 15 min. Successively, the top clear solutions in the centrifuge tubes were filtered through a 0.25 μm filter and then diluted to measure their absorption. Finally, solubility values can be calculated based on the absorbance values and the working curves. The absorption curves are provided in Fig. S10 in ESI† and the solubility results are listed in Table 4. It was found that PBDTTT-C-T shows excellent solubility in o-DCB, i.e. when >70 mg of polymer was dissolved in o-DCB, the solution can still pass through the 0.25 μm filter, but the solution is very sticky so that it would be more reasonable to use >70 mg ml⁻¹ to describe its solubility rather than a precise value. The polymer shows very low solubility in both DIO and NMP, while it can be slightly dissolved into OT, i.e. its solubility in OT is 0.28 mg ml⁻¹. For PC₇₁BM, it shows excellent solubility in o-DCB (>200 mg ml⁻¹), good solubility in DIO (70.3 mg ml⁻¹) and NMP (55.1 mg ml⁻¹), and relatively poor solubility in OT (10.1 mg ml⁻¹). Furthermore, saturated vapor pressure (P^o) of o-DCB and the additives are calculated according to Clausius–Clapeyron equation: and the solubility results are listed in Table 4. Two facts can be extracted from these results: (1) PBDTTT-C-T and PC₇₁BM have extremely asymmetric solubilities in both DIO and NMP compared to in OT; (2) according to P^o values, the drying speeds of these solvents can be arranged as the order of o-DCB > NMP > OT > DIO.

Table 4 The saturated vapor pressure of different solvents 25 °C and the solubilities of PBDTTT-C-T and PC₇₁BM in different solvents

Solvents	Vapor pressure at 25 °C (Pa)	Boiling Point (°C)	PBDTTT-C-T solubility (mg ml^−1)	PC71BM solubility (mg ml^−1)
o-DCB	157	180	>70	225.2
DIO	0.08	330	6.13 × 10^−3	70.3
NMP	47	202	1.83 × 10^−3	55.1
OT	4.29	270	2.8 × 10^−1	10.1

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Discussion

Based on the detailed data obtained from the photovoltaic measurements and morphological characterizations, the mechanism of the influence of the additives on photovoltaic properties of the PBDTTT-C-T/PC₇₁BM blends can be described as below. Overall, since the polymer is almost amorphous, the change of processing solvents has little influence on its crystallinity in the blend films. Since both the polymer and PC₇₁BM have high solubilities in o-DCB, when pure o-DCB was used as processing solvent, the solutes will start to precipitate only if the majority of o-DCB is evaporated; however, at that moment, the solution has very high concentration and o-DCB can be dried quickly, so that it is very hard to allow the sticky solution to evolve a BHJ blend film with suitable phase separation. Consequently, the electron transport channel cannot be effectively formed and the PSC device will show low J_sc and FF. When o-DCB/DIO or o-DCB/NMP was used as processing solvent, o-DCB will be evaporated quickly while the additive will evaporate slowly due to its low P^o; therefore, the polymer will start to precipitate when PC₇₁BM still has good solubility in the solvent mixture, so that discontinuous phase separation will be realized, which is beneficial to enhancing electron transport properties of the blend films and thus improving J_sc and FF. However, since NMP has much higher P^o than DIO, the effect of the asymmetric dissolubility of NMP cannot be as strong as that of DIO. OT has much higher dissolubility to the polymer and much lower dissolubility to PC₇₁BM compared to the other two additives, when the o-DCB/OT mixture is used as processing solvent, it allows the precipitations of both the polymer and PC₇₁BM to grow to very big size aggregations and results in severe phase separation, which will limit the PCE of the device.

According to the surface morphology analysis, the additives also affect D/A composition on the top surfaces of the blend films. When pure o-DCB was used, PC₇₁BM will be enriched on the top surface, which is helpful for getting high contact surface potential and thus good for realizing high V_oc. When solvent additive with low P^o and highly asymmetric dissolubility, like NMP or DIO, was added, PC₇₁BM will be relatively depleted from the top surface so that contact surface potential of the blend and hence V_oc of the corresponding device will be reduced. The comparisons between the devices processed by NMP and DIO reveal that PC₇₁BM can be more depleted from the top surface by using additive with lower P^o. However, when additive with low P^o and relatively higher dissolubility to the polymer was used, PC₇₁BM will be much more depleted from the top surface, and therefore, the device processed with the use of OT showed the lowest V_oc in these four kinds of devices.

Conclusions

In summary, we used PBDTTT-C-T/PC₇₁BM blend active layers as the model system and chose three solvent additives, DIO, NMP and OT, to understand the effects of the solvent additives on the morphologies and photovoltaic performance of the devices. The morphologies at surface and in the bulk of the blend films processed without or with three different additives were systematically investigated by using AFM, XPS, PF-KPFM, TEM, GIWAXS and R-SoXS. The results indicate that the variation of V_oc is directly related to the ratio of polymer and PC₇₁BM and the contact surface potential, while the J_sc and FF are heavily influenced by the sizes and the relative composition fluctuations of the aggregations in the blends of PBDTTT-C-T and PC₇₁BM. Furthermore, we tried to explore the origin of the morphological changes caused by the asymmetric solubilities for the solutes and the varied P^o of the solvent additives. It was also found that the relative solvencies of the additives to the solutes and the large difference of saturated vapor pressure (P^o) values between the host solvent (o-dichlorobenzene, o-DCB) and the guest solvent (additives with high BP) play the key roles in affecting surface and bulk morphologies of the PBDTTT-C-T : PC₇₁BM blend films. This work provides informative and useful guidance to select solvent additive for morphology control of BHJ solar cells.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC) (No. 91333204, 51203168, 51422306, 51503135, 21325419 and 51573120), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Jiangsu Provincial Natural Science Foundation (Grant No. BK20150332), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB430027). R-SoXS and GIWAXS measurements and analysis by WM are supported by the US Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under contract DE-FG02-98ER45737. X-ray data is acquired at beamlines 7.3.3 (WAXS) 5.3.2 (STXM) and 11.0.1.2. (R-SoXS) at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

References

1. G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789.
2. H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.
3. (a) Y. F. Li, Acc. Chem. Res., 2012, 45, 723; (b) J. W. Chen and Y. Cao, Acc. Chem. Res., 2009, 42, 1709.
4. Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868.
5. D. Muhlbacher, M. Scharber, M. Morana, Z. G. Zhu, D. Waller, R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884.
6. (a) Z. He, C. Zhong, X. Huang, W. Y. Wong, H. Wu, L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636; (b) L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180; (c) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li and Y. Yang, Nat. Commun., 2013, 4, 1446.
7. J. Peet, A. J. Heeger and G. C. Bazan, Acc. Chem. Res., 2009, 42, 1700.
8. (a) B. C. Thompson and J. M. J. Fréchet, Angew. Chem., Int. Ed., 2008, 47, 58; (b) J. A. Bartelt, Z. M. Beiley, E. T. Hoke, W. R. Mateker, J. D. Douglas, B. A. Collins, J. R. Tumbleston, K. R. Graham, A. Amassian, H. Ade, J. M. J. Fréchet, M. F. Toney and M. D. McGehee, Adv. Energy Mater., 2013, 3, 363.
9. (a) C. R. McNeill, J. J. M. Halls, R. Wilson, L. G. Whiting, S. Berkebile, M. G. Ramsey, R. H. Friend and N. C. Greenham, Adv. Funct. Mater., 2008, 18, 2309; (b) M. J. Zhang, X. Guo and Y. F. Li, Adv. Energy Mater., 2011, 1, 557.
10. (a) G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4, 864; (b) J. H. Park, J. S. Kim, J. H. Lee, W. H. Lee and K. Cho, J. Phys. Chem. C, 2009, 113, 17579.
11. L. Chang, H. W. A. Lademann, J. B. Bonekamp, K. Meerholz and A. J. Moulé, Adv. Funct. Mater., 2011, 21, 1779.
12. (a) M. M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv. Mater., 2008, 20, 2556; (b) W. W. Li, W. S. C. Roelofs, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2012, 134, 13787; (c) S. Alem, T. Y. Chu, S. C. Tse, S. Wakim, J. P. Lu, R. Movileanu, Y. Tao, F. Belanger, D. Desilets, S. Beaupre, M. Leclerc, S. Rodman, D. Waller and R. Gaudiana, Org. Electron., 2011, 12, 1788; (d) F. Liu, Y. Gu, C. Wang, W. Zhao, D. Chen, A. L. Briseno and T. P. Russell, Adv. Mater., 2012, 24, 3947.
13. J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497.
14. (a) J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. C. Bazan and A. J. Heeger, J. Am. Chem. Soc., 2008, 130, 3619; (b) Y. Yao, J. H. Hou, Z. Xu, G. Li and Y. Yang, Adv. Funct. Mater., 2008, 18, 1783.
15. S. J. Lou, J. M. Szarko, T. Xu, L. P. Yu, T. J. Marks and L. X. Chen, J. Am. Chem. Soc., 2011, 133, 20661.
16. (a) J. T. Rogers, K. Schmidt, M. F. Toney, E. J. Kramer and G. C. Bazan, Adv. Mater., 2011, 23, 2284; (b) X. Guo, C. H. Cui, M. J. Zhang, L. J. Huo, Y. Huang, J. H. Hou and Y. F. Li, Energy Environ. Sci., 2012, 5, 7943; (c) X. Guo, M. J. Zhang, L. J. Huo, C. H. Cui, Y. Wu, J. H. Hou and Y. F. Li, Macromolecules, 2012, 45, 6930; (d) M. S. Su, C. Y. Kuo, M. C. Yuan, U. S. Jeng, C. J. Su and K. H. Wei, Adv. Mater., 2011, 23, 3315.
17. (a) B. Walker, A. Tamayo, D. T. Duong, X. D. Dang, C. Kim, J. Granstrom and T. Q. Nguyen, Adv. Energy Mater., 2011, 1, 221; (b) X. L. Liu, S. Huettner, Z. X. Rong, M. Sommer and R. H. Friend, Adv. Mater., 2012, 24, 669; (c) K. R. Graham, P. M. Wieruszewski, R. Stalder, M. J. Hartel, J. G. Mei, F. So and J. R. Reynolds, Adv. Funct. Mater., 2012, 22, 4801; (d) A. R. Campbell, J. M. Hodgkiss, S. Westenhoff, I. A. Howard, R. A. Marsh, C. R. McNeill, R. H. Friend and N. C. Greenham, Nano Lett., 2008, 8, 3942.
18. (a) L. J. Huo, S. Q. Zhang, X. Guo, F. Xu, Y. F. Li and J. H. Hou, Angew. Chem., Int. Ed., 2011, 50, 9697; (b) X. Guo, M. J. Zhang, W. Ma, L. Ye, S. Q. Zhang, S. J. Liu, H. Ade, F. Huang and J. H. Hou, Adv. Mater., 2014, 26, 4043.
19. Z. Xu, L. M. Chen, G. W. Yang, C. H. Huang, J. H. Hou, Y. Wu, G. Li, C. S. Hsu and Y. Yang, Adv. Funct. Mater., 2009, 19, 1227.
20. A. Hexemer, W. Bras, J. Glossinger, E. Schaible, E. Gann, R. Kirian, A. MacDowell, M. Church, B. Rude and H. Padmore, J. Phys.: Conf. Ser., 2010, 247, 012007.
21. E. Gann, A. T. Young, B. A. Collins, H. Yan, J. Nasiatka, H. A. Padmore, H. Ade, A. Hexemer and C. Wang, Rev. Sci. Instrum., 2012, 83, 045110.
22. W. Ma, L. Ye, S. Zhang, J. Hou and H. Ade, J. Mater. Chem. C, 2013, 1, 5023.
23. (a) H. P. Yan, B. A. Collins, E. Gann, C. Wang, H. Ade and C. R. McNeill, ACS Nano, 2012, 6, 677; (b) A. C. Stuart, J. R. Tumbleston, H. X. Zhou, W. T. Li, S. B. Liu, H. Ade and W. You, J. Am. Chem. Soc., 2013, 135, 1806; (c) W. Ma, J. R. Tumbleston, M. Wang, E. Gann, F. Huang and H. Ade, Adv. Energy Mater., 2013, 3, 864; (d) L. Ye, S. Zhang, W. Ma, B. Fan, X. Guo, Y. Huang, H. Ade and H. J. Hou, Adv. Mater., 2012, 24, 6335.
24. B. A. Collins, Z. Li, J. R. Tumbleston, E. Gann, C. R. McNeill and H. Ade, Adv. Energy Mater., 2013, 3, 65.
25. G. G. Malliaras, J. R. Salem, P. J. Brock and C. Scott, Phys. Rev. B, 1998, 58, 13411.
26. X. Guo, M. J. Zhang, J. H. Tan, S. Q. Zhang, L. J. Huo, W. P. Hu, Y. F. Li and J. H. Hou, Adv. Mater., 2012, 24, 6536.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06020j

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