Effect of structural variation on photovoltaic characteristics of phenyl substituted diketopyrrolopyrroles

Abstract

A comprehensive study has been performed on a series of solution processable phenyl substituted diketopyrrolopyrroles blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) in order to investigate how systematic chemical modifications such as solubilizing groups and conjugation length impact solar cell performance. We find that replacement of linear alkyl chains with bulky ethyl-hexyl groups or the removal of linear alkyl chains on the terminal thiophene units leads to micron scale phase segregation at high donor:acceptor blend ratios. It is found that the conjugation length can be used to simultaneously tune energy levels, solubility, and molecular ordering. We show that over-extending the conjugation length can reduce solubility making film fabrication difficult while decreasing the conjugation length past a critical limit can significantly enhance molecular ordering thereby inducing micron scale phase segregation in blend films. This work shows that a material's potential device performance can be limited by slight chemical modifications which prevent device optimization at high donor:acceptor blend ratios and elevated annealing temperatures where charge mobility is balanced and charge collection is enhanced in the donor and acceptor phase.

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1. Introduction

Organic solar cells have attracted a great deal of interest due to their low cost, solution processibility, light weight, flexibility,^1–4 and recently published efficiencies up to 10%.^5–12 The majority of bulk heterojunction (BHJ) solar cells utilize conjugated polymers as the electron donor. However, conjugated polymers often suffer from batch to batch variation in molecular weights¹³ and end group contamination,¹⁴ which has been shown to adversely influence solar cell performance.^13,14 For these reasons, organic solar cells based on solution-processed small molecules have gained attention since small molecules can be synthetically reproducible.^15–28

Studies on diketopyrrolopyrrole (DPP)-based small molecules have shown that slight modifications in the solubilizing group and thiophene conjugation length can significantly impact the optical properties, morphology, and charge carrier mobility.^29,30 Nanoscale ordering and interchromophore contacts could be enhanced for DPP small molecules by replacing branched alkyl chains with straight chains and by decreasing the length of the alkyl chains.²⁹ Field effect transistors (FETs) fabricated from DPP derivatives showed that decreasing the alkyl chain length from twelve to six carbons promoted molecular ordering while increasing FET hole mobility.³⁰ Efficient solar cells have be fabricated from DPP-containing polymers^31–40 and small molecules.^{29,30,41–57} A DPP derivative incorporating benzofuran units exhibited long range order, near-infrared absorption, and solar cell efficiencies up to 4.8% (ref. 16) or up to 5.5% (ref. 41) using three DPP units when optimized with [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM).

In order to make further advancements in the design of DPP-containing donors, it is necessary to understand the relationship between chemical structure and device performance. Small molecules based on DPP units serve as a good model system to study structure–function relationships due to the ability to tune the physical and optoelectronic properties by incorporating different alkyl and aryl groups. In a previous work,⁵⁸ our group designed a series of compounds (Fig. 1a) to study influence of structural variation on the solid-state properties of DPP-based oligophenylenethiophenes. It was shown that slight systematic changes in chemical structures can result in significant changes in material properties such as the crystallinity, morphology, optical bandgap, and mobility. In this work, we build upon our previous study by utilizing the same class of compounds to investigate how systematic chemical modifications impact solar cell device performance. It is shown that slight chemical modification to functional groups and conjugation length can limit device performance due to undesirable phase segregation, charge carrier mobility imbalance, and large domains which limit exciton harvesting.

Fig. 1 (a) Chemical structures for C6PT1C6, C6PT2C6, C6PT3C6, EHPT2C6, and C6PT2. (b) HOMO–LUMO levels of donor and acceptor materials.

2. Results and discussion

With regards to chemical structure naming for compounds in Fig. 1, the first two letters are either “C6” or “EH” to represent a six-carbon linear alkyl chain or a branched ethyl-hexyl group respectively which is attached to the lactam nitrogen. The third letter, “P”, represents the presence of a phenyl group adjacent to the DPP core. The fourth and fifth letter can be “T1”, “T2” or “T3” to represent one, two or three thiophenes adjacent to the phenyl units. Lastly, “C6” is added at the end of the chemical name if a six carbon alkyl chain is present on the terminal thiophene units.

Solar cells devices of the compounds in Fig. 1a were tested at donor:PC₇₁BM blend ratios ranging from 10 : 90 to 80 : 20 for as cast along with 80, 100, and 120 °C annealed devices. Fig. S1† shows the device parameters vs. blend ratio for each material. For reference, Fig. S3† shows the device parameters vs. molar mass fraction. The optimized conditions are summarized in Fig. 2 and Table 1. Fig. 2 shows that C6PT2C6 yielded the greatest solar cell performance with is attributed to the enhanced current generation as shown from the EQE (Fig. 2b). Modifications to the conjugation length or solubilizing groups result in a decrease in solar cell device performance. These trends are further discussed in the following sections.

Fig. 2 (a) J–V characteristics and (b) external quantum efficiency of optimized DPP donors with PC₇₁BM.

Table 1 Summary of solar cell characteristics of optimized devices

Donor:acceptor	Blend ratio	Annealing (°C)	VOC (V)	JSC (mA cm^−2)	FF	η%
C6PT1C6:PC71BM	60 : 40	80	1.00	3.64	0.35	1.25
C6PT2C6:PC71BM	60 : 40	120	0.90	7.91	0.49	3.45
C6PT3C6:PC71BM	20 : 80	80	0.90	5.76	0.32	1.67
EHPT2C6:PC71BM	10 : 90	80	0.72	3.37	0.33	0.76
C6PT2:PC71BM	20 : 80	80	0.87	4.33	0.30	1.11

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The blend films of C6PT1C6:PC₇₁BM, C6PT2C6:PC₇₁BM, C6PT3C6:PC₇₁BM, EHPT2C6:PC₇₁BM, and C6PT2:PC₇₁BM optimize at mass ratios of 60 : 40, 60 : 40, 20 : 80, 10 : 90, and 20 : 80 which corresponds to donor:acceptor mole ratios of 66 : 36, 62 : 38, 19 : 81, 20 : 80, and 25 : 75 respectively. Both C6PT1C6:PC₇₁BM and C6PT2C6:PC₇₁BM blends optimize at greater donor fractions while C6PT3C6:PC₇₁BM, EHPT2C6:PC₇₁BM, and C6PT2:PC₇₁BM optimize at higher PC₇₁BM fractions. We find that the blend ratio significantly impacts the solar cell device performance.

2.1. Increasing bulkiness of solubilizing group

To investigate the impact of molecular bulkiness, the linear alkyl chains on the lactam nitrogen units of compound C6PT2C6 is substituted with ethyl-hexyl groups to form compound EHPT2C6. Fig. 2a and Table 1 shows that the C6PT2C6:PC₇₁BM device optimizes at a 60 : 40 blend ratio and an annealing temperature of 120 °C with an open circuit voltage (V_OC) of 0.90 V, short circuit current (J_SC) of 7.91 mA cm⁻², fill factor (FF) of 0.49, and power conversion efficiency (PCE) of 3.45% (Table 1). In comparison, the EHPT2C6:PC₇₁BM device optimizes at a 20 : 80 blend ratio and an annealing temperature of 80 °C which yields a V_OC of 0.72 V, J_SC of 3.37 mA cm⁻², FF of 0.33, and PCE of 0.76% (Table 1).

Fig. 2 and Table 1 shows that C6PT2C6:PC₇₁BM and EHPT2C6:PC₇₁BM devices optimize at a 60 : 40 and 10 : 90 blend ratios respectively. To investigate the driving force for device optimization at a specific donor:acceptor blend ratio, the morphology and device performance (S1f–j, ESI†) was measured for each of the compounds in Fig. 1a with donor:PC₇₁BM blend ratios spanning 10 : 90 to 80 : 20. Fig. 3a–e shows the C6PT2C6:PC₇₁BM AFM topography across blend ratios 10 : 90 to 80 : 20. At all C6PT2C6:PC₇₁BM blend ratios the surface morphology is featureless with low root-mean-squared (RMS) roughness around 1 nm. In comparison, blend films of EHPT2C6:PC₇₁BM show micron scale aggregates across blend ratios 20 : 80 to 80 : 20 (Fig. 3g–j). However, the 10 : 90 EHPT2C6:PC₇₁BM blend ratio did not form micron scale aggregates (Fig. 3f). The efficiency vs. blend ratio plot for EHPT2C6:PC₇₁BM blends (Fig. S1s†) shows that the device performance is lowest for blend ratios which exhibited rough films but is greatest for the 10 : 90 blend ratio where the film RMS roughness is around 1 nm. This result shows that micron scale phase segregation at high donor:acceptor ratios can induce device optimization at a low donor:acceptor ratio. Device optimization at low donor:acceptor ratios is likely to lead to current losses from poor exciton harvesting in large fullerenes domains. This may in part explain the low short circuit current observed in the optimized EHPT2C6:PC₇₁BM device.

Fig. 3 Topography images for annealed blend films of C6PT2C6 (a–e), EHPT2C6 (f–j), and C6PT2 (k–o) with PC₇₁BM for donor:acceptor blend ratios 10 : 90 (a, f, k), 20 : 80 (b, g and l), 40 : 60 (c, h and m), 60 : 40 (d, i and n), and 70 : 30 (e, j and o). All films where annealed at 80 °C for 10 minutes. All image scan sizes are 10 × 10 μm.

Fig. 3 showed that 20 : 40 and 40 : 60 EHPT2C6:PC₇₁BM blends exhibited pitted holes and isolated islands respectively. These features are characteristic of a dewetting process. Newly prepared pristine EHPT2C6 films on PEDOT-PSS are relatively flat with 2.2 nm RMS roughness (Fig. S5a†) and do not exhibit any dewetting. Interestingly, micron long fibers (Fig. S5b†) are formed and the RMS roughness increases to 7.0 nm after 24 hours. This result suggests that EHPT2C6 has a tendency to dewet on PEDOT-PSS surfaces. It is possible that the ethyl-hexyl groups on EHPT2C6 enhance the hydrophobicity which leads to dewetting on the hydrophilic PEDOT-PSS surface. A previous work showed that the thin film dewetting morphology can be controlled by modifying the substrate surface energy.⁵⁹ It is interesting to note that the 80 : 20 EHPT2C6:PC₇₁BM blend (Fig. S5c†) also exhibits a similar morphology to the day old pristine EHPT2C6 film. This suggests that PC₇₁BM enhances the dewetting process. Previous works have shown that dewetting can occur in blends where the components have significant differences in surface energies or are immiscible.^59,60 We find that compound EHPT2C6 has a tendency to dewet on PEDOT-PSS surfaces which is further enhanced with the addition of PC₇₁BM. This dewetting process induces device optimization at a low donor:acceptor ratio of 10 : 90 where dewetting is not present.

To further investigate how blend ratio impacts device performance the hole and electron mobilities where measured for the optimized blend films (Table 2). Table 2 shows that optimized blends of C6PT2C6 and EHPT2C6 yield hole mobilities of 1.9 × 10⁻⁵ and 2.2 × 10⁻⁶ cm² V⁻¹ s⁻¹ respectively along with electron mobilities of 1.3 × 10⁻⁴ and 1.5 × 10⁻³ cm² V⁻¹ s⁻¹ respectively. The greater hole mobility in C6PT2C6 blends relative to EHPT2C6, is likely due to the high donor content and greater molecular ordering. In a previous work pristine films of C6PT2C6 showed a greater degree of molecular ordering than EHPT2C6 films. We attribute the greater electron mobility to the significantly greater PC₇₁BM content in the optimized EHPT2C6 film. The ratio of the electron to hole mobility (μ_e/μ_h) for the optimized C6PT2C6:PC₇₁BM and EHPT2C6:PC₇₁BM are calculated at 6.8 and 677, respectively. It is possible that the imbalance in charge mobilities is a contributing factor in the lower FF and V_OC observed in the optimized EHPT2C6:PC₇₁BM device. Previous works have also shown that an imbalance in hole and electron mobilities may enhance charge recombination thereby limiting the fill factor^61–65 and V_OC.^66,67 In summary, increasing the bulkiness of compound C6PT2C6 yields micron scale phase segregation at high donor:acceptor ratios which induces device optimization at a low donor:acceptor ratio where device performance is can be limited by poor exciton harvesting and an imbalance charge carrier mobility.

Table 2 Summary of mobilities for optimized blends

	Hole mobility^a	Electron mobility^a	μe/μh
a All mobility values are in units of cm^2 V^−1 s^−1.
C6PT1C6	1.7 × 10^−6	1.7 × 10^−6	1.0
C6PT2C6	1.9 × 10^−5	1.3 × 10^−4	6.8
C6PT3C6	5.6 × 10^−7	5.4 × 10^−4	970
EHPT2C6	2.2 × 10^−6	1.5 × 10^−3	677
C6PT2	6.60 × 10^−6	5.8 × 10^−5	8.8

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To extend our discussion on the impact of bulkiness it is useful to compare compound EHPT2C6 with SMDPPEH.^46,47,64 Replacement of the phenyl units in EHPT2C6 with thiophenes units yields compound SMDPPEH. As shown from the single crystal structure of EHPT2C6 and SMDPPEH (Fig. S6†), the backbone of compound SMDPPEH is significantly more planar than EHPT2C6. Thin film X-ray diffraction measurements show that the molecular ordering is greater for SMDPPEH⁴⁶ than EHPT2C6 which does not exhibit any X-ray scattering peak. This suggests that replacement of the phenyl groups with thiophene units causes an increase in planarity and molecular ordering. In regards to optical properties, the onset absorption of SMDPPEH is redshifted to 800 nm (ref. 47) in comparison to 626 nm (ref. 58) for EHPT2C6. The onset absorption in these push–pull structures arises from an intermolecular charge transfer state (ICT) between the donor acceptor moieties. In comparison to EHPT2C6, SMDPPEH has a planar backbone and stronger electron donating groups which give rise to a redshifted ICT band. The pristine hole mobilities of SMDPEH and EHPT2C6 are 1.0 × 10⁻⁴ cm² V⁻¹ s⁻¹ (ref. 47) and 4.7 × 10⁻⁶ cm² V⁻¹ s⁻¹ (ref. 58) respectively. The hole mobility is increased by roughly two orders of magnitude when going from EHPT2C6 to SMDPPEH. In solar cells optimized 50 : 50 SMDPPEH:PCBM films yielded a V_OC of 0.75 V, J_SC of 8.0 mA cm⁻², FF of 0.46, and PCE of 2.8%.⁶⁴ As discussed earlier, the poor performance of EHPT2C6:PC₇₁BM devices is due to micron phase segregation which induces device optimization at a low donor:acceptor blend ratio. We find that replacing the thiophene units on SMDPPEH with phenyl groups to form EHPT2C6 induces a twist in the conjugated backbone and increases the molecular bulkiness which reduces molecular ordering and charge transport.

2.2. Removal of linear alkyl chain at the end units

Comparing C6PT2C6 and C6PT2 highlights the impact of removing terminal thiophene linear alkyl chains on the device performance. Table 1 shows that the C6PT2:PC₇₁BM device optimizes at a 20 : 80 blend ratio and an annealing temperature of 80 °C which produces a V_OC of 0.87 V, J_SC of 4.33 mA cm⁻², FF of 0.30, and PCE of 1.11%. Relative to compound C6PT2C6, the optimized C6PT2 device has a nearly equivalent V_OC but a lower J_SC, FF, and therefore PCE. Fig. 3k–o shows the blend film morphology of C6PT2:PC₇₁BM films. Blend films of C6PT2:PC₇₁BM exhibit micron scale phase segregation at high donor:acceptor ratios and device optimization at a low donor:acceptor ratio. A previous work showed that annealing pristine films of C6PT2 induces large plate like structures⁵⁸ and an increase in molecular ordering.⁶⁸ In comparison, blend films of C6PT2C6:PC₇₁BM do no exhibit micron scale phase segregation upon annealing (Fig. 3e). This result suggests that the removal of the linear alkyl chains on C6PT2C6 to form C6PT2 leads to a greater susceptibility for molecular ordering upon annealing.

Blend hole and electron mobility for the optimized C6PT2 films was measured at 1.9 × 10⁻⁶ and 5.8 × 10⁻⁵ cm² V⁻¹ s⁻¹ resulting in the μ_e/μ_h of 8.8 (Table 2). Interestingly, the optimized C6PT2C6 films yielded a similar μ_e/μ_h of 6.8. However, the hole and electron mobility in the optimized C6PT2C6 films are an order of magnitude greater than in the optimized C6PT2 films. In summary, removal of the linear alkyl chains on C6PT2C6 to form C6PT2 results in micron scale phase segregation at high donor:acceptor ratios which induces device optimization at a donor:acceptor ratio where performance is likely limited to a lower charge mobility and lack of the stronger absorbing donor material.

2.3. Conjugation length

As previously discussed, replacement of linear alkyl chains on compound C6PT2C6 with ethyl-hexyl groups or removal of terminal thiophene alkyl chains leads to micron scale phase segregation at high donor:acceptor blend ratios. For this reason, compound C6PT2C6 was used to study the impact of conjugation length by varying the number of thiophene units along the conjugated backbone. Table 1 shows that C6PT1C6 optimizes at a 60 : 40 C6PT1C6:PC₇₁BM blend ratio and an annealing temperature of 80 °C and yields a V_OC of 1.00 V, J_SC of 3.64 mA cm⁻², FF of 0.35, and PCE of 1.25%. In comparison, C6PT3C6 optimizes at a 20 : 80 C6PT3C6:PC₇₁BM blend ratio and an annealing temperature of 80 °C with a V_OC of 0.9 V, J_SC of 5.76 mA cm⁻², FF of 0.32, and PCE of 1.67%.

Fig. 3 shows topography images for annealed blend films of C6PT2C6 (a–e), EHPT2C6 (f–j) and C6PT2 (k–o) with PC₇₁BM. Blend film morphologies of C6PT1C6 and C6PT3C6 with PC₇₁BM are included in the Fig. S4 ESI.† In general, the surface topography for the C6PT1C6, C6PT2C6, and C6PT3C6 blend films is quite smooth with RMS roughness values between 1 and 5 nm. Unlike blend films of EHPT2C6:PC₇₁BM and C6PT2:PC₇₁BM, micron scale phase segregation is not observed at high donor:acceptor blend ratios. One of the main differences between compound C6PT1C6, C6PT2C6, and C6PT3C6 is the solubility which was previously measured at 228, 11, and 1.5 mg mL⁻¹ respectively.⁵⁸ This shows that solubility significantly drops when the conjugated backbone is increased. The low solubility of compound C6PT3C6 has a significant impact on the blend film absorption spectrum as shown in Fig. S7.† Fig. S7† shows that the overall C6PT3C6:PC₇₁BM blend film absorption spectrums decreases with increasing donor:acceptor ratio. The decrease in absorption is attributed to the poor solubility of C6PT3C6 which precipitates at high donor:acceptor ratios thereby limiting the film thickness and overall absorption. As a consequence of the poor solubility, C6PT3C6 optimized at a low donor:acceptor blend ratio of 20 : 80 where the film thickness and overall absorption is greater.

The optimized 20 : 80 C6PT3C6:PC₇₁BM films yielded blend hole and electron mobility values of 5.6 × 10⁻⁷ and 5.5 × 10⁻⁴ cm² V⁻¹ s⁻¹ respectively and a μ_e/μ_h of 970 (Table 2). The imbalance in charge mobilities is likely a contributing factor in the lower J_SC, FF, and PCE observed in the optimized C6PT3C6:PC₇₁BM device. In summary, extending the conjugation length of compound C6PT2C6 by two thiophene units significantly reduces the solubility thereby preventing the formation of thick films at high donor:acceptor ratios. The poor solubility induces C6PT3C6:PC₇₁BM devices to optimize at a low donor:acceptor ratio where device performance is limited by poor exciton harvesting and imbalanced charge mobility.

In contrast to C6PT3C6, compounds C6PT1C6 and C6PT2C6 have a greater solubility and optimized at a 60 : 40 donor:acceptor ratio in devices. As a result, compounds C6PT1C6 and C6PT2C6 serve as a good model system to investigate the impact of conjugation length on device performance when solubility is not a limiting factor.

Fig. 2 and Table 1 show that the optimized C6PT1C6 device has a greater V_OC but lower J_SC and FF relative to the optimized C6PT2C6 device. The greater V_OC can be explained by the lower lying HOMO level of C6PT1C6 at 5.63 V in comparison to the HOMO level of C6PT2C6 at 5.16 V. Relative to compound C6PT1C6, C6PT2C6 has two extra thiophene units. It is possible that the two additional electron donating thiophene units raise the HOMO level of C6PT2C6 thereby reducing the V_OC in C6PT2C6:PC₇₁BM devices.

When the conjugation length of C6PT2C6 is decreased by two thiophene units to make C6PT1C6, the resulting J_SC of the optimized devices drops from 7.91 to 3.64 mA cm⁻². To investigate the differences in current generation between the optimized C6PT1C6 and C6PT2C6 devices, the EQE spectra (Fig. 2b) were divided by the total active layer absorption to yield the internal quantum efficiency (IQE) spectra as shown in Fig. 4. Fig. 4a shows that the IQE of the optimized C6PT1C6 device increases from 30% to 70% when going from the short circuit condition to a −4 V applied reversed bias. In comparison, the IQE of the optimized C6PT2C6 device increases from 60% to 70% when going from short circuit conditions to a −2 V bias. At a strong reverse bias, the IQE of the optimized C6PT1C6 device increases by roughly 40% in comparison to only 10% for the optimized C6PT2C6 device. This result indicates that the charge collection efficiency is weaker in the optimized C6PT1C6:PC₇₁BM device relative to the optimized C6PT2C6PC₇₁BM device. Previous works have shown that charge collection efficiency and field dependent generation can be enhanced by applying a reverse bias.^63,69,70

Fig. 4 Bias dependent internal quantum efficiency of (a) 80 °C annealed 60 : 40 C6PT1C6:PC₇₁BM and (b) 120 °C annealed 60 : 40 C6PT2C6:PC₇₁BM blend. Internal quantum efficiency measured under zero, −1 V (black circle), −2 V (red square), −3 V (blue upward triangle), −4 V (purple ×), and −5 V bias (grey downward triangle).

To further probe charge collection efficiency, the blend hole and electron mobilities were measured for the optimized C6PT1C6 and C6PT2C6 devices. Table 2 shows that the optimized C6PT1C6 and C6PT2C6 devices have hole mobilities of 1.7 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 1.9 × 10⁻⁵ cm² V⁻¹ s⁻¹ respectively in addition to electron mobilities of 1.7 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 1.3 × 10⁻⁴ cm² V⁻¹ s⁻¹ respectively. The optimized C6PT2C6 device has an overall greater electron and hole mobility than the optimized C6P2C6 devices. Previous works have also shown that a greater charge carrier mobility can enhance charge collection efficiency.^62–64,71

In the ESI† we discuss a fluorescence quenching study (Fig. S9†) and XRD measurements (Fig. S10†) which show that extending the conjugation length of C6PT1C6 to form C6PT2C6 reduces molecular ordering thereby allowing for device optimization at higher annealing temperatures where the electron mobility (Fig. S8†) is enhanced.

In order to probe the charge collection in the donor phase, the optimized C6PT1C6 and C6PT2C6 devices where measured with photoconducting atomic force microscopy (pc-AFM) as shown in Fig. 5. Fig. 5a shows that the optimized C6PT1C6 device lacks current contrast in the pcAFM image and has a peak current of about 8 pA. In comparison, Fig. 5b shows that the optimized C6PT2C6 device yields 200–600 nm conductive domains with photocurrents up to 21 pA. Since a high work function gold tip was used, the brighter coloured domains represent hole collection from the donor phase.^72–75 The enhanced surface photocurrent observed in the optimized C6PT2C6 devices suggests that the optimized C6PT2C6 device may have a greater charge collection from the donor phase relative to the C6PT1C6 device.

Fig. 5 Photoconductive AFM images under white light and zero bias for the 80 °C 60 : 40 C6PT1C6:PC₇₁BM blend (a) and the 120 °C 60 : 40 C6PT2C6:PC71BM blend (b). All scan sizes are 2 μm × 2 μm.

In summary, the optimized C6PT2C6 device has a greater performance than the optimized C6PT1C6 device because the extended conjugation length suppresses molecular ordering which reduces phase segregation allowing for device optimization at an elevated annealing temperature where charge mobilities are enhanced for both the donor and acceptor phases thereby increasing the charge collection efficiency, J_SC, FF, and PCE.

3. Conclusions

The solar cell performance of a series of DPP small molecules blended with PC₇₁BM was investigated as a function of chemical structure, the type of solubilizing group, conjugation length, and processing condition. We show that simple chemical modification such as increasing molecular bulkiness and removal of linear alkyl chains can destabilize blend film morphology at high donor:acceptor ratios thereby inducing device optimization at low donor:acceptor blend ratios where device performance is limited by poor exciton harvesting and charge mobility imbalance. We also show that the conjugation length can be used to fine tune the energy level, solubility, and molecular ordering of a compound. Over-extending the conjugation length significantly reduces the solubility thereby preventing the formation of thick films. In contrast, decreasing the conjugation length past a critical limit enhances molecular ordering which may prevent device optimization at an elevated annealing temperature due to large phase segregation. We find that device optimization at higher blend ratios and elevated annealing temperatures can enhance charge collection in the donor and acceptor phase. In this class of materials, the optimal performance arises when using linear alkyl chain solubilizing groups along with an intermittent conjugation length.

4. Experimental

The synthesis for the five donor materials has been previously reported.⁵⁸ Glass substrates coated with 150 nm of Indium tin oxide (ITO, thin film devices) were sonicated for 15 minutes in acetone and isopropanol respectively. The ITO substrates were then baked in an oven overnight and treated with UV/ozone (UVO Cleaner 42, Jelight Co., Inc.) for one hour. PEDOT:PSS (Baytron 4083) was spin casted at 2500 rpm for 60 seconds and subsequently annealed at 140 °C for 15 minutes to yield a thickness of 55 nm. The films were briefly vacuumed while transferring though the small transfer chamber into the nitrogen glove box. Each DPP donor was blended with PC₇₁BM (Nano-C, USA) with an overall concentration of 2% (w/v) in anhydrous chloroform (Sigma-Aldrich) at donor:acceptor ratios of 10 : 90, 20 : 80, 30 : 70, 40 : 60, 50 : 50, 60 : 40, 70 : 30, and 80 : 20. All blend solutions were heated overnight at 60 °C with a stir rate of 300 rpm. In order to obtain high quality films without the presence of comets or foggy halos the hot (60 °C) solution was passed through a 0.45 μm poly(tetrafluoroethylene) (PTFE) filter directly onto the ITO/PEDOT:PSS substrate prior to spin casting. All films were spin casted at 2500 rpm for 15 seconds. PEDOT:PSS and active layer thicknesses were measured with an Ambios XP-100 Stylus profilometer.

After scratching off part of the active layer for the bottom (anode) contact, 110 nm of aluminium was evaporated onto the substrates starting slightly below 1 Å s⁻¹ and increased to 2 Å s⁻¹ once 20 nm of aluminium (Al) was deposited under a pressure of 1 × 10⁻⁷ Torr at room temperature. A shadow mask was used for the Al deposition and resulted in a device area of 19 mm². Each condition was tested with 15 devices. Device efficiencies were measured with a 150 Watt Newport-Oriel AM 1.5G light source calibrated to 100 mW cm⁻² with a National Renewable Energy Laboratory certified silicon diode fitted with a KG1 optical filter. External quantum efficiencies were measured with a Xe lamp, monochromator, optical chopper, and lock-in amplifier. To study the effects of annealing, films were annealed at 80 °C for 10 minutes and then quenched by placing on the glove box metal surface. To study higher annealing temperatures the same film was annealed at a higher temperature for 10 minutes.

The UV-Vis absorption spectra of blend films were measured with a Beckman Coulter DU 800 Spectrometer. Tapping mode AFM images were measured in ambient on an Innova Scanning Probe Microscope (Veeco) using silicon-nitride probes (BudgetSensors) with a spring constant of ∼3 N m⁻¹ and a resonant frequency of 75 KHz. Photoconducting images were measured on an Asylum Research MFP-3D microscope sitting atop of an inverted optical microscopy (Olympus, IX71). To prevent exposure to air, nitrogen was flowed through a fluid cell containing the device while the current was recorded by internal preamplifier (Asylum Research ORCA head model). Gold-coated silicon probes (BudgetSensors) with a spring constant of 0.2 nN m⁻¹ and a resonant frequency of 13 kHz were used. A white light source was focused on the sample with an inverted optical microscope (Olympus) resulting in an illumination spot size approximately 160 μm in diameter. The Gold-coated silicon probe was subsequently position at the center of the illumination spot. All images were scanned under short circuit conditions.

Thin-film XRD spectra were measured on device architectures of ITO/PEDOT:PSS/blend with a X'Pert Philips Material Research Diffractometer. Samples were scanned at 45 kV and 40 mA with a scanning rate of 0.004 degree per second, and Cu K_α radiation (wavelength λ = 1.5405 Å). In the 2θ–ω scan configurations each film was scanned from 4 to 30 2θ.

The diode hole mobility for pristine and optimized blend films were measured by fabricating a hole only device with a ITO/PEDOT:PSS/blend or pristine/MoOx/Au device structure. The MoOx and Au layer had thickness of 10 and 60 nm. Electron mobility was measured by fabricating devices with a glass/Al/blend or pristine/Al geometry. Current density as a function of voltage was measured on a Keithley 4200 in a nitrogen atmosphere. To extract mobility values the current density–voltage curved were fitted to the Mott–Gurney relationship (space charge limited current).⁷⁶

Internal quantum efficiency was determined by dividing the EQE by the active layer absorption. The total absorption was first measured on a Perkin Elmer Lambda 750 using an integrating sphere with the same device structure used in solar cells. A transfer matrix model⁷⁷ was then used to model parasitic absorbance from the electrodes. To calculate the active layer absorption, the modeled parasitic absorption was subtracted from the measured total absorption.

Steady-state fluorescence experiments at room temperature were performed using a Fluorolog Jobin Yvon Spex equipped with a xenon lamp excitation source. All samples were excited at 457 nm and collected in the front face orientation.

Acknowledgements

The authors thank the National Science Foundation (NSF) Division of Materials Research, NSF-SOLAR for the financial support. TQN thanks the Camille Dreyfus Teacher Scholar Award and the Alfred Sloan Research Fellowship program.

Notes and references

1. S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107, 1324–1338.
2. E. Bundgaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2007, 91, 954–985.
3. C. J. Brabec, Sol. Energy Mater. Sol. Cells, 2004, 83, 273–292.
4. A. R. Murphy and J. M. J. Fréchet, Chem. Rev., 2007, 107, 1066–1096.
5. T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, S. Wakim, J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am. Chem. Soc., 2011, 133, 4250–4253.
6. Z. He, C. Zhong, X. Huang, W. Wong, H. Wu, L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636–4643.
7. L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180–185.
8. A. Mishra and P. Bäuerle, Angew. Chem., Int. Ed., 2012, 51, 2020–2067.
9. Z. Li, G. He, X. Wan, Y. Liu, J. Zhou, G. Long, Y. Zuo, M. Zhang and Y. Chen, Adv. Energy Mater., 2012, 2, 74–77.
10. J. Zhou, X. Wan, Y. Liu, Y. Zuo, Z. Li, G. He, G. Long, W. Ni, C. Li, X. Su and Y. Chen, J. Am. Chem. Soc., 2012, 134, 16345–16351.
11. M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovoltaics, 2012, 20, 12–20.
12. J. A. Love, C. M. Proctor, J. Liu, C. J. Takacs, A. Sharenko, T. S. van der Poll, A. J. Heeger, G. C. Bazan and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 1–8.
13. R. C. Coffin, J. Peet, J. Rogers and G. C. Bazan, Nat. Chem., 2009, 1, 657–661.
14. Y. Kim, S. Cook, J. Kirkpatrick, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, M. Heeney, R. Hamilton and I. McCulloch, J. Phys. Chem. C, 2007, 111, 8137–8141.
15. M. T. Lloyd, J. E. Anthony and G. G. Malliaras, Mater. Today, 2007, 10, 34–41.
16. B. Walker, A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tantiwiwat and T.-Q. Nguyen, Adv. Funct. Mater., 2009, 19, 3063–3069.
17. J. Roncali, Acc. Chem. Res., 2009, 42, 1719–1730.
18. G. Wei, S. Wang, K. Renshaw, M. E. Thompson and S. R. Forrest, ACS Nano, 2010, 4, 1927–1934.
19. B. Yin, L. Yang, Y. Liu, Y. Chen, Q. Qi, F. Zhang and S. Yin, Appl. Phys. Lett., 2010, 97, 023303.
20. Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian and Y. Chen, Adv. Mater., 2011, 1, 771–775.
21. B. Walker, C. Kim and T.-Q. Nguyen, Chem. Mater., 2011, 23, 470–482.
22. J. A. Mikroyannidis, D. V. Tsagkournos, S. S. Sharma, Y. K. Vijay and G. D. Sharma, J. Mater. Chem., 2011, 21, 4679–4688.
23. G. Wei, S. Wang, K. Sun, M. E. Thompson and S. R. Forrest, Adv. Energy Mater., 2011, 1, 184–187.
24. H. M. Ko, H. Choi, S. Paek, K. Kim, K. Song, J. K. Lee and J. Ko, J. Mater. Chem., 2011, 21, 7248–7253.
25. G. C. Welch, L. A. Perez, C. V. Hoven, Y. Zhang, X.-D. Dang, A. Sharenko, M. F. Toney, E. J. Kramer, T.-Q. Nguyen and G. C. Bazan, J. Mater. Chem., 2011, 21, 12700–12709.
26. Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang and Y. Chen, Adv. Energy Mater., 2011, 1, 771–775.
27. Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and A. J. Heeger, Nat. Mater., 2012, 11, 44–48.
28. T. S. van der Poll, J. A. Love, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2012, 24, 3646–3649.
29. A. B. Tamayo, M. Tantiwiwat, B. Walker and T.-Q. Nguyen, J. Phys. Chem. C, 2008, 112, 15543–15552.
30. M. Tantiwiwat, A. Tamayo, N. Luu, X.-D. Dang and T.-Q. Nguyen, J. Phys. Chem. C, 2008, 112, 17402–17407.
31. L. Huo, J. Hou, H.-Y. Chen, S. Zhang, Y. Jiang, T. L. Chen and Y. Yang, Macromolecules, 2009, 42, 6564–6571.
32. Y. Zou, D. Gendron, R. Neagu-Plesu and M. Leclerc, Macromolecules, 2009, 42, 6361–6365.
33. J. Jo, D. Gendron, A. Najari, J. S. Moon, S. Cho, M. Leclerc and A. J. Heeger, Appl. Phys. Lett., 2010, 97, 203303.
34. J. C. Bijleveld, V. S. Gevaerts, D. Di Nuzzo, M. Turbiez, S. G. J. Mathijssen, D. M. de Leeuw, M. M. Wienk and R. A. J. Janssen, Adv. Mater., 2010, 22, E242–E246.
35. C. Kanimozhi, P. Balraju, G. D. Sharma and S. Patil, J. Phys. Chem. B, 2010, 114, 3095–3103.
36. R. Badrou Aïch, Y. Zou, M. Leclerc and Y. Tao, Org. Electron., 2010, 11, 1053–1058.
37. G. Zhang, H. Xu, K. Liu, Y. Li, L. Yang and M. Yang, Synth. Met., 2010, 160, 1945–1952.
38. C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and J. M. J. Fréchet, J. Am. Chem. Soc., 2010, 132, 15547–15549.
39. K. H. Hendriks, G. H. L. Heintges, V. S. Gevaerts, M. M. Wienk and R. A. J. Janssen, Angew. Chem., Int. Ed., 2013, 52, 8341–8344.
40. E. Ripaud, D. Demeter, T. Rousseau, E. Boucard-Cétol, M. Allain, R. Po, P. Leriche and J. Roncali, Dyes Pigm., 2012, 95, 126–133.
41. J. Liu, B. Walker, A. Tamayo, Y. Zhang and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 23, 47–56.
42. H. Langhals, S. Demmig and T. Potrawa, Journal für Praktische Chemie, 1991, 333, 733–748.
43. J. Mizuguchi, J. Phys. Chem. A, 2000, 104, 1817–1821.
44. A. B. Tamayo, B. Walker and T.-Q. Nguyen, J. Phys. Chem. C, 2008, 112, 11545–11551.
45. S. Lunák Jr, J. Vynuchal and R. Hrdina, J. Mol. Struct., 2009, 919, 239–245.
46. A. Tamayo, T. Kent, M. Tantitiwat, M. A. Dante, J. Rogers and T.-Q. Nguyen, Energy Environ. Sci., 2009, 2, 1180.
47. A. B. Tamayo, X.-D. Dang, B. Walker, J. Seo, T. Kent and T.-Q. Nguyen, Appl. Phys. Lett., 2009, 94, 103301.
48. M. Weiter, O. Salyk, P. Bednár, M. Vala, J. Navrátil, O. Zmeskal, J. Vynuchal and S. Lunák Jr, Mater. Sci. Eng., B, 2009, 165, 148–152.
49. B. P. Karsten, J. C. Bijleveld and R. A. J. Janssen, Macromol. Rapid Commun., 2010, 31, 1554–1559.
50. P. Sonar, G.-M. Ng, T. T. Lin, A. Dodabalapur and Z.-K. Chen, J. Mater. Chem., 2010, 20, 3626.
51. W. Kylberg, P. Sonar, J. Heier, J.-N. Tisserant, C. Müller, F. Nüesch, Z.-K. Chen, A. Dodabalapur, S. Yoon and R. Hany, Energy Environ. Sci., 2011, 4, 3617.
52. S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp and T. J. Marks, J. Am. Chem. Soc., 2011, 133, 8142–8145.
53. P.-L. T. Boudreault, J. W. Hennek, S. Loser, R. P. Ortiz, B. J. Eckstein, A. Facchetti and T. J. Marks, Chem. Mater., 2012, 24, 2929–2942.
54. J. W. Lee, Y. S. Choi and W. H. Jo, Org. Electron., 2012, 13, 3060–3066.
55. J. Liu, B. Walker, A. Tamayo, Y. Zhang and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 23, 2.
56. W. Shin, T. Yasuda, G. Watanabe, Y. S. Yang and C. Adachi, Chem. Mater., 2013, 25, 2549–2556.
57. O. P. Lee, A. T. Yiu, P. M. Beaujuge, C. H. Woo, T. W. Holcombe, J. E. Millstone, J. D. Douglas, M. S. Chen and J. M. J. Fréchet, Adv. Mater., 2011, 23, 5359–5363.
58. C. Kim, J. Liu, J. Lin, A. B. Tamayo, B. Walker, G. Wu and T.-Q. Nguyen, Chem. Mater., 2012, 24, 1699–1709.
59. J.-N. Tisserant, R. Hany, S. Partel, G.-L. Bona, R. Mezzenga and J. Heier, Soft Matter, 2012, 8, 5804–5810.
60. S. Nilsson, A. Bernasik, A. Budkowski and E. Moons, Macromolecules, 2007, 40, 8291–8301.
61. P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster and D. E. Markov, Adv. Mater., 2007, 19, 1551–1566.
62. W. Tress, A. Petrich, M. Hummert, M. Hein, K. Leo and M. Riede, Appl. Phys. Lett., 2011, 98, 063301.
63. S. Albrecht, W. Schindler, J. Kurpiers, J. Kniepert, J. C. Blakesley, I. Dumsch, S. Allard, K. Fostiropoulos, U. Scherf and D. Neher, J. Phys. Chem. Lett., 2012, 3, 640–645.
64. C. M. Proctor, C. Kim, D. Neher and T.-Q. Nguyen, Adv. Funct. Mater., 2013, 23, 3584–3594.
65. W. Tress, A. Merten, M. Furno, M. Hein, K. Leo and M. Riede, Adv. Energy Mater., 2013, 3, 631–638.
66. J. C. Blakesley and D. Neher, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 075210.
67. D. Credgington and J. R. Durrant, J. Phys. Chem. Lett., 2012, 3, 1465–1478.
68. O. Mikhnenko, J. Lin, Y. Shu, J. E. Anthony, P. W. M. Blom, T.-Q. Nguyen and M. A. Loi, Phys. Chem. Chem. Phys., 2012, 14, 14196–14201.
69. S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kurpiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch and D. Neher, J. Am. Chem. Soc., 2012, 134, 14932–14944.
70. D. Credgington, F. C. Jamieson, B. Walker, T.-Q. Nguyen and J. R. Durrant, Adv. Mater., 2012, 24, 2135–2141.
71. S. R. Cowan, R. A. Street, S. Cho and A. J. Heeger, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 035205.
72. D. C. Coffey, O. G. Reid, D. B. Rodovsky, G. P. Bartholomew and D. S. Ginger, Nano Lett., 2007, 7, 738–744.
73. X.-D. Dang, A. B. Tamayo, J. Seo, C. V. Hoven, B. Walker and T.-Q. Nguyen, Adv. Funct. Mater., 2010, 20, 3314–3321.
74. C. V. Hoven, X.-D. Dang, R. C. Coffin, J. Peet, T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2010, 22, E63–E66.
75. M. Guide, X.-D. Dang and T.-Q. Nguyen, Adv. Mater., 2011, 23, 2313–2319.
76. M. A. Lampert and P. Mark, Current injection in solids, Academic Press, 1970.
77. G. F. Burkhard, E. T. Hoke and M. D. McGehee, Adv. Mater., 2010, 22, 3293–3297.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45662e

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