Adv Func MAterial DPM12 Emilio Toni 2010

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The Origin of the High Voltage in DPM12/P3HT Organic Solar Cells
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By Antonio Sánchez-Díaz, Marta Izquierdo, Salvatore Filippone, Nazario Martin,
and Emilio Palomares*

Organic solar cells made using a blend of DPM12 and P3HT are studied. The results show that higher V oc can be obtained when using DPM12 in comparison to the usual mono-substituted PCBM electron acceptor. Moreover, better device performances are also registered when the cells are irradiated with sun-simulated light of 10–50 mW cm − 2 intensity. Electro-chemical and time-resolved spectroscopic measurements are compared for both devices and a 100-mV shift in the density of states (DOS) is observed for DPM12/P3HT devices with respect to PCBM/P3HT solar cells and slow polaron-recombination dynamics are found for the DPM12/P3HT devices. These observations can be directly correlated with the observed increase in V oc , which is in contrast with previous results that correlated the higher V oc with different ideality factors obtained using dark-diode measurements. The origin for the shift in the DOS can be correlated to the crystallinity of the blend that is infl uenced by the properties of the included fullerene.

1. Introduction

The origin of the open-circuit voltage ( V oc ) of bulk heterojunc-tion solar cells containing a polymer and a mono-substituted fullerene is attracting extensive academic interest. [ 1 , 2 ] To date, a vast majority of the most effi cient devices reported the use of a regioregular poly(3-hexylthiophene) (P3HT) polymer and PCBM ([6,6]-phenyl-C 61 -butyric acid methyl ester) as the electron-donor and electron-acceptor materials, respectively ( Scheme 1 ).

Upon light absorption excitons are formed and those are split into free carriers if they are close enough to the interface that is created when the materials are blended. For effi cient devices, charge transport to the contact electrodes must be faster than

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2010, 20, 2695–2700

[∗] A. Sánchez-Díaz , Prof. E. Palomares Institute of Chemical Research of Catalonia (ICIQ) Avda. Països Catalans 16Tarragona E-43007 (Spain) E-mail: [email protected] Prof. E. Palomares Institut Català de Recerca I Estudis Avançats (ICREA) Avda. Lluís Companys 23, Barcelona E-80810 (Spain) M. Izquierdo , Dr. S. Filippone , Prof. N. Martin Universidad Complutense de Madrid (UCM) Organic Chemistry DepartmentChemistry Faculty Madrid E-28040 (Spain)

DOI: 10.1002/adfm.201000549

the charge-recombination processes occur-ring in the device. Although several charge transfer (CT) processes may limit the device effi ciency under operation, such as exciton quenching at the device contacts, [ 3 , 4 ] low polaron mobility, [ 5 , 6 ] or geminate recombi-nation, [ 7 ] it is widely accepted that the main limiting CT process is the polaron recom-bination in the bulk of the material. [ 8 , 9 ] Indeed, this reaction has been studied in detail using several photo-induced spectro-scopic techniques such as laser transient absorption spectroscopy (L-TAS), [ 10 , 11 ] pho-toinduced absorption (PIA), [ 12 ] etc. More recently, Shuttle et al. have demonstrated that transient photovoltage decay (TPV) can be used to study polaron-recombination kinetics and, moreover, several important cell parameters such as the accumulated charge under irradiation at different light

intensities can also be recorded. [ 8 , 9 ] Organic solar cells based on a mono-substituted fullerene

1,1-bis(4,4 ′ -dodecyloxyphenyl-(5,6)C 61 , diphenylmethanofullerene (DPM12) have been previously reported with effi ciencies reaching 2.3% (80 mW cm − 2 , 1.5 AM G). [ 13 ] Our DPM12 - based organic solar cells showed a V oc that was 100 mV higher under 100 mW cm − 2 sun-simulated light intensity and an overall better device performance than PCBM/P3HT devices when illuminated with light of 10–50 mW cm − 2 intensity, which is in good agreement with the above mentioned results. For this paper, we used the TPV technique with the aim of comparing our DPM12/P3HT devices with standard PCBM/P3HT solar cells and explaining the differences in the observed V oc under normal measurement conditions.

2. Results and Discussion

As previously reported, we estimated the energy values for the lowest unoccupied molecular orbital (LUMO) of the mono-substituted fullerenes by recording the cyclic voltammograms of the pristine materials. [ 14 ] We estimated the LUMO energy level of the fullerene derivatives from the fi rst reduction poten-tial ( Figure 1 ). [ 15–17 ]

The fullerenes studied in this paper, showed similar values for their LUMO energy, namely, –3.7 eV with respect to the vacuum level for both PCBM and DPM12, which is in good agreement with previously published electrochemical measurements.

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Scheme 1 . Chemical structures of the donor and acceptor materials used as light absorbers. From left to right [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), diphenylmethanofullerene (DPM12), and poly(3-hexylthiophene) (P3HT).

S n

C6H13

O

O

P3HTPCBM

OC12H25C12H25O

DPM12

In the model proposed by Scharber et al. [ 18 ] and Koster et al. [ 19 ] the LUMO energy of the acceptor directly affects the V oc in the devices. The model relates the maximum theoretical V oc that can be obtained in an organic solar cell as the energy differ-ence between the highest occupied molecular orbital (HOMO) energy of the donor and the LUMO energy of the acceptor.

In this study, the donor is in both cases the same polymer, P3HT, and the fullerene derivatives used as acceptors seem to have the same LUMO energy. Nevertheless, different V oc values were obtained in the complete, active devices ( Figure 2 , Table 1 ). We focused this study on revealing the origin of the obtained difference in V oc .

Previous studies on DPM12/P3HT solar cells have explained the higher V oc by the fact that there are different ideality fac-tors, as was obtained from dark-diode measurements. More-over, from those measurements the same reduction potential was extracted for DPM12 and PCBM, similarly to the results obtained electrochemically. [ 13 , 20 ] However, we believe that it is very likely that in this case the difference in V oc could arise from the different crystallization properties of the two electron-

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

Figure 2 . A,B) J sc – VP3HT (A) and PCBintensities.

Figure 1 . Cyclic voltammetry of the acceptors DPM12 (straight line) and PCBM (dashed line). Cyclic voltammetry was carried out in a solu-tion of o-dichlorobenzene/acetonitrile 4:1 using 0.1 M tetrabutylammo-nium hexafl uorophosphate as an electrolyte salt and scanning negative potentials with respect to an Ag/AgCl reference electrode at 10 mV s − 1 . The solution was initially outgassed and kept under nitrogen during the measurement.

acceptor materials, which plays a role when the devices are prepared, a fact that has already been observed for different organic photovoltaic (OPV) cells. A difference in crys-tallization may cause a shift or a different distribution of the DOS of the fi lm, which will lead to a change in the V oc . To carry out our experiments and prove or disprove our hypothesis, we used the same electrode material for all electrodes, thus, avoiding small differences in V oc related to the use of different indium tin oxide (ITO) substrates and different anode deposition rates; there-fore, the built-in voltage is expected to be the same. We also used the same donor molecule

for all devices, and mono-substituted acceptors. In doing so, the difference in V oc should only be attributable to factors such as charge recombination on the device, electrode selectivity affecting the injection process, or energetic distribution of the HOMO Donor and LUMO Acceptor levels. It is important to notice that other factors such as the charge-carrier mobility through the materials may also affect the V oc and is strongly related to the recombination kinetics. [ 6 , 21 ]

The J sc – V oc curves of the devices under dark conditions (Figure 2 ) showed signifi cant differences. The main factor

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oc characteristics of devices prepared with DPM12/M/P3HT (B) as the active layer at different light

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Figure 3 . A,B) Polaron recombination lifetime of complete devices versus light bias (A) and versus the fi lm charge density (B).

Table 1. J sc – V oc characteristics of complete devices.

Acceptor V oc [V] J sc [mA cm − 2 ] Fill Factor [%] Effi ciency [%] Light intensity [mW cm − 2 ]

DPM12 0.579 0.693 65.36 2.622 10

DPM12 0.619 2.878 63.35 2.275 50

DPM12 0.629 5.059 62.24 1.981 100

PCBM 0.438 0.930 61.88 2.522 10

PCBM 0.499 3.845 58.20 2.231 50

PCBM 0.524 6.894 56.25 2.031 100

that affects the dark current is the mobility of the charges throughout the material, which is directly related to the mor-phology, the charge injection process from the electrodes, and the recombination reaction at the interface.

Previous studies suggested that the origin of the dark cur-rent can be mainly explained by bimolecular recombination at the interface rather than the selectivity of the electrodes. [ 8 , 9 ] In our case, the high fi ll factors obtained suggest that the charge injection does not play a major role in the current generation. Therefore the differences found in the dark I – V curves sug-gest a higher energy barrier for the recombination process in the DPM12 fullerene derivative samples. DPM12 showed an open circuit voltage that is 100 mV higher than that of PCBM, which may be related to a hindered recombination process for the DPM12 fullerene derivative. To address the polaron-recombination process under device working condi-tions, we measured the transient photo-voltage (TPV) over a range of different light biases ( Figure 3 ).

The transients obtained can be fi tted to an exponential decay

V = V0 + A · exp(−t/τ ) (1) and from there the mean lifetime ( τ ) can be calculated, with A being an exponential prefactor, t is the decay time, and V and V 0 are the voltage and the initial voltage, respectively. The TPV data in Figure 3 is in good agreement with a slower recom-bination process for the DPM12 with respect to the PCBM devices, showing on average an order of magnitude slower polaron recombination. In order to make a fair comparison of the recombination kinetics the accumulated charge in the device must also be measured, because the equilibrium posi-tion between charge formation and recombination is charge dependent.

To monitor the accumulated charge in the device we carried out charge-extraction (CE) measurements. This technique con-sists of applying a white-light pulse that is large enough to reach the steady state under open-circuit conditions in the device. When the light pulse ends, the cell is short circuited through a 39 Ω resistance, so that the total charge fl ow can be moni-tored with a digitalized oscilloscope. By integrating the drop in voltage the charge can be obtained through applying

Q(C) = 1

R(Ω)t0

t∫

t0

V (V )dt

(2)

where R is the resistance connected to the circuit, Q is the charge, and V is the voltage. We would like to stress that this

© 2010 WILEY-VCH Verlag GmAdv. Funct. Mater. 2010, 20, 2695–2700

technique can only be used in effi cient devices, where the slow recombination kinetics ensure a proper measurement of the amount of charge in the device so that only a small loss of charge occurs due to recombination during the discharging of the device.

As the accumulated charge depends on the device volume, we need to normalize this to the corresponding charge density taking into account that the device area is 0.09 cm 2 and the thickness of the active layer is 90 nm and 70 nm for PCBM and DPM12, respectively. Figure 4 shows the charge density in the device for different photo-induced voltages.

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Figure 4 . Charge density of complete devices containing PCBM (fi lled squares) or DPM12 (fi lled circles) acceptors, measured using CE under illumination at different light biases.

Much to our surprise, the exponential curve for the DPM12 device is shifted by about 100 mV to higher potentials at small bias (approaching 1 sun (100 mW cm − 2 )) with respect to the PCBM-based OPV cell.

Our previous results, and several other reports on molecular photovoltaics, have demonstrated that the charge-extraction data can be used to obtain the capacitance of the device, [ 22 ] using a differential charging method, and, thus, a DOS dis-tribution of the Fermi level in the whole device ( Figure 5 ) can be calculated. Recently, it has also been suggested that the V oc in organic devices is governed by the carrier ability to fi ll the DOS and that the upper levels of the DOS are diffi cult to be fi lled with electrons because of the inherent increasing recom-bination dynamics. [ 23 ] Taking all the above experimental obser-vations into account, we can attribute the 100-mV shift in the V oc for the DPM12 devices to either a change in the Gaussian distribution of the energy states or to a shift in the energy-band position. The origin of the change in the DOS distribution could

© 2010 WILEY-VCH Verlag G

Figure 5 . The exponential of the density-of-states distribution in com-plete devices containing PCBM (hollow triangles) or DPM12 (fi lled circles) acceptors. Data fi tted to a single exponential rise.

be explained by a difference in crystallinity of the active layer, which would lead to a difference in dispersion of the HOMO/LUMO energy states (because molecules inside the fi lm have a different surrounding related to their packing), or it can also be explained by a shift in the Fermi-level position because of polymer crystallization.

Differential scanning calorimetry (DSC) can be used to measure the heat energy that is released when a compo-nent in a melt crystallizes. For this experiment, a solution of fullerene and P3HT was prepared using the standard proce-dure described elsewhere and then dried on a crucible in which the DSC was carried out. In addition, pristine P3HT was also measured as a blank. Figure 6 shows the endothermic crystal-lization peaks. For the pristine P3HT sample a peak at 231 ° C with an onset at 160 ° C can be registered corresponding to the crystallization of P3HT. The presence of a second maximum in the peak can be ascribed to impurities. In the P3HT:PCBM case the endothermic peak is shifted to lower temperatures, which means that the presence of PCBM enhances the crystallization of the polymer, as the peak is situated at 212 ° C with its onset at 201 ° C. These results are in agreement with previously reported DSC measurements. [ 24 ] Strikingly, for the DPM12/P3HT mix-ture only a faint broad peak was observed, centered at 190 ° C. This corresponds to a hindered crystallization of the polymer because of the presence of the fullerene derivative DPM12. The lack of crystallization in the presence of the DPM12 moieties can explain the higher V oc observed in the annealed DPM12 devices. The lower current values can also be explained through a smaller hole mobility, which is related to the polymer crystalli-zation. Such behavior has been observed before in P3HT:PCBM devices. [ 25–27 ] Before annealing, the V oc was close to 0.8 V and after annealing it decreased to 0.55 V, whereas the J sc increased by a factor of 5. In the DPM12 case the lack of crystallization leads to a high voltage and lower currents.

The fi lm morphology was also measured using atomic force microscopy (AFM) on the fi lms prepared using the same proce-dure as that used to fabricate the effi cient solar cell devices, in order to further prove the relationship between fi lm crystalliza-tion and the cell effi ciency. As shown in Figure 7 the annealing

mbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2010, 20, 2695–2700

Figure 6 . Differential scanning calorimetry carried out at a rate of 10 ° C min − 1 . Y-axis major intervals represent 1 mW.

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Figure 7 . AFM images of fi lms made of P3HT mixed with PCBM (top images) and DPM12 (bottom images). The left images are the resulting morphology of spin coating the mixture on glass substrates, and the right images corresponds to annealed fi lms at 150 ° C for 20 min. It can be seen that the size of the surface features increases with the annealing process on the P3HT:PCBM fi lms, whereas this effect is much smaller on the P3HT:DPM12 fi lm. The scan size for all images was 5 μ m × 5 μ m.

Figure 8 . Cyclic voltammograms of annealed fi lms on ITO. P3HT/PCBM (1:0.8) (solid line) and P3HT/DPM12 (1:1.1) (dotted line), using a scan rate of 10 mV s − 1 in outgassed acetonitrile.

process of 20 min at 150 ° C, induces a 50% increase of the sur-face roughness on the P3HT:PCBM fi lm, whereas the increase in surface roughness for P3HT:DPM12 fi lms is only 25%. This increase in surface roughness can be ascribed to the crystalli-zation of P3HT, which is aided by the presence of PCBM but much hindered by that of DPM12. The roughness mean square is about 1.5 nm for the as-cast fi lms.

The above results on the difference in voltage are also in good agreement with the cyclic voltammetry data of the blend fi lms on ITO ( Figure 8 ), where a 100-mV shift towards higher oxidation potentials can be observed in the fi rst peak for the P3HT:DPM12 fi lm. Moreover, a shift in the HOMO Donor energy position in blend fi lms related to crystallization of the fullerene moiety has been previously observed for P3HT:PCBM fi lms, combined with a change in color that can be seen by the naked eye or measured by UV-vis spectroscopy corresponding to a bathochromic shift. [ 28 , 29 ]

3. Conclusions

We have studied a new fullerene used as electron acceptor for bulk heterojunction solar cells, the diphenylmethanofullerene derivative DPM12. Although cyclic voltammetry suggests that the LUMO energy level for DPM12 is the same as that for the commonly used fullerene PCBM, different open circuit voltages were observed for standard devices based on these fullerenes. The devices were prepared using P3HT as the

© 2010 WILEY-VCH Verlag GmAdv. Funct. Mater. 2010, 20, 2695–2700

electron-donor polymer in both cases keeping the same cathode and anode composition. For optimized devices, DPM12 showed a slightly lower overall effi ciency than PCBM at full sunlight, however, an increase in V oc of 100 mV for DPM12 over PCBM was observed. The origin of this differ-ence in the V oc has been proven to be related to slower charge-recombination dynamics in the DPM12/P3HT devices and a shift in the HOMO Donor for DPM12/P3HT solar cells, which are directly related to the crystallization properties. Moreover, we have demonstrated that the charge-extraction technique allows changes in the DOS of different fullerene/P3HT devices to be measured and important information about energy-level characteristics of organic photovoltaic devices can, thus, be obtained.

4. Experimental Section Indium tin oxide (ITO, 4 Ω sq − 1 )-covered glass substrates were fi rst cleaned with acetone in order to remove the photo-protective layer (5 min sonication). The glass substrates were then placed twice in an ultrasonic bath in 2-propanol (IPA) for 5 min. Finally, they were dried at 80 ° C for 10 min followed by an ozone treatment of 30 min. Highly conducting poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (Baytron P) was spin-cast (5000 rpm, 1 min) with a thickness < 30 nm from aqueous solution (after passing a 0.20 mm cellulose acetate fi lter). The substrates were dried for 10 min at 110 ° C inside a glove box. The polymer used as the donor material was highly regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) electronic grade, purchased from Rieke → Specialty Polymers. PCBM was purchased from Nano-C → and the chlorobenzene used was purchased from SDS in analytical grade. PCBM was outgassed with an argon fl ux and stored inside a nitrogen-fi lled globe box. The solvent used for the P3HT:fullerene solution was a mixture of chlorobenzene (CB) and 1,2-di-chlorobenzene (ODCB) (50:50) because CB gives good results in terms of solubility and, therefore, homogeneity of the spin-coated fi lm. Using chlorobenzene the sensitivity of the morphology to heat treatment is better than for using chloroform. [ 30 ] ODCB raises the boiling point of the solution allowing for thicker fi lms and better phase segregation. The solution containing P3HT and the fullerene derivative was prepared inside a glove box (MB20G) under nitrogen atmosphere ( < 0.1 ppm O 2 ; < 0.1 ppm H 2 O). The solution was stirred at 65 ° C overnight. The spin coating of the photoactive layer

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was carried out inside the glove box and the conditions used were 900 rpm during 20 s, followed by 1100 rpm for 60 s. These conditions gave thicknesses in the range of 80 nm. Subsequently the devices were placed in a UHV chamber (2 · 10 − 6 mbar), and 10 nm of calcium, followed by 200 nm of silver were deposited on top of the active layer through a shadow mask, which led to 9 mm 2 devices. Thermal annealing was carried out by directly placing the completed devices on a digitally controlled hotplate at 150 ° C for 20 min inside the glove box. The thickness of the fi lms was measured with a stylus profi lometer Ambios Tech. XP-1, from a scratch applied to the middle of the fi lm to have accurate measurements. Following fabrication, the fi lms were maintained under nitrogen atmosphere and stored in the dark until used.

The J sc – V oc measurements were carried out with an ABET 150 W xenon light source equipped with an adequate set of fi lters to achieve a solar spectrum of 1.5 AM G. The light intensity was adjusted to 100 mW cm − 2 using a calibrated Si photodiode. The applied potential and cell current were measured with a Keithley model 2600 digital source meter. The current to voltage ( I – V ) curves were measured automatically with home-built Labview software. For TPV measurements, devices were directly connected to an oscilloscope in open-circuit conditions (1 M Ω ). Then the device was illuminated with white light to set the desired V oc , at this point equilibrium between charge formation, due to the illumination with light, and charge recombination was reached. Under these steady-state conditions, a small pulse of light, which provoked a 2 mV increase in the voltage, was applied. The excess of charges recombine at approximately the same rate setting the equilibrium, which determines the V oc observed. This allowed us to obtain an idea of the polaron-recombination rate under certain illuminated working conditions. Differential scanning calorimetry was conducted under nitrogen at a scan rate of 10 ° C min − 1 with a Mettler Toledo DSC822e instrument. The sample weight was 10 mg. Atomic force microscopy (AFM) of the samples was performed in the tapping mode on a molecular imaging model Pico SPM II (pico + ). The images were collected in air using silicon probes with a typical spring constant of 1–5 nN m − 1 and a resonant frequency of 75 kHz.

Acknowledgements The authors are grateful to the Spanish Ministerio de Ciencia e Innovación (MICINN) for the projects CONSOLIDER-INGENIO Hope CDS0007-2007, CONSOLIDER INGENIO Nanociencia Molecular-CSD2007-00010, CTQ2007-60746/BQU, and CTQ2008-0795/BQ. N.M. and M.I. thank the Comunidad Autónoma de Madrid for the project P-PPQ-000225-0505. E.P. and A.S.D. also thank the ICIQ for fi nancial support.

Received: March 22, 2010 Published online: July 13, 2010

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Adv Func MAterial DPM12 Emilio Toni 2010