effect of charge recombination on the fill factor of small molecule bulk heterojunction solar cells

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Yuan Zhang, Xuan-Dung Dang, Chunki Kim, and Thuc-Quyen Nguyen*

Effect of Charge Recombination on the Fill Factor of Small Molecule Bulk Heterojunction Solar Cells

Solution-processed organic BHJ solar cells based on 3,6-bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu) 2 ) or poly(3-hexylthiophene) blended with [6,6]-phenyl-C 60(70) -butyric acid methyl ester (PC 60(70) BM) behave differently under various irradiation intensities. Small molecule-based DPP(TBFu) 2 :PC 60 BM solar cells show up to 5.2% power conversion effi ciency and a high fi ll factor at low light intensity. At 100 mW cm − 2 illumination, the effi ciency and fi ll factor decrease, resulting in stronger power losses. Impedance spectroscopy at various light intensities reveals that high charge recombination is the cause of the low fi ll factor in DPP(TBFu) 2 :PC 60 BM.

1. Introduction

Organic bulk heterojunction (BHJ) solar cells utilizing con-jugated polymers or small molecules have attracted attention due to the low fabrication costs achievable via solution-processing methods. [ 1 − 6 ] By blending electron acceptor mol-ecules, typically fullerene derivatives, with electron donor materials, [ 7 − 9 ] the photo-generated excitons are effi ciently dissociated into charge carriers at the donor/acceptor inter-faces. Holes and electrons then migrate to the electrodes via a continuous networks of donor and acceptor materials, respectively. While most effi cient polymer BHJ solar cells reach power conversion effi ciencies (PCE) up to 7%, [ 10 − 14 ] devices based on solution-processable small molecules have, to date, only reached PCEs up to 5%. [ 15 , 16 ] The latter class of materials has advantages over conjugated polymers due to the ease of synthesis and purifi cation, together with low batch-to-batch variations. However, incorporation of small molecules into solar cells tends to lead to relatively low fi ll factors, FF , ( < 0.45), even for systems comprising the same chemical moieties as conjugated polymers. [ 16 , 17 ] Although the photoactive fi lm morphology can be improved via thermal or solvent annealing, the associated increase of FF s still lags behind those of polymeric cells. The origins of this differ-ence have not been thoroughly investigated and are poorly understood. [ 18 − 22 ]

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhewileyonlinelibrary.com

Dr. Y. Zhang , Dr. X.-D. Dang , Dr. C. Kim , Prof. T.-Q. Nguyen Center for Polymers and Organic Solids Department of Chemistry and Biochemistry University of California Santa Barbara CA 93106, USA E-mail: [email protected]

DOI: 10.1002/aenm.201100040

The FF value is defi ned by the maximum power divided by product of the short-cir-cuit current ( J sc ) and open-circuit voltage ( V oc ) and is dictated by several parameters. Apart from the shunt resistance, which originates from parasitic resistance at the electrode/photoactive layer interfaces, [ 23 ] unbalanced charge carrier mobilities can also be detrimental. The latter results in an unfavorable space-charge effect that leads to a square-root dependence of the photo-current on the electrical fi eld and a three-fourth dependence on the illumination intensity. [ 24 , 25 ] However, small molecule devices have been reported with small FF s

in which the hole and electron mobilities are similar. [ 17 , 26 ] Thus, imbalanced charge carrier mobilities alone may not account for the differences in FF observed between polymer and small mol-ecule BHJ devices.

Trapping of electrons in lower-lying states within the acceptor phase can also adversely affect the FF . This effect typically leads to a stronger dependence of the V oc on the illumination intensity. [ 27 , 28 ] From the electron transport characteristics of [6,6]-phenyl-C 60(70) -butyric acid methyl ester (PC 60(70) BM) blended with poly(3-hexy-lthiophene) (P3HT), these traps seem to be a minor effect on the solar cell performance, since they are fi lled by photo-generated charges upon illumination at moderate light intensities. [ 28 ]

Bimolecular recombination in BHJ polymer solar cells has been proposed as one of the major mechanisms for power loss. [ 29 − 33 ] This effect is manifested by a strong dependence of the photocurrent on the electrical fi eld, and thereby a low FF . It has been explained either by the fi eld-dependent electron-hole generation rate, [ 34 , 35 ] or by the illumination-dependent parallel resistance. [ 36 ] From recent transient photo-voltage and charge extraction measurements, the photocurrent of a P3HT:PC 60 BM solar cell has been successfully modeled by a recombination current, [ 32 ] assuming that the photocurrent at the V oc condition is canceled out by the recombination current. These studies demonstrate that both the V oc and the FF of P3HT:PCBM devices are primarily determined by the probability of bimo-lecular recombination. [ 32 ] Here a distinction should be made between geminate and non-geminate charge recombination. The fi rst refers to the process whereby electron-hole pairs from the dissociated excitons recombine before they become free carriers. The latter means that recombination occurs between positive and negative free carriers.

A benzofuran-substituted diketopyrrolopyrrole derivative, namely 3,6-bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethyl-hexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu) 2 ), has been

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Figure 1 . Current-voltage characteristics of: a) DPP(TBFu) 2 :PC 60 BM, and, b) P3HT:PC 70 BM solar cells under different illumination intensities.

used as the donor in solution-processable small molecule solar cells. [ 15 ] DPP(TBFu) 2 :PC 70 BM solar cells show PCE values of 4.4% with a FF of 0.48. [ 15 ] For comparison, P3HT:PC 70 BM solar cells show a PCE of 3–5% with a FF of around 0.65. [ 37 − 39 ] In this contribution, we investigate the light intensity dependence of charge recombination in small molecule DPP(TBFu) 2 :PC 60 BM solar cells and compare the results to polymeric P3HT:PC 70 BM devices (see Figure S1 in the Supporting Information for chem-ical structures). We fi nd that the PCE of DPP(TBFu) 2 :PC 60 BM device at low light intensities can reach up to 5.2%, exceeding the PCE measured under 100 mW cm − 2 (1 sun) irradiation. The decrease of the PCE at 1 sun is caused primarily by a saturated V oc and reduced FF . Impedance spectroscopy was applied to investigate whether the charge recombination is the cause of the reduced FF . This technique measures the charge carrier density, effective charge carrier lifetime, and the carrier density-dependent geminate recombination as a function of light inten-sity. Impedance spectroscopy measurements were performed at open-circuit conditions to minimize the driving force for charges to transport to the electrodes, and therefore, increase the probability of charge recombination. The rest of this paper is organized as follows: Section 2 summarizes results and dis-cussion, with Section 2.1 detailing how impedance spectroscopy method can be used to study charge recombination in optimized DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM devices, and Section 2.2 examining the effect of fi lm processing conditions on the charge recombination process; we summarize and conclude in Section 3 and provide experimental details in Section 4.

2. Results and Discussion

2.1. Charge Recombination in Optimized DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM Devices

The current density–voltage ( J–V ) characteristics at various illu-mination intensities of solar cells comprising active layers of DPP(TBFu) 2 :PC 60 BM annealed at 110 ° C and P3HT:PC 70 BM annealed at 150 ° C are displayed in Figure 1 . Under the opera-tion regime, the photocurrent of the DPP(TBFu) 2 :PC 60 BM device shows a stronger dependence on the applied bias when compared to the P3HT:PC 70 BM device at different light intensities, repre-sentative of a relatively low FF . When applying a large reverse bias (not shown), the P3HT:PC 70 BM solar cell displays a saturated photocurrent, whereas that of the DPP(TBFu) 2 :PC 60 BM device continues to increase with the applied bias. This observation implies that the photocurrent of the DPP(TBFu) 2 :PC 60 BM device is more sensitive to the internal electric fi eld and requires a larger fi eld to completely dissociate and sweep out charge carriers.

Figure 2 displays the solar cell parameters of the DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM devices as a func-tion of light intensity. The J sc of both systems scales linearly with the light intensity, yielding a power law of 1; a signature of recombination-limited photocurrent. [ 40 ] The higher J sc of the DPP(TBFu) 2 :PC 60 BM device can be explained by its nar-rower absorption band gap (1.7 eV), compared to that of P3HT (2.0 eV), which allows for better overlap with the solar fl ux. [ 15 ] The V oc of these two devices increases with increasing light

© 2011 WILEY-VCH Verlag GmAdv. Energy Mater. 2011, 1, 610–617

intensity (Figure 2 b). The DPP(TBFu) 2 :PC 60 BM device shows a maximum V oc of around 0.9 V at 100 mW cm − 2 , 0.3 V higher than that of the P3HT:PC 70 BM device. While PC 60 BM and PC 70 BM acceptors have similar lowest unoccupied molecular orbital (LUMO) energies, [ 41 ] the deeper highest occupied molec-ular orbital (HOMO) of DPP(TBFu) 2 (5.2 eV) is responsible for the higher V oc . [ 15 ] A remarkable feature of DPP(TBFu) 2 :PC 60 BM is that the V oc saturates at a low light intensity of approximately 0.1 sun and remains constant upon further increasing the illu-mination. In contrast, the V oc of the P3HT:PC 70 BM device has a linear response to the irradiation intensity, an effect which has been previously predicted with a metal-insulator-metal (MIM) device model or an expanded p – n junction model. [ 31 , 42 ]

Both devices in Figure 2 have similar FF values at low light intensities (around 0.30 at < 1 mW cm − 2 ). For the DPP(TBFu) 2 :PC 60 BM device, the FF reaches the highest value of 0.48 at a light intensity of approximately 23 mW cm − 2 instead of 100 mW cm − 2 and subsequently decreases with increasing irradiation intensity. In contrast, the FF of the P3HT:PC 70 BM device increases with the light intensity and reaches a maximum value of around 0.60 at 100 mW cm − 2 . In Figure 2 d, the PCEs of the DPP(TBFu) 2 :PC 60 BM devices versus the light intensity are plotted together with those of the P3HT:PC 70 BM devices. The PCE of the DPP(TBFu) 2 :PC 60 BM device increases to a maximum value of 5.2% at 11 mW cm − 2 illumination, and then decreases to around 4% at 100 mW cm − 2 , whereas the PCE of the P3HT:PC 70 BM device exhibits a monotonous enhancement with the light intensity and maximizes at 100 mW cm − 2 with an effi ciency of 2.8%. These results reveal that the power loss

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Figure 2 . Light intensity-dependent a) short-circuit current, b) open-circuit voltage, c) fi ll factor, and, d) power conversion effi ciency of DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM solar cells.

of DPP(TBFu) 2 :PC 60 BM cell upon high irradiation is mainly attributable to the reduced FF and saturated V oc .

To further understand why a high illumination intensity does not lead to an enhanced V oc and improved FF and PCE for the DPP(TBFu) 2 :PC 60 BM device, photo-generated charge carrier densities ( ρ ) at the V oc condition were measured by using impedance spectroscopy. At the V oc condition, the applied

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external bias is canceled out by the built-in electric fi eld origi-nating from the work function difference of the electrodes. Under these conditions, one would expect a fl at band condition and zero net current fl ow. Photo-generated charge carriers in the active layer therefore have the highest probability to recom-bine with each other. We made use of impedance analysis with a Schottky equivalent circuit model (see Figure S2 in the Sup-porting Information) to simulate the chemical capacitance ( C ) of the devices. Treating the donor/acceptor interfaces of the photoactive blends as numerous microscopic p – n junc-tions, [ 43 , 44 ] the C value of a working solar cell originates from the photo-induced charge accumulation within the active layer. Using Equation 1 , where q is the elementary charge, A is the device area, and d is the active layer thickness, the charge car-rier concentration ρ at a given V oc was calculated by the integra-tion of C over V oc for different light intensities. [ 45 , 46 ]

D =1

q Ad

Voc∫

dark

C(V )dV

(1)

The effective carrier lifetime ( τ eff ) was extracted from the product of the recombination resistance ( R rec ) and C . In Figure 3 a, the determined ρ at V oc is plotted as a function of light intensity. Carrier densities for both the DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM devices increase with the light intensity due to a stronger photo-absorption. For a light intensity less than 50 mW cm − 2 , the carrier concentra-tion in the DPP(TBFu) 2 :PC 60 BM blends exceeds that of the P3HT:PC 70 BM device, which can be explained by a narrower absorption band gap (1.7 eV) of DPP(TBFu) 2 , which allows for better overlap with the solar fl ux. [ 47 ] Above 50 mW cm − 2 , the ρ of the DPP(TBFu) 2 :PC 60 BM device is lower than that of the P3HT:PC 70 BM device. In such a regime, geminate recombi-nation becomes more pronounced, leading to a larger loss of charge carriers in DPP(TBFu) 2 :PC 60 BM. The amount of the carriers in the DPP(TBFu) 2 :PC 60 BM device saturates at 39 mW cm − 2 , similar to the V oc saturation intensity shown in Figure 2 b. The charge carrier density measured for the P3HT:PC 70 BM solar cell, however, displays a linear scaling with a large range of light intensity (0.55 to 38 mW cm − 2 ). It rapidly increases at a higher illumination up to 100 mW cm − 2 . The sharp increase of ρ for P3HT:PC 70 BM cell suggests a more effi cient charge dis-sociation rate and fewer losses under this regime. Interestingly one can observe that the V oc and carrier density of these two devices show similar trends as functions of applied light inten-sity. This result implies that in addition to the energy offset of the donor-acceptor, the build-up of V oc is also affected by the density of free charge carriers in the photoactive layer. [ 48 ]

The τ eff versus the illumination intensity and the carrier density are plotted in Figure 3 b and c, respectively. For both the small molecule and polymer devices, τ eff decreases with increasing light intensity and carrier density. In general, a shorter τ eff can be ascribed to a higher charge recombination rate, which competes with charge extraction. To understand this observation, we measured τ eff at the J sc condition, where a larger internal electrical fi eld facilitates charge extraction. Under the same illumination intensity, the τ eff at the J sc condi-tion is longer than that at the V oc condition for both devices, as shown in Figure 3 b, consistent with more effi cient charge

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Figure 3 . Illumination-dependent a) free carrier density, and, b) effec-tive carrier lifetime of DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM solar cells measured at open-circuit condition. Also compared in b (closed sym-bols) are the carrier lifetimes at short-circuit condition. c) Effective carrier lifetime as a function carrier concentration in DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM devices.

extraction leading to less charge recombination. This result agrees with a recent study of polymer solar cells using tran-sient photocurrents. [ 46 ] At high light intensity, the τ eff of the DPP(TBFu) 2 :PC 60 BM device measured at J sc and V oc conditions is slightly lower than that of the P3HT:PC 70 BM device, indi-cating more pronounced carrier loss via recombination.

Further insight can be obtained by examination of the charge recombination kinetics. From the relation, [ 43 ]

Jeff =

1

2$D (2)

where β represents the recombination constant, and ρ the car-rier density, the value of β under the solar cell operation regime


can be directly extracted from Equation 2 if it is a constant and is independent of the carrier density. If β is dependent on ρ , β ( ρ ), one needs to perform double-carrier injection measure-ments with an applied bias larger than the built-in potential to inject both electrons and holes into the device. Subsequently, the negative and positive charges will transport along their respective donor and acceptor phases and then recombine at the donor/acceptor interfaces. The resulting current from the charge injection is governed both by the carrier mobility and the recombination constant β ( ρ ).

In Figure 3 c, τ eff values obtained at the V oc condition are compared as a function of carrier concentrations. For P3HT:PC 70 BM, τ eff decreases linearly with ρ in the range 9.3 × 10 15 to 4.8 × 10 16 cm − 3 . Linear fi tting produces provide a slope of 2, which agrees with the results from transient measure-ments, [ 46 ] suggesting a fi rst order dependence of β on the carrier density. The recombination kinetics of the DPP(TBFu) 2 :PC 60 BM device exhibits two regimes. Under low light intensities, τ eff weakly decreases with the carrier concentration, inferring an attenuated dependency of β on ρ . In contrast, for high carrier density, τ eff quickly reduces with ρ leading to a slope of 13, indi-cating that the recombination constant under these conditions strongly depends on the carrier density. The recombination rate ( B ) , describing the absolute intensity of the recombination, is given by: [ 31 ]

B = $(D ) (np − ni pi) (3)

where np is the product of the density of the negative and positive free charges, and n i p i the product of the intrinsic car-rier concentration, which is generally much lower than np . It is clear that under high illumination intensity B becomes stronger because both np and β ( ρ ) increase in this regime. Since the car-rier density near 1 sun is similar for both DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM devices, the strong dependence of β on ρ leads to a higher recombination rate for the DPP(TBFu) 2 :PC 60 BM device and consequently a reduced FF .

To quantitatively determine the recombination constant β ( ρ ), double-carrier injection measurements were performed on the DPP(TBFu) 2 :PC 60 BM device at voltages larger than V oc (Figure S3 in the Supporting Information). With an applied bias larger than the built-in potential, electrons and holes can be injected from the cathode (Al) and anode (ITO), respec-tively. For an average electric fi eld F of 2 × 10 6 V m − 1 , sim-ilar to the solar cell operation regime, the determined β ( ρ ) is 3.3 × 10 − 12 cm 3 s − 1 for the DPP(TBFu) 2 :PC 60 BM device. This value is over one order higher than that of the P3HT:PC 70 BM device (1.0 × 10 − 13 cm 3 s − 1 ) obtained from transient voltage decay measurements. [ 30 , 49 ] Our fi ndings confi rm that the strong carrier density dependence of the recombination constant and the large recombination rate of the DPP(TBFu) 2 :PC 60 BM device are the primary reasons that the FF at 100 mW cm − 2 is low, as compared to that of the P3HT:PC 70 BM solar cell.

2.2. Effect of Film Processing Conditions on the Charge Recombination Process of DPP(TBFu) 2 :PC 60 BM Devices

Thermal annealing has been demonstrated to improve the FF and PCEs for the DPP(TBFu) 2 :PC 60 BM devices. [ 15 ] Next,


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we investigate the effect of thermal annealing on the carrier density, lifetime, and recombination kinetics. Figure 4 a and b show the J–V characteristics measured at irradiation intensi-ties of 100 and 11 mW cm − 2 for as-cast and annealed devices at 80, 100, and 160 ° C for 10 min. Photocurrents reach the highest value (10.7 mA cm − 2 ) after thermal annealing at 110 ° C. [ 15 , 50 ]

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Figure 4 . J – V characteristics of as-cast and thermally annealed DPP(TBFu) 2 :PC 60 BM solar cells measured at: a) 100, and, b) 11 mW cm − 2 . c) Fill factor, and, d) power conversion effi cient of DPP(TBFu) 2 :PC 60 BM solar cells as a function of annealing temperature upon irradiation at 100 and 11 mW cm − 2 .

Upon an irradiation of 11 mW cm − 2 , the short-circuit current of the 110 ° C annealed device drops by roughly one order of mag-nitude from 10.7 mA cm − 2 (100 mW cm − 2 ) to 0.89 mA cm − 2 (Figure 4 b). The V oc measured at 11 mW cm − 2 for the as-cast devices and those annealed at 80 ° C are 0.34 and 0.62 V, respec-tively, much lower than those of the 110 and 160 ° C devices (0.85 and 0.78 V, respectively). At 100 mW cm − 2 , the as-cast devices show the highest V oc (0.9 V).

FF s and PCEs as a function of annealing temperature are shown in Figure 4 c and d, respectively. Under 100 mW cm − 2 illumination, the FF increases with annealing temperature. The highest FF of 0.45 was observed after annealing at 160 ° C, fol-lowed by the value of 0.43 obtained from the device annealed at 110 ° C. The as-cast and 80 ° C annealed devices show a poor FF of approximately 0.25, most reasonably due to an unoptimized fi lm morphology. Under a low light intensity (11 mW cm − 2 ), both the as-cast and the annealed DPP(TBFu) 2 :PC 60 BM cells show a slightly increased FF as compared to those measured at 100 mW cm − 2 . In contrast, the FF of the device annealed at 160 ° C drops from 0.45 at 100 mW cm − 2 to 0.37 at 11 mW cm − 2 . At both illumination intensities, the highest PCE is found for the device annealed at 110 ° C. Favorable phase separation at this temperature yields a higher J sc when compared to other processing conditions. [ 15 , 50 ] The increased J sc combined with the increased FF leads to higher PCEs in the 110 ° C annealed device at all light intensities. Remarkably, with the increase of the light intensity, the most signifi cant power loss from a PCE of 5.2% at 11 mW cm − 2 to 4.0% at 100 mW cm − 2 is mainly due to the decreased FF .

Figure 5 shows the light-dependent solar cell parameters under various annealing conditions. For both as-cast and annealed solar cells, the J sc increases linearly with the light intensity (Figure 5 a). The fi tted curve with a slope of 1 is indic-ative of a recombination-dominant photocurrent. [ 51 , 52 ] The dependence of the V oc on the illumination intensity however behaves differently upon different annealing temperature. For 110 and 160 ° C annealed devices (open diamond and star sym-bols in Figure 5 ), V oc saturates at 23 mW cm − 2 . Conversely, the V oc of as-cast and 80 ° C annealed cells shows a linear enhance-ment as the function of the irradiation. At 100 mW cm − 2 , all devices show a similar V oc (0.8–0.9 V). From the FF versus light intensity plot shown in Figure 5 c, it is noticeable that, except for the 110 ° C annealed device, there is no reduction of the FF s for other devices at high light intensities. As shown in Figure 5 d, only the optimal device (with the highest PCE at 100 mW cm − 2 ) displays a decreased PCE upon high illumina-tion while the PCEs of all other solar cells steadily increase with the light intensity.

The charge carrier densities as a function of light intensity at V oc of the as-cast and annealed DPP(TBFu) 2 :PC 60 BM solar cells are shown in Figure 6 a. Higher carrier densities with increasing light intensities are observed for all devices. Within a wide range of illumination, the carrier density increases with increasing the annealing temperature up to 110 ° C. This trend mimics the changes of the V oc and J sc as a function of light intensity shown in Figure 5 a and b.

From Figure 6 b, which shows the τ eff as a function of light intensity, one can observe a decrease of the carrier lifetime with increasing light intensity. The light-dependent τ eff of the device

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Figure 5 . Illumination-dependent a) short-circuit current, b) open-circuit voltage, c) fi ll factor, and, d) power conversion effi ciency of as-cast and annealed DPP(TBFu) 2 :PC 60 BM solar cells at different annealing temperatures.

Figure 6 . a) Carrier concentration, and, b) effective carrier lifetime of as-cast and thermally annealed DPP(TBFu) 2 :PC 60 BM solar cells as a function of light intensity. c) Effective carrier lifetime versus carrier density of the as-cast and annealed devices at different annealing temperatures.

annealed at 110 ° C is, on average, the shortest, followed by the 160 ° C annealed device. From Equation 2 , shorter-lived carriers are mainly due to the high carrier concentrations, as shown in


Figure 6 a. The longest carrier lifetime is observed in the device annealed at 80 ° C. This observation is in agreement with the double-charge carrier injection measurement (Figure S3 in the Supporting Information), in which the 80 ° C annealed device shows the smallest recombination constant. Figure 6 c displays the effective carrier lifetime as a function of carrier density. Two regimes can be distinguished in the recombina-tion kinetics. While the as-cast device shows a slow decrease of τ eff from 88 to 13 μ s for a carrier concentration from 7 × 10 14 to 2.9 × 10 16 cm − 3 , thermally annealed devices have a rela-tively long-lived carrier lifetime of around 170 μ s at low carrier densities, which rapidly decreases to around 9, 1.2, and 1.7 μ s


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for 80, 110 and 160 ° C annealed devices, respectively, at higher illumination intensities.

The slowly decreased τ eff is indicative of a weak dependence of β on ρ . The fast decay of the carrier lifetime with carrier den-sity under high irradiation (high carrier density) leads to a curve slope of 13 for the 110 ° C annealed device and of 11 for the 80 and 160 ° C counterparts. Although thermal annealing sig-nifi cantly changes the device performance, the recombination kinetics results suggest that the large carrier density-dependent recombination constant may be an intrinsic property of the DPP(TBFu) 2 :PC 60 BM materials.

3. Conclusions

Solution-processed small-molecule solar cells based on DPP(TBFu) 2 :PC 60 BM blends demonstrate high potential for low-cost photovoltaic applications. The recombination con-stant of DPP(TBFu) 2 :PC 60 BM devices displays a stronger dependence on the photo-induced carrier density, com-pared to P3HT:PC 70 BM polymer cells, leading to a faster charge recombination rate under 100 mW cm − 2 irradia-tion. This observation explains the large power losses in the DPP(TBFu) 2 :PC 60 BM system upon high irradiation, which is limited by the decreased FF and saturated V oc . A peak PCE of 5.2% is found for the device annealed at 110 ° C under an illumination intensity of 11 mW cm − 2 . The relatively high carrier density upon 110 ° C annealing leads to the largest recombination rate, which helps explain the loss of the device FF and PCE under 1 sun. Minimizing charge recombina-tion processes at high light intensities is needed to enhance FF and device performance in DPP-based, small-molecule solar cells.

4. Experimental Section For device fabrication, DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM in blend ratios of 60:40 and 55:45 were dissolved in chloroform and chlorobenzene, respectively, with a total concentration of 17 mg mL − 1 . Prior to the casting of active layers, a 60 nm layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) was spin-coated on the pre-cleaned ITO substrates (140 nm) and baked at 140 ° C for 30 min. BHJ active layers were then deposited from the solutions at a spin-speed of 2000 rpm for 1 min leading to a thickness typically of around 90 nm. Next, thermal evaporation of 100 nm of Al was performed to form a top contact. The devices were thermally annealed on a hotplate for 10 min under a N 2 atmosphere at 80, 110, and 160 ° C for DPP(TBFu) 2 :PC 60 BM and 150 ° C for P3HT:PC 70 BM. Three sets of devices were fabricated on different days for the DPP(TBFu) 2 :PC 60 BM and P3HT:PC 70 BM devices. For each set, ten devices were measured and only representative data are shown above. Standard solar cell characterizations (1 sun) were carried out in nitrogen environment under simulated 100 mW cm − 2 AM1.5G irradiation from a 300 W Xe arc lamp with an AM 1.5 global fi lter. For other illumination intensities, a Newport 5215 fi lter wheel with designed optical densities was placed in between the samples and the light source. The light intensity was calibrated with an NREL certifi ed silicon diode with an integrated KG1 optical fi lter. The capacitance of the solar cells were recorded by a Solartron 1260 impedance analyzer with an applied DC bias equal to the open-circuit voltage under corresponding irradiation intensities.

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors thank the Offi ce of Naval Research for supporting the materials synthesis of this work, the Energy Frontier Research Center of the Department of Energy Offi ce of Basic Energy Sciences for supporting these studies, and the Camille Dreyfus Teacher-Scholar Awards Program. YZ thanks Chris Proctor and Bright Walker for stimulating discussion and for proofreading of the manuscript. TQN is an Alfred P. Sloan Foundation Research Fellow.

Received: January 28, 2011 Revised: April 7, 2011

Published online: June 9, 2011

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effect of charge recombination on the fill factor of small molecule bulk heterojunction solar cells

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