Effect of doping on the short-circuit current and open-circuit voltage of polymer solar cells Yong Zhao, Chunjun Liang, Mengjie Sun, Qian Liu, Fujun Zhang, Dan Li, and Zhiqun He Citation: Journal of Applied Physics 116, 154506 (2014); doi: 10.1063/1.4898692 View online: http://dx.doi.org/10.1063/1.4898692 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhancement of short-circuit current density in polymer bulk heterojunction solar cells comprising plasmonic silver nanowires Appl. Phys. Lett. 104, 123302 (2014); 10.1063/1.4869760 Magnetic-field annealing of inverted polymer:fullerene hybrid solar cells with FePt nanowires as additive Appl. Phys. Lett. 103, 253305 (2013); 10.1063/1.4853935 Polymer defect states modulate open-circuit voltage in bulk-heterojunction solar cells Appl. Phys. Lett. 103, 243306 (2013); 10.1063/1.4841475 Origin of the light intensity dependence of the short-circuit current of polymer/fullerene solar cells Appl. Phys. Lett. 87, 203502 (2005); 10.1063/1.2130396 Light intensity dependence of open-circuit voltage of polymer:fullerene solar cells Appl. Phys. Lett. 86, 123509 (2005); 10.1063/1.1889240
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JOURNAL OF APPLIED PHYSICS 116, 154506 (2014)
Effect of doping on the short-circuit current and open-circuit voltage of polymer solar cells Yong Zhao, Chunjun Liang,a) Mengjie Sun, Qian Liu, Fujun Zhang, Dan Li, and Zhiqun He Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
(Received 10 August 2014; accepted 8 October 2014; published online 17 October 2014) The change in doping density in P3HT:PCBM based polymer solar cells (PSCs) with different processing solvents and with/without post-fabrication thermal treatment is investigated with capacitance-voltage measurement and optical microscopic imaging. The results suggest that both slow drying and thermal treatment facilitate the phase-separation and crystallinity of P3HT and PCBM, leading to low defect density and thus low p-type doping. Direct links between the doping density and the performance of the PSCs, specifically the short-circuit current (Jsc) and open-circuit voltage (Voc), are observed. The results show that doping density is one of the decisive factors affecting the photocurrent of the PSCs. Lower doping density leads to a wider depletion region, which is beneficial for carrier collection. The agreement between the calculation and the experiment suggests that the Voc increases monotonically with increasing doping densities in the PSCs. These rules consistently explain our results on the change of Jsc and Voc after thermal annealing in the PSCs with C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898692] different processing solvents. V
I. INTRODUCTION
Despite the fact that organic semiconductors are known to be both structurally and electronically disordered have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities of orders of magnitude lower than inorganic semiconductors, the power conversion efficiency of polymer solar cells (PSCs) has been increasing rapidly in recent years.1–4 Yet despite the high level of activity, the fundamental electronic processes in PSCs, e.g., charge carrier generation, recombination, transport, and collection,5–9 have been the subject of intense discussion. The material properties, such as bandgap, carrier mobility, absorption coefficient, and recombination coefficient have been extensively investigated6,10–12 and are considered to be the key parameters that affect the performance of PSCs. Normally, the first property measured in any new inorganic semiconductor is its doping density, because it directly affects the most important parameters of the semiconductor, such as conductivity, Fermi energy, carrier mobility, carrier lifetime, etc. But the importance of doping in PSCs has not been fully recognized and explored, and it is usually presumed that organic semiconductors are intrinsic, i.e., undoped.13,14 However, all semiconductors have at least some structural or chemical defects in them, and at equilibrium some of the defects will be charged. The defect states in the polymer layer may originate from local structural distortion in the conjugated polymer backbone,15–17 chemical impurities, 18 etc. In almost all cases, there will be more charged defects than the often negligible number of intrinsic charges created by thermal excitation across the bandgap. a)
Author to whom correspondence should be addressed. E-mail address:
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Glatthaar et al.19,20 observed the change in depletion region in the P3HT:PCBM PSCs using electrical impedance spectra; they used the concept of p-type doping to explain the improvement in photocurrent in the experiment. Recently, Nelson et al. discovered that p-type doping has a dominant influence on the photocurrent of PSCs based on some low bandgap polymers.21,22 These discoveries suggest that, as in inorganic semiconductor devices, the level of doping also has profound influence in organic semiconductor devices. However, the doping in the polymer-fullerene blends is always unintentional and investigation into the nature of the doping is necessary. In conventional inorganic semiconductors, the shallow donor (acceptor) gap states induced by chemical impurities or structure defects lead to n-type (p-type) doping. In organic semiconductors, structure defects and chemical impurities are still the major causes of gap states;23 in particular, the shallow gap states could be induced in pentacene thin films by the defects of molecular sliding.24 Overwhelmingly, the gap state acts as a recombination centre and trapping centre for charge carriers.25,26 Little attention is given to the doping effect of the (shallow) gap states induced by defects or impurities in organic semiconductors, although Schafferhans et al. reported an increased charge carrier concentration and a decreased carrier mobility after oxygen exposure in P3HT:PCBM blends and they attributed these additional charge carriers to oxygen doping.27 In addition, more detailed investigation of the effect of doping in polymer semiconductor devices is necessary. Is there any conclusive evidence indicating that the doping density is one of the decisive factors affecting the photocurrent (Jsc) of PSCs? Does the doping density affect the open-circuit voltage (Voc) of PSCs? These questions still remain to be answered. In this paper, we investigate the variation in doping density in P3HT:PCBM PSCs with different processing solvents
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and post-fabrication thermal treatment. Capacitance-voltage measurement is used to determine the doping density of the bulk-heterojunction (BHJ) layer. An optical microscopic image is applied to investigate the evolution of the surface morphology of the BHJ layer. The origin of the p-type doping and the reason for the variation in doping density are discussed in terms of the defect density in the BHJ layer. Direct links between the doping density and the performance of the PSCs, specifically the Jsc and Voc, are observed and discussed. An analytical expression is presented to explain the dependence of Voc on doping density. II. EXPERIMENTAL
Samples were prepared on glass substrates with indium tin oxide (ITO). The PSCs’ architecture is ITO/Cs2CO3/ P3HT:PCBM/MoO3/Al, and the active area is 0.04 cm2. For mobility analysis, the hole-only device structure is ITO/ PEDOT:PSS/P3HT:PCBM/MoO3/Al, and the electronic-only device structure is ITO/Cs2CO3/P3HT:PCBM/LiF/Al. The ITO-coated glass substrates were cleaned in an ultrasonic bath with acetone, detergent, deionized water, and isopropyl alcohol for 20 min in sequence. In a nitrogen-filled glovebox, a solution of 2 mg/ml Cs2CO3 in 2-methoxyethanol was spin-cast onto the clean surface of the ITO substrates with spin speeds at 800 rpm for 40 s and then thermally annealed on a hot plate at 130 C for 10 min. A blend solution of P3HT and PCBM (purchased from Luminescence Technology) with a weight ratio of 1:0.8 (total solids concentration of 36 mg/ ml) were prepared in Chloroform (CF), Chlorobenzene (CB), and orthodichlorobenzene (oDCB), respectively. The solution was then spin-coated onto the Cs2CO3 layer at a speed of 600 rpm for 20 s. The resulting film thicknesses were 240 nm, 153 nm, and 210 nm, respectively, measured with an Ambios XP-2 profilometer. After spin-coating, the samples were placed into petri dishes for 30 min to control the drying time. Once dry, the samples were moved to a thermal evaporator for MoO3 and metal deposition. The chamber was evacuated to a base pressure of 5 104 pa before evaporation began. Eight nanometres of MoO3 were deposited onto the ˚ /s, and 100 nm aluminP3HT:PCBM layer at a rate of 0.2 A ˚ /s. ium was deposited onto the MoO3 layer at a rate of 2 A Once the fabrication was finished, the photocurrent-voltage (J-V) characteristics were recorded by a Keithley source meter under illumination of AM 1.5G 100 mW/cm2 from a solar simulator. The capacitance-voltage (C-V) characteristics were measured on a semiconductor analyser with alternating voltage of 20 mV at 10 kHz. The morphology of the active layers was studied using optical microscopy. Then the sample underwent post thermal treatment on a hot plate at 120 C for 10 min. Finally, the J-V, C-V, and the optical microscopic image of the devices were measured again. III. RESULTS AND DISCUSSION A. Variation of p-doping density in P3HT:PCBM polymer solar cells
The analysis of the capacitance-voltage (C-V) scans in the Schottky junctions with doped semiconductors is based
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on the depletion approximation, which implies that there are no free carriers in the space charge region at the junction under investigation. The width of the space charge region (depletion region) depends on the doping density and the built-in voltage VBI via21,22 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2er e0 ðVBI V Þ w¼ ; (1) qNa where Na is the doping density, e0 is the dielectric constant of the vacuum, and er is the relative dielectric constant of the semiconductor. By changing the width w of the space charge region by varying the applied DC bias, the capacitance C ¼ e0erA/w of the space charge region is changed as well. Thus the slope of the plot of C2 versus applied DC voltage yields the doping density28 1 2 dC2 Na ¼ : (2) qe0 er A2 dV However, Kirchartz et al. warned that the measured doping density with the C-V analysis may be unreliable because that besides the junction capacitance, chemical capacitance also contribute to or even dominate the total capacitance, especially in the case of low doping and small thickness. They present the minimum detectable doping density for different thickness of P3HT:PCBM films. Only the doping higher than the minimum detectable value can be measured reliably by the C-V analysis.29 Fig. 1 shows the measured capacitance-voltage character and the C2-voltage plots of the device with different casting solvents and with/without thermal treatment. The calculated doping densities and the film thicknesses are summarized in Table I. The as-fabricated CF PSCs (short for the PSCs cast in CF solvent) has the highest doping density of 7.0 1016 cm3, after post thermal treatment the doping density decreased to 0.4 1016 cm3. The doping density of the as-fabricated CB device (3.0 1016 cm3) is lower than that of the CF device, and after thermal annealing it decreased mildly to 1.4 1016 cm3. The doping density of the as-fabricated oDCB device (0.3 1016 cm3) is the lowest among all the measured devices and it increased to 1.6 1016 cm3 after thermal annealing. The comparison between the measured doping densities and the minimum detectable values indicates that except the case of the as-fabricated oDCB device in which the actual doping density may be even lower than the measured value of 0.3 1016 cm3, all other measured doping densities are reliable. (See Figure S2 in supporting information for detailed comparison30). The significant difference in the evaporation rate of CF (Chloroform, Boiling point: 61.3 C), CB (Chlorobenzene, Boiling point: 132.2 C), and oDCB (Orthodichlorobenzene, Boiling point: 180.4 C) suggests different growth rates of the active layer. The drying of the film cast from CF solvent is the fastest, leading to smooth surface morphology (Fig. 2(a)), suggesting insufficient phase separation of the two components. Slow drying of the active layer promotes phase separation of the blend, and facilitates crystallization of the P3HT chains and aggregation of the PCBM molecules.31,32
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FIG. 1. The capacitance-voltage characteristics and the C2-voltage plots of the CF, CB, and oDCB devices before and after thermal treatment. The thin straight lines in (d), (e), and (f) illustrate the slope for the calculation of the doping densities.
TABLE I. A summary of the device properties.
Device type CF as-fabricated CF thermal annealed CB as-fabricated CB thermal annealed oDCB as-fabricated oDCB thermal annealed
Film Doping thickness density Jsc Voc (nm) Na (1016 cm3) (mA/cm2) (V) 240 240 153 153 210 210
7.0 0.4 3.0 1.4 0.3 1.6
1.85 4.87 1.87 7.42 8.54 6.84
0.65 0.57 0.68 0.60 0.58 0.59
FF 31% 31% 32% 41% 50% 61%
An optical micrograph shows the appearance of sub-micrometre-sized PCBM crystalline domains33 on the CB film (Fig. 2(b)). The slowest growth rate of the oDCB film leads to very uneven surface morphology (Fig. 2(c)), suggesting sufficient segregation and crystallization of P3HT and PCBM. Thermal treatments change the morphology of the CF blends significantly, evidenced by the clear appearance of the micrometre-sized PCBM aggregates after thermal annealing (Fig. 2(d)). The change in morphology by thermal annealing is mild in the CB film, showing a reduced number but a slightly enlarged size of the PCBM crystallite (Fig. 2(e)). The morphology change in the oDCB film by thermal annealing is not visually distinguishable in the optical
FIG. 2. Surface morphology of P3HT:PCBM films as observed with an optical microscope. A and D, the CF film before and after thermal treatment; B and E, the CB film before and after thermal treatment; C and F, the oDCB film before and after thermal treatment; G (H), schematic showing high (low) density of defects in the film of low (high) crystallinity; the density of the defect states in the grey area represents the p-doping level.
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micrograph, although the contrast of the image becomes more blurring in some sense (Fig. 2(f)). Both slow drying (by different solvents) and thermal treatment lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM.31 These processes improve the crystallinity of the components33–35 and significantly lower the defect states32,36 due to decreased disorder in the blend. In Fig. 2(g), we schematically show the high density of defects in the film of low crystallinity (fast growth and without thermal treatment). In contrast, Fig. 2(h) shows the case of a low density of defects in a film of high crystallinity (slow drying or with thermal treatment). Only the defect states of electron acceptors near the highest occupied molecular orbital (HOMO) level (illustrated by the grey area in the figure) contribute to the p-type doping via thermal excitation of holes in the valence band (HOMO).37 With this notion in mind, one can understand the trend in the variation of the doping density with different casting solvents and with/without thermal treatment, as indicated in Table I. Slow drying increases the phase-separation and cystallinity of P3HT and PCBM, leading to low defect density and thus low p-type doping. It explains the lowest doping density in the oDCB film and the highest doping density in the CF film without thermal treatment. Thermal annealing also increases the cystallinity of the film, thus leading to low defect density and low doping density. This explains the decrease in the doping density in the CF and CB films after thermal annealing. However, it contradicts the oDCB case in which the doping density increased after the thermal treatment. A possible reason may be that the disorder in the as-fabricated oDCB film is already very low and the thermal treatment actually increases the disorder in the film (evidenced by the slightly lower contrast of the surface morphology in Fig. 2(f)). B. The effects of doping on device performance
Fig. 3 shows the J-V characteristics of the device under solar illumination, and a summary of the device properties, including doping density Na, Jsc, Voc, and fill factor, are shown in Table I. There are direct correlations between doping density Na and device performance in all of the PSCs. For the as-fabricated CF device, the doping density Na was as high as 7.0 1016 cm3, and the Jsc and Voc were 1.85 mA/cm2 and 0.65 V, respectively. After thermal treatment, the doping density Na decreased dramatically to
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0.4 1016 cm3, meanwhile the short-circuit current Jsc increased to 4.87 mA/cm2, and the Voc decreased to 0.57 V. The results suggest that low doping density leads to a high short-circuit current Jsc and low open-circuit voltage Voc. The trend still persisted in the CB device in which the doping density decreased from 3.0 1016 cm3 to 1.4 1016 cm3 after thermal treatment, and the Jsc increased from 1.87 mA/cm2 to 7.42 mA/cm2, and the Voc decreased from 0.68 V to 0.60 V. However, in the oDCB case, the p-doping density is already as low as 0.3 1016 cm3 in the asfabricated device; after thermal treatment the doping level increased to 1.6 1016 cm3. In this case, the Jsc decreased from 8.54 mA/cm2 to 6.84 mA/cm2, and the Voc increased from 0.577 V to 0.589 V. The above data consistently suggest that lower doping density leads to higher short-circuit current Jsc and lower open-circuit voltage Voc. 1. The effect of doping on Jsc
The field distribution in PSCs is often described in terms of a metal-insulator-metal model, where relatively uniform electric fields are assumed in the device. However, nonuniform electric fields will develop when there is electronic or chemical doping19,38 or unbalanced charge carrier mobilities in the device.21 In the case of balanced mobilities (the value of electron mobility is close to the hole mobility), the distribution of the electric field is closely related to the doping density of the active layer. For sufficient p-doping density, a depletion region (high electric field) forms near the cathode and a neutral region (low electric field) near the anode (Fig. 4(a), Na ¼ 1.6 1016 cm3).21,22 The carrier mobilities of the as-fabricated and the thermal-annealed oDCB BHJ layer are measured by the space-charge-limited-current (SCLC) method in an electron-only device and a hole-only device. Both electron mobility and hole mobility of the oDCB layer are in the order of 104 cm2/V.s (ln ¼ 2.8 104 cm2/V.s, lp ¼ 4.0 104 cm2/V.s) 3 before thermal treatment, increasing to 10 cm2/V.s (ln ¼ 2.6 103 cm2/V.s, lp ¼ 1.5 103 cm2/V.s) after thermal annealing (see Figure S3 of the supporting information for the J-V characteristics of the electron- and hole-only device30). Fig. 4 shows the distribution of electric potential (a) and the carrier generation rate and recombination rate (b) in the oDCB device at short circuit under solar illumination, calculated from a drift-diffusion simulation. In the simulation, a
FIG. 3. Current-voltage characteristics of the PSCs under solar illumination.
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FIG. 4. The distribution of electric potential (a) and the carrier generation rate and recombination rate (b) at short circuit under illumination, calculated from a drift-diffusion simulation of the as-fabricated oDCB device (Na ¼ 0.3 1016 cm3, ln ¼ 2.8 104 cm2/V.s, and lp ¼ 4.0 104 cm2/ V.s), the thermal annealed oDCB device (Na ¼ 1.6 1016 cm3, ln ¼ 2.6 103 cm2/V.s, and lp ¼ 1.5 103 cm2/V.s) and an ideal device with low doping density and high mobilities (Na ¼ 0.3 1016 cm3, ln ¼ 2.6 103 cm2/V.s, and lp ¼ 1.5 103 cm2/V.s), respectively.
combined optical and electrical model was used to calculate the various parameters in the polymer solar cells. In the optical model, a transfer matrix approach was applied to calculate the photon absorption rate in the active layer. A detailed description of the model and the simulation method is included in our previous work.39 For the as-fabricated device, in which the doping density is low (0.3 1016 cm3), the electric field (indicated by the slope of the potential curve) is uniform throughout most of the active layer (except that at both ends there are strong fields due to the space charge effect40). Consequently, the obvious collection of the carrier by the field occurs throughout the active layer as indicated by the contrast of the carrier recombination rate and the generation rate. But there is still remarkable recombination throughout the layer indicating that the carrier collection is not fully effective; this is because the carrier mobilities are relatively low (104 cm2/V.s) in this case. After thermal annealing, the increase in the doping density to 1.6 1016 cm3 changes the distribution of the electric field significantly. The region on the right of the potential curve (divided by the dashed line) is the neutral region (almost zero electric field) of the device where the charge transport is via diffusion and charge collection is very inefficient, as indicated by the high recombination rate in this region; while the left hand region represents the depletion region with a high electric field where charge collection is highly efficient. After thermal annealing, the improved carrier mobility is
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beneficial for charge collection; but at the same time, the increased doping density narrows the collection region and is unfavourable for charge collection. The overall outcome is that the Jsc is lower in the annealed device due to the high recombination losses in the field-free region. In the figure, we also present a simulation of an ideal device in which the mobilities are assumed to be as high as the thermal-annealed device and the doping density is as low as the as-fabricated device. The simulation shows that very low bulk recombination losses throughout the active layer are possible in the low doping case. The narrow recombination peak at the left side is due to surface recombination. The simple assumption of 100% collection efficiency in the depletion region and 0% collection efficiency in the neutral region was proved to be valid by George et al.22 We can roughly consider the width of the depletion region to be the carrier collection depth of the device. From Eq. (1), it is clear that lower doping density will lead to a wider depletion region which is beneficial for carrier collection. This consistently explains our results on Jsc in CF, CB, and oDCB devices. It is well accepted that the enhanced and balanced carrier mobilities11,12 in the slow drying (or thermal treated) device is the major reason for the Jsc improvement. In our oDCB case, the carrier mobilities are indeed increased after the thermal annealing, but the Jsc still decreased remarkably, suggesting that the increased doping density is indeed the decisive reason here. From the above discussion, it is safe to conclude that the electric doping density is one of the decisive factors affecting the Jsc of the PSCs. Generally, low doping density is required to achieve high quantum efficiency and Jsc, especially for the PSCs with a thick BHJ layer.
2. The effect of doping on Voc
In order to understand the link between doping density and Voc, we deduced an expression for the open-circuit voltage based on the effective medium model.39 In this approach, the BHJ layer is considered as one semiconducting material with the HOMO of the polymer functioning as the valence band and the lowest unoccupied molecular orbital (LUMO) of the PCBM acting as the conduction band. The energy difference between the HOMO and LUMO levels is considered to be the band gap Eg. The quasi-Fermi levels EFn and EFp are Ei EFn kT
n ¼ ni e p ¼ ni e
E E FpkT i
;
(3)
;
(4)
where n (p) is the electron (hole) concentration under illumination, Ei is the intrinsic Fermi level, and ni is the intrinsic concentration of both electrons and holes. The intrinsic carrier concentration ni is given by 1
Eg
ni ¼ ðNc Nv Þ2 e2kT ;
(5)
where Nc(Nv) is the effective density of states of the conductive (valence) band. The product np is known to satisfy n0p0 ¼ ni2 in equilibrium, however,
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np ¼ n0 p0 e
J. Appl. Phys. 116, 154506 (2014) EFn EFp k0 T
¼ n2i e
EFn EFp k0 T
(6)
when the system is not in equilibrium. Using the terms of the quasi-Fermi levels, the expression for the electron (hole) current density, including the diffusion and drift current, is41 Jnð pÞ ¼ lnð pÞ nð pÞ
@ EFnðFpÞ ; @x
(7)
where ln(p) is the electron (hole) mobility. With the open circuit, the current densities are zero; consequently, the quasi-Fermi levels are constant. Since the (ohmic) contacts are in thermal equilibrium, the quasi-Fermi levels have to be equal to the potential at the contacts. This implies that the difference in the quasi-Fermi levels (EFnEFp) is constant throughout the device, and (EFn-EFp)/q is equal to the applied voltage in the open circuit. From Eq. (6), the open-circuit voltage is kT np ln 2 : Voc ¼ (8) q ni Further replace n(p) with n0 þ Dn (p0 þ Dp), where Dn(Dp) is the excess electrons (holes) and n0(p0) is the electron (hole) density in equilibrium. Notice that n0p0 ¼ ni2, p0 ¼ Na, and Dn ¼ Dp, finally we get kT Dn Dn2 DnNa ln 1 þ þ þ : (9) Voc ¼ q Na ni 2 ni 2 In order to compare the theory and the experiment, we first need to estimate the value of Dn and ni. Assuming that Nc ¼ Nv ¼ 2.5 10 19 cm3,14 Eg ¼ 1.05 eV, and T ¼ 293 K, from Eq. (5), the intrinsic carrier density satisfies ni ¼ 2.3 1010 cm3. Under the open-circuit condition, carrier recombination is the only relaxation path of the lightgenerated carriers, thus the carrier generation rate G and the recombination rate R satisfy G ¼ R. Here, we can use the Langevin recombination form R ¼ c(np-ni2) to estimate the carrier density at the open-circuit condition; here the recombination coefficient c is given by c ¼ qlmin/e0er, lmin being the minimum of the electron mobility and the hole mobility.8 Taking G ¼ 4.5 1021 cm3S1, lmin ¼ 1.0 103 cm2/V.s, e0 ¼ 3.15, and p Na ¼ 1.0 1016 cm3, finally we get Dn n ¼ 8.4 1014 cm3. The dependence of Voc on the p-doping density is shown in Fig. 5. The solid line is the calculated curve using Eq. (9) and the above estimated Dn and ni. The straight line in the semi-log plot shows that Voc increases monotonically with increasing doping densities in PSCs. The scatters in the figure are the experimental results summarized in Table I. The experiment results generally agree with the calculated curve, indicating that the doping density is an important factor affecting the Voc. Thus our analysis explains the common trends in PSCs, that the device with low doping density Na usually shows a lower Voc value as indicated in Fig. 5. The discrepancy between the experimental scatters and the calculated curve may be due to variations in Dp or ni in each case. The above analysis suggests that an increase in Jsc is always accompanied by a decrease in Voc if it is caused by
FIG. 5. Voc as a function of the p-doping concentration Na, the solid line is the calculated Voc with Eq. (8) and the scatters are the experimental results summarized in Table I.
a decrease in doping density. Our previous work39 shows that the increase in carrier mobility of the blends also leads to similar changes in Jsc and Voc. Our analysis coincides with a large number of experimental data36,42–45 in which a higher Jsc is always accompanied by a lower Voc. However, there are contradicting results46–48 in the literature in which the devices show an increase in both Voc and Jsc upon annealing of P3HT:PCBM, suggesting the change of other factors (e.g., the electrode interface) may occur in this case (see supporting information for the aging process of our device30). IV. CONCLUSION
We have investigated the change in doping density in P3HT:PCBM BHJ PSCs with different processing solvents and with/without post-fabrication thermal treatment. The doping density is the highest in the as-fabricated PSCs cast from the fast-drying solvent CF. Slow drying CB and oDCB solvents decrease the doping density significantly; in particular, the lowest doping density is observed in the asfabricated oDCB film. Thermal annealing reduces the doping density in the CF and CB films, but increases the doping density in the oDCB case. Evolution in surface morphology suggests that both slow drying and thermal treatment facilitate the phase-separation and crystallinity of P3HT and PCBM, leading to low defect density and thus low p-type doping. Direct links between the doping density and the performance of the PSCs, specifically the Jsc and Voc, are observed. Lower doping density leads to a wider depletion region, which is beneficial for carrier collection. In the oDCB case, despite the increase in carrier mobilities after thermal treatment, the Jsc decreases remarkably, suggesting doping density is the decisive factor for the photocurrent. The agreement between the calculation and the experiment suggests that Voc increases monotonically with increasing doping densities in PSCs. These rules consistently explain our results on the change in Jsc and Voc after thermal annealing in CF, CB, and oDCB devices.
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ACKNOWLEDGMENTS
This work was supported in part by FRFCU under 2013JBZ004 and 2009JBZ019, by NSFC under 60825407 and 21174016, by ISTCP under 2008DFA61420, and by RFDP under 20120009110031. 1
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