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Transient Photocurrent Response of Small-Molecule Bulk Heterojunction Solar Cells Jason Seifter, Yanming Sun, and Alan J. Heeger* Organic semiconductors offer many potential advantages for high performance, low cost “plastic electronics” such as solar cells and light emitting diodes, including solution processability, mechanical flexibility, and the potential for economical, large-scale, roll-to-roll fabrication.[1–7] While power conversion efficiencies (PCEs) have been historically low compared to inorganic counterparts, the last few years have witnessed a dramatic improvement in organic bulk heterojunction (BHJ) solar cell performance, with PCEs recently surpassing 10%,[8–14] which is widely viewed as a critical milestone. The architecture includes a BHJ active layer, where a semiconducting polymer or conjugated small molecule donor is blended with a fullerene derivative electron acceptor and co-deposited from solution.[15–17] Following photoexcitation, a combination of ultrafast charge transfer and diffusion of excitons to a nearby heterojunction yields mobile carriers.[18–21] The result is a self-assembled, phase-separated, complex nanoscale network that transports photoexcited carriers to the electrodes for collection. Extensive research has focused on improving the morphology and performance through solvent and thermal annealing, mixed solvents and solution additives,[22–32] but the mechanisms by which these processes improve the devices remain subjects of ongoing debate. Small molecule BHJ solar cells offer several advantages over the more traditional, polymer-based cells. Compared to polymers, molecular donors have fewer batch-to-batch synthesis variations, are more easily purified with standard techniques, exhibit a greater degree of crystallization, and do not suffer from the effects of non-reproducible molecular weight or polydispersity.[33,34] As a result, the use of small molecule donors leads to greater consistency in material performance. Despite this, however, our understanding of and control over the complex blend morphology in these systems is not as mature as it is for polymer BHJ cells. Since a non-optimal donor-acceptor network can lead to inefficient charge transport or a large number
J. Seifter, Prof. A. J. Heeger Physics Department University of California at Santa Barbara Santa Barbara, CA, 93106, USA E-mail: [email protected] Dr. Y. Sun Center for Polymers and Organic Solids University of California at Santa Barbara Santa Barbara, CA, 93106, USA Prof. A. J. Heeger Materials Department University of California at Santa Barbara Santa Barbara, CA, 93106, USA
DOI: 10.1002/adma.201305160
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of traps, which can act as recombination centers, the importance of correlating molecular structure to photovoltaic performance cannot be overstated. Steady-state measurements such as current-density voltage (J–V) or incident photon to current efficiency (IPCE) are valuable, but insufficient by themselves for fully characterizing the performance of a BHJ solar cell. They typically suffer from the disadvantage of combining several different physical processes, including charge generation, transport, recombination, and collection at the electrodes, effectively obscuring which mechanism(s) may be responsible for changes in figures of merit. Time-dependent measurements, however, offer the possibility to address these processes individually, as they are dominant at different time scales, and can often be manipulated by varying such parameters as internal field strength, light intensity, and temperature. It is important to understand the origin of improvements in performance, and equally important to understand which areas can be improved if we are to continue the current rapid pace of development. Transient photocurrent (TPC) measurements are powerful tools for analyzing BHJ solar cell performance. TPC measurements probe charge carrier transit kinetics, which yield insight into the carrier mobility, different recombination mechanisms, the carrier density, and the density of states.[32,35–42] In order to perform such experiments, a short pulse of light, typically from a laser or fast pulsed LED, is used to photogenerate carriers in a BHJ solar cell. Following excitation and charge transfer, the photogenerated charges are driven toward the respective electrodes by the internal electric field, which can be manipulated with an external voltage bias. While in transit, some fraction of the carriers can propagate relatively unhindered though the solar cell, with a velocity determined by the internal field and a well-defined maximum mobility, μmax, while the transport of other carriers in such an inhomogeneous material must be described by multiple trapping in a distribution of localized states.[43] Unless the trapped charge recombines, the energetic depth of a trap is statistically correlated with the time the carrier is immobilized before being thermally released.[38,40] The resulting distribution of average carrier velocities results in dispersive transport that manifests as a time-dependent mobility.[41] Since organic solar cells are optically thin, carriers are generated throughout the bulk of the active layer, which means that the extracted current is the sum of both electron and hole currents. The simultaneous presence of both carrier species allows for non-geminate carrier recombination. This has the potential to take the form of bimolecular recombination, usually described as Langevin recombination with a significantly reduced rate constant compared to that predicted by the original Langevin theory, or as trap-assisted, Shockley-Read-Hall
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N S
S
N S
N
S
N S
Si S
N
N S
S
p-DTS(PTTh2)2 OMe
I O
DIO
-2
Current Density (mA cm )
I
2 0
PC71BM
b) 0% DIO 0.25% DIO 0.6% DIO
-2 -4 -6 -8 -10 -12 -14 0.0
0.2
0.4
0.6
0.8
Applied Voltage (V) Figure 1. (a) Molecular structures of the materials used in this study. (b) Steady state J–V characteristics of p-DTS(PTTh2)2:PC71BM solar cells processed with different concentrations of DIO in solution.
recombination.[33,36,44–46] The shape of the current transient is encoded with all of these effects, and is therefore complex, but can still yield very useful information which can prove valuable for material design and device construction. Here we report TPC measurements used to investigate the role of the processing additive 1,8-diiodooctane (DIO) in small molecule BHJ solar cells comprised of 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (p-DTS(PTTh2)2), blended with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The molecular structures for all three materials are displayed in Figure 1a. Previous reports[47–49] have shown that a remarkably small amount (0.25% v/v) of DIO additive in the p-DTS(PTTh2)2:PC71BM system significantly improved the solar cell performance by enhancing the short circuit current (Jsc) and fill factor (FF), with external quantum efficiency increasing across the entire visible spectrum. This optimal concentration of DIO served to reduce domain/crystallite length scales in the BHJ active layer to levels more favorable for efficient charge transfer, while simultaneously preserving a high degree of long-range molecular order. It was postulated that the photocarrier collection enhancement was due to either improved carrier generation yields or reduced recombination rates, but the
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authors were unable to conclusively distinguish between these alternatives.[47] Further additions of DIO beyond the optimal concentration quickly became detrimental, however, considerably increasing crystallite length scales and reducing all solar cell figures of merit. Using TPC measurements, we will show that the processing additive has the combined beneficial effects of lowering carrier recombination rates and increasing carrier mobility, both of which enable more charges to reach their respective electrodes and be extracted from the device prior to recombination. Furthermore, carrier generation yields are not improved, but merely unaffected by the use of a small amount of the DIO processing additive. Once excess DIO causes crystallite length scales to get too large, however, charge transfer efficiency drops, resulting in fewer mobile carriers. This implies the existence of a morphological “sweet spot” near 0.25% DIO, where the solar cell enjoys the benefits of improved transport without the detriment of the reduced carrier generation that dominates at higher concentrations. In addition, we will show that geminate recombination via charge transfer excitons plays a negligible role as a carrier loss mechanism in p-DTS(PTTh2)2:PC71BM solar cells regardless of the DIO concentration used during film processing, as field dependent polaron pair dissociation is not observed. Conventional structure p-DTS(PTTh2)2:PC71BM solar cells were fabricated on cleaned Indium Tin Oxide (ITO) coated glass substrates using the same procedure reported in previous publications.[48] A hole transport layer (HTL) consisting of 9 nm of MoOx was first deposited on the ITO anode via thermal evaporation. MoOx was chosen as the HTL over the more common PEDOT:PSS, because it has been documented that the pyridial nitrogen in the [1,2,5]thiadiazolo[3,4-c]pyridine (PT) building block of the p-DTS(PTTh2)2 donor undergoes a detrimental chemical reaction with PEDOT:PSS.[50,51] Specifically, the sulfonic acid protonates the lone pair of the pyridial nitrogen of the PT heterocycle, which severely compromises solar cell performance. Cells fabricated with MoOx suffer from no such effects. p-DTS(PTTh2)2:PC71BM was blended with three concentrations of DIO (0%, 0.25%, 0.6% v/v) and spin-cast from chlorobenzene to form 150 nm thick active layers. Thermal deposition of 100 nm of aluminum directly onto the BHJ with no interfacial electron transport layer completed the device, and the 3.25 mm2 geometric overlap of the Al cathode with the ITO defined the active area. Devices were kept intentionally small to reduce the capacitance of the solar cells. This assists the TPC measurements, which benefit from minimizing RC time constants. Steady state J–V curves for the three solar cells (processed with different amounts of the DIO additive) are shown in Figure 1b, with the relevant device parameters summarized in Table 1. Spectroscopic and morphological analysis of this system, including absorption spectra, X-ray scattering, high resolution TEM, and ultrafast transient absorption have been reported previously.[19,47–49] Performance trends here are consistent with previous work. We see once again that compared to the cell with no additive, the BHJ is optimized with 0.25% DIO, increasing PCE from 4.3% to 5.6%. This can mostly be attributed to the fill factor improving from 43% to 54%, which
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a)
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www.MaterialsViews.com Table 1. p-DTS(PTTh2)2:PC71BM solar cell figures of merit when processed with different concentrations of DIO additive. Jsc [mA cm−2]
Voc [V]
FF
PCE [%]
μ [cm2 V−1s−1]
0
12.1
0.84
0.43
4.3
5.6 * 10−3
0.25
13.6
0.76
0.54
5.6
6.7 * 10−3
0.6
7.8
0.71
0.38
2.1
7.6 * 10−3
DIO [% v/v]
compensates for the loss of open circuit voltage (Voc). Marked deterioration in all figures of merit was observed as DIO concentration was further increased to 0.6%, with PCE dropping precipitously to 2.1%. A simplified schematic of the experimental setup used to perform the TPC measurements is shown in Figure S1 in the Supporting Information. A nitrogen pumped dye laser was used as the excitation source for photocarrier generation. The pulsed laser provided 4 ns wide, λ ≈ 590 nm pulses with a repetition rate of 10 Hz, in order to allow any trapped charges to disperse between measurements, since it has been shown that a very small number of long-lived, trapped carriers can have lifetimes extending out to milliseconds or longer.[38,40] In order to ensure accuracy, it is important that the pulse width be kept as short as possible compared to the transit time of charges in the device, which, as we will show, can be as short as a few tens of nanoseconds. If carriers continue to be photogenerated during extraction, measurement of the transit kinetics becomes much more difficult. A typical measurement consisted of 500 current transients averaged together, where no pulse deviated by more than 2% from the target fluence of 0.05 μJ cm−2. This fluence is low enough to keep space charge effects and series resistance induced transient voltage spikes from significantly perturbing the internal field and impeding the sweep-out of mobile carriers, as is shown in Figure S2. An external voltage bias (Vbias) controls the total internal potential (Vint) across the device, which determines the drift velocity of the carriers, since Vint = VBI − V bias
(1)
where VBI is the built-in voltage of the cell. Finally, the timeresolved photocurrent was recorded with a Tektronix DPO3034 oscilloscope by measuring the voltage drop across a 5 ohm RF sensor resistor in series with the device. The overall RC time constant of the circuit was less than 20 ns, allowing excellent time resolution of the transient kinetics. There are several critical considerations that must be taken into account for accurate TPC measurements, including aggressively minimizing circuit series resistance, optimization of sample geometry, accurate control of laser fluence, and careful selection of RF circuit elements that do not attenuate high-frequency signal components.[52,53] Despite the relatively simple appearance of the apparatus, preserving pulse fidelity in highspeed transient analog electronics with a highly capacitive load is one of the most demanding circuit requirements. As BHJ materials become more efficient, with higher carrier mobilities and shorter carrier transit times, these considerations will become even more important if an accurate measurement of 2488
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transport dynamics is to be performed. A more detailed discussion of the practical considerations required to generate accurate data can be found in the Experimental Section. Transient photocurrent measurements were performed on the aforementioned p-DTS(PTTh2)2:PC71BM solar cells at room temperature in an effort to understand the role of the DIO processing additive. Figure 2a shows bias-dependent TPC traces in the optimized (0.25% DIO) cell, where the internal field of the device is varied by more than an order of magnitude. The same data for the 0% and 0.6% additive cells are given in Figure S3 in the Supporting Information. In order to gain insight into these traces, we first consider the charge density, dρ, contained in a two-dimensional slice of the BHJ after photoexcitation, which is given by q n(x,t) dx, d
dρ(x,t) =
(2)
where n(x, t) is the bulk charge carrier density, and d is the thickness of the device. Note that at this point, we are looking only at one carrier species, but an identical argument holds for the other. If μ(x, t) is the carrier mobility, then the current density due to that slice of charge is given by dJ(x,t) =
dρ q n(x,t) dx q n(x,t) = = v dt d dt d
dJ(x,t) =
q n(x,t) qn(x,t)μ(x,t)Vint μ(x,t)E = . d d2
(3)
By integrating this expression over the entire thickness of the solar cell, we get the current density out of the device at time t. Assume that the mobility is not a function of position, space charge effects are negligible, and the illumination uniformly gives rise to a homogeneous distribution of carriers across the active layer. Then n(x, t) = n(t), Vint is constant, and μ(x, t) = μ(t). The total current density at time t, for each carrier type, is then given by J(t) = ∫
d
μEt
qn(t)μ(t)Vint qn(t)μ(t)Vint dx = d2 d2
μ i (t)Vint t ⎞ ⎛ qn i (t)μ i (t)Vint ⎞ ⎛ J i (t) = ⎜ ⎟⎠ ⎜⎝ 1 − ⎟⎠ d ⎝ d2
d
∫μEt dx (4)
2
d . The total current through the where i = n, p, and t < μ i (t)V int device is the sum of the electron and hole currents. In general, fitting a TPC trace to Equation (4) and extracting device parameters is not a trivial task. To start, ni(t) decays due to carrier recombination, which can be trap-assisted (monomolecular), reduced Langevin (bimolecular), or even higherorder.[43,54,55] The dominant mechanisms are still debated, and in reality, are likely all present to varying degrees at different time scales. In addition, carrier transport is typically dispersive, with the functional form of μi(t) dependent on the density of states. The hallmark of dispersive transport is a long photocurrent tail, as shown in Figure 2b. Detailed modeling of Equation (4) will be the subject of a future publication, but for now, we gain valuable insight without complex numerical simulations. At very short time scales, prior to significant recombination, n i (t) = n i ( 0) = n o . Additionally, the only carriers collected at this
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COMMUNICATION Figure 2. (a) Bias-dependent transient photocurrent measurements of the optimized (0.25% DIO) p-DTS(PTTh2)2:PC71BM solar cell. Note that the displayed voltages in the legend are the total internal potential according to Equation (1), and are valid for all four panels. (b) Photocurrent tail which arises as a result of dispersive transport. (c) TPC measurement of the optimized cell at high internal field (reverse bias) showing a linear fit to the initial decay. (d) Running time integral of the photocurrent trace (the extracted charge) as a function of time for different internal field conditions of the optimized cell.
point will be highly mobile carriers moving with mobility μmax, since the effects of dispersive transport have not yet had time to manifest. By setting μ i (t) = μ i ( 0) = μ max, Equation (4) predicts the current density to fall off linearly at short time scales. This is clearly seen in Figure 2a and Figure 2c (dotted line in Figure 2c). For all measured values of the internal field shown in Figure 2a, the initial photocurrent decay has a linear regime, which is consistent with bulk generation of mobile carriers all moving with an initial constant drift velocity, determined by μmax and the internal field of the device. This holds true for all of the other solar cells as well, as can be seen in Figure S3. In past TPC measurements, the short timescale dynamics were completely inaccessible due to distortions caused by RC time constant limitations, which severely compromises the utility of the technique. Following strict adherence to the considerations outlined in the Experimental Section, we note here that resolution of the linear photocurrent decay is unambiguous proof that there is a population of carriers with well-defined maximum mobility, and that the instrumentation is not RC limited, as otherwise the decay would appear to be exponential in nature with an RC time constant equal to that of the experimental setup. From Figure 2c, which shows an example of a linear fit to the high-field (fastest) transient from Figure 2a, we see the linear regime extending out to 21 ns beyond the peak, all the way down to approximately the “1/e” time of the photocurrent transient. This validates the estimation of our RC limitations as better than 20 ns, and guarantees fidelity of the measurement.
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We can estimate the value of μmax, then, for each solar cell by fitting the slope of the linear regime of the current transient and using Equation (4) with the short timescale considerations outlined above. The resulting μmax values were calculated and are summarized in Table 1. It is important to emphasize that these mobility measurements were derived from vertical charge transport across the bulk of the film, as opposed to the lateral geometry of the FET architecture typically used for mobility measurements. Using Equation (1) to determine the internal potential, the built-in voltage of each device was established by finding the external bias point where the photocurrent changes sign. The initial carrier density, no, was estimated by integrating the current transient to get extracted charge as a function of time, as shown in Figure 2d for each bias condition of the optimized cell. At high internal fields, the collection of carriers is efficient, so the total extracted charge at high reverse bias provides a reasonable estimate of the initial carrier density. Alternatively, the linear regime can be extrapolated to zero photocurrent in order to provide a rough estimation of carrier transit time through the device. Both methods for calculating mobility yielded similar results. As summarized in Table 1, the addition of DIO during processing of the BHJ film improves the maximum carrier mobility in the vertical direction as longrange order is enhanced. With use of the optimal amount (0.25%), the mobility increases from 5.6 * 10−3 cm2 V−1 s−1 to 6.7 * 10−3 cm2 V−1 s−1 compared to the cell without the
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additive. Increasing the concentration to 0.6% further increases the mobility to 7.6 * 10−3 cm2 V−1 s−1, but leads to a less efficient solar cell overall. As we will show later, this is a result of the larger crystallites hindering charge transfer and carrier formation. Since the measured transient is the sum of electron and hole currents, and there is no evidence of well-defined and separate time scales for extraction of the two different carrier species, we are unable to measure distinct electron and hole mobilities from this experiment. Previous FET measurements on this blend system, however, have shown balanced mobilities, consistent with our results.[47,48] Furthermore, in the vertical diode configuration, we would expect the individual carrier mobilities to readjust to be self-consistently comparable in efficient solar cells, as asymmetric carrier extraction would result in charge transport being inhibited by the generation of an internal space charge. As can be seen in Figure 2d and Figure S4, the total extracted charge from the solar cells is highly dependent on the internal field. For example, when operating at the maximum power point, p-DTS(PTTh2)2:PC71BM solar cells fabricated without use of the DIO additive yield only 56% of the number of carriers extracted at high reverse bias. When the DIO additive is used, however, 73% and 71% of the available charge is extracted for the optimized and 0.6% cells, respectively. In order to investigate whether this observation is due to recombination or field-dependent carrier generation, we can follow the method outlined previously by Street and Cowan.[56] By setting t = 0 to be the peak of the photocurrent trace, Equation (4) becomes ⎛ qn i ( 0)μ i ( 0)Vint ⎞ ⎛ qn o μ maxVint ⎞ J i ( 0) = ⎜ ⎟⎠ = ⎜⎝ ⎟⎠ . d d ⎝
(5)
We then normalize each of the TPC traces by their respective internal voltage: (6)
Figure 3. Internal conductance plots for the a) 0.25%, b) 0%, and c) 0.6% solar cells. These are generated by normalizing every transient photocurrent trace by its respective internal field. Note that these graphs show the external bias voltage for each trace instead of the total internal field, in order to allow for easy comparison across samples.
which gives an interesting result. If both the initial carrier density and the maximum carrier mobility are independent of the internal field, then the peaks of the photoconductance traces should all collapse to the same point. This is confirmed in Figure 3 for all three of the solar cells. The major implication of this observation is that it shows geminate recombination from charge transfer (CT) excitons to be almost completely absent in p-DTS(PTTh2)2:PC71BM solar cells, regardless of the use of the processing additive. Initial carrier density, and by extension, polaron pair dissociation, is independent of internal field over an order of magnitude of internal field strength. It has been shown[19] that mobile carriers are produced from a combination of ultrafast, delocalized states (approximately 70%), as well as from CT states populated following exciton diffusion to a heterojunction interface (approximately 30%). While this measurement does not rule out the possibility of geminate recombination by diffusing excitons failing to reach a donor-acceptor interface, it does rule out a tightly-bound charge transfer exciton as an intermediary in
the charge generation process. Charge transfer excitons in this system must have a binding energy comparable to the thermal energy, and recombination via the charge transfer exciton is not an efficient avenue for carrier loss. Ruling out the presence of field-dependent carrier generation is a very powerful result, as it allows direct observation of the competition between sweep-out and recombination in p-DTS(PTTh2)2:PC71BM solar cells, and how that changes with the concentration of DIO used in the film formation. Assuming efficient sweep-out of charge carriers at high internal fields, non-geminate recombination at high internal field is minimal, and the total carrier extraction at high reverse bias provides a measurement of the number of initial charges following photoexcitation. Carrier losses as the internal field is reduced can then be attributed solely to non-geminate recombination, as the lower internal field reduces carrier drift velocity and forces charges to spend more time in the BHJ prior to sweep-out and extraction, allowing greater opportunity for recombination. This is shown in Figure 4a, where total integrated charge is
J i ( 0) qn 0 μ max = Vint d
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a)
-2
Density (nC cm )
12 10 8 6 4
0% DIO 0.25% DIO 0.6% DIO
2 -1000
-500
0
500
Bias Voltage (mV) 1.0
590 nm
0.8
Absorption (AU)
b)
0.6 0.4 0.2 0.0 400
500
600
700
800
Wavelength (nm) Figure 4. (a) Total extracted charge density for each solar cell as a function of bias voltage. The value at –1000 mV (reverse bias) is used as an estimate for the inital carrier density following photoexcitation. (b) Optical absorption spectra of the p-DTS(PTTh2)2:PC71BM films processed with different DIO concentrations. Note that the vertical line corresponds to 590 nm, which is the wavelength of the TPC laser.
plotted as a function of bias voltage. A high reverse bias of 1000 mV (large internal field) results in an equal number of initial carriers for the cell not processed with DIO and the device processed with 0.25% additive, implying that the use of a small amount of DIO does not have a significant effect on the carrier generation profile. In addition, Figure 4b shows the absorption spectra of the three devices. Note that at a wavelength of 590 nm, there are no significant differences in the traces, which means light absorption is unaffected by processing with DIO at the wavelength we are probing. In contrast, the addition of 0.6% DIO results in 37% fewer initial carriers than are present in the other two devices. Since all solar cells were shown to absorb the same amount of light, this reduction of initial carrier density must be due to a reduced probability of charge transfer and generation of mobile charges. Having ruled out the presence of geminate recombination via CT excitons, we attribute this reduction in charge transfer efficiency to an increased number of excitons which fail to reach a donor-acceptor interface prior to recombination. This is consistent with the previous observation of largely increased crystallite size for this system.[47,49]
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The use of the DIO processing additive in this system also has the beneficial effect of reducing non-geminate carrier recombination rates compared to the reference cell, as evidenced by the reduced voltage-dependence on total extracted charge. Comparing the integrated charge at each solar cell’s maximum power point to its initial charge density, we see that the cell without the additive only recovers 56% of its initial mobile carriers while both the cells with the additive recover over 70%. Recombination rates and mobility take on central importance at the maximum power point, where solar cells will ideally be operated, because the internal field in the device, and hence the drift velocity, is relatively low. Since any photoexcited carriers will then be spending a relatively long time in the cell, improvements in these parameters can have a big impact on the final performance of the device. In this paper, the effect of 1,8-diiodooctane as a processing additive in p-DTS(PTTh2)2:PC71BM solar cells has been explored via transient photocurrent techniques. The time-resolved measurements show that the use of the DIO increases vertical carrier mobility through the BHJ and thereby decreases non-geminate carrier recombination. In addition, a small amount of DIO in solution (0.25%) leaves mobile carrier generation relatively unaffected, but larger amounts (0.6%) hinder the formation of charge transfer states as crystallite length scales become too large for efficient charge transfer. The result is a small morphological window where cells with an optimal amount of additive retain efficient mobile carrier generation, while simultaneously enjoying more favorable transport properties. We also demonstrate that field-dependent carrier generation is absent in this system, effectively ruling out geminate recombination via CT excitons as a loss mechanism. This also eliminates tightlybound CT excitons as intermediaries in the charge generation process, implying a low binding energy for CT excitons that is comparable to the thermal energy.
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Total Extracted Charge
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Experimental Section Device Fabrication and Testing: Conventional structure solar cells were fabricated on cleaned ITO coated glass substrates. The substrates were first scrubbed with detergent, and then ultrasonicated in water, acetone, and isopropyl alcohol, sequentially, before drying in an oven overnight. 9 nm of MoOx was deposited on the ITO anode via thermal evaporation at a rate of 0.1 Å s−1. p-DTS(PTTh2)2:PC71BM was blended in a 7:3 ratio of total concentration 40 mg mL−1 with three concentrations of DIO (0%, 0.25%, 0.6% v/v) and spin-cast from chlorobenzene at 1500 rpm to form 150 nm thick BHJs. The active layers were heated to 70 °C for 10 minutes to drive off any residual solvent. Thermal deposition of 100 nm of aluminum directly onto the BHJ with no interfacial electron transport layer completed the device, and the 3.25 mm2 geometric overlap of the Al cathode with the ITO defined the active area. A Keithley 2400 SourceMeter, and a Newport AM 1.5G full spectrum solar simulator with an intensity of 100 mW cm−2 were used to measure steady state J–V curves for the three solar cells. Transient Photocurrent Measurements: TPC measurements were taken with a custom built, fully-automated setup. A Stanford Research Systems NL100 nitrogen laser with an attached dye cell provided the excitation source for photocarrier generation. When pumping a cuvette of Rhodamine 6G laser dye, the pulsed laser provided 4 ns wide, λ ≈ 590 nm pulses with a repetition rate of 10 Hz. A set of computercontrolled variable neutral density filters was used to alter the incident light intensity by up to three orders of magnitude, and a 50/50 beam splitter simultaneously sent every pulse to a Coherent J-10Si-le energy
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www.MaterialsViews.com sensor as well as the solar cell under test. Active monitoring of the fluence of every pulse allowed a typical measurement to consist of 500 current transients averaged together, where no pulse deviated by more than 2% from the target fluence of 0.05 μJ cm−2. The external voltage bias was applied to the cell with an Agilent 33519B waveform generator operating in DC mode, connected to the cell through a wideband bias tee. The bias tee is an essential, but easily overlooked component in these systems, as it provides a high-frequency shunt for the TPC pulse, dramatically reducing the RC time constant of the circuit, and preventing the bias source itself from interfering with the TPC signal. The timeresolved photocurrent was recorded with a 300 MHz Tektronix DPO3034 oscilloscope by measuring the voltage drop across a 5 ohm RF sensor resistor (so as to not attenuate high-frequency signals) in series with the device. 5 ohms was chosen to further minimize the RC time constant of the setup.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank the Department of Energy, Office of Basic Energy Sciences for the financial support under award number DE-FG02– 08ER46535. JS would like to acknowledge Christopher J. Takacs and Robert Pizzi for useful discussions.
Received: October 16, 2013 Revised: November 18, 2013 Published online: January 27, 2014
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[53] S. W. Kettlitz, J. Mescher, N. S. Christ, M. Nintz, S. Valouch, A. Colsmann, U. Lemmer, IEEE Photonics Technol. Lett. 2013, 25, 682. [54] M. Schubert, R. Steyrleuthner, S. Bange, A. Sellinger, D. Neher, Phys. Status Solidi A 2009, 206, 2743. [55] C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. de Mello, J. R. Durrant, Appl. Phys. Lett. 2008, 92, 093311. [56] R. A. Street, S. Cowan, A. J. Heeger, Phys. Rev. B 2010, 82, 121301.
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Transient Photocurrent Response of Small-Molecule Bulk Heterojunction Solar Cells Jason Seifter, Yanming Sun, and Alan J. Heeger* Organic semiconductors offer many potential advantages for high performance, low cost “plastic electronics” such as solar cells and light emitting diodes, including solution processability, mechanical flexibility, and the potential for economical, large-scale, roll-to-roll fabrication.[1–7] While power conversion efficiencies (PCEs) have been historically low compared to inorganic counterparts, the last few years have witnessed a dramatic improvement in organic bulk heterojunction (BHJ) solar cell performance, with PCEs recently surpassing 10%,[8–14] which is widely viewed as a critical milestone. The architecture includes a BHJ active layer, where a semiconducting polymer or conjugated small molecule donor is blended with a fullerene derivative electron acceptor and co-deposited from solution.[15–17] Following photoexcitation, a combination of ultrafast charge transfer and diffusion of excitons to a nearby heterojunction yields mobile carriers.[18–21] The result is a self-assembled, phase-separated, complex nanoscale network that transports photoexcited carriers to the electrodes for collection. Extensive research has focused on improving the morphology and performance through solvent and thermal annealing, mixed solvents and solution additives,[22–32] but the mechanisms by which these processes improve the devices remain subjects of ongoing debate. Small molecule BHJ solar cells offer several advantages over the more traditional, polymer-based cells. Compared to polymers, molecular donors have fewer batch-to-batch synthesis variations, are more easily purified with standard techniques, exhibit a greater degree of crystallization, and do not suffer from the effects of non-reproducible molecular weight or polydispersity.[33,34] As a result, the use of small molecule donors leads to greater consistency in material performance. Despite this, however, our understanding of and control over the complex blend morphology in these systems is not as mature as it is for polymer BHJ cells. Since a non-optimal donor-acceptor network can lead to inefficient charge transport or a large number
J. Seifter, Prof. A. J. Heeger Physics Department University of California at Santa Barbara Santa Barbara, CA, 93106, USA E-mail: [email protected] Dr. Y. Sun Center for Polymers and Organic Solids University of California at Santa Barbara Santa Barbara, CA, 93106, USA Prof. A. J. Heeger Materials Department University of California at Santa Barbara Santa Barbara, CA, 93106, USA
DOI: 10.1002/adma.201305160
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of traps, which can act as recombination centers, the importance of correlating molecular structure to photovoltaic performance cannot be overstated. Steady-state measurements such as current-density voltage (J–V) or incident photon to current efficiency (IPCE) are valuable, but insufficient by themselves for fully characterizing the performance of a BHJ solar cell. They typically suffer from the disadvantage of combining several different physical processes, including charge generation, transport, recombination, and collection at the electrodes, effectively obscuring which mechanism(s) may be responsible for changes in figures of merit. Time-dependent measurements, however, offer the possibility to address these processes individually, as they are dominant at different time scales, and can often be manipulated by varying such parameters as internal field strength, light intensity, and temperature. It is important to understand the origin of improvements in performance, and equally important to understand which areas can be improved if we are to continue the current rapid pace of development. Transient photocurrent (TPC) measurements are powerful tools for analyzing BHJ solar cell performance. TPC measurements probe charge carrier transit kinetics, which yield insight into the carrier mobility, different recombination mechanisms, the carrier density, and the density of states.[32,35–42] In order to perform such experiments, a short pulse of light, typically from a laser or fast pulsed LED, is used to photogenerate carriers in a BHJ solar cell. Following excitation and charge transfer, the photogenerated charges are driven toward the respective electrodes by the internal electric field, which can be manipulated with an external voltage bias. While in transit, some fraction of the carriers can propagate relatively unhindered though the solar cell, with a velocity determined by the internal field and a well-defined maximum mobility, μmax, while the transport of other carriers in such an inhomogeneous material must be described by multiple trapping in a distribution of localized states.[43] Unless the trapped charge recombines, the energetic depth of a trap is statistically correlated with the time the carrier is immobilized before being thermally released.[38,40] The resulting distribution of average carrier velocities results in dispersive transport that manifests as a time-dependent mobility.[41] Since organic solar cells are optically thin, carriers are generated throughout the bulk of the active layer, which means that the extracted current is the sum of both electron and hole currents. The simultaneous presence of both carrier species allows for non-geminate carrier recombination. This has the potential to take the form of bimolecular recombination, usually described as Langevin recombination with a significantly reduced rate constant compared to that predicted by the original Langevin theory, or as trap-assisted, Shockley-Read-Hall
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N S
S
N S
N
S
N S
Si S
N
N S
S
p-DTS(PTTh2)2 OMe
I O
DIO
-2
Current Density (mA cm )
I
2 0
PC71BM
b) 0% DIO 0.25% DIO 0.6% DIO
-2 -4 -6 -8 -10 -12 -14 0.0
0.2
0.4
0.6
0.8
Applied Voltage (V) Figure 1. (a) Molecular structures of the materials used in this study. (b) Steady state J–V characteristics of p-DTS(PTTh2)2:PC71BM solar cells processed with different concentrations of DIO in solution.
recombination.[33,36,44–46] The shape of the current transient is encoded with all of these effects, and is therefore complex, but can still yield very useful information which can prove valuable for material design and device construction. Here we report TPC measurements used to investigate the role of the processing additive 1,8-diiodooctane (DIO) in small molecule BHJ solar cells comprised of 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (p-DTS(PTTh2)2), blended with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The molecular structures for all three materials are displayed in Figure 1a. Previous reports[47–49] have shown that a remarkably small amount (0.25% v/v) of DIO additive in the p-DTS(PTTh2)2:PC71BM system significantly improved the solar cell performance by enhancing the short circuit current (Jsc) and fill factor (FF), with external quantum efficiency increasing across the entire visible spectrum. This optimal concentration of DIO served to reduce domain/crystallite length scales in the BHJ active layer to levels more favorable for efficient charge transfer, while simultaneously preserving a high degree of long-range molecular order. It was postulated that the photocarrier collection enhancement was due to either improved carrier generation yields or reduced recombination rates, but the
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authors were unable to conclusively distinguish between these alternatives.[47] Further additions of DIO beyond the optimal concentration quickly became detrimental, however, considerably increasing crystallite length scales and reducing all solar cell figures of merit. Using TPC measurements, we will show that the processing additive has the combined beneficial effects of lowering carrier recombination rates and increasing carrier mobility, both of which enable more charges to reach their respective electrodes and be extracted from the device prior to recombination. Furthermore, carrier generation yields are not improved, but merely unaffected by the use of a small amount of the DIO processing additive. Once excess DIO causes crystallite length scales to get too large, however, charge transfer efficiency drops, resulting in fewer mobile carriers. This implies the existence of a morphological “sweet spot” near 0.25% DIO, where the solar cell enjoys the benefits of improved transport without the detriment of the reduced carrier generation that dominates at higher concentrations. In addition, we will show that geminate recombination via charge transfer excitons plays a negligible role as a carrier loss mechanism in p-DTS(PTTh2)2:PC71BM solar cells regardless of the DIO concentration used during film processing, as field dependent polaron pair dissociation is not observed. Conventional structure p-DTS(PTTh2)2:PC71BM solar cells were fabricated on cleaned Indium Tin Oxide (ITO) coated glass substrates using the same procedure reported in previous publications.[48] A hole transport layer (HTL) consisting of 9 nm of MoOx was first deposited on the ITO anode via thermal evaporation. MoOx was chosen as the HTL over the more common PEDOT:PSS, because it has been documented that the pyridial nitrogen in the [1,2,5]thiadiazolo[3,4-c]pyridine (PT) building block of the p-DTS(PTTh2)2 donor undergoes a detrimental chemical reaction with PEDOT:PSS.[50,51] Specifically, the sulfonic acid protonates the lone pair of the pyridial nitrogen of the PT heterocycle, which severely compromises solar cell performance. Cells fabricated with MoOx suffer from no such effects. p-DTS(PTTh2)2:PC71BM was blended with three concentrations of DIO (0%, 0.25%, 0.6% v/v) and spin-cast from chlorobenzene to form 150 nm thick active layers. Thermal deposition of 100 nm of aluminum directly onto the BHJ with no interfacial electron transport layer completed the device, and the 3.25 mm2 geometric overlap of the Al cathode with the ITO defined the active area. Devices were kept intentionally small to reduce the capacitance of the solar cells. This assists the TPC measurements, which benefit from minimizing RC time constants. Steady state J–V curves for the three solar cells (processed with different amounts of the DIO additive) are shown in Figure 1b, with the relevant device parameters summarized in Table 1. Spectroscopic and morphological analysis of this system, including absorption spectra, X-ray scattering, high resolution TEM, and ultrafast transient absorption have been reported previously.[19,47–49] Performance trends here are consistent with previous work. We see once again that compared to the cell with no additive, the BHJ is optimized with 0.25% DIO, increasing PCE from 4.3% to 5.6%. This can mostly be attributed to the fill factor improving from 43% to 54%, which
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a)
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www.MaterialsViews.com Table 1. p-DTS(PTTh2)2:PC71BM solar cell figures of merit when processed with different concentrations of DIO additive. Jsc [mA cm−2]
Voc [V]
FF
PCE [%]
μ [cm2 V−1s−1]
0
12.1
0.84
0.43
4.3
5.6 * 10−3
0.25
13.6
0.76
0.54
5.6
6.7 * 10−3
0.6
7.8
0.71
0.38
2.1
7.6 * 10−3
DIO [% v/v]
compensates for the loss of open circuit voltage (Voc). Marked deterioration in all figures of merit was observed as DIO concentration was further increased to 0.6%, with PCE dropping precipitously to 2.1%. A simplified schematic of the experimental setup used to perform the TPC measurements is shown in Figure S1 in the Supporting Information. A nitrogen pumped dye laser was used as the excitation source for photocarrier generation. The pulsed laser provided 4 ns wide, λ ≈ 590 nm pulses with a repetition rate of 10 Hz, in order to allow any trapped charges to disperse between measurements, since it has been shown that a very small number of long-lived, trapped carriers can have lifetimes extending out to milliseconds or longer.[38,40] In order to ensure accuracy, it is important that the pulse width be kept as short as possible compared to the transit time of charges in the device, which, as we will show, can be as short as a few tens of nanoseconds. If carriers continue to be photogenerated during extraction, measurement of the transit kinetics becomes much more difficult. A typical measurement consisted of 500 current transients averaged together, where no pulse deviated by more than 2% from the target fluence of 0.05 μJ cm−2. This fluence is low enough to keep space charge effects and series resistance induced transient voltage spikes from significantly perturbing the internal field and impeding the sweep-out of mobile carriers, as is shown in Figure S2. An external voltage bias (Vbias) controls the total internal potential (Vint) across the device, which determines the drift velocity of the carriers, since Vint = VBI − V bias
(1)
where VBI is the built-in voltage of the cell. Finally, the timeresolved photocurrent was recorded with a Tektronix DPO3034 oscilloscope by measuring the voltage drop across a 5 ohm RF sensor resistor in series with the device. The overall RC time constant of the circuit was less than 20 ns, allowing excellent time resolution of the transient kinetics. There are several critical considerations that must be taken into account for accurate TPC measurements, including aggressively minimizing circuit series resistance, optimization of sample geometry, accurate control of laser fluence, and careful selection of RF circuit elements that do not attenuate high-frequency signal components.[52,53] Despite the relatively simple appearance of the apparatus, preserving pulse fidelity in highspeed transient analog electronics with a highly capacitive load is one of the most demanding circuit requirements. As BHJ materials become more efficient, with higher carrier mobilities and shorter carrier transit times, these considerations will become even more important if an accurate measurement of 2488
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transport dynamics is to be performed. A more detailed discussion of the practical considerations required to generate accurate data can be found in the Experimental Section. Transient photocurrent measurements were performed on the aforementioned p-DTS(PTTh2)2:PC71BM solar cells at room temperature in an effort to understand the role of the DIO processing additive. Figure 2a shows bias-dependent TPC traces in the optimized (0.25% DIO) cell, where the internal field of the device is varied by more than an order of magnitude. The same data for the 0% and 0.6% additive cells are given in Figure S3 in the Supporting Information. In order to gain insight into these traces, we first consider the charge density, dρ, contained in a two-dimensional slice of the BHJ after photoexcitation, which is given by q n(x,t) dx, d
dρ(x,t) =
(2)
where n(x, t) is the bulk charge carrier density, and d is the thickness of the device. Note that at this point, we are looking only at one carrier species, but an identical argument holds for the other. If μ(x, t) is the carrier mobility, then the current density due to that slice of charge is given by dJ(x,t) =
dρ q n(x,t) dx q n(x,t) = = v dt d dt d
dJ(x,t) =
q n(x,t) qn(x,t)μ(x,t)Vint μ(x,t)E = . d d2
(3)
By integrating this expression over the entire thickness of the solar cell, we get the current density out of the device at time t. Assume that the mobility is not a function of position, space charge effects are negligible, and the illumination uniformly gives rise to a homogeneous distribution of carriers across the active layer. Then n(x, t) = n(t), Vint is constant, and μ(x, t) = μ(t). The total current density at time t, for each carrier type, is then given by J(t) = ∫
d
μEt
qn(t)μ(t)Vint qn(t)μ(t)Vint dx = d2 d2
μ i (t)Vint t ⎞ ⎛ qn i (t)μ i (t)Vint ⎞ ⎛ J i (t) = ⎜ ⎟⎠ ⎜⎝ 1 − ⎟⎠ d ⎝ d2
d
∫μEt dx (4)
2
d . The total current through the where i = n, p, and t < μ i (t)V int device is the sum of the electron and hole currents. In general, fitting a TPC trace to Equation (4) and extracting device parameters is not a trivial task. To start, ni(t) decays due to carrier recombination, which can be trap-assisted (monomolecular), reduced Langevin (bimolecular), or even higherorder.[43,54,55] The dominant mechanisms are still debated, and in reality, are likely all present to varying degrees at different time scales. In addition, carrier transport is typically dispersive, with the functional form of μi(t) dependent on the density of states. The hallmark of dispersive transport is a long photocurrent tail, as shown in Figure 2b. Detailed modeling of Equation (4) will be the subject of a future publication, but for now, we gain valuable insight without complex numerical simulations. At very short time scales, prior to significant recombination, n i (t) = n i ( 0) = n o . Additionally, the only carriers collected at this
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COMMUNICATION Figure 2. (a) Bias-dependent transient photocurrent measurements of the optimized (0.25% DIO) p-DTS(PTTh2)2:PC71BM solar cell. Note that the displayed voltages in the legend are the total internal potential according to Equation (1), and are valid for all four panels. (b) Photocurrent tail which arises as a result of dispersive transport. (c) TPC measurement of the optimized cell at high internal field (reverse bias) showing a linear fit to the initial decay. (d) Running time integral of the photocurrent trace (the extracted charge) as a function of time for different internal field conditions of the optimized cell.
point will be highly mobile carriers moving with mobility μmax, since the effects of dispersive transport have not yet had time to manifest. By setting μ i (t) = μ i ( 0) = μ max, Equation (4) predicts the current density to fall off linearly at short time scales. This is clearly seen in Figure 2a and Figure 2c (dotted line in Figure 2c). For all measured values of the internal field shown in Figure 2a, the initial photocurrent decay has a linear regime, which is consistent with bulk generation of mobile carriers all moving with an initial constant drift velocity, determined by μmax and the internal field of the device. This holds true for all of the other solar cells as well, as can be seen in Figure S3. In past TPC measurements, the short timescale dynamics were completely inaccessible due to distortions caused by RC time constant limitations, which severely compromises the utility of the technique. Following strict adherence to the considerations outlined in the Experimental Section, we note here that resolution of the linear photocurrent decay is unambiguous proof that there is a population of carriers with well-defined maximum mobility, and that the instrumentation is not RC limited, as otherwise the decay would appear to be exponential in nature with an RC time constant equal to that of the experimental setup. From Figure 2c, which shows an example of a linear fit to the high-field (fastest) transient from Figure 2a, we see the linear regime extending out to 21 ns beyond the peak, all the way down to approximately the “1/e” time of the photocurrent transient. This validates the estimation of our RC limitations as better than 20 ns, and guarantees fidelity of the measurement.
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We can estimate the value of μmax, then, for each solar cell by fitting the slope of the linear regime of the current transient and using Equation (4) with the short timescale considerations outlined above. The resulting μmax values were calculated and are summarized in Table 1. It is important to emphasize that these mobility measurements were derived from vertical charge transport across the bulk of the film, as opposed to the lateral geometry of the FET architecture typically used for mobility measurements. Using Equation (1) to determine the internal potential, the built-in voltage of each device was established by finding the external bias point where the photocurrent changes sign. The initial carrier density, no, was estimated by integrating the current transient to get extracted charge as a function of time, as shown in Figure 2d for each bias condition of the optimized cell. At high internal fields, the collection of carriers is efficient, so the total extracted charge at high reverse bias provides a reasonable estimate of the initial carrier density. Alternatively, the linear regime can be extrapolated to zero photocurrent in order to provide a rough estimation of carrier transit time through the device. Both methods for calculating mobility yielded similar results. As summarized in Table 1, the addition of DIO during processing of the BHJ film improves the maximum carrier mobility in the vertical direction as longrange order is enhanced. With use of the optimal amount (0.25%), the mobility increases from 5.6 * 10−3 cm2 V−1 s−1 to 6.7 * 10−3 cm2 V−1 s−1 compared to the cell without the
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additive. Increasing the concentration to 0.6% further increases the mobility to 7.6 * 10−3 cm2 V−1 s−1, but leads to a less efficient solar cell overall. As we will show later, this is a result of the larger crystallites hindering charge transfer and carrier formation. Since the measured transient is the sum of electron and hole currents, and there is no evidence of well-defined and separate time scales for extraction of the two different carrier species, we are unable to measure distinct electron and hole mobilities from this experiment. Previous FET measurements on this blend system, however, have shown balanced mobilities, consistent with our results.[47,48] Furthermore, in the vertical diode configuration, we would expect the individual carrier mobilities to readjust to be self-consistently comparable in efficient solar cells, as asymmetric carrier extraction would result in charge transport being inhibited by the generation of an internal space charge. As can be seen in Figure 2d and Figure S4, the total extracted charge from the solar cells is highly dependent on the internal field. For example, when operating at the maximum power point, p-DTS(PTTh2)2:PC71BM solar cells fabricated without use of the DIO additive yield only 56% of the number of carriers extracted at high reverse bias. When the DIO additive is used, however, 73% and 71% of the available charge is extracted for the optimized and 0.6% cells, respectively. In order to investigate whether this observation is due to recombination or field-dependent carrier generation, we can follow the method outlined previously by Street and Cowan.[56] By setting t = 0 to be the peak of the photocurrent trace, Equation (4) becomes ⎛ qn i ( 0)μ i ( 0)Vint ⎞ ⎛ qn o μ maxVint ⎞ J i ( 0) = ⎜ ⎟⎠ = ⎜⎝ ⎟⎠ . d d ⎝
(5)
We then normalize each of the TPC traces by their respective internal voltage: (6)
Figure 3. Internal conductance plots for the a) 0.25%, b) 0%, and c) 0.6% solar cells. These are generated by normalizing every transient photocurrent trace by its respective internal field. Note that these graphs show the external bias voltage for each trace instead of the total internal field, in order to allow for easy comparison across samples.
which gives an interesting result. If both the initial carrier density and the maximum carrier mobility are independent of the internal field, then the peaks of the photoconductance traces should all collapse to the same point. This is confirmed in Figure 3 for all three of the solar cells. The major implication of this observation is that it shows geminate recombination from charge transfer (CT) excitons to be almost completely absent in p-DTS(PTTh2)2:PC71BM solar cells, regardless of the use of the processing additive. Initial carrier density, and by extension, polaron pair dissociation, is independent of internal field over an order of magnitude of internal field strength. It has been shown[19] that mobile carriers are produced from a combination of ultrafast, delocalized states (approximately 70%), as well as from CT states populated following exciton diffusion to a heterojunction interface (approximately 30%). While this measurement does not rule out the possibility of geminate recombination by diffusing excitons failing to reach a donor-acceptor interface, it does rule out a tightly-bound charge transfer exciton as an intermediary in
the charge generation process. Charge transfer excitons in this system must have a binding energy comparable to the thermal energy, and recombination via the charge transfer exciton is not an efficient avenue for carrier loss. Ruling out the presence of field-dependent carrier generation is a very powerful result, as it allows direct observation of the competition between sweep-out and recombination in p-DTS(PTTh2)2:PC71BM solar cells, and how that changes with the concentration of DIO used in the film formation. Assuming efficient sweep-out of charge carriers at high internal fields, non-geminate recombination at high internal field is minimal, and the total carrier extraction at high reverse bias provides a measurement of the number of initial charges following photoexcitation. Carrier losses as the internal field is reduced can then be attributed solely to non-geminate recombination, as the lower internal field reduces carrier drift velocity and forces charges to spend more time in the BHJ prior to sweep-out and extraction, allowing greater opportunity for recombination. This is shown in Figure 4a, where total integrated charge is
J i ( 0) qn 0 μ max = Vint d
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a)
-2
Density (nC cm )
12 10 8 6 4
0% DIO 0.25% DIO 0.6% DIO
2 -1000
-500
0
500
Bias Voltage (mV) 1.0
590 nm
0.8
Absorption (AU)
b)
0.6 0.4 0.2 0.0 400
500
600
700
800
Wavelength (nm) Figure 4. (a) Total extracted charge density for each solar cell as a function of bias voltage. The value at –1000 mV (reverse bias) is used as an estimate for the inital carrier density following photoexcitation. (b) Optical absorption spectra of the p-DTS(PTTh2)2:PC71BM films processed with different DIO concentrations. Note that the vertical line corresponds to 590 nm, which is the wavelength of the TPC laser.
plotted as a function of bias voltage. A high reverse bias of 1000 mV (large internal field) results in an equal number of initial carriers for the cell not processed with DIO and the device processed with 0.25% additive, implying that the use of a small amount of DIO does not have a significant effect on the carrier generation profile. In addition, Figure 4b shows the absorption spectra of the three devices. Note that at a wavelength of 590 nm, there are no significant differences in the traces, which means light absorption is unaffected by processing with DIO at the wavelength we are probing. In contrast, the addition of 0.6% DIO results in 37% fewer initial carriers than are present in the other two devices. Since all solar cells were shown to absorb the same amount of light, this reduction of initial carrier density must be due to a reduced probability of charge transfer and generation of mobile charges. Having ruled out the presence of geminate recombination via CT excitons, we attribute this reduction in charge transfer efficiency to an increased number of excitons which fail to reach a donor-acceptor interface prior to recombination. This is consistent with the previous observation of largely increased crystallite size for this system.[47,49]
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The use of the DIO processing additive in this system also has the beneficial effect of reducing non-geminate carrier recombination rates compared to the reference cell, as evidenced by the reduced voltage-dependence on total extracted charge. Comparing the integrated charge at each solar cell’s maximum power point to its initial charge density, we see that the cell without the additive only recovers 56% of its initial mobile carriers while both the cells with the additive recover over 70%. Recombination rates and mobility take on central importance at the maximum power point, where solar cells will ideally be operated, because the internal field in the device, and hence the drift velocity, is relatively low. Since any photoexcited carriers will then be spending a relatively long time in the cell, improvements in these parameters can have a big impact on the final performance of the device. In this paper, the effect of 1,8-diiodooctane as a processing additive in p-DTS(PTTh2)2:PC71BM solar cells has been explored via transient photocurrent techniques. The time-resolved measurements show that the use of the DIO increases vertical carrier mobility through the BHJ and thereby decreases non-geminate carrier recombination. In addition, a small amount of DIO in solution (0.25%) leaves mobile carrier generation relatively unaffected, but larger amounts (0.6%) hinder the formation of charge transfer states as crystallite length scales become too large for efficient charge transfer. The result is a small morphological window where cells with an optimal amount of additive retain efficient mobile carrier generation, while simultaneously enjoying more favorable transport properties. We also demonstrate that field-dependent carrier generation is absent in this system, effectively ruling out geminate recombination via CT excitons as a loss mechanism. This also eliminates tightlybound CT excitons as intermediaries in the charge generation process, implying a low binding energy for CT excitons that is comparable to the thermal energy.
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Experimental Section Device Fabrication and Testing: Conventional structure solar cells were fabricated on cleaned ITO coated glass substrates. The substrates were first scrubbed with detergent, and then ultrasonicated in water, acetone, and isopropyl alcohol, sequentially, before drying in an oven overnight. 9 nm of MoOx was deposited on the ITO anode via thermal evaporation at a rate of 0.1 Å s−1. p-DTS(PTTh2)2:PC71BM was blended in a 7:3 ratio of total concentration 40 mg mL−1 with three concentrations of DIO (0%, 0.25%, 0.6% v/v) and spin-cast from chlorobenzene at 1500 rpm to form 150 nm thick BHJs. The active layers were heated to 70 °C for 10 minutes to drive off any residual solvent. Thermal deposition of 100 nm of aluminum directly onto the BHJ with no interfacial electron transport layer completed the device, and the 3.25 mm2 geometric overlap of the Al cathode with the ITO defined the active area. A Keithley 2400 SourceMeter, and a Newport AM 1.5G full spectrum solar simulator with an intensity of 100 mW cm−2 were used to measure steady state J–V curves for the three solar cells. Transient Photocurrent Measurements: TPC measurements were taken with a custom built, fully-automated setup. A Stanford Research Systems NL100 nitrogen laser with an attached dye cell provided the excitation source for photocarrier generation. When pumping a cuvette of Rhodamine 6G laser dye, the pulsed laser provided 4 ns wide, λ ≈ 590 nm pulses with a repetition rate of 10 Hz. A set of computercontrolled variable neutral density filters was used to alter the incident light intensity by up to three orders of magnitude, and a 50/50 beam splitter simultaneously sent every pulse to a Coherent J-10Si-le energy
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www.MaterialsViews.com sensor as well as the solar cell under test. Active monitoring of the fluence of every pulse allowed a typical measurement to consist of 500 current transients averaged together, where no pulse deviated by more than 2% from the target fluence of 0.05 μJ cm−2. The external voltage bias was applied to the cell with an Agilent 33519B waveform generator operating in DC mode, connected to the cell through a wideband bias tee. The bias tee is an essential, but easily overlooked component in these systems, as it provides a high-frequency shunt for the TPC pulse, dramatically reducing the RC time constant of the circuit, and preventing the bias source itself from interfering with the TPC signal. The timeresolved photocurrent was recorded with a 300 MHz Tektronix DPO3034 oscilloscope by measuring the voltage drop across a 5 ohm RF sensor resistor (so as to not attenuate high-frequency signals) in series with the device. 5 ohms was chosen to further minimize the RC time constant of the setup.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank the Department of Energy, Office of Basic Energy Sciences for the financial support under award number DE-FG02– 08ER46535. JS would like to acknowledge Christopher J. Takacs and Robert Pizzi for useful discussions.
Received: October 16, 2013 Revised: November 18, 2013 Published online: January 27, 2014
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