Quantitative Analyses of Competing Photocurrent Generation

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Quantitative Analyses of Competing Photocurrent Generation Mechanisms in Fullerene-Based Organic Photovoltaics Liang Xu, Jian Wang, Manuel De Anda Villa, Trey B. Daunis, Yun-Ju Lee, Anton V. Malko, and Julia W.P. Hsu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05044 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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The Journal of Physical Chemistry

Quantitative Analyses of Competing Photocurrent Generation Mechanisms in Fullerene-Based Organic Photovoltaics Liang Xu†, Jian Wang†, Manuel de Anda Villa‡, Trey B. Daunis†, Yun-Ju Lee †, Anton V. Malko‡, and Julia W.P. Hsu† * †

Department of Materials Science and Engineering, The University of Texas at Dallas, 800 W. Campbell Rd., Richardson, TX, 75080, USA

‡

Department of Physics, The University of Texas at Dallas, 800 W. Campbell Rd., Richardson, TX, 75080, USA

Corresponding Author * E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The performance of fullerene-based organic photovoltaic devices (OPVs) with low donor concentrations is not limited by the trade-off between short-circuit current density (Jsc) and open-circuit voltage (Voc), unlike bulk heterojunction OPVs. While the high Voc in this novel type of OPVs has been studied, here we investigate the mechanisms that govern Jsc, which are not well understood. Three mechanisms, diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and non-geminate recombination during charge transport, are studied analytically by combining various experimental techniques and transfer matrix simulation. We find that exciton dissociation at donor/acceptor interfaces is the dominant factor to produce high Jsc, and at low P3HT concentrations exciton relaxation limits photocurrent generation. With more P3HT inclusion, the creation of interfaces promotes exciton dissocation but also reduces fullerene crystallinity, weakening the driving force for charge separation, and introduces non-geminate recombination sites. Quantitative analyses show that the magnitude of measured Jsc and the donor concentration dependence are well accounted for by these three competing mechanisms.


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1. INTRODUCTION With their continually rising power conversion efficiency (PCE) and stability during the past decade, organic photovoltaic devices (OPVs) are serious contenders for low-cost renewable energy applications.1-3 In commonly adapted bulk heterojunction (BHJ) type OPVs, judicious tuning of the energy levels of donor and acceptor materials is critical to balance the intrinsic trade-off between short circuit current (Jsc) and open circuit voltage (Voc). Specifically, a reasonably large offset between the lowest unoccupied molecular orbital (LUMO) of the electron donor and that of the electron acceptor is required to overcome the binding energy of the photogenerated excitons for efficient charge transfer to produce a large short circuit current (Jsc).4,5 However, this large LUMO offset reduces the energy difference between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, diminishing the maximum achievable open circuit voltage Voc.6,7 Despite substantial research efforts,8,9 most highefficiency BHJ systems exhibit Voc values approximately half of the donor’s optical bandgap.2,10,11 Fullerene-based OPVs, consisting of mostly fullerene “acceptor” with a minute amount of organic “donor”, have recently attracted intensive research interests because of their potential in overcoming the Jsc and Voc trade-off of BHJ OPVs.12-18 (The usage of donor and acceptor is following the convention of BHJ.) Voc close to 1 V with significant Jsc (> 10 mA/cm2) has been demonstrated in fullerene-based OPVs incorporated with different donor materials.16,17 The high Voc in fullerene-based OPVs has been interpreted in a Schottky-junction framework with built-in potential determined by the difference between the anode Fermi level and the lowest unoccupied molecular orbital of the fullerene,12 or as the result of less energy loss arising from reduced nongeminate recombination.13 As for Jsc, while fullerene does absorb light,19 pure fullerene devices produce very low photocurrent that exhibits strong field dependence arising from field ionization


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of excitons in the bulk.20,21 Thus, fullerene light absorption alone cannot explain the large photocurrent observed in fullerene-based OPVs with low concentrations of donors. Furthermore, while it has been reported that Jsc displays a maximum value at around 5-10 wt.% donor concentration in several donor-acceptor systems,13,15-17 there is no fundamental understanding on why Jsc peaks at such donor concentrations, what factors determine the measured Jsc values, or what the relative contribution of each factor is on the Jsc values. In this work, we study competing photocurrent generation mechanisms in detail in a series of [6,6]-phenyl-C70-butyric acidmethyl ester (PC71BM) based devices with poly(3-hexylthiophene) (P3HT) concentrations varying from 0 to 30 wt.%. We employ external quantum efficiency (EQE), time-resolved photoluminescence (TR-PL), UV-visible absorption (UV-vis), impedance spectroscopy (IS), and X-ray diffraction (XRD) to probe the changes due to P3HT incorporation, and we compare measured Jsc to ideal values calculated from 1D transfer matrix simulation. The impacts of three competing mechanisms,22 diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and non-geminate recombination during free carrier transport, are considered. We analytically show that these three mechanisms have distinctly different donor concentration dependences. The highest photocurrent is achieved when the competing factors are balanced. Fundamental understanding of the photocurrent loss mechanisms is pivotal for developing fullerene-based OPVs with simultaneously high photocurrent and photovoltage.

2. RESULTS AND DISCUSSIONS In the devices employed herein, fullerene derivative PC71BM was used as the majority of the active layer material due to its high light absorption in the visible range.19 P3HT concentration


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was varied between 0 - 30 wt.%. All devices were fabricated with a standard ITO/PEDOT:PSS/active layer/Ca/Al structure. OPV device parameters obtained from current density–voltage (J-V) measurements (Figure S1) can be found in Table 1. As shown in Figure 1, Jsc in the pure fullerene device without any P3HT is negligible (Figure S2). By adding 1 wt.% of P3HT, Jsc increases almost 20 fold, from 0.15 to 2.7 mA/cm2. With further inclusion of P3HT, Jsc continues to increase, peaking at 7.4 mA/ cm2 with 10 wt.% P3HT, and then declines to 5.2 mA/cm2 at 30 wt.% P3HT. Voc of all devices is ~ 0.9 V, much higher than the 0.67 V found in a P3HT:PCBM BHJ device (J-V results of a typical 70nm BHJ device with 50 wt.% P3HT are provided in Figure S3). There is a minor decrease of fill factor (FF) in devices with higher P3HT concentrations. As a result, the power conversion efficiency (PCE, Table 1) is mainly determined by Jsc, similar to results from a previous study.15 We note that 30 wt.% P3HT is the highest P3HT concentration that can be incorporated in PC71BM while still maintaining 0.9 V Voc, i.e. remaining in the fullerene-based OPV regime that produces substantially higher Voc than P3HT:PCBM BHJ. 12 10 2

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ΔJ

8 6 4

Jsc

2

JTMM

0 0

5 10 15 20 25 30 P3HT Concentration (wt.%)

Figure 1. Jsc (solid squares) and JTMM (empty circles) vs. P3HT concentration. The total photocurrent loss (ΔJ) is defined as JTMM - Jsc, as indicated by the arrow.


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In order to understand the Jsc behavior, 1D transfer matrix method (TMM)23,24 was utilized to calculate the maximum photocurrent density (JTMM) that can be obtained based on the materials’ optical properties and the device architecture (see supporting information for details).25 As seen in Figure 1 JTMM in these PC71BM based devices are overall high (> 10 mA/cm2). The slight decrease of JTMM with increasing P3HT concentration arises from the reduced absorption by PC71BM in films with more P3HT (Figure S4 & S6). Because the absorption features in PC71BM films primarily arise from intermolecular transitions,27,28 reduced absorption is consistent with suppressed formation of PC71BM aggregates (Figure S7).16,26 Note that absorption spectra for all P3HT concentration films are similar (Figure S6), which implies that photocurrent contributions from excitons generated in P3HT are marginal in all devices. Furthermore, since absorption spectra of P3HT critically depend on its morphology (Figure S8),29 the fact that no significant absorption features of ordered P3HT at 570 and 610 nm are observed even in films with 30 wt.% P3HT (Figure S6) indicates that P3HT in these films is largely disordered. Nevertheless, the current reduction due to the absorption change is small, and the overall high JTMM implies that a high PCE can be expected in this system. However, the experimentally measured Jsc values (Figure 1, solid squares) are substantially lower than JTMM (open circles) and display a nonmonotonic dependence on the P3HT concentration, indicating detrimental photocurrent loss in this system. Defining the total photocurrent loss (ΔJ) as JTMM - Jsc (arrow in Figure 1), we study the impact of diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and non-geminate recombination during charge transport22 on ΔJ.

Table 1. Device parameters of fullerene-based OPVs with different P3HT concentrations. The area of each device diode is 0.11 cm2. A 2.5 mm diameter aperture was applied to define the


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illumination area to 0.049 cm2 in J-V and impedance spectroscopy measurements. Jsc, Voc, FF, and PCE were obtained from J-V measurements (Fig. S1, more than 4 devices for each condition were averaged to calculate standard deviation), relative interfacial area was extracted from lowenergy EQE measurements (Fig. 2(a)), exciton lifetimes were obtained from TR-PL decays (Fig. 2(b)), and Cµ & R2 were fitted from IS measurements (Fig. 4(a)).

[P3HT]

V

oc

J

PCE

sc 2

FF (%)

Relative interfacial area

exciton lifetime

(a.u.)

(ps)

Cμ

R2

(F cm-2)

(Ω cm2)

(wt.%)

(mV)

(mA/cm )

0

853 ± 19

0.15 ± 0.01

0.31 ± 0.01

0.04 ± 0.01

/

690

/

/

1

860 ±7

2.70 ± 0.05

0.57 ± 0.01

1.31 ± 0.03

0.02

530

1.31E-07

459

2

883 ± 5

3.95 ± 0.04

0.57 ± 0.01

1.99 ± 0.03

0.04

440

1.28E-07

339

5

883 ± 5

7.26 ± 0.13

0.48 ± 0.01

3.04 ± 0.07

0.15

200

1.58E-07

111

10

890 ± 10

7.40 ± 0.10

0.50 ± 0.01

3.31 ± 0.06

0.19

150

1.66E-07

68

20

908 ± 5

5.87 ± 0.05

0.47 ± 0.01

2.49 ± 0.04

0.22

130

3.08E-07

37

30

898 ± 4

5.18 ± 0.13

0.45 ± 0.01

2.09 ± 0.06

0.22

130

4.07E-07

27

2.1 Diffusion limited exciton relaxation Free carrier generation arises from the dissociation of photo-generated excitons that have diffused to the donor/acceptor interfaces30 and formed charge-transfer (CT) states.31,32 This process competes with relaxation of excitons diffusing within pure domains.33 Here, we applied low-energy EQE spectroscopy and TR-PL measurements34,35 to investigate the effects of P3HT inclusion on the formation of PC71BM/P3HT interfacial area and on exciton quenching, respectively. Figure 2(a) shows the EQE spectra of devices with 1, 2, 5, and 30 wt.% P3HT versus incident photon energy ranging from 1.2 to 3.5 eV. It is clearly evident that, below the


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absorption edge of pure PC71BM (1.7 eV) and P3HT (2.0 eV), EQE signals increase with P3HT concentration, reflecting photocurrent arising from optical absorption from interfacial ground state into the CT states.13,34,36,37 The CT absorption bands are fitted with a Gaussian function, from which the CT state energy (ECT) and relative interfacial area are determined.37 We find an invariant ECT of 1.45 ± 0.05 eV for all P3HT concentrations, which is accompanied by a monotonic increase of relative interfacial area with increasing P3HT concentration up to 20 wt.%, beyond which the interfacial area saturates (Table 1). While it is well known that P3HT preferentially segregates to the top interface in P3HT:PCBM blends due to lower surface energy,38-40 our observation of increasing interfacial area with P3HT concentration (Figure S9) indicates that with increasing P3HT inclusion up to 20 wt.%, bulk P3HT concentration does increase and not all P3HT has segregated to the top surface. As such, we arrive at the conclusion that lowering P3HT inclusion (< 20 wt. %) decreases donor/acceptor interfacial area without affecting CT state energetics, consistent with published results.13 Figure 2(b) shows the TR-PL measurements on the exciton lifetimes in films with different P3HT concentrations, i.e., different donor/acceptor interfacial area. The PL decay is fitted to a single exponential function with the time constant (exciton lifetime) listed in Table 1. In pristine PC71BM films an exciton lifetime of 690 ps is observed, similar to the reported value.28 This lifetime represents the characteristic time for all the excitons to decay back to ground state in the absence of any quencher (P3HT). The decay dynamics become faster with increasing P3HT concentration, indicating an additional quenching channel introduced by P3HT inclusion. The effect saturates beyond 20 wt.% P3HT, similar to the interfacial area change. Figure 2(c) shows a strong positive correlation between the PL exciton decay rate (1/lifetime) and the relative interfacial area, indicating the direct effect on exciton quenching of increased P3HT/PC71BM


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interfaces. Hence, the first current loss mechanism occurs when P3HT concentration is too low, i.e., PC71BM domain sizes are too large, thus excitons are unable to reach a P3HT/PC71BM interface within their diffusion length to form CT states and dissociate into free carriers.33

10 10 10 10

9 -1

1

(a) 1% 2% 5% 30%

-2 -3 -4 -5 -6

8

1.5

2.0 2.5 hv (eV)

3.0

0% 1% 2% 5% 10% 20% 30%

4 3 2

2.0

2.4 2.8 Time (ns)

3.2

10

(c)

6 4 2

(b)

7 6 5

0.1

3.5

2

10

-1

J (mA/cm )

EQE

10

0

Norm. PL Counts

10

PL Decay rate (10 s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Increase P3HT concentration

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Relative interfacial area (a.u.)

(d)

8

ΔJ

JR

6 4 2 0 0


Figure 2. (a) EQE versus incident photon energy (solid lines) of PC71BM-based OPVs with varying P3HT concentrations: 1 wt.% (black), 2 wt.% (green), 5 wt.% (red), and 30 wt.% (blue). The dashed lines show the Gaussian fits to CT absorption bands. (b) Normalized TR-PL decay curves (dots) for PC71BM films with different P3HT concentrations: 0 wt.% (grey), 1 wt.% (black), 2 wt.% (green), 5 wt.% (red), 10 wt.% (dark yellow), 20 wt.% (magenta), and 30 wt.% (blue). The solid lines show the fits of a single exponential decay model. (c) PL decay rate (1/lifetime, obtained from exponential fits in (b)) vs. relative interfacial area (obtained from Gaussian fits in (a)). (d) Total current loss (ΔJ, black squares) and current loss due to diffusion limited exciton relaxation (JR, red triangles) vs. P3HT concentration.


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Assuming 100% dissociation of excitons at CT states to free carriers, the current loss due to !

diffusion limited exciton relaxation (JR) is 𝐽! = 𝐽!"" × !! , where k is the decay rate of each sample and k0 is the decay rate of pure PC71BM.35 The calculation is simplified using only exciton dynamics of PC71BM due to the marginal P3HT contribution to the JTMM discussed above. As seen in Figure 2(d), for 1 wt.% P3HT device, ΔJ can be accounted for by JR alone. In other words, the 20-fold increase in Jsc observed in the 1 wt.% P3HT over that of the pure PC71BM device is completely accounted for by the donor/acceptor interfaces introduced by P3HT inclusion. However, with increasing P3HT concentration, there is a greater discrepancy between ΔJ and JR; JR monotonically decreases but ΔJ reaches its minimum at 10 wt.% P3HT. This means that while introducing interfaces to suppress exciton relaxation is the dominant mechanism for photocurrent generation in fullerene-based OPVs at all P3HT concentrations, in devices with higher P3HT concentrations, additional mechanisms become important. 2.2 Geminate recombination during exciton dissociation When examining the spectral dependence of EQE (Figure 3(a)), we notice the loss of signals at wavelengths from 500 nm to 730nm as P3HT concentration increases. Since P3HT doesn’t produce any photocurrent when excited beyond 650nm, as demonstrated by the EQE of a P3HT:PC61BM (1:1 ratio) BHJ device (Figure 3(a) inset), such EQE loss at long wavelength must originate from changes associated with PC71BM. Increasing P3HT inclusion reduces aggregation of ordered fullerene domains, as evidenced by the decreased grazing incidence Xray diffraction (GIXRD) peak at 17° (Figure S7) and the reduced ordered PC71BM absorbance peak at 400nm with increasing P3HT concentration (Figure S6), consistent with other studies.16,26,27,41 It was reported that the electron affinity of the relatively pure, crystalline PC71BM domains is deeper than that of amorphous, finely mixed P3HT/PCBM blends, providing a


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favorable energy offset (100-200 mV) for charge separation.5,41 Thus, we hypothesize that excitons at CT states would suffer more geminate recombination in devices with high P3HT concentrations than those with low P3HT concentrations due to the reduced driving force for exciton dissociation. To take absorption changes into account, we calculate the internal quantum efficiency spectra (IQE, Figure S10(c)) by following the approach described by Burkhard et. al24 (see supporting information for details). In order to highlight the spectral changes, the IQE spectra are normalized at 400 nm (Figure 3(b)). Clear decreases at wavelengths from 500 nm to 730nm are observed in devices with higher P3HT concentrations, indicating an increasing loss of current generated from longer-wavelength photons when more P3HT is added. Also, the averaged IQE increases linearly with relative interfacial area (Figure 3(b) inset) at low P3HT concentrations (up to 2 wt%), but substantially deviates from the extrapolated values based on the interfacial area at higher concentrations. Thus, additional losses beyond the diffusion limited exciton relaxation must be present. As stated above, the current loss in the 1% device can be completely accounted for by diffusion limited exciton relaxation, which is wavelength independent; thus, its IQE spectrum can be considered ideal and can be used to calculate the ideal EQE spectra (EQEideal) for other P3HT concentration devices if no other loss mechanisms were present. (See supporting information for details.) Discrepancy between ideal and experimentally measured EQE spectra at wavelengths above 500 nm is clearly evident, as shown in Figure 3(c) inset for the 30 wt.% P3HT device (Comparisons for all devices are shown in Figure S11). Since non-geminate recombination of free carriers is wavelength independent, which is confirmed by comparing EQE spectra at different biases (Figure S12), we attribute the observed long-wavelength (500730 nm) photocurrent loss to the geminate recombination of excitons at CT states without


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dissociating into free carriers.42 While currently we do not have a model for the wavelength dependent geminate recombination, the hot exciton effect is unlikely to be the explanation because the thermalization time of excitons is much shorter than their decay time.43 Nevertheless, we can quantify this geminate recombination current loss (JGR) of each device by calculating the difference between integration of ideal and measured EQE spectra with the AM1.5G spectrum (Figure 3(c), see supporting information for details).44 A monotonic increase in JGR with

1% 2% 5% 10% 20% 30%

(a)

1.0

0.6

BHJ

0.4

EQE

0.5

0.2 0.0

0.0

400 500 600 700 Wavelength (nm)

400

500 600 Wavelength (nm)

700

(b) 1.0 0.8

0.5

0.0

Avg. IQE

IQE normalized to 400nm

EQE normalized to 400nm

increasing P3HT concentration is seen, with JGR > 1 mA/cm2 in devices with ≥ 20 wt.% P3HT.

0.6 0.4 0.2 0.0 0.0 0.1 0.2 Relative interfacial area (a.u.)

400

500 600 Wavelength (nm)

700

(c)

1.5 2

JGR (mA/cm )

1.0

Normolized EQE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

1.0 0.5

0.0

0.0 0

30% Exp. Ideal 400 500 600 700 Wavelength (nm)



12

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Figure 3. (a) EQE spectra and (b) IQE spectra (both are normalized at 400 nm) of devices with different P3HT concentrations. All color schemes used here are the same as in Figure 2(b) and the arrow indicates increasing P3HT concentration. The inset of (a) shows the EQE spectrum of a P3HT:PC61BM (1:1 ratio) BHJ device, which has zero signal above 650 nm. The inset of (b) demonstrates the average IQE vs. relative interfacial area (obtained from Fig. 1(c)). The line indicates a linear correlation for P3HT ≤ 2 wt.%. (c) Current loss due to geminate recombination at CT states (JGR, blue diamonds) vs. P3HT concentration. The inset shows ideal EQE (blue dashdotted line) and experimental EQE (blue dotted line) of the device with 30 wt.% P3HT. Both EQE spectra are normalized at 400 nm. 2.3 Non-geminate recombination during charge transport After free carriers are generated from exciton dissociation, non-geminate recombination can prevent them from being collected. Impedance spectroscopy (IS) is a proven technique to study this loss mechanism.45-47 Measurements were performed at short-circuit condition (0 V bias) under equivalent 1 Sun illumination, and the results shown in Nyquist plots (Figure 4(a)) are modeled with an equivalent circuit (Figure 4(b)) consisting of a series resistance Rs, a transport resistance R1, a depletion capacitance C1, a recombination resistance R2, and a chemical capacitance Cμ for the constant phase element (CPE). Best-fit parameters of R2 and Cμ are listed in Table 1. In this equivalent circuit, The R1 || C1 combination contributes to the high-frequency region of the impedance response in the Nyquist plot, while the R2 || Cμ combination is associated with the internal charge transfer events including charge accumulation (Cμ) and recombination (R2) and contributes to the low-frequency response.45-47


13


-Z"(kΩ)

6

1% 2% 5% 10% 20% 30%

(a)

4

(b)

2

0 8

10

12

n τ

0.8

1.0

60 40

0.6 20

0.4 0.2

0 0


(d)

0.8

J NGR

1.3

nid 1.2

0.6

nid

16

-3

1.0

6 Z'(kΩ)

2

(c)

4

τ (µs)

1.2

2

JNGR (mA/cm )

0

n (10 cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

1.1

0.2 0.0

1.0 0


Figure 4. (a) Nyquist plots of impedance response (open circles) measured at short-circuit condition and equivalent 1 Sun illumination. Lines are fitting results based on the equivalent circuit in (b). All color schemes are the same as in Figure 2(b). (c) Carrier density (n, black circles, left axis) calculated according to Equation (2) and minority carrier life (τ, red diamonds, right axis) vs. P3HT concentration. (d) Current loss due to non-geminate recombination (JNGR, green solid circles, left axis) calculated according to Equation (3) and diode ideality factor (nid, black open butterflies, right axis) vs. P3HT concentration. The chemical capacitance Cμ originates from the excess carriers accumulated within the active layer due to inefficient charge transport, and by definition, it is the capability of a system with a carrier density n to accept or release additional carriers due to a change in their chemical potential μ:47,48 !"

𝐶! = 𝑒 ! 𝐿 !!,

(1)


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where e is the magnitude of elementary charge and L is the active layer thickness. Since n is low at short-circuit condition and therefore a Boltzmann distribution applies, we have:48 𝑛=

!! !! ! ! !!

.

(2)

As shown in Figure 4(c) left axis, n increases with P3HT concentration, indicating higher charge accumulation in devices with more P3HT due to less efficient charge collection.45 Meanwhile, carrier recombination time (τ), the product of R2 and Cμ (𝜏 = 𝑅! ∙ 𝐶! ),46,47 decreases with P3HT concentration (Figure 4(c) right axis), signifying enhanced free carrier recombination. Current loss at short circuit due to non-geminate recombination of free carriers (JNGR) can be calculated from n and τ:22 𝐽!"# =

!"# !

.

(3)

As seen in Figure 4(d) on the left axis, JNGR increases monotonically with increasing P3HT concentration up to 1 mA/cm2 at 30 wt.% P3HT, demonstrating significant non-geminate recombination even at short-circuit in devices with high P3HT concentrations. The enhanced non-geminate recombination in devices with higher P3HT concentrations is also supported by the increased diode ideality factor (nid, obtained from light intensity dependence of the Voc49), as shown in Figure 4(d) on the right axis. Band-to-band or Schottky contact type recombination gives nid =1, while trap-assisted Shockley-Read-Hall (SRH) type recombination through midgap states gives nid = 2.49,50 It has been widely demonstrated that non-geminate recombination goes through localized states in OPVs,51-54 and increases in nid were reported for devices with high densities of non-geminate recombination centers.52 In the devices studied here, while higher interfacial areas benefit the charge photogeneration with more P3HT inclusion, they also act as charge recombination sites.36,54,55 In addition, increased non-geminate recombination is


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consistent with the lower fill factor (FF) observed in higher P3HT concentration devices (Figure S13). Similar FF trends in fullerene-based OPVs have also been reported in other works.15,16 The analyses above allow us to quantitatively understand the photocurrent generation mechanisms in fullerene-based OPVs. As shown in Figure 5(a), the experimental Jsc (solid squares), which shows a non-monotonic dependence on P3HT concentration, is well accounted for by subtracting the current loss due to the three mechanisms from the ideal photocurrent generation (open squares, 𝐽!"_!"# = 𝐽!"" − 𝐽! − 𝐽!" − 𝐽!"# ). Each loss mechanism has a distinctively different dependence on P3HT concentration, as indicated by its percentage contribution to the total calculated current loss (Figure 5(b)). We note that exciton relaxation during diffusion is the most important factor limiting photocurrent generation in fullerene-based OPV devices at all P3HT concentrations. P3HT inclusion introduces P3HT/PC71BM interfaces, which facilitate exciton dissociation through forming CT states that effectively compete with exciton relaxation and are critical to achieve high Jsc in these fullerene-based OPVs. However, CT states also act as free carrier recombination sites (SRH type traps) and increase non-geminate recombination loss. In addition, P3HT incorporation results in more amorphous fullerene morphology, which weakens the driving force for exciton dissociation at CT states and increases geminate-recombination loss. Consequently a balance between these three competing mechanisms results in the highest Jsc at 10 wt.% P3HT.


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10

(a)

2

Jsc (mA/cm )

8 6

Exp. Cal.

4 2

0 100

Percentage of ΔJ (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

80 60

JR JGR JNGR

40 20 0 0

5 10 15 20 25 P3HT Concentration (wt.%)

30

Figure 5. (a) A summary showing excellent agreement between experimentally measured Jsc (solid squares) and calculated Jsc (empty squares) for different P3HT concentration devices. (b) The percentage contributions of ΔJ from the three current loss mechanisms with respect to P3HT concentration: JR (red triangles), JGR (blue diamonds), and JNGR (green circles). 3. CONCLUSION AND OUTLOOK In conclusion, the study reported herein focuses on photocurrent generation in fullerene-based OPVs through thorough and quantitative investigations on three mechanisms: diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and non-geminate recombination of free carriers. Distinct behaviors of these three mechanisms have been demonstrated through a series of PC71BM-based devices with varying P3HT concentrations, which show a peak in the Jsc at 10 wt.% P3HT. By quantitatively analyzing the spectral and dynamic changes arising from inclusion of the minority phase (P3HT), this study provides an analytical approach to understand photocurrent generation mechanisms in high photovoltage


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fullerene-based OPVs systems and to provide insight into future material selection. Donor materials that can provide large donor/acceptor interfaces at low concentrations are key to minimize the dominant exciton relaxation loss. Meanwhile, maintaining fullerene crystallinity with donor inclusion will enhance absorption and reduce exciton dissociation loss. Judicious selection of donor/accpetor materials to maximize donor/acceptor interfaces within a relatively ordered fullerene matrix will lead to OPVs with high Voc and Jsc simultaneously.

4. EXPERIMENTAL DETAILS Device Fabrication. 30 nm of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Heraeus Clevios AI 4083) was spin coated onto precleaned ITO substrates (Xinyan, 15 Ω/sq), followed by 140 °C annealing in N2 for 10 min to form the hole transport layer (HTL) of these conventional devices. P3HT (Rieke) and PC71BM (Solenne BV) with a total concentration of 25 mg/mL were dissolved overnight in chlorobenzene solution (Aldrich) at 70 °C. Solutions with P3HT concentrations of 0, 5, and 30 wt.% were first prepared, and solutions of other concentrations were obtained by mixing the two of them accordingly. Active layer thicknesses between 60 - 70 nm (confirmed with profilometry and capacitance measurements, Table S1) were spin-cast at 1500-4000 rpm (to keep film thickness constant for all P3HT concentrations) for 60s. All active layers were annealed at 70 °C in N2 for 10 min after deposition. Finally, Ca (7 nm) and Al (100 nm) were thermally evaporated (Angstrom Engineering) to complete the devices. Current Density–Voltage (J-V) Measurements. The J-V measurements were performed in a N2 filled glovebox under AM1.5G 100 mW/cm2 illumination from a class AAA solar simulator (Abet Technologies) using a low-noise sourcemeter (2635A, Keithley) controlled by a LabView


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program. The solar simulator intensity was set using a NIST-traceable calibrated photodiode (Abet RR_227KG5). A 2.5 mm diameter aperture was placed in front of each device to rigorously define the illumination area of 0.049 cm2. External Quantum Efficiency (EQE) Spectroscopy. EQE spectroscopy measurements were taken at short circuit using a chopped monochromated light (Horiba TRIAX-180, grating 600 groove/mm) from 400 to 1050 nm with a wavelength step of 3.53 nm. A chopper (Tetrahertz, C995) was used to modulate the monochromatic light at 199 Hz, and a lock-in amplifier (Stanford Research System, SR830) was employed to demodulate the signal. To measure the weak subbandgap signal, cutoff filters at 710 nm and 850 nm were used to minimize scattered light from the light source. EQE was quantified using an NREL calibrated Si photodiode. Impedance Spectroscopy (IS). IS measurements were carried out in an O-ring sealed sample holder containing N2 at room temperature using a Zahner IM6 electrochemical workstation. IS was performed at short-circuit condition with an AC bias (20 mV) modulating between 1Hz and 1MHz, and a stable illumination from a white light emitting diode (LED, Zahner WLC01) with intensity of 50 mW/cm2 (equivalent to 1 Sun). Analyses of experimental results using the equivalent circuit model were performed with the software ZMAN 2.0. Time-resolved Photoluminescence (TR-PL) Measurement. The TR-PL measurements were carried out using a microscope-based system. Films with different P3HT concentrations were deposited on cover glass (Fisher 12-540-C, 2 mm thick) for the measurements. Samples are excited by 400 nm/120 fs optical pulses at 7.6 MHz repetition rate produced by doubling the fundamental frequency of a Mira 900 laser followed by pulse-picking (1 out of 10 pulses) using an acousto-optical modulator (NEOS Technologies). Excitation of 50 nW was focused on the sample with beam spot size of 0.6 μm in diameter via a 0.6 NA objective. The TR-PL signals


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were measured at 710nm. The time resolution of the measurement is ~ 60 ps. Spectroscopic Ellipsometry Measurement. The spectroscopic ellipsometry measurements (M2000DI, J. A. Woollam) were performed from 280 nm to 1690 nm at 65°, 70°, and 75° incident angle. P3HT:PCBM films for measurement were prepared on top of ZnO on Si wafers. Fitting was performed using CompleteEASE software. Silicon native oxide thickness was determined by fitting the results of a bare Si wafer using the built-in native oxide optical constants. Next, ZnO on Si films were measured and the optical constants and thickness were fit simultaneously using the “B-Spline” function with a band gap of 3.2 eV and with “Use KK Mode” on to ensure Kramers-Kronig consistency of the optical constants. P3HT:PCBM data were fit using the same procedure, with a band gap of 1.7 eV. Mean squared error (MSE) values for all fits were less than 10. Grazing Incidence X-ray Diffraction Measurement. Grazing incidence X-ray diffraction (GIXRD) analyses were carried out on a Rigaku Ultima III diffractometer (40 kV/44 mA) equipped with Cu Kα radiation (λ = 1.5406 Å). The GIXRD pattern was recorded over a 2θ range from 5° to 35° with an incident angle of 0.5°, a step size of 0.2°, and a scan speed of 2°/min. A straight line between 30° and 35° was fitted on the raw data as background signal, and subtracted. UV–Visible Absorption Spectroscopy. The UV-Vis absorption spectra in transmission mode were measured by an Ocean Optics USB 4000 spectrometer with a DT-mini-2-GS light source, while the total absorption spectra measurements in reflection mode (for IQE calculation) were conducted using an Ocean Optics ISP-R-GT integrating sphere with gloss-trap setup and a HL2000 light source.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. It includes experimental details, J-V results, n&k from ellipsometry measurements, generation rate from TMM, EQE, IQE, total absorption, parasitic absorption, GIXRD results, nid, active layer thickness, dielectric permittivity, low energy EQE fitting results, and other additional discussions. AUTHOR INFORMATION Corresponding Author: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Prof. R. Wallace for the use of the spectroscopic ellipsometer. Device fabrication, characterization, and simulation [L. X, J. W, T. D, Y-J. L, and J.W.P.H.] were sponsored by the National Science Foundation (NSF) DMR-1305893. TR-PL spectroscopy measurements [M. V. A and A. V. M] were supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-SC0010697. J.W.P.H. acknowledges the support from Texas Instruments Distinguished Chair in Nanoelectronics. REFERENCES (1) (2)

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Table of Contents Graphic

12 10

2

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4

limited by exciton diffusion limited by exciton dissociation limited by charge recombination

Jsc_ideal

Jsc_experiment

2 5 10 15 20 25 30 P3HT Concentration (wt.%)


25

Quantitative Analyses of Competing Photocurrent Generation

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