Understanding the Enhanced Open-Circuit Voltage of Polymer Solar

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Understanding the Enhanced Open-Circuit Voltage of Polymer Solar Cells based on a Diketopyrrolopyrrole Small Molecular Acceptor Yuan Xie, Yuanyuan Yu, Quanbin Liang, Jun-Hua Wan, Hongbin Wu, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06717 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Understanding the Enhanced Open-Circuit Voltage of Polymer Solar Cells based on a Diketopyrrolopyrrole Small Molecular Acceptor ¶

¶

Yuan Xie1 , Yuanyuan Yu2 , Quanbin Liang1, Jun-Hua Wan2*, Hongbin Wu1* and Yong Cao1

1

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China E-mail: [email protected] 2

Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education Hangzhou Normal University, Hangzhou 310012, P. R. China E-mail: [email protected]

¶

: These authors contributed equally.

Abstract In this study, polymer solar cells employing poly(3-hexylthiophene) (P3HT) as donor and fullerene

derivative

PC61BM

(Phenyl-C61-butyric

acid

methyl

ester)

or

non-fullerene

diketopyrrolopyrrole (DPP)-based small molecule (SF-DPPEH) as acceptor are investigated. Device based on SF-DPPEH shows remarkably high VOC of 1.20 V, whereas analogous device based on PC61BM only delivers a VOC of 0.64 V. Employing transient photovoltage/photocurrent techniques, we measure charge carrier lifetime and density, non-geminate recombination rate in the photoactive layer, and correlate material energetics and charge recombination dynamics to the change of VOC in the devices, thus the extent to which two factors limit VOC can be quantified.

KEYWORDS: non-fullerene acceptors, open-circuit voltage loss, material energetics, non-geminate

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recombination, transient photovoltage, recombination dynamics.

INTRODUCTION Polymer solar cells (PSCs) have advanced tremendously in the past few years and power conversion efficiency (PCE) of over 11% have been achieved for single-junction cells with fullerene derivative as acceptors.1,2 Nevertheless, the photon energy loss (Eloss= Eg–eVOC, where Eg is the optical bandgap of absorber and VOC is the open-circuit voltage of device) is still very large, with typical value of 0.7–1.0 eV in most fullerene-based PSCs systems. As a consequence, the open-circuit voltage in fullerene-based systems remain relatively low.3,4 In the electronic orbital picture, Eloss is associated with the energetic offset at the donor/acceptor interface acting as driving force for exciton dissociation. Several studies showed that 1.0 V is the upper limit of open-circuit voltage for fullerene-based PSCs, because charge photo-generation become highly inefficient when the energy of charge transfer state (CTS) approaches the singlet excited state energy of fullerene (~1.7 eV).5,6 On the other side, to efficiently harvest solar photons, it is of great importance to use narrow bandgap donor with high-lying highest occupied molecular orbital (HOMO) level as absorber. Thus, a trade-off exists between high open-circuit voltage and high short-circuit current density (JSC), representing a critical limitation for further improvement. As a strategy to circumvent these limitations, non-fullerene polymer solar cells have made substantial progress in recent several years.7,8 It is shown that fine structural modification is an effective approach to tune the optical, electronic properties, as well as aggregation behavior, in these small molecular acceptors. 9 Hence, as compared to PSCs based on fullerene-based electron acceptors, non-fullerene polymer solar cells exhibited comparable device parameters in short-circuit current density (Jsc) and fill factor (FF), and even higher VOC. 10 , 11 As a result, the best single-junction PSCs from non-fullerene acceptors reported so far shows a PCE over 14%, 8,

12

surpassing that of the best PSCs based on fullerene acceptor. Recently, we reported polymer solar


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cells employing poly(3-hexylthiophene) (P3HT) as donor and spiro-diketopyrrolopyrroles (DPP) (SF-DPPEH) as non-fullerene acceptor. The resultant device yields a high VOC of 1.10 V, compared to a VOC of 0.60 V for a control device containing fullerene acceptor.13 And later, we demonstrated that delicate tailoring on SF-DPPEH by endcapping a phenyl unit to each DPP unit can further promote a higher PCE of 5.16%,14 which is among one of the best values for P3HT-based PSCs. While previous efforts were largely focused on the influence of molecular structure and topology control on device performance, there has been comparatively little attention dedicated to attain comprehensive understanding on the physical origin of high VOC in this system. We note that the observed difference in VOC cannot be explained by the difference between the energy levels of two different acceptors. In this study, we quantify the rate of non-geminate recombination in these systems by employing transient photovoltage and charge extraction techniques. Our results reveal that device VOC is not only dependent on the material energetics of the donor/acceptor systems, but also critically correlate to charge dynamics in active layer. In particular we demonstrate a solid relationship between material energetic/recombination dynamics and open-circuit voltage, which we believe to be a powerful tool to provide fundamental insight into the origin of high open-circuit voltage in PSCs based on non-fullerene acceptors. O

O EH

HE

N

S

N S S

N

S N

HE

EH

O

O

O

O

HE

EH

N

N

S

S

S

S

N

N

EH O

HE SF-DPPEH

O

Figure 1. Chemical structures of the polymer donor P3HT and the small molecule acceptor SF-DPPEH.



RESULTS AND DISCUSSION The chemical structures of polymer donor P3HT and acceptor SF-DPPEH used in this study are shown in Fig. 1, while the synthesis of the donor can be referred to literature for more detail. The typical current density versus voltage characteristic (J–V) of devices based on P3HT: PC61BM or P3HT: SF-DPPEH under the illumination of an AM 1.5 G solar simulator (100 mW cm−2) are shown in Fig. 2a. The control device based on P3HT: PC61BM blend shows an open-circuit voltage (VOC) of 0.640 V, a short-circuit current density (Jsc) of 7.98 mA cm−2, and a fill factor (FF) of 57.4%, producing a power conversion efficiency (PCE) of 2.93%. In contrast, the P3HT: SF-DPPEH device shows a reduced Jsc of 5.43 mA cm−2 and a slightly decreased FF of 53.0%, but a significantly increased VOC of 1.200 V, resulting in a PCE of 3.45%. The performance of the devices is consistent with that in previous report,13 while morphological charaterization can also be referred therein. The corresponding external quantum efficiency (EQE) spectra of the devices are shown in Fig. 2b. The P3HT: PC61BM device shows a relatively stronger photoresponse from 300 to 650 nm, while the P3HT: SF-DPPEH device exhibit slightly broader wavelength range extending to 670 nm but with weaker photo-response. As PC61BM possesses lowest unoccupied molecular orbital (LUMO) energy levels of -3.84 eV, while SF-DPPEH exhibits LUMO levels of -3.60 eV, it is apparent that the observed differences in VOC (0.640 V vs. 1.200 V) between the fullerene-based and the non-fullerene devices cannot be explained by the acceptor's LUMO shift (~0.24 eV) alone.


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-2

Current Density (mA cm )

(a) 0

-5 P3HT:SF-DPPEH P3HT:PC61BM

-10

0

0.4

0.8

1.2

Voltage (V)

(b) 60

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


40

20

P3HT:SF-DPPEH P3HT:PC61BM

0 300

400

500

600

700

Wavelength (nm) Figure 2. (a) Current-voltage characteristics for the devices, as obtained under AM 1.5 G illumination, (b) External quantum efficiencies for the solar cells.

Besides the energy levels of the materials, other physical variables that can impact VOC of PSCs



include interfacial charge transfer states,15 energetic disorder in active layers,16,17 film morphology18 and recombination dynamics.19 Since the dynamic equilibrium between the generation and the recombination of charge carriers is established at open-circuit condition, thus understanding the kinetics of charge recombination is critical to unveil the origin of the high Voc in the non-fullerence device. Transient photovoltage and transient photocurrent (TPV/TPC) measurements, especially VOC-dependent TPV measurement, providing direct information on the evolution of free charge population after photo-excitation, were recently employed to determine charge carrier decay in polymer solar cells.20,21 In previous work, an experimentally observed variation in VOC in a series of P3HT/PCBM solar cells was interpreted with variation in the recombination coefficient as well as the charge density within the device.21,22 Hence, we first investigate charge carrier decay dynamics using TPV, TPC methods, and quantify the processes in determining VOC of the devices via these experimental data.

a

-3

Charge Density (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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10

+0.48 V

16

15

5 10

P3HT:PCBM P3HT:SF-DPPEH

0.4

0.6

0.8

1

Open-circuit Voltage (V)


1.2

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Lifetime (us)

b

10

2

10

1


10

0

0.4

0.6

0.8

1

1.2

Open-circuit Voltage (V) c

Lifetime (us)

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


10

2

10

1


10

0

10

15

16

10 -3

Charge Density (cm ) Figure 3. (a). Measured charge carrier density as a function of Voc for the two blend systems. (b).Carrier lifetime versus Voc from TPV measurement at different light intensities. (c).The dependence of carrier lifetime on carrier density for the devices as derived from TPC and TPV measurement. Dashed lines represent the fit to each equation.



Page 8 of 18

In order to quantify recombination process, charge carrier density and its lifetime are both needed. The small perturbation charge lifetime for both devices can be obtained by fitting the transient photovoltage decays at different illumination intensities with a mono-exponential decay course, as shown in Fig. S1 of the Supporting Information. Fig. 3a shows the average carrier density as a function of VOC for the devices. Variation of VOC between 0.4-0.6 V for P3HT: PCBM device and VOC between 0.9-1.15 V for P3HT: SF-DPPEH device were achieved by adjusting the intensity of white light bias.22 The n(VOC) curve visualizes the carrier density in photoactive layer for a certain bulk quasi-Fermi level splitting, and is therefore representing effective electronic bandgap.23,24 It is apparent that the average charge density at open circuit conditions in both devices increase exponentially as a function of VOC, following a relationship n = n 0 e γVOC , where n0 is charge density in the dark and γ is the slope of ln(n)~VOC curves. It is found that cm-3 for P3HT: PCBM device, while device. We note that the values of semiconductor (given by

γ

γ

is ~8.5 V-1 and n0 ~1.0 ×10

14

is ~3.4 V-1 and n0 ~ 2.6×10 cm-3 for P3HT: SF-DPPEH 14

γ of each device is much less than the value expected from ideal

e and is about 19.3 V-1 at room temperature), implying both systems, 2k BT

especially the P3HT: SF-DPPEH system having a high degree of trap states within the photoactive layer.25 At a representative charge density of 1.0 ×10 cm-3, the quasi-Fermi level splitting is larger 16

for the P3HT: SF-DPPEH device and a shift of 0.48 V in effective electronic bandgap can be clearly seen between the two devices, obviously higher than LUMO shift obtained from neat film CV data.13 The observed energetic shift can be also evidenced by the enhanced charge transfer energy (ECT) in the P3HT: SF-DPPEH system. Following the method proposed by Vandewal et al.,26 ECT of P3HT: SF-DPPEH system is determined as ~1.8 eV from its sub-bandgap EQE spectra and electroluminescence (EL) spectra (see Fig. S2 of the Supporting Information), obviously higher than that for the P3HT: PCBM system (~0.99 eV).27 As a result of higher lying charge transfer states, the


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P3HT: SF-DPPEH system can provide higher energy electronic states for charge transfer exciton,16 thus can afford larger quasi-Fermi level splitting for P3HT: SF-DPPEH device upon illumination. Small perturbation charge carrier lifetimes as a function of VOC are plotted in Fig. 3b. For both devices, the lifetime decreases exponentially as a function of VOC via τ ∆n = τ ∆n0 e

− β VOC

,

where τ ∆n0

is the carrier lifetime in the dark and β is the decay constant. The experimentally determined values of β and τ ∆n0 are 16.8 V-1 and 0.10 s for the P3HT: PCBM device, respectively, while the corresponding values are determined as 25.9 V-1 and 6.7 ×10 s for the P3HT: SF-DPPEH device. 6

Despite of its more steep decay order, the τ ∆n0 value of the P3HT: SF-DPPEH device is 7 orders of magnitude larger than that of the P3HT: PCBM device. Fig. 3c shows the carrier lifetimes as a function of average carrier density in a double logarithmic plot for both devices, the slope of which corresponds to β/γ, representing the apparent recombination orders for organic photovoltaics.28 In the P3HT: PCBM device, β/γ is found to be ~2.0, similar to those in previously published literature, which can be assigned to non-geminate recombination where the recombination coefficient is strongly density-dependent.29 On the other side, the value of β/γ is ~7.6 for the P3HT: SF-DPPEH device, implying high apparent recombination order in the device, which can be related to non-geminate recombination in the presence of sub-bandgap localized states in this disordered system.30,31 Taking into account a bimolecular law as the dominant recombination mechanism for these two systems,21 the loss current can be quantified via J Loss = edk ( n ) n 2 = ed

non-geminate recombination rate coefficient and is given by k ( n) =

n (V OC ) where k(n) is the τ (VOC ) ,

1

β (1 + ) nτ ∆n γ

, e the elementary

charge, d the photoactive layer thickness, and n(VOC) is the measured charge carrier density and

τ (VOC ) the measured carrier lifetime obtained under different illumination intensity. The calculated



k(n) as a function of charge density is depicted in Fig. 4. It is clear that the P3HT: SF-DPPEH device shows the lower k(n) at a certain carrier density than the P3HT: PCBM device, consistent with its slower recombination dynamics as shown in Fig. 3c. Comparing at the same equivalent charge density of 1.0 ×10 cm-3, the non-geminate recombination rate coefficient for the P3HT: SF-DPPEH 16

device is an order of magnitude lower than that for the P3HT:PCBM device. Owing to slower non-geminate recombination losses, the P3HT: SF-DPPEH device can afford higher charge accumulation and therefore leading to a larger quasi-Fermi level splitting versus the P3HT:PCBM device. Hence, in addition to the energetic shift of 0.48 V as shown above, the observed 10-fold slower recombination kinetic in the P3HT: SF-DPPEH device is expected to result in a further increase of ~0.09V in VOC.23, 24 Therefore, the transient optoelectronic analysis reveal an overall increase of 0.57 V in VOC for the P3HT: SF-DPPEH, which is in good agreement with the measured

10

-11

10

-12

3 -1

Rate Coefficient (cm s )

value of 0.56 V (Fig. 2a).

Non-geminate Recombination

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10

-13

2 10

15

5 10

15

10

16

2 10

16

-3

Charge Density (cm )

Figure 4. the calculated non-geminate recombination rate coefficient plotted against carrier density for the devices.


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To further verify the accuracy of the above transient optoelectronic analyses on device VOC, one can reconstruct the VOC following a simple device model. With the empirical relationships for charge carrier density and carrier lifetime obtained from Fig. 3a and Fig. 3b, the loss current can be rewritten as: J Loss =

edn 0

β (1 + )τ ∆n γ

e ( β +γ )VOC .21 0

In case that non-geminate recombination of charge

carriers at short-circuit condition is negligible, open-circuit voltage can be derived as:

VOC

β    J SC τ ∆n0 (1 + )  1 γ  ln  =  β +γ  edn 0     after a simple rearrangement on the above equation, thus allowing a

quantitative assessment of impact of changes in engetics and recombination rate on VOC. The open-circuit voltages under AM 1.5G illumination predicted by the above equation are 0.642 V for the P3HT:PCBM device, 1.186 V for the P3HT: SF-DPPEH device, respectively, in excellent agreement with the measured values (0.640 V and 1.200 V, respectively), confirming the validity of transient optoelectronic analyses. It is worthy to note that all of the variables for the VOC calculation are deduced from Fig. 3, thus highlighting the significant impact of material energetics and recombination dynamics on VOC. Hence, this simple empirical relationships based on the transient optoelectronic measurements is capable of describing material energetics and non-geminate recombination and their overall impact on the open circuit voltage of these devices.

CONCLUSIONS In conclusion, we have demonstrated that measurements of material energetics and charge recombination dynamics for polymer solar cells employing poly(3-hexylthiophene) (P3HT) as donor and

fullerene

derivative

PC61BM

(Phenyl-C61-butyric


acid

methyl

ester)

or


spiro-diketopyrrolopyrroles (DPP) (SF-DPPEH) as acceptor. We have found that apparent discrepancy in open-circuit voltage enhancement when replacing fullerene derivative with SF-DPPEH can be resolved by considering the impacts of both energetics and kinetics. The P3HT:SF-DPPEH device shows a energetic shift of 0.48 V in the electronic bandgap with respect to the P3HT:PCBM device, while slower non-geminate recombination further results in an increase of 0.09V in VOC. Thus, the overall impact of material energetics and charge recombination dynamics leads to an increase of 0.57 V in the non-fullerene diketopyrrolopyrrole (DPP)-based device, consistent with the measured high VOC of 1.200 V in the P3HT:SF-DPPEH device. In addition, a solid correlation between calculated and measured VOC is established, allowing an insightful understanding of the impact of material energetics and charge recombination dynamics on the VOC of polymer solar cells.

EXPERIMENTAL SECTION Film and device preparation: Donor material P3HT was purchased from 1-material Inc., while fullerene acceptor was provided from Solenm Inc., both were used as received. Non-fullerene acceptors SF-DPPEH was synthesized in the lab according to reported procedure.13 Pre-cleaned indium tin oxide(ITO) patterned glass substrates were treated with O2-plasma for 4 min before coated with a 40nm PEDOT:PSS (Clevios P AI4083), and baked at 150°C for 15 min. The ITO substrates were then transferred into a glove box filled with N2 gas. Active layers were prepared with optimal chlorobenzene solution following well-established procedure , followed by post-treatment of thermal annealing 100°C for 10 min. A 15 nm layer of calcium and a 100 nm Al cathode were thermally evaporated in a vacuum chamber, device areas of 16mm2 were defined by the shadow mask. Films thickness were measured using surface profiler (Tencor, Alfa Step-500). J–V and characteristics: J–V characteristics were recorded using a Keithley 2400 source meter under


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a 1 sun, AM 1.5G spectrum from a solar simulator (SAN-EI Inc., Model: XES-40S1), Solar-simulator irradiance was calibrated using a standard mono-crystal silicon reference cell (Hamamatsu S1133, with KG-5 visible color filter), which was calibrated by the National Renewable Energy Laboratory (NREL). External quantum efficiency: External quantum efficiency was determined by an integrated system (QE-R, Enlitech Inc.). The sub-bandgap EQE spectra were obtained using the same system operated under high sensitivity mode, with a highly sensitive pre-amplifier. Electroluminescence Spectroscopy: EL spectra were acquired by a Spectroradiometer PR-745(Photo Research, Inc.) with spectral range covering 380 – 1080 nm. Transient optoelectronic analyses: In TPV measurement, solar cell devices were first illuminated by a halogen lamp with 100 mW/cm2 illumination to reach working condition. A set of neutral optical filter were utilized to produce 0.01-1 sun illumination. Perturbed charge carriers are then generated by a 532 nm laser pulse from a Nd: YAG pulse laser (Q-smart 100 of Quantel). The TPV signals were monitored by a Tektronix DPO4014 oscilloscope with high input impedance option to achieve open-circuit condition and the recorded transients were fitted with a mono-exponential decay course to extract the lifetime of photo-generated carriers. For TPC measurement, solar cell devices were in series with a 50 Ω load resistor and voltage transient across the resistor was recorded by the oscilloscope under the same illumination and laser perturbation. The transient is translated into a current transient by Ohm’s law. The photo-generated charge ∆Q by laser perturbation is the time-integrated current transient . All transient data consist of 128 voltage transient averaged together to counteract the fluctuation of laser pulse.

Acknowledgements H.-B.W. and J.-H. W. thank the National Nature Science Foundation of China (Nos. 21372057, 51511130077, 51521002, and 61775061) for the financial support.



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The impact of material energetics and charge recombination dynamics on the open-circuit voltage of polymer solar cells from fullerene derivative or spiro-diketopyrrolopyrroles is quantified. 160x142mm (288 x 288 DPI)


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Understanding the Enhanced Open-Circuit Voltage of Polymer Solar

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