Quantifying Charge Recombination in Solar Cells Based on Donor

Article pubs.acs.org/JPCC

Quantifying Charge Recombination in Solar Cells Based on Donor− Acceptor P3HT Analogues Saptaparna Das,† Petr P. Khlyabich,†,‡ Beate Burkhart,†,‡ Sean T. Roberts,†,§ Elsa Couderc,† Barry C. Thompson,†,‡ and Stephen E. Bradforth*,† †

Department of Chemistry & Center for Energy Nanoscience, University of Southern California, Los Angeles, California 90089, United States ‡ Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *

ABSTRACT: The creation of semi-random donor−acceptor analogues of poly(3hexylthiophene) (P3HT) yields polymers that exhibit pan-chromatic absorption spectra extending into the near-infrared. Despite this extended absorption however, different semirandom polymers exhibit markedly different photovoltaic performance when blended as a bulk-heterojunction with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). To understand the physical origin of these differences, we performed transient absorption (TA) measurements and device characterization of blends of two representative semirandom polymers, poly(3-hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), with PCBM. Although both polymers absorb strongly throughout the visible and near-infrared, devices based on P3HTT-DPP-10%:PCBM exhibit a power conversion efficiency of ∼6%, while films consisting of P3HTT-TP-10%:PCBM blends display values under 1%. TA experiments reveal that polarons generated upon excitation of a P3HTT-TP-10%:PCBM blend undergo a high degree of geminate recombination (survival percentage, ϕS ∼45%) independent of excitation wavelength, explaining its lower efficiency. In contrast, P3HTT-DPP-10%:PCBM blends show excitation wavelength-dependent polaron recombination dynamics. While excitation of the polymer in the visible region leads to less geminate recombination (ϕS ∼65%) compared to P3HTT-TP-10%:PCBM, this loss process is ∼1.5 times more deleterious following near-infrared (NIR) excitation. Despite this observation, a significant fraction (ϕS ∼ 45%) of the charges formed following NIR excitation escape recombination, partly explaining the high performance of P3HTT-DPP-10%:PCBM devices.

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analogues of regioregular poly(3-hexylthiophene) (P3HT)22−26 has been developed, which display exceptionally broad absorption due to the randomized incorporation of only a small amount (∼10% monomer sites) of electron deficient units into the backbone of P3HT. These analogues retain many of the favorable characteristics of P3HT, such as preferable mixing with fullerenes at low w/w ratios and high hole mobility. Unlike alternating donor−acceptor copolymers, these semirandom donor−acceptor copolymers display broad absorption features due to the formation of two classes of chromophores along the polymer backbone.23,24 The first of these two classes can be broadly characterized as chromophores that predominantly consist of P3HT-rich backbone segments whose π→π* transition leads to a strong absorption in the visible (400−600 nm), while the second class consists of intramolecular charge transfer (ICT) chromophores that give rise to absorption in the NIR (700−900 nm).23,24 ICT chromophores contain at least one electron-deficient monomer adjacent to electron-rich thiophene or 3-hexylthiophene units that facilitates the

INTRODUCTION Over the last two decades, organic photovoltaics (OPVs) have shown steady development as a promising new technology.1−4 At present, the best performing OPVs utilize a bulk heterojunction (BHJ) architecture5,6 (a bicontinuous composite of an electron donor and acceptor phase) as this aids excitons in reaching a donor−acceptor interface during their lifetime at which they can undergo charge separation.7−13 The most commonly studied BHJ OPVs consist of high band gap (∼2 eV) polymers14 blended with fullerenes that do not harvest the solar spectrum as efficiently as commercial solar cells that make use of silicon’s indirect bandgap of ∼1.1 eV. In recent studies, near-infrared absorption by OPVs has been achieved by implementing a donor−acceptor approach, where an alternating pattern of an electron deficient monomer and an electron rich monomer is used to obtain low band gap polymers.15,16 These alternating donor−acceptor copolymers have extended near-infrared (NIR) absorption as they primarily shift the polymer’s main absorption band to the red.4 However, this approach tends to lower the polymer’s absorbance in the visible range leading to a reduction in the number of visible solar photons harvested by the polymer.4,17−21 Recently, a new family of semi-random multichromophoric donor−acceptor © 2014 American Chemical Society

Received: January 4, 2014 Revised: March 12, 2014 Published: March 14, 2014 6650

dx.doi.org/10.1021/jp500078b | J. Phys. Chem. C 2014, 118, 6650−6660

The Journal of Physical Chemistry C

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absorption of lower energy photons.27 The average delocalization length of the exciton for either of these chromophore classes is unclear, and there exists the potential that some segments contribute to both optical transitions. The observed spectral characteristics of semi-random polymers make them excellent candidates for photovoltaic devices. While many of the semi-random polymers studied to date show high efficiency in BHJ solar cells with [6,6]-phenylC61-butyric acid methyl ester (PCBM) as an acceptor,22,23,25 there are certain cases where the broadened spectral coverage instead lead to low short circuit current densities (Jsc) and power conversion efficiencies. As a specific comparison, diketopyrrolopyrrole (DPP) based polymers (P3HTT-DPP10%) in BHJ solar cells show efficiencies approaching 6% and a J sc of almost 15 mA/cm 2 , 23 while cells utilizing the thienopyrazine (TP) containing polymer P3HTT-TP display efficiencies below 1% and surprisingly low currents of around 3 mA/cm2.24 In contrast, P3HT:PCBM solar cells were found to give a peak efficiency of 3.89% with a Jsc of 10.22 mA/cm2 under identical device efficiency measurement conditions (i.e., in air).24 The origin of such large differences in Jsc caused by subtle alterations of the polymer chemical composition (for both semi-random polymers only 10% of the total monomer content is the acceptor unit) is unclear, and understanding the origin of such differences is crucial for the design of semirandom donor−acceptor polymers for high efficiency OPVs. The change introduced by adding electron deficient monomers can potentially influence the delocalization of excited states and thus affect charge generation as the extent of exciton delocalization has been suggested to influence the spatial separation of electron−hole pairs following charge transfer.28,29 Moreover, the presence of an acceptor unit may lead to charge trapping and enhanced charge recombination within polymer:fullerene composites. The recombination of charges in OPVs can be broadly classified as either geminate, which is the recombination of the carriers generated from the same exciton, or nongeminate, defined as the recombination of the free mobile carriers originating from different initial excitations.30−32 Recently, transient photocurrent measurements and numerical simulations (based on the Onsager−Braun model) performed by Li et al. on semi-random donor−acceptor copolymer:fullerene devices suggested that low exciton diffusion efficiency and high geminate and nongeminate recombination losses are responsible for poor performance of P3HTT-TP:PCBM.33 They also determined that these losses are negligible for P3HT:PCBM and P3HTT-DPP-10%:PCBM.33 In this paper, we attempt to provide an expanded understanding of the geminate recombination pathways for charges formed by the photoexcitation of semi-random donor−acceptor copolymer:fullerene blends inferred by Li et al. Specifically, we aim to illustrate the correlation between excitation-wavelength dependent geminate recombination dynamics and the internal quantum efficiency (IQE) of the devices. Transient absorption (TA) measurements were performed on composites of two new semi-random P3HT analogues with PCBM to understand the nature of the optically excited states and to investigate how the dynamics of charge generation and recombination are altered by the incorporation of electron withdrawing units into the polymer backbone. Two different electron withdrawing units were studied: DPP and TP, with TP being comparatively stronger in electron withdrawing strength. Since both of the semi-random donor−acceptor polymers studied in this paper

contain only 10% of acceptor unit, from now on they will be referred as P3HTT-TP-10% and P3HTT-DPP-10%. Our results indicate that the poorer performance of P3HTT-TP10%:PCBM compared to P3HT:PCBM is due to rapid and efficient geminate recombination of charges following the creation of excitons from either visible or NIR photons. We conclude that the lower energetic driving force of the exciton and high degree of morphological disorder of P3HTT-TP10%:PCBM facilitates this rapid charge recombination. In contrast, the markedly improved performance of P3HTT-DPP10%:PCBM can be correlated with two observations: slow and lower geminate recombination of charges generated from the excitation of P3HT-like chromophores and some contribution of charges formed by exciting ICT-like chromophores that escape geminate recombination. The results from TA and IQE measurements suggest that although P3HTT-DPP-10%:PCBM solar cells perform better than P3HT:PCBM, there exists further potential for improvement of the devices by overcoming charge recombination in the NIR.

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EXPERIMENTAL METHODS Synthetic Procedures. Procedures for the synthesis of poly(3-hexylthiophene) (P3HT), poly(3-hexylthiophene-thiophene-thienopyrazine) (P3HTT-TP-10%) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) were used without modification as developed and reported earlier.23,24 Preparation of Thin Films for Femtosecond Transient Absorption. Solutions were spin-coated onto precleaned glass slides from 5 mg/mL chlorobenzene (CB) solutions for P3HT and 5 mg/mL o-dichlorobenzene (o-DCB) solutions for P3HTT-TP-10% and P3HTT-DPP-10% films. Polymer:PCBM blends at optimal ratios were spun from CB in the case of P3HT:PCBM (w/w 1:1) and o-DCB for P3HTT-TP10%:PCBM (w/w 1:0.8) and P3HTT-DPP-10%:PCBM (w/w 1:1.3) at 5 mg/mL polymer concentration. P3HT and P3HT:PCBM films were thermally annealed at 150 °C for 30 min in a nitrogen oven. P3HTT-TP-10%, P3HTT-DPP-10%, P3HTT-TP-10%:PCBM, and P3HTT-DPP-10%:PCBM films were placed in a N2 cabinet for 30 min. The processing conditions for neat polymers and polymer:PCBM blends were chosen to match the optimal processing conditions of the corresponding solar cells at which the highest power conversion efficiencies were obtained.23,24 The thickness of each film was chosen such that the peak optical density of the polymer was near 0.2. Solar Cell Fabrication. All steps of device fabrication and testing were performed in air. ITO-coated glass substrates (10 Ω/□, Thin Film Devices Inc.) were sequentially cleaned by sonication in detergent, deionized water, tetrachloroethylene, acetone, and isopropyl alcohol, and dried in a nitrogen stream. A thin layer of PEDOT:PSS (Baytron P VP AI 4083, filtered with a 0.45 μm PVDF syringe filter − Pall Life Sciences) was first spin-coated on precleaned ITO-coated glass substrates and baked at 130 °C for 60 min under vacuum. Separate solutions of P3HT in CB and P3HTT-TP-10%, P3HTT-DPP-10% in oDCB, and PCBM in CB or in o-DCB were prepared. The solutions were stirred for 24 h before they were mixed at desired ratios and stirred for an additional 24 h to form a homogeneous mixture. Subsequently, the polymer:PCBM active layer was spin-coated (with a 0.45 μm PTFE syringe filter - Pall Life Sciences) on top of the PEDOT:PSS layer. Concentrations of the polymer:PCBM solutions for P3HT, 6651

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either a 256-pixel silicon diode array (Hamamatsu) or InGaAs photodiode array (Hamamatsu G9213−256S) for visible or NIR detection, respectively. The spot size of the 500 nm pump at the sample had a fwhm of 410 μm while the 700 nm pump had a slightly larger fwhm of 440 μm. Transient absorption measurements were performed with a range of pump pulse energies between 15 nJ to 200 nJ, and over this range the temporal profile was found to scale linearly with the pump energy (Supporting Information). The data shown for 500 nm excitation in Figures 2, 3 and 4 were measured with excitation densities of 0.8, 1.3, and 1.4 × 1018 cm−3, respectively. The 700 nm excitation data shown in Figures 3 and 4 correspond to excitation densities of 0.87 and 2 × 1018 cm−3, respectively.

P3HTT-TP-10%, and P3HTT-DPP-10%, were 5 mg/mL in polymer. The P3HT:PCBM (1:1) film was spin-coated from CB solution and directly placed in a vacuum chamber for Al deposition. Upon spin-coating of polymer:PCBM solutions based on P3HTT-TP-10% (1:0.8) and P3HTT-DPP-10% (1:1.3), films were first placed into the N2 cabinet for 30 min and then placed in a vacuum chamber for Al deposition. For deposition of the top electrode, substrates were pumped down to high vacuum (2 × 1018 cm−3 (>10 μJ/cm2)44 in TA experiments can influence the polaron rise time in donor− acceptor copolymers.45 However, we did not observe any influence of the excitation density on the polaron band dynamics in the range between 8 × 1017 and 3 × 1018 cm−3 used in our study (Supporting Information, Figure S5). Polymer polarons undergo relatively little charge recombination in the presence of PCBM over our observation window (survival percentage (ϕS) of 75 ± 2% after 1 ns, Figure 2B), which points to the efficient performance of the devices. Although an induced absorption band for the PCBM negative polaron is expected at ∼1040 nm, it is not observed in our measurements, likely due to its low extinction compared to that of P3HT positive polarons.46−49 Using the understanding of the control P3HT:PCBM blend, TA measurements were performed on blends of semi-random polymers by exciting either the P3HT-like absorption feature at 500 nm or the ICT absorption band at 700 nm. Figure 3 shows

like chromophores, respectively. The semi-random donor− acceptor analogues of P3HT we examine here consist of only 10% electron withdrawing units by composition, so P3HT:PCBM (w/w 1:1) BHJ films form a good point of comparison to understand how the presence of the electron withdrawing units affect the blend’s electronic dynamics. Figure 2 shows TA spectra of a P3HT:PCBM blend. Photoexciting the blend at 500 nm leads to an immediate

Figure 2. (A) TA spectra of a P3HT:PCBM blend. The steady-state absorption spectrum is shown as a black dotted line. The induced absorption band attributed to polymer polarons is highlighted by a dashed rectangular box. TA signals for probe wavelengths near 800 nm are excluded because of probe spectral distortion near the driving wavelength used for continuum generation. (B) Normalized TA kinetics of the photoinduced polaron absorption at 870 nm for a P3HT:PCBM blend (red line). A polaron rise time of ∼6.5 ps is observed.

appearance of a ground state photobleach (400−620 nm) due to depopulation of the P3HT ground state and a broad photoinduced absorption near 1200 nm assigned to the singlet exciton of P3HT, 1P3HT*. This assignment is based on TA spectra measured for neat P3HT films that display a similar NIR absorption feature and decays on a time scale identical to that observed in time-resolved photoluminescence measurements (Supporting Information, Figure S1). This conclusion lies in good agreement with spectral assignments made by other groups.38−40 An additional broad induced absorption band appears from 620−750 nm for polaron pairs,41,42 which form either as a direct result of photoexcitation or indirectly from singlet excitons within our experimental time-resolution (180 fs).43 As the time delay between pump and probe is increased, there is a delayed formation of polymer positive polarons (750−1100 nm) that arise with a 6.5 ± 1.5 ps rise time (Figure 2B), indicating charge transfer from photoexcited P3HT to PCBM. This observation is consistent with results from Zhang et al. and Guo et al. who observed a similar delayed rise of polarons following low energy excitation of P3HT:PCBM blend films.40,43 Guo et al. explained the delayed polaron rise by suggesting that exciting the red edge of the P3HT absorption

Figure 3. TA spectra of P3HTT-TP-10%:PCBM following 500 nm excitation (A) and 700 nm excitation (B). The ground state absorption spectrum of each sample is shown as a black dashed line. TA spectra near 800 nm is excluded because of probe spectral distortion near the driving wavelength used for continuum generation. (C) Normalized TA decay of the polaron band at 900 nm following 500 nm (green) and 700 nm (red) excitation. 6653

dx.doi.org/10.1021/jp500078b | J. Phys. Chem. C 2014, 118, 6650−6660

The Journal of Physical Chemistry C

Article

Figure 4. TA spectra of P3HTT-DPP-10%:PCBM following 500 nm excitation (A) and 700 nm excitation (B). TA spectra near 800 nm is excluded because of probe spectral distortion near the driving wavelength used for continuum generation. The ground state absorption spectrum of the blend is shown as a black dashed line. (C) Comparison between the inverted ground state bleach of P3HTT-DPP-10%:PCBM after 700 nm excitation with the ground state absorption spectrum of DPP-T2 oligomer (from ref 62) and PDPP4TP polymer (from ref 63). (D) The structures of DPP-T2 oligomer and PDPP4TP polymer from ref 62 and 63, respectively.

TA spectra of P3HTT-TP-10%:PCBM (w/w 1:0.8) films after excitation of the polymer at either 500 nm (Figure 3A) or 700 nm (Figure 3B). In both cases, immediate formation of a ground state bleach is observed between 400−550 nm and 650−800 nm. Identical bleaching features were also observed for both isolated P3HTT-TP-10% chains in solution (Supporting Information, Figure S2) and neat spun-cast films (Supporting Information, Figure S3) irrespective of the excitation wavelength used. The rapidity (