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Highly Crystalline Low Bandgap Polymer Based on Thieno[3,4-c]pyrrole-4,6-dione for High Performance Polymer Solar Cell with >400 nm-Thick Active Layer Jae Woong Jung, Thomas P. Russell, and Won Ho Jo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Jun 2015 Downloaded from http://pubs.acs.org on June 6, 2015
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Highly Crystalline Low Bandgap Polymer Based on
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Thieno[3,4-c]pyrrole-4,6-dione for High
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Performance Polymer Solar Cell with >400 nm-
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Thick Active Layer
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Jae Woong Jung,†‡ Thomas P. Russell,§ Won Ho Jo†*
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†
Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro,
Gwanak-gu, Seoul 151–744, Korea. §
Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA
01003, USA.
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KEYWORDS. Polymer solar cell, Thienopyrrolodione, Thienylenevinylene, Thick active layer.
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ABSTRACT. Two thieno[3,4-c]pyrrole-4,6-dione (TPD)-based copolymers combined with 2,2’-
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bithiophene (BT) or (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TV) have been designed and
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synthesized to investigate the effect of introduction of vinylene group in the polymer backbone
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on the optical, electrochemical, and photovoltaic properties of the polymers. Although both
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polymers have shown similar optical bandgaps and frontier energy levels regardless of the
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introduction of vinylene bridge, the introduction of π-extended vinylene group in the polymer
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backbone substantially enhances the charge transport characteristics of the resulting polymer due
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to its strong tendency to self-assemble and thus to enhance the crystallinity. An analysis on
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charge recombination in the active layer of solar cell device indicates that the outstanding charge
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transport (µ = 1.90 cm2·V−1·s−1) of PTVTPD with vinylene group effectively suppresses the
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bimolecular recombination, leading to a high power conversion efficiency (PCE) up to 7.16%,
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which is 20% higher than that (5.98%) of the counterpart polymer without vinylene group
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(PBTTPD). More importantly, PTVTPD-based devices do not show a large variation of
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photovoltaic performance with the active layer thickness, i.e., the PCE retains over 6% as the
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active layer thickness increases up to 450 nm, demonstrating that the PTVTPD-based solar cell is
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very compatible with industrial processing.
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1. INTRODUCTION
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Polymer solar cells (PSCs) based on bulk heterojunction (BHJ) structure that consists of
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conjugated polymer and fullerene derivative as electron donor and acceptor, respectively, have
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emerged the next-generation energy resource due to their unique advantages of low-cost and
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large-area production by solution processing.1−4 Over the past decade, alternating conjugated
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copolymers composed of electron-donating (D) and electron-accepting (A) units have been
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considered as the most promising donor material for high performance PSCs, because their
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frontier energy levels can effectively be tuned by proper combination of D and A units.5
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Moreover, molecular properties of D−A type copolymers such as dipole moment, planarity of
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backbone, intermolecular interaction and charge transport characteristics can also be fine-tuned
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by modifying each of D and A units.6−8 Specifically, the molecular design of D−A structure for
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conjugated polymer is the most effective strategy toward harvesting more photons by lowering
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the optical bandgap while the molecular energy levels satisfy the conditions for charge
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separation and transport.9−10 In recent years, several D−A type polymers have demonstrated
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promising photovoltaic performances close to 10% power conversion efficiency (PCE)
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demonstrating their high potentials for commercialization of PSCs.11−13
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Although most of high performance PSCs have achieved their best performance at relatively
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thin active layer (ca. 100 nm), it is very difficult to fabricate reproducibly such thin films without
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pinhole under current industrial processes. When the thickness of active layer is thicker, the
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bimolecular recombination of photo-induced charge carriers increases and therefore the resulting
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space charge decreases the fill factor (FF) mainly due to intrinsic low charge carrier mobility of
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organic semiconductors.14,15 Therefore, one of the most challenging issues for mass-production
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of PSCs is to fabricate thick active layer without large loss of photovoltaic performance.
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Herein, we present highly crystalline D−A type polymers based on thieno[3,4-c]pyrrole-4,6-
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dione (TPD). TPD possesses strong electron-withdrawing property leading to low frontier orbital
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energy levels of the corresponding conjugated polymers, which is beneficial to achieving high
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open-circuit voltage (VOC) of PSCs.16−21 More importantly, highly fused and coplanar structure of
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TPD affords high crystallinity and thus effective charge transport to the corresponding polymer,
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which are prerequisites for suppressing bimolecular recombination of charge carriers in BHJ
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film.20,21 In conjunction with TPD, we adopted 2,2’-bithiophene (BT) and (E)-1,2-di(thiophen-2-
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yl)ethane (simply called thienylenevinylene (TV)) as D unit for constructing D−A type polymers,
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PBTTPD and PTVTPD, respectively. TV has recently been recognized as a promising electron-
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donating unit of low bandgap conjugated polymers for PSC application, because the
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incorporation of vinylene bridge in polymer backbone enhances the planarity of the polymer
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backbone and thus affords strong π–π interaction with large overlapping area between chain
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backbones leading to increased crystallinity of the conjugated polymer.24−28 Hence, the PTVTPD
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with π-extended TV unit shows significantly increased crystallinity and 3-fold larger hole
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mobility (1.90 cm2/V·s) than PTBTTPD without vinylene unit (0.56 cm2/V·s), although the two
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copolymers exhibit similar optical bandgaps and frontier energy levels. Furthermore, the high
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crystallinity of PTVTPD affords superior photo-induced charge generation with suppressed
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bimolecular recombination, which is beneficial for achieving high short circuit current density
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(JSC). Consequently, PTVTPD exhibits a high PCE exceeding 7% at 140 nm-thick active layer. It
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should be noted here that, during the preparation of our manuscript, a polymer with the same
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polymer backbone as our PTVTPD but different alkyl side group on TPD has been reported for
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photovoltaic application.29 They reported low hole mobility (1.72 × 10–2 cm2/V·s) and
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unsatisfactory PCE (4.68%) probably because of low molecular weight (~24 kDa) and low
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crystallinity which are probably due to very long and branched side chains (5-decylheptadecane).
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However, PTVTPD with relatively short side chains (heptadecane) in our study has higher
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molecular weight (~41 kDa) and high crystallinity and thus exhibits much improved hole
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mobility (1.90 cm2/V·s) and higher PCE over 7%. More importantly, PTVTPD retains high
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photovoltaic performance over 6% PCE while the active layer thickness increases from 140 nm
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to 450 nm, suggesting that the PTVTPD is a promising candidate as donor material for industrial
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production of PSCs
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2. RESULTS and DISCUSSION
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The synthetic routes of PBTTPD and PTVTPD are shown in Scheme 1. The polymers were
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synthesized by the Stille coupling reaction using Pd2(dba)3 and P(o-tol)3 as catalyst and ligand,
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respectively. When the number average molecular weights (Mn) and the polydispersity indexes
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(PDI) of the polymers were measured by gel permeation chromatography (GPC), all polymers
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have sufficient molecular weights (Mn>40 kDa) with narrow PDIs for their device applications
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(Table 1).
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When the optical properties of two polymers in solution as measured by UV−Vis absorption
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spectroscopy are compared, it reveals that an identical absorption onset is observed at 695 nm for
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both polymers, corresponding to an optical bandgap of 1.78 eV, while the maximum absorption
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peak (λmax = 584 nm) of PTVTPD is red-shifted as compared to PBTTPD (λmax = 532 nm)
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(Figure 1 and Table 1). PTVTPD exhibits an obvious and intense vibronic shoulder peak at 640
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nm in dilute solution at room temperature, indicating that the polymer chains are aggregated in
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solution. This is further evidenced by disappearance of the shoulder peak at high solution
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temperature (Figure S1). Accordingly for measurement of exact molecular weights of the
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polymers, the molecular weights of the polymers were measured at high temperature (120 °C)
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using a high-temperature GPC. A strong vibronic shoulder is also observed in solid film of
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PTVTPD, indicating that the PTVTPD exhibits strong interaction between polymer chains which
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may arise from increased conjugation length caused by π-extended vinylene unit. It is
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noteworthy that the absorptivity of PTVTPD is higher than that of PBTTPD in both solution and
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film state, which may arise from not only the extended π-conjugation by introduction of vinylene
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bridge but also its better molecular stacking as will be discussed in the following section.
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To gain deeper insight into the effect of vinylene moiety on the molecular structure and
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electronic property of the polymer, the theoretical calculation based on the density functional
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theory (DFT) at the B3LYP/6-31G* level was performed on the repeating unit of each polymer.
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As shown in Figure 1c, both polymers are expected to exhibit excellent planarity through the
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entire polymer backbone with small torsional angles at the optimized geometry. Although both
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polymers possess small dihedral angle close to 0° between TPD and thiophene, there is steric
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hindrance between adjacent thiophenes in PBTTPD (dihedral angle = 14.4°), while PTVTPD
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exhibits a very small torsional angle (
