Bulk heterojunction polymer solar cells made from a novel low‐bandgap polymer show the highest photocurrent response so far for this class of materials (see figure). Efficiencies up to 3.2 % are realized, but this conjugated polymer has the intrinsic capability to reach 7 % efficiency because of its material properties. Possible loss mechanisms and improvements are discussed.

We designed and synthesized a series of conjugated polymers containing alternating electrondonating and electron-accepting units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene), 4,7-(2,1,3)-benzothiadiazole, and 5,5′-[2,2′]bithiophene. These polymers possess an optical band gap as low as 1.4 eV (i.e., in the case of poly [2,6-(4,4-bis(2-ethylhexyl)), and their absorption characteristics can be tuned by adjusting the ratio of the two electrondonating units: (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene) and 5,5′-[2,2′]bithiophene. The desirable absorption attributes of these materials qualify them as excellent candidates for light-harvesting materials in organic photovoltaic applications allowing for high short-circuit current. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high photovoltaic device open-circuit voltage when fullerene derivatives are used as electron transporters. Field-effect transistors made of these materials show hole mobility in the range of 5 × 10 -4 -3 × 10 -3 cm 2 /(V s), which promises good device fill factor. Because of the combination of these characteristics, power conversion efficiencies up to 3.5% and an external quantum efficiency of at least 25% between 400 and 800 nm with a maximum of 38% around 700 nm were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives. Further improvement of the materials will include the modification of both the side chains and the backbone to effect change to the active layer morphology to maintain good charge carrier mobility in the composite.

Bulk heterojunction materials comprising bicontinuous networks of electron donor and acceptor components sandwiched between electrodes with different work functions (e.g., indium-tin oxide, ITO, and a lower work-function metal such as Al) have yielded the best results to date for organic polymer-based solar cells. In such bulk heterojunction materials, ultrafast photoinduced electron transfer occurs at the interface between the polymer (donor) and the fullerene (acceptor) and results in efficient charge separation. Provided that the back transfer rate is sufficiently slow, the photo-generated mobile positive and negative carriers can be collected at the electrodes; electrons at the lower work function electrode and holes at the higher work function electrode.Using this approach, power conversion efficiencies of 5 % have been demonstrated in bulk heterojunction materials comprised of poly(3-hexylthiophene), P3HT and the [6,6]-phenyl-C 61 butyric acid methyl ester fullerene derivative, PCBM. [1,2] Although the quantum efficiency is high within the absorption spectrum, the band gap of this conjugated polymer is too large (the solar radiation spectrum extends into the infrared). Thus, the quest for higher solar cell efficiencies has focused on smaller band gap polymers designed and synthesized to improve the harvesting of solar radiation. [3][4][5][6][7][8][9][10] Here we describe the results of time-resolved spectroscopic studies of the small band gap semiconducting copolymer, poly [2,6-(4,4-bis-(2-ethylhexyl) -4,7-(2,1,3-benzothiadiazole)], PCPDTBT, made of alternating electron-rich and electron-deficient units and in bulk heterojunction materials comprising PCPDTBT and PCBM. The molecular structures of PCPDTBT and PCBM are shown in Scheme 1. Because carrier recombination prior to collection at the electrodes would limit the photovoltaic power conversion efficiency, the goal of our studies was to demonstrate ultrafast photoinduced electron transfer and to study the carrier recombination dynamics following photo excitation.As in most bulk heterojunction materials made from mixtures of conjugated polymers with C 60 and its derivatives (e.g., poly(p-phenylene vinylene)s:C 60 [11][12][13][14][15] and poly(thiophene)s:C 60 [12,16,17] ), we find that in PCPDTBT:PCBM, photo excitation initiates ultrafast (sub-picosecond) electron transfer from the polymer to the PCBM. This electron transfer reaction is faster by two orders of magnitude than the singlet-state lifetime in the pristine polymer. From analysis of the carrier recombination dynamics, we infer the existence of an intermediate charge transferred state (possibly a bound state of an electron on the PCBM and a hole on a nearby PCPDTBT) from which long-lived mobile positive and negative carriers are subsequently generated; see Scheme 2. The yield of longlived mobile carriers in PCPDTBT:PCBM composites is sensitive to the ratio of the components. The time-decay measurements indicate that the 1:3.3 composites exhibit the highest yield of long-lived mobile carriers, cons...

The photoconductive properties of a novel low‐bandgap conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], PCPDTBT, with an optical energy gap of Eg ∼ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.

Carbon bridged (C‐PCPDTBT) and silicon‐bridged (Si‐PCPDTBT) dithiophene donor–acceptor copolymers belong to a promising class of low bandgap materials. Their higher field‐effect mobility, as high as 10−2 cm2 V−1 s−1 in pristine films, and their more balanced charge transport in blends with fullerenes make silicon‐bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased π–π stacking in Si‐PCPDTBT compared to the carbon‐bridged analogue. The fluorescence of this polymer and that of its counterpart C‐PCPDTBT indicates that the higher photogeneration achieved in Si‐PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge‐transfer complex and to a favorable length of the donor–acceptor phase separation. TEM studies of Si‐PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C‐PCPDTBT:fullerene using 1,8‐octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si‐PCPDTBT requires a lower fullerene content (between 50 to 60 wt%) than C‐PCPDTBT (more than 70 wt%). The Si‐PCPDTBT:[C70]PCBM OBHJ solar cells deliver power conversion efficiencies of over 5%.