Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially. Here we investigate the reasons for the discrepancies by determining both ideality factors differentially as a function of voltage and by comparing them with simulations. We find that both the dark and light ideality factors are sensitive to bulk recombination mechanisms at the internal donor:acceptor interface, as is often assumed in the literature. While the interpretation of the dark ideality factor is difficult due to resistive effects, determining the light ideality factor dif ferentially indicates that the open-circuit voltage of many polymer:fullerene solar cells is limited by surface recombination, which leads to light ideality factors decreasing below one at high voltage.

The poor photovoltaic performance of state‐of‐the‐art blends of poly[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl] (PTB7) and [6,6]‐phenyl‐C61‐butyric acid (PCBM) at large active layer thicknesses is studied using space‐charge‐limited current mobility and photovoltaic device measurements. The poor performance is found to result from relatively low electron mobility. This is attributed to the low tendency of PTB7 to aggregate, which reduces the ability of the fullerene to form a connected network. Increasing the PCBM content 60–80 wt% increases electron mobility and accordingly improves performance for thicker devices, resulting in a fill factor (FF) close to 0.6 at 300 nm. The result confirms that by improving only the connectivity of the fullerene phase, efficient electron and hole collection is possible for 300 nm‐thick PTB7:PCBM devices. Furthermore, it is shown that solvent additive 1,8‐diiodooctane (DIO), used in the highest efficiency PTB7:PCBM devices, does not improve the thickness dependence and, accordingly, does not lead to an increase in either hole or electron mobility or in the carrier lifetime. A key challenge for researchers is therefore to develop new methods to ensure connectivity in the fullerene phase in blends without relying on either a large excess of fullerene or strong aggregation of the polymer.

ABSTRACT:We report the synthesis of a novel ladder-type fused ring donor, dithienogermolodithiophene, in which two thieno [3,2-b]thiophene units are held co-planar by a bridging dialkyl germanium. Polymerisation of this extended monomer with N-octylthienopyrrolodione by Stille polycondensation afforded a polymer, pDTTG-TPD, with an optical band gap of 1.75 eV combined with a high ionization potential. Bulk heterojunction solar cells based upon pDTTG-TPD:PC71BM blends afforded efficiencies up to 7.2% without the need for thermal annealing or processing additives.There has been significant recent progress in the development of conjugated polymers for use in organic field effect transistors and bulk heterojunction (BHJ) polymer solar cells. 1 One promising class of polymers for these applications are the so-called ladder polymers 2 , in which linked aromatic units such as thiophene or benzene are forced to be coplanar and fully conjugated by the use of bridging heteroatoms. 3 The enforced co-planarity reduces rotational disorder thereby lowering reorganization energy and potentially increasing charge carrier mobility. 4 The bridging atoms also serve as a point of attachment for the necessary solubilizing groups needed to ensure processable materials.Within the class of donor-acceptor ladder polymers, bridged bithiophenes have proven to be a particularly useful building block. For example donor-acceptor type copolymers of cyclopentadithiophene (a C bridge) with 2,1,3-benzothiadiazole have exhibited FET mobilities up to 3.3 cm 2 V -1 s -1 when substituted with long hexadecyl sidechains. 5 The incorporation of bulky 2-ethylhexyl sidechains affords a more amorphous polymer, which nevertheless showing promising BHJ efficiencies of 5.5% when processed from solutions with high boiling additives. 6 Changing the bridging heteroatom from C to Si (dithienosilole) or Ge (dithienogermole) for analogous benzothiadiazole co-polymers enhances crystallinity, leading to improved charge transport and a reduction in bimolecular recombination. 7 The improvement in crystallinity has been rationalised on the basis of the longer C-Si/Ge bond compared to the C-C bond, which alters the geometry of the fused heterocycle facilitating enhanced intramolecular interactions. 8 In addition the replacement of the C bridge with Si or Ge alters the electronic energy levels of the resultant polymers, generally resulting in a lowering of both the HOMO and LUMO. This has been rationalized by interaction σ* orbital of the silylene/germylene fragment with the π* orbital of the aromatic system. Based upon the promising performance of these bridged dithiophene monomers, we were interested to further extend the conjugation length of the monomer and improve its coplanarity by the incorporation of fused thieno[3,2-]thiophene (TT) instead of thiophene. 10 Thieno[3,2-b]thiophene has been widely utilized as a co-monomer in a variety of high performing polymers, where it has been shown to promote intrachain packing and improve charge carrier mobility. 11 In add...

The choice of electrode for organic photovoltaics is known to be of importance to both device stability and performance, especially regarding the open-circuit voltage (V OC ). Here we show that the work function of a nickel oxide anode, varied using an O 2 plasma treatment, has a considerable influence on the opencircuit voltage V OC of an organic solar cell. We probe recombination in the devices using transient photovoltage and charge extraction to determine the lifetime as a function of charge-carrier concentration and compare the experimental results with numerical drift-diffusion simulations. This combination of experiment and simulations allows us to conclude that the variations in V OC are due to a change in surface recombination, localized at the NiO anode, although only a small change in carrier lifetime is observed.

We explore the interrelation between density of states, recombination kinetics, and device performance in efficient poly[4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl‐alt‐4‐(2‐ethylhexyloxy‐1‐one)thieno[3,4‐b]thiophene‐2,6‐diyl]:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PBDTTT‐C:PC71BM) bulk‐heterojunction organic solar cells. We modulate the active‐layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell efficiency (50–67 wt% PC71BM). Using transient and steady‐state techniques, we find that nongeminate recombination limits the device efficiency and, moreover, that increasing the PC71BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a significant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents significant progress in understanding what limits these high‐efficiency polymer:fullerene systems.