Systematically moving the alkyl-chain branching position away from the polymer backbone afforded two new thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTT-T) polymers. When used as donor materials in polymer:fullerene solar cells, efficiencies exceeding 7% were achieved without the use of processing additives. The effect of the position of the alkyl-chain branching point on the thin-film morphology was investigated using X-ray scattering techniques and the effects on the photovoltaic and charge-transport properties were also studied. For both solar cell and transistor devices, moving the branching point further from the backbone was beneficial. This is the first time that this effect has been shown to improve solar cell performance. Strong evidence is presented for changes in microstructure across the series, which is most likely the cause for the photocurrent enhancement.

Semiconducting polymers play an important role in a wide range of optical and electronic material applications. It is widely accepted that the polymer ordering impacts charge transport in such devices. However, the connection between molecular ordering and device performance is difficult to predict due to the current need for a mathematical theory of the physics that dictate charge transport in semiconducting polymers. We present an analytical and computational description of semicrystalline conjugated polymer materials that captures the impact of polymer conformation on charge transport in heterogeneous thin films. We first develop an analytical theory for the statistical behavior of a polymer chain emanating from a crystallite, predicting the average distance to the first kink that would trap a charge. This analysis is used to define the conditions where percolation would lead to efficient transport through a semicrystalline material. We then establish a model that predicts the multiscale charge transport. This model is used to identify the speed limits of charge transport at short and long time scales for varying fraction of crystallinity. This work provides a rational framework to connect molecular organization to device performance. S emiconducting polymers are currently under extensive investigation for applications in solar cells, 1 light-emitting diodes, 2 flexible electronics, 3 and biointerfacing. 4 The use of polymers offers a number of advantages over traditional silicon devices, including flexibility, inexpensive processing, and the ability to functionalize the materials for various chemical interactions. Performance has substantially improved in recent years due to new materials development and improved processing techniques. 5,6 Films that result in the highest performance typically have a complex semicrystalline morphology, 7 indicating that considerable performance improvement can be achieved through optimization of microstructural properties. To rationally design new materials, it is critical to establish a predictive model that relates the microstructure of a polymer film to its transport properties.Semiconducting polymer thin films usually have ordered phases with varying degrees of crystallinity scattered throughout an amorphous polymer matrix. Crystalline areas of the film are often composed of molecules cofacially stacked in one direction to give overlap of π-orbitals as well as side chains that organize in a perpendicular direction. 8 It is experimentally suggested that efficient transport in such films occurs via connected networks of crystallites. 9 In a previous work, we describe transport in the disordered regions of the polymer starting from a model of chain conformations. 10 Our model distinguishes between on-chain and interchain transport, in contrast to widely used Gaussian disorder models, which describe transport as hopping through a spatially and energetically disordered grid of sites. 11−13 Common features of transport in amorphous polymers such as the Poole-Frenkel electric-fi...

Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.

Here we report on the synthesis of two novel very low band gap (VLG) donor-acceptor polymers (Eg ≤ 1 eV) and an oligomer based on the thiadiazoloquinoxaline acceptor. Both polymers demonstrate decent ambipolar mobilities, with P1 showing the best performance of ∼10(-2) cm(2) V(-1) s(-1) for p- and n-type operation. These polymers are among the lowest band gap polymers (≲0.7 eV) reported, with a neutral λmax = 1476 nm (P2), which is the farthest red-shifted λmax reported to date for a soluble processable polymer. Very little has been done to characterize the electrochromic aspects of VLG polymers; interestingly, these polymers actually show a bleaching of their neutral absorptions in the near-infrared region and have an electrochromic contrast up to 30% at a switching speed of 3 s.

loss compared to the theoretical radiative limit remains a vexing problem. [5][6][7] Within this context, organic ternary heterojunctions are becoming more widely studied as a means to optimize OPV performance. [8][9][10][11][12][13][14][15] In these blends, the use of one acceptor and multiple donors or one donor and multiple acceptors has experimentally been shown to enable improvements over a two-material blend by increasing the open-circuit voltage ( V oc ) without sacrifi cing photocurrent ( J sc ). Furthermore, the use of a second absorbing material often allows a larger portion of the solar spectrum to be harvested, making up for the lessthan-ideal absorption spectra of many common polymer absorbers. [ 16 ] The J sc of the mixed composition will thus be higher than that of the individual subcells due to the expanded external quantum effi ciency (EQE) response of the ternary BHJ. Hence ternary blends may provide a means to circumvent the aforementioned tradeoff between voltage and current when optimizing the optical gap of the solar cell.It is established that the properties of charge transfer (CT) states formed at the donor-acceptor interface are central to the device physics of BHJs. [ 7,[17][18][19] CT states are molecular species where an electron is located in the acceptor phase near the interface leaving a hole behind the donor phase. The extent of delocalization of CT states is still the subject of debate in the organic photovoltaic community. [20][21][22][23] The energy of the CT state ( E CT ) is defi ned as the energy difference between the fully relaxed CT state and the ground state of the blend. This quantity is important as it has been widely observed that E CT correlates well with V oc . [ 18 ] However, the CT energy depends sensitively on the intermolecular distance and relative orientation of the molecules at the interface. [24][25][26] The energy levels of the interfacial molecules are further affected by the relative mesoscale morphology of the phases (e.g., degree of aggregation and order) [27][28][29] and by which molecules surround them (i.e., how intimately mixed the interfacial region is.) [ 30 ] By the very nature of how they form, BHJs exhibit signifi cant variability in molecular confi gurations, which in turn introduces energetic disorder in the distribution of CT states. In ternary systems, all of these effects are compounded, with the added complication that chemical and electronic interactions (i.e., the degree of miscibility and electronic wavefunction overlap) affect the properties of CT states. So far, there is little fundamental Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open-circuit voltage ( V oc ) with increasing amounts of a second donor or an acceptor. The V oc is strongly correlated with the energy of the charge transfer state in the blend, but this value depends on both local and mesoscopic orders. In this work, the behavior of V oc in the presence of a wide range of interfacial electronic states is inv...