Improving Performance of All-Polymer Solar Cells Through Backbone Engineering of Both Donors and Acceptors

All-polymer solar cells (APSCs), composed of semiconducting donor and acceptor polymers, have attracted considerable attention due to their unique advantages compared to polymer-fullerene-based devices in terms of enhanced light absorption and morphological stability. To improve the performance of APSCs, the morphology of the active layer must be optimized. By employing a random copolymerization strategy to control the regularity of the backbone of the donor polymers (PTAZ-TPDx) and acceptor polymers (PNDI-Tx) the morphology can be systematically optimized by tuning the polymer packing and crystallinity. To minimize effects of molecular weight, both donor and acceptor polymers have number-average molecular weights in narrow ranges. Experimental and coarse-grained modeling results disclose that systematic backbone engineering greatly affects the polymer crystallinity and ultimately the phase separation and morphology of the all-polymer blends. Decreasing the backbone regularity of either the donor or the acceptor polymer reduces the local crystallinity of the individual phase in blend films, affording reduced short-circuit current densities and fill factors. This two-dimensional crystallinity optimization strategy locates a PCE maximum at highest crystallinity for both donor and acceptor polymers. Overall, this study demonstrates that proper control of both donor and acceptor polymer crystallinity simultaneously is essential to optimize APSC performance

Structural design of asymmetric diketopyrrolopyrrole polymers for organic solar cells processed from a non-halogenated solvent

Diketopyrrolopyrrole (DPP) polymers possess narrow optical bandgaps and high charge carrier mobilities which make them attractive for solar cell applications. DPP polymers are generally only soluble in chlorinated solvents, which is a drawback for commercial application. Solubility in non-halogenated solvents can be achieved by reducing the translational symmetry of the chain by employing two different aromatic moieties on either side of the DPP units. Here a series of polymers in which thiophene (T) and pyridine (Py) sandwich the DPP units in the main chain is reported. These asymmetric T-DPP-Py units increase the solubility dramatically. The side chain length, nature of the co-monomer, and regioregularity of the main chain are varied to investigate their effect on the solubility in toluene, the active layer morphology and the performance of organic solar cells. We demonstrate that polymers processed from both chloroform and toluene reach very similar power conversion efficiencies and blend morphologies. In general, a small co-monomer, short side chains, a regioregular main chain, and a high molecular weight give the best performance for solar cells processed from toluene

The effect of alkyl side chain length on the formation of two semi-crystalline phases in low band gap conjugated polymers

The effect of the length of solubilizing alkyl side chains, ranging from hexyl to pentadecyl, on the formation and structure of two distinct semi-crystalline semiconductor phases, ß1and ß2, of a single conjugated polymer is investigated for a low band gap poly(diketopyrrolopyrrole-alt-quaterthiophene). Compared to ß1, the ß2phase exhibits a distinct redshifted absorption and an associated near infrared photoluminescence. The length of the alkyl side chains controls the formation of the ß1and ß2phases. Intermediate length alkyl side chains (nonyl and dodecyl) can selectively provide the ß1or ß2phase in solution and in semi-crystalline thin films, depending on the nature of the solvent used. For short side chains (hexyl) the ß2phase forms more readily while for long side chains (pentadecyl) the ß1phase is predominant. The kinetics of ß2phase formation is investigated and reveals a reduced growth rate when long alkyl side chains are present. X-ray diffraction reveals a closer p-p stacking distance for ß2than for ß1, consistent with its redshifted absorption and its higher mobility in field-effect transistors. The polymer with hexyl side chains adopts an edge-on orientation in thin films, while the longer alkyl chains induce a face-on orientation. Photovoltaic devices exhibit an additional near infrared spectral contribution to the photocurrent for the ß2phase. The study shows that the formation of the two polymorphs ß1and ß2is controlled by the alkyl side chains and the solubility that arises from them. Shorter side chains (lower solubility) favor ß2and longer side chains (higher solubility) ß1, and at intermediate lengths both phases can be formed

Polymorphism of a semi-crystalline diketopyrrolopyrrole-terthiophene polymer

Few semiconducting polymers are known that possess more than one semi-crystalline structure. Guidelines for rationalizing or creating polymorphism in these materials do not exist. Two different semi-crystalline polymorphs, β1 and β2, and an amorphous α phase have recently been identified for alternating diketopyrrolopyrrole-quaterthiophene copolymers (PDPP4T). The polymorphs differ structurally by the π–π stacking distance, and electronically by the optical bandgap and charge carrier mobility. Here we investigate the corresponding terthiophene (PDPP3T) derivatives, to study the effect of the relative orientation of adjacent DPP units on the polymorphism. In PDPP3T, the relative orientation of DPP units alternates along the chain, while in PDPP4T it is constant. We show that the two polymorphs, β1 and β2, can also be generated for a PDPP3T polymer in solution and thin film. Interestingly, compared to PDPP4T, more solvents can induce the two distinct semi-crystalline polymorphs for PDPP3T via a β1 → α → β2 polymorphic transition

Impact of π-Conjugated Linkers on the Effective Exciton Binding Energy of Diketopyrrolopyrrole-Dithienopyrrole Copolymers

The effect of the nature of the π-conjugated linker that is positioned between electron-deficient 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and electron-rich dithieno[3,2-b:2′,3′-d]pyrrole (DTP) units in alternating DPP-DTP copolymers on the optical and electrochemical band gaps and the effective exciton binding energy is investigated for six different aromatic linkers. The optical band gap is related to the electron-donating properties of DTP and the electron-withdrawing properties of DPP but likewise strongly affected by the nature of the linker and varies between 1.13 and 1.80 eV for the six different linkers. The lowest optical band gaps are found for linkers that either raise the highest occupied molecular orbital or lower the lowest unoccupied molecular orbital most, while the highest optical band gap is found for phenyl linkers that have neither strong donating nor strong accepting properties. Along with the optical band gap, the electrochemical band gap also changes, but to a lesser extent from 1.46 to 1.89 eV. The effective exciton binding energy (Eb), defined as the difference between the electrochemical and optical band gaps, decreases with an increasing band gap and reaches a minimum of 0.09 eV for the copolymer with the highest band gap, that is, with phenyl linkers. The reduction in Eb with an increasing band gap is tentatively explained by a reduced electronic interaction between the DTP and DPP units when the HOMO localizes on DTP and the LUMO localizes on DPP. Support for this explanation is found in the molar absorption coefficient of the copolymers, which shows an overall decreasing trend with decreasing Eb

Influence of Regioregularity on the Optoelectronic Properties of Conjugated Diketopyrrolopyrrole Polymers Comprising Asymmetric Monomers

Two asymmetric thiophene (T)/pyridine (Py) flanked diketopyrrolopyrrole (DPP) polymers with a regiorandom and regioregular conjugated backbone are synthesized via a Stille polycondensation to investigate the effect of regioregularity on their optoelectronic properties and photovoltaic performance in fullerene-based polymer solar cells. Surprisingly, both polymers possess very similar optical bandgap, energy levels, and photovoltaic performance. These findings, combined with a factor of 19 reactivity difference between the two end groups of the asymmetric DPP monomer, intuitively suggest the formation of regular chain segments in the random polymer. However, by modeling the random polymerization reaction with a kinetic Monte Carlo (KMC) simulation, evidence is obtained for exclusive formation of a fully random polymer structure. UV-vis-NIR absorption spectra of three extended DPP chromophores, containing the donor segments (T-T-T, Py-T-Py, and Py-T-T) present in the regiorandom polymer, confirm that regioregularity of the backbone has a negligible influence on the optical properties

Elements of RAFT navigation:RAFT 20 years later: RAFT-synthesis of uniform, sequence-defined (co)polymers

This paper focuses on one of the next directions in the evolution of reversible addition-fragmentation (RAFT) polymerization, namely, progress towards the synthesis of discrete or uniform, sequence-defined (co)polymers. Following a brief review of RAFT-single unit monomer insertion (RAFT-SUMI), we describe recent developments in the field. We point to difficulties in achieving consecutive RAFT-SUMI and report two strategies for overcoming the issue of initiator-derived by-products. We show that the selection of RAFT agent is critical in selective RAFT-SUMI of N,N-dimethylacrylamide (DMAm) into a trithiocarbonate in aqueous solution. Finally we recount on the use of photoRAFT- or PET-RAFT-SUMI in the high yield synthesis of discrete oligomers comprising two or more consecutive SUMI steps.</p

Effect of main and side chain chlorination on the photovoltaic properties of benzodithiophene-: Alt -benzotriazole polymers

In developing organic semiconductor polymers for photovoltaic applications, chlorine substitution has become an effective strategy in replacing fluorine substitution to overcome the drawbacks of low yield and high cost, commonly associated with fluorination. In general, several molecular positions are available for chlorination. To obtain a clear understanding of the impact of chlorine substitution on the intrinsic polymer properties, an investigation of structure-property relationships is necessary. Herein, four donor-acceptor type polymers with the same conjugated backbone and flexible alkyl chains, but with chlorine atoms in different positions, are employed to systematically investigate the effect of the site of chlorination on the optoelectronic properties and photovoltaic performance. Substitution of fluorine by chlorine in the backbone slightly increases open circuit voltage (Voc) and fill factor (FF) of the solar cells but causes a loss of short-circuit current density (Jsc). The introduction of chlorine in the conjugated side chains, however, significantly improves Voc, FF, and power conversion efficiency, benefiting from a lower HOMO energy level, efficient and well-balanced transport properties, and superior nanoscale morphology

Elements of RAFT navigation: RAFT 20 years later: RAFT-synthesis of uniform, sequence-defined (co)polymers

This paper focuses on one of the next directions in the evolution of reversible addition-fragmentation (RAFT) polymerization, namely, progress towards the synthesis of discrete or uniform, sequence-defined (co)polymers. Following a brief review of RAFT-single unit monomer insertion (RAFT-SUMI), we describe recent developments in the field. We point to difficulties in achieving consecutive RAFT-SUMI and report two strategies for overcoming the issue of initiator-derived by-products. We show that the selection of RAFT agent is critical in selective RAFT-SUMI of N,N-dimethylacrylamide (DMAm) into a trithiocarbonate in aqueous solution. Finally we recount on the use of photoRAFT- or PET-RAFT-SUMI in the high yield synthesis of discrete oligomers comprising two or more consecutive SUMI steps