Molecular Origin of Photovoltaic Performance in Donor-block-Acceptor All-Conjugated Block Copolymers

Abstract

All-conjugated block copolymers may be an effective route to self-assembled photovoltaic devices, but we lack basic information on the relationship between molecular characteristics and photovoltaic performance. Here, we synthesize a library of poly(3-hexylthiophene) (P3HT) block poly((9,9-dialkylfluorene)-2,7-diyl-alt-[4,7-bis(alkylthiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (PFTBT) donorblock-acceptor all-conjugated block copolymers and carry out a comprehensive study of processing conditions, crystallinity, domain sizes, and side-chain structure on photovoltaic device performance. We find that all block copolymers studied exhibit an out-of-plane crystal orientation after deposition, and on thermal annealing at high temperatures the crystal orientation flips to an in-plane orientation. By varying processing conditions on polymer photovoltaic devices, we show that the crystal orientation has only a modest effect (15−20%) on photovoltaic performance. The addition of side chains to the PFTBT block is found to decrease photovoltaic power conversion efficiencies by at least an order of magnitude. Through grazing-incidence X-ray measurements we find that the addition of side chains to the PFTBT acceptor block results in weak segregation and small (<10 nm) block copolymer self-assembled donor and acceptor domains. This work is the most comprehensive to date on all-conjugated block copolymer systems and suggests that photovoltaic performance of block copolymers depends strongly on the miscibility of donor and acceptor blocks, which impacts donor and acceptor domain sizes and purity. Strategies for improving the device performance of block copolymer photovoltaics should seek to increase segregation between donor and acceptor polymer domains.

The synthesis and purification of amphiphilic conjugated donor–acceptor block copolymers

“…However, in a subsequent investigation of 30 closely related P3HT-b-PFTBT BCP derivatives, little or no phase separation was obtained regardless of processing conditions. 23 This supports previous work carried out on P3HT-b-PFT6BT (Figure 2a, R = C 6 H 13 ), in which no phase separation was observed. 24 The researchers in both projects attributed the lack of phase separation to the incorporation of solubilizing alkyl side chains on the PFTBT acceptor materials, and hypothesized that additional alkyl chains led to an increased miscibility of the donor and acceptor.…”

“…In contrast, the addition of solubilizing hexyl side chains to the TBT unit in P3HT-b-PF TEG T6BT increases the monomer's solubility and markedly reduces the amount of triblock copolymer produced. Although the percentages of BCP present in the crude polymer mixtures are low (21 and 32%), they are not unreasonable when compared with previous reports for this type of BCP synthesis 23,35 and emphasize the value of a purification technique capable of isolating the BCP product.…”

“…A recent report found that out of a synthesis of 30 BCPs, the percentage of diblock copolymer in the reaction mixtures ranged from 33-64%. 23 In the amphiphilic BCP system under investigation, however, the differing solubilities of the homopolymers can be exploited to facilitate purification.…”

Role of Solution Structure in Self-Assembly of Conjugated Block Copolymer Thin Films

“…While the spectral signatures for each polymer are similar at the carbon edge, the do provide scattering contrast near the π* resonances from the conjugated backbones and carbonyl group in the DPP block ( Figure S9). 46 Moreover, because only the DPPT-T polymer has a nitrogen atom in the monomer, there is additionally spectroscopic contrast near the nitrogen edge. 45 Resonant soft X-ray scattering (RSoXS) was conducted, in a transmission geometry through the P3HT-b-DPPT-T thin film, for the sample annealed at 225 °C and slowly cooled.…”

“…This goal has been met by BCPs where either one block has a conjugated backbone and the other has an insulating backbone with pendant conjugated molecules, [29][30][31][32][33][34][35][36][37] or by structures where both blocks have conjugated backbones (CBCPs). [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] Because of the relative stiffness of conjugated backbones, the phase separation of their BCP is complex due to energetic interactions between blocks, the volume fraction of the blocks, and potential crystallization.…”