Significance of Average Domain Purity and Mixed Domains on the Photovoltaic Performance of High‐Efficiency Solution‐Processed Small‐Molecule BHJ Solar Cells

Abstract

the donor-acceptor ratio, [7][8][9] the molecular weight, [10][11][12][13] side-chain engineering, [14][15][16] choice of solvent, [17][18][19] fl uorine substitution, [ 20 ] solvent mixtures, [ 21 ] additives, [ 22,23 ] solvent, [ 21,[24][25][26][27] and thermal annealing. [ 28 ] Although the power conversion effi ciency (PCE) of organic BHJ solar cells continues to increase, there has been a growing interest in quantifying the mechanisms and morphology that affect device performance. [29][30][31][32][33][34] Given that many factors infl uence the photovoltaic performance of BHJ devices via charge generation and extraction, it has often been very diffi cult to deduce explicit correlations between a change in morphology and an observed variation in device performance, let alone offer straightforward yet convincing physical causes. Hence, a deeper understanding of these processes may enable the design of materials that are more effi cient and would aid future commercialization efforts.For most BHJ systems, improved solar cell performance has been achieved by varying the fi lm processing conditions often leading to a distinctly different morphology from those seen in lower performing devices. This has led to the model and expectation that pure domains with lateral in-plane dimensions of ≈10 nm need to be matched with the exciton diffusion length alongside high electron as well as hole mobilities. [ 20,35 ] It has also been shown that in bilayer model systems with single planar heterojunctions as well as in polymer-fullerene BHJs the degree of preferential molecular orientation relative to donor-acceptor interface is strongly correlated with the shortcircuit current ( J sc ) and fi ll factor ( FF ). [36][37][38] More recently, it was reported that the miscibility of the fullerene in the donor can be considerable and the mixed component domains lead to excellent exciton quenching and charge generation. [38][39][40][41][42][43][44][45][46][47] The actual morphologies might thus comprise three phases, with relatively pure, aggregated donor and acceptor domains in addition to mixed amorphous regions, as shown in Figure 1 . It has been suggested that indeed this three phase morphology may be favorable as the electronic structure of both the donor and acceptor depends on the level of aggregation, thus providing an electronic landscape that can help to sweep charges out of the mixed domains. [ 33,[48][49][50] Recent studies on polymer:fullerene systems reveal that average domain purity of the mixed phase

Understanding Charge Transport in Molecular Blend Films in Terms of Structural Order and Connectivity of Conductive Pathways

“…[ 30 ] We make no attempt here to discern between the effects of crystallite order and size on the CCL; however, it is worth noting that paracrystalline disorder in small molecules is generally small [ 31 ] and in the case of T1:PC 71 BM, it was recently reported that the cumulative disorder in T1 crystallites is not large enough to affect the connection between the crystallite size and CCL. [ 32 ] The CCLs for the donor π-π stacking peak and alkyl stacking peaks are shown as a function of donor content in ) to highlight the change in π-π stacking distance. From this analysis, it is evident that in each system there is some degree of donor crystallinity over a wide wileyonlinelibrary.com range of donor concentrations.…”

8.0% Efficient All‐Polymer Solar Cells with High Photovoltage of 1.1 V and Internal Quantum Efficiency near Unity

“…Along with the domain sizes, integration of the scattering profiles represents the total scattering intensity (TSI), which is proportional to the square root of the average composition variations over the length-scales probed. [64,66] In general, blend films featuring two pure phases from each component would present the maximum scattering intensity. Two fully molecular-mixed components would provide no scattering.…”

“…A three-phase morphology with two pure phases from each component and an intermixed phase usually gives an intermediate scattering. [66] The PBDT-TPD:PNDI-T and PBDTS-TPD:PNDI-T blends show relative average domain purities of 1 and 0.87, respectively, where the average domain purity of the PBDT-TPD:PNDI-T blend is set to be 1 for comparison. Similarly, the PBDT-TPD:PC 71 BM and PBDTS-TPD:PC 71 BM blends show relative average domain purities of 1 and 0.88, respectively, with the average domain purity of the PBDT-TPD:PC 71 BM blend set to be 1.…”

Non-Equilibrium Charge Motion in Organic Solar Cells

“…This was, at least to some extent, accomplished in paper V, where carefully co-sublimed BHJs, based on small-molecule donor/acceptor mixtures, were investigated. Although similar data, investigating the effect of donor/acceptor mixing on charge transport is available in literature 54,97,98,[101][102][103] , previously reported studies are mainly semi-quantitative, in the sense that they do not answer the question of how pure do the neat domains have to be to facilitate efficient charge transport. The well-defined sample morphology in paper V enabled quantitative analysis.…”

“…Organic bulk heterojunction solar cells are known to have complicated multi-length scale morphologies [97][98][99][100] . However, the experimental data discussed so far (papers I to IV) were analyzed while assuming that the photoactive film can be treated as an effective medium, i.e.…”