Structural Characterization of a Composition Tolerant Bulk Heterojunction Blend

Abstract

Extensive internal electron donor (D)/ acceptor (A) interfaces promote charge separation, while the D and A domains lead to effi cient charge carrier transport. Also worth noting is that the components' domain size should be commensurate with the exciton diffusion length and delocalization (≈10 nm) [ 4 ] to maximize charge generation. [ 5 ] The detailed morphology of the donor and acceptor domains within the BHJ material is therefore a principal factor for effi cient charge generation in the fi lm and current collection to the external circuit.Many cases studied in the past tell us that the performance of an organic photovoltaic device fabricated from blends of D and A can depend strongly on the relative amounts of the components and therefore the D:A blend ratio is an important para meter to optimize the device efficiency. For example, bld ratios signifi cantly infl uence the power conversion effi ciency (PCE) of small molecule organic solar cell, such as the 5,5′-bis-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (DTS(PTTh 2 ) 2 )/[6,6]-phenyl C71 butyric acid methyl ester (PC 71 BM) system, [ 6 ] for which, when deposited from neat solvent one observes changes in PCE from 4.5% at 7:3, to 3.6% at 8:2 or 2.1% at 6:4. PCE values can also be acutely dependent on the blend ratio with conjugated polymer systems. [ 7,8 ] For example, PIDTDTQx/PC 71 BM system shows highest PCE at a 1:4 blend ratio, increasing or decreasing PCBM loading level reduced its PCE to a large extent. [ 7 ] Its PCE is reduced from 7.5% to 4.2% when the weight fraction of PC 71 BM drops from 80% to 66%. Park et al. showed that the connectivity of each phase is sensitive to the blend ratios of poly[N-9′′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) to PC 71 BM. The fi brillar PCDTBT nanostructure becomes clear and well-defi ned at a PC 71 BM content of 80 wt%. [ 9 ] Interestingly, it was found that the performance of the welldefi ned molecule X2 ( Scheme 1 ) with PC 61 BM was relatively insensitive, relative to changes in blend ratio. A similar compositional independence was also found in PTB1/PC 71 BM polymer system, [ 10 ] whose PCE did change by only 0.3% from 7:3 to 5:5 blend ratio. In addition, small molecule DPP(THFu) 2 / PC 71 BM system exhibits a similar compositional tolerance from 7:3 to 5:5 blend ratio. [ 11 ] Despite the obvious advantages of such BHJ systems for tolerating errors during high throughput massThe ratio of the donor and acceptor components in bulk heterojunction (BHJ) organic solar cells is a key parameter for achieving optimal power conversion effi ciency (PCE). However, it has been recently found that a few BHJ blends have compositional tolerance and achieve high performance in a wide range of donor to acceptor ratios. For instance, the X2 :PC 61 BM system, where X2 is a molecular donor of intermediate dimensions, exhibits a PCE of 6.6%. Its PCE is relatively insensitive to the blend ratio over the range from 7:3 to 4:6. The effect of blend ratio of X2 /PC 61 BM on morphol...

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

“…

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.

…”

Green‐Solvent‐Processed Molecular Solar Cells

“…Moreover, the toxicity of 2-MeTHF is lower than that of commonly used halogenated and aromatic solvents, such as chlorobenzene and chloroform (CHCl 3 ). [36] An initial set of tests demonstrated that the solubility of X2 in 2-MeTHF at room temperature is approximately 29 mg mL À1 , which proved to be sufficient for casting films of more than 300 nm thickness. [32][33][34] It seemed reasonable to examine X2 as the molecular donor (see Scheme 1 for molecular structures).…”

“…[38] Analysis of the results shown in Figure 2 provided hole mobilities of 2.1 10 À4 cm 2 V À1 s À1 (CHCl 3 ) and 5.6 10 À4 cm 2 V À1 s À1 (2-MeTHF). An examination of previous work [36] led us to focus on the fabrication of X2:PC 61 BC 8 with 60:40 and 50:50 ratios. [39] BHJ devices fabricated from either CHCl 3 or 2-MeTHF were characterized in parallel to investigate the photovoltaic properties.…”

“…An examination of previous work [36] led us to focus on the fabrication of X2:PC 61 BC 8 with 60:40 and 50:50 ratios. [36] Crystallite correlation lengths (CCL) values determined using the (100) peak at q = 0.39 À1 were found to be 11.9 nm (CHCl 3 ) and 12.4 nm (2-MeTHF). Table 1 provided in Figure 3 b.…”

Analytical transmission electron microscopy at organic interfaces

“…It is evident that these imaging conditions produce completely dissimilar contrast. The analytical TEM techniques which have been employed to enhance contrast in OPV systems include elemental mapping by EDX [47,48] or core-loss EELS [48,49], mapping variations in the plasmon peak position [42,44,45,[50][51][52][53][54] as well as single electron excitations in the low-loss region of the EEL spectrum [45]. …”