Article pubs.acs.org/Macromolecules
Comparative Study of Thermal Stability, Morphology, and Performance of All-Polymer, Fullerene−Polymer, and Ternary Blend Solar Cells Based on the Same Polymer Donor Taesu Kim,† Joonhyeong Choi,† Hyeong Jun Kim,† Wonho Lee,† and Bumjoon J. Kim*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea S Supporting Information *
ABSTRACT: We compared the thermal and morphological stability of all-polymer solar cells (all-PSCs) and fullerene-based PSCs (fullerene-PSCs) having the same polymer donor (PBDTTTPD), which provided comparable peak power conversion efficiencies (PCEs) of >6%. We observed a remarkable contrast in thermal stability dependent upon the acceptor composition in the active layer, with the performance of the fullerene-PSCs completely deteriorating after annealing for 5 h at 150 °C, whereas that of the all-PSCs remained stable even after annealing for 50 h at 150 °C. Pronounced phase separation was observed in the active layer of the fullerene-PSCs at two different length scales. In stark contrast, almost no morphological changes in the allPSCs were observed, likely due to the low diffusion kinetics of the polymer acceptors. To develop a comprehensive understanding of the role of polymer acceptor on the thermal stability of devices, the morphology of ternary blend active layers composed of PBDTTTPD:polymer acceptor:fullerene acceptor with different fullerene contents was examined while annealing at 150 °C. The ternary blends showed two extreme trends of all-PSC- and fullerene-PSC-like behavior in thermal stability depending on the PCBM content. When included in the active layer as 6%).47−49 After annealing for 5 h at 150 °C, the photovoltaic performance of PCBM-PSCs was completely annihilated, as the PCE dropped from 6.12% to 0.05%. Macrophase separation caused the thermal degradation in the PCBM-PSCs. In contrast, all- PSCs retained a PCE of 5.3% even after annealing for 2 days at 150 °C. Next, we studied ternary blend PSCs composed of PBDTTTPD, P(NDI2HD-T), and PCBM with different PCBM contents (ternary-PSCs). At low PCBM contents (50% PCBM after thermal annealing (Figure 3c). In addition, as PCBM content increased, the number density of PCBM crystals increased gradually with annealing time.63 Thus, it is apparent that the large drop in performance of the ternaryPSCs with high PCBM concentrations is associated with the D
DOI: 10.1021/acs.macromol.7b00834 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 5. RSoXS profiles of (a) PBDTTTPD:PCBM blend films (photon energy = 284.4 eV) and (b) PBDTTTPD:P(NDI2HD-T) blend films (photon energy = 285.4 eV) after annealing for 0, 6, and 12 h.
Figure 6. RSoXS profiles acquired with a photon energy at 284.4 eV of PBDTTTPD:P(NDI2HD-T):PCBM blend films with different PCBM contents (a) before and (b) after thermal annealing at 150 °C for 12 h. The inset in (a) shows the relative domain purity from the scattering profiles as a function of PCBM content.
PBDTTTPD:P(NDI2HD-T) films before and after 12 h of annealing remained constant at 2 nm, while that of the PBDTTTPD:PCBM film significantly increased from 10 to 42 nm over the same annealing time. The increased surface roughness may yield poor contact or delamination between the
BHJ active layer and the surrounding electrodes and, by extension, inefficient charge transport in PCBM-PSCs.69,70 The morphological development of PCBM-, all-, and ternaryPSC blends during thermal annealing was investigated by resonant soft X-ray scattering (RSoXS), which provides quantitative information about the degree of BHJ phase E
DOI: 10.1021/acs.macromol.7b00834 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules separation and domain purity.19,71−76 Figure 5 shows the RSoXS profiles for PBDTTTPD:PCBM (D:A = 1:1.5) and PBDTTTPD:P(NDI2HD-T) (D:A = 1.3:1) blends annealed for 0, 6, and 12 h at 150 °C. Two different q ranges, 0.001− 0.005 Å−1 and 0.005−0.03 Å−1, were scanned to observe phase separation at different length scales. Figure 5a shows the RSoXS profiles of the PBDTTTPD:PCBM blend films acquired with a resonant photon energy of 284.4 eV, which afforded the largest contrast between the components of the polymer:PCBM blends. Before annealing, RSoXS of the BHJ blend showed one broad peak at relatively high q (0.0105 Å−1), corresponding to a domain spacing (d = 2π/q) of 60 nm. While the increase of the BHJ domain size from 60 to 69 nm is not significant after annealing for 12 h, the peak intensity increased greatly after annealing, indicating increased domain purity,75 which is also evident in the TEM image in Figure 4e. These observations are consistent with the diffusion of PCBM from the polymer:PCBM intermixed domains into PCBM-rich phases during thermal annealing.1,76−80 A more noticeable change in the RSoXS data due to annealing was the appearance of a new highintensity peak at q = 0.0012 Å−1 (d = 523 nm) in the PCBMPSC film annealed for 6 h, attributed to the formation of PCBM crystallites with several hundred nanometers. After further annealing (12 h), the d spacing decreased to 273 nm (q = 0.0023 Å−1), but the peak intensity increased greatly. These features are consistent with the OM and TEM observations showing an increasing number of PCBM crystallites with annealing time, resulting in a smaller spacing between the crystallites. In stark contrast, RSoXS at 285.4 eV of the PBDTTTPD:P(NDI2HD-T) film showed that the q value remained almost the same as a function of annealing time, while the intensity was slightly enhanced. These results indicated that the domains of the all-polymer blends do not evolve in size or average composition during annealing. RSoXS was also performed on the ternary-PSC systems, using a photon energy of at 284.4 eV to monitor the morphological changes, in particular of PCBM domains, and the results are presented in Figure 6. In our ternary blends, the D/A ratio was kept constant at 1.3:1 w/w while the composition of the P(NDI2HD-T) and PCBM mixture was changed from 0 to 100%. For blends containing PCBM as 50 wt %) (Figure 6b). Both the q value (q = 0.0019 Å−1, d = 340 nm) and the intensity of the peak attributed to PCBM crystallites in the ternary blend with 100% PCBM were larger than those in the ternary blend containing 70% PCBM (q = 0.0016 Å−1, d = 393 nm), indicating the presence of more PCBM crystallites with increasing PCBM content in the ternary blend. Consequently, the RSoXS investigation, taken together with OM and TEM, demonstrated the strong correlation between the morphology and thermal stability of PSC devices. 2.3. Correlation between PCBM Miscibility and Thermal Stability in Ternary Blends. To gain deeper insight into the difference between the morphological stability of the ternary blends with low and high PCBM loadings, we performed RSoXS to quantitatively analyze the blend morphology and miscibility in ternary blends with different PCBM contents. Figure 6a shows the RSoXS profiles of the
ternary blends before thermal annealing. The presence of PCBM domains was observed only for ternary blends with >50% PCBM, and both the peak intensity and d spacing of the PCBM domains increased further at higher PCBM contents. The d spacing of the ternary blends was calculated to be 25.7, 30.9, and 37.5 nm for PCBM loadings of 50, 70, and 100%, respectively. The relative domain purity was determined from the RSoXS profiles by calculating the total scattering intensity (TSI) via integration of the scattering profiles over the q-range measured (Figure S7) according to the previously reported method75 and summarized in the inset of Figure 6a. The relative domain purity of ternary blends with different PCBM contents were estimated by normalizing the TSI of each blend by that of the ternary blend with 100% PCBM. The relative domain purity remained very low (30% PCBM were further increased, indicating that the formation of G
DOI: 10.1021/acs.macromol.7b00834 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
3. CONCLUSIONS We investigated the morphological and thermal stabilities of PBDTTTPD:P(NDI2HD-T), PBDTTTPD:PCBM, and PBDTTTPD:P(NDI2HD-T):PCBM PSCs with similar initial PCE values between 6 and 7%. All-PSCs and ternary-PSCs with PCBM contents below 30% exhibited remarkable thermal stabilities, retaining PCEs of greater than 5% after annealing 50 h at 150 °C, whereas the PCEs of PCBM-PSCs and ternaryPSCs with PCBM contents above 50% rapidly decreased and deteriorated completely after 5 h of annealing. The dramatic differences in the thermal stability of ternary blend devices with different PCBM contents were explained by the mixing behavior of PCBM in the ternary blend. Below the critical loading of PCBM (30%), excess PCBM separated from the intermixed domain. Under thermal stress, the PCBM may diffuse out of the mixed regions of as-cast BHJ films and either contribute to the growth of the large PCBM clusters or nucleate small PCBM clusters within the mixed regions. As a result, the number of PCBM pathways that percolate throughout the mixed domain decreases, resulting in the deterioration of the electron transport and photovoltaic performance of the ternary PSCs. Scenarios of the morphological evolution for the PCBM-, all-, and ternary-PSC active layers are described schematically in Figure 8. The optimized BHJ blend of PBDTTTPD:PCBM undergoes phase separation upon thermal annealing at both the nano- and microscales. Diffusion of PCBM molecules leads to the formation of progressively larger PCBM crystallites, reaching sizes of a few micrometers after annealing at 150 °C. At the nanoscale, i.e.,
