Spectroscopic Investigations of Three-Phase Morphology Evolution in Polymer: Fullerene Solar Cell Blends

Abstract

Nanoscale morphology is critical to determining the device efficiency of bulk heterojunction organic solar cells, and the ideal structure is often described as a threephase network with one well-mixed phase for efficient charge separation and two purer phases for efficient charge transport. In order to understand such nanoscale morphology, we have performed detailed spectroscopic investigations and identified the three-phase morphology evolution in one of the classic blend systems, P3HT:PCBM. The impact of different phases on polymer molecular (chain conformational) order, blend thermal and optical properties were monitored in situ using resonant Raman, absorption and photoluminescence spectroscopy techniques. Semi-crystalline P3HT was found to accommodate up to ~25% PCBM (by weight) in its amorphous phase, with very little impact on either polymer molecular order or aggregation. Higher concentrations of PCBM resulted in a greater proportion of amorphous mixed phase and reduced polymer molecular order and aggregation. On the other hand, the formation of crystalline purer phases via phase separation was evident during in situ thermal annealing, revealing a consistent glass transition temperature (Tg) of ~50 °C in blends with up to 50 %wt PCBM. This indicates similar local chemical compositions in the amorphous mixed phase present in blends despite different overall blend ratios. A much higher Tg (80-100 °C) was observed for blends with >50 %wt PCBM, indicating a stronger impact of PCBM on P3HT molecular order and thermal properties, requiring a higher annealing temperature to ensure formation of the preferred 3-phase morphology.

Twist and Degrade—Impact of Molecular Structure on the Photostability of Nonfullerene Acceptors and Their Photovoltaic Blends

“…Figure 4b comprises the normalized Raman spectra of P3HT within the blends. [34] AFM images ( Figure S18, Supporting Information) of both blends show an increase in roughness upon annealing confirming increased P3HT and IDTBR crystallinity. Tsoi et al show that as P3HT order (i.e., backbone planarity) is reduced, there is a shift of the main CC Raman peak (≈1450 cm −1 ) to higher frequencies, and the relative intensity of the CC peak (≈1380 cm −1 ) decreases as the effective conjugation length and therefore π-electron delocalization is reduced.…”

“…[32] IDTBR is composed of an electron-rich indacenodithiophene (IDT) unit with n-octyl side chains, flanked by the electron-withdrawing units: benzothiadiazole (BT) and 3-ethylrhodanine (Figure 1c). [33] P3HT has often been used as a model system for studying organic solar cells, [34] and is still relevant for use in inexpensive large-scale modules due to it being readily available and having relatively lower cost compared to other higher-efficiency state-of-the-art donor-acceptor polymers. [33] P3HT has often been used as a model system for studying organic solar cells, [34] and is still relevant for use in inexpensive large-scale modules due to it being readily available and having relatively lower cost compared to other higher-efficiency state-of-the-art donor-acceptor polymers.…”

“…Resonant Raman spectroscopy is highly sensitive to the molecular structure and conformation of π-conjugated molecules [35,36] and has been used previously to investigate the morphology [34] and stability of several bulk heterojunction OPV blends. Resonant Raman spectroscopy is highly sensitive to the molecular structure and conformation of π-conjugated molecules [35,36] and has been used previously to investigate the morphology [34] and stability of several bulk heterojunction OPV blends.…”

Multiple Cases of Efficient Nonfullerene Ternary Organic Solar Cells Enabled by an Effective Morphology Control Method

“…As we mentioned, we use the multiphase model [35,[37][38][39][40][41][42] to describe a binary BHJ and we are not intended to describe our SMAs as totally intermixed or totally demixed as we believe most morphology contain relatively pure aggregates and molecularly intermixed domains. As we mentioned, we use the multiphase model [35,[37][38][39][40][41][42] to describe a binary BHJ and we are not intended to describe our SMAs as totally intermixed or totally demixed as we believe most morphology contain relatively pure aggregates and molecularly intermixed domains.…”

“…To maintain the simplicity of the single junction structure and in the meantime broaden the absorption range of the solar cell, ternary OSCs have been developed where two donors and one acceptor or one donor and two acceptors are blended together to form the active layer. [35][36][37][38][39][40][41][42] Apparently, the morphology formed by ternary blends would be even more challenging to evaluate. [11,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] However, one main constraint that slows down the development of ternary OSC is the difficulty in controlling the morphology of ternary blends.…”