The Phase Behavior of a Polymer-Fullerene Bulk Heterojunction System that Contains Bimolecular Crystals

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Entry by Yuhang Jin, AP225 Fall 2011

Reference

Nichole Cates Miller, Roman Gysel, Chad E. Miller, Eric Verploegen, Zach Beiley, Martin Heeney, Iain McCulloch, Zhenan Bao, Michael F. Toney, and Michael D. McGehee, Polym. Phys., 2011, 49, 499.

Keywords

conjugated polymers, differential scanning calorimetry (DSC), phase diagrams, X-ray

Introduction

Fig.1 Chemical structures of (A) pBTTT and (B) PC71BM.

Polymer-fullerene bulk heterojunction (BHJ) solar cells, which show promise as future energy source, consist of an interpenetrating network of an electron-donating conjugated polymer and an electron-accepting fullerene derivative. The phase behavior of these polymer-fullerene blends is very important to the optimization of the solar cell performance. This paper for the first time determines the phase diagram of a polymer-fullerene blend of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT) with phenyl-c71-butyric acid methyl ester (PC71BM) that forms bimolecular crystals due to fullerene intercalation between the polymer side chains. The structures of pBTTT and PC71BM are shown in Fig. 1.

Results and discussion

Fig.2 (A) Key DSC curves for the second heating of the pBTTT-PC71BM blends. (B) The pBTTT-PC71BM phase diagram. Blue diamonds: the peak positions from the DSC scans Red x's: the PC71BM crystallization peak positions . L, LC, s, and BMC stand for liquid, liquid crystal, solid, and bimolecular crystal, respectively.
Fig.3 2D GIXS patterns for pBTTT-PC71BM blends at a variety of compositions and temperatures.

Differential scanning calorimetry (DSC) was used to determine the transition temperatures of pBTTT-PC71BM blends at various compositions. Fig. 2 shows key DSC scans and the phase diagram produced. Two-dimensional grazing incidence X-ray scattering (2D GIXS) with in situ thermal annealing was used to verify the phases, as shown in Fig. 3. The phase diagram showed further agreement with measured and calculated enthalpies.

DSC of pure pBTTT shows phase transitions at 137 and 232 C, due to side-chain melting and complete polymer melting respectively (Fig. 2). The 2D GIXS pattern of the solid polymer at 25 C [Fig. 3(A)] shows a lamellar structure. When heated to 180 C, the polymer exhibits increased angular alignment out of the substrate plane [Fig. 3(B)]. Heating to 250 C causes the diffraction peaks to disappear and be replaced by a broad ring from the liquid polymer [Fig. 3(C)].

DSC of pure PC71BM shows an exothermic crystallizing peak at 204 C and a melting peak at 319 C (Fig. 2). At 25 C the 2D GIXS pattern shows only a broad ring [Fig. 3(M)], indicating that the PC71BM is amorphous. Heating the PC71BM above the crystallization temperature to 250 C causes the formation of randomly oriented large PC71BM crystallites [Fig. 3(N)].

The bimolecular crystal is included on the phase diagram as a line compound at 60 wt % PC71BM. The DSC curve for the 40:60 blend shows transitions at 225 C (the eutectic temperature) and 319 C (the PC71BM melting temperature) [Fig. 2(A)]. The bimolecular crystal has a lamellar structure with a spacing of 30 Å [Fig 3(G,H)]. At 250 C, the 40:60 pBTTT-PC71BM blend exhibits two phases: crystalline PC71BM (sharp rings) and a liquid phase (broad ring) [Fig. 3(I)].

Below the eutectic temperature, 80:20 pBTTT-PC71BM blend consists of pure pBTTT and the bimolecular crystal [Fig. 3(D,E)]. Solid pBTTT coexists with the bimolecular crystal below the side-chain melting temperature (137 C), and liquid crystalline pBTTT coexists with the bimolecular crystal between the side-chain melting temperature and the eutectic temperature (225 C). Only the liquid phase is observed at 250 C [Fig. 3(F)].

The 2D GIXS pattern of the 20:80 pBTTT-PC71BM blend shows a lamellar structure with a spacing of 30 Å (the bimolecular crystal) and an amorphous ring (PC71BM) at room temperature [Fig. 3(J)]. DSC of this blend shows an exothermic PC71BM crystallizing peak at 182 C (Fig. 2), so the PC71BM is crystalline in the 2D GIXS pattern of the blend at 180 C [Fig. 3(K)]. Heating the 20:80 blend to 250 C causes the lamellar structure to disappear while the PC71BM diffraction rings remain, indicating the coexistence of a liquid phase with crystalline PC71BM [Fig. 3(L)].

The 20:80 pBTTT-PC71BM blend is the only blend that at room temperature contains both electron and hole-conducting phases, which are needed for efficient charge extraction and good solar-cell performance. Accordingly, the optimal composition for pBTTT-PC71BM solar cells is 75–80 wt % PC71BM. The pBTTT-PC71BM phase diagram reveals that an electron conducting, pure PC71BM phase coexists with a liquid phase in blends with 50 wt % PC71BM at temperatures above the eutectic temperature (225 C). It is possible to process pure pBTTT and pure PC71BM phases rather than the thermodynamically favorable bimolecular crystal in blends with 50 wt % PC71BM by cooling the blends quickly from above the eutectic temperature. RTA can be used to suppress intercalation in pBTTT-PC71BM blends.