Room to Improve Conjugated Polymer-Based Solar Cells: Understanding How Thermal Annealing Affects the Fullerene Component of a Bulk Heterojunction Photovoltaic Device

Abstract

We examine how thermal annealing affects the fullerene network in conjugated polymer bulk heterojunction (BHJ) solar cells. We begin by creating electron-only devices with a BHJ geometry by blending the fullerene derivative [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) with polystyrene (PS). These electron-only PS:PCBM diodes function even with a poly(ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, indicating that PEDOT:PSS films do not serve as electron blocking layers. Atomic force microscopy shows that the degree of phase segregation in the PS:PCBM blend films is similar to that in the active layer of blends of PCBM with poly(3-hexylthiophene-2,5-diyl) (P3HT), so that the PS:PCBM blends provide a good model for the fullerene part of the BHJ network in P3HT:PCBM solar cells. We find that thermal annealing dramatically decreases the electron current that flows in the PS:PCBM diodes, suggesting that annealing leads to increased phase segregation that lowers the electron mobility on fullerenes in the BHJ geometry. We also find that annealing increases the photoluminesence of P3HT:PCBM blend films, indicating that thermal treatment produces increased phase segregation that leads to decreased exciton harvesting. The fact that annealing decreases both exciton harvesting and electron mobility implies that there is significant room to further improve polymer/fullerene photovoltaics by controlling the amount of phase segregation.

Reappraising the Need for Bulk Heterojunctions in Polymer−Fullerene Photovoltaics: The Role of Carrier Transport in All-Solution-Processed P3HT/PCBM Bilayer Solar Cells

“…We believe that the mobility of electrons in the PCBM layer is the limiting factor because bilayer devices with annealed PCBM layers work better than bilayer devices with as-cast PCBM layers, independently of the state of the P3HT layer ( Figure 6, Table 1). We also have argued in previous work that it is electron conduction among the fullerene component of P3HT: PCBM BHJ devices that is performance-limiting; 19 those arguments are reinforced by the data given here. In other words, the BHJ geometry forces electrons to travel a longer, more tortuous path on a PCBM network that is much less crystalline than is the case in our annealed bilayer devices.…”

“…19 This argument is also supported by the fact that improved crystallinity of the PCBM overlayer is responsible for the improvements in bilayer device performance upon thermal annealing. Thus, we believe that the greatest potential for improving the performance of polymer-based photovoltaics lies in using electron acceptors with higher charge-carrier mobilities and finding a suitable way to optimize electron extraction at the cathode.…”

“…This has made it challenging to fully understand the changes in photovoltaic performance observed upon the thermal annealing of BHJ devices because annealing simultaneously changes the mobilities of both carriers, likely in opposite directions. 19 Annealing also changes the effective carrier pathlengths and transit times, as well as the nature of any extraction barriers at the organic/electrode interfaces. By studying bilayers with a controllably fixed geometry, we have been able to isolate the effect of misbalancing the carrier transit times on device performance.…”

First-principles investigation of organic photovoltaic materialsC60, C70, [C60]PCBM, and bis-Qian1, Umari2, Marzari3 2015Phys. Rev. B

“…The power conversion efficiency of BHJ-OPV has greatly improved over the last decade from 1% to more than 9% by tuning morphology and blending ratio [12][13][14][15][16][17][18][19][20][21][22][23] , reducing interfacial power losses 24,25 , increasing the range of light absorption with tandem cell architecture [26][27][28][29][30][31] , optimizing energy levels, carrier mobility and optical absorption of low-bandgap polymers [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] and fullerenes and their derivatives 43,[49][50][51][52][53][54][55] , and improving device structures 56,57 . In the BHJ type of OPV, low-band gap polymers not only serve as electron donors, but also play severa...…”

Synthesis and photovoltaic properties of 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole-based low-bandgap polymers

“…The development of broad absorbing polymers having extended delocalized p-electrons has been crucial, because of their potential application to the fabrication of flexible large-area devices in polymer solar cells based on the bulk heterojunction (BHJ) model [1][2][3][4][5]. In BHJ solar cells, the interpenetrating network of electron donor (p-conjugated polymer) and electron acceptor (PC 60 BM or PC 70 BM) layer is used for light harvesting and charge separation.…”