Improving the Performance of P3HT–Fullerene Solar Cells with Side-Chain-Functionalized Poly(thiophene) Additives: A New Paradigm for Polymer Design

Abstract

The motivation of this study is to determine if small amounts of designer additives placed at the polymer-fullerene interface in bulk heterojunction (BHJ) solar cells can influence their performance. A series of AB-alternating side-chain-functionalized poly(thiophene) analogues, P1-6, are designed to selectively localize at the interface between regioregular poly(3-hexylthiophene) (rr-P3HT) and PC(n)BM (n = 61, 71). The side chains of every other repeat unit in P1-6 contain various terminal aromatic moieties. BHJ solar cells containing ternary mixtures of rr-P3HT, PC(n)BM, and varying weight ratios of additives P1-6 are fabricated and studied. At low loadings, the presence of P1-6 consistently increases the short circuit current and decreases the series resistance of the corresponding devices, leading to an increase in power conversion efficiency (PCE) compared to reference P3HT/PC(61)BM cells. Higher additive loadings (>5 wt %) lead to detrimental nanoscale phase separation within the active layer blend and produce solar cells with high series resistances and low overall PCEs. Small-perturbation transient open circuit voltage decay measurements reveal that, at 0.25 wt % incorporation, additives P1-6 increase charge carrier lifetimes in P3HT/PC(61)BM solar cells. Pentafluorophenoxy-containing polymer P6 is the most effective side-chain-functionalized additive and yields a 28% increase in PCE when incorporated into a 75 nm thick rr-P3HT/PC(61)BM BHJ at a 0.25 wt % loading. Moreover, devices with 220 nm thick BHJs containing 0.25 wt % P6 display PCE values of up to 5.3% (30% PCE increase over a control device lacking P6). We propose that additives P1-6 selectively localize at the interface between rr-P3HT and PC(n)BM phases and that aromatic moieties at side-chain termini introduce a dipole at the polymer-fullerene interface, which decreases the rate of bimolecular recombination and, therefore, improves charge collection across the active layer.

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

“…Lobez et al presented a new class of additives based on regioregular poly(thiophene)s [169], where the side chains of this polymer have been functionalized at the termini in every other thiophene unit, leading to the enhancement in J SC from 9.32 to 12.2 mA/cm 2 . 4-Bromoanisole (BrAni) is another effective additive for P3HT blends [170].…”

Metal-Polymer Nanocomposites: (Co-)Evaporation/(Co)Sputtering Approaches and Electrical Properties

“…The key limitation of these devices for practical application is, yet, poor efficiency [139][140][141]. A promising methodology to overcome this drawback consists in the organic/c-Si heterojunction cell fabricated by a specific polymer and n-type Si connection.…”

Influence of Physical Load on the Stability of Organic Solar Cells with Polymer : Fullerene Bulk Heterojunction Nanolayers

“…A thermal annealing method has been proposed for crystalline polymerbased BHJ systems [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] , while an additive method is currently used for less-crystalline (amorphous) polymer-based BHJ layers [37][38][39][40][41][42][43][44][45][46][47] . However, it is doubtable that such an optimized BHJ layer (system) can withstand external physical loads such as pressing, hitting, rubbing and the like when it comes to the fullerene nano-domains dispersed in the polymer:fullerene mixture structures.…”