Improving the performance of P3HT-fullerene solar cells with side-chain-functionalized poly(thiophene) additives: a new paradigm for polymer design

Publication Type:

Journal Article

Source:

ACS Nano, ACS Publications, Volume 6, Issue 4, p.3044-3056 (2012)

ISBN:

1936-0851

Keywords:

2012, 2013 and earlier

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.

Notes:

PMID: 22369316