Relating the Morphology of Poly(p‐phenylene vinylene)/Methanofullerene Blends to Solar‐Cell Performance

Abstract

The performance of bulk‐heterojunction solar cells based on a phase‐separated mixture of donor and acceptor materials is known to be critically dependent on the morphology of the active layer. Here we use a combination of techniques to resolve the morphology of spin cast films of poly(p‐phenylene vinylene)/methanofullerene blends in three dimensions on a nanometer scale and relate the results to the performance of the corresponding solar cells. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and depth profiling using dynamic time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) clearly show that for the two materials used in this study, 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐methanofullerene (PCBM) and poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), phase separation is not observed up to 50 wt.‐% PCBM. Nanoscale phase separation throughout the film sets in for concentrations of more than 67 wt.‐% PCBM, to give domains of rather pure PCBM in a homogenous matrix of 50:50 wt.‐% MDMO‐PPV/PCBM. Electrical characterization, under illumination and in the dark, of the corresponding photovoltaic devices revealed a strong increase of power conversion efficiency when the phase‐separated network develops, with a sharp increase of the photocurrent and fill factor between 50 and 67 wt.‐% PCBM. As the phase separation sets in, enhanced electron transport and a reduction of bimolecular charge recombination provide the conditions for improved performance. The results are interpreted in terms of a model that proposes a hierarchical build up of two cooperative interpenetrating networks at different length scales.

On the morphology of polymer‐based photovoltaics

“…24 Janssen and coworkers performed a detailed study of the morphology of MDMO-PPV:PCBM blends with varying PCBM concentration processed from chlorobenzene. 26 No obvious phase separation was observed when the concentration of PCBM was less than 50 wt %. With PCBM concentrations higher than 67 wt %, domains of PCBM began to appear.…”

Origin of size effect on efficiency of organic photovoltaics

“…P in a , where a < 1 [for dominant bimolecular recombination a ¼ 0.5 (Ref. 13) and for dominant space charge limitation a ¼ 0.75]. 11,12 The values G and a are extracted by:…”

“…To the best of our knowledge, for BHJ OPV it was studied only for C 1 sun. 13,32,33 Ultrasmall bilayer C 60 /copper phthalocyanine (CuPc) cells were characterized up to 12 suns of simulated illumination. 34 Recently, 35 we reported an experimental exploitation of concentrated sunlight for such measurements and demonstrated that the PCE of 1 cm 2 P3HT:PCBM BHJ cells peaks at $ 1 sun.…”

“…The immediate consequence is that light absorption does not necessarily lead to the generation of free carriers and photocurrent becomes voltage dependent. [10][11][12][13][14][15] Serious progress has been achieved in the improvement of photovoltaic performance of BHJ solar cells: while the best power conversion efficiency (PCE) reported 8 years ago barely reached values higher than 1%, 3 certified efficiencies beyond 6% and even 8% 16 are state of the art today (Table I).…”

Synthesis and photovoltaic properties of a series of bulk heterojunction solar cells based on interchain-linked conjugated polymers

“…Several recent efforts have focused on the synthesis of a new low-band gap conjugated polymers that improve the absorption of solar radiation and increase charge mobilities. [7][8][9][10][11][12][13][14][15][16][17][18][19][20] In addition to the intrinsic characteristics of the polymer, the amount of PCBM in the blend, 17,18 the compatibility of the polymer and PCBM, the morphology of the photoactive layer [19][20][21] and the deviceprocessing conditions all have important roles affecting the photovoltaic (PV) performance of PSCs. [8][9][10]22,23 Photo-energy conversion efficiencies (Z) of 47% have been achieved for PSCs based on blends of poly(thiophene) derivatives and fullerene derivatives.…”