High-efficiency Bulk-heterojunction Solar Cells Research Articles

Low-Vapor-Pressure Solvent Additives Function as Polymer Swelling Agents in Bulk Heterojunction Organic Photovoltaics

Bulk heterojunction (BHJ) photovoltaics based on blends of conjugated polymers and fullerenes require an optimized nanoscale morphology. Casting BHJ films using solvent additives such as 1,8-diiodooctane (DIO), 1,8-octanedithiol (ODT), chloronapthalene (CN), or diphenyl ether (DPE) often helps achieve this proper morphology: adding just a few volume percent of additive to the casting solution can improve polymer/fullerene mixing or phase separation, so that solvent additives have become staples in producing high-efficiency BHJ solar cells. The mechanism by which these additives improve BHJ morphology, however, is poorly understood. Here, we investigate how these additives control polymer/fullerene mixing by taking advantage of sequential processing (SqP), in which the polymer is deposited first and then the fullerene is intercalated into the polymer underlayer in a second processing step using a quasi-orthogonal solvent. In this way, SqP isolates the role of the additives’ interactions with the polymer an...

Development of non-halogenated binary solvent systems for high performance bulk-heterojunction organic solar cells

Halogenated solvents such as dichlorobenzene (DCB) are commonly used in the fabrication of bulk-heterojunction (BHJ) organic solar cells. However, most halogenated solvents are very toxic. This report provides a route to eliminate halogenated solvents based on Hansen Solubility Parameters (HSPs) for manufacturing high efficiency BHJ solar cells. The device performances fabricated from several different binary solvent systems were compared to that from DCB. For the investigation of different gel behavior of solvent mixtures, Hansen solubility parameters and viscosity were mainly considered for the selection of binary solvent mixture candidates. Upon the addition of 20vol.% benzaldehyde (BA) to p-xylene (p-XL), the device performances were improved in power conversion efficiency (PCE) from 3.1% to 3.8% and external quantum efficiency (EQE) from 63% to 70%. The solar cell devices fabricated using p-XL/BA binary solvent mixtures exhibited PCE comparable to that from DCB.

Balancing Light Absorptivity and Carrier Conductivity of Graphene Quantum Dots for High-Efficiency Bulk Heterojunction Solar Cells

Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDs--graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDs--were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%).

Recent progress of bulk heterojunction solar cells based on small-molecular donors

Organic bulk heterojunction (BHJ) solar cells based on small molecular donors have received great attentions recently, because small molecules possess the merits of high purity, well-defined molecular structures, definite molecular weights and high charge carrier mobility. The highest power conversion efficiency (PCE) of BHJ solar cells based on small molecular donors and fullerene derivative acceptors has reached 7.38%. In this review, we will briefly summarize the development of small molecular donor based BHJ solar cells in the past two years. These results suggest that small molecular donors are promising candidates for high efficiency BHJ solar cells.

Distribution of Crystalline Polymer and Fullerene Clusters in Both Horizontal and Vertical Directions of High-Efficiency Bulk Heterojunction Solar Cells

In this study, we used (i) synchrotron grazing-incidence small-/wide-angle X-ray scattering to elucidate the crystallinity of the polymer PBTC12TPD and the sizes of the clusters of the fullerenes PC61BM and ThC61BM and (ii) transmission electron microscopy/electron energy loss spectroscopy to decipher both horizontal and vertical distributions of fullerenes in PBTC12TPD/fullerene films processed with chloroform, chlorobenzene and dichlorobezene. We found that the crystallinity of the polymer and the sizes along with the distributions of the fullerene clusters were critically dependent on the solubility of the polymer in the processing solvent when the solubility of fullerenes is much higher than that of the polymer in the solvent. In particular, with chloroform (CF) as the processing solvent, the polymer and fullerene units in the PBTC12TPD/ThC61BM layer not only give rise to higher crystallinity and a more uniform and finer fullerene cluster dispersion but also formed nanometer scale interpenetrating network structures and presented a gradient in the distribution of the fullerene clusters and polymer, with a higher polymer density near the anode and a higher fullerene density near the cathode. As a result of combined contributions from the enhanced polymer crystallinity, finer and more uniform fullerene dispersion and gradient distributions, both the short current density and the fill factor for the device incorporating the CF-processed active layer increase substantially over that of the device incorporating a dichlorobenzene-processed active layer; the resulting power conversion efficiency of the device incorporating the CF-processed active layer was enhanced by 46% relative to that of the device incorporating a dichlorobenzene-processed active layer.

High performance bulk-heterojunction organic solar cells fabricated with non-halogenated solvent processing

High efficiency bulk heterojunction (BHJ) organic solar cells based upon poly(3-hexylthiophene) (P3HT)-[6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) are typically solution processed using halogenated solvents like dichlorobenzene (DCB). In this report, we tune the quality of solvent systems using more environmentally friendly, halogen-free solvents to mimic that of DCB based on Hansen solubility parameters (HSPs). A mixture of acetophenone (AP) and mesitylene (MS) can nearly match the HSPs for DCB. Solar cells fabricated using this solvent mixture can exhibit power conversion efficiency (PCE) comparable to that from DCB with a similar surface morphology. It is critical to control the drying process in the mixed solvent system due to significant volatility difference between AP and MS in order to achieve comparable morphology and PCE. This report illustrates a route to fabricate high efficiency BHJ solar cells from halogen-free solvent systems that could eliminate potential hurdles for manufacturing scale-up of organic BHJ solar cells.