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

Abstract

The most efficient organic solar cells produced to date are bulk heterojunction (BHJ) photovoltaic devices based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) with fullerene derivatives such as [6,6]-penyl-C 61 -butyric-acid-methyl-ester (PCBM). The need for blending the two components is based on the idea that the exciton diffusion length in polymers like P3HT is only ∼10 nm, so that the polymer and fullerene components must be mixed on this length scale to efficiently split the excitons into charge carriers. In this paper, we show that the BHJ geometry is not necessary for high efficiency, and that all-solution-processed P3HT/PCBM bilayer solar cells can be nearly as efficient as BHJ solar cells fabricated from the same materials. We demonstrate that o-dichlorobenzene (ODCB) and dichloromethane serve nicely as a pair of orthogonal solvents from which sequential layers of P3HT and PCBM, respectively, can be spin-cast. Atomic force microscopy, various optical spectroscopies, and electron microscopy all demonstrate that the act of spin-coating the PCBM overlayer does not affect the morphology of the P3HT underlayer, so that our spin-cast P3HT/PCBM bilayers have a well-defined planar interface. Our fluorescence quenching experiments find that there is still significant exciton splitting in P3HT/PCBM bilayers even when the P3HT layer is quite thick. When we fabricated photovoltaic devices from these bilayers, we obtained photovoltaic power conversion efficiencies in excess of 3.5%. Part of the reason for this high efficiency is that we were able to separately optimize the roles of each component of the bilayer; for example, we found that thermal annealing has relatively little effect on the nature of P3HT layers spin-cast from ODCB, but that it significantly increases the crystallinity and thus the mobility of electrons through PCBM. Because the carriers in bilayer devices are generated at the planar P3HT/PCBM interface, we also were able to systematically vary the distance the carriers have to travel to be extracted at the electrodes by changing the layer thicknesses without altering the bulk mobility of either component or the nature of the interfaces. We found that devices have the best fill-factors when the transit times of electrons and holes through the two layers are roughly balanced. In particular, we found that the most efficient devices are made with P3HT layers that are about four times thicker than the PCBM layers, demonstrating that it is the conduction and the extraction of electrons through the fullerene that ultimately limit the performance of both bilayer and BHJ devices based on the P3HT/ PCBM material combination. Overall, we believe that polymer-fullerene bilayers provide several advantages over BHJ devices, including reduced carrier recombination and a much better degree of control over the properties of the individual components and interfaces during device fabrication.

Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend

“…direct casting from orthogonal solvents) were unable to form a pure bilayer structure (See Supporting Information). [ 17 ] It should be noted that others have shown the fabrication of bilayer type devices with pure donor-acceptor layers using alternate methods. [ 18 ] Thus, to fabricate a pure bilayer system, it is important to use solvent free techniques which require no additional heating during the fabrication process (as discussed below).…”

On the morphology of polymer‐based photovoltaics

“…56 In fact, the device performances achieved with this postdiffusion route were as good as when the P3HT and PCBM were initially uniformly mixed. [56][57][58][59] Solvent Annealing Approach Solvent annealing is another effective method that has been used to induce phase separation in P3HT:PCBM blends. When the solvent vapor enters the mixture, the mobility of both the P3HTand PCBM increases, enabling the P3HT to crystallize and drive the segregation of the PCBM.…”

Roughening Conjugated Polymer Surface for Enhancing the Charge Collection Efficiency of Sequentially Deposited Polymer/Fullerene Photovoltaics

“…A sequential solution deposition (SqSD) of the electron donating polymer layer and the electron accepting fullerene layer can be utilized to form a polymer/fullerene heterojunction in OPV [4,5]. The SqSD process is more straightforward in facilitating device optimization compared to the BSD process.…”

“…All of these features lead to the excellent electrical properties of these materials [6,7]. Recently, many interesting reports on SqSD-processed polymer/fullerene OPVs based on P3HT and fullerene derivatives have been reported [4,5,[8][9][10][11][12][13][14][15]. However, similar to a BSD-processed P3HT/fullerene OPVs, SqSD-processed P3HT/fullerene OPVs require a thermal annealing step to obtain a satisfactory heterojunction area.…”