We describe a hybrid planar-mixed heterojunction (PM-HJ) organic photovoltaic cell based on tetraphenyldibenzoperiflanthene (DBP) and C 70 with a power conversion efficiency of up to 6.4% 6 0.3%. Optimized cells consist of a DBP:C 70 mixed layer at a volume ratio of 1:8 and a 9-nm thick C 70 cap layer. The external quantum efficiency (EQE) in the visible of the PM-HJ cell is up to 10% larger than the mixed-HJ cell that lacks a C 70 acceptor cap layer. The improvement in EQE is attributed to reduced exciton quenching at the MoO 3 anode buffer layer surface. This leads to an internal quantum efficiency >90% between the wavelengths of k ¼ 450 nm and 550 nm, suggesting efficient exciton dissociation and carrier extraction in the PM-HJ cell. The power conversion efficiency under simulated AM 1.5G, 1 sun irradiation increases from 5.7% 6 0.2% for the mixed-HJ cell to 6.4% 6 0.3% for the PM-HJ cell, with a short-current density of 12.3 6 0.3 mA/cm 2 , open circuit voltage of 0.91 6 0.01 V, and fill factor of 0.56 6 0.01. V C 2013 American Institute of Physics. [http://dx.

We study a family of functionalized squaraine (fSQ) donors for absorbing in the near-infrared (NIR) and green spectral regions. The NIR-absorbing materials are the symmetric molecules 2,4-bis[4-(N-phenyl-1-naphthylamino)-2,6-dihydroxyphenyl]squaraine (1-NPSQ), 2,4-bis[4-(N,N-diphenylamino)-2,6 dihydroxyphenyl]squaraine, and 2,4-bis[4-(N,N-dipropylamino)-2,6-dihydroxyphenyl]squaraine. The green light absorbing donors are asymmetric squaraines, namely, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine and 2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-diphenylamino]squaraine. Substitution of the arylamine groups enhances intermolecular packing, thereby increasing hole transport and the possibility of forming extended nanocrystalline junctions when annealed. Nanocrystalline solar cells based on fSQ and a C(60) acceptor have V(oc) = 1.0 V and fill factors 0.73 ± 0.01. Solar cells incorporating annealed 1-NPSQ films result in a power conversion efficiency of 5.7 ± 0.6% at 1 sun, AM1.5G illumination.

In the PM-HJ structure, there are two principal loss mechanisms leading to the low FF . One is bimolecular recombination of free charge carriers in the extensive donor-acceptor blended region of the PM-HJ structure, [ 15,16 ] whose rate is given by k BM = γ np . Here, γ is the Langevin recombination constant, and n ( p ) is the free electron (hole) density. A second signifi cant loss is due to exciton-polaron quenching in the neat C 70 layer. [ 17,18 ] In previous work, [ 19 ] electron-polaron build-up was observed at the neat acceptor/blocking layer interface that results in quenching and, therefore, a reduction of internal quantum effi ciency ( IQE ). Note that exciton-polaron quenching follows a similar relationship to bimolecular recombination, as both exciton and polaron concentrations are proportional to intensity. Both mechanisms can result in a loss in photocurrent under forward bias that increases the slope of current density-voltage ( J -V ) characteristics in the fourth quadrant, ultimately decreasing both FF and PCE .Recently, an electron conducting/exciton blocking fullerenebased mixed buffer layer placed adjacent to the cathode was shown to increase the effi ciency of bilayer OPV cells. [ 19 ] The buffer consists of a mixture of C 60 that effi ciently conducts electron-polarons and wide energy gap bathocuproine (BCP) that blocks excitons. Exciton-polaron quenching was signifi cantly reduced in bilayer cells employing this electron fi lter due to its ability to spatially separate excitons and polarons at the blocking interface. This led to a signifi cant increase in J SC , while V OC and FF remained unchanged. The PM-HJ cells additionally suffer from bimolecular recombination in the mixed photoactive layer. Using a mixed buffer results in a reduced interfacial fi eld with the active layer due to its increased conductivity compared to a neat, conventional blocking buffer layer. [ 19 ] The resulting increase in fi eld across the photosensitive region leads to more rapid charge extraction. This, in turn, leads to reduced bimolecular recombination in the cell.To disaggregate the sources of loss, we employ the BPhen:C 60 mixed buffer with only a DBP:C 70 mixed HJ as the photoactive region to determine the effects of bimolecular recombination alone. In this structure, excitons rapidly dissociate into charge carriers within the DBP:C 70 blend. [ 7 ] The exciton concentration is negligible in the mixed layer, thereby eliminating excitonpolaron quenching as a signifi cant loss mechanism.The mixed HJ cells were grown by vacuum thermal evaporation (VTE) with the structure: MoO 3 (10 nm)/DBP:C 70 (54 nm, 1:8 volume ratio)/buffer/Ag (100 nm). Two different buffer layers were employed: 8 nm-thick BPhen (control), and 10 nm-thick BPhen:C 60 mixed layer at 1:1 ratio (by volume) capped with a neat, 5 nm-thick BPhen layer. Figure 1 a,b show the J -V characteristics and EQE spectra of mixed HJ devices Small-molecule organic photovoltaic (OPV) cells have attracted interest due to their promise for use as low-cost, light...

We demonstrate the concentration dependence of C60 absorption in solid solutions of C60 and bathocuprione (BCP), revealing a nonlinear decrease of the C60 charge transfer (CT) state absorption. These blends are utilized to study the photocurrent contribution of the CT in bilayer organic photovoltaics (OPVs); 1:1 blends produce 40% less photocurrent. As exciton blocking electron transporting layers, the blends achieve power conversion efficiencies of 5.3%, an increase of 10% compared to conventional buffers.