Unraveling the Morphology of High Efficiency Polymer Solar Cells Based on the Donor Polymer PBDTTT‐EFT

Abstract

advantages, such as solution processability, low cost, and fl exibility. The invention of the bulk heterojunction (BHJ) architecture is regarded as a milestone in the history of OPV. [ 3,4 ] The BHJ architecture consists of nanoscale interpenetrating networks of donor and acceptor phases with phase separation close to the exciton diffusion length (≈10 nm). [ 5 ] The interfacial area between donor and acceptor materials is increased by several orders of magnitude compared to the bilayer architecture resulting in effi cient exciton separation, while also allowing devices thick enough to absorb a sizeable fraction of the solar spectrum. Based on this concept, OPV devices have already achieved effi ciencies of over 9% for single-junction cells. [ 6 ] The development of new polymers has been one of the most critical factors responsible for the realization of high efficiency organic solar cells. The effi ciency of polymer solar cells is related to the shortcircuit current density ( J sc ), open-circuit voltage ( V oc ), and fi ll factor (FF). The basic goal when designing a new polymer is to improve J sc and V oc simultaneously. Ideally, a polymer with low band gap and so a broader absorption spectrum is preferable, as this allows for conversion of more solar photons into electrons. The open-circuit voltage is related to the energy difference between the highest occupied molecular orbital (HOMO) level of the electron donor material and the lowest unoccupied molecular orbital (LUMO) level of the electron acceptor material, [7][8][9][10] depending more directly on the energy of the CT state. [ 11 ] Using a low bandgap donor polymer with LUMO level close to the LUMO of the acceptor is a promising way to improve both J sc and V oc . At the same time, it is necessary to maintain a minimum energy offset between the LUMO level of the donor polymer and the LUMO level of the fullerene acceptor in order to provide a suffi cient driving force for charge separation. [ 12 ] Among the various electron donor polymers employed to date, polymers based on benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,4-b]thiophene (TT) units represent one of the most effi cient classes of electron donor polymers and have shown excellent device performance. [13][14][15][16] In order to tune the band gap and energy levels, much effort has been devoted to modifying the functional groups of the BDT and TT units. [ 14,15,[17][18][19] The modifi cation of side groups in particular has a signifi cant impact on electrochemical and optoelectronic properties. Through the addition of high electron affi nityThe microstructure of the polymer PBDTTT-EFT and blends with the fullerene derivative PC 71 BM that achieve solar conversion effi ciencies of over 9% is comprehensively investigated. A combination of synchrotron techniques are employed including surface-sensitive near-edge X-ray absorption fi ne structure (NEXAFS) spectroscopy and bulk-sensitive grazing-incidence wide angle X-ray scattering (GIWAXS). A preferential "face-on" orientation of PBDTTT-EFT is obse...

High efficiency arrays of polymer solar cells fabricated by spray‐coating in air

“…It has been shown that the surface of a PBDTTT-EFT:PC 71 BM blend film is slightly enriched by the polymer component (containing around 60% of PBDTTT-EFT), and that PC 71 BM aggregation is suppressed due to the presence of DIO during solution casting. [15] For this reason we conclude that the surface structure observed here is unlikely to originate from coarse-scale phase-separation between the polymer and the fullerene.…”

“…[7,8] The design and synthesis of new conjugated polymers has been one of the main drivers in the improvement of OPV efficiency, with polymers based on benzodithiophene (BDT) attracting significant research interest. [9][10][11][12][13][14] Recently, a new polymer,poly [4,8- [15,16]. Notably, such PBDTTT-EFT based devices have been fabricated using spin-coating in a nitrogen-filled glove box.…”

“…It can be seen that devices in which both HTL and active layers were deposited by spin-coating in the glove-box had a maximum PCE of 9.35%; a value consistent with other comparable devices reported in the literature. [15,16] By recording dark J-V measurements ( Figure S1), we determine the electron and hole mobility from control devices in which transport is either - It is apparent that the peak (average) PCE of devices with both HTL and active layer fabricated by spray casting is slightly reduced to 8.06% (7.50%) comparing to devices in which the HTL is deposited by spin casting. We attribute this reduction to the increased conductivity of spray cast PEDOT:PSS layer in which the organic solvent IPA and additive -9 -ethylene glycol (EG) were used to assist the film formation process.…”

Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well‐Known PBDTTT Polymers

“…[ 57,58 ] For instance, Xiao et al [ 55 ] used a comprehensive set of characterization techniques including small-angle neutron/X-ray scattering and neutron refl ectometry to reveal the impact of DIO on the PBDTTT-C-T:PC 71 BM morphology from the cast solution to the solid fi lm; they found that DIO is critical for enhancing wileyonlinelibrary.com crystallinity and strongly improving both lateral and vertical phase segregation in the fi lm solidifi cation stage (see Figure 5 ). Our group and collaborators demonstrated that DIO is helpful to optimize surface composition distributions of PBDTTT-C-T:PC 71 BM fi lm close to electrodes, enabling a fullerene-rich composition near the cathode interface that enhances charge extraction of inverted OPVs.…”

Enhancing the Optoelectronic Performance of Perovskite Solar Cells via a Textured CH3NH3PbI3 Morphology

“…GIWAXS measurements were collected at the SAXS/WAXS beamline at the Australian Synchrotron, with a setup similar to our previously reported studies. [ 32,45 ] The energy of X-ray incident beam was fi xed at 14 KeV with an incident angle of 0.4°. 2D scattering patterns were collected using a Dectris Pilatus 1 M detector with a total exposure time of 3 s. The sample was moved to a new spot after each scan to avoid beam damage.…”