Accurate characterization of triple-junction Polymer Solar Cells

\u3cp\u3eTriple-junction device architectures represent a promising strategy to highly efficient organic solar cells. Accurate characterization of such devices is challenging, especially with respect to determining the external quantum efficiency (EQE) of the individual subcells. The specific light bias conditions that are commonly used to determine the EQE of a subcell of interest cause an excess of charge generation in the two other subcells. This results in the build-up of an electric field over the subcell of interest, which enhances current generation and leads to an overestimation of the EQE. A new protocol, involving optical modeling, is developed to correctly measure the EQE of triple-junction organic solar cells. Apart from correcting for the build-up electric field, the effect of light intensity is considered with the help of representative single-junction cells. The short-circuit current density (J\u3csub\u3eSC\u3c/sub\u3e) determined from integration of the EQE with the AM1.5G solar spectrum differs by up to 10% between corrected and uncorrected protocols. The results are validated by comparing the EQE experimentally measured to the EQE calculated via optical-electronic modeling, obtaining an excellent agreement.\u3c/p\u3

Dichotomous role of exciting the donor or the acceptor on charge generation in organic solar cells

\u3cp\u3eIn organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.\u3c/p\u3

The influence of siloxane side-chains on the photovoltaic performance of a conjugated polymer

\u3cp\u3eThe effect of gradually replacing the branched alkyl side chains of a diketopyrrolopyrrole (DPP) conjugated polymer by linear side chains containing branched siloxane end groups on the photovoltaic performance of blends of these polymers with a common fullerene acceptor is investigated. With an increasing proportion of siloxane side chains, the molecular weight and solubility of the polymers decreases. While the siloxane containing polymers exhibit a higher hole mobility in field-effect transistors, their performance in solar cells is less than the polymer with only alkyl sides chains. Using grazing-incidence wide-angle X-ray scattering, transmission electron microscopy, and fluorescence spectroscopy we identify two main reasons for the reduced performance of siloxane containing polymers in solar cells. The first one is a somewhat coarser phase-separated morphology with slightly wider polymer fibers. This is unexpected as often the fiber width is inversely correlated with polymer solubility. The second one is stronger non-radiative decay of the pristine polymers containing siloxane side chains.\u3c/p\u3

The effects of cross conjugation on the optical absorption and frontier orbital levels of donor-acceptor polymers

The influence of cross-conjugation on the optical and electrochemical properties of donor–acceptor copolymers is investigated. Isoindigo, substituted at the 6,6' or the 5,5' positions, and thieno[3,2-b]thiophene and thieno[2,3-b]thiophene are taken as conjugated and cross-conjugated electron-deficient and electron-rich building blocks from which four isomeric donor–acceptor polymers were synthesized. Introducing cross-conjugation in isoindigo has only a small effect on the electrochemical band gap and on the onset of the absorption, which remains in the near-infrared. The optical absorption spectra, however, differ strongly because cross-conjugation strongly reduces the absorption coefficient. DFT calculations confirm that the transition to the lowest excited singlet state has a small oscillator strength in cross-conjugated isoindigo model compounds. Cross-conjugation in thienothiophene exerts a different effect. It causes a moderate but distinct blue-shift of the optical absorption and a deeper HOMO energy level

2-Methoxyethanol as a new solvent for processing methylammonium lead halide perovskite solar cells

\u3cp\u3eMethylammonium lead halide perovskites used in photovoltaic devices are generally deposited from high boiling point solvents with low volatility such as N,N-dimethylformamide. The slow drying causes the formation of relatively large perovskite crystallites that enhance surface roughness and lead to pin holes between the crystallites. We show that the use of 2-methoxyethanol, which is a more volatile solvent, results in smaller crystals that still span the entire layer thickness. This improves the surface coverage of perovskite films, reduces the leakage current and increases the open-circuit voltage and fill factor of solar cells. P-I-N configuration solar cells, processed under ambient conditions from a triple anion (iodide, chloride, and acetate) lead precursor salt, provide an increase in the power conversion efficiency from 14.1% to 15.3% when N,N-dimethylformamide is replaced by 2-methoxyethanol as the solvent.\u3c/p\u3

Efficient small bandgap polymer solar cells with high fill factors for 300 nm thick films

A high-molecular-weight conjugated polymer based on alternating electron-rich and electron-deficient fused ring systems provides efficient polymer solar cells when blended with C60 and C70 fullerene derivatives. The morphology of the new polymer/fullerene blend reduces bimolecular recombination and allows reaching high fill factors and power conversion efficiencies for films up to 300 nm thickness

Monitoring thermal annealing of Perovskite solar cells with in situ photoluminescence

Layer deposition of organometal halide perovskites for solar cells usually involves tedious experimentation, in which numerous devices are tested to determine the ideal processing conditions. One of the important issues is determining the optimum time and temperature for thermal annealing of the perovskite layer. Here we demonstrate that in-situ photoluminescence allows to determine the optimal annealing procedure without fabricating complete solar cells. We use a deposition method in which dense layers of perovskite crystals are formed within seconds in ambient air by hot casting a mixture of lead acetate, lead chloride, and methylammonium iodide. The as-cast perovskite layers are highly luminescent because charge carriers are unable to reach the charge extraction layers that quench the photoluminescence. Thermal annealing enhances charge transport and quenches the photoluminescence, but deteriorates the photovoltaic performance via decomposition of the perovskite if applied for a too long time. We demonstrate that the optimal annealing time coincides with the time required for the in-situ measured photoluminescence intensity to reach its baseline value for annealing temperatures in the range of 80-100 °C. This results in efficient (>14%) perovskite solar cells and shows that in-situ photoluminescence is a simple but powerful tool for in-line quality monitoring of perovskite films

Homocoupling defects in diketopyrrolopyrrole-based copolymers and their effect on photovoltaic performance

We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects. © 2014 American Chemical Society

All-solution-processed organic solar cells with conventional architecture

All-solution processed organic solar cells with a conventional device structure were demonstrated. The evaporated low work function LiF/Al electrode was replaced by a printed high work function silver electrode combined with an additional electron transport layer (ETL). Two electron transport layers were tested: (I) zinc oxide (ZnO) nanoparticles and (II) poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN). Devices with printed silver nanoparticle inks on top of the ZnO electron transport layer lead to crack formation in the silver layer during the drying and sintering. The crack formation was avoided by using PFN as electron transport layer. The sputtered high work function ITO electrode was substituted by a printed composite electrode containing inkjet-printed silver grids in combination with high conductivity poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). All-solution processed solar cells demonstrated a power conversion efficiency of 1.94%

Efficient thick-film polymer solar cells with enhanced fill factors via increased fullerene loading

\u3cp\u3eDeveloping effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.\u3c/p\u3