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For the past two decades, intensive research has been underway to construct bulk heterojunctions hybrid solar cells using a mixture of inorganic semiconductor quantum dots (QDs) and conjugated polymers. The goal is to combine the advantages of both material classes e.g., flexibility from the polymer, larger photostability and high charge mobility from the QDs and/or the complementary spectral dependence of the absorbance of the polymer and QDs. However, despite its potential for high power conversion efficiency (PCE), the PCE values reported for these solar cells are not currently as high as anticipated. Therefore, the role of different performance limiting processes in hybrid solar cells has to be investigated. We chose to study the bulk heterojunction, and solar cells based thereon, between the bench mark conjugated polymer P3HT and PbS QDs with diameter of 2.4 nm, acting respectively as electron donating and electron accepting material. We also discusses the effect of the incorporation of PbS QDs on the optical properties, morphology and charge transport properties of P3HT. We focus on the effect of the surface modification of the QDs on the morphology, charge separation, charge transport and recombination processes in blends of poly(3-hexythiophene) (P3HT):PbS QDs forming a bulk-heterojunction and on the device performance of P3HT:PbS QDs solar cells.First we investigated the exchange of the as-synthesized capping layer (oleic acid) on PbS QDs by more suitable ligands using different approaches and different ligands, namely solution phase (using octylamine, butylamine, 2’-bithiophene-5-ylmethanethiol, octanethiol) and post-deposition ligand exchange (using acetic acid, 1,4-benzenedithiol). Besides the evaluation of the efficiency of the ligand exchange by NMR and FT-IR spectroscopy, we also studied the consequences of the surface treatment on the electronic spectroscopy of the QDs, the performance of hybrid bulk heterojunction solar cells containing P3HT and PbS QDs and the charge separation mechanism at the interface between P3HT and PbS QDs. As expected, replacing oleic acid (OLA) by shorter chain ligands improves the figures of merit of the solar cells. The best results were obtained for post-deposition ligand exchange by 1,4-benzenedithiol (BDT) which improves the PCE of solar cells to almost 1% which is two orders of magnitude higher than the values previously reported for hybrid solar cells based on a bulk heterojunction of P3HT:PbS QDs where the QDs were capped by acetic acid ligands. Dark current density-voltage (J-V) measurements carried out on the devices show that a larger leakage current and a more efficient recombination are the major factors responsible for the larger losses in the hybrid system compared to organic bulk heterojunctions of P3HT and PCBM.Charge transfer between P3HT and PbS QDs, capped with different ligands, was studied by stationary and time-resolved photoluminescence (PL) quenching and photoinduced absorption (PIA) measurements. Ligand exchange of oleic acid (OLA) by 1,4-benzenedithiol (BDT) clearly led to the most efficient charge transfer. The electron transfer from P3HT to PbS QDs takes place on a time scale from tens of femtoseconds (fs) to hundreds of picoseconds (ps) which can be related to the extensive phase separation between donor and acceptor materials. Further optimization of the photovoltaic devices with the active layer consisting of P3HT:PbS blends treated with BDT allowed us to reach a power conversion efficiency of 1.8 %. However, despite the evidence of electron transfer from P3HT to PbS QDs, it has been observed that the performance of photovoltaic devices based on a layer of neat PbS treated with BDT, i.e. without an electron donor, is higher than that of hybrid devices with P3HT:PbS blends in the active layer. Although in the former device the photocurrent will be mainly generated via direct separation of the electron-hole pair in the QD phase, which can be assisted by the band bending near the contacts, the comparison of the wavelength dependence of the internal quantum yield (IQE) of both devices suggests that the charge generation efficiency by exciton dissociation at the P3HT:PbS interface is more efficient than by direct exciton dissociation in the PbS quantum dots.Furthermore, we investigated the effect of molecular structure of the conjugated polymer P3HT and the sample preparation on the morphology and aggregation of the chromophores in P3HT in view of the preparation of bulk heterojunctions of P3HT and quantum dots of PbS. In addition, we determined the effect of the incorporation of the QDs in the P3HT matrix on the aggregation of the P3HT chains. In order to understand how these factors control the resulting optical and electronic properties of P3HT and P3HT:PbS QDs blends we evaluated the absorption and emission spectra of P3HT and P3HT:PbS QDs blends using Spano’s model while the film morphology was investigated by Atomic Force Microscopy (AFM). In addition, the field and temperature dependent hole mobilities measured by the time of flight method were analysed in the context of Bässler’s Gaussian disorder model. We found a strong correlation between the optical properties and morphological landscape with the hole transport in drop casted films as measured by the time of flight technique. Next, we discussed the charge transport in the P3HT:PbS blends by evaluation of the space-charge limited current in hole-only and electron-only devices prepared by spin coating. When the loading of PbS QDs exceeds the percolation threshold a significant increase of the electron mobility is observed in the blend of P3HTwith PbS QDs. The hole mobility, on the other hand, only slightly decreased upon increasing the loading of PbS QDs. The charge carrier transport is also correlated with device performance and the mechanisms responsible for recombination loss. We show that the photocurrent is limited by the low shunt resistance rather than by space-charge effects. The significant reduction of the fill factor at high light intensity suggests that under these conditions the non-geminate recombination dominates. However, at open-circuit conditions, the trap-assisted recombination dominates over non-geminate recombination.

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The aim of the proposed project is the study of the ultrafast processes that occur in the initial steps after photo-excitation of complex three-dimensional structures like dendrimers and ionsensitive indicators, and on this way obtain a better insight in these processes. From the knowledge of these processes a better understanding of the dynamics and kinetics of the photophysical processes that occur in these molecules will be obtained.