Key Parameters Requirements for Non‐Fullerene‐Based Organic Solar Cells with Power Conversion Efficiency >20%

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10−3 cm2 V−1 s−1 in combination with low nongeminate recombination rate constants of 10−12 cm3 s−1 could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics

Teknologi SMAW Untuk Kebutuhan Proses Fabrikasi Produk Bidang Pengelasan Pada Masyarakat Pedesaan di Sektor Pertanian

Abstrak   Pertanian adalah sektor utama di Desa Bader, Dusun Kayang Makmur, Kabupaten Dolopo, khususnya pada RT 16, RW 06. Alat bantu pertanian mayoritas berbahan logam yang dapat aus, korosi, dan patah. Kerusakan/patah pada alat pertanian berbahan logam, seperti cangkul, bajak, dan sabit lebih efektif dilakukan perbaikan dengan pengelasan SMAW Aplikatif. Kerusakan alat pertanian umumnya bisa diposisikan secara horizontal karena bisa dilepas dari komponen lainnya, sehingga posisi pengelasan yang mudah dan sesuai adalah 1F dan 2F. Penerapan teknologi bidang pengelasan dilakukan secara teori dan praktek dalam kegiatan Program Kemitraan Masyarakat (PKM). Metode pelatihan mengadopsi dari IPTEK yang diterapkan pada perkuliahan di Politeknik Negeri Madiun. Pengabdian untuk menghasilkan luaran peserta yang mampu memperbaiki alat pertanian berbahan logam, dan membuat produk berbahan dasar logam dengan proses pengelasan SMAW.   Kata kunci—  Dusun Kayang Makmur, Pertanian, Pengelasan Aplikatif, SMAW, Program Kemitraan Masyaraka

Using Two Compatible Donor Polymers Boosts the Efficiency of Ternary Organic Solar Cells to 17.7%

The use of ternary organic semiconducting blends is recognized as an effective strategy to boost the performance of polymer solar cells (PSCs) by increasing the photocurrent while minimizing voltage losses. Yet, the scarcity of suitable donors with a deep highest occupied molecular orbital (HOMO) level poses a challenge in extending this strategy to ternary systems based on two polymers. Here, we address this challenge by the synthesis of a new donor polymer (PM7-Si), which is akin to the well-known PM6 but has a deeper HOMO level. PM7-Si is utilized as the third component to enhance the performance of the best-in-class binary system of PM6:BTP-eC9, leading to simultaneous improvements in the efficiency (17.7%), open-circuit voltage (0.864 V), and fill factor (77.6%). These decisively enhanced features are attributed to the efficient carrier transport, improved stacking order, and morphology. Our results highlight the use of two polymer donors as a promising strategy toward high-performance ternary PSCs

Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells

In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here, we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover, sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies, since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending, however, is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends, and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs

Charge generation, transport and recombination in bulk heterojunctionsbetween poly(3-hexylthiophene) and PbS quantum dots

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.Acknowledgements I List of publications V Samenvatting VII Abstract XI List of Abbreviations XIII Contents XVII 1 Introduction 1 1.1 Motivation: Hybrid solar cells 1 1.2 Basic characterization of solar cells 3 1.2.1 Current-voltage characteristics 3 1.2.2 External quantum efficiency 4 1.2.3 Equivalent circuit model 5 1.3 Photocurrent generation in hybrid solar cells 6 1.4 Semiconductor quantum-dots 8 1.4.1 Quantum confinement 8 1.4.2 Basic physical properties of PbS QDs 10 1.5 Semiconducting organic materials 13 1.6 Absorption of light: Exciton formation 17 1.7 Charge-transfer excitons 20 1.7.1 Electron transfer 21 1.7.2 Charge transfer in polymer:QDs systems 22 1.8 Charge transport and recombination 24 1.9 Objectives of the Thesis 29 References 30 2 Surface Modification of PbS Quantum Dots 43 2.1 Introduction 43 2.2 Experimental 44 2.2.1 Synthesis and characterization of 2,2’-bithiophene-5-ylmethanethiol 44 2.2.2 Ligand exchange procedures 44 2.2.3 Spectroscopic verification of ligand exchange 45 2.2.4 Preparation of samples for TEM, TEM experiments 45 2.3 Results and Discussion 46 2.3.1 Solution-phase Ligand Exchange 46 2.3.2 Post-deposition Ligand Exchange 50 2.4 Conclusion 52 References 52 3 Enhancement of the Photovoltaic Performance in P3HT: PbS Hybrid Solar Cells Using Small Size PbS Quantum Dots 55 3.1 Introduction 55 3.2 Experimental 57 3.2.1 Materials 57 3.2.2 Solution-phase ligand exchange 57 3.2.3 Post-deposition ligand exchange 57 3.2.4 Device fabrication 57 3.2.5 Electrical characterization of devices 58 3.3 Results and Discussion 58 3.3.1 P3HT:PbS solar cells 58 3.3.2 Dark current density-voltage characteristics 61 3.3.3 Effect of the size of the PbS QDs 64 3.4 Conclusion 64 References 65 Supporting information (chapter 3) 69 4 Charge Separation Dynamics in P3HT:PbS Solar Cells 73 4.1 Introduction 73 4.2 Experimental 74 4.2.1 Materials 74 4.2.2 Preparation of P3HT and P3HT:PbS blend films for spectroscopic experiments 74 4.2.3 Steady state spectroscopy 75 4.2.4 Characterization of P3HT:PbS active layers 75 4.2.5 Picosecond time-resolved fluorescence spectroscopy 75 4.2.6 Femtosecond fluorescence up-conversion (FFU) spectroscopy 76 4.2.7 Photoinduced Absorption Spectroscopy 77 4.2.8 Device Fabrication and Electrical Characterization 77 4.3 Results and Discussion 78 4.4 Conclusions 93 References 94 Supporting Information (chapter 4) 100 5 Optical Properties, Film Morphology and Hole Transport in P3HT and P3HT:PbS Blends 111 5.1 Introduction 111 5.2 Experimental 112 5.3 Results and Discussion 114 5.3.1 PbS QDs 114 5.3.2 Optical spectra 116 5.3.3 Film morphology 121 5.3.4 Hole transport 127 5.3.5 Analysis of the hole mobility using the disorder model. 131 5.4 Conclusion 135 References 136 Supporting information (chapter 5) 142 6 Charge Transport and Recombination in P3HT:PbS Solar Cells 151 6.1 Introduction 151 6.2 Experimental 152 6.2.1 Materials. 152 6.2.2 Dielectric constant estimation. 152 6.2.3 Deposition of the active layer and ligand exchange. 153 6.2.4 Device fabrication and characterization. 153 6.3 Results and Discussion 154 6.4 Conclusions 164 References 165 Supporting Information (chapter 6) 170 7 Conclusions and Outlook 175 7.1 Conclusions 175 7.2 Outlook 177 7.2.1 Device fabrication optimization 177 7.2.2 Hole transfer 177 7.2.3 New conjugated polymers and different size QDs 179 7.2.4 Surface Modification 179 References 180 Safety Aspects 183 Laser safety 183 General risks for laser spectroscopy 183 General guidelines to help prevent accidents 183 Chemicals 184nrpages: 186status: publishe

Photoinduced Charge Transfer in Hybrid Systems of CuInS2 Nanocrystals and Conductive Polymer

© 2015 American Chemical Society. Colloidal CuInS2 nanocrystals are a promising alternative to toxic cadmium or lead chalcogenide nanocrystals that are widely studied as absorbing material in hybrid solar cells. Photovoltaic devices with colloidal CuInS2 nanoparticles suffer, however, still from low performance. The present study focuses on a detailed investigation of charge transfer as an elemental process involved in the energy conversion process. Therefore, the excited state properties and the process of charge transfer in CuInS2 (CIS) nanocrystal/polymer composites were studied by applying quasi-steady-state photoinduced absorption (PIA) and steady-state photoluminescence (PL) as well as time-resolved photoluminescence (PL) spectroscopy. The excited state dynamics of our systems was studied using time-correlated single photon counting. We examined two different composites, namely, CuInS2 nanocrystals combined with either poly(3-hexylthiophene) (P3HT) or poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Optical absorption and emission spectra of these hybrid material systems exhibit luminescence quenching and polaronic photoinduced absorption indicating photoinduced charge transfer. By systematic variations of the composition of the films, the material ratios favoring efficient charge transfer were determined.status: publishe

Charge transport and recombination in P3HT:PbS solar cells

© 2015 AIP Publishing LLC. The charge carrier transport in thin film hybrid solar cells is analyzed and correlated with device performance and the mechanisms responsible for recombination loss. The hybrid bulk heterojunction consisted of a blend of poly(3-hexylthiophene) (P3HT) and small size (2.4 nm) PbS quantum dots (QDs). The charge transport in the P3HT:PbS blends was determined by measuring the space-charge limited current in hole-only and electron-only devices. When the loading of PbS QDs exceeds the percolation threshold, a significant increase of the electron mobility is observed in the blend with PbS QDs. The hole mobility, on the other hand, only slightly decreased upon increasing the loading of PbS QDs. We also showed 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.status: publishe

Ligand exchange and photoluminescence quenching in organic-inorganic blends poly(3-hexylthiophene) P3HT:PbS

status: publishe

Photoinduced Charge Transfer in Hybrid Systems of CuInS₂ Nanocrystals and Conductive Polymer

Colloidal CuInS₂ nanocrystals are a promising alternative to toxic cadmium or lead chalcogenide nanocrystals that are widely studied as absorbing material in hybrid solar cells. Photovoltaic devices with colloidal CuInS₂ nanoparticles suffer, however, still from low performance. The present study focuses on a detailed investigation of charge transfer as an elemental process involved in the energy conversion process. Therefore, the excited state properties and the process of charge transfer in CuInS₂ (CIS) nanocrystal/polymer composites were studied by applying quasi-steady-state photoinduced absorption (PIA) and steady-state photoluminescence (PL) as well as time-resolved photoluminescence (PL) spectroscopy. The excited state dynamics of our systems was studied using time-correlated single photon counting. We examined two different composites, namely, CuInS₂ nanocrystals combined with either poly­(3-hexylthiophene) (P3HT) or poly­[2,6-(4,4-bis-(2-ethylhexyl)-4<i>H</i>-cyclopenta­[2,1-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Optical absorption and emission spectra of these hybrid material systems exhibit luminescence quenching and polaronic photoinduced absorption indicating photoinduced charge transfer. By systematic variations of the composition of the films, the material ratios favoring efficient charge transfer were determined