Impact of charge transport on current voltage characteristics and power conversion efficiency of organic solar cells

This work elucidates the impact of charge transport on the photovoltaic properties of organic solar cells. Here we show that the analysis of current-voltage curves of organic solar cells under illumination with the Shockley equation results in values for ideality factor, photo-current and parallel resistance, which lack physical meaning. Drift-diffusion simulations for a wide range of charge-carrier mobilities and illumination intensities reveal significant carrier accumulation caused by poor transport properties, which is not included in the Shockley equation. As a consequence, the separation of the quasi Fermi levels in the organic photoactive layer (internal voltage) differs substantially from the external voltage for almost all conditions. We present a new analytical model, which considers carrier transport explicitly. The model shows excellent agreement with full drift-diffusion simulations over a wide range of mobilities and illumination intensities, making it suitable for realistic efficiency predictions for organic solar cells

Linguistic Studies in Latvia

This issue of Humanities and Social Sciences: Latvia introduces a small part of research carried out in the Faculty of Humanities of the University of Latvia

Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism

This article reviews the application of the non-equilibrium Green's function formalism to the simulation of novel photovoltaic devices utilizing quantum confinement effects in low dimensional absorber structures. It covers well-known aspects of the fundamental NEGF theory for a system of interacting electrons, photons and phonons with relevance for the simulation of optoelectronic devices and introduces at the same time new approaches to the theoretical description of the elementary processes of photovoltaic device operation, such as photogeneration via coherent excitonic absorption, phonon-mediated indirect optical transitions or non-radiative recombination via defect states. While the description of the theoretical framework is kept as general as possible, two specific prototypical quantum photovoltaic devices, a single quantum well photodiode and a silicon-oxide based superlattice absorber, are used to illustrated the kind of unique insight that numerical simulations based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape

Male factors determining the outcome of intracytoplasmic sperm injection with epididymal and testicular spermatozoa

Summary. During a period of 8 years, 1079 intracytoplasmic sperm injection (ICSI) procedures with aspirated epididymal or testicular spermatozoa were performed. Epididymal spermatozoa were used in 172 cycles and testicular spermatozoa or spermatids in 907 cycles. Multiple biopsies were obtained from at least two different locations in the testes. Retrieved spermatozoa were used after cryopreservation (frozen) or immediately after aspiration (fresh). Three hundred patients had obstructive azoospermia (OA) or ejaculation failure. In 414 cases, azoospermia was caused by impaired spermatogenesis resulting from maldescended testes, chemotherapy/radiotherapy, or by Sertoli-cellonly syndrome, genetic disorders or unknown aetiology. Transfer rates, pregnancy rates and birth rates per ICSI cycle showed no statistically significant differences between testicular and epididymal spermatozoa in men with OA (28% average birth rates in both cases). However, birth rates differed significantly with regard to the status of spermatogenesis. Treatment of men with nonobstructive azoospermia (NOA) resulted in a birth rate of 19% per cycle. In all patient groups, there was no difference in the birth rates achieved with fresh and cryopreserved spermatozoa. While testicular volume, follicle-stimulating hormone level and age of the male patient are no statistically significant prognostic factors, the underlying cause of azoospermia is the most important factor determining the outcome of ICSI with epididymal and testicular spermatozoa. The pregnancy rate is lower in NOA patients than in those with OA

Perovskite Solar Cells with Carbon-Based Electrodes – Quantification of Losses and Strategies to Overcome Them

Funder: UNIQUEFunder: National University of Ireland Travelling StudentshipFunder: Engineering and Physical Sciences Research Council; Id: http://dx.doi.org/10.13039/501100000266Funder: Cambridge Trust ScholarshipFunder: Robert Gardiner ScholarshipCarbon-based electrodes represent a promising approach to improve stability and up-scalability of perovskite photovoltaics. The temperature at which these contacts are processed defines the absorber grain size of the perovskite solar cell: in cells with low-temperature carbon-based electrodes (L-CPSCs), layer-by-layer deposition is possible, allowing perovskite crystals to be large (>100 nm), while in cells with high-temperature carbon-based contacts (H-CPSCs), crystals are constrained to 10-20 nm size. To enhance the power conversion efficiency of these devices, the main loss mechanisms were identified for both systems. Measurements of charge carrier lifetime, quasi-Fermi level splitting (QFLS) and light-intensity-dependent behavior, supported by numerical simulations, clearly demonstrate that H-CPSCs strongly suffer from non-radiative losses in the perovskite absorber, primarily due to numerous grain boundaries. In contrast, large crystals of L-CPSCs provide long carrier lifetime (1.8 µs) and exceptionally high QFLS of 1.21 eV for an absorber bandgap of 1.6 eV. These favorable characteristics explain the remarkable open-circuit voltage (VOC) of over 1.1 V in hole-selective layer-free L-CPSCs. However, the low photon absorption and poor charge transport in these cells limit their potential. Finally, effective strategies are provided to reduce non-radiative losses in H-CPSCs, transport losses in L-CPSCs and to improve photon management in both cell types.This work has been partially funded within the projects PROPER financed from the German Ministry of Education and Research under funding number 01DR19007 and UNIQUE supported under umbrella of SOLAR-ERA.NET_cofund by ANR, PtJ, MIUR, MINECO-AEI and SWEA, within the EU's HORIZON 2020 Research and Innovation Program (cofund ERA-NET Action No. 691664). D. B. acknowledges the scholarship support of the German Federal Environmental Foundation (DBU) and S. Z. acknowledges the scholarship support of the German Academic Exchange Service (DAAD). B.Y. and A.Ha. acknowledge the funding from the European Union’s Horizon 2020 research and innovation program ESPRESSO under the agreement No.: 764047. This work has also been partially funded by Swiss National Science Foundation with Project No. 200020_185041. T.D. acknowledges a National University of Ireland Travelling Studentship. K.F. acknowledges a George and Lilian Schiff Studentship, Winton Studentship, the Engineering and Physical Sciences Research Council (EPSRC) studentship, Cambridge Trust Scholarship, and Robert Gardiner Scholarship. S.S. acknowledges support from the Royal Society and Tata Group (UF150033). M.A. acknowledges funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No.841386. The authors would like to thank Maryamsadat Heydarian and Laura Stevens for their EQE and AFM measurements. The authors thank the EPSRC (EP/R023980/1) for funding

Perovskite Solar Cells with Carbon-Based Electrodes – Quantification of Losses and Strategies to Overcome Them

Funder: UNIQUEFunder: National University of Ireland Travelling StudentshipFunder: Engineering and Physical Sciences Research Council; Id: http://dx.doi.org/10.13039/501100000266Funder: Cambridge Trust ScholarshipFunder: Robert Gardiner ScholarshipCarbon-based electrodes represent a promising approach to improve stability and up-scalability of perovskite photovoltaics. The temperature at which these contacts are processed defines the absorber grain size of the perovskite solar cell: in cells with low-temperature carbon-based electrodes (L-CPSCs), layer-by-layer deposition is possible, allowing perovskite crystals to be large (>100 nm), while in cells with high-temperature carbon-based contacts (H-CPSCs), crystals are constrained to 10-20 nm size. To enhance the power conversion efficiency of these devices, the main loss mechanisms were identified for both systems. Measurements of charge carrier lifetime, quasi-Fermi level splitting (QFLS) and light-intensity-dependent behavior, supported by numerical simulations, clearly demonstrate that H-CPSCs strongly suffer from non-radiative losses in the perovskite absorber, primarily due to numerous grain boundaries. In contrast, large crystals of L-CPSCs provide long carrier lifetime (1.8 µs) and exceptionally high QFLS of 1.21 eV for an absorber bandgap of 1.6 eV. These favorable characteristics explain the remarkable open-circuit voltage (VOC) of over 1.1 V in hole-selective layer-free L-CPSCs. However, the low photon absorption and poor charge transport in these cells limit their potential. Finally, effective strategies are provided to reduce non-radiative losses in H-CPSCs, transport losses in L-CPSCs and to improve photon management in both cell types.This work has been partially funded within the projects PROPER financed from the German Ministry of Education and Research under funding number 01DR19007 and UNIQUE supported under umbrella of SOLAR-ERA.NET_cofund by ANR, PtJ, MIUR, MINECO-AEI and SWEA, within the EU's HORIZON 2020 Research and Innovation Program (cofund ERA-NET Action No. 691664). D. B. acknowledges the scholarship support of the German Federal Environmental Foundation (DBU) and S. Z. acknowledges the scholarship support of the German Academic Exchange Service (DAAD). B.Y. and A.Ha. acknowledge the funding from the European Union’s Horizon 2020 research and innovation program ESPRESSO under the agreement No.: 764047. This work has also been partially funded by Swiss National Science Foundation with Project No. 200020_185041. T.D. acknowledges a National University of Ireland Travelling Studentship. K.F. acknowledges a George and Lilian Schiff Studentship, Winton Studentship, the Engineering and Physical Sciences Research Council (EPSRC) studentship, Cambridge Trust Scholarship, and Robert Gardiner Scholarship. S.S. acknowledges support from the Royal Society and Tata Group (UF150033). M.A. acknowledges funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No.841386. The authors would like to thank Maryamsadat Heydarian and Laura Stevens for their EQE and AFM measurements. The authors thank the EPSRC (EP/R023980/1) for funding

Roadmap on organic inorganic hybrid perovskite semiconductors and devices

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state of the art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercializatio

Modeling the influence of doping on the performance of bulk heterojunction organic solar cells: One-dimensional effective semiconductor versus two-dimensional donor/acceptor model

This work reports on two different approaches to simulate bulk-heterojunction organic solar cells (BHJ OSCs). The first is a one-dimensional (1D) description which is based on one effective semiconductor and the second is a two-dimensional model (2D) explicitly taking into account both the donor and the acceptor phase. A comparison between the two models is drawn and the influence of either p or n doping on the photovoltaic performance is investigated in detail. Slight differences in the results of the two models are explained with the underlying geometry and it is shown that many features can already be well described with the 1D effective semiconductor model. The influence of doping donor and acceptor phase independently can only be described with the 2D donor/acceptor model. The results indicate that this can be very beneficial for BHJ OSCs. The reason is that upon doping both phases the depletion zone is centered around the donor/acceptor interface and regions of high conductivity are formed in the bulk of the phases, i.e., farthest away from the donor/acceptor interface where charge carriers can recombine. This leads to a reduction of the transport resistance and thus to a significant increase in fill factor. This effect does not occur in a solar cell made of one material only. It is based on the fact that a BHJ OSC is a majority carrier device

Apparent Field-Dependence of the Charge Carrier Generation in Organic Solar Cells as a Result of (Bimolecular) Recombination

Charge carrier generation in organic solar cells is often reported to depend on the electric field. This is, however, not measured directly but derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this work, numerical simulations of TDCF experiments are presented which – when analyzed in the same way as reported in literature – result in a field‐dependence of charge carrier generation despite the fact that a field‐independent generation is used. This discrepancy is shown to be caused by recombination of photogenerated charge carriers occurring in the time range prior to and during extraction. This apparent field‐dependence becomes more pronounced for larger recombination coefficients and decreasing charge carrier mobilities, very much in accordance with experimental TDCF data from literature. Even an apparent voltage‐ and time‐dependence of the bimolecular recombination coefficient is reproduced in the simulations although a constant, voltage‐independent one is used. These findings strongly question whether TDCF is an appropriate method to detect a potential field‐dependence of the photocurrent generation and the recombination coefficient. Our study shows that all experimental results can consistently be explained without the assumption of a field‐dependence of the charge carrier generation and the bimolecular recombination coefficient