Guidelines for the bandgap combinations and absorption windows for organic tandem and triple-junction solar cells

Organic solar cells have narrow absorption windows, compared to the absorption band of inorganic semiconductors. A possible way to capture a wider band of the solar spectrum-and thus increasing the power conversion efficiency-is using more solar cells with different bandgaps in a row, i.e., a multi-junction solar cell. We calculate the ideal material characteristics (bandgap combinations and absorption windows) for an organic tandem and triple-junction solar cell, as well as their acceptable range. In this way, we give guidelines to organic material designers

Asymptotic solution of a model for bilayer organic diodes and solar cells

The current voltage characteristics of an organic semiconductor diode made by placing together two materials with dissimilar electron affinities and ionisation potentials is analysed using asymptotic methods. An intricate boundary layer structure is examined. We find that there are three regimes for the total current passing through the diode. For reverse bias and moderate forward bias the dependency of the voltage on the current is similar to the behaviour of conventional inorganic semiconductor diodes predicted by the Shockley equation and are governed by recombination at the interface of the materials. There is then a narrow range of currents where the behaviour undergoes a transition. Finally for large forward bias the behaviour is different with the current being linear in voltage and is primarily controlled by drift of charges in the organic layers. The size of the interfacial recombination rate is critical in determining the small range of current where there is rapid transition between the two main regimes. The extension of the theory to organic solar cells is discussed and the analogous current voltage curves derived in the regime of interest

A polymer tandem solar cell with 10.6% power conversion efficiency.

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap <1.4 eV), poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2',3'-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] with a bandgap of 1.38 eV, high mobility, deep highest occupied molecular orbital. As a result, a single-junction device shows high external quantum efficiency of >60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm(-2), IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%

A van der Waals pn heterojunction with organic/inorganic semiconductors

van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials have attracted tremendous attention due to their excellent electrical/optical properties and device applications. However, current 2D heterojunctions are largely limited to atomic crystals, and hybrid organic/inorganic structures are rarely explored. Here, we fabricate hybrid 2D heterostructures with p-type dioctylbenzothienobenzothiophene (C8-BTBT) and n-type MoS2. We find that few-layer C8-BTBT molecular crystals can be grown on monolayer MoS2 by vdW epitaxy, with pristine interface and controllable thickness down to monolayer. The operation of the C8-BTBT/MoS2 vertical heterojunction devices is highly tunable by bias and gate voltages between three different regimes: interfacial recombination, tunneling and blocking. The pn junction shows diode-like behavior with rectifying ratio up to 105 at the room temperature. Our devices also exhibit photovoltaic responses with power conversion efficiency of 0.31% and photoresponsivity of 22mA/W. With wide material combinations, such hybrid 2D structures will offer possibilities for opto-electronic devices that are not possible from individual constituents.Comment: 16 pages, 4 figure

Photovoltaic properties of molecules with internal charge transfer

Práce je zaměřena na studium donor-akceptorových molekul s vnitřním přenosem náboje z hlediska jejich použití v organických solárních článcích. V práci bude studován vliv změny chemické struktury těchto molekul na optické a optoelektrické vlastnosti. Dále budou z těchto materiálů připraveny solární články a studováno jejich fotovoltaické chování.The work is focused on the study of donor-acceptor molecules with internal charge transfer in terms of their use in organic solar cells. The work will be studied the effect of changes in the chemical structure of these molecules on the optical and optoelectronic properties. Furthermore, based on these materials prepared solar cells and photovoltaic studied their behavior.

Device physics of organic and perovskite solar cells

We report on fundamental electronic properties of the PTB7:PCBM70 bulk heterojunction solar cells: Sub-gap quantum efficiency measurements determine the Urbach energy of tail states (33 meV) and D/A interfacial bandgap (1.34 eV). Density of deep defects is determined by capacitance spectroscopy, and is ~1016 cm-3.eV-1. By photo-current spectroscopy, we assess surface recombination velocity at D/A interface, which translates to a capture cross-section of ~10-16 cm2 for deep defects. These properties are then used in our analytical modeling. Using a multiple-level trap model, we compute recombination rates in the cells. The model can predict dark saturation currents and ideality factors, and strongly suggests that band tail recombination is the main limiting factor of open-circuit voltage. Additionally, we find that, upon light exposure, photo-induced defects lead to increased trap-assisted recombination and photo-voltage instability. To increase photo-stability, we propose a novel n-i-p hybrid device where n+/a-Si:H is used as the front contact of the bulk heterojunction. Highly energetic photons (blue and UV light), which induce defects in the light-absorbing material, are blocked (absorbed) in the n+/a-Si:H layer. This leads to significant reduction of photo-induced damage in the blend, and thereby enhances photo-stability. Some photo-current is, however, lost due to absorption in n+/a-Si:H. In order to overcome this drawback, we present a novel organic-inorganic hybrid tandem solar cell, in which blue photons are harvested by an a-(SiC):H front cell. A PTB7:PCBM70 cell is used as the back cell. Our results demonstrate a VOC of 1.67 V, JSC of 7.3 mA/cm2 and overall efficiency of 7.6%, which is among the highest reported in the literature. In the last part of this dissertation, we study the electronic properties of methyl-ammonium lead iodide perovskite solar cells. Capacitance spectroscopy shows the existence of a shallow (0.24 eV) and a deep (~0.62 eV) defect band in the bandgap. Moreover, we find that attempt-to-escape frequency is ~1011 Hz. The deeper defect band has a Gaussian distribution with a peak density of ~3×1016 cm-3.eV-1. Ideality factors and dark saturation currents indicate that band-to-band recombination is dominant at high excitation levels, and limits the photo-voltage

Exciton and Charge Dynamics at Hybrid Organic-Inorganic Semiconductor Heterojunctions

The advancements in our fundamental understanding of light-matter interaction in the past century are foundational to our technology-enabled modern lifestyle. While the physics and technology of inorganic semiconductors have been well-developed in the past 60 years, the development of organic semiconductors is in its nascent stages. Combination of the two material systems in organic-inorganic (OI) hybrid semiconductor systems have already found applications in next-generation solar cells, light-emitting diodes, and non-linear optical devices, yet the unique charge and exciton behavior at OI heterojunctions (HJs) remains largely unexplored. The stark differences in the optoelectronic properties of organic and inorganic semiconductors offer a rich and as of yet unexplored territory of charge and energy transfer processes in hybrid semiconductor systems. Expanding the physical understanding of these coupled material systems could potentially lead to major advances in semiconductor applications and science. This thesis presents the first steps toward developing a comprehensive understanding of charge and exciton dynamics in coupled hybrid OI material systems. A theory of optical and electrical behavior of OI-HJ based diodes is outlined. The theory yields a quantitative model for current density versus voltage (J-V) characteristics of OI-HJ based diodes. The existence of a hybrid charge transfer exciton (HCTE) state, composed of a columbically-bound electron in the inorganic semiconductor and hole polaron in the organic semiconductor, is predicted at the hybrid heterointerface. The HCTE is found to be the the fundamental quasi-particle that governs the excited state properties of the diode. A first principles quantum mechanical model of the HCTE is developed to predict its optoelectronic properties. The external quantum efficiency, electroluminescence, photoluminescence, and J-V characteristics for multiple OI diode systems are presented along with model fits to the data. The fits yield insights into the dominant optoelectronic processes in OI material systems, including trap-mediated charge recombination and space-charge-limited current. The ability to systematically manipulate the optoelectronic properties of the HCTE by tuning the dimensionality of electron delocalization in the inorganic semiconductor is demonstrated. Potential novel applications and future directions for exploration that emerge for hybrid material systems as a result of the findings of this thesis are also discussed.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143983/1/apanda_1.pd

Surface doping in T6/ PDI-8CN2 Heterostructures investigated by transport and photoemission measurements

In this paper, we discuss the surface doping in sexithiophene (T6) organic field-effect transistors by PDI-8CN2. We show that an accumulation heterojunction is formed at the interface between the organic semiconductors and that the consequent band bending in T6 caused by PDI-8CN2 deposition can be addressed as the cause of the surface doping in T6 transistors. Several evidences of this phenomenon have been furnished both by electrical transport and photoemission measurements, namely the increase in the conductivity, the shift of the threshold voltage and the shift of the T6 HOMO peak towards higher binding energies.Comment: 5 pages, 5 figure