Additive‐Free Organic Solar Cells with Power Conversion Efficiency over 10%

Nowadays, solvent additives are widely used in organic solar cells (OSCs) to tune the nano‐morphology of the active blend film and enhance the device performance. With their help, power conversion efficiencies (PCEs) of OSCs have recently stepped over 10%. However, residual additive in the device can induce undesirable morphological change and also accelerate photo‐oxidation degradation of the active blend film. Thereby, their involvements are actually unfavorable for practical applications. Here, a donor material PThBDTP is employed, and PThBDTP:PC71BM based OSCs are fabricated. A PCE of over 10% is achieved without using any additives and film post‐treatments. The device displays a high open‐circuit voltage of 0.977 V, a large short‐circuit current density of 13.49 mA cm^‐2, and a high fill factor of 76.3%. These results represent an important step towards developing high‐efficiency additive‐free OSCs.     

Environmentally Printing Efficient Organic Tandem Solar Cells with High Fill Factors: A Guideline Towards 20% Power Conversion Efficiency

The tandem concept involves stacking two or more cells with complementary absorption spectra in series or parallel connection, harvesting photons at the highest possible potential. It is strongly suggested that the roll‐to‐roll production of organic solar cells will employ the tandem concept to enhance the power conversion efficiency (PCE). However, due to the undeveloped deposition techniques, the challenges in ink formulation as well as the lack of commercially available high performance active materials, roll‐to‐roll fabrication of highly efficient organic tandem solar cells currently presents a major challenge. The reported high PCE values from lab‐scale spin‐coated devices are, of course, representative, but not helpful for commercialization. Here, organic tandem solar cells with exceptionally high fill factors and PCE values of 7.66% (on glass) and 5.56% (on flexible substrate), which are the highest values for the solution‐processed tandem solar cells fabricated by a mass‐production compatible coating technique under ambient conditions, are demonstrated. To predict the highest possible performance of tandem solar cells, optical simulation based on experimentally feasible values is performed. A maximum PCE of 21% is theoretically achievable for an organic tandem solar cell based on the optimized bandgaps and achieved fill factors.     

Morphological Characterization of a Low‐Bandgap Crystalline Polymer:PCBM Bulk Heterojunction Solar Cells

Understanding the morphology of polymer‐based bulk heterojunction (BHJ) solar cells is necessary to improve device efficiencies. Blends of a low‐bandgap silole‐containing conjugated polymer, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b;2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (PSBTBT) with [6,6]phenyl‐C61‐butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the “As‐Spun”, “Pre‐Annealed”, and “Post‐Annealed” films were studied by scanning force microscopy, contact angle measurements, X‐ray photoelectron spectroscopy, near‐edge X‐ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron reflectivity. The results showed that PSBTBT was enriched at the cathode interface in the “As‐Spun” films and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing‐incidence X‐ray diffraction and small‐angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ～10 nm in size, leading to an improvement in photovoltaic performance.     

Extracting Microscopic Device Parameters from Transient Photocurrent Measurements of P3HT:PCBM Solar Cells

By including trap‐limited recombination in a time‐domain drift‐diffusion model of a P3HT:PCBM solar cell, experimental transient photocurrent (TPC) measurements across the current–voltage (J–V) curve in both the light and the dark can be reproduced. Using the same set of model parameters, the steady‐state current–voltage curves and charge‐extraction data are also reproduced. The model is validated by predicting the recombination rate at open circuit and comparing results with the rate as measured by the transient photovoltage (TPV) measurement technique. It is demonstrated that the model, which incorporates transport, carrier trapping, carrier de‐trapping, and recombination, is able to reproduce device dynamics correctly. The fit of the model to the experimental results is improved by varying the shape of the electron and hole density of state (DoS) functions. By discretizing the DoS in energy space and allowing its shape to vary, the TPC transients can be closely fitted and the form of the tails of the electron and hole DoS extracted. It is found that the DoS of trapped carriers can be represented by a series of Gaussians offset in energy. This is tentatively attributed to electronic disorder introduced by molecular packing of the molecules.     

Circumventing UV Light Induced Nanomorphology Disorder to Achieve Long Lifetime PTB7‐Th:PCBM Based Solar Cells

Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo‐oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn‐in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies which are presented in here indicate that a plausible triggering for the burn‐in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, a robust nanocrystalline ordering is induced in the PCBM phase prior to UV illumination. In that event, PTB7‐Th:PC₇₁BM cells are shown to exhibit T₈₀ lifetimes larger than 1.6 years under a continuous UV‐filtered 1‐sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.     

Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In

The photoinduced open‐circuit voltage (V_oc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V_oc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the V_oc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the V_oc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against V_oc losses during burn‐in.     

Ultra‐High Voltage Multijunction Organic Solar Cells for Low‐Power Electronic Applications

Low power electronics are an ideal application for organic photovoltaics (OPV) where a low‐cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra‐high voltage small molecule multijunction devices with open circuit voltage (V_OC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi‐layer stacks and ensure current balancing is achieved between sub‐cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub‐cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) and SubPc/Cl₆‐SubPc were selected both for their high V_OC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro‐energy cell type battery under both low illumination intensity white light and monochromatic illumination.     

Quantitative Morphology–Performance Correlations in Organic Solar Cells: Insights from Soft X‐Ray Scattering

Organic/polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. One of the key factors contributing to the rapid and continuous progress of organic photovoltaics (OPVs) is the control and optimization of photoactive‐layer morphology. The impact of morphology on photovoltaic parameters has been widely observed. However, the highly complex and multilength‐scale morphology often formed in efficient OPV devices consisting of compositionally similar components impose obstacles to conventional morphological characterizations. In contrast, due to the high compositional and orientational sensitivity, resonant soft X‐ray scattering (R‐SoXS), and related techniques lead to tremendous progress of characterization and comprehension regarding the complex mesoscale morphology in OPVs. R‐SoXS is capable of quantifying the domain characteristics, and polarized soft X‐ray scattering (P‐SoXS) provides quantitative information on orientational ordering. These morphological parameters strongly correlate the fill factor (FF), open‐circuit voltage (V_oc), as well as short‐circuit current (J_sc) in a wider range of OPV devices, including recent record‐efficiency polymer:fullerene solar cells and 12%‐efficiency fullerene‐free OPVs. This progress report will delineate the soft X‐ray scattering methodology and its future challenges to characterize and understand functional organic materials and provide a non‐exhaustive overview of R‐SoXS characterization and its implication to date.     

Impact of Fullerene Molecular Weight on P3HT:PCBM Microstructure Studied Using Organic Thin‐Film Transistors

The clustering and diffusion of C₇₁‐butyric acid methyl ester (PC₇₁BM) in poly(3‐hexylthiophene) (P3HT) has been studied using single layer blend and bilayer organic field‐effect transistors (OFETs) and by atomic force microscopy (AFM). P3HT:PC₇₁BM blend based OFETs were found to undergo phase‐segregation upon annealing, which was detectable as a fall in electron mobility with increasing annealing temperature. By employing carefully designed bilayer P3HT:PC₇₁BM OFETs, the diffusion‐properties of PC₇₁BM in P3HT could additionally be inferred from electron mobility measurements. It was found that the prerequisite annealing temperatures for detectable PC₇₁BM clustering and diffusion in P3HT was approximately 20 °C higher than for PC₆₁BM. The diffusion coefficient of PC₆₁BM in P3HT was found to be several times higher that that of PC₇₁BM. The present work provides unique insights into the diffusion process of fullerenes in conjugated polymers and could prove highly valuable for future materials development and device optimization.     

Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Charge transport in organic photovoltaic (OPV) devices is often characterized by steady‐state mobilities. However, the suitability of steady‐state mobilities to describe charge transport has recently been called into question, and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated. An experiment to measure the charge extraction time under realistic operating conditions is set up. This experiment is applied to different blends and shows that extraction time is directly related to the geometrical average of the steady‐state mobilities. This demonstrates that under realistic operating conditions the steady‐state mobilities govern the charge extraction of OPV and gives a valuable insight in device performance.     

A Conclusive View on Charge Generation,Recombination, and Extraction in As‐Prepared and Annealed P3HT:PCBM Blends: Combined Experimental and Simulation Work

Time‐delayed collection field (TDCF) and bias‐amplified charge extraction (BACE) are applied to as‐prepared and annealed poly(3‐hexylthiophene):[6,6]‐phenyl C₇₁ butyric acid methyl ester (P3HT:PCBM) blends coated from chloroform. Despite large differences in fill factor, short‐circuit current, and power conversion efficiency, both blends exhibit a negligible dependence of photogeneration on the electric field and strictly bimolecular recombination (BMR) with a weak dependence of the BMR coefficient on charge density. Drift‐diffusion simulations are performed using the measured coefficients and mobilities, taking into account bimolecular recombination and the possible effects of surface recombination. The excellent agreement between the simulation and the experimental data for an intensity range covering two orders of magnitude indicates that a field‐independent generation rate and a density‐independent recombination coefficient describe the current–voltage characteristics of the annealed P3HT:PCBM devices, while the performance of the as‐prepared blend is shown to be limited by space charge effects due to a low hole mobility. Finally, even though the bimolecular recombination coefficient is small, surface recombination is found to be a negligible loss mechanism in these solar cells.     

Light‐Induced Degradation Mechanism in Poly(3‐hexylthiophene)/Fullerene Blend Solar Cells

The mechanism of light‐induced degradation in organic solar cells based on regioregular poly(3‐hexylthiophene) and indene‐C₆₀ bisadduct is studied by transient absorption (TA) and electron spin resonance (ESR) measurements. After 45 h light exposure under simulated solar illumination at 100 mW cm^?2, the short‐circuit current density, open‐circuit voltage, and fill factor are all degraded by about 20%–30% relative to the initial photovoltaic parameters. For the assignment of limiting conversion processes in the degraded solar cells, exciton diffusion into a donor/acceptor interface, charge transfer at the interface, charge dissociation into free charge carriers, and charge collection to each electrode are observed before and after the light exposure by the TA measurement. As a result, it is found that the charge collection deteriorates after the light exposure because of light‐induced charge trap formation in the bulk of the active layer. The origin of charge traps is further discussed on the basis of ESR measurements and density functional theory calculation.