Polymer:Nonfullerene Bulk Heterojunction Solar Cells with Exceptionally Low Recombination Rates

Organic semiconductors are in general known to have an inherently lower charge carrier mobility compared to their inorganic counterparts. Bimolecular recombination of holes and electrons is an important loss mechanism and can often be described by the Langevin recombination model. Here, the device physics of bulk heterojunction solar cells based on a nonfullerene acceptor (IDTBR) in combination with poly(3‐hexylthiophene) (P3HT) are elucidated, showing an unprecedentedly low bimolecular recombination rate. The high fill factor observed (above 65%) is attributed to non‐Langevin behavior with a Langevin prefactor (β/β_L) of 1.9 × 10^?4. The absence of parasitic recombination and high charge carrier lifetimes in P3HT:IDTBR solar cells inform an almost ideal bimolecular recombination behavior. This exceptional recombination behavior is explored to fabricate devices with layer thicknesses up to 450 nm without significant performance losses. The determination of the photoexcited carrier mobility by time‐of‐flight measurements reveals a long‐lived and nonthermalized carrier transport as the origin for the exceptional transport physics. The crystalline microstructure arrangement of both components is suggested to be decisive for this slow recombination dynamics. Further, the thickness‐independent power conversion efficiency is of utmost technological relevance for upscaling production and reiterates the importance of understanding material design in the context of low bimolecular recombination.     

Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

Nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C₆₀ is investigated. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. A combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density is applied. In organic solar cells, charge photogeneration and recombination primarily occur at the donor–acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two‐dimensional heterointerface. In order to study nongeminate recombination dynamics in PHJ devices the charge accumulation at this interface is most relavent. As only the spatially averaged carrier concentration can be determined from extraction techniques, the charge carrier density at the interface n_int is derived from the open circuit voltage. Comparing the experimental results with macroscopic device simulation, the differences of recombination and charge carrier densities in CuPc:C₆₀ PHJ and BHJ devices are discussed with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.     

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

The impact of trapping on the recombination dynamics in polymer:fullerene blends is clarified using the highly ordered bulk heterojunction (BHJ) blend poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) at different weight ratios as a model system. The recombination dynamics are determined using both transient charge extraction and steady‐state techniques. The results show that both the decay of photogenerated charge and the light ideality factor at a polymer:fullerene weight ratio of 1:4 are fully consistent with 2D Langevin recombination; in the 1:1 case the recombination is seen to be affected by electron trapping. The theory of 2D Langevin recombination is extended to the case with high trap density in agreement with the observations in the 1:1 case. The recombination capture coefficients are derived both for trap‐assisted and band‐to‐band recombination and it can be seen that anisotropic charge transport reduces the capture coefficients in both cases resulting in a reduced overall recombination.     

Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge‐Transfer State Electroluminescence

Charge‐transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free‐carrier bimolecular recombination at the donor‐acceptor interface, rather than a charge‐trap‐mediated process. The fingerprint of the presence of nonradiative trap‐assisted recombination, a voltage‐dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination‐intensity dependence of the open‐circuit voltage.     

Electric Field and Mobility Dependent First‐Order Recombination Losses in Organic Solar Cells

The origin of photocurrent losses in the power‐generating regime of organic solar cells (OSCs) remains a controversial topic, although recent literature suggests that the competition between bimolecular recombination and charge extraction determines the bias dependence of the photocurrent. Here the steady‐state recombination dynamics is studied in bulk‐heterojunction OSCs with different hole mobilities from short‐circuit to maximum power point. It is shown that in this regime, in contrast to previous transient extracted charge and absorption spectroscopy studies, first‐order recombination outweighs bimolecular recombination of photogenerated charge carriers. This study demonstrates that the first‐order losses increase with decreasing slower carrier mobility, and attributes them to either mobilization of charges trapped at the donor:acceptor interface through the Poole–Frenkel effect, and/or recombination of photogenerated and injected charges. The dependence of both first‐order and higher‐order losses on the slower carrier mobility explains why the field dependence of OSC efficiencies has historically been attributed to charge‐extraction losses.     

The Evolutionary Advantage of Recombination. II. Individual Selection for Recombination

Based on the Fisher-Muller theory of the evolution of recombination, an argument can be constructed predicting that a recessive allele favoring recombination will be favored, if there are either favorable or deleterious mutants occurring at other loci. In this case there is no clear distinction between individual and group selection. Computer simulation of populations segregating for recessive or dominant recombination alleles showed selection favoring recombination, except in the case of a dominant recombination allele with deleterious background mutants. The relationship of this work to parallel investigations by Williams and by Strobeck, Maynard Smith, and Charlesworth is explored. All seem to rely on the same phenomenon. There seems no reason to assume that the evolution of recombination must have occurred by group selection.     

Recombination and meiosis.

Although exchanges between sister chromatids are common in mitotic cells, those involving homologous chromosomes are rare. Since recombination between homologues is one of the functions of meiosis, it follows that one aspect of the differentiation of the meiocyte involves the synthesis of proteins or enzymes which facilitate synapsis and exchange. Mutants are known which seem to have constitutive levels of mitotic recombination between homologues, and these may be defective in the mechanism which normally represses mitotic recombination. It has been proposed that one component of the synaptonemal complex (s.c.) is a filamentous pairing protein with DNA binding sites which are base sequence specific. Synapsis occurs because the distribution of these sequences is the same in homologues. When only non-homologous chromosomes are present, as in haploid meiosis, only weak pairing can occur, since the base sequences are largely out of register. Although certain features of recombination at the molecular level are known, none of the models so far proposed suggest an explanation for interference between crossovers. It is suggested that interference may depend on the presence of a limited amount of another DNA binding protein which is specifically located within the s.c. A crossover between naked DNA molecules is initially a weak structure, which must be later converted into a visible and mechanically strong chiasma. It is assumed that this stabilization of a crossover is achieved by the DNA binding protein, which can diffuse freely within the s.c. and bind cooperatively to any recombinant DNA molecules within it. Depletion of the binding protein within the vicinity of a crossover makes it unlikely that the second crossover can be formed nearby.     

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States,Charge Transport,and Recombination

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.     

Unifying Charge Generation,Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non‐fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non‐geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid‐state NMR (for atomic level insights on the local ordering and donor:acceptor π? π interactions) and resonant soft X‐ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.     

Region-Specific Recombination in Phage T4. I. a Special Glucosyl-Dependent Recombination System

In this communication, we describe a recombination mechanism in bacteriophage T4D that acts only on glycosylated phage, acts in some regions of the genome, but not others, and is heat sensitive, showing decreasing activity with increasing temperature.     

基于电子回流理论的植物延迟荧光来源理论与实验研究

延迟荧光(DF)技术在植物学、农业和环境等领域有着广泛的应用前景,延迟荧光来源的机理研究对其应用起着重要的作用。基于广泛接受的电子回流理论由一系列的光化学反应方程推导得出多指数衰减的延迟荧光动力学模型,首次提出了一个简化的三指数衰减近似模型,描述了光合电子传递链上的三个最重要成分的电子回流。为了验证近似模型的准确性,以实验室培养的相似的8周大的Jin Dan No.39玉米叶片为模型样品,利用实验室自制的延迟荧光探测系统,得到延迟荧光衰减动力学曲线。分别用三种电子阻断剂:DCMU、DB-MIB和Paraquat处理后的衰减动力学的实验与理论模型进行了对比研究,通过曲线特征来确定不同成分的存在及依赖关系。结果表明延迟荧光衰减动力学三指数拟合非常好(R2=0.99909),而且这三种成分是相互独立的,分别来源于QA,QB和光系统Ⅰ环式电子流。简化模型的提出为延迟荧光的应用题提供一个平台。