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Register a new userDesign and characterization of effective solar cells
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We propose a two-stage multi-objective optimization framework for full scheme solar cell structure design and characterization, cost minimization and quantum efficiency maximization. We evaluated structures of 15 different cell designs simulated by varying material types and photodiode doping strategies. At first, non-dominated sorting genetic algorithm~II (NSGA-II) produced Pareto-optimal-solutions sets for respective cell designs. Then, on investigating quantum efficiencies of all cell designs produced by NSGA-II, we applied a new multi-objective optimization algorithm~II (OptIA-II) to discover the Pareto fronts of select (three) best cell designs. Our designed OptIA-II algorithm improved the quantum efficiencies of all select cell designs and reduced their fabrication costs. We observed that the cell design comprising an optimally doped zinc-oxide-based transparent conductive oxide (TCO) layer and rough silver back reflector (BR) offered a quantum efficiency ($Q_e$) of $0.6031.$ Overall, this paper provides a full characterization of cell structure designs. It derives a relationship between quantum efficiency, $Q_e$ of a cell with its TCO layers doping methods and TCO and BR layers material types. Our solar cells design characterization enables us to perform a cost-benefit analysis of solar cells usage in real-world applications
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We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH3NH3PbI3-xClx solar cells due to oxygen exposure occurs over shorter timescales than FA0.83Cs0.17Pb(I0.8Br0.2)3 mixed-cation devices. We attribute these oxygen-induced losses in the power conversion efficiencies to the formation of electron traps within the perovskite photoactive layer. Our results highlight that the formamidinium-caesium mixed-cation perovskites are much less sensitive to oxygen-induced degradation than the methylammonium-based perovskite cells, and that further improvements in perovskite solar cell stability should focus on the mitigation of trap generation during ageing.
In this perspective, we explore the insights into the device physics of perovskite solar cells gained from modeling and simulation of these devices. We discuss a range of factors that influence the modeling of perovskite solar cells, including the role of ions, dielectric constant, density of states, and spatial distribution of recombination losses. By focusing on the effect of non-ideal energetic alignment in perovskite photovoltaic devices, we demonstrate a unique feature in low recombination perovskite materials - the formation of an interfacial, primarily electronic, self-induced dipole that results in a significant increase in the built-in potential and device open-circuit voltage. Finally, we discuss the future directions of device modeling in the field of perovskite photovoltaics, describing some of the outstanding open questions in which device simulations can serve as a particularly powerful tool for future advancements in the field.
Silicon heterojunction (SHJ) solar cells represent a promising technological approach towards higher photovoltaics efficiencies and lower fabrication cost. While the device physics of SHJ solar cells have been studied extensively in the past, the ways in which nanoscopic electronic processes such as charge-carrier generation, recombination, trapping, and percolation affect SHJ device properties macroscopically have yet to be fully understood. We report the study of atomic scale current percolation at state-of-the-art a-Si:H/c-Si heterojunction solar cells under ambient operating conditions, revealing the profound complexity of electronic SHJ interface processes. Using conduction atomic force microscopy (cAFM), it is shown that the macroscopic current-voltage characteristics of SHJ solar cells is governed by the average of local nanometer-sized percolation pathways associated with bandtail states of the doped a-Si:H selective contact leading to above bandgap open circuit voltages ($V_{mbox{OC}}$) as high as 1.2 V ($V_{mbox{OC}}>e E_{mbox{gap}}^{mbox{Si}}$). This is not in violation of photovoltaic device physics but a consequence of the nature of nanometer-scale charge percolation pathways which originate from trap-assisted tunneling causing dark leakage current. We show that the broad distribution of local photovoltage is a direct consequence of randomly trapped charges at a-Si:H dangling bond defects which lead to strong local potential fluctuations and induce random telegraph noise of the dark current.
We investigate nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C$_{60}$. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. We apply a combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density. 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. To study recombination dynamics in PHJ devices most relevant is the charge accumulation at this interface. As from extraction techniques only the spatially averaged carrier concentration can be determined, we derive the charge carrier density at the interface $n_{int}$ from the open circuit voltage. Comparing the experimental results with macroscopic device simulation we discuss the differences of recombination and charge carrier densities in CuPc:C$_{60}$ PHJ and BHJ devices 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.
Ternary organic solar cells (TOSC) are currently under intensive investigation, recently reaching a record efficiency of 17.1%. The origin of the device open-circuit voltage (VOC), already a multifaceted issue in binary OSC, is even more complex in TOSCs. Herein, we investigate two ternary systems with one donor (D) and two acceptor materials (A1, A2) including fullerene and non-fullerene acceptors. By varying the ratio between the two acceptors, we find the VOC to be gradually tuned between those of the two binary systems, D:A1 and D:A2. To investigate the origin of this change, we employ ultra-violet photoemission spectroscopy (UPS) depth profiling, which is used to estimate the photovoltaic gap in the ternary systems. Our results reveal an excellent agreement between the estimated photovoltaic gap and the VOC for all mixing ratios, suggesting that the energetic alignment between the blend components varies depending on the ratio D:A1:A2. Furthermore, our results indicate that the sum of radiative and non-radiative losses in these ternary systems is independent of the blend composition. Finally, we demonstrate the superiority of UPS over X-ray photoemission spectroscopy (XPS) depth profiling in resolving compositional profiles for material combinations with very similar chemical, but dissimilar electronic structures, as common in TOSCs.
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