Raghu Vamsi Krishna Chavali

Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient ( ηT* ∼ 33%) solar cell

As single junction photovoltaic (PV) technologies, both Si heterojunction (HIT) and perovskite based solar cells promise high efficiencies at low cost. Intuitively, a traditional tandem cell design with these cells connected in series is expected to improve the efficiency further. Using a self-consistent numerical modeling of optical and transport characteristics, however, we find that a traditional series connected tandem design suffers from low JSC due to band-gap mismatch and current matching constraints. Specifically, a traditional tandem cell with state-of-the-art HIT ( η=24%) and perovskite ( η=20%) sub-cells provides only a modest tandem efficiency of ηT∼ 25%. Instead, we demonstrate that a bifacial HIT/perovskite tandem design decouples the optoelectronic constraints and provides an innovative path for extraordinary efficiencies. In the bifacial configuration, the same state-of-the-art sub-cells achieve a normalized output of ηT* = 33%, exceeding the bifacial HIT performance at practical albedo re...

Correlation of Built-In Potential and I–V Crossover in Thin-Film Solar Cells

Thin-film solar cells often show a crossover between the illuminated and dark I-V characteristics. Several device specific reasons for crossover exist and have been discussed extensively. In this paper, we show that a low contact-to-contact built-in potential can produce a voltage-dependent photocurrent that leads to I-V crossover at a voltage that is almost exactly the device built-in potential. This mechanism can produce crossover in the absence of carrier trapping or recombination. It can be a contributing factor to crossover, but when an anomalously low contact-to-contact built-in potential exists, it can be the dominant factor. Using numerical simulations, we examine a variety of model solar cell structures with low contact-to-contact built-in potential and show a strong correlation of the crossover and built-in potential voltages. These simulations also suggest that a plot of the illuminated minus dark current may help identify when a low Vbi is limiting device performance.

Correlated Nonideal Effects of Dark and Light I–V Characteristics in a-Si/c-Si Heterojunction Solar Cells

a-Si/c-Si (amorphous Silcon/crystalline Silicon) heterojunction solar cells exhibit several distinctive dark and light I-V nonideal features. The dark I-V of these cells exhibits unusually high ideality factors at low forward-bias and the occurrence of a “knee” at medium forward-bias. Nonidealities under illumination, such as the failure of superposition and the occurrence of an “S-type” curve, are also reported in these cells. However, the origin of these nonidealities and how the dark I-V nonidealities manifest themselves under illumination, and vice versa, have not been clearly and consistently explained in the current literature. In this study, a numerical framework is used to interpret the origin of the dark I-V nonidealities, and a novel simulation technique is developed to separate the photo-current and the contact injection current components of the light I-V. Using this technique, the voltage dependence of photo-current is studied to explain the failure of the superposition principle and the origin of the S-type light I-V characteristics. The analysis provides a number of insights into the correlations between the dark I-V and the light I-V. Finally, using the experimental results from this study and from the current literature, it is shown that these nonideal effects indeed affect the dark I-V and the light I-V in a predictable manner.

The Frozen Potential Approach to Separate the Photocurrent and Diode Injection Current in Solar Cells

The principle of superposition is commonly utilized in the analysis of current transport under dark and light conditions for several different types of solar cells. However, this principle assumes that the photocurrent is voltage independent and that the diode injection current is generation independent, which restricts its validity. Indeed, the superposition principle cannot be applied to most thin-film solar cells because the above mentioned assumptions are not generally valid. In order to address this issue, a novel numerical modeling approach is described that allows independent computation of the photocurrent and of the diode injection current as components of the total current under illumination. Then, using several test cases, where the principle of superposition breaks down, the usefulness of this modeling approach is demonstrated.

Multiprobe Characterization of Inversion Charge for Self-Consistent Parameterization of HIT Cells

The performance of modern a-Si/c-Si heterojunction (HIT) solar cells is dictated by a complex interplay of multiple device parameters. A single characterization experiment [e.g., light current-voltage (I-V)] can be fitted with a set of parameters, but this set may not be unique and is, therefore, questionable as the basis for future design/optimization. In this paper, we use multiple (quasi-orthogonal) measurement techniques to uniquely identify the key parameters that dictate the performance of HIT cells. First, we study the frequency, voltage, and temperature response of inversion charge (QInv) to create the theoretical basis for characterization of key device parameters, namely, the thickness of the i-layer at the front interface (tia-Si), a-Si/c-Si heterojunction valence band discontinuity (ΔEV), built-in potentials in a-Si (φa-Si) and c-Si (φc-Si) regions, etc. Next, we simulate various characterization measurements, such as capacitance-voltage (C-V) and impedance spectroscopy, which probe QInv and explain the parameter extraction procedure from these measurements. Subsequently, we use the algorithm/procedure just developed to extract the aforementioned parameters for an industrial-grade HIT sample. Finally, we extend this quasi-orthogonal characterization framework by correlating the C-V characteristics with the ubiquitous light and dark I-V characteristics to demonstrate the consistency of the developed theory and uniqueness of the parameter extracted. The unique parameter set thus obtained can simultaneously provide a basis for the interpretation of the experimental measurements and can also be used for the design/optimization of these solar cells.

A Generalized Theory Explains the Anomalous Suns–

Suns-Voc measurements exclude parasitic series resistance effects and are, therefore, frequently used to study the intrinsic potential of a given photovoltaic technology. However, when applied to a-Si/c-Si heterojunction (SHJ) solar cells, the Suns-Voc curves often feature a peculiar turnaround at high illumination intensities. Generally, this turn-around is attributed to extrinsic Schottky contacts that should disappear with process improvement. In this paper, we demonstrate that this voltage turnaround may be an intrinsic feature of SHJ solar cells, arising from the heterojunction (HJ), as well as its associated carrier-transport barriers, inherent to SHJ devices. We use numerical simulations to explore the full current-voltage (J-V) characteristics under different illumination and ambient temperature conditions. Using these characteristics, we establish the voltage and illumination-intensity bias, as well as temperature conditions necessary to observe the voltage turnaround in these cells. We validate our turnaround hypothesis using an extensive set of experiments on a high-efficiency SHJ solar cell and a molybdenum oxide (MoOx) based hole collector HJ solar cell. Our work consolidates Suns-Voc as a powerful characterization tool for extracting the cell parameters that limit efficiency in HJ devices.

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The cell-to-module efficiency gap observed in a-Si/c-Si heterojunction solar cells is a key challenge to the broad adoption of this technology. To systematically address this issue, we describe an end-to-end modeling framework to explore the implications of process and device variations at the module level. First, a process model is developed to connect the a-Si deposition parameters to the material properties. Next, a physics-based device model is presented; the model uses the thermionic emission/diffusion theory to capture the essential features of photocurrent and diode injection current. Using the process and device models, the effects of process conditions on cell performance are explored. Finally, the performance of the module, as a function of device and process parameters, is explored to establish the cell-to-module efficiency gap. The insights developed through this process-to-module modeling framework will help close the cell-to-module efficiency gap of this commercially promising technology.

Response of Si Heterojunction Solar Cells

Understanding the effects of device parameters on carrier transport is essential for the design of high efficiency aSi/c-Si heterojunction solar cells. It is well known that the dark current-voltage (I-V) characteristics are correlated to fundamental PV parameters that dictate cell performance, and therefore an analysis of dark I-V may be a useful diagnostic tool to monitor changes in the device parameters (indirectly correlated to solar cell efficiency). In this paper, we first measure and then interpret the forward bias dark I-V characteristics of a-Si/c-Si solar cells by numerical and analytical models. Several well-known (but poorly understood) features of dark I-V characteristics are shown to be correlated to parameters of fundamental interest to solar cell design and can therefore be used as important markers of device parameter variation during the development process.

A Framework for Process-to-Module Modeling of a-Si/c-Si (HIT) Heterojunction Solar Cells to Investigate the Cell-to-Module Efficiency Gap

The principle of superposition is a well-known method used in the analysis of current transport under dark and light conditions for several different types of solar cells. However, this principle assumes that the photo-current is voltage independent and the contact injection current is generation independent, which restricts its validity. The superposition principle cannot, however, be applied to most thin film solar cells as the above mentioned assumptions are not generally valid. In order to address this issue, we describe a novel numerical modeling approach that allows independent computation of the photo-current and the contact injection current components of the total current under illumination. Then, using several test cases where the principle of superposition breaks down, the usefulness of this modeling approach is demonstrated.

A diagnostic tool for analyzing the current-voltage characteristics in a-Si/c-Si heterojunction solar cells

For commercial solar modules, up to 80% of the incoming sunlight may be dissipated as heat, potentially raising the temperature 20-30°C higher than the ambient. In the long run, extreme self-heating may erode efficiency and shorten lifetime, thereby, dramatically reducing the total energy output by almost ~10% Therefore, it is critically important to develop effective and practical cooling methods to combat PV self-heating. In this paper, we explore two fundamental sources of PV self-heating, namely, sub-bandgap absorption and imperfect thermal radiation. The analysis suggests that we redesign the optical and thermal properties of the solar module to eliminate the parasitic absorption (selective-spectral cooling) and enhance the thermal emission to the cold cosmos (radiative cooling). The proposed technique should cool the module by ~10°C, to be reflected in significant long-term energy gain (~ 3% to 8% over 25 years) for PV systems under different climatic conditions.