Haohui Liu

The realistic energy yield potential of GaAs-on-Si tandem solar cells: a theoretical case study.

Si based tandem solar cells represent an alternative to traditional compound III-V multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs on Si (GaAs/Si) tandem solar cells is performed to assess their energy yield potential under realistic illumination conditions with varying spectrum. We find that the yield of a 4-terminal contact scheme with thick top cell is more than 15% higher than for a 2-terminal scheme. Furthermore, we quantify the main losses that occur for this type of solar cell under varying spectra. Apart from current mismatch, we find that a significant power loss can be attributed to low irradiance seen by the sub-cells. The study shows that despite non-optimal bandgap combination, GaAs/Si tandem solar cells have the potential to surpass 30% energy conversion efficiency.

Numerical Analysis of Radiative Recombination and Reabsorption in GaAs/Si Tandem

We demonstrate a numerical analysis of the device impact of photon reabsorption on single-junction GaAs and tandem GaAs/Si solar cells. A self-consistent optical-electrical model that considers nonideal losses within the devices is developed. For single-junction devices, we find that the impact of photon recycling on the voltage increases monotonically with the injection level. For record-level GaAs solar cells, the voltage boost is 33 mV under open-circuit conditions and 13 mV at the maximum power point. For tandem GaAs/Si solar cells, photon reabsorption moderates the sensitivity of tandem efficiency to both obvious parameters like absorber thickness and implicit parameters like shunt resistance (Rsh) and bulk lifetime. Considering luminescent coupling results in a GaAs top cell that is 9.5% thicker than without luminescent coupling. The tandem device is 50% more sensitive to Rsh changes in the GaAs cell than Rsh changes in the Si cell. The impact of the GaAs top-cell bulk lifetime on tandem efficiency is reduced by 61% if photon reabsorption is not considered. This integrated optoelectronic device model allows one quantification of the implicit effects of photon recycling and luminescent coupling on device parameters for GaAs/Si tandem, providing a valuable tool for high-performance device optimization.

The Impact of Haze on Performance Ratio and Short-Circuit Current of PV Systems in Singapore

The spectral content of sunlight directly affects the power output of solar photovoltaic (PV) devices. The extent of the effect of seasonal and weather-related spectral variations on the power output will depend largely on the semiconductor bandgap. In this study, haze, which is a common weather condition in many parts of the world, is found to affect the power output of PV systems. An analysis of a recent haze event in Singapore in mid-June 2013 reveals that haze has an impact on the performance ratios and short-circuit currents of PV systems. The performance ratio of amorphous silicon thin-film PV systems dropped during the haze event, while that of crystalline silicon wafer-based systems exhibited a slight increase. A detailed analysis showed that the main cause of the observed performance ratio variations was changes in the generated short-circuit currents, which were due to a red shift of the solar spectrum arriving in the module plane during the haze period. Therefore, PV systems with different semiconductor bandgaps were affected differently.

Optical loss analysis of four-terminal GaAs/Si tandem solar cells

In this paper, we present a loss analysis for an in-house fabricated 4-terminal GaAs/Si tandem solar cell with a conversion efficiency of 21.3%. Current losses are broken down into ten loss channels and quantified. In order to predict the current gains from individual improvements, we set each parameter associated with one of the loss channels to its ideal value while keeping the other parameters constant. Using this loss analysis, the three most impactful losses, namely top cell metal grid reflection (3.40 mA/cm2), top cell window collection (2.06 mA/cm2) and reflection of the non-ideal antireflection coatings (1.96 mA/cm2), are identified and further optimization steps for the cell are suggested.

Light management in mechanically-stacked GaAs/Si tandem solar cells: Optical design of the Si bottom cell

Light management in the Si bottom cell of a GaAs/Si tandem system is crucial due to the bandgap mismatch of the two materials, which results in a low current of the bottom cell. To evaluate the light coupling and light trapping, we developed an optical model to simulate the light absorption in the silicon bottom cell. This optical model is an extension of Basores analytical model. By comparing the simulation with the measurement of the prototype tandem solar cell, we find that the Si bottom cell can gain up to 2.4 mA/cm2 photocurrent by improving light coupling. In addition, we study the impact of the rear surface reflectance on the photocurrent in the Si bottom cell. We observed that ~17% relative increase in current generated in this bottom cell can be achieve by changing the rear surface design from a full area metal contact to a local-back-surface-field configuration.

Urban haze and photovoltaics

Urban haze is a multifaceted threat. Foremost a major health hazard, it also affects the passage of light through the lower atmosphere. In this paper, we present a study addressing the impact of haze on the performance of photovoltaic installations in cities. Using long-term, high resolution field data from Delhi and Singapore we derive an empirical relation between reduction in insolation and fine particulate matter (PM2.5) concentration. This approach enables a straightforward way to estimate air pollution related losses to photovoltaic power generation anywhere on the planet. For Delhi, we find that insolation received by silicon PV panels was reduced by 11.5% ± 1.5% or 200 kWh m−2 per year between 2016 and 2017 due to air pollution. We extended this analysis to 16 more cities around the planet and estimated insolation reductions ranging from 2.0% (Singapore) to 9.1% (Beijing). Using spectrum data from Singapore, we projected how other photovoltaic technologies would be affected and found an additional reduction compared to silicon of between 23% relative for GaAs and 42% for a 1.64 eV perovskite material. Considering current installation targets and local prices for electricity, we project that annual losses in revenue from photovoltaic installations could exceed 20 million USD for Delhi alone, indicating that annual economic damage from air pollution to photovoltaic site operators and investors worldwide could be billions of dollars.

Predicted outdoor energy yield of Si based tandem solar cells

Silicon based tandem solar cells have recently received a lot of attention. They are promising candidates to provide the next efficiency boost at moderate fabrication costs. This potentially enables their application in non-concentrating PV systems. It is known that tandem solar cells are sensitive to outdoor irradiation variations, which are more pronounced for non-concentrating applications. Therefore it is important to assess their yield under realistic operating conditions. In this work we analyze the distribution of outdoor conditions measured in Singapore, and calculate the device efficiency losses for these conditions. We find that 2 terminal tandem devices suffer substantially from current mismatch losses, which can amount to 3% to 5% of the rated efficiency under standard testing conditions (STC). Despite this, the predicted performance ratio for tandem solar cell is still on par or better than conventional single-junction Si solar cells. Adjusting the top cell thickness to match the current of the bottom cell under the average Singapore spectrum can decrease the loss due to current mismatch and increase overall yield by 0.8% to 1.7%.

Device impact of photon recycling and luminescent coupling on InGaP/Si tandems

We numerically evaluate the device impact of photon re-absorption on InGaP/Si tandem solar cells using a coupled optical-electronic device model. The presented simulation results provide guidelines for designing high performance InGaP/Si tandem device. We find that including the effects of photon recycling (PR) and luminescent coupling (LC) results in a 12.5% increase in optimum top-cell thickness for a two-terminal configuration. Furthermore, PR and LC affect the sensitivity of the tandems conversion efficiency to various device parameters. As the InGaP bulk lifetime increases, there is an absolute efficiency increase of up to 0.7% for the two-terminal as well as the four-terminal configuration. Considering PR and LC furthermore reduces the power generation sensitivity to shunting in the two-terminal configuration. For the four-terminal configuration, photon re-absorption has a less significant impact.