Voltage-Matched Configurations for Monolithically Series-Connected All-Perovskite Double- and Triple-Tandem Solar Modules

We designed all-perovskite (PVK) double- and triple-tandem solar modules.¹ Monolithically series-interconnected structures were adopted, because these offer high scalability by exploiting the advantages of thin-film modules over wafer-based crystalline-silicon modules. We revealed the voltage-matched configurations and their variants improve the annually averaged conversion efficiencies outdoor with potentially higher durability and lower costs compared with those of the conventional current-matched configurations.<br/>Although the combination of PVK top cells and crystalline-silicon bottom cells has been realized higher conversion efficiencies of double-tandem modules at present, all-PVK tandem modules have various advantages of scalability, lightweight, and suitability for mass production. Therefore, improvements in the efficiencies of all-PVK modules are extremely important. Thus, to find the suitable module configurations, we modeled the photovoltaic performance of single PVK cells with different bandgaps by reference to previous experimental data. Then, we optimized the cell widths and transparent-electrode thicknesses used for the monolithically series-interconnected modules, along with the PVK bandgaps. Finally, we evaluated the annually averaged conversion efficiencies under various meteorological conditions using a database.<br/>The conventional current-matched two-terminal double-tandem (Double-2T) modules have an advantage of a simple structure. The current matching condition requires the use of a large-bandgap PVK top cell. However, the photovoltaic performance currently lowers with increasing bandgap. This lowers the module efficiency. By contrast, the efficiency of another conventional configuration: four-terminal double-tandem (Double-4T) modules is less sensitive to the PVK bandgaps. This allows us to use more efficient and durable PVK compositions. However, Double-4T complicates the power-conditioning system, and hence it is not suitable for practical use. The voltage-matched double-tandem (Double-VM) module consists of the parallel-connected top and bottom modules, in which plural top cells and bottom cells are connected in series, respectively, so that the voltages of the maximal-power points (V_MPP) of these modules are approximately the same as each other. We revealed that Double-VM combines the advantages of Double-2T and Double-4T, and mitigates their shortcomings.<br/>The voltage-matched triple-tandem modules also offer similar advantages to those of Double-VM. However, the use of three substrates covered with transparent conductive oxide films increases the optical loss and ohmic loss, resulting in lower efficiencies than those of the triple-tandem 2T modules. To solve this issue, we proposed a new configuration of the triple-tandem series/parallel-connecting voltage-matched (Triple-SPVM) module, in which the middle-bottom module consisting of directly series-connected middle and bottom cells is connected to the top module in parallel. The use of only two substrates improves the efficiency and lowers the cost. The current mismatch between the series-connected middle and bottom cells is not a great issue, because of slight changes in the relative solar spectrum in the relevant near-infrared range. Thus, Triple-SPVM is the best suited for practical use.<br/>An emerging application of solar modules is power supply to electrochemical reactors for CO₂ conversion to useful organic substances. Direct connection of these two devices is exactly an artificial-photosynthetic device, and stores time-varying solar energy. It is of great importance for efficient conversion of CO₂ that the device operates at V_MPP of the solar modules. Double-VM and Triple-SPVM are suitable for this application, because V_MPP of the module is tunable by changing the series-connected cell numbers in each submodule.^2,3<br/>References<br/>1. Y. Takeda, et al., submitted.<br/>2. Y. Takeda, J. Appl. Phys. 127, 204503 (2020).<br/>3. Y. Takeda, J. Appl. Phys. 132, 075002 (2022).