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Perovskite-based methylammonium lead iodide thin-film solar cells have become a major focus for researchers due to their simple structure and low production costs. Over the past five years, their photoelectric conversion efficiency has rapidly increased from 3.8% to over 15%, surpassing that of amorphous silicon solar cells. In 2013, this technology was recognized by *Science* as one of the top ten scientific breakthroughs. As the technology continues to develop, it is expected that the efficiency will exceed 20%, opening up wide application possibilities.
However, current high-efficiency perovskite solar cells often use expensive organic materials like Spiro-OMeTAD as hole-transport layers, which significantly increases the cost. Additionally, the long-term stability of these organic components remains a concern. Therefore, developing high-efficiency perovskite solar cells without hole-transport materials has become a key research direction.
To date, the highest efficiency of perovskite solar cells without hole-transport layers has reached 8%, which is still far behind that of traditional perovskites with such materials. Moreover, there is ongoing debate about the mechanisms involved in sensitization and heterojunction formation in these devices.
Recently, Meng Qingbo from the Institute of Physics, Chinese Academy of Sciences, and the Beijing National Laboratory for Condensed Matter Physics, made significant progress in improving the thin film deposition process and optimizing the interface. This work led to a breakthrough in the efficiency of perovskite methylammonium lead iodide solar cells, exceeding 10% and achieving an open-circuit voltage above 900 mV (see Figure 1).
For the first time, a single heterojunction ideal diode model was used to systematically analyze the current-voltage characteristics of the device (Figure 2). The results showed a strong agreement between the measured data and the ideal model, indicating that the battery behaves as a typical diode. The ideality factor ranged from 1.85 to 1.93, suggesting that the forward saturation current is primarily determined by carrier recombination in the depletion region. This confirmed the presence of a space charge region in the heterojunction.
Furthermore, the series resistance and forward saturation current were calculated, showing values comparable to those of well-studied high-efficiency thin-film solar cells like Cu(In,Ga)Seâ‚‚ and CdTe.
Impedance spectroscopy further validated the accuracy of the model analysis and provided self-consistent results, directly proving that this type of device is indeed a heterojunction thin-film solar cell. This conclusion has important implications for the design and optimization of future perovskite solar cell devices.
The findings were published in the latest issue of *Applied Physics Letters* (Appl. Phys. Lett. 104, 063901 (2014)). The research was supported by the Beijing Municipal Science and Technology Commission, the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.