Fiber-Shaped Energy Harvesting and Storage Devices by Huisheng Peng

Author:Huisheng Peng
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

5.1.1 Working Mechanism

Many apparently different types of solar cells actually share two common processes during work: light absorption to create electron-hole pairs and charge separation to generate photovoltage and photocurrent. The light absorbers, in general, are photosensitive semiconductors, which classify the solar cells into different categories: sensitized solar cell, polymer solar cell, and, here, perovskite solar cell. The charge separation, which takes place at a contact interphase that is selective for electrons or holes, dictates the energy available inside the device. In a traditional p-i-n solar cell, p-type and n-type semiconductors are selective for holes and electrons, respectively, while in DSC and PSC, the selection is conducted by different electron and hole conductors, and likewise in perovskite.

Particularly in perovskite solar cells, the light is harvested by the perovskite like CH3NH3PbI3 that has a bandgap of 1.55 eV, lower than the semiconducting polymers (~2 eV) and inorganic semiconductors (~3.2 eV), thus promising a broad spectral sensitivity with onset of 800 nm. The strong light absorption of the perovskite enables an outstanding performance of the solar cell since it moderates the requirement for film thickness and facilitates the charge collection. The photoinduced excitons after light absorption have small binding energy (30‒75 meV) that suffices for thermal dissociation into free carriers (electrons and holes). It has been proved that the excited perovskite is mainly populated by free carriers rather than their associated pairs [12]. Then, the free carriers transport through the electron conductors and hole conductors, respectively, and get collected at electrodes.

As illustrated in Fig. 5.2, there are seven processes involved in the illuminated perovskite solar cell: photoexcitation (1), electron transfer to ETM (2), and hole transfer to HTM (3); (4) to (7) represent the unwanted recombination process including the reassociation of free carriers (4), back transfer from ETM and HTM to perovskite (5) and (6), and local short circuit at the absent of perovskite (7). For perovskite solar cell, one of their advantages over DSC and PSC is the long carrier-diffusion length (102‒103 nm) which contributes to a low non-radiative recombination [11].

Fig. 5.2Schematic diagram of energy levels and electron transfer process in an HTM/perovskite/ETM cell

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