Dear Eungkyun,

The structure of the standard mesoscopic perovskite solar cell consists of a glass coated with transparent conducting oxide (TCO) like FTO (F:SnO2) as anode, a compact layer of TiO2 (cl-TiO2) as hole blocking layer, a layer of mesoporous TiO2 (mp-TiO2) as electron transporting layer (ETL) and scaffold, a perovskite light absorbing layer, a hole transporting layer (HTL), and a back electrode (cathode) like gold or silver. The absorbing layer of this type of solar cell contains a compound with a perovskite structure, most commonly a lead metal organic hybrid (Methylammonium lead iodide perovskite, MAPbI3). Methylammonium lead iodide is an ambipolar semiconductor compound that can transmit both electrons and holes to the corresponding collecting electrodes. This is why perovskite solar cells can operate even in the absence of a hole or electron conductor. The most commonly used material as HTL in perovskite solar cells is the Spiro-OMeTAD polymer composition. The properties of this compound, including the appropriate glass transition temperature, solubility, ionization potential and transparency in the range of visible spectra, have made this a convenient option for these applications.

TiO2 compact layer:

Electron–hole pairs that are created in perovskite following light absorption can possibly result in the formation of excitons. Charge separation in perovskite can occur through two possible primary reactions. 1) Injection of photo-generated electrons into an n-type semiconductor such as TiO2; and 2) injection of holes into a p-type hole transporting material such as spiro-OMeTAD. A hole-blocking layer, in most cases, a compact layer of TiO2, is generally used between the FTO conducting substrate and the mesoscopic scaffold and/or perovskite layer to prevent the holes from reaching the FTO substrate (anode), as this would short-circuit the cell. Thus, high-quality film, appropriate conduction band, superior electron mobility and conductivity of compact layer material are responsible for highly efficient PSC device by attenuating excessive charge accumulation and guaranteeing effective electron injection in the compact layer, which can be achieved by TiO2 modification, such as doping. There are many advantages for doping elements in the TiO2 films: 1) improving the electronic properties and reducing the trap-state density of TiO2; 2) inducing a complex interplay of other effects that affect the device performance. For example, it can affect surface morphology of the TiO2 or change the crystallinity of perovskite and thereby influence the performance of PSCs; 3) modifying the conduction band (CB) position of TiO2 through a downward shift to increase electron injection from the perovskite film into the TiO2 or through an upward shift to increase the open-circuit voltage (Voc) of PSCs.

TiO2 scaffold layer:

The mp-TiO2 layer in a typical dye-sensitized solar cell (DSSC) plays three major roles; provides a scaffold to increase the surface area of the absorber layer, acts as a hole blocker, and transports photogenerated electron from the sensitized surface to the front contact (anode). Therefore, the active surface area which has a direct link with a particle size of mesoporous material is a key parameter in DSSC. However, hybrid organic-inorganic perovskite absorber can transport photogenerated electron to the conductive substrate itself. On the other hand, employing the mesoporous TiO2 layer substantially reduces the hysteresis behavior of PSC. In addition, it is widely recognized that the interface between the absorber layer and the carrier transport layers, as well as the inherent carrier transporting properties of the layers, are also important. An ideal carrier transport material is expected to possess a suitable energy level, high conductivity, and low surface recombination rate. TiO2 arguably is the most commonly used ETL has delivered the best photovoltaic performance in PSC thus far. The interface between TiO2 and perovskite material is a crucial factor in determining the crystal growth of perovskite and the effectiveness of charge separation in devices. The strategies used to modify the TiO2 scaffolds can be divided into three directions as follows. (1) surface treatment; (2) incorporation of cooperative materials; (3) elemental doping.

perovskite layer:

There have been many combinations of organic/inorganic cations, metal cations and halide anions used as absorber films in PSCs over the years, MAPbI3 remains the most frequently used absorber film. The major problem with MAPbI3 is that it undergoes an irreversible phase transformation from tetragonal to cubic phase at around 55 °C. Another study showed that the thermal conductivity of MAPbX3 is very low in both single crystalline and poly crystalline form, which leads to concentrated heating due to lack of rapid heat spread causing mechanical stresses and degradation of PSCs. Recently a study provided detail insights into the cellular mechanisms through which MAPbI3 can be potentially bio-accumulated in the body and cause serious health hazards in the long run. Moreover, it is not stable against moisture. Another organic cation formamidinium (FA) with larger cation size has been used in place of methylammonium (MA) which resulted in a reduction of the bandgap. The narrower band gap allows FAI-based perovskites to generate photocurrent over a larger spectral region, causing an increase in Jsc. Additionally, FAI perovskites exhibit improved charge transport characteristics, compared to MAI perovskites. The FAPbI3 undergoes phase transformation to tetragonal crystalline structure at 150 °C, which indicates larger thermal stability as compared to MAPbI3. However, FAPbI3 absorber material is very sensitive to moisture and degrades rapidly because of the formation of large pin holes in the FAPbI3 absorber. Also owing to the large size of FA cations, it is difficult to synthesize high-quality FAPbI3 thin films without the formation of an undesirable yellow phase.

buffer layer:

Several approaches have been reported to increase resistance to moisture-related degradation in PSCs. the first approach is to employ a thin blocking layer (for e.g. Al2O3) between the perovskite and HTM. Another approach is to use moisture blocking HTM. Another approach suggest the use of hydrophobic carbon electrode.

Hole Transporting layer (HTL):

The most commonly used material for the hole transporting Layer (HTL) is Spiro-OMeTAD. This material requires an additive, namely 4-tert-butylpyridine (TBP) or bis (trifluoromethane) sulfonimide lithium salt (Li-TFSI), to improve the conductivity. However, these additives breakdown the perovskite layer, reducing the stability of the device. TBP caused a rapidly decreased absorption intensity of TiO2/perovskite film because the CH3NH3PbI2 is decomposed by dissolution of PbI2 in TBP. Moreover, the acetonitrile, which is the solvent for Li-TFSI, can also corrode perovskite. In addition, researches showed a gradual increase in series resistance Rs when the Spiro-OMeTAD based cells were tested at a constant temperature and moisture because the voids are formed in the Spiro-OMeTAD. Also, it needs p-type doping for optimal cell performance. Therefore, seeking suitable replacement of Spiro-OMeTAD with advantages of low cost, ease of fabrication, and high performance of efficiency and stability, are urgent for future applications.

cathode:

Most of the silver layer as a cathode in perovskite solar cells are prepared in a vacuum environment which is an expensive process.

1. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

2. Device stability of perovskite solar cells – A review. Renewable and Sustainable Energy Reviews 77 (2017) 131–146

3. http://sharifsolar.ir

4. Review of Recent Progress in Chemical Stability of Perovskite Solar Cells. Journal of Materials Chemistry A (2014). DOI: 10.1039/C4TA04994B

5. Enhancing efficiency and stability of perovskite solar cells through Nb-doping of TiO2 at low temperature. Appl. Mater. Interfaces (2017). DOI: 10.1021/acsami.7b01063

6. Impact of Mesoporous Titania-Perovskite Interface on the Performance of Hybrid Organic-Inorganic Perovskite Solar Cells. Phys. Chem. Lett (2016). DOI: 10.1021/acs.jpclett.6b01617

7. Stability of perovskite solar cells. Solar Energy Materials & Solar Cells 147 (2016) 255–275

Dear Mr. Kim,

Hope you are well,

It is so that the perovskite solar cell works as a pin diode with the active absorber pervoskite layer acts as an intrinsic semiconductor layer, the Hole transport layer acts as the P+ layer and the electron transport layer acts as the N+ layer.

The metallization layers act as electrical accessing electrodes.

The built in field is dropped on the I-layer. Then when the device is illuminated the photons penetrate to the I-layer where they will generated electron hole pairs.

These electron hole pairs will be separated by the electric field the electrons will move to the the ETL and the holes will move to the HTL. Finally they reach the corresponding electrode.

l explained the operation of the organic and perovskite solar cells in detail in the paper: Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells

Best wishes

Organic–inorganic halide perovskite (CH3NH3PbI3) was used as a photosensitizer in dye-sensitized solar .Analytical drift-diffusion model for p-i-n perovskite solar cells, where they simplified the photo-generation rate, approximating it as a “priori” (deductive parameter) by analyzing the J–V curves before fitting. The generation rate, therefore, is calculated from the short-circuit current density of a known experimental model, confining their model to the study of only known data. According to standard transfer matrix formalism (TMF), the generation rate shows a fluctuating profile in thin absorbers due to the position dependence. Moreover, the same generation rate profile (i.e., the same average absorption coefficient and therefore the same average optical decay length, λe) regardless of wavelength of light, thickness, and type of absorber (p-i-n, p-p-n, n-i-p, and n-p-p) was used during the optical modeling and surface recombination velocities, active layer thickness, built-in potential, dark diode current were extracted by fitting the model with experimental data .

Dear Eungkyun,

In order to design a Pyrovskite solar cells (PSC) and realize the role of different components we must understand basic steps or processes they are undertaken in the PSC solar cell layers. Following are the processes that undergo to convert electromagnetic energy into electrical energy in a photovoltaic where this phenomenon is known as photovoltaic action:

• absorption of incident light by an absorber i.e., interaction between the photon and an absorber to generate either electron-hole pairs or excitons so, the absorber jumps to the excited state i.e., an electron from valence band to conduction band,

• the separation of charge carrier of opposite types and to move in opposite directions i.e., toward contact the electrodes (anode and cathode),

• the separate extraction of these carriers to an external circuit via contacts where energetic electron leaves an electrode, does something useful at an electrical load, hereby loose energy, and finally returns back to initial position through another electrode to complete one round trip,

• at the end of the round trip, the electron recombine with a hole so that the absorber returns back to the ground state and ready to absorb another photon for next trip.

good luck.

Dear Mr. Eungkyun Kim

In attached I send to you the titles of some scientific contributions that may be useful to you:

1 - https://www.researchgate.net/publication/289706136_Organic_Solar_Cells_organic_solar_cell

2 - https://www.researchgate.net/publication/338313035_Ternary_organic_solar_cells

Dear Kim

You can read the following papers

Article Electron transport material effect on performance of perovsk...

Article Enhancement of efficiency and stability of CH3NH3GeI3 solar ...

Article An optimized perovskite solar cell designs for high conversi...

best regards.