Methodologies for high efficiency perovskite solar cells
© The Author(s) 2016
Received: 31 March 2016
Accepted: 28 April 2016
Published: 30 June 2016
Since the report on long-term durable solid-state perovskite solar cell in 2012, perovskite solar cells based on lead halide perovskites having organic cations such as methylammonium CH3NH3PbI3 or formamidinium HC(NH2)2PbI3 have received great attention because of superb photovoltaic performance with power conversion efficiency exceeding 22 %. In this review, emergence of perovskite solar cell is briefly introduced. Since understanding fundamentals of light absorbers is directly related to their photovoltaic performance, opto-electronic properties of organo lead halide perovskites are investigated in order to provide insight into design of higher efficiency perovskite solar cells. Since the conversion efficiency of perovskite solar cell is found to depend significantly on perovskite film quality, methodologies for fabricating high quality perovskite films are particularly emphasized, including various solution-processes and vacuum deposition method.
Organic–inorganic metal halide perovskites with chemical formula ABX3 (A = CH3NH3, B = Pb or Sn, X = I, Br or Cl) were discovered in 1978 [1, 2]. MAPbX3 (M = CH3NH3) changes its color from colorless to orange and to black as anion changes from Cl to Br and to I, respectively, due to decrease in band gap energy. Low band gap iodide perovskite is expected to be a potential candidate for solar cell light harvester, however little attention has been paid to such a possibility because of being keen on change in electrical property depending on structural dimensionality reported in 1994 . In 2009, Miyasaka et al. used MAPbI3 and MAPbBr3 as light harvesters for the first time in dye-sensitized solar cell structure, in which MAPbI3 deposited on nanocrystalline TiO2 surface demonstrated a power conversion efficiency (PCE) of 3.8 % . To deposit MAPbI3 on TiO2, MAI and PbI2 were dissolved in gamma-butyrolactone (GBL) and the solution was spin-coated, where Miyasaka group prepared 8 wt% coating solution. It was found that the 8 wt% concentration was too low to induce sufficient coverage of TiO2 surface with MAPbI3. In 2011, Park et al. solved this problem by modulating coating solution concentration from 10 to 40 wt% and found that 40 wt% solution was enough to cover the TiO2 surface, leading dark color even at 3–4 μm thick TiO2 film and a PCE of 6.5 % . Absorption coefficient of MAPbI3 deposited on TiO2 film was found to be one order of magnitude higher than the ruthenium-based organometallic dye coded as N719 adsorbed on the same thick TiO2 film. Although these two initial works on perovskite solar cells [4, 5] attracted attention, relatively low PCE values and chemical instability of organic–inorganic hybrid perovskite in polar liquid electrolyte due to ionic characteristics were serious obstacle toward further progress of perovskite solar cell.
In 2012, Park et al. demonstrated a long-term durable high efficiency perovskite solar cell for the first time by replacing a liquid electrolytes with a solid hole-transporting material (HTM), which showed a PCE of 9.7 % at submicron thick TiO2 film covered with 2 nm-sized nano dot MAPbI3 . This solid-state perovskite solar cell confirmed 500 h stability even without encapsulation because nano dot MAPI3 was fully wrapped with hydrophobic spiro-MeOTAD HTM. Two month later, Snaith et al. reported solid-state perovskite solar cell with the same HTM but different oxide Al2O3, which demonstrated a PCE of 10.9 % . Contrary to the MAPbI3–TiO2 combination, electron injection is not expected from MAPbI3 to Al2O3 since the condition band position of Al2O3 is higher than that of MAPbI3. This implies that perovskite acts differently from the organic dye molecules requiring electron injection process.
2.1 Optio-electronic properties of organic lead halide perovskite
Opto-electronic properties of halide perovskites are primarily important in photovoltaics. Absorption coefficient of MAPbI3 was first estimated to be 1.5 × 104 cm−1 at 550 nm in the form of nanodots deposited in the mesoporous TiO2 film . Room temperature absorption coefficients of MAPbI3 and MAPbI3:Cl films were evaluated by several groups using UV–Vis absorbance data combining with the effect of reflection, spectroscopic ellipsometry considering polarized reflection and photothermal deflection spectroscopy [8–17], which was summarized by Green et al. . All the measured data showed that absorption coefficients range from 2.5 × 104 to 8.9 × 104 cm−1 at 620 nm. Among the methods for determining absorption coefficient, ellipsometry may be inaccurate at around band gap transition because high absorption coefficients were shown even below the band edge.
Since dielectric constant determines the magnitude of the coulomb interaction between electron–hole pairs and charge carriers as well as any fixed ionic charges in the lattice, high dielectric constants are required for high efficiency solar cell. Usually inorganic materials have higher dielectric constants than organic materials. Dielectric constant for MAPbI3 is in the range of 5–7 as can be seen in Fig. 2. Higher value for the relative dielectric constant of MAPbI3 was estimated to be about 18 from capacitive measurement . Low effective mass is also required for high efficiency solar cell since effective mass decreases as the carrier becomes more delocalized and its transport becomes more wavelike. Effective mass of electron and hole can be estimated by band structure.
Bandgap (Eg), exciton binding energy (R*), reduced effective mass (µ), effective dielectric constant (ε eff) for MAPbI3 and FAPbI3
E g (meV)
MAPbI3−x Cl x
Charge-carrier mobility plays important role in charge extraction to electrode. Charge-carrier mobility of FAPbI3 was estimated to be about 27 cm2/Vs as measured by THz photoconductivity transient  that is similar to the solution-process MAPbI3 . For the mixed-halide perovskite with fromamidinium cation, charge-carrier mobility was found to decrease as bromide content increases from y = 0 to y = 0.5 in FAPb(BryI1−y)3, significant drop to about 2 cm2/Vs at 0.3 < y < 0.5, but recover the mobility up to 14 for the tri-bromide of y = 1 , where very low carrier mobility found at 0.3 < y < 0.5 was related to amorphous phase.
Internal PL quantum yield (iQY) is important because it affects directly open-circuit voltage (Voc) and photovoltaic performance. The optically implied Voc, reflecting the maximum Voc that can be achieved purely based on the intrinsic material quality, assuming no optical losses nor losses caused by nonideal contact architectures, is defined as qVoc = Eg − T∆S − kT|ln iQY| , where q is the elementary charge, Eg is the band gap, k is the Boltzmann constant, T is the absolute temperature, and S is the entropy. The optically implied Voc was evaluated for MAPbI3−xBrx based on the illumination-intensity-dependent maximum iQY of 30 % for 0.1 < x < 1.4 and the entropy of 260 meV in the band gap range of 1.0–1.8 eV. The Voc deficit (Eg/q − Voc) was estimated to be about 400 mV at 1 sun up to a band gap of 1.97 eV, which means, for instance, electrical Voc of about 1.2 V can be expected for the band gap of 1.6 eV. The Voc deficit will be further reduced by about 60 mV at optimized carrier injection level .
2.2 Methodologies for fabricating high efficiency perovskite solar cells
2.2.1 Solution-processed two-step method
Two-step sequential deposition was first proposed by Mitzi et al. , where PbI2 was deposited on substrate prior to MAI treatment by either vacuum evaporation or spinning coating. The PbI2 coated substrate was dipped in MAI solution. Saturated methanol solution of PbI2 was used as precursor solution for spin-coating process. The PbI2 thin film was immersed in the 2-rpopanol solution containing MAI, which was followed by rinsing with 2-propanol. Dipping time will be crucial to the final product. This two-step method was applied to perovskite solar cell by Gratzel group . Similar procedure was performed, where PbI2 layer was formed on the mesoporous TiO2 film (average particle size of TiO2 was about 20 nm) by spin coating a PbI2 solution in N,N-dimethylformamide (DMF) at 70 °C. The dried PbI2 film was dipped in a solution of MAI in 2-propanol for 20 s. It was described that the best efficiency device was obtained from a slight modified method of prewetting of the PbI2 film by dipping in 2-propanol for one second prior to being dipped in the MAI solution. A certified PCE of 14.1 % was achieved using the two-step method. As mentioned previously, dipping process, such as dipping time and solution concentration, is crucial to the morphology and opto-electronic property of the final MAPbI3 film, associated with the device performance. Two-step spin-coating technology was then proposed to solve the problem occurred by dipping process.
Two-step deposition technique was found to be beneficial to fabrication of perovskite film at relatively high humidity condition. The substrate pre-heating process for PbI2 deposition in the two-step spin-coating procedure was found to be crucial to the final MAPbI3 morphology and photovoltaic performance, where infiltration of PbI2 in the mesoporous TiO2 film was better for the heated substrate than for the substrate without heating . PCE increased with increasing the substrate temperature from room temperature to 50 °C and then decreased upon further increasing temperature to 60 °C, exhibiting optimal substrate temperature of around 50 °C. The pre-heating method was found to be not suitable for one-step coating under high relative humidity environment. Humidity effect in fabrication process was examined, where the relative humidity less than 60 % was hard to affect the overall performance .
Modified two-step deposition methods were proposed. Vapor treatment of organic solvents such as toluene or chlorobenzene on the PbI2 films resulted in better photovoltaic performance because the increased grains size and surface area of the PbI2 layer provided better reaction sites for MAI . Interdiffusion method was proposed to fabricate MAPbI3 without thermal annealing, where a MAPbI3 film formed from the MAI/PbI2 bilayer film in air exposure for 30 min at relative humidity of about 30 % demonstrated a comparable performance to the thermally annealed perovskite . In the two-step process, instead of depositing PbI2 layer, PbO film was electrochemically deposited on a conductive substrate before reaction with MAI at 150 °C for 30 min. . A possible reaction mechanism was proposed as follows. The MAI is decomposed to CH3NH2 and HI at the elevated temperature at the initial stage, and the generated HI is reacted with the PbO to form PbI2. Finally the PbI2 is reacted with MAI to form MAPbI3. The equivalent amount of H2O generated during the conversion process from PbO to PbI2 was argued to have positive effect on the formation of the provskite layer. To prevent volume change in two-step sequential deposition method, a intermediate PbI2(DMSO)x was pre-deposited before treatment of organic ammonium halides, which led to a PCE more than 20 % .
2.2.2 Solution-processed single precursor and anti-solvent method
2.2.3 Solution-processed adduct method
The adduct-induced MAPbI3 layer showed flat surface with large grains. A device employing adduct-induced MAPbI3 demonstrated charge carrier mobility of 3.9 × 10−3 cm2/Vs (the value was measured by photo-CELIV, which was lower than the value (~30 cm2/Vs) obtained by THz method), which was one order of magnitude higher than that (3.2 × 10−4 cm2/Vs) of MAPbI3 prepared by a simple one-step method . Charge extraction characteristics was improved by the adduct method, which may be ascribed to better PL quantum yield. The best PCE of 19.7 % was achieved by using the adduct method.
2.2.4 Vacuum deposition method
Since it is difficult in adjusting stoichiometry in co-deposition vacuum process, stoichiometric control is important. Inductively coupled plasma mass spectrometry (ICP-MS) was used to get the quantitative I/Pb ratio, where omnidirectional MAI evaporation could be controlled using the chamber pressure and incorporated in the film through interaction with the unidirectionally evaporated PbI2 . I/Pb was linearly proportional to chamber pressure, from which a chamber pressure of 1.23 × 10−4 mbar and a perovskite deposition rate of 0.03 nm/s produced stoichiometric MAPbI3. It was noted that UV–Vis spectral feature and PL peak position were almost identical regardless I/Pb ratio, which indicates that UV–Vis and PL are limited to determine the stoichiometry of MAPbI3.
2.2.5 Combined method
3 Summary and outlook
In this review, opto-electronic properties of MAPbI3, FAPbI3 and perovskites with mixed halide anions were investigated. Refractive index, dielectric constant, effective mass and charge diffusion length are important parameters for light absorbing, charge transporting and collecting. MAPbI3 is close to n-type with longer electron diffusion length but FAPbI3 is close to p-type with longer hole diffusion length, which guides design of perovskite layout. Understanding fundamentals of perovskite materials play important role in achieving high efficiency perovskite solar cell. In viewpoint of performance, high quality perovskite layer plays crucial role in overall photovoltaic parameters. Minimizing non-radiative recombination is one of methods to get high quality perovskite layer. High Voc approaching band gap energy is expected if one can reach the theoretical Voc deficit by engineering perovskite layer with highest internal PL quantum yield. A PCE of about 25 % can be realized using MAPbI3 and/or FAPbI3 when Jsc, Voc and FF reach 24 mA/cm2, 1.26 V (Eg = 1.6 V and Voc deficit = 340 mV) and 0.83, respectively.
This work was supported by the National Research Foundation of Korea (NRF) Grants funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea under Contracts No. NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF-2015M1A2A2053004 (Climate Change Management Program), and NRF-2012M3A7B4049986 (Nano Material Technology Development Program).
The authors declare that they have no competing interests.
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