High open-circuit voltage in perovskite solar cells: The role of hole transport layer

doi:10.1016/j.orgel.2018.09.006

Organic Electronics

Volume 63, December 2018, Pages 104-108

https://doi.org/10.1016/j.orgel.2018.09.006 Get rights and content

Highlights

•
Lowering the energy level of HOMO of HTL with respect to the vacuum level increases V_oc of perovskite solar cells.
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Polaron formation decreases the bimolecular recombination coefficient in perovskite solar cells.
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Bimolecular recombination coefficient decreases exponentially with increasing temperature.

Abstract

Using the drift-diffusion model, a new expression for the open-circuit voltage ( $V_{o c}$ ) in perovskite solar cells is derived. The $V_{o c}$ increases with the ratio of the charge carrier mobilities ( $μ_{e} / μ_{h}$ ) and by lowering the HOMO energy level of the hole transport layer (HTL). Using the derived $V_{o c}$ , we have found an analytical expression for the bimolecular recombination coefficient which decreases exponentially with increasing temperature. We have shown that the recombination coefficient thus derived is reduced by the formation of polarons, which may be expected to increase the photocurrent and hence power conversion efficiency in perovskite solar cells.

Graphical abstract

Open-circuit voltage $V_{o c}$ of perovskite solar cells increases by lowering the HOMO energy of HTL and by increasing ratio of charge carrier mobilities $P = μ_{e} / μ_{h}$ .

Introduction

In recent years, organic-inorganic hybrid perovskite solar cells have attracted huge research interests due to the rapid increase in their power conversion efficiency (PCE) [1,2], which is very promising for commercialization in the near future [3]. The semiconducting properties of all-inorganic metal halide perovskites have been studied since the 1950s [4]. It was reported in the 1980s [5] that the alloy PbI₂: KI had a direct band gap between 1.4 and 2.2 eV which matched with the peak of the sun's spectrum and hence was predicted to be a suitable material for photovoltaic applications. It was not until 2009 when Kojima et al. [6] first used organic-inorganic lead halide perovskites CH₃NH₃PbX (X = Br₃ or I₃) in dye-sensitized liquid junction-type solar cells which achieved an appreciable PCE of 3.8% that much attention was paid to the development of solid state perovskite photovoltaics [7,8]. Organo-metal halide perovskites have general formula ABX₃, where A is an organic cation, B is a divalent metal cation, and X is a monovalent halide anion [9]. In less than a decade the efficiency of solar cells fabricated using such organo-metal halide perovskites as the active layer has increased from 3.8% to more than 20% [3,9]. Perovskite materials have high absorption coefficient, direct band gap and absorption of photons generate free electron hole pairs or excitons with small binding energies. Perovskite materials also have moderate (compared to conventional inorganic semiconductors) [10] and balanced charge carrier mobility, long charge carrier diffusion length and lifetime, reduced recombination, and ambipolar transport properties [9,11]. The above properties of perovskite materials make them unique materials for photovoltaic applications.

Although perovskite solar cells (PSCs) have achieved high PCE, the operation mechanism and device physics of PSCs are not yet clearly understood. To be able to further optimize the PCE of PSCs, it is important to gain a better understanding of material properties that influence the PCE [9,11]. Also, as recombination leads to the loss of photogenerated charge carriers in solar cells, understanding the mechanism of recombination in PSCs and the $V_{o c}$ related losses is needed in order to improve their photovoltaic performance [[12], [13], [14]].

One of the prominent photovoltaic parameters that has led to the high efficiency in PSCs is the open-circuit voltage ( $V_{o c}$ ) and there is the need to understand the origins of the $V_{o c}$ in PSCs [15]. The schematic layered structure of the perovskite solar cell considered in this study is shown in Fig. 1. For the structure given in Fig. 1, it is found that when an organic material with lower HOMO level is used as the hole transport layer (HTL) PSCs exhibit higher $V_{o c}$ [7,13,15,16].

In this work, we have derived an expression for $V_{o c}$ for a hybrid heterojunction-like PSC with the structure TiO₂ (Electron transport layer)/perovskite/hole transport layers (HTLs) as shown in Fig. 1 by optimizing the drift-diffusion current density. The $V_{o c}$ thus obtained depends explicitly on the electron and hole mobility ratio, effective energy gap between the conduction band of the perovskite absorber and the HOMO energy level of the hole transport layer (HTL). An analytical expression is also derived for the temperature-dependent bimolecular recombination coefficient. The calculated values of both $V_{o c}$ and the bimolecular recombination coefficient agree reasonably well with experimental results obtained in perovskite solar cells. The recombination expression shows that polaronic effects can help to reduced bimolecular recombination of charge carriers in perovskite solar cells.

Section snippets

Open-circuit voltage and bimolecular recombination coefficient of PSCs

Generally, the open-circuit voltage $(V_{o c})$ of an illuminated solar cell is defined by the energy difference between the quasi-Fermi level of the electron $(E_{F, e})$ in the electron extracting layer and the quasi-Fermi level of the hole $(E_{F, h})$ in the hole extracting layer as [17]: $- q V_{o c} = E_{F, e} - E_{F, h}$ where $q$ is the charge of an electron. For PSCs, the $V_{o c}$ can be written as [15]: $- q V_{o c} = E_{g} - φ_{n} - φ_{p} - Δ E_{l o s s}$ where $φ_{n} (φ_{p})$ is electron (hole) injection barrier height, $Δ E_{l o s s}$ represents energy losses in transporting

Results and discussions

The $V_{o c}$ of a solar cell is generated by the build-up of electrons in the n-type material and holes in the p-type material, resulting in the splitting of the quasi-Fermi levels for electrons and holes [2]. Using Eq. (4), we have calculated $V_{o c}$ for a few PSCs with different perovskite materials and HTLs listed in Table 1, along with the corresponding experimental results for comparison. According to Eq. (4), $V_{o c}$ of PSCs depend on three terms. The first is the effective band gap $E_{g}$ which can be

Conclusions

We have derived $V_{o c}$ for PSCs which is independent of the injection barrier contact potential but depends on the charge carrier mobility ratio. The higher $V_{o c}$ found in PSCs is attributed to the fact that energetic and transport losses are small due to their unique and intrinsic optical and electronic properties. Using HTL with lower HOMO level yields higher $V_{o c}$ and the formation of polarons may help to reduced bimolecular recombination. Our results are expected to assist in optimization of PCEs

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Citation Excerpt :
As shown in Table 1, the open circuit voltage (VOC) of devices without HTLs was similar to devices with PEDOT:PSS, since the VOC in PeSCs is primarily limited by the bandgap of the perovskite material used as the active layer, and the same active layer was used in both cases.[39] Other factors like charge carrier concentration and ambipolar transport properties coupled with the low-lying HOMO level of the HTL can also have significant impact on the VOC and fill factor (FF) of PeSCs,[40,41] We see that the FF and short circuit current density (JSC) were most strongly impacted by the lack of an HTL; the JSC, FF and PCE were very poor in the absence of any HTL. Without an HTL, the energy difference between the valence band of perovskite active layer and conduction band of ITO electrode was found to be approximately 0.8 eV, creating an energy barrier and non-Ohmic contact at the anode.
Interfacial layers are important tools that can be used to control the band structure and performance of perovskite solar cells, however, relatively few p-type materials are known compared to n-type interfacial materials. In this work, we demonstrate that anionic polystyrene sulfonate (PSS) polyelectrolytes, with organic cations as counter ions, are able to function as p-type interfacial layers in methylammonium lead triiodide (MAPbI₃) perovskite solar cells. These polyelectrolytes show interfacial dipole formation resulting in a significant increase in the work function of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and decrease in the Fermi energy of the CH₃NH₃PbI₃ layer (band bending) as evident from x-ray photoelectron spectra (XPS) and ultraviolet photoelectron spectra (UPS) analysis. These effects result in superior electron blocking and hole extracting characteristics at the anode. We show that open circuit voltage, power conversion efficiency and other device parameters can be increased compared to the devices without interfacial layers using these convenient polyelectrolytes.

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High open-circuit voltage in perovskite solar cells: The role of hole transport layer

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Open-circuit voltage and bimolecular recombination coefficient of PSCs

Results and discussions

Conclusions

Sol. Energy Mater.

Nat. Commun.

Science

ACS Energy Lett.

J. Chem. Phys.

J. Am. Chem. Soc.

Energy Environ. Sci.

J. Phys. Chem. Lett.

J. Photon. Energy

J. Phys. Chem. Lett.

Nano Lett.

Front. Nanosci. Nanotech

J. Phys. Chem. Lett.

J. Phys. Chem. Lett.

J. Appl. Phys.

ACS Energy Lett.