Current Density Mismatch in Perovskite Solar Cells

By Michael Saliba, L. Etgar. ACS Energy Letters, 2020

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Perovskite solar cells (PSCs) have sky-rocketed in recent years because of their high-performance, now at a certified 25.2%, as well as readily available fabrication methods and material components. They are now considered one of the main contenders to make a lasting impact in photovoltaics either as a stand-alone solution, for example on flexible substrates, or in multijunction architectures with established technologies such as silicon or CIGS. This tremendous potential has led to an unusually rapid development of perovskite materials by academic and industrial groups. This includes a coordinated effort for certified measurement protocols. For example, efficiency certifications of PSCs now consider the need for a stabilized power output because of the idiosyncratic ion migration in PSCs. Similarly, there is an effort toward the normalization of stability measurements to eventually form an industrial protocol. It is only logical to move toward standardization because it ensures comparability among research groups and industry. More binding measurement protocols also build credibility toward potential investors and are therefore a necessary precursor for industrialization. Another layer of normalizing the measurement of solar cells was recently introduced by several journals in the form of a checklist for reporting performance parameters of solar cell. Here it is typical, among a series of requirements, to report on the discrepancy between the short-circuit current density (Jsc) from external quantum efficiency (EQE) (Jsc,EQE) and current density-voltage (JV) (Jsc,JV) measurements. The EQE is a basic measurement for solar cells. It measures the conversion of an incident photon to an electron by the photovoltaic device and is in general a function of the photon wavelength. The inset of the EQE(λ) can provide the spectral regions which contribute to the photocurrent generation of the solar cell (see Figure 1). Its spectral shape reveals, for example, how efficiently an absorber material transforms light into current and may reveal loss mechanisms within the device stack. Experimentally, the EQE can be measured with a light source, often a Xe lamp, with a monochromator. The light source is used to illuminate the sample through an appropriate shadow mask. The device is kept at short-circuit condition, and the current through the device is measured with a source meter for each wavelength. The raw data of the current needs to be normalized to the number of incident photons, conducted by calibration of the setup with a reference solar cell with a known spectral response for each wavelength. The EQE is the spectral response of the solar cell; therefore, it can be used to calculate the Jsc of the cell under illumination.

Subfields: Electrical and Electronic Engineering, Materials Chemistry, Polymers and Plastics

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