No abstract

External quantum efficiency and transient photocapacitance (TPC) spectra were obtained for perovskite solar cells with methylammonium lead triiodide perovskite absorbers formed by either dip or vapor conversion. These measurements reveal an extended band of sub-gap states in all of the devices studied. The defect band is best fit by a pair of defects, and the appearance of the defect signal in the transient photocapacitance spectra indicates that at least one of the observed defects is in the perovskite absorber. The cells with the largest density of defect states show the lowest short-circuit current density and open-circuit voltage for slow, quasi-steady-state, current density-voltage sweeps and the largest hysteresis in short-circuit current density for fast sweeps. This suggests that defect states in the perovskite absorber limit steady-state device performance, and that these defects or associated mobile charges play a role in the hysteresis observed in current density-voltage measurements.

We analytically calculate fundamental limits on the open-circuit voltage (V oc) of a solar cell imposed by contact selectivity and contact recombination. To do so, we consider a simple model consisting of only carrier generation in an absorber and charge transfer to its contacts enabling an algebraic solution for the relevant partial currents and the calculation of a contact-determined V oc. An expression for V oc is determined assuming the partial currents at the contacts linearly depend on the product of the appropriate equilibrium exchange current density and excess carrier concentration at the contact. Quantitatively defining contact selectivity and contact recombination, we illustrate the roles of the exchange current densities, recombination, and selectivity on V oc. Additionally, we use complete device physics simulations to show that our simplified model is valid in practically relevant situations. The framework we have developed elucidates the physics underlying more qualitative discussions of selectivity often invoked to describe the impact of contacts on V oc, thereby enabling better-motivated improvements in contact design.

ABSTRACT:The thin-film vapor-liquid-solid (TF-VLS) growth technique presents a promising route for high quality, scalable and cost-effective InP thin films for optoelectronic devices. Towards this goal, careful optimization of material properties and device performance is of utmost interest. Here, we show that exposure of polycrystalline Zn-doped TF-VLS InP to a hydrogen plasma (in the following referred to as hydrogenation) results in improved optoelectronic quality as well as lateral optoelectronic uniformity. A combination of low temperature photoluminescence and transient photocurrent spectroscopy were used to analyze the energy position and relative density of defect states before and after hydrogenation. Notably, hydrogenation reduces the relative intra-gap defect density by one order of magnitude. As a metric to monitor lateral optoelectronic uniformity of polycrystalline TF-VLS InP, photoluminescence and electron beam induced current mapping reveal homogenization of the grain versus grain boundary upon hydrogenation. At the device level, we measured more than 260 TF-VLS InP solar cells before and after hydrogenation to verify the improved optoelectronic properties. Hydrogenation increased the average open-circuit voltage (V OC ) of individual TF-VLS InP solar cells by up to 130 mV, and reduced the variance in V OC for the analyzed devices.

We demonstrate that an analytic relationship between coefficients in the Taylor expansion of the junction capacitance can be exploited to yield more precise determinations of carrier densities in drive level capacitance profiling (DLCP). Improvements are demonstrated on data generated according to the DLCP theory and in measurements performed on a CuInxGa1–xSe2 device. We argue that the improved DLCP method is especially important for non-uniform devices, which are more susceptible to noise in the capacitance data used in DLCP because they require that the amplitude of the drive level be restricted. Importantly, the analysis does not require the collection of any data other than what is typically collected during a DLCP measurement while employing fewer independent parameters than the model that is typically used in DLCP. Thus, we expect that it will be readily adoptable by those who perform DLCP measurements.