How Contact Layers Control Shunting Losses from Pinholes in Thin-Film Solar Cells

Abstract

An absorber layer that does not fully cover the substrate is a common issue for thin-film solar cells such as perovskites. However, models that describe the impact of pinholes on solar cell performance are scarce. Here, we demonstrate that certain combinations of contact layers suppress the negative impact of pinholes better than others. The absence of the absorber at a pinhole gives way to a direct electrical contact between the two semiconducting electron and hole transport layers. The key to understand how pinholes impact the solar cell performance is the resulting nonlinear diodelike behavior of the current across the interface between these two layers (commonly referred to as a shunt current). Based on experimentally obtained data that mimic the current−voltage characteristics across these interfaces, we develop a simple model to predict pinhole-induced solar cell performance deterioration. We investigate typical contact layer combinations such as TiO 2 /spiro-OMeTAD, poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate)/phenyl-C 61 -butyric acid methyl ester, and TiO 2 /poly(3-hexylthiophene). Our results directly apply to perovskite and other emerging inorganic thin-film solar cells, and the methodology is transferable to CIGS and CdTe. We find substantial differences between five commonly applied contact layer combinations and conclude that it is not sufficient to optimize the contact layers of any real-world thin-film solar cell only with regard to the applied absorber. Instead, in the context of laboratory and industrial fabrication, the tolerance against pinholes (i.e., the mitigation of shunt losses via existing pinholes) needs to be considered as an additional, important objective.

Copolymer‐Templated Nickel Oxide for High‐Efficiency Mesoscopic Perovskite Solar Cells in Inverted Architecture

“…As is known, pinholes on perovskite are shown to increase the shunting problem and decrease V oc . [ 46 ] Moreover, grain size and crystal structure of the perovskite film are other important parameters for achieving high‐performing devices, which are substantially affected by perovskite film formation. [ 47 ] The corresponding size distributions of perovskite grains, depicted in Figure 5c, display an average perovskite grain size of ≈213 ± 83 and ≈263 ± 83 nm for compact and bilayer NiO x films, respectively.…”

On the Origin of Dark Current in Organic Photodiodes

“…J d at −2 V varies by nearly five orders of magnitude depending on the polymer. It has previously been shown that extrinsic shunt paths caused by substrate defects, dust particles, or localized fluctuations in material properties can result in local increases in leakage currents and thus random variations in J d amongst nominally identical devices . By using relatively thick semiconductor layers (≈280 nm) such effects are reduced, leading to J d variation (at −2 V) of less than a factor of 2 within device batches (Figure S3, Supporting Information).…”

“…It has previously been shown that extrinsic shunt paths caused by substrate defects, dust particles, or localized fluctuations in material properties can result in local increases in leakage currents and thus random variations in J d amongst nominally identical devices. [24,25] By using relatively thick semiconductor layers (≈280 nm) such effects are reduced, leading to J d variation (at −2 V) of less than a factor of 2 within device batches ( Figure S3, Supporting Information). Although extrinsic shunt paths cannot be completely ruled out in these OPDs, their existence cannot explain the five orders of magnitude difference in J d across five different donor:acceptor blends given the comparatively minor intra-batch variation.…”

Functional ZnO/TiO2 Bilayer as Electron Transport Material for Solution‐Processed Sb2S3 Solar Cells

“…Moreover, decreasing the RMS improves the contact with the absorber material, which enhances the performance of planar solar cells. [ 23,28 ] Figure 1e shows the Raman spectra of the different ETM films. All spectra exhibit peaks in the range of 237–330 cm −1 , which are related to the FTO substrate.…”

“…[ 29,30 ] These Raman modes, characteristics of the ZnO crystal with wurtzite structure, agree well with the previous results obtained for ZnO thin films deposited by a sol–gel method. [ 23,28 ] The Raman spectrum of the TiO 2 film shows the E g mode corresponding to the anatase phase at 155 cm −1 . [ 9 ] For the ZnO/TiO 2 sample, the characteristic peaks of both wurtzite ZnO and anatase TiO 2 are present.…”