Voltage Of Solar Cells Research Articles

On the effect of surface recombination in thin film solar cells, light emitting diodes and photodetectors

Radiative and non-radiative charge carrier recombination in thin-film diodes plays a key role in determining the efficiency of electronic devices made of next generation semiconductors such as organic, perovskite and nanocrystals. In this work, we show that lowering the bulk recombination does not necessarily result in enhanced performance metrics of electronic devices. From the perspective of charge carrier extraction and injection, the radiative limit of the open-circuit voltage of solar cells, noise current of photodetectors and lasing threshold of injection lasers cannot be improved if the contacts are not perfectly selective. A numerical drift-diffusion model is used to investigate the interplay between bulk recombination and surface recombination of minority carriers at the contacts in bipolar thin diode devices based on low-mobility semiconductors. The surface recombination becomes prominent in case of reduced bulk recombination strengths when non-selective contacts, i. e. contacts that are either metallic or have imperfect charge-selective interlayer, are employed. Finally, we derive analytical approximations for the case when diffusion-limited surface recombination of minority carriers at Ohmic contacts dominates the dark current. These results indicate that having perfectly selective contacts becomes crucial in systems with suppressed bulk recombination - a challenging requirement for future state-of-the-art thin-film solar cells, light-emitting devices and photodetectors made of next generation semiconductors.

Coevaporated Cd1-xMgxTe thin films for CdTe solar cells

Substantial improvement of the open-circuit voltage of thin film solar cells has been investigated by applying an electron reflector strategy. Cd1-xMgxTe has a strong potential as an electron reflector to keep minority carriers away from the CdTe back surface to reduce the back surface recombination. In this paper, SCAPS simulations were first used to investigate device performance for CdTe solar cells with the electron reflector layers. The theoretical results indicate that the ∼0.4 eV electron barrier height for CdTe solar cells with Cd1-xMgxTe (Eg∼1.85 eV) is sufficient to decrease the back surface recombination and improve device performance, especially the open-circuit voltage. Thus the variation of energy gap of the Cd1-xMgxTe thin films prepared by coevaporation as a function of x was investigated from the transmittance spectra. Then Cd1-xMgxTe (x∼0.3) thin films were used as the electron reflectors for the CdTe thin film solar cells. It is found that CdTe solar cells with Cd1-xMgxTe yielded open-circuit voltage of 804 mV and fill factor more than 70% after an 425 °C anneal, which is higher than those without Cd1-xMgxTe. The electron reflector in CdTe solar cells can effectively reduce the carrier surface recombination, thereby resulting in the increase of the fill factor and the open-circuit voltage.

Improving Carbon Nanotube‐Silicon Solar Cells by Solution Processable Metal Chlorides

Carbon nanotube‐silicon (CNT‐Si) solar cells have the potential of being an alternative to the expensive crystal Si solar cells; however, there are critical limitations, such as the relatively low efficiency, instability, and lack of systematic improvement techniques. Herein, by investigating 17 solution processable, nonprecious metal chlorides, it is shown that appropriate chlorides can significantly improve the behavior of CNT‐Si solar cells individually or as mixtures. The results reveal a close relationship between the standard electrode potential of redox couples and the open‐circuit voltage (VOC) of solar cells, in which high VOC values are usually generated from chlorides with larger potentials. Detailed studies on SnCl2 and a mixture of FeCl3/ZrCl4 reveal mechanisms including doping of CNTs (increase in work function) and acting as antireflection layers, resulting in a substantial increase of both VOC and short‐circuit current density (JSC). As a result, stable power conversion efficiencies of 14.8% and 16.2% in CNT‐Si solar cells are achieved by simply spin‐coating SnCl2 or ZrCl4/FeCl3, respectively. The family of metal chlorides provide many opportunities for overcoming limitations and developing high‐performance CNT‐Si solar cells toward practical applications.

Optical design considerations of rear-side dielectric for higher efficiency of PERC solar cells.

Higher reflectance of the rear-side dielectric stack, at the wavelength of the laser source used for ablation, reduces laser-induced damage and improves the open-circuit voltage of PERC silicon solar cells. The understanding of this correlation increases the working window of cost-effective nanosecond laser ablation of the rear-side dielectric for higher-efficiency industrial PERC-like solar cells.

Effect of seasonal variation on the performance of solar photovoltaic pumping system under field conditions

The present study was undertaken to analyse the performance of solar photovoltaic (SPV) pumping system parameters, under different seasons. The average flow rate in summer, monsoon, post-monsoon and winter season was 3.0, 2.17, 3.44 and 2.93 Lps, respectively. The average conversion efficiency in summer, monsoon, post-monsoon and winter season was 14.36, 15.13, 17.04 and 16.85%, respectively. The range of optimum panel temperature should be between 12–32°C at which conversion efficiency will be maximum varying from 18.39–21%. During smog the average solar irradiance and flow rate decreased from 828 W/m2 to 393 W/m2 and 3.09 Lps to 1.59 Lps, respectively, as compared to before smog conditions. The presence of clouds significantly affected/lowered down the current, RPM and flow rate but the temperature did not change much which reveals that temperature has minimum effect on the development of current and voltage in solar photovoltaic cells.

Tailored emission to boost open-circuit voltage in solar cells

Recently, a lot of research focus has been on how to make solar cells more efficient. One direction is to enhance the open-circuit voltage V oc by optimizing the emission of photons in the cell, where emission is a necessary loss process due to the reciprocity between absorption and emission of light. Here, we performed a Shockley-Queisser detailed balance analysis to predict the benefit of managing emitted photons in a single-junction solar cell. First, at low internal luminescence efficiency η int, non-radiative recombination dominates, and management of emitted photons plays negligible role for V oc. Similarly, for an external luminescence efficiency η ext < 10%, externally emitted photons play negligible role, and V oc is set either by non-radiative recombination; or parasitic absorption of internally emitted photons. For higher η ext, the V oc can be boosted, maximally by 15%, by restricting the external emission to match the incidence cone of the AM1.5D sun light spectrum. Such emission restriction corresponds to lower escape probability of internally emitted photons, enhances photon recycling, drops η ext, and actually makes the solar cell into a worse LED. Finally, for partly diffuse incident light, by restricting the angular emission for photons in a 130 nm wavelength range around the bandgap, we predict a maximum 14% relative boost in solar cell efficiency. The results of this paper are intended to serve as a general guideline on how to utilize emission-tuning possibilities to develop highly efficient photovoltaic devices.

Understanding the open circuit voltage in organic solar cells on the basis of a donor-acceptor abrupt (p-n++) heterojunction

By using electrical characterization and classical solid state semiconductor device theory, we demonstrate that the open circuit voltage (Voc) in organic solar cells based on non-intentional doped semiconductors is fundamentally limited by the built-in potential (Vbi) originated at a donor-acceptor abrupt (p-n++) heterojunction in case of selective contacts. Our analysis is validated using P3HT:PCBM devices fabricated in our research group. We also demonstrate that such a result can be generalized using data already reported in literature for fullerene-based solar cells. Finally, we show that the dependence of Voc on the device contacts can be understood in terms of the potential barriers formed by the Fermi level alignment of semiconductors at the heterojunction and at the Schottky junctions.

Surface passivation of perovskite film for efficient solar cells

In recent years, the power conversion efficiency of perovskite solar cells has increased to reach over 20%. Finding an effective means of defect passivation is thought to be a promising route for bringing further increases in the power conversion efficiency and the open-circuit voltage (VOC) of perovskite solar cells. Here, we report the use of an organic halide salt phenethylammonium iodide (PEAI) on HC(NH2)2–CH3NH3 mixed perovskite films for surface defect passivation. We find that PEAI can form on the perovskite surface and results in higher-efficiency cells by reducing the defects and suppressing non-radiative recombination. As a result, planar perovskite solar cells with a certificated efficiency of 23.32% (quasi-steady state) are obtained. In addition, a VOC as high as 1.18 V is achieved at the absorption threshold of 1.53 eV, which is 94.4% of the Shockley–Queisser limit VOC (1.25 V). Planar perovskite solar cells that have been passivated using the organic halide salt phenethylammonium iodide are shown to have suppressed non-radiative recombination and operate with a certified power conversion efficiency of 23.3%.

Chlorinated Polymers for Efficient Solar Cells with High Open Circuit Voltage: The Influence of Different Thiazole Numbers.

The chlorination strategy has gradually become a promising approach to improve the open circuit voltage (VOC ) in polymer solar cells. In this work, by using an efficient thiazole-induced strategy in a polymer backbone, three thieno[3,4-b]thiophene (TT)-based polymers-PBClTTz-0, PBClTTz-1, and PBClTTz-2-are designed and synthesized with a Cl-substituted benzodithiophene (BDT) moiety and a thiazole unit as a π spacer. As expected, all of the polymers show a desirable open circuit voltage (VOC ) of >0.94 V in the solar cells; specifically, the voltage can reach 1.01 V for polymer PBClTTz-2 with two thiazole moieties. In addition, due to the excellent surface morphology and weak recombination of the active layer, photovoltaic devices based on PBClTTz-1 with one thiazole unit exhibit the highest power conversion efficiency (PCE) of 8.42%, which is noticeably superior to the fluorinated analogue PBClTTz-0 (6.85%). This work reveals the influence of the thiazole unit in a quinoid polymer backbone and confirms that the Donor-Acceptor(π)-Quinoid strategy is a promising construction method in molecular design.

A ring fused N-annulated PDI non-fullerene acceptor for high open circuit voltage solar cells processed from non-halogenated solvents

We report the synthesis and characterization of an arene-fused N-annulated perylene diimide dimer (F-NPDI2) and evaluate the new material’s suitability to be used as a non-fullerene acceptor for organic solar cells. Photovoltaic devices were fabricated using the donor polymer PTB7-Th. A direct comparison of bulk heterojunction active layers processed from halogenated and non-halogenated solvent/additive systems was made. Use of the solvent 1,2,4-trimethylbenzene (TMB) and the solvent additive 1,4-dimethylnaphthalene (DMN) led to best power conversion efficiency of ca. 2.5%. Photovoltaic devices had high open circuit voltages upwards of 1.12 V, representing one of the highest for perylene diimide based systems. The active layer morphology and subsequent device performance was insensitive to the nature and volume of solvent additive used for processing and type of coating method (spin vs slot-die) making for a robust bulk heterojunction for large area organic solar cell applications.

Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells.

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium conceptsappear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.

Determining the output voltage of solar cell using mercury discharging lamp by changing vertical positions

Determining the output voltage of solar cell using mercury discharging lamp by changing vertical positions

Thermal-assisted Voc increase in an indenoindene-based non-fullerene solar system

An indenoindene-cored small molecule acceptor of 3-ethylrodanine terminals, IBR, was synthesized to achieve high open-circuit voltage (Voc) solar cells. Adopting PBDB-T as the donor and IBR as the acceptor, the as-cast bulk-heterojunction non-fullerene solar cells show a promising Voc of 0.88 V. After a thermal annealing process at 160 °C for 10 min, the PBDB-T:IBR solar cells exhibit an enhanced Voc up to 1.16 V, among the highest Voc value reported for single junction organic solar cells, with obtaining a power conversion efficiency of 6.66%. Detailed investigation indicates that this thermal-activated Voc increase is caused by the microstructural rearrangement within polymer:IBR domains, which diminishes morphological defects, contributes to reduced bimolecular recombination and results in small saturation dark current density of the device. The work demonstrates the potential of IBR as efficient molecular acceptor for high Voc devices of special applications on tandem or ternary solar cells.

2D-MoS2 nanosheets as effective hole transport materials for colloidal PbS quantum dot solar cells.

Herein, we demonstrate for the first time matrix-free deposition of two dimensional (2D) MoS2 nanosheets as an efficient hole transport layer (HTL) for colloidal lead sulfide (PbS) quantum dot (QD) solar cells. We have developed all-solution-processed n–p–p+ architecture solar cells where ZnO nanoparticles were used as an n-type window layer, a PbS QD layer acted as a light absorbing p-type layer and 2D-MoS2 nanosheets acted as a p+-type hole transport layer. The MoS2 nanosheets allow better interface with the PbS QD layers. The incorporation of the MoS2 hole transport layer leads to superior fill factor, higher open circuit voltage and better performance in colloidal PbS QD solar cells. These results show that one layer of MoS2 nanosheets improves the power conversion efficiency of the device from 0.92% for a hole transport material free device to 2.48%. The present work reveals the development of 2D-MoS2 nanosheets as a new hole transport layer for the fabrication of cost-effective, durable and efficient colloidal PbS quantum dot solar cells.

Overcoming intrinsic defects of the hole transport layer with optimized carbon nanorods for perovskite solar cells.

To overcome the intrinsic chemical-reduction-activity of highly p-doped PEDOT:PSS and improve the open-circuit voltage (Voc) of planar inverted perovskite solar cells, a kind of oxidized carbon nanorods (OCNRs) is developed by a ball-milling/chemical-oxidation method and incorporated into PEDOT:PSS hole transport layer (HTL). The incorporation of OCNRs can increase the work function of the PEDOT:PSS layer, which avoids the energy-level mismatch between the PEDOT:PSS HTL and the HOMO level of the CH3NH3PbI3 perovskite layer, leading to a relatively high Voc of 1.01 V (vs. 0.92 V for the PEDOT:PSS device). Moreover, the introduction of OCNRs into the PEDOT:PSS HTL increases the grain size and uniformity of the perovskite layer, accompanied by the improved charge transport ability. As a result, the fill factor of perovskite solar cells is increased from 75.4% to 81.7%, and the best power conversion efficiency of 19.02% is achieved.

Vapor transport deposited tin monosulfide for thin-film solar cells: effect of deposition temperature and duration

Proper control of the morphology and preferred orientation of the SnS absorber is crucial for increasing the open-circuit voltage of thin-film solar cells.

Energy alignment and recombination in perovskite solar cells: weighted influence on the open circuit voltage

In this work, we assess the possible reasons for the differences observed in open circuit voltage (VOC) in mixed cation perovskite solar cells when comparing four different hole transport materials (HTMs), namely TAE-1, TAE-3, TAE-4 and spiro-OMeTAD.

High open circuit voltages in pin-type perovskite solar cells through strontium addition

The incorporation of strontium into perovskite solar cells spontaneously modifies the surface increasing the Voc by reducing the non-radiative losses.

Influence of A-site cations on the open-circuit voltage of efficient perovskite solar cells: a case of rubidium and guanidinium additives

The effect of rubidium and guanidinium additives on the morphological, optoelectronic and photovoltaic properties of the state-of-the-art triple A-cation based PSCs is investigated.

Energy-level engineering of the electron transporting layer for improving open-circuit voltage in dye and perovskite-based solar cells

BaSnO3 is designed as an electron transport layer of high-efficiency perovskite and dye-sensitized solar cells by fine-tuning energy levels through substitution of specific amounts of Sr ions.