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We demonstrate a monolithic perovskite/CIGS tandem solar cell with a certified power conversion efficiency (PCE) of 24.2%. The tandem solar cell still exhibits photocurrent mismatch between the subcells thus optical simulations are used to determine the optimal device stack. Results reveal a high optical potential with the optimized device reaching a short-circuit current density of 19.9 mA cm$^{−2}$ and 32% PCE based on semiempirical material properties. To evaluate its energy yield, we first determine the CIGS temperature coefficient, which is at −0.38% K$^{−1}$ notably higher than the one from the perovskite subcell (−0.22% K$^{−1}$), favoring perovskite in the field operation at elevated cell temperatures. Both single-junction cells, however, are significantly outperformed by the combined tandem device. The enhancement in energy output is more than 50% in the case of CIGS single-junction device. The results demonstrate the high potential of perovskite/CIGS tandem solar cells, for which we describe optical guidelines toward 30% PCE.

Monolithic perovskite/silicon tandem solar cells recently surpass the efficiency of silicon single‐junction solar cells. Most tandem cells utilize >250 μm thick, planarized float‐zone (FZ) silicon, which is not compatible with commercial production using

Za zmanjševanje onesnaževanja okolja in omejitev globalnega segrevanja postajajo obnovljivi viri energije vedno pomembnejši. Še posebej svetlo prihodnost ima fotovoltaika, saj je sonce neusahljiv in brezplačen vir energije, ki je dostopen na celotni zemeljski obli. V primerjavi z ostalimi viri energije pridobivanje elektrike iz sončne energije ne povzroča nastajanja izpušnih plinov, ogljični odtis pa je tudi z vključeno proizvodnjo sončnih modulov vsaj dvajsetkrat manjši v primerjavi s fosilnimi gorivi. Optimizacija proizvodnih procesov je tudi znižala energijsko vračilno dobo sončnih elektrarn (PV sistemov), ki je za osrednjo Evropo padla na dve leti za polikristalne silicijeve module in tudi pod dve leti za tankoplastne sončne celice. Celo Kitajska, eden izmed največjih onesnaževalcev na svetu, je prepoznala potrebo po čistejšem okolju. Tako je poleg vodilne vloge v proizvodnji sončnih fotonapetostnih modulov, prevzela tudi vlogo največjega proizvajalca elektrike iz sončne energije. Njen pospešen vstop na fotovoltaični trg je povzročil drastičen padec cen in eksponentno rast kumulativne nameščene moči po svetu. V zadnjem letu smo v svetovnem merilu namestili za 70 GW sončnih elektrarn, skupna moč pa je presegla 300 GW. Potencial in prednosti fotovoltaike jo uvrščajo med najpomembnejše trajnostne energetske tehnologije na področju obnovljivih virov in tudi v energetiki nasploh. Osnovni fotonapetostni gradnik je polprevodniška sončna celica. Lastnosti polprevodniškega materiala, predvsem energijska reža, so tiste, ki najbolj vplivajo na učinkovitost pretvorbe sončne celice. Za enospojne sončne celice je teoretična limita (Shockley-Queisser limita) pri standardnih testnih pogojih 33,7 % za energijsko režo 1,34 eV. Na podlagi polprevodnika ločimo več tipov sončnih celic. Najpogostejše so kristalne silicijeve sončne celice (prva generacija), ki dosegajo visoko učinkovitost pretvorbe (do 26,3 %) in največji tržni delež (> 86 %) na svetovnem trgu. Druga generacija so tankoplastne sončne celice, kjer je aktivna plast debela le par mikrometrov, v primerjavi s 150 ali več μm pri kristalnih silicijevih sončnih celicah. Popularnost tankoplastnih sončnih celic temelji na pričakovanih nižjih stroških izdelave. Zaradi tanjših plasti potrebujemo manj materiala, proizvodni stroški pa so zaradi nizkotemperaturnih procesov nižji in energetsko učinkovitejši. Tipična predstavnika sta CIGS in CdTe, ki sta dosegla tudi masovno industrijsko proizvodnjo. Sončne celice tretje generacije (organske, elektrokemijske, perovskitne) obetajo še nižje proizvodne stroške, a industrijske proizvodnje še niso dosegle. So pa v zadnjem času v znanstveni sferi veliko pozornosti pritegnile perovskitne sončne celice, saj so v le nekaj letih raziskovanja dosegle učinkovitost pretvorbe do 22,1 %, kar je najhitrejši porast do sedaj. Kljub nizkim stroškom proizvodnje in visokim učinkovitostim pretvorbe, pa perovskitne sončne celice vseeno čaka še dolga pot. Predvsem stabilnost in majhna dimenzija do sedaj izdelanih celic predstavljata glavni oviri do množične proizvodnje, a hiter razvoj in obsežne raziskave dajejo upanje na industrijsko proizvodnjo ali primernost za nišne aplikacije. Upravljanje s svetlobo v tankoplastnih sončnih celicah Tankoplastne sončne celice predstavljajo nizkocenovno alternativo običajnim kristalnim silicijevim sončnim celicam, saj so za njihovo izdelavo potrebne nižje temperature, porabi se manj materiala, izdelamo pa lahko celo upogljive celice. V primerjavi s kristalnimi silicijevimi sončnimi celicami so tankoplastne tanjše, kar se v splošnem lahko odrazi v manjši absorpciji svetlobe. Ker je učinkovitost pretvorbe sončnih celic v veliki meri odvisna od absorpcije v aktivni plasti, je za povečanje absorpcije v tanjših absorpcijskih plasteh potrebno skrbno upravljanje svetlobe. Ena izmed najuspešnejših tehnik povečanja absorpcije svetlobe v tankoplastnih sončnih celicah je teksturiranje površine, bodisi na zgornji ali spodnji plasti celice. Teksturirane površine z nanometrsko ali mikrometrsko hrapavastjo povzročajo protiodbojni efekt ali sipanje svetlobe (velikost posameznih struktur enaka ali večja valovni dolžini), kar podaljša optično pot v aktivni plasti. S tem povečamo svetlobno generirani tok celice, in če se ne spremenijo električne lastnosti (napetost odprtih sponk VOC in polnilni faktor FF), tudi učinkovitost pretvorbe. Teksturirane plasti lahko vpeljemo na ali pa v strukturo sončne celice. Lahko na sprednjo stran stekla v superstrat konfiguraciji pritrdimo teksturirano folijo za upravljanje s svetlobo (LM folija) ter s tem zmanjšamo odbojnost in povzročimo sipanje svetlobe. Lahko pa teksture vpeljemo znotraj strukture. V tem primeru so teksturirane površine prozorni prevodni oksidi (TCO, nanešen na steklo v superstrat konfiguraciji) ali teksturirani substrati (teksturirani zadnji odbojnik na plastični ali kovinski foliji), lahko pa v strukturo vključimo tudi dodatne plasti. Za obetaven način vpeljave želenih tekstur v sončne celice se je v zadnjih letih izkazala tehnologija vtisa vnaprej pripravljenih nanostruktur v prozorne in na temperaturo odporne lake. Ena izmed takih replikacijskih tehnik je UV nanovtisna litografija (UV NIL). Vzorci (teksture) so narejeni z mehansko deformacijo viskoznih polimerov (lakov), v katere vtisnemo teksturirani kalup, čemur sledi strjevanje laka z UV svetlobo (UV NIL). V kombinaciji z nanosom TCO na vtisnjeno plast se lahko izognemo dragi izdelavi TCO z naravno ali z dodatno obdelavo dobljeno teksturo. S procesom UV NIL prenesemo hrapavost površine s kalupa na repliko, pri čemer se ohrani morfologija kalupa in posledično sipanje svetlobe. Sipano svetlobo lahko nato karakteriziramo s kotno odvisno spektroskopijo (ARS), ki nam omogoča določiti funkcijo kotne porazdelitve (ADF) prepuščene in odbite sipane svetlobe – kako je svetloba sipana pod različnimi koti. Tipično določimo ADF z goniometričnimi sistemi, ki pa so počasni in merijo odziv le v eni ravnini (1D). Primernejši so sistemi s kamero, kjer sipano svetlobo projiciramo na zaslon in jo nato zajamemo s kamero. To nam omogoča hitrejšo in popolnejšo analizo sipane svetlobe v 3D prostoru. V doktorski disertaciji smo se posvetili izdelavi in karakterizaciji plasti za upravljanje s svetlobo. Plasti smo izdelali s procesom UV NIL za replikacijo teksturiranih površin. Izdelane replike smo testirali in karakterizirali, med drugim tudi s sistemom za merjenje sipane svetlobe. V ta namen smo postavili dva sistema, enega z refleksijskim in enega s transmisijskim zaslonom. Delovanje in potencialne izboljšave z uporabo LM folije smo preverili na primeru perovskitnih sončnih celic. Delovanje brez in z LM folijo smo analizirali tako eksperimentalno kot tudi teoretično z optičnimi simulacijami. Light management is an important aspect of photovoltaics to assure efficient exploitation of solar energy and improve the efficiency of solar cells. Efficient light management is based on anti-reflection and light scattering. The former results in increased light in-coupling, and the latter prolongs the optical path in the active layer of a solar cell consequently, the conversion efficiency increases. Both effects are usually induced by textured surfaces, either within the device structure or on top of the device. In the doctoral dissertation, we focus on fabrication and characterization of textured light management layers. The created light management (LM) foil is then applied on top of perovskite solar cells to enhance the perofromance and analyze the improvements in the fabricated devices. The Ultra-violet Nanoimprint Lithography (UV NIL) is used to create the textured LM foils. It is a novel approach for replicating textured surfaces. The process is cost-effective, simple and faster compared to other texturization techniques. In the replication process, the texture surface from the master is transferred to the replica with the help of the intermediate stamp and the UV sensitive lacquers coated on the substrates. We present the replication process and thoroughly characterize the created replicas using surface morphology and transmittance measurements. Good transfer fidelity and moderate thermal stability up to 200 °C were obtained. During the thermal stability test the samples that were exposed to high temperature (200 °C) for a longer time (>30 min) turned slightly yellow. The yellowing effect resulted in diminished total and diffuse transmittance of light for the wavelengths below 500 nm. Similar effect is also observed during the outdoor testing where different configurations were tested, some samples were placed on a white and some on a black surface. After three-month exposure to outdoor environmental conditions the samples turned yellow. Samples on black surface heated more and the yellowing effect was severer than for the samples on white surface. Additionally, the lacquer slightly melted which is confirmed by lower σRMS values. This shows that replication lacquers with more stable chemical composition are needed for use in real outdoor applications. If the LM foil is used inside the device, an additional conductive layer of transparent conductive oxide (TCO) has to be deposited on top of the LM foil to form electrical contact as the UV NIL lacquers are non-conductive. In our case, a gallium doped ITO (GITO) is used as a TCO. The successfulness of the deposition is confirmed by sheet resistance, optical (transmission) and surface morphology measurements. To fulfill their role, the created replicas should preserve the light scattering properties of the master. For the light scattering characterization, a novel camera-based system is developed. It enables measurements of the spatial angular distribution function (3D ADF) of scattered or emitted light using a digital camera. 3D ADF is determined from the digital image captured from a flat screen. We present two solutions. The first uses a reflective screen and a lens to broaden the angular range. The second uses a transmissive screen positioned at 45°, enabling measurements of all the polar angles. With the developed camera-based systems we can quantify transmitted or reflected light scattered by textured samples or emitted light from light sources in a few seconds. In the dissertation, both setups are described, and main transformations of the acquired digital image to obtain the 3D ADF are explained. The systems are validated on randomly nanotextured transparent samples and a periodically textured non-transparent sample. Good matching is obtained with rigorous simulations, and measurement results carried out with the conventional goniometric angular resolved scattering system. The system with the transmissive screen is used to characterize the created replicas. To test the functionality of the created LM foils in a real application, inorganic-organic perovskites that have proven to be an effective class of materials for fabricating efficient solar cells are used. We apply an LM foil created by UV NIL on the glass side of an inverted (p-i-n) perovskite solar cell with 16.3% efficiency. The obtained 1 mA cm-2 increase in the short-circuit current translates to a relative improvement of 5% in cell performance, which results in a power conversion efficiency of 17.1%. To support the experimental findings, optical 3D simulations based on experimentally obtained parameters are used. A good match between the simulated and experimental data is obtained, validating the model. Optical simulations reveal that the main improvement in device performance is due to a reduction in total reflection and that relative improvement in the short-circuit current of up to 10% is possible for large-area devices. The optical model is also used to analyze the potential of monolithic perovskite/silicon-heterojunction tandem devices that can theoretically overcome the efficiencies of the single junction solar cells. We consider four different device designs in the optical simulations: the planar device, the device built on back- and both-side textured Si wafer, and the device with the textured LM foil. For each of these four designs, the current matching point is simulated to evaluate device efficiencies. The results reveal that the device built on a both-side textured silicon wafer, which is the best performing configuration, can reach 15% relative higher efficiency than planar device. The obtained results show the potential of LM foils for improving the device performance of perovskite solar cells and pave the way for further use of optical simulations in perovskite single junction or tandem solar cells.

Monolithic [Cs0.05(MA0. 17FA0. 83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon.

V diplomskem delu smo izvedli nadgradnjo merilnega sistema za merjenje spektralnega odziva in kvantnega izkoristka silicijskih, perovskitnih in tandemskih sončnih celic ter drugih optoelektronskih elementov. Diplomsko delo najprej opisuje teoretično ozadje svetlobe, osnovne principe in metode, ki jih uporabljamo za meritve z merilnim sistemom, ter sončne celice izmerjene v diplomskem delu. Sledi pregled sistema pred in po nadgradnji, kjer je podrobnejše opisana nadgradnja krmilnega programa v programskem okolju LabVIEW, implementacija predosvetlitvenega in napetostnega vira ter njuni programski in fizični izvedbi. Zadnji del diplomske naloge vsebuje praktične meritve prej omenjenih sončnih celic, uporabnost nadgradnje in končni izgled merilnega sistema po nadgradnji. In the scope of this thesis, we upgraded the measurement system for measur-ing the spectral response and quantum eficiency of solar cells, such as silicon, perovskite and tandem solar cells, and other optoelectronic devices. Firstly, the thesis describes the theoretical background of light, basic principals and methods, which are used for measurements with the measurement system, and the solar cells, which were measured within this thesis. Secondly, we describe a runthrough the system before and after the upgrade, where we focus on upgrades made to the control program, run in LabVIEW, the implementation of the light and voltage bias with their software and hardware executions. Lastly, in the thesis, we evaluate the practical measurements on the solar cells, mentioned previously, efficiency of the upgrade and a last look of the upgraded measurement system.

V magistrski nalogi smo raziskali vpliv dodanega MAPbBr3 k osnovni FACs perovskitni kompoziciji na posamezne fotovoltaične parametre sončne celice ter na njihovo dolgoročno in temperaturno stabilnost. Perovskitne kompozicije brez metilamonija (MA), na primer t.i. FACs kompozicija, se v zadnjem času pogosteje uporabljajo zaradi večje temperaturne stabilnosti. Zaradi hlapenja pri relativno nizkih temperaturah povzroča MA težave pri postopku enkapsulacije in negativno vpliva na dolgoročne stabilnosti sončnih celic. Po drugi strani pa dodatek MA h kompoziciji perovskita omogoča lažjo formacijo filmov, posledica česar je višji PCE. Z višanjem deleža MA narašča tudi širina energijske reže, kar lahko koristno uporabimo za izdelavo tandemskih celic v kombinaciji s silicijem. Zaradi teh pozitivnih lastnosti MA je vredno podrobneje raziskati, v kolikšni meri le-te še pretehtajo nad njegovimi slabostmi. Za namen magistrske naloge smo izdelali dve seriji celic. V prvi smo ugotavljali vpliv dveh različnih anti-topil, uporabljenih v postopku spinskega nanašanja perovskita, v drugi pa vpliv različnih količin dodanega MAPbBr3 na lastnosti celice. V pomoč pri meritvah in obdelavi rezultatov smo uporabili različne merilne sisteme za karakterizacijo celic, razvili pa smo tudi merilni sistem za meritve suns-VOC. Za vsako kompozicijo smo izdelali visoko učinkovite perovskitne sončne celice, z učinkovitostjo pretvorbe (PCE) nad 19 %. V povprečju smo najvišje PCE in faktorje polnjenja (FF) dobili pri celicah z 0-% in 6-% dodatkom MAPbBr3, čeprav imajo tudi pri ostalih dveh kompozicijah nekatere celice PCE nad 19 % in FF nad 75 %. Rezultati I-U meritev so pokazali tudi, da z dodanim MAPbBr3 Voc narašča, Jsc pa upada. Iz rezultatov meritev EQE, fotoluminiscence in transmisije smo potrdili, da se z višanjem deleža MAPbBr3 energijska reža perovskita širi. Za kompozicijo z 10-% deležem MAPbBr3 smo pomerili energijsko režo 1.69 eV, kar je optimalno za perovskitno-silicijeve tandemske sončne celice. Iz meritev suns-VOC smo določili faktor idealnosti n, nato pa smo celice podvrgli še temperaturni obremenitvi in izvedli sledenje maksimalni točki moči (MPP). Ugotovili smo, da celice z višjim deležem MAPbBr3 hitreje degradirajo, sploh v primeru 10-% deleža. V ostalih primerih to ni tako izrazito, še zmeraj pa je razviden trend upada PV parametrov. V magistrskem delu smo tako določili, da je kljub dobrim rezultatom z različnimi dodatki MAPbBr3 še vedno najboljša kompozicija FACs zaradi njene boljše temperaturne stabilnosti. In this thesis, we investigated the influence of MAPbBr3 added to a basic FACs perovskite composition on the PV parameters of a solar cell, and on their long-term and temperature stability. Methylammonium-free (MA) perovskite solar cells have recently gained popularity due to their higher thermal stability. Due to its volatility at relatively low temperatures, MA causes problems in the encapsulation process and has a negative impact on the long-term stability of the solar cells. On the other hand, the addition of MA to the perovskite composition allows for easier film formation, resulting in a higher PCE. As the amount of MA increases, the energy gap also increases, which can be exploited for the fabrication of tandem cells in combination with silicon. All of these usefull properties of MA make it worth investigating further to see to what extent they outweigh its disadvantages. For the purpose of this thesis, two batches of cells have been fabricated. In the first one, the influence of two different anti-solvents used in the spin coating process of perovskite was investigated, and in the second one the influence of different amounts of MAPbBr3 added on the cell properties was determined. Different measurement systems were used for cell characterisation. For each composition, we fabricated highly efficient devices with power conversion efficiencies above 19%. On average, the highest PCE and FF were obtained in cells with 0% and 6% quantity of MAPbBr3, although some cells of other two compositions also mediate PCE and FF above 19% and 75% respectively. The results of the I-V measurements also showed that VOC increases while JSC decreases with the addition of MAPbBr3. From the results of EQE, photoluminescence and transmission measurements, we confirmed that the energy gap widens with increasing MAPbBr3 content. The bandgap of FACs with 10% MAPbBr3 is 1.69 eV, which is otpimal for perovskite/Si tandems. From the Sun-VOC measurements, we determined the ideality factor n. For the final measurements we subjected the cells to high temperature and maximum power point (MPP) tracking. We found that cells with higher MAPbBr3 content degrade more rapidly, especially when the content is 10%. In other cases this is not so pronounced, but there is still a trend towards a decrease in PV parameters. In the master thesis, we have therefore determined that despite overall good results, MA-free FACs is still the best composition due to its better thermal stability.